Unveiling air-induced surface transformation on atomic step-engineered sapphire for oriented growth of transition metal dichalcogenides

Unveiling air-induced surface transformation on atomic step-engineered sapphire for oriented growth of transition metal dichalcogenides | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Unveiling air-induced surface transformation on atomic step-engineered sapphire for oriented growth of transition metal dichalcogenides Wei Fu, Jian Wei Chai, Hiroyo Kawai, Thathsara Maddumapatabandi, and 13 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5529369/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Engineering sapphire substrates with specific surface characteristics is crucial for the epitaxial growth of high-quality wafer-scale transition metal dichalcogenides (TMDs), essential for integration with semiconductor industry processes. Here, we address the gap in understanding the air-induced structure transformation of such engineered sapphire surfaces and their impact on the epitaxial growth of WS 2 . We identified a deactivation pathway for fresh atomic-step-engineered sapphire (α-Al 2 O 3 ) when exposed to air, leading to surface hydrolysis and the formation of aluminum (oxy)hydroxides preferentially along the atomic step edges. These compounds subsequently pyrolyze into oxygen-deficient Al 2 O 3-x under growth conditions. The presence of such an oxygen-deficient non-stoichiometric phase, apart from pristine α-Al 2 O 3 , disrupts the alignment of growth domains by altering the surface chemistry. Additionally, we demonstrate that UV-light irradiation in air prior to growth effectively repairs degraded sapphire surfaces by removing hydroxyls from aluminum oxyhydroxide, forming amorphous stoichiometric Al 2 O 3 , which then transforms into the desired α-Al 2 O 3 phase under growth temperature of 900 o C, favoring orientation-controlled epitaxy. These findings relevant insights toward the consistent production of industrial-scale, high-quality TMD films. Physical sciences/Materials science Physical sciences/Nanoscience and technology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction The scalable epitaxy of single-crystal two-dimensional (2D) transition metal dichalcogenides (TMDs) on insulating substrates that are compatible with semiconductor industry will enable the mass production of next-generation 2D electronics based on TMDs. 1 – 6 Among various growth substrates, c-plane sapphire has emerged as a promising option for TMD epitaxy due to its crystallographic compatibility and its chemical and thermal resilience under typical growth conditions. 7 , 8 In the past few years, significant advances have been made in wafer-scale single-crystal TMD synthesis through strategic engineering of the sapphire surface. This includes optimizing miscut orientation towards the a-axis 8 , creating vicinal a-plane surfaces 9 , developing immature step edges for enhanced nucleation 10 , reconstructing for single atomic plane exposure 7 , and modulating the surface chemistry through controlled growth conditions 11 . Sapphire has the α-phase hexagonal lattice structure, with in-plane unit cell dimensions a = b = 4.76 Å and an out-of-plane dimension c = 12.996 Å, characterized by six layers of alternating Al–O–Al atoms terminating in Al. 12 – 15 The normal termination of clean α-Al 2 O 3 consists of one Al atom for every three O atoms, with the terminal Al above the plane of the close-packed O. 16 Surface atomic features, such as step terrace terminations and step edges orientation have been utilized to modulate the coupling interaction between TMD crystals and a suitably engineered sapphire substrate for achieving well-resolved domain alignment during epitaxial growth. 7 , 8 , 10 , 17 However, in most conventional processes, engineered sapphire substrates are inevitably exposed to air, which may introduce variability in the epitaxial growth. Reports on the structural changes of engineered sapphire exposed to air and its impact on subsequent epitaxial growth remain limited. The α-Al 2 O 3 surface has been shown to react with H 2 O to form aluminum (oxy)hydroxide in carefully controlled experimental as well as theoretical studies. 18 – 22 During the thermal heating at temperatures ranging from 700 o C to 1000 o C, similar to those of typical epitaxial growth conditions, the pyrolysis of aluminum (oxy)hydroxide occurs resulting in oxygen vacancy-rich Al 2 O 3 − x . 23, 24 This process changes the surface chemistry and can significantly impact the alignment of crystal domains during epitaxy. A complete understanding of the atomic step engineered sapphire surface degradation mechanism upon exposure to air, and strategies for limiting/repairing this degradation are crucial for achieving consistent, high-quality TMD epitaxy. Here, we report a systematic study of the impact of air exposure on sapphire substrates and the consequence on the domain orientation of a prototypical 2D monolayer-WS 2 . Utilizing differential charging-enhanced surface analysis, we found significant changes in the surface chemistry of atomic-step-engineered sapphire when exposed to air, which reduced domain alignment during epitaxial growth of the WS 2 monolayers by chemical vapor deposition (CVD), as illustrated in Fig. 1 a. Our characterization indicates that surface hydrolysis occurs during air exposure, resulting in the formation of aluminum (oxy)hydroxide, as shown in Fig. 1 b. This phase subsequently decomposes into oxygen-deficient Al 2 O 3 − x during CVD growth, adversely compromising the WS 2 domain orientation. We also found that increased atomic step heights on engineered sapphire accelerate this surface hydrolysis process. Notably, our experiments demonstrate that UV light irradiation of air exposed sapphire surfaces effectively removes the hydroxyl groups and restores the degraded sapphire surface to crystalline α-Al 2 O 3 during the growth stage. Our findings of deactivation of sapphire by air exposure and its subsequent restoration through UV irradiation provide valuable insights for the commercial production of high-quality 2D materials. Result We selected the commercially available and commonly used c-plane sapphire, cleaved along a \(\:\) direction as our model substrate. Through a custom thermal annealing process (see details in experimental section), the sapphire surface was engineered with atomic steps along m \(\:\) orientation and terminated of half-Al surfaces (see Figure S1). The surface profile of the freshly annealed sapphire measured by atomic form microscopy (AFM), is shown in Fig. 1 c and Figure S2 (Supplementary Section 1). These images display a regular and uniform arrangement of surface atomic steps along the m \(\:\:\) direction with step height of ~0.25 nm (See inset in Figure S2c). The Fig. 1 e shows the cross-section scanning transmission electron microscope (STEM) image of freshly annealed sapphire, viewed from a \(\:\) direction, exhibiting the sharp atomic profile on surface. 8 To elucidate the structural transformations of sapphire surfaces exposed to air, freshly engineered sapphire samples were aged in a cleanroom environment (class 10K, 45% humidity) for varying durations. Figure 1 d is an AFM image of the sapphire surface after aging for 6 weeks, showing the formation of distinct clusters along the edges of atomic surface steps. Previous work noted that surface hydrolysis can occur on the sapphire surfaces when exposed to humid air. 18 Assuming that these clusters are the product of that hydrolysis, our observation could indicate that hydrolysis preferentially targets the engineered atomic step edges of sapphire. In contrast, the AFM images of sapphire aged for shorter periods of 1 ~ 2 weeks (see Figure S3) show no discernible morphological changes, indicating that the early-stage structural transformations were not detectable at the sensitivity level of the AFM system used. To detect these early-stage changes, we therefore used cross-sectional STEM analysis of sapphire surfaces aged for 2, 4 and 6 weeks respectively. Figures 1 e, f, and g show the main morphological change from a fresh sapphire surface to one aged for 6 weeks, revealing the development of an amorphous layer due to air exposure, with thickness ranging from 1 to 4 nm. A more detailed chronological sequence is shown in Figure S4 together with a schematic of the morphological evolution of the sapphire surface under air exposure depicted in Figure S5. This evolution involves the initial structural transformation of an amorphous layer on the atomic step edges, which is subsequently followed by the formation of cluster particles during the surface hydrolysis. X-ray Photoelectron Spectroscopy (XPS) was employed to elucidate the impact of air exposure on the surface chemistry of engineered sapphire. Considering that sapphire is an insulator, it may accumulate a net positive charge due to photoelectron loss when subjected to X-ray irradiation. 25 To counteract this, a low-voltage electron flood gun was utilized to provide charge compensation by directing an electron beam onto the surface. The electron flux intensity was regulated by adjusting the filament current of the electron flood gun. The adventitious carbon C 1s spectrum is recorded and used as internal reference for bias-referencing. 25 High electron flux intensity settings (detailed in Table S1) were used to analyze the Al 2p and O1s XPS spectra of both freshly annealed and 1-week aged sapphire, shown in Fig. 1 h and 1 i, respectively, along with the peak fitting results in Table S2. The high-resolution Al 2p and O 1s spectra of freshly annealed sapphire (Fig. 1 h and Figure S6) revealed an Al 2p peak at 74.1 eV and a primary O 1s peak at 531 eV, indicative of O-Al-O bonding in the α-Al 2 O 3 . 26 An additional O 1s peak at 532.4 eV was observed, attributed to physically adsorbed H 2 O or dissociated OH − radicals on the surface. 27 After 1 week of air exposure, peak deconvolution of the Al 2p and O 1s spectra (Fig. 1 i) revealed the emergence of additional Al-OH bonding peaks, located at 531.4 eV for O 1s and 74.1 eV for Al 2p, attributed to the formation of hydrolysis-induced aluminum (oxy)hydroxide. 26 The slight shifts in Al 2p and O 1s binding energies between the sapphire and its hydrolyzed phases, similar to gibbsite and boehmite (Figure S7), are minimal -around 0.1–0.4 eV (in Table S2), closely approaching the resolution limits of XPS, which poses challenges for analysis. Given that the hydrolyzed phase appears to preferentially accumulate at the atomic step edges of engineered sapphire (in AFM image in Fig. 1 d), and considering the distinct electrical properties of the surface sapphire and the surface hydrolyzed phase, we introduced a planar differential charging effect by optimizing the electron flux intensity through the control of the flood gun filament current. The setup is depicted in Fig. 2 a, where the filament current of the electron flood gun (shown in Fig. 2 b) was set to operate in three distinct modes: no electron flux (i, NEF), low electron flux (ii, LEF) and high electron flux (iii, HEF) (the detailed setting in Table S1). As illustrated in Fig. 2 c, when the electron flux is precisely adjusted slightly beyond what is needed to neutralize the charge during XPS testing (under LEF mode), the excess electrons differentially distribute between the surface sapphire and the hydrolyzed phase due to their differing electrical capacitances. This results in a differential charging effect, creating a larger negative electric field on the hydrolyzed AlO x (OH) y phase. This phenomenon broadens the gap in binding energy shifts for Al 2p and O 1s peaks between the sapphire and hydrolyzed AlO x (OH) y phases, enhancing the clarity and analytical distinction between these two phases in XPS spectra. We firstly conducted the XPS measurement on freshly annealed sapphire under three testing modes: NEF, LEF and HEF. The angle-resolved XPS spectra of Al 2p, O 1s, C 1s of freshly annealed sapphire under NEF, LEF and HEF modes and the detailed analysis are presented in Supplementary section 2. No obvious differential charging effect was observed, indicating that the XPS setup is well-configured and does not contribute to differential charging phenomena. After 2 weeks of air exposure, our XPS survey scan of the sapphire sample (refer to Figure S11) detected only aluminum, oxygen, and adventitious carbon confirming no additional surface contaminants. The detailed angle-resolved XPS spectra of Al 2p, O 1s, and C 1s at three testing modes of NEF, LEF and HEF, as well as the analysis are shown in supplementary section 3. Figure 2 d displayed the XPS spectra obtained at 30 o . In LEF, the binding energies of the O 1s and Al 2p peaks, corresponding to Al-O bonding in α-Al 2 O 3 , are 530.8 eV and 73.9 eV, respectively, aligning with those measured in HEF mode. Notably, distinct low-energy shoulders are observed in both the O 1s and Al 2p peaks, located at 72.3 eV and 529 eV, respectively. The angle-resolved spectra and a summary of the fitting are given in supplementary section 3, revealing an increased ratio of the peak area compared to that of bulk α-Al 2 O 3 as the testing angle increases. This suggests that these additional peaks are indicative of a surface-sensitive phase. Based on the findings from our AFM and cross-sectional STEM experiments, which revealed that air exposure leads to surface hydrolysis and the formation of surface aluminium (oxy)hydroxide, we attribute these observed peaks to Al-OH bonding in surface aluminium (oxy)hydroxide. The ratio of the O1s peak area for Al-O bonding to Al-OH bonding is calculated as 0.167, similar to those observed in NEF and HEF modes. The binding energies for Al-OH in O 1s shift from 531.1 eV in HEF mode to 529 eV in LEF mode, a decrease of 2.1 eV, while the Al 2p peak shifts from 74.1 eV to 72.3 eV, a decrease of 1.8 eV. The binding energies for Al-O bonding in α-Al 2 O 3 remain unchanged across HEF and LEF modes. We attribute these differential shifts in binding energies between α-Al 2 O 3 and AlO x (OH) y in HEF and LEF modes to an enhanced differential charging effect during testing. Considering that this effect was not observed in freshly annealed sapphire but only after two weeks of air exposure, which leads to the formation of AlO x (OH) y clusters on the sapphire surface, we propose that this differential charging is due to the distinct dielectric properties of AlO x (OH) y clusters on the sapphire surface. In LEF mode, with a lower electron flux, more electrons accumulate at the AlO x (OH) y phase due to its greater capacitive nature compared to sapphire, resulting in a more pronounced negative electric field at the AlO x (OH) y phase, which reduces the binding energies of Al-OH in the Al 2p and O 1s peaks. The differential charging effect under low electron flux (LEF) mode proves to be advantageous in distinguishing the chemical structure of mixed phases on insulating surfaces that exhibit similar bonding energies. By selectively shifting the phase with higher dielectric constant to lower binding energies during the XPS measurement, this technique enhances the detection and analysis of subtle structural changes on insulating surfaces. Utilizing this approach, we investigated the air exposure-dependent evolution of the surface phase on the sapphire and found that the sapphire surface undergoes progressive hydrolyzation into aluminium (oxy)hydroxide—from 11% in the first week to 67% after eight weeks. The detailed angle-resolved XPS spectra and a summary of the fitting are shown in Supplementary section 4. Furthermore, we evaluated the air-induced structural transformations on sapphire surfaces engineered with different atomic step heights of 0.2 nm and 1.0 nm and found that the higher the atomic step height on sapphire surfaces, the more pronounced the structural transforms triggered by hydrolysis. The detailed angle-resolved XPS spectra and summary of fitting are shown in Supplementary section 5. The hydrolysis of α-Al 2 O 3 in air, resulting in the formation of amorphous Al 2 O 3 and AlO x (OH) y , significantly influences the surface chemistry of α-Al 2 O 3 during the epitaxial growth of TMD atomic layers. Typically, the epitaxial growth of TMDs occurs at temperatures ranging from 700 to 1000°C in an oxygen-free environment. 11 , 27 – 30 This thermal heating during the growth process can substantially alter the surface structure of sapphire, thereby compromising the controllability of the epitaxy growth. To explore the surface structural changes of aged sapphire under epitaxial growth conditions, we selected a two-week-aged sapphire sample with 21% surface coverage of AlO x (OH) y as a model system. This sample was placed in a tube furnace and subjected to standard growth conditions, including specified temperature settings and gas flow, but without introducing growth precursors (detailed recipe in Figure S28). The detailed XPS analysis of pre-growth heating induced structural transformation of aged sapphire are shown in supplementary section 6. After post-growth heating at 900 o C, the Al 2p and O 1s peaks showed a sharpening corresponding to Al-O bonding in Al 2 O 3 , indicative of a phase transformation from amorphous to crystalline α-Al 2 O 3 at 900 o C, using the underlying α-Al 2 O 3 substrate as a crystalline template. 31 – 33 Moreover, we observed the disappearance of the characteristic AlO x (OH) y peaks (Al 2p at 72.3 eV and O 1s at 529 eV) and a significant increase in the peak area of high-energy O 1s peaks at 534.2 eV which we attributed to surface OH − or H 2 O bonding. 23 These changes suggest the pyrolysis of the AlO x (OH) y phase during the thermal process, leading to the formation of an oxygen-vacancy-rich Al 2 O 3 − x phase, which enhances surface H 2 O adsorption or hydroxylation upon air exposure during sample transfer. Previous studies indicate that heat-induced dehydroxylation of aluminium (oxy)hydroxide, such as Al(OH) 3 , leads to the formation of non-stoichiometric AlO x clusters with abundant oxygen vacancies when heated between 500 o C and 1000 o C. 23 , 34 , 35 These oxygen vacancies on the α-Al 2 O 3 surface can induce energetically favorable surface reconstructions, leading to the formation of various Al-rich surface structures distinct from the original crystalline α-Al₂O₃ surface. 12 , 13 , 36 The increase in the Al/O ratio from 0.57 to 0.76 before and after pre-growth heating at 900 o C, as shown in Figure S30, further supports the formation of an oxygen-vacancy-rich or Al-rich layer on the α-Al 2 O 3 surface. UV irradiation is a well-established surface cleaning method, recognized for its effectiveness in removing organic contaminants and its suitability as a dry-cleaning technique. 37 , 38 Previous research has shown that UV exposure significantly reduces Al-OH bonds in ALD-grown Al₂O₃ through a photo-induced electron transfer process. This involves electrons moving from deeper to shallower traps, accompanied by redox reactions that disrupt and create chemical bonds. 39 , 40 Considering that the transformation of α-Al 2 O 3 surfaces in air primarily results from the accumulation of hydroxyl-rich phases such as AlOOH or Al(OH) 3 , we explored the use of UV light irradiation to remove the aluminium oxyhydroxide layers from aged Al 2 O 3 surfaces. To investigate the effects of UV irradiation on air-exposed sapphire substrates, we selected a sapphire sample with 2 weeks of air exposure as a model. This sample was subjected to UV irradiation (in air, λ = 320 to 400 nm, power density of 0.5 W/cm²) for 30 minutes, as illustrated in Fig. 3 a with the setup given in Figure S31. Subsequent XPS analysis, presented in Supplementary section 7, revealed notable structural transformations induced by UV irradiation. In Fig. 3 b and Figure S32, we observed the disappearance of characteristic AlO x (OH) y peaks at Al 2p 72.3 eV and O 1s 529 eV, alongside a notable broadening at Al 2p 74 eV and O 1s 531 eV, corresponding to Al-O bonding in Al 2 O 3 . These changes suggest a transformation into an amorphous Al 2 O 3 phase. These results confirm that UV light irradiation effectively decomposes the surface AlO x (OH) y phase into amorphous Al 2 O 3 , as depicted in Fig. 3 a. To investigate the surface transformation of the above UV irradiated sample under growth, we put that sample in the pre-growth heating process. In Fig. 3 b, the Al 2p and O 1s peaks after this pre-growth heating exhibited a noticeable sharpening, with the FWHM reduced to 1.19 and 1.25 eV, respectively, closely matching the 1.15 and 1.22 eV. observed in freshly annealed sapphire. The UV irradiated sample displayed a much weaker peak, in contrast with the prominent O 1s peak at 523.4 eV that is typically associated with OH- or H 2 O bonding and indicative of rich oxygen vacancies in the sapphire sample after pre-growth heating of 2-week aged sapphire without UV irradiation. This suggests a reduction in oxygen vacancies for the UV irradiates sample, aligning the spectra characteristics more closely with those of freshly annealed sapphire. As illustrated in Fig. 3 a, UV irradiation before the growth process significantly restores the surface of degraded sapphire by removing AlO x (OH) y , reverting it to a structure similar to that of freshly annealed sapphire in the pre-growth process. This restoration is further supported by the Reflection High-Energy Electron Diffraction (RHEED) diffraction patterns of the 2-week aged sapphire and the repaired sapphire after 2-week aging, UV irradiation and pre-growth heating process. In Fig. 3 c, the aged Al₂O₃ sample initially shows a bright background with a blurred diffraction pattern, indicative of an amorphous layer on the α-Al₂O₃ surface. 41 In contrast, following UV exposure and pre-growth heating, the sample displays distinct and sharp RHEED stripes characteristic of crystalline α-Al₂O₃, as confirmed by the intensity profile comparison in Fig. 3 d. The RHEED pattern of the repaired sapphire closely matches that of freshly annealed sapphire (shown in Figure S33), providing evidence of effectiveness of UV light in restoring the degradation of the sapphire surface. Our investigation into the surface chemistry of sapphire under air exposure reveals that, after air exposure without UV irradiation, the sapphire surface exhibits a significant presence of oxygen vacancies: i.e. an Al-rich layer on the α-Al 2 O 3 surface. We hypothesize that the balance between strain in the Al-rich adlayer and the preference for Al atoms to occupy specific sites on the α-Al 2 O 3 substrate drives the formation of a \(\:\sqrt{31}\) x \(\:\sqrt{31}\) R9°-Al 2 O 3 (0001) surface, consisting of a single well-ordered overlayer of surface Al atoms on bulk Al 2 O. 42 As illustrated in Figure S34, the \(\:\sqrt{31}\) x \(\:\sqrt{31}\) R9° reconstruction emerges from a relaxed (1x1) surface by desorbing the top two O layers along with a fraction of the surface Al. 43 This reconstruction typically forms under ultra-high vacuum annealing at temperatures above 1400 o C 12 , 44 or by depositing 1 monolayer of Al atoms on α-Al 2 O 3 at lower temperature, such as 800 o C 45 , 46 . The formation of the \(\:\sqrt{31}\) x \(\:\sqrt{31}\) R9° structure is influenced by the balance between the surface oxygen vacancies density and the temperature. Specifically, at low temperatures of 800 o C with high oxygen depletion (11%, calculated from the structure model of one monolayer Al deposited on α-Al 2 O 3 surface) and at high temperatures 1450 o C with low oxygen depletion (2%, 47 ) the \(\:\sqrt{31}\) x \(\:\sqrt{31}\) R9° reconstruction can form. In our experiment, the combination of high oxygen depletion and a high growth temperature of 900 o C falls within the range that can facilitate the formation of the \(\:\sqrt{31}\) x \(\:\sqrt{31}\) R9° reconstruction. Additionally, we noted an increased intensity of the OH − peak at 532.4 eV in O 1s spectrum post pre-growth heating, as detailed in Figure S29b and Table S10. This observation, coupled with the absence of a metallic Al layer suggests the reoxidation of the Al metal layer during sample transfer likely induced by exposure to air. This observation aligns with previous research, which demonstrated that the \(\:\sqrt{31}\) x \(\:\sqrt{31}\) R9° reconstructed structure, when exposed to water and hydrogen, forms hydroxide clusters on the surface, as evidenced by scanning force microscopy. 14 To investigate the impact of the reconstructed surface structure of sapphire on the epitaxy growth of TMD, we performed DFT calculations to determine the preferential orientation of TMD on both pristine and reconstructed sapphire surfaces. In Fig. 4 a, the rotating angle was defined as the anticlockwise rotation required to align the zigzag edge of the WS 2 triangular domain, initially parallel to the \(\:\left[1\overline{1}00\right]\) direction of the sapphire crystal, with the specific triangle. On a pristine c-plane sapphire substrate without a reconstructed Al layer, mirror domains prefer orientation angles of 30 o or 90 o . However, with the reconstructed Al layer, the preferred orientation angles shift to 0 o or 60 o . The adsorption energies as a function of orientation angle, plotted in Fig. 4 b, indicate that the energy-favored orientation shifts from 30 o on pristine sapphire to 0 o on the reconstructed Al layer. These findings suggest that surface chemistry variations play a significant role in determining the domain orientation during TMD growth. To experimentally investigate the impact of the reconstructed Al layer on domain orientation, we used two common c-plane sapphire substrates with atomic steps along the \(\:\) (or a-axis) and \(\:\) (or m-axis) directions; the AFM analysis is given in Figure S35. Previous studies reported that on c-plane sapphire with \(\:\) atomic steps, TMD mirror domains tend to align with the step edges due to the similar degenerate energy for mirror domain nucleation. 48 Conversely, on c-plane sapphire with \(\:\:\) atomic steps, unidirectional domain orientation is typically observed, driven by the minimized nucleation energy induced by the step edges. 8 In our experiments, we employed these two sapphire substrates for the CVD growth of WS 2 monolayers. The stacking relationship of the domains on the sapphire crystal surface was determined by AFM. As shown in Fig. 4 c, for the pristine sapphire surface with \(\:\) steps, mirrored domains with orientation angles of 30 o and 90 o were aligned along the \(\:\) step edges. In contrast, on the surface with \(\:\:\) steps, unidirectional domains were observed aligned along the \(\:\:\) steps. This observation is consistent with published research 8 , 48 , indicating that our optimized growth conditions are suitable for identifying the impact of the reconstructed Al layer on growth. We performed a statistical analysis of domain orientation in relation to the evolution of sapphire surface chemistry over a large area. On sapphire substrates with \(\:\) steps, the freshly annealed surface exhibited two mirrored domains with orientation angles of 30 o and 90 o , as shown in the optical image and corresponding statistical distribution of domain orientation in Fig. 4 d. After two weeks of air aging, numerous additional mirrored domains with orientation angles of 0 o and 60 o emerged, as shown in Fig. 4 e. This change is attributed to the formation of the reconstructed Al layer, derived from the aluminum oxyhydroxide precursor upon air exposure. Subsequently, when the aged sapphire was subjected to UV irradiation prior to the growth, the additional 0 o and 60 o domains disappeared, leaving only the original 30 o and 90 o domains similar to epitaxial growth observed on the freshly annealed sapphire (Fig. 4 f). We attribute this to the UV light induced healing of the degraded sapphire surface, which removes the aluminum oxyhydroxide precursor, preventing the formation of the reconstructed Al layer, and promoting the recovery of the pristine crystal surface during growth. For sapphire with \(\:\:\) steps, the freshly annealed surface exhibits unidirectional domain orientation at a rotating angle of 0 o , as shown in the optical image and corresponding statistical distribution of domain orientation in Fig. 4 g. After 2 weeks of air aging, additional domains at an orientation angle of appeared (Fig. 4 h). However, following UV irradiation on the aged sapphire, no additional 60 o domains were detected, and the domains remained unidirectionally aligned with an orientation angle of 0 o (Fig. 4 i). This result demonstrates that the evolution of sapphire surface chemistry significantly impacts the orientation of epitaxially grown domains. The formation of aluminum oxyhydroxide during air exposure leads to the emergence of a reconstructed Al layer on the pristine sapphire substrate, which compromises the epitaxial film by introducing additional domain orientations. However, UV irradiation prior to growth effectively removes this aluminum oxyhydroxide precursor, restoring the sapphire surface to a state comparable to that of a freshly annealed surface. This process recovers the original domain alignment, ensuring high-quality WS 2 growth. The UV light surface repair strategy can be applied for wafer-scale growth of WS 2 . A photograph of a 2-inch c-plane sapphire wafer with a WS 2 film shown in Fig. 5 a, showing a uniform film color that indicates consistent growth with high coverage. Optical images taken from different locations on the 2-inch WS 2 film confirm its high surface quality and uniformity (in Figure S36). Figure 5 b and 5 c present PL and Raman mapping across a 200 µm length of the 2-inch WS 2 wafer, revealing no significant variations in peak position and linewidth. Furthermore, the quality of the WS 2 film was assessed using circularly polarized PL spectra, as shown in Fig. 5 d. The calculated photon energy-dependent circular helicity in Fig. 5 e indicates a very high degree of circular helicity. This serves as strong evidence of the high quality of the film, as circular helicity arises from the valley selective spin excitations at the K and K' points of the Brillouin zone of monolayer WS 2 and is highly sensitive to defects that introduce intervalley scattering which degrades the circular helicity. For high-quality WS 2 monolayer flakes exfoliated from bulk single crystals, the measured circular helicity is typically lower than 40%. Remarkably, our WS 2 film on sapphire exhibited circular helicity up to 60% (Fig. 5 e), which is comparable to the highest reported for exfoliated flakes. 49 , 50 Additionally, high-resolution HAADF STEM demonstrate the uniform local atomic structure of the WS 2 monolayer, as shown in the atomic image in Figure S37, the featured atomic structure with a hexagonal unit cell is clearly visible in the enlarged inset on the left. To further assess the sample quality, the chemical composition and electronic band structure of the WS 2 monolayer film were measured by XPS and angle-resolved photoemission spectroscopy (ARPES). XPS data in Figure S38 indicates a WS 2 stoichiometry comparable with a WS 2 bulk single crystal (from HQ graphene). The valence band dispersion along the ΓM and ΓK high symmetrical directions of the Surface Brillouin Zone of the WS 2 monolayer is shown in Fig. 5 f. The results are in line with previous measurements obtained for single crystal WS 2 monolayers 51 and confirm the single orientation of the WS 2 domain in the probed area (~ 1 mm 2 ). For multiple domain TMD films, the simultaneous observation of GK and GM band dispersion would be expected, reflecting the different in-plane orientation of the TMD domains under the ARPES probe area, as previously reported for MoS 2 and WS 2 thin films; 52 53 this multiple domain signal is not observed in our WS 2 , emphasizing its high level of mono-crystallinity. In summary, we have demonstrated that the surface chemistry of sapphire critically influenced the domain orientation and quality of WS 2 films grown on the sapphire substrate via CVD. When exposed to air, sapphire undergoes hydrolysis, leading to the formation of aluminum oxyhydroxide and subsequent pyrolysis and Al-rich surface reconstruction in the pre-growth stage, which adversely impacts the alignment of crystal orientation epitaxy growth. Moreover, we found that UV irradiation prior to the CVD growth effectively repairs the surface by removing aluminum oxyhydroxide, restoring the sapphire to a state comparable to freshly annealed sapphire. The WS 2 monolayer wafer grown on repaired sapphire exhibits remarkable uniformity and mono-crystallinity. This UV light repair strategy proved effective for both small-scale and wafer-scale growth, offering a valuable method for producing high-performance TMD films for advanced electronic and optoelectronic applications. Methods Thermal annealing of sapphire substrates Single-side polished c-plane sapphire substrates (001) were purchased from Princeton Scientific Corporation and Hefei Crystal Technical Materials. These substrates were fabricated to produce orientations of (0001) off a (11–20) at 1.0° ± 0.1°, and off m (10–10) directions at 0.5° ± 0.1°. For the engineering of atomic step heights of a sapphire, thermal annealing was conducted using a Carbolite RHF muffle furnace located within a cleanroom environment. The annealing process involved heating the substrates to 1250°C at a rate of 5°C/min and holding for 4 hours to achieve an atomic step height of ~ 0.2 nm, and to 1350°C for 4 hours to achieve an atomic step height of ~ 1 nm. Aging sapphires in air The freshly engineered sapphire substrates were exposed to an air environment within a cleanroom. Each substrate was placed on a petri dish lined with filter paper and subjected to various aging durations to assess the effects on surface chemistry and morphology. UV exposure in air The aged sapphires were placed inside a Samco model UV-1 system. The substrates were exposed to UV light (wavelength of 320 nm to 400 nm, power density of 0.5 W/cm 2 ) in an air for 30 minutes. Epitaxial growth of WS monolayer The epitaxial growth was carried out in a three-zone CVD system with a tube diameter of 70 mm. Sulfur powders (6.0 g, 99.998%, ALDRICH) were placed in the upstream of heating zone I and heated between 150–200°C. WO 3 powders (99.995%, ALDRICH) and KCl (99.5%, ALDRICH) were positioned in heating zone II, and sapphire substrates were situated in heating zone III. The CVD process was conducted under low pressure within an argon atmosphere. Specifically, the growth temperatures of zone I, zone II and zone III were set at 180, 630, and 950 o C, respectively, with a gas flow of argon (400 sccm) and hydrogen (10 sccm). The pressure in the growth chamber was kept at 5 torr. The growth time varied, with 5–15 min for individual islands, and 30–40 min for a continuous film. To maintain consistency across different CVD growth runs, freshly annealed sapphire was systematically placed alongside the target sapphire for each session. Sample characterizations. Raman and PL spectra were collected at room temperature using the confocal WiTec Alpha 300R Raman Microscope (laser excitation, 532 nm). The surface profiles of sapphires samples were tested by Bruker’s dimension icon AFM system. The XPS measurements for sapphire samples were conducted by a VG ESCALAB 220i-XL system with a monochromatic Al Kα source and a pass energy of 10 eV, with electron flood gun filament current setting at 0-2.4 A. The XPS data of WS 2 were acquired with a hemispherical electron analyzer (SCIENTA HiPP-2) in normal emission condition, by using a monochromatized Al Kα ( hν = 1486.6 eV) as energy excitation and energy resolution of ~ 0.3 eV. RHEED (STAIB Instruments) was measured at room temperature under an ultrahigh vacuum of 10 − 9 torr. The electron acceleration voltage was 15 kV. Atomic-resolution STEM-ADF imaging was performed on an aberration-corrected JEOL ARM200F, equipped with a cold field-emission gun and an ASCOR corrector operating at 80 kV. The convergence semi-angle of the probe was around 30 mrad. Cross-sectional STEM lamellae of sapphire samples were prepared using a focused ion beam (FIB, FEI Helios Nanolab 600). For low-temperature circular dichroic photoluminescence (CDPL) measurements, the sample was kept in a cryostat on top of a motorized stage, and a 570 nm pulsed laser (∼80 ps) was used for the excitation. The ARPES data were acquired in a custom-designed system, with a hemispherical electron analyzer (SCIENTA DA30L) and monochromatized HeIα ( hν = 21.218 eV) radiation source (SCIENTA VUV5k). Before ARPES measurement the WS 2 ML on HOPG was annealed at 200 o C x 12 h and 400 o C x1 h to remove surface contaminants. Theoretical calculation. The adsorption energies of WS 2 monolayer on a c-plane Al 2 O 3 surface (without and with reconstructed surface) were calculated using density functional theory (DFT) with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional as implemented in the Vienna ab initio simulation package (VASP). 54 – 56 The cut-off energy for plane-wave basis set was set at 500 eV. The system was modeled as a triangular island of a monolayer WS 2 (4 unit cells on each side) on a 3-layer Al 2 O 3 substrate. 6×6 supercell and \(\:\left(\sqrt{31}\times\:\sqrt{31}\right)R9^\circ\:\) supercells were used for crystal and reconstructed Al 2 O 3 surfaces, respectively. The structures were optimized at Γ point until the Hellmann-Feynman forces on the atoms were less than 0.05 eV/Å. The DFT-D2 method of Grimme 57 was used to include the van der Waals interaction. Declarations Acknowledgements We acknowledge the funding support from the Agency for Science, Technology and Research Grant (C230917006). K. E. J. G. acknowledges a Singapore National Research Foundation Grant (CRP21-2018-0001). This research is supported by the Ministry of Education, Singapore, under its AcRF Tier 2 (MOE-T2EP50122-0016). We acknowledge the funding support from the Agency for Science, Technology and Research (C230917006). The computational work was performed on resources of the National Supercomputing Centre (NSCC), Singapore and the A*STAR Computational Resource Centre (A*CRC). References Fiori G., Bonaccorso F., Iannaccone G., Palacios T., Neumaier D., Seabaugh A. , et al. Electronics based on two-dimensional materials. Nat. Nanotechnol. 9 , 768-779 (2014) Briggs N., Subramanian S., Lin Z., Li X., Zhang X., Zhang K. , et al. A roadmap for electronic grade 2D materials. 2D Mater. 6 , 022001 (2019) Lin Y.C., Jariwala B., Bersch B., Xu K., Nie Y., Wang B. , et al. Realizing large-scale, electronic-grade two-dimensional semiconductors. ACS Nano 12 , 965-975 (2018) Akinwande D., Huyghebaert C., Wang C.H., Serna M.I., Goossens S., Li L.J. , et al. Graphene and two-dimensional materials for silicon technology. Nature 573 , 507-518 (2019) Xu Z.Q., Zhang Y.P., Lin S.H., Zheng C.X., Zhong Y.L., Xia X. , et al. Synthesis and transfer of large-area monolayer WS 2 Crystals: moving toward the recyclable use of sapphire substrates. ACS Nano 9 , 6178-6187 (2015) Choi W., Choudhary N., Han G.H., Park J., Akinwande D., Lee Y.H. Recent development of two-dimensional transition metal dichalcogenides and their applications. Mater. Today 20 , 116-130 (2017) Fu J.H., Min J., Chang C.K., Tseng C.C., Wang Q., Sugisaki H. , et al. Oriented lateral growth of two-dimensional materials on c-plane sapphire. Nat. Nanotechnol. 18 , 1289-1294 (2023) Li T., Guo W., Ma L., Li W., Yu Z., Han Z. , et al. Epitaxial growth of wafer-scale molybdenum disulfide semiconductor single crystals on sapphire. Nat. Nanotechnol. 16 , 1201-1207 (2021) Wang J., Xu X., Cheng T., Gu L., Qiao R., Liang Z. , et al. Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS 2 monolayer on vicinal a-plane sapphire. Nat. Nanotechnol. 17 , 33-38 (2022) Zhu H., Nayir N., Choudhury T.H., Bansal A., Huet B., Zhang K. , et al. Step engineering for nucleation and domain orientation control in WSe 2 epitaxy on c-plane sapphire. Nat. Nanotechnol. 18 , 1295-1302 (2023) Zheng P., Wei W., Liang Z., Qin B., Tian J., Wang J. , et al. Universal epitaxy of non-centrosymmetric two-dimensional single-crystal metal dichalcogenides. Nat. Commun. 14 , 592 (2023) Smink S., Majer L.N., Boschker H., Mannhart J., Braun W. Long-range atomic order on double-stepped Al 2 O 3 (0001) Surfaces. Adv. Mater. 36 , 2312899 (2024) Mo J., El Kazzi S., Mortelmans W., Mehta A.N., Sergeant S., Smets Q. , et al. Importance of the substrate’s surface evolution during the MOVPE growth of 2D-transition metal dichalcogenides. Nanotechnology 31 , 125604 (2020) Barth C., Reichling M. Imaging the atomic arrangements on the high-temperature reconstructed α-Al 2 O 3 (0001) surface. Nature 414 , 54-57 (2001). Eng P.J., Trainor T.P., Brown Jr G.E., Waychunas G.A., Newville M., Sutton S.R. , et al. Structure of the hydrated α-Al 2 O 3 (0001) Surface. Science 288 , 1029-1033 (2000) Chambers S.A., Droubay T., Jennison D.R., Mattsson T.R. Laminar growth of ultrathin metal films on metal oxides: Co on hydroxylated α-Al 2 O 3 (0001). Science 297 , 827-831 (2002) Aljarb A., Cao Z., Tang H.-L., Huang J.K., Li M., Hu W. , et al. Substrate lattice-guided seed formation controls the orientation of 2D transition-metal dichalcogenides. ACS Nano 11 , 9215-9222 (2017) Ranea V.A., Carmichael I., Schneider W.F. DFT investigation of intermediate steps in the hydrolysis of α-Al 2 O 3 (0001). J. Phys. Chem. C 113 , 2149-2158 (2009) Chen X., Shan W., Wu D., Patel S.B., Cai N., Li C. , et al. Atomistic mechanisms of water vapor–induced surface passivation. Sci. Adv. 9 , eadh5565 (2023) Liu Q., Tong X., Zhou G. H 2 O dissociation-induced aluminum oxide growth on oxidized Al(111) surfaces. Langmuir 31 , 13117-13126 (2015) Liu Q., Qin H., Boscoboinik J.A., Zhou G. Comparative Study of the oxidation of NiAl(100) by molecular oxygen and water vapor using ambient-pressure X-ray photoelectron spectroscopy. Langmuir 32 , 11414-11421 (2016) Liu P., Kendelewicz T., Brown G.E., Nelson E.J., Chambers S.A. Reaction of water vapor with α-Al 2 O 3 (0001) and α-Fe 2 O 3 (0001) surfaces: synchrotron X-ray photoemission studies and thermodynamic calculations. Surf. Sci. 417 , 53-65 (1998) Malki A., Mekhalif Z., Detriche S., Fonder G., Boumaza A., Djelloul A. Calcination products of gibbsite studied by X-ray diffraction, XPS and solid-state NMR. J. Solid State Chem. 215 , 8-15 (2014) Yue Y., Melani G., Kirsch H., Paarmann A., Saalfrank P., Campen R.K. , et al. Structure and reactivity of α-Al 2 O 3 (0001) surfaces: how do Al–I and gibbsite-like terminations interconvert? J. Phys. Chem. C 126 , 13467-13476 (2022) Greczynski G., Hultman L. Binding energy referencing in X-ray photoelectron spectroscopy. Nat. Rev. Mater. DOI: 10.1038/s41578-024-00743-5 (2024) Moulder J.F., Stickle W.F., Sobol P.E., Bomben K.D. Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data (Perkin-Elmer, 1992) Park Y., Ahn C., Ahn J.G., Kim J.H., Jung J., Oh J. , et al. Critical role of surface termination of sapphire substrates in crystallographic epitaxial growth of MoS 2 using inorganic molecular precursors. ACS Nano 17 , 1196-1205 (2023) Zhou J., Lin J., Huang X., Zhou Y., Chen Y., Xia J. , et al. A library of atomically thin metal chalcogenides. Nature 2018, 556 , 355-359 (2018) Yang P., Liu F., Li X., Hu J., Zhou F., Zhu L. , et al. Highly reproducible epitaxial growth of wafer-scale single-crystal monolayer MoS 2 on sapphire. Small Methods 7 , 2300165 (2023) Cohen A., Mohapatra P.K., Hettler S., Patsha A., Narayanachari K.V.L.V., Shekhter P. , et al. Tungsten oxide mediated quasi-van der waals epitaxy of WS 2 on sapphire. ACS Nano 17 , 5399-5411 (2023) Bagwell R.B., Messing G.L. Effect of seeding and water vapor on the nucleation and growth of α-Al 2 O 3 from γ-Al 2 O 3 . J. Am. Ceram. Soc. 82 , 825-832 (1999) Eklund P., Sridharan M., Singh G., Bøttiger J. Thermal stability and phase transformations of γ-/amorphous-Al 2 O 3 thin films. Plasma Processes Polym. 6 , S907-S911 (2009) Reichel F., Jeurgens L.P.H., Richter G., Mittemeijer E.J. Amorphous versus crystalline state for ultrathin Al 2 O 3 overgrowths on Al substrates. J. Appl. Phys. 103 , 093515 (2008) Wilson S.J. The dehydration of boehmite, γ-AlOOH, to γ-Al 2 O 3 . J. Solid State Chem. 30 , 247-255 (1979) Ramírez L.P., Bournel F., Gallet J.-J., Dudy L., Rochet F. Testing the cabrera–Mott oxidation model for aluminum under realistic conditions with near-ambient pressure photoemission. J. Phys. Chem. C 126 , 2517-2530 (2022) Mortelmans W., El Kazzi S., Groven B., Nalin Mehta A., Balaji Y., De Gendt S. , et al. Epitaxial registry and crystallinity of MoS 2 via molecular beam and metalorganic vapor phase van der Waals epitaxy. Appl. Phys. Lett. 117 , 033101 (2020) Vig J., LeBus J. UV/ozone cleaning of surfaces. IEEE Transactions on Parts, Hybrids, and Packaging 12 , 365-370 (1976) Vig J.R. UV/ozone cleaning of surfaces. J. Vac. Sci. Tech. A 3 , 1027-1034 (1985) Sun C., Zeng R., Zhang J., Qiu Z.J., Wu D. Effects of UV-ozone treatment on sensing behaviours of EGFETs with Al 2 O 3 sensing film. Materials 10 , 1532 (2017) Yoon K.H., Kim H., Koo Lee Y.E., Shrestha N.K., Sung M.M. UV-enhanced atomic layer deposition of Al 2 O 3 thin films at low temperature for gas-diffusion barriers. RSC Adv. 7 , 5601-5609 (2017) Fons P., Iwata K., Niki S., Yamada A., Matsubara K.J.J.o.C.G. Growth of high-quality epitaxial ZnO films on α-Al 2 O 3 . 201 , 627-632 (1999) Lauritsen J.V., Jensen M.C.R., Venkataramani K., Hinnemann B., Helveg S., Clausen B.S. , et al. Atomic-scale structure and stability of the x . surface of Al 2 O 3 (0001). Phys. Rev. Lett. 103 , 076103 (2009) Renaud G., Villette B., Vilfan I., Bourret A. Atomic structure of the α- Al 2 O 3 (0001)( x )R±9 o reconstruction. Phys. Rev. Lett. 73 , 1825-1828 (1994) Milakhin D.S., Malin T.V., Mansurov V.G., Galitsyn Y.G., Kozhukhov A.S., Utkin D.E. , et al. Peculiarities of the AlN crystalline phase formation in a result of the electron-stimulated reconstruction transition( x )R±9 o − (1 × 1). Appl. Surf. Sci. 541 , 148548 (2021) French T.M., Somorjai G.A. Composition and surface structure of the (0001) face of α-alumina by low-energy electron diffraction. J. Phys. Chem. 74 , 2489-2495 (1970) Vermeersch M., Sporken R., Lambin P., Caudano R. The AlAl 2 O 3 interface formation as studied by electron spectroscopies. Surf. Sci. 235 , 5-14 (1990) Richmond E.D. Xps Analysis of the sapphire surface as a function of high temperature vacuum annealing. MRS Online Proc. Lib. 159 , 253-256 (1989) Zhang T., Wang J., Wu P., Lu A.-Y., Kong J. Vapour-phase deposition of two-dimensional layered chalcogenides. Nat. Rev. Mater. 8 , 799-821 (2023) Scrace T., Tsai Y., Barman B., Schweidenback L., Petrou A., Kioseoglou G. , et al. Magnetoluminescence and valley polarized state of a two-dimensional electron gas in WS2 monolayers. Nat. Nanotechnol. 10 , 603-607 (2015) Plechinger G., Nagler P., Arora A., Schmidt R., Chernikov A., del Águila A.G. , et al. Trion fine structure and coupled spin–valley dynamics in monolayer tungsten disulfide. Nat. Commun. 7 , 12715 (2016) Bussolotti F., Yang J., Kawai H., Chee J.Y., Goh K.E.J. Influence of many-body effects on hole quasiparticle dynamics in a WS 2 monolayer. Phys. Rev. B 103 , 045412 (2021) Bussolotti F., Chai J., Yang M., Kawai H., Zhang Z., Wang S. , et al. Electronic properties of atomically thin MoS 2 layers grown by physical vapour deposition: band structure and energy level alignment at layer/substrate interfaces. RSC Adv. 8 , 7744-7752 (2018) Lim Y.F., Priyadarshi K., Bussolotti F., Gogoi P.K., Cui X., Yang M. , et al. Modification of vapor phase concentrations in MoS 2 growth using a NiO foam barrier. ACS Nano 12 , 1339-1349 (2018) Kresse G., Hafner, J. Molecular Dynamics for Liquid Metals. Phys. Rev. B 47 , 558(R) (1993) Kresse G., Hafner J. Ab initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor transition in Germanium. Phys. Rev. B 49 , 14251 (1994) Perdew J. P., Burke K., Ernzerhof M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77 , 3865 (1996) Grimme S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 27 , 1787 (2006) Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformation.docx Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 26 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Goh","email":"","orcid":"https://orcid.org/0000-0003-0599-9696","institution":"Institute of Materials Research and Engineering/Agency for Science, Technology and Research","correspondingAuthor":false,"prefix":"","firstName":"Kuan","middleName":"Eng Johnson","lastName":"Goh","suffix":""}],"badges":[],"createdAt":"2024-11-26 15:56:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5529369/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5529369/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-63452-9","type":"published","date":"2025-09-26T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":81175680,"identity":"701d0b10-32a6-424d-bb9e-443d91e853dd","added_by":"auto","created_at":"2025-04-23 06:13:35","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":369430,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eAir-induced surface evolution on engineered sapphire.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e a, Schematic illustrating the surface structural changes of c-plane sapphire (α-Al\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e) during air exposure and its impact on domain orientation of epitaxy. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003eb\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, Schematic of the formation of aluminum (oxy)hydroxide on sapphire surface during the air exposure. Blue spheres represent Al atoms, red spheres represent O atoms, white spheres represent H atoms. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, AFM image of freshly annealed sapphire with atomic steps towards \u003c/em\u003e\u003cem\u003e\u003cstrong\u003em\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e [1-100] direction. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, AFM image of sapphire, air exposed for 6 weeks, displaying additional cluster formation along atomic steps, which indicates preferential hydrolysis at these locations. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ee-g\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, Cross-section STEM images of sapphire surfaces after freshly annealing and aging in air for 4 and 6 weeks, viewed from the [11-20] direction, revealing a crystalline atomic resolved surface on freshly annealed sapphire and the development of amorphous layers during air exposure. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003eh,i,\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e High-resolution Al 2p and O 1s XPS spectra of freshly annealed and 1-week-aged sapphire samples under high low-energy electron flux settings, identifying the characteristic Al-O bonding spectra of α-Al\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e in freshly annealed sapphire, and spectral shape change becoming detectable by XPS evidenced by additional Al-OH bonding peaks of aluminum (oxy)hydroxide likely arising from surface hydrolysis, as elucidated through XPS peak deconvolution for the 1-week aged sapphire sample.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/d3618540d6ad0f7bef3a15bf.jpeg"},{"id":81175681,"identity":"ae769667-484c-4466-8fc0-5a9cfeccd654","added_by":"auto","created_at":"2025-04-23 06:13:35","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":285945,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eDifferential charging-enhanced XPS analysis of engineered sapphire surface changes after aging in air.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ea,\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e Schematic illustration of XPS setup, equipped with an electron flood gun, which emits low-energy electrons to control the surface charging. The angle of photoemitted electron collection was set at 30\u003c/em\u003e\u003csup\u003e\u003cem\u003eo\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e relative to the substrate normal. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003eb\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, The filament current of flood gun-dependent electron flux intensities and the measured C 1s peak positions for bias-referencing. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, Modulation of surface electrical fields by various electron flood gun settings. At setting of i, defined as no electron flood density (NEF) mode, photoemission leaves the surface with a surplus of positive charges (holes), inducing a positive surface electrical field. At setting of ii, defined as the low electron flood gun (LEF) mode, low-energy electrons partially neutralize these positive holes, and excess electrons accumulate in surficial Al\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and AlO\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(OH)\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e phases, producing a localized negative differential charging effect. At setting of iii, defined as high electron flood gun (HEF) mode, abundant electrons create a uniform negative surface electrical field. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, Al 2p and O 1s spectra of engineered sapphire after two-weeks of air aging, obtained under different electron flood gun settings. The differential charge effect enhanced the separation of Al\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and AlO\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(OH)\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e phases by reducing the Al-OH bonding energy by 1.5 eV, facilitating phase identification and differentiation.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/ef5975e36ac0668ecfa1018f.jpeg"},{"id":81175985,"identity":"846c37e9-2ed2-4b4e-8d2c-c84a1dd2e69d","added_by":"auto","created_at":"2025-04-23 06:21:36","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":215824,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eLight irradiation repairs the surface of aged sapphire.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ea\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, Schematic showing light-irradiation induced surface repair. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003eb\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, XPS analysis of sapphire after UV light exposure and after subsequent pre-growth heating process. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, The RHEED diffraction patterns of aged and repaired sapphires. \u003c/em\u003e\u003cem\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e, The RHEED intensity profiles recorded along the dotted lines in c.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/c44786509fd12ba0614e5e90.jpeg"},{"id":81175682,"identity":"4258965a-fa12-42b5-bd4e-23cb9f0fe9ae","added_by":"auto","created_at":"2025-04-23 06:13:35","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5401861,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/89e7c942cf88b4ad33c01598.png"},{"id":81175986,"identity":"198fb5d4-d649-4549-af14-21109a810a6f","added_by":"auto","created_at":"2025-04-23 06:21:36","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":154813,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eQuality of epitaxially-grown WS2 monolayer.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e a, Photograph of a 2-inch sapphire wafer with WS2 monolayer film; b, Line scan of Raman spectra at the sub-mm scale showing the uniformity of the monolayer film; c, Line scan of PL spectra at the sub-mm scale. d, Circular polarized PL spectra of WS2 grown on c-plane sapphire with \u0026lt;1100\u0026gt; atomic steps. The excitation light is right-handed circularly polarized at 2 eV. Left-handed and right-handed circularly polarized PL spectra are shown in orange and green, respectively. e, Circular helicity calculated form the PL spectra in d. The high value over 60% indicates the high quality of our WS2 monolayer. f, Experimental valence band dispersion of WS2 ML at 295 K along the GK and GM high symmetry direction of ML BZ obtained from a second derivative filter of the ARPES intensity.\u0026nbsp;\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/18b3582ba69b95941772d42b.jpeg"},{"id":92304513,"identity":"a25156a5-b090-4ac0-ae64-bfa6afa58d4c","added_by":"auto","created_at":"2025-09-27 07:06:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8541075,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/989515fc-c6df-42ea-aa08-b53a4cd3d06b.pdf"},{"id":81175687,"identity":"13f35e7a-1e1d-4ace-b78e-a20a2344c07f","added_by":"auto","created_at":"2025-04-23 06:13:36","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5901900,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5529369/v1/3c008e831a29c229e5c4caf8.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Unveiling air-induced surface transformation on atomic step-engineered sapphire for oriented growth of transition metal dichalcogenides","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe scalable epitaxy of single-crystal two-dimensional (2D) transition metal dichalcogenides (TMDs) on insulating substrates that are compatible with semiconductor industry will enable the mass production of next-generation 2D electronics based on TMDs.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Among various growth substrates, c-plane sapphire has emerged as a promising option for TMD epitaxy due to its crystallographic compatibility and its chemical and thermal resilience under typical growth conditions.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e In the past few years, significant advances have been made in wafer-scale single-crystal TMD synthesis through strategic engineering of the sapphire surface. This includes optimizing miscut orientation towards the a-axis\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, creating vicinal a-plane surfaces\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, developing immature step edges for enhanced nucleation\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, reconstructing for single atomic plane exposure\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, and modulating the surface chemistry through controlled growth conditions\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSapphire has the α-phase hexagonal lattice structure, with in-plane unit cell dimensions a\u0026thinsp;=\u0026thinsp;b\u0026thinsp;=\u0026thinsp;4.76 \u0026Aring; and an out-of-plane dimension c\u0026thinsp;=\u0026thinsp;12.996 \u0026Aring;, characterized by six layers of alternating Al\u0026ndash;O\u0026ndash;Al atoms terminating in Al.\u003csup\u003e\u003cspan additionalcitationids=\"CR13 CR14\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e The normal termination of clean α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e consists of one Al atom for every three O atoms, with the terminal Al above the plane of the close-packed O.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e Surface atomic features, such as step terrace terminations and step edges orientation have been utilized to modulate the coupling interaction between TMD crystals and a suitably engineered sapphire substrate for achieving well-resolved domain alignment during epitaxial growth.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e However, in most conventional processes, engineered sapphire substrates are inevitably exposed to air, which may introduce variability in the epitaxial growth. Reports on the structural changes of engineered sapphire exposed to air and its impact on subsequent epitaxial growth remain limited. The α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface has been shown to react with H\u003csub\u003e2\u003c/sub\u003eO to form aluminum (oxy)hydroxide in carefully controlled experimental as well as theoretical studies.\u003csup\u003e\u003cspan additionalcitationids=\"CR19 CR20 CR21\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e During the thermal heating at temperatures ranging from 700 \u003csup\u003eo\u003c/sup\u003eC to 1000 \u003csup\u003eo\u003c/sup\u003eC, similar to those of typical epitaxial growth conditions, the pyrolysis of aluminum (oxy)hydroxide occurs resulting in oxygen vacancy-rich Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003e.\u003csup\u003e23, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e This process changes the surface chemistry and can significantly impact the alignment of crystal domains during epitaxy. A complete understanding of the atomic step engineered sapphire surface degradation mechanism upon exposure to air, and strategies for limiting/repairing this degradation are crucial for achieving consistent, high-quality TMD epitaxy.\u003c/p\u003e \u003cp\u003eHere, we report a systematic study of the impact of air exposure on sapphire substrates and the consequence on the domain orientation of a prototypical 2D monolayer-WS\u003csub\u003e2\u003c/sub\u003e. Utilizing differential charging-enhanced surface analysis, we found significant changes in the surface chemistry of atomic-step-engineered sapphire when exposed to air, which reduced domain alignment during epitaxial growth of the WS\u003csub\u003e2\u003c/sub\u003e monolayers by chemical vapor deposition (CVD), as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. Our characterization indicates that surface hydrolysis occurs during air exposure, resulting in the formation of aluminum (oxy)hydroxide, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. This phase subsequently decomposes into oxygen-deficient Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003e during CVD growth, adversely compromising the WS\u003csub\u003e2\u003c/sub\u003e domain orientation. We also found that increased atomic step heights on engineered sapphire accelerate this surface hydrolysis process. Notably, our experiments demonstrate that UV light irradiation of air exposed sapphire surfaces effectively removes the hydroxyl groups and restores the degraded sapphire surface to crystalline α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e during the growth stage. Our findings of deactivation of sapphire by air exposure and its subsequent restoration through UV irradiation provide valuable insights for the commercial production of high-quality 2D materials.\u003c/p\u003e"},{"header":"Result","content":"\u003cp\u003eWe selected the commercially available and commonly used c-plane sapphire, cleaved along \u003cstrong\u003ea\u003c/strong\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e direction as our model substrate. Through a custom thermal annealing process (see details in experimental section), the sapphire surface was engineered with atomic steps along \u003cstrong\u003em\u003c/strong\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e orientation and terminated of half-Al surfaces (see Figure S1). The surface profile of the freshly annealed sapphire measured by atomic form microscopy (AFM), is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec and Figure S2 (Supplementary Section 1). These images display a regular and uniform arrangement of surface atomic steps along the \u003cstrong\u003em\u003c/strong\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\:\\)\u003c/span\u003e\u003c/span\u003edirection with step height of ~0.25 nm (See inset in Figure S2c). The Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee shows the cross-section scanning transmission electron microscope (STEM) image of freshly annealed sapphire, viewed from \u003cstrong\u003ea\u003c/strong\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e direction, exhibiting the sharp atomic profile on surface.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eTo elucidate the structural transformations of sapphire surfaces exposed to air, freshly engineered sapphire samples were aged in a cleanroom environment (class 10K, 45% humidity) for varying durations. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed is an AFM image of the sapphire surface after aging for 6 weeks, showing the formation of distinct clusters along the edges of atomic surface steps. Previous work noted that surface hydrolysis can occur on the sapphire surfaces when exposed to humid air.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Assuming that these clusters are the product of that hydrolysis, our observation could indicate that hydrolysis preferentially targets the engineered atomic step edges of sapphire. In contrast, the AFM images of sapphire aged for shorter periods of 1\u0026thinsp;~\u0026thinsp;2 weeks (see Figure S3) show no discernible morphological changes, indicating that the early-stage structural transformations were not detectable at the sensitivity level of the AFM system used. To detect these early-stage changes, we therefore used cross-sectional STEM analysis of sapphire surfaces aged for 2, 4 and 6 weeks respectively. Figures \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee, f, and g show the main morphological change from a fresh sapphire surface to one aged for 6 weeks, revealing the development of an amorphous layer due to air exposure, with thickness ranging from 1 to 4 nm. A more detailed chronological sequence is shown in Figure S4 together with a schematic of the morphological evolution of the sapphire surface under air exposure depicted in Figure S5. This evolution involves the initial structural transformation of an amorphous layer on the atomic step edges, which is subsequently followed by the formation of cluster particles during the surface hydrolysis.\u003c/p\u003e\n\u003cp\u003eX-ray Photoelectron Spectroscopy (XPS) was employed to elucidate the impact of air exposure on the surface chemistry of engineered sapphire. Considering that sapphire is an insulator, it may accumulate a net positive charge due to photoelectron loss when subjected to X-ray irradiation.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e To counteract this, a low-voltage electron flood gun was utilized to provide charge compensation by directing an electron beam onto the surface. The electron flux intensity was regulated by adjusting the filament current of the electron flood gun. The adventitious carbon C 1s spectrum is recorded and used as internal reference for bias-referencing.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e High electron flux intensity settings (detailed in Table S1) were used to analyze the Al 2p and O1s XPS spectra of both freshly annealed and 1-week aged sapphire, shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eh and \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ei, respectively, along with the peak fitting results in Table S2. The high-resolution Al 2p and O 1s spectra of freshly annealed sapphire (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eh and Figure S6) revealed an Al 2p peak at 74.1 eV and a primary O 1s peak at 531 eV, indicative of O-Al-O bonding in the \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e An additional O 1s peak at 532.4 eV was observed, attributed to physically adsorbed H\u003csub\u003e2\u003c/sub\u003eO or dissociated OH\u003csup\u003e\u0026minus;\u003c/sup\u003e radicals on the surface.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e After 1 week of air exposure, peak deconvolution of the Al 2p and O 1s spectra (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ei) revealed the emergence of additional Al-OH bonding peaks, located at 531.4 eV for O 1s and 74.1 eV for Al 2p, attributed to the formation of hydrolysis-induced aluminum (oxy)hydroxide.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e The slight shifts in Al 2p and O 1s binding energies between the sapphire and its hydrolyzed phases, similar to gibbsite and boehmite (Figure S7), are minimal -around 0.1\u0026ndash;0.4 eV (in Table S2), closely approaching the resolution limits of XPS, which poses challenges for analysis.\u003c/p\u003e\n\u003cp\u003eGiven that the hydrolyzed phase appears to preferentially accumulate at the atomic step edges of engineered sapphire (in AFM image in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed), and considering the distinct electrical properties of the surface sapphire and the surface hydrolyzed phase, we introduced a planar differential charging effect by optimizing the electron flux intensity through the control of the flood gun filament current. The setup is depicted in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea, where the filament current of the electron flood gun (shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb) was set to operate in three distinct modes: no electron flux (i, NEF), low electron flux (ii, LEF) and high electron flux (iii, HEF) (the detailed setting in Table S1). As illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec, when the electron flux is precisely adjusted slightly beyond what is needed to neutralize the charge during XPS testing (under LEF mode), the excess electrons differentially distribute between the surface sapphire and the hydrolyzed phase due to their differing electrical capacitances. This results in a differential charging effect, creating a larger negative electric field on the hydrolyzed AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e phase. This phenomenon broadens the gap in binding energy shifts for Al 2p and O 1s peaks between the sapphire and hydrolyzed AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e phases, enhancing the clarity and analytical distinction between these two phases in XPS spectra.\u003c/p\u003e\n\u003cp\u003eWe firstly conducted the XPS measurement on freshly annealed sapphire under three testing modes: NEF, LEF and HEF. The angle-resolved XPS spectra of Al 2p, O 1s, C 1s of freshly annealed sapphire under NEF, LEF and HEF modes and the detailed analysis are presented in Supplementary section 2. No obvious differential charging effect was observed, indicating that the XPS setup is well-configured and does not contribute to differential charging phenomena.\u003c/p\u003e\n\u003cp\u003eAfter 2 weeks of air exposure, our XPS survey scan of the sapphire sample (refer to Figure S11) detected only aluminum, oxygen, and adventitious carbon confirming no additional surface contaminants. The detailed angle-resolved XPS spectra of Al 2p, O 1s, and C 1s at three testing modes of NEF, LEF and HEF, as well as the analysis are shown in supplementary section 3. Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed displayed the XPS spectra obtained at 30 \u003csup\u003eo\u003c/sup\u003e. In LEF, the binding energies of the O 1s and Al 2p peaks, corresponding to Al-O bonding in \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, are 530.8 eV and 73.9 eV, respectively, aligning with those measured in HEF mode. Notably, distinct low-energy shoulders are observed in both the O 1s and Al 2p peaks, located at 72.3 eV and 529 eV, respectively. The angle-resolved spectra and a summary of the fitting are given in supplementary section 3, revealing an increased ratio of the peak area compared to that of bulk \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e as the testing angle increases. This suggests that these additional peaks are indicative of a surface-sensitive phase. Based on the findings from our AFM and cross-sectional STEM experiments, which revealed that air exposure leads to surface hydrolysis and the formation of surface aluminium (oxy)hydroxide, we attribute these observed peaks to Al-OH bonding in surface aluminium (oxy)hydroxide. The ratio of the O1s peak area for Al-O bonding to Al-OH bonding is calculated as 0.167, similar to those observed in NEF and HEF modes. The binding energies for Al-OH in O 1s shift from 531.1 eV in HEF mode to 529 eV in LEF mode, a decrease of 2.1 eV, while the Al 2p peak shifts from 74.1 eV to 72.3 eV, a decrease of 1.8 eV. The binding energies for Al-O bonding in \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e remain unchanged across HEF and LEF modes. We attribute these differential shifts in binding energies between \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e in HEF and LEF modes to an enhanced differential charging effect during testing. Considering that this effect was not observed in freshly annealed sapphire but only after two weeks of air exposure, which leads to the formation of AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e clusters on the sapphire surface, we propose that this differential charging is due to the distinct dielectric properties of AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e clusters on the sapphire surface. In LEF mode, with a lower electron flux, more electrons accumulate at the AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e phase due to its greater capacitive nature compared to sapphire, resulting in a more pronounced negative electric field at the AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e phase, which reduces the binding energies of Al-OH in the Al 2p and O 1s peaks.\u003c/p\u003e\n\u003cp\u003eThe differential charging effect under low electron flux (LEF) mode proves to be advantageous in distinguishing the chemical structure of mixed phases on insulating surfaces that exhibit similar bonding energies. By selectively shifting the phase with higher dielectric constant to lower binding energies during the XPS measurement, this technique enhances the detection and analysis of subtle structural changes on insulating surfaces. Utilizing this approach, we investigated the air exposure-dependent evolution of the surface phase on the sapphire and found that the sapphire surface undergoes progressive hydrolyzation into aluminium (oxy)hydroxide\u0026mdash;from 11% in the first week to 67% after eight weeks. The detailed angle-resolved XPS spectra and a summary of the fitting are shown in Supplementary section 4. Furthermore, we evaluated the air-induced structural transformations on sapphire surfaces engineered with different atomic step heights of 0.2 nm and 1.0 nm and found that the higher the atomic step height on sapphire surfaces, the more pronounced the structural transforms triggered by hydrolysis. The detailed angle-resolved XPS spectra and summary of fitting are shown in Supplementary section 5.\u003c/p\u003e\n\u003cp\u003eThe hydrolysis of \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in air, resulting in the formation of amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e, significantly influences the surface chemistry of \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e during the epitaxial growth of TMD atomic layers. Typically, the epitaxial growth of TMDs occurs at temperatures ranging from 700 to 1000\u0026deg;C in an oxygen-free environment.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e This thermal heating during the growth process can substantially alter the surface structure of sapphire, thereby compromising the controllability of the epitaxy growth.\u003c/p\u003e\n\u003cp\u003eTo explore the surface structural changes of aged sapphire under epitaxial growth conditions, we selected a two-week-aged sapphire sample with 21% surface coverage of AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e as a model system. This sample was placed in a tube furnace and subjected to standard growth conditions, including specified temperature settings and gas flow, but without introducing growth precursors (detailed recipe in Figure S28). The detailed XPS analysis of pre-growth heating induced structural transformation of aged sapphire are shown in supplementary section 6. After post-growth heating at 900 \u003csup\u003eo\u003c/sup\u003eC, the Al 2p and O 1s peaks showed a sharpening corresponding to Al-O bonding in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, indicative of a phase transformation from amorphous to crystalline \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e at 900 \u003csup\u003eo\u003c/sup\u003eC, using the underlying \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e substrate as a crystalline template.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Moreover, we observed the disappearance of the characteristic AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e peaks (Al 2p at 72.3 eV and O 1s at 529 eV) and a significant increase in the peak area of high-energy O 1s peaks at 534.2 eV which we attributed to surface OH\u003csup\u003e\u0026minus;\u003c/sup\u003e or H\u003csub\u003e2\u003c/sub\u003eO bonding.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e These changes suggest the pyrolysis of the AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e phase during the thermal process, leading to the formation of an oxygen-vacancy-rich Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003e phase, which enhances surface H\u003csub\u003e2\u003c/sub\u003eO adsorption or hydroxylation upon air exposure during sample transfer. Previous studies indicate that heat-induced dehydroxylation of aluminium (oxy)hydroxide, such as Al(OH)\u003csub\u003e3\u003c/sub\u003e, leads to the formation of non-stoichiometric AlO\u003csub\u003ex\u003c/sub\u003e clusters with abundant oxygen vacancies when heated between 500 \u003csup\u003eo\u003c/sup\u003eC and 1000 \u003csup\u003eo\u003c/sup\u003eC. \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e These oxygen vacancies on the \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface can induce energetically favorable surface reconstructions, leading to the formation of various Al-rich surface structures distinct from the original crystalline \u0026alpha;-Al₂O₃ surface.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e The increase in the Al/O ratio from 0.57 to 0.76 before and after pre-growth heating at 900 \u003csup\u003eo\u003c/sup\u003eC, as shown in Figure S30, further supports the formation of an oxygen-vacancy-rich or Al-rich layer on the \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface.\u003c/p\u003e\n\u003cp\u003eUV irradiation is a well-established surface cleaning method, recognized for its effectiveness in removing organic contaminants and its suitability as a dry-cleaning technique.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e Previous research has shown that UV exposure significantly reduces Al-OH bonds in ALD-grown Al₂O₃ through a photo-induced electron transfer process. This involves electrons moving from deeper to shallower traps, accompanied by redox reactions that disrupt and create chemical bonds.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e Considering that the transformation of \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surfaces in air primarily results from the accumulation of hydroxyl-rich phases such as AlOOH or Al(OH)\u003csub\u003e3\u003c/sub\u003e, we explored the use of UV light irradiation to remove the aluminium oxyhydroxide layers from aged Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surfaces.\u003c/p\u003e\n\u003cp\u003eTo investigate the effects of UV irradiation on air-exposed sapphire substrates, we selected a sapphire sample with 2 weeks of air exposure as a model. This sample was subjected to UV irradiation (in air, \u0026lambda;\u0026thinsp;=\u0026thinsp;320 to 400 nm, power density of 0.5 W/cm\u0026sup2;) for 30 minutes, as illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea with the setup given in Figure S31. Subsequent XPS analysis, presented in Supplementary section 7, revealed notable structural transformations induced by UV irradiation. In Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb and Figure S32, we observed the disappearance of characteristic AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e peaks at Al 2p 72.3 eV and O 1s 529 eV, alongside a notable broadening at Al 2p 74 eV and O 1s 531 eV, corresponding to Al-O bonding in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. These changes suggest a transformation into an amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase. These results confirm that UV light irradiation effectively decomposes the surface AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e phase into amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, as depicted in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea.\u003c/p\u003e\n\u003cp\u003eTo investigate the surface transformation of the above UV irradiated sample under growth, we put that sample in the pre-growth heating process. In Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, the Al 2p and O 1s peaks after this pre-growth heating exhibited a noticeable sharpening, with the FWHM reduced to 1.19 and 1.25 eV, respectively, closely matching the 1.15 and 1.22 eV. observed in freshly annealed sapphire. The UV irradiated sample displayed a much weaker peak, in contrast with the prominent O 1s peak at 523.4 eV that is typically associated with OH- or H\u003csub\u003e2\u003c/sub\u003eO bonding and indicative of rich oxygen vacancies in the sapphire sample after pre-growth heating of 2-week aged sapphire without UV irradiation. This suggests a reduction in oxygen vacancies for the UV irradiates sample, aligning the spectra characteristics more closely with those of freshly annealed sapphire. As illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, UV irradiation before the growth process significantly restores the surface of degraded sapphire by removing AlO\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003ey\u003c/sub\u003e, reverting it to a structure similar to that of freshly annealed sapphire in the pre-growth process. This restoration is further supported by the Reflection High-Energy Electron Diffraction (RHEED) diffraction patterns of the 2-week aged sapphire and the repaired sapphire after 2-week aging, UV irradiation and pre-growth heating process. In Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, the aged Al₂O₃ sample initially shows a bright background with a blurred diffraction pattern, indicative of an amorphous layer on the \u0026alpha;-Al₂O₃ surface.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e In contrast, following UV exposure and pre-growth heating, the sample displays distinct and sharp RHEED stripes characteristic of crystalline \u0026alpha;-Al₂O₃, as confirmed by the intensity profile comparison in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed. The RHEED pattern of the repaired sapphire closely matches that of freshly annealed sapphire (shown in Figure S33), providing evidence of effectiveness of UV light in restoring the degradation of the sapphire surface.\u003c/p\u003e\n\u003cp\u003eOur investigation into the surface chemistry of sapphire under air exposure reveals that, after air exposure without UV irradiation, the sapphire surface exhibits a significant presence of oxygen vacancies: i.e. an Al-rich layer on the \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface. We hypothesize that the balance between strain in the Al-rich adlayer and the preference for Al atoms to occupy specific sites on the \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e substrate drives the formation of a \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e x \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e R9\u0026deg;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (0001) surface, consisting of a single well-ordered overlayer of surface Al atoms on bulk Al\u003csub\u003e2\u003c/sub\u003eO.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eAs illustrated in Figure S34, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e x \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e R9\u0026deg; reconstruction emerges from a relaxed (1x1) surface by desorbing the top two O layers along with a fraction of the surface Al.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e This reconstruction typically forms under ultra-high vacuum annealing at temperatures above 1400 \u003csup\u003eo\u003c/sup\u003eC \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e or by depositing 1 monolayer of Al atoms on \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e at lower temperature, such as 800 \u003csup\u003eo\u003c/sup\u003eC \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. The formation of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e x \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e R9\u0026deg; structure is influenced by the balance between the surface oxygen vacancies density and the temperature. Specifically, at low temperatures of 800 \u003csup\u003eo\u003c/sup\u003eC with high oxygen depletion (11%, calculated from the structure model of one monolayer Al deposited on \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface) and at high temperatures 1450 \u003csup\u003eo\u003c/sup\u003eC with low oxygen depletion (2%, \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e) the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e x \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e R9\u0026deg; reconstruction can form. In our experiment, the combination of high oxygen depletion and a high growth temperature of 900 \u003csup\u003eo\u003c/sup\u003eC falls within the range that can facilitate the formation of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e x \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e R9\u0026deg; reconstruction.\u003c/p\u003e\n\u003cp\u003eAdditionally, we noted an increased intensity of the OH\u003csup\u003e\u0026minus;\u003c/sup\u003e peak at 532.4 eV in O 1s spectrum post pre-growth heating, as detailed in Figure S29b and Table S10. This observation, coupled with the absence of a metallic Al layer suggests the reoxidation of the Al metal layer during sample transfer likely induced by exposure to air. This observation aligns with previous research, which demonstrated that the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e x \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{31}\\)\u003c/span\u003e\u003c/span\u003e R9\u0026deg; reconstructed structure, when exposed to water and hydrogen, forms hydroxide clusters on the surface, as evidenced by scanning force microscopy.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eTo investigate the impact of the reconstructed surface structure of sapphire on the epitaxy growth of TMD, we performed DFT calculations to determine the preferential orientation of TMD on both pristine and reconstructed sapphire surfaces. In Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea, the rotating angle was defined as the anticlockwise rotation required to align the zigzag edge of the WS\u003csub\u003e2\u003c/sub\u003e triangular domain, initially parallel to the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left[1\\overline{1}00\\right]\\)\u003c/span\u003e\u003c/span\u003e direction of the sapphire crystal, with the specific triangle. On a pristine c-plane sapphire substrate without a reconstructed Al layer, mirror domains prefer orientation angles of 30\u003csup\u003eo\u003c/sup\u003e or 90\u003csup\u003eo\u003c/sup\u003e. However, with the reconstructed Al layer, the preferred orientation angles shift to 0\u003csup\u003eo\u003c/sup\u003e or 60\u003csup\u003eo\u003c/sup\u003e. The adsorption energies as a function of orientation angle, plotted in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, indicate that the energy-favored orientation shifts from 30\u003csup\u003eo\u003c/sup\u003e on pristine sapphire to 0\u003csup\u003eo\u003c/sup\u003e on the reconstructed Al layer. These findings suggest that surface chemistry variations play a significant role in determining the domain orientation during TMD growth.\u003c/p\u003e\n\u003cp\u003eTo experimentally investigate the impact of the reconstructed Al layer on domain orientation, we used two common c-plane sapphire substrates with atomic steps along the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e (or a-axis) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e (or m-axis) directions; the AFM analysis is given in Figure S35. Previous studies reported that on c-plane sapphire with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003eatomic steps, TMD mirror domains tend to align with the step edges due to the similar degenerate energy for mirror domain nucleation.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e Conversely, on c-plane sapphire with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\:\\)\u003c/span\u003e\u003c/span\u003eatomic steps, unidirectional domain orientation is typically observed, driven by the minimized nucleation energy induced by the step edges.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e In our experiments, we employed these two sapphire substrates for the CVD growth of WS\u003csub\u003e2\u003c/sub\u003e monolayers. The stacking relationship of the domains on the sapphire crystal surface was determined by AFM. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec, for the pristine sapphire surface with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e steps, mirrored domains with orientation angles of 30\u003csup\u003eo\u003c/sup\u003e and 90\u003csup\u003eo\u003c/sup\u003e were aligned along the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e step edges. In contrast, on the surface with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\:\\)\u003c/span\u003e\u003c/span\u003esteps, unidirectional domains were observed aligned along the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\:\\)\u003c/span\u003e\u003c/span\u003esteps. This observation is consistent with published research\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e, indicating that our optimized growth conditions are suitable for identifying the impact of the reconstructed Al layer on growth.\u003c/p\u003e\n\u003cp\u003eWe performed a statistical analysis of domain orientation in relation to the evolution of sapphire surface chemistry over a large area. On sapphire substrates with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;11\\overline{2}0\u0026gt;\\)\u003c/span\u003e\u003c/span\u003e steps, the freshly annealed surface exhibited two mirrored domains with orientation angles of 30\u003csup\u003eo\u003c/sup\u003e and 90\u003csup\u003eo\u003c/sup\u003e, as shown in the optical image and corresponding statistical distribution of domain orientation in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed. After two weeks of air aging, numerous additional mirrored domains with orientation angles of 0\u003csup\u003eo\u003c/sup\u003e and 60\u003csup\u003eo\u003c/sup\u003e emerged, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee. This change is attributed to the formation of the reconstructed Al layer, derived from the aluminum oxyhydroxide precursor upon air exposure. Subsequently, when the aged sapphire was subjected to UV irradiation prior to the growth, the additional 0\u003csup\u003eo\u003c/sup\u003e and 60\u003csup\u003eo\u003c/sup\u003e domains disappeared, leaving only the original 30\u003csup\u003eo\u003c/sup\u003e and 90\u003csup\u003eo\u003c/sup\u003e domains similar to epitaxial growth observed on the freshly annealed sapphire (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ef). We attribute this to the UV light induced healing of the degraded sapphire surface, which removes the aluminum oxyhydroxide precursor, preventing the formation of the reconstructed Al layer, and promoting the recovery of the pristine crystal surface during growth.\u003c/p\u003e\n\u003cp\u003eFor sapphire with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;10\\overline{1}0\u0026gt;\\:\\)\u003c/span\u003e\u003c/span\u003esteps, the freshly annealed surface exhibits unidirectional domain orientation at a rotating angle of 0\u003csup\u003eo\u003c/sup\u003e, as shown in the optical image and corresponding statistical distribution of domain orientation in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eg. After 2 weeks of air aging, additional domains at an orientation angle of appeared (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eh). However, following UV irradiation on the aged sapphire, no additional 60\u003csup\u003eo\u003c/sup\u003e domains were detected, and the domains remained unidirectionally aligned with an orientation angle of 0\u003csup\u003eo\u003c/sup\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ei).\u003c/p\u003e\n\u003cp\u003eThis result demonstrates that the evolution of sapphire surface chemistry significantly impacts the orientation of epitaxially grown domains. The formation of aluminum oxyhydroxide during air exposure leads to the emergence of a reconstructed Al layer on the pristine sapphire substrate, which compromises the epitaxial film by introducing additional domain orientations. However, UV irradiation prior to growth\u003c/p\u003e\n\u003cp\u003eeffectively removes this aluminum oxyhydroxide precursor, restoring the sapphire surface to a state comparable to that of a freshly annealed surface. This process recovers the original domain alignment, ensuring high-quality WS\u003csub\u003e2\u003c/sub\u003e growth.\u003c/p\u003e\n\u003cp\u003eThe UV light surface repair strategy can be applied for wafer-scale growth of WS\u003csub\u003e2\u003c/sub\u003e. A photograph of a 2-inch c-plane sapphire wafer with a WS\u003csub\u003e2\u003c/sub\u003e film shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea, showing a uniform film color that indicates consistent growth with high coverage. Optical images taken from different locations on the 2-inch WS\u003csub\u003e2\u003c/sub\u003e film confirm its high surface quality and uniformity (in Figure S36). Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec present PL and Raman mapping across a 200 \u0026micro;m length of the 2-inch WS\u003csub\u003e2\u003c/sub\u003e wafer, revealing no significant variations in peak position and linewidth. Furthermore, the quality of the WS\u003csub\u003e2\u003c/sub\u003e film was assessed using circularly polarized PL spectra, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed. The calculated photon energy-dependent circular helicity in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee indicates a very high degree of circular helicity. This serves as strong evidence of the high quality of the film, as circular helicity arises from the valley selective spin excitations at the K and K\u0026apos; points of the Brillouin zone of monolayer WS\u003csub\u003e2\u003c/sub\u003e and is highly sensitive to defects that introduce intervalley scattering which degrades the circular helicity. For high-quality WS\u003csub\u003e2\u003c/sub\u003e monolayer flakes exfoliated from bulk single crystals, the measured circular helicity is typically lower than 40%. Remarkably, our WS\u003csub\u003e2\u003c/sub\u003e film on sapphire exhibited circular helicity up to 60% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee), which is comparable to the highest reported for exfoliated flakes.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e Additionally, high-resolution HAADF STEM demonstrate the uniform local atomic structure of the WS\u003csub\u003e2\u003c/sub\u003e monolayer, as shown in the atomic image in Figure S37, the featured atomic structure with a hexagonal unit cell is clearly visible in the enlarged inset on the left.\u003c/p\u003e\n\u003cp\u003eTo further assess the sample quality, the chemical composition and electronic band structure of the WS\u003csub\u003e2\u003c/sub\u003e monolayer film were measured by XPS and angle-resolved photoemission spectroscopy (ARPES). XPS data in Figure S38 indicates a WS\u003csub\u003e2\u003c/sub\u003e stoichiometry comparable with a WS\u003csub\u003e2\u003c/sub\u003e bulk single crystal (from HQ graphene). The valence band dispersion along the \u0026Gamma;M and \u0026Gamma;K high symmetrical directions of the Surface Brillouin Zone of the WS\u003csub\u003e2\u003c/sub\u003e monolayer is shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ef. The results are in line with previous measurements obtained for single crystal WS\u003csub\u003e2\u003c/sub\u003e monolayers\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e and confirm the single orientation of the WS\u003csub\u003e2\u003c/sub\u003e domain in the probed area (~\u0026thinsp;1 mm\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e). For multiple domain TMD films, the simultaneous observation of GK and GM band dispersion would be expected, reflecting the different in-plane orientation of the TMD domains under the ARPES probe area, as previously reported for MoS\u003csub\u003e2\u003c/sub\u003e and WS\u003csub\u003e2\u003c/sub\u003e thin films;\u003csup\u003e52 53\u003c/sup\u003e this multiple domain signal is not observed in our WS\u003csub\u003e2\u003c/sub\u003e, emphasizing its high level of mono-crystallinity.\u003c/p\u003e\n\u003cp\u003eIn summary, we have demonstrated that the surface chemistry of sapphire critically influenced the domain orientation and quality of WS\u003csub\u003e2\u003c/sub\u003e films grown on the sapphire substrate via CVD. When exposed to air, sapphire undergoes hydrolysis, leading to the formation of aluminum oxyhydroxide and subsequent pyrolysis and Al-rich surface reconstruction in the pre-growth stage, which adversely impacts the alignment of crystal orientation epitaxy growth. Moreover, we found that UV irradiation prior to the CVD growth effectively repairs the surface by removing aluminum oxyhydroxide, restoring the sapphire to a state comparable to freshly annealed sapphire. The WS\u003csub\u003e2\u003c/sub\u003e monolayer wafer grown on repaired sapphire exhibits remarkable uniformity and mono-crystallinity. This UV light repair strategy proved effective for both small-scale and wafer-scale growth, offering a valuable method for producing high-performance TMD films for advanced electronic and optoelectronic applications.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv class=\"Section2\"\u003e\n \u003cdiv class=\"Section3\"\u003e\n \u003ch2\u003eThermal annealing of sapphire substrates\u003c/h2\u003e\n \u003cp\u003eSingle-side polished c-plane sapphire substrates (001) were purchased from Princeton Scientific Corporation and Hefei Crystal Technical Materials. These substrates were fabricated to produce orientations of (0001) off \u003cstrong\u003ea\u003c/strong\u003e (11\u0026ndash;20) at 1.0\u0026deg; \u0026plusmn; 0.1\u0026deg;, and off \u003cstrong\u003em\u003c/strong\u003e (10\u0026ndash;10) directions at 0.5\u0026deg; \u0026plusmn; 0.1\u0026deg;. For the engineering of atomic step heights of a sapphire, thermal annealing was conducted using a Carbolite RHF muffle furnace located within a cleanroom environment. The annealing process involved heating the substrates to 1250\u0026deg;C at a rate of 5\u0026deg;C/min and holding for 4 hours to achieve an atomic step height of ~\u0026thinsp;0.2 nm, and to 1350\u0026deg;C for 4 hours to achieve an atomic step height of ~\u0026thinsp;1 nm.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003ch3\u003eAging sapphires in air\u003c/h3\u003e\n\u003cp\u003eThe freshly engineered sapphire substrates were exposed to an air environment within a cleanroom. Each substrate was placed on a petri dish lined with filter paper and subjected to various aging durations to assess the effects on surface chemistry and morphology.\u003c/p\u003e\n\u003ch3\u003eUV exposure in air\u003c/h3\u003e\n\u003cp\u003eThe aged sapphires were placed inside a Samco model UV-1 system. The substrates were exposed to UV light (wavelength of 320 nm to 400 nm, power density of 0.5 W/cm\u003csup\u003e2\u003c/sup\u003e) in an air for 30 minutes.\u003c/p\u003e\n\u003ch3\u003eEpitaxial growth of WS monolayer\u003c/h3\u003e\n\u003cp\u003eThe epitaxial growth was carried out in a three-zone CVD system with a tube diameter of 70 mm. Sulfur powders (6.0 g, 99.998%, ALDRICH) were placed in the upstream of heating zone I and heated between 150\u0026ndash;200\u0026deg;C. WO\u003csub\u003e3\u003c/sub\u003e powders (99.995%, ALDRICH) and KCl (99.5%, ALDRICH) were positioned in heating zone II, and sapphire substrates were situated in heating zone III. The CVD process was conducted under low pressure within an argon atmosphere. Specifically, the growth temperatures of zone I, zone II and zone III were set at 180, 630, and 950 \u003csup\u003eo\u003c/sup\u003eC, respectively, with a gas flow of argon (400 sccm) and hydrogen (10 sccm). The pressure in the growth chamber was kept at 5 torr. The growth time varied, with 5\u0026ndash;15 min for individual islands, and 30\u0026ndash;40 min for a continuous film. To maintain consistency across different CVD growth runs, freshly annealed sapphire was systematically placed alongside the target sapphire for each session.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSample characterizations.\u003c/strong\u003e Raman and PL spectra were collected at room temperature using the confocal WiTec Alpha 300R Raman Microscope (laser excitation, 532 nm). The surface profiles of sapphires samples were tested by Bruker\u0026rsquo;s dimension icon AFM system. The XPS measurements for sapphire samples were conducted by a VG ESCALAB 220i-XL system with a monochromatic Al K\u0026alpha; source and a pass energy of 10 eV, with electron flood gun filament current setting at 0-2.4 A. The XPS data of WS\u003csub\u003e2\u003c/sub\u003e were acquired with a hemispherical electron analyzer (SCIENTA HiPP-2) in normal emission condition, by using a monochromatized Al K\u0026alpha; (\u003cem\u003eh\u0026nu;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1486.6 eV) as energy excitation and energy resolution of ~\u0026thinsp;0.3 eV. RHEED (STAIB Instruments) was measured at room temperature under an ultrahigh vacuum of 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e torr. The electron acceleration voltage was 15 kV. Atomic-resolution STEM-ADF imaging was performed on an aberration-corrected JEOL ARM200F, equipped with a cold field-emission gun and an ASCOR corrector operating at 80 kV. The convergence semi-angle of the probe was around 30 mrad. Cross-sectional STEM lamellae of sapphire samples were prepared using a focused ion beam (FIB, FEI Helios Nanolab 600). For low-temperature circular dichroic photoluminescence (CDPL) measurements, the sample was kept in a cryostat on top of a motorized stage, and a 570 nm pulsed laser (\u0026sim;80 ps) was used for the excitation. The ARPES data were acquired in a custom-designed system, with a hemispherical electron analyzer (SCIENTA DA30L) and monochromatized HeI\u0026alpha; (\u003cem\u003eh\u0026nu;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;21.218 eV) radiation source (SCIENTA VUV5k). Before ARPES measurement the WS\u003csub\u003e2\u003c/sub\u003e ML on HOPG was annealed at 200 \u003csup\u003eo\u003c/sup\u003eC x 12 h and 400 \u003csup\u003eo\u003c/sup\u003eC x1 h to remove surface contaminants.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTheoretical calculation.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe adsorption energies of WS\u003csub\u003e2\u003c/sub\u003e monolayer on a c-plane Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface (without and with reconstructed surface) were calculated using density functional theory (DFT) with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional as implemented in the Vienna \u003cem\u003eab initio\u003c/em\u003e simulation package (VASP).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e The cut-off energy for plane-wave basis set was set at 500 eV. The system was modeled as a triangular island of a monolayer WS\u003csub\u003e2\u003c/sub\u003e (4 unit cells on each side) on a 3-layer Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e substrate. 6\u0026times;6 supercell and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\sqrt{31}\\times\\:\\sqrt{31}\\right)R9^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003e supercells were used for crystal and reconstructed Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surfaces, respectively. The structures were optimized at \u0026Gamma; point until the Hellmann-Feynman forces on the atoms were less than 0.05 eV/\u0026Aring;. The DFT-D2 method of Grimme\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e was used to include the van der Waals interaction.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eWe acknowledge the funding support from the Agency for Science, Technology and Research Grant (C230917006). K. E. J. G. acknowledges a Singapore National Research Foundation Grant (CRP21-2018-0001). This research is supported by the Ministry of Education, Singapore, under its AcRF Tier 2 (MOE-T2EP50122-0016). We acknowledge the funding support from the Agency for Science, Technology and Research (C230917006). The computational work was performed on resources of the National Supercomputing Centre (NSCC), Singapore and the A*STAR Computational Resource Centre (A*CRC).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eFiori G., Bonaccorso F., Iannaccone G., Palacios T., Neumaier D., Seabaugh A.\u003cem\u003e, et al.\u003c/em\u003e Electronics based on two-dimensional materials. \u003cem\u003eNat. Nanotechnol.\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, 768-779 (2014)\u003c/li\u003e\n \u003cli\u003eBriggs N., Subramanian S., Lin Z., Li X., Zhang X., Zhang K.\u003cem\u003e, et al.\u003c/em\u003e A roadmap for electronic grade 2D materials. \u003cem\u003e2D Mater.\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 022001 (2019)\u003c/li\u003e\n \u003cli\u003eLin Y.C., Jariwala B., Bersch B., Xu K., Nie Y., Wang B.\u003cem\u003e, et al.\u003c/em\u003e Realizing large-scale, electronic-grade two-dimensional semiconductors. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 965-975 (2018)\u003c/li\u003e\n \u003cli\u003eAkinwande D., Huyghebaert C., Wang C.H., Serna M.I., Goossens S., Li L.J.\u003cem\u003e, et al.\u003c/em\u003e Graphene and two-dimensional materials for silicon technology. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e573\u003c/strong\u003e, 507-518 (2019)\u003c/li\u003e\n \u003cli\u003eXu Z.Q., Zhang Y.P., Lin S.H., Zheng C.X., Zhong Y.L., Xia X.\u003cem\u003e, et al.\u003c/em\u003e Synthesis and transfer of large-area monolayer WS\u003csub\u003e2\u003c/sub\u003e Crystals: moving toward the recyclable use of sapphire substrates. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, 6178-6187 (2015)\u003c/li\u003e\n \u003cli\u003eChoi W., Choudhary N., Han G.H., Park J., Akinwande D., Lee Y.H. Recent development of two-dimensional transition metal dichalcogenides and their applications. \u003cem\u003eMater. Today\u003c/em\u003e \u003cstrong\u003e20\u003c/strong\u003e, 116-130 (2017)\u003c/li\u003e\n \u003cli\u003eFu J.H., Min J., Chang C.K., Tseng C.C., Wang Q., Sugisaki H.\u003cem\u003e, et al.\u003c/em\u003e Oriented lateral growth of two-dimensional materials on c-plane sapphire. \u003cem\u003eNat. Nanotechnol.\u003c/em\u003e \u003cstrong\u003e18\u003c/strong\u003e, 1289-1294 (2023)\u003c/li\u003e\n \u003cli\u003eLi T., Guo W., Ma L., Li W., Yu Z., Han Z.\u003cem\u003e, et al.\u003c/em\u003e Epitaxial growth of wafer-scale molybdenum disulfide semiconductor single crystals on sapphire. \u003cem\u003eNat. Nanotechnol.\u003c/em\u003e \u003cstrong\u003e16\u003c/strong\u003e, 1201-1207 (2021)\u003c/li\u003e\n \u003cli\u003eWang J., Xu X., Cheng T., Gu L., Qiao R., Liang Z.\u003cem\u003e, et al.\u003c/em\u003e Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS\u003csub\u003e2\u003c/sub\u003e monolayer on vicinal a-plane sapphire. \u003cem\u003eNat. Nanotechnol.\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 33-38 (2022)\u003c/li\u003e\n \u003cli\u003eZhu H., Nayir N., Choudhury T.H., Bansal A., Huet B., Zhang K.\u003cem\u003e, et al.\u003c/em\u003e Step engineering for nucleation and domain orientation control in WSe\u003csub\u003e2\u003c/sub\u003e epitaxy on c-plane sapphire. \u003cem\u003eNat. Nanotechnol.\u003c/em\u003e \u003cstrong\u003e18\u003c/strong\u003e, 1295-1302 (2023)\u003c/li\u003e\n \u003cli\u003eZheng P., Wei W., Liang Z., Qin B., Tian J., Wang J.\u003cem\u003e, et al.\u003c/em\u003e Universal epitaxy of non-centrosymmetric two-dimensional single-crystal metal dichalcogenides. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 592 (2023)\u003c/li\u003e\n \u003cli\u003eSmink S., Majer L.N., Boschker H., Mannhart J., Braun W. Long-range atomic order on double-stepped Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001) Surfaces. \u003cem\u003eAdv. Mater.\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2312899 (2024)\u003c/li\u003e\n \u003cli\u003eMo J., El Kazzi S., Mortelmans W., Mehta A.N., Sergeant S., Smets Q.\u003cem\u003e, et al.\u003c/em\u003e Importance of the substrate\u0026rsquo;s surface evolution during the MOVPE growth of 2D-transition metal dichalcogenides. \u003cem\u003eNanotechnology\u003c/em\u003e \u003cstrong\u003e31\u003c/strong\u003e, 125604 (2020)\u003c/li\u003e\n \u003cli\u003eBarth C., Reichling M. Imaging the atomic arrangements on the high-temperature reconstructed \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001) surface. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e414\u003c/strong\u003e, 54-57 (2001).\u003c/li\u003e\n \u003cli\u003eEng P.J., Trainor T.P., Brown Jr G.E., Waychunas G.A., Newville M., Sutton S.R.\u003cem\u003e, et al.\u003c/em\u003e Structure of the hydrated \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (0001) Surface. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e288\u003c/strong\u003e, 1029-1033 (2000)\u003c/li\u003e\n \u003cli\u003eChambers S.A., Droubay T., Jennison D.R., Mattsson T.R. Laminar growth of ultrathin metal films on metal oxides: Co on hydroxylated \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001). \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e297\u003c/strong\u003e, 827-831 (2002)\u003c/li\u003e\n \u003cli\u003eAljarb A., Cao Z., Tang H.-L., Huang J.K., Li M., Hu W.\u003cem\u003e, et al.\u003c/em\u003e Substrate lattice-guided seed formation controls the orientation of 2D transition-metal dichalcogenides. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e11\u003c/strong\u003e, 9215-9222 (2017)\u003c/li\u003e\n \u003cli\u003eRanea V.A., Carmichael I., Schneider W.F. DFT investigation of intermediate steps in the hydrolysis of \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001). \u003cem\u003eJ. Phys. Chem. C\u003c/em\u003e \u003cstrong\u003e113\u003c/strong\u003e, 2149-2158 (2009)\u003c/li\u003e\n \u003cli\u003eChen X., Shan W., Wu D., Patel S.B., Cai N., Li C.\u003cem\u003e, et al.\u003c/em\u003e Atomistic mechanisms of water vapor\u0026ndash;induced surface passivation. \u003cem\u003eSci. Adv.\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, eadh5565 (2023)\u003c/li\u003e\n \u003cli\u003eLiu Q., Tong X., Zhou G. H\u003csub\u003e2\u003c/sub\u003eO dissociation-induced aluminum oxide growth on oxidized Al(111) surfaces. \u003cem\u003eLangmuir\u003c/em\u003e \u003cstrong\u003e31\u003c/strong\u003e, 13117-13126 (2015)\u003c/li\u003e\n \u003cli\u003eLiu Q., Qin H., Boscoboinik J.A., Zhou G. Comparative Study of the oxidation of NiAl(100) by molecular oxygen and water vapor using ambient-pressure X-ray photoelectron spectroscopy. \u003cem\u003eLangmuir\u003c/em\u003e \u003cstrong\u003e32\u003c/strong\u003e, 11414-11421 (2016)\u003c/li\u003e\n \u003cli\u003eLiu P., Kendelewicz T., Brown G.E., Nelson E.J., Chambers S.A. Reaction of water vapor with \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001) and \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001) surfaces: synchrotron X-ray photoemission studies and thermodynamic calculations. \u003cem\u003eSurf. Sci.\u003c/em\u003e \u003cstrong\u003e417\u003c/strong\u003e, 53-65 (1998)\u003c/li\u003e\n \u003cli\u003eMalki A., Mekhalif Z., Detriche S., Fonder G., Boumaza A., Djelloul A. Calcination products of gibbsite studied by X-ray diffraction, XPS and solid-state NMR. \u003cem\u003eJ. Solid State Chem.\u003c/em\u003e \u003cstrong\u003e215\u003c/strong\u003e, 8-15 (2014)\u003c/li\u003e\n \u003cli\u003eYue Y., Melani G., Kirsch H., Paarmann A., Saalfrank P., Campen R.K.\u003cem\u003e, et al.\u003c/em\u003e Structure and reactivity of \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001) surfaces: how do Al\u0026ndash;I and gibbsite-like terminations interconvert? \u003cem\u003eJ. Phys. Chem. C\u003c/em\u003e \u003cstrong\u003e126\u003c/strong\u003e, 13467-13476 (2022)\u003c/li\u003e\n \u003cli\u003eGreczynski G., Hultman L. Binding energy referencing in X-ray photoelectron spectroscopy. \u003cem\u003eNat. Rev. Mater.\u003c/em\u003e DOI: 10.1038/s41578-024-00743-5 (2024)\u003c/li\u003e\n \u003cli\u003eMoulder J.F., Stickle W.F., Sobol P.E., Bomben K.D. Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data (Perkin-Elmer, 1992)\u003c/li\u003e\n \u003cli\u003ePark Y., Ahn C., Ahn J.G., Kim J.H., Jung J., Oh J.\u003cem\u003e, et al.\u003c/em\u003e Critical role of surface termination of sapphire substrates in crystallographic epitaxial growth of MoS\u003csub\u003e2\u003c/sub\u003e using inorganic molecular precursors. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 1196-1205 (2023)\u003c/li\u003e\n \u003cli\u003eZhou J., Lin J., Huang X., Zhou Y., Chen Y., Xia J.\u003cem\u003e, et al.\u003c/em\u003e A library of atomically thin metal chalcogenides. \u003cem\u003eNature\u003c/em\u003e 2018, \u003cstrong\u003e556\u003c/strong\u003e, 355-359 (2018)\u003c/li\u003e\n \u003cli\u003eYang P., Liu F., Li X., Hu J., Zhou F., Zhu L.\u003cem\u003e, et al.\u003c/em\u003e Highly reproducible epitaxial growth of wafer-scale single-crystal monolayer MoS\u003csub\u003e2\u003c/sub\u003e on sapphire. \u003cem\u003eSmall Methods\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 2300165 (2023)\u003c/li\u003e\n \u003cli\u003eCohen A., Mohapatra P.K., Hettler S., Patsha A., Narayanachari K.V.L.V., Shekhter P.\u003cem\u003e, et al.\u003c/em\u003e Tungsten oxide mediated quasi-van der waals epitaxy of WS\u003csub\u003e2\u003c/sub\u003e on sapphire. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 5399-5411 (2023)\u003c/li\u003e\n \u003cli\u003eBagwell R.B., Messing G.L. Effect of seeding and water vapor on the nucleation and growth of \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e from \u0026gamma;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. \u003cem\u003eJ. Am. Ceram. Soc.\u003c/em\u003e \u003cstrong\u003e82\u003c/strong\u003e, 825-832 (1999)\u003c/li\u003e\n \u003cli\u003eEklund P., Sridharan M., Singh G., B\u0026oslash;ttiger J. Thermal stability and phase transformations of \u0026gamma;-/amorphous-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e thin films. \u003cem\u003ePlasma Processes Polym.\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, S907-S911 (2009)\u003c/li\u003e\n \u003cli\u003eReichel F., Jeurgens L.P.H., Richter G., Mittemeijer E.J. Amorphous versus crystalline state for ultrathin Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e overgrowths on Al substrates. \u003cem\u003eJ. Appl. Phys.\u003c/em\u003e \u003cstrong\u003e103\u003c/strong\u003e, 093515 (2008)\u003c/li\u003e\n \u003cli\u003eWilson S.J. The dehydration of boehmite, \u0026gamma;-AlOOH, to \u0026gamma;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. \u003cem\u003eJ. Solid State Chem.\u003c/em\u003e \u003cstrong\u003e30\u003c/strong\u003e, 247-255 (1979)\u003c/li\u003e\n \u003cli\u003eRam\u0026iacute;rez L.P., Bournel F., Gallet J.-J., Dudy L., Rochet F. Testing the cabrera\u0026ndash;Mott oxidation model for aluminum under realistic conditions with near-ambient pressure photoemission. \u003cem\u003eJ. Phys. Chem. C\u003c/em\u003e \u003cstrong\u003e126\u003c/strong\u003e, 2517-2530 (2022)\u003c/li\u003e\n \u003cli\u003eMortelmans W., El Kazzi S., Groven B., Nalin Mehta A., Balaji Y., De Gendt S.\u003cem\u003e, et al.\u003c/em\u003e Epitaxial registry and crystallinity of MoS\u003csub\u003e2\u003c/sub\u003e via molecular beam and metalorganic vapor phase van der Waals epitaxy. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e117\u003c/strong\u003e, 033101 (2020)\u003c/li\u003e\n \u003cli\u003eVig J., LeBus J. UV/ozone cleaning of surfaces. \u003cem\u003eIEEE Transactions on Parts, Hybrids, and Packaging\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 365-370 (1976)\u003c/li\u003e\n \u003cli\u003eVig J.R. UV/ozone cleaning of surfaces. \u003cem\u003eJ. Vac. Sci. Tech. A\u003c/em\u003e \u003cstrong\u003e3\u003c/strong\u003e, 1027-1034 (1985)\u003c/li\u003e\n \u003cli\u003eSun C., Zeng R., Zhang J., Qiu Z.J., Wu D. Effects of UV-ozone treatment on sensing behaviours of EGFETs with Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sensing film. \u003cem\u003eMaterials\u0026nbsp;\u003c/em\u003e\u003cstrong\u003e10\u003c/strong\u003e, 1532 (2017)\u003c/li\u003e\n \u003cli\u003eYoon K.H., Kim H., Koo Lee Y.E., Shrestha N.K., Sung M.M. UV-enhanced atomic layer deposition of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e thin films at low temperature for gas-diffusion barriers. \u003cem\u003eRSC Adv.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 5601-5609 (2017)\u003c/li\u003e\n \u003cli\u003eFons P., Iwata K., Niki S., Yamada A., Matsubara K.J.J.o.C.G. Growth of high-quality epitaxial ZnO films on \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. \u003cstrong\u003e201\u003c/strong\u003e, 627-632 (1999)\u003c/li\u003e\n \u003cli\u003eLauritsen J.V., Jensen M.C.R., Venkataramani K., Hinnemann B., Helveg S., Clausen B.S.\u003cem\u003e, et al.\u003c/em\u003e Atomic-scale structure and stability of the x . surface of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001). \u003cem\u003ePhys. Rev. Lett.\u003c/em\u003e \u003cstrong\u003e103\u003c/strong\u003e, 076103 (2009)\u003c/li\u003e\n \u003cli\u003eRenaud G., Villette B., Vilfan I., Bourret A. Atomic structure of the \u0026alpha;- Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(0001)( x )R\u0026plusmn;9\u003csup\u003eo\u003c/sup\u003e reconstruction. \u003cem\u003ePhys. Rev. Lett.\u003c/em\u003e \u003cstrong\u003e73\u003c/strong\u003e, 1825-1828 (1994)\u003c/li\u003e\n \u003cli\u003eMilakhin D.S., Malin T.V., Mansurov V.G., Galitsyn Y.G., Kozhukhov A.S., Utkin D.E.\u003cem\u003e, et al.\u003c/em\u003e Peculiarities of the AlN crystalline phase formation in a result of the electron-stimulated reconstruction transition( x )R\u0026plusmn;9\u003csup\u003eo\u003c/sup\u003e \u0026minus; (1 \u0026times; 1). \u003cem\u003eAppl. Surf. Sci.\u003c/em\u003e \u003cstrong\u003e541\u003c/strong\u003e, 148548 (2021)\u003c/li\u003e\n \u003cli\u003eFrench T.M., Somorjai G.A. Composition and surface structure of the (0001) face of \u0026alpha;-alumina by low-energy electron diffraction. \u003cem\u003eJ. Phys. Chem.\u003c/em\u003e \u003cstrong\u003e74\u003c/strong\u003e, 2489-2495 (1970)\u003c/li\u003e\n \u003cli\u003eVermeersch M., Sporken R., Lambin P., Caudano R. The AlAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e interface formation as studied by electron spectroscopies. \u003cem\u003eSurf. Sci.\u003c/em\u003e \u003cstrong\u003e235\u003c/strong\u003e, 5-14 (1990)\u003c/li\u003e\n \u003cli\u003eRichmond E.D. Xps Analysis of the sapphire surface as a function of high temperature vacuum annealing. \u003cem\u003eMRS Online Proc. Lib.\u003c/em\u003e \u003cstrong\u003e159\u003c/strong\u003e, 253-256 (1989)\u003c/li\u003e\n \u003cli\u003eZhang T., Wang J., Wu P., Lu A.-Y., Kong J. Vapour-phase deposition of two-dimensional layered chalcogenides. \u003cem\u003eNat. Rev. Mater.\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 799-821 (2023)\u003c/li\u003e\n \u003cli\u003eScrace T., Tsai Y., Barman B., Schweidenback L., Petrou A., Kioseoglou G.\u003cem\u003e, et al.\u003c/em\u003e Magnetoluminescence and valley polarized state of a two-dimensional electron gas in WS2 monolayers. \u003cem\u003eNat. Nanotechnol.\u003c/em\u003e \u003cstrong\u003e10\u003c/strong\u003e, 603-607 (2015)\u003c/li\u003e\n \u003cli\u003ePlechinger G., Nagler P., Arora A., Schmidt R., Chernikov A., del \u0026Aacute;guila A.G.\u003cem\u003e, et al.\u003c/em\u003e Trion fine structure and coupled spin\u0026ndash;valley dynamics in monolayer tungsten disulfide. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 12715 (2016)\u003c/li\u003e\n \u003cli\u003eBussolotti F., Yang J., Kawai H., Chee J.Y., Goh K.E.J. Influence of many-body effects on hole quasiparticle dynamics in a WS\u003csub\u003e2\u003c/sub\u003e monolayer. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cstrong\u003e103\u003c/strong\u003e, 045412 (2021)\u003c/li\u003e\n \u003cli\u003eBussolotti F., Chai J., Yang M., Kawai H., Zhang Z., Wang S.\u003cem\u003e, et al.\u003c/em\u003e Electronic properties of atomically thin MoS\u003csub\u003e2\u003c/sub\u003e layers grown by physical vapour deposition: band structure and energy level alignment at layer/substrate interfaces. \u003cem\u003eRSC Adv.\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 7744-7752 (2018)\u003c/li\u003e\n \u003cli\u003eLim Y.F., Priyadarshi K., Bussolotti F., Gogoi P.K., Cui X., Yang M.\u003cem\u003e, et al.\u003c/em\u003e Modification of vapor phase concentrations in MoS\u003csub\u003e2\u003c/sub\u003e growth using a NiO foam barrier. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 1339-1349 (2018)\u003c/li\u003e\n \u003cli\u003eKresse G., Hafner, J. Molecular Dynamics for Liquid Metals. Phys. Rev. B \u003cstrong\u003e47\u003c/strong\u003e, 558(R) (1993)\u003c/li\u003e\n \u003cli\u003eKresse G., Hafner J. \u003cem\u003eAb initio\u003c/em\u003e Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor transition in Germanium. Phys. Rev. B \u003cstrong\u003e49\u003c/strong\u003e, 14251 (1994)\u003c/li\u003e\n \u003cli\u003ePerdew J. P., Burke K., Ernzerhof M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. \u003cstrong\u003e77\u003c/strong\u003e, 3865 (1996)\u003c/li\u003e\n \u003cli\u003eGrimme S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. \u003cstrong\u003e27\u003c/strong\u003e, 1787 (2006)\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5529369/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5529369/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEngineering sapphire substrates with specific surface characteristics is crucial for the epitaxial growth of high-quality wafer-scale transition metal dichalcogenides (TMDs), essential for integration with semiconductor industry processes. Here, we address the gap in understanding the air-induced structure transformation of such engineered sapphire surfaces and their impact on the epitaxial growth of WS\u003csub\u003e2\u003c/sub\u003e. We identified a deactivation pathway for fresh atomic-step-engineered sapphire (α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) when exposed to air, leading to surface hydrolysis and the formation of aluminum (oxy)hydroxides preferentially along the atomic step edges. These compounds subsequently pyrolyze into oxygen-deficient Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3-x \u003c/sub\u003eunder growth conditions. The presence of such an oxygen-deficient non-stoichiometric phase, apart from pristine α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, disrupts the alignment of growth domains by altering the surface chemistry. Additionally, we demonstrate that UV-light irradiation in air prior to growth effectively repairs degraded sapphire surfaces by removing hydroxyls from aluminum oxyhydroxide, forming amorphous stoichiometric Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, which then transforms into the desired α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase under growth temperature of 900 \u003csup\u003eo\u003c/sup\u003eC, favoring orientation-controlled epitaxy. These findings relevant insights toward the consistent production of industrial-scale, high-quality TMD films.\u003c/p\u003e","manuscriptTitle":"Unveiling air-induced surface transformation on atomic step-engineered sapphire for oriented growth of transition metal dichalcogenides","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-23 06:13:31","doi":"10.21203/rs.3.rs-5529369/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9984c9fe-75f0-4396-9557-7a40c85f4753","owner":[],"postedDate":"April 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":42099737,"name":"Physical sciences/Materials science"},{"id":42099738,"name":"Physical sciences/Nanoscience and technology"}],"tags":[],"updatedAt":"2025-09-27T07:06:40+00:00","versionOfRecord":{"articleIdentity":"rs-5529369","link":"https://doi.org/10.1038/s41467-025-63452-9","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-09-26 04:00:00","publishedOnDateReadable":"September 26th, 2025"},"versionCreatedAt":"2025-04-23 06:13:31","video":"","vorDoi":"10.1038/s41467-025-63452-9","vorDoiUrl":"https://doi.org/10.1038/s41467-025-63452-9","workflowStages":[]},"version":"v1","identity":"rs-5529369","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5529369","identity":"rs-5529369","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}