Giant Flexoelectricity in Bent Semiconductor Thinfilm.

We elucidate the flexoelectricity of semiconductors in the high strain gradient regime, the underlying mechanism of which is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high-strain-gradient-induced band gap closure. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film. Therefore, upon the band gap closure, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to a pronounced change of polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

Methods for Conducting Electron Backscattered Diffraction (EBSD) on Polycrystalline Organic Molecular Thin Films.

Electron backscattered diffraction (EBSD) is a technique regularly used to obtain crystallographic information from inorganic samples. When EBSD is acquired simultaneously with emitting diodes data, a sample can be thoroughly characterized both structurally and compositionally. For organic materials, coherent Kikuchi patterns do form when the electron beam interacts with crystalline material. However, such patterns tend to be weak due to the low average atomic number of organic materials. This is compounded by the fact that the patterns fade quickly and disappear completely once a critical electron dose is exceeded, inhibiting successful collection of EBSD maps from them. In this study, a new approach is presented that allows successful collection of EBSD maps from organic materials, here the extreme example of a hydrocarbon organic molecular thin film, and opens new avenues of characterization for crystalline organic materials.

Thermally Evaporated Metal Halide Perovskites and Their Analogues: Film Fabrication, Applications and Beyond.

Metal halide perovskites emerge as promising semiconductors for optoelectronic devices due to ease of fabrication, attractive photophysical properties, their low cost, highly tunable material properties, and high performance. High-quality thin films of metal halide perovskites are the basis of most of these applications including solar cells, light-emitting diodes, photodetectors, and electronic memristors. A typical fabrication method for perovskite thin films is the solution method, which has several limitations in device reproducibility, adverse environmental impact, and utilization of raw materials. Thermal evaporation holds great promise in addressing these bottlenecks in fabricating high-quality halide perovskite thin films. It also has high compatibility with mass-production platforms that are well-established in industries. This review first introduces the basics of the thermal evaporation method with a particular focus on the critical parameters influencing the thin film deposition. The research progress of the fabrication of metal halide perovskite thin films is further summarized by different thermal evaporation approaches and their applications in solar cells and other optoelectronic devices. Finally, research challenges and future opportunities for both fundamental research and commercialization are discussed.

Application of Laser Treatment in MOS-TFT Active Layer Prepared by Solution Method.

The active layer of metal oxide semiconductor thin film transistor (MOS-TFT) prepared by solution method, with the advantages of being a low cost and simple preparation process, usually needs heat treatment to improve its performance. Laser treatment has the advantages of high energy, fast speed, less damage to the substrate and controllable treatment area, which is more suitable for flexible and large-scale roll-to-roll preparation than thermal treatment. This paper mainly introduces the basic principle of active layer thin films prepared by laser treatment solution, including laser photochemical cracking of metastable bonds, laser thermal effect, photoactivation effect and laser sintering of nanoparticles. In addition, the application of laser treatment in the regulation of MOS-TFT performance is also described, including the effects of laser energy density, treatment atmosphere, laser wavelength and other factors on the performance of active layer thin films and MOS-TFT devices. Finally, the problems and future development trends of laser treatment technology in the application of metal oxide semiconductor thin films prepared by solution method and MOS-TFT are summarized.

Ferromagnetism above Room Temperature in a Ni-Doped Organic-Based Magnetic Semiconductor.

Ferromagnetic semiconductors with structural flexibility are an indispensable feature for future flexible spin-electronic applications. In this case, we introduce magnetic ingredients into an organic semiconductor, namely, pentacene, to form a ferromagnetic organic semiconductor (FOS). The first observation for ferromagnetic Ni-doped pentacene semiconductors at room temperature in the field of semiconductor spintronics is reported in this article. To date, the mechanism of FOSs with ferromagnetism is not understood yet, especially when their Curie temperature is enhanced above room temperature. Here, we demonstrate dopants of Ni atoms and the modulation of the growth temperature in the FOS films to achieve room-temperature ferromagnetic properties in a series of FOS films, one of which has a maximum coercivity of 257.6 Oe. The spin-exchange interaction between a Ni atom and a pentacene molecule is detected through the magnetic hysteresis obtained using a superconducting quantum interference device magnetometer. We verify the effectiveness of this spin coupling through magnetic force microscopy, Raman spectroscopy, scanning Kelvin probe microscopy, and theoretical simulation. A model for the indirect spin coupling between Ni atoms is proposed for the mechanism of room-temperature ferromagnetic ordering of spins due to the exchange force indirectly. We believe that the π-electrons of pentacene molecules at the triple state for this model can support the spin coupling of electrons of Ni atoms. Our findings facilitate the development of brand-new spintronic devices with structural flexibility and room-temperature ferromagnetism.

Inversion of Circular Dichroism Signals in Chiral Polythiophene Films Induced by End-On-Oriented Surface-Segregated Monolayers.

A chiral polythiophene surfactant based on poly(3-(S)-2-methylbutylthiophene) ((S)-P3MBT) with a semifluoroalkyl group at one end of the main chain was synthesized and used to form surface-segregated monolayers (SSMs). Films of pure (S)-P3MBT mainly adopted the edge-on orientation, whereas (S)-P3MBT films with a SSM of the polymer surfactant (S)-P3MBT-F17 contained a large proportion of end-on-oriented polythiophene, both at the surface and inside the films. The thin films with the SSM showed circular dichroism signals, with the sign opposite to those observed in (S)-P3MBT films. These findings suggest that the orientation-controlled SSM layers induced changes in the packing of the polymer aggregates in the films, resulting in a dramatic change in the excitonic interactions of the chiral semiconducting polymers.