Showing papers on "Potential well published in 1999"
TL;DR: In this paper, the relationship between the energy gap and microstructure of ZrO2:16%Y has been determined and discussed, and the quantum confinement effect was observed at the grain size lower than 100 nm with the band gap energy shift of 0.62±0.05
Abstract: The results of optical absorption measurements on nanocrystalline ZrO2:16%Y thin films are presented. Dense 0.7 μm thick films with 1–300 nm grain size have been obtained on sapphire substrate using a polymeric precursor spin coating technique. The relationship between the energy gap and microstructure of ZrO2:16%Y has been determined and discussed. The quantum confinement effect was observed at the grain size lower than 100 nm with the band gap energy shift of 0.25 eV when the microstructure was changed up to 1 nm. Some limitation of the model has been observed and discussed. The band gap energy of 5.62±0.05 eV has been determined as microstructure independent value.
129 citations
TL;DR: In this paper, the broken translation symmetry due to structural defects may play a more important role than the quantum confinement effect in the Raman features of optical phonons in polar semiconductor quantum wires such as SiC nanorods.
114 citations
TL;DR: In this article, the authors predict an efficient electronic energy transfer from an excited semiconductor quantum well to optically active organic molecules of the nearby medium (substrate and/or overlayer).
Abstract: We predict an efficient electronic energy transfer from an excited semiconductor quantum well to optically active organic molecules of the nearby medium (substrate and/or overlayer). The energy transfer mechanism is of the Forster type and, at semiconductor-organic distances of about 50 A, can easily be as fast as 10-100 ps, which is about an order of magnitude shorter than the effective exciton lifetime in an isolated quantum well. In such conditions, the Wannier-Mott exciton luminescence is quenched and the organic luminescence is efficiently turned on. We consider both free as well as localized quantum well excitons discussing the dependence of the energy transfer rate on temperature and localization length. A similar mechanism for the non-radiative energy transfer to the organic overlayer molecules from unbound electron-hole pairs excited in the 2D continuum is shown to be much less competitive with respect to other relaxation channels inside the inorganic quantum well (in particular, 2D exciton formation).
113 citations
TL;DR: In this paper, the x-ray-absorption spectra of a series of nanodiamond thin films with grain diameters ranging from 3.5 nm to $5\ensuremath{\mu}\mathrm{m}$ at the C $K$-edge using the sample drain current mode at room temperature were measured.
Abstract: This study measures the x-ray-absorption spectra of a series of nanodiamond thin films with grain diameters ranging from 3.5 nm to $5\ensuremath{\mu}\mathrm{m}$ at the C $K$-edge using the sample drain current mode at room temperature. Resonance peaks resembling the C $1s$ core exciton are observed. The exciton state and conduction band edge are found to shift to higher energies with the decrease of the grain size indicative of the presence of the quantum confinement effect.
95 citations
TL;DR: In this article, the photoluminescence (PL) characteristics of pure nanoscale silicon quantum wires (SiQW's) were evaluated under ultraviolet photoexcitation.
Abstract: The recent success of bulk synthesis of pure nanoscale silicon quantum wires (SiQW's) enables us to evaluate their photoluminescence (PL) characteristics under ultraviolet photoexcitation. Intensive multiple light emissions ranging from dark red to blue regions were revealed for as-grown and partially oxidized SiQW samples. The red light emission was ascribed to a quantum confinement effect originating from the crystalline core of the SiQW's that is closely mediated by the interface. However, the PL emission from green to blue is found to be definitely unrelated to quantum confinement; instead they are attributed to the radiative recombination from defect centers in the overcoating layer of the amorphous silicon oxide outside the SiQW's.
73 citations
TL;DR: In this article, the authors calculate the variation of the potential and confinement energy due to inter-diffusion of a spherical InxGa1−xAs quantum dot embedded in GaAs, and show that the spread in confinement energies due to variation in dot size and composition decreases with the diffusion-induced decrease in confinement energy.
Abstract: We calculate the variation of the potential and confinement energy due to inter-diffusion of a spherical InxGa1−xAs quantum dot embedded in GaAs. The potential drops off more quickly with increasing diffusion length about a quantum dot than is the case for a quantum well. This results in a far more rapid variation of confinement energy for a dot than for a well, until eventually no electron states are confined in the diffused dot. We show that the spread in confinement energies due to variation in dot size and composition decreases with the diffusion-induced decrease in confinement energy. Our results indicate that the recently observed narrowing of photoluminescence line-width, that occurs as a result of rapid thermal annealing of self-assembled InAs quantum dot structures, can be explained in terms of normal inter-diffusion processes between the dots and surrounding matrix material.
35 citations
TL;DR: In this paper, a lateral effective potential is proposed to calculate the electron and hole states in V-groove quantum wires, which is used together with a suitable coordinate transformation which results in two decoupled one-dimensional Schrodinger equations.
Abstract: We propose a lateral effective potential which allows a straightforward calculation of electron and hole states in V-groove quantum wires. This effective potential is used together with a suitable coordinate transformation which results in two decoupled one-dimensional Schrodinger equations which are readily solved. The energy levels and wave functions calculated by this method are in close agreement with several previous results, which indicates that this method may be valuable for the study of physical properties in V-shaped quantum wires which require analytically calculated wave functions. Also, the proposed effective potential gives rise to a shallow vertical quantum well which is found in some V-groove quantum wire structures.
33 citations
TL;DR: In this paper, a quantum-mechanical microscopic description of a Wannier-Mott exciton in a quantum dot is employed together with a macroscopic description of the organic medium.
Abstract: A study is made of the Forster energy transfer from an excited semiconductor quantum dot to an organic material surrounding the dot. A quantum-mechanical microscopic description of a Wannier-Mott exciton in a quantum dot is employed together with a macroscopic description of the organic medium. The calculations show that for II–VI semiconductors such transfer can occur in a time of the order of tens of picoseconds, which is much shorter than the lifetime of an excitation in a quantum dot in the absence of transfer. Therefore, just as for quantum wells, the energy transfer mechanism considered can be a quite efficient means for pumping organic sources of radiation.
23 citations
TL;DR: In this article, the quantum confinement of phonon modes with different symmetries was investigated and it was shown that the modes with the A(1) symmetry have the strongest quantum confinement effect and the T-1 modes the weakest.
20 citations
TL;DR: In this article, the effects of surface and quantum confinement on nano-LaFeO 3 surface adsorbed by the sensor gas are probed by vacuum adsorptive surface photovoltage technique.
19 citations
TL;DR: In this article, the authors studied the optical properties of free-standing porous silicon (PS) films during thermal oxidation at 200°C in air by measuring of the PL, IR, optical absorption and Raman scattering spectra.
Abstract: We have systematically studied the evolution of the optical properties of free-standing porous silicon (PS) films during thermal oxidation at 200 °C in air by measuring of the PL, IR, optical absorption and Raman scattering spectra. After thermal oxidation for 200 h, the PL peak energies of free-standing PS films focus on a small energy range centered around 1.61 eV. In this case, a conclusion that the sizes of nanometer silicon particles (NSPs) decrease with increasing time of thermal oxidation is obtained by theoretical fitting for Raman scattering spectra. The evolution of transmission curve is quite complicated (which redshifts first and then blueshifts during thermal oxidation), and can be explained by a model including the quantum confinement effect in the NSPs and the influence of the Si–O bonds on the surface of NSPs. Meanwhile, no focusing of the optical absorption edge of free-standing PS films is found. Experimental results clearly indicate that there is no simple correlation between the PL ene...
TL;DR: In this article, femtosecond pump-probe spectroscopy was used to investigate the gain dynamics of strongly confined CdSe quantum dots by femto-spectroscopy and concluded that the gain mechanism is governed by biexciton to exciton transitions.
Abstract: II - VI semiconductor (CdS, CdSe) nanocrystals with an average size of approximately one bulk exciton Bohr radius are embedded in a glass matrix. Due to the quantum confinement effect, they act as a quasi zero-dimensional system (quantum dots). Under strong nanosecond and femtosecond optical excitation, these quantum dots exhibit optical amplification (gain). We investigate the ultrashort gain dynamics of the strongly confined CdSe quantum dots by femtosecond pump-probe spectroscopy. From multiple-beam pump-probe measurements, we conclude that the gain mechanism is governed by biexciton to exciton transitions. Femtosecond dephasing measurements reveal a constant scattering rate across the gain region and confirm the two-electron-hole pair gain model. Nanosecond pump-probe measurements on CdS quantum dots in sol-gel glasses show optical gain up to room temperature. In all cases, the gain region is broad and stretches below the fundamental absorption of the nanocrystals. The reason is the multitude...
TL;DR: In this article, the effect of surface-bond saturation on the luminescence of Si nanocrystals is studied by the tight-binding cluster model, and it is found that the energy gaps increase when the diameter decreases, in accordance with the quantum confinement effect.
Abstract: The effect of surface-bond saturation on the luminescence of Si nanocrystals is studied by the tight-binding cluster model. It is found that the energy gaps increase when the diameter decreases, in accordance with the quantum confinement effect. The energy gap becomes smaller in the case of incomplete saturation of surface bonds, which are closer to the experimental results. The lifetimes decrease simultaneously one or two orders of magnitude, which explains the high luminescence efficiency of porous Si and Si nanocrystals. The lowest unoccupied molecular orbital (LUMO) $A1$ state turns into a surface state as the surface saturation strength is weakened, while the highest occupied molecular orbital $T2$ state changes little. The lowest LUMO state may be the $A1$ or $T2$ state, depending on the cluster shape. The energy difference between the two states is several or several tenths of a meV, which may explain the high luminescence efficiency of porous Si at room temperature. It seems that our theory can qualitatively unify the quantum confinement model and the surface-localized-state model for the luminescence mechanism.
TL;DR: In this paper, three kinds of cluster-based materials are prepared by evaporation and inert gas condensation method, and their structures and properties are examined by transmission electron microscopy, Raman scattering, STM/STS, optical spectroscopy.
Abstract: Three kinds of cluster-based materials are prepared by evaporation and inert gas condensation method. Their structures and properties are examined by transmission electron microscopy, Raman scattering, STM/STS, optical spectroscopy, etc. Some important results are obtained: (1) surface phonon modes of quasi-free Si clusters are observed when Si clusters softly land onto the mother skeleton of the porous silicon and/or through grazing angle collisions with the walls of the pores; (2) very sharp peaks of conductance resonances are obtained when the STM tip is right on the top of the Au cluster deposited on the H-terminated silicon crystal; and (3) large blue shifts and photoluminescence from violet to orange with main peaks in the blue range are observed from Ge cluster-based nanofilms at an excitation wavelength of 370 nm. Mechanisms are discussed including the quantum confinement effect of the Ge cluster cores, radiation transition from oxygen difficiency centers in the oxide surface layers, and exciton confinement in the interfacial layers between the crystalline cores and the oxide shells.
29 Aug 1999
TL;DR: In this paper, the authors report results from an experimental study as well as theoretical modeling of the quantum confinement effect on the enhancement of the thermoelectric figure of merit for superlattice quantum wells with small widths.
Abstract: In bulk form, Si/sub 1-x/Ge/sub x/ is a promising thermoelectric material for high temperature applications. In this paper, we report results from an experimental study as well as theoretical modeling of the quantum confinement effect on the enhancement of the thermoelectric figure of merit. The experimental results provide demonstration of proof of principle for the theoretical model which predicts a large enhancement in the thermoelectric figure of merit for superlattice quantum wells with small widths. Thermoelectric transport properties are measured as a function of temperature. An enhancement of S/sup 2/n within the quantum well over the bulk value is observed experimentally, where S is the Seebeck coefficient and n is the carrier density.
TL;DR: In this article, a theoretical study of the electron states in a quantum well with finite barrier height in a magnetic field applied parallel to the quantum well is presented. And the influence of well depth, mass ratio and magnetic field is investigated.
Abstract: We present a theoretical study of the electron states in a quantum well with finite barrier height in a magnetic field applied parallel to the quantum well. For large electron mass mismatch between the quantum well and the barrier, we found the surprising result that the electron energy for zero wave vector decreases with decreasing well width and the energy spectrum has a local minimum at a non-zero wave vector. The influence of well depth, mass ratio and magnetic field are investigated.
TL;DR: In this article, the mechanism for the growth of Si nanowires and the growth model for different morphologies of Si-nodes are described, and the quantum confinement effect of the Si-nanowires is presented.
Abstract: The methods for synthesizing one-dimensional Si nanowires with controlled diameter are introduced. The mechanism for the growth of Si nanowires and the growth model for different morphologies of Si nanowires are described, and the quantum confinement effect of the Si nanowires is presented.
TL;DR: In this article, the authors investigated the absorption spectra of nanocrystalline silicon embedded SiO2 matrix and found that the absorption presents an exponential dependance absorption coefficient on photon energy, and a sub-band appears in the the range of 1.0-1.5 eV.
Abstract: Nanocrystalline silicon embedded SiO2 matrix is formed by annealing the SiO2 films fabricated by plasma enhanced chemical vapor deposition technique. In conjunction with the micro-Ramam spectra, the absorption spectra of the films have been investigated. The blue-shift of absorption edge with decreasing size of silicon crystallites is due to quantum confinement effect. It is found that nanocrystalline silicon is of an indirect band structure, and that the absorption presents an exponential dependance absorption coefficient on photon energy ii! the range of 2.0-3.0 eV, and a sub-band appears in the the range of 1.0-1.5 eV. We believe that the exponential absorption is due to the indirect band-to-band transition of electrons in silicon nanocrystallites, while the Sub-band absorption is ascribed to transitions between the amorphous silicon states existing in the films.
TL;DR: In this article, a review on studies of exciton-phonon interaction in single quantum wells by means of a nonequilibrium phonon technique is given, showing that the exciton energy distribution is sensitive to the phonon energy and momentum distribution and also depends on the quantum well width.
Abstract: A review is given on studies of exciton–phonon interaction in single quantum wells by means of a nonequilibrium phonon technique. It is shown that in the presence of nonequilibrium phonons the exciton energy distribution is non-Boltzmann. It is sensitive to the phonon energy and momentum distribution and also depends on the quantum well width. The results clearly show the role of 2D exciton confinement in the exciton–phonon interaction. In slightly asymmetric double quantum wells a rapid phonon-assisted relaxation of coupled excitons is observed.
TL;DR: The synthesis of II-VI semiconductor nanoparticles by mechanochemical reaction has been reviewed in this article, where a simple washing process employed to remove the chloride byproduct resulted in separated particles of less than 10 nm and the resulting particles and crystallite sizes were controlled by changing the milling conditions, starting materials, annealing temperature and the presence of a diluent.
Abstract: The synthesis of II-VI semiconductor nanoparticles by mechanochemical reaction has been reviewed. Solid-state displacement reactions between ZnCl/sub 2/ or CdCl/sub 2/ and alkali or alkaline earth chalcogenides were induced during mechanical milling in a steady-state manner, leading to the formation of II-VI semiconductor nanoparticles in a chloride salt matrix. A simple washing process employed to remove the chloride byproduct resulted in separated particles of less than 10 nm. The resulting particles and crystallite sizes were controlled by changing the milling conditions, starting materials, annealing temperature and the presence of a diluent. With decreasing particle size, the band-gap energy and plasmon peak energy of CdS increased due to the quantum confinement effect.
TL;DR: In this paper, a GaInAsP/InP multiple-layered quantum-wire laser with the wire width of 21 nm in the period of 100 nm was realized by CH4/H2 reactive-ion etching followed by slight wet chemical etching and embedding growth by organometallic vapor phase epitaxy.
Abstract: GaInAsP/InP multiple-layered quantum-wire lasers with the wire width of 21 nm in the period of 100 nm were realized by CH4/H2 reactive-ion etching followed by slight wet chemical etching and embedding growth by organometallic vapor phase epitaxy. A threshold current density as low as 1.45 kA/cm2 was obtained with the cavity length of 980 µm. To our knowledge, this is the lowest value reported for 1.55 µm GaInAsP/InP quantum-wire lasers fabricated by the etching and regrowth method. Because of the temperature dependence of the lasing wavelength, a relatively large blue shift of 47 meV in the quantum-wire laser was observed, which can be attributed to not only a lateral quantum confinement effect but also a three-dimensional compressive strain effect. Finally, we improved the initial wafer structure in order to suppress over-etching of the active region, and obtained lasers consisting of a five-layered wirelike active region with good size uniformity (wire width of 42 nm, period of 120 nm). A threshold current density as low as 540 A/cm2 was obtained with the cavity length of 1.38 mm.
TL;DR: In this article, a spatial structure of porous silicon formed by anodization was simulated using a lattice model considering the quantum confinement effect, and it was shown that columnar and dendritic structures can be produced in degenerately doped p-type Si wafers by self-organization of nanowires with quantum confinement effects.
Abstract: A spatial structure of porous silicon formed by anodization was simulated using a lattice model considering the quantum confinement effect. Calculation starting from a random distribution of null cells at the surface yielded the final granular, columnar or flat surface structures depending on the magnitude of the anodizing bias voltage in concordance with previously reported structures. Our calculations suggest that columnar and dendritic structures can be produced in degenerately doped p-type Si wafers by self-organization of nanowires with quantum confinement effects.