Laser-coupled tunneling junction devices can lead to various phenomena such as electromagnetic field local enhancement and optical rectification effects, which have significant applications in fields such as plasmon optical tweezers, single-molecule imaging, and single-photon light sources. Using optical fields to drive tunneling junction nanodevices may also reduce the size of electronic devices and improve their speed. The main methods to construct tunneling junction devices include tunneling junctions with adjusted nanogap through dynamic methods and tunneling junction devices with fixed nanogap through methods like electromigration and feedback electrodeposition. However, the fabrication of tunneling junction devices by electromigration still has the issues of high cost, time consumption, and low success rate. In previous works, our group successively fabricated novel solid-state tunneling junction nanodevices with stable nanogap through feedback electrodeposition, and these devices are appropriate for such research. In the present study, we couple continuous laser to the characteristic solid-state tunneling junction nanodevices and systematically study the cause of photoinduced tunneling current. We hope that our results may provide a reference for the optical manipulation and optoelectronic coupling of solid-state tunneling junction nanodevices, and contribute to the development of optically coupled solid-state tunneling junction-related devices and technologies.
The fabrication of solid-state tunneling junction nanodevices includes seven steps. Firstly, pull θ-shaped double-hole quartz glass tubes into nanoprobes with conical tips through external forces at both sides while heated at the center using a cone puller. Secondly, introduce butane gas into the double holes of the nanoprobes and use a butane spray gun to heat the tip, causing the butane gas to undergo pyrolysis and carbon deposition at the tip. Thirdly, insert copper wires with a diameter of 0.5 mm into the two holes, so that the front ends of the copper wires are in contact with the carbon inside the nanoprobes. The copper wires are fixed using a hot melt adhesive. Fourthly, etch the exposed carbon material at the tip of the nanoprobes using an electrochemical workstation, to form deposition sites as the preparation for subsequent gold electrodes. Fifthly, pre-electrodeposit gold electrodes at the tip of the nanoprobes using the constant current method. Sixthly, feedback-electrodeposit gold electrodes at the tip of the nanoprobes using the constant potential method. Finally, soak the fabricated electrodes in deionized water for more than 12 h, so that the gold atoms on the tip surface of the electrode reach a stable state through a self-resetting effect, and solid-state tunneling devices with sub-5 nm nanogaps are obtained.
The current in the devices increases when the laser is switched on, and decreases when the laser is switched off at zero bias voltage, which proves the presence of photocurrent. The tunneling junction nanodevices exhibit quick optical response, and the photocurrent at zero bias voltage shows that a significant spontaneous thermal current exists in such devices. Influencing factors of the photocurrent are then researched. When the laser power grows, the photocurrent grows linearly within the power range of 0‒1000 μW. And the photocurrent decreases when the power exceeds 1000 μW due to irreversible optical damage. When the modulation frequency grows, the photocurrent decreases inversely within the frequency range of 250‒6000 Hz. This result shows that thermal expansion effects play an important role in photocurrent (Fig.3). The relation between photocurrent and polarization angle follows a square-of-cosine rule, and the photocurrent has a period of 180°. This result shows that plasmon resonance effects contribute to the photocurrent, which includes plasmon-induced thermal expansion current and hot carrier current. And the photocurrent does not decrease to zero whatever the polarization angle is, which proves the presence of thermal voltage current. When the bias voltage grows, the photocurrent grows linearly, which shows that optical rectification effects are not significant in our experimental conditions (Fig.4). The simulation results also show the polarization dependence of electrical field intensity, which is positively correlated with the temperature rise. When the devices are put in the air and illuminated by the laser with a power density of 106 W/m2, the temperature in the devices rises from 300 K to 312.6 K in less than 5 ms (Fig.5).
In the present study, the influencing factors and causes of the photocurrent in solid-state tunneling junction devices are systematically studied. Under laser illumination, significant photocurrent is generated in the tunneling junction nanodevices. The experiment and simulation results show that the local thermal expansion effects, thermal voltage effects, and hot carrier effects are the main reasons for photocurrent generation, while optical rectification effects are not significant. Additionally, the optical rectification effect is not significantly limited by the laser peak power. To get the maximum photocurrent, we can increase laser power and bias voltage (under threshold), decrease modulation frequency, and choose an appropriate polarization angle. And it is possible to amplify the optical rectification effects using a pulsed laser. Our study may contribute to the study of the interaction mechanism between laser and nanostructures, as well as provide a reference for the control of photocurrent.
