Identification and Analysis of Hazardous Materials Using Optical Spectroscopy

This investigation is aimed towards using optical spectroscopy for remote identification and quantitative analysis of hazardous substances for safety and security applications. We introduce a new model employing portable photosensor devices that are based on the double-barrier and vertically placed silicon structure, for such applications. The different absorption depths of individual waves allow us to carry out their spectral selection using an algorithm developed for this specific objective. We tested the proposed model on experimental Ag-p-Si-n-Si structures. The algorithm is developed for the spectral analysis without the preliminary calibration. The low dark currents (several dozens of pA) permit us to carry out the spectral analysis of the integral flux of the electromagnetic radiation of low intensity. The quantitative data from light current-voltage characteristics allow us to obtain an intensity distribution spectrum characteristic of the material by using red LED and the green laser. The results of this investigation divulge new possibilities for the creation of a new type of the portable semiconductor spectrophotometer and due to its stand-off detection capability, offer potential pathways to evaluate hazardous substances.

Wafer-scale arrayed p-n junctions based on few-layer epitaxial GaTe

Two-dimensional （2D） materials have attracted substantial attention in electronic and optoelectronic applications with the superior advantages of being flexible, transparent, and highly tunable. Gapless graphene exhibits ultra-broadband and fast photoresponse while the 2D semiconducting MoS2 and GaTe exhibit high sensitivity and tunable responsivity to visible light. However, the device yield and repeatability call for further improvement to achieve large-scale uniformity. Here, we report a layer-by-layer growth of wafer-scale GaTe with a high hole mobility of 28.4 cm^2/（V.s） by molecular beam epitaxy. The arrayed p-n ）unctions were developed by growing few-layer GaTe directly on fhree-inch Si wafers. The resultant diodes reveal good rectifying characteristics and a high photovoltaic external quantum efficiency up to 62% at 4.8 μW under zero bias. The photocurrent reaches saturation fast enough to capture a time constant of 22 μs and shows no sign of device degradation after 1.37 million cycles of operation. Most strikingly, such high performance has been achieved across the entire wafer, making the volume production of devices accessible. Finally, several photoimages were acquired by the GaTe/Si photodiodes with reasonable contrast and spatial resolution, demonstrating the potential of integrating the 2D materials with silicon technology for novel optoelectronic devices.

High-performance solar-blind ultraviolet photodetector based on electrospun TiO2-ZnTiO3 heterojunction nano- wires

High-performance solar-blind UV （ultraviolet） photodetectors （PDs） based on low-dimension semiconducting nanostructures with high sensitivity, excellent cycle stability, and the ability to operate in harsh environments are critical for solar observations, space communication, UV astronomy, and missile tracking. In this study, TiO2-ZnTiO3 heterojunction nanowire-based PDs are successfully developed and used to detect solar-blind UV light. A photoconductive analysis indicates that the fabricated PDs are sensitive to UV illumination, with high sensitivity, good stability, and high reproducibility. Further analysis indicates that the rich existence of grain boundaries within the TiO2-ZnTiO3 nanowire can greatly decrease the dark current and recombination of the electron-hole pairs and thereby significantly increase the device＇s photosensitivity, spectra responsivity （1.1 ～ 106）, and external quantum efficiency （4.3 ～ 108 %）. Moreover, the PDs exhibit good photodetective performance with fast photoresponse and recovery and excellent thermal stability at temperatures as high as 175 ℃. According to these results, TiO2-ZnTiO3 heterojunction nanowires exhibit great potential for applications in high-performance optical electronics and PDs, particularly next-generation photodetectors with the ability to operate in harsh environments.

Strain modulation on graphene/ZnO nanowire mixed- dimensional van der Waals heterostructure for high- performance photosensor

The mixed-dimensional van der Waals （vdW） heterostructure is a promising building block for strained electronics and optoelectronics because it avoids the bond fracture and atomic reconstruction under strain. We propose a novel mixed-dimensional vdW heterostructure between two-dimensional graphene and a one-dimensional ZnO nanowire for high-performance photosensing. By utilizing the piezoelectric properties of ZnO, strain modulation was accomplished in the mixed-dimensional vdW heterostructure to optimize the device performance. By combining the ultrahigh electrons transfer speed in graphene and the extremely long life time of holes in ZnO, an outstanding responsivity of 1.87 ×10^5 A/W was achieved. Under a tensile strain of only 0.44% on the ZnO nanowire, the responsivity was enhanced by 26%. A competitive model was proposed, in which the performance enhancement is due to the efficient promotion of the injection of photogenerated electrons from the ZnO into the graphene caused by the strain-induced positive piezopotential. Our study provides a strain-engineering strategy for controlling the behavior of the photocarriers in the mixed-dimensional vdW heterostructure, which can be also applied to other similar systems in the future.