Controlled growth and field emission properties of zinc oxide nanopyramid arrays

Single-crystalline, pyramidal zinc oxide nanorods have been synthesized in a large quantity on p-Si substrate via catalyst-free thermal chemical vapor deposition at low temperature. SEM investigations showed that the nanorods were vertically aligned on the substrate, with diameters ranging from 60 to 80 nm and lengths about 1.5 μm. A self-catalysis VLS growth mechanism was proposed for the formation of the ZnO nanorods. The field emission properties of the ZnO nanopyramid arrays were investigated. A turn-on field about 3.8 V/μm was obtained at a current density of 10 μA/cm², and the field emission data was analyzed by applying the Fowler-Nordheim theory. The stability of emission current density under a high voltage was also tested, indicating that the ZnO nanostructures are promising for an application such as field emission sources.     

Field electron emission improvement of ZnO nanorod arrays after Ar plasma treatment

Vertically well-aligned single crystal ZnO nanorod arrays were synthesized and enhanced field electron emission was achieved after radio-frequency (rf) Ar plasma treatment. With Ar plasma treatment for 30 min, flat tops of the as-grown ZnO nanorods have been etched into sharp tips without damaging ZnO nanorod geometrical morphologies and crystallinity. After the Ar ion bombardment, the emission current density increases from 2 to 20 μA cm⁻² at 9.0 V μm⁻¹ with a decrease in turn-on voltage from 7.1 to 4.8 V μm⁻¹ at a current density of 1 μA cm⁻², which demonstrates that the field emission of the as-grown ZnO nanorods has been efficiently enhanced. The scanning electron microscopy (SEM) results, in conjunction with the results of transmission electron microscopy (TEM), Raman spectroscopy and photoluminescence observation, are used to investigate the mechanisms of the field emission enhancement. It is believed that the enhancements can be mainly attributed to the sharpening of rod tops, and the decrease of electrostatic screening effect.     

Effect of growth time on the structure, Raman shift and photoluminescence of Al and Sb codoped ZnO nanorod ordered array thin films

Al and Sb codoped ZnO nanorod ordered array thin films have been deposited on glass substrate with a ZnO seed layer by hydrothermal method at different growth time. The effect of growth time on structure, Raman shift, and photoluminescence (PL) was studied. The thin films at growth time of 5 h consist of nanorods growth vertically oriented with ZnO seed layer, and the nanorods with an average diameter of 27.8 nm and a length of 1.02 μm consist of single crystalline wurtzite ZnO crystal and grow along [0 0 1] direction. Raman scattering analysis demonstrates that the thin films at the growth time of 5 h have great Raman shift of 15 cm⁻¹ to lower wavenumber and have low asymmetrical factor Г_a/Г_b of 1.17. Room temperature photoluminescence reveals that there is more donor-related PL in films with growth time of 5 h.     

Field emission properties of polymer-converted carbon films by heat treatment

Carbon films were prepared on single crystal silicon substrates by heat-treatment of a polymer-poly(phenylcarbyne) at 800 °C in Ar atmosphere. The heat-treatment caused the change of the polymer into carbon film, which exhibited good field emission properties. Low turn-on emission field of 4.3 V/μm (at 0.1 μA/cm²) and high emission current density of 250 μA/cm² (at 10 V/μm) were observed for the polymer-converted carbon films. This behavior was demonstrated to be mainly related to the microstructure of the carbon films, which consisted of fine carbon nanoparticles with high sp² bonding. The carbon films, which can be deposited simply with large areas, are promising for practical applications in field emission display.     

Fabrication and electrical characterization of nanocrystalline ZnO/Si heterojunctions

Nanocrystalline zinc oxide (nc-ZnO) films were prepared by a sol-gel process on p-type single-crystalline Si substrates to fabricate nc-ZnO/p-Si heterojunctions. The structure and morphology of ZnO films on Si substrates, which were analyzed by X-ray diffraction (XRD) spectroscopy and atomic force microscopy (AFM), showed that ZnO films consisted of 50-100 nm polycrystalline nanograins with hexagonal wurtzite structure. The electrical transport properties of the nc-ZnO/p-Si heterojunctions were investigated by temperature-dependent current-voltage (I-V) measurements and room temperature capacitance-voltage measurements. The temperature-dependent I-V characteristics revealed that the forward conduction was determined by multi-step tunneling current, and the activation energy of saturation current was about 0.26 eV. The 1/C²-V plots indicated the junction was abrupt and the junction built-in potential was 1.49 V at room temperature.     

Modulation of field emission properties of vertically aligned ZnO nanorods with aspect ratio and number density

Vertically aligned ZnO nanorod arrays with different aspect ratios were synthesized by hybrid wet chemical route. Modulation of the field emission properties (FE) with aspect ratio of ZnO nanorods was examined. With the increase in the aspect ratio, the emission current density increases from 0.02 to 8 μA/cm² at 7.0 V/μm. Turn-on voltage was seen to decrease from 9.6 to 7 V/μm at a current density of 10 μA/cm² with the increase in aspect ratio in the ZnO films. The interrelation between the FE characteristics (emission thresholds, current density, surface uniformity, etc.) and microstructure of the ZnO nanostructure obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) was discussed. Quality of the ZnO nanorods was also examined by using Raman spectroscopy and Fourier transformed infrared spectroscopy (FTIR). It was found that the observed enhancements of FE characteristics could mainly be attributed to the increase in aspect ratio and associated number density of ZnO nanorods.     

Particulate assisted growth of ZnO nanorods and microrods by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on the gallium nitride (GaN) and sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) without using any metal catalyst. The experiment was carried out at three different laser wavelengths of Nd:YAG laser (λ = 1064 nm, λ = 532 nm) and KrF excimer laser (λ = 248 nm). The ZnO films grown at λ = 532 nm revealed the presence of ZnO nanorods and microrods. The diameter of the rods varies from 250 nm to 2 μm and the length varies between 9 and 22 μm. The scanning electron microscopy (SEM) images of the rods revealed the absence of frozen balls at the tip of the ZnO rods. The growth of ZnO rods has been explained by vapor-solid (V-S) mechanism. The origin of growth of ZnO rods has been attributed to the ejection of micrometric and sub-micrometric sized particulates from the ZnO target. The ZnO films grown at λ = 1064 nm and λ = 248 nm do not show the rod like morphology. X-ray photoelectron spectroscopy (XPS) has not shown the presence of any impurity except zinc and oxygen.     

Green electroluminescence from an n-ZnO: Er/p-Si heterostructured light-emitting diode

Erbium-doped ZnO (ZnO:Er) thin films with various doping concentrations were deposited on p-Si substrates by ultrasonic spray pyrolysis (USP). The n-ZnO:Er/p-Si heterojunctions were further employed to fabricate light-emitting diodes (LEDs). The devices showed diode-like rectifying current–voltage characteristics with a low reverse breakdown voltage, attributed to the avalanche breakdown. A distinct green electroluminescence peaking at 537 nm and 558 nm were observed at room temperature under reverse bias. The green electroluminescence originated from the electron impact excitation of Er³⁺ ions doped in ZnO films.     

Well-aligned ZnO rod arrays grown on glass substrate from aqueous solution

Well-aligned ZnO rod arrays have been successfully synthesized on glass substrate from the aqueous solution of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ (HMT). Some critical issues such as seed layers, concentration and reaction time were investigated. The results show that ZnO seed layers were pre-requisite for the aligned growth of ZnO rod arrays. The length of rods is tunable in a range from 2 μm to 3 μm by varying the solution concentration and reaction time. X-ray diffraction results demonstrate that ZnO rods are wurtzite crystal structures preferentially orienting in the direction of the c-axis. Microstructure observation by scanning electron microscope confirms that ZnO rods grew up perpendicular to the substrate. Room-temperature photoluminescence (PL) spectrum of rod arrays shows a strong emission band at about 396 nm.     

Enhanced field emission from ZnO nanopencils by using pyramidal Si(1 0 0) substrates

Zinc oxide nanopencil arrays were synthesized on pyramidal Si(1 0 0) substrates via a simple thermal evaporation method. Their field emission properties have been investigated: the turn-on electric field (at the current density of 10 μA/cm²) was about 3.8 V/μm, and the threshold electric field (at the current density of 1 mA/cm²) was 5.8 V/μm. Compared with similar structures grown on flat Si substrates, which were made as references, the pyramidal Si-based ZnO nanopencil arrays appeared to be superior in field emission performance, thus the importance of the non-flat substrates has been accentuated. The pyramidal Si substrates could not only suppress the field screening effect but also improve the field enhancement effect during the field emission process. These findings indicated that using non-flat substrates is an efficient strategy to improve the field emission properties.     

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400-800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.     

Synthesis, field emission and glucose-sensing characteristics of nanostructural ZnO on free-standing carbon nanotubes films

Free-standing multiwall carbon nanotubes (MWNTs) films were coated, using chemical vapor deposition method, with a thin layer of nanostructural ZnO. The morphology and crystal structure of the as-grown products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman scattering analyses. Field emission (FE) results demonstrated that the needle-like and spherical ZnO-MWNTs composite structure films possessed good performance with a turn-on field of 1.3, 2.2 V μm⁻¹ and a threshold field of 2.6, 4.5 V μm⁻¹, respectively. The glucose-sensing characteristic has also been studied. The multi-layer electrode (PDDA/GOx/ZnO/MWNTs) exhibited significant electrocatalysis to the oxidation and reduction of H₂O₂ than the PDDA/GOx/MWNTs electrode, which provided wide potential applications in clinical, environmental, and food analysis.     

Zn-interstitial-enhanced ferromagnetism in Cu-doped ZnO films

Copper-doped ZnO (ZnO:Cu) films exhibiting room-temperature (RT) ferromagnetism were prepared by filtered cathodic vacuum arc (FCVA) technique. The ZnO:Cu films deposited at RT showed the strongest magnetic moment of 0.40 μ_B/Cu as compared with the samples prepared at elevated temperatures. The observed strong ferromagnetism in the RT-deposited ZnO:Cu films could be partly associated with Zn-interstitial defects. The degradation of magnetic moment in the ZnO:Cu prepared at high temperatures and annealed at elevated temperatures might be attributed to the out-diffusion of Zn interstitials to the ZnO lattice.     

ZnO nanostructures with different morphologies and their field emission properties

Zinc oxide (ZnO) products with the morphologies of balls, nunchakus and belts have been synthesized from aqueous solutions by adjusting the reagent concentration and reaction time. The X-ray diffraction (XRD) peaks of the products were indexed to ZnO materials, but exhibited different relative intensities for the (0 0 2) diffraction peak. Field emission (FE) measurements showed that the turn-on and threshold field for the ZnO nanonunchakus were 3.01 ± 0.005 and 5.47 ± 0.005, 3.71 ± 0.005 and 6.43 ± 0.005 V/μm, respectively, for the ZnO nanobelts, revealing that the products have comparable FE properties with those of the reported ZnO nanowires and carbon nanotubes (CNTs).     

ZnO:Ag film growth on Si substrate with ZnO buffer layer by rf sputtering

ZnO buffer layers were deposited on n-Si (1 0 0) substrate by rf magnetron sputtering at a lower power of 40 W. Then Ag-doped ZnO (SZO) films were deposited on buffered and non-buffered Si at a higher sputtering power of 100 W. The effects of buffer layer on the structural, electrical and optical properties of SZO films were investigated. The three-dimensional island growth process of ZnO buffer layer was discussed. The energy band diagram of p-SZO/n-Si heterojunction was constructed based on Anderson's model. Results show the ZnO buffer layer leads to better properties of SZO film, including larger grain size, smoother surface, higher carrier mobility, better rectifying behavior, lower interface state density, and weaker deep-level emission. It is because the ZnO buffer layer effectively relaxes the partial stress induced by the large lattice mismatch between SZO and Si.     

1.54 μm emission mechanism of Er-doped zinc oxide thin films

Zinc oxide (ZnO) and Er-doped zinc oxide (ZnO:Er) thin films were formed by pulsed laser deposition, and characterized by photoluminescence (PL) and X-ray diffraction (XRD) in order to clarify the 1.54 μm emission mechanism in the ZnO:Er films. Er ions were excited indirectly by the 325 nm line of a He-Cd laser, and the comparison of the ultraviolet to infrared PL data of ZnO and ZnO:Er films showed that the 1.54 μm emission of Er³⁺ in ZnO:Er film appears at the expense of the band edge emission and the defect emission of ZnO. The crystallinity of the films was varied with the substrate temperature and post-annealing, and it was found that the intensity of the 1.54 μm emission is strongly related with the crystallinity of the films. There are three processes leading to the 1.54 μm emission; absorption of excitation energy by the ZnO host, energy transfer from ZnO to Er ions, and radiative relaxation inside Er ions, and it is suggested that the crystallinity plays an important role in the first two processes.     

Growth and electrical properties of ZnO nanorod arrays prepared by chemical spray pyrolysis

Chemical spray pyrolysis was applied to grow ZnO nanorod arrays from zinc chloride solutions with pH=2 and 5 on glass/ITO substrate at 480 and 550 °C. The obtained structures were characterized by their morphological, electrical and PL properties. According to SEM, deposition of acidic solutions retards coalescence of the growing crystals. The charge carrier density in ZnO nanorods was determined from the C-V characteristics of ZnO/Hg Schottky barrier. Carrier densities ∼10¹⁵ cm⁻³ and slightly above 10¹⁶ cm⁻³ were recorded for ZnO deposited at 550 and 480 °C, respectively. According to PL studies, intense UV-emission is characteristic of ZnO independent of growth temperature, the concentration of oxygen vacancy related defects is lower in ZnO nanorods deposited at 550 °C. Solution pH has no influence on carrier density and PL properties.     

Field emission behavior of cuboid zinc oxide nanorods on zinc-filled porous silicon

Single-crystalline zinc oxide (ZnO) nanorods with cuboid morphology have been prepared on the zinc-filled porous silicon substrate using a vapor phase transport method. Field-emission measurements showed that the turn-on field and threshold field of the cuboid ZnO nanorods film were about 3.2 and 8.2 V/μm respectively. From the emitter surface, a homogeneous emission image was observed with emission site density (ESD) of ∼10⁴ cm⁻². The better emission uniformity and the high ESD may be attributed to a large number of ZnO nanocrystallites as emitter on the surface of the nanorod end contributing to emission.     

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO₃ (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42μ_B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21μ_B/Co and 0.65μ_B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.     

The structural and optical properties of ZnO nanorod arrays

High quality vertical-aligned ZnO nanorod arrays were synthesized by a simple vapor transport process on Si (111) substrate at a low temperature of 520 °C. Field-emission scanning electron microscopy (FESEM) showed the nanorods have a uniform length of about 1 μm with diameters of 40-120 nm. X-ray diffraction (XRD) analysis confirmed that the nanorods are c-axis orientated. Selected area electron diffraction (SAED) analysis demonstrated the individual nanorod is single crystal. Photoluminescence (PL) measurements were adopted to analyze the optical properties of the nanorods both a strong UV emission and a weak deep-level emission were observed. The optical properties of the samples were also tested after annealing in oxygen atmosphere under different temperatures, deep-level related emission was found disappeared at 600 °C. The dependence of the optical properties on the annealing temperatures was also discussed.