Effect of the dangling bond on the electronic and magnetic properties of BN nanoribbon

The effect of the dangling bond on the electronic and magnetic properties of BN nanoribbon with zigzag edge (ZBNNR) and armchair edge (ABNNR) have been studied using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. Though ZBNNR or ABNNR with H atom terminated at both edges is nonmagnetic semiconductor, the dangling bond induces magnetism for the ZBNNR with bare N edge, bare B edge, bare N and B edges, the ABNNR with bare N edge and bare B edge. However, the ABNNR with bare N and B edges is still nonmagnetic semiconductor due to the strong coupling of the dangling bonds of dimeric N and B atoms at the same edge. The magnetic moment of ZBNNR with bare N(B) edge is nearly half the magnetic moment of ABNNR with bare N(B) edge. Such a half relationship is also existed in the number of the dangling bond states appeared around the Fermi level in the band structures. Furthermore, the dangling bond states also cause both ZBNNR and ABNNR with bare N edge a transition from semiconducting to half-metallic and thus a completely (100%) spin-polarization, while cause both ZBNNR and ABNNR with bare B edge as well as ABNNR with bare N and B edges only a decrease in their band gap.     

Structural and electronic properties of a single Si chain doped zigzag AlN nanoribbon

The first-principles projector-augmented wave (PAW) potentials within the density function theory (DFT) framework have been used to determine the geometry structures and electronic properties of the zigzag edge AlN nanoribbons (ZAlNNRs) doped with a single Si chain under generalized gradient approximation (GGA). The average Al–Si, Si–Si, Al–N, Si–N, Al–H and N–H bond lengths are 2.39, 2.16, 1.83, 1.74, 1.59 and 1.03 Å, respectively. Pure 7-ZAlNNR is an indirect semiconductor with a large band gap of 2.235 eV, while a semiconductor to metal transformation is taken place after a single Si chain substituting for a single Al–N chain at various positions. In pure 7-ZAlNNR, the HVB and LCB are mainly attributed to the edge N and Al atoms, respectively, while in a single Si chain substituting doped 7-ZAlNNR, the HVB and LCB are mainly attributed to the Si atoms. The Al–N, Al–H and Al–Si bonds are ionic bond, the Si–Si and Si–H bonds are covalent bond, the N–H and N–Si bonds are covalent bond modified ionic bond.     

硅烯饱和吸附碱金属原子的第一性原理研究

基于密度泛函理论的第一性原理计算, 研究了硅烯饱和吸附碱金属元素原子的稳定性、微观几何结构和电子性质, 并与纯硅烯及其饱和氢化结构进行了对比分析. 研究发现复合物SiX(X=Li, Na, K, Rb)的形成能都是负的, 相对于纯硅烯来说可以稳定存在. Bader电荷分析表明, 电荷从碱金属原子转移至硅原子. 从成键方式来看, 硅烯与氢原子形成共价键, 而与碱金属原子之间形成的键主要是离子性成键, 但还存在部分共价关联成分. 能带计算表明, 锂原子饱和吸附在硅烯形成的复合物SiLi是直接带隙的半导体, 带隙大小为0.34 eV. 其他碱金属饱和吸附在硅烯上形成的复合物都表现为金属性.     

Electronic structures of the F-terminated AlN nanoribbons

Using the first-principles calculations, electronic properties for the F-terminated AlN nanoribbons with both zigzag and armchair edges are studied. The results show that both the zigzag and armchair AlN nanoribbons are semiconducting and nonmagnetic, and the indirect band gap of the zigzag AlN nanoribbons and the direct band gap of the armchair ones decrease monotonically with increasing ribbon width. In contrast, the F-terminated AlN nanoribbons have narrower band gaps than those of the H-terminated ones when the ribbons have the same bandwidth. The density-of-states (DOS) and local density-of-states (LDOS) analyses show that the top of the valence band for the F-terminated ribbons is mainly contributed by N atoms, while at the side of the conduction band, the total DOS is mainly contributed by Al atoms. The charge density contour analyses show that Al–F bond is ionic because the electronegativity of F atom is much stronger for F atom than for Al atom, while N–F bond is covalent because of the combined action of the stronger electronegativity and the smaller covalent radius.     

Tuning the electronic and magnetic properties of the Si nanoribbons through dangling bond

Combined with three spin configurations, the effects of the dangling bonds on the electronic and magnetic properties of both zigzag edge and armchair edge Si nanoribbions (ZSiNR and ASiNR) have been investigated systematically by the first-principles calculations in the local spin-density function theory. The dangling bonds at one edge or both edges make ZSiNR to transform from ferromagnetic state of the perfect ZSiNR to antiferromagnetic state. However, the dangling bonds at one edge and both edges make ASiNR to transform from nonmagnetic semiconductor of the perfect ASiNR to ferromagnetic and antiferromagnetic metals, respectively. Furthermore, the magnetic moment of the ferromagnetic state increases for the perfect bare one edge and bare both edges successively for either ZSiNR or ASiNR.     

第一性原理研究[112]硅锗异质结纳米线的电子结构与光学性质

基于密度泛函理论体系下的广义梯度近似,本文利用第一性原理方法着重研究了[112]晶向硅锗异质结纳米线的电子结构与光学性质.能带结构计算表明:随着锗原子数的增加,[112]晶向硅锗纳米线的带隙逐渐减小;对Si_(36)Ge_(24)H_(32)纳米线施加单轴应变,其能量带隙随拉应变的增加而单调减小.光学性质计算则表明:随着锗原子数的增加,[112]硅锗纳米线介电函数的峰位和吸收谱的吸收边均向低能量区移动;而随着拉应变的增大,吸收系数峰值呈现出逐渐减小的趋势,且峰位不断向低能量区移动,上述结果说明锗原子数的增加与施加拉应变均导致[112]硅锗纳米线的吸收谱产生红移.本文的研究为硅锗异质结纳米线光电器件研究与设计提供一定的理论参考.     

双层h-BN/Graphene结构稳定性及其掺杂特性的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法研究了双层h-BN/Graphene的稳定性及其掺杂特性.研究发现,双层h-BN/Graphene能带结构在K点处有一个小的带隙,在费米能处有类Graphene的线性色散关系.通过施加应变和掺杂来调节带隙,发现掺杂后费米能级附近引入的新能级,主要是N原子的贡献,掺杂后的Na原子和N,C之间存在电荷转移,材料转变为金属性.电荷的转移、载流子密度的增加,在电子元器件中有重要的应用前景.     

Electronic and magnetic properties of SiC nanoribbons by F termination

By using the first-principles calculations, the electronic properties are studied for the F-terminated SiC nanoribbons (SiCNRs) with either zigzag edges (ZSiCNRs) or armchair edges (ASiCNRs). The results show that the broader F-terminated ZSiCNRs are metallic and the edge states appear at the Fermi level, while the F-terminated ASiCNRs are always semiconductors independent of their width but the edge states do not appear due to the Si-C dimer bonds at the edges. The charge density contours analyses shows that the Si-F and Si-C bonds are all ionic bonds due to the much stronger electronegativities of the F and C atoms than that of the Si atom. However, the C-F bonds display a typical non-polar covalent bonding feature because of the electronegativity difference between the F and C atoms of 1.5 is a much smaller than that of between the F and Si atoms of 2.2, as well as the tighter bounded C 2s ²2p ² electrons with smaller orbital radius than the Si 3s ²3p ² electrons. For both the F- and the H-terminated ZSiCNRs, the ground state is a ferromagnetic semiconductor.     

Role of Al atom on structural and magnetic properties of β-SiC:Cr

The electronic structure, magnetic properties and also the bonding mechanism of the pure SiC and compounds SiC:Cr and SiC:Cr:Al have been studied using the Quantum SPRESSO Software within the density-functional theory (DFT). β-SiC, which is a nonmagnetic semiconductor, has more applications in industry. For the magnetic property, it is alloyed with transition metal. One of the transition metal is chromium. The calculations showed that its alloy at both Si site and C site (SiC:Cr) changes the physical properties of the host material and contributes in the molecular bond. It is seen that if the Al atom was doped in the compounds, SiC:Cr will produce hole carriers and the magnetic properties will thus increase to the considerable values due to the mediation effect. The magnetic property will create the up and down spin band gap to filter carriers. The charge density distribution illustrates that the Al atom has the atomic behavior in the compounds Sic:Cr:Al and does not contribute in the molecular bond. For comparison, the calculations were performed for the pure β-SiC.     

Electronic structure of SixGe1−x semiconductor solid solutions

The electronic structure of Si_xGe_1?x solid solutions is studied by means of a semi-ab initio method without involving the virtual crystal approximation. The calculations are performed on large cubic cells of the diamond lattice which contain a total of 64 Si and Ge atoms homogeneously distributed in different compositional ratios. The basis functions and the potentials used in the calculation give good band structures of elemental Si and Ge respectively. The calculated variation of band gap with x is in agreement with optical experiment: the two linear curves of gap vs concentration have different slopes at high and low x values with a crossing-over at about x=0.28. The band gaps are direct for xε (0, 0.28) and indirect for x ≥ 0.28. The density of states (DOS) of the solid solution can be very well approximated by the weighted average of the bulk Si and Ge DOS. There is a very slight charge transfer of about 0.05 electron per atom from Si to Ge.     

Fe_9Si的电子结构及铁磁性质的第一性原理研究

采用第一性原理的密度泛函理论(Density Functional Theory)赝势平面波方法,对Fe_9Si的电子结构和铁磁性质进行理论计算.计算结果表明:(1)Fe_9Si具有负的形成热-0.1094 eV/atom,结合能5.124eV/atom,表明Fe_9Si合金具有强结合力和结构稳定性;(2)Fe_9Si具有典型的金属能带特征,穿过Fermi能级的能带最主要是Fe的3d态电子的贡献,其次是来自Si的3p态电子的贡献.结合键不是单一金属键,而是金属键和共价键组成的混合键;(3)Fe_9Si的铁磁性主要来自Fe原子的未满层壳的3d态电子的自旋.计算结果为Fe_9Si铁磁性材料的设计与应用提供了理论依据.     

Fe9Si的电子结构及铁磁性质的第一性原理研究

采用第一性原理的密度泛函理论（Density Functional Theory）赝势平面波方法, 对Fe9Si的电子结构和铁磁性质进行理论计算。 计算结果表明： (1) Fe9Si具有负的形成热-0.1094 eV/atom, 结合能5.124 eV/atom, 表明Fe9Si合金具有强结合力和结构稳定性; (2) Fe9Si具有典型的金属能带特征, 穿过Fermi能级的能带最主要是Fe的3d态电子的贡献, 其次是来自Si的3p态电子的贡献。 结合键不是单一金属键, 而是金属键和共价键组成的混合键; (3) Fe9Si的铁磁性主要来自Fe原子的未满层壳的3d态电子的自旋。 计算结果为Fe9Si铁磁性材料的设计与应用提供了理论依据。     

Magnetism in transition-metal-doped silicon nanotubes

Using first-principles density functional calculations, we show that hexagonal metallic silicon nanotubes can be stabilized by doping with 3d transition metal atoms. Finite nanotubes doped with Fe and Mn have high local magnetic moments, whereas Co-doped nanotubes have low values and Ni-doped nanotubes are mostly nonmagnetic. The infinite Si24Fe4 nanotube is found to be ferromagnetic with nearly the same local magnetic moment on each Fe atom as in bulk iron. Mn-doped nanotubes are antiferromagnetic, but a ferrromagnetic state lies only 0.03 eV higher in energy with a gap in the majority spin bands near the Fermi energy. These materials are interesting for silicon-based spintronic devices and other nanoscale magnetic applications.     

First principles calculations of armchair graphene nanoribbons interacting with Cu atoms

We have investigated the electronic properties of bare, H-terminated, Cu-terminated and Cu-doped armchair graphene nanoribbons (AGNRs) using ab-initio approach. We found that H-termination enhances the stability and band gap whereas H extraction introduces dangling bands and lowers the band gap making bare ribbons indirect band gap semiconductors. The calculations revealed that strong hybridization between Cu atoms and AGNRs, lessen the band gap for Cu-terminated ribbons and gives rise to metallicity in Cu-doped AGNRs irrespective of their widths. Formation energy of considered ribbons yield that H-terminated AGNRs with lowest formation energy are most energetically favored, next are one edge Cu-terminated ribbons followed by bare ones whereas both edges Cu-doped ribbons are least energetically plausible. We predict that presence of Cu atoms in GNRs, significantly alter the band gap and can be used in band gap engineering of nanoribbons.     

Effects of the edge shape and the width on the structural and electronic properties of silicene nanoribbons

Under the generalized gradient approximation (GGA), the structural and electronic properties are studied for H-terminated silicene nanoribbons (SiNRs) with either zigzag edge (ZSiNRs) or armchair edge (ASiNRs) by using the first-principles projector-augmented wave potential within the density function theory (DFT) framework. The results show that the length of the Si-H bond is always 1.50 Å, but the edge Si-Si bonds are shorter than the inner ones with identical orientation, implying a contraction relaxation of edge Si atoms. An edge state appears at the Fermi level E_F in broader ZSiNRs, but does not appear in all ASiNRs due to their dimer Si-Si bond at edge. With increasing width of ASiNRs, the direct band gaps exhibit not only an oscillation behavior, but also a periodic feature of Δ_3n > Δ_3n+1 > Δ_3n+2 for a certain integer n. The charge density contours analysis shows that the Si-H bond is an ionic bond due to a relative larger electronegativity of H atom. However, all kinds of the Si-Si bonds display a typical covalent bonding feature, although their strength depends on not only the bond orientation but also the bond position. That is, the larger deviation of the Si-Si bond orientation from the nanoribbon axis as well as the closer of the Si-Si bond to the nanoribbon edge, the stronger strength of the Si-Si bond. Besides the contraction of the nanoribbon is mainly in its width direction especially near edge, the addition contribution from the terminated H atoms may be the other reason.     

锯齿型石墨烯纳米带的能带研究

运用π电子紧束缚模型,具体研究了锯齿型石墨烯纳米带(ZGNRs)的边界结构对能带,特别是费米面附近的导带和价带电子的影响.计算了七种不同边界结构的ZGNRs的能带色散关系及费米面附近价带电子在原胞中各原子上的分布情况.计算结果表明:两边界都无悬挂原子的NN-ZGNRs,只有一边界有悬挂原子的DN-ZGNRs,两边界都有五边形环的SPP-ZGNRs和ASPP-ZGNRs为金属性.两边界都有悬挂原子的DD-ZGNRs,一边界为五边形环另一边界无悬挂原子的PN-ZGNRs和一边界为五边形环另一边界有悬挂原子的P 关键词： 锯齿型石墨烯纳米带 紧束缚模型 电子密度分布 缺陷结构     

Scanning tunneling microscopy chemical signature of point defects on the MoS2(0001) surface

Using ab initio calculations, we have studied the modification of the electronic structure of the MoS2(0001) surface by several point defects: a surface S vacancy and different transition metal atoms substituting a S atom (Pd, Au, Fe, and V). With a S vacancy, a gap state appears with weight mostly on the Mo and S atoms surrounding the vacancy. The substitutional atoms of complete d band (Pd and Au) do not present magnetic polarization and slightly modify the DOS near the Fermi energy. On the other hand, the incomplete d band atoms (Fe and V) present spin polarization and modify significantly the states near the band edges. From calculated STM images and STS curves, we show that this chemical signature can be measured and used to characterize the surface defects of the substrate which are suitable nucleation centers for nanocluster growth.     

Density functional investigation of structural, electronic and optical properties of Ge-doped ZnO

Recent experiments reported fascinating phenomenon of photoluminescence (PL) blueshift in Ge-doped ZnO. To understand it, we examined the structural, electronic and optical properties of Ge-doped ZnO (ZnO:Ge) systematically by means of density functional theory calculations. Our results show that Ge atoms tend to cluster in heavily doped ZnO. Ge clusters can limit the conductivity of doped ZnO but reinforce the near-band-edge emission. The substitutional Ge for Zn leads to Fermi level pinning in the conduction band, which indicates Ge-doped ZnO is of n-type conductivity character. It is found that the delocalized Ge 4s states hybridize with conduction band bottom, and is dominant in the region near the Fermi level, suggesting that Ge-4s states provides major free carriers in ZnO:Ge crystal. The observed blueshift of PL in Ge-doped ZnO originates from the electron transition energy from the valence band to the empty levels above Fermi level larger than the gap of undoped ZnO. The electron transition between the gap states induced by oxygen vacancy and conduction band minimum may be the origin of the new PL peak at 590 nm.     

First-principle study of nanostructures of functionalized graphene

We present first-principle calculations of 2D nanostructures of graphene functionalized with hydrogen and fluorine, respectively, in chair conformation. The partial density of states, band structure, binding energy and transverse displacement of C atoms due to functionalization (buckling) have been calculated within the framework of density functional theory as implemented in the SIESTA code. The variation in band gap and binding energy per add atom have been plotted against the number of add atoms, as the number of add atoms are incremented one by one. In all, 37 nanostructures with 18C atoms, 3 × 3 × 1 (i.e., the unit cell is repeated three times along x-axis and three times along y-axis) supercell, have been studied. The variation in C–C, C–H and C–F bond lengths and transverse displacement of C atoms (due to increase in add atoms) have been tabulated. A large amount of buckling is observed in the carbon lattice, 0.0053–0.7487 Å, due to hydrogenation and 0.0002–0.5379 Å, due to fluorination. As the number of add atoms (hydrogen or fluorine) is increased, a variation in the band gap is observed around the Fermi energy, resulting in change in behaviour of nanostructure from conductor to semiconductor/insulator. The binding energy per add atom increases with the increase in the number of add atoms. The nanostructures with 18C+18H and 18C+18F have maximum band gap of 4.98 eV and 3.64 eV, respectively, and binding energy per add atom –3.7562 eV and –3.3507 eV, respectively. Thus, these nanostructures are stable and are wide band-gap semiconductors, whereas the nanostructures with 18C+2H, 18C+4H, 18C+4F, 18C+8F, 18C+10F and 18C+10H atoms are small band-gap semiconductors with the band gap lying between 0.14 eV and 1.72 eV. Fluorine being more electronegative than hydrogen, the impact of electronegativity on band gap, binding energy and bond length is visible. It is also clear that it is possible to tune the electronic properties of functionalized graphene, which makes it a suitable material in microelectronics.     

Ba8Ga16Si30与Sr8Ga16Ge30电子特性和热性质的研究

采用基于密度泛函理论和平面波赝势技术的CASTEP程序计算了笼状化合物Ba8Ga16Si30与Sr8Ga16Ge30的电子结构,并以此为基础进一步研究了它们的热性质。态密度和布居分析显示,Ba8Ga16Si30中笼上原子间成键趋于共价性,Sr8Ga16Ge30中笼上原子间成键趋于离子性,可以认为Ba8Ga16Si30的稳定性比Sr8Ga16Ge30强。带结构显示Ba8Ga16Si30与Sr8Ga16Ge30均为金属。热性质研究中提出一个新的模型,模型中认为Ba8Ga16Si30的笼内Ba原子和笼上Ga原子做爱因斯坦振荡,笼上Si原子做德拜运动,这种模型能够更好地描述Ba8Ga16Si30的热导。