Weiguang Yang

Ferroelectric Polarization-Enhanced Photoelectrochemical Water Splitting in TiO2–BaTiO3 Core–Shell Nanowire Photoanodes

The performances of heterojunction-based electronic devices are extremely sensitive to the interfacial electronic band structure. Here we report a largely enhanced performance of photoelectrochemical (PEC) photoanodes by ferroelectric polarization-endowed band engineering on the basis of TiO2/BaTiO3 core/shell nanowires (NWs). Through a one-step hydrothermal process, a uniform, epitaxial, and spontaneously poled barium titanate (BTO) layer was created on single crystalline TiO2 NWs. Compared to pristine TiO2 NWs, the 5 nm BTO-coated TiO2 NWs achieved 67% photocurrent density enhancement. By numerically calculating the potential distribution across the TiO2/BTO/electrolyte heterojunction and systematically investigating the light absorption, charge injection and separation properties of TiO2 and TiO2/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. The ferroelectric polarization could be switched by external electric field poling and yielded PEC performance gain or loss based on the direction of the polarization. This study evidence that the piezotronic effect (ferroelectric or piezoelectric potential-induced band structure engineering) holds great promises in improving the performance of PEC photoelectrodes in addition to chemistry and structure optimization.

Simultaneous Enhancement of Charge Separation and Hole Transportation in a TiO2–SrTiO3 Core–Shell Nanowire Photoelectrochemical System

Efficient charge separation and transportation are key factors that determine the photoelectrochemical (PEC) water-splitting efficiency. Here, a simultaneous enhancement of charge separation and hole transportation on the basis of ferroelectric polarization in TiO2 -SrTiO3 core-shell nanowires (NWs) is reported. The SrTiO3 shell with controllable thicknesses generates a considerable spontaneous polarization, which effectively tunes the electrical band bending of TiO2 . Combined with its intrinsically high charge mobility, the ferroelectric SrTiO3 thin shell significantly improves the charge-separation efficiency (ηseparation ) with minimized influence on the hole-migration property of TiO2 photoelectrodes, leading to a drastically increased photocurrent density ( Jph ). Specifically, the 10 nm-thick SrTiO3 shell yields the highest Jph and ηseparation of 1.43 mA cm-2 and 87.7% at 1.23 V versus reversible hydrogen electrode, respectively, corresponding to 83% and 79% improvements compared with those of pristine TiO2 NWs. The PEC performance can be further manipulated by thermal treatment, and the control of SrTiO3 film thicknesses and electric poling directions. This work suggests a material with combined ferroelectric and semiconducting features could be a promising solution for advancing PEC systems by concurrently promoting the charge-separation and hole-transportation properties.

Graphene/SrTiO3 nanocomposites used as an effective electron-transporting layer for high-performance perovskite solar cells

Organic–inorganic perovskite solar cells based on binary oxides have been studied for a long time and have obtained an impressive advance in performance. However, studies using ternary oxides as the electron-transporting layer are scarce and there are still many problems to be solved. The ternary oxide SrTiO3 with a perovskite structure is well matched with the perovskite absorber layer in the crystal structure. Although the device based on mesoporous-SrTiO3 (mp-SrTiO3) showed a high Voc, its average Jsc is still too low compared with mp-TiO2 based devices. In this work, we used graphene/SrTiO3 nanocomposites as an effective electron-transporting layer. Due to the superconductivity of the graphene combined with tuning the amount of the starting graphene to increase the light harvesting of the absorber layer and decrease recombination centers, we got a great achievement. The device based on graphene/SrTiO3 nanocomposites exhibited a PCE of 10% with a Jsc of 18.08 mA cm−2, increased by 46.0 and 45.6% respectively compared with the mp-SrTiO3 based device, indicating incorporation of graphene is an effective way to improve the Jsc of mp-SrTiO3 based perovskite solar cells.

Three-dimensional self-branching anatase TiO2 nanorods: morphology control, growth mechanism and dye-sensitized solar cell application

Complex three-dimensional (3D) hierarchical nanostructures based on well-defined low-dimensional nanobranches of different sizes and specific exposed facets are highly desirable to obtain tunable physicochemical properties. Herein, a facile, one-step hydrothermal method is employed to construct self-branching anatase TiO2 (SBAT) 3D hierarchical nanostructures. By simply controlling the reaction time and weight ratio of F127/TBAH, SBAT nanorods can be obtained with a large percentage of exposed {010} facets. Based on X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis, a growth mechanism is proposed for the formation of such self-branching 3D nanostructures, which involves the formation of the L-shaped step edges on the [103] surfaces and the alignment of the crystal facets (103) of anatase nanocrystals with the (103) face on the tips of the main anatase TiO2 nanorods. The dye-sensitized solar cell assembled with the SBAT nanorods exhibits an outstanding power conversion efficiency of 7.17%, which is superior to that of the devices based on the 1D anatase TiO2 nanorods and P25 TiO2. This high performance can be attributed to the high dye-uptake density, large size and unique self-branching 3D hierarchical nanostructures built from 1D nanobranches growing epitaxially from the main rod.

Single-crystalline self-branched anatase titania nanowires for dye-sensitized solar cells

The morphology of the anatase titania plays an important role in improving the photovoltaic performance in dye-sensitized solar cells. In this work, single-crystalline self-branched anatase TiO2 nanowires have been synthesized by hydrothermal method using TBAH and CTAB as morphology controlling agents. The obtained self-branched TiO2 nanowires dominated by a large percentage of (010) facets. The photovoltaic conversion efficiency (6.37%) of dye-sensitized solar cell (DSSC) based on the self-branched TiO2 nanowires shows a significant improvement (26.6%) compared to that of P25 TiO2 (5.03%). The enhanced performance of the self-branched TiO2 nanowires-based DSSC is due to heir large percent of exposed (010) facets which have strong dye adsorption capacity and effective charge transport of the self-branched 1D nanostructures.

Simulation Analysis of RuT450 Drilling Force Based on LS-DYNA Gun Drilling

Cutting force is one of the most important parameters in the machining process, it significantly influenced machining precision of the workpiece, power consumed in the machining process, wear of the cutting tools and so on. There are many factors that affect the cutting force, such as the performance of the workpiece material, cutting speed, usage of the cutting fluid, etc. Single factor variable method was used in this paper, RuT450 was used as workpiece, welded cemented carbide gun drill was used as cutting force and LS-DYNA was used as simulation platform to established the cutting simulation model to analyzed the impact of the cutting speed and feed rate to the drilling force. Simulation results show that, at the low speed drilling stage, drilling force increases with the increase of the feed rate and decreases with the increase of the rotation feed, from the stress cloud it could be seen that the equivalent stress near the drill tip reached the maximum in the drilling process.

Effect of surface morphology on laser-induced crystallization of amorphous silicon thin films

The effect of surface morphology on laser-induced crystallization of hydrogenated intrinsic amorphous silicon (a-Si:H) thin films deposited by PECVD is studied in this paper. The thin films are irritated by a frequency-doubled (λ=532 nm) Nd:YAG pulsed nanosecond laser. An effective melting model is built to identify the variation of melting regime influenced by laser crystallization. Based on the experimental results, the established correlation between the grain growth characterized by AFM and the crystalline fraction (Xc) obtained from Raman spectroscopy suggests that the crystallized process form amorphous phase to polycrystalline phase. Therefore, the highest crystalline fraction (Xc) is obtained by a optimized laser energy density.

Electrical properties of polycrystalline mercuric iodide detector

Potentially low cost and large area polycrystalline mercuric iodide is one of the preferred materials for the fabrication of room temperature X-ray and gamma-ray detectors. The properties of the contact between electrode and film play an important role in the performance of the polycrystalline mercuric iodide detector. The crystalline structure of the as-deposited polycrystalline α-HgI2 films were characterized by XRD. The surface morphology of the films was obtained by optical microscope and scan electron microscopes (SEM). And the I-V curve and the response to 241Am were measured after evaporating Au electrode. The energy resolution of 241Am α particles at room temperature was obtained.

Investigation of Se supply for the growth of CZTSSe thin films for photovoltaics

In this work, the selenization growth of Cu2ZnSn(SxSe1−x)4 (CZTSSe) films was optimized by two groups of experiments in vacuum chamber. The selenization of CZTSSe is strongly dependent on the Se supply in the vacuum chamber. Insufficient Se supply left the selenization incomplete. Higher Se supply to CZTSSe either by increasing the Se powder usage or by increasing the external pressure resulted in the degradation of CZTSSe films with lower degree of crystallinity.The characterization of XRD, Raman and SEM confirmed the films obtained under the best conditions were well-developed CZTSSe films with compact faceted grains and good crystallinity. Additionally, theCZTSSe film grown using 500°C showed more orientation along (220).

Solution to wafer edge silicon needle defect of deep trench process

In design and manufacturing of power MOS microelectronic device, deep trench process is used for some special request. The trench depth reaches to scores of micrometer. some are often used in deep trench etching step. Wafer edge generates silicon grass defect. It becomes the major source of particle during the post wet clean steps, which impact line yield and contaminate wet etching tools. The paper introduces two solutions for solving this defect by optimizing trench-photo process. One is inverted trapezoid process and the other is negative resist process. Both solutions in lithography are aimed at wafer edge protection. The deep trench etching step is to protect the underground material from being damaged, and results to solve the wafer edge silicon grass defect of deep trench process.