Polycrystalline ZnO Research Articles

Polycrystalline ZnO nanorods for lasing applications

Single and double mode random lasing were observed in a polycrystalline ZnO nanorod array. The double mode random lasing showed mode competition when the mode spacing was 2.3 nm or below. Structurally, X-ray diffraction measurements confirmed the formation of the polycrystalline phase, and photoluminescence measurements revealed a broad visible peak due to point defects, suggesting enhanced oxygen diffusion due to annealing. Our results suggest polycrystalline nanorods prepared by chemical bath deposition as a material system for obtaining random lasing for optoelectronic applications and devices.

Quantifying Competitive Grain Overgrowth in Polycrystalline ZnO Thin Films

The grain size evolution of polycrystalline thin films, which form by competitive grain overgrowth as commonly interpreted using the van der Drift model, is understood to follow a power-law scaling of the average grain size d with film thickness h, i.e. d ∝ hα. While simulations have identified a growth exponent α = 0.4 for three-dimensional growth, previous experimental studies did not confirm this value, instead finding α values in the range of ~0.5-0.7. Here we study competitive grain overgrowth using a system of ZnO thin films grown by low-pressure metal-organic chemical vapor deposition. We present quantitative grain data on the evolution of grain size and orientation across the thickness of thin films, obtained by automated crystal orientation mapping of double-wedge transmission electron microscopy samples. The data from a-textured ZnO films, grown under three different conditions, are compared against van der Drift model predictions of self-similarity of the grain size distribution and the power-law scaling. The results are further interpreted by comparing to simulations of facetted polycrystalline film growth, which we adapt to the ZnO system by including idiomorphic growth shapes with a six-fold symmetry and random or biased nuclei orientations. As well as showing the predicted self-similarity of grain size distributions during growth, for the first time our experimental data confirm a power-law growth exponent of α = 0.4, as also predicted by the simulations using randomly oriented nuclei. Nevertheless, interpretation of this result is contingent on the absence of factors such as textured nucleation and renucleation during film growth. Indeed, only one film, grown at a higher ratio of H2O/DEZ precursor gases, displaying random initial nucleation, and minimal grain renucleation during growth, shows a proper conformance to the model nature and predictions.

Enhancement in optoelectrical properties of polycrystalline ZnO thin films by Ar plasma

In this study, the effects of Ar plasma treatment on the electrical, optical, morphological, and crystalline structural properties of polycrystalline ZnO thin films have been investigated. The simultaneous enhancements in conductivity and near band edge emissions (NBEs), accompanied by a suppression of deep level emissions (DLEs), suggest a passivation effect of the Ar plasma. The dependences on plasma treatment time showed correlations between the DLE and Hall mobility as well as between carrier concentration and the NBE. The results suggested that not only radiative defects but also nonradiative defects could be passivated. The plasma treatment also led to a distortion and deterioration of crystalline structure, as well as damage on the surface, which may account for the decrease in Hall mobility for long treatment time. The incorporation of hydrogen into the ZnO film after Ar plasma treatment has been confirmed using a secondary ions mass spectroscopy. The hydrogen incorporation could be the most probable factor accounting for the significant improvement in optoelectrical properties of ZnO thin film. In addition to the role of passivant, hydrogen atoms can also act as shallow donors, contributing to the eventual increase in carrier concentration.

Optimization of Ge substrates for ZnO deposition and their application for CO2 detection

The present work focuses on the deposition of ZnO thin films on crystalline and porous Germanium substrates for hazardous gas detection. The porous Ge substrates were obtained by means of surface exfoliation by using H+ implantation and subsequent annealing. Morphological analysis revealed the formation of pores in the range between 1 and 5 µm. The deposition of ZnO thin films has been made on crystalline and porous Ge substrates utilizing the dip coating method. Structural and morphological characterizations confirmed the formation of highly porous polycrystalline ZnO film exclusively on porous Ge substrates. All the obtained samples were tested for their sensitivities at 0.1, 0.5, 1 and 2% of CO2 concentrations. The ZnO films deposited on porous Ge exhibited an increment of sensing response by a factor of 2, which is correlated to the increment of the superficial area induced by the substrate. Maximum sensing response ~ 50% was achieved for 2% of CO2 when measured at 300 °C. ZnO over porous Ge films showed complete resistance recovery, whereas ZnO over crystalline Ge showed a drift in the recovery resistance. ZnO deposited porous Ge showed the fastest response and recovery time ~ 25 and 10 s, respectively.

3D core-multishell piezoelectric nanogenerators

The thin film configuration presents obvious practical advantages over the 1D implementation in energy harvesting systems such as easily manufacturing and processing, and long-lasting and stable devices. However, ZnO-based piezoelectric nanogenerators (PENGs) generally rely on the exploitation of single-crystalline nanowires because of their self-orientation in the c-axis direction and ability to accommodate long deformations resulting in high piezoelectric performance. Herein, we show an innovative approach to produce PENGs by combining polycrystalline ZnO layers fabricated at room temperature by plasma-assisted deposition with supported small-molecule organic nanowires (ONWs) acting as 1D scaffolds. Such hybrid nanostructures present convoluted core-shell morphology, formed by a single-crystalline organic nanowire conformally surrounded by a poly-crystalline ZnO shell and combine the organic core mechanical properties with the ZnO layer piezoelectric response. In a step forward towards the integration of multiple functions within a single wire, we have also developed ONW-Au-ZnO nanoarchitectures including a gold shell acting as inner electrode achieving output piezo-voltages up to 170 mV. The synergistic combination of functionalities in the ONW-Au-ZnO devices promotes an enhanced performance generating piezo-currents one order of magnitude larger than the ONW-ZnO nanowires and superior to the thin film nanogenerators for equivalent and higher thicknesses.

Erratum: “Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films” [J. Appl. Phys. 124, 065304 (2018)

Erratum: “Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films” [J. Appl. Phys. 124, 065304 (2018)

Frequency-Dependent Impedance Responses of ZnO Using UV Light

Impedance spectroscopy data show that polycrystalline ZnO films can show either increases or decreases in their effective resistances after UV exposure, depending on the frequency of the applied AC excitation. Simple equivalent circuit models, based on resistance (R) and capacitance (C) in parallel, are sufficient to confirm the observed experimental trends. Simulated data demonstrate that that arbitrary R and C values will not produce the sign change, but that typical resistance and capacitance characteristics for photoconductive semiconductors like ZnO can cause the sign change. These results suggest that it could be desirable to manipulate the R and C values of photodetector materials to either control – or eliminate – such frequency-dependent UV responses.

Resistive switching in sub-micrometric ZnO polycrystalline films

Resistive switching (RS) devices are considered as the most promising alternative to conventional random access memories. They interestingly offer effective properties in terms of device scalability, low power-consumption, fast read/write operations, high endurance and state retention. Moreover, neuromorphic circuits and synapse-like devices are envisaged with RS modeled as memristors, opening the route toward beyond-Von Neumann computing architectures and intelligent systems. This work investigates how the RS properties of zinc oxide thin films are related to both sputtering deposition process and device configuration, i.e. valence change memory and electrochemical metallization memory (ECM). Different devices, with an oxide thickness ranging from 50–250 nm, are fabricated and deeply characterized. The electrical characterization evidences that, differently from typical nanoscale amorphous oxides employed for resistive RAMs (HfOx, WOx, etc), sub-micrometric thicknesses of polycrystalline ZnO layers with ECM configuration are needed to achieve the most reliable devices. The obtained results are deeply discussed, correlating the RS mechanism to material nanostructure.

Zinc oxide nanoparticles synthesized by Suaeda japonica Makino and their photocatalytic degradation of methylene blue

Green chemistry gained special attention for the environmental safety. In addition, green synthesized plant based nanoparticles for the degradation of industrial pollutant dye also received special focuses. Thus, photocatalytic activity of nanosized zinc oxide (Sj-ZnONps) synthesized by a simple co- precipitation method using sodium hydroxide, zinc nitrate, and Suaeda japonica extract as starting materials was carried out. The reaction was implemented in comparatively low temperature (50 °C) without further calcination. The absorption spectrum demonstrated an extinction peak at ∼362 nm, which is characteristic to the ZnO nanoparticles. Field-emission transmission electron microscope revealed smaller agglomeration of hexagonal Sj-ZnONps (∼100 nm). X-ray diffraction patterns exposed polycrystalline ZnO with hexagonal wurtzite structure. The self-assembly of Sj-ZnONps was achieved due to the capping of phytoconstituents present in extract as evident from Fourier-transform infrared spectroscopy analysis. The photocatalytic degradation of Sj-ZnONps was estimated in reduction of methylene blue (MB). The reaction mixtures comprising of MB, S. japonica extract and Sj-ZnONps had the most significant decrease of MB by 54%. In conclusion, the Sj-ZnONps can be used as a photocatalyst for decomposition of organic pollutions present in water.

Core-Shell Electrospun Polycrystalline ZnO Nanofibers for Ultra-Sensitive NO2 Gas Sensing.

This Research Article discusses the growth of polycrystalline, self-supporting ZnO nanofibers, which can detect nitrogen dioxide (NO2) gas down to 1 part per billion (ppb), one of the smallest detection limits reported for NO2 using ZnO. A new and innovative method has been developed for growing polycrystalline ZnO nanofibers. These nanofibers have been created using core-shell electrospinning of inorganic metal precursor zinc neodecanoate, where growth occurs at the core of the nanofibers. This process produces contamination-free, self-supporting, polycrystalline ZnO nanofibers ofan average diameter and grain size 50 and 8 nm, respectively, which are ideal for gas sensing applications. This process opens up an exciting opportunity for creating nanofibers from a variety of metal oxides, facilitating many new applications especially in the areas of sensors and wearable technologies.

Effects of Polyethylenimine and Its Molecular Weight on the Chemical Bath Deposition of ZnO Nanowires.

The addition of polyethylenimine (PEI) in the standard chemical bath deposition (CBD) of ZnO nanowires has received an increasing interest for monitoring their aspect ratio, but the physicochemical processes at work are still under debate. To address this issue, the effects of PEI are disentangled from the effects of ammonia and investigated over a broad range of molecular weight (i.e., chain length) and concentration, varying from 1300 to 750 000 and from 1.5 to 10 mM, respectively. It is shown that the addition of PEI strongly favors the elongation of ZnO nanowires by suppressing the homogeneous growth at the benefit of the heterogeneous growth as well as by changing the supersaturation level through a pH modification. PEI is further found to inhibit the development of the sidewalls of ZnO nanowires by adsorbing on their nonpolar m-planes, as supported by Raman scattering analysis. The inhibition proceeds even in the low pH range, which somehow rules out the present involvement of electrostatic interactions as the dominant mechanism for the adsorption. Furthermore, it is revealed that PEI drastically affects the nucleation process of ZnO nanowires on the polycrystalline ZnO seed layer by presumably adsorbing on the nanoparticles oriented with the m-planes parallel to the surface, reducing in turn their nucleation rate as well as inducing a significant vertical misalignment. These findings, specifically showing the effects of the PEI molecular weight and concentration, cast light onto its multiple roles in the CBD of ZnO nanowires.

Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition

Controlling the formation of ZnO nanowire (NW) arrays on a wide variety of substrates is crucial for their efficient integration into nanoscale devices. While their nucleation and growth by chemical bath deposition (CBD) have intensively been investigated on non-polar and polar c-plane ZnO surfaces, their formation on alternatively oriented ZnO surfaces has not been addressed yet. In this work, the standard CBD technique of ZnO is investigated on and semipolar ZnO single crystal surfaces. A uniform nanostructured layer consisting of tilted ZnO NWs is formed on the surface while elongated nanostructures are coalesced into a two-dimensional compact layer on the surface. By further combining the CBD with selective area growth (SAG) using electron beam-assisted lithography, highly tilted well-ordered ZnO NWs with high structural uniformity are grown on the patterned surface. The structural analysis reveals that ZnO NWs are homoepitaxially grown along the polar c-axis. The occurrence of quasi-transverse and -longitudinal optical phonon modes in Raman spectra is detected and their origin and position are explained in the framework of the Loudon’s model. These results highlight the possibility to form ZnO NWs on original semipolar ZnO surfaces. It also opens the way for comprehensively understanding the nucleation and growth of ZnO NW arrays on poorly and highly textured polycrystalline ZnO seed layers composed of nanoparticles with a wide range of non-polar, semipolar, and polar plane orientations. Eventually, the possibility to tune both the inclination and dimensions of well-ordered ZnO NW arrays by using SAG on semipolar surfaces is noteworthy for photonic and optoelectronic nanoscale devices.

Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films

We investigated the effects of the erosion zone of magnetron sputtering (MS) targets on the deposition rates of magnetron-sputtered Al-doped ZnO (AZO) polycrystalline films and on the structural and electrical properties of the resulting AZO films. We deposited AZO films on glass substrates at a substrate temperature of 200 °C, which were placed parallel to the target surface, by radio frequency (RF), direct current (DC) or RF-superimposed DC (RF/DC) MS. Sintered AZO targets with an Al2O3 content of 2.0 wt. % were used. The structural and electrical properties were investigated by X-ray diffraction and Hall-effect measurements, respectively. For RF-magnetron-sputtered AZO films, the results of the electron probe microanalyzer and photo luminescence measurements show that the effects of the erosion zone of the targets on the properties were very small at any given substrate position, resulting in small differences in the structural and electrical properties among the AZO films grown at different substrate positions. In the case of DC-magnetron-sputtered AZO films, residual damage owing to recoiling Ar atoms was found, particularly for the films grown at substrate positions in the area opposite to the erosion zone of the targets. This gives rise to the formation of nonradiative recombination centers, low carrier concentration, and high contribution of grain boundary scattering to carrier transport owing to poor alignments between columnar grains, resulting in low Hall mobility. In this study, we prove that the use of the RF/DC MS technique is effective in reducing the amount of residual damage.

Two-Dimensional Polycrystalline ZnO Hierarchical Structures as Single-atom Catalyst Supports

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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.

Effects of a surfactant on the morphology and photocatalytic properties of polycrystalline Fe-doped ZnO powders

In this study, we compared the structural, morphological, and optical properties of a series of Zn1-xFexO (x = 0.00, 0.01, and 0.03) powders synthesized via a hydrothermal route with and without cetyltrimethylammonium bromide as a cationic surfactant. Our results highlighted the critical effects of the surfactant and the iron concentration on the structure, morphology, and optical properties of ZnO. X-ray diffraction, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy analyses indicated the presence of a single phase comprising ZnO with a hexagonal wurtzite structure in all samples and a single oxidation state (+3) for the iron (Fe3+) that replaced Zn2+ in the ZnO structure. Morphological investigations by scanning electron microscopy showed that the surfactant and Fe3+ dopant greatly affected the shape of the ZnO grains, which varied between sheets and rod-like flowers. We found that the morphological and photocatalytic properties of the two series of samples comprising iron-doped ZnO with and without the surfactant were in opposition. We propose a possible growth mechanism for the ZnO polycrystalline grains in the presence of the Fe dopant and/or surfactant. The photocatalytic properties increased for the samples prepared with surfactant as the iron content increased, which was confirmed by ultraviolet-visible reflection measurements. The photocatalytic activities of the samples prepared without surfactant decreased in both the ultraviolet and visible spectral regions as the iron content increased in the samples.

Influence of ZnO thin film crystallinity on in vitro biocompatibility.

This study evaluated the cytocompatibility of single- and poly-crystalline ZnO thin films using extract and direct contact methods. Exposure to poly-crystalline ZnO extract resulted in reduced cell viability, on average 82%/70% as measured by MTS/LDH assays, respectively. Direct exposure to both single- and poly-crystalline ZnO thin films resulted in reduced cell viability, which was attributed to anoikis due to inhibition of cell adhesion to the substrate by zinc. Intracellular zinc imaging suggests that single crystalline ZnO thin films do not result in a significant change in intracellular zinc concentrations. Overall, the results suggest that single-crystalline ZnO thin films have better short-term (24 h) cytocompatibility and support their potential to serve as a biocompatible sensor material.

Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces

We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds timescales using time-resolved terahertz spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ∼150 nm crystallites reaches 750 cm2/V s, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO in...

Highly Dispersible Buckled Nanospring Carbon Nanotubes for Polymer Nano Composites

We propose the unique structure of highly dispersible single-walled carbon nanotubes (SWCNTs) in various solvents and polymers using the ZnO nano particle template. Buckled nanospring-shaped carbon nanotubes (NS-CNTs) were synthesized by a chemical reaction of ZnO nanoparticles with acid-treated SWCNTs and then dissolving ZnO through chemical etching. The unique structure of distorted hexagonal NS-CNTs encircled around ZnO nanoparticles was formed by the bending of SWCNTs caused by the agglomeration of chemically adsorbed Zn(OH)2, which is further crystallized as the polycrystalline ZnO inner core. The highly dispersible NS-CNTs could be incorporated in the poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)] copolymer, one of widely studied ferro- and piezo-electric polymer, up to the value of 15 wt% as nanofillers. The relative dielectric constant (K) of polymer nanocomposite, at 1 kHz, was greatly enhanced from 12.7 to the value of 62.5 at 11 wt% of NS-CNTs, corresponding to a 492% increase compared to that of pristine P(VDF-TrFE) with only a small dielectric loss tangent (D) of 0.1.

Surface-Modified CdS/ZnO Material: Single-Reactor Synthesis and Mechanism of Formation in Aqueous Solution

Method of chemical precipitation from aqueous solutions was used to cover the surface of polycrystalline ZnO nanotubes with a nanostructured CdS layer. The thus synthesized CdS/ZnO composite material was studied by the methods of X-ray diffraction analysis, electron microscopy, and optical spectroscopy. The fundamental time-related aspects of the process of CdS formation on the ZnO surface were examined. It was found that the amount of deposited CdS nanoparticles is independent of the deposition duration. The morphological specific features of ZnO nanotubes are preserved upon a prolonged keeping of ZnO in solution. The photocatalytic activity of CdS/ZnO under visible and UV light was examined in the reaction of hydroquinone oxidation. A possible mechanism of how the CdS/ZnO composite is formed in an aqueous solution in the course of growth of a layer constituted by CdS nanoparticles on the surface of ZnO nanotubes is suggested on the basis of the experimental data. It is demonstrated that the chemical-precipitation method can be used to obtain surface-active composite materials that are photoactive in the visible spectral range.