Karel Král

Optical line width in semiconductor quantum dots

The line-width of the optical transitions in quantum dots is studied theoretically on the basis of the electron coupling to the longitudinal optical phonons in polar semiconductors. With using the self-consistent Born approximation to the electronic self-energy, we are able to reproduce one of the main experimental results obtained on CdSe and CuBr quantum dots, namely the linear dependence of the width of the optical line on the inverse of the quantum dot diameter. In addition to it, the theory allows to expect certain resonance features on the linear dependence of line width. We remind that perhaps extensions of the present line-width theory might be suitable for to describe adequately the behavior of the line width in CdSe and InAs quantum dots.

A Study of PbTiO3 Crystallization in Pure and Composite Nanopowders Prepared by the Sol-Gel Technique

In this investigation the crystallization of PbTiO3 upon annealing of pure nanopowders and PbTiO3–SiO2 (1:1 v/v) nanocomposite powders prepared by the sol-gel technique was studied. Using X-ray diffraction phase analysis, the start of PbTiO3 crystallization in pure PbTiO3 powders was detected at 400°C. Distinct crystallization of PbTiO3 in PbTiO3–SiO2 nanocomposites starts at 700°C, whereas SiO2 remains amorphous. There are indications that an interface interaction between the PbTiO3 and the SiO2 phase plays an important role in hindering the crystallization of PbTiO3. The particle size (size of coherently scattering regions) was estimated from the broadening of the X-ray diffraction line profiles. The average size of PbTiO3 nanocrystallites increases with temperature and time of annealing, the influence of temperature being more significant than that of the annealing time. Differential scanning calorimetry confirmed the results of the X-ray diffraction with respect to the start of the crystallization. Laser beam scattering and scanning electron microscopy provided the statistical distribution of the grain size and the morphology of the powder grains, showing that each grain of the powders contains several nanocrystallites (coherently scattering regions).

Charge-carrier/exciton transfer between two quasi-zero-dimensional nanostructures

Quantum transport theory is applied to obtain formula for the irreversible transfer of electronic excited state (exciton), or a charge carrier, between two quasi-zero-dimensional nanostructures. The electron-phonon interaction is included in a non-perturbative way. The formula is expected to be suitable for analysis of experimental data on exciton or charge transfer. We emphasize the use of the transfer formula in the problem of the electrical conduction of polymers. Menšík.

Photoluminescence of nanostructures with indirect band gap

Photoluminescence properties of the low-dimensional semiconductor nanostructures of materials with indirect electronic band gap are studied theoretically with an emphasis put on the coupling of charge carriers to the atomic lattice vibrations. The photoluminescence intensity decay time dependence, the temperature dependence of the photoluminescence as well as the dependence of the photoluminescence on the lateral dimensions of the nanoparticles are considered theoretically together with their comparison with experimental data. These properties are treated using the example of the photoluminescence properties of small InAs nanoparticles. The results obtained for the small particles of InAs are expected to be interesting also in connection with the properties of e. g. silicon nanostructures.

Quantum dots with indirect band gap: power-law photoluminescence decay

In this work the experimental effect of a slow decay of the photoluminescence is studied theoretically in the case of quantum dots with an indirect energy band gap. The slow decay of the photoluminescence is considered as decay in time of the luminescence intensity, following the excitation of the quantum dot sample electronic system by a short optical pulse. In the presented theoretical treatment the process is studied as a single dot property. The inter-valley deformation potential interaction of the excited conduction band electrons with lattice vibrations is considered in the self-consistent Born approximation to the electronic self-energy. The theory is built on the non-equilibrium electronic quantum transport theory. The time dependence of the photoluminescence decay is estimated upon using a simple effective mass model. The numerical calculation of the considered model shows the power-law time characteristics of the photoluminescence decay in the long-time limit of the decay. We demonstrate that th...

Power-law photoluminescence decay in quantum dots

Some quantum dot samples show a long-time (power-law) behavior of their luminescence intensity decay. This effect has been recently explained as being due to a cooperation of many tunneling channels transferring electrons from small quantum dots with triplet exciton to quantum dots at which the electrons can recombine with the holes in the valence band states. In this work we show that the long-time character of the sample luminescence decay can also be caused by an intrinsic property of a single dot, namely, by a non-adiabatic effect of the electron occupation up-conversion caused by the electron-phonon multiple scattering mechanism.

Electron transfer between quasi-zero-dimensional nanostructures

The electric charge transport is an important phenomenon in the systems like interacting quantum dots and molecules, and in polymers, including DNA molecules. We expect that in these nanostructure systems the key role is played by the interaction of the charge carriers with the optical phonons. We show the role of the multiple scattering of the charge carriers on the optical phonons in the inter-molecular transfer. The charge carrier transport based on this mechanism will be discussed theoretically and compared with the earlier experimental results on the charge transport in molecular Donor-Acceptor charge transfer crystals and also in other systems. In order to treat theoretically the electron transfer between two zero-dimensional nanostructures, we will use for simplicity the model of two interacting quantum dots coupled by the electron inter-dot tunneling mechanism. A connection with the popular Marcus semiclassical charge transfer theory, between two quasi-zero-dimensional reactants in chemistry, is also shown. We will use the nonequilibrium quantum electronic transport theory based on the nonequilibrium Greens functions.

Dependence of Resonant Effects in Excited-State Decay on the form of Inter-State Coupling

Time dependence of excited state decay is studied theoretically. The model system is assumed to consist of two electronic levels coupled to a single vibrational mode and dissipating its energy to a thermal bath. The form of inter-state coupling is assumed to be either a) constant (independent on vibrational coordinate) or b) linear in vibrational coordinate. Generally, the rate of excited state decay resonantly increases when the ratio r of the excitation energy and vibrational quantum takes integer values (r = 1, 2, 3, …). The effect is stronger and appears even for r ≥ 2 when the Huang-Rhys factor is larger and/or the inter-state coupling is explicitly dependent on vibrational coordinate.