Porosity-induced full-range visible-light photodetection via ultrahigh broadband antireflection in ZnO nanowires

This study investigates graded porosity in nanowire arrays to realize a new graded refractive index mechanism that results in unprecedented broadband antireflection and extends the photodetection properties to cover the entire visible-light range. In this work, ZnO nanowires (NWs) are chosen as a model system to demonstrate the porosity-induced antireflection and photodetection properties for visible light. Porous ZnO NW arrays (PZNA) were synthesized by the hydrothermal method followed by controlled hydrogen annealing for different durations. The surface pores of the PZNA were formed with a gradient distribution from the top to the bottom of the nanowires. This pore gradient distribution serves as a new mechanism to achieve a graded refractive index, which provides improved broadband antireflection in PZNA with a minimum reflectance of less than 5% at 800 nm. Moreover, the cathodoluminescence spectra suggest the evolution of many defects in PZNA, which contribute to defect-state excitation phenomena. Based on their unique features with regard to antireflection, multiple scattering and defect state excitation, the PZNA devices exhibit an exceptional capability for steady photodetection over the entire visible-light range. The corresponding unique photodetection mechanisms for the phenomena are discussed. These new physical phenomena can be readily extended to other 1D materials and used to develop other optoelectronic devices. Creating graded porosity in semiconductor nanostructures improves the performance of photodetectors, shows research from Taiwan. Getting light into optoelectronic devices can be difficult because the large difference in the refractive indices of air and a semiconductor causes some of the light to be reflected backwards. Now, Chuan-Pu Liu of National Cheng Kung University and co-workers have created graded porosity in zinc oxide nanowires by annealing them in a hydrogen atmosphere. The pore density gradually decreased deeper into the nanostructure, which caused the refractive index to increase from that of air at the surface to that of solid zinc oxide further in. This graded porosity reduced the proportion of light reflected at the surface. The researchers anticipate that the method can be applied to other one-dimensional materials and be used in various optoelectronic devices. A new mechanism of porosity-induced graded refractive index, which results in enhanced broadband anti-reflection properties that can extend the inherent properties of 1D materials, for example, wide band gap ZnO in this work, and modify it to achieve an unprecedented photodetection capability covering the entire visible-light range, is demonstrated.