Systematic Photoluminescence Study Research Articles

X-ray photoelectron spectroscopy and tunable photoluminescence study of gold nanoparticles embedded in PVA films.

This article reports the systematic photoluminescence study of the various contents of gold nanocomposites in polyvinyl alcohol (PVA) films. The variations in the gold content in PVA film were 0.2, 0.5, 1.0, and 1.5wt%. All the samples were excited at two selected wavelengths; those are at 400 nm and 532 nm. On exciting the gold-PVA nanocomposite films at 400 nm the photoluminescence was observed in the region of 430-500 nm in comparison to pure PVA films that show an emission at 400 nm. However, on exciting the gold-PVA nanocomposites at 532 nm, the emission was observed at 560-650 nm with a long tail till 700 nm that is unlike the pure PVA films that do not show any emission peak in this region. This suggests that emission between 430 and 500 nm regions is due to the coordination of PVA with gold nanoparticles because PVA has an emission at 400 nm. However, the emission peak between 560 and 650 nm is entirely due to the gold nanocomposite particle. The peak also shows a smaller red-shift that is usually with the increasing nanoparticles size with the increasing content in the PVA films. The formation of gold nanoparticles was justified by X-ray diffraction (XRD) analysis which is further supported by X-ray photoelectron spectroscopy (XPS) analysis.

Photoluminescence from Be‐Doped GaN Grown by Metal‐Organic Chemical Vapor Deposition

A systematic photoluminescence study of Be‐doped GaN grown by metal‐organic chemical vapor deposition is presented. All Be‐doped samples show the ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV and the yellow luminescence (YLBe) band with a maximum at ≈2.15 eV in GaN:Be having various concentrations of Be. The UVLBe band is attributed to the shallow state of the BeGa acceptor with a delocalized hole. The YLBe band is caused by a Be‐related defect, possibly the polaronic state of the BeGa acceptor with the charge transition level at 0.3 eV above the valence band. This broad band exhibits unusual properties. In particular, it always shows two steps in its thermal quenching. The second step at T ≈ 200 K is attributed to the emission of holes from the 0.3 eV level to the conduction band. The origin of the first step remains unexplained.

Insights into structural and spectroscopic characterization of Sm3+ doped orange rich red emitting CsMgPO4 phosphors

In the present study, Sm3+ activated inorganic orthophosphate CsMgPO4 (CSMP) phosphors were prepared by adopting a solid-state reaction method. The structural phase purity and morphological features were studied by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. The molecular structure and vibrational modes were substantiated with the Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy characterization. The optical bandgap of the host and Sm3+ doped phosphors was deduced from the diffused reflectance (DR) spectra with a typical value of 5.72 eV and a small variation is observed with increasing concentrations. A systematic study of photoluminescence (PL) properties of Sm3+ doped CSMP phosphors was carried out. From the room temperature excitation and emission spectra, it is found that the phosphor emits in the orange rich red light under the suitable excitation of 402 nm in the UV region and concentration quenching occurs at x = 0.02 doping level. The emission peaks observed at around 562, 598 and 644 nm confirm the characteristic Sm3+ 4f-4f transitions. The temperature-dependent photoluminescence (TD-PL) of the x = 0.02 (optimum doping) is recorded from 30 to 210 °C, showing good thermal stability even at 150 °C. The thermal quenching mechanisms are discussed based on the configuration coordinate model of excitation and emission. The prepared phosphors are found to exhibit near thermal stability compared to the commercially available red phosphors. PL decay time and quantum efficiency were measured. The colour coordinates are found to lie in the orangish-red region of the colour space. Thus the prepared phosphors CSMP:x Sm3+ can be useful as a red component in designing UV excitable chip-based phosphor-converted white LED applications.

Prevention of surface recombination by electrochemical tuning of TiO2-passivated photocatalysts

We present a systematic study of photoluminescence (PL) spectroscopy of TiO2-passivated GaAs as a function of electrochemical potential in an ionic liquid solution. We observe a 7X increase in the PL intensity as the GaAs transitions from accumulation to depletion due to the applied potential. We attribute this to the excellent control over the surface Fermi level enabled by the high capacitance of the electrochemical double layer and TiO2. This allows us to control the surface carrier concentration and corresponding non-radiative recombination rate. In addition to photoluminescence (PL) spectroscopy, we also measured the capacitance-potential (i.e., C-V) characteristics of these samples, which indicate flat band potentials that are consistent with these regimes of ion accumulation observed in the photoluminescence measurements. We have also performed electrostatic simulations of these C-V characteristics, which provide a detailed and quantitative picture of the conduction and valence band profiles and charge distribution at the surface of the semiconductor. These simulations also enable us to determine the range of potentials over which the semiconductor surface experiences depletion, inversion, and accumulation of free carriers. Based on these simulations, we can calculate the Shockley-Read-Hall recombination rate and model the PL intensity as a function of voltage. We show that this approach allows us to explain our experimental data well.

Eu3+ ions imminence impact on its photoluminescence in Y2O3 host

A systematic photoluminescence study on 2% Eu-doped Y2O3 particles has been carried out. It is observed that 1300°C annealed samples show higher non-radiative contributions, poor lifetime and quantum efficiency in comparison to the samples annealed at lower temperatures (700°C and 900°C), even though the structural symmetry and emission profile remains almost unaffected by the increasing annealing temperature. Based on empirical exploration it has been established that annealing at 1300°C, reduces the average distance between Eu ions to ~6.7Å on. Odd behavior of 1300°C heated Y2O3:Eu sample has been attributed to the decreased interatomic separation i.e. increased proximity and associated increase in interaction between Eu ions. The inferences has been further supported by luminescence investigation of Y2O3 sampled co-doped with Yb–Er.

Comparison of magneto-optical properties of various excitonic complexes in CdTe and CdSe self-assembled quantum dots

We present a comparative study of two self-assembled quantum dot (QD) systems based on II–VI compounds: CdTe/ZnTe and CdSe/ZnSe. Using magneto-optical techniques we investigated a large population of individual QDs. The systematic photoluminescence studies of emission lines related to the recombination of neutral exciton X, biexciton XX, and singly charged excitons (X+, X−) allowed us to determine average parameters describing CdTe QDs (CdSe QDs): X–XX transition energy difference 12 meV (24 meV); fine-structure splitting meV ( meV); g-factor g = 2.12 (g = 1.71); diamagnetic shift (). We find also statistically significant correlations between various parameters describing internal structure of excitonic complexes.

Random Lasing in Novel Dye‐Doped White Paints with Shape Memory

Although enormous advances are accomplished using shape memory polymers (SMPs) and their derived composites, the application of these media in optics and photonics is still restricted to a few systems. The present work addresses the fabrication and characterization of novel composites based on polydiolcitrates doped with titania nanoparticles, aiming at expanding the capabilities of the original elastomer. They constitute advanced photonic white paints, which incorporate the shape memory effect as an additional functionality. In these materials, light propagation is diffusive and it is possible to tailor its transport mean free path by acting on the titania fillers concentration. Optical gain is introduced by additionally doping the composites with organic dyes. This allows the shape recovery process to be monitored by luminescence and, conversely, the composites can perform as configurable shape active media. Their application in nonresonant feedback random lasers is demonstrated by systematic photoluminescence studies. The threshold for stimulated emission is shown to depend on both the fillers and dye concentrations. The so‐obtained multifunctional composites are promising candidates for establishing synergies between applications from the domain of SMPs with those typically restricted to random photonic media, leading to novel self‐reporting sensors/actuators or responsive illumination devices, for example.

Influence of the core size on biexciton quantum yield of giant CdSe/CdS nanocrystals

We present a systematic study of photoluminescence (PL) emission intensity and biexciton (BX) quantum yields (QYBX) in individual "giant" CdSe/CdS nanocrystals (g-NCs) as a function of g-NC core size and shell thickness. We show that g-NC core size significantly affects QYBX and can be utilized as an effective tuning parameter towards higher QYBX while keeping the total volume of the g-NC constant. Specifically, we observe that small-core (2.2 nm diameter) CdSe/CdS NCs with a volume of ∼200 nm(3) (shell comprises 4 CdS monolayers) show very low average and maximum QYBX's of ∼3 and 7%, respectively. In contrast, same-volume medium-core (3 nm diameter) NCs afford higher average values of ∼10%, while QYBX's of ∼30% are achieved for same-volume large-core (5.5 nm diameter) CdSe/CdS NCs, with some approaching ∼80%. These observations underline the influence of the g-NC core size on the evolution of PL emissive states in multi-shell NCs. Moreover, our study also reveals that the use of long anneal times in the growth of CdS shells plays a critical role in achieving high QYBX.

Pump-intensity- and shell-thickness-dependent evolution of photoluminescence blinking in individual core/shell CdSe/CdS nanocrystals.

We report a systematic study of photoluminescence (PL) intensity and lifetime fluctuations in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. We show that while at low pump intensities PL blinking in thin-shell (4-7 monolayers, MLs) NQDs can be described by random switching between two states of high (ON) and low (OFF) emissivities, it changes to the regime with a continuous distribution of ON intensity levels at high pump powers. A similar behavior is observed in samples with a medium shell thickness (10-12 MLs) without, however, the PL intensity ever switching to a complete "OFF" state and maintaining ca. 30% emissivity ("gray" state). Further, our data indicate that highly stable, blinking-free PL of thick-shell (15-19 MLs) NQDs ("giant" or g-NQDs) is characterized by nearly perfect Poisson statistics, corresponding to a narrow, shot-noise limited PL intensity distribution. Interestingly, in this case the PL lifetime shortens with increasing pump power and the PL decay may deviate from monoexponential. However, the PL intensity distribution remains shot-noise limited, indicating the absence of significant quantum yield fluctuations at a given pump power intensity during the experimental time window.

Dependence of energy transfer and photoluminescence on tailored defects in Eu-doped ZnO nanosheets-based microflowers

Eu-doped ZnO nanosheets-based microflowers have been synthesized successfully by using a hydrothermal method. Eu 3+-related red emission results from energy transfer are observed in the microflowers under UV laser excitation. Further systematic photoluminescence studies on the samples upon oxygen and vacuum annealing treatments suggest that there exists a strong correlation between the intrinsic defects and the efficiency of the Eu 3+-related emission. It is demonstrated that these defects may act as the media in the energy transfer process from the ZnO host to Eu 3+ ions.

Luminescence study of grown sapphire: from starting material to single crystal

AbstractThe results of a systematic study of photoluminescence (PL) properties of un‐doped sapphire crystals grown using different starting materials on various crystal growth methods (CZ, EFG, HDSM, Kyropoulus, HEM) are presented. Narrow line of emission has been attributed to radiation transition in Cr3+ ions (R‐line), while the nature of red wide band (RWB) of emission (600‐850 nm) is still under question. Integral intensities of R‐line and RWB were analyzed and their dependences on type of starting raw materials and of the method of crystal growth were shown. The crystals grown using Vernuil starting material exhibited significant PL in all utilized methods. On the contrary, sapphire samples grown by the same technologies wherein EMT HPDAR (High Purity Densified Alumina) was the starting material revealed much lower PL. HPDAR is produced by EMT, Inc. with proprietary and patented technology. The concentrations of various metal impurities such as Ca, Mg, Zn, Cu, Cr, Fe, Ni, H were measured. Effect of these impurities on luminescence intensity and especially the role of hydrogen incorporated into sapphire lattice are discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence from Si nanocrystals induced by high-temperature implantation in SiO2

A systematic study of photoluminescence (PL) behavior of Si nanocrystals in SiO2 obtained by ion implantation in a large range of temperatures (−200 up to 800 °C), and subsequent furnace annealing in N2 ambient was performed. A PL signal in the wavelength range 650–1000 nm was observed. The PL peak wavelength and intensity are dependent on the fluence, implantation and annealing temperatures. It was found that after annealing at 1100 °C, both implantations of 1.5×1017 Si/cm2 at room temperature or 0.5×1017 Si/cm2 at 400 °C result in the same PL peak intensity. By varying the implantation temperature we can achieve the same PL efficiency with lower fluences showing that hot implantations play an important role for initial formation of the nanocrystals. The PL intensity evolution as a function of the annealing time was also studied. As the implantation temperature was increased, larger mean size Si nanocrystals were observed by means of dark-field transmission electron microscopy analysis.

Self-assembled nanoholes, lateral quantum-dot molecules, and rolled-up nanotubes

We present a detailed investigation of novel strain-driven semiconductor nanostructures. Our examinations include self-assembled nanoholes, lateral quantum-dot (QD) molecules, and rolled-up nanotubes. We overgrow InAs QDs with GaAs and apply atomically precise in situ etching to fabricate homogeneous arrays of nanometer-sized holes with diameters of 40 to 60 nm and depths up to 6.2 nm. The structural properties of the nanoholes can be precisely tuned by changing the QD capping thickness and the in situ etching time. We show that strain fields surrounding the buried quantum dots drive the nanohole formation process. We overgrow the nanoholes with 0.2- to 2.5-ML InAs and observe the formation of compact lateral InAs QD molecules. The number of QDs involved in a lateral QD molecule can be tuned from two to six by changing the growth temperature. Our systematic photoluminescence study documents the QD molecule formation process step by step and helps to interpret our structural results. We also present the fabrication of laterally aligned lateral QD bimolecules by growing InGaAs on a GaAs [001] substrate patterned with a square array of nanometer sized holes. Charge carriers in such bimolecules might serve as quantum gates in a future semiconductor based quantum computer. Furthermore, we release strained semiconductor bilayers from their surface to fabricate individual rolled-up semiconductor micro- and nanotubes. We control the diameter of strain-driven In(Ga)As-GaAs tubes from the nanometer to micrometer range by simply changing the layer thicknesses and built-in strain. We propose to roll in metal strip lines to fabricate nanocoils and nanotransformers. To support our proposition, we fabricate homogeneous single and twin GaInP tubes. We present a straight GaInP microtube of more than 2 mm in length and a length-to-diameter ratio of about 2000, thus, elucidating the great potential of this technology.

Time-Resolved Photoluminescence Measurements of In0.15Ga0.85N/In0.015Ga0.985N Quantum Wells with Si-doped Barriers

ABSTRACTA systematic study of photoluminescence (PL) and time-resolved PL spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si doping concentration in the barriers has been carried out. As the Si doping concentration increases, the PL emission intensity was increased and the PL peak energy was blueshifted. The energy separation between the spontaneous emission (SPE) and stimulated emission (SE) peaks decrease with increasing Si doping concentration. We also observed that the slow decay time τ2 in the QWs decreases with increasing Si doping concentration, from ∼ 130 ns for [Si] = 2 × 1018 cm−3 to ∼ 30 ns for [Si] = 1 × 1019 cm−3. The PL emission peak shifts to lower energies with delay time after a pulsed excitation and this shift decreases with increasing Si doping concentration. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the SPE and SE peaks with increasing Si doping concentration are attributed to the screening of piezoelectric field by carriers originated from Si doped barriers.

Optical properties of GaAs/AlGaAs selectively doped quantum well structures

In this work, we present a systematic study of photoluminescence (PL), photoluminescence excitation (PLE) and resonant photoluminescence on AlGaAs/GaAs selectively doped quantum well structures, in order to better understand the optical properties of these systems. To support the interpretation of our experimental results we have calculated the confined states for the systems in the effective mass approximation, taking into account many body effects, such as exchange and correlation in the local density approximation.

Low temperature photoluminescence of tellurium-doped GaSb grown by molecular beam epitaxy

We present the first systematic study of photoluminescence (PL) of GaSb grown by molecular beam epitaxy and Te doped at levels ranging from non-intentional to high. The PL spectra show features that gradually evolve with the doping level. The main structure of the PL spectra of the nominally undoped sample is a band attributed to the native double acceptor defect, which determines the p-type conductivity. In doped samples, the evidence of levels directly connected with tellurium has been found. At low doping levels we observe the recombination of free holes with electrons bound to the Te hydrogenic donor, while in heavily doped samples the dominant feature is a deep PL band attributed to a complex involving incorporated Te and structural defects. The observed blue shift of the PL bands as the doping level increases is consistent with the occurrence of band-filling effects.

Annealing Study of Erbium and Oxygen Implanted Gallium Nitride

AbstractA systematic study of photoluminescence (PL) of Er and O ion implanted and annealed n-type GaN grown on R-plane sapphire (A12O3) was performed. The Er implants ranged from 2 × 1013 to 1 × 1015 Er++/cm2, and the O co-implants ranged from 1014 to 1016 O+/cm2. The resulting nine different combinations of GaN:Er,O were annealed at 600 °C (4 hrs. in N2), 700 °C (1.5 hrs. in N2), 800 °C (0.75 hr. in NH3), and 900 °C (0.5 hr. in NH3) Following each annealing step, the Er3 -related PL at 1.54 μm was measured from each sample at 77K, when pumped directly with 135 mW of power at 980 nm. The three samples with the highest dose of Er (1 × 1015 Er++/ cm 2), regardless of O co-dopant dose, yielded the strongest PL peak intensity at 1.54 μm after all the anneals. The integrated PL from 1.52 to 1.58 μm was reduced by 62 % when going from 77 K to room temperature (RT).

Investigation of the photoluminescence linewidth broadening in symmetric and asymmetric [formula omitted] n-type δ-doped quantum wells

We have performed a systematic study of photoluminescence (PL) line shape in n-type δ-doped In 0.15Ga 0.85As GaAs quantum wells (δ QWs). Samples grown by molecular beam epitaxy with well widths of 60, 100, 300 Å, with sheet doping concentration ranging from 10 11 to 10 13 cm −2, were analysed. Two positions were considered for the doping planes, at the centre (symmetric) and at the edge (asymmetric) of the QW. The luminescence spectrum from the symmetric 60 Å δ QW exhibits a dominating broad structure that at low excitation condition and low temperature has an almost rectangular form. The square shape of this spectrum is explained by the collective recombination of electrons from the conduction band with thermalized photogenerated holes. For the symmetric L W = 300 A ̊ samples, the PL spectra display an almost symmetric peak followed by a small shoulder on the higher or lower energy side depending on the carrier concentrations. For the asymmetric 300 Å δ QW sample two distinct peaks were observed. They were correlated to transitions involving the fundamental and first excited electronic subbands.

Photoluminescence properties of ZnGa2O4:Mn powder phosphors

The results of a systematic photoluminescence study of ZnGa2O4:Mn powder phosphor are reported. At room temperature this phosphor exhibits bright green luminescence with a spectral peak at 2.46 eV and Commission International de l’Eclairage chromaticity coordinates of x=0.073 and y=0.696. At low temperatures the luminescence was found to consist of three components assigned to the 4T1–6A1 inner transition of the 3d electrons of Mn2+ ions located on three different sites of the host crystal. Selective excitation and lifetime measurements were used to investigate the assignment of these features. The photoluminescence lifetime showed a single exponential decay of about 4 ms and at T=1.6 K an optical-phonon-related fine structure [Ephonon=(8.2±0.2) meV] of the main photoluminescence line was observed.

Growth and Photoluminescence of Strained < 110 > Si/SilxGex/Si Quantum Wells Grown by Rapid Thermal Chemical Vapor Deposition

ABSTRACTWe report the first chemical vapor deposition growth and systematic photoluminescence study of strained Si1–xGex alloy layers on < 110 > Si substrates. Compared to < 100 > Si substrates, the same growth conditions yielded a slightly lower Ge composition, but a much lower growth rate. For thick layers, the relaxation along [110] direction is insufficient and lead to predominantly uniaxial strain in the the films. From the photoluminescence, the bandgap of strained Si1–x.Gex on < 110 > Si for 0.16 ≤ x ≤ 0.43 is determined and compared to theory. A strong “no-phonon” emission process in photoluminescence spectra of strained < 110 > Si/Si1–x.Gex/Si was observed as compared with that observed in < 100 > layers. Finally, quantum confinement shift of Si/Si0.71 Ge0.29/Si wells with a confinement energy up to 110 meV has been observed by varying the well width from 133 Å to 17 Å.