Diameter Of Quantum Dots Research Articles

Quantum-confinement effect in individual Ge1−xSnx quantum dots on Si(111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy

The authors observed a quantum-confinement effect in individual Ge1−xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ∼1.5eV with a decrease in QD diameter from 35to4nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two dimensionality in density of states of quasi zero-dimensional QDs with the QD flattening.

Preparation and Characterization of Thiacalix[4]arene Coated Water-Soluble CdSe/ZnS Quantum Dots as a Fluorescent Probe for Cu2+ Ions

Highly fluorescent water-soluble CdSe/ZnS (core/shell) quantum dots (QDs) as a fluorescent Cu2+ ion probe were synthesized using thiacalix[4]arene carboxylic acid (TCC) as a surface coating agent. Hydrophobic trioctylphosphine oxide (TOPO) capped CdSe/ZnS QDs were overcoated with TCC in tetrahydrofuran at room temperature, and deprotonation of the carboxyl groups of TCC resulted in the formation of water-soluble QDs. The surface structure of the QDs was characterized by using transmission electron microscopy (TEM) and fluorescence correlation spectroscopy (FCS). TEM images showed that TCC-coated QDs were monodispersed with the particle size (core-shell moiety) of approximately 5 nm. Hydrodynamic diameter of the TCC-coated QDs was determined to be 8.9 nm by FCS, showing that the thickness of the surface organic layer of the QDs was approximately 2 nm. These results indicate that the surface layer of TCC-coated QDs forms a bilayer structure consisting of TOPO and TCC molecules. TCC-coated CdSe/ZnS QDs were highly fluorescent (quantum yield, 0.21) compared to the QDs surface-modified with mercaptoacetic acid and mercaptoundecanoic acid. Fluorescence of the TCC-coated QDs was effectively quenched by Cu2+ ions even in the presence of other transition metal ions such as Cd2+, Zn2+, Co2+, Fe2+, and Fe3+ ions in the same solution. The Stern-Volmer plot for the fluorescence quenching by Cu2+ ions showed a linear relationship up to 30 microM of Cu2+ ions. The ion selectivity of TCC-coated QDs was determined by measurements of fluorescence responses towards biologically important transition metal ions (50 microM) including Fe2+, Fe3+, Co2+>Zn2+, Cd2+. The fluorescence of TCC-coated QDs was almost insensitive to other biologically important ions such as Na+, K+, Mg2+, and Ca2+, suggesting that TCC-coated QDs can be used as a fluorescent Cu2+ ion probe for biological samples. A possible quenching mechanism by Cu2+ ions was also discussed on the basis of a Langmuir-type adsorption isotherm.

Crystal Growth of CdSe Quantum Dots Adsorbed on Nanoparticle, Inverse Opal, and Nanotube TiO2 Photoelectrodes Characterized by Photoacoustic Spectroscopy

The crystal growth of semiconductor quantum dots (QDs) adsorbed on nanostructured TiO2 photoelectrodes is important not only for crystallographic studies but also for improving the photovoltaic efficiency of semiconductor-sensitized solar cells. In this study, nanoparticle, inverse opal, and nanotube TiO2 electrodes are prepared. These electrodes are then adsorbed with CdSe QDs as photosensitizers to investigate crystal growth. The average diameters of the CdSe QDs can be estimated from the photoacoustic (PA) spectra. When the adsorption time increases, the CdSe QD diameter increases and then shows saturation. Normal solution growth plus suppression (negative growth) can be obtained by PA analysis. Both of them depend on the morphology of TiO2 electrodes and the final solution concentration for the CdSe QD preparation.

Correlation between crystal growth and photosensitization of nanostructured TiO 2 electrodes using supporting Ti substrates by self-assembled CdSe quantum dots

Crystal growth of semiconductor quantum dots (QDs) adsorbed on nanostructured TiO 2 electrodes is important not only for crystallographic studies but also for improving the photovoltaic efficiency of semiconductor-sensitized solar cells. In this study, nanostructured TiO 2 electrodes using supporting Ti substrates were prepared. These electrodes are then adsorbed with self-assembled CdSe QDs as photosensitizers to investigate the crystal growth and photoelectrochemical current properties. Average diameters of the CdSe QDs can be estimated from optical absorption spectra by using photoacoustic (PA) technique. PA technique is a powerful tool for evaluating the optical absorption of opaque and scattered samples because of the detection by photothermal phenomenon. When the adsorption time increases, the CdSe QDs diameter increases and then shows saturation for all the cases. Normal solution growth plus suppression (negative growth) contributions can be derived by PA spectroscopic analysis. Both of them depend on adsorption temperatures for CdSe QDs formation. Photosensitization of the nanostructured TiO 2 electrodes in the visible region resulting from CdSe QDs deposition can be clearly observed. Incident photon to current conversion efficiency (IPCE) of CdSe QDs adsorbed at high temperature formation is smaller than that adsorbed at low temperature one, indicating the increase of recombination centers with increasing adsorption temperature. This implies that negative growth, or dissolving effect, produces much more recombination centers inside of CdSe QDs and/or interface between the QDs and TiO 2.

Electron transport in quantum dot solids: Monte Carlo simulations of the effects of shell filling, Coulomb repulsions, and site disorder

A Monte Carlo model is developed for the hopping conductance in arrays of quantum dots (QDs). Hopping is simulated using a continuous time random walk algorithm, incorporating all possible transitions, and using a nonresonant electron-hopping rate based on broadening of the energy levels through quantum fluctuations. Arrays of identical QDs give rise to electronic conductance that depends strongly upon level filling. In the case of low charging energy, metal insulator transitions are observed at electron occupation levels, $⟨n⟩$, that correspond to the complete filling of an $S$, $P$, or $D$ shell. When the charging energy becomes comparable to the level broadening, additional minima in conductance appear at integer values of $⟨n⟩$, as a result of electron-electron repulsion. Disorder in QD diameters leads to disorder in the energy levels, resulting in washing out of the structure in the dependence of conductance on $⟨n⟩$ and a net reduction in conductance. Simulation results are shown to be consistent with experimental measurements of conductance in arrays of zinc oxide and cadmium selenide QDs that have different degrees of size disorder, and the degree of size disorder is quantified. Simulations of the temperature dependence of conductance show that both Coulombic charging and size disorder can lead to activated behavior and that size disorder leads to conductance that is sublinear on an Arrhenius plot.

Self‐organized quantum dot superlattice inside silica mesoporous material

AbstractThree‐dimensional quantum dot superlattices were synthesized by self‐assemble process method with silica mesoporous thin films. The diameter (3.8 nm) and position of metal quantum dots can be designed by the crystal morphology of silica mesoporous film which have Pm3n cubic symmetry. The diameter and arrangement of dots were strictly controlled by host materials. The I‐V characteristics and time‐dependent current oscillation were measured with sub‐micron contacts to metal superlattices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of MOVPE InAs/GaAs quantum dots overgrown by InGaAs

The effects of covering InAs quantum dot (QD) layer by In x Ga 1− x As thin strain-reducing layer (SRL) on QD properties were studied by photoluminescence (PL) and atomic force microscopy (AFM). Samples were grown by low-pressure metalorganic vapor phase epitaxy. The In content in SRL was changed in the range 0–29%. The red shift of room temperature PL maxima from 1.28 to 1.46 μm with increasing In content was observed. The energy difference between PL transitions of the ground state and the first excited state (Δ E SP) was almost unchanged (∼85 meV) up to 23% of In in SRL. AFM results show an increase of QD height and no change in QD diameter with increasing In content in SRL. This can be the cause of the red shift of emission with increasing concentration of In in SRL and the conservation of Δ E SP.

Theoretical Study on Controllability of Quantum State Energy in an InGaAs/GaAs Quantum Dot Buried in InGaAs

We theoretically studied the relationship between quantum energy states and structural parameters of an InGaAs/GaAs quantum dot (QD) buried in strained InGaAs, emitting at 1.1 to 1.4 em. The crystal distortion of the buried QD structure in three dimensions was computed based on the three-dimensional finite element method. Under the computed strain fields, the Schrödinger equation was solved to obtain wavefunctions and eigenenergies. By calculating the dependence on structural parameters, we investigated the controllable range of the ground state energy and the energy separation between the ground state and the first excited state. We found that the energy separation exhibited a maximum value as a function of QD composition, enabling us to identify the composition of the QD structure. The effects of the burying layer composition and QD diameter were also investigated to expand the controllable range of the state energy. We also showed that the wavefunction symmetry was improved by burying the QD in the InGaAs layer. Our results will be useful in developing advanced devices for optical telecommunications and quantum information technology.

Statistical particle tracking velocimetry using molecular and quantum dot tracer particles

We present a statistical approach to particle tracking velocimetry developed to treat the issues associated with nanometer-sized tracer particles such as fluorescent molecules and quantum dots (QDs) along with theory and experimental results. Extremely small tracers pose problems to traditional tracking methods due to high levels of thermal motion, high levels of intensified camera noise, high drop-in/drop-out rates and, in the case of QDs, fluorescence intermittency (“blinking”). The algorithm presented here compensates for these problems in a statistical manner and determines the physical velocity distributions from measured particle displacement distributions by statistically removing randomly distributed, non-physical tracking events. The algorithm is verified with both numerically simulated particle trackings and experiments using 54 nm diameter fluorescent dextran molecules and 6 and 16 nm diameter QDs.

Size dependence of photoluminescence and resonant Raman scattering from ZnO quantum dots

ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction.

Density controllable growth of ZnO quantum dots by MOCVD

High quality self-assemble ZnO quantum dots (QDs) have been successfully grown on the Si(111) substrates by metalorganic chemical vapor deposition (MOCVD). The diameter of ZnO QDs is about 10 nm in average, and the densities and the sizes of ZnO QDs can be well controlled by adjusting the growth temperature, which were evident in the SEM images. The properties and stress involved in ZnO QDs are studied by X-ray diffraction. In addition, room temperature photoluminescence spectra reveal that the ZnO QDs exhibit a band gap blue shift because of the quantum confinement effects.

Blue-green-red LEDs based on InGaN Quantum Dots by Plasma-assisted MBE using GaN QDs for Dislocation Filtering

ABSTRACTIn this paper, we report the development of blue-green-red LEDs based on InGaN quantum dots (QDs) and quantum wells in the active region, and GaN QDs in the nucleation layer for dislocation filtering, by plasma assisted molecular beam epitaxy. Self-assembled InGaN QDs and GaN QDs were grown in the Stranski-Krastanov mode. For the GaN QDs grown at 770 °C, the height distribution of the dots shows a bimodal distribution, which can be attributed to the interaction of the GaN QDs with the threading dislocations. TEM and XRD studies indicate that GaN QDs in the nucleation region help threading dislocations to deviate and annihilate. The average dot height, diameter and density of the InGaN QDs were estimated to be 3 nm, 30 nm and 7×1010 cm−2, respectively. The cathodoluminescence emission peak of the InGaN/GaN multiple layer quantum dots (MQDs) was found to red shift 330 meV with respect to the emission peak of the uncapped single layer of InGaN QDs due to Quantum Confined Stark effect. Blue LEDs based on InGaN/GaN multiple quantum wells (MQWs) as well as green and red LEDs based on InGaN MQDs emitting at 440 nm, 560 nm and 640 nm with FWHM of 30 nm, 87 nm and 97 nm, respectively, were grown and fabricated. The electroluminescence peak positions of both the green and red InGaN MQD LEDs are shown to be more blue-shifted with increasing injection current than that of the blue InGaN/GaN MQW LEDs.

Photoluminescence of single, site-selected, InAs∕InP quantum dots in high magnetic fields

Photoluminescence measurements in magnetic fields up to 16T are reported from single InAs∕InP quantum dots nucleated at the apex of individual InP pyramidal nanotemplates. Clear Zeeman splitting and diamagnetic shifts of the s-shell exciton peak are observed and are correlated with the quantum-dot dimension. This behavior is interpreted in terms of a template-controlled quantum-dot diameter and is corroborated by the observed diamagnetic shifts.

Nanostructures and Optical Properties of Mesoporous Composite Nanofibers Containing CdS Quantum Dots

Mesoporous silica nanofibers containing CdS quantum dots (QDs) were fabricated by the confined self-assembly of a lyotropic mesophase within the nanochannels of porous alumina. The resulting mesoporous nanofibers show unique internal mesostructures that depend on the size of the incorporated CdS QDs. Porous nanochannels of the composite nanofibers are shown to form a circularly wound assembly perpendicular to the long fiber axis with small QDs incorporated. As the diameter of the CdS QDs within the silica nanofibers increases, the circularly wound nanochannels become disordered. This unique mesoporous structure is further perturbed by increasing the amount of incorporated CdS QDs. The perturbed mesostructure inside the nanofibers results in an enhanced efficiency of nitrogen gas adsorption and spectral changes of the incorporated QD emissions. Such perturbed nanochannels inside the nanofibers can be utilized as a preferred mesostructure for adsorbing gas molecules and removing surface emission states of the CdS QDs.

Role of thin InP cap layer and anion exchange reaction on structural and optical properties of InAs quantum dots on InP (001)

Structural and optical properties of InAs quantum dots (QDs) or InP (001) substrates were studied during low-pressure metalorganic chemical vapor deposition (MOCVD), using photoluminescence (PL) and transmission electron microscopy (TEM). To clarify how the properties of QDs can be controlled through surface reactions, two kinds of source gas supply sequence were compared. With a double-capping procedure, PL changed into a series of multiple peaks arising from different integer numbers of InAs monolayers. PL studies revealed that the spectra of double-capped samples are not affected by a long phosphine (PH3) exposure during growth interruption, indicating reactions relevant to arsenic (As)/phosphorus (P) exchange at the QD surface is self-limited. TEM observations clarified that the shape of double-capped QDs dramatically changes into a thin plate-shape with flat upper and lower interfaces while the density of QDs remains unchanged. In contrast, under the normal capping procedure, the structural and optical properties on QDs are sensitive to the duration of PH3 before InP capping. Results suggest that As/P exchange is not self-limiting, leading to decreased QD density and enlarged QD diameters. We discuss the role the thin InP initial cap layer plays in the anion exchange reaction during the double-capping procedure and the technical importance of controlling the emission wavelength of QDs.

Structural and optical characterizations of multi-layered and multi-stacked PbSe quantumdots

A multi-layered nanocomposite thin film was fabricated to increase the density of PbSequantum dots. It consisted of low-loss polymer layers and multi-stacked PbSe quantum dotlayers in series. The solutions of a UV-curable low-loss polymer and PbSe quantum dotswere spin-coated layer-by-layer. An arrayed and multi-stacked PbSe quantum dot layer wasself-assembled by spin-coating and solvent evaporation. The multi-stacked layers of PbSequantum dots have a face-centred cubic structure with an average {111} plane distance of∼6.1 nm, determined by small-angle x-ray scattering. The average diameter of PbSe quantum dots is∼5 nm and the average interparticle spacing distance is∼2.5 nm,characterized by transmission electron microscopy. The density of PbSe quantum dots in the single-arrayedlayer is ∼1.8 × 1012 cm−2 and the density in the three-layered thin film of multi-stacked PbSe QDs is>1.6 × 1013 cm−2. The peak intensities of absorption and photoluminescence were increased withincreasing number of PbSe quantum dot layers. The time of luminescence decay toe−1 is 138 ns at room temperature. We investigated the structures of multi-layered andmulti-stacked PbSe quantum dot thin films as well as their optical properties, and thensuggested their photonic applications.

Nanogel-quantum dot hybrid nanoparticles for live cell imaging

We report here a novel carrier of quantum dots (QDs) for intracellular labeling. Monodisperse hybrid nanoparticles (38 nm in diameter) of QDs were prepared by simple mixing with nanogels of cholesterol-bearing pullulan (CHP) modified with amino groups (CHPNH 2). The CHPNH 2–QD nanoparticles were effectively internalized into the various human cells examined. The efficiency of cellular uptake was much higher than that of a conventional carrier, cationic liposome. These hybrid nanoparticles could be a promising fluorescent probe for bioimaging.

Growth of InN quantum dots on N-polarity GaN by molecular-beam epitaxy

We investigated the growth behaviors of InN quantum dots (QDs) on N-polarity GaN by molecular-beam epitaxy. The N-polarity growth has been intentionally used to raise the temperature to facilitate formation of high-quality dots. It was found that the InN QDs could be grown up to 550°C the Stranski–Kastanov growth mode with the wetting layer thickness of about 1 monolayer, which was confirmed by the simultaneous in situ observations of reflection high-energy electron diffraction and spectroscopic ellipsometry. The density and the diameter of typical InN QDs grown at 450–550°C were the order of 1011cm−2 and 15–20nm, respectively.

Hole spin relaxation in semiconductor quantum dots

Hole spin relaxation time due to the hole-acoustic phonon scattering in GaAs quantum dots confined in quantum wells along (001) and (111) directions is studied after the exact diagonalization of Luttinger Hamiltonian. Different effects such as strain, magnetic field, quantum dot diameter, quantum well width and the temperature on the spin relaxation time are investigated thoroughly. Many features which are quite different from the electron spin relaxation in quantum dots and quantum wells are presented with the underlying physics elaborated.

The determination of the size and shape of buried InAs/InP quantum dots by transmission electron microscopy

Bright-field, diffraction-contrast imaging in the transmission electron microscope has been applied to the determination of the diameter and height populations of a single layer of buried, pure, InAs/InP quantum dots (QDs). Plan-view diffraction contrast from the QDs was observed to increase significantly when the sample was tilted away from the [0 0 1] growth direction to near the [1 1 1] zone-axis orientation. This added contrast was a result of contributions to the displacement of atoms in a direction perpendicular to the electron beam arising from strain in the growth direction. Since the strain in the growth direction was about an order of magnitude larger than the strain perpendicular to the growth direction, as the sample is tilted away from the [0 0 1] zone-axis condition, the larger strain component increases the projected strain thereby increasing the QD contrast in the image. For the sample studied, both of the populations for the QD diameter and the image contrast were observed to be multimodal with the seven peaks in the contrast distribution correlating with seven distinct populations of QDs each differing in height by one monolayer (ML), from 3 to 9 MLs. An analysis of the theoretically expected and experimentally observed standard deviations in the Gaussian fits to the QD diameter and height distributions provided an additional constraint in the selection of the optimal model for the multimodal distributions.