金属辅助化学刻蚀法制备硅纳米线的研究进展

硅纳米线(Si NWs)由于具有独特的一维结构、热电导率、光电性质、电化学性能等特点,被广泛应用于热电与传感器件、光电子元器件、太阳能电池、锂离子电池等领域。金属辅助化学刻蚀法(MACE)是制备Si NWs的常用方法之一,具有操作简便、设备简单、成本低廉和高效等优点,可大规模商业化应用,因而近年来被广泛研究。金属辅助化学刻蚀制备硅纳米线的过程可以分为两步:首先在洁净的硅衬底表面沉积一层金属(Ag、Au、Pt等)纳米颗粒,以催化、氧化它附近的硅原子;然后利用HF溶解氧化层,从而对硅晶片进行刻蚀,形成纳米线阵列。然而,这种简单高效的制备硅纳米线的方法存在一些难以控制的缺点:(1)金属纳米颗粒聚集、相连后造成Si NWs之间的缝隙比较大,从而导致Si NWs密度较低;(2)由于金属纳米颗粒沉积的随机性,在硅晶片表面分布不均匀,不仅导致刻蚀出的纳米线直径范围(50～200 nm)较宽,而且使制得的纳米线阵列排列无序且间距不易调控;(3)当刻蚀出的硅纳米线太长时,范德华力等作用会造成纳米线顶端出现严重的团簇现象。针对常规法存在的一些问题以及不同的器件对硅纳米线的形貌、类型和直径等的要求,近年来的研究主要集中在如何减少纳米线顶端团簇、调控纳米表面粗糙度和直径、低成本制备有序硅纳米线等方面。目前一些改进常规金属辅助化学刻蚀的方法取得了进展,比如:(1)用酸溶液或UV/Ozone对硅晶片预处理,在表面形成氧化层,可以使纳米线的均匀性得到改善并增大其密度(从18%提高到38%);(2)使用物理气相沉积法在硅晶片表面沉积一层金属纳米薄膜,然后再刻蚀,这种方法能够减少纳米线顶端团簇和有效调控纳米线直径;(3)利用模板法(聚苯乙烯小球模板、氧化铝模板、二氧化硅模板和光刻胶模板等)可以制备出有序的硅纳米线阵列。本课题组用离子束刻蚀的方法制备了直径范围可以控制在30～90 nm的聚苯乙烯小球模板,为小尺寸有序硅纳米线的制备打下了坚实的基础。本文简要介绍了常规MACE的原理和制备流程,总结了硅晶片的类型、刻蚀溶液的浓度、温度和刻蚀时间等因素对Si NWs形貌、尺度、表面粗糙度、刻蚀方向以及刻蚀速率的影响,用相关的机制解释了H2O2过量时刻蚀路径偏离垂直方向的机理以及刻蚀速率随溶液浓度变化的原因,重点综述了氧化层预处理、物理法沉积贵金属纳米薄膜、退火处理和模板法等改进方法在减少纳米线顶部团簇、改善均匀性、制备有序且直径和间距可控纳米线中的研究进展。     

非晶Co及CoP纳米阵列的制备及研究

采用交流电化学沉积法,在多孔阳极氧化铝(AAO)模板孔洞中成功制备出直径50nm的Co纳米线阵列.采用透射电子显微镜(TEM)和选区电子衍射(SAED)观察了样品的微观形貌结构.结果显示,阵列中纳米线均匀有序,形状各向异性较大,其晶体结构为非晶.在沉积Co纳米线阵列的电解液中添加一定的还原剂(NaH2PO2·H2O),制备出非晶CoP纳米阵列,由TEM观察可知,阵列形貌发生变化,沉积产物为离散的纳米粒子而非纳米线.探讨了交流电沉积纳米阵列的生长机理.     

图形化硅纳米线阵列的制备

本文主要研究了在常态(常温、常压等)条件下,利用金属催化化学腐蚀方法在硅片表面上大面积制备排列整齐、取向一致的硅纳米线阵列.同时,出于对后续制作硅纳米线传感器考虑,利用微电子标准加工工艺,以氮化硅做掩膜,通过选择合适的实验参数,在硅片表面选择性生长纳米线阵列,得到图形化的硅纳米线阵列.     

硅基径向纳米线阵列的制备及其机理研究

采用金属辅助化学刻蚀(MACE)的方法在硅衬底上制备出尺寸可控、表面光滑的硅纳米线阵列.利用扫描电子显微镜等检测仪器对所制备的硅纳米线样品进行了表征,研究了附银时间和双氧水浓度等工艺参数对硅纳米线生长的影响.结果表明,在附银时间为60 s、双氧水浓度为1.0 mol/L条件下,能够得到较为均匀、规则的硅纳米线阵列.此外,探讨了银辅助刻蚀硅纳米线的形成机理.     

不同结构镍纳米线阵列的制备及其磁性

采用直流电沉积在多孔有序氧化铝模板中制备了不同结构的有序镍纳米线阵列。采用扫描电子显微镜和透射电子显微镜对所制备的镍纳米线的形貌和结构进行了表征。研究了镍纳米线不同结构对镍纳米线阵列磁性性能的影响规律。当电沉积电压为2.5V时制备的镍纳米线为多晶结构;电沉积电压4V时,镍纳米线为沿[220]择优取向的单晶结构;电沉积电压大于5V时,择优取向由[220]转为[111]方向。磁滞回线结果表明,单晶镍纳米线阵列与多晶纳米线阵列相比具有更高的矩形度,沿[111]择优取向的单晶纳米线相比沿[220]取向的单晶镍纳米线具有更大的矩形度,表现出显著的磁各向异性。     

化学气相沉积法制备GaN纳米线及其形貌结构和生长机理研究

采用化学气相沉积(CVD)法,分别以Ni、Au为催化剂,氨化金属Ga制备出GaN纳米线。运用SEM,EDX,TEM等表征手段分析了GaN纳米线的形貌与结构。通过改变氨化温度、生长时间、催化剂、衬底以及Ga源和衬底间的距离等生长条件,研究了其对GaN纳米线形貌和结构的影响,通过分析探讨纳米线的生长过程与机制,得到了生长GaN纳米线的最佳工艺。     

基于金属催化化学腐蚀的硅纳米线可控制备

硅纳米线由于其独特的光电特性,被认为是最具前景的光电材料之一。本文关注于使用金属催化化学腐蚀法制备硅纳米线当中各种可控参数的影响,成功地完成了硅纳米线制备。研究发现,高电镀液浓度与长电镀时间都会形成小密度的硅纳米线阵列;而腐蚀液浓度控制着腐蚀速率与硅纳米线的形貌。腐蚀时间对硅纳米线长度和密度都具有一定的影响;而使用氮气干燥能够改善硅纳米线的聚集情况。     

利用电沉积Ni纳米晶制备无缠绕阵列碳纳米管

以金属Cu基板上脉冲电沉积Ni纳米晶薄膜作为催化剂,在乙醇火焰中制备了直立、无缠绕阵列碳纳米管.利用原子力显微镜(AFM)、扫描电子显微镜(SEM)、透射电镜(TEM)和激光拉曼谱仪(Raman)对不同工艺制备的Ni纳米晶薄膜和阵列碳纳米管的形貌进行了表征.结果表明:通过综合控制脉冲电沉积参数、电沉积时间和火焰中的合成时间,可以获得大面积、密集、均匀、直立、无缠绕、形态良好、重复性高的阵列碳纳米管.脉冲电沉积Ni纳米晶和合成无缠绕阵列碳纳米管的最佳工艺条件是:脉冲电沉积正、负脉冲的工作频率为154Hz、占空比为38.5%、电沉积时间为1min、基板预热至600℃、火焰中停留1min.通过对生长机理的研究发现:当电沉积时间较短时,获得的Ni纳米晶薄膜较薄,具有较高的局域粗糙程度和催化活性,有利于碳纳米管的同时大面积"拥挤生长"形成阵列结构;另外,通过调整脉冲电沉积参数,可以控制Ni纳米晶的大小,从而控制碳纳米管的长径比,当长径比较小时即可获得无缠绕的阵列碳纳米管.     

交流电沉积制备SnO2纳米线及其电沉积条件研究

采用阳极氧化的方法制备了孔径为30～50nm多孔阳极氧化铝(AAO)模板,用交流(AC)电沉积法在氧化铝模板孔洞内沉积金属锡,然后在空气中750℃加热含锡的模板10h,热氧化制备了直径在30～50nm SnO2纳米线．用扫描电子显微镜,透射电子显微镜,X射线衍射仪对所制备的SnO2纳米线进行了表征,表明所制备的纳米线为多晶结构．并通过改变交流电沉积过程中的电压,频率和热氧化时间来考察不同沉积条件对沉积结果的影响,发现采用交流沉积制备金属纳米线的过程中,频率和电压是影响沉积结果的关键因素．     

不同结构镍纳米线阵列的制备及其磁性

采用直流电沉积在多孔有序氧化铝模板中制备了不同结构的有序镍纳米线阵列。采用SEM和TEM对所制备的镍纳米线的形貌和结构进行了表征。研究了镍纳米线不同结构对镍纳米线阵列磁性性能的影响规律．当电沉积电压为2．5V时制备的镍纳米线为多晶结构；电沉积电压4V时，镍纳米线为沿[220]择优取向的单晶结构；电沉积电压＞5V时，择优取向由[220]转为[111]方向．磁滞回线结果表明，单晶镍纳米线阵列与多晶纳米线阵列相比具有更高的矩形度，沿[111]择优取向的单晶纳米线相比沿[220]取向的单晶镍纳米线具有更大的矩形度，表现出显著的磁各向异性。     

Extended arrays of vertically aligned sub-10 nm diameter [100] Si nanowires by metal-assisted chemical etching

Large-area high density silicon nanowire (SiNW) arrays were fabricated by metal-assisted chemical etching of silicon, utilizing anodic aluminum oxide (AAO) as a patterning mask of a thin metallic film on a Si (100) substrate. Both the diameter of the pores in the AAO mask and the thickness of the metal film affected the diameter of SiNWs. The diameter of the SiNWs decreased with an increase of thickness of the metal film. Large-area SiNWs with average diameters of 20 nm down to 8 nm and wire densities as high as 10 (10) wires/cm (2) were accomplished. These SiNWs were single crystalline and vertically aligned to the (100) substrate. It was revealed by transmission electron microscopy that the SiNWs were of high crystalline quality and showed a smooth surface.     

Large-area silicon nanowires from silicon monoxide for solar cell applications

Large-area upstanding silicon nanowires (SiNWs) were synthesized by hot-filament chemical vapor deposition (HFCVD) using silicon monoxide (SiO) powder as Si source under high vacuum (1.2 x 10(-5) Torr). Gold nanoparticles (AuNPs) were employed as catalyst, which were formed on Si substrate by in-situ reduction of gold chloride (AuCl3). The size and distribution of the Au nanoparticles can be easily controlled through chemical reaction conditions. Consequently, the diameter, length and density of SiNWs could be varied in certain range. The SiNWs obtained are single crystalline with growth directions predominantly along [01-1]. Silicon nanowires in large-scale and diameter less than 10 nm can be grown on different Si substrates with this method. Organic inorganic hybrid solar cells based on SiNWs arrays have been demonstrated.     

Synthesis, morphology and compositional evolution of silicon nanowires directly grown on SnO(2) substrates

We here propose an all-in situ method for growing vapor-liquid-solid (VLS) silicon nanowires (SiNWs) directly on SnO(2) substrates in a plasma-enhanced chemical vapor deposition system. The tin catalysts are formed by a well-controlled H(2) plasma treatment of the SnO(2) layer. The lowest temperature for the tin-catalyzed VLS SiNWs growth in a silane plasma is ～250?°C. The effects of substrate temperature and H(2) dilution of silane on the morphology and compositional evolution of the SiNWs were systematically investigated. The catalyst content in the SiNWs can be effectively controlled by the deposition temperature. Moreover, enhanced absorption (down to ～1.1?eV) is achieved due to the strong light trapping and anti-reflection effects in the straight and long tapered SiNWs.     

Conjugated polymer-silicon nanowire array hybrid Schottky diode for solar cell application

The hybrid Schottky diode based on silicon nanowire arrays (SiNWs) and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) has been fabricated for high performance solar cells. The length of SiNWs on a silicon substrate, which is prepared by metal-assisted chemical etching, can be tuned by adjusting the length of the etching time. In addition, the average distances between the adjacent silicon nanowires can be controlled by changing the immersing time in a saturated PCl(5) solution. The hybrid devices are made from the SiNWs with different wire lengths and various distances between adjacent wires by spin-casting PEDOT:PSS on the silicon substrates. It is found that the length and density play leading roles in the electric output characteristics. The device made from SiNWs with optimum morphology can achieve a power conversion efficiency of 7.3%, which is much improved in comparison with that of the planar one. The measurement of the transient photovoltage decay and the analysis of the current versus voltage curve indicate that the charge recombination process is a dominant factor on the device performance.     

Investigation of optical properties of core–shell silicon nanowires

The ability to control the size, orientation, composition and morphology of silicon nanowires (SiNWs) presents an ideal platform for exploring a wide range of potential technological applications. In this work, we demonstrated the detail study of optical properties of highly disordered core–shell SiNWs that were grown by atmospheric pressure chemical vapor deposition. The microstructure of SiNWs was characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The TEM study shows that the SiNWs consists of crystalline core silicon surrounded by thick amorphous silicon oxide. The total diameter including the outer SiO₂ sheath was 60–80 nm. The reflection and absorption of a-SiO₂/c-SiNWs were affected by process parameter like silane flow rate and hydrogen dilution. The optical reflection of SiNWs decreased with increasing photon energy across the visible and near the ultraviolet range, approaching moth's eye antireflection. Specifically, a minimum reflection of 2–3% was observed at 400 nm. The band gap is estimated at ∼1.32 eV by quasi-direct band Tauc's plot. The sum of localized states at the band edge is ∼0.53 eV. Straight SiNWs have lower reflection than those of nanoparticles mixed SiNWs and coil mixed SiNWs. The reflection and absorption of SiO₂/SiNWs were confirmed to respond strongly to infrared with increasing H₂ flow rate.     

热蒸发铜粉法制备硅纳米线的研究

研究发现,热蒸发铜粉即可在硅衬底上直接生长出硅纳米线.场发射电子扫描电镜和透射电镜分析表明,纳米线的形貌、结构及生长机制,随沉积区域的不同而变化.在高温沉积区,硅纳米线高度弯曲且相互缠绕,按气-液-固机制生长;在低温沉积区,高度定向生长的直硅纳米线,规整地排列在硅衬底表面,其生长机制是氧化辅助生长机制.     

Vapor-solid-solid growth of crystalline silicon nanowires using anodic aluminum oxide template

Silicon nanowires (SiNWs) were grown at low temperatures close to metal silicon eutectic point on a silicon substrate using gold catalyst coupled with assistance of the aluminum anodic oxide template. Either a vapor-solid-solid (VSS) growth process below metal silicon eutectic temperature or a vapor-liquid-solid (VLS) process at slightly higher temperatures was observed. The transmission electron microscopy coupled with both the X-ray energy dispersive spectroscopy and the selected area electron diffraction was adopted to characterize the SiNWs. Although the mechanism triggering the VSS process is still not clear, both the geometric and morphological characteristics of the SiNWs grown by the VSS process are discussed and compared with the SiNWs grown by the VLS process. The VSS SiNWs have a much slower growth rate (less than 100 nm/h), a smaller and more uniform diameter (in the range of 15.22 nm) due to a much slower rate of silicon diffusion and much smaller amount of silicon (6.8 wt.%) dissolved in the solid nanocatalyst.     

Segregation behaviors and radial distribution of dopant atoms in silicon nanowires

Gaining an understanding the dynamic behaviors of dopant atoms in silicon nanowires (SiNWs) is the key to achieving low-power and high-speed transistor devices using SiNWs. The segregation behavior of boron (B) and phosphorus (P) atoms in B- and P-doped SiNWs during thermal oxidation was closely observed using B local vibrational peaks and Fano broadening in optical phonon peaks of B-doped SiNWs by micro-Raman scattering. Electron spin resonance (ESR) signals from conduction electrons were used for P-doped SiNWs. Our results showed that B atoms preferentially segregate in the surface oxide layer, whereas P atoms tend to accumulate in the Si region around the interface of SiNWs. The radial distribution of P atoms in SiNWs was also investigated to prove the difference segregation behaviors between of P and B atoms.     

Functionalization of silicon nanowire surfaces with metal-organic frameworks

Metal-organic frameworks (MOFs) and silicon nanowires (SiNWs) have been extensively studied due to their unique properties; MOFs have high porosity and specific surface area with well-defined nanoporous structure, while SiNWs have valuable one-dimensional electronic properties. Integration of the two materials into one composite could synergistically combine the advantages of both materials and lead to new applications. We report the first example of a MOF synthesized on surface-modified SiNWs. The synthesis of polycrystalline MOF-199 (also known as HKUST-1) on SiNWs was performed at room temperature using a step-by-step (SBS) approach, and X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy elemental mapping were used to characterize the material. Matching of the SiNW surface functional groups with the MOF organic linker coordinating groups was found to be critical for the growth. Additionally, the MOF morphology can by tuned by changing the soaking time, synthesis temperature and precursor solution concentration. This SiNW/MOF hybrid structure opens new avenues for rational design of materials with novel functionalities.      

Synthesis of silicon nanowires using laser ablation method and their manipulation by electron beam

Size control of silicon nanowires (SiNWs) synthesized by laser ablation of a Si target with iron or nickel as catalysts were investigated by changing the synthesis parameters such as the content of catalyst in targets and laser power during synthesis. The diameter and length of SiNWs significantly depended on the synthesis parameters, i.e. the size of SiNWs can be controlled by the synthesis parameters. Manipulation of SiNWs was also performed during the observation of scanning electron microscope. By changing the degree of charge-up for free-standing adjacent intertwined SiNWs at an edge of Si substrate, the distance and speed of opening motion of them can be controlled. This motion is probably caused by the Coulomb repulsive interaction between them.