| AVS 61st International Symposium & Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS+2D-WeA |
| Session: | Plasma Processing for 2D Materials, Coating, and Surface Modification |
| Presenter: | Rachel Morrish, Colorado School of Mines |
| Authors: | R.M. Morrish, Colorado School of Mines C.D. Sentman, Colorado School of Mines T. Haak, Colorado School of Mines C.A. Wolden, Colorado School of Mines |
| Correspondent: | Click to Email |
Two-dimensional metal dichalcogenides (WS2, MoS2) have attracted great interest due to their extraordinary optical properties, catalytic performance, and electronic structure. Synthesis of WS2 has been accomplished by a variety of methods, but the high temperature (> 800 ºC) and/or harsh S atmosphere required by many of these procedures precludes deposition onto conductive layers and low-temperature glass substrates needed for many applications. Previously we demonstrated the advantages of plasma processing for low temperature synthesis of a related metal dichalcogenide, pyrite (FeS2). It was shown that stoichiometric FeS2 films could be fabricated either by plasma-assisted sulfurization of hematite [1] or by direct deposition using pulsed plasma-enhanced chemical vapor deposition (PECVD) with mixtures of H2S and Fe(CO)5. [2] In this talk we describe how these two approaches may be readily extended to the synthesis of WS2 using WO3 and W(CO)6 , respectively.
Thin tungsten disulfide (WS2) films were prepared on FTO coated glass substrates by H2S plasma sulfurization of sputtered WO3. The reactive environment provided by the plasma enabled the complete transformation of oxide films to stoichiometric WS2 within one hour at 500 °C. An apparent activation energy of 63.6 ± 1.9 kJ/mol was calculated for the plasma conversion process, which is less than half the barrier reported for the reaction of WO3 with H2S. The conversion followed Deal-Grove behavior, with the growing WS2 overlayer hindering diffusion to/from the reactive interface. Calibrated light absorption and relative intensity of the second order Raman 2LA(M) peak were identified as two additional methods to progressively monitor the thickness of the WS2. The semiconducting WS2 layers exhibited n-type behavior with an indirect band gap at 1.4 eV and an absorption coefficient of ~5 × 104 cm-1. Self-limiting growth of WS2 thin films was accomplished by pulsed PECVD with continuous delivery of tungsten hexacarbonyl diluted in a mixture of H2S and argon. The growth rate per cycle was controlled between 0.1 – 1 Å/pulse by adjusting the precursor flowrate. It was found that the morphology and orientation of the films was a complex function of film thickness, substrate temperature, and plasma parameters. Preliminary electrochemical measurements showed that these WS2 films reduced the overpotential required for the hydrogen evolution reaction by several hundred mV relative to FTO while displaying good stability.
[1] R. Morrish, R. Silverstein and C. A. Wolden, JACS134 17854 (2012).
[2] C. D. Sentman, M. O'Brien and C. A. Wolden, JVSTA32 021201 (2014).