Journal
ACS NANO
Volume 11, Issue 8, Pages 8223-8230Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.7b03242
Keywords
transition metal dichalcogenide; molybdenum disulfide; tungsten disulfide; molybdenum selenide; band alignment; ionization energy; photoemission electron microscopy
Categories
Funding
- U.S. Department of Energy (DOE) Office of Science [DE-AC04-94AL85000]
- Army Research Office MURI [W911NF-11-1-0362]
- U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy SunShot Initiative award for Bridging Research Interactions through Collaborative Development Grants in Energy (BRIDGE) [DE-FOA-0000654 CPS25859]
- LANL LDRD program
- CINT user program and Sandia LDRD
- Sandia National Laboratories
- U.S. Department of Energy's National Nuclear Security Administration [DE-NA0003525]
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The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with-deep ultraviolet illumination. The ionization:energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in-agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered-(type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron hole separation in photovoltaics.
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