Shape Of Quantum Dots Research Articles

Electronic states and shapes of silicon quantum dots

A curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in the bandgap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, a Si—O—Si bridge bond on curved surface provides localized levels in bandgap and its bonding energy is shallower than that on the facet. The red-shifting of the photoluminescence spectrum on smaller silicon quantum dots can be explained by the curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels provided by the curved surface effect.

Wavelength and polarization variations of InAs/GaAs quantum dots emission at liquid Helium temperature via microphotoluminescence spectroscopy

In this paper, we investigate variation of the wavelength, intensity and polarization of the self-assembled InAs/GaAs quantum dots emission by microphotoluminescence spectroscopy at the liquid helium temperature. The microcavity wafer sample is grown by molecular beam epitaxy (MBE) and chemically etched into the micropillar structure (with elliptical cross section - long and short axis 2µm×1.5µm respectively). Two polarized modes of the micropillar and also quantum dot in the orthogonal axis of long and short diameters are considered and marked by H and V respectively. The analysis of the spectra shows that by increasing temperature, the quantum dot emission wavelength will be shifted towards longer wavelength, and at resonance wavelength of quantum dot and cavity confined modes which are red shifted less than quantum dot, enhancement would happen. Changes in emission wavelength and coupling of four individual quantum nanoparticles in the sample (with different spatial and spectral conditions) were studied in the temperature range ~ 5-50 K. Also, experiments show that the polarization amount and fine structure splitting for different quantum dots are different. The difference can be attributed to the deviations from the symmetric shape of quantum dots and the capping stage during next epitaxial growth. Polarization variability on- and off- resonance by changing temperature is also reported.

Population Transfer in Disk Shaped Quantum Dot in Delayed Half Cycle Pulses and Magnetic Field

Population Transfer in Disk Shaped Quantum Dot in Delayed Half Cycle Pulses and Magnetic Field

Magnetic properties of graphene quantum dots

Using the tight-binding approximation we calculated the magnetic susceptibility of graphene quantum dots (GQD) of different geometrical shapes and sizes, smaller than the magnetic length, when the magnetic properties are governed by the electron edge states. Two types of edge states can be discerned: the zero-energy states (ZES) located exactly at the zero-energy Dirac point, and the dispersed edge states (DES) with the energy close, but not exactly equal to zero. DES are responsible for the temperature independent diamagnetic response, while ZES provide the temperature dependent spin Curie paramagnetism. The hexagonal, circular and randomly shaped GQDs contain mainly DES and, as a result, they are diamagnetic. The edge states of the triangular GQDs are ZES and these dots reveal the interplay between the spin paramagnetism, dominating for small dots and at low temperatures, and bulk orbital diamagnetism, dominating for large dots and at high temperatures.

Optical nonlinearities associated to applied electric fields in parabolic two-dimensional quantum rings

The linear and nonlinear optical absorption as well as the linear and nonlinear corrections to the refractive index are calculated in a disc shaped quantum dot under the effect of an external magnetic field and parabolic and inverse square confining potentials. The exact solutions for the two-dimensional motion of the conduction band electrons are used as the basis for a perturbation-theory treatment of the effect of a static applied electric field. In general terms, the variation of one of the different potential energy parameters – for a fixed configuration of the remaining ones – leads to either blueshifts or redshifts of the resonant peaks as well as to distinct rates of change for their amplitudes.

Microwave-assisted synthesis of l-glutathione capped ZnSe QDs and its interaction with BSA by spectroscopy

Stable, water-soluble and biologically compatible ZnSe quantum dots (QDs) with l-glutathione (GSH) as a capping agent were synthesized in aqueous medium by microwave irradiation. The GSH/Zn/Se molar ratios, reaction temperature, time and pH are the crucial factors for properties of QDs. Fluorescence (FL) spectra, absorption spectra, transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and Fourier transform infrared (FTIR) spectra studies showed that the optical properties of QDs were strong, shape of QDs was similar to spherical and the particle size was about 2–3nm. The 42% quantum yield (QY) of QDs can be achieved without any post-preparative treatment. The interaction of QDs bioconjugated to bovine serum albumin (BSA) was also studied by absorption and FL spectra experiments. With addition of QDs, the FL intensity of BSA was largely quenched, which can be explained by static mechanism. The results suggested the QDs-BSA binding reaction was a static quenching.

Preparation of water‐soluble CdSe quantum dots and its application for nitrite detection in the anodic electrochemiluminescence

Water-soluble CdSe quantum dots (QDs) have been prepared by using L-cysteine as the stabilizer in an aqueous phase under the optimized conditions. The characteristics and shapes of CdSe QDs have been proposed on the basis of UV-Vis and fluorescence spectra. A rapid analytical method for electrochemiluminescence (ECL) determination of nitrite has been developed on the basis of the quenching effect on anodic ECL emission of CdSe QDs under the optimum experimental conditions. In a neutral system and at a relatively low potential (+0.960 V), the ECL emission of CdSe QDs could be greatly enhanced by sulfite and could be gradually quenched by nitrite at an indium tin oxide (ITO) electrode. The proposed method may allow the measurement of nitrite ranging from 1 μM to 0.5 mM with a correlation coefficient of 0.9956 (n = 10) and a detection limit of 0.2 μM (3σ), and the relative standard deviation for 10 μM nitrite (n = 9) is 1.72 %. The proposed method could be adopted for the sensitive detection of ECL quenchers by using nitrite as a model molecule.

Preparation of novel hetero-nanostructures and high efficient visible light-active photocatalyst using incorporation of CNT as an electron-transfer channel into the support TiO2 and PbS

A series of novel visible light-active photocatalysts of TiO2/CNT and PbS/CNT were prepared by simple wet-chemical method and the resulting materials were used for degradation of methyl orange (MO) as probe pollutant. The reactive substances used to decorate the PbS quantum dots (QDs) with the diameters of ca. 6–10nm located onto the CNT substrate, were lead acetate (Pb(CH3COOH)) and Thiourea (NH2CSNH2) aqueous solution at 90°C. The physic-chemical properties of the resulting photocatalysts were characterized by SEM-EDX, TEM, XRD, FTIR, and UV–vis DRS spectroscopy. It was found that the size, shape, and coverage density of the PbS QDs and TiO2 nanoparticles (NPs) onto the CNT can be controlled through varying synthesis conditions. As a result, the experiments demonstrated photodegradation efficiency of ∼100% of MO, for TiO2/CNT and PbS/CNT, under 20 and 45min UV light irradiation time, respectively, and also 30 and 80min under visible light irradiation time, respectively. In addition, the possible decomposing mechanisms were also discussed. The results of the current study showed the feasible and potential use of high efficient CNT:TiO2:PbS photocatalyst in photodecomposition of toxic organic pollutants. As the above-mentioned results show, the use of CNT in photocatalyst systems is not only enhances the optical properties of TiO2 and PbS photocatalysts, but also improve their capability in photodegradation of organic pollutants.

Proposed Quenching of Phonon-Induced Processes in Photoexcited Quantum Dots due to Electron-Hole Asymmetries

Differences in the confinement of electrons and holes in quantum dots are shown to profoundly impact the magnitude of scattering with acoustic phonons. Using an extensive model that includes the non-Markovian nature of the phonon reservoir, we show how the effect may be addressed by photoluminescence excitation spectroscopy of a single quantum dot. We also investigate the implications for cavity QED, i.e., a coupled quantum dot-cavity system, and demonstrate that the phonon scattering may be strongly quenched. The quenching is explained by a balancing between the deformation potential interaction strengths and the carrier confinement and depends on the quantum dot shape. Numerical examples suggest a route towards engineering the phonon scattering.

Ge quantum dot lattices in Al2O3 multilayers

In this article, we show how to produce materials consisting of regularly ordered Ge quantum dot lattices in an amorphous alumina matrix with a controllable Ge quantum dot size, shape, spacing, crystalline structure, and degree of regularity in their ordering. The production of such materials is achievable already at room temperature by magnetron sputtering deposition of a (Ge + Al2O3)/Al2O3 multilayer. The materials show photoluminescence in the visible and ultraviolet light range, a size-dependent blue shift of the photoluminescence peak and an enhancement of its intensity by size reduction, indicating the quantum dot origin of the photoluminescence. The materials also exhibit excellent mechanical properties due to the alumina matrix. Their internal structure is shown to be highly resistive to irradiation with energetic particles for a large range of the irradiation parameters.

Rim formation on non-elongated InAs quantum dots grown by partial cap and annealing process at low temperature

We have used atomic force microscopy to study in detail the shape of InAs quantum dots (QDs) grown on GaAs (001) substrates. The QDs are fabricated by partial capping and a subsequent annealing process at low temperature. We observe circular, non-elongated QDs, with rim structures on top. The majority of the QDs located on a step edge have an anisotropic rim structure with a crescent shape, whereas approximately 50% of QDs located on terraces have isotropic rim structures with a ring shape. These results indicate that the rims of non-elongated QDs are not always isotropic, and the formation of an isotropic or anisotropic rim structure is dependent on the location of the QD.

Shape evolution of low density InAs quantum dots in the partial capping process by using As2 source

Shape changes in the capping process of low density InAs quantum dots (QDs) with a thin GaAs layer under As4 and As2 conditions were investigated by atomic force microscopy. The shapes of the capped QDs differ considerably depending on the used arsenic source, As4 or As2. In the case of the As4 source, the QD shape elongated along the [1¯10] direction with the increase in the capping temperature, whereas the elongation along the [1¯10] direction is small compared to that along the [1¯10] direction. On the other hand, in the case of As2, the elongation along the [1¯10] direction is lesser than that in the As4 case. In addition, the elongation along the [110] direction is promoted even at a low capping temperature. In the case of As2, isotropic capping of the QDs can be achieved at a higher capping temperature compared to the case using As4.

Virtual-bound, filamentary and layered states in a box-shaped quantum dot of square potential form the exact numerical solution of the effective mass Schrödinger equation

The effective mass Schrödinger equation of a QD of parallelepipedic shape with a square potential well is solved by diagonalizing the exact Hamiltonian matrix developed in a basis of separation-of-variables wavefunctions. The expected below bandgap bound states are found not to differ very much from the former approximate calculations. In addition, the presence of bound states within the conduction band is confirmed. Furthermore, filamentary states bounded in two dimensions and extended in one dimension and layered states with only one dimension bounded, all within the conduction band—which are similar to those originated in quantum wires and quantum wells—coexist with the ordinary continuum spectrum of plane waves. All these subtleties are absent in spherically shaped quantum dots, often used for modeling.

Three-dimensional imaging for precise structural control of Si quantum dot networks for all-Si solar cells

All-Si tandem solar cells based on Si quantum dots (QDs) are a promising approach to future high-performance, thin film solar cells using abundant, stable and non-toxic materials. An important prerequisite to achieve a high conversion efficiency in such cells is the ability to control the geometry of the Si QD network. This includes the ability to control both, the size and arrangement of Si QDs embedded in a higher bandgap matrix. Using plasmon tomography we show the size, shape and density of Si QDs, that form in Si rich oxide (SRO)/SiO2 multilayers upon annealing, can be controlled by varying the SRO stoichiometry. Smaller, more spherical QDs of higher densities are obtained at lower Si concentrations. In richer SRO layers ellipsoidal QDs tend to form. Using electronic structure calculations within the effective mass approximation we show that ellipsoidal QDs give rise to reduced inter-QD coupling in the layer. Efficient carrier transport via mini-bands is in this case more likely across the multilayers provided the SiO2 spacer layer is thin enough to allow coupling in the vertical direction.

Role of As in the Anisotropic Positioning of Self-Assembled InAs Quantum Dots

ABSTRACTProgress in tailoring the size, shape and positioning of Quantum Dots on the substrate is crucial for their potential applications in new optoelectronic devices for nano-photonics as well as in quantum information and computation. Using Molecular Beam Epitaxy in pulsed deposition mode we demonstrate that the nucleation of InAs Quantum Dots can be selectively guided on the GaAs(001) surface by a suitable choice of the kinetic parameters for the growth of both the GaAs buffer layer and the InAs Quantum Dots. By developing a two-species rate-equation kinetic model we show that the positioning of the Quantum Dots on only one side of mounds of the GaAs buffer can be traced back to the very small As flux gradient between the two mound slopes $\left( {\Delta F_A /F_A \approx 1 - 5\% } \right)$ caused by the proper tilting of the incoming As flux. Such gradient originates, at the relatively high growth-temperature, a net cation flow from one slope of the mound to the other that is responsible for the selective growth.

Influence of external factors on the self-organization of lead and tin telluride nanostructures on the BaF2(111) surface under conditions close to the thermodynamic equilibrium

The morphology of PbTe and SnTe nanostructures grown on BaF2(111) substrates from the vapor phase in a vacuum under conditions close to the thermodynamic equilibrium has been investigated using atomic force microscopy. The equilibrium shape of PbTe and SnTe quantum dots and the statistical parameters of arrays of these quantum dots have been studied as a function of the thermodynamic conditions of growth, the crystal lattice mismatch between the materials of the quantum dots and substrate, and elastic properties of these materials. It has been established that, when the BaF2(111) substrate is deformed under external mechanical loading, the self-organization of dislocations on the BaF2(111) surface can result in the formation of a nanoscale ordered strain relief, which can be used for the fabrication of nanostructures. The morphology of this relief depends on the external load and on the temperature at which the substrate is deformed. It has been shown that the deformation effect on the surface of the substrate and light irradiation of the growth zone of nanostructures affect the nucleation of islands and kinetic processes occurring on the surface of the substrate during their growth. Using the influence of external factors on the BaF2(111) surface under certain thermodynamic conditions, it is possible to grow SnTe and PbTe nanostructures with different morphologies: continuous epitaxial layers with a thickness of less than 10 nm, homogeneous arrays of quantum dots with a high lateral density (more than 1011 cm2), quasi-periodic lateral nanostructures (nanowires), “single” and “coupled” quantum dots, and “molecules” of quantum dots.

Shape and curved surface effect on silicon quantum dots

Curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, an 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. The red-shifting of PL spectrum on smaller silicon quantum dots can be explained by curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels formed in the band gap.

Effect of quantum dot shape of the GaAs/AlAs heterostructure on interlevel hole light absorption

The effect of quantum dot shape on the hole energy spectrum and optical properties caused by the interlevel charge transition based on the 4x4 Hamiltonian has been studied for the GaAs quantum dot in the AlAs semiconductor matrix. Calculations have been carried out in perturbation theory taking into account the hybridization of states for cubic, ellipsoidal, cylindrical and tetrahedral shapes by changing the volume of a quantum dot. Based on the energy calculations and on the determined wave functions of the hole states we have defined the selection rules and the dependence of the interlevel hole absorption coefficient on QD shapes and volumes.

Shape and Size Dependent Electronic Properties of GaAs/AlGaAs Quantum Dots

We have theoretically investigated the effect of shape anisotropy and size on electronic structure of GaAs/AlGaAs quantum dots (QDs). The quantum dot is modeled by anisotropic parabolic confinement potential in the plane perpendicular to the growth direction while the confinement along the growth direction is modeled as quantum well potential. The Luttinger Hamiltonian formulation has been used to account for the valence subband mixing. The electronic structure is calculated by numerical diagonalization of Luttinger Hamiltonian using the harmonic oscillator basis functions. The calculations for hole energies and transition energies have been carried out over wide range of size and shape of QDs. The results show that transition energy of QDs decreases with the height of QDs. Significant variation in the hole energy is observed with the change in anisotropy. We also observe that shape anisotropy and mixing have significant effect on the energy states.

High efficient luminescence in type-II GaAsSb-capped InAs quantum dots upon annealing

The photoluminescence efficiency of GaAsSb-capped InAs/GaAs type II quantum dots (QDs) can be greatly enhanced by rapid thermal annealing while preserving long radiative lifetimes which are ∼20 times larger than in standard GaAs-capped InAs/GaAs QDs. Despite the reduced electron-hole wavefunction overlap, the type-II samples are more efficient than the type-I counterparts in terms of luminescence, showing a great potential for device applications. Strain-driven In-Ga intermixing during annealing is found to modify the QD shape and composition, while As-Sb exchange is inhibited, allowing to keep the type-II structure. Sb is only redistributed within the capping layer giving rise to a more homogeneous composition.