Biexponential Decay Behavior Research Articles

In-situ synthesis of samarium activated MgO–LaAlO3 nanocomposite for enhanced and prolonged phosphorescence

A series of MgO–La1-xAlO3:xSm (x = 0–5 mol.%) nanocomposite phosphors have been synthesized in-situ using Pechini sol-gel method. The rhombohedral and face-centered crystal structures of LaAlO3 and MgO, respectively, were corroborated utilizing X-Ray Diffractograms and their Rietveld refinement. The optical band gaps of these nanocomposites have been observed in the range of 5.20–5.63 eV. Upon excitation by n-UV 405 nm radiation, the emission characteristics of these phosphors spanning 450–750 nm have been thoroughly investigated. The most notable emission occurred at 599 nm in the orange region, confirmed by CIE-1931 chromaticity coordinates. The MgO–La1-xAlO3:xSm (x = 2 mol.%) is found to be the optimum concentration exhibiting maximum luminescence with a high color purity of 90 %. Insights from Riesfeld's theory pointed towards d-q multipolar interactions as the underlying mechanism behind concentration quenching observed beyond this optimum concentration. Bi-exponential decay behavior was observed via Time-Resolved Photoluminescence (TRPL) technique, with an Internal Quantum Efficiency of 63 % calculated utilizing Auzel's formalism. The study extensively explored the impact of MgO addition on the crystallite size, band-gap, luminescence, and life-time properties, attributing the enhanced and prolonged luminescence behavior to the MgO–La1-xAlO3:xSm (x = 2 mol.%) nanocomposite. Assessment of additional parameters such as CCT (Correlated Color Temperature) and energy transfer efficiencies further reinforced the suitability of these orange emissive phosphors for potential applications in photonics and forensics, underlining their promising prospects in various technological domains.

Laser Ablation of Silicon Nanoparticles and Their Use in Charge-Coupled Devices for UV Light Sensing via Wavelength-Shifting Properties.

This study explores the controlled laser ablation and corresponding properties of silicon nanoparticles (Si NP) with potential applications in ultraviolet (UV) light sensing. The size distribution of Si NPs was manipulated by adjusting the laser scanning speed during laser ablation of a silicon target in a styrene solution. Characterization techniques, including transmission electron microscopy, Raman spectroscopy, and photoluminescence analysis, were employed to investigate the Si NP structural and photophysical properties. Si NP produced at a laser scanning speed of 3000 mm/s exhibited an average diameter of ~4 nm, polydispersity index of 0.811, and a hypsochromic shift in the Raman spectrum peak position. Under photoexcitation at 365 nm, these Si NPs emitted apparent white light, demonstrating their potential for optoelectronic applications. Photoluminescence analysis revealed biexponential decay behavior, suggesting multiple radiative recombination pathways within the nanoscale structure. Furthermore, a thin film containing Si NP was utilized as a passive filter for a 2nd generation CCD detector, expanding the functionality of the non-UV-sensitive detectors in optics, spectrometry, and sensor technologies.

Eu3+ incorporated Bi4MgO4(PO4)2: Derivation of the novel nanophosphor by solution combustion and investigation in to crystallographic and photometric characteristics

A bright red color emanating novel nanophosphor series is augmented through urea assisted solution combustion synthesis (SCS) by incorporating trivalent europium ions into Bi4MgO4(PO4)2 host lattice. The energy dispersive spectroscopy (EDS) and elemental mapping mark the uniform presence of Bi, Eu, Mg, P and O elements in the prepared samples. The Eu3+ doped Bi4MgO4(PO4)2 nanophosphor has orthorhombic crystal structure with Pbca(61) space group and contains [Bi2O2]2+ planes and [(PO4)2MgBi2O2]2- slabs and is efficiently developed and thoroughly investigated for insights into the remarkable structural and opto-electronic properties. Rietveld refinement is employed to obtain crystallographic parameters for optimal luminescent nano-composition and photoluminescent spectral evaluation disclosed the optoelectronic behavior of the Bi4(1-x)Eu4xMgO4(PO4)2 (x = 0.01–0.08) nanophosphor series. The Bi3.8Eu0.2MgO4(PO4)2 nanophosphor is confirmed for phase purity with respect to the parent phosphate-based host through powder X-ray diffraction patterns. Kubelka- Munk function estimated 3.873 eV as the band-gap for Bi4MgO4(PO4)2 pure host. The transmission (TEM) and scanning electron microscopy (SEM) establish the nano-ranged conduct of particles as calculated from the Scherrer equation. The trivalent europium ions acting as dopant exhibit bi-exponential decay behavior whereas the Auzel's model makes way to achieve the value of radiative lifetime (3.395 ms) for 5D0 quantum mechanical state. The PL as well as decay curves are inspected to gain the quantum efficiencies and non-radiative rates for Eu3+ doped Bi4MgO4(PO4)2 nanophosphors. The CIE coordinates (x = 0.5964, y = 0.3905), quantum efficiency (66%) and photoluminescent exhibition mark Bi3.8Eu0.2MgO4(PO4)2 as a potential nanophosphor for solid state lighting (SSLs) and imaging applications.

Sm3+ doped Bi4MgO4(PO4)2:crystal and optoelectronic investigation of the solution combustion derived bright orange emanating novel nanophosphor for SSLs

Photoluminescent investigation and Rietveld refinement has been performed on trivalent samarium fused Bi4MgO4(PO4)2 -a novel nanophosphor series. The series from 0.5 mol% to 6 mol% dopant concentrations has been developed by means of a time efficacious path of solution combustion synthesis (SCS) fueled by urea. The X-ray powder diffraction patterns for complete Bi4(1-x)Sm4xMgO4(PO4)2 (x = 0.005–0.06) nanophosphor series being in accordance with that of pure host comply of phase purity. The arrangement in space group Pbca(61) and crystallization in orthorhombic cell of Sm3+ doped Bi4MgO4(PO4)2 is recognized by Rietveld refinement. The bivalent Mg ions as a part of [(PO4)2MgBi2O2]2- slabs are present alongside [Bi2O2]2+ planes in the unit cell. In host Bi4MgO4(PO4)2, the diffuse reflectance spectrum led calculations give an optical band-gap of 3.87 eV. The PL emission spectra mark 2 mol% as the optimal fluorescent composition and the emission intensity declines for further compositions. It is credited to concentration quenching phenomena due to dipole-dipole energy exchanges as observed by Dexter's theoretical model. The decay curves for Sm3+ doped phosphate-based nanophosphor bear bi-exponential decay behavior and hence, the average PL lifetime values derived are used in the Auzel's model to obtain the magnitude of radiative lifetime. The transmission electron microscopy operated at high -resolution (HRTEM) and field emission scanning electron microscopy (FE-SEM) inform about the polycrystalline and morphological behavior of the synthesized nanophosphor. The elemental purity of the sample is confirmed by the energy dispersive spectroscopy (EDS). The PL as well as decay curves are inspected to gain the quantum efficiencies and non-radiative rates for Sm3+ doped Bi4MgO4(PO4)2 nanophosphors. The analysis related to different parameters as well as chromaticity coordinates (x = 0.517, y = 0.480) and quantum efficiency (94%) espouse Bi3.92Sm0.08MgO4(PO4)2 for relevance in solid state lighting (SSLs) expedients.

Photoluminescence and time resolved photoluminescence properties in as grown ZnO thin films prepared by DC reactive sputtering for optoelectronic devices

Results of steady-state and time-resolved Photoluminescence (PL) measurements performed on ZnO thin films, deposited on (001) p-doped silicon substrate, by DC reactive sputtering technique for different growth times are presented. In all cases, two emission bands are observed. One is an exciton emission band and the second an intense and broad visible emission band.As the growth time increases, the intensity of the exciton emission decreases while that of the visible emission increases. Time-resolved PL (TRPL) measurements at the UV emission peak show a bi-exponential decay behavior. As the growth time increases, the fast and slow decay lifetimes increase together with a decrease of the amplitude ratio (AR) of the slow decay component to the rapid one due to the reduction of the non-radiative pathways when the oxygen deposition time is short. Moreover, slower rise time of the TRPL signal is observed when the growth time increases, proving the increase of the trap states density in the ZnO band gap. These results suggest that the amount of oxygen deposited as reactive gas in DC reactive sputtering technique must be optimized to improve the performance of UV light-emitting applications.

Ultra-high-b radial diffusion-weighted imaging (UHb-rDWI) of human cervical spinal cord.

Injury in the cervical spinal cord (CSC) can lead to varying degrees of neurologic deficit and persistent disability. Diffusion tensor imaging (DTI) is a promising method to evaluate white matter integrity and pathology. However, the conventional DTI results are limited with respect to the specific details of neuropathology and microstructural architecture. In this study we used ultrahigh-b radial-DWI (UHb-rDWI) with b-values ranging from 0 to ∼7500 s/mm2 and calculated decay constant (DH ) at the high b-values, which gives much deeper insight about the microscopic environment of CSC white matter. To evaluate a novel diffusion MRI, UHb-rDWI technique for imaging of the CSC. Longitudinal. Four healthy controls, each scanned twice. 3T/2D single shot diffusion-weighted stimulated echo planar imaging with reduced field of view. The signal from each pixel of b0 (b = 0) and b-value (b ≠ 0) images were fitted to a biexponential function and normalized. The signal-b curve is obtained by dividing the latter curve by the former. DH was obtained from the curve at b >4000 s/mm2 . A Monte-Carlo Simulation (MCS) was performed to investigate how DH changes upon the increased water-exchange at the CSC. The signal-b curves plotted at multiple levels of healthy CSC are almost identical on two successive scans and show a biexponential decay behavior: fast exponential decay at lower b-values and much slower decay at UHb-values. The mean values of DH were measured as (0.0607 ± 0.02531) ×10-3 and (0.0357 ± 0.02072) ×10-3 s/mm2 at the lateral funiculus and posterior column, respectively. MCS of diffusion MRI shows that the DH is elevated by increased water exchange between the intra- and extraaxonal spaces. UHb-rDWI signal-b plots of the normal CSC were highly reproducible on successive scans and their biexponential decay behavior can be used to characterize normal spinal white matter. 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:204-211.

Direct Chemical-Vapor-Deposition-Fabricated, Large-Scale Graphene Glass with High Carrier Mobility and Uniformity for Touch Panel Applications

In this work, we report the transfer-free measurement of carrier dynamics and transport of direct chemical vapor deposition (CVD) grown graphene on glass with the aid of ultrafast transient absorption microscopy (TAM) and demonstrate the use of such graphene glass for high-performance touch panel applications. The 4.5 in.-sized graphene glass was produced by an optimized CVD procedure, which can readily serve as transparent conducting electrode (TCE) without further treatment. The graphene glass exhibited an intriguing optical transmittance and electrical conductance concurrently, presenting a sheet resistance of 370-510 Ω·sq-1 at a transmittance of 82%, much improved from our previous achievements. Moreover, direct measurement of graphene carrier dynamics and transport by TAM revealed the similar biexponential decay behavior to that of CVD graphene grown on Cu, along with a carrier mobility as high as 4820 cm2·V-1·s-1. Such large-area, highly uniform, transparent conducting graphene glass was assembled to integrate resistive touch panels that demonstrated a high device performance. Briefly, this work aims to present the great feasibility of good quality graphene glass toward scalable and practical TCE applications.

Photophysics and Rotational Dynamics of a Fluoroquinolone Drug Norfloxacin in Biomimetic Reverse Micellar Nanocavities

The present work describes the study of the photophysics and dynamics of a prospective anti-cancer antibacterial drug norfloxacin (NFX) within the micro-heterogeneous environment of Aerosol OT (AOT) reverse micelle (RM). The environment sensitive fluorescence spectroscopic behavior of NFX is found to exhibit a significant red-shift with increasing water pool size (w 0 = [water]/[surfactant]) of the RM. A prominent decrease of fluorescence anisotropy of NFX with increasing w 0 demonstrates a relaxation of the motional restrictions imposed on the bound drug molecules with enhanced hydration. These results are further substantiated from our time-resolved measurements. Study of the rotational dynamics of NFX within the RM as a function of w 0 exhibits a typical biexponential decay behavior which has been adequately described on the basis of the wobbling-in-cone model. The rotational dynamical parameters are also calculated for wobbling motion of the drug within the RM as a function of w 0 .

Long-lived coherence in pentafluorobenzene as a probe of ππ(*) - πσ(*) vibronic coupling.

The dynamics of pentafluorobenzene after femtosecond laser excitation to the optically bright ππ(*) first excited electronic state have been investigated by femtosecond time-resolved time-of-flight mass spectrometry and femtosecond time-resolved photoelectron imaging spectroscopy. The observed temporal profiles exhibit a bi-exponential decay behavior with a superimposed, long-lived, large-amplitude oscillation with a frequency of νosc = 78-74 cm(-1) and a damping time of τD = 5-2 ps. On the basis of electronic structure and quantum dynamics calculations, the oscillations have been shown to arise due to vibronic coupling between the optically bright ππ(*) state and the energetically close-lying optically dark πσ(*) state. The coupling leads to a pronounced double-well character of the lowest excited adiabatic potential energy surface along several out-of-plane modes of b1 symmetry. The optical electronic excitation initiates periodic wavepacket motion along these modes. In the out-of-plane distorted molecular configuration, the excited state acquires substantial πσ(*) character, thus modulating the ionization probability. The photoelectron spectra and the anisotropy of their angular distribution confirm the periodically changing electronic character. The ionizing probe laser pulse directly maps the coupled electron-nuclear motion into the observed signal oscillations.

Photophysics and Rotational Dynamics of a Fluoroquinolone Drug Norfloxacin in Biomimetic Reverse Micellar Nanocavities

The present work describes the study of the photophysics and dynamics of a prospective anti-cancer antibacterial drug norfloxacin (NFX) within the micro-heterogeneous environment of Aerosol OT (AOT) reverse micelle (RM). The environment sensitive fluorescence spectroscopic behavior of NFX is found to exhibit a significant red-shift with increasing water pool size (w 0 = [water]/[surfactant]) of the RM. A prominent decrease of fluorescence anisotropy of NFX with increasing w 0 demonstrates a relaxation of the motional restrictions imposed on the bound drug molecules with enhanced hydration. These results are further substantiated from our time-resolved measurements. Study of the rotational dynamics of NFX within the RM as a function of w 0 exhibits a typical biexponential decay behavior which has been adequately described on the basis of the wobbling-in-cone model. The rotational dynamical parameters are also calculated for wobbling motion of the drug within the RM as a function of w 0 .

Asante Calcium Green and Asante Calcium Red--novel calcium indicators for two-photon fluorescence lifetime imaging.

For a comprehensive understanding of cellular processes and potential dysfunctions therein, an analysis of the ubiquitous intracellular second messenger calcium is of particular interest. This study examined the suitability of the novel Ca2+-sensitive fluorescent dyes Asante Calcium Red (ACR) and Asante Calcium Green (ACG) for two-photon (2P)-excited time-resolved fluorescence measurements. Both dyes displayed sufficient 2P fluorescence excitation in a range of 720–900 nm. In vitro, ACR and ACG exhibited a biexponential fluorescence decay behavior and the two decay time components in the ns-range could be attributed to the Ca2+-free and Ca2+-bound dye species. The amplitude-weighted average fluorescence decay time changed in a Ca2+-dependent way, unraveling in vitro dissociation constants K D of 114 nM and 15 nM for ACR and ACG, respectively. In the presence of bovine serum albumin, the absorption and steady-state fluorescence behavior of ACR was altered and its biexponential fluorescence decay showed about 5-times longer decay time components indicating dye-protein interactions. Since no ester derivative of ACG was commercially available, only ACR was evaluated for 2P-excited fluorescence lifetime imaging microscopy (2P-FLIM) in living cells of American cockroach salivary glands. In living cells, ACR also exhibited a biexponential fluorescence decay with clearly resolvable short (0.56 ns) and long (2.44 ns) decay time components attributable to the Ca2+-free and Ca2+-bound ACR species. From the amplitude-weighted average fluorescence decay times, an in situ K D of 180 nM was determined. Thus, quantitative [Ca2+]i recordings were realized, unraveling a reversible dopamine-induced [Ca2+]i elevation from 21 nM to 590 nM in salivary duct cells. It was concluded that ACR is a promising new Ca2+ indicator dye for 2P-FLIM recordings applicable in diverse biological systems.

Effect of surface on the optical structure and thermal properties of organically capped CdS nanoparticles

Cadmium sulfide (CdS) nanoparticles were synthesized by a novel wet chemical route with various organic thiol stabilizers. Systematic experimental studies, including X-ray diffraction (XRD), ultraviolet–visible (UV–vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), photoluminescence (PL) spectroscopy, and time-resolved photoluminescence (TRPL), have evidenced that the stability, crystallinity, and optical properties of the CdS nanoparticles are affected by the organic groups which generate significant effects in surface reconstruction. Particle size was evaluated from UV–vis spectroscopy using the effective mass approximation (EMA) method and from XRD patterns based on Scherrer׳s formula. The S–H vibrations are not detectable in the infrared (IR) spectra of any of the bound ligands, which are expected for thiols covalently bound to the surface of nanoparticles. PL studies reveal that the emission from the nanostructures is not much influenced by the surface states, indicating a good passivation of the particle׳s surface. The time-resolved measurements reveal a biexponential decay behavior. The fast decay component is attributed to the recombination of core states, while the slow decay component of PL is associated with the charge-carrier recombination process with the involvement of surface states.

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.

Microheterogeneity of Some Imidazolium Ionic Liquids As Revealed by Fluorescence Correlation Spectroscopy and Lifetime Studies

The microscopic structure and dynamics of the room temperature ionic liquids (RTILs) that are responsible for some of the peculiar properties of this class of solvents continue to intrigue the researchers and stimulate new investigations. Herein, we use the fluorescence correlation spectroscopy (FCS) technique to study the diffusion of some probe molecules in RTILs, the results of which, when combined with those obtained from fluorescence lifetime studies, provide insights into the microscopic structural details of this class of novel solvents. Experiments performed with three charged and neutral probe molecules in five carefully selected 1-alkyl-3-methylimidazolium ionic liquids reveal that unlike in conventional solvents these probes exhibit a bimodal diffusion behavior in RTILs thus indicating the presence of two distinct environments. It is found that the contribution of the slow component of the diffusion increases with increasing alkyl chain length of the cation. Not only are these results supported by the biexponential decay behavior of the fluorescence intensity of the systems, but the individual values of the lifetime components and their weight allow determination of the nature of the two major environments. In essence, the results point to the potential of the two combined techniques in unraveling some of the complex features of the ionic liquids.

Well vertically aligned ZnO nanowire arrays with an ultra-fast recovery time for UV photodetector

Well-crystallized ZnO nanowire arrays were grown on GaN/sapphire by one-step chemical vapor deposition under control of the fabrication pressure of 1000–2500 Pa and the best-aligned arrays were obtained at 1000 Pa. A photoluminescence study shows a red shift with nanowire diameter increase. Under 365-nm UV irradiation of 0.3 mW/cm2, the photoresponse study of the best ZnO arrays shows an ultra-fast tri-exponential rise with three constants of 0.148, 0.064 and 0.613 s, and a bi-exponential decay behavior with two recovery constants of 30 and 270 ms. The ZnO/GaN heterojunction barriers could be responsible for the ultra-fast tri-exponential rise and bi-exponential decay behavior.

Femtosecond excited-state dynamics of 4-nitrophenyl pyrrolidinemethanol: evidence of twisted intramolecular charge transfer and intersystem crossing involving the nitro group.

Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-(-)-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the -NO(2) torsional coordinate, which revealed the perpendicular orientation of -NO(2) in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized time-resolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm(-1) over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the -NO(2) group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the -NO(2) group torsional motion as the main excited-state relaxation coordinate.

Identification of Hydroxyl Protons, Determination of Their Exchange Dynamics, and Characterization of Hydrogen Bonding in a Microcrystallin Protein

Heteronuclear correlation experiments employing perdeuterated proteins enable the observation of all hydroxyl protons in a microcrystalline protein by MAS solid-state NMR. Dipolar-based sequences allow magnetization transfers that are >50 times faster compared to scalar-coupling-based sequences, which significantly facilitates their assignment. Hydroxyl exchange rates were measured using EXSY-type experiments. We find a biexponential decay behavior for those hydroxyl groups that are involved in side chain-side chain C-O-H...O horizontal lineC hydrogen bonds. The quantification of the distances between the hydroxyl proton and the carbon atoms in the hydrogen-bonding donor as well as acceptor group is achieved via a REDOR experiment. In combination with X-ray data and isotropic proton chemical shifts, availability of (1)H,(13)C distance information can aid in the quantitative description of the geometry of these hydrogen bonds. Similarly, correlations between backbone amide proton and carbonyl atoms are observed, which will be useful in the analysis of the registry of beta-strand arrangement in amyloid fibrils.

Spectroscopic Identification of Ternary Cm−Carbonate Surface Complexes

The influence of dissolved CO(2) on the sorption of trivalent curium (Cm) on alumina (gamma-Al(2)O(3)) and kaolinite was investigated by time resolved laser fluorescence spectroscopy (TRLFS) using the optical properties of Cm as a local luminescent probe. Measurements were performed at T < 20 K on Cm loaded gamma-Al(2)O(3) and kaolinite wet pastes prepared in the absence and presence of carbonate in order to pictorially illustrate any changes through a direct comparison of spectra from both systems. The red-shift of excitation and emission spectra, as well as the increase of fluorescence lifetimes observed in the samples with carbonate, clearly showed the influence of carbonate and was fully consistent with the formation of Cm(III) surface species involving carbonate complexes. In addition, the biexponential decay behavior of the fluorescence lifetime indicated that at least two different Cm(III)-carbonate species exist at the mineral-water interface. These results provide the first spectroscopic evidence for the formation of ternary Cm(III)-carbonate surface complexes.

Thermal stability and vibrational spectroscopy of N–O shallow donor centers in silicon

N–O-related shallow donors in nitrogen-doped Czochralski silicon have been studied by infrared spectroscopy. Quasithermal equilibrium states were established by long-term thermal annealing in the temperature range from 600to1000°C. By quantitative analysis of the 1s→2p± far-infrared electronic transitions between 230 and 250cm−1, it is found that the formation and decay characteristics of these centers do not correspond to theoretical predictions. All complexes investigated show a monotonic decrease for annealing temperatures above 600°C. In particular, the dominant NO2 complex exhibits a pronounced biexponential decay behavior. Based on the characteristic thermal fingerprint of the individual shallow donor species, associated local vibrational modes in the midinfrared were investigated. Two bands at 1070 and 860cm−1 can be assigned to NO2, the center with the highest concentration variation in the relevant temperature range between 600 and 800°C. These frequencies match favorably with recent calculations for this complex in the symmetrical O–N–O configuration.

Optical and structural properties of self-assembled ZnO QD chains by L-MBE

ZnO complex 3D nano-structures have been self-organized on Al 2O 3 (0 0 0 1) substrate by laser molecular beam epitaxy (L-MBE). It is shown by AFM morphology that the structure is composed of 1D quantum dot chains (QDCs) and larger nano-islands at the nodes of QDCs. The formation mechanism of the nano-structure is also investigated. XRD results indicate that the nano-structure is highly c-axis oriented, with the aligned in-plane oriented domains. Time-integrated photoluminescence (TIPL) of the sample shows obvious blue-shift and broadening of the near band-edge (NBE) emission at room temperature, which are related to the quantum confinement effects. Time-resolved PL (TRPL) result shows bi-exponential decay behavior of ZnO QDCs, with a fast decay time of 38.21 ps and a low decay time of 138.19 ps, respectively, which is considered to be originated from the interdot coupling made by coherent emission and reabsorption of the photons in QDCs.