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 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.     

碳纳米管包裹的硅纳米线复合结构的热稳定性研究

采用经典分子动力学方法模拟一定直径[111]晶向的硅纳米线填充不同扶手椅型单壁碳纳米管复合结构的加热过程, 通过可视化和能量分析的方法判断复合结构中硅纳米线和碳纳米管的热稳定性. 通过讨论碳纳米管的空间限制作用和分子间相互作用力的关系, 对碳纳米管和硅纳米线的热稳定性变化进行初步解释. 研究发现碳纳米管中硅纳米线的热稳定性和碳纳米管的直径关系密切: 当管径较小时, 硅纳米线的热稳定性有所提高, 当管径增大到一定大小时, 硅纳米线的热稳定性会突然显著地下降, 直到硅纳米线与管壁不存在分子间相互作用力, 硅纳米线的热稳定性才会恢复. 而硅纳米线填充到碳纳米管中对碳纳米管的热稳定性有着明显的降低作用.     

Surface effects on the thermal conductivity of silicon nanowires

Thermal transport in silicon nanowires (SiNWs) has recently attracted considerable attention due to their potential applications in energy harvesting and generation and thermal management. The adjustment of the thermal conductivity of SiNWs through surface effects is a topic worthy of focus. In this paper, we briefly review the recent progress made in this field through theoretical calculations and experiments. We come to the conclusion that surface engineering methods are feasible and effective methods for adjusting nanoscale thermal transport and may foster further advancements in this field.     

Sulfide-assisted growth of silicon nano-wires by thermal evaporation of sulfur powders

Silicon nanowires (SiNWs) with a diameter of 20 nm were synthesized by the thermal evaporation of sulfur powders on silicon wafers. The source of the SiNWs came from the silicon substrates. It is considered that the generated SiS compound assisted the formation of SiNWs. Finally, the Raman shift of SiNWs was discussed.     

碳纳米管电缆式复合材料的热导率

通过非平衡分子动力学方法, 对单壁碳管填充金纳米线的碳纳米管电缆式复合材料开展热导率的模拟分析. 采用Tersoff势函数描述碳-碳原子间的相互作用, Lennard-Jones长程作用势描述碳-金原子间的相互作用, 嵌入原子势函数描述金-金原子间相互作用. 研究结果表明: 相同尺寸下, 金纳米线的电子热导率相较于空碳管以及电缆式复合材料的声子热导率小很多, 对复合材料总热导率的贡献可以忽略; 由于管内金纳米线的存在, 其与碳管的相互作用使得碳管碳原子倾向于沿着轴向振动, 声子间U散射随之减少, 声子平均自由程增加, 导致复合材料热导率明显大于空碳管, 在100–500 K温度范围内高出约20%–45%, 但增大幅度随温升呈降低趋势; 复合材料热导率随着管长增加而增大, 变化趋势和空碳管相似, 但其增长幅度更大; 复合材料和空碳管的热导率随管径增大而减小, 且变化幅度基本一致. 关键词： 碳纳米管 纳米线 电缆式复合材料 导热     

Growth of single-walled carbon nanotubes on silicon nanowires

Single-walled carbon nanotubes (SWNTs) have been grown on silicon nanowires (SiNWs) by ethanol chemical vapor deposition (CVD) with Co catalysts. We have found that a surface SiO_x layer of SiNWs is necessary for the formation of active Co catalysts. In fact, the yield of the SWNT/SiNW heterojunctions gradually decreases as the thickness of the surface SiO_x layer decreases. Since thin SiNWs are transparent to an electron beam, the Co nanoparticles on SiNWs can be easily observed as well as SWNTs by TEM. Therefore, the relationship between the diameters of each SWNT and its catalyst nanoparticle has been investigated. The diameters of SWNTs are equal to or slightly smaller than those of the catalyst nanoparticles.     

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.     

Au5Si2/Si Heterojunction Nanowires Formed by Combining SiO Evaporation with Vapour--Liquid--Solid Mechanism

Crystalline AusSi2/Si heterojunetion nanowires （AusSi2/SiNWs） are obtained by thermal evaporating SiO pow- ders on thick gold-coated silicon substrates in a low vacuum system. Structure analysis of the produced AusSh/Si heterojunetions is performed by employing a transmission electron microscope （TEM） and a selected area electric diffraetometer. The chemical compositions axe studied by a energy-dispersive x-ray spectroscope attached to the TEM. A two-step growth model is proposed to describe the formation of the AusSi2/SiNWs. During the first step, crystalline SiNWs are formed via a growth mechanism combining the oxide-assisted growth process with the vapour-liquid-solid model at relatively high temperature. In the second step, the temperature decreases and one segment of the preformed SiNWs reacts with the remnant Au to form single crystalline AusSi2 nanowires by a solid-liquid-solid process. The present work should be useful for the future synthesis and research of high-quality gold silicide nanowires and microelectronic devices based on the nanowires.     

硅纳米线阵列的光学特性

在常温常压条件下,采用改进的金属催化化学腐蚀方法在n型单晶硅片(100)上制备了大面积垂直于硅衬底、直径均匀、排列整齐的硅纳米线阵列。分析了样品的表面形貌和反射谱,纳米线直径为10~50 nm。在腐蚀时间分别为15,30,60 m in时,纳米线长度分别为9,17,34μm。样品的减反射性能优异,在300~1 000nm波段,得到了2.4%的反射率。初步分析了纳米线阵列的减反射机制和不同腐蚀时间样品的反射率差异。     

Heat conduction in extended X-junctions of single-walled carbon nanotubes

Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which are responsible for the larger temperature gradient and reduction in thermal conductivity when compared to standard X-junction. Quantum corrected results show that the longitudinal thermal resistance of the X-junction and EX-junction decreases monotonically with increasing temperature which makes the longitudinal thermal conductivity of the tube with junction less sensitive to temperature above 400 K comparing with the individual pristine tube. The origin of the significant decrease of thermal conductivity of EX-junction is discussed through phonon spectra analysis.     

碳纳米管中点缺陷对热导率影响的正交试验模拟分析

在碳纳米管的制备过程中, 各种点缺陷不可避免地存在于其晶格结构中, 对于碳管的热输运性质造成不可忽视的影响. 使用非平衡分子动力学方法, 选用反应经验键序势能, 模拟计算含有缺陷的碳纳米管的热导率. 尝试采用正交试验方法设计算例, 不但减少了计算量, 并且利于分析缺陷类型、 管长和管径三种结构因素对缺陷造成的热导率下降影响的主次和趋势. 重点研究了掺杂、 吸附和空位三类点缺陷的影响, 与无缺陷完整碳纳米管进行比较, 开展缺陷效应分析, 并进一步考察了环境温度等因素的影响. 模拟结果表明, 相对完整无缺陷碳管, 含有点缺陷的碳管热导率显着下降; 在有缺陷存在的情况下, 缺陷的类型对碳管热导率的影响最大, 管径次之, 管长影响相对最小; 缺陷类型对热导率影响力从大到小依次为: 空位 > 掺杂 > 吸附; 不同环境温度下, 点缺陷对碳管热导率的影响不尽相同.     

Si纳米线的固-液-固可控生长及其形成机理分析

以Au膜作为金属催化剂,直接从n-(111)Si单晶衬底上制备了直径为30—60nm和长度从几微米到几十微米的高质量Si纳米线.实验研究了Au膜层厚、退火温度、N2气流量和生长时间对Si纳米线形成的影响.结果表明,通过合理选择和优化组合上述各种工艺条件,可以实现直径、长度、形状和取向可控的纳米线生长.基于固-液-固生长机理,定性阐述了Si纳米线的形成过程.     

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.     

Fabrication and field emission properties of multi-walled carbon nanotube/silicon nanowire array

Silicon nanowire (SiNW) arrays were fabricated on silicon wafers by the metal-assisted chemical etching method. Varied average diameters of SiNW arrays were realized through further treatment in a mixed agent of HF and HNO₃ of certain concentrations. After the treatment, there were more than 93% SiNWs with diameters smaller than 100 nm. The tip of each SiNW was subsequently wrapped with multi-walled carbon nanotubes (MWCNTs) with chemical vapor deposition method. The as-fabricated MWCNT/SiNW arrays were fabricated into electric field emitters, with turn-on field of 2.0 V/μm (current density: 10 μA/cm²), much lower than that of SiNW array (5.0 V/μm). The turn-on electric field of MWCNT/SiNW array decreased with the decreasing of the average diameter of SiNWs, indicating the performance of the field emission is relative to the morphology of SiNWs. As the SiNW array is uniform in height and easy to fabricate, the MWCNT/SiNW array shows potential applications in flat electric display.     

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.     

Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.     

Phonon heat transport in silicon nanowires

Thermal conductivity of silicon and porous silicon nanowires based on the equation of phonon radiative transport is theoretically evaluated. The thermal conductivities of silicon nanowires with square cross-sections are found to match molecular dynamics simulation results reasonably well. It is shown that the results of meso-porous silicon nanowires are about two orders of magnitude lower than that of silicon nanowires in a wide range of temperature (50 K-300 K). Received 24 April 2001 and Received in final form 23 December 2001     

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.     

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.