Yiping Wang

Cl‐Doped ZnO Nanowire Arrays on 3D Graphene Foam with Highly Efficient Field Emission and Photocatalytic Properties

An environmentally friendly, low-cost, and large-scale method is developed for fabrication of Cl-doped ZnO nanowire arrays (NWAs) on 3D graphene foam (Cl-ZnO NWAs/GF), and investigates its applications as a highly efficient field emitter and photocatalyst. The introduction of Cl-dopant in ZnO increases free electrons in the conduction band of ZnO and also leads to the rough surface of ZnO NWAs, which greatly improves the field emission properties of the Cl-ZnO NWAs/GF. The Cl-ZnO NWAs/GF demonstrates a low turn-on field (≈1.6 V μm(-1)), a high field enhancement factor (≈12844), and excellent field emission stability. Also, the Cl-ZnO NWAs/GF shows high photocatalytic efficiency under UV irradiation, enabling photodegradation of organic dyes such as RhB within ≈75 min, with excellent recyclability. The excellent photocatalytic performance of the Cl-ZnO NWAs/GF originates from the highly efficient charge separation efficiency at the heterointerface of Cl-ZnO and GF, as well as improved electron transport efficiency due to the doping of Cl. These results open up new possibilities of using Cl-ZnO and graphene-based hybrid nanostructures for various functional devices.

Band gap engineering of a soft inorganic compound PbI2 by incommensurate van der Waals epitaxy

Van der Waals epitaxialgrowth had been thought to have trivial contribution on inducing substantial epitaxial strain in thin films due to its weak nature of van der Waals interfacial energy. Due to this, electrical and optical structure engineering via van der Waals epitaxial strain has been rarely studied. In this report, we show that significant band structure engineering could be achieved in a soft thin film material PbI2 via van der Waals epitaxy. The thickness dependent photoluminescence of single crystal PbI2 flakes was studied and attributed to the substrate-film coupling effect via incommensurate van der Waals epitaxy. It is proposed that the van der Waals strain is resulted from the soft nature of PbI2 and large van der Waals interaction due to the involvement of heavy elements. Such strain plays vital roles in modifying the band gap of PbI2. The deformation potential theory is used to quantitatively unveil the correlation between thickness, strain, and band gap change. Our hypothesis is confirmed by the subsequent mechanical bending test and Raman characterization.

Epitaxial Halide Perovskite Lateral Double Heterostructure

Epitaxial III-V semiconductor heterostructures are key components in modern microelectronics, electro-optics, and optoelectronics. With superior semiconducting properties, halide perovskite materials are rising as promising candidates for coherent heterostructure devices. In this report, spinodal decomposition is proposed and experimentally implemented to produce epitaxial double heterostructures in halide perovskite system. Pristine epitaxial mixed halide perovskites rods and films were synthesized via van der Waals epitaxy by chemical vapor deposition method. At room temperature, photon was applied as a knob to regulate the kinetics of spinodal decomposition and classic coarsening. By this approach, halide perovskite double heterostructures were created carrying epitaxial interfaces and outstanding optical properties. Reduced Fröhlich electron-phonon coupling was discovered in coherent halide double heterostructure, which is hypothetically attributed to the classic phonon confinement effect widely existing in III-V double heterostructures. As a proof-of-concept, our results suggest that halide perovskite-based epitaxial heterostructures may be promising for high-performance and low-cost optoelectronics, electro-optics, and microelectronics. Thus, ultimately, for practical device applications, it may be worthy to pursue these heterostructures via conventional vapor phase epitaxy approaches widely practised in III-V field.

Self-heating–induced healing of lithium dendrites

Healing away the dendrites The formation of lithium dendrites during charge-discharge cycles limits the development of lithium metal batteries, because the dendrites can cause electrical shorting of the cells. A number of tricks have been used to try to prevent dendrite formation. Li et al. took the opposite approach (see the Perspective by Mukhopadhyay and Jangid). They operated their cells at higher current densities, under which one would expect dendrites to form owing to the higher nucleation rates. However, under these conditions, the dendrites that started to form heated up and annealed, leading to their disappearance. Science, this issue p. 1513; see also p. 1463 Lithium metal dendrites can be healed in situ by Joule self-heating of the dendritic particles. Lithium (Li) metal electrodes are not deployable in rechargeable batteries because electrochemical plating and stripping invariably leads to growth of dendrites that reduce coulombic efficiency and eventually short the battery. It is generally accepted that the dendrite problem is exacerbated at high current densities. Here, we report a regime for dendrite evolution in which the reverse is true. In our experiments, we found that when the plating and stripping current density is raised above ~9 milliamperes per square centimeter, there is substantial self-heating of the dendrites, which triggers extensive surface migration of Li. This surface diffusion heals the dendrites and smoothens the Li metal surface. We show that repeated doses of high-current-density healing treatment enables the safe cycling of Li-sulfur batteries with high coulombic efficiency.

van der Waals epitaxy of CdTe thin film on graphene

van der Waals epitaxy (vdWE) facilitates the epitaxial growth of materials having a large lattice mismatch with the substrate. Although vdWE of two-dimensional (2D) materials on 2D materials have been extensively studied, the vdWE for three-dimensional (3D) materials on 2D substrates remains a challenge. It is perceived that a 2D substrate passes little information to dictate the 3D growth. In this article, we demonstrated the vdWE growth of the CdTe(111) thin film on a graphene buffered SiO2/Si substrate using metalorganic chemical vapor deposition technique, despite a 46% large lattice mismatch between CdTe and graphene and a symmetry change from cubic to hexagonal. Our CdTe films produce a very narrow X-ray rocking curve, and the X-ray pole figure analysis showed 12 CdTe (111) peaks at a chi angle of 70°. This was attributed to two sets of parallel epitaxy of CdTe on graphene with a 30° relative orientation giving rise to a 12-fold symmetry in the pole figure. First-principles calculations reveal that,...

High-Temperature Ionic Epitaxy of Halide Perovskite Thin Film and the Hidden Carrier Dynamics

High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III-V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated-a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology.

Nonlinear Electron-Lattice Interactions in a Wurtzite Semiconductor Enabled via Strongly Correlated Oxide.

With VO2 , a classic strongly correlated oxide material, a model semiconductor CdS is stretched and its electron-lattice interaction in a nonlinear manner is modulated. Optical spectroscopy is applied to probe the electronic band structure-associated parameters which is explained by the theoretical prediction based on k·p method and microscopy study. The research provides a new avenue on dynamic straining engineering.

van der Waals epitaxy of CdS thin films on single-crystalline graphene

van der Waals epitaxy (vdWE) of three-dimensional CdS thin films on both single-crystalline graphene/Cu(111)/spinel(111) and single-crystalline graphene/SiO2/Si substrates is achieved via thermal evaporation. X-ray and electron backscatter diffraction pole figures reveal that the CdS films are a Wurtzite structure with a weak epitaxy on graphene and accompanied with a fiber texture background. The epitaxial alignment between CdS and graphene is observed to be an unusual non-parallel epitaxial relationship with a 30° rotation between the unit vectors of CdS and graphene. A geometrical model based on the minimization of superlattice area mismatch is employed to calculate possible interface lattice arrangement. It is found that the 30° rotation between CdS and graphene is indeed the most probable interface epitaxial lattice alignment. The vdWE of CdS on graphene, transferrable to arbitrary substrates, may represent a step forward for the growth of quality CdS thin films on arbitrary substrates through a grap...

Decoupling interface effect on the phase stability of CdS thin films by van der Waals heteroepitaxy

Wurtzite (W) and zinc-blende (ZB) polytypism has long been observed in epitaxial CdS thin films. The present work, based on van der Waals epitaxial CdS thin films, is an attempt to explain which crystal modification, W or ZB, is favored under different growth conditions. In this van der Waals epitaxy system where the substrate influence is considered weak, it is found that the substrate temperature plays a crucial role in determining the crystal modification of CdS, that is, W and ZB CdS are more stable at low and high ends of substrate temperature, respectively. We attribute this temperature effect to the entropy difference (SWu2009<u2009SZB), a conclusion well supported by the thermodynamic hard sphere model formulation of the entropy difference between hexagonal close-packed and face-centered cubic structures. By summarizing other works, we find that the entropy difference model can also be applied to large mismatched (≳3%) CdS-substrate chemical epitaxy systems but not for small mismatched (≲3%) ones. In the ...

Tunable optical properties and stability of lead free all inorganic perovskites (Cs2SnIxCl6−x)

Organic–inorganic hybrid lead-based perovskites experience significant environmental instability under ambient moist air and are not environmentally benign due to the usage of toxic Pb. Here, we report a new approach to synthesize lead-free all inorganic perovskites (Cs2SnIxCl6−x) using hydriodic acid (HI) demonstrating greatly enhanced environmental stability and tunable optical properties by controlling the I−/Cl− ratios. Single phase perovskites can be achieved with a low iodine or chlorine content, and a phase separation occurs in the binary system with closer iodine and chlorine dopings. UV-vis diffuse reflectance and photoluminescence measurements reveal tunable band gaps of Cs2SnIxCl6−x perovskites from the UV to the infrared region. The mixed halide perovskite with a lower chloride content shows significantly higher photoluminescence intensity. The thermal stability of mixed halide all-inorganic perovskites is continuously improved as the Cl content increases. The synthesis of Sn-based perovskites with tunable optical properties and environmental stability represents one step further toward the realization of the stable lead-free all inorganic perovskites.