Defect-related Band Research Articles

Self-assembly epitaxial growth of nanorods on nanowalls in hierarchical ZnO hexagonal nanocastle

Three-dimensional (3D) hierarchical por- ous nanostructure materials with controlled morphol- ogy have attracted much attention for their unique properties and wide applications. In the present work, a novel 3D hierarchical ZnO nanostructure was prepared through chemical vapor deposition, which is self-assembled by an array of vertical 1D nanorods on top of 2D nanowalls network in 3D hexagonal nanocastle. As revealed by transmission electron microscopy, the hierarchical nanostructures are com- posed of single crystal ZnO along the (0001) direction, and the preferred surfaces of nanowalls are f10 10g. Based on those experimental results, a growth mech- anism which involves a Zn-self-catalytic vapor- liquid-solid and vapor-solid mode is proposed to explain the formation of nanorods on nanowalls in 3D nanocastle. Raman and photoluminescence spectrum demonstrated that these 3D hierarchical ZnO nanostructures are nearly free of strain and exhibit an intense ultraviolet emission at 377.6 nm with negligible defect-related green bands.

Structural, morphological and optical properties of sol gel processed CdZnO nanostructured films: effect of precursor solvents

Cadmium-doped zinc oxide (CZO) has been the potential candidate for the fabrication of low-cost transparent conductors having applications in energy conversion devices such as thin-film solar cells and light-emitting diodes. In this work, the effect of different precursor solvents on the structural, morphological, and optical properties of sol?gel spin-coated CZO films was investigated. The X-ray diffraction study has revealed the hexagonal wurtzite crystal structure with crystallite size ranging from 45 to 49 nm for these films. The surface morphology of these films was analyzed by the atomic force microscopy. The precursor solvent effect was also analyzed using the UV?vis spectroscopy and fluorescence spectroscopic techniques. All the prepared films show the larger values of optical transparency (>80%) along with the shift in the fundamental absorption edge and the direct optical gap was found to be between 3.14 and 3.19 eV. The highly intense band edge emission for the sample was synthesized using the 2-ethoxyethanol while the presence of other defect-related bands in the other samples has been observed. The change in the precursor solvent volatility influences the crystal structure to favor the change in the density/type of defect states in the bandgap is used to discuss the present results. The formation of highly porous films will be suitable for gas sensor applications while the larger value of optical transparency and lower surface roughness have their probable application as the transparent electrodes in optoelectronics.

Multiple kinds of emission modes in semiconductor microcavity coupled with plasmon

The photoluminescence emission of the ZnO microcavity coupled with the plasmon has three kinds of modes: the emission modes in the band-edge emission band, the wide emission modes and the thin emission modes in the defect-related emission band. The emission modes in the band-edge emission band are the plasmon coupled exciton polariton with the effective refractive index of 2.04. The exciton coupling equation with the effective refractive index can describe the coupling between the exciton, plasmon and microcavity very well. The wide emission modes in the visible band are the plasmon coupled transverse Fabry–Perot modes with the effective refractive index of 1.84 at the wavelength of 580nm (2.14eV). The thin emission modes in the visible band are the plasmon coupled WGM with the effective refractive index of 1.87 at the wavelength of 566.1nm (2.190eV). Because of the phase difference of the two polarizations in the total internal reflections, doubled modes are observed for the WGM, which leads to the thin modes in the visible band.

Solvothermal preparation of flower-like ZnS microspheres, their photoluminescence and hydrogen absorption characteristics

The complex 3D flower-like ZnS microspheres have been synthesized via a one-pot template-free solvothermal method in mixed solvents of ethylenediamine, ethanolamine and ethanol without any surface-active agent. The prepared nanostructures have been characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The FESEM, TEM and HRTEM results exhibit that the 3D ZnS architectures present flower-like microstructures self-assembled with aligned high-density nanosheets. The XRD results show that the flower-like ZnS microspheres are wurtzite structure and highly crystalline. The photoluminescence spectrum of the flower-like ZnS microspheres exhibits a strong broad defect related band at 400–600nm which almost covered the whole visible region. The first ever study on hydrogen absorption characteristics of the ZnS microspheres performed at 373K shows an absorption capacity of 0.84wt%.

Fabrication and Electroluminescence of N-ZnO Nanorods/p-Si Nanowires Heterostructured Light-Emitting Diodes

The n-ZnO nanorods/p-Si nanowires heterojunction was fabricated by chemical method. The microstructure of the epitaxially grown ZnO nanorod was characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The photoluminescence of the nanostructure showed a typical UV emission band and a defect-related emission band. Further, the electroluminescence of the nanostructure were measured, and the mechanism was discussed based on the band diagram of the n-ZnO nanorods/p-Si nanowires heterojunction.

Structural and optical characterization of GaN nanowires

Optical properties of GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on Si(111) substrates were studied with the use of micro-Raman spectroscopy and photoluminescence. Two types of NWs, hereafter labeled as A and B, grown with different values of Ga flux were studied. Morphology of the samples was probed by high resolution scanning electron microscopy. It was found that large Ga flux has led to a partial coalescence of nanowires in sample A. Reduction of Ga flux during growth of sample B resulted in an ensemble of separated nanowires. Micro-Raman and photoluminescence spectra were taken under illumination of 325 nm He-Cd laser line. Micro-Raman data reveal hexagonal phase of GaN NWs as well as a slight strain of Si substrate. Photoluminescence data yield that ensembles of separated NWs in sample B are defect free, whereas the spectra for coalesced wires in sample A exhibit both broad yellow luminescence band and defect-related band centered around 3.36 eV. Moreover, it was found that photoluminescence spectra are sensitive to UV illumination in agreement with the model of photoinduced surface desorption of oxygen. The effect is stronger for sample B due to larger surface-to-volume ratio for separated NWs as compared to the coalesced NWs in sample A.

Concentration dependent structural and optical properties of electrochemically grown ZnO thin films and nanostructures

ZnO thin films and nanostructures have been grown on ITO and Si (100) substrates with varying concentration of electrolyte by electrodeposition at low temperatures. The structural analysis showed an evolution of directional growth along (002) crystallographic plane with the change of electrolyte concentrations. The peak position of defect-related bands depends upon the electrolyte concentration, and at higher molar concentration the dominant luminescent center corresponds to green emission. Furthermore, the nature of substrate also contributes to the origin of the different luminescent center. These results demonstrate that the electrolyte concentration and choice of substrate can tune the optical properties of ZnO thin films and nanostructures.

Enhancement of the photoluminescence in CdSe quantum dot–polyvinyl alcohol composite by light irradiation

The effect of photo-induced enhancement (more than a tenfold) of room temperature deep-trap photoluminescence (PL) in CdSe quantum dots (QDs) embedded in polyvinyl alcohol (PVA) film has been found and investigated by the PL and X-ray diffraction methods. The effect is observed under illumination of the QD/PVA composite with LED's light of 409 or 470nm at elevated temperatures and is shown to be caused by an increase of the activation energy of thermal quenching of defect-related PL. It is shown that thermal annealing of the composite by itself stimulates polymer crystallization and produces a small increase in the intensity of both the band-edge and defect-related PL bands of CdSe QDs. It is found that the effect of illumination decreases when the annealing temperature increases from 90°C to 120°C because thermal annealing at 120°C per se results in strong enhancement of room temperature deep-trap PL. The effect of photo-induced enhancement of defect-related PL is found to be irreversible and is assumed to be related to the change of QD surface defect passivation or surface defect rearrangement. This is ascribed to partial destruction of PVA matrix as a result of interaction of QD/PVA interface with photocarriers generated in the QDs due to LED's light absorption.

Temperature-sensitive growth kinetics and photoluminescence properties of CdS quantum dots

As a critical issue in “green” synthesis, the mechanism by which the growth temperature influences the photoluminescence (PL) properties of quantum dots (QDs) remains unclear. In this work, temperature sensitive growth kinetics and PL evolution of thioglycolic acid (TGA) capped CdS QDs were systematically investigated. This revealed that TGA capped CdS QDs coarsened below and above 90 °C experienced completely different growth kinetics, i.e. a two-stage growth from oriented attachment to Ostwald ripening (OA–OR) and a hybrid growth of the two mechanisms (OA + OR), respectively. With the evolution of the OA–OR growth, the corresponding PL shows characteristics of a gradual weakness of the defect-related emissions centered at ∼650 nm and a steadily enhanced band-edge emission peaking at ∼450–470 nm. For the OA + OR hybrid growth, the defect-related emission band steadily coexists with the band-edge emission after an initial decrease. In-depth kinetics and high-resolution transmission electron microscopy (HRTEM) investigation indicate that a low temperature favors a long-term OA self-integration stage, by which internal defects could be relaxed or even eliminated. For a high temperature, this tendency could be greatly inhibited by the concurrent OR growth just around these defects. The FTIR analysis further revealed that the adsorption–desorption equilibrium of capping ligands was temperature-sensitive, resulting in different growth kinetics and PL characteristics of CdS QDs.

Structural and Photoluminescence Properties of ZnO Nanowires

Arrays of ZnO nanowires are grown by the vapor-liquid-solid method on a silicon substrate. The results of XRD, SEM, and AFM studies show that the diameters of nanowires vary in the range (50–300) nm, and their length is up to 40 μm. The wires exhibit bright photoluminescence: the band corresponding to the near band edge region and one or two (depending on the growth conditions) defect-related bands. The intensity ratio of the bands reflects the non-stoichiometry of the material and can be controlled by the zinc evaporation temperature and the temperature in the growing zone.

Surface plasmon driven enhancement in UV-emission of electrochemically grown Zn1−xCdxO nanorods using Au nanoparticles

Zn1−xCdxO (0<x<0.16) alloys semiconductor nanorods having a diameter of 80–150nm and length ∼2μm has been synthesized by electrodeposition. Au-nanoparticles (Au-NPs) have been sputtered on the Zn1−xCdxO nanorods for different sputtering times (from 0 to 100s). The photoluminescence (PL) spectra of bare Zn1−xCdxO nanorods showed a weak bandgap emission in UV region and a broad defect-related emission band in the visible region of the electromagnetic spectrum. The UV-emission attains maximum intensity for the 50s Au sputtered ZnCdO nanostructures and this enhancement in intensity is due to the transfer of electrons from defect states within the bandgap of ZnCdO to the Au-NPs that cause an increase of resonant electron density. These resonant electrons escape from the surface of the 99 Au-NPs to conduction band of ZnCdO nanorods leading to the suppression of defect related emission intensity. The suppression of defects related emission decreased with higher cadmium concentration in Zn1−xCdxO nanorods.

Enhancement of optical properties and donor-related emissions in Y-doped ZnO

The Zn1−xYxO nanoparticles with good optical properties have been prepared by sol–gel method. The yttrium doping effect on the structures and optical properties were investigated by XRD, SEM, XPS and low temperature photoluminescence. The UV emission intensity of yttrium doped ZnO was over 300 times stronger than that of pure ZnO, which was an exciting result in enhancing the ultraviolet near band edge emission in photoluminescence from ZnO nanoparticles. The UV emission band of doped ZnO nanoparticles exhibits a red shift from 388 to 398nm, indicating a shallow energy level near valence band has been formed due to the yttrium doping into ZnO lattices. The defect-related band is suppressed (ID/IUV=1–0.83) considerably in Zn1−xYxO nanoparticles, revealing the quenching of the broad yellow-orange emission. The doping effect on the optical properties is investigated by temperature dependent photoluminescence. The experimental results indicated that the donor level of yttrium is deeper than that of undoped ZnO.

Photoluminescence Properties of Self-Assembled Monolayers of CdSe and CdSe/ZnS Quantum Dots

The photoluminescence (PL) properties of self-assembled monolayers (SAMs) of CdSe and CdSe/ZnS quantum dots (QDs) were investigated. From measurements of the temperature dependence of PL spectra, it is demonstrated that the thiol group, which is a constituent of the reagent for formation of the SAM, degrades the PL properties during the SAM formation process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH = 10, the band-edge PL intensity is increased remarkably. The improvement of PL properties is attributed to QD surface modification with a Cd(OH)2 layer. On the other hand, the defect-related PL band is not observed at all, even at 10 K, in the SAMs of CdSe/ZnS core/shell QDs. This fact indicates that the ZnS shell layer prevents degradation of the PL properties during the SAM process.

Evidence of charge delocalization in Ce1−xFex2+(3+)O2−ynanocrystals(x=0, 0.06, 0.12)

We have measured Raman scattering spectra of pure and iron-doped Ce${}_{1\ensuremath{-}x}$Fe${}_{x}$${}^{2+(3+)}$O${}_{2\ensuremath{-}y}$ ($x=0$, 0.06, and 0.12) nanocrystals. According to the x-ray diffraction study, Fe doping produces contraction of the CeO${}_{2}$ unit cell, leading to the Raman mode hardening. Contrary to expectation, the ${F}_{2g}$ Raman mode exhibits softening and broadening by changing the valence state of Fe dopant, as a consequence of the electron-molecular vibration coupling. This finding supports the assumption that additional charge in highly oxygen-deficient pure and Fe-doped CeO${}_{2\ensuremath{-}y}$ samples are not only localized at Ce${}^{3+}$ ions but also delocalized onto Ce(Fe)-O(V${}_{O}$)-Ce(Fe) orbitals. Delocalization of electrons from Ce${}^{3+}$ ions causes insulator-to-metal transition in highly oxygen-deficient nanoceria. The far-infrared reflectivity spectrum of nanoceria shows metalliclike reflectivity, which in the low-frequency region is well fitted with the Hagen-Rubens approximation for metals. Photoluminescence measurements revealed the existence of a defect-related band in the energy gap of CeO${}_{2\ensuremath{-}y}$. The electron-molecular vibration (phonon) coupling constants $\ensuremath{\lambda}$ and density of electron states at the Fermi level per spin and molecule $N(0)$ were determined within the framework of Allen's theory. The proposal of an energy band structure of nanoceria is also presented.

Effect of biocompatible glutathione capping on core–shell ZnS quantum dots

Glutathione capped quantum dots are a potential candidate for different applications like ligand exchange in living cells, cell imaging and detection of glucose levels. Keeping these in mind, glutathione capped ZnS quantum dots were synthesized by using the thiol group of the capping agent by chemical precipitation method. Morphological characterizations were done by XRD and TEM. X-ray diffraction (XRD) measurements showed that the nanocrystals have Zinc Blende structure. Grain size and particle size shows a little variation with glutathione capping. Optical characterizations were done by UV–visible absorption, FTIR and energy resolved photoluminescence. UV–visible studies shows that the band gap also shows a small variation with glutathione capping. FTIR studies confirm glutathione capping on the surface of ZnS quantum dots. Room temperature energy resolved photoluminescence spectrum of samples exhibited a defect-related blue emission band. However, the PL properties seem to start tunability at higher concentration of glutathione which is a very good sign for extending this research further.

Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles

Electrochemical deposition of ZnO nanorods having a diameter of 80–150nm and length ~2μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375nm and a broad defect-related emission band centered at ~596nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50s. The enhancement factor reached a maximum value for the Au sputtering time of 50s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity.

Dynamics and microscopic origin of fast 1.5μm emission in Er-doped SiO2sensitized with Si nanocrystals

We investigate the origin of fast 1.5 $\ensuremath{\mu}$m photoluminescence from Er-doped SiO${}_{2}$ sensitized with silicon nanocrystals, which appears and decays within the first microsecond after a short laser excitation pulse. Time-resolved and temperature-dependent measurements on the 1.5 $\ensuremath{\mu}$m emission from Er-doped and Er-free samples reveal that the major part of this emission is Er related. A possible contribution from other photoluminescence bands, specifically of the defect-related band centered around 1.3 $\ensuremath{\mu}$m, has also been considered. All the results obtained indicate the dominant contribution of Er${}^{3+}$ ions to the fast 1.5 $\ensuremath{\mu}$m emission in the investigated materials. We propose two possible mechanisms behind the fast excitation and quenching of Er${}^{3+}$ 1.5 $\ensuremath{\mu}$m emission, which are both facilitated by Er-related trap centers with ionization energy of $E$${}_{A}$ $\ensuremath{\approx}$ 60 meV.

Characteristics of Zr-Doped ZnO Thin Films Grown by Atomic Layer Deposition

In the study, we present the electrical and optical properties of zirconium-doped zinc oxide (ZnO:Zr) thin films grown by atomic layer deposition (ALD) on sapphire substrates. The ZnO:Zr films exhibited low resistivity (1.3 × 10−3 Ω cm), high carrier concentration (2.2 × 1020 cm−3), and high transparency (> 92%) in the visible spectrum. The photoluminescence spectra consisted of a strong spontaneous emission at 380 nm associated with the near-band-edge emission and a weak defect-related band. Optically-pumped stimulated emission in ZnO:Zr films was achieved with a low threshold intensity of 105 kW/cm2 at room temperature. The results indicate that the ZnO:Zr films prepared by ALD are applicable to transparent conductive oxide and ultraviolet light-emitting materials.

Catalyst-free synthesis of ZnO microrod arrays on SiC substrate and their tunable photoluminescence by erosion process

We present here the results on the preparation and characterization of highly oriented ZnO microrods fabricated on SiC substrate via a vapor solid route. These ZnO microrods grow along the c-axis perpendicularly to the substrate and are well separated from each other. Photoluminescence (PL) due to the band-gap emission (centered at 380 nm) and defect-related deep band emission (centered at 530 nm) were observed. Tunable PL intensities of these two bands were achieved by acid etching for varying durations.

Trap-assisted conduction in Pt-gated Gd2O3/Si capacitors

The barrier height at the Pt/Gd2O3 interface is determined by current–voltage measurements. Current conduction is found to be governed by carrier injection from the electrode, with a barrier height of 0.6 ± 0.1 eV. This value, which was verified by the method suggested by Zafar et al. [Appl. Phys. Lett. 80, 4858 (2002)], is much smaller than the difference between the metal work function (5.6 eV) and the oxide electron affinity (1.95–2.05 eV). As Fermi-level pinning is not dominant at Pt/Gd2O3 interfaces, it is proposed that electrons are injected into a defect-related energy band in the oxide. The existence of such a defect, as well as its position in the oxide forbidden energy bandgap, agrees with results obtained by magnetic resonance measurements.