Large Quantum Dots Research Articles

Indium phthalocyanine–CdSe/ZnS quantum dots nanocomposites showing size dependent and near ideal optical limiting behaviour

Indium phthalocyanine–CdSe/ZnS quantum dots (QDs) nanocomposites (InPc–CdSe/ZnS) of three sizes (5.57, 8.12 and 8.75nm) were synthesized according to known procedures. The particle size of the CdSe/ZnS QDs alone are 3.95, 6.02, and 6.66nm, and are denoted as QD1, QD2 and QD3 respectively. The nonlinear absorption (NLA) properties of the nanoconjugates (InPc–CdSe/ZnS) were investigated with nanosecond laser radiation at 532nm wavelength. Enhanced NLA properties compared to the InPc alone were observed in the conjugates. The NLA was found to increase with the size of the CdSe/ZnS particles attached to the phthalocyanine. The observed increase was due to the availability of more free-carrier ions in the larger QDs, thus giving rise to the enhanced free-carrier absorption. The measured free-carrier absorption cross-sections (σFCA) are 1.10, 1.65 and 1.95 (×10−19cm2) for InPc-QD1, InPc-QD2 and InPc-QD3 respectively. The nanoconjugates (InPc–CdSe/ZnS) showed a much lower threshold for optical limiting together with a much lower transmission at high fluences, than the previously reported nanocomposite limiters.

Composition profiling of GaAs/AlGaAs quantum dots grown by droplet epitaxy

Droplet epitaxy (DE) is a growth method which can create III-V quantum dots (QDs) whose optoelectronic properties can be accurately controlled through the crystallisation conditions. In this work, GaAs/AlGaAs DE-QDs have been analyzed with the complimentary techniques of cross-sectional scanning tunneling microscopy and atom probe tomography. Structural details and a quantitative chemical analysis of QDs of different sizes are obtained. Most QDs were found to be pure GaAs, while a small proportion exhibited high intermixing caused by a local etching process. Large QDs with a high aspect ratio were observed to have an Al-rich crown above the GaAs QD. This structure is attributed to differences in mobility of the cations during the capping phase of the DE growth.

Anisotropy effect on the linear and nonlinear optical properties of a lased dressed donor impurity in a GaAs/GaAlAs nanowire superlattice

We have developed a finite difference method as well as have used compact density-matrix approach to analyze the effect of laser radiation and geometrical anisotropy on the optical properties of a GaAs/Ga1−xAlxAs nanowire superlattice (NWSL). In this regards the transition between ground and first excited states is considered to obtain the linear, third order nonlinear and total optical absorption coefficients (ACs) and refractive index (RI) changes. Our calculation shows that (I) presence of laser radiation causes an increment in ACs and RI changes and shifts optical spectrum toward lower energies. (II) For small values of volume of QDs the magnitude of optical ACs and RI changes is increased (decreased) by an oblate (a prolate) deviation from spherical QDs whereas for large QDs’ volumes an opposite behavior is observed. (III) The nonlinear terms of ACs and RI changes are very sensitive to laser radiation. Additionally, the saturation in optical spectrum can be adjusted by magnitude of laser radiation and shape of QDs involved in the NWSL.

Toward weak confinement regime in epitaxial nanostructures: Interdependence of spatial character of quantum confinement and wave function extension in large and elongated quantum dots

In this paper we present a comprehensive and detailed analysis of carrier/exciton wave function extension in large low-strain ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}$As quantum dots (QDs). They exhibit rather shallow confinement potential with electron/hole localization energy below 30 meV and confinement strength substantially weakened in comparison to typical epitaxial quasi-zero-dimensional semiconductor nanostructures. The aim of this study is to investigate the influence of different factors on the wave function (probability density distribution) for carriers or excitons in this regime, i.e., object shape anisotropy as well as strain, piezoelectricity, and Coulomb interactions, and to identify the physical mechanisms determining the properties of optical emission. To probe the wave function symmetry, polarization-resolved photoluminescence has been performed, and the spatial extensions of the corresponding probability densities have been verified in magneto-optical measurements. The observed diamagnetic coefficients in the range of (15--31) $\ensuremath{\mu}\mathrm{eV}/{\mathrm{T}}^{2}$ reflect large in-plane QD size. These studies also enable us to investigate the importance of light hole states admixture to the valence band ground state in such nanostructures, which can be addressed via the degree of linear polarization of emission as well as the exciton ${g}_{X}$ factor. The linear-polarization-resolved measurements revealed an exceptionally low exciton fine structure splitting of $5 \ensuremath{\mu}\mathrm{eV}$ on average as well as a low emission polarization degree of \ensuremath{-}0.05, with the polarization perpendicular to the QD elongation direction dominating. The increased light hole contribution to the lowest energy hole level is reflected in the decreased exciton ${g}_{X}$ factor (in the range of 0--1) and is consistent with the results of the eight-band k\ifmmode\cdot\else\textperiodcentered\fi{}p modelling. Based on the temperature dependence of the diamagnetic coefficient, the problem of individual QD uniformity has additionally been discussed. To evaluate the impact of the confinment potential and the structure geometry on the optical properties of the QDs, a comparison between the investigated dots and InAs/InGaAlAs/InP quantum dashes exhibiting a much deeper confining potential is presented.

CdSe Quantum Dot-sensitized, Nanoporous p-type NiO Photocathodes for Quantum Dot-sensitized Solar Cells

CdSe quantum dots (QDs) were deposited on a p-type nanoporous NiO surface by repeating the successive ionic layer adsorption and reaction (SILAR) procedure 6, 9, 12, and 15 times. The NiO/CdSe prepared using 12 SILAR cycles, which is denoted as the NiO/CdSe(12), exhibited a larger QD size and better QD coverage on the nanoporous NiO than the NiO films with CdSe(6) and CdSe(9), leading to better light absorption over longer wavelengths and a significantly increased hole lifetime. The resulting quantum dot-sensitized solar cell (QD-SSC) exhibited significantly enhanced cell efficiency (0.35%) compared to that (0.013%) of the QD-SSC with the NiO/CdSe(6). NiO/CdSe(15), however, exhibited a lower hole lifetime and hindered ion transport as a result of the agglomeration of QDs as well as the severely blocked pores of the nanoporous NiO, causing a significantly reduced cell efficiency for QD-SSCs with NiO/CdSe(15).

CdSe/ZnS quantum dot thin film formation by an electrospray deposition process for light-emitting devices.

About 30 nm quantum-dot thin films are formed by electrospray deposition (ESD) process and quantum-dot-light-emitting-diodes (QD-LEDs) are demonstrated. Maximum brightness of 23 000 cd m(-2) and current efficiency of 5.9 cd A(-1) are achieved with the ESD process. The ESD process can be a potential solution for large area quantum dot layers with simple and flexible control.

Effect of Excited-State Structural Relaxation on Midgap Excitations in Co2+-Doped ZnO Quantum Dots

The unique midgap excited states of Co2+-doped ZnO quantum dots have given rise to new applications in photocatalysis, sensing, magneto-optics, and magnetoelectronics. However, the electronic characteristics of these midgap transitions are not fully understood, and the uncertain interplay between these transitions has led to disagreement in the literature. In this work, midgap excited states of Co2+-doped ZnO quantum dots are analyzed using linear response time-dependent density functional theory and the effective mass theory with a focus on the geometry relaxation in the donor-type photoionization excited state. Relaxation of the excited-state geometry lowers the charge-transfer transition energy to be in the vicinity of the prominent spin-allowed 4A2 → 4T1 Co2+ d–d transition that gives this material its characteristic color. For large quantum dots, the excited-state population distribution between this Co2+ d–d excited state and the charge-transfer excited state can be tuned by thermal energy, resultin...

7.3: Influence of Layer Thickness on the Performance of Quantum Dots Light Emitting Devices

AbstractThe large area quantum dots light diode assays were fabricated by using CdSe/ZnS core/shell quantum dots (QDs) as light emitting layer, ZnO nanoparticles (NPs) as an electron transport/injection layer (ETL) and organic polymers as hole transport layer (HTL). The performance of the device was measured for samples of different thicknesses of QDs layer. By optimizing the constituent layers thicknesses, light emitting diode arrays (10 cm×10 cm active area, 32×32 pixel) were fabricated with improved electroluminance efficiency (1 to 1.8 Cd/A), low turn‐on voltages (3.2 V) and pure QDs emission. These findings suggest that such simple tuning techniques in these LEDs will be promising for use in practical flat panel displays (FPDs).

Electron spin relaxation due to charge noise

We study decoherence of an electron spin qubit in a quantum dot due to charge noise. We find that at the lowest order in spin-orbit interaction, the pure dephasing channel is suppressed for both $1/f$ charge noise and Johnson noise, so that charge noise leads to a pure relaxation channel of decoherence. Because of the weaker magnetic field dependence, the spin relaxation rate due to charge noise could dominate over phonon noise at low magnetic fields in a gate-defined GaAs or Si quantum dot or an InAs self-assembled quantum dot. Furthermore, in a large InAs self-assembled quantum dot, spin relaxation due to phonon noise could be suppressed in high magnetic fields due to the suppression of the coupling matrix element, so that the spin relaxation due to charge noise could dominate in both low and high magnetic fields. Numerically, in a 1 T magnetic field, the spin relaxation time due to typical charge noise is about 100 s in Si, $0.1$ s in GaAs for a gate-defined quantum dot with a 1 meV confinement, and 10 $\ensuremath{\mu}$s (or 1 s) in InAs self-assembled quantum dot with a 4 meV (or 30 meV) confinement.

Transport through side-coupled multilevel double quantum dots in the Kondo regime

We analyze the transport properties of a double quantum dot device in the side-coupled configuration. A small quantum dot (QD), having a single relevant electronic level, is coupled to source and drain electrodes. A larger QD, whose multilevel nature is considered, is tunnel-coupled to the small QD. A Fermi liquid analysis shows that the low temperature conductance of the device is determined by the total electronic occupation of the double QD. When the small dot is in the Kondo regime, an even number of electrons in the large dot leads to a conductance that reaches the unitary limit, while for an odd number of electrons a two stage Kondo effect is observed and the conductance is strongly suppressed. The Kondo temperature of the second stage Kondo effect is strongly affected by the multilevel structure of the large QD. For increasing level spacing, a crossover from a large Kondo temperature regime to a small Kondo temperature regime is obtained when the level spacing becomes of the order of the large Kondo temperature.

The dynamics of energy and charge transfer in lead sulfide quantum dot solids

We report on a systematic time-resolved photoluminescence study of the competing energy and charge transfer rates in PbS QDs of differing sizes in the same QD solid as a function of both temperature and ligand-controlled different inter-QD average separations. This complements previous studies that typically varied only one parameter and reveals new aspects while also confirming some known features. For the smallest PbS QDs, the dominant decay process is nonradiative resonant energy transfer (NRET) to adjacent larger dots for all separations but at a rate that increases with decreasing temperature. For the largest QDs, NRET being forbidden, the decay is found to be exponential in the inter-QD separation consistent with carrier tunneling but, for each fixed tunneling distance, exhibiting a thermally activated tunneling carrier population with the activation energy dependent upon the ligand length controlling the inter-QD separation. A consistent understanding of this expanded and rich decay rate behavior of both large and small QDs, we show, can be obtained by accounting for the ligand length dependent (a) dielectric environment of the QD solid modeled using an effective medium description, (b) the energy cost of dissociating the exciton into electron and hole in neighboring QDs, and (c) the potential participation of midgap states. Implications of the findings for NRET based photovoltaics are discussed.

Diffusion-Controlled Synthesis of PbS and PbSe Quantum Dots with in Situ Halide Passivation for Quantum Dot Solar Cells

We developed a simple non-hot-injection synthetic route that achieves in situ halide-passivated PbS and PbSe quantum dots (QDs) and simplifies the fabrication of Pb-chalcogenide QD solar cells. The synthesis mechanism follows a temperature-dependent diffusion growth model leading to strategies that can achieve narrow size distributions for a range of sizes. We show that PbS QDs can be produced with a diameter as small as 2.2 nm, corresponding to a 1.7 eV band gap, while the resulting size distribution (6-7%) is comparable to that of hot-injection syntheses. The in situ chloride surface passivation is demonstrated by X-ray photoelectron spectroscopy and an improved photostability of both PbS and PbSe QDs when stored under air. Additionally, the photoluminescence quantum yield of the PbS QDs is ∼30% higher compared to the traditional synthesis. We show that PbS QD solar cells with 6.5% power conversion efficiency (PCE) can be constructed. Finally, we fabricated PbSe QD solar cells in air (rather than in inert atmosphere), achieving a PCE of 2.65% using relatively large QDs with a corresponding band gap of 0.89 eV.

Resonance states in a cylindrical quantum dot with an external magnetic field

Bound and resonance states of quantum dots play a significant role in photo-absorption processes. In this work, we analyse a cylindrical quantum dot, its spectrum and, in particular, the behaviour of the lowest resonance state when a magnetic field is applied along the symmetry axis of the cylinder. To obtain the energy and width of the resonance we use the complex rotation method. As expected, the structure of the spectrum is strongly influenced by the Landau levels associated to the magnetic field. We show how this structure affects the behaviour of the resonance state and that the binding of the resonance has a clear interpretation in terms of the Landau levels and the probability of localization of the resonance state. The localization probability and the fidelity of the lowest energy state allow identification of two different physical regimes, a large field–small quantum dot radius regime and a small field–large quantum dot radius, where the binding of the resonance is dominated by the field strength or the potential well, respectively.

Study of hole mobility in poly(N-vinylcarbazole) films doped with CdSe/ZnS quantum dots encapsulated by 11-(N-carbazolyl) undecanoic acid (C11)

We report the experimental study of hole transport in poly(vinylcarbazole) (PVK) films doped with colloidal CdSe/ZnS core-shell quantum dots (QDs) determined using the Time-of-Flight (TOF) method. The miscibility between PVK and the QDs is improved by capping the QDs with a novel 11-(N-carbazolyl) undecanoic acid (C11) ligand instead of commonly used organic ligands, such as oleic acid. The study of the hole mobility of the pristine and doped PVK films with a doping concentration of the C11 capped QDs ranging from 1.61 × 1017 to 7.10 × 1018 dots/cm3 was performed as a function of electric field and temperature in the range of 105–106 V/cm and 298–338 K, respectively. Upon increasing the QD concentration, a decrease of hole mobility was observed by up to nearly 2 orders in magnitude at a doping concentration of 3.87 × 1018 dots/cm3 at T = 298 K. This suggests that the QDs induce shallow hole traps. The field and temperature dependence of the hole mobility was characterized using the Bassler disorder model and showed an increase of the energetic disorder (σ) from 124 to 204 meV as well as of the spatial disorder (Σ) from 0.95 to 5 when the concentration of the QDs was increased to 3.87 × 1018 dots/cm3. At higher concentration of the QDs (7.10 × 1018 dots/cm3), an increase of the hole mobility was observed suggesting hopping of the holes through the QD clusters. In addition, we also found that for this high doping concentration, the field dependence of the hole mobility was no longer in agreement with the Bassler disorder model. One should consider that at this doping concentration, the volume occupied by the inorganic (CdSe + ZnS) and organic (C11) components of the QDs in the doped film was estimated to be 14.6 and 15.8 volume %, respectively. This implies that the volume fraction of the inorganic material is very close to the percolation threshold, which amounts to 17 volume % for small spherical particles embedded in a three dimensional matrix. Furthermore, the conductivity data suggest a qualitative change in film properties between the samples with 3.87 × 1018 and 7.10 × 1018 dots/cm3. The study of film morphology by atomic force microscopy (AFM) experiment shows that while for the film with 3.87 × 1018 dots/cm3 the surface of the film has still the same features as that of a pristine PVK film, this is no longer the case for the film with 7.10 × 1018 dots/cm3, where shallow holes with a diameter of 100 to 200 nm become visible. These holes with the size much larger than the diameter of an individual QD likely correspond to clusters of the QDs. Upon further increasing the QD concentration to 9.68 × 1018 dots/cm3, the density of these holes is also increased. A correlation between the conductivity data and the film morphologies indicates that the presence of these QD clusters in the sample with 7.10 × 1018 dots/cm3 does not only change the homogeneity and roughness of the film but also leads to a significant change in the shape of the density of states of the energy sites for hopping holes resulting in a field and temperature dependence of the hole mobility that is no longer compatible with the Gaussian disorder model for this sample. Furthermore, the presence of these “hole” structures observed with AFM might imply a formation of large QD clusters in the polymer film, which form continuous pathways for charge carrier hopping between the opposite electrodes.

The influence of the shape and elastic strains of quantum dots on diffuse X-ray scattering

The influence of the shape and elastic-strain fields of buried quantum dots (QDs) on the angular distribution of diffuse X-ray scattering is studied. Elastic lattice displacements were calculated using the Green function method. Diffuse scattering from a semiconductor matrix with QDs having the shape of a cylinder, truncated cone, and spheroid was numerically simulated. It is shown that angular distributions of diffuse scattering for QDs with different shapes have significant distinctions at a small volume density of nanostructures in the crystal. At a large QD density, which is typical for superlattices, these distinctions become weak.

Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

Broadband enhancement and inhibition of single quantum dot emission in plasmonic nano-cavities operating at telecommunications wavelengths

A plasmonic microcavity providing broadband control of spontaneous emission for large and sparse semiconductor quantum dots emitting at telecommunications wavelengths is proposed. By designing and fabricating such a cavity, we demonstrate a broadband Purcell effect with spontaneous emission enhancement over a broad spectral range of Δλ≃20 nm with a 3.9-fold maximum enhancement, as well as inhibition over Δλ≃100 nm around 1.3 μm. The broadband feature relaxes the constraint on spectral matching between the dot emission and the cavity mode, favourable for implementing efficient non-classical light sources or nanoscale lasers.

Charge density mapping of strongly-correlated few-electron two-dimensional quantum dots by the scanning probe technique

We perform a numerical simulation of the mapping of charge confined in quantum dots by the scanning probe technique. We solve the few-electron Schrödinger equation with the exact diagonalization approach and evaluate the energy maps as a function of the probe position. Next, from the energy maps we try to reproduce the charge density distribution using an integral equation given by the perturbation theory. The reproduced density maps are compared with the original ones. This study covers two-dimensional quantum dots of various geometries and profiles with the one-dimensional (1D) quantum dot as a limiting case. We concentrate on large quantum dots for which strong electron–electron correlations appear. For circular dots the correlations lead to the formation of Wigner molecules that in the presence of a tip appear in the laboratory frame. The unperturbed rotationally-symmetric charge density is surprisingly well reproduced by the mapping. We find in general that the size of the confined droplet as well as the spatial extent of the charge density maxima is underestimated for a repulsive tip potential and overestimated for an attractive tip. In lower symmetry quantum dots Wigner molecules with single-electron islands nucleate for some electron numbers even in the absence of a tip. These charge densities are well resolved by the mapping. These single-electron islands appear in the laboratory frame provided that the classical point charge density distribution is unique, in the 1D limit of confinement in particular. We demonstrate that for electron systems which possess a few equivalent classical configurations the repulsive probe switches between the configurations. In consequence the charge density evades mapping by the repulsive probe.

The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots

A combination of FT-IR, (1)H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chemical shifts of their protons, and their diffusion coefficients. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb(2+) ion or bridges two Pb(2+) ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is approximately 1:1 for all sizes of the QDs we studied. The second oleate species in these samples bridges two Pb(2+) ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ~1.4 nm), which are byproducts of the QD synthesis. The concentration of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concentration of the oleic acid ligand in the reaction mixture. The oleate molecules on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Additional washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quantitatively characterizes the distribution of binding geometries at the inorganic/organic interface of the nanocrystals and demonstrates the utility of using organic ligands as probes for the composition of a colloidal QD sample as a function of the preparation procedure.

Variation of the carrier lifetime across the ground-state band of ensemble quantum dots: An indication for quantum-dot characteristics at high temperatures

High-temperature characteristics of InAs/GaAs quantum dots (QDs) with well-separated ground and excited state photoluminescence peaks were studied by time-resolved photoluminescence. The carrier lifetime across the ground state band of the ensemble QDs showed a large variation (> 2 ns) at 250 K while the lifetimes were almost the same at low temperatures. The large variation in the carrier lifetime, shorter at the high energy side and longer at the low energy side, is a clear indication of carrier transport from smaller QDs to larger QDs via the wetting layer. The variation is a good indicator to determine whether the QD characteristics are remained at high temperatures, which is important for device applications.