碳纳米管的电子能带结构和态密度研究

为了研究单壁碳纳米管的电子能带特性及其态密度,本文采用紧束缚法和态密度函数,对无限长碳纳米管进行了理论计算和模拟。结果表明:碳纳米管的电子能带结构与其几何结构密切相关,一般认为扶手型管呈现出金属性;而锯齿管则不同,当n=3k(k为整数)时,其能级存在一个较小的禁带宽度,具有半金属性质;而其它锯齿型管均存在较大的禁带宽度,且禁带宽度随管径的增大而减小,具有半导体性质,这为碳纳米管的电子结构的研究提供了必要且有价值的理论依据。     

石墨烯带长度变化与电子结构和电子输运的关系

利用基于非平衡格林函数的密度泛函方法,对长度变化石墨烯带的电子结构和输运特性进行了研究,得出在同一长度的石墨烯带内,系统态密度的峰值与电子传输概率的峰值具有较好的对应关系,但是电子传输概率的大小与电子态密度值的大小没有明显的正比关系。随着长度的增加,电子的传输更倾向于集中在某几个能量态,传输曲线呈现梳齿状,这是由于石墨烯带越长,电子态呈现越稳定的分布。研究还得出不同长度的石墨烯带的电导值和HOM O-LUM O差值成反比,且随着石墨烯带长度增大,其电导出现非线性变化,且呈现规则的振荡状态。     

三代半导体材料构成光子晶体能态密度的研究

基于光子晶体广泛应用,本文利用平面波展开法研究了三代半导体材料构成三角晶格光子晶体能态密度分布规律。得到了当Si作为背景材料构成光子晶体在f=0.25时构成最大光子禁带。AlP作为背景材料构成光子晶体在f=0.24时构成最大光子禁带。GaN作为背景材料构成光子晶体在f=0.3时构成最大光子禁带。三代半导体构成光子晶体对应最大光子晶体禁带宽度较第一二代逐渐增大,研究结果为光子晶体器件的构造提供理论依据。     

碱金属掺杂SnO2材料的电子结构和光学性质

文章基于第一性原理研究了碱金属掺杂的SnO2的能带结构以及态密度.研究结果表明:掺杂能够使能级增多,很好地调节带隙值.Li,Na,K,Rb掺杂的SnO2材料,价带顶有能级穿过费米线,材料呈现出半导体特性.其中Rb掺杂使材料在费米面附近产生杂质能级.而Cs,Fr掺杂的SnO2材料,价带顶向低能级方向移动,费米能级不再穿过价带,费米线附近出现轨道杂化,两者相比Fr掺杂使费米面附近能级分布更加离散.掺杂后SnO2的反射率变化主要体现在可见光以及紫外区域,吸收边发生了红移,对实现SnO2光催化起很大作用.     

相因子对GHZ态结构和自旋相互能量的影响

根据多粒子相十纠缠态得到一般的多粒子GHZ态模型，并利用Wigner函数绘出了相空间的准概率分布图像。然后通过各个直积态的本征态的本征值来计算总的自旋相互作用能量。此外，当相因子变化时，GHZ的结构和能量的变化得到观察与分析。最后，根据稳定性与能量的关系，从能量的角度进一步验证了全同性原理费米子的特征。计算Wigner函数时，我们利用了Matlab语言的Qotoolbox工具包在向量空间中绘出其图形，这比解析的方法更快捷、方便。     

基于石墨烯的CO和CO2气体传感器研究进展

室温下石墨烯具有电子迁移率高、比表面积大、机械强度高、化学稳定性和热稳定性优异、导电性好等独特性能,是当今最受关注的二维材料之一。与传统无机氧化物材料相比,石墨烯气体传感器具有工作温度低、能耗小、恢复性高的优点。文章对两种石墨烯气体传感器的研究进展进行了综述。根据气体选择性不同,将石墨烯气体传感器分为检测CO和CO₂气体传感器。分别对其灵敏度、气体响应灵敏度和响应时间等特性进行分析对比。此研究对此类传感器的应用与推广具有一定的指导意义。     

基于石墨烯的NO2和NH3气体传感器研究进展

室温下石墨烯具有较大的分子吸附比表面积、低噪声、高载流子迁移率等优异电学性能,是一种性能极佳的传感材料。与传统无机氧化物气体传感器相比,石墨烯气体传感器具有工作温度低、能耗小、恢复性高的优点。文章对两种石墨烯气体传感器的研究进展进行了综述。首先根据气体选择性不同,研究了NO₂和NH₃两种石墨烯气体传感器。然后对它们的灵敏度、气体响应灵敏度、响应时间等特性进行了分析对比。该项分析研究工作对气体传感器的实际应用与推广具有一定参考价值。     

沟道层带尾态密度和厚度对a-IGZO薄膜晶体管特性影响的数值仿真

建立了非晶铟镓锌氧化物(a-IGZO)薄膜晶体管(thin-film transistor, 简称TFT)的仿真模型,仿真分析了IGZO半导体层带尾态密度和沟道层厚度两个参数对IGZO TFTs特性的影响规律.研究结果表明,当半导体层带尾态密度增加到1.5×1019 cm-3·eV-1及以上时,器件电学特性受深陷阱态影...     

基底温度对电子束沉积SiO_2薄膜的影响

利用热力学统计理论及薄膜生长理论,给出了薄膜堆积密度、折射率与基底温度之间的关系。在实验中采用电子束热蒸发技术,在不同的沉积速率和基底温度下制备了单层二氧化硅薄膜。研究了沉积速率与薄膜表面均匀度及折射率的关系,并着重分析了基底温度对薄膜折射率、透射率、表面形貌及微观结构的影响。实验结果表明:基底温度升高,薄膜表面粗糙度减小,晶粒间隙缩小,折射率增加,透射率提高,吸收度降低。且当基底温度为500℃时,在可见光区域SiO2薄膜的透射率可达99.4%以上。对实验数据进行拟合,理论计算与实验结果符合得很好。     

掺Cr,Ni对S在Fe(100)面吸附的第一性原理研究

基于密度泛函理论(DFT)的第一性原理方法,在广义梯度近似下计算了Fe中掺Cr或Ni时S原子在Fe(100)面吸附的结构和电子性质,并计算了其分子轨道和吸附能。结果表明:S原子均是吸附在H位最稳定;纯铁时S在Fe(100)面H位的吸附能为-7.70 eV,掺Ni时S原子在H位的吸附能为-7.35 eV,吸附能的相对变化为4.5%;掺Cr时S原子在H位的吸附能为-5.79 eV,吸附能相对纯铁时变化为24.8%,表明掺Cr对S原子在Fe表面的吸附抑制作用更大。对比分析了每种吸附情况下的分波态密度,结果发现掺Cr时具有较高的局域电子云重叠,从而产生的排斥作用抑制了S原子的吸附。     

On the specific features of the density of states of epitaxial graphene formed on metal and semiconductor substrates

Analytical expressions for the local densities of states of epitaxial graphene formed on metal and semiconductor substrates are derived on unified grounds. The conditions of strong and weak graphene-substrate coupling are considered. It is shown that, in the case of strong coupling (the interaction of carbon atoms of graphene with the substrate is much stronger than that of carbon atoms with each other), the local density of states of graphene is close to the density of states of an individual carbon adatom on both metal and semiconductor substrates. In the opposite case of weak graphene-(semiconductor substrate) coupling (the interaction of carbon atoms of graphene with the substrate is much weaker than that of carbon atoms with each other), there is no gap in the local density of states of graphene, and the Dirac point is in the band gap of the semiconductor substrate and coincides in energy with the local level of the separated (individual) carbon adatom. Graphene formed on a metal substrate also exhibits a zero-gap density of states. The problem of the band gap induced in graphene by a semiconductor substrate is considered in the general case. It is shown that, depending on the relation between the parameters of the problem, either one or two band gaps overlapping in energy with the band gap of the substrate can exist in the spectrum of graphene. The dependence of the band gaps on the strength of the graphene-substrate interaction is constructed. Numerical estimations are performed for epitaxial graphene formed on 6H-SiC {0001} faces.     

Adsorption-induced energy gap in the density of states of single-sheet graphene

On the basis of a simple model, it is shown that a monolayer of adatoms bound by an indirect exchange interaction causes the appearance of an energy gap in the density of states of a two-dimensional substrate. The character of the dependence of the π and π* bands and band gap on the position of the adatom level is analyzed in detail. The occupation numbers for the monolayer of adatoms and graphene are calculated. Numerical calculations are carried out for graphane.     

Third-order susceptibility of alkali metal vapors

This paper presents calculations of the third-order susceptibility in the alkali metal vapors involving spin-orbit coupling in the energy spectrum of the alkali atoms and some power transfer between the atoms in the ensemble and their surroundings, including the radiation field. The results are plotted as a function of wavelength in the vicinity of the neodymium and iodine laser lines, and their second, third, and fourth harmonics for each of the alkalies; i.e., 10 520 Å < (lambda, 2lambda, 3lambda, 4lambda) < 10 720 Å, and 13 070 Å < (lambda, 2lambda, 3lambda, 4lambda) < 13 230 Å, respectively.     

Electronic states in epitaxial graphene fabricated on silicon carbide

An analytical expression for the density of states of a graphene monolayer interacting with a silicon carbide surface (epitaxial graphene) is derived. The density of states of silicon carbide is described within the Haldane-Anderson model. It is shown that the graphene-substrate interaction results in a narrow gap of ∼0.01–0.06 eV in the density of states of graphene. The graphene atom charge is estimated; it is shown that the charge transfer from the substrate is ∼10⁻³–10⁻² e per graphene atom.     

Optical transparency of graphene layers grown on metal surfaces

It is shown that, in contradiction with the fundamental results obtained for free graphene, graphene films grown on the Rh(111) surface to thicknesses from one to ~(12–15) single layers do not absorb visible electromagnetic radiation emitted from the surface and influence neither the brightness nor true temperature of the sample. At larger thicknesses, such absorption occurs. This effect is observed for the surfaces of other metals, specifically, Pt(111), Re(1010), and Ni(111) and, thus, can be considered as being universal. It is thought that the effect is due to changes in the electronic properties of thin graphene layers because of electron transfer between graphene and the metal substrate.     

Epitaxial graphene gas sensors on SiC substrate with high sensitivity

2D material of graphene has inspired huge interest in fabricating of solid state gas sensors.In this work,epitaxial graphene,quasi-free-standing graphene,and CVD epitaxial graphene samples on SiC substrates are used to fabricate gas sensors.Defects are introduced into graphene using SF6 plasma treatment to improve the performance of the gas sensors.The epitaxial graphene shows high sensitivity to NO2 with response of 105.1%to 4 ppm NO2 and detection limit of 1 ppb.The higher sensitivity of epitaxial graphene compared to quasi-free-standing graphene,and CVD epitaxial graphene was found to be related to the different doping types of the samples.     

The adsorption effect of C6H5 on density of states for double wall carbon nanotubes by tight binding model

A theoretical approach based on a tight-binding model is developed to study the effects of the adsorption of finite concentrations of C₆H₅ gas molecules on double-walled carbon nanotube (DWCNT) electronic properties. To obtain proper hopping integrals and random on-site energies for the case of one molecule adsorption, the local density of states for various hopping integrals and random on-site energies are calculated. Since C₆H₅ molecule is a donor with respect to the carbon nanotubes and their states should appear near the conduction band of the system, effects of various hopping integral deviations and on-site energies for one molecule adsorption are considered to find proper hopping and on-site energies consistent with expected n-type semiconductor. We found that adsorption of C₆H₅ gas molecules could lead to a (8.0)@(20.0) DWCNT n-type semiconductor. The width of impurity adsorbed gas states in the density of states could be controlled by adsorbed gas concentration.     

衬底温度对CZTSSe薄膜结构特性的影响

采用热蒸发法制备铟锌锡硫(CZTSSe)薄膜。采用低 温一步 法在300℃衬底温度下制备CZTSSe薄膜;采用两步法,即在衬底温度 分别为300℃制备CZTSSe 薄膜;将衬底温度设定为480℃不变,一步蒸发沉积CZTSSe薄膜。通 过对X射线衍射(XRD)、 扫描电镜(SEM)、拉曼谱对比发现,在300℃低温下一步法和300℃ 、480℃两 步法沉积的薄膜表面粗糙,碎小晶粒较多;在480℃一步高温法制备 的薄膜表面平整, 晶粒大小均匀,3个衍射峰的半高峰宽变窄,薄膜的结晶质量得到 改善,且没有发现其它杂相的拉曼特征峰,沉积出适合作为制备CZTSSe薄膜太阳电池的 吸收层。     

Energy gaps in the density of states of a graphene buffer layer on silicon carbide: Consideration for the irregularity of layer-substrate coupling

Green’s function and the density of states of epitaxial graphene formed on the surface of a semiconductor are derived in the diagonal approximation. A graphene buffer layer on silicon carbide is considered in detail. It is assumed that, in the buffer layer, there are two types of states, specifically, states weekly and strongly coupled with the substrate. It is shown that, if there is an energy gap in the density of states of the buffer layer, the existence of the gap and its width are defined by the states providing weak graphene-substrate coupling.     

Effect of a surface pre-treatment on graphene growth using a SiC substrate

This study reports surface pre-treatment techniques for the formation of a high-quality graphene layer on a SiC surface. It is demonstrated that silicon passivation of SiC surface using a silane flow and subsequent sacrificial oxidation can significantly improve the surface condition of a graphene layer on SiC by ensuring much fewer carbon dumps and wrinkles, reducing the electrical resistance, and providing smoother surface roughness and a larger domain size. The effect of in situ cleaning by a SF₆ treatment before graphitization was also studied. It was found that in situ cleaning using SF₆ gas can be a simple and effective means of improving the quality of a graphene layer grown on SiC. The results of this study suggest that a surface treatment before graphitization is the key to synthesize high-quality epitaxial graphene layer.