Helicity-dependent photocurrent (HDPC) has been observed in epitaxial thin films of the three-dimensional topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ with a thickness of 7, 20, and 30 quintuple layers (QLs). By analyzing the incident angle dependence of the HDPC, we successfully extract the photocurrent induced by the circular photon drag effect (CPDE) of even parity from the HDPC. It is found that the circular photogalvanic effect (CPGE) is larger than the odd parity of CPDE. The CPGEs of the top and bottom surface states are successfully distinguished at room temperature. As the thickness of the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ film is increased from 7 to 20 QLs, the dominant contribution of the CPGE current switches from the top surface states to the bottom surface states under the front illumination of a 1064-nm laser. Additionally, it is found that the CPGE currents of the 20- and 30-QL samples reverse sign when the temperature is increased from 77 to 300 K, which may be attributed to the HDPC current induced by the combined action of the inverse spin Hall effect and the vertical thermoelectric effect overwhelming the CPGE current at low temperature in the samples. The insights we obtained are significant for engineering optospintronic devices based on three-dimensional topological insulators with a strong thermoelectric effect.

ZnO nanowires (NWs) were grown on Si(100) substrates at 975 °C by a vapor-liquid-solid method with ~2 nm and ~4 nm gold thin films as catalysts, followed by an argon plasma treatment for the as-grown ZnO NWs. A single ZnO NW–based memory cell with a Ti/ZnO/Ti structure was then fabricated to investigate the effects of plasma treatment on the resistive switching. The plasma treatment improves the homogeneity and reproducibility of the resistive switching of the ZnO NWs, and it also reduces the switching (set and reset) voltages with less fluctuations, which would be associated with the increased density of oxygen vacancies to facilitate the resistive switching as well as to average out the stochastic movement of individual oxygen vacancies. Additionally, a single ZnO NW–based memory cell with self-rectification could also be obtained, if the inhomogeneous plasma treatment is applied to the two Ti/ZnO contacts. The plasma-induced oxygen vacancy disabling the rectification capability at one of the Ti/ZnO contacts is believed to be responsible for the self-rectification in the memory cell.

The circular (CPGE) and linear photogalvanic effect (LPGE) of a three-dimensional topological insulator Bi ₂ Se ₃ thin film of seven quintuple layers excited by near-infrared (1064 nm) and mid-infrared (10.6 m) radiations have been investigated. The comparison of the CPGE current measured parallel and perpendicular to the incident plane, together with the comparison of the CPGE current under front and back illuminations, indicates that the CPGE under front illumination of 1064 nm light is dominated by the top surface states of the Bi ₂ Se ₃ thin film. The CPGE current excited by 10.6 m light is about one order larger than that excited by 1064 nm light, which may be attributed to the smaller cancelation effect of the CPGE generated in the two-dimensional electron gas when excited by 10.6 m light. Under the excitation of 1064 nm light, the LPGE current is dominated by the component which shows an even parity of incident angles, while the LPGE current excited by 10.6 m light is mainly contributed by the component which is an odd parity of incident angles. Both of the CPGE and LPGE currents excited by 1064 nm decrease with increasing temperature, which may be owing to the decrease of the momentum relaxation time and the stronger electron–electron scattering with increasing temperature, respectively.