Density functional theory investigation of the mechanical properties of single-walled carbon nanotubes

We detail the results of our first-principles study based on density functional theory on the elastic properties of (6, 6) single-walled carbon nanotubes (SWCNTs) in both periodic and non-periodic systems. The Young’s modulus and the shear modulus of nanotubes were evaluated through applying axial and torsion strains on periodic, H-, and C-capped nanotubes. Based on our first-principles calculations, the Young’s modulus of the periodic nanotube tens to increase as the nanotube’s length increases, and finally approaches a constant value at long tube lengths. It was found that the Young’s modulus characteristic of H- and C-capped nanotubes exhibit contradictory behaviors during compression with the periodic nanotube. Our calculations also predict that the Young’s and Shear moduli of C-capped nanotubes are larger than those of other types of nanotubes.     

Experimental and theoretical investigation of silver-coated ZnO nanorod arrays as antennas for the visible and near-IR spectral range

A new design of optical antennas consisting of zinc oxide (ZnO) nanorods covered by a thin metal film is proposed. Arrays of highly oriented ZnO nanorods perpendicular to a substrate and covered by a thin silver film have been obtained using methods of carbothermal synthesis and magnetron sputtering. The problems of electromagnetic wave diffraction on a single metal/dielectric nanovibrator (situated at the interface of dielectrics) and on a two-dimensional periodic array of these nanovibrators have been solved. The results of calculations of the electrodynamic characteristics of optical antennas with various lengths have been compared to experimental data.     

Controllable synthesis of periodic flower-like ZnO nanostructures on Si subwavelength grating structures

We report on the periodic well-defined flower-like zinc oxide (ZnO) nanostructures (NSs) self-assembled through a simple hydrothermal method using silicon (Si) subwavelength grating (SWG) structures. The Si SWGs serve as building blocks for constructing a two-dimensional (2D) periodic architecture to integrate the one-dimensional (1D) ZnO NSs. Various controlled morphologies of ZnO NSs with high crystallinity are obtained by changing the growth conditions. For 1D ZnO NSs integrated on periodic hexagonal Si SWG structures, the reflection characteristics are investigated in comparison with the conventional ZnO nanorod (NR) arrays. For a three-dimensional (3D) flower-like ZnO NS on Si SWGs, a relatively low total reflectance of < 8% at wavelengths of 300-1050 nm is achieved compared to the ZnO NRs on Si substrate.     

Titania-assisted dispersion of carboxylated single-walled carbon nanotubes in a ZnO sol for transparent conducting hybrid films

We report a facile chemical route for stabilizing a dispersion of carboxylated single-walled carbon nanotubes (SWCNTs) in a ZnO sol. The dispersion is stabilized via capping of the carboxyl groups on the SWCNT surface by a titania layer, which was confirmed by Fourier transform infrared spectroscopy and transmission electron microscopy. We also demonstrate that the conductivity of the films prepared from the SWCNT/TiO(x)/ZnO sol is dramatically enhanced by thermal treatment and that the thermal stability of the hybridized films with the ZnO sol is notably improved relative to that of a pristine SWCNT film. The structural and chemical changes of the fabricated films were characterized by Raman spectroscopy. As one application, it was presented that thermally treated SWCNT/TiO(x)/ZnO hybrid thin film sensors showed hydrogen sensing characteristics even at room temperature.     

Controllable Synthesis of [11−2−2] Faceted InN Nanopyramids on ZnO for Photoelectrochemical Water Splitting

Indium nitride (InN) is one of the promising narrow band gap semiconductors for utilizing solar energy in photoelectrochemical (PEC) water splitting. However, its widespread application is still hindered by the difficulties in growing high‐quality InN samples. Here, high‐quality InN nanopyramid arrays are synthesized via epitaxial growth on ZnO single‐crystals. The as‐prepared InN nanopyramids have well‐defined exposed facets of [0001], [11?2?2], [1?212], and [?2112], which provide a possible routine for understanding water oxidation processes on the different facets of nanostructures in nanoscale. First‐principles density functional calculations reveal that the nonpolar [11?2?2] face has the highest catalytic activity for water oxidation. PEC investigations demonstrate that the band positions of the InN nanopyramids are strongly altered by the ZnO substrate and a heterogeneous n–n junction is naturally formed at the InN/ZnO interface. The formation of the n–n junction and the built‐in electric field is ascribed to the efficient separation of the photogenerated electron–hole pairs and the good PEC performance of the InN/ZnO. The InN/ZnO shows good photostability and the hydrogen evolution is about 0.56 µmol cm^?2 h^?1, which is about 30 times higher than that of the ZnO substrate. This study demonstrates the potential application of the InN/ZnO photoanodes for PEC water splitting.     

First-principles study of ZnO cluster-decorated carbon nanotubes

We have investigated the structural, electronic and carbon monoxide (CO) detection properties of the ZnO cluster-decorated single-walled carbon nanotubes (SWCNTs) by using density functional theory (DFT). The stable structures of hybrid ZnO/SWCNT materials are that the ZnO cluster plane is perpendicular to the surface of SWCNTs with the Zn atoms towards the SWCNTs (Zn atom above axial C-C bond or above the C atom). For the ZnO cluster-decorated semiconducting SWCNTs, the SWCNTs present p-type characteristics which may lead to the decrease of conductance upon illumination with ultraviolet (UV) light. The CO can be adsorbed on the hybrid ZnO/SWCNT materials due to the charge transfer between them. Compared with isolated ZnO clusters or bare SWCNTs, the ZnO/SWCNT network would have excellent CO detection ability due to their suitable adsorption energy and conductivity.     

Atomic and electronic structure of [0001]/($$\bar{1}\bar{2}30$$) Σ7 symmetric tilt grain boundary in ZnO bicrystal with linear current-voltage characteristic

The atomic and electronic structures of [0001]/( ) Σ 7 symmetric tilt grain boundary in an undoped ZnO bicrystal were investigated by high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. HRTEM imaging and atomistic calculations revealed that the grain boundary was composed of at least two types of structural units. It was also found that one of the structural units has two threefold-coordinated atoms per a unit and the other has two fivefold-coordinated atoms. First-principles calculations indicated that these atoms with various coordination numbers do not form deep unoccupied electronic states in the band gap of ZnO, which is in consistency with a linear current-voltage characteristic observed for the bicrystal with the Σ 7 boundary.     

A biocompatible chitosan composite containing phosphotungstic acid modified single-walled carbon nanotubes

Surface modification of carbon nanotubes is crucial for the dispersion and interfacial adhesion of carbon nanotubes in polymer composites. Here we present a novel method to construct single-walled carbon nanotube/chitosan composites using phosphotungstic acid as an anchor reagent to modify single-walled carbon nanotubes. The most direct benefit from this method is that this modification is mild but effective: the induced defects on single-walled carbon nanotubes are negligible based on Raman and transmission electron microscopy observations; and homogeneous dispersion of single-walled carbon nanotubes in chitosan matrices and strong binding between single-walled carbon nanotubes and chitosan are achieved. Moreover, according to the results of tetrazolium-based colorimetric assays in vitro, we demonstrate that the produced phosphotungstic-acid-modified single-walled carbon nanotube/chitosan composites have good biocompatibility. Thus, our study provides a feasible route to fabricate biocompatible composites containing single-walled carbon nanotubes for potential application in bone tissue engineering.     

Accurate calculations of the Peierls stress in small periodic cells

The Peierls stress for a [111]-screw dislocation in bcc Tantalum is calculated using an embedded atom potential. More importantly, a method is presented which allows accurate calculations of the Peierls stress in the smallest periodic cells. This method can be easily applied to ab initio calculations, where only the smallest unit cells capable of containing a dislocation can be conviently used. The calculation specifically focuses on the case where the maximum resolved shear stress is along a {110}-plane. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fabrication of two-dimensional zinc oxide nanorod patterns and their application for optical diffraction grating effect

We present a novel method for fabricating two-dimensional arrays of zinc oxide (ZnO) nanorod patterns. Vertically aligned ZnO nanostructure patterns were synthesized by hydrothermal growth on a two-dimensional seed layer pattern formed by atomic layer deposition. Various characteristics of ZnO seed films were experimentally studied in order to optimize the fabrication of ZnO rod structures in terms of their uniformity and vertical alignment on the two-dimensional-patterned surfaces. Using these ZnO structures as hierarchical electrodes, we demonstrate that localized electric fields in the proximity of ZnO rod patterns can be used for periodic alignment of liquid crystal molecules, resulting in the optical diffraction grating effect. Process conditions for creating ZnO rod patterns that best enhance diffraction efficiency are further discussed. The current study is the first demonstration of hierarchical ZnO rod patterns as electrodes for optical modulation of a medium. We believe this will be beneficial for future optical applications.     

空气中热氧化电沉积Zn纳米晶制备一维ZnO纳米针及其生长机制与场发射效应的研究

在金属基板表面电沉积一层金属Zn纳米晶，将该纳米晶置于高温炉中，通过热氧化法成功制备了一维ZnO纳米针。研究了不同的热氧化温度因素对一维ZnO纳米针的制备及其形貌的影响。在本方法中，一维ZnO纳米针材料对应于初始电沉积层的Zn纳米晶颗粒，这与其他方法中ZnO的生长机制不同，可以认为本方法的ZnO的生长遵从自催化扩散机制。同时，研究了一维ZnO纳米针薄膜的场发射效应。     

Analysis of SAW propagation in gratings on ZnO/diamond substrates

The space harmonic method is used to analyze surface acoustic wave (SAW) propagation under an infinite periodic metal grating on ZnO/diamond composite layered substrates. Dispersion properties for shorted and open gratings are derived as a function of the thickness of the grating electrodes. From these dispersion relations, the coupling of modes (COM) parameters are derived. Energy profiles inside ZnO/diamond show that the energies contained in each of the ZnO and diamond layers are of the same order when the thickness of the ZnO layer is P/pi (P = grating period) and that the energy is contained within two wavelengths below the ZnO/diamond interface.     

Evidence for Fe(2+) in wurtzite coordination: iron doping stabilizes ZnO nanoparticles

First-principles calculations are used to investigate the structural and electronic properties of Fe-doped ZnO nanoparticles. Based on extensive validation studies surveying various density functionals, the hybrid functional PBE0 is employed to calculate the structures, formation energies, and electronic properties of Fe in ZnO with Fe concentrations of 6.25, 12.5, and 18.75 at%. Substitution of Zn by Fe, zinc vacancies, and interstitial oxygen defects is studied. High-resolution inner-shell electron energy loss spectroscopy measurements and X-ray absorption near-edge structure calculations of Fe and O atoms are performed. The results show that Fe-doped ZnO nanoparticles are structurally and energetically more stable than the isolated FeO (rocksalt) and ZnO (wurtzite) phases. The Fe dopants distribute homogeneously in ZnO nanoparticles and do not significantly alter the host ZnO lattice parameters. Simulations of the absorption spectra demonstrate that Fe(2+) dominates in the Fe-doped ZnO nanoparticles reported recently, whereas Fe(3+) is present only as a trace.     

铝掺杂单壁扶手型(6,6)硅纳米管电子结构和光电性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算,研究了铝掺杂对单壁扶手型(6,6)硅纳米管电子结构和光电性质的影响。结果表明,本征态硅纳米管属于直接带隙半导体,其禁带宽度为0.42eV,而铝掺杂硅纳米管为间接带隙半导体,其禁带宽度为0.02eV。单壁扶手型(6,6)硅纳米管的价带顶主要由Si-3p态电子构成,而其导带底则主要由Si-3p态电子决定。同时通过铝掺杂,使硅纳米管的禁带宽度变窄,吸收光谱产生红移,研究结果为硅纳米管在光电器件方面的应用提供了理论基础。     

Large‐Scale Horizontally Aligned ZnO Microrod Arrays with Controlled Orientation,Periodic Distribution as Building Blocks for Chip‐in Piezo‐Phototronic LEDs

A novel method of fabricating large‐scale horizontally aligned ZnO microrod arrays with controlled orientation and periodic distribution via combing technology is introduced. Horizontally aligned ZnO microrod arrays with uniform orientation and periodic distribution can be realized based on the conventional bottom‐up method prepared vertically aligned ZnO microrod matrix via the combing method. When the combing parameters are changed, the orientation of horizontally aligned ZnO microrod arrays can be adjusted (θ = 90° or 45°) in a plane and a misalignment angle of the microrods (0.3° to 2.3°) with low‐growth density can be obtained. To explore the potential applications based on the vertically and horizontally aligned ZnO microrods on p‐GaN layer, piezo‐phototronic devices such as heterojunction LEDs are built. Electroluminescence (EL) emission patterns can be adjusted for the vertically and horizontally aligned ZnO microrods/p‐GaN heterojunction LEDs by applying forward bias. Moreover, the emission color from UV‐blue to yellow‐green can be tuned by investigating the piezoelectric properties of the materials. The EL emission mechanisms of the LEDs are discussed in terms of band diagrams of the heterojunctions and carrier recombination processes.     

Enhanced ferromagnetism in ZnO nanoribbons and clusters passivated with sulfur

Inspired by recent experimental results, the electronic and magnetic properties of sulfur-passivated ZnO clusters and zigzag nanoribbons have been studied using first principles calculations in the framework of the local spin density approximation. In the case of the ZnO nanoribbons, the sulfur atoms or thiol groups were attached in different ways to the zinc or oxygen atoms located at the edges, whereas in clusters, the sulfur atoms were set on the surface, mainly interacting with atoms with low-coordinate number. After an exhaustive atomic relaxation, we found that a magnetic moment emerges in zigzag nanoribbons both with and without sulfur-passivation on the edges. However, the magnitude of the magnetic moment is very sensitive to sulfur passivation. In particular, we found that when sulfur is attached to the zinc atoms in an alternating fashion along the ribbon edges, the magnetic moment is a maximum (1.4 μ_B/unit cell). In the case of clusters, we found that the Zn₁₅O₁₅ cluster exhibits a high spin moment of 5.5 μ_B when capped with sulfur atoms. Our calculations indicate that sulfur-passivating of ZnO nanosystems could be responsible for recently observed ferromagnetic responses. This article is published with open access at Springerlink.com     

Synthesis of one-dimensional ZnO nanoneedles using thermal oxidation process in the air and its application as filed emitters

In the present work, a novel route for synthesizing one-dimensional ZnO nanoneedles were introduced; that is, a pure Zinc nanocrystalline layer was firstly plated by using a periodic reverse pulse plating process, and then the ZnO nanoneedles grew upon the layer through thermal oxidation process in the air. The results showed that the oxidation temperature played a key role for growth, morphology and density of ZnO nanoneedles, which leads to the difference of their filed emission properties due to the various field enhancement factors β. A novel self-catalyzed diffusion model was proposed for interpreting the nanoneedles growth mechanism.     

碳锗掺杂对硅纳米管电子结构和光电性质的影响

采用基于密度泛函理论的第一性原理计算, 研究了C/Ge掺杂对单壁扶手型硅纳米管电子结构和光电性质的影响。结果表明, 本征态和C/Ge掺杂的硅纳米管均属于直接带隙半导体, 其价带顶主要由Si-3p态电子构成, 而导带底则主要由Si-3p态电子决定。通过C掺杂, 可使硅纳米管的禁带宽度减小, 静态介电常数增大, 吸收光谱产生红移; 而通过Ge掺杂, 可使硅纳米管的禁带宽度增大, 静态介电常数减小, 吸收光谱产生蓝移。研究结果为硅纳米管在光电器件方面的应用提供了理论基础。     

Energetic trends of single-walled carbon nanotube ab initio calculations

Hartree–Fock (HF) calculations for a variety of single-walled carbon nanotube (SWCNT) systems indicate linear relationships between electronic energies and changes in length and circumference for both armchair and zigzag type nanotubes. A simple protocol to predict energies for large SWCNT (C atoms >500) is developed through a set of structural parameters and AM1 optimized geometries from small SWCNTs. The energetic trends shown by the calculations are used to support the theory of SWCNT nucleation from a preformed carbon, or graphene with six 5-member rings, cap.     

ZnO准一维纳米结构生长形态的演化机理

利用气相生长系统,通过调控实验参数,制备了多种形貌的ZnO准一维结构,如纳米条带、[011-0]和[21-1-0]取向的单侧生齿的梳状纳米条带、微米尺度的梳状结构,由多节状六角棱柱和八角棱柱组装成的微米条带等.通过X射线衍射、扫描电子显微镜及其所加载的能谱分析和背散射电子衍射仪、高分辨透射电子显微镜等分析技术, 对其中具有代表性的介观结构进行了系统的形貌分析和细致的结构解析.分析出基本的结构单元及其复合体, 揭示了显微尺度下ZnO晶体的外形多样性以及其形态演化中的关联和规律,即ZnO纳米条带、梳状结构和多节状微米条带具有晶体结构上的同一性.