Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Dynamics of electron-hole pair recombination in semiconductor clusters

Abstract: The kinetics of radiative electron-hole pair recombination in CdS and Cd{sub 3}As{sub 2} clusters (where the radius of the cluster is smaller than the de Broglie wavelength of photogenerated excitons) were studied with picosecond photon counting luminescence decay measurements over wide temperature and energy ranges. The decay profiles were quantitatively examined with several models. The decays are composed of two distinct time regimes, each with very different temperature and emission energy dependence. The first (fast) regime is attributed to an unusually efficient thermal repopulation mechanism. The second (slow) component is well described by a distributed kinetic model. The kinetic behavior of wide (CdS) and narrow (Cd{sub 3}As{sub 2}) band gap materials was remarkably similar when composed of clusters in the quantum confined regime.

Kinematics of Exciton-Exciton Annihilation in Molecular Crystals

Abstract: A theory is developed for calculating the kinematic part of the exciton-exciton annihilation rate in molecular crystals. The spatial and spin motion of the excitons, as well as the annihilation process itself, is treated phenomenologically. Exciton propagation is assumed to take place as in the hopping model. The importance of the dimensionality of the exciton motion is pointed out; in nearly one- or two-dimensional cases, certain lifetime processes control the exciton collision rate, in contrast to the three-dimensional case. These lifetime processes include motion out of the one- or two-dimensional subspace and, for excitons with spin, spin relaxation. The theory leads to a description of magnetic field effects on the annihilation rate of triplet excitons at room temperature. When applied to triplet excitons in anthracene, this description gives a satsfactory fit to the observed effects and leads to the determination of the nearest-neighbor exciton annihilation rate, the singlet-channel annihilation rate constant, and the exciton diffusion constant for motion perpendicular to the $\mathrm{ab}$ plane of anthracene.

The Oxygen Vacancy as the Origin of a Green Emission in Undoped ZnO

Abstract: We have investigated undoped ZnO single crystals, which are commercially available from Eagle-Picher, by photoluminescence (PL) and optically detected magnetic resonance (ODMR) spectroscopy. The electrical properties of this material are very similar to the samples investigated. The total residual shallow donor concentration is about 1 x 10{sup 17} cm{sup -3}. The low temperature emission is dominated by the donor bound exciton (D{sup 0}X) at 3.366 eV. At 2.45 eV the broad, unstructured ''green'' emission is located, its full width at half maximum is 320 meV. The temperature dependence of the PL reveals that this green band maintains its peak energy up to 450 K, which is a feature typical of a recombination within a localised defect, while the D{sup 0}X emission follows the shrinkage of the bandgap with increasing temperature. (orig.)

Rapid radiative decay and enhanced optical nonlinearity of excitons in a quantum well.

Abstract: An exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field, which results in a polariton in a bulk crystal, brings about the rapid radiative decay of the exciton in low-dimensional systems due to breakdown of the translational symmetry. This large decay constant at the same time makes the excitons deviate from ideal bosons so that we have a large third-order optical susceptibility enhanced by the macroscopic transition dipole moment under near-resonant excitation. The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicosecond in CdS quantum wells through the short lifetime of excitons.

Exciton Multiplication and Relaxation Dynamics in Quantum Dots: Applications to Ultra-High Efficiency Solar Photon Conversion

Abstract: We show how semiconductor quantum dots may greatly increase photon conversion efficiencies by producing multiple excitons from a single photon. This is possible because quantization of energy levels in quantum dots enhances Auger processes, eliminates the requirement to conserve crystal momentum, and thus promotes multiple exciton generation, Quantum yields of 300% for exciton formation in PbSe, PbS, and PbTe quantum dots have been reported at photon energies about 4 times the HOMO-LUMO transition energy (QD bandgap), indicating the formation of 3 excitons/photon for all photoexcited quantum dots.