Search

Several aspects concerning optical absorption in 3D assemblies of semiconductor quantum dots (QD solids) were studied. Considering the numerically simulated spectral dependences of the absorption coefficient in the case 3D QD assemblies with finite crystal size distribution, described by log-normal probability distribution functions (PDFs), several fundamental predictions were derived. First of all, it is predicted that the band gap energy of the QD solid should exhibit a certain ‘red-shift’ upon enlargement of the dispersion of the crystal size at a fixed average value thereof when the size-quantization regime in individual QDs has been entered. Furthermore, very large Urbach energy values are predicted, comparable to those characteristic for amorphous materials, for semiconductor QD solids with finite dispersion of the crystal size when the average QD size falls within the size-quantization interval. The Urbach energy in 3D assemblies composed by strongly quantized QDs with large dispersions of the PDFs characterizing the size distributions could be 100 times larger than the values in the non-quantized case (regardless of the size distribution in the latter case). Such large values are not due to the structural disorder inherent to individual QDs constituting the array, but a consequence of the disorder on the superlattice scale. Analogous arguments could be applied to predict large Urbach energies corresponding to the absorption tails in absorption spectra constructed from the spectral response of stationary nonequilibrium conductivity (photoconductivity). All these predictions are in excellent agreement with the available experimental data. Moreover, the presented approach could enable exact prediction of the optical absorption of a semiconductor QD solid if the PDF of the crystal size is known from the experiment. The smearing of excitonic absorption peaks in QD solids induced as a consequence of particle size distribution is considered and analyzed to some extent as well. [ABSTRACT FROM AUTHOR]

Electron bombardment of gaseous N2 produces N2+ and N3, which are subsequently trapped in the N2 matrix at 10 K. Both the infrared and ultraviolet absorption spectra of the matrix sample at various stages of electron irradiation were recorded. Apart from a progression observed below 192 nm, with intervals ∼900 cm–1, corresponding to the transition of D 2Πg ← X 2Σg+ of N2+, three new progressions were recorded in the range 225-192 nm, with intervals ∼1000 cm–1, that correlated well with variations in intensities of the electronic absorption band of N3 at 272.7 nm; an absorption coefficient of 3.76 × 10–17 cm molecule–1 for the transition A2Σu+← X2Πg of N3 was estimated for the first time. These newly observed progressions were characterized and the vertical excitation energy and oscillator strength were calculated using time-dependent, density-functional theory. This was based on assigning the three progressions to electronic transitions of N3 from the ground state to 2 2Πu, 1 2Σg+, and 1 2Σg–, respectively. [ABSTRACT FROM AUTHOR]

The magnetic field-dependent heavy hole excitonic states in a strained Ga0.2In0.8As/GaAs quantum dot are investigated by taking into account the anisotropy, non-parabolicity of the conduction band, and the geometrical confinement. The strained quantum dot is considered as a parabolic dot of InAs embedded in a GaAs barrier material. The dependence of the effective excitonic g-factor as a function of dot radius and the magnetic field strength is numerically measured. The interband optical transition energy as a function of geometrical confinement is computed in the presence of a magnetic field. The magnetic field-dependent oscillator strength of interband transition under the geometrical confinement is studied. The exchange enhancements as a function of dot radius are observed for various magnetic field strengths in a strained Ga0.2In0.8As/GaAs quantum dot. Heavy hole excitonic absorption spectra, the changes in refractive index, and the third-order susceptibility of third-order harmonic generation are investigated in the Ga0.2In0.8As/GaAs quantum dot. The result shows that the effect of magnetic field strength is more strongly dependent on the nonlinear optical property in a low-dimensional semiconductor system. [ABSTRACT FROM AUTHOR]

Absorption spectra toward Herschel 36 (Her 36) for the Ã1∏ ← ~X1∑ transitions of CH+ in the J = 1 excited rotational level and for the Ã2Δ ← ~X2∏ transitions of CH in the J = 3/2 excited fine structure level have been analyzed. These excited levels are above their ground levels by 40.1 K and ~25.7 K and indicate high radiative temperatures of the environment of 14.6 K and 6.7 K, respectively. The effect of the high radiative temperature is more spectacular in some diffuse interstellar bands (DIBs) observed toward Her 36; remarkable extended tails toward red (ETRs) were observed. We interpret these ETRs as being due to a small decrease of the rotational constants upon excitation of the excited electronic states. Along with radiative pumping of a great many high-J rotational levels, this causes the ETRs. In order to study this effect quantitatively, we have developed a model calculation in which the effects of collisions and radiation are treated simultaneously. The simplest case of linear molecules is considered. It has been found that the ETR is reproduced if the fraction of the variation of the rotational constant, α ≡ (B'-B)/B, is sufficiently high (3%-5%) and the radiative temperature is high (Tr > 50 K). Although modeling for general molecules is beyond the scope of this paper, the results indicate that the prototypical DIBs λ5780.5, λ5797.1, and λ6613.6 which show the pronounced ETRs are due to polar molecules that are sensitive to the radiative excitation. The requirement of high α favors relatively small molecules with three to six heavy atoms. DIBs λ5849.8, λ6196.0, and λ6379.3 that do not show the pronounced ETRs are likely due to non-polar molecules or large polar molecules with small α. [ABSTRACT FROM AUTHOR]

The absorption of ultraviolet light creates excitations in DNA, which subsequently start moving in the helix. Their fate is important for an understanding of photodamage, and is determined by the interplay of electronic couplings between bases and the structure of the DNA environment. We model the effect of dynamical fluctuations in the environment and study correlation, which is present when multiple base pairs interact with the same mode in the environment. We find that the correlations strongly affect the exciton dynamics, and show how they are observed in the decay of the anisotropy as a function of coherence and population time in a nonlinear optical experiment. [ABSTRACT FROM AUTHOR]

We present the intrinsic optical properties over a broad spectral range of TiN thin films deposited on an Si/SiO2 substrate. We analyze the measured reflectivity spectra of the film-substrate multilayer structure within a well-establish procedure based on the Fresnel equation and extract the real part of the optical conductivity of TiN. We identify the metallic contribution as well as the finite energy excitations and disentangle the spectral weight distribution among them. The absorption spectrum of TiN bears some similarities with the electrodynamic response observed in the normal state of the high-temperature superconductors. Particularly, a mid-infrared feature in the optical conductivity is quite reminiscent of a pseudogap-like excitation. [ABSTRACT FROM AUTHOR]

We theoretically study the excitonic optical absorption in semiconductor superlattices irradiated by an intense terahertz (THz) laser polarized along the growth direction. We calculate the linear excitonic absorption spectra using the finite-difference time-domain method in real space. Our numerical results and qualitative analysis show that when tuning the frequency or intensity of the THz laser, the dynamical localization (DL) and the ac Stark effect (ACSE) are either jointly or individually responsible for the exciton peak shift. Moreover, with the onset of miniband collapse, the excitonic dimensionality crossover and the DL will counteract each other in shifting the exciton peak. The direct experimental verification of the DL through semiconductor optical spectra is still a challenge. Regarding this fact, we propose a method to single out the DL effect from the spectral shift dominated by the ACSE. [ABSTRACT FROM AUTHOR]

Two new two-photon absorption (TPA) molecules, named SK-G1 and NT-G1, are synthesized and the photo-physical characteristics are investigated by using linear absorption spectra, one-photon fluorescence spectra and two-photon excited fluorescence spectra. Both the compounds exhibit TPA properties, and the TPA values determined by z-scan measurement are 10GMand 39GMfor SK-G1 and NT-G1, respectively, at wavelength 800nm. Time-resolved spectroscopic techniques are employed to further explore the excited state dynamics of NT-G1with larger TPA cross section. The research results show that there is an ultrafast intraband energy transfer process (about 3ps) before the formation of charge transfer state with a relatively long lifetime. [ABSTRACT FROM AUTHOR]

Polarization-dependent linear absorption, second-harmonic generation (SHG) and 3rd-order nonlinearities of well-aligned ZnO nanorod arrays have been investigated with ps pulses. The depressed spectral width and the enhanced intensity of reflective SHG along the long axis of ZnO nanorods were observed by using p-polarized pulses, which is explained by the optical confinements. The nonlinear absorption coefficient measured with s-polarization reached the maximum 4.0 × 104cm/GW at the wavelength [?]750 nm, which revealed a large two-photon resonance absorption attributed to the quantum confined exciton when the polarization is vertical to the long axis of ZnO nanorod. [ABSTRACT FROM AUTHOR]

Lead iodide nanoparticles are synthesized in reverse micelle solution of AOT/H2O/n-heptane. Optical absorption spectra and TEM analysis indicated the formation of crystalline particles with an average radius of 1.5 nm, which is less than the Bohr radius of the exciton (1.9 nm) in bulk PbI2. Using theoretical models and optical spectra of quantum confined PbI2 nanoparticles, a radius of 1.5 nm and a thickness of 1.7 nm was calculated, which are in full agreement with the TEM results. Particles were isolated from the dispersed medium and were analysed by powder XRD and Raman spectroscopy, indicating the formation of a predominantly 2H-PbI2 polytype. This work presents the first case of fully isolated, fully characterized solid nanoparticles of PbI2. It also presents XRD and Raman spectrum for the first time for PbI2 nanoparticles of intermediate quantum confinement. [ABSTRACT FROM AUTHOR]

ZnS nanoribbons with variable widths ranging from a few nanometres to micrometre orderhave been fabricated in a controlled way. The ultra-long nanoribbons were singlecrystalline, smooth and uniform throughout their lengths. Growth of the nanoribbons wasinitiated by the vapour–liquid–solid process and subsequently their widths wereregulated by the influence of the vapour–solid process. These nanoribbons possesseda bulk-like bandgap as determined from the optical absorption spectra. Roomtemperature photoluminescence was observed in the UV–visible region from theZnS nanoribbons with the emission peaks at 398 and 458 nm attributed to thetransitions from the sulfur vacancy related defect states and surface states respectively. [ABSTRACT FROM AUTHOR]

A blue organic light-emitting diode (OLED) with silica-coated silver nanocubes (Ag@SiO2 NCs) inserted at the hole transporting layer/emission layer interface is reported. The localized surface plasmon resonance (LSPR) properties of the Ag@SiO2 NCs were characterized by the measured absorption spectra, stable-stated and transient photoluminescence, and calculated dipole radiation power. The results suggest that the Ag NCs significantly improved the radiation intensity of the nearby excitons due to their sharp corners and edges, but had less impact on the radiation rate of the excitons. The exciton recombination zone in the blue OLED was confirmed by a group of devices with an ultra-thin yellow emission layer located at different places, which helped to figure out the distribution of the excitons around the Ag@SiO2 NCs and deeply understand the coupling between the excitons and the Ag@SiO2 NCs. In our blue OLED, an appropriate distance between the Ag NCs and the excitons was realized by the SiO2 coating layer and the exciton distribution, which greatly improved the energy transfer between the excitons and the Ag NCs. In addition, the LSPR enhanced electric field around the Ag@SiO2 NCs improved the carrier injection at the hole transporting layer/emission layer interface and increased the current density of the blue OLED. Finally, the blue OLED with a simple triple layer structure achieved a high current efficiency of 51.1 cd A−1. [ABSTRACT FROM AUTHOR]

Transparent conducting oxides keep attracting strong scientific interest not only due to their promising potential for ‘transparent electronics’ applications but also due to their intriguing optical absorption characteristics. Materials such as In2O3 and Ga2O3 have complicated unit cells and, consequently, are interesting systems for studying the physics of excitons and anisotropy of optical absorption. Since currently no experimental data is available, for instance, for their dielectric functions across a large photon-energy range, we employ modern first-principles computational approaches based on many-body perturbation theory to provide theoretical-spectroscopy results. Using the Bethe–Salpeter framework, we compute dielectric functions and we compare to spectra computed without excitonic effects. We find that the electron–hole interaction strongly modifies the spectra and we discuss the anisotropy of optical absorption that we find for Ga2O3 in relation to existing theoretical and experimental data. [ABSTRACT FROM AUTHOR]

Excitons in high-purity crystals of Cu2O undergo a density-dependent lifetime that opposes Bose–Einstein condensation (BEC). This rapid decay rate of excitons at a density n has generally been attributed to Auger recombination having the form , where A is an exciton-Auger constant. Various measurements of A, however, have reported values that are orders-of-magnitude larger than the existing theory. In response to this conundrum, recent work has suggested that excitons bind into excitonic molecules, or biexcitons, which are short-lived and expected to be optically inactive. Of particular interest is the case of excitons confined to a parabolic strain well—a method that has recently achieved exciton densities approaching BEC. In this paper we report time- and space-resolved luminescence data that supports the existence of short-lived biexcitons in a strain well, implying an exciton loss rate of the form with a biexciton capture coefficient C(T) proportional to , as predicted by basic thermodynamics. This alternate theory will be considered in relation to recent experiments on the subject. [ABSTRACT FROM AUTHOR]

Considering the exciton effect, the excitation energy and its binding energy of the metallic single-walled carbon nanotubes (SWNTs) are theoretically studied by using the simple tight-binding model, based on which the linear absorption spectra are also calculated. It is found that due to the trigonal warping effect, the excitation energies of the linear optical spectra all are split into two corresponding ones. Additionally, the splitting depends on both the chirality and the transition energy: (1) the splitting is maximal for the zigzag tubes, the splitting decreases with the increasing chiral angle; (2) the higher the transition energy is, the larger the splitting is. It is very interesting to find that the obtained results are in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Homogeneous monolith of A1PO4 gel doped with rhodamine 6G (Rh6G) at different dye loadings is prepared by a one-step process with sol-gel method using the precursors A1(lact)3 and H3PO4. The optical properties of A1PO4 gel doped with Rh6G are characterized by UV-Visible absorption spectra and fluorescence spectra. Rh6G molecular J-dimers and H-dimers even multimers are analyzed by excitation spectra based on exciton theory. The A1PO4 gel doped with Rh6G dye with molar ratio of Rh6G/A1(lact)3 of 1 x 10[?]4 has excellent optical properties without obvious aggregates. [ABSTRACT FROM AUTHOR]

According to ultraviolet (UV)-vis absorption spectra recorded in the DNA metallization process, DNA-templated Co/Cu binary nanoparticle chains are fabricated by incubating genome DNA of paralichthys olivaceus muscle in CoCl2 and CuCl2 mixture solution for 20 hours and reducing the complex for 2 hours. Transmission electron microscopy observation indicates that Co and Cu nanoparticles with 20 nm in diameter were randomly dispersed on the DNA template. The superconducting quantum interference device (SQUID) measurements display that the magnetic interaction between cobalt particles is greatly decreased by the copper particle. With increasing copper content, the coercivity of the systems enhance from 9 Oe to 100 Oe (1 Oe = 79.5775 A/m). [ABSTRACT FROM AUTHOR]

The properties of meso-tetraphenylporphine (TPP) aggregates formed in acidic aqueous-organic solutions are investigated by UV-vis spectroscopy. According to the absorption spectra, the Q band absorption of the aggregated TPP shows red shift and intensity enhancement, and a model that includes the participation of water molecules in a porphyrin aggregation complex is proposed, then a qualitative explanation based on Gouterman's excition coupling theory is presented. Calculations including eigenenergies, eigenstates and the transition dipole strength of the coupled states are carried out. [ABSTRACT FROM AUTHOR]

We consider excitons in a quantum dot as q-deformed systems. The interaction of some excitonic systems with one cavity mode is considered. The dynamics of the system is obtained by diagonalizing the total Hamiltonian, and the emission spectrum of a quantum dot is derived. The physical consequences of a q-deformed exciton on the emission spectrum of a quantum dot are given. It is shown that when the exciton system deviates from Bose statistics, the emission spectra will become multi-peak. With our investigation we try to find the origin of the q-deformation of the exciton. The optical response of excitons, which is affected by the nonlinear nature of q-deformed systems, up to the second order of approximation is calculated and the absorption spectrum of the system is given. [ABSTRACT FROM AUTHOR]

ZnO nanorod close-packed arrays are successfully fabricated in a substrate-free manner by a citric acid assisted annealing process at a low growth temperature of 400 °C. Each nanorod of ZnO nanorod close-packed arrays grows along the [0001] direction and is single crystalline with an average diameter of 50 nm, and an average length of 0.5 µm. The aspect ratio is 10. The ZnO nanorod close-packed arrays show a strong exciton absorption peak at 372 nm in UV-visible absorption spectra, exhibiting a blue-shift relative to the bulk exciton absorption (387 nm). Finally, a new growth mechanism is proposed for the substrate-free preparation of ZnO nanorod close-packed arrays by a citric acid assisted annealing process. [ABSTRACT FROM AUTHOR]