Centimeter-Scale MoS₂ Thin Films As a Temperature Sensor

Abstract

Backgrounds/Introduction Transition-metal dichalcogenides (TMDs) is an atomically-thin semiconducting material family, exhibiting unique physical and multi-functional properties. Despite the great promises in flexible electronics, the practical adoption of TMDs in a wider variety of far-reaching application domains, including high-temperature applications such as automobiles and aircrafts, still requires TMDs to be prepared at a larger scale and tailor-researched to the needs of a specific application. In this work, molybdenum disulfide (MoS2), one of the most widely studied TMD materials, is synthesized at a centimeter-scale by sulfurizing the transition metal seed layer (Mo) in a CVD (chemical vapor deposition) reactor, and comprehensively characterized to explore MoS2’s potential as the next-generation temperature sensor. Key Results: Fig. 1 depicts both the schematic drawing (1a) and the transmission electron microscope (TEM) image (1b) of few-layer MoS2 thin films grown on a SiO2/Si substrate by the CVD process described in our earlier work [1]. As seen in the figure, 2D MoS2 layers were successfully grown in a planar direction with their basal planes parallel to the growth substrates. The horizontal growth of few-layer MoS2 thin films (thicknesses of less than 5 nm) was achieved by precisely controlling the thickness of the metal seed layer (Mo). We first investigated the Raman characteristics of our centimeter-scale (2cm x 2cm), CVD-grown MoS2 thin films at varying temperatures (from 26°C to 206°C) in Fig. 2. It is clearly seen that both E2g (out-of-plane vibration modes at ~ 383 cm-1) and A1g (in-plane vibration modes at ~ 408 cm-1) characteristic peaks appeared in the temperature-dependent Raman measurement. Since these peak positions are known to be strongly dependent on materials or external parameters (e.g., thickness, mechanical strain, charge transfer), observing any notable change in the peak position at varying conditions can lead to discovery of MoS2’s novel functionality. The overall trend of red shift (i.e., decrease of Raman shift) with an increase in temperature observed in Fig. 3 is in good agreement with what has been already reported for an exfoliated single-layer MoS2 flake [2]. This has been attributed to either charge transfer from the substrate (doping effect) or compressive strain resulted from thermal expansion coefficient mismatch between MoS2 and the substrate. In order to further elucidate the physical mechanism while examining the potential of MoS2 to become a temperature sensor of the RTD (resistance temperature detector) type, we carefully measured the conductivity vs. temperature characteristics by using the temperature controller-attached probe station (Figs. 4 and 5). Firstly, a clear trend of increase in electrical conductivity with temperature indicates the semiconducting nature of our MoS2 thin films (Fig. 4). More importantly, Fig. 5 suggests that electronic transport in the temperature range of room temperature to about 300°C follows an Arrhenius behavior, implying a variable-range hopping mechanism [3]. Table 1 summarizes the temperature coefficient (cm-1/°C, measured from Raman) and the activation energy (eV, measured from the Arrhenius plot). Significance: Previously, researchers have studied the temperature dependence of electrical conductivity for exfoliated MoS2 flakes (either undoped [4] or doped [5]). These methods may not be best suited for temperature sensing applications because of the small areal size and lack of tight control on the thickness. This work significantly advances the field by demonstrating the temperature-induced modulation of spectroscopic and electrical transport characteristics of a relatively large-area MoS2 thin film. References: Jung, Y., Shen, J., Liu, Y., Woods, J. M., Sun, Y., & Cha, J. J. (2014). Metal seed layer thickness-induced transition from vertical to horizontal growth of MoS2 and WS2. Nano letters, 14(12), 6842-6849. Taube, A., Judek, J., Jastrzębski, C., Duzynska, A., Świtkowski, K., & Zdrojek, M. (2014). Temperature-dependent nonlinear phonon shifts in a supported MoS2 monolayer. ACS applied materials & interfaces, 6(12), 8959-8963. Mukherjee, S., Biswas, S., Ghorai, A., Midya, A., Das, S., & Ray, S. K. (2018). Tunable optical and electrical transport properties of size-and temperature-controlled polymorph MoS2 nanocrystals. The Journal of Physical Chemistry C, 122(23), 12502-12511. Garadkar, K. M., Patil, A. A., Hankare, P. P., Chate, P. A., Sathe, D. J., & Delekar, S. D. (2009). MoS2: Preparation and their characterization. Journal of Alloys and Compounds, 487(1-2), 786-789. El Beqqali, O., Zorkani, I., Rogemond, F., Chermette, H., Chaabane, R. B., Gamoudi, M., & Guillaud, G. (1997). Electrical properties of molybdenum disulfide MoS2. Experimental study and density functional calculation results. Synthetic Metals, 90(3), 165-172. Figure 1