Excitonic PL Research Articles

Synthesis of PbMoO4 nanoparticles by microwave-assisted hydrothermal process and their photocatalytic activity.

Lead molybdate (PbMoO4) was successfully synthesized using a microwave-assisted method and characterized by XRD, Raman spectroscopy, SEM, PL and DRS. We also investigated the photocatalytic activity of these materials for the decomposition of Rhodamin B under UV-light irradiation. The XRD and Raman results revealed the successful synthesis of 42-52 nm, well-crystallized PbMoO4 crystals with the microwave-assisted hydrothermal method. The PbMoO4 catalysts prepared using the microwave-assisted process enhanced the photocatalytic activity compared to that prepared by hydrothermal method and the catalysts prepared at a solution pH = 11 and temperature of 105 degrees C showed the highest photocatalytic activity. The PL peaks appeared at about 540 nm for all catalysts and the excitonic PL signal was proportional to the photocatalytic activity for the decomposition of Rhodamin B.

Two-step fabrication of R-PbI_4(1-y)Br_4y type light emitting inorganic-organic hybrid photonic structures

An easy fabrication approach for highly c-axis oriented Inorganic-Organic (IO) mixed hybrid semiconductor thin films has been demonstrated for the first time. A simple vacuum deposition technique followed by chemical processing is utilized to fabricate controlled thin films of wide range of selective compositions from R-PbI4(1-y)Br4y (R = organic; y = 0 to 1) type IO hybrids. These thin films are alternative stack of inorganic and organic nanoscale layers, resembles multiple quantum wells. As a result, they show several novel features including tunable strong room-temperature excitons over a broad spectral range from 380 nm to 520 nm. Structural and in situ optical studies confirm that the methodology can be directly adoptable in top-down technology and also extendable for many varieties of inorganic-organic frameworks. The device quality fabrication process and optoelectronic multi-functionality has been demonstrated by template based 3D microstructures, exciton PL imaging and photodetector response.

Origin of Indirect Optical Transitions in Few-Layer MoS2, WS2, and WSe2

It has been well-established that single layer MX2 (M = Mo, W and X = S, Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX2, there are two valleys along the Γ-K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in few-layer MX2 by examining the temperature-dependent shift of indirect exciton photoluminescence peak. Highly anisotropic thermal expansion of the lattice and the corresponding evolution of the band structure result in a distinct peak shift for indirect transitions involving the K and Λ (midpoint along Γ-K) valleys. We identify the origin of the indirect emission and concurrently determine the relative energy of these valleys.

Removal of cationic Rhodamine-B dye using nano-titania with anatase crystalline structure and kinetic analysis of the photocatalytic reaction

Heterogeneous photocatalytic removal of Rhodamine-B (RhB) dye from liquid phase was done using anatase-phase nanocrystalline TiO2 synthesized via a modified sol-gel process. The anatase-phase nanocrystalline TiO2 was characterized using various analytical techniques including XRD, UV-vis DRS, PL, and FTIR to investigate its phase composition and structure, nanocrystalline size, band gap energy, photoluminescence and surface properties of the prepared systems. The photocatalytic discoloration efficiency of anatase-phase nanocrystalline titania was investigated by monitoring the decomposition of RhB dye as target compounds in an aqueous solution. The results showed that the as-prepared anatase-phase nanocrystalline TiO2 was excellent for degradation of RhB molecule, and the crystallite size, excitonic PL and surface hydroxyl content have intimate relationship with the decomposition efficiency of RhB. The reaction mechanism was proposed and the results demonstrate that the role of direct photolysis on RhB dye degradation can be neglected. Meanwhile, the Langmuir-Hinshelwood kinetic model describes the photodecay date of RhB in consistent with a first order powder law and thus photocatalytic oxidation reaction followed a pseudo-first-order kinetics.

Fabrication and optical properties of Mn2+‐doped CdS/ZnS core/shell nanocrystals

AbstractWe have fabricated Mn2+‐doped CdS/ZnS core/shell nanocrystals (NCs) where Mn ions exist within ZnS shell layer, and have studied optical properties of the samples with different Mn concentrations. The exciton and Mn PL are clearly observed in the Mn2+‐doped CdS/ZnS core/shell NCs. The intensity of the exciton PL decreases with the Mn concentration, while that of the Mn PL increases. Furthermore, the Stokes shift of the exciton PL is suppressed by doping of the Mn ions. Our findings reveal that the Mn ions within the ZnS shell is excited by energy transfer from excitons of the CdS cores. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of large Zeeman splitting in GaGdN/AlGaN ferromagnetic semiconductor double quantum well superlattices

Symmetric GaGdN/AlGaN (Gd concentration: 2%) and GaN/AlGaN double quantum well superlattices (DQW-SLs) were grown by radio-frequency plasma-assisted molecular-beam epitaxy on GaN (0001) templates. Atomic steps were observed on all the sample surfaces by atomic force microscope. X-ray diffraction θ/2θ scan curves exhibited well-defined satellite structures. Room temperature ferromagnetism was confirmed for the GaGdN/AlGaN DQW-SL samples by using alternating gradient magnetometer. Strong photoluminescence was observed from both GaGdN and GaN QWs at higher energy side of GaN excitonic peak. Magneto-photoluminescence spectra for GaGdN/AlGaN DQW-SL samples showed a large magnetic field dependence of the excitonic energy by applying a magnetic field up to 7T. The observed strong redshift of excitonic PL indicated an enhancement of Zeeman splitting of the free carrier energy levels in magnetic GaGdN/AlGaN DQW-SL. Enhanced g-factor was estimated to be about 60 for GaGdN/AlGaN DQW-SL sample with QW thickness of 1nm.

Pool–Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals

Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz–Keldysh and Pool–Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp– β√ E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as Δ E∼ β√ E. From these findings, the Pool–Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz–Keldysh effects contribute.

Synthesis of TiO2 Supported on SBA-15 Using Chelating Method and Their Photocatalytic Decomposition of Methylene Blue

SBA-15 mesoporous materials were successfully prepared by the conventional hydrothermal method and used as the support for TiO2 loaded SBA-15 photocatalysts. The synthesized materials were characterized by XRD, PL, FT-IR, BET and TEM. We also examined the activity of these materials as photocatalysts for the decomposition of methylene blue. The loading of titanium dioxide on the framework of SBA-15 makes the pore diameter and the pore volume decrease and decreases the surface area compared to that of SBA-15. For the TiO2 loaded SBA-15 photocatalysts, the IR absorption at approximately 960 cm(-1) is commonly accepted as the characteristic vibration of the Ti-O-Si bond. The PL peaks appears at about 410 nm at a loading ratio of less than 5% but moves to 430 nm at higher loading ratios. It was also shown that the excitonic PL signal is proportional to the photocatalytic activity for the decomposition of methylene blue. The photocatalytic activity increases with increasing TiO2 loading ratio, shows a maximum value at 7% TiO2/SBA-15, and then decreases at 10% TiO2/SBA-15.

Enhanced exciton photoluminescence in the selectively Si-doped GaAs/AlxGa1−xAs heterostructures

Experimental results examining the photoluminescence spectra of selectively Si-doped GaAs/AlxGa1−xAs heterostructures is presented. Possible mechanisms of carrier recombination are discussed with a special emphasis on the peculiarities of excitonic photoluminescence. Strong intensity lines in photoluminescence spectra are associated with the formation and enhancement of free exciton and exciton-polariton emission in the flat band region of an active i-GaAs layer. The excitonic PL intensity is sensitive to the excitation intensity indicating high nonlinear behavior of spectral-integrated photoluminescence intensity and exciton line narrowing. These observed phenomena may be related to the collective interaction of excitons and the interaction of excitons with emitted electromagnetic waves. The gain of the amplification of the excitonic photoluminescence intensity in the heterostructure was found to be more than 1000 times larger than the intensity of i-GaAs active layer. The quality factor of the exciton line emission and the exciton-polariton line was found to be 3800 and 7600, respectively.

Spectroscopy of shallow InAs/InP quantum wire nanostructures

A comprehensive investigation of the optical properties of InAs/InP(001) quantum wires(QWrs) and their parent quantum well system formed by the deposition of 4 ML(monolayers) of InAs on InP is carried out by means of temperature dependentphotoluminescence (PL) and Fourier transform infrared spectroscopy. Unusual two-branchswitching of the excitonic PL band maxima is revealed in the temperature dependence forboth wires and wells. This is interpreted in terms of the thermal activation ofexcitonic ground states of the confined nanostructures. Strong modification of theabsorbance line shape leading to the appearance of flat spectral regions in theroom temperature spectrum of a QWr sample is interpreted in terms of thermallyinduced change of the dimensionality: from 1D to anisotropic 2D. This change ofdimensionality is detected also in the polarized absorbance measurements through thedisappearance or significant reduction of the polarization anisotropy in the regions of thehh1–e1 (hh: heavy hole; e: electron) and lh1–e1 (lh: light hole) transitions inQWrs.

Complete Quenching of CdSe Nanocrystal Photoluminescence by Single Dye Molecules

A study was conducted to investigate the energy transfer (ET) between CdSe-based nanocrystals and organic dye molecules. The study used cores, core/shell, and rods nanocrystals with Texas Red Cadavarine and Lissamine Rhodamine B Ethylenediamine dyes. The absorption and emission spectra showed that the nanocrystals has a large absorption cross-section in the spectral region. It was observed during the study that a single dye molecule can effectively perform quenching of exciton PL on CdSe nanocrystals. The study also found that the energy transfer can be changed by changing the surface chemistry of the CdSe nanocrystals. The study concluded that the performance of the dye-based solar cells can be improved by utilizing semiconductor nanocrystals as supersensitizers. The study used UV-vis-NIR to obtained the absorption spectra of the nanocrystals.

Exciton related photoluminescence stimulation in SiC nanocrystals

The paper presents the results of porous SiC characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence spectroscopy techniques. XRD study shows the investigated porous 6H-SiC layers contain inclusions of 4H-SiC and 15R-SiC polytypes as well as the amorphous graphite phase. Photoluminescence study of PSiC layers with different thicknesses and SiC NC sizes reveals the intensity stimulation for exciton and defect-related PL bands. The intensity stimulation of defect-related PL bands is due to the increase, at the etching process, of the concentration of surface defects, apparently to deal with carbon. The intensity enhancement for exciton-related PL bands is attributed to the exciton recombination rate increasing due to the realization of exciton confinement and exciton–polariton effects in big size SiC NCs of different polytypes (6H-PSiC with inclusions of 15R- and 4H-PSiC).

PHOTOLUMINESCENCE OF A HIGH MOBILITY 2DEG IN THE FRACTIONAL QUANTUM HALL EFFECT REGIME

The magneto-PL spectra of modulation-doped, ultra-high mobility GaAs/AlGaAs single heterojunctions (HJs) were studied under a perpendicularly applied magnetic field up to 33 T and at temperatures of 0.3 and 1.2 K. The spectra show remarkable intensity redistribution between free (bulk) exciton and 2DEG-hole PL channels occurring at electron filling factors, ν = 2 and 1. At 0.3 K, significant 2DEG-hole PL spectral changes are observed near ν = 2/3 and 1/3. Several heterojunctions with 2DEG density in the range of n2D - (1 - 2.7) · 1011 cm -2 display similar features. These spectral peculiarities are attributed to the modification of the 2DEG energy spectrum caused by the e-e interaction, in particular, the recombination of valence hole with the composite (fractionally-charged) particles of the magnetized 2DEG. In HJs with lower n2D < 1011 cm -2, the observed PL evolution at ν < 1 is mainly determined by an intensity redistribution between the σ+ and σ- circularly-polarized free exciton PL components. In this case, the exciton energy is lower than the energy of the 2DEG-hole system, so that the free excitons do not dissociate near the magnetized 2DEG and thus, the 2DEG-hole PL is barely observed.

DYNAMICS OF SPATIALLY INDIRECT EXCITONS IN SEMICONDUCTOR HETEROSTRUCTURES IN MAGNETIC FIELDS

Magnetophotoluminescence from spatially separated excitons in type-II semiconductor heterostructures, GaP/AlP neighboring confinement structures (NCSs) have been investigated. The sample studied consists of adjacent GaP and AlP quantum wells sandwiched between AlGaP barriers. The excitation power dependence and the well thickness dependence of the exciton PL are studied in magnetic fields applied perpendicular to the heterointerfaces. Compared to the thinner NCS, the magneto-PL of the wider NCS is strongly modified by the excitation power; the magnetic field dependence of the PL peak energy clearly changes from the anomalous red-shifts into the diamagnetic shifts with increasing excitation laser power.

Magnetophotoluminescence study of GaxIn1−xP quantum wells with CuPt-type long-range ordering

Magnetophotoluminescence (PL) measurements are performed on GaxIn1−xP∕AlGaInP quantum wells with CuPt-type long-range ordering. Curve-fitting and second-order derivative operations are conducted, respectively, on the PL spectra, and two different PL components are identified with an energetic separation of about 5∼25meV. Obvious diamagnetic shift can be observed only for the PL component with higher energy, indicating the excitonic feature of the related PL transition. The in-(001)-plane exciton reduced effective mass and binding energy are evaluated based solely on the excitonic PL component, by a quasi-two-dimensional exciton effective mass approximation. The results suggest that the ordering competes with compressive strain, and enlarges the exciton reduced effective mass and binding energy. The enlargement is discussed and a comparison with previous reports is given.

Magneto‐optical properties in hybrid nanostructures of II–VI diluted magnetic semiconductors with perpendicular ferromagnetic films

AbstractHybrid nanostructures of a diluted magnetic semiconductor quantum well (DMS‐QW) of Zn1–x –y Cdx Mny Se have been fabricated with a ferromagnetic Co/Pt‐multilayered film, by electron beam lithography. The disk structure of the DMS‐QW with the diameter of 100 nm was embedded in the magnetic thin film with the thickness of 50 nm. Giant Zeeman effects of excitons in the DMS‐QW in the hybrid structure are studied by micro‐photoluminescence (PL) spectroscopy. The Co/Pt film shows perpendicular magnetization without external field, and therefore it can generate magnetic fields nearly perpendicular to the DMS‐QW plane in the hybrid structure. As the result, the circularly polarized excitonic PL arises in zero external field, indicating that the perpendicular magnetic field applied from the Co/Pt film aligns the exciton spins in the DMS‐disk in the present hybrid structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd1−xMnxSe

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd1−xMn x Se. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 μm−2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn1−xMn x Se layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.

Dynamics of relaxation and trapping of excitons in Al x Ga 1– x As films

Carrier relaxation in high quality AlxGa1–xAs (x up to 5%) epilayers has been investigated by means of time resolved photoluminiscence. The time for the excitonic PL to reach the maximum intensity shows a non-monotonic behaviour with increasing excitation density, which is attributed to the competition between exciton localization and carrier-carrier scattering. The increase of the Al content enhances the alloy scattering and increases the number of localization sites, moderately accelerating the dynamics. A four-level rate equation model qualitatively describes the influence of trapping in the exciton dynamics at low excitation densities. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Remote hydrogen plasma doping of single crystal ZnO

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I4 for a variety of ZnO single crystals–bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I4 intensity in conducting samples annealed at 500 and 600 °C and partially restores emission in the I4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity.

Photoluminescence of a two-dimensional electron gas in a modulation-dopedGaAs/AlxGa1−xAsquantum well at filling factorsν<1

The evolution of the 2DEG-hole magneto-photoluminescence (PL) with decreasing 2DEG density is studied in a high quality 25 nm-wide modulation-doped GaAs/AlGaAs quantum well in magnetic fields $B<~7\mathrm{T}$ and lattice temperature ${T}_{L}=1.9\mathrm{K}.$ The 2DEG density was varied by optical depletion (with He--Ne laser illumination) in the range of ${n}_{2\mathrm{D}}=(1--7)\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\ensuremath{-}2}.$ As the filling factor decreases below $\ensuremath{\nu}=1,$ a high-energy PL band (H) emerges; its evolution with B and electron density ${n}_{2\mathrm{D}}$ is studied. At $\ensuremath{\nu}\ensuremath{\sim}0.4,$ two additional PL lines split off the H-band, and these are assigned to the charged triplet ${X}_{t}^{\ensuremath{-}}$ and neutral exciton ${X}^{0}$ PL. The evolution from free-hole--2DEG to charged exciton PL with decreasing ${n}_{2\mathrm{D}}$ and with increasing magnetic field (at $\ensuremath{\nu}<0.4)$ is attributed to the appearance of regions containing localized electrons in the photoexcited quantum well. Localization results in simultaneous presence of the free-hole--2DEG PL from the electron puddles (H-band) and the charged exciton PL lines (spin-singlet, ${X}_{s}^{\ensuremath{-}}$ and spin-triplet, ${X}_{t}^{\ensuremath{-}})$ from areas containing localized electrons.