Raman scattering by confined optical phonons in Si and Ge nanostructures

A microscopic theory of the Raman scattering based on the local bond-polarizability model is presented and applied to the analysis of phonon confinement in porous silicon and porous germanium, as well as nanowire structures. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green's function and the Born model, including central and non-central interatomic forces. For the porous case, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p--type (001)-oriented crystalline Si wafers in a hydrofluoric acid bath.     

Physical and Electrical Performance of Vapor–Solid Grown ZnO Straight Nanowires

Physical and electrical properties of wurtzitic ZnO straight nanowires grown via a vapor–solid mechanism were investigated. Raman spectrum shows four first-order phonon frequencies and a second-order Raman frequency of the ZnO nanowires. Electrical and photoconductive performance of individual ZnO straight nanowire devices was studied. The results indicate that the nanowires reported here are n-type semi-conductors and UV light sensitive, and a desirable candidate for fabricating UV light nanosensors and other applications.     

Synthesis and Photoluminescence Properties of Porous Silicon Nanowire Arrays

Herein, we prepare vertical and single crystalline porous silicon nanowires (SiNWs) via a two-step metal-assisted electroless etching method. The porosity of the nanowires is restricted by etchant concentration, etching time and doping lever of the silicon wafer. The diffusion of silver ions could lead to the nucleation of silver nanoparticles on the nanowires and open new etching ways. Like porous silicon (PS), these porous nanowires also show excellent photoluminescence (PL) properties. The PL intensity increases with porosity, with an enhancement of about 100 times observed in our condition experiments. A “red-shift” of the PL peak is also found. Further studies prove that the PL spectrum should be decomposed into two elementary PL bands. The peak at 850 nm is the emission of the localized excitation in the nanoporous structure, while the 750-nm peak should be attributed to the surface-oxidized nanostructure. It could be confirmed from the Fourier transform infrared spectroscopy analyses. These porous SiNW arrays may be useful as the nanoscale optoelectronic devices.     

Strain modification in epitaxial 2H-AlN layers on 3C-SiC/Si(111) pseudo-substrates

Optical measurements are used to investigate the crystalline quality and the stress in thin AlN layers; these thin films are grown on cubic silicon carbide layers which are in turn grown on silicon (111) substrates. Different Ge amounts were deposited at the silicon substrate in order to reduce the lattice parameters mismatch between Si and SiC grown layers. The residual stress of the hexagonal AlN layers is derived from the phonon frequency shifts of the E₁(TO) phonon mode. The crystalline quality of AlN films is investigated by considering the intensity of E₁(TO) mode of the 2H-AlN and its full width of the half maximum (FWHM). Ge deposition at low temperature 325 °C, before the carbonization process leads to an improved crystalline quality and a reduced residual stress in the AlN/SiC/Si heterostructures. The best crystalline quality and the lowest stress value are found in the case where 1ML Ge amount was predeposited. The E₁(TO) mode, phonon frequency shifts-down by 3 cm^? 1/GPa with respect to an unstrained layer. The obtained values for the phonon deformation are in reasonable agreement with theoretical calculations.     

Initial Growth of Single-Crystalline Nanowires: From 3D Nucleation to 2D Growth

The initial growth stage of the single-crystalline Sb and Co nanowires with preferential orientation was studied, which were synthesized in porous anodic alumina membranes by the pulsed electrodeposition technique. It was revealed that the initial growth of the nanowires is a three-dimensional nucleation process, and then gradually transforms to two-dimensional growth via progressive nucleation mechanism, which resulting in a structure transition from polycrystalline to single crystalline. The competition among the nuclei inside the nanoscaled-confined channel and the growth kinetics is responsible for the structure transition of the initial grown nanowires.     

Phonon spectra of selenium and gallium dispersed in a porous glass

The Mössbauer investigation has revealed that the transition of selenium from the crystalline state to the vitreous state is accompanied by a shift of the phonon spectrum of ¹¹⁹Sn impurity atoms toward the low-frequency range. A similar shift of the phonon spectrum is observed for vitreous selenium dispersed in a porous glass with a pore diameter of ～70 Å. The “hardening” of the phonon spectrum is observed for ¹¹⁹Sn impurity atoms in gallium dispersed in the porous glass. The possible factors responsible for the increase in the critical temperature of superconductivity of gallium in the porous glass are discussed.     

Sn-induced low-temperature growth of Ge nanowire electrodes with a large lithium storage capacity

We herein present the synthesis of germanium (Ge) nanowires on Au-catalyzed low-temperature substrates using a simple thermal Ge/Sn co-evaporation method. Incorporation of a low-melting point metal (Sn) enables the efficient delivery of Ge vapor to the substrate, even at a source temperature below 600 °C. The as-synthesized nanowires were found to be a core/shell heterostructure, exhibiting a uniform single crystalline Ge sheathed within a thin amorphous germanium suboxide (GeO(x)) layer. Furthermore, these high-density Ge nanowires grown directly on metal current collectors can offer good electrical connection and easy strain relaxation due to huge volume expansion during Li ion insertion/extraction. Therefore, the self-supported Ge nanowire electrodes provided excellent large capacity with little fading upon cycling (a capacity of ～900 mA h g(-1) at 1C rate).     

Laser Ablation Synthesis and Optical Characterization of Silicon Carbide Nanowires

Silicon carbide (SiC) nanowires were synthesized at 900°C by the laser ablation technique. The growth morphology, microstructure, and defects in SiC nanowires were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The Raman scattering study indicated that the Raman peaks corresponding to the TO and LO phonon modes of the SiC nanowires had larger red shifts compared to those of bulk SiC material. The red shift, broadening peak, and the asymmetry of the Raman peak could be explained by the size confinement effect in the radial and growth directions. The growth mechanism of SiC nano-wires was discussed based on the vapor–liquid–solid reaction.     

Raman Study of Glasses of Ba2TiSi2O8 and Ba2TiGe2O8

Raman spectra are reported for fresnoite (Ba₂Ti(Si,Ge)₂O₈ glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba₂TiSi₂O₈ and Ba₂TiGe₂O₈. The Ba₂TiGe₂O₈ glass spectra show correspondence with the Ba₂TiGe₂O₈ crystalline Raman spectra; the v _s(Ge–O–Ge) mode occurs at 518 cm⁻¹ in the glass and at 521 cm⁻¹ in the crystalline material. Five-fold coordinated titanium is the majority species present in the Ba₂TiGe₂O₈ glass as revealed by a strong band at 824 cm⁻¹ in the I glass spectrum. The Ba₂TiSi₂O₈ glass spectra are similar to the Ba₂TiSi₂O₈ crystalline spectrum; the strongest band is found at 836 cm⁻¹ in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba₂TiSi₂O₈ glass has a structure similar to the crystalline phase.     

Porous silicon nanowires

In this mini-review, we summarize recent progress in the synthesis, properties and applications of a new type of one-dimensional nanostructures-single crystalline porous silicon nanowires. The growth of porous silicon nanowires starting from both p- and n-type Si wafers with a variety of dopant concentrations can be achieved through either one-step or two-step reactions. The mechanistic studies indicate the dopant concentration of Si wafers, oxidizer concentration, etching time and temperature can affect the morphology of the as-etched silicon nanowires. The porous silicon nanowires are both optically and electronically active and have been explored for potential applications in diverse areas including photocatalysis, lithium ion batteries, gas sensors and drug delivery.     

Kinetics of Si and Ge nanowires growth through electron beam evaporation

Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360°C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450°C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted.     

Influence of lead concentration on morphology and optical properties of Pb-doped ZnO nanowires

Undoped and Pb-doped ZnO nanowires with different lead concentrations were grown on Si(111) substrates using a thermal evaporation method. Scanning electron microscopy (SEM) results showed that, the undoped ZnO nanowires were well aligned with uniform diameters and lengths. On the other hand, the Pb-doped ZnO nanowires were tapered and not aligned in a unique direction. X-ray diffraction patterns and Raman measurements clearly indicated hexagonal structures for all of the products. In addition, the Raman results demonstrated that the Pb-doped ZnO nanowires had a lower crystalline quality than the undoped ZnO nanowires. Photoluminescence (PL) studies also confirmed the Raman results and showed a lower optical property for the Pb-doped ZnO nanowires compared to the undoped ZnO nanowires. Moreover, the PL results showed a smaller band-gap for the Pb-doped ZnO nanowires compared to the undoped ZnO.     

Characterization and Optical Properties of the Single Crystalline SnS Nanowire Arrays

The SnS nanowire arrays have been successfully synthesized by the template-assisted pulsed electrochemical deposition in the porous anodized aluminum oxide template. The investigation results showed that the as-synthesized nanowires are single crystalline structures and they have a highly preferential orientation. The ordered SnS nanowire arrays are uniform with a diameter of 50 nm and a length up to several tens of micrometers. The synthesized SnS nanowires exhibit strong absorption in visible and near-infrared spectral region and the direct energy gap E _g of SnS nanowires is 1.59 eV.     

An alternative route for the synthesis of silicon nanowires via porous anodic alumina masks

Amorphous Si nanowires have been directly synthesized by a thermal processing of Si substrates. This method involves the deposition of an anodic aluminum oxide mask on a crystalline Si (100) substrate. Fe, Au, and Pt thin films with thicknesses of ca. 30 nm deposited on the anodic aluminum oxide-Si substrates have been used as catalysts. During the thermal treatment of the samples, thin films of the metal catalysts are transformed in small nanoparticles incorporated within the pore structure of the anodic aluminum oxide mask, directly in contact with the Si substrate. These homogeneously distributed metal nanoparticles are responsible for the growth of Si nanowires with regular diameter by a simple heating process at 800°C in an Ar-H₂ atmosphere and without an additional Si source. The synthesized Si nanowires have been characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman.     

Synthesis of 6H-SiC nanowires on bamboo leaves by carbothermal method

6H silicon carbide (SiC) nanowires were fabricated on bamboo leaves infiltrated with tetraethyl orthosilicate (TEOS) by carbothermal method at 1300–1400 °C. The bamboo leaves were the carbon source and template for the growth of SiC. The TEOS was the silicon source. The crystalline structure, morphology, and the distribution of the prepared SiC were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy. The SiC had a hexagonal 6H- structure with diameter of 60 to 160 nm and length up to tens of microns. The yield of SiC grown on the top surface of the bamboo leaf was higher and had a branched structure. The SiC on the bottom surface showed a bamboo-like structure. The nanowires were mainly 6H phase, however cubic 3C-SiC phase was found on the divisions in the branch structure and the nodes in the bamboo-like structure. The difference in density of SiC nanowires between the two surfaces is proposed to be related to the structural and compositional differences of the two surfaces. TEOS was preferred to attach the hydrophilic top surface, which led to larger amount of SiC. Meanwhile, the Al content inside the bottom surface prohibited the growth of the SiC nanowires. The growth mechanism of the SiC nanowires is also discussed.     

Coupling of Semiconductor Nanowires with Neurons and Their Interfacial Structure

We report on the compatibility of various nanowires with hippocampal neurons and the structural study of the neuron–nanowire interface. Si, Ge, SiGe, and GaN nanowires are compatible with hippocampal neurons due to their native oxide, but ZnO nanowires are toxic to neuron due to a release of Zn ion. The interfaces of fixed Si nanowire and hippocampal neuron, cross-sectional samples, were prepared by focused ion beam and observed by transmission electron microscopy. The results showed that the processes of neuron were adhered well on the nanowire without cleft.     

Fabrication of Coaxial Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Heterostructure Nanowires by O<Subscript>2</Subscript> Flow-Induced Bifurcate Reactions

We report on bifurcate reactions on the surface of well-aligned Si_1−x Ge _x nanowires that enable fabrication of two different coaxial heterostructure nanowires. The Si_1−x Ge _x nanowires were grown in a chemical vapor transport process using SiCl₄ gas and Ge powder as a source. After the growth of nanowires, SiCl₄ flow was terminated while O₂ gas flow was introduced under vacuum. On the surface of nanowires was deposited Ge by the vapor from the Ge powder or oxidized into SiO₂ by the O₂ gas. The transition from deposition to oxidation occurred abruptly at 2 torr of O₂ pressure without any intermediate region and enables selectively fabricated Ge/Si_1−x Ge _x or SiO₂/Si_1−x Ge _x coaxial heterostructure nanowires. The rate of deposition and oxidation was dominated by interfacial reaction and diffusion of oxygen through the oxide layer, respectively.     

Facile synthesis and superior ethyl acetate sensing performance of Au decorated ZnO flower-like architectures

Au decorated ZnO flower-like architectures assembled from single crystal nanowires have been successfully synthesized. A facile thermal treatment route was employed, utilizing the composite gel of zinc acetate and polyvinyl pyrrolidone (PVP) as raw materials and was followed by a subsequent Au reduction process. PVP served as a surfactant and played a critical role in the generation of crystalline nanowires as well as for the formation of ZnO flower-like structure. Based on control experiments, the growth mechanism of ZnO flower-like structures was proposed. The diameter of ZnO crystalline nanowires was approximately 50–80 nm and the size of Au particles deposited on the surface of ZnO nanowires was approximately 5 nm. When tested as gas sensing material, the as-prepared Au decorated ZnO flower-like architecture exhibited superior gas sensing performance compared to ethyl acetate in terms of high response (approximately 102 at 100 ppm), short response and recovery times (10 s and 13 s, respectively), and low operating temperature (240 °C). The superior gas sensing performances are mainly attributed to the synergistic effects of ZnO crystalline nanowires and Au nanoparticles, as well as to the flower-like structure.     

Synthesis of Indium Nanowires by Galvanic Displacement and Their Optical Properties

Single crystalline indium nanowires were prepared on Zn substrate which had been treated in concentrated sulphuric acid by galvanic displacement in the 0.002 mol L⁻¹ In₂(SO₄)₃-0.002 mol L⁻¹ SeO₂-0.02 mol L⁻¹ SDS-0.01 mol L⁻¹ citric acid aqueous solution. The typical diameter of indium nanowires is 30 nm and most of the nanowires are over 30 μm in length. XRD, HRTEM, SAED and structural simulation clearly demonstrate that indium nanowires are single-crystalline with the tetragonal structure, the growth direction of the nanowires is along [100] facet. The UV-Vis absorption spectra showed that indium nanowires display typical transverse resonance of SPR properties. The surfactant (SDS) and the pretreatment of Zn substrate play an important role in the growth process. The mechanism of indium nanowires growth is the synergic effect of treated Zn substrate (hard template) and SDS (soft template).     

Thermal conductivity in porous silicon nanowire arrays

The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.