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Influence of Gas Adsorption and Gold Nanoparticles on the Electrical Properties of CVD-Grown MoS2 Thin Films Yunae Cho,† Ahrum Sohn,† Sujung Kim,† Myung Gwan Hahm,‡,§ Dong-Ho Kim,‡ Byungjin Cho,*,‡ and Dong-Wook Kim*,† †
Department of Physics, Ewha Womans University, Seoul 03760, Korea Department of Advanced Functional Thin Films, Korea Institute of Materials Science (KIMS), Changwon 51508, Korea § Department of Materials Science & Engineering, Inha University, Incheon 22212, Korea ‡
S Supporting Information *
ABSTRACT: Molybdenum disulfide (MoS2) has increasingly attracted attention from researchers and is now one of the most intensively explored atomic-layered two-dimensional semiconductors. Control of the carrier concentration and doping type of MoS 2 is crucial for its application in electronic and optoelectronic devices. Because the MoS2 layers are atomically thin, their transport characteristics may be very sensitive to ambient gas adsorption and the resulting charge transfer. We investigated the influence of the ambient gas (N2, H2/N2, and O2) choice on the resistance (R) and surface work function (WF) of trilayer MoS2 thin films grown via chemical vapor deposition. We also studied the electrical properties of gold (Au)-nanoparticle (NP)-coated MoS2 thin films; their R value was found to be 2 orders of magnitude smaller than that for bare samples. While the WF largely varied for each gas, R was almost invariant for both the bare and Au-NP-coated samples regardless of which gas was used. Temperature-dependent transport suggests that variable range hopping is the dominant mechanism for electrical conduction for bare and Au-NP-coated MoS2 thin films. The charges transferred from the gas adsorbates might be insufficient to induce measurable R change and/or be trapped in the defect states. The smaller WF and larger localization length of the Au-NP-coated sample, compared with the bare sample, suggest that more carriers and less defects enhanced conduction in MoS2. KEYWORDS: MoS2, gas adsorption, charge transfer, chemical vapor deposition, work function
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other 2D semiconductors.13−21 Such an ambient dependence allows us to use MoS2 for high-sensitivity chemical and gas sensors.13,14 The sensing capabilities of MoS2, however, suggest that reliable and reproducible control of the carrier concentration and dopant type requires careful consideration of the ambient conditions during device fabrication and operation. There have been many reports that oxygen, hydrogen, and water molecules in air may vary the resistance (R) of MoS2 thin films.15−21 Theoretical work on the issue proposes that charge transfer between the gas adsorbates and MoS2 occurs.15 Qiu et al. found that the on-state current of field-effect transistors (FETs) with exfoliated bilayer MoS2 as channel layers increased by 1000% in high vacuum compared with those in air.16 Late et al., however, reported that the onstate current of their exfoliated monolayer MoS2 FETs was more or less the same in air compared with those in vacuum.20 Late et al. observed a reduction of the hysteresis width of the
INTRODUCTION Atomic-layered two-dimensional (2D) semiconductors are expected to usher in a new era of electronics and optoelectronics with the aid of van der Waals heterostructures, which have a far more notable tolerance in the choice of material combinations compared with conventional covalentbond-type semiconductors.1 Among the numerous 2D semiconductors, molybdenum disulfide (MoS2), in particular, has attracted tremendous research interest owing to its unique physical properties, including high carrier mobility, sizable band-gap energy, and mechanical flexibility.1−5 Optimal band profiles in heterostructures are crucial to achieving the desirable functionality for specific electronic and optoelectronic applications. Therefore, we needed to find a way to control the carrier concentration and dopant type in MoS2. For instance, the deposition of chemical species and metal nanoparticles (NPs) has been widely used to modify the majority carrier type and carrier concentration in MoS2.6−12 Because MoS2 layers are atomically thin and have extremely large surface-to-volume ratios, their electrical properties are very sensitive to the gas environment, which is also the case in © 2016 American Chemical Society
Received: July 4, 2016 Accepted: August 4, 2016 Published: August 4, 2016 21612
DOI: 10.1021/acsami.6b08104 ACS Appl. Mater. Interfaces 2016, 8, 21612−21617
Research Article
ACS Applied Materials & Interfaces
at least 3 h after loading a sample and then filled with either H2 (2%)/ N2 (98%) or O2. In the vacuum probe station, the system was evacuated to a base pressure of
