Photoluminescence Centers Research Articles

The effect of annealing on the photoluminescent and optical properties of porous anodic alumina films formed in sulfamic acid

Photoluminescent and optical properties of porous oxide films formed by two-step aluminum anodization at a constant potential of 30 V in sulfamic acid have been investigated after their annealing, ranging from room temperature up to 600 °C. X-ray diffraction reveals the amorphous nature of porous oxide films. Infrared and energy dispersive spectroscopy indicates the presence of sulfuric species incorporated in oxide films during the anodization. Photoluminescence (PL) measurements show PL bands in the range from 320 to 600 nm. There are two peaks in emission and excitation spectra. One emission peak is at constant wavelength centered at 460 nm and the other shifts from 390 to 475 nm, depending on excitation wavelength. For excitation spectra, one spectral peak is at constant wavelength at 270 nm and the other also shifts to longer wavelengths while increasing emission wavelength. Upon annealing of the as-prepared oxide films PL increases reaching maximum value at about 300 °C and then decreases. The results indicate the existence of two PL centers, one placed at surface of the pore wall, while the other positioned inside the oxide films.

Comparison of the photoluminescence properties of semiconductor quantum dots and non-blinking diamond nanoparticles. Observation of the diffusion of diamond nanoparticles in living cells

Long-term observations of photoluminescence at the single-molecule level were until recently very diffcult, due to the photobleaching of organic ?uorophore molecules. Although inorganic semiconductor nanocrystals can overcome this diffculty showing very low photobleaching yield, they suffer from photoblinking. A new marker has been recently introduced, relying on diamond nanoparticles containing photoluminescent color centers. In this work we compare the photoluminescence of single quantum dots (QDs) to the one of nanodiamonds containing a single-color center. Contrary to other markers, photoluminescent nanodiamonds present a perfect photostability and no photoblinking. At saturation of their excitation, nanodiamonds photoluminescence intensity is only three times smaller than the one of QDs. Moreover, the bright and stable photoluminescence of nanodiamonds allows wide field observations of single nanoparticles motion. We demonstrate the possibility of recording the tra jectory of such single particle in culture cells.

Effect of etch-treatment upon the intensity and peak position of photoluminescence spectra for anodic alumina films with ordered nanopore array

Porous anodic alumina membranes (AAMs) were prepared in oxalic acid and then carried on an etch-treatment in phosphoric acid. Using the etch-treatment the photoluminescence (PL) intensity of AAMs increases by a factor of 1/3. The effect of etch-treatment upon the intensity and peak position of photoluminescence (PL) spectra was investigated. It was found that the intensity of the photoluminescence (PL) spectra increased with the etching time increasing. A PL spectrum can be divided into two subbands with the peak at 434 and 460 nm, respectively. As the etching time prolongs, the intensity of the peak of 434 nm subband increases and that of the 460 nm subband rises firstly and then decreases. It can be explained by that two luminescence centers (F and F+ centers) coexist in AAMs. F centers are concentrated in the surface layer and F+ centers are enriched in the depth of pore wall. The increment of the PL intensity comes from the contribution of F+ photoluminescence centers concentrated in the depth of pore wall in AAMs. This work will be beneficial to improving the photoluminescence intensity and understanding the light-emitting mechanisms for related materials.

Dicarbon antisite defect inn-type4H-SiC

We identify the negatively charged dicarbon antisite defect (${\text{C}}_{2}$ core at silicon site) in electron-irradiated $n$-type $4H\text{-SiC}$ by means of combined electron paramagnetic resonance (EPR) measurements and first-principles calculations. The HEI5 and HEI6 EPR centers ($S=1/2$; ${C}_{1h}$ symmetry) are associated with cubic and hexagonal dicarbon antisite defects, respectively. This assignment is based on a comparison of the measured and calculated hyperfine tensors of $^{13}\text{C}$ and $^{29}\text{S}\text{i}$ atoms as far as the second neighborhood around the defects. Theoretically, the dicarbon antisites are stable in a single negative charge state under a wide range of $n$-type samples. We found that the defects can be created under a wide range of irradiation conditions, and our measurements strongly suggest the existence of carbon antisite defects in the as-grown samples. Annealing studies revealed several atomistic processes such as recombination of carbon interstitials with vacancies and formation of carbon aggregates. These processes were activated at about $1000\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, and as theoretically predicted, the dicarbon antisite is much more stable than the dicarbon interstitial defect (${\text{C}}_{2}$ core at carbon site). The measured activation temperature is consistent with the temperature range for forming various carbon aggregate-related photoluminescence centers.

Photoluminescence properties of porous anodic aluminium oxide membranes formed in mixture of sulfuric and oxalic acid

Porous anodic aluminium oxide (AAO) membranes with ordered nanopore arrays were fabricated electrochemically in the mixture electrolytes with various volume ratios of 0.3 M H2SO4 and 0.3 M C2H2O4. Photoluminescence (PL) properties of as-prepared AAO membranes were investigated. It was found that a 290 nm emission was observed in the AAO samples formed in the H2SO4 electrolyte and a 325 nm emission in the samples formed in the C2H2O4 electrolyte, respectively. For the AAO membranes anodized in the mixture electrolytes, both 290 and 325 nm emissions appeared together; moreover, the 290 nm peak was weakened, and the 325 nm peak was enhanced dramatically. It is suggested that the 290 and 325 nm emissions could be attributed to the luminescent centres transformed from sulfuric and oxalic impurities, respectively, and that nonradiative energy transfer occurs between these two kinds of PL centres.

Diffusion-Temperature-Dependent Formation of Cu Centers in Cu-Saturated Silicon Crystals Studied by Photoluminescence and Deep-Level Transient Spectroscopy

Using photoluminescence (PL) and deep-level transient spectroscopy (DLTS) we observed the concentration changes of Cu-related centers in silicon crystals saturated with Cu at various temperatures (600–1000 °C), in which each sample was measured at the same location for both methods. The DLTS peak assigned to have the same origin as the 1.014 eV PL center (CuPL center) was by far the strongest component among the DLTS peaks, and no DLTS peak due to isolated substitutional Cu (Cus) was observed for any of the diffusion temperatures used. The PL and DLTS intensities of the CuPL center increased with increasing diffusion temperature of Cu from 600 to 700 °C, reached a maximum between 700 and 800 °C, then sharply decreased at higher temperatures than 800 °C. This behavior was reasonably explained by considering the outdiffusion and precipitation of Cu in addition to the solubility of Cu. The maximum DLTS concentration of the CuPL center (∼1014 cm-3) observed in the present study far exceeded the estimated concentration of Cus formed through a vacancy-mediated reaction. From this finding we concluded that a model containing Cus was invalid for the CuPL center and that another model containing unique Cu atom at the center of the Si–Si bond was realistic.

Diamond nanocrystals hosting single nitrogen-vacancy color centers sorted by photon-correlation near-field microscopy

Diamond nanocrystals containing highly photoluminescent color centers are attractive, nonclassical, and near-field light sources. For near-field applications, the size of the nanocrystal is crucial, since it defines the optical resolution. Nitrogen-vacancy (NV) color centers are efficiently created by proton irradiation and annealing of a nanodiamond powder. Using near-field microscopy and photon statistics measurements, we show that nanodiamonds with sizes down to 25 nm can hold a single NV color center with bright and stable photoluminescence.

Effect of Annealing on the Visible Photoluminescence Characteristics of Octadecyltrichlorosilane Monolayers on Silica Surfaces

We report the effect of annealing conditions, including annealing environment, annealing temperature, and annealing time, on the visible photoluminescence (PL) at ∼450 nm from octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on silica surfaces. When the OTS SAMs are annealed at 200 °C in air, the PL and PL excitation peak wavelengths show considerable redshifts with increasing annealing time. We have also found that the annealing temperature and time affect the PL decay dynamics. The PL decay of the sample annealed at 150 °C for 2 h in air hardly depends on the measurement temperature. However, the PL decay of the samples annealed at 200 °C for more than 1 h starts to show the measurement temperature dependence, indicating that the thermally activated nonradiative decay is involved in the entire recombination processes. These annealing-dependent PL features can be interpreted in terms of the progress of thermal decomposition of alkyl chains in OTS SAMs during annealing and the concomitant changes in the microscopic and electronic structures of the relevant PL centers. Density functional theory calculations are also used to evaluate the changes in the energy levels of the emission centers during annealing.

Oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation and their effects on the photoluminescence wavelength

Natural oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation under various hydrogen gas pressures are discussed by measuring the vibrational frequency of Si–Hn units on the surface and intensity of Si–O–Si stretching vibration. The surfaces of nanocrystallites are predominantly composed of Si–H bonds and oxidation starts from backbonds of these bonds. The deposited nanocrystal films have a porous secondary structure which depends on the background gas pressure. The oxidation rate observed by infrared absorption measurements depended on this porous secondary structure. The oxidation process is discussed by the correlation between oxidation rate and porous structure of nanocrystal film. We found that Si–O bond density increases with covering the surface of the nanocrystallites during the diffusion of oxygen-related molecules through the void spaces in the porous structure. The surface oxidation of each nanocrystallite is not homogeneous; after the coverage of easy-to-oxidize sites, oxidation continues to gradually progress at the post-coverage stage. We point out that the oxidation process at coverage and post-coverage stages result in different photoluminescence (PL) wavelengths. Adsorption of the water molecule before oxidation also affects the PL wavelength. Defect PL centers which have light emission around 600 and 400 nm are generated during the coverage and post-coverage oxidation processes, respectively.

Thermal stability of CdZnO∕ZnO multi-quantum-wells

The thermal stability of CdZnO∕ZnO multi-quantum-well (MQW) structures was studied using rapid thermal annealing in nitrogen from 300to750°C. Photoluminescence (PL) emission from the MQWs was studied while varying the annealing temperature and time. For 15min annealings, the PL center wavelength showed a 7nm reduction for temperatures up to 650°C. Above 650°C, the wavelength changed rapidly, with a 50nm reduction at 750°C. Annealing at 700°C for up to 20min produced a systematic reduction in PL wavelength up to 39nm. The data suggest that CdZnO∕ZnO MQWs are relatively stable for nitrogen annealing below 650°C for times up to 15min.

Modeling the influence of the porosity of laser-ablated silicon films on their photoluminescence properties

Nanostructured porous Si-based films produced by pulsed laser ablation (PLA) from a silicon target in residual helium gas can exhibit both size-dependent (1.6–3.2 eV) and fixed photoluminescent (PL) bands (1.6 and 2.2 eV) with their relative contributions depending on the film porosity. We study the influence of prolonged oxidation in ambient air on properties of the fixed PL bands, associated with oxidation phenomena, and their correlation with structural properties of the films. In addition, we propose a model describing the appearance of surface radiation states for oxidized films of various porosities. Our experiments and numerical simulations led to a conclusion that the 1.6 eV PL is due to a mechanism involving a recombination through the interfacial layer between Si core and an upper oxide of nanocrystals. This mechanism gives the optimal porosity of 73% for the most efficient production of 1.6 eV PL centers that is in excellent agreement with our experimental results.

Spectroscopic study of milled MnF2 nanoparticles. Size-and-strain-induced photoluminescence enhancement

This work presents a correlated structural and spectroscopic study on ball-milledMnF2. The aims are to produce impurity-lean particles through particle-size reduction leading toroom-temperature photoluminescence (PL) and to modify the electronic states of theemitting centres. Despite non-radiative centres being still present, the PL quenchingtemperature was increased nearly 80 K, from 120 to 200 K, following this method. MilledMnF2 has particle sizes down to several nanometres, and structural changes from the initialα-TiO2 structureto the α-PbO2 phase. Milling favours the presence of adsorbed water on the nanoparticlesurface. Time-resolved spectroscopy indicates that the nanoparticle PL consistsof a significantly inhomogeneous broadened band with respect to the initialMnF2 PL. The temperature dependence of the lifetime measured at different wavelengths of theemission spectrum indicates the presence of several PL centres, the population of which iscontrolled by exciton migration and trapping. The widespread occurrence of emittingcentres is explained in terms of milling-induced strains, the coexistence of two differentstructural phases, and the presence of adsorbed water molecules.

Optical properties of Zr5Ti7O24:Eu3+ prepared by the sol–gel technique

Thermal processes can lead to the creation of photoluminescence centers in sol–gel derived Zr5Ti7O24:Eu3+ (ZT:Eu3+) systems. Photosensitivity starts with heat treatments at 700 °C in air. The photoluminescence excitation occurs in the 225–550 nm range. The emission spectra covers the 550–700 nm range and consist of several sharp lines associated with transitions between the stark components of the excited states of the 4f 6 configuration to the 7 F j states of the Eu3+ ion. The crystalline structure of the compound was obtained by X-ray diffraction techniques and is associated with a space group Pbcn. The thermal treatments in the 700–960 °C range have corresponding effects on both, the crystalline parameters and the optical properties of the europium ions. Time-resolved experiments were performed and the results are presented and discussed in order to get a better understanding of the effects of the thermal treatments on the Eu3+ transition lifetimes.

Ab Initio atomic simulations of antisite pair recovery in cubic silicon carbide

The thermal stability of an antisite pair in cubic silicon carbide (3C-SiC) is studied using ab initio molecular dynamics within the framework of density functional theory. The lifetime of the antisite pair configuration is calculated for temperatures between 1800 and 2250K, and the effective activation energy for antisite pair recombination is determined to be 2.52eV. The recombination energy path and static energy barrier are also calculated using the nudged elastic band method along with the dimer method to accurately locate the transition states. The consistency of the results suggests that the antisite pair cannot be correlated with the DI photoluminescence center, as proposed previously by theoretical interpretations. An extended exchange mechanism is found for the antisite pair recombination.

Photoluminescence of SiO2 layers prepared on β-SiC films and an analysis of their elemental composition

The spectra of pulsed time-resolved photoluminescence of oxidized films prepared through electrochemical oxidation on SiC/Si films are investigated. The elemental composition of the films is analyzed using Rutherford backscattering and nuclear reactions. It is revealed that a certain part of the carbon atoms remain in the oxide. The specific features of the parameters of the spectral bands and their kinetics with time are explained by the presence of carbon uniformly distributed throughout the oxide. The structure of oxides at different oxidation times is examined with the use of electron microscopy. Conclusions are drawn regarding possible photoluminescence centers.

Electric current induced anti-traps for indirect excitons

Abstract In this work we study, both experimentally and theoretically, in-plane localized ring patterns of photoluminescence (PL) from indirect excitons in GaAs/AlGaAs coupled quantum wells (QWs). These spatially pinned PL centers (PLCs) are attributed to transverse current filaments which cross the structure and inject hot electrons in the QWs. We model the experimental data with a set of drift–diffusion and Poisson equations, a thermalization equation and a quantum mass action law. The quantum statistical corrections are explicitly included in the description.

Two characteristic photoluminescence states and their metastability in hydrogenated amorphous silicon and its alloys

Photoluminescence (PL) lifetime distributions, G(τ), in hydrogenated amorphous silicon (a-Si:H) and its alloys are studied by frequency resolved spectroscopy. Two lifetime components, which are characterized with respective specific peak lifetimes of about 1ms and 10μs, are commonly observed in G(τ) throughout the a-Si:H based materials irrespective of their difference in a localized tail-state distribution. The ratio of magnitudes of the two lifetime components greatly changes in the low temperature range where the steady-state PL intensity is almost constant. This is called lifetime conversion and is a kind of metastability characterizing disordered materials. However, a system-dependent difference is observed in its temperature dependence. By careful examination of the temperature variation in G(τ), we have confirmed that the PL corresponding to those lifetime components is caused by a certain localized PL center that will be localized at some special structural unit around the Si–H bond. Another measurement of the modulated IR absorption that can detect structural instability in the vicinity of the Si–H bond is used for examining the self-trapping nature of the localized PL center. Based on these results, it is inferred that the PL center takes on two different states in accordance with the two lifetime components. The PL state associated with the 1ms lifetime component exhibits a much stronger self-trapping nature as compared to the other state associated with the 10μs lifetime component. The lifetime conversion is a change between these two PL states with different degrees of self-trapping. Those characteristic features of the PL in a-Si:H based materials are discussed in relation to the distinctive nature of the disordered network structure, i.e., the local strain.

The nature of emission centers in CdS nanocrystals

Photoluminescence (PL) of CdS nanocrystals (NCs) grown in different polymeric matrices is investigated. It is shown that chemical nature of the matrix does not affect the nature of PL centers in NCs. PL centers of NCs are identical to the emission centers in CdS single crystals and are related to their intrinsic defects.

Antisite defects in a chemical compound crystal caused by ion irradiation

We have studied the ion-induced defects in a chemical compound semiconductor (3C–SiC) using an empirical molecular dynamics calculation. The produced defects were analysed by the Pixel mapping method, focusing on antisite defects (replacement). For amorphization, interstitial atoms are more important than antisites. Unless ion dose is high, with increasing temperature and impact energy, antisites appeared in large numbers. This result is consistent with an experimental and theoretical model for the photoluminescence center DI that survives even after 2000K annealing and is supposed to be an antisite pair. Moreover, specific (311) antisite planes and (311) interstitial ones could be produced, as if the (311)Si and (311)C double layer shifted up/down.

Annealing of multivacancy defects in4H−SiC

The annealing behavior of defects observed in electron paramagnetic resonance (EPR) and photoluminescence (PL) is discussed. We consider the divacancy (the P6/P7 EPR centers) and a previously unreported EPR center that we suggest is a ${V}_{\mathrm{C}}\text{\ensuremath{-}}{V}_{\mathrm{Si}}\text{\ensuremath{-}}{V}_{\mathrm{C}}$ trivacancy and their relationship with each other and with the UD1--3 series of PL lines near $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We observe that the divacancy and the UD2 PL lines annealing behavior is strongly correlated further establishing the relationship between the EPR and PL centers. We present a detailed discussion of this center and its $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ hyperfine spectra which supports our assignment as a trivacancy. The intensity of this center increases with annealing temperature as the divacancy decreases and there is a sample-to-sample correspondence between the overall intensities of the two centers. However, the details of their annealing suggest a more complex relationship than a simple one-to-one transformation. In addition, while the UD1 PL increases with annealing temperature, sample-to-sample variations indicate it is not related to this EPR center.