ZnO Single Crystals Research Articles

Surface modification of ZnO on the Zn-polar 0001 face by self-assembled triptycene-based polar molecules along with an increase of electric conductance

The application of self-assembled monolayers (SAMs) is a representative method of surface modification for tuning material properties. In this study, we examine the influence of the surface modification by coating the Zn-polar 0001 surface of ZnO single crystal with a SAM of triptycene-based polar molecules in our own technique and investigated temperature dependences of the sheet conductance of the surface with and without the SAM. The sheet conductance at 70 K with the SAM is increased by an order of magnitude, compared to the case without the SAM. We infer that the additional electrons are introduced at the surface by the polar triptycene molecules, whose electropositive hydroxyl groups are supposed to face toward the Zn-polar surface of ZnO. The present result implies that the molecular orientation of the triptycene SAM plays a critical role in the surface properties of oxide semiconductors.

In situ analysis of the nucleation of O- and Zn-polar ZnO nanowires using synchrotron-based X-ray diffraction.

The selection of the polarity of ZnO nanowires grown by chemical bath deposition offers a great advantage for their integration into a wide variety of engineering devices. However, the nucleation process of ZnO nanowires and its dependence on their polarity is still unknown despite its importance for optimizing their morphology and properties and thus to enhance the related device performances. To tackle this major issue, we combine an in situ analysis of the nucleation process of O- and Zn-polar ZnO nanowires on O- and Zn-polar ZnO single crystals, respectively, using synchrotron radiation-based grazing incidence X-ray diffraction with ex situ transmission and scanning electron microscopy. We show that the formation of ZnO nanowires obeys three successive phases from the induction, through nucleation to growth phases. The characteristics of each phase, including the nucleation temperature, the shape and dimension of nuclei, as well as their radial and axial development are found to depend on the polarity of ZnO nanowires. A comprehensive description reporting the dominant physicochemical processes in each phase and their dependence on the polarity of ZnO nanowires is presented, revisiting their formation process step-by-step. These findings provide a deeper understanding of the phenomena at work during the growth of ZnO nanowires by chemical bath deposition and open the perspective to develop a more accurate control of their properties at each step of the formation process.

Controlled crystal growth of ZnO nanobelts by chemical vapor deposition method

With the development expansion of environmental sensors in recent years, nanogenerators utilizing small amounts of mechanical energy in surrounding environment to generate electricity have been frequently studied. In order to convert mechanical energy into electrical energy more efficiently, it is necessary to fabricate single crystal ZnO nanostructures with specific crystal facet that exhibits piezoelectricity in the direction of applied strain. Since ZnO nanobelts have three pairs of crystal facets, controlling crystal growth can increase the size of specific crystal facet and their piezoelectricity is greater than that of ZnO nanowires, which is advantageous for power generation using piezoelectricity. ZnO nanobelts have been fabricated by chemical vapor deposition method, and we attempted to control the crystal growth by changing the heating temperature and pressure. Morphologies were observed by field emission scanning electron microscopy. Diffraction patterns were observed by transmission electron microscopy. ZnO nanobelts with the growth direction of [0001] and [0110] have been successfully fabricated in this research.

Features of the crystal structure of ZnO:Mn nanocrystals obtained by ultrasonic spray pyrolysis

Samples of ZnO and ZnO:Mn nanocrystals with Mn 2 and 4 at.% impurity concentrations obtained by ultrasonic spray pyrolysis were studied by X-ray diffraction. It was determined that the reflexes of the X-ray diffractions of the samples had a shift relative to the standard position towards large diffraction angles. This indicated the nonequilibrium state of the crystal lattice, which caused the appearance of strain stresses. Debye–Scherrer and Williamson–Hall methods were used to calculate the parameters of the crystal lattice, the size of nanocrystals, and strain stresses with different concentrations of Mn. When the concentration of Mn increased to 4 at.%, there was an increase in strain stresses in nanocrystals by 10 times in comparison with undoped ZnO. The unit cell volume of ZnO:Mn nanocrystals was significantly smaller in comparison with single-crystal ZnO, which indicated that they had their intrinsic defects.

Peculiarities of specular infrared reflection spectra of ZnO-based ceramics

Undoped and Mn-doped ZnO ceramics were theoretically and experimentally investigated using specular infrared reflection method. It was shown that infrared reflection spectra can be modeled using the parameters explored for ZnO single crystals. For ceramic samples, it was shown that ZnO grains with orientation of the C-axis along the normal to the electric field ( ) give the main contribution to IR reflection spectra. It has been ascertained that the surface roughness is manifested in these spectra mainly within the range 450…550 cm–1 giving negligible effect for the frequencies above longitudinal phonon frequency. This allowed the electrophysical parameters of ZnO crystallites to be evaluated. In the case of undoped ceramics, the obtained results were found to be consistent with the values of direct current measurements. This finding supports the utility of infrared spectroscopy for determination of the electrophysical parameters of polycrystalline ceramic materials. For Mn-doped ceramic samples, the conductivity value measured using the direct current method was found to be essentially lower than those determined from simulation of infrared reflection spectra. This phenomenon was explained by barrier formation at the grain boundaries in Mn-doped ZnO ceramics.

Microwave synthesis of ZnO microcrystals with novel asymmetric morphology

This paper reports a discovery of novel morphological features of ZnO asymmetric micron-sized single crystals synthesized by microwave irradiation of W/O emulsions containing a Zn source in the inner aqueous phase. The influence of the zinc ion concentration on the size and shape is studied in detail. The morphological and structural properties show that the obtained ZnO powders are almost mono-dispersed ZnO single crystals with a novel morphology of truncated hexagonal pyramids ca. 1 μm in diameter. A scanning precession electron diffraction analysis reveals that the asymmetrically shaped ZnO single crystals thus obtained have a crystallographic c-axis perpendicular to the bottom hexagonal flat surfaces, and annular bright-field imaging indicates that the bottom surfaces are Zn-terminated ZnO(0001)-Zn polar faces. These new findings are led from a combination of well-controllable liquid-phase emulsion synthesis of the oxide powders and highly advanced atomic-resolution TEM imaging technique, suggesting for the first time a highly probable relationship between the polarity of the wurtzite-type crystal structure and the novel asymmetric morphology of the ZnO microcrystals obtained in this study.

Reversible control of intrinsic shear strength of a ZnO single crystal through electron-beam-induced hole state

Reversible control of intrinsic shear strength of a ZnO single crystal through electron-beam-induced hole state

Preparation and characterization of self-assembled ZnO nanowire devices: nanowire strain sensor and homogeneous p–n junction

In this work, intrinsic and p-type ZnO nanowires (NWs) have been synthesized. Pure intrinsic ZnO nanowires have been fabricated by direct oxidation method and their aspect ratio reached up to 271.3. Sb-doped ZnO nanowires were uniformly grown on Si substrates by chemical vapor deposition with diameters ranging from 0.5 to 5 μm and lengths ranging from 100 μm to 3 mm. Directional arrangement of nanowires has been realized by two self-assembly methods, pulling method and water flow method, and two kinds of ZnO nanodevices (strain sensor and homogenous p–n junction) were prepared and characterized based on the directional arranged nanowires. According to the current response of ZnO nanowire strain sensor, the deformation quantities of elastic plate under the action of external forces in orthogonal X and Y direction were calculated respectively. The ZnO nanowire homogenous p–n junction was made of two vertical Sb-doped and intrinsic ZnO nanowires. The I–V characteristic curve showed good rectification characteristics, and the forward turn-on voltage was about 10 V. However, since the current was too small due to the small carrier concentration in the ZnO single crystal, it is difficult to achieve electroluminescence at present.

ZnO[formula omitted] is unstable in moderate pressures of water

A ZnO(101¯0) single crystal was investigated using in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. In roughly 1 mbar water the surface roughens within minutes. Hereby, the formation of (0001)- or (0001¯)-type steps is favored over the formation of (12¯10)-type steps. The roughened surface is stable in ultra-high vacuum and does not exhibit a different amount of hydroxylation or adsorbed water compared to the as-prepared surface. The speed of the roughening is related to the total volume of water supplied to the surface rather than the water pressure.

Two-dimensional suprawavelength periodic surface structuring of a ZnO single crystal with a UV femtosecond laser.

Herein, we report on the one-step formation of a novel microstructure on the surface of crystalline ZnO in ambient air excited by a single femtosecond laser beam (central wavelength 400 nm, pulse duration 35fs), which has photon energy close to the bandgap of ZnO. A two-dimensional surface structure with a controlled period of ∼2-6 μm is observed, with its orientation independent on the status of laser polarization (linear, circular, or elliptical polarization). We find that the orientation of this two-dimensional structure is defined by the direction of the crystal a and c axes. This structural period of ∼2-6 micrometers and the independence of its orientation on the laser polarization are in sharp contrast with the traditional laser induced periodic surface structure (LIPSS). In the meantime, surface cracks with a feature size of ∼30 nm are observed at the bottom of the valley of the two-dimensional structure and theoretical results show there exists strong electric field enhancement on the cracks under 400 nm femtosecond laser irradiation. In view of these unusual features, we attribute the formation of this two-dimensional structure to the mechanical cracking of the ZnO crystal along its (11-20) and (0001) planes induced by the multiple-cyclic heating due to linear absorption of the femtosecond pulses.

One-step synthesis CdS/single crystal ZnO nanorod heterostructures with high photocatalytic H2 production ability

CdS/single crystal ZnO nanorod heterostructure (CdS/ZnO) was fabricated via microwave synthesis method, and the structural, morphological and photocatalytic hydrogen generation was tested. Studied results demonstrated that CdS particles were decorated on the surface of single crystal ZnO nanorod with crystallite size about 43 nm, and CdS nanoparticle significantly enhanced the photocatalytic hydrogen generation ability of the heterostructure by increasing the specific surface area, promoting the separation, migration of photoinduced carriers and increasing the light absorption. Pure ZnO exhibits bandgap energy of 3.09 eV, while the CdS/ZnO sample possessed a more narrow band gap (2.62 eV) than ZnO nanorod. Under the irradiation of visible light, the highest photocatalytic hydrogen production rate of 4076 µmolg−1h−1 is observed over the CdS/ZnO with the content of CdS is 38 wt%, which is about 13.59 and 76.91 times more active than pure ZnO nanorod and CdS nanoparticle. In addition, the specific surface area of the composite sample reaches 43.375 m2/g, and the instantaneous photocurrent reaches 130 μA /cm2. The present microwave radiation method is simple, easy to control, and timesaving, thereby providing a efficient route for the preparation of metal oxide-based photocatalytic material with high hydrogen evolution activity.

Co-catalyst-free large ZnO single crystal for high-efficiency piezocatalytic hydrogen evolution from pure water

Piezocatalytic materials have been widely used for catalytic hydrogen evolution and purification of organic contaminants. However, most studies focus on nano-size and/or polycrystalline catalysts, suffering from aggregation and neutralization of internal piezoelectric field caused by polydomains. Here we report a single crystal ZnO of large size and few bulk defects crafted by a hydrothermal method for piezocatalytic hydrogen generation from pure water. It is noteworthy that single-side surface areas of both original as-prepared ZnO and Ga-doped ZnO bulk crystals are larger than 30 cm2. The high quality of ZnO and Ga-doped ZnO bulks are further uncovered by high-resolution transmission electron microscope (HRTEM), photoluminescence (PL) and X-ray diffraction (XRD). Remarkably, an outstanding hydrogen production rate of co-catalyst-free Ga-doped ZnO bulk crystal (i.e., a maximum rate of 5915 μmol h−1 m−2) is observed in pure water triggered by ultrasound in dark, which is over 100 times higher than that of its powder counterpart (i.e., 52.54 μmol h−1 m−2). The piezocatalytic performance of ZnO bulk crystal is systematically studied in terms of varied exposed crystal facet, thickness and conductivity. Different piezocatalytic performances are attributed to magnitude and distribution of piezoelectric potential, revealed by the finite element method (FEM) simulation. The density functional theory (DFT) calculations are employed to investigate the piezocatalytic hydrogen evolution process, indicating a strong H2O adsorption and a low energy barrier for both H2O dissociation and H2 generation on the stressed Zn-terminated (0001) ZnO surface.

Low-Cost Electrodeposition of Size-Tunable Single-Crystal ZnO Nanorods

In this paper we report a low cost, simple, electrochemical method for large-area growth of single crystal ZnO nanorods. The method utilizes a metallic zinc foil as the source of the necessary zinc ions for ZnO growth on indium-doped tin oxide (ITO) glass slides. The method is thoroughly discussed and investigated varying all the parameters involved. The resulting ZnO nanorods are highly oriented along c-axis and densely packed, while their length and diameter can be tuned by varying the growth parameters. Two different types of seed layers on the ITO glass slides are tested. A seed layer made by spin coating of ZnO nanoparticles results in a twofold increase of the ZnO nanorod surface density as compared with a ZnO thin film seed layer by physical vapor deposition. Additionally, the effect of oxygen supply during electrodeposition was investigated as a crucial regulatory parameter not only for the geometrical and topological characteristics of the ZnO nano-arrays but for their physical properties as well.

Neutron irradiation induced defects in oxides and their impact on the oxide properties

Understanding the irradiation-induced defects in oxides is of interest for a wide range of applications. ZnO is an interesting oxide with mixed ionic and covalent bonding that contains a variety of point defect structures—making it an excellent model for studying irradiation-induced defects and their impact on properties. Here, we investigate the effects of neutron irradiation on the formation of defects and on the structural, optical, and electrical properties of ZnO single crystals. We observe the formation of vacancies and voids via positron annihilation spectroscopy. Neutron irradiation led to a significant deterioration of the ZnO structure and formed a high concentration of point defects, vacancy clusters, and voids with large disparities in their structure across variable irradiation times. It also led to significant changes in the optical properties and sample color. Irradiation for 444 h induced a high concentration of Cu acceptors as well as a high concentration of Ga donors. Temperature-dependent Hall effect measurements revealed the competing production of donors and acceptors and showed an increase in the slope of the carrier freeze-out curve with increasing irradiation dose. This work demonstrates the combined effects of neutron irradiation in producing a wide range of structural defects, impurities, and dopants in oxides and their enormous impact on modifying the oxide structure and both the optical and electronic properties. It particularly emphasizes the importance of considering the production of new impurities and dopants during the neutron irradiation of oxides.

NO2 Gas Sensing Characters of Polycrystaline ZnO Nanorods

ZnO has attracted much attention as an important semiconductor material, since it has a wide band gap (3.4eV), and excellent stability. It has been extensively researched to detect many gases, such as H2, C2H5OH, NH3 and VOC(volatile organic compounds). Herein, we developed a MOF derived ZIF-8 coated ZnO nanorods(ZnO@ZIF) as a precursor by solvethermal route, then calcined at 500 ºC in air, during the process, the exposure facets of ZnO nanorods have changed. The polycrystaline ZnO nanorods shows higher response to 1-5 ppm NO2 at 140ºC compared with single crystal ZnO nanorods. This provides a new etching way to change the surface facets by using ZIF coated and calcined in air.

Electroreduction of Aryldiazonium Ion at the Polar and Non-Polar Faces of ZnO: Characterisation of the Grafted Films and Their Influence on Near-Surface Band Bending.

ZnO is a strong candidate for transparent electronic devices due to its wide band gap and earth-abundance, yet its practical use is limited by its surface metallicity arising from a surface electron accumulation layer (SEAL). The SEAL forms by hydroxylation of the surface under normal atmospheric conditions, and is present at all crystal faces of ZnO, although with differing hydroxyl structures. Multilayer aryl films grafted from aryldiazonium salts have previously been shown to decrease the downward bending at O-polar ZnO thin films, with Zn-O-C bonds anchoring the aryl films to the substrate. Herein we show that the Zn-polar (0001), O-polar (000 ), and non-polar m-plane (10 0) faces of ZnO single crystals, can also be successfully electrografted with nitrophenyl (NP) films. In all cases, X-ray photoelectron spectroscopy (XPS) measurements reveal that the downward surface band bending decreases after modification. XPS provides strong evidence for Zn-O-C bonding at each face. Electrochemical reduction of NP films on O-polar ZnO single crystals converts the film to a mainly aminophenyl layer, although with negligible further change in band bending. This contrasts with the large upward shifts in band bending caused by X-ray induced reduction.

Observation of the Polaron Excited State in a Single-Crystal ZnO

Elucidating the property of the polaronic state at ultrafast temporal resolution is of great significance, especially for photocatalytic and solar-energy materials. In this work, femtosecond transient absorption (TA) spectroscopy with state selectivity is utilized to monitor the dynamics of the polaron generated by exciting the mid-gap states from the oxygen vacancy into the polaronic state in a single-crystal ZnO. The photoexcited polaronic state is verified by the observed coherent acoustic phonon with an oscillation on the kinetics at the initial several picoseconds at a frequency of about 50 GHz. We found that the excited state of polaron is located inside the conduction band (CB), which injects an electron into the CB within about 1 ps observed via a broad-band TA spectrum ranging from 400 to 4000 nm. Compared with that of free carriers generated by direct band gap excitation, the lifetime of the carriers resulted from the electron injection from the polaronic states is significantly prolonged, an important feature beneficial to photocatalysts.

Long-lived electron spin coherence in Ga-doped ZnO at room temperature

Electron spin dynamics are studied in Ga-doped ZnO single crystals by time-resolved Faraday and Kerr rotation spectroscopies. Long-lived spin coherence with two dephasing processes is discovered where the characteristic time is up to 5.2 ns at room temperature. Through the dependence measurements of laser wavelength and temperature, the room-temperature long-lived spin signal is attributed to localized electrons. The spin dephasing (relaxation) processes are independent of transverse (longitudinal) magnetic fields, indicating the spin dephasing not resulting from the g-factor inhomogeneity and electron-nuclear hyperfine interaction. It reveals that the two spin dephasing processes originate from two types of localized electrons, both of which are dominated by the anisotropic exchange Dzyaloshinskii-Moriya interaction between adjacent localized electrons.

Solution-based fabrication of high-entropy Ba(Ti,Hf,Zr,Fe,Sn)O3 films on fluorine-doped tin oxide substrates and their piezoelectric responses

This paper reports the first fabrication of high-entropy Ba(Ti,Hf,Zr,Fe,Sn)O3 films on fluorine-doped tin oxide substrates through spin coating and hydrothermal synthesis. A homogeneous distribution of all constituent elements was observed through energy-dispersive x-ray spectroscopy mapping. High distortion (approximately 1.7%) of the lattice structure was caused by the underlying substrate and partial substitution of Ti with other larger elements of Hf, Zr, Fe, and Sn on B sites. Two types of Ba(Ti,Hf,Zr,Fe,Sn)O3 films were studied: high-density and slightly textured polycrystalline particles (Sample A), and randomly distributed [110]-oriented nanorods with marginal crystallinity (Sample B). In the piezoelectric experiment, Samples A and B substantially outperformed the medium-entropy oxide (MEO) samples and single-crystal ZnO bulk. Sample A also exhibited the highest piezoelectric coefficient (d33 ∼ 60 pm/V) among all studied samples. Its superior performance was attributable to a combination of high crystallinity, strong out-of-plane distortion, slightly textured polycrystalline structures, and cocktail effects. Furthermore, enhanced piezoelectricity was achieved after poling for the high-entropy oxide (HEO) and MEO samples, indicating their ferroelectric effect. The d33 of Sample A more than doubled (≈126.7 pm∙V−1), and the increase was substantially higher than that of Sample B. Our HEO samples have promise for use in piezoelectric- and ferroelectric-related applications.

The growth mechanism of zinc oxide micro-rods via vapor-phase method

Two different ZnO micro-structures were deposited by a Chemical Vapour Deposited method in a small quartz tube. The micro-rods and micro-wires were found on the edge of the tube respectivly. The morphology and structural properties were texted by scanning electronic microscopy technique and X-ray diffraction. The results shows that the micro-rods have typical hexagonal wurtzite structure without any defect and the micro-wires have two types, the single ZnO crystal and the double-like one. The X-ray diffraction gives some information through the intense 002 peak which means the samples have a great wurtzite structure and grown up along the c-axis. The mechanism of the two samples also discussed, it is in relation to the substrate temperature and the concentration of the mixed gas.