Showing papers in "SPIE milestone series in 2005"

Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer

Abstract: ELECTROLUMINESCENT devices have been developed recently that are based on new materials such as porous silicon' and semiconducting polymers 2,3 . By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals 4-6 , and of electroluminescent polymers7, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV) 2-10 with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage" of only 4 V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect 19-24 the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.

Optical gain and stimulated emission in nanocrystal quantum dots

Abstract: The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

Fluorescence intermittency in single cadmium selenide nanocrystals

Abstract: SEMICONDUCTOR nanocrystals offer the opportunity to study the evolution of bulk materials properties as the size of a system increases from the molecular scale 1,2 . In addition, their strongly size-dependent optical properties render them attractive candidates as tunable light absorbers and emitters in optoelectronic devices such as light-emitting diodes 3,4 and quantum-dot lasers 5, 6 and as optical probes of biological systems'. Here we show that light emission from single fluorescing nanocrystals of cadmium selenide under continuous excitation turns on and off intermittently with a characteristic timescale of about 0.5 seconds. This intermittency is not apparent from ensemble measurements on many nanocrystals. The dependence on excitation intensity and the change in on/off times when a passivating, high-bandgap shell of zinc sulphide encapsulates the nanocrystal 8,9 suggests that the abrupt turning off of luminescence is caused by photoionization of the nanocrystal. Thus spectroscopic measurements on single nanocrystals can reveal hitherto unknown aspects of their photophysics.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases

Abstract: We report on calculations of inlrasubband and intersubband phonon scattering in quantum-confined electron gases based on lattice-matched In x Ga 1-x As/InP quantum wells. Dimensionality effects on the emission of acoustic phonons are studied comparing the scattering times of two-, one-, and zero-dimensional electron gases as a function of the lateral confinement. Optical phonon scattering in quantum wells and wires is discussed using a phenomenological broadening of the one-dimensional density of states. The energy relaxation rates of heated electron gases due to phonon emission and absorption have been calculated for lattice temperatures T 1 between 0.3 and 20 K. For a given healing power per electron, the electron temperature T e in a quantum wire can be greater or smaller than that in the corresponding quantum well, depending on the electron density n s , while the energy relaxation in quantum dots with significant quantization energies is always slower than in the corresponding wells and wires.

Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites

Abstract: A simple route to the production of high-quality CdE (E = S, Se, Te) semiconductor nanocrystallites is presented. Crystallites from ∼ 12 A to ∼ 115 A in diameter with consistent crystal structure, surface derivatization, and a bigh degree of monodispersity are prepared in a single reaction. The synthesis is based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and permits controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of crystallites from portions of the growth solution isolates samples with narrow size distributions (<5% rms in diameter). High sample quality results in sharp absorption features and strong "band-edge" emission which is tunable with particle size and choice of material. Transmission electron microscopy and X-ray powder diffraction in combination with computer simulations indicate the presence of bulk structural properties in crystallites as small as 20 A in diameter.

Calculation of the band gap for small CdS and ZnS crystallites

Abstract: The tight-binding approximation and the recursion method are used to study the size dependence of the band gap for small CdS and ZnS crystallites (20-2500 atoms). Because of the lack of accurate experimental data, a simple model of the crystal is considered; one which has no dangling bonds and a symmetrical shape. It is then possible to have a good evaluation of the band gap, even for the largest crystallites. The optical-absorption spectra exhibit an excitonic peak; we determine the peak position from a simple evaluation of the binding energy. The results are compared with the results of other calculations based upon the effective-mass approximation and some experimental data.

Interband absorption of light in a semiconductor sphere

Abstract: A very simple model is used to allow for the influence of size quantization on interband absorption in a semiconductor sphere. Expressions are obtained for the absorption coefficients of light in three limiting cases; when the radius of a semiconductor sphere a is less than the Bohr radii of electrons a, and holes a h ; when a h

New results on optical phase conjugation in semiconductor-doped glasses

Abstract: We have observed a darkening effect and the formation of permanent volume gratings in CdSSe semiconductor-doped glasses, both phenomena being certainly due to the same photochemical mechanism. This allowed us to reconcile contradictory results concerning the speed of their nonlinear response. We also studied the frequency dependence of X (3) below the gap and the intensity dependence of the reflectivity of these phase-conjugate mirrors. Our results can be interpreted in terms of the band-filling model with Boltzmann statistics. Finally, frequency- and. time-resolved luminescence measurements helped us to interpret various results.

Solving Schrodinger's equation around a desired energy : Application to silicon quantum dots

Abstract: We present a simple, linear-in-size method that enables calculation of the eigensolutions of a Schrodinger equation in a desired energy window. We illustrate this method by studying the near-gap electronic structure of Si quantum dots with size up to Si 1315 H 460 ≈37 A in diameter) using a plane wave pseudopotential representation.

Crystal structure and optical properties of Cd32S14(SC6H5)36-DMF4, a cluster with a 15 angstrom CdS core

Abstract: Recrystallization of the solid Cd 10 S 4 (SC 6 H 5 ) 12 from a solution of pyridine and N,N-di-methytformamide (DMF) results in the formation of the cluster Cd 32 S 1 4 (SC 6 H 5 ) 36 -DMF 4 as pale yellow cubes. The structure consists of an 82-atom CdS core that is a roughly spherical piece of the cubic sphalerite lattice -12 angstroms in diameter. The four corners of the lattice are capped by hexagonal wurtzite-like CdS units, which results in an overall tetrahedral cluster ∼ 15 angstroms in diameter. This cluster dissolves intact in tetrahydrofuran where its absorption spectrum reveals a sharp peak at 358 nanometers at room temperature and its emission spectra show a strong broad band at 500 nanometers.

Effects of dielectric confinement and electron-hole exchange interaction on excitonic states in semiconductor quantum dots

Abstract: A general scheme is established within the effective-mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect. This effect is found to appear most pronounced in the quantum-dot structure in comparison with the quantum-well and quantum-wire structures. A formula of the lowest exciton energy in the strong confinement regime is derived and the significance of the dielectric confinement effect is clarified. We investigate the dependence of the binding energy and the oscillator strength of the lowest-energy excitonic state on the quantum-dot radius, the electron-to-hole mass ratio, and the dielectric-constant ratio between the quantum dot and the surrounding medium. The subband mixing effect due to the electron-hole Coulomb interaction gives a finite oscillator strength to excitonic transitions which are forbidden in the absence of the Coulomb interaction. This effect is shown unambiguously in the calculated excitonic energy spectra. Furthermore, the electron-hole exchange interaction in a quantum dot is discussed. The short-range part of the exchange energy is shown to increase in proportion to the inverse of the volume of the quantum dot as the quantum-dot size is reduced. On the other hand, the long-range part of the exchange energy is found to be sensitively dependent on the shape of the quantum dot. In particular, it vanishes for the optically allowed excitonic states in a spherical quantum dot.

Electronic states of semiconductor clusters : Homogeneous and inhomogeneous broadening of the optical spectrum

Abstract: The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35-50 ? diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm -1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron-vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm -1 , or seven times the homogeneous width, in the most monodisperse sample.

Exciton-LO-phonon couplings in spherical semiconductor microcrystallites

Abstract: Exciton-LO-phonon couplings in CdS x Sc 1-x semiconductor microcrystallites (x=0.12±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 Harniltonian is constructed for a microcrystallite. It is found that the Huang-Rhys parameters have a minimum at a radius of 70 A for CdSe and 270 A 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,S 3/2 ) state in the trapped state is found to have small transition probability and g values of 1 in CdSe (R =30 A) and 0.01 in GaAs? = 100 A). The higher states are found to have larger transition probability and g values of 0.7 in CdSe (R =30 A) and 0.01 in GaAs (R = 100 A). 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.

Analytical formalism for determining quantum-wire and quantum-dot band structure in the multiband envelope-function approximation

Abstract: We describe a new formalism for determining energy eigenstates of spherical quantum dots and cylindrical quantum wires in the multiple-band envelope-function approximation. The technique is based upon a reformulation of the K·P theory in a basis of eigenstates of total angular momentum. Stationary stales are formed by mixing bulk energy eigenvectors and imposing matching conditions across the heterostructure interface, yielding dispersion relations for eigenenergies in quantum wires and quantum dots. The bound states are studied for the conduction band and the coupled light and heavy holes as a function of radius for the GaAs/Al 2 Ga 1-x As quantum dot. Conduction-band-valence-band coupling is shown to be critical in a "type-II" InAs/GaSb quantum dot, which is studied here for the first time. Quantum-wire valence-subband dispersion and effective masses are determined for GaAs/Al x Ga 1-x As wires of several radii. The masses are found to be independent of wire radius in an infinite-well model, but strongly dependent on wire radius for a finite well, in which the effective mass of the highest-energy valence subband is as low as 0.16m o . Implications of the band-coupling effects on optical matrix elements in quantum wires and dots are discussed.

Polar optical vibrational modes in quantum dots

Abstract: A macroscopic continuum model coupling the mechanical vibrational amplitude and electrostatic potential is applied to obtain the optical vibrational modes in quantum-dot structures. A unified method of solution (valid for any type of nanostructure) that reduces the four coupled second-order differential equations to Helmholtz's and Laplace's equations is described. Analytical solutions for the vibrational amplitude and the Frohlich-type electron-phonon interaction are given for quantum dots with spherical geometry. The existence of surface modes and their relation to the matching boundary conditions are studied. A qualitative discussion of the ir and Raman activity of the calculated modes is given together with a comparison with the few existing experimental data.

Electronic structures of zero-dimensional quantum wells

Abstract: The electronic structures of zero-dimensional quantum wells are studied with a spherical model in the framework of the effective-mass theory. The mixing effect of the heavy and light holes is taken into account, and the symmetry classification and the energy levels of hole states are obtained. The energies of the donor and acceptor states are calculated. The difference between the shallow-impurity states and the eigenstates for the small semiconductor sphere disappears. The selection rules for the optical transition between the conduction- and valence-band states are obtained. The An =0 selection rule is not followed strictly because of the mixing of the L- and (L +2)-orbital wave functions in the wave functions of the hole states. The exciton binding energies are calculated for the small GaAs spheres. The energy levels of the ZnSe spheres are given as functions of the radius and compared with the experiments.

Phonon broadening and spectral hole burning in very small semiconductor particles

Abstract: We report on experimental evidence of the important role of phonon broadening in very small CdSSe crystallites. At low temperature, spectral hole burning is observed. These results are in agreement with the special conditions of quantum confinement and with the polar character of this semiconductor.

Photochemistry of colloidal metal sulfides. 7. Absorption and fluorescence of extremely small ZnS particles (The world of the neglected dimensions)

Abstract: Extremely small colloidal ZnS particles (diameter -1.7 nm) were made by either photo-degradation of 3 nm particles or rapid precipitation in phosphate containing solution at pH = 7-8. The absorption spectra of these particles are different from that of macrocrystalline ZnS, and the changes are regarded as an indication for the transition from semiconductor ZnS to polymolecular ZnS with decreasing particle size. - The dependence of the intensity of the fluorescence on particle size, temperature, photoanodic corrosion and the quenching of fluorescence were investigated for colloids on a silicon dioxide carrier, in phosphate solution, and without a stabilizer. Photo-anodic corrosion strongly improves the fluorescence properties. - One adsorbed Cd 2+ ion per colloidal particle is sufficient for efficient quenching of the fluorescence. However, a new fluorescence band appears which is explained by the formation of a layer of 1:1 co-colloid at the surface of the ZnS particles. - Methylviologen was also found to be a very efficient quencher. The decay of the fluorescence is wavelength dependent, i.e. the fraction of long-lived fluorescence is greater at longer wavelengths. - A mechanism is discussed, where the fluorescence centers are anion vacancies, and fluorescence is emitted when electrons trapped in states of different energies and exhibiting different life-times tunnel to the localized positive holes.

Electron-phonon interactions and excitonic dephasing in semiconductor nanocrystals

Abstract: The size dependence of the contribution to the excitonic dephasing rate in semiconductor nanocrystals is clarified for various electron-phonon coupling mechanisms. On the basis of these dependencies, the commonly observed linearly temperature-dependent term of the excitonic dephasing rate and the proportionality of its magnitude to the inverse square of the nanocrystal size are attributed to pure dephasing due to deformation-potential coupling. The calculated coefficients of the linearly temperature-dependent term are quantitatively in good agreement with the experimental results on CdSe and CuCI nanocrystals.

Investigation of femtosecond electronic dephasing in CdSe nanocrystals using quantum-beat-suppressed photon echoes

Abstract: We report the first direct measurements of femtosecond electronic dephasing in CdSe nanocrystals using three-pulse photon echoes and a novel mode-suppression technique. We are able to separate the dynamics of the coherently excited LO phonons from the underlying electron-hole dephasing by suppressing the quantum beats. The homogeneous linewidth of these materials at 15 K results from electronic dephasing in -85 fs, approximately half of which is due to acoustic phonon modes. Contributions from acoustic phonons dominate the homogeneous linewidth at room temperature.

Observation of coupled vibrational modes of a semiconductor nanocrystal

Abstract: Raman scattering and far-infrared absorption spectra of PbS nanocrystals with radii of 2 nm are presented. The experimental results are consistent with theoretical calculations which account correctly for the mechanical boundary conditions at the nanocrystal-host interface, and include the first observation of the mixed or coupled vibrational modes predicted by the theory.

Quantization of multiparticle auger rates in semiconductor quantum dots

Abstract: We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.

The one phonon raman spectrum in microcrystalline silicon

Abstract: The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

Intraband transitions in semiconductor nanocrystals

Abstract: Intraband spectra of CdSe nanocrystal colloids are observed. The photoexcited nanocrystals, of average diameters 31.5, 38, and 43 A, exhibit an isolated and strong infrared-induced absorption with peak energy between 0.5 and 0.3 eV. The energies and cross sections of the resonance are consistent with a one-electron transition between the delocalized IS e and IP e states of the strongly confined quantum dots.

Optical lattice vibrations in finite ionic crystals: I

Abstract: A theory for the properties of long-wave optical vibration modes in finite ionic crystals of arbitrary shape is given. Neglecting retardation effects, it is found that in finite specimens there exist transverse and longitudinal bulk modes as well as surface modes, which are neither transverse nor longitudinal and which have intermediate frequencies. Explicit solutions are given for diatomic crystal samples with one, two and three dimensions finite. The modifications which arise from more complex unit cells and polarizable ions are given. A detailed calculation of the properties of long-wave optical phonons in finite specimens of strontium titanate is presented.

Quantum size effect in three-dimensional microscopic semiconductor crystals

Abstract: The exciton absorption spectrum of microscopic CuCl crystals grown in a transparent dielectric matrix has been studied. The size of the microscopic crystals was varied in a controlled manner from several tens of angstroms to hundreds of angstroms. There is a short-wave shift (of up to 0.1 eV) of the exciton absorption lines, caused by a quantum size effect.

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots : assignment of the first electronic transitions

Abstract: CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

Observation of the dark exciton in CdSe quantum dots

Abstract: We use external magnetic fields to identify the band edge emitting statc in CdSe quantum dms. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk. is strongly enhanced by quantum confinement and allows the observation of an optically passive "dark" excitonic state.

Self-organization of CdSe nanocrystallites into three-dimensional quantum dot superlattices

Abstract: The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superiattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.