Search results for "Exciton"
showing 10 items of 317 documents
Quantum Chemical Parametrization and Spectroscopic Characterization of the Frenkel Exciton Hamiltonian for a J-Aggregate Forming Perylene Bisimide Dye
2012
Quantum chemical and quantum dynamical calculations are performed for a bay-substituted perylene bisimide dye up to its hexameric aggregate. The aggregate structure is determined by employing the self-consistent charge density functional tight-binding (SCC-DFTB) approach including dispersion corrections. It is characterized by a stabilization via two chains of hydrogen bonds facilitated by amide functionalities. Focusing on the central embedded dimer, the Coulomb coupling for this J-aggregate is determined by means of the time-dependent density functional theory (TDDFT) to be -514 cm(-1). Exciton vibrational coupling is treated within the shifted oscillator model from which five strongly co…
Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates
2011
3 figuras, 3 páginas.
Morphology and Band Structure of Orthorhombic PbS Nanoplatelets: An Indirect Band Gap Material
2021
PbS quantum dots and nanoplatelets (NPLs) are of enormous interest in the development of optoelectronic devices. However, some important aspects of their nature remain unclear. Recent studies have revealed that colloidal PbS NPLs may depart from the rock-salt crystal structure of bulk and form an orthorhombic (Pnma) modification instead. To gain insight into the implications of such a change over the optoelectronic properties, we have synthesized orthorhombic PbS NPLs and determined the lattice parameters by means of selected area electron diffraction measurements. We have then calculated the associated band structure using density functional theory with Perdew–Burke–Ernzerhof functional fo…
Low-temperature exciton absorption in InSe under pressure.
1992
We have investigated the effect of pressure on the lowest direct band-edge exciton of the layered semiconductor InSe by optical-absorption measurements at 10 K and for pressures up to 4 GPa. The Elliott-Toyozawa formalism is used to analyze the line shape of the exciton absorption spectra. In this way we determine the pressure dependence of the lowest direct band gap, the exciton binding energy, and the exciton linewidth. The band gap exhibits a pronounced nonlinear shift with pressure, which is a consequence of the strong anisotropy of intralayer and interlayer bonding. The exciton binding energy decreases with pressure, mainly due to the large increase of the low-frequency dielectric cons…
Dynamics of exciton creation and decay processes in composition – disordered InGaN thin films
2011
In the GaN-based ternary alloys, InGaN crystals have been recognized as key materials for e-h plasmas-exciton dynamics, because of large exciton binding energies (24.8 meV in GaN). We report investigations of creating and recombination dynamics of excitons in commercially important InxGa1-xN composition range from x = 0.1 to 0.18 in which nanoscale indium composition fluctuation occurs and formation of indium rich clusters acting as quantum dots (QD) can be expected. Three MOCVD grown samples having x = 0.1; 0.14 and 0.18 have been investigated using 3D picosecond transient PL spectroscopy. It has been found that the band to band photo excitation at 15 K in whole composition range results i…
Spectral characterization of bulk and nanostructured aluminum nitride
2009
Spectral characteristics including photoluminescence (PL) spectra and its excitation spectra for different AlN materials (AlN ceramics, macro size powder and nanostructured forms such as nanopowder, nanorods and nanotips) were investigated at room temperature. Besides the well known UV-blue (around 400 nm) and red (600 nm) luminescence, the 480 nm band was also observed as an asymmetric long-wavelength shoulder of the UV-blue PL band. This band can be related to the luminescence of some kind of surface defects, probably also including the oxygen-related defects. The mechanisms of recombination luminescence and excitation of the UV-blue luminescence caused by the oxygen-related defects were …
Surfactant-Dependent Exciton Mobility in Single-Walled Carbon Nanotubes Studied by Single-Molecule Reactions
2010
Measurements of stepwise photoluminescence quenching in individual, (n,m)-selected single-walled carbon nanotubes (SWCNTs) undergoing chemical reaction have been analyzed to deduce mobilities of optically generated excitons. For (7,5) nanotubes, the mean exciton range varies between approximately 140 and 240 nm for different surfactant coatings and correlates weakly with nanotube PL intensity. The results are consistent with a model of localized SWCNT excitons having substantial diffusional mobility along the nanotube axis.
F centre production in CsI and CsI–Tl crystals under Kr ion irradiation at 15 K
2000
Abstract We present results of simultaneous in situ luminescence and optical absorption studies in scintillator CsI and CsI–Tl crystals, exposed to very dense electronic excitations induced by 86Kr ions (8.63 MeV/amu). Irradiation at 15 K leads to the formation of the prominent F absorption band. In addition, several other features of the broad absorption between exciton and F bands were ascribed to an anion vacancy, α centre (240 nm), self-trapped hole, Vk centre (410 nm) and interstitials, H centres (560 nm). We have found that low doping of thallium (∼1017 cm−3) causes the F centre formation to proceed more rapidly than in pure crystal. On the other hand, we were not able to create any a…
Exciton-created defects and their participation in energy transfer from excitons to Tl ions in KI–Tl crystal
2000
Abstract Luminescence properties (spectra, yields, kinetics) of Tl ions induced by energy transfer from different types of anion excitons were studied and compared with those observed under direct Tl ion excitation by light quanta from A and C absorption bands in KI–Tl crystal at low temperature. The mechanism of energy transfer from excitons to the impurity ions was proposed based on the exciton-created defects participating in the impurity ion excitation.
The experimental observation of the potential barrier for self-trapped exciton decay into F-H pair in KCl-Na crystals
1995
Abstract The optical absorption induced by the electron pulse irradiation of Na+ doped KCl has been measured. Transient optical absorption band of FA centers was observed at 80 K (LNT). The temperature dependence of FA center formation was studied. It is proposed that the obtained activation energy originates from the potential barrier between the STE perturbed by the cation impurity and the nearest neighbour FA-H pair. The mechanism of the suppression of the defect formation by the monovalent cation impurity in alkali halide crystals is discussed.