Showing papers on "Potential well published in 2005"

Experimental evidence for the quantum confinement effect in 3C-SiC nanocrystallites.

Abstract: Using electrochemical etching of a polycrystalline 3$C$-SiC target and subsequent ultrasonic treatment in water solution, we have fabricated suspensions of 3$C$-SiC nanocrystallites that luminesce. Transmission electron microscope observations show that the 3$C$-SiC nanocrystallites, which uniformly disperse in water, have sizes in the range of 1\char21{}6 nm. Photoluminescence and photoluminescence excitation spectral examinations show clear evidence for the quantum confinement of 3$C$-SiC nanocrystallites with the emission band maximum ranging from 440 to 560 nm. Tunable, composite polystyrene/SiC film can be made by adding polystyrene to a toluene suspension of the 3$C$-SiC nanocrystallites and then coating the resulting solution onto a Si wafer.

Electronic and Excitonic Structures of Inorganic–Organic Perovskite-Type Quantum-Well Crystal (C4H9NH3)2PbBr4

Abstract: The electronic and excitonic structures of an inorganic–organic perovskite-type quantum-well crystal (C4H9NH3)2PbBr4 have been investigated by optical absorption, photoluminescence, electroabsorption, two-photon absorption, and magnetoabsorption spectroscopies. Excitons in (C4H9NH3)2PbBr4 are of the Wannier-type, and ns (n≥2) excitons form an ideal two-dimensional Wannier exciton system. The binding energy, longitudinal–transverse splitting energy, and exchange energy of 1s excitons have been determined to be 480, 70 and 31 meV, respectively. These high values originate from both a strong two-dimensional confinement and the image charge effect. These values are larger than those in (C6H13NH3)2PbI4, owing to the smaller dielectric constant of the well layer in (C4H9NH3)2PbBr4 than that in (C6H13NH3)2PbI4. The seemingly unusual electric-field dependence of excitons resonance is also reasonably understood by taking the image charge effect into account.

White-light emission from organics-capped ZnSe quantum dots and application in white-light-emitting diodes

Abstract: Organics-capped ZnSe quantum dots were synthesized by a colloidal chemical approach using ZnO and Se powder as precursors. The photoluminescence of the specimens showed strong white emission (∼200nm full width at half maximum) in the visible range under ambient conditions. The white emission was attributed to the mixing of blue emission of ZnSe nanocrystals exhibiting quantum confinement effect with green-red emission of radiative deep levels from ZnSe surface strained lattice. Based on organic-capped ZnSe quantum dots, the white-light-emitting diodes were fabricated using a near-UV InGaN chip as the excitation source. The diodes emitted white light with CIE chromaticity coordinates of (0.38 and 0.41) and show great potential for use in lighting applications.

Observation of the quantum-confinement effect in individual Ge nanocrystals on oxidized Si substrates using scanning tunneling spectroscopy

Abstract: Scanning tunneling spectroscopic studies revealed the quantum-confinement effects in Ge nanocrystals formed with ultrahigh density (>1012cm−2) by Ge deposition on ultrathin Si oxide films. With decreasing crystal size, the conduction band maximum upshifted and the valence band minimum downshifted. The energy shift in both cases was about 0.7 eV with the size change from 7 to 2 nm. This shows that the energy band gaps of Ge nanocrystals increased to ∼1.4eV with decreasing size. This size dependence can be explained by the quantum-confinement effect in Ge nanocrystals.

Effect of Microstructure of TiO2 Thin Films on Optical Band Gap Energy

Abstract: TiO2 coatings are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for four hours, the spectra and XRD patterns of TiO2 thin film are obtained. XRD patterns reveal that only anatase phase can be observed in TiO2 coatings regardless of the different annealing temperatures, and with the increasing annealing temperature, the grain size gradually increases. The relationship between the energy gap and microstructure of anatase is determined and discussed. The quantum confinement effect is observed that with the increasing grain size of TiO2 thin film, the band gap energy shifts from 3.4 eV to 3.21 eV. Moreover, other possible influence of the TiO2 thin-film microstructure, such as surface roughness and thin film absorption, on band gap energy is also expected.

Structural and chemical trends in doped silicon nanocrystals: First-principles calculations

Abstract: Electronic and structural properties of substitutional group-V donors (N, P, As, Sb) and group-III acceptors (B, Al, Ga, In) in silicon nanocrystals with hydrogen passivation are explored using first-principles calculations based on hybrid density functional theory with complete geometrical optimization. The bonding near the impurity is similar to that found for the impurity in bulk crystalline silicon, with some quantitative differences. The N case shows large local distortions, as it does in the bulk, characteristic of a deep trap. For the other impurities, no evidence is found for a transition to atomic scale localization induced by the small size of the nanocrystal. The chemical trends of the donor and acceptor binding energies and the donor excited state energies in doped nanocrystals are similar to those in the bulk; however, the absolute magnitudes are substantially larger. The increase in the magnitude of the binding energy is mainly due to the quantum confinement effect combined with the reduced screening of the impurity potential in small nanocrystals. The screening of the impurity potential is carefully examined using the self-consistent electrostatic potential from the full calculations. Strong chemical and local-field effects are seen within the radius of the first neighbor bonds to the impurity atom. This explains the large increase in the donor excited state energy level splittings and the relative importance of the central cell contributions to the binding energies. The acceptor and donor orbitals have different atomic character on the impurity site, leading to substantially different acceptor and donor energy level splittings.

Electronic states in quantum dots with ellipsoidal symmetry

Abstract: In the framework of perturbation theory and for any limiting potential with ellipsoidal symmetry energy states of electron in weakly oblate (prolate) ellipsoidal quantum dot has been investigated. Analytical expressions for particle energy spectrum have been obtained taking into account that electron effective masses are different in medium and in quantum dot. Obtained results were applied for the case of rectangular quantum well of finite height. The investigation of energy level dependence on geometrical parameters of quantum dot was performed. In particular, within the first order of perturbation theory it has been shown, that the dependence of energy correction on reduced radius of quantum dot has a maximum.

Synthesis and field emission properties of aluminum nitride nanocones

Abstract: One-dimensional aluminum nitride nanostructures have displayed superior field emission due to the combination of small or negative electron affinity and one-dimensional quantum confinement effect. Herein we report on the synthesis of quasi-aligned AlN nanocones via chemical vapor deposition on the Ni-coated silicon wafer at 750 °C through the reaction between AlCl 3 vapor and NH 3 /N 2 gas. The as-prepared hexagonal AlN nanocones grow preferentially along c -axis with the tips’ sizes of about 60 nm and the lengths up to several microns. The field emission measurement exhibits a notable electron emission with the apparent turn-on field of 17.8 V/μm, indicating their potential applications as the field emitters. Due to space charge effect, the corresponding Fowler–Nordheim plot shows a two-sectional characteristic with the field enhancement factors of 1450 and 340 at low and high electric fields, respectively.

Luminescent silicon carbide nanocrystallites in 3C-SiC∕polystyrene films

Abstract: We report optical emission of SiC nanocrystallite films, which clearly shows the quantum confinement effect. Bulk polycrystalline 3C-SiC was first electrochemically etched and then the fabricated porous silicon carbide was ultrasonically treated in water or toluene suspension to disperse into colloidal nanoparticles. Transmission electron microscopy images clearly show that the colloidal nanoparticles have 3C-SiC lattice structure with sizes varying from about 6nm down to below 1nm. The suspension of 3C-SiC nanocrystallites exhibits ultrabright emission with wavelengths ranging from 400to520nm when the excitation wavelength varies from 250to480nm, in accordance with the quantum confinement effect. By adding polystyrene to the toluene suspension containing SiC nanoparticles and coating the mixing solution onto a Si wafer, we obtain the SiC∕polystyrene films that luminesce.

Quantum confinement in Volmer–Weber-type self-assembled ZnO nanocrystals

Abstract: We have studied the quantum confinement effect on Volmer–Weber-type self-assembled ZnO nanocrystals. Volmer–Weber-type self-assembled ZnO nanocrystals were grown on the Pt(111) substrate by using a rf-magnetron sputtering method and were confirmed by the Auger electron spectroscopy. The free exciton transition energies of 57-, 38-, and 24‐nm-size nanocrystals were found to be roughly 3.298, 3.311, and 3.337eV, respectively, by photoluminescence measuremnets at room temperature. The blueshift of the photoluminescence peak energy of ZnO nanocrystals of 24nm in diameter roughly varied by 40meV compared to bulk ZnO.

Characterization of InGaN/GaN Multiple Quantum Well Nanorods Fabricated by Plasma Etching with Self-Assembled Nickel Metal Nanomasks

Abstract: High-density (3.0×1010 cm-2) InGaN/GaN multiple quantum well (MQW) nanorods were fabricated from an as-grown bulk light-emitting diode structure by inductively coupled plasma dry etching with self-assembled nickel metal nanomasks. The self-assembled nickel metal nanomasks were formed by rapid thermal annealing of a nickel metal film at 850°C for 1 min. The influence of the thicknesses of the Ni metal film on the dimensions and density of the nanorods was also investigated. The structural and optical properties of the InGaN/GaN MQW nanorods were established using field emission scanning electron microscopy, transmission electron microscopy and photoluminescence measurements. The diameters and heights of nanorods were estimated to be 60 to 100 nm and more than 0.28 µm, respectively. The peak emission wavelength of the nanorods showed a blue shift of 5.1 nm from that of the as-grown bulk. An enhancement by a factor of 5 in photoluminescence intensity of the nanorods compared with that of the as-grown bulk was observed. The blue shift is attributed to strain relaxation in the wells after dry etching, the quantum confinement effect, or a combination of the two, which results in the enhancement of emission intensity.

The self-consistent calculation of a spherical quantum dot: A quantum genetic algorithm study

Abstract: In this study, we have calculated the subband energy level, potential profile, and the corresponding wavefunction and chemical potential for different temperatures and donor concentrations in a spherical quantum dot self-consistently. We have also investigated the effect of exchange-correlation potential on the energy levels. In addition, we have checked the applicability of quantum genetic algorithm to a realistic self-consistent quantum dot problem. In all computations, the penetration of wavefunction to the barrier region is taken into account.

Influence of matrix on third order optical nonlinearity for semiconductor nanocrystals embedded in glass thin films prepared by Rf-sputtering

Abstract: CdSe nanocrystals were successfully embedded in high index glass films by rf-sputtering technique. All films showed the shift of absorption edge to shorter wavelength compared with that of bulk CdSe, so-called, blue shift, and it was evident that there is a quantum confinement effect in the films. The amount of the blue shift depended on the kind of matrix glass as well as the size of embedded CdSe nanocrystals. The third-order optical nonlinearity evaluated by Z-scan technique also depended on the matrix glass. The larger nonlinearity was observed from the matrix glass with higher refractive index. In addition, it was found that the value of shift of absorption edge, or the quantum confinement effect was linearly related to the third-order nonlinearity for CdSe nanocrystals embedded in glass thin films.

Valley Splitting in V-Shaped Quantum Wells

Abstract: The valley splitting (energy difference between the states of the lowest doublet) in strained silicon quantum wells with a V-shaped potential is calculated variationally using a two-band tight-binding model. The approximation is valid for a moderately long (approximately 5.5–13.5nm) quantum well with a V-shaped potential which can be produced by a realistic delta-doping on the order of nd≈1012cm−2. The splitting versus applied field (steepness of the V-shaped potential) curves show interesting behavior: a single minimum and for some doublets, a parity reversal as the field is increased. These characteristics are explained through an analysis of the variational wave function and energy functional.

Quantum confinement in nanocrystalline silicon

Abstract: Quantum confinement effects in different kinds of nanocrystalline silicon systems are experimentally and theoretically investigated. Porous silicon structured as a nanowire network and silicon nanodots embedded in amorphous silicon dioxide are studied. The main quantum confinement effect in both cases is represented by the appearance of new energy levels in the silicon band gap. The corresponding energies can be experimentally determined from the current ‐ temperature characteristics, which show an Arrhenius-like behavior. The curves present several activation energies between liquid nitrogen temperature and room temperature. The energy levels can be evaluated from a quantum well model. The fundamental level is located at the top of the valence band. The change of the activation energy is then related with the filling of the levels. The ratios of the consecutive activation energies in the current ‐ temperature characteristics prove that the excitation undergoes the angular momentum conservation law imposed by the applied electric field. The estimation of the mean size of the nanocrystals from the values of the activation energies is in good agreement with the microstructure investigations performed on the samples. The confinement levels are also in good agreement with the photoluminescence measurements.

Self-assembly of a novel beta-In2S3 nanostructure exhibiting strong quantum confinement effects.

Abstract: The 3D beta-In2S3 flowerlike architecture assembled from nanoflakes was prepared via a novel complex-precursor assisted (CPA) solvothermal route. The as-prepared beta-In2S3 powder was characterized by X-ray diffraction pattern (XRD), X-ray photoelectron spectra (XPS), transition electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), ultraviolet-visible light (UV-vis) spectra, and photoluminescence spectrum. The novel 3D beta-In2S3 nanostructure exhibit a strong quantum confinement effect. FT-IR spectra were used to investigate the coordinative chemical effect in the complex. A possible mechanism was discussed.

Energy spectrum analysis of donor-center quantum dots

Abstract: Within the effective mass approximation, the energy spectra of the donor- center quantum dot system in Gaussian confining potential are calculated by using the method of numerical matrix diagonalization. The results show that the property of the ground and low-lying states of the quantum dot is rather sensitive to the size and the strength of confinement potential.

Complex scaling approach for the quantum confined Stark effect in quantum wells

Abstract: A computationally efficient approach to calculate characteristics of quantum well excitons in an external electric field is introduced. The nonstationary nature of eigenstates in the presence of an electric field is taken into account with the help of the complex scaling approach. The method allows one to simultaneously obtain the field-induced shift and the broadening of the exciton resonance. The method is applied to a shallow quantum well in the regime of strong confinement. It is shown that in this case the field-induced broadening is strongly affected by the effective electron-hole interaction at small to moderate electric fields.

Photoluminescence and optical limiting properties of silicon nanowires.

Abstract: Si nanowires (SiNWs) have been produced by thermal vaporization on Si(111) substrate without catalysts added. The grown SiNWs have been characterized by Raman scattering, SEM, XRD, and electron diffraction and shown to be highly crystalline with only little impurities such as amorphous Si and silicon oxides. Photoluminescence (PL) study has illustrated that the Si band-to-band gap increases from 1.1 eV for bulk Si to 1.56 eV for the as-grown SiNWs due to quantum confinement effect. A strong PL peak at 521 nm (2.37 eV) is attributed to the relaxation of the photon-induced self-trapped state in the form of surface Si-Si dimers, which may also play an important role in optical limiting of SiNWs with 532-nm nanosecond laser pulses. With the observation of optical limiting at 1064 nm, nonlinear scattering is believed to make a dominant contribution to the nonlinear response of SiNWs.

Deep quantum well electro-absorption modulator

Abstract: Double well structures in electro-absorption modulators are created in quantum well active regions by embedding deep ultra thin quantum wells. The perturbation introduced by the embedded, deep ultra thin quantum well centered within a conventional quantum well lowers the confined energy state for the wavefunction in the surrounding larger well and typically results in the hole and electron distributions being more confined to the center of the conventional quantum well. The extinction ratio provided by the electro-absorption modulator is typically increased.

Electronic states in a step quantum well in a magnetic field

Abstract: The quantum states and energy spectrum of an electron in a rectangular step quantum well in a magnetic field parallel to the plane of two-dimensional electronic gas are investigated. It is shown that the joint effect of a magnetic field and confining potential of quantum well results in radical change of the electron energy spectrum. The energy dependencies on the parameters of the quantum well and magnetic field induction are investigated. Numerical calculations are carried out for an AlAs/ GaAlAs/ GaAs/ AlAs step quantum well.