Chemical Treatment-Induced Indirect-to-Direct Bandgap Transition in MoS₂: Impact on Excitonic Emission

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Nevertheless, realising the full potential of TMDs in electronic and optoelectronic devices, such as transistors, light-emitting diodes (LEDs), and photodetectors, is constrained by high contact resistance. This limitation arises from their low intrinsic carrier concentrations and the current insufficiency of doping strategies for atomically thin materials. Notably, chemical treatment with 1,2-dichloroethane (DCE) has been demonstrated as an effective post-growth method to enhance the n-type electrical conductivity of TMDs. Despite the well-established electrical improvements post-DCE treatment, its effects on optical properties, specifically the retention of optical characteristics and excitonic behaviour, are not yet clearly understood. Here, we systematically investigate the layer- and time-dependent optical effects of DCE on molybdenum disulfide (MoS₂) using photoluminescence (PL) spectroscopy and Density Functional Theory (DFT) simulations. Our PL results reveal a rapid reduction in the indirect bandgap transition, with the direct transition remaining unaffected. DFT confirms that chlorine (Cl) atoms bind to sulphur vacancies, creating in-gap states that facilitate non-radiative recombination, explaining the observed indirect PL suppression. This work demonstrates DCE's utility beyond n-type doping, showcasing its ability to engineer the optical band structure in MoS₂ by selectively suppressing indirect transitions. This capability directly paves the way for enhanced efficiency in 2D optoelectronic devices. Physical sciences/Materials science Physical sciences/Nanoscience and technology Physical sciences/Optics and photonics Physical sciences/Physics Transition metal dichalcogenides molybdenum disulfide chlorine doping density functional theory Photoluminescence sulphur vacancies Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Over the past decades, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention due to their ultrathin structures (thickness < 1 nm for monolayers), high carrier mobilities, tuneable bandgaps (in the range of 1–2 eV), and excellent sensing capabilities 1 – 4 . Their atomically thin structure leads to high surface-to-volume ratios, making their electrical conductivity highly sensitive to environmental changes - a desirable characteristic for sensing applications 5 . Key examples of TMDs such as WS₂, WSe 2 , MoS₂, and MoTe₂ are alternative active materials for optoelectronic devices, including LEDs and lasers, and their bandgaps, which can be precisely tuned via thickness, strain and doping 6 – 10 , offering an additional degree of freedom to tailor their properties. A unique feature of layered TMDs is their characteristic transition from an indirect to a direct bandgap in the monolayer limit, attributed to quantum confinement 11 . This is particularly beneficial for optoelectronic devices, as direct bandgaps allow efficient radiative recombination of electrons and holes without requiring phonon assistance, resulting in stronger light emission and absorption. However, despite their remarkable structural, electrical and optical properties, current experimental results show mobilities of ~ 50 cm²/V·s 12 . This significant discrepancy from theoretical predictions points to dominant scattering mechanisms that limit electrical conductivity, hindering their competitiveness with widely adopted electronic and optoelectronic materials such as Si and GaAs 13 – 21 . The successful deployment of TMD materials as active components in high-performance devices critically depends on the availability of both n-type and p-type doping. Due to the strong covalent bonds within each layer and their atomically thin structure, traditional doping techniques used for bulk semiconductors are not suitable for 2D materials, as they often cause significant structural damage or introduce deep-level defects. For instance, ion implantation can create disordered lattices, while high-temperature diffusion doping can lead to interlayer delamination or unintentional phase transitions. These effects severely degrade the optoelectronic performance by reducing carrier mobility, quenching photoluminescence, or increasing trap-assisted recombination 22 – 25 . On the other hand, due to the atomic thickness of TMDs, their optical and structural properties, as well as carrier dynamics, can be effectively engineered by using various post-growth methods 26 – 28 . Several prominent post-growth doping techniques have been reported in the literature, notably chemical treatment, ion implantation, plasma doping, thermal annealing, electron beam irradiation, and ultraviolet-ozone treatment. These methods differ in their mechanisms of introducing dopants or modifying carrier concentrations. Chemical treatment typically relies on surface adsorption or substitutional doping through reaction with precursor molecules. Ion implantation involves accelerating dopant ions into the material, allowing precise control but often causing lattice damage. Plasma doping introduces energetic ions or radicals that can modify the surface or embed dopants. Thermal annealing promotes the diffusion of dopants or activates existing ones by repairing defects. Electron beam irradiation induces defects or modulates local electronic structure through energy transfer, while ultraviolet-ozone treatment introduces oxygen-related species that chemically modify the surface or passivate defects 29 – 36 . The main challenge in post-growth doping of 2D materials lies in maintaining their excellent optical properties while consistently controlling the doping concentration. In this study, we specifically addressed this challenge in the context of chemical treatment based on 1,2-dichloroethane (DCE). The chemical formula of DCE is C₂H₄Cl₂. When MoS₂ is submerged in DCE solution, chlorine (Cl) atoms interact with MoS 2 , acting as electron donors and leaving behind ethylene gas (C₂H₄). It has been employed as an effective tool for defect engineering and doping in graphene and 2D TMDs. Several studies in the literature have demonstrated the n-type doping of TMDs using DCE solution. L. Yang et al. reported n-type doping in WS 2 and MoS 2 via a 12-hour DCE treatment, resulting in reduced contact resistance from ~ 10 2 kΩ·µm to 0.7 kΩ·µm and 0.5 kΩ·µm, respectively 37 . In a recent study by A. Roy et al., it is demonstrated that the Schottky barrier height of WS 2 decreased from 1.02 eV to 0.8 eV with a DCE treatment applied as a function of treatment time, from 6 to 24 hours 38 . T.Y. Kim et al. proposed using varying molar concentrations of DCE solvent for improved control over the doping process in MoS 2 39 . They reported a well-defined correlation between carrier density and the molar concentration of DCE during the 45-minute treatment 39 . While the effects of the DCE treatment on the electrical properties of the aforementioned TMDs have been comprehensively studied and optimised by different research groups, its impact on the optical properties of TMDs remains unclear. In this study, we systematically investigate the dependence of the optical properties of semiconducting TMDs on both DCE treatment time and layer number. We exfoliated MoS 2 flakes with different numbers of layers on polydimethylsiloxane (PDMS), from monolayer to bulk, and transferred the flakes to fused silica substrates. PL spectroscopy is carried out to investigate the effect on their optical properties with a wide range of DCE treatment durations. To elucidate the energetically favoured Cl-doping mechanisms in MoS₂ and the influence of intrinsic sulphur defects on its optical properties, density functional theory (DFT) calculations are performed. Methods Experimental Bulk single-crystal MoS 2 are purchased from HQ Graphene and 2D Semiconductors, which are slightly n-type in their pristine state (see supplementary material) 30 . The Scotch tape and PDMS-assisted mechanical exfoliation method is used to obtain MoS 2 flakes with different numbers of layers 40 . Flakes larger than 10 × 10 µm are transferred onto fused silica substrates using a custom-built viscoelastic transfer system. The photoluminescence (PL) spectra of the flakes are measured using a micro-PL setup equipped with a 500 mm monochromator (Shamrock 500i, Andor), a Si CCD (Newton BEX2-DD, Andor) and a 532 nm excitation laser, before and after DCE treatment. The laser beam is focused to be a spot of ~ 0.8 µm in diameter with a 100x objective (numerical aperture, NA = 0.7). After obtaining PL measurements for pristine flakes, the samples are soaked in the DCE solution for varying durations, ranging from 30 seconds to 24 hours, in a clean room atmosphere. Immediately after DCE treatment, the flakes are dried with N 2 gas. Finally, PL measurements are conducted on DCE-treated samples at room temperature. For each sample, the intensity ratio of the PL peaks before and after DCE treatment is calculated to assess the effect of the DCE treatment. For electrical characterisation, the flake is transferred onto the SiO 2 (300 nm)-on-Si substrate, which is cleaned with acetone (ACE) and isopropyl alcohol (IPA) before the transfer process. The electrodes are patterned on the sample by EBL with a bilayer resist to obtain an undercut structure (MMA/PMMA, Allresist GmbH) and Au/Cr (60 nm/10 nm) is deposited as contact electrodes (Figure S2a). The lift-off process is carried out in warm ACE. The fabricated FET device is mounted onto a ceramic chip holder with wires bonded for electrical characterization. Electrical characterisation is carried out on the FET devices before and after 12h of DCE treatment. Output and transfer characterisations are measured using Agilent B2902A source measure unit at room temperature. Computational DFT calculations are conducted using the Quantum Espresso software package 41 , 42 , to unveil the complex interplay between MoS 2 and the Cl-doping process. The projected augmented wave pseudo-potentials 43 , 44 and the Perdew–Burke–Ernzerhof parametrisation of the generalised gradient approximation exchange-correlation functional are employed 45 . A 6×6×1 K-point mesh within the Gamma-centred Monkhorst-Pack scheme is applied to Brillouin Zone integration with a kinetic energy cut-off of 60 Ry and a density cut-off of 480 Ry. A Gaussian smearing scheme is utilised with a broadening of 0.01 Ry. In the geometry optimisations, the system is allowed to relax fully using a Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm along with the total energy threshold of 10 − 5 Ry and the force threshold of 10 − 3 Ry/Å. In the calculations, a 4 × 4 × 1 hexagonal supercell of a pristine MoS2, with the dimensions of 1.24 nm × 1.74 nm × 3.0 nm, is used to construct six models: two substitutional and four interstitial doping models. In substitutional doping models of T3 and B3, an isolated Cl atom substitutes for an S atom on either the upper (T3) or the lower S layer (B3) of MoS 2 . Note that, in the substitutional doping models, metal vacancies are not considered as their formation is energetically highly unfavourable 46 . In the interstitial doping models, an isolated Cl atom is introduced via interstitial insertion at different symmetry-allowed sites: T1, H3, and the van der Waals (vdW) gap within the MoS 2 lattice. T1 is a one-fold coordinated site on the topmost Mo atom. H3 is a three-fold coordinated site at the centre of the honeycomb formed by three Mo atoms. VdW gap site is a one-fold coordinated site at the vdW gap of a MoS 2 bilayer. Substitutional doping process (i.e. T3 and B3) mainly consists of two elementary steps: (i) a vacancy formation in MoS 2 after the immersion in a solution, leading to the generation of dangling bonds, and (ii) chemical adsorption of Cl on MoS 2 . The formation energy associated with Cl doping, E form can be computed as a sum of vacancy formation energy, E vac , and adsorption energy of Cl, E ads . $$\:{E}_{form}={E}_{\:vac}+{E}_{ads}$$ 1 $$\:{E}_{ads}=\:{E}_{doped}+({E}_{system}+\:{\eta\:}_{Cl}\times\:{\mu\:}_{Cl})$$ 2 where E doped is the total energy of a Cl-doped MoS 2 system, η Cl is the number of Cl atoms doped in MoS 2 and µ Cl is the chemical potential of a Cl atom in a Cl 2 gas molecule. For the interstitial doping models, the Cl adsorption energy is computed using only Eq. 2 . $$\:{E}_{\:vac}=\:{E}_{system}-({E}_{pristine}\:-\:{\eta\:}_{s}\times\:{\mu\:}_{s})$$ 3 where E system and E pristine are the total energies of a MoS 2 sheet with and without vacancy, respectively. η S is the number S atoms removed from the sheet and µ s is the chemical potential of a S atom, referenced to the energy per atom in α-S bulk. Following full relaxation, density of states and electronic band calculations are performed on the most energetically favourable model. The models depicted in the figures are visualised using VESTA software 47 . Results and Discussion It is well understood that the electronic band structure and bandgap of TMDs depend on the number of layers. Therefore, the number of layers in the TMD flakes can be precisely determined using the PL peak wavelength/energy 48 . The PL spectrum of each layer is utilised to confirm their number of layers both on the PDMS and fused silica substrate (Figure S1 ). The peak value for monolayer MoS 2 occurs at approximately 655 nm, representing the direct bandgap. The indirect transition peak of bilayer MoS 2 starts to appear around 802 nm, while its direct transition peak is around 665 nm. From a monolayer to a thicker flake, the PL intensity dramatically decreases as a function of increased thickness, and the bandgap of MoS 2 red shifts up to 1.35 eV, i.e. 921 nm, as shown in the inset of Figure S1 . However, distinguishing the indirect band gaps beyond 7 layers becomes challenging. We refer to more than 7-layer flakes as bulk. In the literature, T.Y. Kim et al. 39 demonstrated that a 45-minute treatment effectively dopes MoS₂, and this finding is used as a reference point for this study. The PL spectra of different layers are characterised before and after a 60-minute DCE treatment (Fig. 1 ). After 60 minutes in the DCE solution, the PL peak intensity of monolayer MoS₂ decreases to ~ 80% of its initial value. Under the same treatment time, the PL peak intensity of the direct transition in bilayer (2L) MoS₂ retains 70 ± 15% of its initial intensity, while the indirect transition retains only 18 ± 11%. Repeated 60-minute DCE treatments show a layer-dependent response, where the suppression of the indirect transition diminishes with increasing thickness. The same experiments are repeated with the pristine flakes for 2 minutes of DCE treatment, and it is observed that even with 2 minutes of treatment time, the PL intensity of the indirect transition for 2L and beyond reduces significantly, while their direct transition intensity remains almost unaffected. The same DCE treatment is processed on more than a hundred samples with various layer numbers and under different time durations. Figure 2 shows the overview of the PL ratios of DCE-treated to pristine MoS₂ samples for both direct and indirect transitions of 1L (direct only), 2L, 4L, 6L, 7L, and bulk, over periods ranging from 30 seconds to 24 hours. The effect of DCE treatment on the PL intensity of 2D MoS₂ is assessed for each flake individually by taking the ratio of the peak intensities before and after treatment (R = I dope /I pristine ). The data points with error bars in Figs. 3 and 4 represent the average values of R for all (> 100) samples exposed to the same duration in the DCE solution. It is clear that the impact of DCE treatment is not the same on the direct and indirect transitions of MoS₂ samples - the indirect transitions are affected more than the direct transitions for all layers. In other words, the indirect transitions in the multilayer MoS 2 can be damped, while the direct transition is maintained. The effect of DCE on the optical properties of MoS₂, under the same treatment time, also depends on the number of layers. Figure 3 shows the intensity ratio R as a function of the number of layers for different doping durations. The impact of DCE on the PL intensity across all treatment durations reduces as the number of layers increases. The layer dependency of DCE treatment in MoS₂ flakes eventually saturated. The dashed line fittings are to highlight the difference in the effect between the direct and indirect transitions. This trend may indicate that the doping process affects only the surface layers. To understand the mechanisms behind the DCE treatment resulting in n-type doping (Figure S2) and its effect on the optical properties of the MoS 2 , the energetically favourable adsorption mechanisms of Cl and the electronic band structures with density of state (DOS) are calculated in pristine monolayer and bilayer MoS 2 flakes for the most possible interaction mechanism scenario. Figure 4 shows the energetic preference of symmetry-permitted doping sites for Cl in MoS 2 , where the defect formation (E vac ), Cl binding (E ads ), and total formation (E form ) energies, are indicated with orange, blue and green bars. The DFT calculations show that the Cl binding at an interstitial site of a MoS 2 lattice is energetically highly endothermic (7 eV for H3, Fig. 4 d), while one adsorbed on S-vacancy is exothermic, meaning that Cl binding to defects is an energetically driven process (-1.9 eV for both T3 and B3, Fig. 5 b, c). Even with a preceding defect formation in a pristine MoS 2 , the total energy required for substitutional doping of Cl for an S atom is still lower (1.8 eV for both T3 and B3) than those for interstitial doping (H3 and vdW gap, Fig. 4 e). When comparing the total formation energies, Cl adsorption on a pristine surface is energetically most favourable (0.9 eV for T1) – which is the essential step to attract Cl to the surface (Fig. 4 a). This is followed by the energies for the replacement of a surface S atom by a Cl atom in T3 and B3 models. Cl doping at the vdW gap ( VDG in Fig. 4 e), is energetically more expensive than substitutional doping, while cheaper than the interstitial doping. These results indicate that the most likely scenario is that the Cl atoms are filling the S-vacancies or replacing S atoms in MoS 2 during the doping process. Moreover, the DFT results suggest that, depending on the local chemical environment and doping conditions, it is also likely for Cl atoms to migrate at the vdW gap in the MoS 2 multilayers – this indeed increases the interlayer spacing while reducing the interlayer coupling, which can alter the electronic band structure of MoS 2 . The band structure analysis in Fig. 5 highlights the effect of treatment on the electronic band structure of materials. In the presence of substitutional doping of Cl in a bilayer MoS 2 , the bandgap still exhibits indirect bandgap characteristics with a narrower bandgap (Fig. 5 c, d) compared to the pristine layer (Fig. 5 a, b) while the insertion of Cl at the vdW gap, i.e. VDG, causes less dispersive (flatter) bands resulting in both lowering the bandgap and transition from indirect to direct bandgap (Fig. 5 e). Density-of-state (DOS) calculations give additional support to our hypothesis and reveal the effect of dopant contribution near the Fermi level energy states (Fig. 5 f-h). This leads to an increase in electrical conductivity. The emergence of an additional nearly flat band around the Fermi energy level indicates strong electronic localisation due to the VDG dopant (Fig. 5 e, h). Also, the indirect bandgap feature significantly diminishes upon vdW gap doping, compared to the substitutional doping in T3 and B3 models (Fig. 5 c, d, g). This suggests, while both substitutional and interstitial doping are likely to suppress the indirect bandgap feature as observed in experiments, the vdW gap type doping is likely to induce the bandgap transition. All these results together indicate that Cl-doping is a candidate mechanism for engineering the bandgap. Chlorine (Cl) atoms are found to settle at defect sites near the MoS₂ indirect band level. This facilitates non-radiative relaxation from the conduction to the valence band, thereby quenching the PL intensity of the indirect transition. We attribute the unaffected direct transition to its occurrence solely at the K-point, which is relatively isolated in momentum space and thus less sensitive to defect-induced scattering or trapping. Defects arising from Cl-doping introduce localised states that primarily affect extended states across the band structure, leaving the highly localised direct transition largely unaffected. Conclusion In this study, we systematically investigated the impact of chlorine-based chemical treatment using 1,2-dichloroethane (DCE) on the optical properties of two-dimensional MoS₂ with varying layer numbers and treatment durations. Through comprehensive PL measurements, a suppression in the indirect transition peak intensity is observed, even after short DCE exposures (i.e., 2 minutes), while the direct transition remains largely unaffected across all layer numbers. This effect becomes more pronounced with increased treatment duration and diminishes with increased layer thickness, indicating a layer- and time-dependent doping behaviour. Density functional theory calculations revealed that chlorine atoms preferentially occupy sulphur vacancy sites in MoS₂, forming energetically favourable configurations. These Cl dopants introduce in-gap states near the indirect band edge, promoting non-radiative recombination pathways that selectively reduce the indirect PL transitions. This insight suggests a transition from an indirect to direct bandgap character in multilayer MoS₂, effectively enabling bandgap engineering through surface chemical doping. It is demonstrated that DCE treatment is a tuneable post-growth strategy not only for n-type doping but also for manipulating the optical response of MoS₂. The ability to selectively suppress indirect transitions while maintaining the integrity of direct transitions provides a powerful strategy for enhancing excitonic emission in TMD-based optoelectronic devices, particularly in applications where robust direct-gap emission is crucial. Declarations Acknowledgements This study was supported by the Scientific Research Projects Coordination Unit of Istanbul University, project numbers FBA-2023-39412 and FYL-2023-39742, and the Scientific and Technological Research Council of Turkey (TÜBİTAK) 1001 – Scientific and Technological Research Projects Support Program, project number 121F169. Y.W. acknowledges a Research Fellowship awarded by the Royal Academy of Engineering RF/201718/17131 and an EPSRC grant EP/V047663/1. C.S. and N.N. acknowledge the EuroHPC Joint Undertaking for awarding this project access to the EuroHPC supercomputer LEONARDO-BOOSTER, hosted by CINECA (Italy) and the LEONARDO consortium through an EuroHPC Benchmark Access call (EHPC-BEN-2024B11-048). They also thank the National Centre for High-Performance Computing (UHEM/ITU) (https://en.uhem.itu.edu.tr/) for the provision of additional computational resources under the grant number 1016652023. The authors would like to thank Dr. Alexander Armstrong and Prof. Keith McKenna for their valuable discussions on DFT. Author Contributions Y.K.B., E.H. and F.S. fabricated the samples, performed the steady-state photoluminescence measurements; C.S. performed the DFT calculations and supervised by N.N; Y.K.B., N.N., A.E., Y.W. and F.S., analysed the results; A.E., Y.W and F.S. managed various aspects and funded the project; Y.K.B., and F.S. wrote the manuscript with contributions from all co-authors; Y.W. and F.S. oversaw the entire project. All authors have read and agreed to the published version of the manuscript. Data and code availability The authors declare that all the data and code supporting the findings of this study are available within the article or upon request from the corresponding author. Corresponding Author * Yue Wang: School of Physics, Engineering and Technology, University of York, York, YO10 5DD, United Kingdom, orcid.org/0000-0002-2482-005X, email: [email protected] * Fahrettin Sarcan: Department of Physics, Faculty of Science, Istanbul University, Vezneciler, 34134, Istanbul, Turkey, orcid.org/0000-0002-8860-4321, email: [email protected] References Wang, J., Han, J., Chen, X. & Wang, X. Design strategies for two-dimensional material photodetectors to enhance device performance. InfoMat vol. 1 33–53 Preprint at https://doi.org/10.1002/inf2.12004 (2019). Manzeli, S., Ovchinnikov, D., Pasquier, D., Yazyev, O. V. & Kis, A. 2D transition metal dichalcogenides. Nature Reviews Materials vol. 2 Preprint at https://doi.org/10.1038/natrevmats.2017.33 (2017). Jariwala, D., Sangwan, V. K., Lauhon, L. J., Marks, T. J. & Hersam, M. C. Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. 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Momma, K. & Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J Appl Crystallogr 44 , 1272–1276 (2011). Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: A new direct-gap semiconductor. Phys Rev Lett 105 , (2010). Additional Declarations No competing interests reported. Supplementary Files supplementaryInformation.docx Cite Share Download PDF Status: Published Journal Publication published 30 Nov, 2025 Read the published version in npj 2D Materials and Applications → Version 1 posted Editorial decision: Revision requested 11 Sep, 2025 Reviews received at journal 20 Aug, 2025 Reviews received at journal 05 Aug, 2025 Reviewers agreed at journal 01 Aug, 2025 Reviewers agreed at journal 01 Aug, 2025 Reviewers agreed at journal 31 Jul, 2025 Reviewers agreed at journal 30 Jul, 2025 Reviewers invited by journal 30 Jul, 2025 Editor assigned by journal 30 Jul, 2025 Submission checks completed at journal 30 Jul, 2025 First submitted to journal 25 Jul, 2025 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. 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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-7212840","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":494474225,"identity":"59f9bffb-ec38-4ede-a43f-ddfdc4a57dc5","order_by":0,"name":"Yusuf Kerem Bostan","email":"","orcid":"","institution":"Istanbul University","correspondingAuthor":false,"prefix":"","firstName":"Yusuf","middleName":"Kerem","lastName":"Bostan","suffix":""},{"id":494474227,"identity":"cf6ae9a4-8719-487f-a3e2-e161b89ebae4","order_by":1,"name":"Elanur Hut","email":"","orcid":"","institution":"Istanbul University","correspondingAuthor":false,"prefix":"","firstName":"Elanur","middleName":"","lastName":"Hut","suffix":""},{"id":494474228,"identity":"f8cbe500-a755-4f4d-86da-7a36663d6230","order_by":2,"name":"Cem Sanga","email":"","orcid":"","institution":"Istanbul Technical University","correspondingAuthor":false,"prefix":"","firstName":"Cem","middleName":"","lastName":"Sanga","suffix":""},{"id":494474230,"identity":"51bfb581-0d07-4310-b08e-100b8e4f7420","order_by":3,"name":"Nadire Nayir","email":"","orcid":"","institution":"Istanbul Technical University","correspondingAuthor":false,"prefix":"","firstName":"Nadire","middleName":"","lastName":"Nayir","suffix":""},{"id":494474233,"identity":"58a4108a-6583-430a-bc9e-264531513486","order_by":4,"name":"Ayse Erol","email":"","orcid":"","institution":"Istanbul University","correspondingAuthor":false,"prefix":"","firstName":"Ayse","middleName":"","lastName":"Erol","suffix":""},{"id":494474236,"identity":"b335f787-a495-4b6d-80fd-d6990f6dcf31","order_by":5,"name":"Yue Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3UlEQVRIiWNgGAWjYDADfiBmbAAz2YjUItlAshaDA8RqMbiRfICZp+aO3eYbycc+zmCwk2eQSEsgoCUtgZnn2LPkbTfSkmduYEg2bJBIO0BAS44Bcw7b4WSzGznGjA8YmBMYJNIbCGjJ/8Cc8+9wsvEMsJZ6YrTkMDDnth22M5AAatnAcBiohYDDJM88Mzj8t+9wgsSZZ8mMMwyOG7bxPEvAq4XvePLDhzO+Hbbnb08+zNhTUS3Pz55mgFeLAtARIHckNgiAzDYgIiLlGyC0PQM/fh+MglEwCkbBCAYAVf1HoUnTqw4AAAAASUVORK5CYII=","orcid":"","institution":"University of York","correspondingAuthor":true,"prefix":"","firstName":"Yue","middleName":"","lastName":"Wang","suffix":""},{"id":494474238,"identity":"674ef417-a3f4-4b1b-bffa-ad03ad6833b7","order_by":6,"name":"Fahrettin Sarcan","email":"","orcid":"","institution":"Istanbul University","correspondingAuthor":false,"prefix":"","firstName":"Fahrettin","middleName":"","lastName":"Sarcan","suffix":""}],"badges":[],"createdAt":"2025-07-25 09:38:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7212840/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7212840/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41699-025-00639-0","type":"published","date":"2025-11-30T15:58:10+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":88272656,"identity":"38e7cd00-8ec8-4c57-8961-e0d4fcc309d8","added_by":"auto","created_at":"2025-08-04 17:22:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":277116,"visible":true,"origin":"","legend":"\u003cp\u003eThe PL spectra of (a, b) 1L, (c, d) 2L, (e, f) 4L, (g, h) 6L and (i, j) 7L MoS₂ samples before (dark blue) and after (orange) exposed to DCE solution for 2 minutes (left column) and 60 minutes (right column).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/432b32c98e71170c34e58ec7.png"},{"id":88271424,"identity":"1b52d25a-d6e4-4ce2-971d-0069ae2bc906","added_by":"auto","created_at":"2025-08-04 17:14:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":230672,"visible":true,"origin":"","legend":"\u003cp\u003eTime dependence of the PL intensity ratios before and after DCE treatment for both direct (dark blue) and indirect (orange) transitions of (a) 1L, (b) 2L, (c) 4L, (d) 6L, (e) 7L and (f) Bulk MoS\u003csub\u003e2\u003c/sub\u003e under various treatment time in logarithmic scale, ranging from 30 s to 1440 min (24 hours).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/8594696c1f55d93ff4f36fac.png"},{"id":88271423,"identity":"d9bf62a5-e864-41bb-af3b-da33cd1b83d4","added_by":"auto","created_at":"2025-08-04 17:14:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":254016,"visible":true,"origin":"","legend":"\u003cp\u003e2D MoS\u003csub\u003e2\u003c/sub\u003e layer dependence of the PL intensity ratios before and after DCE treatment in both direct (dark blue) and indirect (orange) transitions under (a) 1 min, (b) 2 min, (c) 4 min, (d) 10 min, (e) 60 min and (f) 480 min DCE treatment.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/479ba8cbbc66e7d66ea5c426.png"},{"id":88272658,"identity":"a8d11c2c-663c-419f-a9cc-0884d85f222f","added_by":"auto","created_at":"2025-08-04 17:22:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2677946,"visible":true,"origin":"","legend":"\u003cp\u003eOptimised configurations of a MoS\u003csub\u003e2\u003c/sub\u003e bilayer with Cl (a) adsorbed on the surface (T1), (b) replacing a S atom on the upper S (T3) and (c) the lower S layer of MoS\u003csub\u003e2\u003c/sub\u003e (B3), (d) inserted at the centre of honeycomb neighbouring by three Mo atoms (H3) and (e) inserted at the vdW gap. (f) A graph depicting S-vacancy formation energy E\u003csub\u003ef\u003c/sub\u003e, which is the sum of E\u003csub\u003evac\u003c/sub\u003e and E\u003csub\u003eads\u003c/sub\u003e, along with E\u003csub\u003evac\u003c/sub\u003e and E\u003csub\u003eads\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/bfd13f4eb31a6cec5732bcb6.png"},{"id":88271431,"identity":"63883c16-6ae3-44e2-a08d-50154cef327a","added_by":"auto","created_at":"2025-08-04 17:14:08","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":6476701,"visible":true,"origin":"","legend":"\u003cp\u003eDFT computed electronic bandgap structures of a pristine (a) monolayer and (b) bilayer MoS\u003csub\u003e2\u003c/sub\u003e; and a bilayer MoS\u003csub\u003e2 \u003c/sub\u003ewith Cl doped on (c) T3, (d) B3 and (e) at the vdW gap. Density of state (DOS) calculations of (f) a pristine bilayer, Cl-doped on (g) T3 and (h) at the vdW gap.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/b12bf340ed7ea29183d8b04d.png"},{"id":97178753,"identity":"7daa1d71-9f9a-495b-82e6-715e00b3c759","added_by":"auto","created_at":"2025-12-01 16:13:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":10465177,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/291507c3-9218-4696-9c59-a236d2ba2625.pdf"},{"id":88272657,"identity":"20743d77-98bc-4b5e-8aed-83b520029758","added_by":"auto","created_at":"2025-08-04 17:22:08","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1251970,"visible":true,"origin":"","legend":"","description":"","filename":"supplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7212840/v1/1f392dc871fbd105918b4dc8.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Chemical Treatment-Induced Indirect-to-Direct Bandgap Transition in MoS₂: Impact on Excitonic Emission","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOver the past decades, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention due to their ultrathin structures (thickness \u0026lt; 1 nm for monolayers), high carrier mobilities, tuneable bandgaps (in the range of 1–2 eV), and excellent sensing capabilities \u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e–\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Their atomically thin structure leads to high surface-to-volume ratios, making their electrical conductivity highly sensitive to environmental changes - a desirable characteristic for sensing applications \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Key examples of TMDs such as WS₂, WSe\u003csub\u003e2\u003c/sub\u003e, MoS₂, and MoTe₂ are alternative active materials for optoelectronic devices, including LEDs and lasers, and their bandgaps, which can be precisely tuned via thickness, strain and doping \u003csup\u003e\u003cspan additionalcitationids=\"CR7 CR8 CR9\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e–\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, offering an additional degree of freedom to tailor their properties. A unique feature of layered TMDs is their characteristic transition from an indirect to a direct bandgap in the monolayer limit, attributed to quantum confinement \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. This is particularly beneficial for optoelectronic devices, as direct bandgaps allow efficient radiative recombination of electrons and holes without requiring phonon assistance, resulting in stronger light emission and absorption. However, despite their remarkable structural, electrical and optical properties, current experimental results show mobilities of ~ 50 cm²/V·s \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. This significant discrepancy from theoretical predictions points to dominant scattering mechanisms that limit electrical conductivity, hindering their competitiveness with widely adopted electronic and optoelectronic materials such as Si and GaAs \u003csup\u003e\u003cspan additionalcitationids=\"CR14 CR15 CR16 CR17 CR18 CR19 CR20\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e–\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. The successful deployment of TMD materials as active components in high-performance devices critically depends on the availability of both n-type and p-type doping. Due to the strong covalent bonds within each layer and their atomically thin structure, traditional doping techniques used for bulk semiconductors are not suitable for 2D materials, as they often cause significant structural damage or introduce deep-level defects. For instance, ion implantation can create disordered lattices, while high-temperature diffusion doping can lead to interlayer delamination or unintentional phase transitions. These effects severely degrade the optoelectronic performance by reducing carrier mobility, quenching photoluminescence, or increasing trap-assisted recombination \u003csup\u003e\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e–\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. On the other hand, due to the atomic thickness of TMDs, their optical and structural properties, as well as carrier dynamics, can be effectively engineered by using various post-growth methods \u003csup\u003e\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e–\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Several prominent post-growth doping techniques have been reported in the literature, notably chemical treatment, ion implantation, plasma doping, thermal annealing, electron beam irradiation, and ultraviolet-ozone treatment. These methods differ in their mechanisms of introducing dopants or modifying carrier concentrations.\u003c/p\u003e\u003cp\u003eChemical treatment typically relies on surface adsorption or substitutional doping through reaction with precursor molecules. Ion implantation involves accelerating dopant ions into the material, allowing precise control but often causing lattice damage. Plasma doping introduces energetic ions or radicals that can modify the surface or embed dopants. Thermal annealing promotes the diffusion of dopants or activates existing ones by repairing defects. Electron beam irradiation induces defects or modulates local electronic structure through energy transfer, while ultraviolet-ozone treatment introduces oxygen-related species that chemically modify the surface or passivate defects \u003csup\u003e\u003cspan additionalcitationids=\"CR30 CR31 CR32 CR33 CR34 CR35\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e–\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. The main challenge in post-growth doping of 2D materials lies in maintaining their excellent optical properties while consistently controlling the doping concentration. In this study, we specifically addressed this challenge in the context of chemical treatment based on 1,2-dichloroethane (DCE).\u003c/p\u003e\u003cp\u003eThe chemical formula of DCE is C₂H₄Cl₂. When MoS₂ is submerged in DCE solution, chlorine (Cl) atoms interact with MoS\u003csub\u003e2\u003c/sub\u003e, acting as electron donors and leaving behind ethylene gas (C₂H₄). It has been employed as an effective tool for defect engineering and doping in graphene and 2D TMDs. Several studies in the literature have demonstrated the n-type doping of TMDs using DCE solution. L. Yang et al. reported n-type doping in WS\u003csub\u003e2\u003c/sub\u003e and MoS\u003csub\u003e2\u003c/sub\u003e via a 12-hour DCE treatment, resulting in reduced contact resistance from ~ 10\u003csup\u003e2\u003c/sup\u003e kΩ·µm to 0.7 kΩ·µm and 0.5 kΩ·µm, respectively \u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. In a recent study by A. Roy et al., it is demonstrated that the Schottky barrier height of WS\u003csub\u003e2\u003c/sub\u003e decreased from 1.02 eV to 0.8 eV with a DCE treatment applied as a function of treatment time, from 6 to 24 hours \u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. T.Y. Kim et al. proposed using varying molar concentrations of DCE solvent for improved control over the doping process in MoS\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e39\u003c/sup\u003e. They reported a well-defined correlation between carrier density and the molar concentration of DCE during the 45-minute treatment \u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. While the effects of the DCE treatment on the electrical properties of the aforementioned TMDs have been comprehensively studied and optimised by different research groups, its impact on the optical properties of TMDs remains unclear.\u003c/p\u003e\u003cp\u003eIn this study, we systematically investigate the dependence of the optical properties of semiconducting TMDs on both DCE treatment time and layer number. We exfoliated MoS\u003csub\u003e2\u003c/sub\u003e flakes with different numbers of layers on polydimethylsiloxane (PDMS), from monolayer to bulk, and transferred the flakes to fused silica substrates. PL spectroscopy is carried out to investigate the effect on their optical properties with a wide range of DCE treatment durations. To elucidate the energetically favoured Cl-doping mechanisms in MoS₂ and the influence of intrinsic sulphur defects on its optical properties, density functional theory (DFT) calculations are performed.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eExperimental\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eBulk single-crystal MoS\u003csub\u003e2\u003c/sub\u003e are purchased from HQ Graphene and 2D Semiconductors, which are slightly n-type in their pristine state (see supplementary material) \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The Scotch tape and PDMS-assisted mechanical exfoliation method is used to obtain MoS\u003csub\u003e2\u003c/sub\u003e flakes with different numbers of layers \u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Flakes larger than 10 × 10 µm are transferred onto fused silica substrates using a custom-built viscoelastic transfer system. The photoluminescence (PL) spectra of the flakes are measured using a micro-PL setup equipped with a 500 mm monochromator (Shamrock 500i, Andor), a Si CCD (Newton BEX2-DD, Andor) and a 532 nm excitation laser, before and after DCE treatment. The laser beam is focused to be a spot of ~ 0.8 µm in diameter with a 100x objective (numerical aperture, NA = 0.7). After obtaining PL measurements for pristine flakes, the samples are soaked in the DCE solution for varying durations, ranging from 30 seconds to 24 hours, in a clean room atmosphere. Immediately after DCE treatment, the flakes are dried with N\u003csub\u003e2\u003c/sub\u003e gas. Finally, PL measurements are conducted on DCE-treated samples at room temperature. For each sample, the intensity ratio of the PL peaks before and after DCE treatment is calculated to assess the effect of the DCE treatment.\u003c/p\u003e\u003cp\u003eFor electrical characterisation, the flake is transferred onto the SiO\u003csub\u003e2\u003c/sub\u003e (300 nm)-on-Si substrate, which is cleaned with acetone (ACE) and isopropyl alcohol (IPA) before the transfer process. The electrodes are patterned on the sample by EBL with a bilayer resist to obtain an undercut structure (MMA/PMMA, Allresist GmbH) and Au/Cr (60 nm/10 nm) is deposited as contact electrodes (Figure S2a). The lift-off process is carried out in warm ACE. The fabricated FET device is mounted onto a ceramic chip holder with wires bonded for electrical characterization. Electrical characterisation is carried out on the FET devices before and after 12h of DCE treatment. Output and transfer characterisations are measured using Agilent B2902A source measure unit at room temperature.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eComputational\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eDFT calculations are conducted using the Quantum Espresso software package \u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, to unveil the complex interplay between MoS\u003csub\u003e2\u003c/sub\u003e and the Cl-doping process. The projected augmented wave pseudo-potentials \u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e and the Perdew–Burke–Ernzerhof parametrisation of the generalised gradient approximation exchange-correlation functional are employed \u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. A 6×6×1 K-point mesh within the Gamma-centred Monkhorst-Pack scheme is applied to Brillouin Zone integration with a kinetic energy cut-off of 60 Ry and a density cut-off of 480 Ry. A Gaussian smearing scheme is utilised with a broadening of 0.01 Ry. In the geometry optimisations, the system is allowed to relax fully using a Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm along with the total energy threshold of 10\u003csup\u003e− 5\u003c/sup\u003e Ry and the force threshold of 10\u003csup\u003e− 3\u003c/sup\u003e Ry/Å.\u003c/p\u003e\u003cp\u003eIn the calculations, a 4 × 4 × 1 hexagonal supercell of a pristine MoS2, with the dimensions of 1.24 nm × 1.74 nm × 3.0 nm, is used to construct six models: two substitutional and four interstitial doping models. In substitutional doping models of T3 and B3, an isolated Cl atom substitutes for an S atom on either the upper (T3) or the lower S layer (B3) of MoS\u003csub\u003e2\u003c/sub\u003e. Note that, in the substitutional doping models, metal vacancies are not considered as their formation is energetically highly unfavourable \u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. In the interstitial doping models, an isolated Cl atom is introduced via interstitial insertion at different symmetry-allowed sites: T1, H3, and the van der Waals (vdW) gap within the MoS\u003csub\u003e2\u003c/sub\u003e lattice. T1 is a one-fold coordinated site on the topmost Mo atom. H3 is a three-fold coordinated site at the centre of the honeycomb formed by three Mo atoms. VdW gap site is a one-fold coordinated site at the vdW gap of a MoS\u003csub\u003e2\u003c/sub\u003e bilayer.\u003c/p\u003e\u003cp\u003eSubstitutional doping process (i.e. T3 and B3) mainly consists of two elementary steps: (i) a vacancy formation in MoS\u003csub\u003e2\u003c/sub\u003e after the immersion in a solution, leading to the generation of dangling bonds, and (ii) chemical adsorption of Cl on MoS\u003csub\u003e2\u003c/sub\u003e. The formation energy associated with Cl doping, E\u003csub\u003eform\u003c/sub\u003e can be computed as a sum of vacancy formation energy, E\u003csub\u003evac\u003c/sub\u003e, and adsorption energy of Cl, E\u003csub\u003eads\u003c/sub\u003e.\u003c/p\u003e\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{E}_{form}={E}_{\\:vac}+{E}_{ads}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{E}_{ads}=\\:{E}_{doped}+({E}_{system}+\\:{\\eta\\:}_{Cl}\\times\\:{\\mu\\:}_{Cl})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cp\u003ewhere E\u003csub\u003edoped\u003c/sub\u003e is the total energy of a Cl-doped MoS\u003csub\u003e2\u003c/sub\u003e system, η\u003csub\u003eCl\u003c/sub\u003e is the number of Cl atoms doped in MoS\u003csub\u003e2\u003c/sub\u003e and µ\u003csub\u003eCl\u003c/sub\u003e is the chemical potential of a Cl atom in a Cl\u003csub\u003e2\u003c/sub\u003e gas molecule. For the interstitial doping models, the Cl adsorption energy is computed using only Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{E}_{\\:vac}=\\:{E}_{system}-({E}_{pristine}\\:-\\:{\\eta\\:}_{s}\\times\\:{\\mu\\:}_{s})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cp\u003ewhere E\u003csub\u003esystem\u003c/sub\u003e and E\u003csub\u003epristine\u003c/sub\u003e are the total energies of a MoS\u003csub\u003e2\u003c/sub\u003e sheet with and without vacancy, respectively. η\u003csub\u003eS\u003c/sub\u003e is the number S atoms removed from the sheet and µ\u003csub\u003es\u003c/sub\u003e is the chemical potential of a S atom, referenced to the energy per atom in α-S bulk.\u003c/p\u003e\u003cp\u003eFollowing full relaxation, density of states and electronic band calculations are performed on the most energetically favourable model. The models depicted in the figures are visualised using VESTA software \u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eIt is well understood that the electronic band structure and bandgap of TMDs depend on the number of layers. Therefore, the number of layers in the TMD flakes can be precisely determined using the PL peak wavelength/energy \u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. The PL spectrum of each layer is utilised to confirm their number of layers both on the PDMS and fused silica substrate (Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The peak value for monolayer MoS\u003csub\u003e2\u003c/sub\u003e occurs at approximately 655 nm, representing the direct bandgap. The indirect transition peak of bilayer MoS\u003csub\u003e2\u003c/sub\u003e starts to appear around 802 nm, while its direct transition peak is around 665 nm. From a monolayer to a thicker flake, the PL intensity dramatically decreases as a function of increased thickness, and the bandgap of MoS\u003csub\u003e2\u003c/sub\u003e red shifts up to 1.35 eV, i.e. 921 nm, as shown in the inset of Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. However, distinguishing the indirect band gaps beyond 7 layers becomes challenging. We refer to more than 7-layer flakes as bulk.\u003c/p\u003e\u003cp\u003eIn the literature, T.Y. Kim et al. \u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e demonstrated that a 45-minute treatment effectively dopes MoS₂, and this finding is used as a reference point for this study. The PL spectra of different layers are characterised before and after a 60-minute DCE treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). After 60 minutes in the DCE solution, the PL peak intensity of monolayer MoS₂ decreases to ~\u0026thinsp;80% of its initial value. Under the same treatment time, the PL peak intensity of the direct transition in bilayer (2L) MoS₂ retains 70\u0026thinsp;\u0026plusmn;\u0026thinsp;15% of its initial intensity, while the indirect transition retains only 18\u0026thinsp;\u0026plusmn;\u0026thinsp;11%. Repeated 60-minute DCE treatments show a layer-dependent response, where the suppression of the indirect transition diminishes with increasing thickness. The same experiments are repeated with the pristine flakes for 2 minutes of DCE treatment, and it is observed that even with 2 minutes of treatment time, the PL intensity of the indirect transition for 2L and beyond reduces significantly, while their direct transition intensity remains almost unaffected.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe same DCE treatment is processed on more than a hundred samples with various layer numbers and under different time durations. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the overview of the PL ratios of DCE-treated to pristine MoS₂ samples for both direct and indirect transitions of 1L (direct only), 2L, 4L, 6L, 7L, and bulk, over periods ranging from 30 seconds to 24 hours. The effect of DCE treatment on the PL intensity of 2D MoS₂ is assessed for each flake individually by taking the ratio of the peak intensities before and after treatment (R\u0026thinsp;=\u0026thinsp;I\u003csub\u003edope\u003c/sub\u003e/I\u003csub\u003epristine\u003c/sub\u003e). The data points with error bars in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e represent the average values of R for all (\u0026gt;\u0026thinsp;100) samples exposed to the same duration in the DCE solution. It is clear that the impact of DCE treatment is not the same on the direct and indirect transitions of MoS₂ samples - the indirect transitions are affected more than the direct transitions for all layers. In other words, the indirect transitions in the multilayer MoS\u003csub\u003e2\u003c/sub\u003e can be damped, while the direct transition is maintained.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe effect of DCE on the optical properties of MoS₂, under the same treatment time, also depends on the number of layers. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the intensity ratio R as a function of the number of layers for different doping durations. The impact of DCE on the PL intensity across all treatment durations reduces as the number of layers increases. The layer dependency of DCE treatment in MoS₂ flakes eventually saturated. The dashed line fittings are to highlight the difference in the effect between the direct and indirect transitions. This trend may indicate that the doping process affects only the surface layers.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo understand the mechanisms behind the DCE treatment resulting in n-type doping (Figure S2) and its effect on the optical properties of the MoS\u003csub\u003e2\u003c/sub\u003e, the energetically favourable adsorption mechanisms of Cl and the electronic band structures with density of state (DOS) are calculated in pristine monolayer and bilayer MoS\u003csub\u003e2\u003c/sub\u003e flakes for the most possible interaction mechanism scenario. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the energetic preference of symmetry-permitted doping sites for Cl in MoS\u003csub\u003e2\u003c/sub\u003e, where the defect formation (E\u003csub\u003evac\u003c/sub\u003e), Cl binding (E\u003csub\u003eads\u003c/sub\u003e), and total formation (E\u003csub\u003eform\u003c/sub\u003e) energies, are indicated with orange, blue and green bars. The DFT calculations show that the Cl binding at an interstitial site of a MoS\u003csub\u003e2\u003c/sub\u003e lattice is energetically highly endothermic (7 eV for H3, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), while one adsorbed on S-vacancy is exothermic, meaning that Cl binding to defects is an energetically driven process (-1.9 eV for both T3 and B3, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, c). Even with a preceding defect formation in a pristine MoS\u003csub\u003e2\u003c/sub\u003e, the total energy required for substitutional doping of Cl for an S atom is still lower (1.8 eV for both T3 and B3) than those for interstitial doping (H3 and vdW gap, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). When comparing the total formation energies, Cl adsorption on a pristine surface is energetically most favourable (0.9 eV for T1) \u0026ndash; which is the essential step to attract Cl to the surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). This is followed by the energies for the replacement of a surface S atom by a Cl atom in T3 and B3 models. Cl doping at the vdW gap \u003cb\u003e(\u003c/b\u003eVDG in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee), is energetically more expensive than substitutional doping, while cheaper than the interstitial doping.\u003c/p\u003e\u003cp\u003eThese results indicate that the most likely scenario is that the Cl atoms are filling the S-vacancies or replacing S atoms in MoS\u003csub\u003e2\u003c/sub\u003e during the doping process. Moreover, the DFT results suggest that, depending on the local chemical environment and doping conditions, it is also likely for Cl atoms to migrate at the vdW gap in the MoS\u003csub\u003e2\u003c/sub\u003e multilayers \u0026ndash; this indeed increases the interlayer spacing while reducing the interlayer coupling, which can alter the electronic band structure of MoS\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe band structure analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e highlights the effect of treatment on the electronic band structure of materials. In the presence of substitutional doping of Cl in a bilayer MoS\u003csub\u003e2\u003c/sub\u003e, the bandgap still exhibits indirect bandgap characteristics with a narrower bandgap (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, d) compared to the pristine layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b) while the insertion of Cl at the vdW gap, i.e. VDG, causes less dispersive (flatter) bands resulting in both lowering the bandgap and transition from indirect to direct bandgap (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee).\u003c/p\u003e\u003cp\u003eDensity-of-state (DOS) calculations give additional support to our hypothesis and reveal the effect of dopant contribution near the Fermi level energy states (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef-h). This leads to an increase in electrical conductivity. The emergence of an additional nearly flat band around the Fermi energy level indicates strong electronic localisation due to the VDG dopant (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee, h). Also, the indirect bandgap feature significantly diminishes upon vdW gap doping, compared to the substitutional doping in T3 and B3 models (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, d, g). This suggests, while both substitutional and interstitial doping are likely to suppress the indirect bandgap feature as observed in experiments, the vdW gap type doping is likely to induce the bandgap transition. All these results together indicate that Cl-doping is a candidate mechanism for engineering the bandgap.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eChlorine (Cl) atoms are found to settle at defect sites near the MoS₂ indirect band level. This facilitates non-radiative relaxation from the conduction to the valence band, thereby quenching the PL intensity of the indirect transition. We attribute the unaffected direct transition to its occurrence solely at the K-point, which is relatively isolated in momentum space and thus less sensitive to defect-induced scattering or trapping. Defects arising from Cl-doping introduce localised states that primarily affect extended states across the band structure, leaving the highly localised direct transition largely unaffected.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, we systematically investigated the impact of chlorine-based chemical treatment using 1,2-dichloroethane (DCE) on the optical properties of two-dimensional MoS₂ with varying layer numbers and treatment durations. Through comprehensive PL measurements, a suppression in the indirect transition peak intensity is observed, even after short DCE exposures (i.e., 2 minutes), while the direct transition remains largely unaffected across all layer numbers. This effect becomes more pronounced with increased treatment duration and diminishes with increased layer thickness, indicating a layer- and time-dependent doping behaviour. Density functional theory calculations revealed that chlorine atoms preferentially occupy sulphur vacancy sites in MoS₂, forming energetically favourable configurations. These Cl dopants introduce in-gap states near the indirect band edge, promoting non-radiative recombination pathways that selectively reduce the indirect PL transitions. This insight suggests a transition from an indirect to direct bandgap character in multilayer MoS₂, effectively enabling bandgap engineering through surface chemical doping. It is demonstrated that DCE treatment is a tuneable post-growth strategy not only for n-type doping but also for manipulating the optical response of MoS₂. The ability to selectively suppress indirect transitions while maintaining the integrity of direct transitions provides a powerful strategy for enhancing excitonic emission in TMD-based optoelectronic devices, particularly in applications where robust direct-gap emission is crucial.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThis study was supported by the Scientific Research Projects Coordination Unit of Istanbul University, project numbers FBA-2023-39412 and FYL-2023-39742, and the Scientific and Technological Research Council of Turkey (T\u0026Uuml;BİTAK) 1001 \u0026ndash; Scientific and Technological Research Projects Support Program, project number 121F169. Y.W. acknowledges a Research Fellowship awarded by the Royal Academy of Engineering RF/201718/17131 and an EPSRC grant EP/V047663/1. C.S. and N.N. acknowledge the EuroHPC Joint Undertaking for awarding this project access to the EuroHPC supercomputer LEONARDO-BOOSTER, hosted by CINECA (Italy) and the LEONARDO consortium through an EuroHPC Benchmark Access call (EHPC-BEN-2024B11-048). They also thank the National Centre for High-Performance Computing (UHEM/ITU) (https://en.uhem.itu.edu.tr/) for the provision of additional computational resources under the grant number 1016652023. The authors would like to thank Dr. Alexander Armstrong and Prof. Keith McKenna for their valuable discussions on DFT.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Author Contributions\u003c/p\u003e\n\u003cp\u003eY.K.B., E.H. and F.S. fabricated the samples, performed the steady-state photoluminescence measurements; C.S. performed the DFT calculations and supervised by N.N; Y.K.B., N.N., A.E., Y.W. and F.S., analysed the results; A.E., Y.W and F.S. managed various aspects and funded the project; Y.K.B., and F.S. wrote the manuscript with contributions from all co-authors; Y.W. and F.S. oversaw the entire project.\u0026nbsp;All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eData and code availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that all the data and code supporting the findings of this study are available within the article or upon request from the corresponding author.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Corresponding Author\u003c/p\u003e\n\u003cp\u003e* Yue Wang: School of Physics, Engineering and Technology, University of York,\u0026nbsp;York, YO10 5DD, United Kingdom, orcid.org/0000-0002-2482-005X, email: [email protected]\u003c/p\u003e\n\u003cp\u003e* Fahrettin Sarcan: Department of Physics, Faculty of Science, Istanbul University, Vezneciler, 34134, Istanbul, Turkey, orcid.org/0000-0002-8860-4321, email: [email protected]\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eWang, J., Han, J., Chen, X. \u0026amp; Wang, X. 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Atomically thin MoS2: A new direct-gap semiconductor. \u003cem\u003ePhys Rev Lett\u003c/em\u003e \u003cstrong\u003e105\u003c/strong\u003e, (2010).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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":"npj-2d-materials-and-applications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npj2dmaterials","sideBox":"Learn more about [npj 2D Materials and Applications](http://www.nature.com/npj2dmaterials/)","snPcode":"41699","submissionUrl":"https://submission.springernature.com/new-submission/41699/3","title":"npj 2D Materials and Applications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Transition metal dichalcogenides, molybdenum disulfide, chlorine doping, density functional theory, Photoluminescence, sulphur vacancies","lastPublishedDoi":"10.21203/rs.3.rs-7212840/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7212840/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe unique electrical and optical properties of emerging two-dimensional transition metal dichalcogenides (TMDs) present compelling advantages over conventional semiconductors, including Si, Ge, and GaAs. Nevertheless, realising the full potential of TMDs in electronic and optoelectronic devices, such as transistors, light-emitting diodes (LEDs), and photodetectors, is constrained by high contact resistance. This limitation arises from their low intrinsic carrier concentrations and the current insufficiency of doping strategies for atomically thin materials. Notably, chemical treatment with 1,2-dichloroethane (DCE) has been demonstrated as an effective post-growth method to enhance the n-type electrical conductivity of TMDs. Despite the well-established electrical improvements post-DCE treatment, its effects on optical properties, specifically the retention of optical characteristics and excitonic behaviour, are not yet clearly understood. Here, we systematically investigate the layer- and time-dependent optical effects of DCE on molybdenum disulfide (MoS₂) using photoluminescence (PL) spectroscopy and Density Functional Theory (DFT) simulations. Our PL results reveal a rapid reduction in the indirect bandgap transition, with the direct transition remaining unaffected. DFT confirms that chlorine (Cl) atoms bind to sulphur vacancies, creating in-gap states that facilitate non-radiative recombination, explaining the observed indirect PL suppression. This work demonstrates DCE's utility beyond n-type doping, showcasing its ability to engineer the optical band structure in MoS₂ by selectively suppressing indirect transitions. This capability directly paves the way for enhanced efficiency in 2D optoelectronic devices.\u003c/p\u003e","manuscriptTitle":"Chemical Treatment-Induced Indirect-to-Direct Bandgap Transition in MoS₂: Impact on Excitonic Emission","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-04 17:14:03","doi":"10.21203/rs.3.rs-7212840/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-11T12:06:54+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-20T14:42:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-05T08:35:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"124904086341535077612169884495601819811","date":"2025-08-01T19:41:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"220190984308799033522064990241518921441","date":"2025-08-01T16:32:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"245997982502158034903818721825336669980","date":"2025-07-31T08:54:16+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"238167296266223351909252884742588244553","date":"2025-07-30T17:52:22+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-30T17:03:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-30T15:00:12+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-30T10:05:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj 2D Materials and Applications","date":"2025-07-25T09:36:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-2d-materials-and-applications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npj2dmaterials","sideBox":"Learn more about [npj 2D Materials and Applications](http://www.nature.com/npj2dmaterials/)","snPcode":"41699","submissionUrl":"https://submission.springernature.com/new-submission/41699/3","title":"npj 2D Materials and Applications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5841ef6c-5c74-41e5-8389-22d81620f621","owner":[],"postedDate":"August 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":52520538,"name":"Physical sciences/Materials science"},{"id":52520539,"name":"Physical sciences/Nanoscience and technology"},{"id":52520540,"name":"Physical sciences/Optics and photonics"},{"id":52520541,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2025-12-01T16:06:59+00:00","versionOfRecord":{"articleIdentity":"rs-7212840","link":"https://doi.org/10.1038/s41699-025-00639-0","journal":{"identity":"npj-2d-materials-and-applications","isVorOnly":false,"title":"npj 2D Materials and Applications"},"publishedOn":"2025-11-30 15:58:10","publishedOnDateReadable":"November 30th, 2025"},"versionCreatedAt":"2025-08-04 17:14:03","video":"","vorDoi":"10.1038/s41699-025-00639-0","vorDoiUrl":"https://doi.org/10.1038/s41699-025-00639-0","workflowStages":[]},"version":"v1","identity":"rs-7212840","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7212840","identity":"rs-7212840","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}