Temperature-Dependent Phonon Shifts in Mono-layer, Few-layer, and Bulk WS2 Films

Two-dimensional transition metal disulfides (TMDs) have recently attracted significant research attention due to their rich physical and chemical properties. Graphene has also been studied intensively due to its high electron mobility of similar to 200000 cm(2).V-1.s(-1). Since there is no band gap, it is difficult for a graphene-based device to achieve high current on/off ratio. For TMDs, such as MoS2, MoSe2, WSe2, and WS2, the band gaps of these materials can be adjusted according to the number of layers. Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects, it is also attractive for use as a channel material in Si complementary metal oxide semiconductor (CMOS) devices larger than 22 nm. Among them, MoS2 in single-layer and multi-layer films have been intensively researched for many years. MoS2-based field effect transistors (FETs) with excellent electrical properties have been reported. WS2 has lower in-plane electronic mass than MoS2, MoSe2, and MoTe2, and therefore has potential for higher carrier mobility or higher output current for WS2-based FETs. Experimental research on WS2 is limited compared to MoS2, and more work is needed to further exploit the full potential of WS2-based FETs. Therefore, the electron-phonon interaction and vibration properties of WS2 used in nano-electronic applications and FETs must be investigated. To this end, mono-layer (1L), few-layer (FL), and bulk WS2 films were prepared using mechanical exfoliation from a WS2 crystal. 3M scotch-tape was used for transferring the WS2 films. Detailed temperature-dependent Raman study on 1L, FL, and bulk WS2 films has been conducted using a 514-nm excitation laser. Raman spectroscopy, as an effective and non-destructive approach for phonon vibration study, has been used to evaluate TMDs. The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change. With the film thickness increasing to bulk, the A(1g)(Gamma) and E-2g(1)(Gamma) modes show blue-shift and red-shift, respectively, with respect to 1L WS2. Moreover, when the dominant Raman vibration modes swaps between E-2g(1)(Gamma) and A(1g)(Gamma), the "cross-over" temperature was identified for 1L, FL, and bulk WS2 films. WS2 shows smaller frequency change Delta between the E-2g(1)(Gamma) and A(1g)(Gamma) modes than MoS2, with varying film thickness. The temperature coefficient of the Raman peak position was one magnitude lower for WS2 than MoS2, implying that WS2 has better thermal stability than MoS2. The results of this systematic study provide a physical guidance for WS2-based device design.