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Title: Inter-layer coupling induced valence band edge shift in mono- to few-layer MoS 2

Abstract

In this study, recent progress in the synthesis of monolayer MoS 2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here,we report a study of highly crystalline islands of MoS 2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS 2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS 2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS 2.

Authors:
 [1];  [1];  [1];  [2];  [3];  [1];  [1];  [3];  [4];  [5];  [6];  [7];  [1];  [1];  [8];  [3];  [1]
  1. Temple Univ., Philadelphia, PA (United States)
  2. Tampere Univ. of Technology, Tampere (Finland)
  3. Northeastern Univ., Boston, MA (United States)
  4. National Tsing Hua Univ., Hsinchu (Taiwan)
  5. National Tsing Hua Univ., Hsinchu (Taiwan); Academic Sinica, Taipei (Taiwan)
  6. National Univ. of Singapore (Singapore)
  7. Helmholtz-Zentrum Berlin fur Materialien und Energie, Berlin (Germany)
  8. Temple Univ., Philadelphia, PA (United States); Northeastern Univ., Boston, MA (United States)
Publication Date:
Research Org.:
Temple Univ., Philadelphia, PA (United States); Northeastern Univ., Boston, MA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1347388
Grant/Contract Number:
SC0012575; FG02-07ER46352
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Scientific Reports
Additional Journal Information:
Journal Volume: 7; Journal ID: ISSN 2045-2322
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; electronic properties and materials; two-dimensional materials

Citation Formats

Trainer, Daniel J., Putilov, Aleksei V., Di Giorgio, Cinzia, Saari, Timo, Wang, Baokai, Wolak, Mattheus, Chandrasena, Ravini U., Lane, Christopher, Chang, Tay -Rong, Jeng, Horng -Tay, Lin, Hsin, Kronast, Florian, Gray, Alexander X., Xi, Xiaoxing X., Nieminen, Jouko, Bansil, Arun, and Iavarone, Maria. Inter-layer coupling induced valence band edge shift in mono- to few-layer MoS2. United States: N. p., 2017. Web. doi:10.1038/srep40559.
Trainer, Daniel J., Putilov, Aleksei V., Di Giorgio, Cinzia, Saari, Timo, Wang, Baokai, Wolak, Mattheus, Chandrasena, Ravini U., Lane, Christopher, Chang, Tay -Rong, Jeng, Horng -Tay, Lin, Hsin, Kronast, Florian, Gray, Alexander X., Xi, Xiaoxing X., Nieminen, Jouko, Bansil, Arun, & Iavarone, Maria. Inter-layer coupling induced valence band edge shift in mono- to few-layer MoS2. United States. doi:10.1038/srep40559.
Trainer, Daniel J., Putilov, Aleksei V., Di Giorgio, Cinzia, Saari, Timo, Wang, Baokai, Wolak, Mattheus, Chandrasena, Ravini U., Lane, Christopher, Chang, Tay -Rong, Jeng, Horng -Tay, Lin, Hsin, Kronast, Florian, Gray, Alexander X., Xi, Xiaoxing X., Nieminen, Jouko, Bansil, Arun, and Iavarone, Maria. Fri . "Inter-layer coupling induced valence band edge shift in mono- to few-layer MoS2". United States. doi:10.1038/srep40559. https://www.osti.gov/servlets/purl/1347388.
@article{osti_1347388,
title = {Inter-layer coupling induced valence band edge shift in mono- to few-layer MoS2},
author = {Trainer, Daniel J. and Putilov, Aleksei V. and Di Giorgio, Cinzia and Saari, Timo and Wang, Baokai and Wolak, Mattheus and Chandrasena, Ravini U. and Lane, Christopher and Chang, Tay -Rong and Jeng, Horng -Tay and Lin, Hsin and Kronast, Florian and Gray, Alexander X. and Xi, Xiaoxing X. and Nieminen, Jouko and Bansil, Arun and Iavarone, Maria},
abstractNote = {In this study, recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here,we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2.},
doi = {10.1038/srep40559},
journal = {Scientific Reports},
number = ,
volume = 7,
place = {United States},
year = {Fri Jan 13 00:00:00 EST 2017},
month = {Fri Jan 13 00:00:00 EST 2017}
}

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  • Room-temperature photoreflectance (PR) and reflectance (R) spectroscopy are utilized to investigate the near-band-edge transitions of molybdenum disulfide (MoS{sub 2}) thin films grown on sapphire substrates by a hot-wall chemical vapor deposition system. The layer thickness and optical properties of the MoS{sub 2} thin films are confirmed by Raman spectroscopy, atomic force microscope, and photoluminescence (PL) analysis. The B exciton shows relatively weak PL intensity in comparing with the A exciton even for monolayer MoS{sub 2} films. In the R spectrum of few‐layer MoS{sub 2}, it is not possible to clearly observe exciton related features. The PR spectra have two sharp,more » derivative-like features on a featureless background. Throughout the PR lineshape fitting, the transition energies are designated as the A and B excitons at the K-point of the Brillouin zone, but at room temperature there seems to be no distinguishable feature corresponding to an H‐point transition for the mono- and few-layer MoS{sub 2} films unlike in bulk. These transition energies are slightly larger than those obtained by PL, which is attributed to the Stokes shifts related to doping level. The obtained values of valence-band spin-orbit splitting are in good agreement with those from other experimental methods. By comparing the PR lineshapes, the dominant modulation mechanism is attributed to variations of the exciton transition energies due to change in the built-in electric field. On the strength of this study, PR spectroscopy is demonstrated as a powerful technique for characterizing the near-band-edge transitions of MoS{sub 2} from monolayer to bulk.« less
  • Few-layer MoS{sub 2} prepared by the chemical vapor deposition method was treated with nitrogen plasma under different radio-frequency (rf) power conditions in order to experimentally study the change in the electrical property. Control of the rf power could change the work function of MoS{sub 2} from 5.40 eV to 5.06 eV. It is shown that the increased rf power leads to the increased (reduced) number of nitrogen (oxygen) atoms, increasing the electron concentration and shifting the Fermi level toward conduction band. The sensitivity of the work function to the rf power provides an opportunity to tune the work function of MoS{sub 2}.
  • The angle dependence of the valence-band photoemission from the trigonal prismatic layered MoS{sub 2} shows both the forward-scattering features normally observed in core-level photoelectron diffraction and, in addition, the {ital initial-state orbital character associated with partially occupied, nonbonding} {ital Mo}{sup {ital IV}}(4{ital d}{sub {ital z}{sup 2}}+4{ital d}{sub {ital x}{sup 2}{minus}{ital y}{sup 2}} +4{ital d}{sub {ital xy}}) {ital orbitals near the top of the valence band}. The difference in forward scattering between the Mo and S emitters is also used to assess relative contributions from the Mo and S atomic orbitals at specific binding energies within the valence band. Deposition ofmore » cesium (0.23 ML Cs with 1 ML equal to the Cs saturation coverage) onto the basal plane of MoS{sub 2} introduces a density of states at 1.25 eV above the top of the valence-band maximum. The intensity anisotropy for this Cs-induced valence level is interpreted via the angle dependence of the electric dipole matrix element as due to the initial-state orbital character at the bottom of the conduction band of the Cs/MoS{sub 2} heterostructure. {copyright} {ital 1996 The American Physical Society.}« less