Optical anisotropy and the direction of polarization of exciton emissions in a semiconductor quantum dot:Effect of heavy-and light-hole mixing

The dependence of the directions of polarization of exciton emissions, fine structure splittings(FSS), and polarization anisotropy on the light-and heavy-hole(LH–HH) mixing in semiconductor quantum dots(QDs) is investigated using a mesoscopic model. In general, all QDs have a four-fold exciton ground state. Two exciton states have directions of polarization in the growth-plane, while the other two are along the growth direction of the QD. The LH–HH mixing does affect the FSS and polarization anisotropy of bright exciton states in the growth-plane in the low symmetry QDs(e.g., C_(2V),C_S, C_1), while it has no effect on the FSS and polarization anisotropy in high symmetry QDs(e.g., C_(3V), D_(2d)). When the hole ground state is pure HH or LH, the bright exciton states in the growth-plane are normal to each other. The LH–HH mixing affects the relative intensities and directions of bright exciton states in the growth-plane of the QD. The polarization anisotropy of exciton emissions in the growth-plane of the QD is independent of the phase angle of LH–HH mixing but strongly depends on the magnitude of LH–HH mixing in low symmetry QDs.     

Electronic energy alignment at the PbSe quantum dots/ZnO(101̅0) interface

A number of solar energy conversion strategies depend on exciton dissociation across interfaces between semiconductor quantum dots (QDs) and other electron or hole conducting materials. A critical factor governing exciton dissociation and charge transfer in these systems is the alignment of electronic energy levels across the interface. We probe interfacial electronic energy alignment in a model system, sub-monolayer films of PbSe QDs adsorbed on single crystal ZnO(101?0) surfaces using ultraviolet photoemission spectroscopy. We establish electronic energy alignment as a function of quantum dot size and surface chemistry. We find that replacing insulating oleic-acid capping molecules on the QDs by the short hydrazine or ethanedithiol molecules results in pinning of the valence band maximum (VBM) of QDs to ZnO substrate states, independent of QD size. This is in contrast to similar measurements on TiO₂(110) where the alignment of the PbSe QD VBM to that of the TiO₂ substrate depends on QD size. We interpret these findings as indicative of strong electronic coupling of QDs with the ZnO surface but less with the TiO₂ surface. Based on the measured energy alignment, we predict that electron injection from the 1s_e level in photo-excited PbSe QDs to ZnO can occur with small QDs (diameter ? = 3.4 nm), but energetically unfavorably for larger dots (? = 6.7 nm). In the latter, hot electrons above the 1s_e level are necessary for interfacial electron injection.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Anomalous strain effect on the thermal conductivity of borophene: a reactive molecular dynamics study

Borophene, an atomically thin, corrugated, crystalline two-dimensional boron sheet, has been recently synthesized. Here we investigate mechanical properties and lattice thermal conductivity of borophene using reactive molecular dynamics simulations. We performed uniaxial tensile strain simulations at room temperature along in-plane directions, and found 2D elastic moduli of 188 N m⁻¹ and 403 N m⁻¹ along zigzag and armchair directions, respectively. This anisotropy is attributed to the buckling of the borophene structure along the zigzag direction. We also performed non-equilibrium molecular dynamics to calculate the lattice thermal conductivity. Considering its size-dependence, we predict room-temperature lattice thermal conductivities of 75.9 ± 5.0 W m⁻¹ K⁻¹ and 147 ± 7.3 W m⁻¹ K⁻¹, respectively, and estimate effective phonon mean free paths of 16.7 ± 1.7 nm and 21.4 ± 1.0 nm for the zigzag and armchair directions. In this case, the anisotropy is attributed to differences in the density of states of low-frequency phonons, with lower group velocities and possibly shorten phonon lifetimes along the zigzag direction. We also observe that when borophene is strained along the armchair direction there is a significant increase in thermal conductivity along that direction. Meanwhile, when the sample is strained along the zigzag direction there is a much smaller increase in thermal conductivity along that direction. For a strain of 8% along the armchair direction the thermal conductivity increases by a factor of 3.5 (250%), whereas for the same amount of strain along the zigzag direction the increase is only by a factor of 1.2 (20%). Our predictions are in agreement with recent first principles results, at a fraction of the computational cost. The simulations shall serve as a guide for experiments concerning mechanical and thermal properties of borophene and related 2D materials.     

Luminescence properties of InAs quantum dots formed by a modified self-assembled method

The luminescence properties of self-assembled InAs quantum dots (QDs) on GaAs (1 0 0) substrates grown by molecular beam epitaxy have been investigated using temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL). InAs QDs were grown using an In-interruption growth technique, in which the indium flux was periodically interrupted. InAs QDs grown using In-interruption showed reduced PL linewidth, redshifted PL emission energy, increased energy level spacing between the ground state and the first excited state, and reduced decay time, indicating an improvement in the size distribution and size/shape of QDs.     

Polarized absorption spectra of highly oriented two-dimensional aggregates of tetrachlorobenzimidazolocarbocyanine in thin films

Reaching a control on the mesoscopic morphology and internal molecular arrangement of cyanine aggregates is an important step for realization of devices with tailor-made optical properties. Despite a wealth of research, understanding of the relationship between molecular organization, excitonic states and dynamics of aggregates is still preliminary. To this end, we have employed polarized absorption spectroscopy to investigate the relationship between internal molecular organization and excitonic states of J-aggregates in 1,1′,3,3′tetraethyl-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine (TTBC) thin films in poly-vinyl alcohol (PVA). Angular dependence of the UV–vis spectra has been measured at 11 different orientations between the electric field polarization and the macroscopic alignment axis. Aggregate spectral response consisted of an asymmetrically split Davydov pair of bands exhibiting opposite polarization: an H-band (505 nm, Lorentzian-like, polarized along the macroscopic film axis) and a J-band (594 nm, one-dimensional J-aggregate like band shape, polarized perpendicular to the macroscopic film axis). The polarized absorption observations were found to be consistent with a herringbone model for which the internal molecular arrangement, the excited state structure and dynamics have recently been detailed by us upon interpretation of isotropic absorption data in ionic aqueous solution.     

Intrinsic luminescence in oriented BeO crystals under VUV and inner-shell excitation

Beryllium oxide (BeO) crystals were investigated by time-resolved low temperature VUV-spectroscopy at the SUPERLUMI station and BW3 beam line of HASYLAB (DESY, Hamburg). Photoluminescence spectra (3–10.5 eV), luminescence decay kinetics upon selective photoexcitation, as well as luminescence excitation (50–650 eV) and reflectivity (9–35 eV) spectra were measured and analyzed for oriented BeO crystals. It was shown that study of oriented crystals makes the traditional time-resolved spectroscopy method essentially more informative. Formation of the self-trapped exciton excited states of different multiplicity was found to sensitively depend on excitation energy and mutual orientation of the crystal's C optical axis and electric vector E of exciting polarized synchrotron radiation.     

Re-charging the luminescence states in CdSe/ZnS quantum dots at the conjugation to Osteopontin antibodies

The paper presents the original study of photoluminescence (PL) and Raman scattering spectra of core–shell CdSe/ZnS quantum dots (QDs) covered by the amine-derivatized polyethylene glycol (PEG) with luminescence interface states. First commercially available CdSe/ZnS QDs with emission at 640 nm (1.94 eV) covered by PEG polymer have been studied in nonconjugated states. PL spectra of nonconjugated QDs are characterized by a superposition of PL bands related to exciton emission in a CdSe core and to the hot electron–hole recombination via high energy luminescence states. The study of high energy PL bands in QDs at different temperatures has shown that these PL bands are related to luminescence interface states at the CdSe/ZnS or ZnS/polymer interface. Then CdSe/ZnS QDs have been conjugated with biomolecules—the Osteopontin antibodies. It is revealed that the PL spectrum of bioconjugated QDs changed essentially with decreasing hot electron–hole recombination flow via luminescence interface states. It is shown that the QD bioconjugation process to Osteopontin antibodies is complex and includes the covalent and electrostatic interactions between them. The variation of PL spectra due to the bioconjugation is explained on the basis of electrostatic interaction between the QDs and biomolecule dipoles that stimulates re-charging QD interface states. The study of Raman scattering of bioconjugated CdSe/ZnS QDs has confirmed that the antibody molecules have the electric dipoles. It is shown that CdSe/ZnS QDs with luminescence interface states are promising for the study of bioconjugation effects with specific antibodies and can be a powerful technique in biology and medicine.     

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures have been studied from luminescence spectra obtained at T=2 K. The energy and phase relaxation times of localized exciton states have been determined from a study of the destruction of exciton optical alignment by an external magnetic field and direct measurements of the polarized-radiation decay kinetics in the picosecond range. The exciton polarization lifetimes measured by two independent techniques are found to be in a good agreement. Fiz. Tverd. Tela (St. Petersburg) 40, 809–810 (May 1998)     

Auger-free luminescence due to interatomic p–d transition and self-trapped exciton luminescence in Rb2ZnCl4 crystals

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb₂ZnCl₄. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

Spectroscopic and nonlinear characteristics of anthracene-containing silicon-organic polymers in solution

Spectroscopic characteristics, i.e. absorption, fluorescence, fluorescence excitation spectra, fluorescence decay time, fluorescence polarization degree of novel silicon-containing organic polymers including main chain anthracene groups were investigated. Three kinds of emission spectra were revealed and assigned to polaron–exciton, anthracene and anthracene dimer. The measured fluorescence polarization spectra gave evidence of directed excitation energy migration along the disordered polymeric chain. Strong quenching of anthracene fluorescence during the polaron–exciton lifetime was interpreted as a result of the interaction between two excitations that causes anthracene anion-radical formation. The third-order nonlinear susceptibility of the polymers in solution measured by the Z-scan technique at 1054 nm is 190×10⁻¹⁴ cm² W⁻¹.     

Size dependence of ground-state energy of the excitons in spherical Y2O3

The exciton energies of rare earth oxides (Ln₂O₃) have rarely been calculated by the theory. Experimentally, the blue-shift of exciton energy in nanocrystals deviates from the traditional size confinement effect. Herein, the dependence of the ground-state energy of an exciton in Y₂O₃ spheres on particle radius was calculated by using a variational method. In the model, an exciton confined in a sphere surrounded by a dielectric continuum shell was considered. The ground-state energy of exciton comprises kinetic energy, coulomb energy, polarization energy and exciton–phonon interaction energy. The kinetic and coulomb energy were considered by the effective mass and the dielectric continuum and the exciton–phonon interaction energy was given by the intermediate coupling method. The numerical results demonstrate that the present model is roughly consistent with the experimental results. The confinement effect of the kinetic energy is dominant of the blue-shift of the exciton energy in the region of R < 5 nm, while confinement effect of the coulomb energy is dominant of the blue-shift of the exciton energy in the region of R > 5 nm. The polarization energy contributes largely to the exciton energy as the particle size is smaller than ~ 10 nm, while the exciton–phonon interaction energy takes only a little contribution in all the range.     

Optical characterization of Er3+and Yb3+ co-doped barium fluorotellurite glass

Spectroscopic studies of Er³⁺/Yb³⁺ co-doped (Ba,La)-fluorotellurite glass composition have been carried out using standard experimental and theoretical methods. Quantitative analyses of the room temperature absorption and emission spectra as well as the emission lifetimes yield various important spectroscopic parameters such as the radiative decay rates, fluorescence branching ratios, and emission/absorption cross sections. In addition, internal radiative quantum yields have been determined for the infrared emission at 1571 nm and for the upconversion emission at 547 nm. The influence of various non-radiative properties such as multiphonon relaxation, concentration quenching, and quenching by hydroxyl radicals have also been quantitatively estimated and correlated with the observed spectral properties. The comparative studies with the other composition of tellurite and different glasses showed that present glass composition could be a potential candidate for the broadband amplifier.     

Photoluminescence of Lu3Al5O12:Bi and Y3Al5O12:Bi single crystalline films

Single crystalline films of Lu₃Al₅O₁₂:Bi and Y₃Al₅O₁₂:Bi have been studied at 4.2–450 K by the time-resolved luminescence spectroscopy method. Their emission spectrum consists of two types of bands with strongly different characteristics. The ultraviolet band consists of two components, arising from the electronic transitions which correspond to the ³P₁ → ¹S₀ and ³P₀ → ¹S₀ transitions in a free Bi³⁺ ion. At T < 80 K, mainly the lower-energy component with the decay time ～10^?3 s is observed, arising from the metastable ³P₀ level. At T > 150 K, the higher-energy component prevails, arising from the thermally populated emitting ³P₁ level. The visible emission spectrum consists of two dominant strongly overlapped broad bands with large Stokes shifts. At 4.2 K, their decay times are ～10^?5 s and ～10^?4 s and decrease with increasing temperature. Both of the visible emission bands are assumed to be of an exciton origin. The lower-energy band is ascribed to an exciton, localized near a single Bi³⁺ ion. The higher-energy band, showing a stronger intensity dependence on the Bi³⁺ content, is assumed to arise from an exciton, localized near a dimer Bi³⁺ center. The structure of the corresponding excited states is considered, and the processes, taking place in these states, are discussed.     

Quantum dot p-i-n structure in an electric field

Time-resolved photoluminescence (PL), steady-state PL, and electroluminescence (EL) techniques have been used to characterize the carrier relaxation processes and carrier escape mechanisms in self-assembled InAs/GaAs quantum dot (SAQD) p-i-n structures under reverse bias. The measurements were performed between 5 K and room temperature on a ring mesa sample as a function of bias. At 100 K, the PL decay time originating from the n  =  1 SAQD decreases with increasing reverse bias from ∼3 ns under flat band condition to∼ 400 ps for a bias of −3 V. The data can be explained by a simple model based on electron recombination in the quantum dots (QDs) or escape out of the dots. The escape can occur by one of three possible routes: direct tunneling out of the distribution of excited electronic levels, thermally assisted tunneling of ground state electrons through the upper excited electronic states or thermionic emission to the wetting layer.     

Synthesis,biocompatibility and luminescence properties of quantum dots conjugated with amino acid-functionalized β-cyclodextrin

A series of CdSe and CdSe/CdS quantum dots (QDs) labeled with amino acid-modified β-cyclodextrin (β-CD) was prepared by a simple ultrasonic method. These amino acid-modified β-CD-coated QDs are very soluble and stable in biological buffer. They also have high colloidal stability and strong optical emission properties that are similar to those of untreated tri-n-octylphosphine oxide (TOPO)-coated QDs. The quantum yields (QYs) of these amino acid-modified β-CD-coated CdSe and CdSe/CdS QDs in biological buffer were found to be very high. In particular, the QYs of the positively charged l-His-β-CD-coated CdSe/CdS QDs were as high as 33.5±1.8%. In addition, the fluorescence lifetime of these QDs was also very long in PBS solutions as determined by frequency domain spectroscopy. For example, the lifetime of l-His-β-CD-coated CdSe/CdS QDs was 8.6 ns. The in vitro cytotoxicity of these QDs in ECV-304, SH-SY5Y and HeLa cells was found to be lower. l-His-β-CD-coated CdSe/CdS QDs were the least cytotoxic (IC₅₀ 95.6±3.2 mg mL^?1 in ECV-304 cells after 48 h). The flow cytometry results show that the positively charged amino acid led to a considerable increase in biocompatibility of QDs. This may be attributed to the presence of an amino acid-modified β-CD outer layer, which enhanced the biocompatibility.     

Decay of orientational grating of weakly confined excitons in GaAs thin films

We report the dynamical properties of the exciton orientation in GaAs thin films using the orientational grating (OG) technique. From the results of excitation-power dependence of OG signal, we confirmed that the OG signal comes from the optical nonlinearity of weakly confined excitons. In addition, the OG-decay time decreases with an increase of excitation power due to exciton–exciton interaction, and the shortest decay time is below 1 ps. Our results may imply the potential application of optical nonlinearity of weakly confined exciton to ultrafast switching devices operating at 1 Tbit/s.     

Intra-dot relaxation and dephasing rates from time-resolved photoluminescence from InAs quantum dot ensembles

An ensemble of InAs quantum dots with ground state transition energies centered at 1.216 eV and density 10¹¹dots/cm² has been studied by time-resolved photoluminescence (PL). The wavelength of the 100-fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon sideband) PL. The decay of the PL was time resolved with a streak camera in the interval 1.5–3 ns to avoid scattered laser light. The intensity of the PL was recorded with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal’s cleave axes); the ratio is 2.22 at low temperatures and low excitation. A phenomenological rate equation analysis is made, separating the excitations into two classes, one polarized along the excitation polarization and the other unpolarized (either that way immediately after the excitation pulse or scattered from the first class). Excellent fits to the data lead to the conclusion that both classes decay radiatively with a lifetime of 1 ns, and a transfer from the polarized to the unpolarized species takes place with a distribution time of 12 ns at low temperatures and low excitation, dropping rapidly toward zero for temperatures above 30 K and for intense excitation levels. The polarization of a coherently excited ground state exciton should dephase with a rate equal to the sum of the radiative rate plus the inverse of this distribution time.     

Pump–probe absorption/gain spectroscopy of strongly confined semiconductor nanocrystals

The density matrix approach has been employed to analyze the pump–probe spectroscopic absorption spectra of small semiconductor nanocrystals popularly known as quantum dots (QDs) under the strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Coulombic interaction energy becomes negligible in comparison with the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near-resonant pump and a broadband weak probe. Numerical estimations have been made for: (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in the case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III–V semiconducting microscopic structures.