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Rafik Addou1 Christopher Cormier1 Christopher Smyth1 Robert Wallace1

1, The University of Texas at Dallas, Richardson, Texas, United States

Two-dimensional transition metal dichalcogenides (TMDs) are currently considered as promising materials to complement or even supplant current Si-based device technology [1].The electronic and optoelectronic properties of MoSe2, have been extensively studied in recent years. MoSe2 has been implemented in FETs, exhibiting a promising performance (i.e. μFE 150-200 eV) according to previous reports [2,3]. We have studied the surface of bulky MoSe2 crystals, grown by chemical vapor transport by means of various nanometrology methods such as X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning spectroscopy microscopy/spectroscopy (STM/S) [4]. The “as-exfoliated” surface shows the presence of surface imperfections caused by defects and impurities with an inherent n-type conductivity. The air stability of MoSe2 semiconductor has been investigated from first-principles calculations based on DFT and XPS measurements of the elemental oxygen concentration and work function as a function of exposure time to air and O2 at room temperature. We find that the surface chemistry of the sulfides and selenides is relatively stable in air [5]. Moreover, surface defects were induced by either vacuum annealing or ions sputtering and then studied in-situ by XPS, LEED, and STM/S. Both microscopy and spectroscopy data reveal a change in the intrinsic electronic properties caused by the creation of metallic defects. The interfacial chemistry between contact metals and MoSe2 and its dependence upon the deposition chamber ambient was monitored in-situ using XPS. Significant variation in interfacial reactivity between contact metal and MoSe2 were observed. Particularly, Pd and Au form a van der Waals interface (no alloying) with MoSe2 under either ultra-high vacuum (UHV) or high vacuum (HV) conditions, whereas Sc and Cr aggressively react with the substrate under both UHV and HV conditions as shown previously with MoS2 and WSe2 [6,7].

This work is supported part by NSF Award No. 1407765 under the US/Ireland UNITE collaboration, the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor
Research Corporation program sponsored by MARCO and DARPA, and by the Southwest Academy on Nanoelectronics (SWAN) sponsored by the Nanoelectronic Research Initiative and NIST.

References.
[1] D. Jena, Proc. IEEE 101 1585–160 (2013).
[2] N. R. Pradhan et al. ACS Nano, 8 (8), 7923–7929 (2014).
[3] B. Chamlagain et al. ACS Nano 8 5079-88 (2014).
[4] R. Addou et al. ACS Appl. Mater. Interfaces 7, 11921–11929 (2015).
[5] R. C. Longo et al. 2D Mater. 4, 025050 (2017).
[6] C. M. Smyth et al., J. Phys Chem. C. 120, 14719-14729 (2016).
[7] C. M. Smyth et al., 2D Mater. 4, 025084 (2017).

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