多晶Si薄膜表面金催化Si纳米线生长

以Au膜作为催化剂和大晶粒多晶Si薄膜为衬底,利用固-液-固生长机制,制备出直径在30～ 100 nm和长度为几百微米的高密度Si纳米线.实验研究了退火温度、生长时间和N2流量对Si纳米线生长的影响.结果表明,随着退火温度的升高,生长时间的延长和N2流量的增加,Si纳米线的长度和密度都显著增加.对不同生长时间下获得的Si纳米线样品进行了X射线衍射测量,结果显示随着生长时间的延长,多晶Si薄膜和表面的Au膜成分都在减少.光致发光谱则显示出弱的蓝光发射和强的红光发射特性,前者应是由非晶SiOx壳层中的氧空位发光中心引起,后者则应归因于Si纳米线芯部与非晶SiOx壳层之间界面区域附近中的Si =O双键态或非桥键氧缺陷中心.     

Zn掺杂Z形GaN纳米线的制备及表征

本文以氧化镓、氧化锌和氨气为原料,通过常压化学气相沉积法(APCVD)在Au/Si(100)衬底上成功生长出了Zn掺杂的"Z"形GaN纳米线。利用场发射扫描电镜(FESEM)、X-射线衍射仪(XRD)、透射电子显微镜(TEM)、光致发光谱(PL)等测试方法对样品的形貌、晶体结构及光学性质进行了表征。结果表明:在温度为950℃,氧化镓和氧化锌的质量比为8∶1的条件下,制备出的Zn掺杂Z形GaN单晶纳米线直径为70 nm、长度为数十个微米,生长机理遵循VLS机制。Zn元素的掺杂使GaN纳米线在420 nm处出现了光致发光峰,发光性能有所改善。     

氧化锌纳米线场发射第一性原理研究

本文采用基于密度泛函理论的第一性原理计算方法,研究了不同横截面氧化锌纳米线(ZnONW)的结构稳定性和场致发射特性.结果表明:ZnONW的直径越大,原子弛豫后Zn-O键键长变化越小,体系越稳定;随外加电场增强,长径比较大的纳米线体系态密度(DOS)和分波态密度(PDOS)均向低能方向移动,最高占据分子轨道(HOMO)-最低未占据分子轨道(LUMO)能隙减少,体系电荷移向NW顶端.DOS/PDOS,HOMO/LUMO,能隙及有效功函数分析结果表明,具有较小横截面积的NW-1场发射性能最佳.     

泡沫镍衬底上生长二氧化锡纳米线的生长行为及光致发光性能研究

采用热蒸发法,在镀有Au(金)的多孔泡沫镍基底上生长了SnO2纳米线,生长温度分别为750℃、800℃和850℃,分别利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)研究了生长温度对纳米线生长行为的影响,采用X射线光电子能谱仪(XPS)研究了样品的元素结合态,采用荧光光谱仪研究了样品的室温光致发光(PL)性能.结果证明,生长温度对SnO2纳米线形貌、尺寸、密度等形态学特征影响显著.在750℃下,SnO2为颗粒状;800℃生成了较高密度的SnO2纳米线,且随温度升高纳米线产量增加、直径显著增大;在850℃下其直径增大至微米级.对室温光致发光性能的研究发现,随生长温度升高,在SnO2产量增加和氧缺陷浓度的提高的共同作用下,导致其发光性能随之显著提高.     

温度对Ga掺杂ZnS纳米结构的形貌及光致发光性能的影响

分别以Zn粉和S粉为原材料,Ga为掺杂剂,Au纳米颗粒为催化剂,采用低温化学气相沉积法(CVD),在Si(l00)衬底上制备了Ga掺杂的ZnS纳米结构.利用X射线衍射仪(XRD)、能量弥散X-ray谱(EDS)、场发射扫描电子显微镜(SEM)和光致发光光谱(PL)等测试手段对样品的结构、成分、形貌和发光性能进行了分析.结果表明:随着温度的升高(450～ 550℃),Ga掺杂ZnS纳米结构的形貌发生了从蠕虫状纳米线到光滑纳米线再到纳米棒的演变,所制备的Ga掺杂的ZnS纳米结构均为六方纤锌矿结构,分别在波长为336 nm和675 nm处存在一个较强的近带边紫外发射峰和一个Ga掺杂引起的微弱红光峰,而其它发光峰均是缺陷引起的.此外,本文还对Ga掺杂ZnS纳米结构的形成过程进行了探讨,并提出了可能的形成机理.     

IB族金属掺杂单层MoS_2电子结构的研究

本文基于密度泛函理论框架下的第一性原理,采用数值基组研究了IB族金属Cu,Ag,Au掺杂单层MoS_2的键长畸变、稳定性、能带结构和态密度。探讨了Cu,Ag,Au掺杂对单层MoS_2电子结构的影响;结果表明:Cu,Ag,Au在Mo位掺杂都会导致杂质原子附近的键长发生畸变,但畸变程度略有差异,Cu掺杂体系的稳定性强于Ag,Au在Mo位掺杂的体系;Cu,Ag,Au掺杂致使单层MoS_2的禁带中出现新能级;Cu,Ag,Au在S位吸附的稳定性强于Mo位吸附,对于MoS_2在S位吸附Cu,Ag,Au对其禁带的宽度影响较小;但稳定性最强的都是Au元素的吸附体系。     

CVD法制备高质量ZnO纳米线及生长机理

以金做催化剂,采用化学气相沉积(CVD)方法在Si(100)衬底上生长了整齐紧密排列的ZnO纳米线.XRD图谱上只有ZnO的(002)衍射峰,说明ZnO纳米线沿[001]择优生长;扫描电子显微镜分析表明:ZnO纳米线整齐排列在Si(100)衬底上,直径在100nm左右,平均长度为4μm.研究发现ZnO纳米线的生长机理与传统的V-L-S机理有所不同:生长过程中,在Si(100)衬底上先生长了大约500nm厚的ZnO薄膜,而ZnO纳米线生长在薄膜之上.     

Mg掺杂浓度对GaN电子结构和光学性质的影响

基于密度泛函理论的第一性原理,分析了Mg掺杂浓度对GaN晶格参数、能带结构、电子态密度和光学性质的影响.研究表明:Mg掺杂GaN体系,晶格常数增大,禁带宽度增加,而且禁带宽度随着Mg含量的增加而增加,同时N2p和Mg2p态电子轨道的相互杂化,从而在费米能级附近引入受主能级,随着Mg含量的增加,费米能级进入价带的位置加深,同时Mg掺杂浓度越高,价带和导带带宽越窄.掺Mg后在介电函数和光学吸收谱的低能区和高能区均出现了新的介电峰,这些峰的出现和禁带中的杂质能级到导带底的跃迁有关,由于带隙的增加使介电峰向高能量方向发生偏移.     

La掺杂量对ZnO光电性能影响的第一性原理研究

采用密度泛函理论下的平面波超软赝势方法和杂化泛函理论下的模守恒赝势方法,分别计算了未掺杂ZnO和两种La掺杂浓度的ZnO模型,其中对较高La掺杂浓度的计算还设置了两种不同的掺杂位置.结构优化后,首先通过计算形成能、系统总能量和电荷布居值,对掺杂后体系的稳定性进行了分析;而后结合自旋基态能量与自旋电子态密度对掺杂体系的磁性状态进行了说明;最后通过计算得到的电子结构及吸收光谱讨论了La掺杂量对ZnO光电性能的影响.结果表明:随La掺杂量增加,ZnO体系稳定性有所降低;La掺杂ZnO无磁性,电子结构不会受到自旋能级分裂的影响;与纯ZnO相比,La掺杂ZnO的禁带宽度增大,吸收光谱蓝移,然而通过控制La浓度与掺杂方式可以有效增强La-5d与Zn-4s电子态的交换关联作用而减小ZnO的最小光学带隙,提高ZnO对可见光的吸收系数,使光生空穴-电子对有效分离的影响.     

在硅衬底上制备Si纳米线及其表征

采用以硅衬底为硅源的简单方法,在蘸有一层催化剂(0.05 M FeCl2·6H2O)薄膜的硅基片上直接制备了大量的单晶Si纳米线.用扫描电子显微镜(SEM)、透射电子显微镜(TEM)分别观察了硅纳米线的形貌及单根Si纳米线的显微结构.试验结果表明:随着温度的升高,纳米线的直径增大;催化剂是Si纳米线生成的重要因素,没有沉积催化剂的硅基片上不会有硅纳米线的生成,且该硅纳米线的生长机理为典型的气-液-固模式.     

Silicon nanowires grown on a pre-annealed Si substrate

Polycrystalline Si nanowires (poly SiNWS) were successfully synthesized by plasma-enhanced chemical vapor deposition (PECVD) at 440°C using silane as the Si source and Au as the catalyst. The diameters of Si nanowires range from 15 to 100 nm. The growth process indicates that to fabricate SiNWS by PECVD, pre-annealing at high temperature is necessary. A few interesting nanowires with Au nanoclusters uniformly distributed in the body of the wire were also produced by this technique.     

Growth of hexagonal and cubic InN nanowires using MOCVD with different growth temperatures

We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(1 1 1) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10 nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700 °C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200 nm and lengths varying between 3 and 5 μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E₂ (high) at 491 cm⁻¹ and A₁ (LO) at 591 cm⁻¹.     

Statistical considerations on electronic behavior of the gap states in amorphous semiconductors

The equilibrium distribution functions of the gap states taking into account the effective intrasite electronic correlation energy have been employed to discuss the electronic behaviors of amorphous semiconductors, especially to interpret the temperature dependence of dc and ac conductivity and also the effect of foreign impurity atoms on the conductivity in chalcogenide glasses. The dc conductivity of thermal activation type has been explained without assuming a negligible density of states near the Fermi level, and the gradual decrease in activation energy of conductivity with increasing impurity concentration, even in melt-quenched chalcogenide glasses, has been deduced.     

Growth of SrFe12O19 nanowires under an induced magnetic field

Nanowires of SrFe₁₂O₁₉ with diameters of 100 nm and lengths of 2.5 μm have been successfully synthesized in a hydrothermal cell at 180 °C with an 0.35 T magnetic field applied. The growth behavior of the nanoparticles was compared with that under zero magnetic field. The X-ray diffraction patterns indicate that both of the two processes result in formation of pure SrFe₁₂O₁₉, however transmission electron microscope observations show that the morphology of the particles changed from flake-like in zero magnetic field into nanowires in a magnetic field. Compared to the sample obtained under zero magnetic field, the as-prepared one exhibits a higher saturation magnetization. The possible underlying mechanism responsible for the morphology change and the magnetic properties improvement were discussed.     

Growth mechanisms of GaAs nanowires by gas source molecular beam epitaxy

GaAs nanowires were grown on GaAs (1 1 1)B substrates in a gas source molecular beam epitaxy system, using self-assembled Au particles with diameters between 20 and 800 nm as catalytic agents. The growth kinetics of the wires was investigated for substrate temperatures between 500 and 600 °C, and V/III flux ratios of 1.5 and 2.3. The broad distribution of Au particles enabled the first observation of two distinct growth regimes related to the size of the catalyst. The origins of this transition are discussed in terms of the various mass transport mechanisms that drive the wire growth. Diffusion of the growth species on the 2-D surface and up the wire sidewalls dominates for catalyst diameters smaller than ∼130 nm on average, while direct impingement on the catalyst followed by bulk diffusion through the Au particle appears to sustain the wire growth for larger catalyst diameters. A change in wire sidewall facets, indicating a probable transition in the crystal structure, is found to be primarily dependent on the V/III flux ratio.     

Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods

The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO₂/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO₂/Si heterostructures, which were previously formed by the modified “hot wall” technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO₂/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride.     

Bandwidth narrowing in n-type many-valley semiconductors

A simple method that takes into account the many-valley effects of the host conduction band is outlined for the calculation of the impurity density of states. N random impurity sites were simulated with a computer, within a diamond lattice as a host. A Kohn-Luttinger donor wave function was associated with each impurity. It was found that the low energy side (or the high energy side when the effect of non-orthogonality is taken into account) of the density of states is strongly reduced to a small tail when compared to the case of neglecting many-valley effects. At the Fermi energy, the density of states is considerably enhanced which leads to an enhancement of the extrinsic specific heat. The inverse participation ratio varies with the impurity concentration, presenting different states which are responsible for different physical properties. An analytical calculation taking into account these effects was carried out for the sake of comparison.     

Growth of nanowires from annealing SiBONC nanopowders

Silicon carbide (SiC) nanowires were prepared by the gas pressure annealing of SiBONC powders, which were synthesized by pyrolysis of a polymeric precursor. The yield, morphology and composition of the nanowires were influenced by the Si/B ratio in the original ceramic powders, annealing temperature and atmosphere. Annealing temperatures between 1500 and 1600 °C and Si/B molar ratios between 70:30 to 60:40 were suitable for growth of the nanowires. When annealing in an argon (Ar) atmosphere, the SiC nanowires contained little oxygen (O); and the diameters ranged from 20 to 200 nm. Then annealing in a nitrogen (N₂) atmosphere, the nanowires were thicker and rougher, and consisted of a relatively high level of nitrogen. Varied shapes and morphologies of the nanowires were observed for different synthesis conditions. The present novel method makes possible the large-scale fabrication of β-SiC nanowires.