An analysis on synthesizing large scales of one-dimensional silicon nano-structures by simple evaporation of sulfur-contained powders

The growth mechanism for synthesizing large scales of one-dimensional silicon nano-structures (silicon nano-wires (SiNWs) or silicon oxide nano-wires (SiO₂-NWs)) by a simple evaporation of sulfur-contained powders on silicon wafer is discussed. A novel sulfide-assisted mechanism referring to oxygen-assisted mechanism is proposed. Amongst this simple method, sulfide or pure sulfur can both assist the formation of SiNWs. The growth is fast and some SiNWs are easily oxidized to be amorphous structure of SiO₂-NWs under the low-vacuum system. The simple method suggests a useful route to achieve plenty of one-dimensional silicon nano-structures for further research.     

Selective formation of silicon nanowires on pre-patterned substrates

In this paper, the selective growth of silicon nanowires (SiNWs) was studied. With the aid of photolithography, the vertically aligned silicon nanowires were selectively formed on the patterned substrates via an electroless metal deposition (EMD) method under normal conditions (room temperature, 1 atm). Low-pressure chemical vapor deposition (LPCVD) silicon nitride was used as the masking layer for SiNWs preparation. The scanning electron microscope was used to examine the etching results. Both the patterned and the unpatterned silicon substrates were used for study. The results indicated that the growth rates of the SiNWs upon the patterned and the unpatterned substrates are different. For the patterned substrates, the growth rate of SiNWs is dependent upon the pattern shape. The influence of length-to-width ratio for the rectangular-shaped patterns was studied. It is concluded that by designing the proper length-to-width ratio, the nanowires with different lengths can be fabricated simultaneously on the same substrate.     

ON THE STUDY OF SILICON NANO-WIRES SELF-ASSEMBLED AS PARTICLES

Two different types of Silicon nano-wires (SiNWs) have been observed by scanning and transmission electron microscopy. One are of free standing SiNWs deposited uniformly on the surface of silicon substrates, and the other are self-assembled into special shaped particles. These SiNWs were synthesized by thermal evaporation of SiO amorphous powders without any metal catalysts in the temperature range of 900-1250℃. Growth history reveals that the self-assembled SiNWs are formed by original nucleation from the surface of amorphous SiO_{x particle matrices through phase separation and silicon precipitation followed by further growth through oxide-assisted vapor-solid reactions. The above results provide a solid experimental support for the oxide-assisted growth model of SiNWs.}     

Microstructural characterization of Li insertion in individual silicon nanowires

The lithiation and delithiation process of silicon nanowire arrays (SiNWs) on silicon substrates has been studied with high-resolution electron microscopy. The composition of lithiated SiNWs was revealed, consisting of the unreacted crystalline silicon core and the reacted amorphous Li–Si shell. In particular, the Li–Si shell was comprised of a mixture of amorphous silicon oxide and crystalline silicon, leading to hindrance during Li–Si alloying/dealloying upon cycling.     

Silicon nano-wires fabricated by thermal evaporation of silicon wafer

Thin silicon nano-wires (SiNWs) with a diameter of 10–20 nm were fabricated by a simple thermal evaporation of silicon wafer at 1523 K. The gold produced by an electrochemical method was covered on the wafer surface as catalyst. It was found that the SiNWs are amorphous and its Raman peak shifted down maybe due to the effect of laser heating and quantum confinement. Finally, a temperature gradient growth model is suggested to explain the growth direction of SiNWs.     

Effects of silver and gold catalytic activities on the structural and optical properties of silicon nanowires

The metal-assisted chemical etching of silicon in an aqueous solution of hydrofluoric acid and hydrogen peroxide is established for the fabrication of large area, uniform silicon nanowire (SiNW) arrays. In this study, silver (Ag) and gold (Au) are considered as catalysts and the effect of different catalysts with various thicknesses on the structural and optical properties of the fabricated SiNWs is investigated. The morphology of deposited catalysts on the silicon wafer is characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). It is shown that the morphology of the fabricated silicon nanostructures remarkably depends upon the catalyst layer thickness, and the catalyst etching time directly affects the structural and optical properties of the synthesized SiNWs. FESEM images show a linear increment of the nanowire length versus time, whereas the etching rate for the Au-etched SiNWs was lower than the Ag-etched ones. Strong light scattering in SiNWs caused the total reflection to decrease in the range of visible light, and this decrement was higher for the Ag-etched SiNW sample, with a longer length than the Au-etched one. A broadband visible photoluminescence (PL) with different peak positions is observed for the Au- and Ag-etched samples. The synthesized optically active SiNWs can be considered as a promising candidate for a new generation of nano-scale opto-electronic devices.     

Silicon nano-wires fabricated by a novel thermal evaporation of zinc sulfide

Silicon nano-wires (SiNWs) with diameter of 30 nm and length of tens of micrometers on silicon wafers were synthesized by a novel thermal evaporation of zinc sulfide. After thermal evaporation at 1080°C for 1 h, crystalline SiNWs were produced. It was found that the tip of SiNWs contained sulfur, while the other places of SiNWs did not. It is considered that the decomposition of SiS resulted in the formation of SiNWs. On the basis of the facts, a sulfide-assisted growth model of SiNWs was suggested.     

Recent Research on One-Dimensional Silicon-Based Semiconductor Nanomaterials: Synthesis,Structures, Properties and Applications

The field of silicon nanowires (SiNWs) and silicon-based 1D nanostructured heterostructures represent one of the most important research subjects within the nanomaterials family. A series of synthesis approaches of SiNWs and silicon-based 1D nanostructured heterostructures have been developed, and have garnered the greatest attention in the past decades for a variety of applications. This article provides an overview on recent research on the synthesis, properties and applications of SiNWs, silicon nanotubes (SiNTs) and complex silicon-based 1D nanostructures.     

Formation of Si nanowires by the electrochemical reduction of porous Ni/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) were prepared by the electrochemical reduction of solid Ni/SiO₂ blocks in molten CaCl₂ at 1173 K. The SiNWs have diameter distributions ranging from 80 to 350 nm, and the nickel–silicon droplets are found on the tips of the nanowires. The growth mechanism of SiNWs was investigated, which confirmed that the nano-sized nickel–silicon droplets formed at the Ni/SiO₂/CaCl₂ three-phase interline. The droplets lead to the oriented growth of SiNWs. Formation of nano-sized nickel–silicon droplets suggests that this method could be a potential way to produce nano-sized metal silicides.     

Coherent anti-Stokes Raman scattering in silicon nanowire ensembles

In this letter, we, for the first time, report on coherent anti-Stokes Raman scattering (CARS) spectroscopy of an ensemble of silicon nanowires (SiNWs) formed by wet chemical etching of crystalline silicon with a mask of silver nanoparticles. The fabricated SiNWs have diameter ranged from 30 to 200 nm and demonstrate both visible and infrared photolumine cence (PL) and spontaneous Raman signal, with their intensities depending on presence of silver nanoparticles in SiNWs. The efficiency of CARS in SiNW ensembles is found to be significantly higher than that in crystalline silicon. The results of CARS and PL measurements are explained in terms of resonant excitation of the electron states attributed to silicon nanoparticles.     

Effects of vacancy defects and axial strain on thermal conductivity of silicon nanowires: A reverse nonequilibrium molecular dynamics simulation

Thermal conductivity of silicon nanowires (SiNWs) is evaluated using the reverse nonequilibrium molecular dynamics simulation. The Stillinger–Weber (SW) and Tersoff interatomic potentials are employed to simulate thermal conductivity of SiNWs. In this work, the influence of random vacancy defects, axial strain, temperature and length on thermal conductivity and effective mean free path of SiNWs is investigated. It is found that by raising the percent of random vacancy defects, thermal conductivity of SiNWs decreases linearly for the results obtained form SW potential and nonlinearly for those obtained from Tersoff interatomic potential. Dependence of the thermal conductivity on axial strain is also studied. Results show that thermal conductivity increases as compressive strain increases and decreases as tensile strain increases. Influence of temperature is also predicted. It is found that the thermal conductivity of SiNWs decreases with increasing the mean temperature. Most of the simulations are performed for 4 UC×4 UC×40 UC silicon nanowires using ssp boundary condition.     

The Influence of B<Subscript>2</Subscript>H<Subscript>6</Subscript> on the Growth of Silicon Nanowire

The un-doped and boron-doped silicon nanowires (SiNWs) were grown via vapor–liquid–solid (VLS) mechanism by low pressure chemical deposition (LPCVD). The diameters of un-doped and boron-doped SiNWs varied from 18.5 to 75.3 nm and 26.6 to 66.1 nm, respectively. The critical growth temperature of boron-doped SiNWs is 10°C lower than that of un-doped ones and the diameters of the boron-doped SiNWs is always larger than that of the un-doped ones under different growth temperatures. This is because that the introduction of diborane enhanced the dissociation of SiH₄ which determines the growth process of SiNW. A growth process of silicon nanowire is proposed to describe the influence of B₂H₆.     

Investigation of growth behavior and properties of Si nanowires grown at various supply times of Ar gas current

Silicon nanowires (SiNWs) were synthesized on gold-coated silicon substrates by a very high frequency plasma enhanced chemical vapor deposition technique. The influence of supply time of Ar gas current on morphology, microstructure and optical properties of the SiNWs was investigated by means of field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and photoluminescence. The lengths and density of the SiNWs decrease with an increase in the supply time of flowing Ar gas. The results revealed that the morphologies of the SiNWs strongly depended on the time of flowing Ar gas.     

Preparation and surface modification of silicon nanowires under normal conditions

Preparation and surface modification of silicon nanowires (SiNWs) grown by the metal catalyzed solution method under normal conditions (room temperature, 1 atm) had been studied in this paper. Firstly, SiNWs using a simple solution method via electroless metal deposition (EMD) of silver under room temperature, standard pressure had been prepared. The influence of the growth parameters such as solution concentration, etching time on the SiNWs formation had been studied. Secondly, the surface modification of SiNWs with platinum and copper had been investigated. The results indicated that the SiNWs modified with Pt and Cu showed different surface morphologies. Pt modification on SiNWs presented in the form of nanoparticles, whereas Cu modification in the form of membrane. Therefore, the Pt modified SiNWs have more vast surface-to-bulk ratio than the unmodified ones, and SiNWs modified with copper nanoparticles will lead to the smaller surface-to-bulk ratio. So the platinum-modified SiNWs have a promising application in sensors’ field.     

Oxide-assisted growth of silicon nanowires by carbothermal evaporation

Silicon nanowires (SiNWs) have successfully been synthesized by carbothermal evaporation method. By ramping-up the furnace system at 20 °C min⁻¹ to 1100 °C for 6 h, the vertically aligned coexist with crooked SiNWs were achieved on the silicon substrate located at 12 cm from source material. The processing parameters such as temperature, heating rate, duration, substrate position and location are very important to produce SiNWs. Morphology and chemical composition of deposited products were investigated by field-emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray analysis (EDX). The existence of small sphere silicon oxide capped nanowires suggested that the formation of SiNWs was governed by oxide-assisted growth (OAG) mechanism.     

碱液修饰对硅纳米线阵列电池光谱响应的影响

采用金属催化化学腐蚀法在p型(100)硅基底上制备了硅纳米阵列,然后用碱溶液对纳米线阵列进行修饰。分别研究了碱液修饰对硅纳米线阵列形貌、光电性质的影响。研究表明: 与绒面及纳米线阵列相比,碱修饰30 s硅纳米线阵列的表面分散均匀,反射率降低;光谱响应度显著提高,并且出现最大量子效率对应波长红移现象。最后,详细讨论了碱液修饰硅纳米线阵列电池对光谱响应的影响机制。     

Impurity doping in silicon nanowires synthesized by laser ablation

Boron (B) or phosphorus (P) doped silicon nanowires (SiNWs) were synthesized by laser ablation. Local vibrational modes of B were observed in B-doped SiNWs by micro-Raman scattering measurements at room temperature. Fano broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the Si optical phonon peak for B-doped SiNWs. An electron spin resonance signal due to conduction electrons was observed only for P-doped SiNWs. These results prove that B and P atoms were doped in substitutional sites of the crystalline Si core of SiNWs during laser ablation and electrically activated in the sites.     

The effect of differently sized Ag catalysts on the fabrication of a silicon nanowire array using Ag-assisted electroless etching

A simple and low cost method to generate single-crystalline, well-aligned silicon nanowires (SiNWs) of large area, using Ag-assisted electroless etching, is presented and the effect of differently sized Ag catalysts on the fabrication of SiNWs arrays is investigated. The experimental results show that the size of the Ag catalysts can be controlled by adjusting the pre-deposition time in the AgNO₃/HF solution. The optimum pre-deposition time for the fabrication of a SiNWs array is 3 min (about 162.04 ± 38.53 nm Ag catalyst size). Ag catalysts with smaller sizes were formed in a shorter pre-deposition time (0.5 min), which induced the formation of silicon holes. In contrast, a large amount of Ag dendrites were formed on the silicon substrate, after a longer pre-deposition time (4 min). The existence of these Ag dendrites is disadvantageous to the fabrication of SiNWs. Therefore, a proper pre-deposition time for the Ag catalyst is beneficial to the formation of SiNWs.SiNWs were synthesized in the H₂O₂/HF solution system for different periods of time, using Ag-assisted electroless etching (pre-deposition of the Ag catalyst for 3 min). The length of the SiNWs increases linearly with immersion time. From TEM, SAED and HRTEM analysis, the axial orientation of the SiNWs is identified to be along the [001] direction, which is the same as that of the initial Si wafer. The use of HF may induce Si–H_x bonds onto the SiNW array surface. Overall, the Ag-assisted electroless etching technique has advantages, such as low temperature, operation without the need for high energy and the lack of a need for catalysts or dopants.     

The effect of the electron-phonon coupling on the thermal conductivity of silicon nanowires

The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire.     

硅纳米线拉曼光谱的研究

声子限制效应会引起本征硅纳米线拉曼光谱红移及不对称宽化,但研究发现其并非引起硅纳米线拉曼光谱改变的主要因素。研究表明,由于在拉曼光谱测量中,通常使用的入射激光功率都在5 mW以上,激光加热会导致很高的局部温度,从而引起拉曼光谱大幅度红移并对称宽化,这是硅纳米线拉曼光谱红移的主要影响因素。另外,激光功率很高时,由激光激发的载流子会与声子发生Fano型干涉,从而使硅纳米线拉曼光谱发生Fano型红移和不对称宽化。除此之外,对小直径本征硅纳米线,声子限制效应导致波矢选择定弛则弛豫,使不在布里渊区中心的声子也可以参与拉曼散射,因而其拉曼光谱中除常见的几个拉曼峰外还会出现新拉曼峰。