Si is an important anode material for the next generation of Li ion batteries. Here the energetics and dynamics of Li atoms in bulk Si have been studied at different Li concentrations on the basis of first principles calculations. It is found that Li prefers to occupy an interstitial site as a shallow donor rather than a substitutional site. The most stable position is the tetrahedral (T(d)) site. The diffusion of a Li atom in the Si lattice is through a T(d)-Hex-T(d) trajectory, where the Hex site is the hexagonal transition site with an energy barrier of 0.58 eV. We have also systematically studied the local structural transition of a Li(x)Si alloy with x varying from 0 to 0.25. At low doping concentration (x = 0-0.125), Li atoms prefer to be separated from each other, resulting in a homogeneous doping distribution. Starting from x = 0.125, Li atoms tend to form clusters induced by a lattice distortion with frequent breaking and reforming of Si-Si bonds. When x ≥ 0.1875, Li atoms will break some Si-Si bonds permanently, which results in dangling bonds. These dangling bonds create negatively charged zones, which is the main driving force for Li atom clustering at high doping concentration.

The ultrahigh specific lithium ion storage capacity of Si nanowires (SiNWs) has been demonstrated recently and has opened up exciting opportunities for energy storage. However, a systematic theoretical study on lithium insertion in SiNWs remains a challenge, and as a result, understanding of the fundamental interaction and microscopic dynamics during lithium insertion is still lacking. This paper focuses on the study of single Li atom insertion into SiNWs with different sizes and axis orientations by using full ab initio calculations. We show that the binding energy of interstitial Li increases as the SiNW diameter grows. The results show that the Li surface diffusion has a much higher chance to occur than the surface to core diffusion, which is consistent with the experimental observation that the Li insertion in SiNWs is layer by layer from surface to inner region. After overcoming a large barrier crossing surface-to-intermediate region, the diffusion toward center has a higher possibility to occur than the inverse process.KEYWORDS Silicon nanowire, anode, lithium ion battery, ab initio simulation, binding energy, diffusion barrier S ilicon nanowires (SiNWs) have attracted much attention for many applications, such as field effect transistors, 1-3 nanosensors, 4-6 and solar cells. 7-9 These applications take advantage of the high crystallinity and/or large surface area of SiNWs. Excitingly, SiNWs have recently been demonstrated as ultrahigh capacity lithium ion battery negative electrodes, 10 which opens up exciting opportunities for energy storage devices. Silicon has the highest known specific charge capacity (4200 mAh/g), which is ∼10 times larger than that of the graphite carbon used in existing technology. 11 However, the 300% volume expansion upon lithium insertion has caused pulverization or mechanical fracture in micrometer particle and bulk Si. The success of using SiNWs lies in their facile strain relaxation without mechanical breaking, efficient electron transport along their long axis, and large lithium ion flux due to their large surface area. Since the important demonstration of SiNWs as lithium ion battery anodes, a variety of Si nanostructure morphologies has been demonstrated to overcome the mechanical breaking issues and perform well as anodes. Si nanostructures shown to exhibit good performance include crystallineamorphous core-shell Si NWs, 12 carbon-amorphous Si core-shell NWs, 13 Si nanotubes, 14 porous Si particles, 15 and an ordered macroporous carbon-Si composite. 16 However, experimental investigation has mainly been focused on electrochemical cycling of Si-Li compounds and phase transitions during Li insertion; all the Si-Li phases were found when the system is rich in Li atoms. 10,17 Although the Si-Li compound can be accurately analyzed, the fundamental interaction between Li and Si atoms and the microscopic dynamic process during Li insertion still remain unknown.On the theoretical side, there have been studies on electronic properties, 18-20 surface effects, 21,22 B or P...

Holography is one of the most attractive approaches for reconstructing optical images, due to its capability of recording both the amplitude and phase information on light scattered from objects. Recently, optical metasurfaces for manipulating the wavefront of light with well-controlled amplitude, phase, and polarization have been utilized to reproduce computer-generated holograms. However, the currently available metasurface holograms have only been designed to achieve limited colors and record either amplitude or phase information. This fact significantly limits the performance of metasurface holograms to reconstruct full-color images with low noise and high quality. Here, we report the design and realization of ultrathin plasmonic metasurface holograms made of subwavelength nanoslits for reconstructing both two- and three-dimensional full-color holographic images. The wavelength-multiplexed metasurface holograms with both amplitude and phase modulations at subwavelength scale can faithfully produce not only three primary colors but also their secondary colors. Our results will advance various holographic applications.

The effects of biaxial and uniaxial strains on electron-phonon coupling and superconductivity in monolayer phosphorene are systematically investigated by first-principles calculations. It is found that the electron-phonon coupling primarily comes from the low frequency optical phonon modes around B g 3 1 , and the biaxial strain gives rise to more a obvious increase in density of states around the Fermi level and phonon softening in the low frequency regime compared to the other two types of uniaxial strain. Therefore, the electron-phonon coupling is more significantly enhanced by the biaxial strain than the uniaxial strains and the superconducting transition temperature T c increases sharply from 3 K to 16 K at the typical doping concentration n 2D = 3.0 × 10 14 cm −2 when the biaxial strain reaches 4.0%.

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) Group-IV tellurides XTe (X=Si, Ge and Sn), with a focus on GeTe. 2D Group-IV tellurides energetically prefer an orthorhombic phase with a hinge-like structure and an in-plane spontaneous polarization. The intrinsic Curie temperature T c of monolayer GeTe is as high as 570 K and can be raised quickly by applying a tensile strain. An out-of-plane electric field can effectively decrease the coercive field for the reversal of polarization, extending its potential for regulating the polarization switching kinetics. Moreover, for bilayer GeTe the ferroelectric phase is still the ground state. Combined with these advantages, 2D GeTe is a promising candidate material for practical integrated ferroelectric applications.

that of the input beam) of amplitude holograms is very low, since much of the incident power is reflected or scattered. Fortunately, the interference intensity pattern can also be translated into phase variations to generate a phase hologram, [6] which is usually preferred because it has a higher diffraction efficiency and can substantially increase the brightness of the reconstructed images. The phase profile in traditional phase-only hologram is controlled by etching different depths into a transparent substrate. Two-level binary phase holograms have been widely used because of their ease of fabrication, but they have a theoretical diffraction efficiency of only 40.5% and suffer the twinimage issue. Multilevel-phase holograms promise high diffraction efficiency and avoid the twin-image problem, but require expensive and complicated grayscale, variable-dose, or multistep lithography processes. [7] Later on, computer-generated holography (CGH) [8,9] was proposed to numerically calculate the phase and amplitude distributions at the hologram plane, which are then encoded into spatial light modulators (SLMs) [10,11] or specific optical elements such as dielectric gratings [12] and dielectric optical antennas. [13] However, since conventional phase holograms rely on light propagation over distances much longer than the wavelength in order to accumulate enough phase variation for effective wavefront shaping, the optical elements for building the phase hologram will have large unitcell size and film thickness due to the limited refractive indices of natural materials. The smallest achievable hologram pixel size usually is as large as several micrometers (several times of the wavelength for visible light), which makes the hologram suffer the highorder diffraction, twin-image issue, and limited resolution. [14] Therefore, compact holograms with subwavelength features in both the thickness and the pixel size are highly demanded, which will not only improve the quality of holographic imaging reconstruction but also reduce the hologram footprint for nanophotonic integration.One significant breakthrough to bring new capabilities in confining light to subwavelength scale is plasmonics, [15,16] where the coupling between electromagnetic waves and collective electron oscillations at metal-dielectric interfaces is studied. Due to the strong interaction with the incident light, plasmonic metamaterials provide more freedom in controlling light at visible and near-infrared frequencies due to their arbitrarily designed permittivity and permeability. [17][18][19] However, the fabrication challenges at nanoscale and the absorption loss of bulk metamaterials at optical frequencies are usually As a revolutionary three-dimensional (3D) optical imaging technique, optical holography has attracted wide attention for its capability of recording both the amplitude and phase information of light scattered from objects. Holograms are designed to transform an incident wave into a desired arbitrary wavefront in the far field, which requires ultim...

We experimentally demonstrate a wide field surface plasmon (SP) assisted super-resolution imaging technique, plasmonic structured illumination microscopy (PSIM), by combining tunable SP interference (SPI) with structured illumination microscopy (SIM). By replacing the laser interference fringes in conventional SIM with SPI patterns, PSIM exhibits greatly enhanced resolving power thanks to the unique properties of SP waves. This PSIM technique is a wide field, surface super-resolution imaging technique with potential applications in the field of high-speed biomedical imaging.

Black phosphorus (BP) has drawn great attention owing to its tunable band gap depending on thickness, high mobility, and large I/ I ratio, which makes BP attractive for using in future two-dimensional electronic and optoelectronic devices. However, its instability under ambient conditions poses challenge to the research and limits its practical applications. In this work, we present a feasible approach to suppress the degradation of BP by sulfur (S) doping. The fabricated S-doped BP few-layer field-effect transistors (FETs) show more stable transistor performance under ambient conditions. After exposing to air for 21 days, the charge-carrier mobility of a representative S-doped BP FETs device decreases from 607 to 470 cm V s (remained as high as 77.4%) under ambient conditions and a large I/ I ratio of ∼10 is still retained. The atomic force microscopy analysis, including surface morphology, thickness, and roughness, also indicates the lower degradation rate of S-doped BP compared to BP. First-principles calculations show that the dopant S atom energetically prefers to chemisorb on the BP surface in a dangling form and the enhanced stability of S-doped BP can be ascribed to the downshift of the conduction band minimum of BP below the redox potential of O/O. Our work suggests that S doping is an effective way to enhance the stability of black phosphorus.