ZnO Nanodots Research Articles

Self-organized ZnO nanodot arrays: Effective control using SiNx interlayers and low-temperature plasmas

An advanced inductively coupled plasma (ICP)-assisted rf magnetron sputtering deposition method is developed to synthesize regular arrays of pear-shaped ZnO nanodots on a thin SiNx buffer layer pre-deposited onto a silicon substrate. It is shown that the growth of ZnO nanodots obey the cubic root-law behavior. It is also shown that the synthesized ZnO nanodots are highly-uniform, controllable by the experimental parameters, and also feature good structural and photoluminescent properties. These results suggest that this custom-designed ICP-based technique is very effective and highly-promising for the synthesis of property- and size-controllable highly-uniform ZnO nanodots suitable for next-generation light emitting diodes, energy storage, UV nanolasers, and other applications.

Selective Area Atomic Layer Deposited ZnO Nanodot on Self-Assembled Monolayer Pattern Using a Diblock Copolymer Nano-Template

ZnO nanodots were prepared by selective area atomic layer deposition (SA-ALD) on an octadecyltrichlorosilane (ODTS) self-assembly monolayers (SAMs) patterns formed using a diblock co-polymer (DBC) nanotemplate. In order to transfer well-ordered nanaotemplate in SAMs, SiO2 sacrificial layer was inserted between DBC and SAMs. Cylindrical nanoholes under 16 nm diameters were well-formed on SiO2 layer. SA-ALD of ZnO was successfully performed on by ODTS SAMs.

Effect of Annealing on the Ultraviolet Sensing Properties of the Chemically Synthesized n-Type Nanodots of ZnO

Isolated nanodots of ZnO of at least 100 nm in size were deposited on transparent conducting substrate (Indium Tin Oxide coated quartz) by aqueous chemical growth technique. The nanodots have polycrystalline character and their crystallinity improves due to annealing in vacuum and oxygen atmosphere. The ultraviolet sensing property of the ZnO nanodot was investigated by current-voltage measurement. Under ultraviolet irradiation, the photo current was observed to be higher for the nanodots having higher volume density of point defects. Keywords: Nanodots, Atomic Force Microscope, Photocurrent, oxygen Vacancies, semiconductors, ZnO, GaAs PMT detector

Atomic layer deposition of ZnO on thermal SiO2 and Si surfaces using N2-diluted diethylzinc and H2O2 precursors

ZnO nanodots are attracting more and more attention in various photoelectrical applications due to multiple excition generation. In this article, atomic layer deposition (ALD) growth of ZnO nanodots has been realized for the first time on both thermal SiO2 and Si surfaces using N2-diluted gaseous DEZn and H2O2 precursors. The experimental results indicate that the ALD ZnO exhibits a nano-crystalline film with corrugated surfaces in the case of the deposition temperature of 200°C, likely due to concrescence among ZnO nanodots. When the deposition temperature is increased up to 300°C, ZnO is grown in the form of well-discrete nanodots. This is due to increased desorption of the reacting molecules and a reduction of nucleation sites on the growing surfaces at 300°C, thus leading to the reaction between DEZn and OH groups only on some favorable sites from thermodynamic and energy points of view. In terms of the thermal SiO2 surface, ZnO nanodots with a density of around 5×1010cm−2 are obtained for 100 cycles. As for the Si surface, ZnO nanodots with a density as high as ∼1×1011cm−2 are achieved for 50 cycles. Finally, the X-ray photoelectron spectroscopy and X-ray diffraction analyses reveal that the ALD ZnO at 300°C is dominated by ZnO bonds together with a small quantity of ZnOH bonds, and the deposition temperature of 300°C can result in preferential growth of ZnO (002) orientation and a bigger crystallite size.

Fabrication of ZnO Nanodot Arrays by Sol-Gel within AAO Template

ZnO nanodot arrays were fabricated by using AAO template combining with sol-gel method. Structure and optical properties of ZnO/AAO composite structure were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The SEM results indicated that the pores in the template were arranged in a regular hexagonal lattice and ZnO nanodots were assembled into the AAO template. The XRD patterns demonstrated that the AAO template had an amorphous structure, and the ZnO nanodots had wurtzite structures. The PL spectrum revealed that ZnO/AAO composite structure had a strong ultraviolet emission at 390 nm.

Synthesis and characterization of self-assembled ZnO nano-dots grown on SiN x/Si(001) substrates by radio frequency magnetron sputtering

This paper reports the synthesis and characterization of self-assembled ZnO nano-dots deposited on SiN x /Si(001) substrates by radio frequency magnetron sputtering. The effect of the working pressure on the microstructure of the as-grown ZnO thin films was examined. At a working pressure of 6 × 10 − 3 Torr, a flat layered structure was dominant with a preferred orientation of the ZnO(0002) plane, while ZnO nano-structures were observed on the samples grown at 2 × 10 − 2 Torr. This was attributed to the columnar growth that facilitated the nucleation of ZnO nano-structures on the growing surfaces. Hexagonal nano-pyramids were formed, which then transformed into nano-dots as the film became thicker. The ZnO nano-dots were uniform and well dispersed, exhibiting distinct photoluminescence spectra due to the quantum confinement effect.

Amplified spontaneous emission from ZnO in n-ZnO/ZnO nanodots–SiO2 composite/p-AlGaN heterojunction light-emitting diodes

This study demonstrates amplified spontaneous emission (ASE)of the ultraviolet (UV) electroluminescence (EL) from ZnO atλ∼380 nm in the n-ZnO/ZnOnanodots–SiO2 composite/p-Al0.12Ga0.88N heterojunctionlight-emitting diode. A SiO2 layer embedded with ZnO nanodots was prepared on the p-typeAl0.12Ga0.88N using spin-oncoating of SiO2 nanoparticles followed by atomic layer deposition (ALD) of ZnO. Ann-type Al-doped ZnO layer was deposited upon the ZnO nanodots–SiO2 composite layer also by the ALD technique. High-resolution transmissionelectron microscopy (HRTEM) reveals that the ZnO nanodots embedded in theSiO2 matrix have diameters of 3–8 nm and the wurtzite crystal structure,which allows the transport of carriers through the thick ZnO nanodots–SiO2 composite layer. The high quality of the n-ZnO layer was manifested by the wellcrystallized lattice image in the HRTEM picture and the low-threshold opticallypumped stimulated emission. The low refractive index of the ZnO nanodots–SiO2 composite layer results in the increase in the light extraction efficiency from n-ZnO and theinternal optical feedback of UV EL into n-ZnO layer. Consequently, significantenhancement of the UV EL intensity and super-linear increase in the EL intensity, as wellas the spectral narrowing, with injection current were observed owing to ASE in the n-ZnOlayer.

Pulsed i‐PVD of Dielectric Nanodot Arrays Using a Nanoporous Template

AbstractThe results of multi‐scale numerical simulations of pulsed i‐PVD template‐assisted nanofabrication of ZnO nanodot arrays on a silicon substrate are presented. The ratios and spatial distributions of the ion fluxes deposited on the lateral and bottom surfaces of the nanopores are computed as a function of the external bias and plasma parameters. The results show that the pulsed bias plays a significant role in the ion current distribution inside the nanopores. The results of numerical experiments of this work suggest that by finely adjusting the pulse voltage, the pulse duration and the duty cycle of the external pulsed bias, the nanopore clogging can be successfully avoided during the deposition and the shapes of the deposited ZnO nanodots can be effectively controlled.magnified image

Ultraviolet Electroluminescence From n-ZnO–SiO$_{2}$–ZnO Nanocomposite/p-GaN Heterojunction Light-Emitting Diodes at Forward and Reverse Bias

Ultraviolet (UV) light-emitting diodes composed of n-ZnO:Al-SiO2-ZnO nanocomposite/p-GaN:Mg heterojunction were fabricated on the (0002) Al2O3 substrate. A SiO2 layer embedded with ZnO nanodots was prepared on the p-type GaN using spin-on coating of SiO2 nanoparticles together with atomic layer deposition (ALD). An n-type Al-doped ZnO layer was deposited also by ALD. The SiO2-ZnO nanocomposite layer accomplishes a role of the current blocking layer and also causes, by its low refractive index, the increase in the light extraction efficiency from n-ZnO. Significant UV electroluminescence from n-ZnO was achieved at a low forward-bias current of 1.8 mA. Strong UV emission arising from impact ionization in GaN, ZnO, and GaN:Mg states was also observed at reverse breakdown bias.

Fabrication and characterization of p-Si/n-ZnO heterostructured junctions

In this paper ZnO nanorods and nanodots (with and without a SiO2 buffer layer) were grown on p-Si, forming p–n heterojunctions. The nanorod devices showed no visible electroluminescence (EL) emission but showed rectifying behavior. Covering around 60% of the length of the nanorods with PMMA produced an ideality factor of 3.91±0.11 together with a reverse saturation current of 6.53±4.2×10-8A. Up to two orders of magnitude rectification was observed for the current at bias -3 and 3V. The nanodot devices showed EL emission under forward bias conditions. It seems that the buffer layer increased both the stability and efficiency of the devices, since the buffer layer device could operate at larger applied voltage and showed EL emission under reverse bias.

Synthesis of horizontally aligned ZnO nanowires localized at terrace edges and application for high sensitivity gas sensor

We developed step edge decoration method for the fabrication of semiconductor ZnO nanodots and nanowires using pulsed laser deposition. We synthesized high quality ZnO nanowires with the small diameter of about 20nm and the uniform interval of about 80nm between each nanowire, which has a simple structure for the formation of contact electrodes. The ZnO nanowire-based sensor was prepared only with the simple process of a gold electrode formation. The ZnO nanowire-based sensor exhibited the high surface-to-volume ratio of 58.6μm−1 and the significantly high sensitivity of about 10 even for the low ethanol concentration of 0.2ppm.

Controllable Synthesis of Ordered ZnO Nanodots Arrays by Nanosphere Lithography

Controllable ordered hexagonally patterned ZnO nanodots were achieved by polystyrene nanosphere lithography at room temperature. First a self-assembled monolayer of polystyrene spheres was formed as a mask. Then ZnO was deposited through the mask by e-beam vaporization. After the polystyrene spheres were etched away, ordered ZnO nanodots arrays were formed on the substrate. The obtained ZnO nanodots are of acceptable quality with high chemical purity and preferential c-axis orientation. A photoluminescence measurement shows a blue shift in free-exciton emission derived from low-dimensional quantum characteristics. The size of the nanodots can be controlled by varying the deposition time. The ZnO nanodots with growth times of 1, 2, and 3 min exhibit different free-exciton emission at 3.378, 3.356, and 3.314 eV. Therefore this is a technique with precise control of the size and geometry of the dots, which has a great promise for applications in nanoscale optoelectronic devices.

The impact of morphology upon the radiation hardness of ZnO layers

It is shown that ZnO nanorods and nanodots grown by MOCVD exhibit enhancedradiation hardness against high energy heavy ion irradiation as comparedto bulk layers. The decrease of the luminescence intensity induced by 130 MeVXe23+ irradiationat a dose of 1.5 × 1014 cm−2 in ZnO nanorods is nearly identical to that induced by a dose of6 × 1012 cm−2 in bulk layers. The damage introduced by irradiation is shown to change thenature of electronic transitions responsible for luminescence. The change ofexcitonic luminescence to the luminescence related to the tailing of the densityof states caused by potential fluctuations occurs at an irradiation dose around1 × 1014 cm−2 and 5 × 1012 cm−2 in nanorods and bulk layers, respectively. More than one order of magnitude enhancementof radiation hardness of ZnO nanorods grown by MOCVD as compared to bulk layersis also confirmed by the analysis of the near-bandgap photoluminescence bandbroadening and the behavior of resonant Raman scattering lines. The resonant Ramanscattering analysis demonstrates that ZnO nanostructures are more radiation-hardas compared to nanostructured GaN layers. High energy heavy ion irradiationfollowed by thermal annealing is shown to be a way for the improvement of thequality of ZnO nanorods grown by electrodeposition and chemical bath deposition.

Dot-Height Dependence of Photoluminescence from ZnO Quantum Dots

We report on the optical properties of ZnO nanodots and how they are influenced by the height of the dot configuration. The nanodots were grown on SiO2/Si substrates by remote-plasma enhanced metalorganic chemical vapor deposition (RPE-MOCVD). The dot configuration was regulated by controlling the growth time. The photoluminescence (PL) spectra of nanodots having average dot height of 2.5, 3.5, and 4.5 nm grown on SiO2/Si substrates were measured at 16 K. A blue shift in the excitonic emission was observed for shorter dots due to a larger quantum confinement effect. The fractional-dimensional space model was used to estimate a dimensionality of 2.35 for ZnO nano dots with an average dot height of 2.5 nm.

Probing Molecular Binding Effect from Zinc Oxide Nanocrystal Doping in Surface-Stabilized Ferroelectric Liquid Crystal with Two-Dimensional Infrared Correlation Technique

Doping liquid crystal (LC) with nanomaterials has been shown to yield some degrees of freedom for tailoring LC properties. This approach can be employed to produce new LC materials with high application potentials by blending instead of synthesizing new mesogenic molecules. In this paper, we show that doping ferroelectric liquid crystal (FLC) with ZnO nanocrystals improves the alignment order of a surface-stabilized FLC (SSFLC) in both steady-state and field-induced reorientation processes. We used the two-dimensional infrared (2D IR) correlation technique to reveal that the ZnO nanocrystals were uniformly dispersed into the FLC medium. The homogeneous dispersion of ZnO nanodots produces stronger correlations among the IR-active molecular normal modes of FLC molecules, which then leads to more concerted reorientation process at the submolecular level. A molecular binding effect originating from a dipolar interaction of the ZnO nanodot with surrounding CO groups of FLC molecules was proposed to illustrate our measured results. We estimated the total energy reduced by the doping to be about 1000 J/m3. The alignment stability gained is similar to that experienced by FLC molecules within 100-nm distance to an alignment surface with a strong anchoring strength of 1 × 10-4 J·m-2.

Monochromatic photoluminescence obtained from embedded ZnO nanodots in an ultrahard diamond-like carbon matrix

At room temperature, we observe the self assembly of nanoclusters in an amorphous matrix using a vacuum deposition technique. Self-assembled ZnO nanoclusters embedded in hard diamond-like amorphous carbon thin films, deposited by high vacuum Filtered Cathodic Vacuum Arc (FCVA) technique at room temperature without post-processing, have been observed. A selective self assembly of metal and oxygen ions in a 3-element plasma was observed. XPS distinctly showed presence of ZnO and DLC-mixture in 5, 7 and 10 at.% Zn (in target) films while maintaining high sp 3 content. This in turn improved the Young's modulus value of the ZnO nanoclusters embedded in DLC film (~ 220 GPa) compared to bulk ZnO (~ 110 GPa). Films with ZnO detected were observed to exhibit absorption edge at 377 nm monochromatic UV light emissions. This corresponded to a band gap value of about 3.30 eV. The emission with greatest intensity (after normalization) was detected from 10 at.% Zn (in target) film where presence of ZnO nanoclusters (~ 40 nm) in DLC matrix were confirmed by TEM. This showed that well-defined crystalline ZnO nanoclusters contributed to strong PL signal. Strong monochromatic emissions detected hinted that no defect states were present.

Self-assembly and optical properties of patterned ZnO nanodot arrays

Patterned ZnO nanodot (ND) arrays and a ND-cavity microstructure were realized on ananodic alumina membrane (AAM) surface through a spin-coating sol–gel process, whichbenefits from the morphology and localized negative charge surface of AAM as well as theoptimized sol concentration. The growth mechanism is believed to be a self-assemblyprocess. This provides a simple approach to fabricate semiconductor quantumdot (QD) arrays and a QD-cavity system with its advantage in low cost andmass production. Strong ultra-violet emission, a multi-phonon process, and itsspecial structure-related properties were observed in the patterned ZnO ND arrays.

Optical quenching of NiO∕Ni coated ZnO nanowires

Low temperature photoluminescence (PL) measurements were carried out to investigate optical transition characteristics of ZnO nanowires, Ni nanodot, and NiO coated ZnO nanowires (NWs). PL emission spectra show emission peaks of distinctive bound exciton to neutral donor (DX0) and to acceptor (AX0) transition. The authors found that the optical quenching was drastic with increasing temperature and the activation energies are unusually small. The major PL emission peak for ZnO NW was switched from DX0 to AX0 for Ni–ZnO nanodot NWs. This is due to the reason that diffused hydrogen atoms into ZnO NWs during thermal annealing played as acceptors.

Electrical conduction and photoluminescence properties of solution-grown ZnO nanowires

We report on the optical and electrical properties of zinc oxide nanorods synthesized in solution using Oswald ripening of ZnO nanodots with the addition of ethylenediamene growth directing agent. This method results in high quality, single crystalline ZnO nanorods that extend up to 3μm in length and have an average diameter of 25±7nm, compared to ∼75nm diameter for similarly prepared nanorods but without the addition of the growth directing agent. Furthermore, we find that the higher aspect ratio nanorods exhibit strong size-dependent electrical characteristics, with a critical diameter of about 27nm delimiting nonconductive and conductive behaviors. Theoretical calculations indicate that the origin of this size-dependent conductivity is the presence of surface states that deplete the carriers in the smaller diameter nanorods, and an estimate of the density of these states is provided.

Monodisperse ZnO Nanodots: Synthesis, Charaterization, and Optoelectronic Properties

An effective and reliable synthetic route has been developed for the preparation of almost perfectly monodisperse 3 to ∼4 nm ZnO nanocrystals. In addition, a PL peak with a blue-shift of 0.16 eV has been observed in the PL spectrum, showing good size uniformity of the nanodots. The band gap has also been verified in the analysis of I−V characteristics of an ITO/ZnO/SiO2/Al structure, from which the band alignment of the ZnO nanodots on ITO has been determined. This work demonstrates an efficient synthesis of ZnO nanodots and an easy approach to studying physical properties of nanocrystals that will help the material optimization in device application.