Zi-Yi Wang

Lead-Free Perovskite Nanowire Array Photodetectors with Drastically Improved Stability in Nanoengineering Templates

Organometal halide perovskite materials have triggered enormous attention for a wide range of high-performance optoelectronic devices. However, their stability and toxicity are major bottleneck challenges for practical applications. Substituting toxic heavy metal, that is, lead (Pb), with other environmentally benign elements, for example, tin (Sn), could be a potential solution to address the toxicity issue. Nevertheless, even worse stability of Sn-based perovskite material than Pb-based perovskite poses a great challenge for further device fabrication. In this work, for the first time, three-dimensional CH3NH3SnI3 perovskite nanowire arrays were fabricated in nanoengineering templates, which can address nanowire integration and stability issues at the same time. Also, nanowire photodetectors have been fabricated and characterized. Intriguingly, it was discovered that as the nanowires are embedded in mechanically and chemically robust templates, the material decay process has been dramatically slowed down by up to 840 times, as compared with a planar thin film. This significant improvement on stability can be attributed to the effective blockage of diffusion of water and oxygen molecules within the templates. These results clearly demonstrate a new and alternative strategy to address the stability issue of perovskite materials, which is the major roadblock for high-performance optoelectronics.

All Inorganic Cesium Lead Iodide Perovskite Nanowires with Stabilized Cubic Phase at Room Temperature and Nanowire Array-Based Photodetectors

Alluring optical and electronic properties have made organometallic halide perovskites attractive candidates for optoelectronics. Among all perovskite materials, inorganic CsPbX3 (X is halide) in black cubic phase has triggered enormous attention recently owing to its comparable photovoltaic performance and high stability as compared to organic and hybrid perovskites. However, cubic phase stabilization at room temperature for CsPbI3 still survives as a challenge. Herein we report all inorganic three-dimensional vertical CsPbI3 perovskite nanowires (NWs) synthesized inside anodic alumina membrane (AAM) by chemical vapor deposition (CVD) method. It was discovered that the as-grown NWs have stable cubic phase at room temperature. This significant improvement on phase stability can be attributed to the effective encapsulation of NWs by AAM and large specific area of these NWs. To demonstrate device application of these NWs, photodetectors based on these high density CsPbI3 NWs were fabricated demonstrating decent performance. Our discovery suggests a novel and practical approach to stabilize the cubic phase of CsPbI3 material, which will have broad applications for optoelectronics in the visible wavelength range.

The thickness-dependent band gap and defect features of ultrathin ZrO2 films studied by spectroscopic ellipsometry

The band gap and defect features of ultrathin ZrO2 films with varying thicknesses have been investigated by spectroscopic ellipsometry through the point-by-point data inversion method. The ε2-sprectra in the 3-6 eV range are extracted based on an optical model consisting of a Si substrate/effective ZrO2 film/air ambient structure where the effective ZrO2 film is a combination of interfacial layers and ZrO2. Evident widening of the band gap with a reducing size is observed when the effective ZrO2 films are below a critical thickness, somewhere between 8.80 nm and 17.13 nm. This is due to quantum-confinement and amorphous effects. Moreover, the sub-band-gap defects at interfacial layers and in bulk ZrO2 are identified and present strong thickness dependence as well. The interfacial defects at 3.26, 4.13, 4.43, and 4.77 eV mainly exist below the critical thickness and exhibit a significant suppression with increasing film thickness. The bulk defects at 4.15 eV and 4.46 eV dominate in ZrO2 films once they are over the critical thickness. The evolution of the band gap and defects is closely related to variance in the electronic structure of amorphous ZrO2. Our results may be helpful in understanding controversial problems concerning the size effect on ultrathin high-k oxide films and exploring the further miniaturization of electronic devices based on them.

Temperature-Dependent Optical Properties of Titanium Oxide Thin Films Studied by Spectroscopic Ellipsometry

The electron-beam evaporation method was devoted to fabricate anatase-phase TiO2 thin films on silicon substrate. The optical constants of the thin films determined by spectroscopic ellipsometry in the spectral range from 300 to 800 nm were studied in a temperature range from 293 to 533 K. The refractive indices decrease apparently with increasing temperature, and the thermal expansion and electron–phonon interaction can be introduced to elucidate this phenomenon. The absorption edge in extinction coefficient spectra shows a redshift at elevated temperature, which is attributed to thermally driven band gap shrinkage and electron lifetime loss of optical electron transition.

Influence of hydration water on CH 3 NH 3 PbI 3 perovskite films prepared through one-step procedure

Organic-inorganic perovskites were fabricated through a one-step procedure with different levels of hydration water in precursor solutions. The optical properties of CH3NH3PbI3 films were investigated through spectroscopic ellipsometry and photoluminescence measurements. With the measured optical constants, the efficiency limit of perovskite solar cells is predicted with a detailed balance model. By comparing the optical measurement to that of planar heterojunction solar cells, we conclude that the radiative efficiency and porosity of the perovskite film significantly influence the performance of perovskite solar cells. An optimized hydration-water concentration is obtained for the 3CH3NH3I:1PbAc2•xH2O precursor solution. The results can provide guidance for further optimization of the device performance of perovskite solar cells by utilizing hydration water.

Substantial influence on solar energy harnessing ability by geometries of ordered Si nanowire array

The reflectance of the controlled periodic Si nanowire (NW) arrays is systematically explored, which characterizes the influence on the solar energy harnessing ability by the geometries of the NW. A unique dependence of the reflectance of the Si NW array on the diameter, the height, and the bending of the NW are disclosed. The solar energy loss caused by the reflection of the Si NW array exhibits the minimum for the NW with intermediate diameter and length. A plane-wave-based transfer-matrix method (TMM) simulation is performed, which is well consistent with the experimental results. Our results demonstrate the design principle to optimize the Si NW arrays for high-efficiency solar cells.PACS81.07.-b; 78.67.-n; 81.16.-c

Thickness-dependent free-electron relaxation time of Au thin films in near-infrared region

Abstract. The free-electron relaxation time is a crucial property to be considered in the design of optical devices, because it determines the dielectric function. Thus, an accurate understanding of this relaxation time is essential for design optimization. Some simulations showed that the relaxation times of Au thin films with thicknesses below 30 nm are different from those of the bulk material. Therefore, we deposited films with four different thicknesses below this value and used near-infrared spectroscopic ellipsometry to show that the relaxation time is dependent on the film thickness. We fitted the ellipsometry spectrum of Au thin films with a thickness <30  nm and found the imaginary part of the dielectric function of the thin films to vary with the film thickness in the near-infrared region. Furthermore, different relaxation times were used to simulate the reflectance of a Fabry–Pérot absorber and a plasmonic metamaterial absorber. The simulation results indicated that the obtained relaxation time enables a more reliable evaluation of optical device design.

Study on electrical defects level in single layer two-dimensional Ta2O5 *

Two-dimensional atomic-layered material is a recent research focus, and single layer Ta2O5 used as gate dielectric in field-effect transistors is obtained via assemblies of Ta2O5 nanosheets. However, the electrical performance is seriously affected by electronic defects existing in Ta2O5. Therefore, spectroscopic ellipsometry is used to calculate the transition energies and corresponding probabilities for two different charged oxygen vacancies, whose existence is revealed by x-ray photoelectron spectroscopy analysis. Spectroscopic ellipsometry fitting also calculates the thickness of single layer Ta2O5, exhibiting good agreement with atomic force microscopy measurement. Nondestructive and noncontact spectroscopic ellipsometry is appropriate for detecting the electrical defects level of single layer Ta2O5.

Si-centered capped trigonal prism ordering in liquid Pd82Si18 alloy study by first-principles calculations

First-principles molecular dynamic (MD) simulation and X-ray diffraction were employed to study the local structures of Pd–Si liquid at the eutectic composition (Pd82Si18). A strong repulsion is found between Si atoms, and Si atoms prefer to be evenly distributed in the liquid. The dominate local structures around Si atoms are found to be with of a trigonal prism capped by three half-octahedra and an archimedean anti-prism. The populations of these clusters increase significantly upon cooling, and may play an important role in the formation of Pd82Si18 alloy glass.

Enhancement of solar absorption by a surface-roughened metal–dielectric film structure

A solar selective absorber with a multilayered SiO2 (87.0 nm)/Cr (8.3 nm)/SiO2 (96.3 nm) film structure was designed and fabricated by magnetron sputtering on a surface-roughened copper (Cu) substrate. The proposed structure can enhance solar absorption by combining both the typical solar absorption designs of the textured surface and metal–dielectric multilayer film structure. The measured solar absorptance is about 94%, which yields an enhancement of about 2% accompanied by a slightly higher thermal emittance than that observed for the surface-smoothed structure. The increasing thermal emittance of the surface-roughened film structure is expected to markedly cancel the advantage of absorptance enhancement as the temperature increases to 600 K, implying that the proposed film structure functions more efficiently at low or intermediate temperatures (<600 K).