Nanowire Ultraviolet Photodetectors and Optical Switches

Nanowires and nanotubes may become important building blocks for nanoscale optoelectronics, since they can function as miniaturized devices as well as electrical interconnects. Nano-devices such as field-effect transistors, single-electron transistors, metal±semiconductor junctions, and intermolecular crossed junctions have been demonstrated. Many of these devices rely on binary switching, which is critical for important applications such as memory storage and logic circuits. Switching on the nanometer and molecular level has been predominantly achieved through proper electrical gating, as exemplified by nanotube transistors. However, no attention has been given to the photoconducting properties of nanowires despite the exciting possibilities for use in optoelectronic circuits. Here, we show the possibility of creating highly sensitive nanowire switches by exploring the photoconducting properties of individual semiconductor nanowires. The conductivity of the ZnO nanowires is extremely sensitive to ultraviolet light exposure. The light-induced conductivity increase allows us to reversibly switch the nanowires between aOFFo and aONo states, an optical gating phenomenon analogous to the commonly used electrical gating. The ZnO nanowires used in the experiments were grown by a vapor phase transport process developed in our lab. The diameters of these wires range from 50 to 300 nm. To characterize their photoconducting properties, the nanowires were dispersed directly on pre-fabricated gold electrodes. Alternatively, electron-beam lithography was used to fabricate gold electrodes on top of the nanowires. Field-emission scanning electron microscopy (FE-SEM) was used to image the ZnO nanowire devices. Electrical resistivity measurements were performed in a four-terminal configuration in air, nitrogen, or vacuum environments. Four-terminal measurements of individual ZnO nanowires indicate that they are highly insulating in the dark with a resistivity above 3.5 MX cm. When the nanowires are exposed to ultraviolet (UV)-light with wavelengths below 380 nm (handheld UV-lamp, 0.3 mW cm, 365 nm), the nanowire resistivity decreases by typically 4 to 6 orders of magnitude. Figure 1 compares the current±voltage (I±V) curves measured on a 60 nm nanowire in the dark and upon UV-light exposure. A larger photoresponse was detected at higher bias. We notice that the I±V curve for the UV-exposed nanowire exhibits nonlinear behavior. The same nonlinear I±V has been observed for both the wire-on-electrode and electrode-on-wire configurations. The four-terminal and two-terminal measurements show essentially identical resistivity values, which suggests that the Au/ZnO contacts may not contribute to the I±V nonlinearity. The exact reason for this nonlinearity remains unknown at this stage. The high sensitivity of the nanowire photoconductors can be seen in Figure 2, which shows the power dependence of the photoresponse. The third harmonic of a Nd:YAG laser was used as the UV light source. Neutral density filters were used to change the incident UV light power. It was found that the photoresponse (Ipc) can be expressed by a simple power law