Size Distribution Of Quantum Dots Research Articles

(Invited) Structural and Optical Properties of InN Quantum Dots Grown by an Alternating Supply of Source Precursors

The high density 2×1010 cm− 2 InN quantum dots (QDs) was grown by the two-step growth method. The InN nucleation, InN decomposition and In adatom diffusion in the step-1, annealing and the step-2 process, respectively, determined the density, shape and size distribution of the InN QDs. Especially in the InN nucleation density in step-1 process dominate the size distribution of the InN QDs. The bimodal size distribution of the two-step growth InN QDs can be suppressed by increasing the InN QDs nucleation density. The photoluminescence (PL) peak energy at 1.137 eV for InN QDs with average height of 6 nm the calculated confinement energy is ~ 356 meV and the electron concentration is estimated to be 2×1018 cm− 3. The two-step growth method can maintain both the InN QDs density and the optical quality, indicating a potential technique for future optoelectronic device applications.

Ultrafast Dynamics within the 1S Exciton Band of Colloidal PbSe Quantum Dots Using Multiresonant Coherent Multidimensional Spectroscopy

The simple particle-in-a-sphere model of quantum dot excitons is the basis for understanding the excitonic peak positions, line widths, and relaxation dynamics in many spectroscopic experiments. Recent multiresonant coherent multidimensional spectroscopy (CMDS) with picosecond excitation pulses measured the two-dimensional spectra of PbSe quantum dots and successfully used this simple model of an inhomogeneous distribution of spherically confined exciton and biexciton states and rate constants to describe the dephasing and population relaxation dynamics. The long excitation pulses prevented resolution of faster dynamics. This work reports the development of multiresonant CMDS with femtosecond excitation pulses to resolve the spectra and dynamics associated with the 1S exciton line shape of PbSe quantum dots. The experiments use different combinations of excitation frequencies, excitation pulse time delays, and a monochromator to display and measure correlations between the spectral features and their dynamics. Line-narrowing of the inhomogeneous distribution occurs at short time delays where the excitation excites a subset of the quantum dots within the 1S line shape and the last pulse probes this subset. The line-narrowing disappears at longer delay times. Three pulse photon echo peak shifts (3PEPS) also occur when the line-narrowing is present, but the shifts disappear as the correlation between the first and last coherence frequencies disappears. Wigner plots reveal the spectral dynamics accompanying the peak shift and the disappearance of the line-narrowing. This work shows there is rapid relaxation dynamics occurring within the line profile of the quantum confined excitonic states that is not consistent with current understanding of the excitonic line broadening. The data suggest that the relaxation dynamics play a more dominant role in defining the excitonic line widths than the inhomogeneous broadening of the quantum dot size distribution. These observations are consistent with other spectroscopic experiments on CdSe and PbS quantum dots. The experiments also show the presence of a higher energy feature that lies outside the 1S line shape and undergoes very rapid relaxation.

Quantum Confinement-Tunable Ultrafast Charge Transfer at the PbS Quantum Dot and Phenyl-C61-butyric Acid Methyl Ester Interface

Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C61-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion.

Effect of the lower and upper interfaces on the quality of InAs/GaAs quantum dots

The aim of this work is to influence quantum dot (QD) formation by improving the lower and upper InAs/GaAs QD interface quality. QD properties were studied by reflectance anisotropy spectroscopy, atomic force microscopy and high resolution transmission electron microscopy. All structures were prepared by low pressure metal organic vapor phase epitaxy.Concerning the lower interface, a good epitaxial surface planarity is required for QD formation with high QD density and narrow size distribution. Therefore the growth conditions of the QD buffer layer are very important. We demonstrate the improvement of the QD size distribution and homogeneity, when the growth rate of the buffer layer was decreased.The upper QD interface is formed during the covering process. InAs quantum dots were capped by GaAs or by GaAsSb. The presence of Sb atoms in covering layer strongly influences the interface abruptness. In the case of GaAs covering layer, an InGaAs layer with gradual decrease of In concentration is unintentionally formed at the interface between InAs and GaAs. The presence of Sb in GaAsSb covering layer helps to form abrupt interface between InAs and covering layer. However, enhanced surfacting of In atoms was observed for GaAsSb SRL. An optimal GaAsSb composition profile is suggested to prevent dissolution of QDs during the covering process and to minimize the amount of surfacting In atoms.

Experimental observation of exciton splitting and relaxation dynamics from PbS quantum dots in a glass matrix

Multiple optical transitions from PbS quantum dots (QDs) in glass matrix are observed. Energy separations between them amount up to $\ensuremath{\sim}$200 meV. Instead of being due to a size distribution of QDs, they are found to be related to a splitting of the lowest exciton levels. Systematic analysis of the relaxation dynamics reveals the lifetimes of the split states ranging from sub-100-ns to $\ensuremath{\sim}$$\ensuremath{\mu}$s. Moreover, we observe excited and ``intrinsic'' states having a sub-100-ps and $\ensuremath{\sim}$ns lifetime, respectively. The behavior of the split structure can be modeled by a phonon-assisted relaxation mechanism. This investigation offers a conclusive interpretation for the wide distribution of experimentally observed nonequilibrium carrier lifetimes in PbS QDs.

Preparation of monodispersed PbS quantum dots on nanoporous semiconductor thin film by two-phase method

The novel two-phase method was developed to in situ prepare monodispersed PbS quantum dots (QDs) on nanoporous semiconductor thin film. In this method, the two phases refer to an oil phase consisting of S2− ions (phase I) and an aqueous solution (phase II). Nanoporous SnO2 thin film adsorbed by Pb2+ ions was immersed in the aqueous solution. The S2− ions contained in the oil phase prefered to transfer through the oil/aqueous interfaces, and contacted with Pb2+ ions on the nanoporous thin film within the phase II to form QDs. The particle size and size distribution of QDs were controlled by the diffusion of S2− ions within the phase II. Volume ratio of ethylene glycol and water in the phase II can change the diffusion rate of S2− ions in the phase II, thus PbS QDs with different particle sizes and size distribution were prepared. The SnO2 thin film with PbS QDs deposited by the two-phase method was used to fabricate QDs-sensitized solar cells and the corresponding photoelectric conversion efficiency attained to 1.05% measured under simulated illumination with intensity of 100mWcm−2 (AM 1.5). These results demonstrated that the reported two-phase method is a facile and environment-friendly approach to fabricate monodispersed PbS QDs on the nanoporous SnO2 thin film.

Growth kinetics study revealing the role of the MPA capping ligand on adjusting the growth modes and PL properties of CdTe QDs

So far, the photoluminescence (PL) properties optimization of mercaptopropionic acid (MPA) capped CdTe has yet to be understood from a perspective of growth kinetics. In this work, the growth kinetics, size distribution and PL properties of CdTe quantum dots (QDs) grown in MPA aqueous solutions of two extreme concentrations, i.e. 0 mM and 50 mM, were systematically investigated. The studies revealed that CdTe QDs coarsen in low and high concentrations of MPA followed different growth kinetics, i.e. a mixed OA (Oriented Attachment)–OR (Ostwald Ripening) growth and an OA dominated growth, respectively. Moreover, the OR growth in a low MPA concentration solution was found to be disadvantageous for narrowing the size distribution but favourable for enhancing fluorescence, while the OA growth in a high MPA concentration solution is beneficial for focusing and controlling the particle size. Further activation energy calculations indicate that the extra MPA that forbids the OR growth and retards the OA growth is favourable for slowing the particle growth, thus achieving CdTe QDs with a controlled size and narrow distribution. Furthermore, the kinetics study reveals how the introduction of concentrated surface ligands affects the PL properties (FWHM and intensity) of the material.

Determination of particle size distribution of water-soluble CdTe quantum dots by optical spectroscopy

In the present study, we report the synthesis of glutathione (GSH) capped CdTe quantum dots (QDs) using the one-pot approach as well as their optical properties.

Vertical correlation and miniband formation in submonolayer Zn(Cd)Te/ZnCdSe type-II quantum dots for intermediate band solar cell application

High resolution x-ray diffraction based reciprocal space mapping is employed to investigate vertical correlation in submonolayer Zn(Cd)Te/ZnCdSe type-II quantum dots (QDs). The average lateral deviation from one dot to another is found to decrease from 13%–17% to 8%–11% with an increase in QD size. Narrower photoluminescence with a better yield is obtained for the sample with improved vertical correlation, indicating smaller QD size distribution along with partial suppression of non-radiative recombination paths. Observed reduction in radiative lifetimes and supportive calculations demonstrate enhanced hole-hole wavefunction overlap pointing towards possibility of miniband formation, an advantageous feature for an intermediate band solar cell.

Size Distribution of SnO2 Quantum Dots Studied by UV–Visible, Transmission Electron Microscopy and X-Ray Diffraction

Measuring the size of the quantum dots (QDs) with accuracy is crucial considering its effect on the physical and chemical properties. Size determination of SnO2 QDs prepared by a soft chemical method using various techniques and their correlation is reported here. Direct method like high resolution transmission electron microscopy (HRTEM), and indirect method like X-ray diffraction and ultra violet-visible (UV-Vis) techniques used for size determination and then correlated. Effective crystallite size found from TEM morphological analysis is 2.4 ± 0.1 nm, which matches closely with the crystallite size of 2.3 ± 0.1 nm as calculated using Williamson-Hall plot. Particle size is also calculated from UV-Vis spectroscopy following quantum confinement effect in SnO2. The obtained slopes from the Tauc's plot provide a distri- bution of particle sizes which matches well with the result from TEM analysis.

Chemical beam epitaxy growth and optimization of InAs/GaAs quantum dot multilayers

This paper reports on an in-situ growth process used to optimize InAs/GaAs quantum dot (QD) multilayer structures grown on (001) GaAs substrate by chemical beam epitaxy (CBE). Defects related to the incoherently relaxed InAs clusters are found to alter the QD nucleation mechanism on the subsequent layers, leading to reduced QD density and photoluminescence intensity. The formation of poor crystalline quality clusters is avoided by growing the GaAs spacer layers in a two-step process. The technique consists in covering the InAs QD layer with a 10nm-thick GaAs layer grown at 465°C, and then removing the excess indium contained in the uncapped portion of the clusters by increasing the temperature to 565°C for 10min before the deposition of the remaining GaAs spacer layer. Morphological investigation shows that the QD density and size distribution obtained in the first layer are preserved up to the tenth layer. The QD integrated photoluminescence intensity is found to increase linearly with the number of stacked layers. These results are very promising for chemical beam growth of high performance intermediate-band solar cells.

Annealing-induced bimodal size distribution of small CdSe quantum dots with white-light emission

White-light emitting (WLE) CdSe quantum dots (QDs) capped with oleic acid (OA) were synthesized and the effects of thermal annealing and photoactivation on the small CdSe QDs in (polymethyl methacrylate) (PMMA) were investigated. A bimodal size-distribution phenomenon induced by annealing at temperatures of ∼200 °C was observed. The phenomenon observed here is different from the result of large CdSe QDs with the similar treatment. The size-dependent property arises from the fact that smaller QDs have a higher surface energy than larger ones. Annealing-induced bimodal size distribution results in a luminescence change of small CdSe QDs.

Effect of thickness on the photoluminescence of silicon quantum dots embedded in silicon nitride films

In this work, the effect of film thickness on the photoluminescence (PL) spectra of Si quantum dots embedded in silicon nitride films is investigated experimentally and theoretically. The films were deposited by remote plasma enhanced chemical vapor deposition using the same SiH2Cl2/H2/Ar/NH3 mixture and deposition conditions, in order to obtain films with similar composition and approximately equal average size (∼3.1 nm) of Si quantum dots. Only the deposition times were varied to prepare five samples with different thicknesses ranging from ∼30 nm to 4500 nm. Chemical characterization by Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy were carried out in order to check that the composition in all films was the same. The structure, average size, and size distribution of the Si quantum dots were deduced from High-Resolution Transmission Electron Microscopy. The thickness of the films was determined by ellipsometry and interferometry of UV-Vis transmission spectra. It was found experimentally that the increase of the thickness above a few hundreds of nanometers produces significant distortions of the PL spectra of the films, such as peak shifts and the appearance of shoulders and multiple peaks suggesting interference effects. Comparing the experimental results with theoretical simulations, it is shown that these distortions are mainly due to interference effects and not to intrinsic changes in the films. The approximation used to simulate the PL spectra as a function of film thickness allows improving the fitting between simulated and experimental spectra by changing some optical parameters and can be helpful to further investigate the intrinsic optical properties of the films.

Multi-spectral lasing characteristics of InAs/GaAs quantum dot laser

The emission wavelength and mode structure of InAs/GaAs quantum dot (QD) lasers are reported. The active region consists of five-stacked QD layers with each QD layer formed by varying thickness of InAs gown by gas source molecular beam epitaxy. At operation temperature of 20°C, the ridge waveguide laser emits more than 33mW optical power from one of the cleaved facets under continuous-wave mode and broad gain spectra are obtained due to a large size distribution of InAs QDs. The lasing spectra are found to be a series of complicated longitudinal modes separated by non-lasing spectral regions. We proved experimentally that those behaviors are not relative to the cavity dimension or the supper radiance modulated by photon reflecting in vertical facets. The lasing characteristics can be explained in terms of spatially discrete nature from the non-uniformity size distribution of quantum dots.

Assembling Tin Dioxide Quantum Dots to Graphene Nanosheets by a Facile Ultrasonic Route

Nanocomposites have significant potential in the development of advanced materials for numerous applications. Tin dioxide (SnO2) is a functional material with wide-ranging prospects because of its high electronic mobility and wide band gap. Graphene as the basic plane of graphite is a single atomic layer two-dimensional sp(2) hybridized carbon material. Both have excellent physical and chemical properties. Here, SnO2 quantum dots/graphene composites have been successfully fabricated by a facile ultrasonic method. The experimental investigations indicated that the graphene was exfoliated and decorated with SnO2 quantum dots, which was dispersed uniformly on both sides of the graphene. The size distribution of SnO2 quantum dots was estimated to be ranging from 4 to 6 nm and their average size was calculated to be about 4.8 ± 0.2 nm. This facile ultrasonic route demonstrated that the loading of SnO2 quantum dots was an effective way to prevent graphene nanosheets from being restacked during the reduction. During the calcination process, the graphene nanosheets distributed between SnO2 nanoparticles have also prevented the agglomeration of SnO2 nanoparticles, which were beneficial to the formation of SnO2 quantum dots.

Regulating properties of quantum dots: effect of methyl side groups of mercapto acids

A series of mercapto acids with and without methyl side groups, namely 3-mercaptopropanoic acid (MPA), 4-mercaptobutyric acid (MBA), 5-mercaptovaleric acid (MVA), 3-mercaptobutyric acid (3MBA), 3-mercaptoisobutyric acid (MIBA), 3-mercapto-2-methylbutyric acid (MMBA) and 3-mercapto-2,2-dimethylpropanoic acid (MDMPA), were synthesized and used as capping agents to prepare CdTe quantum dots (QDs) by a modified hydrothermal method with Na2TeO3 as Te source. By comparing the growth rate, size distribution, fluorescence and solution stability of QDs capped with these mercapto acids, the effect of the position and amount of the methyl side groups of mercapto acids in the aqueous synthesis of CdTe QDs was revealed. The use of branched mercapto acids with one methyl side group (3MBA and MIBA) was superior to obtain QDs with narrow size distribution, strong fluorescence and good long-term solution stability than the use of linear mercapto acids (MPA, MBA and MVA) whereas the use of branched mercapto acids with two methyl side groups (MMBA and MDMPA) was not equally successful. Among these mercapto acids, 3MBA-QDs had the strongest fluorescence and MIBA-QDs had the narrowest size distribution. Our results are helpful for synthesizing QDs with good properties by selecting and designing appropriate capping agents.

High-performance InAs/GaAs quantum dot laser with dot layers grown at 425 oC

We investigate InAs/GaAs quantum dot (QD) lasers grown by gas source molecular beam epitaxy with different growth temperatures for InAs dot layers. The same laser structures are grown, but the growth temperatures of InAs dot layers are set as 425 and 500 °C, respectively. Ridge waveguide laser diodes are fabricated, and the characteristics of the QD lasers are systematically studied. The laser diodes with QDs grown at 425 °C show better performance, such as threshold current density, output power, internal quantum efficiency, and characteristic temperature, than those with QDs grown at 500 °C. This finding is ascribed to the higher QD density and more uniform size distribution of QDs achieved at 425 °C.

On the missing links in quantum dot solar cells: a DFT study on fluorophore oxidation and reduction processes in sensitized solar cells

The possibility of achieving many electrons per absorbed photon of sufficient energy by quantum dots (QDs) drives the motivation to build high performance quantum dot solar cells (QDSCs). Although performance of dye-sensitized solar cells (DSCs), with similar device configuration as that of QDSCs, has significantly improved in the last two decades QDSCs are yet to demonstrate impressive device performances despite the remarkable features of QDs as light harvesters. We investigated the fundamental differences in the optical properties of QDs and dyes using DFT calculations to get insights on the inferior performance of QDSCs. The CdSe QDs and the ruthenium bipyridyl dicarboxylic acid dye (N3) were used as typical examples in this study. Based on a generalized equation of state correlating material properties and photoconversion efficiency, we calculated ground and excited state properties of these absorbers at the B3LYP/lanl2dz level of DFT and analyzed them on the basis of the device performance. Five missing links have been identified in the study which provides numerous insights into building high efficiency QDSCs. They are (i) fundamental differences in the emitting states of the QDs in the strong and weak confinement regimes were observed, which explained successfully the performance differences; (ii) the crucial role of bifunctional ligands that bind the QDs and the photo-electrode was identified; in most cases use of bifunctional ligands does not lead to a QD enabled widening of the absorption of the photo-electrode; (iii) wide QDs size distribution further hinders efficient electron injections; (iv) wide absorption cross-section of QDs favours photon harvesting; and (v) the role of redox potential of the electrolyte in the QD reduction process.

Improved Precursor Chemistry for the Synthesis of III–V Quantum Dots

The synthesis of III-V quantum dots has been long known to be more challenging than the synthesis of other types of inorganic quantum dots. This is attributed to highly reactive group-V precursors. We synthesized molecules that are suitable for use as group-V precursors and characterized their reactivity using multiple complementary techniques. We show that the size distribution of indium arsenide quantum dots indeed improves with decreased precursor reactivity.

Tunneling conduction in dense silicon quantum dot/poly (methyl methacrylate) composites

The conduction mechanism of organically passivated silicon quantum dots (SiQDs) embedded in Poly(methyl methacrylate) (PMMA) has been investigated. This architecture forces the charge carriers onto the QDs, eliminating the need for charge transfer between the host matrix PMMA and the SiQDs. Octene was determined to be the shortest aliphatic ligand for electroluminescent films, as mixtures of 5:1 SiQDs:PMMA were readily synthesized without precipitation. At low fields, the current is limited by space charge effects with a weak temperature dependence which is believed to be due to the quantum dot size distribution. The lack of a temperature dependence on the current–voltage characteristics in the high voltage regime suggests that trap assisted tunneling is the primary conduction mechanism rather than transport through the PMMA. An analysis of the results suggests that the tunneling effective mass in PMMA is approximately 0.23mo. Thus, preparing SiQDs in a PMMA matrix may yield a recombination pathway for generated excitons allowing for the creation of efficient light emitting devices.