CdSe Microcrystallites Research Articles

Electron Dynamics in CdSe Microcrystallites in Porous Glass

We embedded CdSe nanocrstallites into a porous glass with a diffusion process. Using picosecond time resolved luminescence spectrum technique, we investigated the electron dynamics in the CdSe microcrystallites in porous glass. The luminescence decay of our sample has a fast component decay sensitive to temperature. For a sample of CdSe microcrystallites with an average size of 25 Å diameter, the fast decay is 50 ps at 77 K, and is 100 ps at 2 K. With a simple three level model, the luminescence decay is simulated. The temperature dependence of luminescence decay results from nonradiative process.

Quantum size effects and optical nonlinearity of confined excitons in semiconducting microcrystallites

We report linear and nonlinear optical response of CdSe and CuCl microcrystallites embedded in glasses. In CdSe microcrystallites, we observe both the individual confinement of the electron-hole pair for R < 68 Å and the exciton confinement for R > 120 Å. Furthermore, we have investigated the size-dependent enhancement of the third-order nonlinear susceptibility χ3 in CuCl microcrystallites with radii of 15 to 80 Å. The absolute values of χ3 measured at 80 K is increased as a function of the radius and subsequently decreased after reaching a maximum value at ∼ 55 Å. These results clearly confirm the mesoscopic enhancement of the nonlinear optical response of the quantum confined exciton system.

Linear and nonlinear optical properties of CdSe microcrystallites in glasses

We investigated linear and nonlinear properties of CdSe microcrystallites embedded in glass matrices and with radii ranging from 2 to 12 nm. The transient absorption spectra measured by the femtosecond pump-probe method gave us information on the relaxation processes of the confined electrons and holes in the microcrystallites. When the microcrystallite radius is smaller than the exciton radius, the optical transition between the quantized levels of an electron and of a hole is blocked by the existence of photoexcited carriers in these levels and in the shallow trap centers. In contrast, when the crystallite radius is larger than the exciton radius, relaxation of hot carriers with emission of phonons is observed, as in bulk crystals.

Exciton-LO-phonon couplings in spherical semiconductor microcrystallites.

Exciton--LO-phonon couplings in ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ semiconductor microcrystallites (x=0.12\ifmmode\pm\else\textpm\fi{}0.05) are investigated by measuring the temperature dependence of the width and energy of excitons by electroabsorption. The LO phonons are shown semiquantitatively to contribute to the experimentally obtained temperaure dependencies of the width and energy of excitons. The dependence of the coupling constant (the Huang-Rhys parameter) on the radius of microcrystallites is calculated for CdSe and GaAs microcrystallites. The phonon confinement effects are considered with ``free-standing'' and ``rigid''-boundary conditions. As for the exciton state, nonparabolicity of the conduction band and the valence-band mixing are considered in order to obtain a precise exciton wave function, which is crucially important in calculating the Huang-Rhys parameter in a microcrystallite. The exciton--confined-optical-phonon interaction Hamiltonian is constructed for a microcrystallite. It is found that the Huang-Rhys parameters have a minimum at a radius of 70 \AA{} for CdSe and 270 \AA{} for GaAs microcrystallites. The size dependence of the Huang-Rhys parameter is also calculated for a microcrystallite with an extra charge at the spherical-particle center. The lowest (s,${\mathit{S}}_{3/2}$) state in the trapped state is found to have small transition probability and g values of 1 in CdSe (R=30 \AA{}) and 0.01 in GaAs (R=100 \AA{}). The higher states are found to have larger transition probability and g values of 0.7 in CdSe (R=30 \AA{}) and 0.01 in GaAs (R=100 \AA{}). These results suggest that large g values observed experimentally in CdS and CdSe microcrystallites originate from extrinsic effects such as the presence of charged point defects inside the microcrystallite.

Spectral hole-burning in quantum confined semiconductor microcrystals

We present ns — hole burning experiments of the quantum confined transitions in CdSe microcrystallites. By means of respective model calculations we deduce the contribution of inhomogeneous and homogeneous broadening of the spectra at 77 K and room temperature. Tuning the excitation wavelength over a broad spectral range we find common blocking of altogether three electronic states about 160 meV apart the origin of which is discussed.

Optical nonlinearity of CdSe microcrystallites in a sputtered SiO2 film

Optical nonlinearity of a sputtered SiO2 film containing CdSe microcrystallites with an average diameter of 15 Å is measured by degenerate four-wave mixing (DFWM) experiments. An effective nonlinear cross section σeff of 1.4×10−16 cm2 is obtained. The DFWM signal as a function of the probe delay time shows biexponential behavior with a fast decay time of 10 ps and a slow decay time of 60 ps, which give fast and slow carrier recombination times as 20 ps and 120 ps. The third-order susceptibility ‖χ(3)‖ is estimated to be 1.3×10−8 esu. The photodarkening and laser annealing effects which are serious problems for device applications are not observed.

Nonlinear optical properties of quantum-confined CdSe microcrystallites

Quantum-confinement effects in CdSe microcrystallites in a glass matrix are investigated in the nanosecond time domain. The results of pump–probe and single-beam absorption saturation measurements show strong evidence for electron–hole quantization in the semiconductor microstructures, a different bleaching behavior from those of bulk semiconductors and multiple quantum wells, and a trend of increased saturation intensity with decreased microcrystallite size. A theory for the one- and two-pair states in quantum dots is developed that fully includes the relevant Coulomb interactions. The theory is evaluated numerically, and results are presented for intrinsic quantum dots and for quantum dots with impurities. The experimentally observed nonlinearities are attributed to saturation of one-electron–hole-pair resonances and induced absorption caused by two-pair resonances in the presence or absence of impurities (traps).

Influence of Hydrogenation of Defects at the Microcrystallite‐Glass Interface on the Nonlinear Optics of CdSe Microcrystallites

On montre que la non-linearite optique de l'absorption microcristalline peut etre controlee par chimie de surface. Les etats de l'interface microcristalline-verre sont responsables de la desoccupation partielle du niveau microcristallin excite

Experimental studies of the non-linear optical properties of cadmium selenide quantum-confined microcrystallites

The non-linear optical properties of CdSe microcrystallites exhibiting three-dimensional quantum confinement effects are investigated using (i) nanosecond pump-probe spectroscopic techniques and (ii) single beam saturation experiments to study the dependence on particle size. The experimental data show unique non-linear behaviour arising from state filling and Coulomb effects as well as trap-related phenomena exceedingly different from other non-linear optical materials.

Ultrafast optical nonlinearities in II–VI compounds

We review our experimental and theoretical results on the femtosecond laser excited CdS and CdSe bulk and quantum dots. Optical Stark effect in bulk CdS is reported where the electron-hole continuum states in addition to the bound-exciton states are blue shifted. The nonlinear optical properties of CdSe microcrystallites embedded in glass matrices are investigated using femtosecond and nanosecond laser pulses. Spectral hole-buring is observed which is accompanied by an induced absorption feature on the high-energy side. A 25±10 fs dephasing time is obtained from these measurements. It is concluded that Coulomb effects are important in quantum dots. They give rise to a ground state two-pair resonance on the low energy side of the one-pair state and higher energy two-pair resonances which are spectrally located between the one-pair states. Single-beam saturation experiments show that the intensities required for bleaching are larger for smaller crystallites.

Femtosecond optical nonlinearities of CdSe quantum dots

Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>