Gate tunable p-type multilayer tin mono-sulfide (SnS) field-effect transistor (FET) devices with SnS thickness between 50 and 100 nm were fabricated and studied to understand their performance. The devices showed anisotropic inplane conductance and room temperature field effect mobilities ∼5-10 cm V s. However, the devices showed an ON-OFF ratio ∼10 at room temperature due to appreciable OFF state conductance. The weak gate tuning behavior and finite OFF state conductance in the depletion regime of SnS devices are explained by the finite carrier screening length effect which causes the existence of a conductive surface layer from defect induced holes in SnS. Through etching and n-type surface doping by CsCO to reduce/compensate the not-gatable holes near the SnS flake's top surface, the devices exhibited an order of magnitude improvement in the ON-OFF ratio, and a hole Hall mobility of ∼100 cm V s at room temperature is observed. This work suggests that in order to obtain effective switching and low OFF state power consumption, two-dimensional (2D) semiconductor based depletion mode FETs should limit their thickness to within the Debye screening length of the carriers in the semiconductor.

Graphitic carbon nitride (g-CN) has been widely studied as a metal-free photocatalyst, leading to some excellent results; however, the rapid recombination of photogenerated charge carriers substantially limits its performance. Here, we establish two types of g-CN-based heterojunction (type II and nonmediator assisted Z-scheme) photoanodes on a transparent conducting substrate via coupling with rod-like and nanoparticulate WO, respectively. In these composites, g-CN film grown by electrophoretic deposition of exfoliated g-CN serves as the host or guest material. The optimized type II WO/g-CN composite exhibits an enhanced photocurrent of 0.82 mA cm at 1.23 V vs. RHE and an incident photo-to-current conversion efficiency (IPCE) of 33% as compared with pure WO nanorods (0.22 mA cm for photocurrent and 15% for IPCE). Relative to pure g-CN film (with a photocurrent of several microampere and an IPCE of 2%), a largely improved photocurrent of 0.22 mA cm and an IPCE of 20% were acquired for the Z-scheme g-CN/WO composite. The enhancement can be attributed to accelerated charge separation in the heterointerface because of the suitably aligned band gap between WO and g-CN, as confirmed by optical spectroscopy and ultraviolet photoelectron spectroscopy (UPS) analysis. The photocatalytic process and mechanism of the g-CN-based heterojunctions are proposed herein, which potentially explain the origin of the enhanced photoelectrochemical performance. This achievement and the fundamental information supplied here indicate the importance of rationally designing heterojunction photoelectrodes to improve the performance of semiconductors. This is particularly important for materials such as pure g-CN and WO, as their photoactivities are strongly restricted by high recombination rates.

Zinc oxide is regarded as a promising candidate for application in photoelectrochemical water oxidation due to its higher electron mobility. However, its instability under alkaline conditions limits its application in a practical setting. Herein, we demonstrate an easily achieved wet-chemical route to chemically stabilize ZnO nanowires (NWs) by protecting them with a thin layer Fe O shell. This shell, in which the thickness can be tuned by varying reaction times, forms an intact interface with ZnO NWs, thus protecting ZnO from corrosion in a basic solution. The reverse energetic heterojunction nanowires are subsequently activated by introducing an amorphous iron phosphate, which substantially suppressed surface recombination as a passivation layer and improved photoelectrochemical performance as a potential catalyst. Compared with pure ZnO NWs (0.4 mA cm ), a maximal photocurrent of 1.0 mA cm is achieved with ZnO/Fe O core-shell NWs and 2.3 mA cm was achieved for the PH -treated NWs at 1.23 V versus RHE. The PH low-temperature treatment creates a dual function, passivation and catalyst layer (Fe PO ), examined by X-ray photoelectron spectroscopy, TEM, photoelectrochemical characterization, and impedance measurements. Such a nano-composition design offers great promise to improve the overall performance of the photoanode material.