Two-dimensional (2D) layered materials exhibit many unique properties, such as near-atomic thickness, electrical tunability, optical tunability, and mechanical deformability, which are characteristically distinct from conventional materials. They are particularly promising for next-generation biologically inspired optoelectronic artificial synapses, offering unprecedented opportunities beyond the current complementary metal–oxide–semiconductor-based computing device technologies. This Research update article introduces the recent exploration of various 2D materials for optoelectronic artificial synapses, such as graphene, transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes, and metal oxides. Material property suitability and advantages of these 2D materials in implementing optoelectronic artificial synapses are discussed in detail. In addition, recent progress demonstrating 2D materials-enabled optoelectronic artificial synaptic devices is reviewed along with their device operation principles. Finally, pending challenges and forward-looking outlooks on this emerging research area are suggested.

Crystallographically anisotropic two-dimensional (2D) molybdenum disulfide (MoS2) with vertically aligned (VA) layers is attractive for electrochemical sensing owing to its surface-enriched dangling bonds coupled with extremely large mechanical deformability. In this study, we explored VA-2D MoS2 layers integrated on cellulose nanofibers (CNFs) for detecting various volatile organic compound (VOC) gases. Sensor devices employing VA-2D MoS2/CNFs exhibited excellent sensitivities for the tested gases of ethanol, methanol, ammonia, and acetone; e.g., a high response rate up to 83.39 % for 100 pm ethanol, significantly outperforming previously reported sensors employing horizontally aligned 2D MoS2 layers. Furthermore, VA-2D MoS2/CNFs were identified to be completely dissolvable in buffer solutions such as phosphate-buffered saline (PBS) solution and baking soda buffer (BSB) solution without releasing toxic chemicals. This unusual combination of high sensitivity and excellent biodegradability inherent to VA-2D MoS2/CNFs offers unprecedented opportunities for exploring mechanically reconfigurable sensor technologies with bio-compatible transient characteristics.

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

Transition metal dichalcogenides (TMDs) is an emerging 2D semiconducting material group which has excellent physical properties in the ultimately scaled thickness dimension. Specifically, van der Waals (vdW) heterostructures hold the great promise in further advancing both the fundamental scientific knowledge and practical technological applications of 2D materials. Although 2D materials have been extensively studied for various sensing applications, temperature sensing still remains relatively unexplored. In this work, we experimentally study the temperature-dependent Raman spectroscopy and electrical conductivity of molybdenum disulfide (MoS2) and its heterostructures with platinum dichalcogenides (PtSe2 and PtTe2) to explore their potential to become the next-generation temperature sensor. It is found that the MoS2-PtX2 heterostructure greatly enhances the temperature sensitivity.