Metal-assisted chemical etching of silicon and the behavior of nanoscale silicon materials as Li-ion battery anodes

This review outlines the developments and recent progress in metal-assisted chemical etching of silicon, summarizing a variety of fundamental and innovative processes and etching methods that form a wide range of nanoscale silicon structures. The use of silicon as an anode for Li-ion batteries is also reviewed, where factors such as film thickness, doping, alloying, and their response to reversible lithiation processes are summarized and discussed with respect to battery cell performance. Recent advances in improving the performance of silicon-based anodes in Li-ion batteries are also discussed. The use of a variety of nanostructured silicon structures formed by many different methods as Li-ion battery anodes is outlined, focusing in particular on the influence of mass loading, core-shell structure, conductive additives, and other parameters. The influence of porosity, dopant type, and doping level on the electrochemical response and cell performance of the silicon anodes are detailed based on recent findings. Perspectives on the future of silicon and related materials, and their compositional and structural modifications for energy storage via several electrochemical mechanisms, are also provided.

Lattice vibrations and Raman scattering in two- dimensional layered materials beyond graphene

We review lattice vibrational modes in atomically thin two-dimensional （2D） layered materials, focusing on 2D materials beyond graphene, such as group VI transition metal dichalcogenides, topological insulator bismuth chalcogenides, and black phosphorus. Although the composition and structure of those materials are remarkably different, they share a common and important feature, i.e., their bulk crystals are stacked via van der Waals interactions between ＂layers＂, while each layer is comprised of one or more atomic planes. First, we review the background of some 2D materials （MX2, M = Mo, W; X = S, Se, Te. Bi2X3, X = Se, Te. Black phosphorus）, including crystalline structures and stacking order. We then review the studies on vibrational modes of layered materials and nanostructures probed by the powerful yet nondestructive Raman spectroscopy technique. Based on studies conducted before 2010, recent investigations using more advanced techniques have pushed the studies of phonon modes in 2D layered materials to the atomically thin regime, down to monolayers. We will classify the recently reported general features into the following categories： phonon confinement effects and electron-phonon coupling, anomalous shifts in high-frequency intralayer vibrational modes and surface effects, reduced dimensionality and lower symmetry, the linear chain model and the substrate effect, stacking orders and interlayer shear modes, polarization dependence, and the resonance effect. Within the seven categories, both intralayer and interlayer vibrational modes will be discussed. The comparison between different materials will be provided as well.

Te-seeded growth of few-quintuple layer Bi2Te3 nanoplates

We report on a Te-seeded epitaxial growth of ultrathin Bi2Te3 nanoplates （down to three quintuple layers （QL）） with large planar sizes （up to tens of micrometers） through vapor transport. Optical contrast has been systematically investigated for the as-grown Bi2Te3 nanoplates on the SiO2/Si substrates, experimentally and computationally. The high and distinct optical contrast provides a fast and convenient method for the thickness determination of few-QL Bi2Te3 nanoplates. By aberration-corrected scanning transmission electron microscopy, a hexagonal crystalline structure has been identified for the Te seeds, which form naturally during the growth process and initiate an epitaxial growth of the rhombohedral- structured Bi2Te3 nanoplates. The epitaxial relationship between Te and Bi2T% is identified to be perfect along both in-plane and out-of-plane directions of the layered nanoplate. Similar growth mechanism might be expected for other bismuth chalcogenide layered materials.

Theoretical study of defect impact on two-dimensional MoS_2

Our theoretical findings demonstrate for the first time a possibility of band-gap engineering of monolayer MoS2 crystals by oxygen and the presence of vacancies. Oxygen atoms are revealed to substitute sulfur ones, forming stable MoS2-xOx ternary compounds, or adsorb on top of the sulfur atoms. The substituting oxygen provides a decrease of the band gap from 1.86 to 1.64 eV and transforms the material from a direct-gap to an indirect-gap semiconductor. The surface adsorbed oxygen atoms decrease the band gap up to 0.98 eV depending on their location tending to the metallic character of the electron energy bands at a high concentration of the adsorbed atoms. Oxygen plasma processing is proposed as an effective technology for such band-gap modifications.

Exciton dynamics in luminescent carbon nanodots： Electron-hole exchange interaction

The electron-hole exchange interaction significantly influences the optical properties of excitons and radiative decay. However, exciton dynamics in luminescent carbon dots （Cdots） is still not clear. In this study, we have developed a simple and efficient one-step strategy to synthesize luminescent Cdots using the pyrolysis of oleylamine. The sp^2 clusters of a few aromatic rings are responsible for the observed blue photoluminescence. The size of these clusters can be tuned by controlling the reaction time, and the energy gap between the π-π＊ states of the sp^2 domains decreases as the sp^2 cluster size increases. More importantly, the strong electron-hole exchange interaction results in the splitting of the exciton states of the sp^2 clusters into the singlet-bright and triplet-dark states with an energy difference ΔE, which decreases with increasing sp^2 cluster size owing to the reduction of the confinement energy and the suppression of the electron-hole exchange interaction.

Valley polarization in stacked MoS2 induced by circularly polarized light

Manipulation of valley pseudospins is crucial for future valleytronics. lhe emerging transition metal dichalcogenides （TMDs） provide new possibilities for exploring the interplay among the quantum degrees of freedom, including real spin, valley pseudospin, and layer pseudospin. For example, spin-valley coupling results in valley-dependent circular dichroism in which electrons with particular spin （up or down） can be selectively excited by chiral optical pumping in monolayer TMDs, whereas in few-layer TMDs, the interlayer hopping further affects the spin-valley coupling. In addition to valley and layer pseudospins, here we propose a new degree of freedom--stacking pseudospin--and demonstrate new phenomena correlated to this new stacking freedom that otherwise require the application of external electrical or magnetic field. We investigated all possible stacking configurations of chemical-vapor-deposition-grown trilayer MoS2 （AAA, ABB, AAB, ABA, and 3R）. Although the AAA, ABA, 3R stackings possess a sole peak with lower degree of valley polarization than that in monolayer samples, the AAB （ABB） stackings exhibit two distinct peaks, one similar to that observed in monolayer MoS2 and findings provide a more future valleytronics. an additional unpolarized complete understanding of peak at lower energy. Our valley quantum control for

A large scale perfect absorber and optical switch based on phase change material(Ge_2Sb_2Te_5) thin film

In the conventional optical coating, the minimum thickness required to achieve anti-reflection should be quarter wavelength(λ/4n, where n is the refractive index). However, it was demonstrated that a lossy thin film with the thickness below quarter wavelength can also sustain the interference effect leading to a high absorption close to unity. In this paper we show that by depositing a phase change material Ge2Sb2Te5(GST) thin film(amorphous) on a metal reflector, unity absorption is attainable. We attribute the high absorption to the interference effect within the GST thin film even though its thickness is less than a quarter wavelength. The wavelength of the absorption peak can be tuned by adjusting the GST film thickness. In addition, the performance of this perfect absorber is insensitive to the incidence angle variation. Relying on the fact that GST is a phase change material, the absorption band can be tuned by inducing the phase transition of GST. The huge difference of the reflectivity between amorphous and crystalline phase leads to a high optical contrast ratio as high as 400 at the specific wavelength, suggesting the potential in the application of optical switch and rewritable data storage.

Self-assembly of single walled carbon nanotubes onto cotton to make conductive yarn

A simple, economical and scalable technique is demonstrated to make conductive yarn. Single wailed carbon nanotubes （SWCNTs） are non-covalently functionalized with dye （Acid Red 91） and dispersed in water; while cotton yarn is treated with poly （ethylene imine）. When the resulting yarn is immersed in the SWCNT dispersion, SWCNTs self-assemble onto the yarn due to electrostatic forces between the functionalJzed nanotubes and yarn. Scanning electron microscopy, transmission electron microscopy and Raman spectroscopy indicate the assembly of carbon nanotubes. The SWCNT functionalized yarn exhibits reasonable electrical conduction behaviour and are then used to make chemiresistors. The electrical resistance of the chemiresistors used as sensors increases on exposure to ammonia gas, which can be explained in terms of electron transfer between gas molecules and SWCNTs.