ZnO Rod Arrays Research Articles

Preparation of Oriented ZnO Rod Arrays Using Hexagonal Plate-Like Particles as a Seed Layer

ZnO rod film is a promising material for electrodes and sensors due to its large surface area and high electrical conductivity. One of the drawbacks of conventional ZnO rod film is the random orientation of rods. In this study, an oriented ZnO seed layer composed of hexagonal plate-like ZnO particles was prepared by dip-coating. An oriented ZnO rod film was then synthesized by growing this seed layer using a hydrothermal synthesis method. We optimized the concentration of the precursor and the hydrothermal treatment time to synthesize homogeneous ZnO rod arrays. The uniformity of the rod arrays was improved by applying a strong magnetic field (12 T) during hydrothermal treatment.

Cold crystallization and morphology control of ZnO nanostructures for chemical sensors

AbstractCold crystallization and morphology control of ZnO nanostructures have been achieved. Various ZnO nanostructures, such as ZnO rod arrays, high c‐axis‐oriented stand‐alone ZnO self‐assembled films, ZnO particulate films, ZnO nanorod arrays, semicircular ZnO nanowhisker assemblies, bunched rose‐ and core–shell‐like ZnO particles, twin ZnO nanoarray assemblies, ZnO nanotubes, and ZnO nanoflowers, have been developed using the hexagonal crystal structure of ZnO. These structures have advantages, such as high anisotropy, high specific surface area, high crystallinity, and characteristic crystal facets, and can be applied in devices such as photoelectric conversion sensors, chemical sensors, gas sensors, batteries, and catalysts. Additionally, various nanostructures using other oxide materials are expected to be developed by applying the scientific knowledge obtained from the cold crystallization and morphology control of ZnO. Cold crystallization, which is a long‐standing challenge in the field of ceramics, has been realized, which is a historical turning point in the field of ceramics research.

Optimization of Synthesizing Upright ZnO Rod Arrays with Large Diameters through Response Surface Methodology

The deposition parameters involved in chemical bath deposition were optimized by a response surface methodology to synthesize upright ZnO rod arrays with large diameters. The effects of the factors on the preferential orientation, aspect ratio, and diameter were determined systematically and efficiently. The results demonstrated that an increased concentration, elevated reaction temperature, prolonged reaction time, and reduced molar ratio of Zn2+ to tri-sodium citrate could increase the diameter and promote the preferential oriented growth along the [002] direction. With the optimized parameters, the ZnO rods were grown almost perfectly vertically with the texture coefficient of 99.62. In the meanwhile, the largest diameter could reach 1.77 μm. The obtained rods were merged together on this condition, and a dense ZnO thin film was formed.

Low cost inorganic white light emitting diode based on submicron ZnO rod arrays and electrodeposited Cu2O thin film

We present here the fabrication and characterization of low cost inorganic white light emitting diode (LED) based on submicron ZnO rod arrays and Cu2O thin film. Firstly, n-ZnO rod arrays were grown on FTO substrate by hydrothermal method at 150 °C. Then, p-Cu2O thin film was electrodeposited on top of ZnO arrays from alkaline cupric lactate solution at 60 °C. X-ray diffraction and scanning electron microscopy were employed to investigate the structural and the morphology of the prepared samples as well as the diode device. The results indicate the formation of hexagonal structure of ZnO arrays and cubic structure of polycrystalline Cu2O thin film. The performance of LED device was investigated employing I-V and electroluminescence measurements. The heterojunction device showed good rectification behavior and a broad emission covering visible spectrum for all applied voltage which indicates the white electroluminescence of the heterostructure device. Therefore, the inorganic ZnO arrays/Cu2O thin film is a promising heterojuniction for low cost and large-scale production of white LED.

Photovoltaic Response of Transparent Schottky Ultraviolet Detectors Based on Graphene-on-ZnO Hexagonal Rod Arrays

Herein, we report the ultraviolet (UV) photoresponse characteristics of vertically grown ZnO rod arrays and graphene hybrid detectors. Graphene serves as a UV transparent electrode for efficient light absorption and as a Schottky electrode for promoting the separation of charge carriers photogenerated in ZnO. In particular, when the ZnO rods were passivated by polymer encapsulation, the proposed device (device I) exhibited improved Schottky junction characteristics and photovoltaic response to UV light when compared to the device based on bare ZnO rods (device II). These improvements, which originated from the reduced photodesorption effect, enable device I to operate in a zero-bias condition, producing a substantial photovoltaic output current with a photoresponsivity of 5.14 × 10–4 A/W, which is not achievable using device II. More importantly, compared to the photoconductive response of device II operating in bias conditions, the photovoltaic response of device I yields a higher on/off ratio of ∼280 an...

One step to fabricate vertical submicron ZnO rod arrays by hydrothermal method without seed layer for optoelectronic devices

Reducing production cost by minimizing growth steps of ZnO microstructures is an important key for large scale fabrication of optoelectronic devices. Here, we present the growth of submicron ZnO arrays on FTO substrates by only one step without seed layers using hydrothermal method. The effect of KOH mass concentration on the physical properties of ZnO submicron rods has been investigated employing Uv–vis spectroscopy, X-ray diffraction, and scanning electron microscopy. The results indicate the formation of hexagonal structure of ZnO submicron arrays with different surface morphology, different orientations and diameters dependent on KOH concentration. Uniform ZnO submicron rod arrays with (002) preferred orientation were obtained when the KOH concentration was 0.24g/ml. Beside the uniformity and good alignment of ZnO arrays along the c-axis of FTO substrate, they have the smallest diameter which make them suitable to photo-electronic applications.

Growth and physical properties of ZnO rod arrays that integrally match the geometries of photoresists with hole sizes larger than rods' habit diameters

Bottom-up and top-down methods can be combined to create well-designed two-dimensional rod arrays for use in optoelectronics. Toward this end, the bottom-up material growth should follow the exact input of the top-down surface patterning, for example, the growth morphologies should integrally match to the geometries of the patterning. Through the repeated use of appropriate amounts of sodium citrate and polyethylenimine, ZnO rod arrays, in which each rod integrally matched with restriction holes in the surfaces, were formed even when the diameters of restriction holes were much larger than the original habit diameters of the ZnO rods. The diameters and lengths of the ZnO rods were varied by adjusting the diameters of photoresist holes, the concentrations of precursors in solution, and the reaction time. This method was successfully extended the diameter of integrally matching growth of ZnO rods to micrometer scale. The effects of the restriction diameters, the concentration of adjuvants, and the crystallinity of the seeds on integrally matching growth of ZnO rods are discussed. The photoluminescence spectra and crystallinities of the ZnO rods produced in different environments were characterized, revealing that a smaller photoresist aperture improved crystallization quality. The photoluminescence spectra of ZnO rods with different growth times and adjuvant concentrations were also explored. Fibrous ZnO/Cu2O heterostructures were successfully synthesized via the growth of ZnO rods. The electrical properties of ZnO/Cu2O and ZnO rods/Si heterojunctions were investigated, and the crystallinities of the ZnO rods were shown to change based on the different synthesis conditions used. This method for the integrally matching growth of ZnO using photoresist patterns was also generalized to ZnO tubes and rings, which individually match to single restriction hole.

Improvement and optimization of the growth quality of upright ZnO rod arrays by the response surface methodology

Response surface methodology was employed to guide the growth of ZnO rod arrays. With its assistance, the growth quality of the obtained samples was improved dramatically. The morphologies evolved from sparsely, randomly oriented rods to dense and upright rod arrays. Moreover, the significant parameters for ZnO rod arrays growth and their optimal regions were also determined. Furthermore, with the optimized parameters, the obtained aspect ratio of the vertically aligned ZnO rod arrays reached a value of 31. This is relatively a high value of ZnO rod arrays synthesized by wet chemical methods without capping agents in neutral solutions.

Optimization of high-quality vertically aligned ZnO rod arrays by the response surface methodology

Optimization of the deposition parameters was conducted by the response surface methodology to synthesize high-quality ZnO rod arrays with a high texture coefficient, a large aspect ratio and a narrow bandgap. In addition, mathematical models based on statistical analysis were also developed to predict the texture coefficient, aspect ratio and bandgap of the ZnO rod arrays. With the optimized parameters, all of the three involved responses obtained the desired optimum values. The results show that the texture coefficient can be elevated up to a value of 0.998, which represents an almost perfect value. Moreover, wide range of aspect ratios was obtained for various applications and the obtained maximum value of 21.3 is relatively high value by wet chemical method, especially when no capping agent and no refreshing growth solution in a nearly neutral solution is used.

Fabrication of gas sensor based on mesoporous rhombus-shaped ZnO rod arrays

Mesoporous rhombus-shaped ZnO rod arrays based gas sensors were successfully fabricated by a post-thermal conversion of rhombic Zn(OH)F precursors after a low temperature fluorine-mediated hydrothermal method. The gas sensors annealed at 350, 450, 550 and 650°C showed different responses for ethanol detection. Increasing of the annealing temperatures resulted in the decline of gas response, which is due to the effect of grain size distributions, specific surface area and defect concentration. At low temperature (less than 350°C), the incomplete decomposition of Zn(OH)F led to the instability of the sensors. The sensor annealed at 450°C showed the best performance, and the response to 100ppm ethanol reached ∼11.8 at 300°C. The sensor also exhibited good response/recovery speed (4s and 7s), excellent gas response and good stability. The fabrication of the high-performance gas sensors possesses the capability of constructing complex architectures with mesoporous rhombus-shaped ZnO rod arrays as the building block.

Back Cover: Micropatterned ZnO rod arrays prepared by Au‐catalyzed electroless deposition (Phys. Status Solidi RRL 7/2014)

ZnO nanostructures represent an interesting choice for the fabrication of optoelectronic micro/nanodevices. In their Letter on pp. 648–652, Camelia Florica et al. were using easily scalable, low-cost and highly reproducible techniques such as photolithography and electroless deposition in order to prepare ZnO functionalized micropatterns for applications in sensing and self-cleaning surfaces. The morphological, structural, optical, wetting and electrical properties of the ZnO rods arrays were investigated. A conversion from hydrophilic to superhydrophobic was observed for the samples, depending on the ZnO substrate covering. A change in ZnO samples electrical resistance was evidenced from current–voltage measurements after their exposure to ammonia, such micropatterned ZnO rod arrays being effective for designing microdevices for potential applications in various areas where chemical sensing or superhydrophobicity properties are required.

Micropatterned ZnO rod arrays prepared by Au‐catalyzed electroless deposition

AbstractMicropatterned ZnO was synthesized by an electroless deposition process using Au stripes as catalytic surfaces. The Au‐patterned electrodes were prepared on SiO2/Si wafers using photolithography. The site‐selective deposition of patterned ZnO hexagonal rod arrays is confirmed by scanning electron microscopy. The ZnO micropatterned surface revealed a conversion of wettability from hydrophilic to superhydrophobic depending on the deposition reaction param‐ eters. The electrical measurements carried out at room temperature before and after exposure to ammonia vapors of the patterned ZnO arrays show a resistance variation with exposure time. Highly reproducible, easy scalable and low‐cost, photolithography and electroless deposition techniques could provide a facile approach to fabricate functionalized micropatterns, for a wide range of applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication and electrical characterization of ZnO rod arrays/CuSCN heterojunctions

ZnO rod arrays/CuSCN heterojunctions are fabricated by depositing ZnO rod arrays films using two-step chemical bath deposition (CBD) and CuSCN thin films using successive ionic layer adsorption and reaction (SILAR) on ITO substrate successively. The structures and morphologies of ZnO films and CuSCN films, analyzed by X-ray diffraction (XRD) spectroscopy and metallurgical microscope, show that ZnO films are hexagonal wurtzite structure and consisted of vertical polycrystalline rods with diameter of 1μm, CuSCN films are β-phase structure and consisted of elongated grains with length of 3μm. Current–voltage (I–V) measurements of ZnO/CuSCN heterojunctions show good diode characteristics with rectification ratio about 48.3 at 3V. The forward conduction is, respectively, determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of ZnO/CuSCN heterojunctions has been proposed to explain the transport mechanism.

Growth of vertically aligned ZnO nanowalls for inverted polymer solar cells

This paper reports a facile fabrication of vertical ZnO nanowalls on ITO coated glass substrates by using an aqueous solution growth method at low temperature. The formation of nanowalls is ascribed to selective dissolution of (001) planes of the chemical bath deposited dense ZnO rods. The morphology of the etched ZnO nanowalls is determined by the structure of the as-grown ZnO rod arrays, which can be readily controlled by tuning aqueous solution parameters such as: initial pH value of chemical bath solution and the growth temperature. With verticaly aligned ZnO nanowalls as electrode in inverted polymer solar cells, the average performance of devices with open circuit voltage, short circuit current density, fill factor, and power conversion efficiency are measured as 0.56V, 7.56mWcm−2, 0.49 and 2.14%, respectively. The results indicate that the two-dimensional structure of ZnO nanowalls can effectively serve as an electrode for inverted polymer solar cells.

ROOM-TEMPERATURE FERROMAGNETIC PROPERTIES OF Co-DOPED ZnO NAOROD ARRAYS

Co -doped ZnO rod arrays were fabricated on zinc foils by a hydrothermal method. X-ray diffraction results indicate that the samples have wurtzite crystalline structure without metallic Co or other secondary phases detected. The nanorod arrays exhibit room-temperature ferromagnetic behavior for different Co concentration. The influence of Co on the magnetic properties of ZnO was studied by the first-principles calculations. The exchange interaction between local spin-polarized electrons and conductive electrons is proposed as a cause of the room-temperature ferromagnetism.

Comparison of photocatalytic activity of ZnO rod arrays with various diameter sizes and orientation

Abstract ZnO micro/nanorod arrays were successfully grown on different substrates (such as Si/seeds, Si, glass and Zn) through chemical bath deposition (CBD) method. The substrates were found to have effect not only on the diameter size but also on the orientation of ZnO rod arrays. By further varying deposition times, different diameter sizes of ZnO rod arrays were also obtained. Photoluminescence (PL) and photocatalysis properties were studied at room temperature. The photocatalytic results showed that the photocatalytic activities of ZnO rod arrays not only depend on diameter size, but also depend on other factors (such as orientation, morphology and amount of photocatalyst). ZnO rod arrays with the smallest diameter and highest orientation exhibited the maximum photocatalytic activity.

Preparation and enhanced ferromagnetic, semi-conductive, and optical properties of Co-doped ZnO rod arrays

In this study, co-doped ZnO rod arrays with wurtzite crystal structure were successfully prepared on zinc substrate by the co-precipitation method. The results of crystal analysis indicated that the dopant Co2+ was incorporated into ZnO crystal. Photoluminescence (PL) spectra and magnetization curves proved that their PL intensity and ferromagnetism were increased with the increase of Co2+ concentrations in a ZnO host. Current–voltage characteristic curves of Co-doped ZnO rod arrays indicated that as-prepared samples were n-type semiconductors. Co-doped ZnO rod arrays could be envisioned to detect diseases and bacteria based on these properties.

Low-Temperature Facile Synthesis of ZnO Rod Arrays and Their Device Applications

Different morphologies of ZnO rod arrays with tunable photoluminescence (PL) were synthesized by vapor-phase transport deposition. The morphology and luminescence properties of ZnO rod arrays were found to be dependent on the deposition temperature and can be further tuned by postgrowth annealing treatment. Crystalline ZnO rods with intense UV emission at room temperature (RT) can be synthesized under a relatively lower temperature of 500 °C in a conventional tube furnace. ZnO rods grown at higher or lower temperature regions, however, showed additional defect-related emissions. Besides the commonly observed green emission from vapor-deposited ZnO rods, our samples also showed broad orange-red emission centered at ca. 620 nm. The origin of the different emission bands from the ZnO rod arrays was analyzed based on the annealing treatment in conjunction with the PL characterizations. Furthermore, two prototypes of optoelectronic devices (i.e., LEDs and FETs) were fabricated based on the as-grown ZnO rods. UV electroluminescence was achieved from ZnO rod arrays/p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si heterojunctions at RT, and FET based a single ZnO rod showed comparable transistor properties with previous reports on the ZnO nanorods grown at higher temperatures.

One-minute microwave-assisted chemical bath deposition of nanostructured ZnO rod-arrays

Microwave-assisted chemical bath deposition (MACBD) is an emerging route for rapid synthesis of films and nanostructured particles. In this paper we report MACBD of ZnO rod-array films on bare glass substrates from an aqueous bath of tetra ammonium zinc complex. The deposition time is reduced to about 1 min as compared to around 60 min for conventional CBD. X-ray diffraction study shows that as-deposited films are uniaxially out-of-plane textured along the c-axis. Scanning Electron Microscopy reveals that the films consist of elongated elliptical tapered rods of diameters 250 to 350 nm. Atomic Force Microscopy shows that the films consist of about 350 nm grains. The RMS roughness is about 60 nm. The energy band gap is 3.27 eV as estimated from optical data. The films are n-type with electrical conductivity of 1 × 10 − 4 S/cm.

Influence of Architecture-Controlled GaN Rod Arrays on the Output Power of GaN LEDs

We report the influence of controlled lengths and densities of GaN rod arrays on the output power of GaN light-emitting diodes (LEDs). The morphology-controlled GaN rod arrays are fabricated via using ZnO rod arrays as a dry etching mask. Our investigation indicates that the output power of GaN LEDs has a strong dependence on the lengths and densities of GaN rod arrays. The variation of output power of GaN LEDs with GaN rod arrays is caused by the Fabry-Pérot resonance of the film composed by GaN rod arrays. The theoretical analysis also shows a good agreement with the measurement results.