Increasing Dot Size Research Articles

Confined energy of two electrons in a spherical quantum dot with interaction effects

The confined energy and its interaction effects in a spherical quantum dot are calculated using the perturbation method within effective mass approximation. The interaction energy is computed for GaAs/GaxIn1−xSb as a function of the dot size. Results are presented for the spherical quantum dot with square well confinement corresponding to different values of x. Our result shows that confined energies and interaction energies decrease as the dot size increases. Interaction energies contribute 6–10 % in singlet state and 4–8 % in triplet state to total confined energy of lower dot radii. All calculations are carried out with finite models and the results are compared with existing literature data.

Diffraction of quantum dots reveals nanoscale ultrafast energy localization.

Unlike in bulk materials, energy transport in low-dimensional and nanoscale systems may be governed by a coherent "ballistic" behavior of lattice vibrations, the phonons. If dominant, such behavior would determine the mechanism for transport and relaxation in various energy-conversion applications. In order to study this coherent limit, both the spatial and temporal resolutions must be sufficient for the length-time scales involved. Here, we report observation of the lattice dynamics in nanoscale quantum dots of gallium arsenide using ultrafast electron diffraction. By varying the dot size from h = 11 to 46 nm, the length scale effect was examined, together with the temporal change. When the dot size is smaller than the inelastic phonon mean-free path, the energy remains localized in high-energy acoustic modes that travel coherently within the dot. As the dot size increases, an energy dissipation toward low-energy phonons takes place, and the transport becomes diffusive. Because ultrafast diffraction provides the atomic-scale resolution and a sufficiently high time resolution, other nanostructured materials can be studied similarly to elucidate the nature of dynamical energy localization.

Effect of external magnetic field on the ground state properties of D− centres in a Gaussian quantum dot

The binding energies of a negatively charged hydrogenic donor ion (D−) trapped in a three-dimensional quantum dot with Gaussian confinement in the presence of an external magnetic field are obtained as a function of the quantum dot size and the confinement strength and also as a function of the magnetic field by a variational method with a simple wave function. A Jastrow-like factor is used to take care of the correlation effect. The results are obtained. The magnetic moment and susceptibility are also obtained for the donor complex. One of the most important objectives of this work is to study the effect of the magnetic field on the resultant dipole moment of a D− complex in a Gaussian QD. It is shown that the resultant dipole moment of a D− complex increases as the dot size increases and decreases as the magnetic field increases. The effect of the confinement strength on the resultant dipole moment is also investigated for different values of the magnetic field.

Electronic properties of hemispherical quantum dot/wetting layer with and without hydrogenic donor impurity

We obtain the electronic structure of an InAs hemispherical quantum dot with a wetting layer embedded in a GaAs barrier, with and without hydrogenic impurity located at the origin, using finite element method in the effective mass approximation. The binding energy is calculated as a function of the shape and size of the dot. The system is considered to interact with a weak, pulsed probe field and a strong continuous-wave control field. We obtain the expressions for the linear and nonlinear absorptions and dispersions of the probe pulse. According to results, applying the control field results in the reduction of the probe field absorption and group velocity at transparency window. Results show that as the dot size increases, the absorptions and dispersions increase and exhibit red shifts. Adding impurity to the system results in the blue shifts and the reduction of the absorptions and dispersions.

The role of surface and deep-level defects on the emission of tin oxide quantum dots

This paper reports on the role of surface and deep-level defects on the blue emission of tin oxide quantum dots (SnO2 QDs) synthesized by the solution-combustion method at different combustion temperatures. X-ray diffraction studies showed the formation of a single rutile SnO2 phase with a tetragonal lattice structure. High resolution transmission electron microscopy studies revealed an increase in the average dot size from 2.2 to 3.6 nm with an increase of the combustion temperature from 350 to 550 °C. A decrease in the band gap value from 3.37 to 2.76 eV was observed with the increase in dot size due to the quantum confinement effect. The photoluminescence emission was measured for excitation at 325 nm and it showed a broad blue emission band for all the combustion temperatures studied. This was due to the creation of various oxygen and tin vacancies/defects as confirmed by x-ray photoelectron spectroscopy data. The origin of the blue emission in the SnO2 QDs is discussed with the help of an energy band diagram.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Effect of Dot Size on Exciton Binding Energy and Electron-Hole Recombination Probability in CdSe Quantum Dots.

Exciton binding energy and electron–hole recombination probability are presented as two important metrics for investigating effect of dot size on electron–hole interaction in CdSe quantum dots. Direct computation of electron–hole recombination probability is challenging because it requires an accurate mathematical description of the electron–hole wave function in the neighborhood of the electron–hole coalescence point. In this work, we address this challenge by solving the electron–hole Schrodinger equation using the electron–hole explicitly correlated Hartree–Fock (eh-XCHF) method. The calculations were performed for a series of CdSe clusters ranging from Cd20Se19 to Cd74608Se74837 that correspond to dot diameter range 1–20 nm. The calculated exciton binding energies and electron–hole recombination probabilities were found to decrease with increasing dot size. Both of these quantities were found to scale as Ddot–n with respect to the dot diameter D. One of the key insights from this study is that the electron–hole recombination probability decreases at a much faster rate than the exciton binding energy as a function of dot size. It was found that an increase in the dot size by a factor of 16.1, resulted in a decrease in the exciton binding energy and electron–hole recombination probability by a factor of 12.9 and 4.55 × 105, respectively.

Epitaxial Silicon Dots Self-Assembled on Aluminum Nitride/Si (111)

Si nanoscale dots are synthesized on AlN/Si(111) by molecular beam epitaxy. A dot density of 2.2 × 10(11) cm(-2) with a mean radius of 5.6 ± 2.8 nm is obtained in Volmer-Weber growth mode. A double Si coverage leads to a decrease in dot density and increase in dot size. The dot orientations are [11[overline]0](Si) (or [1[overline]10](Si))//[112[overline]0](AlN) and (111)(Si)//(0001)(AlN), which are similar (or identical) to the orientation of AlN relative to the Si substrate.

Dot size effects of nanocrystalline germanium on charging dynamics of memory devices

The dot size of nanocrystalline germanium (NC Ge) which impacts on the charging dynamics of memory devices has been theoretically investigated. The calculations demonstrate that the charge stored in the NC Ge layer and the charging current at a given oxide voltage depend on the dot size especially on a few nanometers. They have also been found to obey the tendency of initial increase, then saturation, and lastly, decrease with increasing dot size at any given charging time, which is caused by a compromise between the effects of the lowest conduction states and the capacitance of NC Ge layer on the tunneling. The experimental data from literature have also been used to compare and validate the theoretical analysis.

Experimental and Analytical Study of Dot Gain Between Elastic and Deformable Drums

During the printing process on liquid electrostatically printing (LEP), liquid ink is transferred from the ink tank to the media passing through a number of drums. First, the ink is transferred to the photoconductor drum, and then the inked image is transferred to the blanket drum by electrical forces, after drying the image on blanket drum it is transferred to the paper. Most of the printed images contain mid tone areas. These areas are composed of screened dots. When a dot is transferred from the photoconductor to the blanket, the dot is squeezed and as a result, the dot size increases. This phenomenon is called dot gain. The dot approximately expanded by 10% during the transfer between the photoconductor and blanket. In this work we present analytical model that simulate dot gain phenomena, a good agreement between analytical and experimental results are shown as well.

InAs/GaAs 양자점의 발광특성에 대한 InGaAs 캡층의 영향

The optical properties of InAs quantum dots (QDs) grown on a GaAs substrates by migration enhanced molecular beam epitaxy method have been investigated by using photoluminescence (PL) and time-resolved PL measurements. The luminescence properties of InAs/GaAs QDs have been studied as functions of temperature, excitation laser power, and emission wavelength. The PL peak of InAs QDs capped with layer (QD2) measured at 10 K is redshifted about 80 nm compared with that of InAs QDs with no InGaAs layer (QD1). This redshift of QD2 is attributed to the increase in dot size due to the diffusion of In from the InGaAs capping layer. The PL decay times of QD1 and QD2 at 10 K are 1.12 and 1.00 ns taken at the PL peak of 1,117 and 1,197 nm, respectively. The reduced decay time of QD2 can be explained by the improved carrier confinement and enhanced wave function overlap due to increased QD size. The PL decay times for both QD1 and QD2 are independent on the emission wavelength, indicating the uniformity of dot size.

The effect of spatial symmetry on the nonlinear optical rectification of lens-shaped quantum dots

The one-band k·p model, the compact density matrix approach and the iterative method are used to calculate the nonlinear optical rectification coefficient (ORC) of an electron confined in an asymmetric lens-shaped quantum dot (ALQD). Numerical results are presented for the typical GaAs ALQD. The effects of the size of the ALQD, the incident photon energy and the symmetry of the QD on the ORC are investigated. It is found that there is a scaling rule that connects the ORC to the size of the ALQD. Using this scaling rule, it is shown that the ORC increases, and its peak shifts to lower energies, with increasing dot size. Also, the ORC decreases with increasing mirror symmetry of the QD.

Langmuir–Blodgett films composed of amphiphilic double-decker shaped polyhedral oligomeric silsesquioxanes

New amphiphilic polyhedral oligomeric silsesquioxanes (POSSs) were synthesized, and their monolayer behavior on a water surface and Langmuir–Blodgett (LB) film formation were studied. Two kinds of amphiphilic POSS molecules, which have two or four di(ethylene glycol) units (2OH-DDSQ and 4OH-DDSQ, respectively), were synthesized by direct hydrosilylation of di(ethylene glycol) vinyl ether with double-decker shaped polyhedral oligomeric silsesquioxanes (DDSQs). Surface pressure ( π)–area ( A) isotherms and Brewster angle microscope (BAM) measurements indicated that both amphiphilic DDSQs form a stable monolayer at the air–water interface. In addition, 4OH-DDSQ can be deposited on a solid substrate by the LB technique. Atomic force microscope (AFM) images of a one-layer 4OH-DDSQ film showed a homogenous uniform surface on a hydrophilic silicon substrate, whereas nanometer scale dots were formed on a hydrophobic silicon substrate. Multilayer deposition on a hydrophobic substrate resulted in an increase of dot size with increasing deposition number of layers. Moreover, homogenous multilayer films with a few voids were obtained on a hydrophilic substrate. The results indicate that 4OH-DDSQ is a good candidate for preparing hybrid nanoassemblies.

Strain-Engineered Low-Density InAs Bilayer Quantum Dots for Single Photon Emission

We investigate the growth of strain-engineered low-density InAs bilayer quantum dots (BQDs) on GaAs by molecular beam epitaxy. Owing to increasing dot size and In composition of the upper QDs, low-density BQDs in a GaAs matrix with an emission wavelength up to 1.4 μm at room temperature are achieved. Such a wavelength is larger than that of conventional QDs in a GaAs matrix (generally of about 1.3 μm). The optical properties of the BQDs are sensitive to annealing temperature used after spacer layer growth. Significant decrease of integrated PL intensity is observed as the annealing temperature increases. At 10K, single photon emission from the BQDs with wavelength around 1.3 μm is observed.

Persistent template effect in InAs/GaAs quantum dot bilayers

The dependence of the optical properties of InAs/GaAs quantum dot (QD) bilayers on seed layer growth temperature and second layer InAs coverage is investigated. As the seed layer growth temperature is increased, a low density of large QDs is obtained. This results in a concomitant increase in dot size in the second layer, which extends their emission wavelength, reaching a saturation value of around 1400 nm at room temperature for GaAs-capped bilayers. Capping the second dot layer with InGaAs results in a further extension of the emission wavelength, to 1515 nm at room temperature with a narrow linewidth of 22 meV. Addition of more InAs to high density bilayers does not result in a significant extension of emission wavelength as most additional material migrates to coalesced InAs islands but, in contrast to single layers, a substantial population of regular QDs remains.

Biexciton binding energy control in site-selected quantum dots

A unique nanotemplate deposition technique is utilized in growth of semiconductor quantum dots which enables precise control over the dot dimensions and nucleation site. Here, we demonstrate tuning of the biexciton binding energy in a single, site-selected InAs/InP quantum dot through manipulation of the nanotemplate dimensions and thus, dot size. A monotonic decrease of the biexciton binding energy from the binding to anti-binding regime through zero is observed with increasing dot size. Piezoelectric fields in large quantum dots are suggested as the mechanism to obtain an unbound biexciton state. The tunability of the biexciton binding energy demonstrated here for a deterministically positioned quantum dot is an important step towards a scalable route in the generation of entangled photon pairs that emit around the telecommunications band of 1.55 μm.

Local Probing of Magnetization Reversal in Ni–Fe Elliptical Dots With Variable Geometry

We have investigated the detailed magnetization reversal in 10-nm-thick Ni-Fe elliptical dots with several dot sizes using magnetic field sweeping (MFS)-magnetic force microscopy (MFM). At the dot edges, the shape of the phase (the stray field) curve versus magnetic field changes from a stepped hysteresis loop to a almost nonstepped hysteresis loop with increasing dot size. On the other hand, at the center within the dot, sharp or weak decreases in phase are observed as the magnetic field is varied. From these results, it is found that, in the magnetization reversal of a 10-nm-thick Ni-Fe elliptical dot with several dot sizes, the dominant factor changes from the change in the magnetic domain configuration to the domain wall motion as the length of major axis increases.

Photoluminescence spectroscopy of trions in quantum dots: A theoretical description

We present a full configuration-interaction study of the spontaneous recombination of neutral and singly charged excitons trions in semiconductor quantum dots from weak- to strong-coupling regimes. We find that the enhancement of the recombination rate of neutral excitons with increasing dot size is suppressed for negative trions and even reversed for positive trions. Our findings agree with recent comprehensive photoluminescence experiments in self-assembled quantum dots P. Dalgarno et al., Phys. Rev. B 77, 245311 2008 and confirm the major role played by correlations in the valence band. The effect of the temperature on the photoluminescence spectrum and that of the ratio between the electron and hole wave-function length scales are also described.

Structural and optical properties of cubic-InN quantum dots prepared by ion implantation in Si (100) substrate

The authors have synthesized InN quantum dots by ion implantation into a Si (100) substrate followed by a postannealing process. X-ray photoemission spectroscopy data verified the formation of In–N bonding in both as-implanted and postannealed samples. Diffraction patterns from transmission electron microscopy (TEM) confirm that the dots are of cubic crystal (zinc-blende phase) with no presence of wurtzite InN. The silicon matrix provides a constraint for the formation of the InN cubic metastable phase. However, dislocations were revealed by high resolution TEM at the interfaces between the dots and the silicon. In addition, the authors found that as the annealing temperature or time increases, dot size increases and dot density decreases. Furthermore, they demonstrate that the main emission energy of zinc-blende InN dots is about 0.736eV.