Atomically thin sheets such as graphene and monolayer transition metal dichalcogenides hosts rich structural variety and desirable properties that derive from their 2D form, such as strong light-matter interaction and unconventional thermomechanics. My research employs accurate first-principles theory to model their growth and characterization, in close collaboration with several experimental teams.
Point defects in 2D materials
The thermodynamics of defects in a solid determines their population and what are the common species. But comparing with experiments can be difficult and time-consuming – an individual defect could be hard to unambiguously pinpoint from electron microscopy. We therefore calculate defect properties, say optical ones, to compare with experimentally measured fingerprints of defects. This procedures establishes efficient and non-invasive optical probes to interrogate sample quality.
Defects can also be exploited to improve material properties rather than degrade them, e.g. detrimental defects can be neutralized by introducing another type of innocuous defect to pair with it; nucleation of 2D materials at substrate defects could paradoxially improve the orientational uniformity of the grown 2D layer.
- F. Zhang, Y. Wang, C. Erb, K. Wang, P. Moradifar, V. H. Crespi, N. Alem, “Full orientation control of epitaxial MoS2 on hBN assisted by substrate defects“, Phys. Rev. B 99, 155430 (2019)
- X. Zhang*, F. Zhang*, Y. Wang, D. S. Schulman, T. Zhang, A. Bansal, N. Alem, S. Das, V. H. Crespi, M. Terrones, J. M. Redwing, “Defect-controlled nucleation and orientation of WSe2 on hBN – a route to single crystal epitaxial monolayers”, ACS Nano 13, 3341 (2019)
- Y. Wang, V. H. Crespi, “Nanovelcro: theory of guided folding in atomically thin sheets with regions of complementary doping”, Nano Lett. 17, 6708 (2017)
- A. Azizi, Y. Wang, G. Stone, A. L. Elias, Z. Lin, M. Terrones, V. H. Crespi, and N. Alem, “Defect Coupling and Sub-Angstrom Structural Distortions in W1–xMoxS2 Monolayers“, Nano Lett. 17, 2802 (2017).
- V. Carozo, Y. Wang, K. Fujisawa, B. R. Carvalho, A. McCreary, S. Feng, Z. Lin, C. Zhou, N. Perea-López, A. L. Elías, B. Kabius, V. H. Crespi, and M. Terrones, “Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide“,Sci. Adv. 3, (2017).
Excited-state properties
The calculation of resonance Raman intensities in 2D materials requires knowledge of the dielectric response including excitonic effects using many-body perturbation theory. We develop a computational framework for calculating Raman intensities from first-principles following a diagrammatic approach, which scales favorably with respect to the number of Raman modes compared to existing implementations using finite displacements.
- Y. Wang, B. Carvalho, V. H. Crespi, “Strong exciton regulation of Raman scattering in monolayer dichalcogenides”, Phys. Rev. B (Rapid Commn.) 98, 161405 (2018)
- A. McCreary, J. R. Simpson, Y. Wang, D. Rhodes,K. Fujisawa, L. Balicas, M. Dubey, V. H. Crespi, M. Terrones, A. R. H. Walker, “Intricate Resonant Raman Response in Anisotropic ReS2“, Nano Lett. 17, 5897 (2017)
- B. R. Carvalho*, Y. Wang*, S. Mignuzzi, D. Roy, M. Terrones, C. Fantini, V. H. Crespi, L. M. Malard, and M. A. Pimenta, “Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy“, Nat. Commun. 8, 14670 (2017).
2D Growth control
Increasingly successful efforts are being directed to the synthesis of millimeter-scale 2D monolayers with high sample qualities. These advances motivate the nanoscale control over 2D growth by properly designing the topographies and tailoring the surface chemistry of the substrate. We employ density functional theory and empirical forcefield calculations to predict grain boundary control and epitaxial control as applicable to a broad class of sheet-substrate combinations.
- Y. Wang, V. H. Crespi, “Theory of Finite-length Grain Boundaries of Controlled Misfit Angle in Two-dimensional Materials”, Nano Lett. 17, 5297 (2017)
- A. Azizi, Y. Wang, Z. Lin, K. Wang, A. L. Elias, M. Terrones, V. H. Crespi, and N. Alem, “Spontaneous Formation of Atomically Thin Stripes in Transition Metal Dichalcogenide Monolayers“, Nano Lett. 16, 6982 (2016).
- Z. Zhang, Y. Wang, X. Leng, V. H. Crespi, F. Kang, R. Lv, “Controllable Edge Exposure of MoS2 for Efficient Hydrogen Evolution with High Current Density”, ACS Appl. Energy Mater. 1, 1268 (2018)