Highly conductive group VI transition metal dichalcogenide films by solution-processed deposition
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David L. Young and Yong Zhang National Renewable Energy Laboratory, Golden, Colorado 80401 (Received 18 October 2006; accepted 13 February 2007)
A new soluble synthetic route was developed to fabricate thin films of layered structure transition metal dichalcogendies, MoS2 and WS2. High-quality thin films of the dichalcogenides were prepared using new soluble precursors, (CH3NH3)2MS4 (M ⳱ Mo, W). The precursors were dissolved in organic solvents and spun onto substrates via both single- and multistep spin coating procedures. The thin films were formed by the thermal decomposition of the coatings under inert atmosphere. Structural, electrical, optical absorption, thermal, and transport properties of the thin films were characterized. Surface morphology of the films was analyzed by atomic force microscopy and scanning electron microscopy. Highly conductive and textured n-type MoS2 films were obtained. The measured room temperature conductivity ∼50 ⍀−1 cm−1 is substantially higher than the previously reported values. The n-type WS2 films were prepared for the first time using solution-processed deposition. WS2 displays a conductivity of ∼6.7 ⍀−1 cm−1 at room temperature.
I. INTRODUCTION
Solution-based deposition techniques, such as printing1 and spin casting,2,3 have been well developed to fabricate thin-film transistors (TFTs). Spin-coating techniques have apparent advantages for fabrication of lowcost, flexible, and large-area devices and have been widely used to obtain uniform and continuous films. Most metal chalcogenide semiconductors exhibit poor solubility in common solvents, which limits the utility of solution-processed deposition. Recently, Mitzi et al. have demonstrated that by use of soluble precursors such as hydrazinium salts, binary metal chalcogenide TFTs (e.g., SnS2 and In2Se3) can be processed via spin-coating techniques.2,4 Transition metal dichalcogenides (TMDCs) are a group of inorganic materials with both fundamental and technological importance. They have been used in numerous applications, such as lubricants, catalysis, and solar cells.5–7 It has been demonstrated that micro- and/ or nano-structures of TMDCs can be obtained from thermal decomposition of ammonium thiosalts at various temperatures.8–10 Synthetic routes used to produce TMDC thin films include sulphurization,11 electrodepo-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0179 1390 J. Mater. Res., Vol. 22, No. 5, May 2007 http://journals.cambridge.org Downloaded: 07 Jan 2015
sition,12 sputtering/annealing,13,14 chemical bath depositions,15 and thermal decomposition.16 Most thin film fabrication methods mentioned above require high vacuum or high temperatures. Very little has been reported on solution-based deposition of Group-VI metal dichalcogenides. For example, spin-coated MoSx thin films have been reported only for tribological applications,17 and to the best of our knowledge, no study has been reported on solution-processed tungsten dichalcogenide thin films. In this work,
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