TiO$lt;sub$gt;2$lt;/sub$gt;纳米管电子结构和光学性质的第一性原理研究

运用基于密度泛函理论的第一性原理方法, 系统研究了小尺寸锐钛矿相(n,0)型TiO₂纳米管(D<16 Å)的几何构型、电子结构和光学性质. 结果表明: 随着管径增大, 体系单位TiO₂分子的形成能降低, 体系趋于稳定; 在管径14 Å左右, (n,0)型TiO₂纳米管会发生一次构型的转变. 能带分析显示, TiO₂纳米管的电子态比较局域化, 小管径下(D<14 Å)其导电性更好; 随着构型的转变, TiO₂纳米管由直接带隙转变为间接带隙, 并且带隙值随着管径的增大而增大, 这是由于π轨道重叠效应的影响大于量子限域效应所导致的结果. 两种效应的竞争, 使得TiO₂纳米管的介电函数虚部ε₂ (ω)谱的峰值位置随管径增大既可能红移也可能蓝移, 管径大于9 Å (即(8, 0)管)之后, TiO₂纳米管的光吸收会出现明显的增强. 关键词： 2纳米管')" href="#">TiO₂纳米管 第一性原理 电子结构 光学性质     

萤石结构TiO2的电子结构和光学性质

利用第一性原理计算了立方相萤石TiO₂的晶胞参数,能带结构和电子态密度.结果显示萤石TiO₂属于间接带隙半导体材料,其间接禁带宽度(Γ→X)E_g为2.07eV,比常见的金红石和锐钛矿TiO₂的禁带宽度窄.为了更清楚地了解萤石的光学性质,利用Kramers-Kronig色散关系,分别对萤石和金红石TiO₂的复介电常数、吸收率等参数进行了计算,并将二者结果做了 关键词： 2')" href="#">萤石结构TiO₂ 密度泛函理论 能带结构 光学性质     

拉伸形变下BC3纳米管的能带结构

运用紧束缚能带理论,研究拉伸形变下BC₃纳米管的能带结构. 研究表明:随着拉伸和压缩强度的不断增加,BC₃纳米管的导带能级和价带能级逐渐靠近,最终发生能带交叠. 压缩形变下能带的交叠程度可达05 eV,而拉伸形变下只有02 eV. 对于扶手椅型BC₃纳米管,随着拉伸和压缩的不断增加,BC₃纳米管首先由直接半导体转化为间接半导体,进而发生能带的交叠,表现出金属性. 在无形变时,扶手椅型BC₃纳米 关键词： 3纳米管')" href="#">BC₃纳米管 能隙 拉伸形变 半导体     

溅射制备Ge,Nb共掺杂窄光学带隙和低电阻率的TiO2薄膜

采用射频磁控溅射技术制备了Ge,Nb共掺杂的锐钛矿结构TiO₂薄膜,详细探讨了薄膜的结构、电阻率及光学带隙等性质随Ge,Nb掺杂量、溅射功率和热处理温度等参数的变化,发现Ge,Nb共掺杂可以同时调节TiO₂薄膜的光学带隙和电阻率.体积分数约为6%Nb和20%Ge的共掺杂TiO₂薄膜电阻率由10⁴Ω/cm减小至10^-1Ω/cm,光学带隙由3.2 eV减小至1.9 eV.退火后掺杂TiO₂薄膜不仅显示更低的电阻率,还表现出更强的可见-红外光吸收.结果表明,改变Ge,Nb的掺杂量和退火条件能够制备出电阻率和带隙都可调的TiO₂薄膜.     

C掺杂锐钛矿相TiO2吸收光谱的第一性原理研究

运用第一性原理,对C掺杂锐钛矿相TiO₂的电子结构进行了研究,从能带结构理论解释了C掺杂TiO₂吸收光谱的一些实验现象.发现在C掺杂后的锐钛矿相TiO₂的禁带宽度增大,并且在带隙中出现了杂质能级,这些杂质能级主要是由C 2p轨道上的电子构成的,它们之间是独立的,正是这些独立的杂质能级使TiO₂掺杂后可以发生可见光响应.价带上的电子可以吸收一定能量的光子跃迁到杂质能级,而杂质能级上的电子也可以吸收一定能量的光子跃迁到导带,所以从理论上可以计算出掺杂后的TiO₂在可见光范围内存在两个吸收边,与实验中所得到的现象相一致. 关键词： C掺杂 2')" href="#">锐钛矿TiO₂ 能带结构 吸收光谱     

OsSi2电子结构和光学性质的研究

采用基于第一性原理的密度泛函理论赝势平面波方法,对正交相OsSi₂的电子结构、态密度和光学性质进行了理论计算,能带结构计算表明它是一种间接带隙半导体,禁带宽度为0813 eV;其价带主要由Os的5d和Si的3p态电子构成;导带主要由Si的3s,3p以及Os的5d态电子构成;静态介电常数ε₁（0）=1543; 折射率n＝393并利用计算的能带结构和态密度分析了OsSi₂的介电函数、吸收系数、折射率、反射率、 关键词： 2')" href="#">OsSi₂ 第一性原理 电子结构 光学性质     

超长(n,n)型碳纳米管的密度泛函理论研究

用密度泛函B3LYP/3-21G(d)方法,并利用周期边界条件,研究了n=2—20不同管径的超长(n, n)型单壁碳纳米管的结构、能量、能带结构和能隙.结果表明,管径和能量(或生成焓)都随n有很好的变化规律,并可拟合成很好的解析函数.当n为2和3时,碳纳米管的能隙分别为1.836eV和0.228eV,呈半导体特征,且具有间接带隙;当n=4—20时,能隙介于0.027 eV和0.079 eV之间,呈较强的金属性,且具有直接带 关键词： 扶手椅型碳纳米管 周期边界条件(PBC) 超长模型 能带     

V+注入锐钛矿TiO2第一性原理研究

用金属离子注入方法在锐钛矿TiO₂薄膜中掺杂了V⁺，采用全势线性缀加平面波方法计算了锐钛矿TiO₂及V⁺掺杂TiO₂超原胞的电子结构，通过紫外-可见吸收光谱测试方法检测了注入不同剂量的V⁺对TiO₂薄膜吸收光谱的影响.理论计算和实验结果表明，锐钛矿TiO₂薄注入V⁺后，带隙宽度变小，吸收光谱发生红移，并且TiO 关键词： +注入')" href="#">V⁺注入 2')" href="#">TiO₂ 全势线性缀加平面波方法 能带结构     

关于Zn1-xBexO电子结构和光学性质的研究

采用基于密度泛函理论和平面波赝势技术的CASTEP程序对Zn_1-xBe_xO合金电子结构和光学性质进行了计算.当0≤x≤1,其带隙从0.963 eV变化到7.293 eV.分析了晶格畸变和能带间排斥效应对带隙的影响.当Be含量x=0.125,0.25,0.375,0.5,0.625,0.75时,a/b轴压应变控制着带隙变化;当x=0.875,1时,c轴压应变控制着带隙变化.能带间的p-d排斥影响价带顶变动,Γ_1v与Γ_1c之间排斥影响导带底变动.这些能带间的排斥效应被用来分析Zn_1-xBe_xO带隙变动.另外,也分析了Zn_1-xBe_xO介电函数虚部ε₂. 关键词： 带结构 光学性质 应变 排斥     

N掺杂锐钛矿TiO2光学性能的第一性原理研究

用平面波赝势方法(PWP)计算了N掺杂锐钛矿型TiO₂前后的光学特性,即介电函数虚部ε₂(ω)，光学吸收系数I(ω)和反射率R(ω). 并从能带结构上解释了为什么掺N后锐钛矿型TiO₂的光学谱在2.93，3.56和3.97eV处相对掺杂前会出现3个峰值的原因. 从光谱图上分析得出，掺杂后TiO₂要发生红移现象，实验现象证实了这一结果. 关键词： N掺杂 2')" href="#">锐钛矿型TiO₂ 光学性能 第一性原理     

Structure and properties of BeO nanotubes

The structure of a new non-carbon (beryllium oxide BeO) nanotube consisting of a rolled-up graphene sheet is proposed, and its physical properties are described. Ab initio calculations of the binding energy, the electronic band structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y for BeO nanotubes of different diameters are performed in the framework of the density functional theory (DFT). From a comparison of the binding energies calculated for BeO nanotubes and crystalline BeO with a wurtzite structure, it is inferred that BeO nanotubes can be synthesized by a plasma-chemical reaction or through chemical vapor deposition. It is established that BeO nanotubes are polar dielectrics with a band gap of ～5.0 eV and a stiffness comparable to that of the carbon nanotubes (the Young’s modulus of the BeO nanotubes Y _BeO is approximately equal to 0.7Y _C, where Y _C is the Young’s modulus of the carbon nanotubes). It is shown that, for a nanotube diameter D > 1 nm, the (n, n) armchair nanotubes are energetically more favorable than the (n, 0) zigzag nanotubes.     

Synthesis of Different TiO_2 Nanostructures and Their Physical Properties

Titanium dioxide(TiO_2) nanosheet, nanorod and nanotubes are synthesized using chemical vapor deposition(CVD) and anodizing processes. TiO_2 nanosheets are grown on Ti foil which is coated with Au catalyst in CVD,TiO_2 nanorods are synthesized on treated Ti foil with HCI by CVD, and TiO_2 nanotubes are prepared by the three-step anodization method. Scanning electron microscopy shows the final TiO_2 structures prepared using three processes with three different morphologies of nanosheet, nanorod and nanotube. X-ray diffraction verifies the presence of TiO_2. TiO_2 sheets and rods are crystalized in rutile phase, and TiO_2 tubes after annealing turn into the anatase crystal phase. The optical investigations carried out by diffuse reflection spectroscopy reveal that the morphology of TiO_2 nanostructures influencing their optical response and band gap energy of TiO_2 is changed for different TiO_2 nanostructures.     

Energy and electronic properties of non-carbon nanotubes based on silicon dioxide

The geometric, energy, and electronic characteristics of new non-carbon nanotubes based on silicon dioxide are investigated in the framework of the local electron density functional formalism. Nanotubes are classified according to the type of rolling-up of the SiO₂ sheet. It is shown that, among the entire set of considered nanotubes with different symmetries, the (6, 0) nanotubes are energetically more favorable. The densities of states for nanotubes are calculated. It is established that all nanotubes are dielectrics with a wide band gap. The band gap varies over a wide range with a change in the longitudinal strain of the nanotube.     

Structure and electronic properties of the double-wall nanotubes constructed from SiO2 nanotubes encapsulated inside zigzag carbon nanotubes

This paper presents ab initio self-consistent field crystal orbital calculations on the structures, stabilities, elastic and electronic properties of the double-wall nanotubes made of SiO(2) nanotubes encapsulated inside zigzag carbon nanotubes based on density functional theory. It is found that formation of the combined systems is energetically favorable when the nearest distance between the two constituents is in the area of the van der Waals effect. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. Based on the deformation potential theory and effective mass approximation, the mobilities of charge carriers are calculated to be in the range of 10(2)-10(4) cm(2) V(-1) s(-1), the same order of magnitude as those of the corresponding zigzag carbon nanotubes. The Young's moduli are also calculated for the combined systems.     

Reduction in the electronic band gap of titanium oxide nanotubes

The structural and electronic properties of individual titanium oxide nanotubes have been studied using both empirical and ab initio calculations. Two different types of titanium oxide nanotubes (A-nanotube and B-nanotube) have been constructed and energy-minimized by molecular mechanics calculations. We found that the A-nanotubes are energetically more favorable than the B-nanotubes. The electronic band structure of the titanium oxide nanotubes was also calculated with respect to the tubule diameter and the tubule type using the ab initio method. The band gap of the A-nanotube was reduced by up to 60% as the tubule diameter decreases from 1.2 nm to 0.5 nm.     

Optical properties of anatase and rutile TiO_2 studied by GGA + U

The optical properties of thermally annealed TiO_2 samples depend on their preparation process, and the TiO_2 thin films usually exist in the form of anatase or rutile or a mixture of the two phases. The electronic structures and optical properties of anatase and rutile TiO_2 are calculated by means of a first-principles generalized gradient approximation(GGA) +U approach. By introducing the Coulomb interactions on 3d orbitals of Ti atom(U~d) and 2p orbitals of O atom(U~p), we can reproduce the experimental values of the band gap. The optical properties of anatase and rutile TiO_2 are obtained by means of the GGA+U method, and the results are in good agreement with experiments and other theoretical data. Further, we present the comparison of the electronic structure, birefringence, and anisotropy between the two phases of TiO_2. Finally,the adaptability of the GGA+U approach has been discussed.     

Structural and electronic properties of carbon nanotubes under hydrostatic pressures

We studied the structural and electronic properties of carbon nanotubes under hydrostatic pressures based on molecular dynamics simulations and first principles band structure calculations. It is found that carbon nanotubes experience a hard-to-soft transition as external pressure increases. The bulk modulus of soft phase is two orders of magnitude smaller than that of hard phase. The band structure calculations show that band gap of (10, 0) nanotube increases with the increase of pressure at low pressures. Above a critical pressure (5.70GPa), band gap of (10, 0) nanotube drops rapidly and becomes zero at 6.62GPa. Moreover, the calculated charge density shows that a large pressure can induce an {sp}²-to-{sp}³ bonding transition, which is confirmed by recent experiments on deformed carbon nanotubes.     

Encapsulation of lamivudine into single walled carbon nanotubes: A vdW-DF study

To explore a suitable carrier for lamivudine drug, the incorporation of lamivudine inside the single walled carbon nanotubes (SWCNTs) has been investigated by using first-principles van der Waals density functional (vdW-DF) calculations. The obtained binding energies reveal that lamivudine prefers to be encapsulated into the metallic nanotubes with diameter of about 13 Å. Semiconducting SWCNTs exhibit slightly weaker interaction strength with the lamivudine in comparison with the metallic counterparts. However, the calculated binding energies for both considered nanotubes are typical for the physisorption. The influence of nanotube length on the lamivudine incorporation inside the various considered nanotubes has also been investigated and the results show that it plays an important role in the encapsulation process. The electronic structures analysis for the energetically most favorable complexes reveal that incorporated lamivudine changes slightly the electronic properties of SWCNTs. This indicates that there is no considerable hybridization between the corresponding orbitals and the weak interaction obtained quantitatively in terms of binding energies.     

Structure and electronic properties of MoS2 nanotubes

Structural and electronic properties as well as the stability of MoS2 nanotubes are studied using the density-functional-based tight-binding method. It is found that MoS2 zigzag ( n,0) nanotubes exhibit a narrow direct band gap and MoS2 armchair ( n,n) possess a nonzero moderate direct gap. Interestingly, the ( n,n) tubes show a small indirect gap similar to the direct gap of ( n,0) nanotubes. Simulated electron diffraction patterns confirm the existence of armchair and zigzag disulphide nanotubes. The structure of the MoS2 nanotube tips is explained by introducing topological defects which produce positive and negative curvature.