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Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy

ACS Nano

Keyshar, Keyshar; Kunttal, Kunttal; Berg, Morgann B.; Zhang, Zhang; Xiang, Xiang; Vajtai, Vajtai; Robert, Robert; Gupta, Gupta; Gautam, Gautam; Chan, Calvin C.; Beechem, Thomas E.; Ajayan, Ajayan; Pulickel, Pulickel; Mohite, Mohite; D., Aditya D.; Ohta, Taisuke O.

Here, 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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TYPE Journal Article YEAR 2017
OSTIDOI

Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

Journal of Crystal Growth

Ahmed, Adam S.; Wen, Hua; Ohta, Taisuke O.; Pinchuk, Igor V.; Zhu, Tiancong; Beechem, Thomas E.; Kawakami, Roland K.

We report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by X-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of <1.5 Å. Transport measurements of exfoliated graphene after SrO deposition show a strong dependence between the Dirac point and Sr oxidation. Subsequently, the SrO is leveraged as a buffer layer for more complex oxide integration via the demonstration of (001) oriented SrTiO3 grown atop a SrO/HOPG stack.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI

Oxidation of ultrathin GaSe

Applied Physics Letters

Beechem, Thomas E.; Kowalski, Brian M.; Brumbach, Michael T.; McDonald, Anthony E.; Spataru, Dan C.; Howell, Stephen W.; Ohta, Taisuke O.; Pask, Jesse A.; Kalugin, Nikolai G.

Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga2Se3 and amorphous Se. Photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

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TYPE Journal Article YEAR 2015
ScopusOSTIDOI
Results 51–75 of 141
Results 51–75 of 141