Showing papers on "Potential well published in 2006"
TL;DR: In this paper, the size-dependent band gap of colloidal quantum dots is calculated using a recently developed method that predicts accurately the eigenstates and eigenenergies of nanostructures by utilizing the adiabatic theorem of quantum mechanics.
Abstract: The size-dependent band gap of semiconductor quantum dots is a well-known and widely studied quantum confinement effect. In order to understand the size-dependent band gap, different theoretical approaches have been adopted, including the effective-mass approximation with infinite or finite confinement potentials, the tight-binding method, the linear combination of atomic orbitals method, and the empirical pseudopotential method. In the present work we calculate the size-dependent band gap of colloidal quantum dots using a recently developed method that predicts accurately the eigenstates and eigenenergies of nanostructures by utilizing the adiabatic theorem of quantum mechanics. We have studied various semiconductor (CdS, CdSe, CdTe, PbSe, InP, and InAs) quantum dots in different matrices. The theoretical predictions are, in most cases, in good agreement with the corresponding experimental data. In addition, our results indicate that the height of the finite-depth well confining potential is independent ...
323 citations
TL;DR: In this paper, the photoluminescence peak energy was fitted to the relationship, E(eV)=1.13+13.9∕d2, where d is the diameter of the Si QD in nanometers.
Abstract: Crystalline silicon quantum dots (Si QDs) were spontaneously grown in the silicon nitride films by plasma-enhanced chemical vapor deposition using SiH4 and NH3 as precursors. When the size of the Si QDs was reduced from 4.9 to 2.9nm, the photoluminescence peak energy was shifted from 1.73 to 2.77eV. The photoluminescence peak energy was fitted to the relationship, E(eV)=1.13+13.9∕d2, where d is the diameter of the Si QD in nanometers. The measured band-gap energies of the Si QDs were in good agreement with the quantum confinement model for crystalline Si QDs. These results suggest that the hydrogen dissociated from NH3 plays an important role in improving the crystallinity and surface passivation of Si QDs.
239 citations
TL;DR: The size dependence of electron-phonon coupling is principally a result of the Frohlich interaction as discussed by the authors, and the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second-to-first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter.
Abstract: ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Frohlich interaction.
150 citations
TL;DR: In this article, band gap engineering and spatial confinement of optical phonons were observed depending upon the size of ZnO quantum dots at room temperature, and the size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect.
Abstract: Both band gap engineering and spatial confinement of optical phonon were observed depending upon the size of ZnO quantum dots at room temperature. Size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect. The measured Raman spectral shift and asymmetry for the E2(high) mode caused by localization of optical phonons agree well with that calculated by using the modified spatial correlation model.
106 citations
TL;DR: In this article, the dielectric response function of quantum electron plasmas was used to calculate the potential distributions around a moving test charge, and the near field potential follows the modified Debye-Huckel potential, while the far-field potential turns out to be oscillatory.
Abstract: By using the dielectric response function of quantum electron plasmas, potential distributions around a moving test charge are calculated. The near-field potential follows the modified Debye-Huckel potential, while the far-field potential turns out to be oscillatory. Both the Debye-Huckel and wake potentials strongly depend on the Fermi energy and the electron quantum correlation strength. The relevance of the present investigation to semiconductor plasmas is discussed.
59 citations
TL;DR: In this paper, photoluminescence measurements of Mg0.2Zn0.8O multishell layers on ZnO core nanorods are reported.
Abstract: We report on photoluminescence measurements of Mg0.2Zn0.8O∕ZnO∕Mg0.2Zn0.8O multishell layers on ZnO core nanorods. Dominant excitonic emissions in the photoluminescence spectra show a blueshift depending on the ZnO shell layer thickness attributed to the quantum confinement effect in the nanorod heterostructure radial direction. Furthermore, near-field scanning optical microscopy clearly shows sharp photoluminescence peaks from the individual nanorod quantum structures, corresponding to subband levels.
55 citations
TL;DR: An inorganic-organic hybrid semiconductor, ZnS/CHA (CHA = cyclohexylamine) nanocomposites was successfully synthesized via a solvothermal method using CHA as solvent, which yielded uniform and ultralong nanowires with widths of 100-1000 nm and lengths of 5-20 microm.
Abstract: An inorganic−organic hybrid semiconductor, ZnS/CHA (CHA = cyclohexylamine) nanocomposites was successfully synthesized via a solvothermal method using CHA as solvent, which yielded uniform and ultralong nanowires with widths of 100−1000 nm and lengths of 5−20 μm. Changing the reaction conditions could alter the morphology and optical properties of the nanocomposites. The periodic layer subnanometer structures were identified by high-resolution transmission electron microscopy (HR-TEM) images, with thickness of ∼2 nm. The composites exhibited a very large blue-shift in their optical absorption edge as well as an exciton excitation band due to a strong quantum confinement effect caused by the internal subnanometer-scale structures. The pure hexagonal wurtzite ZnS nanowires were also obtained by extracting the ZnS/CHA nanocomposites with dimethyl formamide (DMF). In addition, the luminescent properties of exciton and defect-related transitions in different samples of ZnS/CHA were discussed in detail.
47 citations
TL;DR: In this article, the fabrication and photoluminescent (PL) properties of ZnO∕Mg0.2Zn0.8O coaxial nanorod quantum structures with various quantum well and barrier layer thicknesses were reported.
Abstract: The authors report on fabrication and photoluminescent (PL) properties of ZnO∕Mg0.2Zn0.8O coaxial nanorod quantum structures with various quantum well and barrier layer thicknesses. Employing catalyst-free metal-organic vapor-phase epitaxy, coaxial nanorod single quantum well structures were fabricated by the alternate heteroepitaxial growth of ZnO and Mg0.2Zn0.8O layers over the entire surfaces of the ZnO nanorods with fine thickness controls of the layers. The quantum confinement effect of carriers in coaxial nanorod quantum structures depends on the Mg0.2Zn0.8O quantum barrier layer thickness as well as the thickness of the ZnO quantum well layer. The temperature-dependent PL characteristics of the coaxial nanorod quantum structures are also discussed.
44 citations
TL;DR: A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature.
Abstract: A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature. The optoelectrical and structural properties of this novel material have been investigated by Raman and UV-VIS transmission spectroscopy measurements indicating that these films are composed of nanocrystallites embedded in amorphous SiHx matrix and with a widened bandgap. The observed downshift of the optical phonon Raman spectra (514.4 cm(-1)) from crystalline Si peak (521 cm(-1)) and the widening of the bandgap indicate a quantum confinement effect from the Si nanocrystallites. By using this kind of p-layer, a-Si:H solar cells on bare stainless steel foil in nip sequence have been successfully prepared with a V c of 0.90 V, a fill factor of 0.70 and an efficiency of 9.0%, respectively. (c) 2006 Elsevier B.V. All rights reserved.
40 citations
TL;DR: In this article, the voids in artificial opal latex are filled with nanosized TiO2 particles by adding a drop of TiCl4 into the latex matrix, hydrolyzing, and heating.
Abstract: Inverse opal TiO2 may offer a novel and promising solution for enhancing the light harvesting efficiency of dye-sensitized solar cells (DSSCs). Its large interconnected pores enable a better penetration of the dye sensitizers via the matrix pores, making this material surpasses the efficiency of conventional TiO2 electrodes. Moreover, it also exhibits a photonic band gap that may enable a significant change in its dye absorbance by the adjustment of the photon localization near the red edge of the photonic band gap to the position of dye absorption. In this study, we report a simple method of fabrication of inverse opal TiO2, wherein the voids in artificial opal latex are filled with nanosized TiO2 particles by adding a drop of TiCl4 into the latex matrix, hydrolyzing, and heating. In this process, we investigate the effect of different heat treatment times on the properties of inverse opal TiO2. Photoacoustic (PA) characterization shows that longer heat treatment times could produce more defect sites. The presence of defects causes the inhibition of electron transfer and results in a decrease in incident photon-to-current conversion efficiency (IPCE). CdSe quantum dots were adsorbed onto inverse opal TiO2 by chemical deposition. The blue shift of PA spectra relative to the bulk CdSe and the gain in IPCE were clearly observed. This result indicates the quantum confinement effect and photosensitization of CdSe quantum dots.
40 citations
Patent•
09 Oct 2006TL;DR: In this article, a phonon engineering approach is used in which a band structure of the potential well and/or light generating structure is designed to facilitate the emission of polar optical phonons by electrons.
Abstract: A light emitting heterostructure and/or device in which the light generating structure is contained within a potential well is provided. The potential well is configured to contain electrons, holes, and/or electron and hole pairs within the light generating structure. A phonon engineering approach can be used in which a band structure of the potential well and/or light generating structure is designed to facilitate the emission of polar optical phonons by electrons entering the light generating structure. To this extent, a difference between an energy at a top of the potential well and an energy of a quantum well in the light generating structure can be resonant with an energy of a polar optical phonon in the light generating structure material. The energy of the quantum well can comprise an energy at the top of the quantum well, an electron ground state energy, and/or the like.
TL;DR: In this paper, high-resolution transmission electron microscopy was used to measure the photoluminescence of ZnO quantum dots (QDs) embedded in silicon substrates, and it was shown that the emission of QDs embedded film ranged from 3.0 to 3.6 eV.
TL;DR: In this article, the authors reported the fabrication of the nanopillars with single InGaAs∕GaAs quantum well by selective-area metal organic vapor phase epitaxy.
Abstract: The authors report the fabrication of the nanopillars with single InGaAs∕GaAs quantum well by selective-area metal organic vapor phase epitaxy. The standard diameter deviation of the nanopillars is about 8% and the standard deviation in their height about 5%. Their photoluminescence peak positions shift to the longer wavelength with an increase in the diameter of the nanopillars, which is not due to the quantum confinement effect in the radial or axial direction but due to the stoichiometry difference of the indium content in the nanopillars with different diameters.
TL;DR: The quantum-confinement effect in two-dimensional β-FeSi2 nanoislands epitaxially grown on Si (111) by codeposition of Fe and Si was observed using scanning tunneling spectroscopy at room temperature as discussed by the authors.
Abstract: The quantum-confinement effect in two-dimensional β-FeSi2 nanoislands epitaxially grown on Si (111) by codeposition of Fe and Si was observed using scanning tunneling spectroscopy at room temperature. The energy band gaps of the H-terminated β-FeSi2 nanoislands increased by approximately 0.4eV when island height decreased from 5to2nm. This size dependence was explained by the quantum-confinement effect in β-FeSi2 nanoislands.
TL;DR: In this article, the relationship between the band gap energy of ZnO films and the grain size is discussed, and the shift of the energy can be attributed to the quantum confinement effect in nanocrystal ZnOs.
TL;DR: In this article, a self-consistent variational treatment of the electron-hole Coulomb interaction is used to calculate the energy and wave function of a single photon in a shallow quantum well.
TL;DR: Both blue shifts of valence band maximum energy and absorption peaks with decreasing particle size provide a unique pathway to reveal the quantum confinement effect of uncapped CdTe nanocrystals.
Abstract: CdTe nanocrystals have been successfully fabricated by a mechanical alloying process. X-ray diffraction (XRD) patterns demonstrate that a single-phase CdTe compound with a zinc blende structure has been formed after ball milling elemental Cd and Te mixture powders for 27 h. The large broadening effect for the width of the {111} diffraction peak of uncapped CdTe nanocrystals on smaller size was observed in slowly scanned XRD patterns. The X-ray photoelectron spectrum was used to study the surface of the uncapped CdTe nanocrystals within both core level and valence band regions. The presence of tellurium oxide film on the surface of the uncapped CdTe nanocrystals has been detected in the X-ray photoelectron spectrum of the Te 3d core level, which was comparable to the observed amorphous oxide thin layer on the surface of uncapped CdTe nanocrystals in a high resolution transmission electron microscopy (HRTEM) image. The energy of the valence band maximum for uncapped CdTe powders blue shifts to the higher energy side with smaller particle sizes. In UV-visible optical absorption spectra of the suspension solution containing uncapped CdTe nanocrystals, the absorption peaks were locating within the ultraviolet region, which shifted toward the higher energy side with prolonged ball milling time. Both blue shifts of valence band maximum energy and absorption peaks with decreasing particle size provide a unique pathway to reveal the quantum confinement effect of uncapped CdTe nanocrystals.
TL;DR: In this paper, an exciton trapped by a Gaussian confining potential quantum dot has been investigated and the dependences of binding energies of the ground state and the first excited state on the size of the potential and the strength of the magnetic field are analyzed explicitly.
Abstract: In this paper, an exciton trapped by a Gaussian confining potential quantum dot has been investigated. Calculations are made by using the method of numerical diagonalization of Hamiltonian in the effective-mass approximation. The dependences of binding energies of the ground state and the first excited state on the size of the confining potential and the strength of the magnetic field are analysed explicitly.
TL;DR: In this article, a mixed mechanism combining the surface bonding states with widened bandgaps of alumina nanoparticles by the quantum confinement effect is presented, which agrees well with the observed photoluminescence results.
Abstract: We report on blue luminescence from alumina nanoparticles suspended in toluene solution. They were fabricated through ultrasonic treatment of porous anodic alumina membrane. The photoluminescence in the suspension of alumina nanoparticles shows considerable blueshift as larger particles precipitate. Transmission electron microscopy observations confirm the occurrence of the precipitation. A mixed mechanism combining the surface bonding states with widened bandgaps of alumina nanoparticles by the quantum confinement effect is presented. It agrees well with the observed photoluminescence results.
TL;DR: In this article, the authors compared pure and hydrogen terminated silicon carbide (SiC) nanowires with ab initio density functional theory calculations and showed that the pure wires preserve their crystalline-like topology with only small relaxations on their surface.
Abstract: Pure and hydrogen terminated silicon carbide (SiC) nanowires grown along [0001] direction in the wurtzite structure are studied using ab initio density functional theory calculations. The pure wires preserve their crystalline-like topology with only small relaxations on their surface. Also, the pure wires are semiconducting, but their band gap energy decreases with decreasing diameter due to the presence of surface states. As expected, hydrogen saturation induces a broadening of the band gap energy because of the quantum confinement effect. SiC nanowires are also predicted to be very rigid. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: The theoretical calculations strongly indicate that the exciton peak is red-shifted in the metal-semiconductor composite quantum dots due to a strong local field, i.e., the quantum confined Stark effect.
Abstract: Local field-induced optical properties of Ag-coated CdS quantum dot structures are investigated. We experimentally observe a clear exciton peak due to the quantum confinement effect in uncoated CdS quantum dots, and surface plasmon resonance and red-shifted exciton peak in Ag-coated CdS composite quantum dot structures. We have calculated the Stark shift of the exciton peak as a function of the local field for different silver thicknesses and various sizes of quantum dots based on the effective-mass Hamiltonian using the numerical-matrix-diagonalization method. Our theoretical calculations strongly indicate that the exciton peak is red-shifted in the metal-semiconductor composite quantum dots due to a strong local field, i.e., the quantum confined Stark effect.
TL;DR: In this article, the electronic structure and optical properties of freestanding GaN wurtzite quantum wires are studied in the framework of six-band effective-mass envelope function theory.
Abstract: The electronic structure and optical properties of freestanding GaN wurtzite quantum wires are studied in the framework of six-band effective-mass envelope function theory. It is found that the electron states are either twofold or fourfold degenerate. There is a dark exciton effect when the radius R of GaN wurtzite quantum wires is in the range of [0.7, 10.9] nm. The linear polarization factors are calculated in three cases, the quantum confinement effect (finite long wire), the dielectric effect and both effects (infinitely long wire). It is found that the linear polarization factor of a finite long wire whose length is much less than the electromagnetic wavelength decreases as R increases, is very close to unity (0.979) at R = I nm, and changes from a positive value to a negative value around R = 4.1 nm. The linear polarization factor of the dielectric effect is 0.934, independent of radius, as long as the radius remains much less than the electromagnetic wavelength. The result for the two effects shows that the quantum confinement effect gives a correction to the dielectric effect result. It is found that the linear polarization factor of very long (treated approximately as infinitely long) quantum wires is in the range of [0.8, 1]. The linear polarization factors of the quantum confinement effect of CdSe wurtzite quantum wires are calculated for comparison. In the CdSe case, the linear polarization factor of R = I nm is 0.857, in agreement with the experimental results (Hu et al 2001 Science 292 2060). This value is much smaller than unity, unlike 0.979 in the GaN case, mainly due to the big spin-orbit splitting energy Delta(so) of CdSe material with wurtzite structure.
TL;DR: In this article, a numerical scheme has been proposed to calculate the screened Coulomb potential, profile of charge density with various quantum wire widths and relative renormalization of gap energy.
Abstract: Bandgap renormalization for the geometry of T-shaped quantum wires is calculated as a function of the electron–hole plasma density and quantum wire width in the random phase approximation. Considering a suitable confinement potential, a numerical scheme has been proposed to calculate the screened Coulomb potential, profile of charge density with various quantum wire widths and relative renormalization of gap energy. We will show that carrier concentration, screened confinement potential and relative bandgap renormalization are functions of the ratio of well width in the x and y directions. We also show that increasing temperature leads to more relative renormalization of gap energy.
TL;DR: In this paper, the localization of two-dimensional electron-hole complexes at the attractive potential of arbitrary shape is treated theoretically and a general method of construction of simple descriptive trial functions is suggested to calculate the binding energy of the ground state of such complexes.
Abstract: The localization of two-dimensional electron-hole complexes at the attractive potential of arbitrary shape is treated theoretically A general method of construction of simple descriptive trial functions is suggested to calculate the binding energy of the ground state of such complexes The limiting cases corresponding to different relations between the characteristic parameters of the system are analyzed The developed approach is illustrated by particular calculations for the exciton in a two-dimensional quantum well with an additional lateral potential
TL;DR: In this paper, the electronic structure and electron g factors of HgTe quantum dots are investigated, in the framework of the eight-band effective-mass approximation, and it is found that the electron states of quantum spheres have aspheric properties due to the interaction between the conduction band and valence band.
Abstract: The electronic structure and electron g factors of HgTe quantum dots are investigated, in the framework of the eight-band effective-mass approximation. It is found that the electron states of quantum spheres have aspheric properties due to the interaction between the conduction band and valence band. The highest hole states are S (l = 0) states, when the radius is smaller than 9.4 nm, the same as the lowest electron states. Thus strong luminescence from HgTe quantum dots with radius smaller than 9.4 nm has been observed (Rogach et al 2001 Phys. Status Solidi b 224 153). The bandgap of HgTe quantum spheres is calculated and compared with earlier experimental results (Harrison et al 2000 Pure Appl. Chem. 72 295). Due to the quantum confinement effect, the bandgap of the small HgTe quantum spheres is positive. The electron g factors of HgTe quantum spheres decrease with increasing radius and are nearly 2 when the radius is very small. The electron g factors of HgTe quantum ellipsoids are also investigated. We found that as some of the three dimensions increase, the electron g factors decrease. The more the dimensions increase, the more the g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension.
TL;DR: In this article, the bound state energies in various quantum wires under potential modulation, including crossed, T-shaped and L-shaped quantum wires, were calculated and the relation and evolution of bound states in different quantum wire systems were shown.
Abstract: We calculate the bound state energies in various quantum wires under potential modulation, including crossed, T-shaped and L-shaped quantum wires. It is found that the bound state energy rapidly increases with the potential height in the lower potential region while it slowly approaches a limit in the higher potential region. A fit formula that describes the dependence of the bound state energy and the potential height is obtained. Based on the formula and the contour plots of electron probability density, the relation and evolution of bound states in different quantum wire systems are shown. We also find that the bound state in a quantum dot, in some potential-modulation region, turns into a quasibound state with finite lifetime.
Patent•
02 Oct 2006
TL;DR: A spin coherent single photon detector has a body of semiconductor material with a quantum well region formed in barrier material in the body as mentioned in this paper, and the body has first and second electrodes formed thereon, the first electrode forming an isolation electrode for defining, when negatively energized, an extent of the quantum well, and the second electrode being positioned above a location where an electrostatic quantum dot is defined in said quantum well in response to positive energization of the second electrodes.
Abstract: A spin coherent, single photon detector has a body of semiconductor material with a quantum well region formed in barrier material in the body. The body has first and second electrodes formed thereon, the first electrode forming an isolation electrode for defining, when negatively energized, an extent of the quantum well in the body and the second electrode being positioned above a location where an electrostatic quantum dot is defined in said quantum well in response to positive energization of the second electrode. The quantum well occurs in three layers of material. An effective, weighted g-factor for the detector is sufficiently close to zero that the Zeeman energy is less than a linewidth, expressed in terms of energy, of photons to be detected by the detector.
TL;DR: In this article, a general description of finite electron and exciton systems in semiconductor quantum dots or quantum wells with external electrostatic confinement is presented, where the energy spectrum and wave functions are constructed by exact diagonalization of the Hamiltonian in terms of the N -particle eigenmodes.
Abstract: In the limit of strong correlations, we present a general description of finite electron and exciton systems in semiconductor quantum dots or quantum wells with external electrostatic confinement. We are able to analytically obtain the energy spectrum and wave functions which are constructed by exact diagonalization of the Hamiltonian in terms of the N -particle eigenmodes. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the influence of the coupling effect on quantum confinement energy levels and on the Coulomb charging effect in double-layer GeSi quantum dots (QDs) is investigated by admittance spectroscopy.
Abstract: The influence of the coupling effect on quantum confinement energy levels and on the Coulomb charging effect in double-layer GeSi quantum dots (QDs) is investigated by admittance spectroscopy. The coupling effect depends on the thickness of the space layer between QD layers. The increasing Coulomb charging energy observed in QD samples with different thickness (4.5, 6, and 7.5nm) indicates that the coupling effect can weaken the quantum confinement effect and Coulomb charging interaction. When the space layer is thicker than 7.5nm, the influence of the coupling effect can be neglected.