Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of nanostructured ZnO

The dielectric properties of nanostructured wurtzite-type ZnO are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. Characteristic peaks at about 12 and 32 eV in the imaginary part of the dielectric function shift to lower energies as particle size decreases. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA), GGA+U and modified Becke-Johnson (mBJ) is presented. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states of Zn and O.     

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.     

不同浓度Ag掺杂ZnO电子结构和光学性质的第一性原理研究

基于密度泛函理论框架下的第一性原理计算方法,研究了不同浓度Ag掺杂ZnO体系的电子结构和光学性质。计算结果表明,不同浓度Ag原子代替Zn原子后会导致电子结构和光学性质有显著的改变,能带随掺杂浓度的增大带隙渐渐变窄,光吸收、反射等也随银掺杂浓度的增大先是向高能端偏移再向低能端移动。这暗示Ag掺杂ZnO对其电子结构及光学性质有很大的影响,为进一步研究掺杂对ZnO性质的影响提供理论基础。     

Al，Ga掺杂ZnO电子结构和光电特性的第一性原理计算

采用基于密度泛函理论(DFT)框架下广义梯度近似(GGA)的PBE平面波超软赝势方法,计算了本征ZnO,Al掺杂ZnO(ZnAlO)和Ga掺杂ZnO(ZnGaO)的能带结构、态密度、复介电函数和复电导率. 其中Al或Ga是以替位杂质的形式进入ZnO晶格. 计算结果表明纤锌矿型ZnO,ZnAlO和ZnGaO都是直接带隙半导体材料,掺杂后ZnO的带隙变小,且ZnAlO的带隙略大于ZnGaO. 掺杂后ZnO的电子结构发生变化,费米能级由本征态时位于价带顶上移进入导带,ZnO表现为n型掺杂半导体材料,掺杂后在导带底出现大量由掺杂原子贡献的自由载流子—电子,明显提高了电导率和介电函数,改善了ZnO的导电性能,并且ZnAlO的导电性能要略好于ZnGaO.     

Electron energy-loss spectroscopy of V2O5 nanofibers synthesized by electro-spinning

The dielectric properties of V₂O₅ nanofibers, synthesized by the electrospinning method, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the Generalized Gradient Approximation (GGA) is presented, having found an excellent agreement between them. Although the experimental EELS has been acquired for the nanoparticles composing the fibers, and numerical calculations were carried out for bulk material, agreement between experimental and calculated results shows that no difference exists between the electronic structure in calculated bulk material and the nanoparticles. Furthermore, our results from EELS confirm that we accomplished the expected crystalline phase. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states.     

Al，Ga掺杂ZnO电子结构和光电特性的第一性原理计算

采用基于密度泛函理论(DFT)框架下广义梯度近似(GGA)的PBE平面波超软赝势方法，计算了本征ZnO，Al掺杂ZnO(ZnAlO)和Ga掺杂ZnO(ZnGaO)的能带结构、态密度、复介电函数和复电导率. 其中Al或Ga是以替位杂质的形式进入ZnO晶格. 计算结果表明纤锌矿型ZnO，ZnAlO和ZnGaO都是直接带隙半导体材料，掺杂后ZnO的带隙变小，且ZnAlO的带隙略大于ZnGaO. 掺杂后ZnO的电子结构发生变化，费米能级由本征态时位于价带顶上移进入导带，ZnO表现为n型掺杂半导体材料，掺杂后在导带底出现大量由掺杂原子贡献的自由载流子—电子，明显提高了电导率和介电函数，改善了ZnO的导电性能，并且ZnAlO的导电性能要略好于ZnGaO.     

Local structure investigation of (Co,Cu) co-doped ZnO nanocrystals and its correlation with magnetic properties

Pure, Co doped and (Co, Cu) co-doped ZnO nanocrystals have been prepared by wet chemical route at room temperature to investigate the effect of Cu doping in Co doped ZnO nanocrystals . The nanocrystals have initially been characterized by X-ray diffraction, FTIR, Raman, optical absorption and EPR spectroscopy and the results were corroborated with DFT based electronic structure calculations. Magnetic properties of the samples have been investigated by studying their magnetic hysteresis behavior and temperature dependence of susceptibilities. Finally the local structure at the host and dopant sites of the nanocrystals have been investigated by Zn, Co and Cu K edges EXAFS measurements with synchrotron radiation to explain their experimentally observed magnetic properties.     

Electron energy loss spectroscopy of TiC,ZrC and HfC

The dielectric properties of commercial TiC, ZrC and HfC powders were determined by analyzing the low loss region of the EELS spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) were obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions across the energy gap, these spectra can be interpreted on the basis of existing energy-band calculations. Comparison between experimental results and theory shows good agreement.     

Growth of ZnO nanostructures by vapor–liquid–solid method

Zinc oxide nanorods have been grown by vapor–liquid–solid (VLS) catalytic growth. The optical properties and structures properties of the grown ZnO nanostructures have been studied by photoluminescence, high resolution X-ray diffraction and scanning electron microscopy. The results show that the formation of ZnO nanostructures is strongly influenced by the growth conditions and used substrates. It was found that oriented ZnO nanorods are grown more easily on a substrate with a similar crystalline structure as ZnO. By optimizing growth conditions, oriented-ZnO nanorods grown on Si(001) substrate with a diameter of around 300 nm and lengths of 20 to 35 μm have been achieved, and they show excellent optical properties. Laser action has been observed at room temperature by using optical pumping. PACS 81.05.Dz; 81.10.Bk; 81.16.Hc     

Nanoscale EELS analysis of dielectric function and bandgap properties in gaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.     

Electrical and optical properties of ZnO films grown by molecular beam epitaxy

Zinc oxide (ZnO) films have been grown on sapphire by molecular beam epitaxy (MBE), and it is found that the grain size of the ZnO films increased with increasing the growth temperature. Photoluminescence (PL) study shows that the intensity ratio of near-band-edge emission to deep-level-related emission (NBE/DL) of the ZnO is significantly enhanced with increasing the growth temperature, and the dependence of the carrier mobility on the growth temperature shows very similar trend, which implies that there is a community factor that determines the optical and electrical properties of ZnO, and this factor is suggested to be the grain boundary. The results obtained in this paper reveal that by reducing the grain boundaries, ZnO films with high optical and electrical properties may be acquired.     

Theoretical study of structural, optical and electrical properties of zirconium-doped zinc oxide

The structural, optical and electrical properties of zirconium-doped zinc oxide have been investigated by first principle calculations. Three possible structures including substitutional Zr for Zn (Zr_Zn), interstitial Zr (Zr_i) and substitutional Zr for O (Zr_O) are considered. The results show that the formation energy of Zr_Zn defect is the lowest, which indicates that Zr_Zn defect forms easier and its concentration may be the highest in the samples. It is also found that as the proportion of Zr increases, the lattice constants increase while the optical band gap first becomes larger and then smaller, which are consistent with our recently experimental results. The electronic structure calculations display that as Zr_Zn defect is introduced into ZnO, the Fermi-level shifts to the conduction band, and there are excess electrons in the conduction band, which may be a possible reason of the good conductivity of Zr doped ZnO film.     

Influence of mesoporous substrate morphology on the structural, optical and electrical properties of RF sputtered ZnO layer deposited over porous silicon nanostructure

Morphological, optical and transport characteristics of the RF sputtered zinc oxide (ZnO) thin films over the mesoporous silicon (PS) substrates have been studied. Effect of substrate porosity on the grain growth and transport properties of ZnO has been analyzed. Physical and optical properties of ZnO-PS structures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. Our experimental results indicate that on changing porosity of the PS substrates, regularity of the spatial distribution of the ZnO nanocrystallites can be controlled. While the morphology and grain size of ZnO depended strongly on the morphology and pore size of the PS substrates, the rectifying factors of the metal semiconductor junction were found to be different by a factor of 3. The deposition of semiconducting oxides on such mesoporous substrates/templates offers the possibility to control their properties and amplify their sensing response.     

Photochemical synthesis of ZnO/Ag nanocomposites

Composite ZnO/Ag nanoparticles have been formed via the photocatalytic reduction of silver nitrate over the ZnO nanocrystals, their optical, electrophysical and photochemical properties have been investigated. Mie theory has been applied to analyze the structure of the absorption spectra of ZnO/Ag nanocomposite. The irradiation effects upon the optical properties of ZnO/Ag nanostructure have been investigated. It has been found that the irradiation of ZnO/Ag nanoparticles results in electrons accumulation by both the semiconductor and the metallic components of the nanocomposite. It has been found that silver nitrate can be photochemically deposited onto the surface of ZnO nanoparticles under the illumination with the visible light in the presence of the sensitizer – methylene blue. Kinetics of the sensitized Ag(I) photoredution has been studied. It has been concluded that the key stage of this process is the electron injection from singlet-excited methylene blue molecule into ZnO nanoparticle.     

Measuring the aspect ratios of ZnO nanobelts

Nanobelts are new materials that have a rectangular cross-section and are characterized by widths and width-to-thickness aspect ratios. In this paper, the thickness and aspect ratios of ZnO nanobelts are measured by a conjunction application of convergent beam electron diffraction (CBED) and electron energy-loss spectroscopy (EELS). The thicknesses of thicker nanobelts are first determined by CBED under two-beam diffracting condition, then they are used to determine the electron inelastic mean-free-path (MFP) length, which is 161±15 nm for ZnO at 200 kV. The thicknesses of the thinner nanobelts are then determined by EELS using the calibrated MFP. The results show that the aspect ratio depends on conditions under which the sample was synthesized.     

Uniaxial stress influence on lattice, band gap and optical properties of n-type ZnO:first-principles calculations

The lattice, the band gap and the optical properties of n-type ZnO under uniaxial stress are investigated by first-principles calculations. The results show that the lattice constants change linearly with stress. Band gaps are broadened linearly as the uniaxial compressive stress increases. The change of band gap for n-type ZnO comes mainly from the contribution of stress in the c-axis direction, and the reason for band gap of n-type ZnO changing with stress is also explained. The calculated results of optical properties reveal that the imaginary part of the dielectric function decreases with the increase of uniaxial compressive stress at low energy. However, when the energy is higher than 4.0 eV, the imaginary part of the dielectric function increases with the increase of stress and a blueshift appears. There are two peaks in the absorption spectrum in an energy range of 4.0-13.0 eV. The stress coefficient of the band gap of n-type ZnO is larger than that of pure ZnO, which supplies the theoretical reference value for the modulation of the band gap of doped ZnO.     

Comment on "Investigation on optical properties of ZnO nanowires by electron energy-loss spectroscopy"

Recently, Zhang et al. have published a paper [Zhang, Z.H., Qi, X.Y., Jian, J.K., Duan, X.F., 2006. Micron 37, 229–233] in which – among others – the determination of the optical properties of a semiconductor by use of electron energy-loss spectroscopy (EELS) is performed with 200 keV electrons and a collection angle of only 0.3 mrad. The authors do not take into account relativistic effects such as Čerenkov losses (CL) before performing Kramers–Kronig Analysis (KKA) on the EELS spectra obtaining erroneous results. Although the positions of features within the optical properties are consistant with the simulated ones, the relative hights or absolute values differ a lot.     

Modeling absorption spectrum and saturation intensity of ZnO nano-colloid

In this work, the complex magnitude of the dielectric constant of a thin film of ZnO nano-particles has been obtained in the range of visible spectrum. Using this spectrum, the UV–visible absorptive spectrum of ZnO spherical nano-colloid is obtained using Mie theory. The absorption spectrum of ZnO nano-colloid has been related to the optical properties of the prepared thin film with a simulation model based on the Mie theory and the findings are corroborated by fitting with the experimental results. Based on this curve fitting, the volume fraction of nanoparticles in the nano-colloid is obtained and the diameter (and the Bohr radius) of the molecules contributing in a nanoparticle crystal structure has been obtained. In this paper, frequency dependence of the optical properties coefficients and also local field considerations are applied. Finally the precise magnitude of saturation intensity of ZnO nano-colloid is measured.