Showing papers on "Potential well published in 2012"

Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities

Abstract: Graphitic (g)-C3N4 with a layered structure has the potential of forming graphene-like nanosheets with unusual physicochemical properties due to weak van der Waals forces between layers. Herein is shown that g-C3N4 nanosheets with a thickness of around 2 nm can be easily obtained by a simple top-down strategy, namely, thermal oxidation etching of bulk g-C3N4 in air. Compared to the bulk g-C3N4, the highly anisotropic 2D-nanosheets possess a high specific surface area of 306 m2 g-1, a larger bandgap (by 0.2 eV), improved electron transport ability along the in-plane direction, and increased lifetime of photoexcited charge carriers because of the quantum confinement effect. As a consequence, the photocatalytic activities of g-C3N4 nanosheets have been remarkably improved in terms of OH radical generation and photocatalytic hydrogen evolution.

One-dimensional quantum confinement effect modulated thermoelectric properties in InAs nanowires.

Abstract: We report electrical conductance and thermopower measurements on InAs nanowires synthesized by chemical vapor deposition. Gate modulation of the thermopower of individual InAs nanowires with a diameter around 20 nm is obtained over T = 40–300 K. At low temperatures (T < ∼100 K), oscillations in the thermopower and power factor concomitant with the stepwise conductance increases are observed as the gate voltage shifts the chemical potential of electrons in InAs nanowire through quasi-one-dimensional (1D) subbands. This work experimentally shows the possibility to modulate semiconductor nanowire’s thermoelectric properties through 1D subband formation in the diffusive transport regime for electron, a long-sought goal in nanostructured thermoelectrics research. Moreover, we point out the scattering (or disorder) induced energy level broadening as the limiting factor in smearing out the 1D confinement enhanced thermoelectric power factor.

Novel attractive force between ions in quantum plasmas.

Abstract: We report a new attractive force between ions that are shielded by degenerate electrons in quantum plasmas. Specifically, we show that the electric potential around an isolated ion has a hard core negative part that resembles the Lennard-Jones–type potential. Physically, the new electric potential is attributed to the consideration of the quantum statistical pressure and the quantum Bohm potential, as well as the electron exchange and electron correlations due to electron-1/2 spin within the framework of the quantum hydrodynamical description of quantum plasmas. The shape of the attractive potential is determined by the ratio between the Bohr radius and the Wigner-Seitz radius of degenerate electrons. The existence of the hard core negative potential will be responsible for the attraction of ions forming lattices and atoms or molecules, as well as for critical points and phase transitions in quantum plasmas at nanoscales.

Wavefunction engineering in quantum confined semiconductor nanoheterostructures for efficient charge separation and solar energy conversion

Abstract: Colloidal quantum-confined semiconductor nanoheterostructures (SNHs) that are composed of multiple component materials in rationally designed spatial arrangements are promising light harvesting and charge separation materials for solar energy conversion. SNHs can be engineered to exhibit type I, quasi-type II and type II carrier localization, affecting their photophysical properties and photochemical performances. Unlike bulk semiconductor heterostructures, the electron and hole energy levels and spatial distributions in SNHs can be continuously tuned by adjusting the material dimension through the quantum confinement effect, providing additional control of their properties through wavefunction engineering. In this article, we review recent progress in using wavefunction engineering to control the absorption and emission spectra, single and multiple exciton dynamics and charge transfer properties of SNHs (core/shell QDs and dot-in-rod nanorods) as well as to improve their performance as light harvesting and charge separation materials for solar energy conversion.

Theoretical Studies on the Effect of Confinement on Quantum Dots Using the Brus Equation

Abstract: Quantum confinement effect in semiconductor quantum dots (QD's) of CdSe, ZnS and GaAs has been studied using the Brus Equation. It is found that the simple models obtained for the three different semiconductor nanocrystals exhibit the size dependence predicted by the particle-in-a-box model. The result shows that ground state confinement energy is inversely proportional to the size (radius). Thus, as one increases the radius (size), the confinement energy decreases, but never reaches zero. i.e., the lowest possible energy for the quantum dot sample is not zero.

Tuning the potentials of "extra" electrons in colloidal n-type ZnO nanocrystals via Mg2+ substitution.

Abstract: Colloidal reduced ZnO nanocrystals are potent reductants for one-electron or multielectron redox chemistry, with reduction potentials tunable via the quantum confinement effect. Other methods for tuning the redox potentials of these unusual reagents are desired. Here, we describe synthesis and characterization of a series of colloidal Zn(1-x)Mg(x)O and Zn(0.98-x)Mg(x)Mn(0.02)O nanocrystals in which Mg(2+) substitution is used to tune the nanocrystal reduction potential. The effect of Mg(2+) doping on the band-edge potentials of ZnO was investigated using electronic absorption, photoluminescence, and magnetic circular dichroism spectroscopies. Mg(2+) incorporation widens the ZnO gap by raising the conduction-band potential and lowering the valence-band potential at a ratio of 0.68:0.32. Mg(2+) substitution is far more effective than Zn(2+) removal in raising the conduction-band potential and allows better reductants to be prepared from Zn(1-x)Mg(x)O nanocrystals than can be achieved via quantum confinement of ZnO nanocrystals. The increased conduction-band potentials of Zn(1-x)Mg(x)O nanocrystals compared to ZnO nanocrystals are confirmed by demonstration of spontaneous electron transfer from n-type Zn(1-x)Mg(x)O nanocrystals to smaller (more strongly quantum confined) ZnO nanocrystals.

Influence of confinement regimes on magnetic property of pristine SnO2 quantum dots

Abstract: We demonstrate a clear correlation between the quantum confinement effect and magnetization of pristine tin dioxide (SnO2) quantum dots (QDs). We have synthesized single crystalline QDs of SnO2 above and below the exciton Bohr radius (2.7 nm). Such fine control over the size of the QDs is a challenging task. The 2 nm QDs belong to strong confinement regimes and are found to be ferromagnetic in nature, whereas 3 nm QDs are diamagnetic like bulk SnO2. To the best of our knowledge, so far no experimental studies on the influence of confinement effect on the magnetic behaviour of SnO2 QDs have been reported. We propose two possible mechanisms based on the theory of localization of holes due to a strong confinement effect to explain room temperature ferromagnetism in 2 nm QDs. The localization of holes is confirmed from photoluminescence and UV visible spectroscopy.

Size dependent optical properties of the CdSe-CdS core-shell quantum dots in the strong confinement regime

Abstract: We have investigated the third-order optical nonlinearity in CdSe and CdSe-CdS core-shell quantum dots with particle sizes in the range 4.4–5.2 nm by the z-scan technique. Optical absorption and fluorescence properties show quantum confinement effect and the fluorescence peak is shifted when the diameter of the core-shell quantum dots becomes large. Their nonlinear absorption and nonlinear refraction were observed with nanosecond laser radiation of 532-nm wavelength. At an excitation irradiance of 392 MW/cm2, the CdSe-CdS core-shell quantum dots exhibit a reverse saturation and a negative nonlinear refractive index indicating a clear nonlinear behavior. The introduction of CdS shell around CdSe core gives large third-order nonlinearity and third-order optical susceptibility decreases with increasing particle core size in the case of core-shell quantum dots within the range of our investigations. The third-order susceptibility of these quantum dots vary from 3.39 × 10−11 to 5.77 × 10−11 esu. The optical limiting threshold of the quantum dots varies in the range from 0.298 to 0.345 GW/cm2 and this therefore makes CdSe-CdS core-shell quantum dots a promising candidate for reverse-saturable absorption based devices at high laser intensities such as optical limiters.

A first-principles study of II–VI (II = Zn; VI = O, S, Se, Te) semiconductor nanostructures

Abstract: We present a systematic investigation of the structural, electronic and optical properties of wurtzite-like ZnX (X = O, S, Se, Te) nanostructures at the DFT/TDDFT level of theory. To provide a direct comparison with the experiment, realistic 1.0–1.5 nm quantum dots have been built up from the bulk. Low-lying computed excitation energies agree well with the available experimental data. The broad excitation profiles and narrow emission spectra typical of semiconductor quantum dots could be explained by the fact that the LUMO is the state accepting the electron of the low-lying TDDFT excitations calculated. Calculated binding energies for the Zn 3d shell have been found to be 0.5 eV lower than those of the corresponding bulk materials. Anion vacancies can explain the visible light emission of ZnX by introducing a trap state into the bandgap of the nanostructures, in agreement with previous theoretical and experimental works on ZnO. Calculations on rod- and sheet-like prototype clusters point to a significant quantum confinement effect on the optoelectronic properties of Zn-based nanomaterials.

Raman investigation of silicon nanocrystals: quantum confinement and laser-induced thermal effects

Abstract: We present a detailed experimental and theoretical Raman investigation of quantum confinement and laser-induced local thermal effects on hydrogenated nanocrystalline silicon with different nanocrystal sizes (3.6–6.2 nm). The local temperature was monitored by measuring the Stokes/anti-Stokes peak ratio with the laser power density range from ~120 to 960 kW/cm2. In combination with the three-dimensional phonon confinement model and the anharmonic effect, which incorporates the three-phonon and four-phonon decay processes, we revealed an asymmetrical decay process with wavenumbers ~170 and 350 cm–1, an increasing anharmonic effect with nanocrystal sizes, and a shortening lifetime with enhanced temperature and decreasing nanocrystal dimension. Furthermore, we demonstrated experimentally that for Si nanocrystals smaller than 6 nm, the quantum confinement effect is dominant for the peak shift and line broadening. Copyright © 2011 John Wiley & Sons, Ltd.

Electronic states and curved surface effect of silicon quantum dots

Abstract: The calculation results show that the bonding energy and electronic states of silicon quantum dots (Si QDs) are different on various curved surfaces (CS), for example, a Si-O-Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. Curved surface breaks symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The red-shifting of photoluminescence spectra on smaller silicon quantum dots can be explained by CS effect. In CS effect, surface curvature is determined by the shape of Si QDs or silicon nanostructures, which is independent of their sizes. The CS effect has the interesting fundamental physical properties in nanophysics as that of quantum confinement effect.

Production of nanometer-size GaAs nanocristals by nanosecond laser ablation in liquid.

Abstract: This paper reports the formation and characterization of spherical GaAs quantum dots obtained by nanosecond pulsed laser ablation in a liquid (ethanol or methanol). The produced bare GaAs nanoparticles demonstrate rather narrow size distribution which depends on the applied laser power density (from 4.25 to 13.9 J/cm 2 in our experiments) and is as low as 2.5 nm for the highest power used. The absolute value of the average diameter also decreases significantly, from 13.7 to 8.7 nm, as the laser power increases in this interval. Due to the narrow nanoparticle size dispersion achieved at the highest laser powers two absorption band edges are clearly distinguishable at about 1.72 and 3.15 eV which are ascribed to E0 and E1 effective optical transitions, respectively. A comparison of the energies with those known for bulk GaAs allows one to conclude that an average diameter of the investigated GaAs nanoparticles is close to 10 nm, i.e., they are quantum dots. High resolution transmission electron microscopy (HRTEM) images show that the bare GaAs nanoparticles are nanocrystalline, but many of them exhibit single/multiple twin boundary defects or even polycrystallinity. The formation of the GaAs crystalline core capped with a SiO2 shell was demonstrated by HRTEM and energy dispersive X-ray (EDX) spectroscopy. Effective band edges can be better distinguished in SiO2 capped nanoparticles than in bare ones, In both cases the band edges are correlated with size quantum confinement effect.

Quantum size effect in the photoluminescence properties of p-type semiconducting transparent CuAlO2 nanoparticles

Abstract: Photoluminescence properties of CuAlO2 nanoparticles, deposited by a cost-effective direct current sputtering technique, have been studied. The nanoparticles show room-temperature photoluminescence peaks of near-band-edge emission due to recombination of free excitons. A blue-shift in the emission peaks as a decreasing function of the nanoparticle sizes is observed, which is attributed to the quantum confinement effect within the CuAlO2 nanoparticles. Theoretical calculations of bandgap enhancement values are found to be matching fairly well with that of the experimentally obtained values, confirming the existence of the quantum size effect within the nanomaterial. Approximate calculations show that the confinement effect falls within moderate-to-weak confinement regime. X-ray diffraction and electron microscopic measurements confirm the proper phase formation and nanocrystalline structure of the as-deposited nanoparticles. The room-temperature and size-dependent photoluminescence properties of this nanomaterial will be very useful for light emitting diode and similar optoelectronic applications.

Excitation dependent photoluminescence study of Si-rich a-SiNx:H thin films

Abstract: We report photoluminescence (PL) investigations on Si-rich amorphous hydrogenated silicon nitride (a-SiNx:H) thin films of different compositions, using three different excitation lasers, viz., 325 nm, 410 nm, and 532 nm. The as-deposited films contain amorphous Si quantum dots (QDs) as evidenced in high resolution transmission electron microscopy images. The PL spectral shape is in general seen to change with the excitation used, thus emphasizing the presence of multiple luminescence centres in these films. It is found that all the spectra so obtained can be deconvoluted assuming Gaussian contributions from defects and quantum confinement effect. Further strength to this assignment is provided by low temperature (300 °C) hydrogen plasma annealing of these samples, wherein a preferential enhancement of the QD luminescence over defect luminescence is observed.

Room temperature direct-bandgap electroluminescence from n-type strain-compensated Ge/SiGe multiple quantum wells

Abstract: N-type strain-compensated Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) were grown on a Si0.1Ge0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition on a n+-Si(001) substrate. Under low forward bias voltage ranging from 0.6 to 1.2 V, narrow direct-bandgap electroluminescence (EL) peak from MQWs light emitting diode was observed at room temperature. The quantum confinement effect of the direct-bandgap transitions and the temperature dependent EL peak redshift are in good agreement with the calculated results.

Nonlinear optical characterization and measurement of optical limiting threshold of CdSe quantum dots prepared by a microemulsion technique

Abstract: CdSe quantum dots prepared by micro emulsion technique shows quantum confinement effect and broad emission at 532 nm. These quantum dots have about 4.35 nm size, and they exhibit good nonlinear effects which are measured using z-scan technique. The samples have a reverse saturation in the nonlinear absorption as nonlinear optical absorption coefficient β is 2.545 × 10−10 W m−1 and nonlinear optical refraction coefficient n 2 is −1.77 × 10−10 esu. The third-order nonlinear optical susceptibility is found to be 4.646 × 10−11 esu and also the figure of merit is 2.01 × 10−12 esu m. The optical limiting threshold which is found to be 0.346 GW/cm2 makes it a good candidate for device fabrication.

Effect of Cr-doping on the structural and optical properties of CdS nanoparticles prepared by chemical precipitation method

Abstract: Undoped and Cr doped CdS nanoparticles have been prepared by chemical precipitation method. X-ray diffraction analysis reveals that the undoped and Cr doped CdS nanoparticles exhibit hexagonal structure and the average particle size of the nanoparticles is in the range of 2.2–3.8 nm. The HRTEM studies show that the average particle size of undoped and Cr doped CdS nanoparticles is in the range of 2–3.7 nm. The compositional analysis results indicates that Cd, S and Cr are present in the samples. From the optical studies it is observed that the absorption edge of the prepared CdS and Cr doped CdS nanoparticles are shifted towards the shorter wavelength region (blue shift) when compared to that of bulk CdS and this shift is due to the quantum confinement effect present in the samples.

Influence of Reaction Temperature on Crystal Structure and Band Gap of ZnO Nanoparticles

Abstract: Zinc oxide nanoparticles have been synthesized at room temperature and at 50°C via precipitation method for the present study. The effect of synthesis temperature on the crystalline size, microstructure, optical properties, and energy gap of the synthesized zinc oxide nanoparticles has been investigated. The particle size was found to reduce from 26 nm to 17 nm when the reaction temperature was increased from room temperature to 50°C. Energy gap of the prepared samples has been theoretically determined using effective mass model equation and compared with the experimental value. It depicts that the energy gap of the smaller nanoparticles is higher than the bigger one. The recorded blue shift confirmed the quantum confinement effect. Compositional quality and functional group of the zinc oxide nanoparticles have been studied, and the frequency of zinc oxide nanoparticles has been observed at two different regions, 566 cm−1 and 432 cm−1. The recorded ultraviolet-visible spectrum shows absorption peaks at 36...

Synthesis and photoluminescence properties of HMT passivated Dy2O3 nanoparticles

Abstract: Dy2O3 nanoparticles were synthesized at room temperature through soft chemical route using dysprosium acetate hexahydrate and hexamethylenetetramine as starting materials. As-synthesized Dy2O3 nanoparticles were calcined up to 600 °C and the samples were subjected to various characterizations. The decomposition course of as-prepared particles and the formation process of Dy2O3 were studied by TG/DTA. The cubic bixbyite phase formation of Dy2O3 nanoparticles was confirmed by XRD results. The particle size and morphology were investigated by TEM. The composition and surface passivation were confirmed by EDS and FTIR studies, respectively. The possible formation mechanism has been discussed based on the experimental results. Bandgap of the material was estimated from the UV–vis absorption spectra. A blue-shift was observed in the absorbance as a result of quantum confinement effect, which was supported by PL spectra.

Exciton states in quasi-zero-dimensional semiconductor nanosystems

Abstract: The variational method in the context of the modified effective mass approximation is used to calculate the dependence of exciton ground-state energy for a quantum dot embedded in a borosilicate glassy matrix on the quantum dot radius. It is shown that the peaks in the absorption and low-temperature luminescence spectra of such a nanosystem are shifted to shorter wavelengths due to size quantization of the exciton ground-state energy in the quantum dot.

Quantum confinement effect of CdSe induced by nanoscale solvothermal reaction

Abstract: We report a novel method, nanoscale solvothermal reaction (NSR), to induce the quantum confinement effect of CdSe on nanostructured TiO2 by solvothermal route. The time-dependent growth of CdSe is observed in solution at room temperature, which is found to be accomplished instantly by heat-treatment in the presence of solvent at 1 atm. However, no crystal growth occurs upon heat-treatment in the absence of solvent. The nanoscale solvothermal growth of CdSe quantum dot is realized on the nanocrystalline oxide surface, where Cd(NO3)2·4H2O and Na2SeSO3 solutions are sequentially spun on nanostructured TiO2, followed by heat-treatment at temperatures ranging from 100 °C to 250 °C. Size of CdSe increases from 4.4 nm to 5.3 nm, 8.7 nm and 14.8 nm, which results in decrease in optical band gap from 2.19 eV to, 1.95 eV, 1.74 eV and 1.75 eV with increasing the NSR temperature from 100 °C to 150 °C, 200 °C and 250 °C, respectively, which is indicative of the quantum confinement effect. Thermodynamic studies reveal that increase in the size of CdSe is related to increase in enthalpy, for instance, from 3.77 J mg−1 for 100 °C to 8.66 J mg−1 for 200 °C. Quantum confinement effect is further confirmed from the CdSe-sensitized solar cell, where onset wavelength in external quantum efficiency spectra is progressively shifted from 600 nm to 800 nm as the NSR temperature increases, which leads to a significant improvement of power conversion efficiency by a factor of more than four. A high photocurrent density of 13.7 mA cm−2 is obtained based on CdSe quantum dot grown by NSR at 200 °C.

Structure and optical properties of a-plane ZnO/Zn0.9Mg0.1O multiple quantum wells grown on r-plane sapphire substrates by pulsed laser deposition

Abstract: A series of 10-period ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with well widths varying from 2.2 to 5.6 nm have been grown on r-plane sapphire substrates by pulsed laser deposition. A good periodic structure with clear interfaces was observed by transmission electron microscopy. In a-plane ZnO/Zn0.9Mg0.1O MQWs, the luminescence was dominated by localized exciton emissions at low temperatures, while the free exciton (FE) transition was dominating emissions at temperatures above 100 K. The thermal quenching behavior of exciton emission has been analyzed. A rate equation assuming two nonradiative recombination channels is used to describe the quenching of the transitions observed. Moreover, the FE emission energy in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum confinement effect.

Surface passivation dependent photoluminescence from silicon quantum dot phosphors.

Abstract: We demonstrate wavelength-tunable, air-stable and nontoxic phosphor materials based on silicon quantum dots (SiQDs). The phosphors, which are composed of micrometer-size silicon particles with attached SiQDs, are synthesized by an electrochemical etching method under ambient conditions. The photoluminescence (PL) peak wavelength can be controlled by the SiQD size due to quantum confinement effect, as well as the surface passivation chemistry of SiQDs. The red-emitting phosphors have PL quantum yield equal to 17%. The SiQD-phosphors can be embedded in polymers and efficiently excited by 405 nm light-emitting diodes for potential general lighting applications.

Optical properties of Mg-doped ZnO nanoparticles

Abstract: Mg-doped ZnO nanoparticles with hexagonal structure were synthesis in basic chemical solution method without using capping agent. The microstructure and optical response of prepared nanoparticles were characterized by using XRD, EDX, optical absorption, photoluminescence and TEM measurements. Semiconductor nanoparticles show unique size dependent optical properties due to quantum confinement effect and there is interest for application in optoelectronics. The ZnO and its composition have considerable attention due to its potential in wideband gap (3.3eV) optoelectronic applications. In most of these methods various capping agents have been used to prevent the growth of particles with time. One of the major challenges in the optimizing of the optical properties of ZnO is the incorporation of doping ions into the ZnO lattice. The peak position of doped nanoparticles shows shift as effect of doping incorporation. In this work, we present a simple chemical method for synthesis mono-disperse Mg-doped ZnO nanoparticles with semi-spherical shape, without using surfactant.

Ultrasound assisted synthesis and properties of ZnO:B nanorods and micro flowers.

Abstract: Nanopowders of ZnO pure and doped with boron have been synthesized through sonochemical method using acetate of the material as starting reagent. The incorporation of boron has been confirmed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis. Continuous (CS) and pulsed (PS) mode syntheses have shown interesting structural and optical properties such as photoluminescence (PL) and ultra-violet (UV) absorption. X-ray diffraction (XRD) studies showed broadening and shifting of peaks with boron incorporation in ZnO leading to size reduction in doped samples. The structure of the nanoparticles was found to be rod like and these rod like structure coalesce in boron doped ZnO. This has been explained on the basis of nucleation of octahedral units in the beginning leading it to tetrahedral structure. Electron microscopy has been used to explain these results. Definite peak shifts towards low wavelength side in absorption band and photoluminescence spectra have been observed for smaller particle size in pulsed mode powder. The blue shift due to particle size has been explained in the light of quantum confinement effect.