Abstract
Realizing the potential for 2D SnSe optoelectronics requires understanding the thickness dependence of structure, defects, and optical properties. We investigate the thickness-dependent crystal structure, band gap, and carrier lifetime of SnSe films deposited by molecular beam epitaxy (MBE) on (100) MgO. MBE enables stoichiometric (2h00)-oriented SnSe films with tunable thicknesses from 80 nm down to 4 nm. As thickness decreases, out-of-plane covalent bonds contract, while in-plane bonding and the van der Waals gap expand with a concurrent increase in stacking fault density, consistent with theoretical predictions of reduced stacking fault energies. Below 8 nm, the band gap transitions from indirect to direct, increasing from 1.4 eV to 1.8 eV, primarily driven by a combination of structural changes and confinement effects. Our results demonstrate how the thickness and structural distortion of 2D materials can be used to modulate the optical properties relevant to optoelectronics.
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Acknowledgements
The authors acknowledge support from the Air Force Office of Scientific Research under award no. FA9550-24-1-0263. L.M.G. acknowledges the Air Force Office of Scientific Research (AFOSR) Young Investigator Award under Grant No. FA9550-22-1-0237. This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology (IEN) under the SEED Grant support. The IEN is a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-1542174). J.R.C. acknowledges the support of the National Science Foundation (NSF) Graduate Research Fellowship Program under Grant No. DGE-2039655. The STEM characterization by J.J.C. and S.D.F. was supported by the Department of Energy (DOE) Basic Energy Sciences (BES) DE-SC0023905. The work presented has been facilitated by the Materials Innovation Platform of The Pennsylvania State University, the Two-Dimensional Crystal Consortium (2DCC-MIP), supported by NSF through cooperative agreements no. DMR-1539916 and DMR-2039351. C.P. acknowledges support from the Arkansas High Performance Computing Center, which is funded through multiple National Science Foundation grants and the Arkansas Economic Development Commission.
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M.B.F., L.M.G., and A.M.O. wrote the main manuscript text. M.B.F. prepared the figures and led the analysis and characterization. M.B.F. and A.G. led the characterization, analysis, and discussion of the optical properties, and M.B.F., J.R.C., and J.W. led the deposition of the films used in this study. S.D.F. prepared cross-sectional STEM samples using focused ion beam milling and characterized the atomic structures using STEM under the supervision of J.J.C. W.J.S. measured the Raman spectra and time-resolved photoluminescence under the supervision of T.B. L.M.G. developed the idea and led the study. C.P. performed all computational results in this study. All authors reviewed the manuscript.
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Frye, M.B., Chin, J.R., Smith, W.J. et al. Thickness-modulated crystal structure and band gap of 2D SnSe deposited by molecular beam epitaxy. npj 2D Mater Appl (2026). https://doi.org/10.1038/s41699-025-00655-0
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DOI: https://doi.org/10.1038/s41699-025-00655-0
