First-principles study of narrow single-walled GaN nanotubes

Structural and electronic properties of narrow single-walled GaN nanotubes with diameter from 0.30 to 0.55 nm are investigated using the density functional method with generalized-gradient approximation. The calculations of total energies predict that the most likely GaN nanotubes in our calculation are (2,2), (3,2) and (3,3) nanotubes. From a detailed analysis we find that these narrow single-walled GaN nanotubes are all semiconductors, of which the armchair and chiral tubes are indirect-band-gap semiconductors whereas the zigzag ones have a direct gap except for (4,0) tube. The indirect band gap of (4,0) tube can stem from band sequence change induced by curvature effect. Our results show that the π-π hybridization effect and the formation of benign buckling separations play a key role in the band sequence changes of (4,0) tube.     

The calculation of energy gaps in small single-walled carbon nanotubes within a symmetry-adapted tight-binding model

This paper studies in detail the electronic properties of the semimetallic single-walled carbon nanotubes by applying the symmetry-adapted tight-binding model.It is found that the hybridization of π-σ states caused by the curvature produces an energy gap at the vicinity of the Fermi level.Such effects are obvious for the small zigzag and chiral single-walled carbon nanotubes.The energy gaps decrease as the diameters and the chiral angles of the tubes increase,while the top of the valence band and the bottom of the conduction band of armchair tubes cross at the Fermi level.The numeral results agree well with the experimental results.     

Tuning electronic and magnetic properties of AlN nanosheets with hydrogen and fluorine: First-principles prediction

We performed first-principles calculations to study the electronic structures and magnetic properties of the two-dimensional AlN nanosheet decorated with hydrogen and fluorine. The results show that the band gap of AlN nanosheet can be tuned significantly, and they can be a direct or an indirect semiconductor when decorated with H or F atoms on AlN surface. Spin-polarized calculations show that semi-decorated AlN sheets with H or F atoms can present a half-metal or p-type ferromagnetic (FM) semiconductor with Curie temperatures above room temperature. More interestingly, when AlN nanosheet co-decorated with H and F on each side, they show anisotropic semiconducting characters with a band gap of 1.02 eV. Our studies demonstrate that the decoration III-V group semiconductor nanosheets with foreign atoms might be an efficient route to tune the electronic and magnetic properties in low-dimensional materials.     

Stability and electronic structure of single-walled InN nanotubes

Based on density functional theory calculations, we predict the stability and electronic structures of single-walled indium nitride (InN) nanotubes. Compared with other group III-nitride nanotubes with a similar diameter, strain energies of InN nanotubes relative to their graphitic sheet are the lowest, suggesting the possibility of the formation of InN nanotubes. Considering the stability of a graphitic InN sheet, InN nanotubes are in metastable states with the stability between GaN nanotubes and AlN nanotubes. Contrary to the case of carbon nanotubes and BN nanotubes, the bond-length of both horizontal and vertical In–N bonds in InN nanotubes decreases as the tube diameter increases. InN nanotubes are all semiconductors with an almost constant band gap of about 1 eV. The existence of a direct gap in zigzag InN nanotubes and the small band gap indicate that they may have potential applications in light emitting devices and solar cells.     

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.     

MOVPE 生长GaN薄膜的 光致黄带发光与激发光源的相关性

用四种不同光源作为激发光源,研究了蓝宝石衬底金属有机物汽相外延方法生长的氮化镓薄膜的光致发光特性。结果发现用连续光作为激发光源时,光致发光谱中除出现365 nm的带边发射峰外,同时观察到中心波长位于约550 nm 的较宽黄带发光;而用脉冲光作为激发光源时其发光光谱主要是365 nm附近的带边发光峰,未观察到黄带发光。氮化镓薄膜的光致发光特性依赖于所用的激发光源性质。     

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.     

Cu、Ag和Au掺杂AlN的电磁性质的第一性原理研究

采用基于密度泛函理论（DFT）的平面波超软赝势法,研究了Cu、Ag、Au掺杂AlN的晶格常数、磁矩、能带结构和态密度。电子结构表明,Cu、Ag、Au的掺杂使在带隙中引入了由杂质原子的d态与近邻N原子的2p态杂化而成的杂质带,都为p型掺杂,增强了体系的导电性。Cu掺杂AlN具有半金属铁磁性,半金属能隙为0.442eV,理论上可实现100%的自旋载流子注入;Ag掺杂AlN具有很弱的半金属铁磁性;而Au掺杂AlN不具有半金属铁磁性。因此,与Ag、Au相比,Cu更适合用来制作AlN基稀磁半导体。     

Common electronic band gaps and similar optical properties of ZnO nanotubes

Electronic and optical properties of single-walled zinc oxide （ZnO） nanotubes are investigated from the firstprinciples calculations. Electronic structure calculations show that ZnO nanotubes are all direct band gap semiconducting nanotubes and the band gaps are relatively insensitive to the diameter and chirality of tubes. The origin of the common electronic band gaps of ZnO nanotubes is explained in terms of band-folding from the two-dimensional band structure of graphite-like sheet. Moreover, the optical properties such as dielectric function and energy loss function spectra of different ZnO nanotubes are very similar, relatively independent of diameter and chirality of tubes. The calculated dielectric function and loss function spectra show a moderate optical anisotropy with respect to light polarization.     

Aqueous chemical growth of free standing vertical ZnO nanoprisms,nanorods and nanodiskettes with improved texture co-efficient and tunable size uniformity

Tuning the morphology, size and aspect ratio of free standing ZnO nanostructured arrays by a simple hydrothermal method is reported. Pre-coated ZnO seed layers of two different thicknesses (≈350 nm or 550 nm) were used as substrates to grow ZnO nanostructures for the study. Various parameters such as chemical ambience, pH of the solution, strength of the Zn²⁺ atoms and thickness of seed bed are varied to analyze their effects on the resultant ZnO nanostructures. Vertically oriented hexagonal nanorods, multi-angular nanorods, hexagonal diskette and popcorn-like nanostructures are obtained by altering the experimental parameters. All the produced nanostructures were analysed by X-ray powder diffraction analysis and found to be grown in the (002) orientation of wurtzite ZnO. The texture co-efficient of ZnO layer was improved by combining a thick seed layer with higher cationic strength. Surface morphological studies reveal various nanostructures such as nanorods, diskettes and popcorn-like structures based on various preparation conditions. The optical property of the closest packed nanorods array was recorded by UV-VIS spectrometry, and the band gap value simulated from the results reflect the near characteristic band gap of ZnO. The surface roughness profile taken from the Atomic Force Microscopy reveals a roughness of less than 320 nm.     

Be, O共掺杂实现p型AlN的第一性原理研究

基于密度泛函理论框架下的第一性原理平面波超软赝势方法,研究了掺杂和非掺杂AlN体系的晶格参数、 能带结构、总体态密度、分波态密度、差分电荷分布及电荷集居数.计算结果表明: Be掺杂AlN晶体能够在能隙中形成深受主能级,空穴载流子局域于价带顶, 而引入了激活施主O原子的Be, O共掺杂方法,能使受主能带变宽、非局域化特征明显. 同时,受主能级向低能方向移动,形成了浅受主能级, 从而提高了Be原子的掺杂浓度和系统的稳定性. Be, O共掺杂更有利于获得p型AlN.     

Determination of the exciton binding energy in single-walled carbon nanotubes

We report that measurements of the Raman intensity versus applied voltage are sensitive to filling of the density of states and enable us to measure the second band gap in specific semiconducting single-walled carbon nanotubes (SWNTs). Raman scattering preferentially selects sets of SWNTs whose excitonic transitions are resonant with the incident or scattered photon energies. Simultaneous measurement of the electronic gap and exciton resonance allows us to infer binding energies for the exciton of 0.49+/-0.05 and 0.62+/-0.05 eV for tubes of (10, 3) and (7, 5), respectively. Metallic SWNTs exhibit no excitonic feature.     

Topotactical Route to Multiwalled Cerium Oxide Nanotubes from MWCNTs

The development of new strategies for synthesizing 1D cerium oxide (CeO₂) hollow nanostructures has attracted much attention in recent years due to the importance of their superior properties and highly anisotropic geometry. This study reports an unpublished route of fabricating novel multiwalled CeO_2-δ nanotubes (CeO_2-δ NTs) in which the entire volume of functionalized multiwalled carbon nanotubes (f-MWCNTs) is converted into the CeO_2-δ pseudomorph through oxidation and dehydration topotactic reactions. The stable CeO_2-δ (111) planes are topotactically grown on the curved C (002) planes, preferentially exposed along the nanotube axis. In their initial condition, the novel nanotubes consist of Ce oxyhydroxide (CeO_2-x(OH)_x) with residual carbon. When heating the air up to 500 °C, CeO_2-x(OH)_x transforms gradually by dehydration into CeO_2-δ, while the residual carbon is oxidized. Despite compositional changes, nanotubes maintain their multiwalled structural integrity up to ≈550 °C. The CeO_2-δ NTs exhibit an unusually high presence of Ce³⁺ ions and surface O vacancies, contributing to a low direct band gap ranging from 2.67 to 2.32 eV compared to their NPs counterparts (3.2 eV).     

Pressure dependence of the optical-absorption edge of AlN and graphite-type BN

Effect of pressure on the band gaps on AlN and graphite-type BN (g-BN) has been studied up to 2.7 GPa at room temperature by measuring the optical-absorption edge of single crystals of each substance pressurized in a sapphire-anvil cell. The direct band gap of AlN shifted towards higher energy at a rate of 49±1 meV/GPa, whereas in g-BN the pressure dependence of the band gap was −36±1 meV/GPa. The results are compared with existing first-principles calculations.     

C与Na掺杂AlN电子结构与光学性质的第一性原理研究

基于密度泛函理论(DFT)的平面波超软赝势法,计算了本征AlN,C-AlN,Na-AlN以及C-Na-AlN四种体系的电子结构和光学性质.得出结论：掺杂后各体系与本征AlN相比发生了晶格畸变,C-Na-AlN体系的结合能最小,体系最稳定.掺杂体系相比于本征AlN,禁带宽度都有不同程度的减小,导致电子在体系内跃迁时的概率增大,其中C-Na-AlN体系尤为明显,电子跃迁所需要的能量更小.掺杂后吸收带边发生了红移,拓宽了AlN体系对光的响应范围,增强了光吸收,并且C-Na-AlN体系在可见光区域内光吸收能力最强,在介电函数图的分析中可以得到,C-Na-AlN体系的介电常数最大,表明其电荷束缚能力最强,体系稳定性强,极化能力最好.     

Cu掺杂的AlN铁磁性和光学性质的第一性原理研究

采用基于密度泛函理论（DFT）的总体能量平面波超软赝势方法,结合广义梯度近似（GGA）,对Cu掺杂AlN 32原子超原胞体系进行了几何结构优化,计算了Cu掺杂AlN的晶格常数,能带结构,电子态密度和光学性质.结果表明,Cu掺杂AlN会产生自旋极化状态,能带结构显示半金属性质,掺杂后带隙变窄,长波吸收加强,能量损失明显减小.同传统的稀磁半导体（DMS）相比,Cu掺杂AlN不会有铁磁性沉淀物的问题,因为Cu本身不具有磁性.因而,Cu掺杂的AlN也许是一种非常有前途的稀磁半导体. 关键词： AlN 第一性原理 铁磁性 光学性质     

锯齿型碳纳米管的结构衍生及电子特性

利用密度泛函理论研究锯齿型单、双壁碳纳米管从核到管状团簇直至纳米管的逐层结构衍生.研究结果表明五边形结构在管状团簇生长中发挥关键作用.此外,基于管状团簇的研究,运用周期性边界条件得到锯齿型单、双壁碳纳米管,并通过计算能带和态密度研究其电子特性.对单壁(n,0)和双壁(n,0)@(2n,0)碳纳米管,当n=3q(q为整数)时,具有金属或窄带隙半导体特性;n?=3q时,具有较宽带隙半导体特性,且带隙随管径的增加而减小.然而,小管径碳纳米管受曲率效应的明显影响,n?=3q的(4,0),(4,0)@(8,0)和(5,0)@(10,0)均呈现金属性;n=3q的(6,0)@(12,0)则表现出明显的半导体特性.     

Electronic properties of single-walled V2O5 nanotubes

Atomistic models of quasi-one-dimensional vanadium pentoxide nanostructures—single-walled nanotubes formed by rolling (010) layers of V₂O₅ are constructed and their electronic properties and bond indices are studied using the tight-binding band method. We show that all zigzag (n,0)- and armchair (n,n)-like nanotubes are uniformly semiconducting, and the band gap trends to vanish as the tube diameters decrease. The V-O covalent bonds were found to be the strongest interactions in V₂O₅ tubes, whereas V-V bonds proved to be much weaker.     

Molecular dynamic investigation of mechanical properties of armchair and zigzag double-walled carbon nanotubes under various loading conditions

Using molecular dynamic simulation (MDS), effects of chirality and Van der Waals interaction on Young's modulus, elastic compressive modulus, bending, tensile, and compressive stiffness, and critical axial force of double-walled carbon nanotube (DWCNT) and its inner and outer tubes are considered. Achieving the highest safety factor, mechanical properties have been investigated under applied load on both inner and outer tubes simultaneously and on each one of them separately. Results indicate that as a compressive element, DWCNT is more beneficial than single-walled carbon nanotube (SWCNT) since it carries two times higher compression before buckling. Except critical axial pressure and tensile stiffness, in other parameters zigzag DWCNT shows higher amounts than armchair type. Outer tube has lower strength than inner tube; therefore, most reliable design of nanostructures can be attained if the mechanical properties of outer tube taken as the properties of DWCNT.     

Plasma induced by pulsed laser and fabrication of silicon nanostructures

It is interesting that in preparing process of nanosilicon by pulsed laser, the periodic diffraction pattern from plasmonic lattice structure in the Purcell cavity due to interaction between plasmons and photons is observed. This kind of plasmonic lattice structure confined in the cavity may be similar to the Wigner crystal structure. Emission manipulation on Si nanostructures fabricated by the plasmonic wave induced from pulsed laser is studied by using photoluminescence spectroscopy.The electronic localized states and surface bonding are characterized by several emission bands peaked near 600nm and 700nm on samples prepared in oxygen or nitrogen environment. The electroluminescence wavelength is measured in the telecom window on silicon film coated by ytterbium. The enhanced emission originates from surface localized states in band gap due to broken symmetry from some bonds on surface bulges produced by plasmonic wave in the cavity.