拉伸作用对单层石墨片电子能隙的影响

基于紧束缚方法,在考虑最近邻相互作用的情况下,研究了拉伸锯齿型边和扶手型边单层石墨的能带结构,得到了两种类型单层石墨片的π电子能带及带隙与拉力的解析关系式.通过数值计算能够发现：拉力不但使单层石墨产生带隙,而且带隙随着拉力的增大而变宽,并且锯齿型比扶手型的带隙更易变宽. 关键词： 单层石墨片 拉伸形变 电子能隙     

B与N掺杂对单层石墨纳米带自旋极化输运的影响

通过第一性原理计算研究了具有锯齿状边沿并且具有反铁磁构型的单层石墨纳米带的自旋极化输运.研究发现,在中心散射区同一位置掺入单个B和N原子,尽管对整个体系磁矩的影响完全相同,但对两个自旋分量电流的影响却完全相反.掺B时,自旋向上的电流显著大于自旋向下的电流;而掺N时,自旋向下的电流显著大于自旋向上的电流.这是由于不管掺B还是掺N都将打破自旋简并,使得导带和价带中自旋向上的能级比自旋向下的能级更高.掺B引入空穴,使完全占据的价带变为部分占据,从而自旋向上的能级正好处于费米能级,使得电子透射能力更强、电流更大,而自旋向下的能级则离费米能级较远使电子透射的能力较弱.掺N则引入电子,使得原来全空的导带变为部分占据,从而费米能级穿过导带中自旋向下的能级,使得自旋向下的电子比自旋向上的电子透射能力更强. 关键词： 自旋极化输运 单层石墨纳米带 第一性原理 非平衡格林函数     

堆叠石墨片对锯齿型石墨纳米带电子输运的影响

采用格林函数方法研究了堆叠石墨片对锯齿型石墨纳米带电子输运性质的影响,计算了两种不同堆叠方式下锯齿型石墨纳米带的电导.研究发现,由于堆叠石墨片与石墨纳米带的耦合作用,锯齿型石墨纳米带的电导谱出现了电导谷.在远离费米能处,两种堆叠方式下的电导谷位置相近甚至重合;而在费米能附近,两种堆叠方式下的电导谷存在差异.此外,讨论了堆叠石墨片的几何尺寸对锯齿型石墨纳米带电子输运的影响.结果显示,随石墨片几何尺寸的增大,锯齿型石墨纳米带在两种堆叠方式下远离费米能处的电导谷逐渐向费米能方向移动,同时其费米能附近的电导谷在两种堆叠方式下的差异随石墨片尺寸的增大变得更为明显.研究结果表明,堆叠石墨片能够有效地调制锯齿型石墨纳米带的电子输运性质.     

爆轰制备纳米石墨片的微观形成过程研究

 石墨的层状结构使外来物质可进入层间,形成插入层;插入层在爆轰条件下分解后可将石墨层片推开,形成纳米石墨薄片。通过建立石墨层间化合物有限元模型,利用γ拟合法获取层间化合物的相关参数,基于Thomas-Fermi方程推导出相邻石墨层片分离时的层间作用力与层间距的函数关系;通过LS-DYNA模拟了石墨层片的分离过程,并给出石墨层间距随时间的变化关系,从而阐明薄片状无粘连纳米石墨的形成过程。     

基于片层光照明的新型单分子横向磁镊

磁镊是一种高精度的单分子技术,它用磁场对连有生物大分子的超顺磁球产生磁力,通过追踪磁球的位置来测量生物大分子的长度信息.磁镊包括横向磁镊和纵向磁镊.纵向磁镊空间精度高,但昂贵;横向磁镊简单便宜,但由于受其成像原理的限制,一般情况下只能连接较长的DNA等生物大分子,且其空间精度较差,进而限制了其应用范围.为了解决这个问题,本文改进了横向磁镊,用片层光照明的方法使光线主要被磁球散射,从而能够直接观察到吸附在样品槽侧壁上的磁球,这使得测量短连接的底物成为可能.对于实际应用的检测,首先测试了包含270 bp发卡结构的0.5μm双链DNA,用其中发卡结构的"折叠-去折叠"跳变过程证明了改进后的横向磁镊的确可以追踪短DNA等生物大分子.然后,进一步用16μm的λ-DNA检验了实验系统.最后,将新型横向磁镊与普通横向磁镊及纵向磁镊在小力和大力条件下拉伸不同长度DNA的噪声进行了比较,发现改进后的横向磁镊在空间精度上明显优于普通横向磁镊,与纵向磁镊相比也无明显差异.以上结果证明了改进后的横向磁镊的精度优势,并扩展了横向磁镊的应用范围.     

尘埃粒子电势对等离子体电导率的影响

本文通过提出电势影响因子的概念描述了尘埃粒子与背景等离子体之间的电势差对等离子体电导率的影响. 电势影响因子与尘埃粒子的电荷数、数密度、半径以及背景等离子体电子数密度成正比, 而与背景等离子体电子温度成反比. 在考虑尘埃粒子电势影响下, 推导和完善了尘埃等离子体的复电导率模型. 选取火箭喷焰为典型实例, 分析比较了微波和近红外波段范围内, 考虑和不考虑电势差影响两种情况下的复电导率. 结果表明, 在给定的尘埃等离子体参数条件下, 随着入射电磁波频率的增大, 电势差对复电导率的影响在减小, 当入射频率增加至给定的近红外区域时, 电势差对复电导率虚部的影响可以忽略.     

预处理石墨对合成金刚石性能的影响

 本文介绍了对合成金刚石用石墨片进行预处理的方法。研究表明，用已处理过的石墨片合成出粒度组、强度高、完整单晶多的金刚石，金刚石中Ni、Mn、Co、Si的含量明显减少，而氮的含量明显增加。并观察到触媒片两面都能均匀地生长理想的金刚石晶体。该预处理方法不仅提高金刚石的质量，而且有可能为认识石墨转化为金刚石的机理提供一些依据。     

单原子层纳米片新技术

英国牛津大学的尼克罗西（Valeria Nicolosi）和爱尔兰三一学院的科尔曼（Jonathan Coleman）领导的国际研究小组研发出以超声波脉冲和普通溶剂制备单原子层纳米片的新技术。该技术糅合了最近荣获诺贝尔奖的石墨烯技术,简单、廉价、高效,可实现大规模工业化生产。
他们以该技术开发了大量的纳米片新材料,包括氮化硼、二硫化钼、碲化铋,它们在新型电子设备、超强复合材料、能量生产储存中显示出卓越的性能。这些新材料将成为重要的热电材料,以其制造的热电设备可从燃气、石油、煤电站的废热中提取热量,效率高达70%,而且成本低廉、过程简单。这些新材料还可用于称为“超级电容器”的下一代电池,其效率比目前的电池要高成千上万倍,从而使电动汽车等工业技术的水平获得极大地提升。     

纳米单原子链的热传导

根据Hardy能流密度公式、Kubo热导率公式,推导了纳米单原子链的热传导系数公式,并进行了数值计算.研究结果表明,纳米原子链的热传导系数小于无限长原子链的热传导系数,并且纳米原子链的长度越短,则热传导系数越小.这些现象可以作如下解释:原子链可以看作是一维晶格,格波在到达原子链端点时会发生反射,而改变了格波的能量传播方向,使能流密度降低,从而使纳米原子链的热导率小于无限长原子链的热导率.并且原子链越短,格波在到达原子链端点的过程中衰减越小,从而使反射格波的能流密度越接近于入射格波的能流密度,使能流密度更为降低,从而使纳米原子链的热导率更小.     

Optical conductivity of single-layer graphene induced by temporal mass-gap fluctuations

We consider the dynamics of charge carriers in single-layer graphene that are subject to random temporal fluctuations of their mass gap. The optical conductivity is calculated by incorporating the quantum-stochastic time evolution into the standard linear-response (Kubo) theory. We find that, for an intermediate range of frequencies below the average gap size, electron transport is enhanced by fluctuations. At the same time, in the limit of high as well as low frequencies, the conductivity is suppressed as the variance of gap fluctuations increases. In particular, the dc conductivity is always suppressed by a random temporal mass with nonvanishing mean value and vanishes in the zero-temperature limit. Our results are complementary to those obtained recently for static random-gap disorder in finite-size systems.     

Integer quantum Hall effect in Kekulé-patterned graphene

Y-shaped Kekulébond textures in a honeycomb lattice on a graphene-copper superlattice have recently been experimentally revealed.In this paper,the effects of such a bond modulation on the transport coefficients of Kekulé-patterned graphene are investigated in the presence of a perpendicular magnetic field.Analytical expressions are derived for the Hall and longitudinal conductivities using the Kubo formula.It is found that the Y-shaped Kekulébond texture lifts the valley degeneracy of all Landau levels except that of the zero mode,leading to additional plateaus in the Hall conductivity accompanied by a split of the corresponding peaks in the longitudinal conductivity.Consequently,the Hall conductivity is quantized as±ne²/h for n=2,4,6,8,10,...,excluding some plateaus that disappear due to the complete overlap of the Landau levels of different cones.These results also suggest that DC Hall conductivity measurements will allow us to determine the Kekulébond texture amplitude.     

Effect of non-uniform exchange field in ferromagnetic graphene

We have presented here the consequences of the non-uniform exchange field on the spin transport issues in spin chiral configuration of ferromagnetic graphene. Taking resort to the spin–orbit coupling (SOC) term and non-uniform exchange coupling term we are successful to express the expression of Hall conductivity in terms of the exchange field and SOC parameters through the Kubo formula approach. However, for a specific configuration of the exchange parameter we have evaluated the Berry curvature of the system. We also have paid attention to the study of SU(2) gauge theory of ferromagnetic graphene. The generation of anti damping spin–orbit torque in spin chiral magnetic graphene is also briefly discussed.     

Optical conductivity of ABA-stacked trilayer graphene

The optical conductivity of a trilayer graphene is studied using the Kubo-Greenwood formula. We calculate the real part of the diagonal optical conductivity of an ABA-stacked trilayer graphene with different Fermi energies. The optical conductivity arises from interband matrix elements of the electric current operator involving the transitions from the occupied states to the unoccupied ones. We study the dependence of the real part of the diagonal optical conductivity on the photon energy, and the role of the transitions.     

空位缺陷对金属基外延石墨烯态密度和电导率的影响

考虑到空位缺陷的存在和原子非简谐振动,以铜、镍基外延石墨烯为例, 研究了金属基外延石墨烯空位缺陷浓度和态密度以及电导率随温度的变化规律,探讨了空位缺陷的影响。结果表明：(1) 空位缺陷浓度随温度升高而非线性增大,外延石墨烯的空位缺陷浓度及其随温度的变化率均大于石墨烯; (2) 与石墨烯相同,金属基外延石墨烯的态密度变化曲线对电子能量为0为对称,但空位缺陷的存在使态密度在电子能量为零时的值不为零,空位缺陷对导带态密度的影响大于价带;态密度随空位缺陷浓度的增大而线性减小,但减小幅度不大,而温度对石墨烯态密度几乎无影响;（3）金属基外延石墨烯的电导率近似等于电子声子相互作用贡献的电导率,并随温度升高而非线性减小;空位缺陷的存在使电导率有所减小,但只在较高温度下才明显。原子非简谐振动情况的电导率稍大于简谐近似的电导率,温度愈高,两者电导率的差愈大,即非简谐效应愈显著。     

Influence of interlayer coupling and intra-layer Coulomb interaction on electronic transport in bilayer graphene

Calculations of renormalized perpendicular conductivity within Kubo formula employing single particle temperature dependent Green's function formalism for bilayer graphene has been attempted. On the basis of numerical analysis, perpendicular conductivity as a function of temperature, interlayer coupling, onsite Coulomb interaction and carrier concentration per site has been analyzed for both AA- and AB-stacked bilayer graphene. It is found that perpendicular conductivity increases with interlayer coupling and also with temperature at low temperatures while at higher temperatures, there is saturation in perpendicular conductivity. Influences of onsite Coulomb interaction and carrier concentration per site on perpendicular conductivity is just opposite to each other while onsite Coulomb energy suppresses the rate of increase of σ_⊥/σ_⊥0 with temperature, on the other hand increase in carrier density per site enhance this rate significantly. Finally, theoretically obtained results on temperature dependent perpendicular conductivity are viewed in terms of electronic transport data as well as recent theoretical works available in bilayer graphene.     

硅功能化石墨烯热导率的分子动力学模拟

采用Tersoff势函数与Lennard-Jones势函数,结合速度形式的Verlet算法和Fourier定律,对单层和两层硅功能化石墨烯沿长度方向的导热性能进行了正向非平衡态分子动力学模拟.通过模拟发现,硅原子的加入改变了石墨烯声子的模式、平均自由程和移动速度,使得单层硅功能化石墨烯模型的热导率随着硅原子数目的增加而急剧地减小.在300 K至1000 K温度变化范围内,单层硅功能化石墨烯的热导率呈下降趋势,具有明显的温度效应.对双层硅功能化石墨烯而言,少量的硅原子嵌入,起到了提高热导率的作用,但当硅原子数目达到一定数量后,材料的导热性能下降.     

Effect of triangular vacancy defect on thermal conductivity and thermal rectification in graphene nanoribbons

We investigate the thermal transport properties of armchair graphene nanoribbons (AGNRs) possessing various sizes of triangular vacancy defect within a temperature range of 200–600 K by using classical molecular dynamics simulation. The results show that the thermal conductivities of the graphene nanoribbons decrease with increasing sizes of triangular vacancy defects in both directions across the whole temperature range tested, and the presence of the defect can decrease the thermal conductivity by more than 40% as the number of removed cluster atoms is increased to 25 (1.56% for vacancy concentration) owing to the effect of phonon–defect scattering. In the meantime, we find the thermal conductivity of defective graphene nanoribbons is insensitive to the temperature change at higher vacancy concentrations. Furthermore, the dependence of temperatures and various sizes of triangular vacancy defect for the thermal rectification ration are also detected. This work implies a possible route to achieve thermal rectifier for 2D materials by defect engineering.     

Dynamical thermal conductivity of bilayer graphene in the presence of bias voltage

We study dynamical thermal conductivity of doped biased bilayer graphene for both AA and AB-stacking in the context of tight binding model Hamiltonian. The effects of bias voltage and chemical potential on the behavior of dynamical thermal conductivity are discussed for different stacking of bilayer graphene. Green's function approach has been implemented to find the behavior of thermal conductivity of bilayer graphene within linear response theory. We have found that thermal conductivity decreases with chemical potential for different values of temperature and frequency. Also thermal conductivity of AB stacked bilayer graphene versus bias voltage includes a peak for each value of chemical potential. Furthermore we study the frequency dependence of thermal conductivity of AA stacked bilayer graphene for different values of temperature and bias voltage.