ZnO Single Crystals Research Articles

Photoluminescence and Electrical Properties of ZnO Nanopowders Prepared by a Solution Combustion Method for Optoelectronic Device Application.

Nano-sized powders of ZnO phosphor were prepared by a solution combustion method (SCM). The ZnO powder prepared using Zn(OH)₂ and glycine as the oxidant and fuel, respectively, (fuel/oxidant = 0.8), show good powder characteristics such as an average grain size of 30 nm and specific surface area of 120 m²/g. Single-phase ZnO powders were obtained regardless of the fuel/ oxidant ratio. The photoluminescence spectra of the obtained ZnO powders show a single sharp peak near 390 nm corresponding to a deep blue color. This was confirmed by cathodoluminescence measurement and CIE color coordinate values. The PL spectra, powder characteristics and electrical properties show very good consistency. Furthermore, the electron carrier concentration of the ZnO nanopowders prepared by SCM is more than 3 orders of magnitude higher than that of the purest ZnO single crystal, which is commercially available. These powders could be potentially applied as blue phosphors of displays such as A.C. powder electroluminescent devices and plasma display panels.

Investigation of humidity sensor based on Au modified ZnO nanosheets via hydrothermal method and first principle

In this paper, Au modified ZnO (Au/ZnO) nanosheets with high humidity performance were prepared by hydrothermal method. The position and the energy band of Au atoms in ZnO single crystals were simulated by first-principles based on density-functional theory (DFT), and the mechanism of Au/ZnO humidity sensors was studied further. The calculations show that Au atoms are adsorbed on the surface of ZnO supercell to provied more oxygen vacancies. Therefore, more water molecules can be adsorbed on the Au/ZnO humidity sensor to react with more electrons and hence to enhance the performance of the humidity sensor. Experiments show that the performance of humidity sensor based on Au/ZnO can be improved by changing the concentration of the modifier Au nanoparticles. Au/ZnO (0.5 mM HAuCl4) humidity sensor exhibits approximately 5 orders of magnitude variation in impedance realization with RH in a range from 11% to 95%, and it exhibits good linearity and narrow hysteresis loop (˜3.0%), as well as short response (16 s) time and recovery time (28 s). The performance of Au/ZnO humidity can be improved by modifier Au nanoparticles, because the Au/ZnO have more surface oxygen defects and special electronic storage capabilities. These results indicate that Au/ZnO nanosheets have a potential in the manufacture of humidity sensors and provide a new idea for high performance of ZnO humidity sensors.

Synthesis and gas-sensing properties of ZnO@NiCo2O4 core@shell nanofibers

In this paper, 1D ZnO nanofibers were selected as the backbones, which were prepared by a hydrothermal route. Afterward, porous NiCo2O4 nanosheets with large surface area were further grown on the surface of single-crystal ZnO nanofibers via a chemical bath deposition. The structures and compositions of the as-prepared gas-sensing materials were characterized. It was found that uniform porous NiCo2O4 nanosheets were anchored on the surface of ZnO nanofibers backbones with a low lattice mismatch at the interface of heterostructures. Gas-sensing tests showed that this novel ZnO-core@NiCo2O4-shell nanocomposite exhibited improved gas-sensing performances towards methanol, such as high response, low detection limit and excellent stability. The response to 5 ppm methanol was about 1.96 (Rg/Ra), whereas pristine NiCo2O4 and ZnO showed negligible response. The enhanced gas-sensing properties were attributed to the unique core-shell heterojunctions between NiCo2O4 nanosheets and ZnO nanofibers resulting in an improved receptor function as well as transducer function.

Surface ferromagnetism in ZnO single crystal

We report on the room temperature surface ferromagnetism in undoped ZnO bulk single crystal. Typical ferromagnetic saturation behavior is clearly demonstrated by magnetization loops, and the saturated magnetization can be manipulated by annealing under argon atmosphere. A strongly correlation, between the saturated magnetization and the concentration of singly occupied oxygen vacancies on the surface of ZnO single crystal, is established by means of the Raman spectra, photoluminescence, electron paramagnetic resonance and atomic force microscope. It is suggested that the observed room temperature ferromagnetism in ZnO single crystal should be attributed to the effect of surface oxygen vacancy defects. This finding may help to get further insight into the ferromagnetic origin in other undoped oxide semiconductor materials.

Conversion mechanism of conductivity and properties of nitrogen implanted ZnO single crystals induced by post-annealing

In this paper, the conversion mechanism of conductivity and properties of nitrogen (70 keV, 1 × 1016 cm−2) implanted ZnO single crystals induced by post-annealing in a temperature ranging from 500 °C to 800 °C have been investigated by Hall, Raman, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and low temperature photoluminescence (PL) measurements. The results indicate that the sample converts to p-type conductivity as annealed at 650 °C. For the as-implanted sample, different type of nitrogen (N) local states are detected, including substitutional N atom and N2 molecules on O site [No and (N2)o], N–O, N–H, and N–C complexes. However, their thermal stabilities display significant difference upon thermal annealing. Differing from the initial ZnO bulk, the low-temperature PL spectrum of the post-annealed N implanted ZnO sample consists of two dominant peaks located at 3.105 eV and 3.220 eV, respectively. The former is attributed to radiative electron transition from the conduction band to the Zn vacancy (VZn) acceptor level, and the latter is assigned to recombination of the donor–acceptor pair (DAP). Our results suggest that the VZn together with the No acceptors are responsible for p-type conductivity of N doped ZnO.

Antenna-enhanced high harmonic generation in a wide-bandgap semiconductor ZnO

We demonstrate high harmonic generation (HHG) into deep-UV range in a ZnO single crystal with resonant nanoantennas. Non-perturbative HHG is successfully induced by optical excitation of as low as 20 GW/cm2. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.

Ionic liquid/ZnO(0001̄) single crystal and epitaxial film interfaces studied through a combination of electrochemical measurements and a pulsed laser deposition process under vacuum

O-Polar ZnO(0001[combining macron]) single crystals and ZnO and Mg-doped ZnO (MgZnO) films which were subsequently deposited on the ZnO crystals by a pulsed laser deposition (PLD) method were electrochemically investigated through the interfaces with ionic liquid (IL) in a vacuum. The sample surfaces were confirmed to be atomically clean and flat by reflection high energy electron diffraction (RHEED) observation, prior to their electrochemical measurements. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were then performed, and the donor density, flat band potential of these ZnO samples, and the electric double layer capacitance at the IL/ZnO interfaces were successfully evaluated. The flat band potentials of ZnO and MgZnO films were found to shift to more negative potentials relative to those of the single crystal ZnO, with different values for thicker films, respectively. Some possible origins of the different flat band potentials between ZnO and MgZnO films, and their film thickness dependence of the flat band potential will be discussed in this paper.

Microstructure property study of ZnO single crystal irradiated with 200 MeV 86Kr17+ ions

ZnO single crystal materials irradiated with 200 MeV energy 86Kr17+ ions have been analyzed by a series of theoretical calculations, Raman spectroscopy and TEM tests, etc. After irradiation by 200 MeV 86Kr17+ ions, two broad vibrational absorption peaks appeared and there is no obvious change in other characteristic peaks. By measuring the Raman spectra of the incident light perpendicular to and parallel to the z axis of the crystal, it was proved that the vibration absorption peak of 526 cm−1 ∼ 600 cm−1 is caused by defects related to oxygen vacancy (VO). The experimental data fully proved that ZnO single crystal has good radiation resistance. These data are of great significance for the application of ZnO materials in various new devices in the future.

Enhanced NO2 gas-sensing performance of Pd/ZnO-codecorated SnO2 nanorod sensors

SnO2 nanorods codecorated with Pd and ZnO were synthesized using a three-step process involving: the synthesis of SnO2 nanorods by the thermal evaporation of Sn powders followed by the sol–gel deposition of ZnO and Pd nanoparticles. The NO2 gas-sensing properties of the nanorods were examined. The nanorods were composed of primitive tetragonal-structured single crystal SnO2, while the ZnO and Pd nanoparticles were composed of wurtzite-structured ZnO single crystal and face-centered cubic-structured Pd single crystal, respectively. The Pd/ZnO-codecorated SnO2 nanorod sensors showed a remarkably enhanced response to NO2 compared to either ZnO or Pd-decorated SnO2 nanorod sensors. The responses of multiple networked Pd/ZnO-decorated SnO2 nanorod sensors were increased four to fivefold at NO2 concentrations ranging from 1 to 5 ppm Pd/ZnO-decorated SnO2 nanorod sensors which also showed shorter response and recovery times and higher selectivity than with ZnO or Pd-decorated SnO2 nanorod sensors. The NO2 sensing mechanism of the Pd/ZnO-codecorated SnO2 nanorods is also discussed.

Influence of metal/semiconductor interface on attainable piezoelectric and energy harvesting properties of ZnO

The piezoelectric coefficient is a measure to quantify the potential use of a material in energy harvesting and sensor applications. High concentration of free charge carriers in piezoelectric materials can significantly impede the use of generated piezoelectric charge. In this study, undoped semiconducting ZnO single crystals with both Ohmic and Schottky contacts were prepared to quantify the effective piezoelectric response at temperatures from 20 °C to −140 °C and frequencies of mechanical loading from 0.5 Hz to 160 Hz. It was demonstrated that the formation of an electrostatic potential barrier at the metal-semiconductor interface increases the overall resistance, which provides access to unbiased piezoelectric coefficients of ZnO single crystals even at room temperature. These findings were verified using semiconducting ZnO for energy harvesting at room temperature and relatively low loading frequency.

X-ray absorption, photoemission and electron spin resonance studies of Ar+ ion implanted ZnO

Defect induced room temperature ferromagnetism (RT-FM) in Ar+ implanted ZnO single crystal (ZnO-SC) is observe from the electron Spin Resonance (ESR) spectra and is correlated with their electronic structure studies using synchrotron radiation based X-ray absorption near edge structure (XANES) spectroscopy and valence band photoemission spectroscopy (VB-PES). Enhancement of absorption intensity at the edge of XANES spectra of Ar+ implanted ZnO indicates the increase of local density of states (DOS); which arises from the surface defects and/or dangling bonds in ZnO. VB-PES spectra change upon Ar+ implantation, becoming broader, implying the induced surface defects in ZnO-SC. X-ray photoelectron spectroscopy studies confirm that the Zn 2p3/2 core level peak shifts in lower energy side in Ar+ implanted ZnO-SC that is associated with the formation of oxygen vacancies/defects in ZnO-SC which is responsible for the RT-FM.

Zn Vacancy Formation Energy and Diffusion Coefficient of CVT ZnO Crystals in the Sub-Surface Micron Region

By using positron annihilation spectroscopy methods, we have experimentally demonstrated the creation of isolated zinc vacancy concentrations >1020 cm−3 in chemical vapor transport (CVT)-grown ZnO bulk single crystals. X-ray diffraction ω-rocking curve (XRC) shows the good quality of ZnO single crystal with (110) orientation. The depth analysis of Auger electron spectroscopy indicates the atomic concentrations of Zn and O are almost stoichiometric and constant throughout the measurement. Boltzmann statistics are applied to calculate the zinc vacancy formation energies (Ef) of ~1.3–1.52 eV in the sub-surface micron region. We have also applied Fick’s 2nd law to calculate the zinc diffusion coefficient to be ~1.07 × 10−14 cm2/s at 1100 °C. The zinc vacancies began annealing out at 300 °C and, by heating in the air, were completely annealed out at 700 °C.

Topographic evolution and scaling study of ZnO (0001) single crystal after low-energy atom beam irradiation

Abstract The topographic evaluation of the nano-scale pattern, on ZnO (001) surface, and its scaling properties have been investigated here after low energy atom beam irradiation. Preferential sputtering promotes the formation of Zn-rich zones near the surface that serve as the nucleation centers for the spontaneous nanostructure growth. The rms roughness ( σ ) however increases with irradiation fluence and displays power law behaviour, σ ∼ t β , for t t c (crossover time) but saturates at longer time scale. In the case of t > t c , σ is independent of irradiation time but interestingly follows the power law behaviour σ ∼ L α , where L is the size of image used in analysis. The morphology of the surfaces has also been investigated using the height-height correlation and power spectral density methods. The observed values of growth ( β ) and roughness ( α ) exponents indicate a scaling behavior that is induced by the disordered but well-defined geometry of the nanostructures.

Nonlinear optical response of bulk ZnO crystals with different content of intrinsic defects

The nonlinear optical (NLO) properties of native defect-rich ZnO single crystals were studied in details within the excitation of the continuous wave (CW) and pulsed laser radiation at 532 nm (2.33 eV). Analysis of the experimental data of optical elastic scattering, Fourier transform infrared (FTIR), near infrared–visible–ultraviolet (NIR–Vis–UV) spectra recorded in reflection and absorption modes, and data of photoluminescence (PL) spectroscopy confirmed the contribution of both intrinsic defects and their clusters, being determined before by neutron diffraction and XRD analysis. It was shown that the high sensitivity of the NLO diagnostics via self-action of a laser beam is due to the resonant excitation of the deep defects states at wavelength 532 nm. It was also demonstrated the correlation of the real and imaginary parts of the cubic NLO susceptibility with PL and FTIR spectroscopies data dealing with the defect bands manifestation, while the elastic optical scattering and Vis-NIR spectroscopy data – with the intrinsic defect content. High photoinduced polarizabilities of the excited states provide application of the refractive NLO response and the optical elastic scattering indicatrices analysis as an express nondestructive technique to reveal the deep defects content in the bulk ZnO.

Temperature-dependent thermal behavior of impurity hydrogen trapped in vacancy-type defects in single crystal ZnO

Interacting nature between impurity hydrogen atoms and vacancy-type defects in single crystal ZnO was investigated by means of positron annihilation lifetime spectroscopy. In order to clarify the observation of their thermal behavior, the sample was implanted with 1H+ using an electrostatic accelerator. After the implantation, the positron lifetime became shorter, which suggests that the hydrogen atoms were captured by zinc vacancies (VZn) to form vacancy-hydrogen complexes (VZn + nH). The complexes decompose by heat treatment: most of the hydrogen atoms gradually dissociate from VZn + nH in the temperature range 393–773 K. It was also suggested that large vacancy clusters were formed by the agglomeration of smaller clusters during the process of stepwise isochronal annealings at temperatures from 773 to 1073 K, and their decomposition took place at 1173–1373 K. Temperature-dependent thermal behaviors of hydrogen atoms and vacancy-type defects in ZnO are discussed.

Halide-carbon vapor transport of ZnO and its application perspectives for doping with multivalent metals

The growth of ZnO single crystals in closed ampoules using HCl+C mixture as a chemical vapor transport agent (TA) was studied. The influence of growth temperature, TA density, C/HCl ratio, and undercooling on the ZnO mass transport rate was investigated theoretically and experimentally. The influence of growth medium composition on the features of crystal growth, such as the minimization of growth nucleus density, the increase in the lateral growth rate of up to 1 mm per day, the suppression of the attachment effect, and stable growth of non-polar, semi-polar and polar planes of a hexagonal structure was analyzed. The structural, photoluminescent, optical and electrical properties were investigated. The doping efficiency of ZnO by oxides of metals was analyzed for various TAs and Zn pressure in the growth medium. The possibility of increase in the doping efficiency by several orders of magnitude for multivalent metals was predicted for HCl+C TA.

Surface-dependent formation of Zn clusters in ZnO single crystals by electron irradiation

In sharp contrast to the well-documented irradiation-hardness property of ZnO, in situ transmission electron microscopy investigations reveal an unexpected nucleation and rotation of Zn clusters in single-crystal ZnO nanosheets (enclosed by the ${0001}$ planes) subjected to the 200-keV electron beam irradiation. Detailed classical molecular dynamics simulations mirror the experimental findings and suggest that the nucleation of Zn clusters is dependent on the surface planes. More importantly, a Schottky barrier may form at the Zn/ZnO interface, whereas the Zn can act as an electron sink and thus may facilitate the $p$-type doping of the ZnO, consistent with the former experimental findings. In addition, the as-nucleated Zn cluster shows a transient hexagonal phase with an expansion in the a-b plane (${a}_{\mathrm{Zn}}\ensuremath{\sim}2.72\phantom{\rule{0.16em}{0ex}}\AA{}$) compared with its bulk counterpart (${a}_{\mathrm{Zn}}\ensuremath{\sim}2.66\phantom{\rule{0.16em}{0ex}}\AA{}$), and exhibits an unreported relationship with ZnO: ${[0001]}_{\mathrm{Zn}}\ensuremath{\parallel}{[0001]}_{\mathrm{ZnO}},(01\overline{1}0{)}_{\mathrm{Zn}}\ensuremath{\parallel}(1\underline{25}\underline{\overline{26}}0{)}_{\mathrm{ZnO}}$. It is interesting to note that the enlarged value of ${a}_{\mathrm{Zn}}\ensuremath{\sim}2.72\phantom{\rule{0.16em}{0ex}}\AA{}$ has been predicted by the theoretical calculation [N. Gaston et al., Phys. Rev. Lett. 100, 226404 (2008)], while never reported in the experiments. Our results contribute to the basic understanding of point defects behavior in ZnO and shed light on controllable structural modification of Zn/ZnO heterostructures for industrial applications.

Effect of ZnO grain boundaries on non-linearity: first-principles calculations

The non-linearity (NL) of monocrystal and polycrystal ZnO materials was studied on the basis of density functional theory. Firstly, wurtzite ZnO was selected as a candidate material for selector devices. As indicated by the calculated I-V curves on different ZnO surfaces, non-linearity and electrical conductivity varied among the (), (0001) and () directions. The calculating models for wurtzite ZnO in three directions were named SF1, SF2 and SF3, respectively. SF1 and SF2 showed poor non-linearity although they demonstrated high electrical conductivity. SF3 exhibited excellent NL, but its electrical conductivity was low. Polycrystal ZnO material was studied, and three grain boundary (GB) models, namely, ZnO∑7()[0001]GB (GB1), 45° rotating GB(GB2) and ∑1()[]GB (GB3), were constructed. The GB barrier was found between two ZnO grains by calculating the I-V curves of GB and analysing the electrostatic potential, which resulted in NL improvement of GB compared with SF. By comparing the electrostatic potential analysis, we found that the electrical conductivity of GB3 was significantly enhanced to greater extent than that of SF3 because the Fermi level moved towards the conduction band. Therefore, the corresponding structure of polycrystalline ZnO should have better performance compared with single crystal ZnO to meet the selector design requirements. This work may be instructive and valuable for the design and optimization of ZnO-based gate selectors.

Vapor-Phase Synthesis of ZIF-8 MOF Thick Film by Conversion of ZnO Nanorod Array.

ZIF-8 metal organic framework "micrometer-thick" films were constructed from ZnO precursor by a vapor-phase synthesis. The ZnO-to-ZIF-8 crystal transformation proceeded in the presence of 2-methylimidazole (Hmim) vapor. Continuous coatings of intergrown ZIF-8 crystals require control of a nucleation density. The dependence of ZnO crystal plane on the ZnO-to-ZIF-8 crystal transformation was investigated using four bulk ZnO single crystals: a-plane (11-20), c-plane (0001), m-plane (10-10), and r-plane (10-11). It was revealed that the m-plane (10-10) of ZnO is more effectively transformed into ZIF-8. In this work, highly oriented ZnO nanorod array film was used to provide the transport pathway of Hmim molecules and volume expansion space of ZnO-to-ZIF-8 crystal transformation for nucleation and crystal intergrowth. The high conversion of ZnO nanorod array into ZIF-8 in a short time could be achieved because (1) such mass transfer is easy due to the uniform internanorod distance being maintained during reaction and (2) the surface of the nanorod array is dominated by the highly reactive m-plane (10-10).

Optical and Structural Study of ZnO Thin Films Deposited by RF Magnetron Sputtering at Different Thicknesses: a Comparison with Single Crystal

Optical characterizations of preferred orientation (002) ZnO thin films, prepared by RF magnetron sputtering under pure argon plasma, with different thicknesses have been investigated, where grain size and resistivity increase with thickness. Energy band gap and refractive indices have been calculated. A correlation between band gap values and crystalline quality, which is improved with thickness, has been discussed. The calculated refractive indices of the thicker deposited films have increased as the thickness increases. Full width Half Maxima (FWHM) of band gap Photoluminescence (PL) emission decreases with thickness increasement due to quality improvement as it has been monitored by X-Ray Diffraction (XRD). A comparison between PL spectra of 1200 nm film and ZnO single crystal at low temperature is found to be similar.