Defect-related Emission Band Research Articles

UV emission from self-assembled ZnS nanowires on DNA templates

We report on the self-assembled growth of 8–10 nm ZnS nanoparticles and subsequently nanowires of size ~100 nm onto DNA templates driven by the electrostatic interaction between mobile cations (Zn 2+ ) and the negative charges of the phosphate group in DNA backbone. The nanowires show free excitonic ultra-violet (UV) emission at 310 nm along with some defect related emission bands at 345–364 nm. The observed excitonic emission is slightly red-shifted from the optical absorption edge at 300 nm plausibly due to splitting of valence band into hole states. The growth kinetics and relatively lesser internal strain are plausibly responsible for the improved emission spectra of the ZnS nanowires.

Effect of Cu- and Y-Codoping on Structural and Luminescent Properties of Zirconia Based Nanopowders

Zirconia (ZrO2) has attracted considerable attention because of its mechanical, electric, thermal and luminescent properties offering diverse applications such as catalysts, high temperature and corrosion resistant coatings, sensors, radiation detectors, laser techniques, biological labeling, etc. Different defect-related emission bands can be observed in visible spectral range from pure and/or Y-stabilized ZrO2materials allowed an application of zirconia for white light emitting devices. ZrO2 nanopowders doped with Y and/or Cu were synthesized by a co-precipitation technique using Zr, Y and/or Cu nitrates and calcined at 500-1000оС for 1 h. The Y content in the samples was fixed at 3 mol%, whereas Cu concentration was varied from 1 to 8 mol%. Structural, optical and light emitting properties were studied by Raman scattering, PL and PL excitation methods at room temperature. Raman scattering spectra of Y-doped and (Y,Cu)-codoped ZrO2 samples annealed at 500-800 °C showed the presence of broad phonon bands peaked at 145, 255, 325, 475 and 630 cm-1 corresponded to tetragonal ZrO2 phase. The peak’s broadening is caused by a structural distortion. For (Y,Cu)-codoped samples the increase of annealing temperature up to 900°C results in the shift of Eg modes from 475 cm-1 and 630 cm-1 towards 466 cm-1 and 636 cm-1, respectively, due to stress relaxation. Besides, the appearance of several phonons corresponding to monoclinic ZrO2 phase was observed that was the evidence of the t-m phase transformation. An annealing at 1000°C causes the formation of m-ZrO2 phase only that was confirmed by the quenching of E2a phonon mode (259 cm-1). The comparison of these data with those obtained for Y-doped ZrO2 powders showed that Cu-codoping results in the formation of m-ZrO2 phase in (Y,Cu) codoped ZrO2 samples at 900-1000°C contrary to stable t-ZrO2 phase in Y-doped only materials. It should be noted that the increase of Cu content can lead to the decrease of the annealing temperature of ZrO2 t-mphase transition. The analysis of PL spectra of (Y,Cu) codoped samples showed the presence of two main PL components peaked at about 640 and 550 nm in the most samples. The PL excitation spectra demonstrated that the former PL component is excited by the 290-300nm light preferably, whereas 540-nm PL emission is excited by the light with shorter wavelength (250-260nm). An annealing at 500-900 °C results in the slight variation of total PL intensity, while the PL component peaked at 640 nm increases mainly. At the same time the treatment at higher temperature (1000°C) leads to an enhancement of green emission mainly. Besides an appearance of 300-nm peak in PLE spectra demonstrates that the shorter PL component is excited by the longer-wavelength light. The main reason of the transformation of PL spectra of (Y,Cu) codoped samples is ZrO2 t-mphase transition. This phenomenon as well as PL and PL excitation mechanisms will be discussed in details. This work was supported by National Academy of Sciences of Ukraine

Study on growth mechanism and optical properties of ZnSe nanoparticles

ZnSe nanoparticles with the cubic zinc blende structure were synthesized by a triethanolamine (TEA)-assisted solvothermal method without surfactants and templates. The effects of the TEA on the growth mechanism of ZnSe nanoparticles were analyzed. Photoluminescence measurements showed that the intensity of the defect-related emission band varied with the volume of TEA, as different types of defects were dominant in the ZnSe nanoparticles. Moreover, the emission band centered at 545 nm was attributed to the recombination of a donor–acceptor pair involving Zn vacancies, while the emission band centered at 668 nm was due to the stacking faults, twinning defects and some point defects. In Raman spectra, a clear shift of the transverse and longitudinal optical phonon modes, as well as a broadening of the longitudinal optical phonon peak were observed for ZnSe nanoparticles.

Surface plasmon-enhanced UV emission of ZnO nanorods using Ag nanoparticles

The emission properties of Ag-decorated ZnO nanorods were studied. The widths and lengths of the nanorods ranged from 60 nm to 200 nm. The photoluminescence (PL) spectra of bare ZnO nanorods showed a weak bandgap emission at around 370 nm and a broad defect-related emission band centred at ∼570 nm. After adding Ag, it resulted in an increase and decrease of the near band edge (NBE) and defect-related emission intensities, respectively. The reasons for the enhancement of the band edge emission which caused by the surface plasmon resonance coupling of the near band edge emission between Ag and ZnO are discussed. The visible emission has been suppressed significantly leading to a UV/visible emission ratio of 6 compared with a value of 1.6 for the pristine ZnO nanorods. The enhancement is due to the surface plasmon resonance effect.

Photoluminescence and Raman scattering study in ZnO:Cu nanocrystals

Photoluminescence (PL), scanning electronic microscopy (SEM), Raman scattering, X ray diffraction (XRD) and energy dispersion spectroscopy (EDS) have been applied for the comparative study of ZnO and ZnO:Cu nanocrystals (NCs). NCs were created by the electrochemical (anodization) method with different NC sizes (from 300×550nm down to 170×320nm) and were annealed at 400°C for 2h in ambient air. Raman and XRD studies have shown that thermal annealing stimulates the ZnO crystallization with the creation of wurtzite crystal lattice. XRD and EDS methods have been used for the confirmation doping of the ZnO NCs by Cu atoms and to show that the metallic Cu nanoparticles have been embedded in the porous ZnO NC films. It is shown that the Raman intensity for all Raman peaks in ZnO Cu NC systems is higher threefold in comparison with those in ZnO NC films owing to, apparently, the surface enhanced Raman scattering (SERS) effect. In crystalline ZnO and ZnO Cu NCs four PL bands appear with the PL peaks at 1.54, 2.08, 2.50 and 3.08eV. The reasons of emission transformation in different samples and the nature of optical transitions have been discussed. It is shown that the plasmon generation in metallic Cu nanoparticles stimulates the SERS effect at Raman scattering, the PL intensity enhancement of defect related emission bands, but, at the same time, leads to decreasing the PL intensity of near band edge emission in ZnO Cu NC systems.

Nature of yellow luminescence band in GaN grown on Si substrate

High-optical-quality GaN was grown on a (111)Si substrate by metal–organic vapor phase epitaxy using an AlInN buffer layer and an indium doped AlN nucleation layer. The photoluminescence spectra at room temperature showed a strong and narrow edge-emission peak and weak defect-related emission bands. We found four spectral peaks in the green and yellow luminescence bands at G0: 514.5 nm (2.410 eV), G1: 546.5 nm (2.269 eV), Y1: 553.5 nm (2.240 eV), and Y0: 584.5 nm (2.121 eV), independent of the growth methods/conditions. The results suggest that the emission is associated with an intrinsic defect such as a Ga vacancy.

Study on the synthesis and excitation-powerdependent photoluminescence spectrum of ZnSe nanoparticles

Zinc selenide (ZnSe) nanoparticles with the cubic zinc blende structure were successfully prepared by a solvothermal method without any surface-active agents. The as-obtained sample was characterized by X-ray diffraction, transmission electron microscopy (TEM), selected area electron diffraction, high-resolution TEM, and room-temperature photoluminescence (PL) techniques. It was proved that EDTA played a significant role during the synthesis of ZnSe nanoparticles. The room-temperature PL spectrum of the ZnSe nanoparticles showed a strong near-band-edge emission peak at 472 nm and a weak defect-related emission band in the range of 600–650 nm. Excitation-powerdependent PL spectrum of the ZnSe nanoparticles showed that the near-band-edge emission peak displayed an evident redshift with increasing the excitation power, and the corresponding energy shift might be as large as 51 meV. In addition, the integrated intensity of the near-band-edge emission peak increased with increasing the excitation power, which indicated the competition between the radiative recombination process and the nonradiative recombination process of photogenerated carriers.

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.

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.

Photoluminescence of nanoparticles in vapor phase of colliding plasma

We report photoluminescence observed from ZnO nano particles/clusters formed in gaseous phase of two colliding plumes ZnO plasma using fourth harmonic 266 nm of Nd:YAG laser. The two expanding plumes generated using 1.06 μm of Nd:YAG laser interact with each other, resulting in an overlap region where the collision front in lateral direction of two plumes forms an overlapping region starting at ∼20 ns lasting ∼5 μs with respect to ablating pulse. The blue shift in peak position (∼30–5 meV) of photoluminescence profile of ZnO clusters to that observed in bulk ZnO shows the effect of quantum confinement. The dynamical growth of nanoparticles is ascertained using Rayleigh scattered second harmonic radiation at 532 nm of Nd:YAG laser. A combined photoluminescence and light scattering at longer time delays ∼1.5 ms where plasma emission is absent confirms the formation of the nano-particles/clusters of ZnO. Observed intensity variation in the Rayleigh scattered signal and blue shift in photoluminescence peak position at different temporal delays with respect to the ablation pulse corroborates the presence and size variation of nano-particles/clusters. The defect related emission band arising due to oxygen vacancy is not observed from the ZnO clusters.

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.

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.

Investigation of defect levels in Cs2Hg6S7 single crystals by photoconductivity and photoluminescence spectroscopies

The heavy element semiconductor compound Cs2Hg6S7 is of interest as a potential wide gap semiconductor for gamma ray detection. To determine electrically active defects and their energy levels, photoconductivity (PC) spectroscopy was carried out over the temperature range of 90-295 K. The low temperature spectrum exhibits photoconductive transitions at 1.495, 1.61, 1.66, and 1.68 eV. The optical transitions are tentatively attributed to defects with levels located at energies of 50, 70, 120, and 240 meV from the band edge. A superlinear dependence of photocurrent on illumination intensity is observed that is attributed to a two-center recombination process that involves shallow traps and recombination centers. Near band edge photoluminescence (PL) was observed over the temperature range of 24–80 K. The spectrum revealed three defect related emission bands located at 1.68, 1.66, and 1.62 eV, whose ionization energies are 57 meV, 78 meV, and 115 meV, respectively. From the temperature and excitation dependencies of the observed peak intensities and energies, the radiative recombination mechanisms of the bands were attributed to transitions involving excitons bound to neutral and ionized acceptors. Good agreement of the defect level energies determined by PL and PC were noted, indicating that they were of the same origin. The defects were tentatively attributed to metal vacancies that form shallow acceptor levels.

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.

EPR, thermo and photoluminescence properties of ZnO nanopowders

Nanocrystalline ZnO powders have been synthesized by a low temperature solution combustion method. The photoluminescence (PL) spectrum of as-formed and heat treated ZnO shows strong violet (402, 421, 437, 485 nm) and weak green (520 nm) emission peaks respectively. The PL intensities of defect related emission bands decrease with calcinations temperature indicating the decrease of Zn i and V o + caused by the chemisorptions of oxygen. The results are correlated with the electron paramagnetic resonance (EPR) studies. Thermoluminescence (TL) glow curves of gamma irradiated ZnO nanoparticles exhibit a single broad glow peak at ∼343 °C. This can be attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centers. The trapping parameters of ZnO irradiated with various γ-doses are calculated using peak shape method. It is observed that the glow peak intensity increases with increase of gamma dose without changing glow curve shape. These two characteristic properties such as TL intensity increases with gamma dose and simple glow curve structure is an indication that the synthesized ZnO nanoparticles might be used as good TL dosimeter for high temperature application.

I2basal plane stacking fault in GaN: Origin of the 3.32 eV luminescence band

We investigate the $3.32 \mathrm{eV}$ defect-related emission band in GaN correlating transmission electron microscopy and spatially and spectrally resolved cathodoluminescence at low temperature. The band is unambiguously associated with basal plane stacking faults of type ${I}_{2}$, which are a common defect type in semi- and nonpolar GaN grown on foreign substrates. We ascribe the luminescence to free-to-bound transitions. The suggested intrinsic acceptors involved have an ionization energy of $\ensuremath{\approx}$$0.17 \mathrm{eV}$, and are located at the ${I}_{2}$-type basal plane stacking faults.

Influence of doping on optical properties of catalyst‐ and mask‐free grown gallium nitride nanorods

AbstractWe present a comparison of the optical properties of Si and Mg doped GaN nanorods with respect to an undoped sample. The reference sample is characterized by a high crystalline quality as indicated by the spectral width of the dominant emission peaks being comparable for the ensemble and single nanorods as well as by the absence of the yellow photoluminescence. Even after the incorporation of n‐dopants this defect related emission band is still not detectable.The p‐doped sample shows several modifications of the spectrum. The near band edge emission is dominated by an additional band at 3.41 eV related to partial coalescence of the nanorods, while the excitonic emission is very weak. The DAP band emission is obviously evolved into a double band series with two zero‐phonon peaks at 3.26 eV and 3.18 eV indicating the involvement of two acceptor related states, and is further superimposed by a weak yellow luminescence band around 2.5 eV. These results are important as basis for applications of such nanorods as electrically driven semiconductor devices with excellent crystalline quality as well as with very high surface sensitivity, which is useful for solar cells or chemical sensors, for example. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Crystal quality and conductivity type of (0 0 2) ZnO films on (1 0 0) Si substrates for device applications

Undoped high quality (0 0 2) zinc oxide thin films were grown on (1 0 0) silicon substrates by pulsed laser deposition (PLD). The films were developed at low growth temperatures between 200 and 300 °C, and a range of oxygen pressures from 8.5 × 10 −5 to 2.6 × 10 −4 Torr. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and room temperature photoluminescence (PL) have been carried out in order to verify the formation of ZnO layers and evaluate the structural and optical properties of the grown layers. Four-point Van Der Pauw Hall effect measurements were performed to evaluate the electrical conduction in the films. The grown ZnO films on Si were found to be perfectly (0 0 2) aligned polycrystalline structures with a best full-width at half-maximum (FWHM) value of 0.24° obtained for the optimum growth conditions. The PL study showed high optical quality with a substantially suppressed defect related emission band for the optimized conditions. Hall effect measurements showed that samples prepared at oxygen pressures of 1.0 × 10 −4 Torr and above had p-type conductivity at room temperature, whereas samples grown at lower pressures were n-type. All samples showed n-type conductivity at low temperature (77 K) regardless of the growth conditions, which suggests a thermal competition between donor-like oxygen vacancies and/or oxygen vacancy complexes with hydrogen, and acceptor-like oxygen interstitials and possibly Zn vacancies.

Formation of Hollow Gallium Nitride Spheres via Silica Sphere Templates

We report the formation of hollow GaN spheres using silica sphere templates. First, silica spheres with an average diameter of ∼130 nm were synthesized. A mixture of GaCl3, silica spheres, water, and urea in 2-propanol was prepared and heated to 100 °C for 24 h to generate silica spheres with γ-Ga2O3 nanoparticle shells. Decomposition of urea and its reaction with water slowly increased the solution pH to ∼8; this controlled reaction is the key to forming uniform γ-Ga2O3 shells with a thickness of about 13 nm. The amounts of urea and water have been varied to find the optimal conditions for the preparation of the oxide shells. Transfer of the colloidal particle solution to silicon substrates and ammonolysis at 850 °C for 6 h produced the SiO2−GaN core−shell nanostructures. Immersion of the silicon substrates in an HF solution removed the silica cores, and hollow GaN spheres with a shell thickness of around 8 nm were formed. The morphologies and crystal structures of the oxide and nitride shells have been carefully examined. The GaN shell materials show an absorption band at 350−360 nm and a broad defect-related emission band centered at around 570 nm.