Deep level emission of ZnO nanoparticles deposited inside UV opal

The temperature-dependent photoluminescence spectra of zinc oxide (ZnO) nanocrystals deposited inside the ultraviolet (UV) opal were studied. ZnO was grown in the voids between FCC packed SiO₂ spheres using spray pyrolysis under ultrasonic vibration in the solution containing a zinc nitrate precursor. The ZnO nanoparticles inside opal matrix with UV photonic band-gap exhibit suppression of the excitonic emission and enhancement of the deep level emission. Suppression of the excitonic lines is due to the inhibition of spontaneous emission, while enhancement and broadening of the DL emission in the green spectral region is due to Purcell effect. The infiltration of ZnO nanoparticles inside the photonic crystal may be a useful technique to increase its emission efficiency in the selected spectral region.     

Photoluminescence properties of bare and ZnO infilled artificial opal

Photoluminescence of bare and ZnO infilled artificial opals was investigated. A presence of a photonic band gap results in distortion of the photoluminescence spectra of both the bare and ZnO infilled opal nanocomposite. Filling of the opal with ZnO resulted in a shift of the Bragg diffraction peak from 430 to 460 nm. The emission from ZnO infilled opal contains no UV photoluminescence from ZnO nanocrystals, while the ZnO nanocrystals deposited on substrate by the same method exhibit strong excitonic UV emission. Although a high temperature treatment in ambient air results in an increase in the photoluminescence intensity of the ZnO nanocrystals, the quenched behavior of the excitonic emission from ZnO nanocrystals embedded in the opal matrix remains. A domination of the artificial opal matrix intrinsic emission in the photoluminescence spectra from the untreated as well as heat treated ZnO filled opal nanocomposites is observed.     

ZnO filled opal arrays: Photo and cathodoluminescence studies

The photo and cathodoluminescence of zinc oxide (ZnO) filled opal arrays, as well as ZnO nanocrystals prepared by a chemical deposition method were investigated. The photoluminescence (PL) from the arrays was studied using angular resolution. The PL spectra of the filled opal exhibited a dip corresponding to the array’s photonic band gap. ZnO nanocrystals embedded in the opal matrix demonstrated quenched excitonic emission while the opal matrix showed enhanced emission. This effect is explained by an energy transfer from ZnO nanocrystals to the opal matrix.     

Enhanced photoluminescence from three-dimensional ZnO photonic crystals

We have fabricated optically active ZnO inverse opals by infiltrating polystyrene (PS) opal templates using an electrodeposition process. Compared with bare ZnO films also prepared by electrodeposition, the three-dimensional (3D) ordered ZnO structure exhibits markedly enhanced photoluminescence. The effect of photonic band gap on PL spectra is also clearly observed from the ZnO inverse opal structure.     

Suppression of the green photoluminescence band in ZnO embedded into porous opal by spray pyrolysis

The photoluminescence (PL) and transmittance characteristics of the zinc oxide embedded into voids of FCC sub-micron packed silicon dioxide spheres by using technologically simple and inexpensive spray pyrolysis are reported. The uniform formation of ZnO nanocrystalline particles inside of the porous opal takes place after deposition in aqueous solution with zinc nitrite hexahydride precursor followed by thermal annealing. The decrease of green PL is observed due to the inhibition of spontaneous emission through oxygen vacancies in ZnO. The strong red shift of the transmittance characteristics signifies the essential filling of voids in the opal matrix.     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Luminescence of ZnO nanocrystals capped with an organic dye

ZnO nanocrystals capped with an organic dye Rhodamine 6G (Rh6G) were investigated by photoluminescence (PL) and cathodoluminescence (CL) techniques. PL and CL spectra showed a remarkable decrease in visible emission intensity after ZnO nanocrystals were capped with Rh6G, indicating that dangling bonds and defect states existing at the surface of ZnO nanocrystals were significantly passivated. Rh6G on the ZnO surface exhibited a monomer-like emission, and the intensity and the position of the emission were dependent on the dye concentration.     

Novel properties of nanoscale organic–inorganic systems and photonic crystals — conducting polymers in nanoscale periodic structures,microcavities and photonic crystals

Conducting polymer/C₆₀and C₆₀doped conducting polymer/C₆₀heterojunctions have been fabricated and found to exhibit remarkably enhanced photoresponse due to the highly effective photoinduced charge transfer at the interface. In conducting polymer/C₆₀alkali metal nanoscale composite systems, multiphase superconductivity has been clarified and explained by taking the coupling of nanoscale grains by Josephson junctions into consideration. As examples of intramolecular organic-inorganic combined systems, unique electrical and optical characteristics have been revealed in oligosilanylene oligophenylene polymers. Electroluminescence has been demonstrated in organic-inorganic junction devices such as conducting polymer/porous Si and conducting polymer/diamond junctions. Conducting, polymer-based nanoscale multilayer systems have been studied utilizing molecular self-assembly method and novel photosensitive characteristics have been revealed. \indent Novel optical and electrical properties of conducting polymers infiltrated in a photonic crystal, synthetic opal made of SiO₂spheres of several hundred nm in diameter, and also a conducting polymer replica have been revealed. A clear diffraction pattern was observed in a photonic crystal infiltrated with conducting polymers, and transmission spectra are dependent on various ambient conditions. Their photoluminescence (PL) spectra, spectral narrowing of PL and lasing characteristics at relatively low optical excitation have also been clarified. Novel conducting characteristics of conducting polymers in a photonic crystal that was prepared by pyrolysis of a polymer replica of opal have also been observed.     

Synthesis and optical properties of In-doped GaN nanocrystals

Indium-doped GaN nanocrystals with 5% and 10% In have been prepared by a low temperature solvothermal method using hexamethyldisilazane as the nitriding reagent. The nanocrystals show Raman bands at lower frequencies compared to GaN. Photoluminescence spectra of the In-doped GaN nanocrystals exhibit an increase in the FWHM with the decrease in the PL band energy, the band energy itself decreasing with increase in the In content.     

Photoluminescence from ZnO-SiO2 opals with different sphere diameters and thicknesses

We systematically investigated the photoluminescence (PL) and transmittance characteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325 nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stop-bands center of 410 nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570 nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals.     

Equations for filling factor estimation in opal matrix. Addendum to “Deep level emission of ZnO nanoparticles deposited inside UV opal”, [Opt. Commun. 259 (1) (2006) 378-384]

We consider two equations for the filling factor estimation of infiltrated zinc oxide (ZnO) in silica (SiO₂) opal and gallium nitride (GaN) in ZnO opal. The first equation is based on the effective medium approximation, while the second one—on Maxwell-Garnett approximation. The comparison between two filling factors shows that both equations can be equally used for the estimation of the quantity of infiltrated nanoparticles inside opal photonic crystal.     

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 10^19-20 cm⁻³. Current-voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A⁰X) emission and a donor-acceptor pair (DAP) emission due to doped nitrogen, respectively.     

Photoluminescence and absorption in sol-gel-derived ZnO films

Highly c-axis-oriented ZnO films were obtained on corning glass substrate by sol-gel technique. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption in the absorption (UV) spectra are closely related to the post-annealing treatment. The difference between PL peak position and the absorption edge, designated as Stokes shift, is found to decrease with the increase of annealing temperature. The minimum Stokes shift is about 150 meV. The decrease of Stokes shift is attributed to the decrease in carrier concentration in ZnO film with annealing. X-ray diffraction, surface morphology and refractive index results indicate an improvement in crystalline quality with annealing. Annealed films also exhibit a green emission centered at ∼520 nm with activation energy of 0.89 eV. The green emission is attributed to the electron transition from the bottom of the conduction band to the antisite oxygen O_Zn defect levels.     

Photoluminescence of a ZnO/GaN Heterostructure Interface

Growth of a ZnO/GaN heterostructure is carried out using pulsed laser deposition. By etching the ZnO layer from the ZnO/GaN structure, the photoluminescence （PL） of the associated GaN layer shows that the donor- acceptor luminescence of CaN shifts to about 3.27eV, which is consistent with the electroluminescence （EL） of n-ZnO/p-GaN already reported. XPS shows that oxygen diffuses into the CaN crystal lattice from the surface to 20nm depth. The PL spectra at different temperatures and excitation densities show that oxygen plays the role of potential fluctuation. The associated PL results of the interface in these LEDs could be helpful to understand the mechanism of EL spectra for ZnO/CaN p-n junctions.     

A simple growth route towards ZnO thin films and nanorods

Highly orientated ZnO thin films and the self-organized ZnO nanorods can be easily prepared by a simple chemical vapor deposition method using zinc acetate as a source material at the growth temperature of 180 and 320 °C, respectively. The ZnO thin films deposited on Si (100) substrate have good crystallite quality with the thickness of 490 nm after annealing in oxygen at 800 °C. The ZnO nanorods grown along the [0001] direction have average diameter of 40 nm with length up to 700 nm. The growth mechanism for ZnO nanorods can be explained by a vapor-solid (VS) mechanism. Photoluminescence (PL) properties of ZnO thin films and self-organized nanorods were investigated. The luminescence mechanism for green band emission was attributed to oxygen vacancies and the surface states related to oxygen vacancy played a significant role in PL spectra of ZnO nanorods.     

Synthesis and optical properties of Co-doped ZnO nanofibers prepared by electrospinning

In this work, Co-doped ZnO nanofibers have been fabricated successfully by an electrospinning technique. The as-prepared nanofibers are characterized by themogravimetric analysis (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectra and photoluminescence spectroscopy (PL). Results have showed that a wurtzite ZnO nanofibers were obtained and the PL spectrum showed a red-shift by 10 nm due to narrowing of the ZnO band gap (∼3.29 eV) as a result of Co doping. Meanwhile, Raman scattering spectra exhibited an unusual peak at 540 cm⁻¹.     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Dopant induced morphology changes in ZnO nanocrystals

Zinc oxide (ZnO) nanocrystals doped with different groups of impurities, e.g., Li, Na, Cu, Pr and Mg synthesized by solid-state reaction method under similar conditions exhibit different morphology. XRD showed monophasic wurtzite structure but change in lattice parameters and Zn-O bond length indicates incorporation of dopant ion in ZnO lattice. The morphology of ZnO nanocrystals exhibited striking dependence on type of dopant ion with the shape changing from nanorods, spherical to petal like particles. Photoluminescence (PL) shows pronounced UV emission and negligible visible emission for Li, Na and Cu doped ZnO nanocrystals with peak positions coinciding with that of undoped ZnO. Whereas signature emission of Pr³⁺ ion as well as broad visible emission from Mg doped ZnO revealed the role of intra gap metastable states formed by the dopant ion in the emission process.     

Opals with a thin-film metallic defect-hybrid colloidal plasmonic photonic crystals

Hybrid photonic crystals consisting of a thin-film opal and a thin profiled gold film situated on the surface or inside the opal have been prepared. The optical transmittance spectra of hybrid crystals have been studied. It has been found that the spectra exhibit minima due to diffraction resonances in the photonic crystal and bands of enhanced transmission due to transfer of radiation through the metal film surface by plasmon polaritons. It has been shown that the transmittance spectra of hybrid crystals with a metal film on the surface are subjected to a stronger modification than the hybrids having a metal film inside the crystal.     

Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts

Novel porous ZnO nanobelts were successfully synthesized by heating layered basic zinc acetate (LBZA) nanobelts in the air. The precursor of LBZA nanobelts consisted of a lamellar structure with two interlayer distances of 1.325 and 0.99 nm were prepared using a low-temperature, solution-based method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy are used to characterize the as-products. PL measurements show that the porous ZnO nanobelts have strong ultraviolet emission properties at 380 nm, while no defect-related visible emission is detected. The good performance for photoluminescence emission makes the porous ZnO nanobelts promising candidates for photonic and electronic device applications.