Nanoarchitectonics of Ultraporous Nanoparticle Networks for High Performance UV Photodetectors

Abstract

Accurate detection of ultraviolet radiation is critical to many technologies including wearable devices for skin cancer prevention, optical communication systems and missile launch detection. Si-based photodetectors, relying on n-p type semiconductor homojunction technology, are the most established commercial solution for measurement of ultraviolet light. These devices have some significant shortcomings including high operation voltage, the requirement of longpass filters to block low energy photons and cooling systems to reduce noise and leakage current. This significantly hinders their integration in wearable technologies and alternative solutions are intensively sought. Here, we report a hierarchical design and a rapid synthesis approach for the fabrication of highly performing visible-blind photodetectors based on wide bandgap semiconductors. Combined nano- and micro-scale fine-tuning of the film optical and electrical properties results in record-high photo-currents (milliampere) while preserving pico-ampere dark-currents and excellent selectivity to ultra-low ultraviolet (UV) light densities. In addition, we show that structural engineering of the nanoparticle grain boundaries can drastically enhance the performance of ultraporous nanoparticle network (UNN) photodetectors leading to gigantic photo to dark current ratios with low operation voltages (< 1 V). This is attributed to the optimal interplay of surface depletion and carrier conduction resulting in the formation of an open-neck grain boundary morphology. This is a significant improvement over state-of-the-art devices where a compromise is necessary between high photo-current and low dark-currents. As a result, these photodetectors do not require bulky and costly read-out circuitry and can be directly integrated in portable Complementary metal–oxide–semiconductor (CMOS) based electronics that is currently utilized in many wearable devices. Furthermore, we present a highly performing nanoscale architecture for band-selective UV-photodetectors that features unique tunability and miniaturization potential. The device layout relies on the three dimensional (3D) integration of ultraporous layers of tailored nanoparticles. By tailoring the transmittance window between the indirect band gap of titanium dioxide (TiO2) nanoparticles and the sharp edge of the direct band gap of zinc oxide (ZnO), we achieve a band-selective photoresponse with tunable bandwidth to less than 30 nm. However, a standing challenge with wide bandgap photodetectors is to drastically improve the sluggish response time of these nanostructured devices. In this research, we also present a three-dimensional nanoscale heterojunction architecture for fast-responsive visible-blind UV photodetectors. The device layout consists of p-type nickel oxide (NiO) clusters densely packed on the surface of an ultraporous network of electron-depleted n-type ZnO nanoparticles featuring a significant decrease in the rise and decay times compared to the pure ZnO device. These drastic enhancements in photoresponse dynamics are attributed to the stronger surface band bending and improved electron-hole separation of the nanoscale NiO/ZnO interface. These findings demonstrate a superior architecture for the engineering of miniaturized wearable UV-photodetectors with largely suppressed dark-currents, fast photo-current dynamics and ultra-low power consumption.