Aggregation Of Quantum Dots Research Articles

Scanning Tunneling Spectroscopy of Individual PbSe Quantum Dots and Molecular Aggregates Stabilized in an Inert Nanocrystal Matrix

The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a two-dimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates.

Formation of nanopores in suspended lipid bilayers using quantum dots

In this work, nanopores are formed in lipid (DOPC:DOPE) membranes suspended across 150 micron apertures by oligomeric aggregation of 12 nm diameter CdSe quantum dots. The bilayer and quantum dot nanopores are simultaneously characterized by low noise electrical current monitoring and epifluorescence microscopy. Suspended lipid bilayers form high resistance gigaseals (>10 GOhm) that serve as barriers to the migration of charged ions and particles. Oligomeric aggregation of quantum dots is observed on the surface of the suspended lipid bilayer in the presence of charge stabilized quantum dot suspensions, The aggregate forms a nanometer scale pore (~2 nm in diameter) in the bilayer resulting in non-quantal ion current bursts. Migration of net neutral Rhodamine B dye (1.6 nm molecular diameter) across the bilayer is measured only in the presence of the aggregates. Potential applications for the non-lithographic fabrication of bilayer nanopores include biochemical detection, DNA sequencing, or cellular drug delivery.

Ultrasensitive Pb2+ Detection by Glutathione-Capped Quantum Dots

Water-soluble and stable quantum dots (QDs), CdTe and CdZnSe, are applied for ultrasensitive Pb(2+) detection. These QDs are capped with glutathione (GSH) shells. GSH and its polymeric form, phytochelatin, are employed by nature to detoxify heavy metal ions. As a result of specific interaction, the fluorescence intensity of GSH-capped QDs is selectively reduced in the presence of heavy metal ions such as Pb(2+). The detection limit of Pb(2+) is found to be 20 nM due to the superior fluorescence properties of QDs. Detailed studies by spectroscopy, microscopy, and dynamic light scattering show that competitive GSH binding of Pb(2+) with the QD core changed both the surface and photophysical properties of the QDs. Fluorescence of QDs is quenched, and QD aggregation occurs. Coupling the GSH-capped QDs with a high-throughput detection system, we have developed a simple scheme for quick and ultrasensitive Pb(2+) detection without the need for additional electronic devices. In the presence of ionic mixtures, our system is still capable of Pb(2+) detection with a detection limit as low as 40 nM. The system only becomes less sensitive when the ionic mixture is present at a very high concentration (i.e., > or =50 microM).

Quantized Growth of CdTe Quantum Dots; Observation of Magic-Sized CdTe Quantum Dots

This work presents experimental observation of the quantized growth of CdTe quantum dots (QD) in the presence of hexadecylamine (HDA), hexylphosphonic acid (HPA), and trioctylphosphine oxide (TOPO) above 200 °C. The crystal growth of CdTe QDs is monitored by in situ UV−vis absorption spectroscopy. The high-temperature absorption spectra indicate the evolution of multiple peaks corresponding to various sizes of QDs. Analysis of the growth kinetics suggests quantized growth of the CdTe QDs in the coordinating solvent mixture. The high-resolution transmission electron microscopy (HRTEM) images and electron diffraction pattern show that most of the QDs have the zinc blende crystal structure. The HRTEM images indicate nanotwinning and stacking faults in larger CdTe QDs. Domain sizes in the HRTEM images correlate well with the smallest observed magic-sized CdTe QDs, in agreement with the proposed aggregation growth mechanism under the experimental conditions. The smallest observed zinc blende CdTe QDs with the diameter of 1.9 ± 0.3 nm are isolated by quenching the reaction mixture during the initial phase of the QD synthesis. The experimental observation suggests that the surprising stability of the magic-sized CdTe QDs is the result of the surface stabilization of the QDs of the HDA and/or HPA. As previously suggested, the aggregation is driven by dipole−dipole interaction between CdTe nanoparticles. The results show that the aggregation of quantum dots could be very important at the early stage of the growth. The magic-sized QDs can be dissolved in either methanol or toluene, which suggests heterogeneity of their surface chemistry. The QDs dissolved in the methanol phase exhibit relatively strong white light emission from 400 to 650 nm with an emission quantum yield of approximately 4%. The QDs dissolved in the toluene phase exhibit very weak emission.

Photochemical Instability of Thiol-Capped CdTe Quantum Dots in Aqueous Solution and Living Cells: Process and Mechanism

The process and mechanism of photochemical instability of thiol-capped CdTe quantum dots (QDs) in aqueous solution were experimentally studied. After laser irradiation, the corresponding Raman bands of the Cd-S bond decreased obviously, indicating bond breaking and thiol detachment from the QD surfaces. Meanwhile, a photoinduced aggregation of QDs occurred with the hydrodynamic diameter <Dh> increased to hundreds of nanometers from an initial 20 nm, as detected with dynamic light scattering measurements. The bleaching of the photoluminescence of QDs under laser irradiation could be attributed to the enhanced nonradiative transfer in excited QDs caused by increased surface defects due to the losing of thiol ligands. Singlet oxygen (1O2) was involved in the photooxidation of QDs, as revealed by the inhibiting effects of 1O2 quenchers of histidine or sodium azide (NaN3) on the photobleaching of QDs. The linear relationship in Stern-Volmer measurements between the terminal product and the concentration of NaN3 demonstrated that 1O2 was the main pathway of the photobleaching in QD solutions. By comparing the photostability of QDs in C2C12 cells with and without NaN3 treatment, the photooxidation effect of 1O2 on photobleaching of cellular QDs was confirmed.

On-Line Investigation of Laser-Induced Aggregation and Photoactivation of CdTe Quantum Dots by Fluorescence Correlation Spectroscopy

In this work, the laser-induced aggregation and subsequent photoactivation process of mercaptopropionic acid (MPA)-capped CdTe QDs was on-line studied with fluorescence correlation spectroscopy technique (FCS). It was observed that under laser irradiation, certain QDs facilely aggregated together and photoactivation process occurred. The aggregation process was dependent on sizes of QDs. Smaller QDs were more apt to aggregate together. The buffer types and concentrations markedly affected the photoactivation process. The higher concentration of salt would suppress the aggregation and photoactivation process of QDs significantly. Meanwhile, the photoactivation process was also closely associated with the concentration of QDs. Only when the concentrations of QDs were over the certain concentration (called as “photoactivation critical concentration”) could the photoactivation of QDs occur under laser irradiation. Finally, a simple model was summarized to explain the photoactivation process of MPA-capping CdTe QDs.

Quantum dot labeling of mesenchymal stem cells

BackgroundMesenchymal stem cells (MSCs) are multipotent cells with the potential to differentiate into bone, cartilage, fat and muscle cells and are being investigated for their utility in cell-based transplantation therapy. Yet, adequate methods to track transplanted MSCs in vivo are limited, precluding functional studies. Quantum Dots (QDs) offer an alternative to organic dyes and fluorescent proteins to label and track cells in vitro and in vivo. These nanoparticles are resistant to chemical and metabolic degradation, demonstrating long term photostability. Here, we investigate the cytotoxic effects of in vitro QD labeling on MSC proliferation and differentiation and use as a cell label in a cardiomyocyte co-culture.ResultsA dose-response to QDs in rat bone marrow MSCs was assessed in Control (no-QDs), Low concentration (LC, 5 nmol/L) and High concentration (HC, 20 nmol/L) groups. QD yield and retention, MSC survival, proinflammatory cytokines, proliferation and DNA damage were evaluated in MSCs, 24 -120 hrs post QD labeling. In addition, functional integration of QD labeled MSCs in an in vitro cardiomyocyte co-culture was assessed. A dose-dependent effect was measured with increased yield in HC vs. LC labeled MSCs (93 ± 3% vs. 50% ± 15%, p < 0.05), with a larger number of QD aggregates per cell in HC vs. LC MSCs at each time point (p < 0.05). At 24 hrs >90% of QD labeled cells were viable in all groups, however, at 120 hrs increased apoptosis was measured in HC vs. Control MSCs (7.2% ± 2.7% vs. 0.5% ± 0.4%, p < 0.05). MCP-1 and IL-6 levels doubled in HC MSCs when measured 24 hrs after QD labeling. No change in MSC proliferation or DNA damage was observed in QD labeled MSCs at 24, 72 and 120 hrs post labeling. Finally, in a cardiomyocyte co-culture QD labeled MSCs were easy to locate and formed functional cell-to-cell couplings, assessed by dye diffusion.ConclusionFluorescent QDs label MSC effectively in an in vitro co-culture model. QDs are easy to use, show a high yield and survival rate with minimal cytotoxic effects. Dose-dependent effects suggest limiting MSC QD exposure.

Electronic Coupling and Exciton Energy Transfer in CdTe Quantum-Dot Molecules

Stable dispersions of molecularlike aggregates of CdTe quantum dots are prepared by chemical cross-linking. Cryo-TEM images confirm the presence of cross-linked quantum dots and show that the size of the small aggregates can be controlled by the amount of cross-linker added. Optical measurements reveal two types of interdot interactions within these quantum-dot molecules: exciton energy transfer and electronic coupling. Quantitative information on the energy transfer rates in quantum-dot molecules is obtained by photoluminescence lifetime measurements. The degree of electronic coupling is dependent on the size of the quantum dots, which is supported by quantum mechanical calculations.

Shake and Break—Shaking-Induced Changes of Size Quantization in Aggregated Quantum Dots

We show how simple mechanical agitation of precipitated CdSe quantum dot aggregates causes partially reversible color changes (clearly visible to the eye) in the absorption spectrum of the CdSe (about 4 nm size). The color changes, which are due to changes in size quantization, are not accompanied by change in quantum dot size. This phenomenon is explained by partial deaggregation of the precipitates, leading to reduced charge overlap between neighboring dots. Shaking was shown to result in a looser aggregate structure. It is suggested that CdSO3 particles (an initial product of the CdSe formation reaction) act as weak bridges between CdSe quantum dots, mediating the interparticle interactions and allowing the deaggregation to occur on shaking.

Oscillating fluorescence in an unstable colloidal dispersion of CdSe/ZnS core/shell quantum dots.

Fluorescence oscillation is observed in an ensemble of colloidal CdSe/ZnS core/shell quantum dots (QDs) dispersed in nonpolar solvent under continuous irradiation. The QDs dispersed in toluene gradually aggregate and change their fluorescence intensity, even in the dark. During the aggregation, the QD/toluene suspension is unstable, that is, overdispersed. The fluorescence oscillation is found only in this unstable state before the system reaches steady state. In addition, the aggregation rate is promoted by irradiation and strongly correlates with the oscillation amplitude. Our experimental results indicate that the dispersion instability plays an important role in both linear and nonlinear dynamics of the fluorescence. It is inferred from the experimental results and previous studies that the complex time evolution of fluorescence in the QD/toluene dispersion is possibly due to adsorption and desorption of surface ligand molecules over the course of QD aggregation.

Optical properties of CdS quantum‐dot superlattices self‐organized with electrostatic interaction

Quantum-dot superlattices (QDSLs) of CdS were grown via a self-organization process with electrostatic interaction between positively- and negatively-charged CdS quantum dots (QDs). The positively- and negatively-charged CdS QDs were prepared separately by modifying a QD surface with cationic and anionic thiol compounds, respectively. The absorption peak of mixed solutions of oppositely charged QDs is red-shifted compared with that of each QD sample: The red-shift is due to a coupling of QDs through the electrostatic interaction. Millimeter-sized QD aggregates were obtained with the elapse of ∼20 hours after the mixing. The peak energies of photoluminescence (PL) and PL excitation spectra of the aggregates correspond well to those of PL and absorption spectra of the mixed solution samples. Furthermore, in x-ray diffraction patterns, a diffraction peak is clearly observed at the low 2θ angle of ∼4°. From the results, we conclude that the obtained aggregates are ordered structures of QDs, that is, QDSLs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of LO Phonons in Optical Spectra of Quantum Dot Aggregates in Sub-Wetting Layer Region of Energy

Samples of the self-assembled polar semiconductor quantum dots, grown by the Stranski–Krastanow growth method, contain the wetting layer, representing the lowest barrier for the electronic excitations to get to the extended motion states. The scattering of electrons between the extended states and the electronic states localized in the individual quantum dots, with emission or absorption of optical phonons via Fröhlich's coupling, may influence the optical spectra of such samples in the sub-wetting layer region of the energy of electronic excitations. The contribution of this interaction to the appearance of the sub-wetting layer continuum in the optical spectra and to the level broadening of the localized states, is studied.

Optical Spectra of Quantum Dot Aggregates in the Sub-Wetting Layer Region

Optical Spectra of Quantum Dot Aggregates in the Sub-Wetting Layer Region

Nonlinear Time-Series Analysis of Photoinduced Fluorescence Oscillation in a Water Dispersion of Colloidal Quantum Dots

We report on photoinduced fluorescence oscillation observed in a water dispersion of CdSe/ZnS core/shell quantum dots (QDs). The fluorescence intensity spontaneously oscillates under continuous excitation in a temperature-sensitive manner and emerges at 29−30 °C. The experimental results suggest that the fluorescence oscillation occurs only in the presence of QD aggregates. The oscillation is characterized by wavelet and nonlinear time-series analyses of the fluorescence intensity data at 30 °C. The results reveal self-similarity and nonlinearity, suggesting that the fluorescence oscillation is governed by nonlinear dynamics and is a special class of photoinduced chemical oscillation.

Electronic spectral densities and optical spectra of quantum dot aggregates in sub-wetting layer region

Samples with self-organized quantum dots, made of polar semiconductors and grown by Stranski–Krastanow growth method, contain the so-called wetting layer. The lowest-energy extended states of the electronic excitations are expected to be the wetting-layer states. Coupling between these extended states and the electronic states localized in the individual quantum dots may have an influence on the optical spectra of such samples in energy region corresponding to the sub-wetting layer. Here we study this effect, focusing first of all on Fröhlich coupling between the electrons and polar optical phonons. Contribution of this interaction to the appearance of the sub-wetting layer continuum in the electronic spectral densities, and to the level broadening of the localized states, under rather strong population of the wetting-layer states, is estimated. Relation of these results to the effects observed in photoluminescence excitation experiments is discussed.

The J-band of organic dyes: lineshape and coherence length

Self-organised J-aggregates of dye molecules, known for over 60 years, are emerging as remarkably versatile quantum systems with applications in photography, opto-electronics, solar cells, photobiology and as supra-molecular fibres. Recently there has been much effort to achieve quantum entanglement and coherence on the nanoscale in atom traps and quantum dot aggregates (for use in quantum computing). We point out that the excitonic state of the J-aggregate is a text-book case of mesoscopic quantum coherence and entanglement. The establishment of coherence can literally be seen since the dye changes colour dramatically on aggregation due to strong shifts in the absorption spectrum. Here we reproduce in a simple theory the shifts and shapes of optical absorption spectra upon aggregation to a polymer and calculate the coherence length of quantum entanglement of monomer wavefunctions.

Application of Hot Press Sintering to Na2O-B2O3-SiO2 Gels Containing Quantum Dots for Lowering of Densification Temperature

Na2O-B2O3-SiO2 (NBS) gels containing a large amount of CdS quantum-dots (10 wt%) were densified using the hot press (HP) sintering method. By HP treatment, full-densification temperature could be lowered by about 40°C than that of the normal non-pressing (NP) heat treatment. Exciton absorption of CdS quantum-dots in HP-sample showed a large blue shift compared with that in the NP sample, and the size-distribution of CdS dots remained very sharp, with a mean particle diameter d = 3.66 nm and a standard deviation of σ = 0.72. HP pressure had a large effect on the reduction of sintering temperature and time, resulting in the suppression of the aggregation and growth of CdS quantum-dots in NBS glasses.

Atomic Force Microscopy and Near-Field Scanning Optical Microscopy Study of Quantum-Dot Assemblies and Fractal Films

Quantum dots and nanocrystalline films are interesting materials due to their novel properties, not achievable from the bulk materials [1]. New materials fabricated by assembling of quantum dots and nanostructured materials exhibit, for example, high optical nonlinearities. Fractal structures [2] can be formed from quantum-dot aggregates or films. Scanning probe microscopy is the essential technique to characterize these nanometer-scaled materials?Gold nanocluster colloids are synthesized in the interior of surfactant aggregates known as inverse micelles, without the use of water to solubilize the metal salt [3]. The size of the Au quantum dots is well controlled in the colloid synthesis and selected using chromatography. The Au clusters are then sprayed onto a glass slides to form thin films made of Au quantum dots. We have also used the laser ablation technique [4] to grow Ag nanoparticles, fractal aggregates and thin films. These samples have been studied using atomic force microscopy (AFM) and nearfield scanning optical microscopy.

Optical properties of InAs quantum dots: Common trends

We compare results obtained in several tens of samples grown by molecular-beam epitaxy under different growth conditions with a substantial amount of data found in the literature. By plotting the photoluminescence (PL) peak energy ${(E}_{p})$ of the quantum dot (QD) bands as a function of the nominal thickness of deposited InAs (L) three regions are clearly evidenced in the ${(E}_{p},L)$ plane. Below the so-called critical thickness ${(L}_{c}),$ three-dimensional precursors of QD's show a smooth dependence of their emission energy on L. Around ${L}_{c},$ QD's show a steep dependence of ${E}_{p}$ on L, independent of the growth conditions. Finally, for $L\ensuremath{\gtrsim}2$ ML one observes a saturation of the PL energy. This energy assumes only discrete values dependent on the growth conditions, which is attributed to the aggregation of quantum dots with different faceting.