Atomistic study of structures and elastic properties of single crystalline ZnO nanotubes

The structural stability and Young's modulus of single crystalline ZnO nanotubes are investigated using atomistic simulations. Unlike the case for conventional layered nanotubes, the energetic stability of single crystalline ZnO nanotubes is related to the wall thickness. The potential energy of ZnO nanotubes with fixed outer and inner diameters decreases with increasing wall thickness, while the nanotubes with the same wall thickness are independent of the outer and inner diameters. The transformation of single crystalline ZnO nanotubes with a double layer from wurtzite phase to graphitic phase suggests the possibility of wall-typed ZnO nanotubes. The size-dependent Young's modulus of ZnO nanotubes is also investigated. The wall thickness plays a significant role in the Young's modulus of single crystalline ZnO nanotubes, whereas the variation of outer and inner diameters slightly affects the Young's modulus of nanotubes with same wall thickness.     

Molecular dynamics study on size-dependent elastic properties of silicon nanoplates

Using molecular dynamics simulations, the size-dependent Young's moduli of silicon nanoplates due to surface effects are investigated at intrinsic scale. The transformations of surface reconstructions are discussed in terms of the difference between the total strain energy densities (u_T) of the system from the MD and the specified strain energy densities (u_C). An analytical prediction for the effective Young's modulus is derived for the intrinsic scale specimens, and it agrees well with MD simulations.     

Modeling the size-dependent elastic properties of polymeric nanofibers

We present a strain gradient (SG) theory to explain the strongly inverse size dependence between the elastic modulus and fiber diameter in polymeric nanofibers. For centrosymmetric and isotropic materials we showed that the three length-scale parameters can be combined into a single parameter that can be used to predict the onset of the size-dependent trend when the fiber diameter is reduced past its critical size. To address the issue of whether the SG offers a plausible explanation of the size-dependent behavior we conducted a series of uniaxial tensile and static bending tests involving polycaprolactone nanofibers. Since the elastic modulus is highly sensitive to the fiber diameter, it is necessary to correct the experimental data to account for the lack of circularity in the cross-section of the real fiber. Additionally, we applied the SG model to study the size-dependent elastic properties of polypyrrole nanotubes. By approaching the SG theory from a dynamics point of view, our model is able to capture size-dependent effects in the mechanics of fine-scale materials for both static and dynamic responses.     

Low-temperature synthesis and photocatalytic properties of ZnO nanotubes by thermal oxidation of Zn nanowires

A low-cost and catalyst-free two-step approach has been developed to produce ZnO nanotubes (ZNTs) by simple thermal oxidation of Zn nanowires under 20?Pa at a low temperature of 400?°C. The growth mechanism of ZNTs is discussed in detail. The formation of these tubular structures is closely linked to the oxidation pressure and temperature, which involves a process consisting of the deposition of Zn nanowires, cracking of the Zn nanowires and sublimation of the Zn cores, and subsequent oxidation to ZNTs. The optical properties were studied by using Raman and photoluminescence spectra, where a strong green emission related to the single ionized oxygen vacancy appears. The photocatalytic activity measurement indicates an enhanced photocatalytic activity of the prepared ZNTs due to their high surface-to-volume ratios and abundant oxygen vacancies near the surfaces of the ZNTs. This type of high surface area structural ZNTs could find promising potential for optoelectronic and environmental applications.     

A generalized description of the elastic properties of nanowires

We report a model of nanowire (NW) mechanics that describes force vs displacement curves over the entire elastic range for diverse wire systems. Due to the clamped-wire measurement configuration, the force response in the linear elastic regime can be linear or nonlinear, depending on the system and the wire displacement. For Au NWs the response is essentially linear since yielding occurs prior to the onset of the inherent nonlinearity, while for Si NWs the force response is highly nonlinear, followed by brittle fracture. Since the method describes the entire range of elastic deformation, it unequivocally identifies the yield points in both of these materials.     

ZnO纳米锥光致发光性及印刷ZnO纳米锥场发射性能研究

利用物理热蒸发法制备大规模的蒲公英状的ZnO纳米锥,利用荧光光谱仪对ZnO纳米锥进行了光致发光性能测试.针对现有的丝网印刷碳纳米管(CNTs)薄膜需要各种后处理工艺后才能改善其场发射特性的问题,提出了一种不需任何后处理丝网印刷ZnO纳米锥的浆料配制工艺.用该工艺制备的丝网印刷ZnO纳米锥的场发射特性测试表明,ZnO纳米锥与制浆剂质量比为3∶5的薄膜的开启场强最低为2.25V/μm(电流密度为1μA/cm2),在4.6V/μm场强下,阳极荧光粉的发光点亮度高且分布均匀.说明该方法成本低,工艺简单,无需任何后处理,在ZnO纳米锥场发射显示器的制作中有很好的实际应用价值.     

ZnO nanosquids: branching nanowires from nanotubes and nanorods

One-dimensional (1D) semiconductor nanostructures are promising building blocks for future nanoelectronic and nanophotonic devices. ZnO has proven to be a multifunctional and multistructural nanomaterial with promising properties. Here we report the growth of ZnO nanosquids which can be directly grown on planar oxidized Si substrates without using catalysts and templates. The formation of these nanosquids can be explained by the theory of nucleation, and the vapor-solid crystal growth mechanism. The branching nanowires of these ZnO nanosquids could have potential application in multiplexing future nanoelectronic devices. The sharp band-edge emission at approximately 380 nm indicates that these ZnO nanosquids are also applicable for interesting optoelectronic devices.     

A study of the growth mechanism of CVD-grown ZnO nanowires

ZnO nanowires were grown by CVD process using both pure Zn powder and a mixture of ZnO and graphite powders as the Zn source, and the key factors controlling nanowire growth were identified. In both processes, the partial pressure of zinc vapor determines the prevailing growth morphology and is sensitive to the growth conditions. In the case of Zn powder as the source, the predominant growth mechanism is driven by self-catalyzed growth on the Si substrate, and in the case of a mixture of ZnO and graphite used as the source, the formation of ZnO nanowires is controlled by the vapor–liquid-solid mechanism, where the gold particles serve as catalyst.     

Theoretical study of elastic properties of SiC nanowires of different shapes

The atomic structure and elastic properties of silicon carbide nanowires of different shapes and effective sizes were studied using density functional theory and classical molecular mechanics. Upon surface relaxation, surface reconstruction led to the splitting of the wire geometry, forming both hexagonal (surface) and cubic phases (bulk). The behavior of the pristine SiC wires under compression and stretching was studied and Young's moduli were obtained. For Y-shaped SiC nanowires the effective Young's moduli and behavior in inelastic regime were elucidated.     

Luminescent properties of ZnO nanowires and as-grown ensembles

Vertically aligned ZnO nanowires were synthesized on a sapphire ([Formula: see text]) substrate by vapour deposition and their light-emitting properties were characterized using photoluminescence and cathodoluminescence spectroscopies. Besides the nanowires, there exists a mosaic ZnO film on the substrate surface. Comparison of the luminescent properties of the as-grown ensemble and the nanowires extracted from it shows that the mosaic film is the major source of the defect-related green emission while the nanowires possess highly crystalline quality with virtually no defects. Photoemission spectroscopy shows that the valence band states associated with O 2p from the as-grown sample are diminished compared with those from the extracted nanowires. These findings suggest that the green emission partly arises from oxygen vacancies located on the surface of the mosaic film.     

Mn掺杂ZnO纳米线的拉曼散射和光致发光特性

研究了不同Mn掺杂含量的ZnO纳米线在室温条件下的拉曼散射和光致发光性能,发现Mn掺杂入ZnO后引入了部分应力,在其拉曼光谱中表现出拉曼峰的位置发生偏移,Mn的掺杂含量越高,峰偏移得越明显.Mn的掺杂对ZnO纳米线的发光性能也有影响,尽管掺杂后仍保持有较为明显的紫外发光峰,但是,随着Mn含量的增加,紫外发光峰的强度降低,并且半峰宽逐渐增大.此外,Mn的掺杂明显地改变了ZnO紫外发光峰的位置.     

TiO2纳米管与纳米线的光电化学研究

利用在钛箔表面沉积一层TiO2纳米粒子作为晶种,与NaOH反应,通过改变反应温度制备了TiO2纳米管与纳米线.制备了TiO2纳米管和纳米线膜电极,并进行了光电化学测试.光电化学实验表明,混晶结构TiO2纳米管和纳米线显示出优良的光电转化性能.     

Bending properties of carbon nanotubes encapsulating solid nanowires

Using empirical potentials and atomistic simulations, we model three-point bend tests of single-walled carbon nanotubes encapsulating metal nanowires. The presence of a metal nanowire inside the nanotube greatly suppresses the tube-buckling instability. Increasing tube diameter leads to an increase in the bending strength; however, in contrast to hollow tubes, there is no decrease in the maximum deflection before buckling. Analysis of the principal bending vibrational mode shows a lowering of the frequency, associated with increased tube inertia. Remarkably, metal-filled tubes exhibit strong damping of oscillations whereas unfilled single-walled and multiwalled tubes show no damping. Our studies demonstrate the benefits of filling tubes with solids to modify bending strength and flexibility, suggesting applications for nanotube-based elements in micromechanical devices or nanoprobes.     

The magnetic properties of Gd doped ZnO nanowires

We present a study of the ferromagnetic properties of Gd doped ZnO nanowires (Nws) fabricated by means of a chemical vapor deposition process. The sample was grown with a Gd mole ratio 5% in a mixed Zn/Mn source under a constant O₂/Ar gas mixture flowing at 580 °C followed by annealing at 800 °C. We found that the magnetic properties of ZnO:Gd Nws are a function of the external magnetic field and temperature. An average value of the moment per Gd atom is as high as 3278 μB as compared to its atomic moment of 8 μB, showing that the ZnO:Gd Nws are an intrinsic diluted magnetic semiconductor. The unprecedented colossal moment is attributed to the effective Ruderman-Kittel-Kasuya-Yosida exchanging interaction.     

Different mechanical properties of the pristine and hydrogen passivated ZnO nanowires

We comparatively calculate the Young’s moduli of the pristine and the hydrogen passivated ZnO nanowires using the first-principles approaches. It is found that the pristine nanowire has the higher Young’s modulus, but the corresponding hydrogen passivated nanowire has the lower Young’s modulus than that of bulk ZnO. The physical origin of the opposite tendency can be attributed to both the different surface relaxations of the two kinds of ZnO nanowires and the core nonlinear effect.     

Pd-nanoparticle-decorated ZnO nanowires: ultraviolet photosensitivity and photoluminescence properties

ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O(2) molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.     

Crabwise ZnO nanowires: growth and field emission properties

Crabwise ZnO nanowires with an average length of 5 microm and an average diameter of 30 nm were selectively grown on ZnO:Ga/glass templates. Cathodoluminescence measurement indicated that the crystal quality of the crabwise ZnO nanowires was good. With an applied voltage of 120 V, the crabwise ZnO nanowire field emitters gave an emission current of 0.1 mA/cm2. Moreover, the field enhancement factor, beta, of the crabwise ZnO nanowires was approximately 980.     

A comprehensive study on the size-dependent hygrothermal analysis of exponentially graded microplates on elastic foundations

Abstract

In this paper, the effects of hygrothermal conditions on various behaviors, such as bending, free vibration, mechanical and thermal buckling, of exponentially graded microplates lying on two-parameter elastic foundations are investigated. The trigonometric four-variable plate theory incorporated to the modified couple stress theory (MCST) is employed to derive the equations of motion. The present MCST contains an internal material length scale parameter, thus it can capture the size effect. The microplate is assumed to be subjected to a temperature rise and moisture concentration which are varied linearly through the thickness of the plate. Based on an exponential law, the material properties of the microplate are graded only in z direction. The equations of motion are solved analytically to obtain the displacements, stresses, eigenfrequencies and critical buckling load and temperature of the microplates. The present results are validated by comparing them with those previously published. The numerical examples reveal that considering the size effect and/or the elastic foundations leads to an increment in plate stiffness and thereby leads to a decrement in the deflection and an increment in eigenfrequency and buckling loads. It is also shown that the size effect is negligible for the thicker plate.     

A study on rapid growth and piezoelectric effect of ZnO nanowires array

This work presents a rapid and simple synthesis procedure for ZnO nanowires (NWs) array by using the vapor–solid (VS) method. Experimental results indicate that the length and diameter of the grown ZnO NWs are associated with the temperature effect, while the growth density of NWs is strongly related to gas flux during the VS process. Additionally, the synthesized ZnO NWs possess specific crystalline qualities, making them highly promising for piezoelectric device applications. Therefore a piezoelectric type nanogenerator based on the ZnO NWs is also designed in this work, with a high output of piezoelectric current of 0.6 μA cm⁻² obtained as well. Our results further demonstrate the feasibility of applying piezoelectric energy via the rapidly grown ZnO NWs array.