Showing papers on "Potential well published in 2009"

Identification of surface structures on 3C-SiC nanocrystals with hydrogen and hydroxyl bonding by photoluminescence.

Abstract: SiC nanocrystals (NCs) exhibit unique surface chemistry and possess special properties. This provides the opportunity to design suitable surface structures by terminating the surface dangling bonds with different atoms thereby boding well for practical applications. In this article, we report the photoluminescence properties of 3C-SiC NCs in water suspensions with different pH values. Besides a blue band stemming from the quantum confinement effect, the 3C-SiC NCs show an additional photoluminescence band at 510 nm when the excitation wavelengths are longer than 350 nm. Its intensity relative to the blue band increases with the excitation wavelength. The 510 nm band appears only in acidic suspensions but not in alkaline ones. Fourier transform infrared, X-ray photoelectron spectroscopy, and X-ray absorption near-edge structure analyses clearly reveal that the 3C-SiC NCs in the water suspension have Si−H and Si−OH bonds on their surface, implying that water molecules only react with a Si-terminated surface...

Quantum confinement effect in ZnO nanoparticles synthesized by co-precipitate method

Abstract: Quantum mechanical effects such as an increased bandgap of semiconductors with reduction of size are viewed as having strong potential for future applications. In the present work, zinc oxide (ZnO) nanoparticles (NPs) were synthesized via the co-precipitate method. Very narrow particle size distribution of the ZnO nanoparticles was achieved through careful control of the synthesis conditions. The structural, morphological, and optical characterization was carried out using X-ray diffraction, atomic force microscopy, and UV–vis reflectance techniques, respectively. The results indicated that increasing the temperature from 60 to 65 °C caused a subsequent increase in particle size from ∼4 to 12 nm. An associated increase in bandgap with decrease in particle size was also noticed which is a strong indication of the quantum confinement effect.

On the thermodynamic origin of the quantum potential

Abstract: In a new thermodynamic interpretation, the quantum potential is shown to result from the presence of a subtle thermal vacuum energy distributed across the whole domain of an experimental setup. Explicitly, its form is demonstrated to be exactly identical to the heat distribution derived from the defining equation for classical diffusion wave fields. For a single free particle path, this thermal energy does not significantly affect particle motion. However, in between different paths, or at interfaces, the accumulation–depletion law for diffusion waves provides an immediate new understanding of the quantum potential’s main features.

Ultra-fine β-SiC quantum dots fabricated by laser ablation in reactive liquid at room temperature and their violet emission

Abstract: A simple and flexible route is presented for the fabrication of ultrafine β-SiC quantum dots (QDs) based on laser ablation of silicon wafers immersed in ethanol and subsequent etching. The obtained β-SiC QDs are nearly monodispersed and about 3.5 nm in size. The relative content of β-SiC after laser ablation depends on the liquid phase's ability to supply carbon atoms at a certain laser fluence. Proper liquid media with appropriate carbon atoms supply capacity can lead to nearly pure β-SiC in the as-prepared sample. The obtained β-SiC QDs exhibit strong and stable emission in the violet region, significantly blue-shifting relative to that of bulk SiC. This big blue shift of emission is attributed to the significant quantum confinement effect induced by their ultrafine size. This method can be extended to produce some other ultrafine Si compounds which are usually formed at high temperature and/or high pressure. This study could present the building blocks of nanostructured devices as violet light sources and new materials in biological molecular labels.

Relationship between oxygen defects and the photoluminescence property of ZnO nanoparticles: A spectroscopic view

Abstract: The present paper deals with the synthesis and characterization of ZnO nanoparticles obtained by coprecipitation method at three different pH conditions. Samples characterizations aimed at understanding their spectroscopic properties are carried out using a variety of experimental techniques. X-ray diffraction and transmission electron microscopic studies show significant increase in the particle size of the synthesized ZnO nanoparticles ranging from 3–40 nm with increasing pH values. Absorption spectra show particle size dependent blueshift in the energy band, which may be ascribed to quantum confinement effect. Fourier transform infrared (FTIR) spectroscopy reveals enhancement in the surface defects of the synthesized ZnO nanoparticles with increasing pH values. Electron paramagnetic resonance studies at room temperature (300 K) and several liquid helium temperatures (including the lowest temperature 13 K for all samples) indicate the presence of singly ionized oxygen vacancy defects (VO+) and O2− super...

Reduced quantum confinement effect and electron-hole separation in SiGe nanowires

Abstract: Received 9 April 2009; published 11 May 2009Using first-principles methods, we investigate the structural and electronic properties of SiGe nanowires-based heterostructures, whose lattice contains the same number of Si and Ge atoms but arranged in a differentmanner. Our results demonstrate that the wires with a clear interface between Si and Ge regions not only formthe most stable structures but show a strongly reduced quantum confinement effect. Moreover, we, with theinclusion of many-body effects, prove that these nanowires—under optical excitation—display a clear electron-hole separation property which can have relevant technological applications.DOI: 10.1103/PhysRevB.79.201302 PACS number s : 73.22. f, 73.20.At, 78.67.Bf

SiGe nanowires: Structural stability, quantum confinement, and electronic properties

Abstract: We report first-principles calculations of [110] SiGe NWs; we discuss the effect of geometry and composition on their thermodynamic stability, on their electronic properties, and on the nature of the quantum confinement effect. The analysis of formation enthalpy reveals that ${\text{Ge}}_{core}/{\text{Si}}_{shell}$ NWs represent the most stable structure at any diameter, as a confirmation of the results of many experimental works. The study of the dependence of the energy band gap on the composition and geometry shows how abrupt NWs (wires with a clear flat interface between Si and Ge) present strongly reduced quantum confinement effect and offer a very easy way to predict and to engine energy band gap, which can have a strong relevance from a technological point of view. A careful analysis of the influence of composition on the wave-function localization and quantum confinement effect is also presented, in particular, for core-shell NWs.

Synthesis of GaAs nanowires with very small diameters and their optical properties with the radial quantum-confinement effect

Abstract: We report the synthesis and optical properties of GaAs nanowires with very small diameters. We grew the GaAs nanowires by using size-selective gold particles with nominal diameters of 5, 10, 20, 40, and 60 nm. The diameter-controlled nanowires enable us to observe blueshifts of the free exitononic emission peak from individual nanowires with decreasing gold-particle size due to the two-dimensional radial quantum-confinement effect. We also analyze the absorption and emission polarization anisotropies of these bare GaAs quantum nanowires.

Structural and Optical Characteristics of Crystalline Silicon Carbide Nanoparticles Synthesized by Carbothermal Reduction

Abstract: Silicon carbide (SiC) nanoparticles were successfully synthesized by using carbothermal reduction method. Nanoparticles with zincblende structure (3C-SiC) could be prepared using polysiloxane as silicon source and phenol resin (MH) or xylene resin (YN) as carbon source. The sample YN has an average grain size of 22 nm, larger than that of the sample MH (8 nm). Raman spectroscopy revealed that all nanoparticle samples contain graphitic surface carbon layers. Oxygen contamination on the nanoparticle surface could be reduced by postfluorine treatment (MH-F). But the sample MH-F showed reduced SiC crystallinity compared with the sample MH. The nanoparticle samples exhibited an intensive emission band in the blue region observed by photoluminescence (PL) spectroscopy. The bandgap energy of the nanoparticle samples is estimated to be ∼ 3 eV from the PL spectra, blueshifted by ∼0.6 eV from that of bulk 3C-SiC due to the quantum confinement effect.

Quantum confinement effect on the effective mass in two-dimensional electron gas of AlGaN/GaN heterostructures

Abstract: We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2±0.01)m0.

Phase Contribution of Image Potential on Empty Quantum Well States in Pb Islands on the Cu(111) Surface

Abstract: We use scanning tunneling spectroscopy to explore the quantum well states in the Pb islands grown on a Cu(111) surface. Our observation demonstrates that the empty quantum well states, whose energy levels lie beyond 1.2 eV above the Fermi level, are significantly affected by the image potential. As the quantum number increases, the energy separation between adjacent states is shrinking rather than widening, contrary to the prediction for a square potential well. By simply introducing a phase factor to reckon the effect of the image potential, the shrinking behavior of the energy separation can be reasonably explained with the phase accumulation model. The model also reveals that there exists a quantum regime above the Pb surface in which the image potential is vanished. Moreover, the quasi-image-potential state in the tunneling gap is quenched because of the existence of the quantum well states.

Photoluminescence and photoabsorption blueshift of nanostructured ZnO: Skin-depth quantum trapping and electron-phonon coupling

Abstract: Although the size- and shape-induced blueshift in the photoluminescence and photoabsorption of nanostructured ZnO has been extensively investigated, the underlying mechanism remains yet unclear. Here we show that theoretical reproduction of the observed trends clarifies that the blueshift originates from the Hamiltonian perturbation due to the broken-bond-induced local strain and quantum trapping and electron-phonon coupling in the surface skin up to two atomic layers in depth while bonds in the core interior retain their bulk nature. The extent of the blue shift depends on the tunable fraction of undercoordinated atoms in the surface skin. Therefore, the quantum confinement effect is indeed more “superficial” than first thought [H. Winn, OE Mag. 8, 10 (2005)].

Synthesis of ZnO nanoparticles by solvothermal method and their ammonia sensing properties.

Abstract: ZnO nanoparticles were prepared by a simple solvothermal route by using a ethylene glycol (EG)-ethanol (E) solvent system. The shape and size of ZnO nanoparticles were tuned by changing the volume composition of the solvents and the synthesis temperature. Phase purity was confirmed by X-ray diffraction (XRD) and crystal size was determined by transmission electron microscopy (TEM). Ellipsoidal and spherical nanoparticles were obtained by varying the volume ratio of the solvents. Mostly spherical nanoparticles with average diameter of 4 nm was synthesized with higher volume fraction of EG at a lower synthesis temperature. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed for the smallest nanoparticle size that is ascribed to the quantum confinement effect. The ammonia gas sensing characteristics of the smallest diameter nanoparticles showed better sensitivity at a relatively lower temperature than that of large diameter particles.

Quantum dot solar cell with quantum dot bandgap gradients

Abstract: Efficient photovoltaic devices with quantum dots are provided Quantum dots have numerous desirable properties that can be used in solar cells, including an easily selected bandgap and Fermi level In particular, the size and composition of a quantum dot can determine its bandgap and Fermi level By precise deposition of quantum dots in the active layer of a solar cell, bandgap gradients can be present for efficient sunlight absorption, exciton dissociation, and charge transport Mismatching Fermi levels are also present between adjacent quantum dots, allowing for built-in electric fields to form and aid in charge transport and the prevention of exciton recombination

Oblique-Angle Sputtering Effects on Characteristics of Nanocolumnar Structure Anisotropic Indium Tin Oxide Films

Abstract: We present a nanocolumnar structure anisotropic indium tin oxide (ITO) film deposited at different oblique angles by a radiofrequency magnetron sputtering system. Three dominant diffraction peaks were observed. The location of peaks SnO 2 (110), In 2 O 3 (222), and In 2 O 3 (400) increases from 29.39, 31.34, and 36.34° to 29.45, 31.60, and 36.48°, respectively, as the sputtering oblique angle increases from 0 to 80°. This may be attributed to a high incorporation of oxygen into the film deposited at a high oblique angle. An anisotropic ITO film with a higher oxygen content has a smaller lattice constant, a larger bandgap, a higher resistivity, and a degradation of crystallization. As the flux arrival angle α increases, the absorption edge of the spectra demonstrated a blueshift. The blueshift may result from the Burstein―Moss effect due to the increase in the oxygen content in the anisotropic ITO film or to the quantum confinement effect caused by the exciton quantization in the ITO film.

Efficient luminescence and photocatalytic behaviour in ultrafine TiO2 particles synthesized by arrested precipitation

Abstract: We have demonstrated optical and photophysical properties of newly synthesized ultrasmall TiO2nanoparticles. The particles are synthesized by arrested precipitation method in trioctylphosphine oxide (TOPO) and benzophenone as the capping agent and also as solvent at high temperature in Ar atmosphere. High-resolution TEM and XRD measurements revealed that the particles are in the size range of 3.3–3.5 nm. Optical absorption spectra of these particles show large blue shift indicating a quantum confinement effect. The particles are highly luminescent, which is very unusual for an indirect band gap material. The emission quantum yield has been measured to be >2%. Again the photocatalytic activity of newly synthesized TiO2 particles was found to be much higher as compared to TiO2nanoparticles, indicating a better photocatalytic material arising due to size quantization.

Intrinsic optical gain of ultrathin silicon quantum wells from first-principles calculations

Abstract: Optical gains of ultrathin Si(001) quantum wells are calculated from first principles, and found to be positive because of an intrinsic quantum confinement effect. The gain of the ultrathin silicon film is comparable to that of the bulk GaAs if the carrier density is large enough. The impact of surface structure of the silicon film on the efficiency of light emission is also investigated and we found that ${\text{SiO}}_{2}$ crystal that forms a strainless connection with a Si(001) surface such as quartz enhances optical gain.

Optical properties of well-crystallized and size-tuned ZnO quantum dots

Abstract: Well-crystallized and size-tuned ZnO quantum dots (QDs) were prepared by pulsed laser ablation and following size classification using a differential mobility analyzer. Photoluminescence (PL) spectra of the ZnO QDs showed predominant ultraviolet (UV) emission. The finely size-tuned ZnO QDs allowed us to precisely evaluate the quantum confinement effect, i.e., the size-driven color-tunable behavior in the UV emission range. The faint green luminescence found in the PL spectra and the low Stokes shifts of several tens meV demonstrated that the present QDs are almost free from defects. These features are quite favorable for use in future optoelectronic devices.

Engineering Quantum Confined Silicon Nanostructures: Ab-Initio Study of the Structural, Electronic and Optical Properties

Abstract: Silicon nanostructures, in the form of nanodots, nanowires and nanoslabs have attracted a lot of interest in recent years. Quantum confinement effects play an important role with respect to both the electronic and optical properties. We review and summarize here our results concerning the properties of semiconductor nanocomplexes. Total energy calculations within the density functional theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped Si nanosystems of different dimensionality, size and surface termination. Single-particle and Many- Body perturbation theory calculations have been carried out in order to study the optical properties, both in ground and excited state configuration, of Si nanodots. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order both, to understand the physical properties of these systems, and to provide an explanation for both the Stokes shift and the near-visible PL experimentally observed. For Si nanocrystals embedded in a SiO2 matrix, the strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in good agreement with the experimental results. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced, favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals, showing the possibility of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals. We also discuss here the case of multiple doping. In the case of one-dimensional systems we have calculated the structural, electronic and optical properties of hydrogenated Ge and Si nanowires of different sizes and different spatial orientations. We have analyzed how the geometrical relaxation affects the optoelectronic properties. Moreover for the smallest structures, we have calculated the electronic and optical properties overcoming the one-particle approach. Large self-energy corrections, compared to the bulk ones, have been found together with strong excitonic effects. In particular in the case of Si nanowires the calculated electronic and optical gaps compare well with the available experimental data. Indeed we show that freshly etched porous Si is better described as a distribution of interacting nanowires than as an ensemble of isolated nanoparticles. Concerning the two-dimensional nanoslab systems we show how the calculated optoelectronic properties of Si superlattices and multiple quantum wells, where CaF2 and SiO2 are the barrier mediums, are in good agreement with the experimental outcomes and we discuss the comparison between Si and Ge nanofilms.

ZnO quantum dots embedded in a SiO2nanoparticle layer grown by atomic layer deposition

Abstract: We have fabricated ZnO quantum dots embedded in an amorphous silicon oxide layer by atomic layer deposition (ALD). SiO2 nanoparticles with diameters of approximately 10 nm dispersed in isopropyl alcohol solution were spin-on coated on the Si substrate and dried in an oven. Subsequently, ALD of ZnO was performed using two precursors, Zn(C2H5)2 and H2O, which can infiltrate into the small voids between SiO2 nanoparticles. It is revealed that the deposited ZnO was uniformly embedded in the SiO2 layer as crystalline ZnO quantum dots with diameters in the range of about 3–8 nm after a high-temperature post-deposition annealing treatment. The quantum confinement effect of the ZnO dots is well manifested by a significant blue-shift of about 80 meV in the photoluminescence spectrum at room temperature. This technique is applicable to a new fabrication route of optoelectronic devices, such as UV light emitting diodes and lasers. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab initio study of energy band structures of GaAs nanoclusters

Abstract: Electronic states and optical transitions of hydrogen terminated GaAs nanoclusters up to 16.9 nm in diameter were studied using a large-scale quantum chemistry approach called central insertion scheme by which the quantum confinement effect is shown to quantitatively agree with experimental results. The ab initio study further reveals that the effective mass of the conduction-band electron (valence-band hole) in the GaAs nanocluster is larger (smaller) than the bulk material value.

Exciton quantum confinement effect in nanostructures formed by laser radiation on the surface of CdZnTe ternary compound

Abstract: Self-organizing structures of nanometer size are observed on the surface of CdZnTe crystal irradiated by strongly absorbed Nd:YAG laser radiation (LR) at intensities within 4–12 MW/cm2. The effect of exciton quantum confinement manifested by a shift to higher energies of the A0,X exciton band of the photo-luminescent spectrum is present in structures of 10–15 nm in diameter at the top of nano-hills. A graded band gap structure with optical window is formed at the top of nano-hills. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)