Nanoamorphous-Surface-State Controlled Pseudo-Negative-Photoconductivity Of Tiny SnO₂ Nanoparticles

When light illuminates on the surface of a semiconductor or an insulator, bound electron within will be excited, leading to an increase in the conductance. This kind of optoelectronic phenomenon is called internal photoelectric effect. On the basis of fast and efficient optoelectronic conversion, how to control and utilize the inevitable heating effect become a challenge. There are two difficulties in applying light heating effect. One is the influences of heat on the structure of the device materials. If the structure changes during temperature increasing, the performance of the device will decline or vanish. The other is long response and recover time of temperature change.Based on the above consideration, tiny SnO2 nanoparticles coated muti-walled carbon nanotubes are synthesized. Their resistance increases under light illumination, which is very abnormal. The response is stable and repeatable. This negative photoconductivity effect is originated from the intensive electron scattering of nano-amorphous-surface-state (NASS) of the tiny SnO2 nanoparticles resulting in smaller inner current and the large adsorption of moisture by NASS at dark leading to large surface current. When temperature increase under light illumination, the moisture adsorbed on the NASS of the tiny SnO2 nanoparticles desorbs, resulting in a decrease in the Fermi level. Thus the conductance drops. So we call it pseudo-negative-photoconductivity (PsdNPC) effect. The current change keeps in step with the temperature change theoretically.In situ transmission electron microscopy experiment demonstrates the stability of the nanoparitcles below 200℃ and their amorphization aggregation at their contact sites above-300℃. By utilizing PsdNPC effect and normal positive photoconductivity, ultraviolet detector with voltage output is fabricated. Instead of traditional photoconductive ultraviolet detector, this detector with voltage output gets rid of precious and high-precision amperemeter, thus has a wider application. Moreover, logic gates "NAND" and "NOR" are fabricated. Based on these logic gates, all functions of optoelectronic conversion can be realized.The response and recovery time of the PsdNPC effect are relatively long, because of the slow heat conduction speed. After analyzing a theoretical model based on the effect, we optimize the molar mass of the working materials, the distance between the working material and the heat source, and thermal conductivity of the substrate. The response and recovery time shorten a lot after optimization. What’s more, graphene as the inner skeleton of the devices can shorten the response and recovery time to-0.24 s and-0.44 s respectively, by utilizing its superior heat conductivity to keep SnO2 nanoparticles heat-coordinated.In summary, graphene skeleton heat-coordinated and NASS controlled PsdNPC effect of tiny SnO2 nanoparticles are confirmed. It’s stable and has short response and recovery time. It may be utilized in optoelectronic devices with new functions.