Single Crystalline ZnO Nanorods Research Articles

Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide.

Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Effect of the seed layer on the UV photodetection properties of ZnO nanorods

The ZnO seed layer, acting as nucleation center for the growth of ZnO nanorods (NRs), has strong impact on the optical and photodetection properties of ZnO-based UV photodetectors (PDs). In this paper, vertically aligned ZnO NRs were grown by varying the thickness of the seed layer in the range 50–125 nm, to investigate its influence on the recovery time of the PD. Single crystalline ZnO NRs were obtained as indicated by combined electron microscopy and X-ray diffraction analysis. The photoluminescence (PL) spectra proved that the lowest PL intensity (i.e.: the lowest recombination) belongs to the sample with seed layer thickness of 100 nm (labeled as NR-7). The carrier concentration of ZnO NR films was estimated from the slope of the Mott–Schottky plot. It was 1.49 × 10+20 cm−3 for seed layer thickness of 65 nm (NR-5), which was dramatically reduced to 5.44 × 10+17 cm−3 in the sample NR-9 (seed layer thickness 125 nm). Furthermore, the current-voltage (I-V) and chronoamperometric (I-t) analysis indicate a high UV responsivity under a UV irradiation. The fastest recovery time (0.1 s time decay constant) occurs in sample NR-7 (seed layer 100 nm thick). These results indicate that effective control of the electronic and optical properties in ZnO NRs can be obtained by proper tuning of the seed layer, enabling a simple and straightforward strategy to optimize NR functionality, depending on their planned use.

High-performance textile piezoelectric pressure sensor with novel structural hierarchy based on ZnO nanorods array for wearable application

With the increasing demand for smart wearable clothing, the textile piezoelectric pressure sensor (T-PEPS) that can harvest mechanical energy directly has attracted significant attention. However, the current challenge of T-PEPS lies in remaining the outstanding output performance without compromising its wearing comfort. Here, a novel structural hierarchy T-PEPS based on the single-crystalline ZnO nanorods are designed. The T-PEPS is constructed with three layers mode consisting of a polyvinylidene fluoride (PVDF) membrane, the top and bottom layers of conductive rGO polyester (PET) fabrics with self-orientation ZnO nanorods. As a result, the as-fabricated T-PEPS shows low detection limit up to 8.71 Pa, high output voltage to 11.47 V and superior mechanical stability. The sensitivity of the sensor is 0.62 V·kPa−1 in the pressure range of 0–2.25 kPa. Meanwhile, the T-PEPS is employed to detect human movements such as bending/relaxation motion of the wrist, bending/stretching motion of each finger. It is demonstrated that the T-PEPS can be up-scaled to promote the application of wearable sensor platforms and self-powered devices.

Effect of oxygen nonstoichiometry on the photoelectrochemical performance of oxide-nanorod based TiO2/Sb2S3 and ZnO/Sb2S3 heterostructured photoanodes

Single crystalline ZnO and TiO2 nanorods are grown on fluorine-doped tin oxide (FTO) substrates by hydrothermal method. The nanorods are annealed under air and reducing conditions to alter the oxygen nonstoichiometry and hence the mid-bandgap defect states. Such an annealing process is shown to impart significant change on the photoelectrochemical (PEC) performance of the photoelectrodes. Large photocurrent densities (J) of 0.78 mA cm−2 are obtained for air annealed (AA) TiO2 nanorods (TNR) compared to hydrogen annealed (HA) TNR (J = 0.36 mA cm−2). ZnO nanorods (ZNR), on the contrary, shows photocurrent density of 0.76 mA cm−2 and 0.36 mA cm−2 for ZNR-HA and ZNR-AA photoanodes, respectively. The contrasting difference in the PEC performance is attributed to the synergetic effect of interfacial impedance with electrolytes and the oxygen nonstoichiometry. Further, to overcome the limitation of light absorption by these materials owing to their wide bandgap, TiO2 and ZnO nanorods are coated with Sb2S3 by chemical bath deposition to form heterostructured TNR-AA/Sb2S3(CBD) and ZNR-HA/Sb2S3(CBD) thin films. Such heterostructures exhibit enhanced photocurrent values of ∼1.39 mA cm−2 and 3.36 mA cm−2 (at 1.6 V versus Ag/AgCl), respectively. The PEC performances of the nanorods are analyzed in terms of the annealing conditions and subsequent introduction of defect states in the bandgap. The present study shows the importance of oxygen defect control at the interface between the oxide and chalcogenide, and its role in the betterment of PEC performance in TiO2/Sb2S3 and ZnO/Sb2S3 heterostructure photoanodes.

Tower-Like ZnO Nanorod Bundles Grown on Freestanding Diamond Wafers for Electron Field Emission Improvement

Tower-like ZnO nanorod (ZNR) bundles were grown on freestanding diamond (FSD) wafers by a low-temperature hydrothermal method without any catalysts, and the corresponding electron field emission property was investigated. The structural characterizations disclosed that these tower-like bundles were composed of single crystalline ZNRs with hexagonal wurtzite structure. They grew perpendicularly to each crystal planes of the diamond grains. Different hydrothermal reaction times (e.g., 1.5, 3, 6, 8 h) were carried out to investigate the morphology features of ZNRs grown on the growth surface of FSD wafers. The ZNR bundles with a growth time of 6 h showed the highest crystallinity and aspect ratio. Such ZNR bundles/FSD hybrid exhibited a high emission current density of 0.2 mA at an applied field of 11.8 V/μm and a reduced turn-on field of 6.8 V/μm. The field enhancement factor β was calculated to be ~4979 based on the Fowler-Nordheim theory. The geometry and structure of polycrystalline diamond were responsible for the formation of ZNR bundles and the high β-value.

Template-free synthesis and photocatalytic activity of hierarchical hollow ZnO microspheres composed of radially aligned nanorods

In this work, hierarchical hollow ZnO microspheres with an average diameter of 3.5 μm are successfully synthesized via a simple template-free solvothermal route. And these microspheres are constructed by anisotropic single-crystalline ZnO nanorods which grows along the [0001] c-axis direction because of the selective adsorption of ethylene glycol. It is noted that ethylene glycol plays a crucial role in promoting the growth and aggregation of nanorod by comparing the effect of other solvents, such as water, ethanol, isopropanol and glycerol. Furthermore, the photocatalytic activity of as-obtained hollow ZnO microspheres is evaluated by the degradation of methylene blue (MB) in aqueous solution. The photocatalytic performance reveals that these hollow ZnO microspheres can not only efficiently degradate the dye, but also show good stability during the cycling photodegradation.

Photocatalytic performance of single crystal ZnO nanorods and ZnO nanorods films under natural sunlight

To solve the low catalytic activity and recycling difficulty of ZnO nanomaterials, a simple and large-scale synthesized method was used to prepare single crystalline ZnO nanorods with visible light response, and then ZnO membrane was fabricated using carbon fiber (CF) cloth as support. The resultant ZnO nanorods were carriedout with X-ray diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), Raman spectra, X-ray photoelectron spectroscopy (XPS), Ultraviolet spectrophotometer, and fluorescence spectrum. The experimental results display that the prepared ZnO nanorods own single crystalline structure and outstanding photocatalytic performance, which the removal time of all RhB is 15 min at UV light, 60 min at visible light and 100 min at natural sunlight, respectively. Moreover, ZnO nanorod membrane can completely remove RhB within 6 h under natural sunlight, meaning that the synthesized ZnO nanorods and ZnO nanorod membrane have potential industrial application prospect.

Enhanced visible photocatalytic activity of tree-like ZnO/CuO nanostructure on Cu foam

Novel tree-like ZnO/CuO composite, composed of CuO nanowire trunks and ZnO nanorod branches, have been successfully synthesized on Cu foam substrate. Single-crystalline CuO nanowires and ZnO nanorods are obtained by thermal oxidation and hydrothermal processes, respectively. The morphological, crystal and optical properties of the as-prepared samples have been studied by field-emission scanning microscopy, X-ray diffractometer, transmission electron microscopy, and UV–vis spectrophotometer. The resultant tree-like ZnO/CuO composites on Cu foam show excellent photocatalytic activity with a rate constant of about 0.0073 min−1 in the photo-degradation of Rhodamine B (RhB) under visible light (λ > 420 nm). The Cu foam not only provides extra-large surface area for the growth of tree-like CuO/ZnO composites, but also acts as an effective frame to support the catalysts. Furthermore, tree-like ZnO/CuO composite catalysts show high recovery yield for the recycling during the degradation of organic pollutants. This work provides an insight view into the design and synthesis of composite photocatalyst effectively supported by a substrate.

High response and selectivity of single crystalline ZnO nanorods modified by In2O3 nanoparticles for n-butanol gas sensing

In2O3-nanoparticle-modified single crystalline ZnO nanorods with enhanced gas-sensing performance were prepared using a facile sol-gel and chemical precipitation process. In2O3 nanoparticles obtained good dispersibility and were modified on the surface ZnO nanorods. One-dimensional ZnO nanorods served as a matrix for In2O3 nanoparticles, which effectively restrained the agglomeration of the In2O3 nanoparticles and increased the active sites for the sensing reaction. The sensing performance of as-prepared In2O3/ZnO and ZnO sensors was systematically tested using n-butanol as the probe analyte. Test results revealed the In2O3/ZnO sensor response was 104.3 at a concentration of 100 ppm, 3.5 and 5.3 times higher than that of the pure ZnO and In2O3 sensor, respectively. The influence of added In2O3 on sensing performance of the In2O3/ZnO nanocomposites was discussed. Test results showed that the optimal atomic ratio of indium was 3.3%. The response of the In2O3/ZnO sensor reached 6.1 at a concentration of 1 ppm, suggesting good detection of n-butanol. The In2O3/ZnO sensor displayed rapid response-recovery speed along with good selectivity and stability. A possible sensing mechanism for In2O3/ZnO was thus proposed based on experimental data and band structure analysis.

A facile method for bright, colour-tunable light-emitting diodes based on Ga-doped ZnO nanorods

Bottom-up fabrication of nanowire-based devices is highly attractive for oxide photonic devices because of high light extraction efficiency; however, unsatisfactory electrical injection into ZnO and poor carrier transport properties of nanowires severely limit their practical applications. Here, we demonstrate that ZnO nanorods doped with Ga donors by in situ dopant incorporation during vapour–solid growth exhibit superior optoelectronic properties that exceed those currently synthesised by chemical vapour deposition, and accordingly can be electrically integrated into Si-based photonic devices. Significantly, the doping method was found to improve the nanorod quality by decreasing the concentration of point defects. Light-emitting diodes (LEDs) fabricated from the Ga-doped ZnO nanorod/p-Si heterojunction display bright and colour-tunable electroluminescence (EL). These nanorod LEDs possess a dramatically enhanced performance and an order of magnitude higher EL compared with equivalent devices fabricated with undoped nanorods. These results point to an effective route for large-scale fabrication of conductive, single-crystalline ZnO nanorods for photonic and optoelectronic applications.

Optimization of the CVD parameters for ZnO nanorods growth: Its photoluminescence and field emission properties

Vertically aligned single crystalline ZnO nanorods arrays were grown on a silicon substrate in a CVD reactor using Au as a catalyst. The CVD parameters such as substrate temperature, catalyst layer thickness/morphology and reaction time play a crucial role in the synthesis of nanostructures by the vapor-liquid-solid process. By optimizing these various CVD parameters, highly controlled guided growth of ZnO nanorods was achieved. The structural and morphological properties of resultant ZnO nanorods were studied by means of X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction and field emission scanning electron microscopy. The photoluminescence spectroscopy was also done to investigate the defects in grown ZnO nanorods. The field emission results of nanorods array indicated that the properties of the field emission follow Fowler–Nordheim law. The nanorods (grown for 20 min) are vertically aligned on the substrate surface without entanglements and with good crystal quality as well which is very important for several applications.

Rapid Synthesis of Semipolar ZnO Nanorod Arrays on M-Sapphire by Microwave-Assisted Chemical Bath Deposition

Microwave-assisted chemical bath deposition (CBD) is a newly developed technique for fast growth of single-crystalline ZnO nanorods in a solution containing zinc nitrate at temperature below 100°C. ZnO nanorods have many applications for devices, however, semipolar oriented ZnO nanorods have been rarely reported. In this study, epitaxial growth of well-aligned arrays of inclined semipolar ZnO nanorods on a ZnO seed layer on m-plane sapphire substrates is successfully fabricated by using microwave-assisted chemical bath deposition for 10 min at 90 °C. TEM analysis reveals that The ZnO nanorods have an inclined angle about 30° with m-sapphire surface and an average length about 500 nm which gives a growth rate of 3 μm/h. Micro-photoluminescence measurements show that ZnO nanorods are of a reasonably good quality.

Transformation from Film to Nanorod via a Sacrifical Layer: Pulsed Laser Deposition of ZnO for Enhancing Photodetector Performance

A novel fabrication method for single crystalline ZnO nanorods by pulsed laser deposition (PLD) using a chemical-bath-deposited ZnS seed layer is proposed. For the substrate temperature (Ts) lower than 700 °C, the PLD-ZnO showed a polycrystalline phase and film-type morphology, resulting from the ZnS seed layer with a cubic phase. However, the ZnS film became a sacrifical layer and single crystalline ZnO(002) nanorods can be achieved at Ts of 900 °C, where ZnS was decomposed to zinc metals and sulfur fumes. The transformation from ZnO film to nanorod microstructure was demonstrated with the change of ZnS layer into Zn grains. Enhanced performance of the metal-semiconductor-metal photodetectors were fabricated with ZnO/ZnS samples grown at Ts of 500, 700, and 900 °C. The responsivities (@1 V and 370 nm) of these three devices were 1.71, 6.35, and 98.67 A/W, while their UV-to-visible discrimination ratios were 7.2, 16.5, and 439.1, respectively. Obviously, a higher light-capturing efficiency was obtained in the 900 °C-grown ZnO/ZnS device owing to its one-dimensional nanostructure with high crystal quality. The results indicate PLD combined with a sacrifical nanostructure is a promising method for obtaining high-quality ZnO nanorods, which paves the way for the fabrication of high performance ZnO-based devices.

Self-templating faceted and spongy single-crystal ZnO nanorods: Resistive switching and enhanced piezoresponse

A template-free, cost-effective, hydrothermal procedure is used to synthesize large areas of either faceted or spongy self-standing single-crystalline ZnO nanorods (NRs) from electrodeposited Zn films. The morphology of the NRs (faceted versus spongy) can be easily adjusted by simply varying the electrodeposition parameters of the parent Zn film. The obtained NRs exhibit an enhanced piezoelectric response (compared to bulk ZnO) and resistive switching properties which depend on their intrinsic morphology. This combination of properties together with the simplicity of the synthetic approach is particularly appealing for the fabrication of large arrays of nanosensors, nanoactuators and other applications that could benefit from an enhanced surface area in single-crystalline semiconductors.

Synthesis and optoelectronic properties of Ga-doped ZnO nanorods by hydrothermal method

High-density single-crystalline Ga-doped ZnO (GZO) nanorods were grown on glass substrate by hydrothermal method. The morphological and structural characteristics of the GZO were characterized by scanning electron microscopy (and X-ray diffraction. Results showed that the peaks related to the wurtzite structure ZnO were the (100), (002), and (101) diffraction peaks. The (002) peak indicated that the nanorods were preferentially oriented in the c-axis direction. Energy-dispersive spectroscopy indicated that Ga atom entered into the ZnO lattice. Moreover, the optical properties of GZO were measured by photoluminescence spectroscopy. All GZO nanorod arrays showed two different emissions, namely, ultraviolet (UV) and green emissions. A meta–semiconductor–metal UV photodetector (PD) based on GZO nanorods was also fabricated. The photo-current and dark-current constant ratio of the fabricated PD was approximately 15.2 at applied bias of 1 V.

Evolution of CuO poly-crystalline layers to coherent single-crystalline dots on ZnO nanorods upon annealing

ZnO/CuO p-n heterojunctions have attracted much attention for device applications, but coherent junctions, which are crucial for controlling electrical properties, still remain a challenge due to different crystal structure. In this work, CuO single-crystalline dots are coherently synthesized on ZnO nanorods by using a proposed two-step process. Transmission electron microscopy images confirm the formation of CuO coherent dots on single-crystalline ZnO nanorods upon annealing the nanorods covered with a poly-crystalline CuxO layer. The coherent dots exhibit two types of epitaxial orientations: CuO [002] ǀǀ ZnO [101¯1], CuO [111] ǀǀ ZnO [0002], and CuO [002] ǀǀ ZnO [101¯1¯], CuO [111] ǀǀ ZnO [0002¯]. As the thickness of the as-deposited CuxO layer increases from 10 to 30nm, the aspect ratio of the resulting CuO dots decreases from 0.43 to 0.21, approaching a film-like morphology. This work provides a route to prepare CuO coherent single-crystalline structures on ZnO, which is one step further toward fabricating excellent CuO/ZnO nanodevices.

Catalytic growth of one-dimensional single-crystalline ZnO nanostructures on glass substrate by vapor transport

Novel one-dimensional single-crystalline ZnO nanorod and nanoneedle arrays on a Cu catalyst layer-coated glass substrate were investigated via a simple physical vapor-deposition method by thermal evaporation of Zn powder in the presence of O2 gas. The ZnO nanorods and nanoneedles were synthesized along the c-axis growth direction of the hexagonal crystal structure. The diameter and growth rate of the high-quality and well-oriented one-dimensional ZnO nanostructures is achieved as a function of varying growth temperature and growth time. This study serves as the basis for further research on the growth of 1D ZnO nanostructures on a cost-effective glass substrate and at low temperature.

Position controlled and seed/catalyst free growth of ZnO nanorod arrays on reduced graphene oxide nanosheets

A new type of seed/catalyst-free and selective-growth process is developed for in situ grown vertically-aligned ZnO nanorods on the reduced graphene oxide (rGO) nanosheets. The dense and single-crystalline ZnO nanorods have been successfully grow on rGO nanosheets with the position- and size-controlled. The effect of rGO layer on the ZnO nanorods and the growth mechanism were investigated. The position of ZnO nanorod arrays correlates well with the size and position of rGO nanosheets, which is determined by dangling bonds such as oxygen-containing groups on rGO surface. Meanwhile, ZnO nanorods could grow laterally to form a continuous arrays with a large coverage on rGO substrates, which is mainly due to the step edges in rGO boundaries. The diameter and density of ZnO nanorods were readily determined by the growth temperature, and the length was easily controlled by the growth time. The absorption and PL spectrums suggest that ZnO nanorods/rGO have a strong UV absorption ranging from 330 to 375 nm and no obvious UV PL emission at room temperature, which indicates that the ZnO nanorods/rGO nanohybrids could be an excellent candidate for application in UV photodetectors. This work represents the success in seed/catalyst-free fabrication of aligned ZnO nanorod arrays directly on rGO nanosheets and the process can be readily scaled up for industrial applications of optoelectronic devices.

Constructing MnO2/single crystalline ZnO nanorod hybrids with enhanced photocatalytic and antibacterial activity

In order to improve the photocatalytic and antibacterial activity of ZnO nanorods, ZnO nanorods decorated with MnO2 nanoparticles (MnO2/ZnO nanorod hybrids) were prepared by using microwave assisted coprecipitation method under the influence of hydrogen peroxide, and the structure, photocatalytic activity and antibacterial property of the products were studied. Experimental results indicated that MnO2 nanoparticles are decorated on the surface of single crystalline ZnO nanorods. Moreover, the resultant MnO2/ZnO nanorod hybrids have been proven to possess good photocatalytic and antibacterial activity, which their degradated efficiency for Rhodamin B (RhB) is twice as the pure ZnO nanorods. Enhancement for photocatalytic and antibacterial activity is mainly attributed to the low band gap energy and excellent electrochemical properties of MnO2 nanoparticles.

Magnetic and optical properties of Mn-doped ZnO vertically aligned nanorods synthesized by hydrothermal technique

In this paper we have reported the synthesis of high quality vertically aligned undoped and Mn-doped ZnO single crystalline nanorods arrays on Si (100) substrates using two steps process, namely, initial slow seed layer formation followed by solution growth employing wet chemical hydrothermal method. The shapes of the as grown single crystalline nanorods are hexagonal. The diameter and length of the as grown undoped ZnO nanorods varies in the range of 80-150 nm and 1.0 - 1.4 μm, respectively. Along with the lattice parameters of the hexagonal crystal structure, the diameter and length of Mn doped ZnO nanorods are found to increase slightly as compared to the undoped ZnO nanorods. The X-ray photoelectron spectroscopy confirms the presence of Mn atoms in Mn2+ state in the single crystalline ZnO nanorods. The recorded photoluminescence spectrum contains two emissions peaks having UV exciton emissions along with a green-yellow emission. The green-yellow emissions provide the evidence of singly ionized oxygen vacancies. The magnetic field dependent magnetization measurements [M (H)] and zero field cooled (ZFC) and field cooled (FC) magnetization [M(T)] measurements have been carried out at different isothermal conditions in the temperature range of 5-300 K. The Mn doped ZnO nanorods clearly show room temperature ferromagnetic ordering near room temperature down to 5 K. The observed magnetization may be attributed to the long range ferromagnetic interaction between bound magnetic polarons led by singly charged oxygen vacancies.