Strong Quantum Confinement Effect Research Articles

Photocatalytic degradation of organic dye on porous iron sulfide film surface

Thin films of nanocrystalline and porous FeS2 with marcasite phase have been deposited from a greenish-blue iron nitroprusside precursor film, which readily gives FeS2 on reacting with an aqueous solution of sodium sulfide. High resolution X-ray diffraction (HRXRD) pattern indicated the formation of polycrystalline and orthorhombic (marcasite) phase of FeS2, whereas the field emission scanning electron microscopy (FESEM) showed the morphology of the films to be consisted of grains of average 25nm diameter with unevenly distributed numerous pores. Optical characterization (UV–Vis and photoluminescence) revealed significant amount of blueshift in the band gap energy of the deposited material, which is attributed to the strong quantum confinement effect exerted by the FeS2 nanocrystals. The deposited FeS2 films showed good photocatalytic activity toward the degradation of Rose Bengal dye and could be found efficient for wastewater treatment.

Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots

A facile and rapid microwave irradiation method was developed to prepare ZnS nanoparticles (NPs) using a set of ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The phases, structures, and optical absorption properties of the NPs were determined in depth with X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV–vis absorption spectroscopy (UV–vis), diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy (PL). The average crystallite size of the ZnS NPs calculated from the XRD pattern was of the order of 2.8nm which exhibits cubic zinc blende structure. The energy band gap measurements of NPs were carried out by UV and DRS. The results revealed that the ZnS NPs exhibit strong quantum confinement effect. The optical band gap energy increases significantly compared with those of the bulk ZnS. The refractive indices for different ZnS nanosamples and different concentrations of ZnS NPs for a typical sample suspended in deionized water were also measured.

An atomic ordering based AlInP unicompositional quantum well grown by MOVPE

An AlInP unicompositional single quantum well (QW) structure has been grown and studied by using metal-organic vapor phase epitaxy (MOVPE). The energy bandgap offset of the quantum well is achieved by taking the advantage of the AlInP epilayer’s bandgap energy reduction induced by its CuPt–B type spontaneous atomic ordering instead of by changing the QW layers’ compositions during the MOVPE growth. The degree of the CuPt–B atomic ordering of the AlInP epilayers of the QW structure was controlled by adjusting the input V/III flux ratio during the MOVPE growth. Low temperature photoluminescence (PL) measurement shows strong quantum confinement effect of the grown AlInP unicompositional QWs. After annealing at 900°C for 30s, the peak emission wavelength of the QW was measured. It remained at the same wavelength as that of the AlInP barrier layer, which showed the disappearance of the quantum confinement effect of the QW structure. This is attributed to the elimination of the CuPt–B atomic ordering of the AlInP epilayers of the QW sample under a high temperature annealing. The QW structure became a thick disordered AlInP bulky layer after the thermal annealing.

Structural, optical and electrical properties of solvothermally synthesized PbTe nanodisks

The present study investigates the properties of mechanically compacted pellets of nanosized lead telluride powders synthesized by using ethylene glycol as solvent and PVP as surfactant. XRD analysis indicates the formation of PbTe nanocrystals which exhibit both cubic and orthorhombic phases. Morphological study reveals that the prepared PbTe nanocrystals have disk-like shape. Electrical analysis reveals that the electrical conductivity of PbTe nanocrystals can be enhanced by the formation of nanodisks. Optical absorption and PL results show a large blue-shift which implies that the prepared PbTe nanodisks show strong quantum confinement effect. The possible formation mechanism of the PbTe nanodisks was also discussed.

Wurtzite ZnSe quantum dots: synthesis, characterization and PL properties

A facile method for synthesis of monodispersed, starch-capped ZnSe nanoparticles at room temperature is being reported. The nanoparticles exhibited strong quantum confinement effect with respect to the bulk ZnSe. The transmission electron microscopy image indicated that the particles were well dispersed and spherical in shape. The X-ray diffraction analysis showed that the ZnSe nanoparticles were of the wurtzite structure, with average particle diameter of about 3.6 nm. The Fourier transform infrared spectrum confirmed the presence of starch as passivating agent.

Quantum Sized Gold Nanoclusters with Atomic Precision

Gold nanoparticles typically have a metallic core, and the electronic conduction band consists of quasicontinuous energy levels (i.e. spacing δ ≪ k(B)T, where k(B)T is the thermal energy at temperature T (typically room temperature) and k(B) is the Boltzmann constant). Electrons in the conduction band roam throughout the metal core, and light can collectively excite these electrons to give rise to plasmonic responses. This plasmon resonance accounts for the beautiful ruby-red color of colloidal gold first observed by Faraday back in 1857. On the other hand, when gold nanoparticles become extremely small (<2 nm in diameter), significant quantization occurs to the conduction band. These quantum-sized nanoparticles constitute a new class of nanomaterial and have received much attention in recent years. To differentiate quantum-sized nanoparticles from conventional plasmonic gold nanoparticles, researchers often refer to the ultrasmall nanoparticles as nanoclusters. In this Account, we chose several typical sizes of gold nanoclusters, including Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60), to illustrate the novel properties of metal nanoclusters imparted by quantum size effects. In the nanocluster size regime, many of the physical and chemical properties of gold nanoparticles are fundamentally altered. Gold nanoclusters have discrete electronic energy levels as opposed to the continuous band in plasmonic nanoparticles. Quantum-sized nanoparticles also show multiple optical absorption peaks in the optical spectrum versus a single surface plasmon resonance (SPR) peak at 520 nm for spherical gold nanocrystals. Although larger nanocrystals show an fcc structure, nanoclusters often have non-fcc atomic packing structures. Nanoclusters also have unique fluorescent, chiral, and magnetic properties. Due to the strong quantum confinement effect, adding or removing one gold atom significantly changes the structure and the electronic and optical properties of the nanocluster. Therefore, precise atomic control of nanoclusters is critically important: the nanometer precision typical of conventional nanoparticles is not sufficient. Atomically precise nanoclusters are represented by molecular formulas (e.g. Au(n)(SR)(m) for thiolate-protected ones, where n and m denote the respective number of gold atoms and ligands). Recently, major advances in the synthesis and structural characterization of molecular purity gold nanoclusters have made in-depth investigations of the size evolution of metal nanoclusters possible. Metal nanoclusters lie in the intermediate regime between localized atomic states and delocalized band structure in terms of electronic properties. We anticipate that future research on quantum-sized nanoclusters will stimulate broad scientific and technological interests in this special type of metal nanomaterial.

Band-gap tuning at the strong quantum confinement regime in magnetic semiconductor EuS thin films

Ultraviolet-visible absorption spectra of nanoscaled EuS thin films reveal a blue shift of the energy between the top-valence and bottom-conduction bands. This band-gap tuning changes smoothly with decreasing film thickness and becomes significant below the exciton Bohr diameter ∼3.5 nm indicating strong quantum confinement effects. The results are reproduced in the framework of the potential morphing method in Hartree Fock approximation. The large values of the effective mass of the holes, due to localization of the EuS f-states, limit the blue shift to about 0.35 eV. This controllable band-gap tuning of magnetic semiconductor EuS renders it useful for merging spintronics and optoelectronics.

Experimental Study of Silicon Monolayers for Future Extremely Thin Silicon-on-Insulator Devices: Phonon/Band Structures Modulation Due to Quantum Confinement Effects

We have experimentally studied Si monolayers, fabricated by thermal oxidation of silicon-on-insulator (SOI) substrates at high temperature, for future extremely thin SOI (ETSOI) complementary metal oxide semiconductor (CMOS) devices, and have shown the strong quantum confinement effects in the ETSOIs. We have successfully formed 0.52-nm Si monolayers, as confirmed by transmission electron microscopy (TEM) and a UV/visual reflection method. We have experimentally shown the asymmetric broadening and the peak downshift of the Raman peak of ETSOIs evaluated by UV-Raman spectroscopy, which is enhanced in the ETSOI thickness TSOI of less than about 5 nm. These results are due to the quantum phonon confinement effects in ETSOIs. Using the TEM observation and UV-Raman spectroscopy of ETSOIs, we have also shown the tensile strain of ETSOIs due to the Si bending and the TSOI variations in ETSOI substrates. In addition, we have observed photoluminescence (PL) from the ETSOIs with a TSOI of less than about 5 nm and the PL intensity strongly depends on the TSOI. However, the peak photon energy of about 1.85 eV in the PL spectrum is independent of the TSOI. We cannot explain the PL results perfectly at present, but we have introduced a possible three-region model of electron/hole pair generation in a two-dimensional Si layer and electron/hole pair recombination at the Si/SiO2 interface state region.

Experimental Study of Silicon Monolayers for Future Extremely Thin Silicon-on-Insulator Devices: Phonon/Band Structures Modulation Due to Quantum Confinement Effects

We have experimentally studied Si monolayers, fabricated by thermal oxidation of silicon-on-insulator (SOI) substrates at high temperature, for future extremely thin SOI (ETSOI) complementary metal oxide semiconductor (CMOS) devices, and have shown the strong quantum confinement effects in the ETSOIs. We have successfully formed 0.52-nm Si monolayers, as confirmed by transmission electron microscopy (TEM) and a UV/visual reflection method. We have experimentally shown the asymmetric broadening and the peak downshift of the Raman peak of ETSOIs evaluated by UV-Raman spectroscopy, which is enhanced in the ETSOI thickness TSOI of less than about 5 nm. These results are due to the quantum phonon confinement effects in ETSOIs. Using the TEM observation and UV-Raman spectroscopy of ETSOIs, we have also shown the tensile strain of ETSOIs due to the Si bending and the TSOI variations in ETSOI substrates. In addition, we have observed photoluminescence (PL) from the ETSOIs with a TSOI of less than about 5 nm and the PL intensity strongly depends on the TSOI. However, the peak photon energy of about 1.85 eV in the PL spectrum is independent of the TSOI. We cannot explain the PL results perfectly at present, but we have introduced a possible three-region model of electron/hole pair generation in a two-dimensional Si layer and electron/hole pair recombination at the Si/SiO2 interface state region.

Room-temperature solution synthesis of Ag2Te hollow microspheres and dendritic nanostructures, and morphology dependent thermoelectric properties

Ag2Te hollow microspheres and dendritic nanostructures were synthesized by a simple room-temperature solution method using AgNO3, TeO2, hydrazine hydrate and ethylenediamine. The as-prepared Ag2Te hollow microspheres and dendritic nanostructures were formed by the self-assembly of nano-building blocks of nanoparticles and nanosheets, respectively. The chemical reaction and formation mechanism of Ag2Te nanostructures were proposed. The products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrical conductivity (σ), Seebeck coefficient (S) and power factor (S2σ) of the as-prepared Ag2Te nanostructures were measured and were found to be strongly dependent on the morphology of Ag2Te nanostructures. The S and σ of Ag2Te hollow microspheres were higher than those of Ag2Te dendritic nanostructures, and S2σ of Ag2Te hollow microspheres (9.01 × 10−4 W m−1K−2) was 5 times that of dendritic nanostructures (1.80 × 10−4 W m−1K−2). The observed thermoelectric properties of Ag2Te nanostructures may be due to the strong quantum confinement effect introduced by decreasing the dimensionality and size of nano-building blocks from dendritic nanostructures to hollow microspheres.

Fabrication of Luminescent Silver Doped PbS Nanowires in Polymer

We report here fabrication of silver (0 to 1.76 mol%) doped PbS nanowires (radius r approximately 1.75 nm) in polymer by a simple wet chemical process. An X-ray photoelectron spectroscopy study clearly confirms the possibility of silver (Ag) doping in PbS nanowires. Both absorption and photoluminescence spectra reveal very strong quantum confinement effect in PbS nanowires as expected for a r/Bohr radius ratio approximately 0.0972 nm. Visible excitonic emission is observed at room temperature in the photoluminescence spectra of undoped and silver doped PbS nanowires in polymer. The excitonic emission is appreciably blue-shifted when doped by silver (1.76 mol%) indicating strong modification of the electronic states by magnetic silver ions. While Ag2+ centers at the substitutional lattice site show an emission band around 525 nm, Ag0 at the interstitial site act as nonradiative recombination centers. Effect of silver doping on the luminescence intensity is also discussed.

Growth and Experimental Evidence of Quantum Confinement Effects in Cu&lt;sub&gt;2&lt;/sub&gt;O and CuO Thin Films

Thin Cu films of thickness 0.4 – 150 nm were deposited via radio frequency magnetron sputtering on Si(100) wafers, corning glass and quartz. Subsequently the Cu films were oxidized in ambient air at 230oC and 425oC in order to produce single-phase Cu2O and CuO, respectively. Selected samples were measured in the transmission geometry with the help of an ultraviolet – visible spectrophotometer. From the absorption spectra of the films, it was found that the gap EB for the dipole allowed transitions showed blue shifts of about 1.2 eV for the Cu2O thinnest film (0.75 nm), whereas the Edirect for the direct gap transitions showed blue shifts of about 0.16 eV for the CuO thinnest film (0.7 nm). The blue shift of the energy gap in the copper-oxide semiconductors is an indication of the presence of strong quantum confinement effects.

Dl‐Aspartic Acid‐Mediated Hydrothermal Synthesis of β‐ZnS Microspheres and Their Optical Properties

AbstractZnS hollow microspheres were synthesized by a dl‐aspartic acid mediated hydrothermal route. dl‐aspartic acid plays an important role as crystal growth soft template, which regulates the release of Zn2+ ions for the formation of ZnS hollow spheres. The formation of these hollow spheres was mainly attributed to an Ostwald ripening process. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), electron diffraction (ED), UV/Vis spectroscopy (UV), and photoluminescence (PL). The shells of the microspheres were composed of ZnS quantum dots (QDs) with the average size of 2.31 nm. The average microspheres diameter is 0.5–3.5 μm. The shell thickness of the hollow sphere is ≈︁300 nm. The optical bandgap energy increased significantly compared to the bulk ZnS material due to the strong quantum confinement effect. Two strong emissions at ≈︁425 nm and ≈︁472 nm in the photoluminescence (PL) spectrum of ZnS hollow microspheres indicate strong quantum confinement because of the presence of QDs.

CONTROLLED SYNTHESIS AND CHARACTERIZATION OF CERIUM-DOPED ZnS NANOPARTICLES IN HMTA MATRIX

Cerium-doped ZnS nanoparticles have been synthesized through hydrothermal method. The nanoparticles were stabilized using hexamethylenetetramine (HMTA) as surfactant in aqueous solution. The average particle size of the prepared samples is about 2 nm. The structure of the as-prepared ZnS nanoparticles is cubic (zinc blende) as demonstrated by X-ray powder diffraction (XRD) and selected area electron diffraction (SAED) analysis. TEM results showed that the synthesized nanoparticles were uniformly dispersed in the HMTA matrix without aggregation. The UV–Vis absorption spectra of the prepared ZnS nanoparticles show a considerable blueshift in the absorption band edge compared to bulk ZnS indicating a strong quantum confinement effect. Formation of HMTA-capped ZnS nanoparticles was confirmed by FTIR studies. Photoluminescence studies showed that the relative emission intensity of Ce3+ -doped ZnS nanoparticles is higher than that of undoped ZnS nanoparticles, which is due to the enhancement of radiative recombination in the luminescence process. The PL spectra showed two emission peaks at around 420 nm and 442 nm, which may be attributed to deep-trap emission or defect-related emission of ZnS and presence of various surface states.

CdS quantum dots in a novel glass with a very low activation energy and its variation of diffusivity with temperature

CdS quantum dots of different average sizes in the range 2 to 3.8 nm were grown by diffusion-limited growth process in indigenously made silicate glass. The absorption spectra showed a strong quantum confinement effect with a blue shift of the order of ~500 meV depending on the average size. Critical radius of quantum dots was found to be 1.8 nm. The size dispersion decreased from 15.2 to 12.5% with a 20% increase in the particle size. The activation energy for diffusion was found to be very low i.e. 193 kJ mol−1 and the diffusion coefficient increased by 60% for 10 K rise in temperature. The PL emission spectra showed the presence of only deep traps around ~600 nm with a red shift of 200 nm. No shallow traps or band edge emission was observed. The PL peak position changed from 560 to 640 nm with a 35 K increase in annealing temperature.

Lateral electrical transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2.

In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.

Colloidal PbS nanocrystals with narrow photoluminescence spectra

Luminescent PbS nanocrystals have been synthesized by the colloidal method. PVA has been employed to modify the surface of prepared PbS nanocrystals and improve their optical properties. Optical and morphological characteristics of lead sulfide nanocrystals have been studied by high resolution electron microscopy and spectrophotometer. Optical absorption and photoluminescence (PL) studies of the PbS nanocrystals have shown the strong quantum confinement effects. For the first time, the prepared lead sulfide nanocrystals have emitted at 608 nm wavelength with narrow band width (21 nm) and high Stokes shift. Experimental results have shown that the surface charge traps have higher contribution to the optical properties of colloidal PbS nanocrystals and in our sample photoluminescence was due to hole relaxation. These properties made them a material with potential application in nanophotonics.

Oriented Attachment Growth of Quantum-Sized CdS Nanorods by Direct Thermolysis of Single-Source Precursor

Quantum-sized CdS nanorods were synthesized by direct thermal decomposition of a single-source precursor in a monosurfactant system. The CdS nanorods were uniform, had high crystallinity, and exhibited strong quantum confinement effect. The nanorod growth was controlled by an oriented attachment mechanism, and the morphology was determined by the competition between dipole attraction and steric repulsion of nanodots. Increasing precursor concentration and prolonging reaction time were favorable for the formation of CdS nanorods.

Investigation on the synthesis and quantum confinement effects of pure and Mn 2+ added Zn (1− x) Cd xS nanocrystals

Zn (1− x) Cd x S and Zn (1− x) Cd x S:Mn 2+ semiconductor quantum dots (2–4 nm) have been prepared by a novel solvothermal route assisted microwave heating method. The growth parameters governing the smaller size and higher yield have been optimized. The synthesized QDs exhibit a significant blue shift as compared to their corresponding bulk counterpart in the UV–vis optical absorption spectrum. The dielectric constant value varies from 2.79 to 6.17 (at 40 °C, 1 kHz) depending upon the composition of the alloy; lower value corresponds to Zn 0.75Cd 0.25S:Mn 2+ and the higher value corresponds to Zn 0.25Cd 0.75S:Mn 2+. The crystallite size to exciton bohr radius ratio being <1 indicates a strong quantum confinement effect in both CdS and ZnS QDs. The quantum confinement effect exists in the sequence of ZnS:Mn 2+ < Zn (1− x) Cd x S:Mn 2+ ( x < 0.5) < ZnS < Zn (1− x) Cd x S < CdS < CdS:Mn 2+.

Quantum dots of lead selenide: evolution of size and geometry and influence on photovoltaic effect

Lead selenide quantum dots were synthesised by the colloidal chemical synthesis route at different reaction temperatures. The nanocrystals were characterised by X–ray diffraction (XRD), High–resolution transmission electron microscopy (HRTEM) and FTIR spectroscopy. Diameter of the PbSe quantum dots increased with reaction temperature, with average diameter varying from 30 nm to 200 nm at reaction temperatures ranging from 160ºC to 250ºC. Geometries such as sphere and square with relatively uniform size distribution have been obtained at lower reaction temperature, whereas higher reaction temperatures resulted in mixtures of pentagons and hexagons and also elongated geometries. Organic–inorganic hybrid bulk hetero junction (BHJ) thin film photovoltaic devices were designed and fabricated with PbSe quantum dots and the conducting polymer–poly 3–hexyl thiophene (P3HT). Open circuit voltages obtained for these cells confirmed existence of strong quantum confinement effect for the devices containing PbSe quantum dots with diameters below the exciton Bohr diameter.