{"paper_id":"2a68a9a8-e8fa-4ad3-b992-b90a61f6ec6d","body_text":"Mass Production of Gradient Perovskite Nanowires via Microscale Thermal Engineering | 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 Mass Production of Gradient Perovskite Nanowires via Microscale Thermal Engineering Yiming Yang, Jianliang Li, Jing Li, Jiao Xu, Weili Liu, Conghui Tan, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5739231/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The bandgap engineering of halide perovskites at microscopic level is challenging due to fabrication complexity, environmental sensitivity and material stability. Here we report a facile approach to high-yield compositional graded perovskite nanowires (NWs) via vapor-phase anion exchange methods. Using rationally engineered thermal inhomogeneity along the length of single NWs, arrays of NWs with compositional gradient across tens of micrometers can be readily mass-produced via bottom-up as well as top-down exchange strategies. These exchanged NWs exhibit well-preserved single-crystallinity for efficient optical and electrical transport, while their halogen stoichiometry, fluorescence, and energy band structure demonstrate apparent axial gradient. Detailed analysis of elemental distribution and thermal simulation reveal that the ultralow thermal conductivity together with reduced dimensionality leads to microscale temperature gradient, which is further converted to compositional gradient upon anion exchange. In addition, the gradient NWs show excellent optoelectronic features suitable for further integration into functional devices. This work provides guidelines for composition manipulation of perovskites through thermal engineering, extending their applications in ultracompact microspectrometers, spectral imaging sensors, and other miniaturized optoelectronic devices. Physical sciences/Nanoscience and technology/Nanoscale materials/Nanowires Physical sciences/Materials science/Nanoscale materials/Nanowires halide perovskites nanomaterials bandgap engineering ionic transport photoluminescence Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 INTRODUCTION The success of halide perovskites in photovoltaics brings about extensive interest of their broader applications for semiconductor devices including light-emitting diodes 1 – 4 , photodetectors 5 – 8 , lasers 9 – 12 , and memristors 13 – 16 . Particularly, the soft lattice of perovskites enables exceptional tunability of their composition, optical and electrical properties via bandgap engineering. Aside from conventional methods including doping or quantum confinement effect, the bandgap engineering in perovskites can be conveniently achieved through exchange of their halogen anions in the liquid 17 – 19 , vapor 5 , 20 , 21 , and solid phase 22 – 24 . However, at microscale, the tuning of local band structure becomes formidable and uncontrolled, which is largely due to the chaotic nature of anionic thermal diffusion. To increase the precision and spatial resolution of bandgap engineering, a variety of methods have been developed to combine with anion exchange, such as nanofabrication 6 , 17 , micromanipulation 23 , 25 , and mechanical motion 18 , 19 . These approaches essentially transform the artificially created spatial or environmental uniformities to compositional variation in perovskites. Nevertheless, the increased complexity of fabrication procedures results in surging cost of devices and difficulty for mass-production. Alternatively, it might be possible to utilize the intrinsic thermodynamic features of perovskites for microscale bandgap engineering. Halide perovskites possess one of the lowest thermal conductivity found in natural crystalline-state semiconductors 26 – 28 , which originates from the loosely bonded lattice as well as heavy element such as Pb. In nanostructured all-inorganic perovskites, ultralow thermal conductivity down to 0.38 W·m − 1 ·K − 1 has been reported 29 . The inefficient thermal transport in perovskites leads to short thermal decay length, providing opportunities for local spatial-resolved anion exchange. Moreover, thermal transport can be further suppressed by reducing dimensionality of the material 30 – 33 . As one-dimensional (1D) systems, perovskite NWs possess both single-crystallinity and conducting channels for thermal and electrical transport, thus offering an ideal platform for studying the microscale bandgap engineering. In this work, we accomplished microscopic compositional gradient in tilted perovskite NW arrays via a facile vapor-phase anion exchange method. Both top-down and bottom-up experimental approaches are feasible for the mass production of gradient NWs. Detailed analysis of halogen profiles along individual NWs suggests that the microscale gradient originates from local temperature inhomogeneity, which is uniquely found in this class of materials with ultralow thermal conductivity, and further supported by thermal simulations. In addition, the gradient NWs demonstrate promising optoelectronic performance for spectral-resolved optoelectronic devices. These results offer insight into utilizing thermal management for bandgap engineering, which may find wide application prospects in fabrication strategies for microelectronic devices. RESULTS AND DISCUSSION The single-crystalline CsPbBr 3 NWs were prepared via a solution-processed method assisted by anti-solvent (see Methods for details). Curiously, the outcome of synthesis is substrate-dependent, that on SiO 2 /Si substrates the synthesized NWs are lying on the surface, while on tin fluoride oxide (FTO) substrates, high-density NW arrays are spontaneously formed (Fig. 1 a and S1). As shown in the scanning electron microscopy (SEM) images (Fig. 1 b&c), the NWs exhibit well-defined geometry and crystal facets with length reaching tens of micrometers and diameter ranging from 500 nm to 1 µm. The density of NWs is estimated up to 10 6 NWs/cm 2 . The majority of the as-grown NWs are inclined to the FTO substrate with varying tilting angle (Fig. 1 d). Such formation of tilted NW array may be attributed to the polycrystalline surface of FTO (Figure S2), which provides nucleation sites correlated to the local morphology. We next conducted optical and fluorescence microscopy measurements on the NW array (Fig. 1 e to g). The NWs exhibit bright photoluminescence with narrow emission peak centered at 528 nm (Fig. 1 f&g). In addition, the energy dispersive X-ray spectroscopy (EDS) measurements of single NWs confirm uniform distribution of Cs, Pb, and Br (Fig. 1 h), and the X-ray diffraction (XRD) pattern of the NWs is consistent with orthorhombic phase of CsPbBr 3 (Fig. 1 i). These characterizations suggest that high-quality single-crystalline NW array can be produced for the subsequent compositional manipulation. To conduct bandgap engineering on the NW array, the NW substrate and iodine source were placed in a glass vial, and heated on a hotplate for anion exchange, as shown in Fig. 2 a. The experiment was conducted inside a N 2 -filled glovebox, where the concentration of water and oxygen is below 0.01 ppm. The butylammonium iodide (BAI) powder was employed as iodine source, which sublimates to gaseous state upon heating and diffuses upward to the substrate. The anion exchange between the CsPbBr 3 NWs and BAI vapor occurs when the heating temperature is above 175°C. Figure 2 b demonstrates an optical microscope image of the exchanged NW array, which shows apparently altered color compared to that of pristine NWs (Figure S4). Strikingly, the corresponding micro-PL image (Fig. 2 c) reveals red-to-green gradient emission along the length of individual NWs. The bottom of the exchanged NWs consistently exhibits red emission, indicating that the iodine exchange is most intensive near the substrate-end of the NWs. The specific gradient structure is dependent on the heating temperature and duration, as well as diameter and length of individual NWs. In sufficient time or at elevated temperature, the emission of the entire NW array can be exchanged to red color (Figure S5), suggesting complete conversion from CsPbBr 3 to CsPbI 3 . Subsequently, single NWs were transferred onto a FTO substrate by a micromanipulator for spatial-resolved characterizations. Figure 2 d&e demonstrate a typical exchanged NW with length of 30 µm, which shows gradually varied emission from green of pure CsPbBr 3 to yellow and red of CsPbBr 3 − 3x I 3x (0 < x ≤ 1). Note that the red and yellow emission from the green-side of the NW is likely due to waveguide effect from the single-crystalline NW cavity 34 , 35 . The PL spectra of five consecutive locations along an exchanged NW demonstrate apparent redshift from 564 to 660 nm (Fig. 2 f). Additionally, the morphology and elemental distribution of exchanged NWs were examined by SEM and EDS. As shown in Fig. 2 g&h, the surface of the exchanged NW remains smooth with no apparent modification of its overall geometry. Despite of a large volume mismatch of 18% between pure CsPbBr 3 and CsPbI 3 , the gradual variation in composition exerts minimal impact on the crystallinity. Figure 2 i concludes the elemental mapping of Pb, Br, and I along an exchanged NW. The Br content gradually decreases from top of the NW, while iodine concentration exhibits exactly opposite trend, as demonstrated in the line cross-sections of the EDS images (Fig. 2 j). Moreover, uniform distribution of lead is maintained in the exchanged NWs. The above characterizations confirm that high-density compositional gradient NW array can be achieved, while the quality of individual NWs is maintained upon exchange. We further examined the local morphology and electronic structure of single gradient NWs by atomic force microscopy (AFM) and Kelvin probe force microscope (KPFM). As shown in Fig. 3 a&b and S6, the height of the gradient NW, equally the NW diameter, exhibits no apparent change from the Br-rich to the I-rich end. On the contrary, the surface potential of the NW, denoted by the contact potential difference (CPD), demonstrates significant contrast along the gradient NW. In the detailed line-profile of height (Fig. 3 c), a slight increase of NW diameter is revealed at the I-rich side, which can be attributed to the larger radius of iodine atoms. The CPD of I-rich end increases by approximately 50 mV compared to that of Br-rich end (Fig. 3 d). In previously reported Cl-Br system 17 , it is shown that the pristine CsPbBr 3 NW consistently exhibits higher CPD than Cl-exchanged NWs. Combining these observations, the shrinking of bandgap from 2.34 eV of CsPbBr 3 to 1.75 eV of CsPbI 3 may be the main contributing factor of CPD difference. In addition, to quantitatively obtain detailed halogen distribution, we performed confocal scanning micro-PL measurements to simultaneously acquire the intensity and spectrum of fluorescence in a point-by-point manner along the NW (Fig. 3 e). As shown in Fig. 3 f, the PL spectrum continuously redshift along the NW with well-preserved intensity and slight broadening near the I-rich end. By applying linear Vegard's law 36 , 37 , the iodine content (x of CsPbBr 3 − 3x I 3x ) along the NW is calculated based on the center of emission peak. As shown in Fig. 3 g, the iodine concentration exhibits linear decrease with distance away from the substrate. Such distribution of halogen deviates from that in solid-state exchange via a point iodine source, where Gaussian distribution is commonly observed 38 – 40 . This result suggests that the anion exchange occurs globally at a large section of the NW rather than from a local halogen source (Fig. 3 h). The linear distribution of iodine is likely attributed to the monotonically varied exchange rate along the NW. The formation of gradient NWs suggests there might exist local temperature inhomogeneity along the axial direction within single NWs, which affects the microscale exchange rate and results in compositional gradient. In the heating-from-bottom setup (Fig. 4 a), the substrate acts as a heat source with constant temperature, which conducts heat upward along the NWs. Due to the ultralow thermal conductivity and sub-micrometer diameter of the NWs, the heat transfer process is considerably inefficient, which may lead to local temperature gradient along individual NWs. Such local temperature gradient greatly affects the reaction rate, and is ultimately converted to compositional gradient upon exchange. To verify this assumption, we employed another top-down exchange strategy, which utilizes a heat sink to reverse the microscopic temperature gradient (Fig. 4 b). As shown in the experimental setup (Fig. 4 c, more details in the Supporting Information), the iodine source was placed in the hot zone of a tube furnace with NW substrate downstream in the low-temperature region. During anion exchange, a carrier gas of nitrogen was flowed, transferring vapor of iodine source to the NW substrate. In this scenario, the substrate serves as a heat sink, oppositely to the bottom-up setup. As gas from hot zone approaches the NW array, the top section of individual NWs is quickly heated, while the temperature of their bottom section is pinned to that of the substrate (Fig. 4 b). Therefore, a reversed temperature gradient is formed along individual NWs, and the exchange occurs in a top-down manner in contrast to the bottom-up approach. Figure S7 and 4d demonstrate the optical microscope and corresponding micro-PL images of an I-exchanged NW array, respectively. The top section of the NW array consistently exhibits red emission, confirming the discussed exchange mechanism. Following the same approach, we employed Phenethylammonium chloride (PEACl) as chlorine source for further production of Cl-gradient NWs. Compared to iodine, chlorine anions possess smaller radius and thereby diffuse faster in CsPbBr 3 NWs. As a result, at the same heating temperature, uniformly exchanged NW array rather than gradient structures tend to form upon exchange (Figure S9). To achieve gradient NWs, the central temperature of furnace is reduced to 130 ℃ to slow down the exchange rate. Figure S10 and 4e show the optical microscope and micro-PL images of the Cl-exchanged NW array, respectively. The top section of each NW consistently demonstrates blue emission, while the bottom section exhibits broadband blue-to-green light, indicating formation of Br-Cl gradient. These results strongly suggest that the local temperature gradient along individual NWs is responsible for the mass production of compositional gradient NW arrays. To quantitatively evaluate the microscale thermal inhomogeneity, we further conducted thermal simulations to analyze the temperature distribution in the bottom-up setup (more details in the Supporting Information). An infinitely-large FTO/glass substrate was employed as the heat source with constant surface temperature of 171.5°C, and tilted CsPbBr 3 NWs with length of 25 µm were thermally connected to the substrate. All simulation parameters (Table S1 ) were consistent with the experimental values, and the simulations were performed under natural convection conditions and at steady state. Figure 4 f illustrates the simulated temperature profile of the NW array. The NWs with diameter of 300 and 500 nm demonstrate axial temperature difference of 30.2 and 19.8°C between the two ends, respectively. The temperature gradient is dependent on the length, diameter, and tilting angle of individual NWs. Such temperature gradient is well within the experimental conditions for conducting anion exchange, thus confirming the origin of compositional gradient. As a comparison, we also simulated Si NWs with the same geometry but greatly enhanced thermal conductivity of 150 W·m − 1 ·K − 1 .The resulting temperature profile of Si NW is rather uniform with no apparent inhomogeneity (Figure S14). These results suggest that unlike conventional semiconductors, the suppressed heat transfer in halide perovskites may exert substantial influence even at the microscale. In Fig. 4 g, the line-profiles of temperature along individual NWs were plotted, which shows apparent diameter-dependent decay away from the heat source. The corresponding temperature gradient, represented by the derivation of the temperature distribution, exhibits values up to 3.6°C/µm near the substrate and average of 1.2°C/µm over the entire NW with diameter of 300 nm (Fig. 4 h). These temperature profiles can be understood by considering the 1D heat transport Eq. 4 1 (details in the Supporting Information), the solution of which takes the form: where T is the local temperature, x represents the distance away from the substrate. T L and T H denote the low and high temperature at the two ends of the NW, respectively. L is the length of NW, and L T is temperature decay length. By fitting the temperature profiles with Eq. ( 1 ), the L T of NWs with various diameters can be obtained. As shown in Fig. 4 i, the temperature decay length demonstrates apparent reduction from 10.7 to 8.5 µm as the NW diameter decreases from 1µm to 300 nm, respectively. Such diameter-dependent local temperature decay can be attributed to the combined heat transport from thermal conduction with substrate and convection to the air. In the free-standing NW geometry, the temperature decay length takes the form , where κ is the thermal conductivity, and λ is the heat dissipation into the air, which is inversely proportional to the NW diameter. Thus, for smaller CsPbBr 3 NWs, the low κ and increased λ lead to significantly reduced L T down to the sub-10 µm regime, creating realistic effect on the outcome of anion exchange. On the contrary, for Si NWs, the thermal conduction is the dominating factor, resulting in much longer decay length than the NW itself. Therefore, the temperature distribution is generally uniform in Si NWs as well as conventional semiconductors. Furthermore, the optoelectronic properties of individual gradient NWs were evaluated by transferring single NWs onto pre-patterned gold electrodes (Fig. 5 a). Figure 5 b&c demonstrate the optical microscope and corresponding micro-PL images of the two-terminal NW devices incorporating single NWs with Br-I and Br-Cl gradient. The current-voltage characteristics of typical NW devices were measured in dark and under illumination of a 405 nm diode laser (Fig. 5 d and S15). The NW device exhibits ultra-low dark current down to 1 pA, limited by the noise level of the source meter (Keithley 2450). Under illumination, the gradient NW device demonstrates rectifying I-V characteristics in contrast to pure CsPbBr 3 NW devices with symmetric I-V curves (Figure S16). Such transport anisotropy is possibly due to the funnel-like energy band structure (Fig. 5 e), which facilitates transfer of photo-generated carriers to the narrow-band side of the NW 22 , 42 . Under various illumination intensity, the NW device consistently demonstrate asymmetric behavior (Fig. 5 f and S17), while the photocurrent magnitude is proportional to light intensity, leading to calculated linear dynamic range (LDR) of approximately 60 dB (Fig. 5 g). Note that the LDR of the NW device is limited by the measurement range, and is potentially higher thanks to the extremely low dark current. The responsivity and detectivity of the device are also calculated, as depicted in Fig. 5 h. At incident intensity of 0.08 mW/cm 2 , the NW device exhibits responsivity of 2.37 AW − 1 and detectivity of 1.026×10 11 Jones, highlighting its capability of weak light detection. In addition, the response speed of the NW device was also assessed (Fig. 5 i). The device shows fast photoresponse over the entire intensity range, with rise and fall speed of 17 ms and 18.5 ms, respectively (Figure S20). These results demonstrate promising potentials of the gradient NWs for applications in optoelectronic nano-devices. By further patterning multiple electrodes along a single NW, spectral-resolved photodetectors or reconstructive spectrometers 43 can be readily fabricated based on these gradient NWs. CONCLUSION In summary, we show that gradient perovskite NWs can be mass-produced via facile bottom-up or top-down vapor-phase anion exchange methods. As the two methods generate reversed outcome to each other, further combining of the bottom-up and top-down setups for simultaneous iodine and chlorine exchange may extend the band engineering in single NWs to the entire visible spectra. The local temperature gradient is found to be responsible for creating compositional gradient along the NWs. Therefore, by intentionally manipulating the local temperature distribution, such as adding micro-heaters, it is possible to achieve more precise and versatile control of the micro- and nanoscale band structure. The long compositional gradient over tens of microns along single NWs offers possibilities for a variety of microscale optoelectronic devices that require spectral resolution, including miniaturized spectrometers, spectral imaging sensors, tunable lasers, etc. METHODS Chemicals and reagents. Lead (Ⅱ) bromide (PbBr 2 , > 99.999%) and Cesium bromide (CsBr, 99.999% trace metal basis) were purchased from Sigma Aldrich. N, N-Dimethylformamide (DMF, anhydrous, 99.8%) and isopropanol (IPA, anhydrous, 99.8%) were purchased from Aladdin. Butylammonium iodide (BAI, > 97.0%) was purchased from TCI. Phenethylammonium Chloride (PEACl) was purchased from Xi'an Polymer Light Technology Corp. Tin fluoride oxide (FTO) substrates were purchased from Advanced Election Technology CO.,Ltd. All Chemicals and reagents were used without further purification. Synthesis of CsPbBr 3 NW array . The CsPbBr 3 NWs were grown by a modified solution recrystallization method assisted by anti-solvent. The precursor solution of 0.05 M was prepared by dissolving PbBr 2 and CsBr powders with molar ratio of 1:1 into DMF, followed by vigorous stirring at 25°C for 4 hours. After the powders were completely dissolved, the solution was filtered by a nylon 66 filter head with 0.22 µm porous. Subsequently, 10 µL precursor solution was dropped onto a clean FTO glass, placed in a capped breaker with small amount of isopropanol as anti-solvent. The CsPbBr 3 NWs were formed on the substrate after a few hours. Measurements and Characterizations. The micro-PL spectra were acquired using an Olympus BX53F microscope equipped with a spectrometer (Princeton Instruments, SpectraPro, HRS-500s). The optical microscope and PL images were collected by an Olympus camera (Olympus DP 74) under a 100X objective lens. The confocal micro-PL images were captured by an Olympus FV1000 inverted microscope with a 405 nm laser for excitation. The multi-channel PL emission imaging and lambda scans were conducted with 20 nm spectral windows and 2 nm step size. In addition, the PL intensity mappings along the NW long axis were extracted from the lambda scans. The morphology and the composition analysis of the NWs were performed by a scanning electron microscopy (TESCAN, MAGNA) equipped with energy dispersive X-ray spectroscopy (EDS). The XRD patterns were acquired by a Smart Lab 9 KW (Nippon Neotoku Corporation) with Cu Kα radiation under operating voltage of 240 kV. The AFM and KPFM measurements were conducted by an atomic force microscope (Bruker, Dimension Icon). The optoelectronic properties were measured by a Keithley 2450 Source Meter with a 405 nm diode laser. The light intensity was adjusted by optical neutral density filters. Declarations NOTES The authors declare no conflicts of interest. ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grant No. 62004022 and 12304209). Y. Yang acknowledges support from the Science and Technology Plan of Liaoning Province (Grant No. 2024-MSBA-23). J. Li acknowledges support from the China Postdoctoral Science Foundation (Grant No. 2023M730479). This work is also supported by the Fundamental Research Funds for the Central Universities. The authors acknowledge assistance from DUT Instrumental Analysis Center. References Miao Y et al (2019) Stable and bright formamidinium-based perovskite light-emitting diodes with high energy conversion efficiency. Nat Commun 10:3624 Fu Y et al (2022) Strongly Quantum-Confined Perovskite Nanowire Arrays for Color-Tunable Blue-Light-Emitting Diodes. ACS Nano 16:8388–8398 Kim JS et al (2022) Ultra-bright, efficient and stable perovskite light-emitting diodes. 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Supplementary Files SupportingInformation.docx Supporting Information Cite Share Download PDF Status: Posted 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-5739231\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Article\",\"associatedPublications\":[],\"authors\":[{\"id\":403541381,\"identity\":\"23c4e3f9-e4e9-4bec-a2c1-398f7b2828e7\",\"order_by\":0,\"name\":\"Yiming Yang\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyUlEQVRIiWNgGAWjYPACG8Y+MM1GvJY0xjYGZtK0HCZBi8G1M4afC36dl22TyD/A8KHsMAP/7AYCWm7nGEvP7Ltt3CaRzMA449xhBok7BwhqMZDm7bmdCNLCzNt2mMFAIoGwLb95e85BtPwlUouZNM+PAxAtjMRokbydVmbN25Bs3Mbz2OBgz7l0HokbBLTw3U7efJvnj51sP3viwwc/yqzl+GcQ0KJwgMOAARgpYHAAiHnwqwcC+Qb2BwwMfwiqGwWjYBSMgpEMAK6TQ5wTGmNrAAAAAElFTkSuQmCC\",\"orcid\":\"https://orcid.org/0000-0002-3841-3519\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Yiming\",\"middleName\":\"\",\"lastName\":\"Yang\",\"suffix\":\"\"},{\"id\":403541382,\"identity\":\"eb8b6fb7-6340-4133-88ba-c72f90c24649\",\"order_by\":1,\"name\":\"Jianliang Li\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jianliang\",\"middleName\":\"\",\"lastName\":\"Li\",\"suffix\":\"\"},{\"id\":403541383,\"identity\":\"8c279fc3-dfdd-482d-bfb8-4df299954421\",\"order_by\":2,\"name\":\"Jing Li\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jing\",\"middleName\":\"\",\"lastName\":\"Li\",\"suffix\":\"\"},{\"id\":403541384,\"identity\":\"25a6e931-62bf-4372-b7fe-ff4fee05a111\",\"order_by\":3,\"name\":\"Jiao Xu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jiao\",\"middleName\":\"\",\"lastName\":\"Xu\",\"suffix\":\"\"},{\"id\":403541385,\"identity\":\"f31d22bd-8dfc-42a6-9464-e991e37e0bfc\",\"order_by\":4,\"name\":\"Weili Liu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Weili\",\"middleName\":\"\",\"lastName\":\"Liu\",\"suffix\":\"\"},{\"id\":403541386,\"identity\":\"fa72af26-613d-4225-a8ff-f39f2906f8a5\",\"order_by\":5,\"name\":\"Conghui Tan\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Conghui\",\"middleName\":\"\",\"lastName\":\"Tan\",\"suffix\":\"\"},{\"id\":403541387,\"identity\":\"164a4d03-cdcf-4976-860a-0bc7bfba4bb1\",\"order_by\":6,\"name\":\"Meiqi An\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Meiqi\",\"middleName\":\"\",\"lastName\":\"An\",\"suffix\":\"\"},{\"id\":403541388,\"identity\":\"96323407-7f24-4354-a527-22be9d310246\",\"order_by\":7,\"name\":\"Shuai Yang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Shuai\",\"middleName\":\"\",\"lastName\":\"Yang\",\"suffix\":\"\"},{\"id\":403541389,\"identity\":\"dd438c76-38a7-4b56-8eb3-b6076f45c480\",\"order_by\":8,\"name\":\"Yanan Bao\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yanan\",\"middleName\":\"\",\"lastName\":\"Bao\",\"suffix\":\"\"},{\"id\":403541390,\"identity\":\"cc4037ae-4842-4175-9302-0f2c08caf488\",\"order_by\":9,\"name\":\"Huayi Tang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Huayi\",\"middleName\":\"\",\"lastName\":\"Tang\",\"suffix\":\"\"},{\"id\":403541391,\"identity\":\"028fbdc7-ad50-4118-aa65-76d1c62dd180\",\"order_by\":10,\"name\":\"Han Bao\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Han\",\"middleName\":\"\",\"lastName\":\"Bao\",\"suffix\":\"\"},{\"id\":403541392,\"identity\":\"634edc4f-6f8d-4784-92cf-8e0f797746da\",\"order_by\":11,\"name\":\"Yingmin Luo\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yingmin\",\"middleName\":\"\",\"lastName\":\"Luo\",\"suffix\":\"\"},{\"id\":403541393,\"identity\":\"8c7a41f3-8a8b-45e6-9a13-314f97fbe301\",\"order_by\":12,\"name\":\"Yurui Fang\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-3098-7681\",\"institution\":\"Dalian University of Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yurui\",\"middleName\":\"\",\"lastName\":\"Fang\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2024-12-31 05:05:14\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-5739231/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-5739231/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":74294322,\"identity\":\"f64f2841-bdd1-46be-9895-2edc56a7b105\",\"added_by\":\"auto\",\"created_at\":\"2025-01-20 17:46:36\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":988382,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003ePreparation and characterizations of CsPbBr\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e3\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003e NWs\\u003c/strong\\u003e. \\u003cstrong\\u003e(a)\\u003c/strong\\u003e Schematic drawing of the synthesis setup and substrate-dependent NW growth. \\u003cstrong\\u003e(b)\\u003c/strong\\u003e SEM image of NW array on FTO substrate captured at tilted view of 45°. Scale bar: 10µm. \\u003cstrong\\u003e(c) \\u003c/strong\\u003eZoom-in SEM image showing the cross section of a single NW. Scale bar: 500 nm.\\u003cstrong\\u003e (d)\\u003c/strong\\u003e Cross-sectional SEM image of the NWs. Scale bar: 10 µm. \\u003cstrong\\u003e(e) \\u003c/strong\\u003eTop-view optical microscope image of the NW array. Scale bar: 20 µm. \\u003cstrong\\u003e(f)\\u003c/strong\\u003e PL spectrum of a typical NW. \\u003cstrong\\u003e(g) \\u003c/strong\\u003eTop-view micro-PL image of the NW array. Scale bar: 20 µm.\\u003cstrong\\u003e (h)\\u003c/strong\\u003e SEM and corresponding elemental mapping of Cs, Pb, and Br along a NW. Scale bar: 5 µm. \\u003cstrong\\u003e(i)\\u003c/strong\\u003e XRD pattern of the NW array compared to standard diffraction pattern of PDF#18-0364.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"image3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/baaced466642cb552dfb1ae1.png\"},{\"id\":74295220,\"identity\":\"4fb0b7e3-027f-4052-bc3e-a0ee661be3fd\",\"added_by\":\"auto\",\"created_at\":\"2025-01-20 17:54:36\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":835421,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eAnion exchange of NW array and characterizations of single exchanged NWs. (a)\\u003c/strong\\u003e Schematic drawing of the experimental setup. \\u003cstrong\\u003e(b)\\u003c/strong\\u003e Top-view optical microscope image of the exchanged NW array. Scale bar: 20 µm.\\u003cstrong\\u003e (c)\\u003c/strong\\u003e The corresponding micro-PL image of (b). The white arrows mark the top and bottom of a typical NW.\\u003cstrong\\u003e (d)\\u003c/strong\\u003e Optical microscope image of a horizontal exchanged NW. Scale bar: 5 µm. \\u003cstrong\\u003e(e) \\u003c/strong\\u003eThe corresponding micro-PL image of (d). \\u003cstrong\\u003e(f)\\u003c/strong\\u003e PL spectra of five points marked by white dashed circles along the NW in (e).\\u003cstrong\\u003e \\u003c/strong\\u003eThe spectra of pure CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e and CsPbI\\u003csub\\u003e3\\u003c/sub\\u003e NWs are also presented in dashed lines for comparison. \\u003cstrong\\u003e(g)\\u003c/strong\\u003e SEM image of an exchanged NW. Scale bar: 5 µm. \\u003cstrong\\u003e(h) \\u003c/strong\\u003eZoom-in image from the white dashed box in (g). Scale bar: 500 nm. \\u003cstrong\\u003e(i)\\u003c/strong\\u003e SEM and corresponding elemental mapping of Pb, Br, and I along the NW. Scale bar: 5 µm.\\u003cstrong\\u003e (j) \\u003c/strong\\u003eLine cross-sections of (i) showing Br and I distribution profiles.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"image4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/1ac74f0bbedb8072ff3deb33.png\"},{\"id\":74294323,\"identity\":\"e66d8084-7bf9-4085-b270-37b81f26dfcc\",\"added_by\":\"auto\",\"created_at\":\"2025-01-20 17:46:36\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":520879,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eScanning probe characterizations of individual gradient NWs.\\u003c/strong\\u003e \\u003cstrong\\u003e(a) \\u003c/strong\\u003e3D topographical graph of a typical gradient NW on SiO\\u003csub\\u003e2\\u003c/sub\\u003e/Si. The left side of the NW corresponds to the I-rich end. \\u003cstrong\\u003e(b) \\u003c/strong\\u003eThe corresponding surface potential mapping of the NW in (a). \\u003cstrong\\u003e(c-d)\\u003c/strong\\u003e Height (c) and contact potential difference (d) profiles along the NW. \\u003cstrong\\u003e(e) \\u003c/strong\\u003eSchematic diagram of the confocal scanning micro-PL setup. \\u003cstrong\\u003e(f) \\u003c/strong\\u003eMapping of PL spectra and intensity along a gradient NW. \\u003cstrong\\u003e(g)\\u003c/strong\\u003e Calculated iodine concentration along the NW. The black curve shows the linear fitting of data. \\u003cstrong\\u003e(h)\\u003c/strong\\u003e Schematic drawing of global (left) and local (right) anion exchange and resulting iodine concentration profiles (middle).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"image5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/2013e9b00a7c41a3a0c849bf.png\"},{\"id\":74294329,\"identity\":\"3ff3a9f9-e5de-4bb9-9dc2-b9deea596c1b\",\"added_by\":\"auto\",\"created_at\":\"2025-01-20 17:46:36\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":587996,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eAnion exchange mechanism and simulation of local temperature profiles for gradient NW array. (a-b)\\u003c/strong\\u003e Schematic drawing of the bottom-up (a) and top-down (b) exchange mechanism. \\u003cstrong\\u003e(c)\\u003c/strong\\u003e Illustration of the top-down exchange setup inside a tube furnace. \\u003cstrong\\u003e(d-e)\\u003c/strong\\u003e Micro-PL images of I-exchanged (d) and Cl-exchanged (e) NW arrays. The arrows mark the top and bottom of a typical NW. The inset shows the PL image of a gradient NW transferred onto a SiO\\u003csub\\u003e2\\u003c/sub\\u003e/Si substrate. Scale bars: 10 µm (d) and 20 µm (e). \\u003cstrong\\u003e(f) \\u003c/strong\\u003eSteady-state temperature distribution along tilted NWs on FTO substrate with constant temperature of 171.5 ℃. \\u003cstrong\\u003e(g)\\u003c/strong\\u003e Temperature distribution along NWs with diameter ranging from 300 nm to 1 mm. The zero point of x-axis refers to the substrate. \\u003cstrong\\u003e(h)\\u003c/strong\\u003e Calculated temperature gradient along various NWs. \\u003cstrong\\u003e(i)\\u003c/strong\\u003e Fitted thermal decay length as a function of NW diameter.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"image6.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/c800865962c680e7f83ae45a.png\"},{\"id\":74294340,\"identity\":\"f2998e11-e033-420e-ae43-fe6daf66a3bc\",\"added_by\":\"auto\",\"created_at\":\"2025-01-20 17:46:37\",\"extension\":\"png\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":806672,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eOptoelectronic measurements of individual gradient NWs.\\u003c/strong\\u003e \\u003cstrong\\u003e(a)\\u003c/strong\\u003e Schematic drawing of a single NW device with illumination from above. \\u003cstrong\\u003e(b-c) \\u003c/strong\\u003eOptical microscope (upper) and corresponding micro-PL (bottom)\\u003cstrong\\u003e \\u003c/strong\\u003eimages of single Br-I (b) and Br-Cl (c) gradient NW devices with channel length of 10 mm. Scale bar: 5 µm.\\u003cstrong\\u003e (d)\\u003c/strong\\u003e I-V curves of the Br-I NW device in dark and under illumination from a 405 nm laser with intensity of 77.4 mW/cm\\u003csup\\u003e2\\u003c/sup\\u003e. \\u003cstrong\\u003e(e)\\u003c/strong\\u003e Energy band structure of gradient NWs. \\u003cstrong\\u003e(f)\\u003c/strong\\u003e I-V curves of a typical Br-I NW device at various light intensity. \\u003cstrong\\u003e(g)\\u003c/strong\\u003e Photocurrent as a function of illumination intensity at bias of 2 V. The red line shows the linear fitting of data. \\u003cstrong\\u003e(h)\\u003c/strong\\u003e Calculated responsivity (blue curve) and detectivity (red curve) of the Br-I NW device at bias of 2 V. \\u003cstrong\\u003e(i)\\u003c/strong\\u003e Photoresponse of the Br-I NW device under various light intensity and bias of 2 V.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"image7.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/8916e040e99788e5fe6c25fb.png\"},{\"id\":80055043,\"identity\":\"171c82a3-0a03-4ac4-915e-8ec49ba2829b\",\"added_by\":\"auto\",\"created_at\":\"2025-04-07 11:14:16\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":5366697,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/b74b609c-1744-430c-8364-115d4c7db46e.pdf\"},{\"id\":74294327,\"identity\":\"0893b90a-f16c-4334-b496-5dde99a33e44\",\"added_by\":\"auto\",\"created_at\":\"2025-01-20 17:46:36\",\"extension\":\"docx\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":11306994,\"visible\":true,\"origin\":\"\",\"legend\":\"Supporting Information\",\"description\":\"\",\"filename\":\"SupportingInformation.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5739231/v1/a3a328f3ff0b68d81b9d5233.docx\"}],\"financialInterests\":\"There is \\u003cb\\u003eNO\\u003c/b\\u003e Competing Interest.\",\"formattedTitle\":\"Mass Production of Gradient Perovskite Nanowires via Microscale Thermal Engineering\",\"fulltext\":[{\"header\":\"INTRODUCTION\",\"content\":\"\\u003cp\\u003eThe success of halide perovskites in photovoltaics brings about extensive interest of their broader applications for semiconductor devices including light-emitting diodes \\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR2 CR3\\\" citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e\\u003c/sup\\u003e, photodetectors \\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR6 CR7\\\" citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e\\u003c/sup\\u003e, lasers \\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR10 CR11\\\" citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e\\u003c/sup\\u003e, and memristors\\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR14 CR15\\\" citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e\\u003c/sup\\u003e. Particularly, the soft lattice of perovskites enables exceptional tunability of their composition, optical and electrical properties via bandgap engineering. Aside from conventional methods including doping or quantum confinement effect, the bandgap engineering in perovskites can be conveniently achieved through exchange of their halogen anions in the liquid\\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR18\\\" citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e\\u003c/sup\\u003e, vapor\\u003csup\\u003e\\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e\\u003c/sup\\u003e, and solid phase \\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR23\\\" citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e\\u003c/sup\\u003e. However, at microscale, the tuning of local band structure becomes formidable and uncontrolled, which is largely due to the chaotic nature of anionic thermal diffusion. To increase the precision and spatial resolution of bandgap engineering, a variety of methods have been developed to combine with anion exchange, such as nanofabrication\\u003csup\\u003e\\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e\\u003c/sup\\u003e, micromanipulation\\u003csup\\u003e\\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR25\\\" class=\\\"CitationRef\\\"\\u003e25\\u003c/span\\u003e\\u003c/sup\\u003e, and mechanical motion\\u003csup\\u003e\\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e\\u003c/sup\\u003e. These approaches essentially transform the artificially created spatial or environmental uniformities to compositional variation in perovskites. Nevertheless, the increased complexity of fabrication procedures results in surging cost of devices and difficulty for mass-production.\\u003c/p\\u003e \\u003cp\\u003eAlternatively, it might be possible to utilize the intrinsic thermodynamic features of perovskites for microscale bandgap engineering. Halide perovskites possess one of the lowest thermal conductivity found in natural crystalline-state semiconductors\\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR27\\\" citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR28\\\" class=\\\"CitationRef\\\"\\u003e28\\u003c/span\\u003e\\u003c/sup\\u003e, which originates from the loosely bonded lattice as well as heavy element such as Pb. In nanostructured all-inorganic perovskites, ultralow thermal conductivity down to 0.38 W\\u0026middot;m\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e\\u0026middot;K\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e has been reported\\u003csup\\u003e\\u003cspan citationid=\\\"CR29\\\" class=\\\"CitationRef\\\"\\u003e29\\u003c/span\\u003e\\u003c/sup\\u003e. The inefficient thermal transport in perovskites leads to short thermal decay length, providing opportunities for local spatial-resolved anion exchange. Moreover, thermal transport can be further suppressed by reducing dimensionality of the material \\u003csup\\u003e\\u003cspan additionalcitationids=\\\"CR31 CR32\\\" citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR33\\\" class=\\\"CitationRef\\\"\\u003e33\\u003c/span\\u003e\\u003c/sup\\u003e. As one-dimensional (1D) systems, perovskite NWs possess both single-crystallinity and conducting channels for thermal and electrical transport, thus offering an ideal platform for studying the microscale bandgap engineering.\\u003c/p\\u003e \\u003cp\\u003eIn this work, we accomplished microscopic compositional gradient in tilted perovskite NW arrays via a facile vapor-phase anion exchange method. Both top-down and bottom-up experimental approaches are feasible for the mass production of gradient NWs. Detailed analysis of halogen profiles along individual NWs suggests that the microscale gradient originates from local temperature inhomogeneity, which is uniquely found in this class of materials with ultralow thermal conductivity, and further supported by thermal simulations. In addition, the gradient NWs demonstrate promising optoelectronic performance for spectral-resolved optoelectronic devices. These results offer insight into utilizing thermal management for bandgap engineering, which may find wide application prospects in fabrication strategies for microelectronic devices.\\u003c/p\\u003e\"},{\"header\":\"RESULTS AND DISCUSSION\",\"content\":\"\\u003cp\\u003eThe single-crystalline CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs were prepared via a solution-processed method assisted by anti-solvent (see \\u003cem\\u003eMethods\\u003c/em\\u003e for details). Curiously, the outcome of synthesis is substrate-dependent, that on SiO\\u003csub\\u003e2\\u003c/sub\\u003e/Si substrates the synthesized NWs are lying on the surface, while on tin fluoride oxide (FTO) substrates, high-density NW arrays are spontaneously formed (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ea and S1). As shown in the scanning electron microscopy (SEM) images (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003eb\\u0026amp;c), the NWs exhibit well-defined geometry and crystal facets with length reaching tens of micrometers and diameter ranging from 500 nm to 1 \\u0026micro;m. The density of NWs is estimated up to 10\\u003csup\\u003e6\\u003c/sup\\u003e NWs/cm\\u003csup\\u003e2\\u003c/sup\\u003e. The majority of the as-grown NWs are inclined to the FTO substrate with varying tilting angle (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ed). Such formation of tilted NW array may be attributed to the polycrystalline surface of FTO (Figure S2), which provides nucleation sites correlated to the local morphology. We next conducted optical and fluorescence microscopy measurements on the NW array (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ee to g). The NWs exhibit bright photoluminescence with narrow emission peak centered at 528 nm (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ef\\u0026amp;g). In addition, the energy dispersive X-ray spectroscopy (EDS) measurements of single NWs confirm uniform distribution of Cs, Pb, and Br (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003eh), and the X-ray diffraction (XRD) pattern of the NWs is consistent with orthorhombic phase of CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ei). These characterizations suggest that high-quality single-crystalline NW array can be produced for the subsequent compositional manipulation.\\u003c/p\\u003e\\n\\u003cp\\u003eTo conduct bandgap engineering on the NW array, the NW substrate and iodine source were placed in a glass vial, and heated on a hotplate for anion exchange, as shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ea. The experiment was conducted inside a N\\u003csub\\u003e2\\u003c/sub\\u003e-filled glovebox, where the concentration of water and oxygen is below 0.01 ppm. The butylammonium iodide (BAI) powder was employed as iodine source, which sublimates to gaseous state upon heating and diffuses upward to the substrate. The anion exchange between the CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs and BAI vapor occurs when the heating temperature is above 175\\u0026deg;C. Figure \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eb demonstrates an optical microscope image of the exchanged NW array, which shows apparently altered color compared to that of pristine NWs (Figure S4). Strikingly, the corresponding micro-PL image (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ec) reveals red-to-green gradient emission along the length of individual NWs. The bottom of the exchanged NWs consistently exhibits red emission, indicating that the iodine exchange is most intensive near the substrate-end of the NWs. The specific gradient structure is dependent on the heating temperature and duration, as well as diameter and length of individual NWs. In sufficient time or at elevated temperature, the emission of the entire NW array can be exchanged to red color (Figure S5), suggesting complete conversion from CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e to CsPbI\\u003csub\\u003e3\\u003c/sub\\u003e. Subsequently, single NWs were transferred onto a FTO substrate by a micromanipulator for spatial-resolved characterizations. Figure \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ed\\u0026amp;e demonstrate a typical exchanged NW with length of 30 \\u0026micro;m, which shows gradually varied emission from green of pure CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e to yellow and red of CsPbBr\\u003csub\\u003e3\\u0026thinsp;\\u0026minus;\\u0026thinsp;3x\\u003c/sub\\u003eI\\u003csub\\u003e3x\\u003c/sub\\u003e (0\\u0026thinsp;\\u0026lt;\\u0026thinsp;x\\u0026thinsp;\\u0026le;\\u0026thinsp;1). Note that the red and yellow emission from the green-side of the NW is likely due to waveguide effect from the single-crystalline NW cavity\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e34\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e\\u003c/sup\\u003e. The PL spectra of five consecutive locations along an exchanged NW demonstrate apparent redshift from 564 to 660 nm (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ef). Additionally, the morphology and elemental distribution of exchanged NWs were examined by SEM and EDS. As shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eg\\u0026amp;h, the surface of the exchanged NW remains smooth with no apparent modification of its overall geometry. Despite of a large volume mismatch of 18% between pure CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e and CsPbI\\u003csub\\u003e3\\u003c/sub\\u003e, the gradual variation in composition exerts minimal impact on the crystallinity. Figure \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ei concludes the elemental mapping of Pb, Br, and I along an exchanged NW. The Br content gradually decreases from top of the NW, while iodine concentration exhibits exactly opposite trend, as demonstrated in the line cross-sections of the EDS images (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ej). Moreover, uniform distribution of lead is maintained in the exchanged NWs. The above characterizations confirm that high-density compositional gradient NW array can be achieved, while the quality of individual NWs is maintained upon exchange.\\u003c/p\\u003e\\n\\u003cp\\u003eWe further examined the local morphology and electronic structure of single gradient NWs by atomic force microscopy (AFM) and Kelvin probe force microscope (KPFM). As shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ea\\u0026amp;b and S6, the height of the gradient NW, equally the NW diameter, exhibits no apparent change from the Br-rich to the I-rich end. On the contrary, the surface potential of the NW, denoted by the contact potential difference (CPD), demonstrates significant contrast along the gradient NW. In the detailed line-profile of height (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ec), a slight increase of NW diameter is revealed at the I-rich side, which can be attributed to the larger radius of iodine atoms. The CPD of I-rich end increases by approximately 50 mV compared to that of Br-rich end (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ed). In previously reported Cl-Br system\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e\\u003c/sup\\u003e, it is shown that the pristine CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NW consistently exhibits higher CPD than Cl-exchanged NWs. Combining these observations, the shrinking of bandgap from 2.34 eV of CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e to 1.75 eV of CsPbI\\u003csub\\u003e3\\u003c/sub\\u003e may be the main contributing factor of CPD difference. In addition, to quantitatively obtain detailed halogen distribution, we performed confocal scanning micro-PL measurements to simultaneously acquire the intensity and spectrum of fluorescence in a point-by-point manner along the NW (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ee). As shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ef, the PL spectrum continuously redshift along the NW with well-preserved intensity and slight broadening near the I-rich end. By applying linear Vegard\\u0026apos;s law\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e\\u003c/sup\\u003e, the iodine content (x of CsPbBr\\u003csub\\u003e3\\u0026thinsp;\\u0026minus;\\u0026thinsp;3x\\u003c/sub\\u003eI\\u003csub\\u003e3x\\u003c/sub\\u003e) along the NW is calculated based on the center of emission peak. As shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eg, the iodine concentration exhibits linear decrease with distance away from the substrate. Such distribution of halogen deviates from that in solid-state exchange via a point iodine source, where Gaussian distribution is commonly observed\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e38\\u003c/span\\u003e\\u0026ndash;\\u003cspan class=\\\"CitationRef\\\"\\u003e40\\u003c/span\\u003e\\u003c/sup\\u003e. This result suggests that the anion exchange occurs globally at a large section of the NW rather than from a local halogen source (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eh). The linear distribution of iodine is likely attributed to the monotonically varied exchange rate along the NW.\\u003c/p\\u003e\\n\\u003cp\\u003eThe formation of gradient NWs suggests there might exist local temperature inhomogeneity along the axial direction within single NWs, which affects the microscale exchange rate and results in compositional gradient. In the heating-from-bottom setup (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ea), the substrate acts as a heat source with constant temperature, which conducts heat upward along the NWs. Due to the ultralow thermal conductivity and sub-micrometer diameter of the NWs, the heat transfer process is considerably inefficient, which may lead to local temperature gradient along individual NWs. Such local temperature gradient greatly affects the reaction rate, and is ultimately converted to compositional gradient upon exchange. To verify this assumption, we employed another top-down exchange strategy, which utilizes a heat sink to reverse the microscopic temperature gradient (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eb). As shown in the experimental setup (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ec, more details in the Supporting Information), the iodine source was placed in the hot zone of a tube furnace with NW substrate downstream in the low-temperature region. During anion exchange, a carrier gas of nitrogen was flowed, transferring vapor of iodine source to the NW substrate. In this scenario, the substrate serves as a heat sink, oppositely to the bottom-up setup. As gas from hot zone approaches the NW array, the top section of individual NWs is quickly heated, while the temperature of their bottom section is pinned to that of the substrate (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eb). Therefore, a reversed temperature gradient is formed along individual NWs, and the exchange occurs in a top-down manner in contrast to the bottom-up approach. Figure S7 and 4d demonstrate the optical microscope and corresponding micro-PL images of an I-exchanged NW array, respectively. The top section of the NW array consistently exhibits red emission, confirming the discussed exchange mechanism. Following the same approach, we employed Phenethylammonium chloride (PEACl) as chlorine source for further production of Cl-gradient NWs. Compared to iodine, chlorine anions possess smaller radius and thereby diffuse faster in CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs. As a result, at the same heating temperature, uniformly exchanged NW array rather than gradient structures tend to form upon exchange (Figure S9). To achieve gradient NWs, the central temperature of furnace is reduced to 130 ℃ to slow down the exchange rate. Figure S10 and 4e show the optical microscope and micro-PL images of the Cl-exchanged NW array, respectively. The top section of each NW consistently demonstrates blue emission, while the bottom section exhibits broadband blue-to-green light, indicating formation of Br-Cl gradient. These results strongly suggest that the local temperature gradient along individual NWs is responsible for the mass production of compositional gradient NW arrays.\\u003c/p\\u003e\\n\\u003cp\\u003eTo quantitatively evaluate the microscale thermal inhomogeneity, we further conducted thermal simulations to analyze the temperature distribution in the bottom-up setup (more details in the Supporting Information). An infinitely-large FTO/glass substrate was employed as the heat source with constant surface temperature of 171.5\\u0026deg;C, and tilted CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs with length of 25 \\u0026micro;m were thermally connected to the substrate. All simulation parameters (Table \\u003cspan class=\\\"InternalRef\\\"\\u003eS1\\u003c/span\\u003e) were consistent with the experimental values, and the simulations were performed under natural convection conditions and at steady state. Figure \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ef illustrates the simulated temperature profile of the NW array. The NWs with diameter of 300 and 500 nm demonstrate axial temperature difference of 30.2 and 19.8\\u0026deg;C between the two ends, respectively. The temperature gradient is dependent on the length, diameter, and tilting angle of individual NWs. Such temperature gradient is well within the experimental conditions for conducting anion exchange, thus confirming the origin of compositional gradient. As a comparison, we also simulated Si NWs with the same geometry but greatly enhanced thermal conductivity of 150 W\\u0026middot;m\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e\\u0026middot;K\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e.The resulting temperature profile of Si NW is rather uniform with no apparent inhomogeneity (Figure S14). These results suggest that unlike conventional semiconductors, the suppressed heat transfer in halide perovskites may exert substantial influence even at the microscale. In Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eg, the line-profiles of temperature along individual NWs were plotted, which shows apparent diameter-dependent decay away from the heat source. The corresponding temperature gradient, represented by the derivation of the temperature distribution, exhibits values up to 3.6\\u0026deg;C/\\u0026micro;m near the substrate and average of 1.2\\u0026deg;C/\\u0026micro;m over the entire NW with diameter of 300 nm (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eh). These temperature profiles can be understood by considering the 1D heat transport Eq.\\u0026nbsp;4\\u003csup\\u003e1\\u003c/sup\\u003e (details in the Supporting Information), the solution of which takes the form:\\u003c/p\\u003e\\n\\u003cdiv id=\\\"Equ1\\\" class=\\\"Equation\\\"\\u003e\\n \\u003cdiv class=\\\"EquationNumber\\\"\\u003e\\u003cimg src=\\\"data:image/png;base64,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\\\" width=\\\"292\\\" height=\\\"41\\\"\\u003e\\u003c/div\\u003e\\n\\u003c/div\\u003e\\n\\u003cp\\u003ewhere \\u003cem\\u003eT\\u003c/em\\u003e is the local temperature, \\u003cem\\u003ex\\u003c/em\\u003e represents the distance away from the substrate. \\u003cem\\u003eT\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eL\\u003c/em\\u003e\\u003c/sub\\u003e and \\u003cem\\u003eT\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eH\\u003c/em\\u003e\\u003c/sub\\u003e denote the low and high temperature at the two ends of the NW, respectively. \\u003cem\\u003eL\\u003c/em\\u003e is the length of NW, and \\u003cem\\u003eL\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eT\\u003c/em\\u003e\\u003c/sub\\u003e is temperature decay length. By fitting the temperature profiles with Eq. (\\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e), the \\u003cem\\u003eL\\u003c/em\\u003e\\u003csub\\u003eT\\u003c/sub\\u003e of NWs with various diameters can be obtained. As shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ei, the temperature decay length demonstrates apparent reduction from 10.7 to 8.5 \\u0026micro;m as the NW diameter decreases from 1\\u0026micro;m to 300 nm, respectively. Such diameter-dependent local temperature decay can be attributed to the combined heat transport from thermal conduction with substrate and convection to the air. In the free-standing NW geometry, the temperature decay length takes the form \\u003cimg src=\\\"data:image/png;base64,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\\\" height=\\\"20\\\" width=\\\"68\\\"\\u003e, where \\u003cem\\u003e\\u0026kappa;\\u003c/em\\u003e is the thermal conductivity, and \\u003cem\\u003e\\u0026lambda;\\u003c/em\\u003e is the heat dissipation into the air, which is inversely proportional to the NW diameter. Thus, for smaller CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs, the low \\u003cem\\u003e\\u0026kappa;\\u003c/em\\u003e and increased \\u003cem\\u003e\\u0026lambda;\\u003c/em\\u003e lead to significantly reduced \\u003cem\\u003eL\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eT\\u003c/em\\u003e\\u003c/sub\\u003e down to the sub-10 \\u0026micro;m regime, creating realistic effect on the outcome of anion exchange. On the contrary, for Si NWs, the thermal conduction is the dominating factor, resulting in much longer decay length than the NW itself. Therefore, the temperature distribution is generally uniform in Si NWs as well as conventional semiconductors.\\u003c/p\\u003e\\n\\u003cp\\u003eFurthermore, the optoelectronic properties of individual gradient NWs were evaluated by transferring single NWs onto pre-patterned gold electrodes (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003ea). Figure \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003eb\\u0026amp;c demonstrate the optical microscope and corresponding micro-PL images of the two-terminal NW devices incorporating single NWs with Br-I and Br-Cl gradient. The current-voltage characteristics of typical NW devices were measured in dark and under illumination of a 405 nm diode laser (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003ed and S15). The NW device exhibits ultra-low dark current down to 1 pA, limited by the noise level of the source meter (Keithley 2450). Under illumination, the gradient NW device demonstrates rectifying I-V characteristics in contrast to pure CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NW devices with symmetric I-V curves (Figure S16). Such transport anisotropy is possibly due to the funnel-like energy band structure (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003ee), which facilitates transfer of photo-generated carriers to the narrow-band side of the NW\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e42\\u003c/span\\u003e\\u003c/sup\\u003e. Under various illumination intensity, the NW device consistently demonstrate asymmetric behavior (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003ef and S17), while the photocurrent magnitude is proportional to light intensity, leading to calculated linear dynamic range (LDR) of approximately 60 dB (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003eg). Note that the LDR of the NW device is limited by the measurement range, and is potentially higher thanks to the extremely low dark current. The responsivity and detectivity of the device are also calculated, as depicted in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003eh. At incident intensity of 0.08 mW/cm\\u003csup\\u003e2\\u003c/sup\\u003e, the NW device exhibits responsivity of 2.37 AW\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e and detectivity of 1.026\\u0026times;10\\u003csup\\u003e11\\u003c/sup\\u003e Jones, highlighting its capability of weak light detection. In addition, the response speed of the NW device was also assessed (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003ei). The device shows fast photoresponse over the entire intensity range, with rise and fall speed of 17 ms and 18.5 ms, respectively (Figure S20). These results demonstrate promising potentials of the gradient NWs for applications in optoelectronic nano-devices. By further patterning multiple electrodes along a single NW, spectral-resolved photodetectors or reconstructive spectrometers\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e43\\u003c/span\\u003e\\u003c/sup\\u003e can be readily fabricated based on these gradient NWs.\\u003c/p\\u003e\"},{\"header\":\"CONCLUSION\",\"content\":\"\\u003cp\\u003eIn summary, we show that gradient perovskite NWs can be mass-produced via facile bottom-up or top-down vapor-phase anion exchange methods. As the two methods generate reversed outcome to each other, further combining of the bottom-up and top-down setups for simultaneous iodine and chlorine exchange may extend the band engineering in single NWs to the entire visible spectra. The local temperature gradient is found to be responsible for creating compositional gradient along the NWs. Therefore, by intentionally manipulating the local temperature distribution, such as adding micro-heaters, it is possible to achieve more precise and versatile control of the micro- and nanoscale band structure. The long compositional gradient over tens of microns along single NWs offers possibilities for a variety of microscale optoelectronic devices that require spectral resolution, including miniaturized spectrometers, spectral imaging sensors, tunable lasers, etc.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e\"},{\"header\":\"METHODS\",\"content\":\"\\u003cp\\u003e \\u003cb\\u003eChemicals and reagents.\\u003c/b\\u003e Lead (Ⅱ) bromide (PbBr\\u003csub\\u003e2\\u003c/sub\\u003e, \\u0026gt;\\u0026thinsp;99.999%) and Cesium bromide (CsBr, 99.999% trace metal basis) were purchased from Sigma Aldrich. N, N-Dimethylformamide (DMF, anhydrous, 99.8%) and isopropanol (IPA, anhydrous, 99.8%) were purchased from Aladdin. Butylammonium iodide (BAI, \\u0026gt;\\u0026thinsp;97.0%) was purchased from TCI. Phenethylammonium Chloride (PEACl) was purchased from Xi'an Polymer Light Technology Corp. Tin fluoride oxide (FTO) substrates were purchased from Advanced Election Technology CO.,Ltd. All Chemicals and reagents were used without further purification.\\u003c/p\\u003e \\u003cp\\u003e \\u003cb\\u003eSynthesis of CsPbBr\\u003c/b\\u003e \\u003csub\\u003e \\u003cb\\u003e3\\u003c/b\\u003e \\u003c/sub\\u003e \\u003cb\\u003eNW array\\u003c/b\\u003e. The CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs were grown by a modified solution recrystallization method assisted by anti-solvent. The precursor solution of 0.05 M was prepared by dissolving PbBr\\u003csub\\u003e2\\u003c/sub\\u003e and CsBr powders with molar ratio of 1:1 into DMF, followed by vigorous stirring at 25\\u0026deg;C for 4 hours. After the powders were completely dissolved, the solution was filtered by a nylon 66 filter head with 0.22 \\u0026micro;m porous. Subsequently, 10 \\u0026micro;L precursor solution was dropped onto a clean FTO glass, placed in a capped breaker with small amount of isopropanol as anti-solvent. The CsPbBr\\u003csub\\u003e3\\u003c/sub\\u003e NWs were formed on the substrate after a few hours.\\u003c/p\\u003e \\u003cp\\u003e \\u003cb\\u003eMeasurements and Characterizations.\\u003c/b\\u003e The micro-PL spectra were acquired using an Olympus BX53F microscope equipped with a spectrometer (Princeton Instruments, SpectraPro, HRS-500s). The optical microscope and PL images were collected by an Olympus camera (Olympus DP 74) under a 100X objective lens. The confocal micro-PL images were captured by an Olympus FV1000 inverted microscope with a 405 nm laser for excitation. The multi-channel PL emission imaging and lambda scans were conducted with 20 nm spectral windows and 2 nm step size. In addition, the PL intensity mappings along the NW long axis were extracted from the lambda scans. The morphology and the composition analysis of the NWs were performed by a scanning electron microscopy (TESCAN, MAGNA) equipped with energy dispersive X-ray spectroscopy (EDS). The XRD patterns were acquired by a Smart Lab 9 KW (Nippon Neotoku Corporation) with Cu Kα radiation under operating voltage of 240 kV. The AFM and KPFM measurements were conducted by an atomic force microscope (Bruker, Dimension Icon). The optoelectronic properties were measured by a Keithley 2450 Source Meter with a 405 nm diode laser. The light intensity was adjusted by optical neutral density filters.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003ch2\\u003eNOTES\\u003c/h2\\u003e \\u003cp\\u003eThe authors declare no conflicts of interest.\\u003c/p\\u003e\\u003ch2\\u003eACKNOWLEDGMENTS\\u003c/h2\\u003e \\u003cp\\u003eThis work is financially supported by the National Natural Science Foundation of China (Grant No. 62004022 and 12304209). Y. Yang acknowledges support from the Science and Technology Plan of Liaoning Province (Grant No. 2024-MSBA-23). J. Li acknowledges support from the China Postdoctoral Science Foundation (Grant No. 2023M730479). This work is also supported by the Fundamental Research Funds for the Central Universities. The authors acknowledge assistance from DUT Instrumental Analysis Center.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eMiao Y et al (2019) Stable and bright formamidinium-based perovskite light-emitting diodes with high energy conversion efficiency. Nat Commun 10:3624\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eFu Y et al (2022) Strongly Quantum-Confined Perovskite Nanowire Arrays for Color-Tunable Blue-Light-Emitting Diodes. ACS Nano 16:8388\\u0026ndash;8398\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eKim JS et al (2022) Ultra-bright, efficient and stable perovskite light-emitting diodes. Nature 611:688\\u0026ndash;694\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eCao YB et al (2023) High-efficiency, flexible and large-area red/green/blue all-inorganic metal halide perovskite quantum wires-based light-emitting diodes. 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Science 365:1017\\u0026ndash;1020\\u003c/span\\u003e\\u003c/li\\u003e\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":true,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"halide perovskites, nanomaterials, bandgap engineering, ionic transport, photoluminescence\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-5739231/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-5739231/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eThe bandgap engineering of halide perovskites at microscopic level is challenging due to fabrication complexity, environmental sensitivity and material stability. Here we report a facile approach to high-yield compositional graded perovskite nanowires (NWs) via vapor-phase anion exchange methods. Using rationally engineered thermal inhomogeneity along the length of single NWs, arrays of NWs with compositional gradient across tens of micrometers can be readily mass-produced via bottom-up as well as top-down exchange strategies. These exchanged NWs exhibit well-preserved single-crystallinity for efficient optical and electrical transport, while their halogen stoichiometry, fluorescence, and energy band structure demonstrate apparent axial gradient. Detailed analysis of elemental distribution and thermal simulation reveal that the ultralow thermal conductivity together with reduced dimensionality leads to microscale temperature gradient, which is further converted to compositional gradient upon anion exchange. In addition, the gradient NWs show excellent optoelectronic features suitable for further integration into functional devices. This work provides guidelines for composition manipulation of perovskites through thermal engineering, extending their applications in ultracompact microspectrometers, spectral imaging sensors, and other miniaturized optoelectronic devices.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Mass Production of Gradient Perovskite Nanowires via Microscale Thermal Engineering\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-01-20 17:46:30\",\"doi\":\"10.21203/rs.3.rs-5739231/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"d72c1b82-1da3-4864-941b-1d8756adf491\",\"owner\":[],\"postedDate\":\"January 20th, 2025\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[{\"id\":43026454,\"name\":\"Physical sciences/Nanoscience and technology/Nanoscale materials/Nanowires\"},{\"id\":43026455,\"name\":\"Physical sciences/Materials science/Nanoscale materials/Nanowires\"}],\"tags\":[],\"updatedAt\":\"2025-04-07T11:06:07+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2025-01-20 17:46:30\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-5739231\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-5739231\",\"identity\":\"rs-5739231\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}