Photoluminescence Excitation Results Research Articles

Novel red colour emitting Ca0.995Mg2(SO4)3:0.5Eu2+ phosphor under ultraviolet, blue, and green excitation for plant growth LEDs

In the recent few years, Eu2+ - and Mn4+ -activated phosphors are widely used as potential colour converters for indoor plant cultivation lighting application due to their marvellous luminescence characteristics as well as low cost. In this investigation, we synthesized novel red colour-emitting Ca(2-x) Mg2 (SO4 )3 :xmol% Eu2+ (x = 0-1.0mol%) phosphors via a solid-state reaction method in a reducing atmosphere. The photoluminescence (PL) excitation spectra of synthesized phosphors exhibited a broad excitation band with three excitation bands peaking at 349 nm, 494 nm, and 554 nm. Under these excitations, emission spectra exhibited a broad band in the red colour region at ~634 nm. The PL emission intensity was measured for different concentrations of Eu2+ . The maximum Eu2+ doping concentration in the Ca2 Mg2 (SO4 )3 host was observed for 0.5mol%. According to Dexter theory, it was determined that dipole-dipole interaction was responsible for the concentration quenching. The luminous red colour emission of the sample was confirmed using Commission international de l'eclairage colour coordinates. The results of PL excitation and emission spectra of the prepared phosphors were well matched with excitation and emission wavelengths of phytochrome PR . Therefore, from the entire investigation and obtained results it was concluded that the synthesized Ca0.995 Mg2 (SO4 )3 :0.5mol%Eu2+ phosphor has huge potential for plant cultivation application.

Room temperature operation and low dark current of In0.15Ga0.85As/InAs/In0.15Ga0.85As dot-in-well short-wave infrared photodetector: Experimental and theoretical correlation

Photoluminescence (PL) and Photoluminescence excitation (PLE) measurements have been carried out to investigate the carrier recombination pathways in a GaAs capped InAs quantum dot (QD) and an InAs dot in InGaAs well (DWELL) structure. The presence of an InGaAs quantum well (QW) results in an anomalous behavior in the temperature dependent PL spectra. The carrier trapping in the InGaAs well at low temperature and the redistribution of these carriers as the temperature rises is well explored through the temperature dependent PL spectra. The existence of additional energy states due to the incorporation of an InGaAs QW has been observed through the PLE result and explained schematically. Cross-sectional transmission electron microscopy provides a direct evidence of the formation of hybrid thick QW structure due to the intermixing of InGaAs well and the InAs wetting layer (WL). The strain relaxation in the DWELL structure is substantiated from the simulated strain profile as well as the high-resolution X-ray diffraction (HRXRD) result. Moreover, the DWELL p-i-p quantum dot infrared detector (QDIP) exhibits a room temperature spectral response in the short-wavelength infrared (SWIR) region. A two-order reduced dark current and one order improved detectivity is obtained for the DWELL QDIP compared to the conventional GaAs capped InAs QDIP. • Infrared photodetector with InAs quantum dot in InGaAs well as the active layer. • The carrier recombination process along with temperature is discussed. • PL and PLE results show the dominance of the InGaAs wells at low temperature. • The device showed spectral response in short-wave IR range till room temperature. • The dark current obtained for the QDIP is also very low.

White light emission in alkali metal ion co-doped single host lattice phosphor Sr3B2O6:Ce3+, Eu2+, A+ [A=Li, Na and K

White light emitting alkali metal ion co-doped single host lattice phosphors Sr2.955B2O6:0.02Ce3+, 0.005Eu2+, 0.02A+ [A=Li, Na and K] were synthesized by high temperature solid state reaction. The synthesized phosphors were characterized by powder X-ray diffraction and Fourier transform infrared spectroscopic techniques. The optical properties were studied using diffuse reflectance UV–vis reflectance spectroscopy and photoluminescence (PL) spectroscopy. The role of Ce3+ as a sensitizer of Eu2+ was found from PL excitation results. The PL white light emission depends on the nature of co-doped alkali metal ion charge compensator.

Controlling the size and optical properties of ZnO nanoparticles by capping with SiO2

The size and shape of the ZnO nanoparticles synthesized through sol–gel method were controlled by capping with SiO2. X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) and High Resolution Transmission Electron Microscope (HR-TEM) results demonstrated that the particle growth of the ZnO nanoparticles has been restricted to 5nm with SiO2 capping. As a result, the absorption spectra of ZnO nanoparticles capped with SiO2 got blue shifted (toward lower wavelength side) due to strong quantum confinement effects. BET (Brunauer–Emmet–Teller) surface area pore size analyzer results showed that surface area of samples increased monotonously with increase of SiO2 concentration. It was observed that the absorption spectra of ZnO capped with SiO2 broadened with increase of SiO2 concentration. Absorption and photoluminescence excitation results (PLE) confirmed that this broadening is due to the absorption of non-bridging oxygen hole centers (NBOHC) of SiO2. These results also indicated that ZnO nanoparticles capped with SiO2 are insensitive to Raman scattering. Maximum UV emission intensity was achieved with 353nm excitation wavelength compared to 320nm in ZnO as well as in SiO2 capped ZnO nanoparticles. Furthermore, there is an enhancement in the intensities of emission peaks related to oxygen vacancies and interstitials with SiO2 capping. The enhancement in the UV intensity is attributed to the surface passivation of ZnO nanoparticles and excitation processes in SiO2.

Synchrotron-based photoluminescence excitation spectroscopy applied to investigate the valence band splittings in AlN and Al0.94Ga0.06N

We demonstrate that synchrotron-based photoluminescence excitation (PLE) spectroscopy is a versatile tool for determining valence band splittings of AlN and high aluminum content AlGaN. PLE results are independently confirmed by synchrotron-based spectroscopic ellipsometry. The splittings between the ordinary and the extraordinary absorption edges are found to be −240 meV and −170 meV for AlN and Al0.94Ga0.06N, respectively. These values differ from the crystal field energy due to residual strain.

Highly efficient orange emission in ZnO:Se nanorods

The origin of the visible emissions in ZnO nanostructures is a long-standing issue. In this work, a strong orange emission around 2.1 eV in ZnO:Se nanorods is reported. Temperature-dependent photoluminescence (PL) and PL decay results indicate that radiative recombination dominates the overall decay process, leading to highly efficient orange emission at room temperature. It is proposed that Se acts as isoelectronic centers and no evidence of ZnO1−xSex alloy formation is found. Based on the PL excitation results, we propose a Se-induced impurity band model to describe the photophysics of the orange emission.

Carrier-mediated 155 μm photoluminescence from single Er^3+ center in SnO_2 nanocrystals

We report the successful incorporation of Er3+ ions into SnO2 nanocrystals via a solvothermal method, with a resulting Er3+ concentration of the order of 1019 cm−3. Upon excitation above the SnO2 bandgap at 300 nm, intense and well-resolved Er3+-related photoluminescence (PL) at 1.55 μm was observed at room temperature. The results of PL excitation and ultraviolet/visible diffuse reflectance spectra indicate that the excitation is a carrier-mediated process with an energy transfer from the SnO2 host to Er3+. For the above-gap excitation, only a single type of Er3+ luminescence center located at a centrosymmetric site was identified. The near-infrared luminescence dynamics and the weak PL thermal quenching of Er3+ were also revealed.

Investigation of Luminescence from Dy3 + in AlN

The preparation of powder samples of aluminum nitride activated by dysprosium, , was achieved for the first time using a low-temperature, solution-based approach. Aqueous solutions of aluminum and dysprosium nitrates was first converted to hydroxides and then to ammonium–metal hexafluorides (. Finally, this complex ammonium salt was converted to finely divided powders of via a pyrolysis reaction in a high-purity ammonia flow inside a tubular quartz furnace at . The phase purity and crystallinity of the material were determined using X-ray diffraction and energy dispersive spectroscopy. The intraconfigurational transitions of were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) measurements. The excited states of in this large-gap nitride were identified from a careful analysis of PL and PLE results. It was also observed from the PLE measurements that the ions in this host are excited through energy transfer from both the excited state of the host lattice and defects. Measurements using site-selective spectroscopy suggest multiple sites for ions in AlN. A theoretical model describing the excitation process and saturation of excited states of at higher intensity of the exciting radiation is also proposed.

Optical and structural properties of InGaN/GaN multiple quantum well structure grown by metalorganic chemical vapor deposition

InGaN/GaN multiple quantum well light emitting diode structures have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated, in this study, by high-resolution X-ray diffraction, high-resolution transmission electron microscopy, photoluminescence, and photoluminescence excitation. HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. These indicate the high quality of layer interface structures of this sample. Excitation power-dependent photoluminescence shows that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties of this sample. Temperature-dependent photoluminescence of this sample has also been studied. The peak position of the PL exhibits a monotonic red-shift and the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the photoluminescence excitation results, a large energy difference, so-called Stokes shift, between the band-edge absorption and emission was observed.

Photoluminescence excitation spectroscopy of InAs/GaAs quantum dots in high magnetic field

Results of photoluminescence excitation (PLE) measurements of quantum dots (QDs) in high magnetic field are presented in this work. A structure with a single layer of InAs/GaAs QDs of a relatively small inter-level spacing and inhomogeneous broadening has been investigated at T=4.2 K in magnetic field up to 28 T . It has been found that the PLE spectrum at zero magnetic field, with detection energy set at the ground state (s-shell) of the QDs, is dominated by two features. These have been attributed to absorption in the QDs p- and d-shell, respectively. The peaks split in magnetic field following the pattern of the Fock–Darwin potential and features, attributed to the lowest Landau level in the QDs can be observed in high magnetic field.

Photoluminescence excitation spectroscopy of InGaN epilayers

Photoluminescence (PL) has been reported from InGaN-based heterostructures, including thick epilayers on GaN, InGaN/GaN quantum wells and InGaN/GaN quantum boxes, with peak energies ranging from 3.44 to 1.31 eV at low temperature. The corresponding absorption spectra are not always easy to obtain, but photoluminescence excitation (PLE) spectroscopy provides an efficient means of obtaining comparable information. We describe here a comprehensive investigation of PLE spectra from a wide range of InGaN samples. Variation of the measured bandgap energy with the detection energy for individual samples suggests that the InGaN emission spectrum is inhomogeneously broadened. The PLE spectrum obtained at the peak emission energy of a particular sample is equivalent to the absorption spectrum of that sample. The data range of the band edge measurements is extended to lower energies by the PLE results. In general, the PLE data confirm the existence of a linear relationship between the optical bandgap and the emission energy.

Characterization of CdSe and CdSe/CdS core/shell nanoclusters synthesized in aqueous solution

CdSe nanoclusters overcoated with CdS shell were prepared with macapoacetic acid as stabilizer. The optical properties of CdSe nanoclusters and the influence of CdS shell on the electronic structures of CdSe cores were studied by optical absorption, photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Based on PL and PLE results and the theoretical calculation on fine structure of bandedge exciton, a model of formation of excimer within the small clusters was proposed to explain the large Stokes shift of luminescence from absorption edge observed in PL results.

Optical spectroscopy of titanium-doped CdZnTe

The energy level diagram of titanium in CdZnTe grown by the vertical Bridgman method has been investigated by using a variety of optical spectroscopy techniques: photoluminescence (PL), PL excitation (PLE) and deep level optical spectroscopy (DLOS). By PL spectroscopy, a broad emission band around 0.8 eV, with a double-peaked shape, is reported for the first time. PL and PLE results, combined with the DLOS data of the eV titanium-related electron trap lead us to (i) attribute the 0.8 eV emission band to the internal transition of the single donor and (ii) demonstrate that the state is split by the combined action of spin-orbit coupling and a strong dynamic Jahn-Teller effect. The double-peaked shape of the 0.8 eV emission band is interpreted as a combination of two recombination processes involving . A configuration coordinate diagram of ions is proposed. Finally the F centre has been identified in CdZnTe:Ti by PL; the threshold ionization energy of this centre is determined by PLE at 1.441 eV.

Photoluminescence spectroscopy of erbium implanted gallium nitride

Results of a photoluminescence (PL) and photoluminescence excitation (PLE) study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 μm Er3+ PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ∼10% for above gap excitation (λex=325 nm) and ∼50% for below gap excitation (λex=488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er3+ excitation mechanisms in the GaN host. The PLE results show that Er3+ can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er3+ ions can be excited either through carrier-mediated processes involving defects in the host or through resonant pumping into Er3+ 4f energy levels. With respect to these two excitation schemes, distinct Er3+ PL properties were observed for resonant and off-resonant Er3+ excitation indicating the presence of different subsets of Er3+ ions in GaN.

High Temperature Photoluminescence and Photoluminescence Excitation Spectroscopy of Er Doped Gallium Nitride

AbstractWe report results of high temperature photoluminescence and photoluminescence excitation studies of Er doped GaN. Er was doped into GaN either by ion-implantation or during growth by metalorganic molecular beam epitaxy (MOMBE). Using above gap excitation (λex=325nm), GaN:Er showed strong 1.54 μm Er3+ luminescence up to 550K which indicates the potential of this material for high temperature opto-electronic applications. In addition, we performed timeresolved photoluminescence excitation (PLE) measurements over the wavelength range 420 to 680 nm using a Nd:YAG pumped Optical Parametric Oscillator (OPO). Similar to our previous PLE results of Er doped AIN, we observed that Er3+ ions in GaN can be excited either through resonant pumping of Er3+ energy levels or through energy-transfer processes involving defect states. The relative contribution of direct and indirect Er3+ excitation, however, seems to depend on the material preparation method.

Optical and transport studies of highly acceptor doped GaAs/AlGaAs quantum wells

We present a theoretical and experimental study of the properties of highly acceptor-doped quantum wells (QWs), with doping levels varying from 9 × 10 9 to 6 × 10 12 cm −2. From variable temperature Hall measurements it is found that the temperature dependence of the mobility differs remarkably from p-type bulk GaAs, as well as n-type QWs. For the moderately doped ( N A < 3 × 10 11cm −2) sample, the mobility is almost constant with temperature up to ∼ 100K, while the 2D hopping conduction dominates in the same temperature range for the degenerate samples. The activation mechanism involved in the investigated structure can be understood in terms of the change of the subband structure due to the central doping. Steady-state photoluminescence (PL) and PL excitation results are compared with theoretical predictions involving exchange and correlation effects for the electron-hole system and their interaction with acceptor ions. We finally report on the survival of the excitons well above the degenerate limit, due to inefficient screening in the 2D case.

Photoluminescence and Photoluminescence Excitation Spectra of AlAs/GaAs Disordered Superlattices with Various Disordered Lengths

A disordered superlattice (d-SL) is a superlattice with artificial disordering. Temperature dependence of photoluminescence (PL) and PL excitation (PLE) spectra are measured for AlAs/GaAs short-period d-SLs with various disordered lengths and are compared with those of ordered superlattices (o-SLs). X-ray satellite peaks are split and PL peak energies are red-shifted with increase of the unit length of disorder. From the PLE results, radiative recombination processes in the AlAs/GaAs d-SLs are inferred to be type-II. The d-SL with a longer unit length of disorder shows more complicated variations in the PL spectrum with changing temperature. The variations are intensified with increase of the disorder lengths. These variations indicate that the localized strength which corresponds to the disorder length is different among the states induced by disordering. The dominant radiative recombination states at higher temperature would correspond to stronger localized states in the d-SLs.

Optical studies of acceptor centre doped [formula omitted] quantum wells

We present a theoretical and experimental study of the optical properties of acceptor centre doped quantum wells. We have performed theoretical calculations for the dependence of the band structure with doping level. Steady state photoluminescence and photoluminescence excitation results are compared with theoretical calculations involving exchange and correlation effects for the electron-hole system and the interaction between charge carriers and acceptor ions. We have studied the intensity, energy peak position, and broadening effects for excitons at doping level between 10 8 and 10 13 cm −2. Theoretical calculations that only consider band filling effects are not sufficient to describe the effect on the band structure due to the doping. A much better agreement is achieved when exchange and correlation effects for the electron-hole system are taken into account. Excitons can still be detected at high hole concentrations, above the degenerated limit. They survive due to the inefficiency of screening in the two-dimensional system.

High exciton binding energies in GaAs/GaAlAs quantum wells

Binding energies of excitons in GaAs-Ga 1− x Al x As quantum wells are calculated including valence band mixing and also Coulomb coupling between excitons belonging to different subbands, nonparabolicity of the bulk conduction band, and the dielectric mismatch between well and barrier materials. All these effects are of a comparable importance, tend to increase the binding energy, and taken together result in very high binding energies, particularly in narrow GaAs/AlAs quantum wells. A comparison with photoluminescence excitation (PLE) and magneto-optical (MO) experiments shows better agreement with the PLE results, and suggests that binding energies deduced from MO experiments have a larger intrinsic error.

Free carrier effects on the optical properties of InGaAsInP quantum wells

In this paper we present photoluminescence (PL) and photoluminescence excitation (PLE) spectra as a function of electron sheet carrier density (n s) in a Schottky gated 110 Å wide InGaAsInP quantum well. Major free carrier effects are observed on both the PL and PLE spectra. The PL spectra show a high energy cut-off at the electron Fermi energy for n s = 1.4 × 10 11 cm −2. As n s tends to zero the PL linewidth decreases from the high energy side, as expected for free carrier broadening. At the same time the n=1 transitions observed in PLE show a marked increase in oscillator strength and narrowing as n s tends to zero. The PLE results are discussed in terms of phase space filling and the variation of many body effects as a function of n s. The many body nature of the PLE transitions at the Fermi energy, for finite n s, is further demonstrated by the anomalous temperature dependence of the n=1 heavy hole transition.