Si Nanosheets Research Articles

Si/Ti2O3/Reduced Graphene Oxide Nanocomposite Anodes for Lithium-Ion Batteries with Highly Enhanced Cyclic Stability.

Silicon (Si) has attracted tremendous attention as a high-capacity anode material for next generation Li-ion batteries (LIBs); unfortunately, it suffers from poor cyclic stability due to excessive volume expansion and reduced electrical conductivity after repeated cycles. To circumvent these issues, we propose that Si can be complexed with electrically conductive Ti2O3 to significantly enhance the reversible capacity and cyclic stability of Si-based anodes. We prepared a ternary nanocomposite of Si/Ti2O3/reduced graphene oxide (rGO) using mechanical blending and subsequent thermal reduction of the Si, TiO2 nanoparticles, and rGO nanosheets. As a result, the obtained ternary nanocomposite exhibited a specific capacity of 985 mAh/g and a Coulombic efficiency of 98.4% after 100 cycles at a current density of 100 mA/g. Furthermore, these ternary nanocomposite anodes exhibited outstanding rate capability characteristics, even with an increased current density of 10 A/g. This excellent electrochemical performance can be ascribed to the improved electron and ion transport provided by the Ti2O3 phase within the Si domains and the structurally reinforced conductive framework comprised of the rGO nanosheets. Therefore, it is expected that our approach can also be applied to other anode materials to enable large reversible capacity, excellent cyclic stability, and good rate capability for high-performance LIBs.

Silicon growth at the two-dimensional limit on Ag(111).

Having fueled the microelectronics industry for over 50 years, silicon is arguably the most studied and influential semiconductor. With the recent emergence of two-dimensional (2D) materials (e.g., graphene, MoS2, phosphorene, etc.), it is natural to contemplate the behavior of Si in the 2D limit. Guided by atomic-scale studies utilizing ultrahigh vacuum (UHV), scanning tunneling microscopy (STM), and spectroscopy (STS), we have investigated the 2D limits of Si growth on Ag(111). In contrast to previous reports of a distinct sp(2)-bonded silicene allotrope, we observe the evolution of apparent surface alloys (ordered 2D silicon-Ag surface phases), which culminate in the precipitation of crystalline, sp(3)-bonded Si(111) nanosheets. These nanosheets are capped with a √3 honeycomb phase that is isostructural to a √3 honeycomb-chained-trimer (HCT) reconstruction of Ag on Si(111). Further investigations reveal evidence for silicon intermixing with the Ag(111) substrate followed by surface precipitation of crystalline, sp(3)-bonded silicon nanosheets. These conclusions are corroborated by ex situ atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Even at the 2D limit, scanning tunneling spectroscopy shows that the sp(3)-bonded silicon nanosheets exhibit semiconducting electronic properties.

Two-dimensionally grown single-crystal silicon nanosheets with tunable visible-light emissions.

Since the discovery of graphene, growth of two-dimensional (2D) nanomaterials has greatly attracted attention. However, spontaneous growth of atomic two-dimensional (2D) materials is limitedly permitted for several layered-structure crystals, such as graphene, MoS2, and h-BN, and otherwise it is notoriously difficult. Here we report the gas-phase 2D growth of silicon (Si), that is cubic in symmetry, via dendritic growth and an interdendritic filling mechanism and to form Si nanosheets (SiNSs) of 1 to 13 nm in thickness. Thin SiNSs show strong thickness-dependent photoluminescence in visible range including red, green, and blue (RGB) emissions with the associated band gap energies ranging from 1.6 to 3.2 eV; these emission energies were greater than those from Si quantum dots (SiQDs) of the similar sizes. We also demonstrated that electrically driven white, as well as blue, emission in a conventional organic light-emitting diode (OLED) geometry with the SiNS assembly as the active emitting layers. Tunable light emissions in visible range in our observations suggest practical implications for novel 2D Si nanophotonics.

Cubic lattice nanosheets: thickness-driven light emission.

Silicon has a diamond-like cubic crystal lattice for which two-dimensional (2D) nanometer thickness nanosheet crystallization appears not to be trivial. However, in this issue of ACS Nano, the group led by Heon-Jin Choi demonstrates the gas-phase dendritic growth of Si nanosheets, only 1 to 13 nm thick. Moreover, such nanosheets display strong thickness-dependent photoluminescence in a visible range with red, green, and blue emission each documented.

Direct Measurements of Surface Scattering in Si Nanosheets Using a Microscale Phonon Spectrometer: Implications for Casimir-Limit Predicted by Ziman Theory

Thermal transport in nanostructures is strongly affected by phonon-surface interactions, which are expected to depend on the phonon's wavelength and the surface roughness. Here we fabricate silicon nanosheets, measure their surface roughness (∼ 1 nm) using atomic force microscopy (AFM), and assess the phonon scattering rate in the sheets with a novel technique: a microscale phonon spectrometer. The spectrometer employs superconducting tunnel junctions (STJs) to produce and detect controllable nonthermal distributions of phonons from ∼ 90 to ∼ 870 GHz. This technique offers spectral resolution nearly 10 times better than a thermal conductance measurement. We compare measured phonon transmission rates to rates predicted by a Monte Carlo model of phonon trajectories, assuming that these trajectories are dominated by phonon-surface interactions and using the Ziman theory to predict phonon-surface scattering rates based on surface topology. Whereas theory predicts a diffuse surface scattering probability of less than 40%, our measurements are consistent with a 100% probability. Our nanosheets therefore exhibit the so-called "Casimir limit" at a much lower frequency than expected if the phonon scattering rates follow the Ziman theory for a 1 nm surface roughness. Such a result holds implications for thermal management in nanoscale electronics and the design of nanostructured thermoelectrics.

Scalable synthesis of silicon nanosheets from sand as an anode for Li-ion batteries

The silicon nanostructure is a promising candidate for an anode of Li-ion batteries due to its high theoretical capacity. In this work, we have demonstrated the scalable synthesis of Si nanosheets from natural sand by magnesiothermic reduction, and suggested a new formation mechanism for Si nanosheets. In the suggested mechanism, an Mg₂Si intermediate phase was formed at an early stage of the reduction process, which leads to the two-dimensional Si nanostructure. The synthesized Si nanosheets have a leaf-like sheet morphology ranging from several ten to several hundred nanometers, and show comparable electrochemical properties to the commercial Si nanopowder as an anode for lithium ion batteries. For the improved electrochemical performance, Si nanosheets are encapsulated with reduced graphene oxide (RGO), and the RGO-encapsulated Si nanosheet electrode exhibits high-reversible capacity and excellent rate capability.

Two‐Dimensional Si Nanosheets with Local Hexagonal Structure on a MoS2 Surface

The structural and electronic properties of a Si nanosheet (NS) grown onto a MoS2 substrate by means of molecular beam epitaxy are assessed. Epitaxially grown Si is shown to adapt to the trigonal prismatic surface lattice of MoS2 by forming two-dimensional nanodomains. The Si layer structure is distinguished from the underlying MoS2 surface structure. The local electronic properties of the Si nanosheet are dictated by the atomistic arrangement of the layer and unlike the MoS2 hosting substrate they are qualified by a gap-less density of states.

Getting through the Nature of Silicene: An sp2–sp3 Two-Dimensional Silicon Nanosheet

By combining experimental techniques with ab initio density functional theory calculations, we describe the Si/Ag(111) 2D systems in terms of a sp2–sp3 form of silicon characterized by a vertically distorted honeycomb lattice provided by the constraint imposed by the substrate. The Raman spectrum reflects the multihybridized nature of the 2D Si nanosheets (NSs) resulting from a buckling-induced distortion of a purely sp2 hybridized structure. We show that vibrational and electronic properties of 2D Si-NSs are tightly linked to the buckling arrangement.

A computational study of the insertion of Li, Na, and Mg atoms into Si(111) nanosheets

Based on first principles calculations, we study the interaction of metal atoms (Li, Na, and Mg) with Si(111) nanosheets of different thicknesses. We show that the chemistry of the interactions is sensitive to both the nanosheet thickness and the dopant–surface distance. Both Li and Na atoms adsorb strongly on the nanosheet surface, accompanied by large charge transfers (∼0.9e) from the metal atoms to surrounding atoms. In contrast, Mg atoms have weak adsorption. Compared to bulk Si, we show that nanosheet Si is expected to improve the charge/discharge rate of Li/Na/Mg-ion batteries. Nevertheless, due to large insertion barriers (up to the prohibitive ∼2.1 and ∼3.1eV for Mg and Na, respectively) and significant energy differences between surface and sub-surface sites (∼1.0 and ∼1.9eV for Mg and Na, respectively), the theoretical capacities of Si for both Na-ion and Mg-ion batteries cannot be achieved at realistic charge/discharge rates.

Enhanced Li adsorption and diffusion in silicon nanosheets based on first principles calculations

First-principles density functional theory calculations are employed to investigate novel ultrathin silicon nanosheets (SiNSs) for their potential application as the anode material for Li-ion batteries. We find that Li has a higher tendency to bind on the surface of SiNS rather than penetrating through inside. The binding energies of Li show a strong dependence on the thickness of the nanosheets. The results suggest that insertion/deinsertion of Li can be controlled by using nanosheets of different thickness. More importantly, we show that there is a large increase of diffusivity in Si nanosheets as compared with the bulk case. In addition, Li diffusion shows strong dependence on the chemical functionalization of SiNSs, in which the diffusion rate is the fastest on H passivated surface as compared with the halogen passivated surfaces. Our results suggest that SiNSs are potential materials for Li-ion battery applications.

Stability and electronic properties of ultrathin films of silicon and germanium

Recent studies have examined the possibility of growing honeycomb silicene and germanene, the silicon and germanium analogues of graphene. Here we use first-principles calculations to examine the relative stability of a number of other single-layer structures that are derived from prominent surface reconstructions of group-IV semiconductors. We find that Si single-layers with the geometry of the √3 × √3 reconstruction are more stable than honeycomb silicene. Likewise, honeycomb germanene is less stable than germanium ultrathin films with a √3 × √3 or 7 × 7 arrangement. √3 × √3 Si and Ge single layers are metals with strong peaks at the Fermi level, and, in the case of Ge, they bear a magnetic moment. Overall, the results demonstrate that free-standing Si and Ge nano-sheets differ in key structural and electronic properties from graphene, and may thus provide systems with alternative functionalities.

Reconstruction and electronic properties of silicon nanosheets as a function of thickness

We have shown, using density functional theory calculations, that the properties of Si nanosheets change as a function of thickness. While Si(111) oriented nanosheets that are 0.56 nm thick (2-layers) display a novel reconstruction, classified as Si(111)-2 × 2 on both surface layers (T. Morishita, M. J. S. Spencer, S. P. Russo, I. K. Snook and M. Mikami, Chem. Phys. Lett., 2011, 506, 221), nanosheets that are up to a thickness of 1.42 nm show the Si(111)-2 × 1 surface reconstruction, that is seen on the bulk Si(111) surface, on both sides of the nanosheet. For these thicker nanosheets, the relative orientation of the π-chain structure on each surface of the nanosheet can either be the same or different, resulting in unique electronic properties. When the orientation is the same, there is a widening of the band gap, indicating that the interaction between the surface π-chains is not present when they are oriented in different directions. The electronic properties of the nanosheets approach those of the bulk by 1.42 nm thick. The variation in structural and electronic properties of Si nanosheets with different thicknesses, as shown in this study, highlights the novelty of these materials and their significance for applications in electronic device technologies.

Synthesis of Ultrathin Silicon Nanosheets by Using Graphene Oxide as Template

A facile method was developed to synthesize free-standing Si nanosheets by using graphene oxide as a template. The resulting Si nanosheets were well-crystallized and showed high and stable Li ion storage performance compared with Si nanoparticles because of the dimensional feature.

Strain Induced Band Dispersion Engineering in Si Nanosheets

The electronic properties of strained hydrogen-passivated Si nanosheets are investigated using the first-principles density functional theory. Asymmetrical strain has been found to cause a direct-t...

Electron transport properties of Si nanosheets: Transition from direct tunneling to Fowler-Nordheim tunneling

We have characterized the electron transport properties of n-decylamine-functionalized Si nanosheets (NSs) using atomic force microscopy with a conductive cantilever under vacuum conditions at room temperature. Electrons are transported from the cantilever to the substrate through Si NSs. The Si NSs exhibit nonresonant tunneling; the transport mechanisms are based on direct tunneling at low bias voltages and Fowler–Nordheim tunneling at high bias voltages.

Band structure of hydrogenated Si nanosheets and nanotubes

The band structures of fully hydrogenated Si nanosheets and nanotubes are elucidated bythe use of an empirical tight-binding model. The hydrogenated Si sheet is a semiconductorwith an indirect band gap of about 2.2 eV. The symmetries of the wavefunctions allow us toexplain the origin of the gap. We predict that, for certain chiralities, hydrogenated Sinanotubes represent a new type of semiconductor, one with coexisting directand indirect gaps of exactly the same magnitude. This behavior is different fromthat governed by the Hamada rule established for non-hydrogenated carbon andsilicon nanotubes. A comparison to the results of an ab initio calculation is made.

Surface reconstruction of ultrathin silicon nanosheets

Si nanosheets are a promising candidate material for nanoscale devices but a thorough understanding of their surface properties is still lacking. Here we find, using first-principles molecular-dynamics calculations, a new surface structure specific to a double-layer Si nanosheet. The new surface structure, which can be labeled as Si(1 1 1)-2 × 2 in the same fashion as the bulk Si surface, is formed during surface reconstruction at finite temperature. The reconstructed nanosheet surface consists of highly distorted tetrahedra as on the Si(1 1 1)-2 × 1 surface, but its electronic band structure is markedly different from that of Si(1 1 1)-2 × 1. The vibrational properties of the reconstructed Si nanosheet are also found to reflect the distortion in the tetrahedral order.

Synthesis of Si Nanosheets by a Chemical Vapor Deposition Process and Their Blue Emissions

We synthesized free-standing Si nanosheets (NSs) with a thickness of about <2 nm using a chemical vapor deposition process and studied their optical properties. The Si NSs were formed by the formation of frameworks first along six different <110> directions normal to [111], its zone axis, and then by filling the spaces between the frameworks along the <112> directions under high flow rate of processing gas. The Si NSs showed blue emission at 435 nm, and absorbance and photoluminescence (PL) excitation measurements indicate that enhanced direct band transition attributes to the emission. Time-resolved PL measurement, which showed PL emission at 435 nm and a radiative lifetime of 1.346 ns, also indicates the enhanced direct band gap transition in these Si NSs. These outcomes indicate that dimensionality of Si nanostructures may affect the band gap transition and, in turn, the optical properties.

The electronic and structural properties of novel organomodified Si nanosheets

We show that the properties of a new class of functional materials, silicon nanosheets modified with phenyl groups and H atoms, are highly promising for applications such as electronic devices. This novel material retains the sp(3) structure after functionalisation, resulting in a wide (direct) band gap of 1.92 eV.

First‐Principles Simulation on Piezoresistive Properties in Doped Silicon Nanosheets

AbstractWe have simulated the electronic states and piezoresistance coefficients in single‐crystal silicon (Si) nanosheets on the basis of first‐principles calculations of model structures. The carrier conductivities of the hydrogen‐terminated Si nanosheet models with (001) surface orientation have been calculated using band carrier densities and their corresponding effective mass tensors derived from the two‐dimensional (2D) band diagram by our original approach for a small amount of carrier occupation. The [110] uniaxial tensile stress causes band deformation, leading to the redistribution of carriers, and a drastic change of conductivity can be observed for nanosheet models. The p‐type longitudinal and transverse piezoresistance coefficients about the [110] tensile stress swell up as the nanosheet becomes thinner, and we have obtained high piezoresistance coefficients of 343 × 10−11 Pa−1 of πl[110] and − 141 × 10−11 Pa−1 of πt [110] for about 1 nm thickness. It is expected that p‐type ultra‐thin Si(001) nanosheet will be a suitable candidate for nanoscale piezoresistors due to its giant piezoresistivity. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.