Longitudinal Optical Phonon Replicas Research Articles

Photoluminescence characteristics of catalyst free ZnO nanowires

Catalyst free ZnO nanowires are grown by thermal evaporation, and are characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and photoluminescence (PL). The ultra-violet (UV) emission band observed in the PL of ZnO nanowires grown on ZnO buffer layer is investigated at different temperatures and excitation intensities. The prominently observed transitions at low temperatures are bound exciton (BX), first longitudinal optical (LO) phonon replica of free exciton () and donor–acceptor pair (DAP). Electron–hole plasma (EHP) emission is observed above 175 K of temperature and above 1.27 MW cm−2 of excitation intensity which is associated with the thermal ionization of bound excitons. The observed intensity and temperature dependent EHP emission is attributed to spontaneous emission.

Temperature dependence of the energy bandgap of two-dimensional hexagonal boron nitride probed by excitonic photoluminescence

Hexagonal boron nitride (hBN) is an emerging material for the exploration of new physics in two-dimensional (2D) systems that are complementary to graphene. Nanotubes with a diameter (∼60 nm) that is much larger than the exciton binding energy in hBN have been synthesized and utilized to probe the fundamental optical transitions and the temperature dependence of the energy bandgap of the corresponding 2D hBN sheets. An excitonic transition at 5.901 eV and its longitudinal optical phonon replica at 5.735 eV were observed. The excitonic emission line is blue shifted by about 130 meV with respect to that in hBN bulk crystals due to the effects of reduced dimensionality. The temperature evolution of the excitonic emission line measured from 300 to 800 K revealed that the temperature coefficient of the energy bandgap of hBN nanotubes with large diameters (or equivalently hBN sheets) is about 0.43 meV/0K, which is a factor of about 5 times smaller than the theoretically predicted value for the transitions between the π and π* bands in hBN bulk crystals and 6 times smaller than the measured value in AlN epilayers with a comparable energy bandgap. The observed weaker temperature dependence of the bandgap than those in 3D hBN and AlN is a consequence of the effects of reduced dimensionality in layer-structured hBN.

Investigation of Room Temperature Photoluminescence of ZnO Films Induced by Different Laser Fluence Irradiation

In this paper, we investigated the photoluminescence property of ZnO films which were irradiated by KrFexcimer laser. Through the analysis of photoluminescence and UV-VIS spectra, it is shown that the red shift of band gap forthe irradiated sample. Room temperature UV photoluminescence of ZnO film is composed of contribution from the FX, the neutral donor bound exciton (D0X) emission and the longitudinal optical (LO) phonon replicas of the bound exciton transition (D0X-1LO). The visible emission band is ascribed toVO+, VZn-, Oiand Vo++. It shows that KrF laser irradiation could effectively modulate the exciton emission, which is important for the application of high performance of emitting optoelectronic devices.

Temperature dependence of exciton localization in ZnO/Zn1−xMgxO multiple quantum wells with different barrier compositions

Temperature-dependent photoluminescence (PL) measurements are used to study two ZnO/Zn1−xMgxO multiple quantum well (MQW) structures with different barrier compositions. The PL band in the well layers is dominated by localized excitons (LEs), free excitons (FEs), and two longitudinal optical (LO) phonon replicas of the LE emission. The LE emission of higher Mg compositional barrier exhibits a significant blue shift of about 70meV with respect to the lower one. The mechanisms of carrier dynamics and localization are investigated within the temperature range 14–300K. As the temperature increases, luminescence from the excitons localized in the well layers shows an ‘S-shaped’ shift in the high-barrier MQWs, whereas a monotonic red shift is observed in the low-barrier MQWs. The ‘S-shaped’ shift behavior is associated with delocalization of the excitons in the potential minima induced by interface fluctuations or alloy disorder. Large exciton binding energies of 64 and 76meV were deduced, demonstrating efficient quantum confinement in the ZnO/ZnMgO MQWs.

Temperature-dependent Photoluminescence of Boron-doped ZnO Nanorods

Boron-doped ZnO (BZO) nanorods were grown on quartz substrates using hydrothermal synthesis, and the temperature-dependence of their photoluminescence (PL) was measured in order to investigate the origins of their PL properties. In the UV range, near-band-edge emission (NBE) was observed from 3.1 to 3.4 eV; this was attributed to various transitions including recombination of free excitons and their longitudinal optical (LO) phonon replicas, and donor-acceptor pair (DAP) recombination, depending on the local lattice configuration and the presence of defects. At a temperature of 12 K, the NBE produces seven peaks at 3.386, 3.368, 3.337, 3.296, 3.258, 3.184, and 3.106 eV. These peaks are, respectively, assigned to free excitons (FX), neutral-donor bound excitons (D o X), and the first LO phonon replicas of D o X, DAP, DAP-1LO, DAP-2LO, and DAP-3LO. The peak position of the FX and DAP were also fitted to Varshni’s empirical formula for the variation in the band gap energy with temperature. The activation energy of FX was about ~70 meV, while that of DAP was about ~38 meV. We also discuss the low temperature PL near 2.251 eV, related to structural defects.

Effects of annealing temperature on optical properties of ZnO nanorods with Mg0.2Zn0.8O capping layers

ZnO nanorods were grown on spin-coated ZnO seed layers by the hydrothermal method. The Mg0.2Zn0.8O capping layers were deposited on ZnO nanorods by the sol-gel method. The ZnO nanorods with Mg0.2Zn0.8O capping layers were annealed at 600°C. Temperature-dependent photoluminescence (PL) spectroscopy was carried out to investigate the mechanism governing the quenching behavior of the PL spectra. For the 12 K PL spectra, the peaks of the Mg0.2Zn0.8O capping layers, excitons bound to neutral donors (D0X), two-electron satellite transitions, donor-acceptor pairs, and free-to-neutral-acceptors and their longitudinal optical (LO) phonon replicas were observed at annealing temperatures from 600°C. At 12 K, the peak of the Mg0.2Zn0.8O capping layers was blue-shifted as the annealing temperature increased. The peaks of the D0X, free excitons and Mg0.2Zn0.8O capping layers merged and the PL intensity of the peaks decreased while the temperature increased from 12 to 300 K.

Temperature-dependent luminescence dynamics for ZnO thin films

The paper presents the photoluminescence investigation of zinc oxide thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz wafers. The films with different thickness (number of layers) were annealed at different temperatures after the preparation process. It was found that high quality, transparent ZnO thin films could be produced on quartz substrates at relatively low annealing temperature (450–550 \(^{\circ }\mathrm{C}\)). The dependence of the ZnO thin film quality was studied by X-ray diffraction and atomic force microscopy techniques. Optical properties were investigated by classic and time-resolved photoluminescence (TRPL) measurements. Photoluminescence spectra allowed us to estimate energy of the free excitons, bond excitons and their longitudinal optical (LO) phonon replicas as a function of the annealing temperature. An innovative TRPL technique let us precisely measure the decay time of the free- and bond excitons’ in the real time. TRPL measurements as a function of temperature reveal a biexponential decay behavior with typical free/bound exciton decay constants of 970/5310 ps for the as-grown sample and 1380/5980 ps after annealing process. Presented spectra confirm high structural and optical quality of investigated films. We proved that the thermal treatment improve both optical and structural quality and extend the photoluminescence’s lifetimes. The obtained experimental results are important for identification of exciton’s peaks and their LO phonon replicas for the investigated ZnO films.

Phonon Line Emission Revealed by Self-Assembly of Colloidal Nanoplatelets

We show that colloidal nanoplatelets can self-assemble to form a 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, greatly enhanced when the NPLs are self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton, and its presence in self-assembled nanoplatelets is explained using a model based on an efficient photons reabsorption between neighboring nanoplatelets. The presence of phonon replica at low temperatures in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets.

Studies on temperature- and excitation-power-dependent photoluminescence of ZnO thin film grown by plasma-assisted molecular beam epitaxy

The temperature- and excitation-power-dependence of near-edge emission in a ZnO thin film were studied. The near-band-edge emission peaks at 3.370, 3.355, 3.338, 3.305, 3.254, 3.184, 3.112, and 3.042 eV were attributed to the emission of free excitons (FX); neutral-donor-bound excitons (D0X); neutral-acceptor or deep donor excitons; two-electron satellites (TES); donor–acceptor pairs (DAP); and the first-order longitudinal optical (1LO) phonon replica of DAP (DAP-1LO), DAP-2LO, and DAP-3LO, respectively. According to Haynes's rule, the factors a and b are 0.3 and 5 meV, respectively (Eloc = 0.3ED + 5 meV). The temperature dependence of the FX transition energy was analyzed using both Varshni's formula and Cody's formula for the variation in the band gap with temperature. Varshni's formula yielded a good fit for FX emission. The dependence of the photoluminescence intensity I on the excitation power L can be described by a power law, i.e., I ∼ Lα, where α is an exponent. Our results show that α is 0.86 for D0X and 0.63 for DAP.

Temperature-dependent Photoluminescence Study on Aluminum-doped Nanocrystalline ZnO Thin Films by Sol-gel Dip-coating Method

The photoluminescence (PT) properties of Al-doped ZnO thin films grown by the sol-gel dip-coating method have been investigated. At 12 K, nine distinct PL peaks were observed at 2.037, 2.592, 2.832, 3.027, 3.177, 3.216, 3.260, 3.303, and 3.354 eV. The deep-level emissions (2.037, 2.592, 2.832, and 3.027 eV) were attributed to native defects. The near-band-edge (NBE) emission peaks at 3.354, 3.303, 3.260, 3.216, and 3.177 eV were attributed to the emission of the neutral-donor-bound excitons (<TEX>$D^0X$</TEX>), two-electron satellite (TES), free-to-neutral-acceptors (e,<TEX>$A^0$</TEX>), donor-acceptor pairs (DAP), and second-order longitudinal optical (2LO) phonon replicas of the TES (TES-2LO), respectively. According to Haynes' empirical rule, we calculated the energy of a free exciton (FX) to be 3.374 eV. The thermal activation energy for <TEX>$D^0X$</TEX> in the nanocrystalline ZnO thin film was found to be ~25 meV, corresponding to the thermal dissociation energy required for <TEX>$D^0X$</TEX> transitions.

Microstructure and enhanced exciton–phonon coupling in Fe doped ZnO nanoparticles

We report the microstructure and exciton–phonon coupling properties of Fe doped ZnO nanoparticles. Particles are prepared through sol–gel method at room temperature. Doping of Fe3+ induces strain in the host lattice. Microstructural properties are analysed through Williamson–Hall analysis. Optical absorption studies show strong free excitonic absorption band at 369nm. The photoluminescence (PL) studies reveal that ultraviolet, blue and green emission bands are located at 380, 445 and 500nm respectively. Fe doped ZnO nanoparticles exhibits only ultraviolet and blue emission bands. Increase of Fe concentration makes green emission gradually disappeared. Gaussian fitted photoluminescence spectra show the emission is composed of free exciton (FX) recombination and its higher orders of longitudinal optical (LO) phonon replicas. Doping induced blue shift in FX peak and also increases the exciton–phonon coupling.

Fabrication and low-temperature photoluminescence spectra of Mn-doped ZnO nanowires

Mn-doped ZnO nanowires have been fabricated through a high temperature vapor-solid deposition process. The low-temperature photoluminescence spectra of the samples show that there are multipeak emissions at the ultraviolet (UV) region (about 3.4–3.0 eV). The excitonic and phonon-assisted transitions in Mn-doped ZnO nanowires were investigated. The results show that there is an obvious oscillatory structure emission at the UV region under low temperature from 12–125 K. The oscillatory structure has an energy periodicity about 70 meV and the oscillatory structure is mainly attributed to longitudinal optical (LO) phonon replicas of free excitons (FX). The multipeak emissions at 12 K are attributed to a donor-bound exciton (DBX, 3.3617 eV), 1LO-phonon replicas of a free exciton (FX-1LO, 3.3105 eV), 2LO-phonon replicas of a free exciton (FX-2LO, 3.2396 eV), and 3LO-phonon replicas of a free exciton (FX-3LO, 3.1692 eV), respectively. The intensity of UV emission and the efficiency of emission from the Mn-doped ZnO nanowires are improved.

Optical Properties of Undopeda-Plane GaN Grown with Different Initial Growth Pressures

The optical properties of undoped a-plane GaN films grown by metal organic vapor phase epitaxy (MOVPE) with different initial growth pressures were investigated using photoluminescence (PL) measurements. Compared to GaN sample grown with higher initial grown pressure, which exhibited the dominant emission band at 3.423 eV, the dominant PL spectra for GaN sample grown with lower initial growth pressure was the donor-acceptor pair (DAP) band at 3.268 eV. Interestingly, the PL intensity of DAP longitudinal optical (LO) phonon replica was stronger than DAP emission above 50 K, indicating strong phonon coupling. The emission band at 3.359 eV observed for the sample grown with higher initial growth pressure was not observed for the sample grown with lower initial growth pressure. Based on the results obtained from Si doping, it was suggested that this band might be related with the improved crystalline quality through Si doping.

Optical Properties of Undoped a-Plane GaN Grown with Different Initial Growth Pressures

The optical properties of undoped a-plane GaN films grown by metal organic vapor phase epitaxy (MOVPE) with different initial growth pressures were investigated using photoluminescence (PL) measurements. Compared to GaN sample grown with higher initial grown pressure, which exhibited the dominant emission band at 3.423 eV, the dominant PL spectra for GaN sample grown with lower initial growth pressure was the donor-acceptor pair (DAP) band at 3.268 eV. Interestingly, the PL intensity of DAP longitudinal optical (LO) phonon replica was stronger than DAP emission above 50 K, indicating strong phonon coupling. The emission band at 3.359 eV observed for the sample grown with higher initial growth pressure was not observed for the sample grown with lower initial growth pressure. Based on the results obtained from Si doping, it was suggested that this band might be related with the improved crystalline quality through Si doping.

Influence of initial growth pressure on the optical properties of Si-doped nonpolar a-plane GaN grown with different doping levels

The effect of crystalline quality of undoped a-plane GaN buffer layers on the properties of Si-doped GaN layers grown with different doping levels was investigated. The dominant photoluminescence (PL) emission at about 3.42eV (D1 band) for all Si-doped GaN samples was associated with excitons bound to basal stacking faults (BSFs). The intensity ratio of the D1 band to near band edge (NBE) emission was reduced with increasing doping concentration, associated with the decrease in the density of BSFs. Interestingly, it was also found that the intensity of the DAP–longitudinal optical (LO) phonon replica exceeded the DAP emission above 50K, which might be related to the structural defect-related emission in Si-doped a-plane GaN samples.

Optical properties and crystallinity of ZnO thin films grown on porous silicon by using plasma-assisted molecular beam epitaxy

Zinc oxide (ZnO) thin films were grown on porous silicon (PS) at different growth temperature by using plasma-assisted molecular beam epitaxy (PA-MBE). From photoluminescence (PL), the asymmetric-shaped near-band-edge emission (NBE) peaks comprised of a free exciton (FX) and a longitudinal-optical phonon replica of the FX (FX-1LO) were observed from the ZnO thin films at room temperature (RT). The intensity ratio of the FX and the FX-1LO peaks decreased with increasing the temperature. The Huang-Rhys factor S of the ZnO thin films increased linearly from 0.576 to 1.519 with increasing the temperature, resulting from a decrease in the probability of exciton-phonon scattering. The crystal quality of the ZnO thin films was degraded with increasing the temperature. Nonetheless, the degradation of the crystal quality for the ZnO thin films grown on PS was much smaller than that of the ZnO thin films grown on Si substrates at higher temperatures.

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300K, which consisted of two emissions at 3.338 and 3.279eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10−3eV/K and β=4.9×103K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.

Photoluminescence characterization of vertically aligned ZnO microrods

A detailed optical characterization of vertically aligned ZnO microrods (μRs) grown using a Ni-based catalyst was carried out by excitation-power- and temperature-dependent photoluminescence (PL) measurements. Low-temperature PL spectra of ZnO μRs are dominated by near-band-edge (NBE) emission consisted of a series of sharp lines typical for the bulk ZnO. Starting from the higher energy free exciton (FX) emission feature, the majority of them can be explained by radiative recombination of excitons bound to neutral donors (D0X), defect bound exciton (DBX), two-electron satellites emission, free-to-bound, i.e. free electrons to the neutral acceptors (eA0) transition, as well as their longitudinal-optical phonon replicas. An additional excitonic line located in between the FX and D0X lines, denoted as the surface excitons (SX) for ZnO μRs is observed. The intensity of the SX line is found to be smaller than that of the nanosized counterpart and has been attributed to the surface–volume ratio effects. The excitation-power-dependent results of FX line at low and high power regimes show quite close values corresponding to, respectively, p=2 and p=1 limits of the theoretical power law expression I∼Lp and larger deviations for the D0X, SX and DBX lines. The temperature-dependent measurements confirmed the presence of eA0 line showing kT/2 influence to the position of eA0 emission line in comparison with FX. FX emissions persist up to 300K and together with the dominant eA0 emission govern the line shape of the NBE emission range, while D0X and SX lines are quenched completely at 150K.

Different temperature dependence of excitonic and defect-related photoluminescence spectra in ZnS nanobelts and nanowires

In this paper, both excitonic and defect-related information of ZnS nanobelts and nanowires have been investigated by a temperature-dependent photoluminescence (PL) spectrum. PL spectra of ZnS nanobelts and nanowires differ significantly in the ultraviolet (UV) and visible emission regions. In UV emission regions, due to high-quality crystals, free exciton B (FXB), free exciton A (FXA), FXA-one longitudinal optical (LO) phonon replica are observed in ZnS nanobelts, as well as free-to-bound (e, A) with its one LO phonon replica, while neutral-donor bound exciton (Do, X) and free-to-bound (e, A) are observed in ZnS nanowires at 10 K. The peak and relative intensity of the FX and (Do, X) versus temperature follow well with conventional empirical relations. In the visible emission regions, weak donor–acceptor pair (DAP) and self-activated (SA) emission from ZnS nanowires are commonly observed, but the Y band emission is only observed at 10 K in ZnS nanobelts. The Y band emission disappears at some temperature lower than 50 K. The peak position and full width at half maximum of DAP and SA emission bands display different temperature dependences. Detailed study on temperature-dependent PL spectra of ZnS nanobelts and nanowires provides crucial information on the nature of the electronic states and recombination mechanisms in these nanostructures.

Optical Properties of ZnO Soccer-Ball Structures Grown by Vapor Phase Transport

ZnO soccer balls were grown on an Au-catalyzed Si(100) substrate by vapor phase transport (VPT) with a mixture of zinc oxide and graphite powders. Temperature-dependent PL was carried out to investigate the mechanism governing the quenching behavior of the PL spectra. From the PL spectra of the ZnO soccer balls at 10 K, several PL peaks were observed at 3.365, 3.318, 3.249, and 3.183 eV corresponding to excitons bound to neutral donors (DoX), a donor–acceptor pair (DAP), first-order longitudinal optical phonon replica of donor–acceptor pair (DAP-1LO), and DAP-2LO, respectively. The mixed system composed of the free exciton (FX) and DoX and the DAP radiative lifetimes were estimated with a theoretical relation between the lifetime and the spectral width. The exciton radiative lifetimes were observed to increase linearly with temperature.