n-ZnO Nanorods Research Articles

Fabrication and improved photocatalytic activity of n-ZnO nanorod arrays/p-CuO thin film heterojunction

ZnO nanorod arrays (NRs) are successfully synthesized on CuO thin film by hydrothermal method. The result of scanning electron microscopy demonstrates that the ZnO NRs produce great changes on morphology after deposited on CuO thin film. The current–voltage measurement of ZnO NRs/CuO film heterostructures show an evident rectifying behavior. Furthermore, the photocatalytic activity is investigated by degradation of methyl orange dye. It is found that compared with ZnO NRs, ZnO NRs/CuO film heterostructures display a better photocatalytic activity.

PMMA interlayer-modulated memory effects by space charge polarization in resistive switching based on CuSCN-nanopyramids/ZnO-nanorods p-n heterojunction.

Resistive switching (RS) devices are commonly believed as a promising candidate for next generation nonvolatile resistance random access memory. Here, polymethylmethacrylate (PMMA) interlayer was introduced at the heterointerface of p-CuSCN hollow nanopyramid arrays and n-ZnO nanorod arrays, resulting in a typical bipolar RS behavior. We propose the mechanism of nanostructure trap-induced space charge polarization modulated by PMMA interlayer. At low reverse bias, PMMA insulator can block charges through the heterointerface, and and trapped states are respectively created on both sides of PMMA, resulting in a high resistance state (HRS) due to wider depletion region. At high reverse bias, however, electrons and holes can cross PMMA interlayer by Fowler-Nordeim tunneling due to a massive tilt of energy band, and then inject into the traps of ZnO and CuSCN, respectively. and trapped states are created, resulting in the formation of degenerate semiconductors on both sides of PMMA. Therefore, quantum tunneling and space charge polarization lead to a low resistance state (LRS). At relatively high forward bias, subsequently, the trapped states of and are recreated due to the opposite injection of charges, resulting in a recovery of HRS. The introduction of insulating interlayer at heterointerface, point a way to develop next-generation nonvolatile memories.

Three-dimensional ordered ZnO/Cu2O nanoheterojunctions for efficient metal-oxide solar cells.

Interface modulation for broad-band light trapping and efficient carrier collection has always been the research focus in solar cells, which provides the most effective way to achieve performance enhancement. In this work, solution-processed 3D ordered ZnO/Cu2O nanoheterojunctions, consisting of patterned n-ZnO nanorod arrays (NRAs) and p-Cu2O films, are elaborately designed and fabricated for the first time. By taking advantage of nanoheterojunctions with square patterned ZnO NRAs, solar cells demonstrate the maximum current density and efficiency of 9.89 mA cm(-2) and 1.52%, which are improved by 201% and 127%, respectively, compared to that of cells without pattern. Experimental analysis and theoretical simulation confirm that this exciting result originates from a more efficient broad-band light trapping and carrier collection of the 3D ordered ZnO/Cu2O nanoheterojunctions. Such 3D ordered nanostructures will have a great potential application for low-cost and all oxide solar energy conversion. Furthermore, the methodology applied in this work can be also generalized to rational design of other efficient nanodevices and nanosystems.

Contact light-emitting diodes based on vertical ZnO nanorods

We report vertical contact light-emitting diodes (VCLEDs), that are based on heterojunctions formed by using the point contacts of n-ZnO nanorods (NRs) to the p-type semiconductor substrate and that are fabricated using a new approach to the formation of LEDs (Appl. Phys. Lett. 98, 093110 (2011)). A p-type GaN film grown on a sapphire substrate was used to form n-ZnO NRs/p-GaN VCLEDs on a large area of about 4 cm2. The VCLEDs emitted a pure blue electroluminescence with high efficiency. Electroluminescence at 470 nm, which is visible to the naked eye, started at small current of about 50 μA and is attributed to the good optical properties of the structurallyperfect heterojunctions in the point contacts. The VCLED configuration allows the creation of ZnO/p-GaN nano-LEDs of high density and high-quality with a greatly reduced concentration of nonradiative defects in the active regions. The VCLEDs showed the high brightness of light required for active matrix displays and general solid-state lighting.

Electroluminescence of ZnO nanorods/ZnMgO films/p-SiC structure heterojunction LED

Through a facile low-temperature hydrothermal process, ZnO nanorod arrays were grown on ZnMgO films/p-SiC to form a heterojunction LED. ZnMgO films were grown on p-SiC by a simple sol–gel method. In this heterojunction structure, ZnMgO films works as the seeds film for the growth of ZnO nanorods. In particular, ZnMgO films can work as barrier layer between n-ZnO nanorods and p-SiC, which controls the movement of holes and electrons. Thus, with this introduced ZnMgO films, the electroluminescence (EL) from ZnO can be observed in ZnO/SiC heterojunction. Under a forward bias larger than 18V, the emission band in electroluminescence (EL) spectrum is considered as a combination of a peak centered at 388nm and a yellow band emission peak around 450nm. As the injection current increased, the intensity of ultraviolet emission was also increased. At last, the function of ZnMgO films in the heterojunction structure was discussed.

Electrochemical growth and characterization of a p-Cu2O thin film on n-ZnO nanorods for solar cell application

Heterostructural p-Cu2O/n-ZnO solar cells are fabricated by the consecutive electrochemical deposition of an n-ZnO nanorod layer, followed by a p-Cu2O layer on a transparent conductive substrate. The growth parameters, growth temperature and time of applied potential, which determine the crystalline structure and film thickness of Cu2O, are fine-tuned to investigate their effects on the performance of heterojunction solar cells. From XRD and SEM analyses, the best crystalline quality and the optimum Cu2O film thickness are accomplished at the potential of −0.4 V and the growth temperature of 60 °C for 10 min. In addition, the electrolyte concentration plays an important role in the optical absorbance of Cu2O film, which significantly exhibits two-time enhancement as adjusting the CuSO4 concentration of 0.4 M to 0.02 M. The best performance of the heterojunction cell is able to achieve the conversion efficiency of 0.33% with Voc, Jsc, and FF being 0.15 V, 7.03 mA cm−2, and 0.33, respectively. Hence, this study demonstrates that the proposed strategy to improve the solar cell performance realized by electrochemical deposition has the potential to produce cheap and environmentally friendly solar cells.

Tunable charge transport through n-ZnO nanorods on Au coated macroporous p-Si

Diode-like and Ohmic charge transport through aligned ZnO nanorods on Au coated macroporous Si.

Growth and characterization of p-Cu2O/n-ZnO nanorod heterojunctions prepared by a two-step potentiostatic method

p-Cu2O/n-ZnO nanorod heterojunctions were fabricated by a two-step process. The process was performed with potentiostatic deposition of n-ZnO nanorods on conductive indium-tin-oxide (ITO) glasses followed by potentiostatic deposition of p-Cu2O to form p-Cu2O/n-ZnO nanorod heterojunctions. The deposition condition required to form the cuprous oxide layer affected significantly the formation and microstructure of the p-Cu2O/n-ZnO nanorod heterojunctions. In particular, the high-quality p-Cu2O/n-ZnO nanorod heterojunctions were obtained only at relatively high temperatures of 90 and 100°C. The p-Cu2O/n-ZnO nanorod heterojunctions exhibited a well-defined p–n diode characteristic with an ideality factor of about 4.3.

Electrical transport behavior of n-ZnO nanorods/p-diamond heterojunction device at higher temperatures

The n-ZnO nanorods (NRs)/p-diamond heterojunction structures are fabricated by thermal vapor growing ZnO NRs on chemical vapor deposited boron-doped diamond film. Temperature dependent current-voltage (I-V) characteristics of the p-n heterojunction are examined from 25 °C to 220 °C. The turn-on voltage and ideality factor of the devices decrease with increasing temperature, whereas the reverse saturation current increases at higher temperatures. The carrier injection efficiency is effectively enhanced at high temperatures. The electrical transport behaviors are investigated at various temperatures and bias voltages.

Near-Ultraviolet Light-Emitting Devices Using Vertical ZnO Nanorod Arrays

Reports on electroluminescence (EL) in solid-state, nanomaterial-based devices emitting in the lower wavelength range of the visible spectrum are limited, and the emission stability of these devices is rarely reported. We have fabricated light-emitting devices (LEDs) based on integration of n-ZnO nanorods and p-GaN films, which emit in the violet to near-ultraviolet (NUV) region. We also present data on the stability of EL in fabricated devices. Vertical arrays on ZnO nanorods, with estimated ZnO nanorod density ~108 cm−2, were grown on p-GaN films with typical length of ~4 μm and width of ~120 nm. The NUV LEDs show low turn-on voltage (~3.0 V), small reverse saturation current (~10 μA), and more than two orders of magnitude rectification ratio, all of which indicates a good-quality p–n junction at the p-GaN/n-ZnO nanorod interface. The EL spectra of LEDs present an emission band centered at ~403 nm. Gaussian fitting of the EL peak revealed three emission peaks at 378 nm, 405 nm, and 431 nm with dominant emission in the NUV region. Significantly, the fabricated NUV LEDs present stable and repeatable EL characteristics, as revealed by bias-stress stability tests. The good electrical properties and stable EL performance make these nanostructure-based NUV LEDs potential candidates for mass production of next-generation lighting devices.

Temperature Dependant Characteristics of Zinc Oxide Nanorods Based Heterojunction Diode

Temperature-dependant characteristics of heterojunction diode made by n-ZnO nanorods grown on p-silicon substrates has been characterized and demonstrated in this paper. ZnO nanorods were grown onto the silicon substrate via simple thermal evaporation process by using metallic zinc powder in the presence of oxygen at approximately 550 degrees C without the use of any metal catalysts or additives. The as-grown ZnO nanorods were characterized in terms of their structural and optical properties. The detailed structural studies by XRD, TEM, HRTEM and SAED revealed that the grown nanorods are well-crystalline with the wurtzite hexagonal phase and preferentially grown along the [0001] direction. The as-grown n-ZnO nanorods grown on p-Si substrate were used to fabricate p-n heterojunction diode. The fabricated p-n junction diode attained almost similar turn-on voltage of approximately 0.6 V. The values of turn-on voltage and least current are same with the variations of temperature (i.e., 27 degrees C, 70 degrees C and 130 degrees C).

Investigation of Electrodeposited and Solution Grown ZnO Nanorod Based UV Photodetector

In this work solution growth technique as well as electrodepostion technique was employed to directly grow ZnO nanorod arrays on the p-GaN layer for fabricating n-ZnO nanorod/ p-GaN heterojunction diodes. Microstructure and room temperature photoluminescence (PL) measurement confirmed the growth of ZnO nanorod arrays with near perfect microstructure, stoichiometry and excellent optical quality. Current-voltage (I-V) measurements showed the formation of diode structure with a typical diode characteristic having a turn on voltage of 2.5V and 6.4V respectively, for solution grown and electrodeposited ZnO nanorod based p-n diode. A low resistive ITO coated glass was used to make contact with the top nanorod arrays and spectral response of the p-n diode was investigated. UV response particularly from solution grown ZnO nanorod based p-n diode was very promising.

Studying trivalent/bivalent metal ion doped TiO 2 as p-TiO 2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO 2 thin films (M x Ti 1− x O 2, M = Cr 3+, Fe 3+, Ni 2+, Co 2+, Mn 2+ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti 4+ oxidation state of the Ti2p band in the doped p-TiO 2. The homogenous M x Ti 1− x O 2 was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current ( I)–voltage ( V) characteristics for the Ag/n-ZnO/M x Ti 1− x O 2/ITO/glass assembly showed rectifying behavior with small turn-on voltages ( V 0) < 1 V. The ideality factor ( η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.

Vertical ZnO nanorod/Si contact light-emitting diode

Blue-white light emission was obtained from glass/indium tin oxide (ITO)/n-ZnO nanorod array (NRA)/p+-Si vertical contact light emitting diodes (VCLEDs). The nanoscale p-n heterojunction VCLEDs were formed by direct engagement n−-tips of n-ZnO NRA grown vertically on an ITO/glass substrate with p+-Si wafer. Proposed configuration of the VCLED allows creating a high density (∼109 cm−2) of self-assembled ZnO/Si nanodiodes with point junctions of high quality due to structural perfection of the Si wafer and the tips of ZnO nanorods as well as providing a high injection current and light emission from the NRA VCLED required for solid state lighting.

Synthesis and characterization of zinc oxide nanorods on silicon for the fabrication of p-Si/ n-ZnO heterojunction diode

Hexagonal-shaped ZnO nanorods were successfully grown on p-silicon substrate via thermal evaporation of metallic zinc powder in the presence of oxygen. The grown nanorods were characterized in terms of their structural and optical properties by using X-ray diffraction, FESEM, HRTEM and room-temperature photoluminescence (PL) spectroscopy. It is confirmed from the observed structural and optical properties, that the grown nanorods are well-crystalline with the wurtzite hexagonal phase and are preferentially grown along the [0 0 0 1] direction and exhibiting good optical properties. The as-grown n-ZnO nanorods on p-silicon ( n-ZnO/ p-Si) substrate were used to fabricate p– n heterojunction diode which attains a turn-on voltage of ∼0.5 V. Temperature-dependant (298, 323, 343, 363, 383, and 423 K) I– V characteristics for the fabricated diode were also examined and demonstrated in this paper. It was found that the fabricated heterojunction diode was reasonably stable at higher temperatures and the series resistance slightly increases with increasing the temperature.

Color tunable light-emitting diodes based on p+-Si/p-CuAlO2/n-ZnO nanorod array heterojunctions

Wide-range color tuning from red to blue was achieved in phosphor-free p+-Si/p-CuAlO2/n-ZnO nanorod light-emitting diodes at room temperature. CuAlO2 films were deposited on p+-Si substrates by sputtering followed by annealing. ZnO nanorods were further grown on the annealed p+-Si/p-CuAlO2 substrates by vapor phase transport. The color of the p-CuAlO2/n-ZnO nanorod array heterojunction electroluminescence depended on the annealing temperature of the CuAlO2 film. With the increase of the annealing temperature from 900 to 1050 °C, the emission showed a blueshift under the same forward bias. The origin of the blueshift is related to the amount of Cu concentration diffused into ZnO.

Fabrication and characterization of high-brightness light emitting diodes based on n-ZnO nanorods grown by a low-temperature chemical method on p-4H-SiC and p-GaN

Light emitting diodes (LEDs) based on n-ZnO nanorods (NRs)/p-4H-SiC and n-ZnO (NRs)/p-GaN were fabricated and characterized. For the two LEDs the ZnO NRs were grown using a low temperature (<100 °C) aqueous chemical growth (ACG) technique. Both LEDs showed very bright nearly white light electroluminescence (EL) emission. The observed luminescence was a result of the combination of three emission lines composed of violet-blue, green and orange-red peaks observed from the two LEDs. Room temperature photoluminescence (PL) was also measured and consistency with EL was observed. It was found that the green and violet-blue peaks are red-shifted while the orange peak is blue-shifted in the EL measurement. It was also found that due to the effect of the GaN substrate the violet-blue peak in the EL measurement is more red-shifted in n-ZnO (NRs)/p-GaN LEDs as compared to n-ZnO (NRs)/p-4H-SiC LEDs.

Luminescence from Zinc Oxide Nanostructures and Polymers and their Hybrid Devices

Zinc oxide (ZnO) is a strong luminescent material, as are several polymers. These two materials have distinct drawbacks and advantages, and they can be combined to form nanostructures with many important applications, e.g., large-area white lighting. This paper discusses the origin of visible emission centers in ZnO nanorods grown with different approaches. White light emitting diodes (LEDs) were fabricated by combining n-ZnO nanorods and hollow nanotubes with different p-type materials to form heterojunctions. The p-type component of the hybrids includes p-SiC, p-GaN, and polymers. We conclude by analyzing the electroluminescence of the different light emitting diodes we fabricated. The observed optical, electrical, and electro-optical characteristics of these LEDs are discussed with an emphasis on the deep level centers that cause the emission.

Junction temperature in n-ZnO nanorods/(p-4H–SiC, p-GaN, and p-Si) heterojunction light emitting diodes

The junction temperature of n-ZnO nanorods/(p-4H–SiC, p-GaN, and p-Si) heterojunction light emitting diodes (LEDs) at built-in potential was modeled and experiments were performed at various temperatures (15–65 °C) to validate the model. As the LEDs operate near the built-in potential that’s why it is interesting to investigate the temperature coefficient of forward voltage near the built-in potential (∼ V o). The model and experimental values of the temperature coefficient of forward voltage near the built-in potential (∼ V o) were compared. We measured the experimental temperature coefficient of the series resistance. By including the temperature coefficient of the series resistance in the model, the theoretical and experimental values become very close to each other. It was found that the series resistance has the main contribution in the junction temperature of our devices. We also measured the junction temperature above the built-in potential and found that the model deviates at higher forward voltage. From this observation we concluded that the model is applicable for low power devices, operated near the built-in potential.

Inorganic-organic ZnO Based Heterostructures for Lighting

Here we present results on different organic and inorganic light emitting diodes (LED). The use of n-ZnO nanorods grown on different substrates forms the base of this new technology. p-type semiconducting as well as polymeric based substrates were used. The combination of n-ZnO nanorods and light emitting polymer(s) will lead to a LED with a broad emission spectrum and hence the possibility of producing white LEDs. Different issues regarding this hybrid new technology for lighting are to be discussed. Among them, we will present electrical, optical and electro-optical characteristics of different configurations of this hybrid technology regarding the polymer layer(s) choice. The light quality of the fabricated n-ZnO nanorods/p-type substrates LEDs will be shown and analyzed. Finally a comparison between this hybrid technology and other LEDs based on ZnO nanorods in combination with other inorganic p-type semiconductor is to be discussed.