Intrinsic crystal defects play a major role in tailoring the electrical and optical properties of two-dimensional (2D) materials. Here, we probe the impact of planar crystal defects on the electrical characteristics of germanium sulfide (GeS) field effect transistor (FET) at different operating temperatures varying from 300 to 575 K. Our results show that the measured mobility of the GeS field effect transistor was 0.04 × 10 −3 cm 2 /(V s) at 300 K, and this value reached 58 × 10 −3 cm 2 /(V s) at 575 K. It is important to note that the mobility of GeS FET at elevated temperatures in this study is greater than the mobilities in the recently reported GeS photodetector studies. Furthermore, evidence that the threshold voltage (V th ) decreases and carrier concentration increases with increasing temperature in the GeS channel is provided. We demonstrate an Arrhenius-like relation of the carrier transport as a function of temperature, a behavior that we attribute to nearest-neighbor-hopping (NNH) conduction. The existence of planar defects is revealed using transmission electron microscopy (TEM) while density functional theory (DFT) analysis supports the hypothesis that the formation of localized energy states governs hopping conduction. This study reports hopping conduction at the temperature above 300 K for the first time, whereas previous investigations on 2D materials have reported a hopping mechanism in the low-temperature (<200 K) range. These observations give insight into the fundamental charge conduction mechanisms at high temperature in other 2D materials systems which are expected to aid in the development of applications for harsh environments.

The wetting behavior of homogeneous systems is now well understood at the macroscopic scale. However, this understanding offers little predictive power regarding wettability when mesoscopic chemical and morphological heterogeneities come into play. The fundamental interest in the effect of heterogeneity on wettability is derived from its high technological relevance in several industries, including the petroleum industry where wettability is recognized as a key determinant of the overall efficiency of the water-flooding-based enhanced oil recovery process. Here, we demonstrate the use of the atomic force microscopy force curve measurements to distinguish the roles of chemistry and morphology in the wetting properties of rock formations, thus providing a clear interpretation and deeper insight into the wetting behavior of heterogeneous formations. Density functional theory calculations further prove the versatility of our approach by establishing benchmarks on ideal surfaces that differ in chemistry and morphology in a predefined manner.

Atomically thin, two-dimensional (2D) indium selenide (InSe) has attracted considerable attention because of the dependence of its bandgap on sample thickness, making it suitable for small-scale optoelectronic device applications. In this work, by the use of Raman spectroscopy with three different laser wavelengths, including 488, 532, and 633 nm, representing resonant, near-resonant, and conventional nonresonant conditions, a conclusive understanding of the thickness dependence of lattice vibrations and electronic band structure of InSe and InSe/graphene heterostructures is presented. Combining our experimental measurements with first-principles quantum mechanical modeling of the InSe systems, we identified the crystal structure as ε-phase InSe and demonstrated that its measured intensity ratio of Raman peaks in the resonant Raman spectrum evolves with the number of layers. Moreover, graphene coating enhances Raman scattering of few-layered InSe and also makes its photoluminescence stable under higher intensity laser illumination. The optically induced charge transfer between van der Waals graphene/InSe heterostructures is observed under excitation of the E′ transition in InSe, where the observed mechanism may potentially be a route for future integrated electronic and optoelectronic devices.

Two-dimensional metal dichalcogenide/monochalcogenide thin flakes have attracted much attention owing to their remarkable electronic and electrochemical properties; however, chemical instability limits their applications. Chemical vapor transport (CVT)-synthesized SnTiS3 thin flakes exhibit misfit heterojunction structure and are highly stable in ambient conditions, offering a great opportunity to exploit the properties of two distinct constituent materials: semiconductor SnS and semi-metal TiS2. We demonstrated that in addition to a metal-like electrical conductivity of 921 S/cm, the SnTiS3 thin flakes exhibit a strong bandgap emission at 1.9 eV, owing to the weak van der Waals interaction within the misfit-layer stackings. Our work shows that the misfit heterojunction structure preserves the electronic properties and lattice vibrations of the individual constituent monolayers and thus holds the promise to bridge the bandgap and carrier mobility discrepancy between graphene and recently established 2D transition metal dichalcogenide materials. Moreover, we also present a way to identify the top layer of SnTiS3 misfit compound layers and their related work function, which is essential for deployment of van der Waals misfit layers in future optoelectronic devices.

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