Intra-dot Auger relaxation in quantum dots

We report on calculations of relaxation in InAs/GaAs self assembled dots. We point out that the intra-dot Auger relaxation is extremely fast in these structures. This suggests that energy relaxation in InAs/GaAs self organized quantum dots is dominated by capture effects.     

Photoinduced charge transfer between CdSe nanocrystal quantum dots and Ru-polypyridine complexes

In this communication, we demonstrate a new approach to sensitization of Ru-polypyridine complexes by using semiconductor nanocrystal quantum dots (NQDs). When mixed in solution, the complexes functionalized by carboxylic groups adsorb onto the surface of the NQDs. Excitation of NQDs by 400 nm light leads to fast, 5 ps hole transfer from the photoexcited NQDs to the surface-adsorbed complexes. This result indicates that Ru complexes can be sensitized by CdSe NQDs, which opens interesting opportunities for designing new types of photocatalytic materials for solar energy conversion applications. These materials will take advantage of broad size-controlled absorption spectra and large extinction coefficients of NQDs as well as the unique property of NQDs to respond to absorption of a single photon by producing multiple electron-hole pairs.     

Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots

The light-induced spectral diffusion and fluorescence intermittency (blinking) of semiconductor nanocrystal quantum dots are investigated theoretically using a diffusion-controlled electron-transfer (DCET) model, where a light-induced one-dimensional diffusion process in energy space is considered. Unlike the conventional electron-transfer reactions with simple exponential kinetics, the model naturally leads to a power-law statistics for the intermittency. We formulate a possible explanation for the spectral broadening and its proportionality to the light energy density, the -32 power law for the blinking statistics of the fluorescence intermittency, the breakdown of the power-law behavior with a bending tail for the "light" periods, a lack of bending tail for the "dark" periods (but would eventually appear at later times), and the dependence of the bending tail on light intensity and temperature. This DCET model predicts a critical time t(c) (a function of the electronic coupling strength and other quantities), such that for times shorter than t(c) the exponent for the power law is -12 instead of -32. Quantitative analyses are made of the experimental data on spectral diffusion and on the asymmetric blinking statistics for the "on" and "off" events. Causes for deviation of the exponent from the ideal value of -32 are also discussed. Several fundamental properties are determined from the present experimental data, the diffusion correlation time, the Stokes shift, and a combination of other molecular-based quantities. Specific experiments are suggested to test the model further, extract other molecular properties, and elucidate more details of the light-induced charge-transfer dynamics in quantum dots.     

Oligomeric ligands for luminescent and stable nanocrystal quantum dots

We report a new family of oligomeric alkyl phosphine ligands for nanocrystal quantum dots. These oligomeric phosphines show effective binding affinity to quantum dot surfaces. They form thin and secure organic shells that stabilize quantum dots in diverse environments including serum and polymer matrices. They maintain the initial as-grown photoluminescence quantum yield of the quantum dots and enable homogeneous incorporation into various matrices. They present a chemically flexible structure that can be used for further chemistry, such as cross-linking, copolymerization, and conjugation to biomolecules.     

Self-assembly of PbTe quantum dots into nanocrystal superlattices and glassy films

Monodisperse lead telluride (PbTe) nanocrystals ranging from approximately 4 to 10 nm in diameter are synthesized to provide quantum dot building blocks for the design of novel materials for electronic applications. Two complementary synthetic approaches are developed that enable either (1) isolation of small quantities of nanocrystals of many different sizes or (2) the production of up to 10 g of a single nanocrystal size. PbTe nanocrystals are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical absorption. Assembly of PbTe nanocrystals is directed to prepare nanocrystal solids that display either short-range (glassy solids) or long-range (superlattices) packing order by varying deposition conditions. Film order and average interparticle spacing are analyzed with grazing-incidence small-angle X-ray scattering (GISAXS) and high-resolution scanning electron microscopy (HRSEM). We perform the first optical and electronic studies of PbTe solids and demonstrate that chemical activation of these films enhances conductivity by approximately 9-10 orders of magnitude while preserving their quantum dot nature.     

"Giant" multishell CdSe nanocrystal quantum dots with suppressed blinking

Semiconductor nanocrystal quantum dots (NQDs) comprise an important class of inorganic fluorophores for applications from optoelectronics to biology. Unfortunately, to date, NQD optical properties (e.g., their efficient and particle-size-tunable photoluminescence) have been susceptible to instabilities at the bulk and single-particle levels. Specifically, ensemble quantum yields (QYs) in emission are dependent upon NQD surface chemistry and chemical environment, while at the single-particle level, NQDs are characterized by significant fluorescence intermittency (blinking) that hinders applications as single-photon light sources for quantum informatics and biolabels for real-time monitoring of single biomolecules. Furthermore, while NQDs are significantly more photostable than their organic dye counterparts, traditional NQDs photobleach over periods of seconds to many minutes. Here, we demonstrate for the first time that by encapsulating the NQD core in a sufficiently thick inorganic shell, we are able to divorce NQD function from NQD surface chemistry and chemical environment. We show that our "giant" NQDs (g-NQDs) are functionally distinct from standard core-only, core/shell and even core/multishell NQDs. g-NQDs are substantially less sensitive to changes in surface chemistry. They do not photobleach under continuous laser excitation over periods of several hours repeated over several days, and they exhibit markedly different blinking behavior; >20% of the g-NQDs do not blink, while >40% have on-time fractions of >80%. All of these observations are in stark contrast with control samples comprising core-only and standard, thinner core/multishell NQDs.     

Aptamer-capped nanocrystal quantum dots: a new method for label-free protein detection

We demonstrate that aptamer-capped near-infrared PbS quantum dots (QDs) can detect a target protein based on selective charge transfer. The water-soluble QDs are synthesized with the thrombin-binding aptamer, which retains the secondary quadruplex structure necessary for binding to thrombin. These QDs have diameters of 3-6 nm and fluoresce around 1050 nm. When the aptamer-functionalized QD binds to its target, a fluorescence quenching occurs due to charge transfer from amine groups on the protein to the QD. Thrombin is detected within 1 min with a detection limit of approximately 1 nM. This selective detection is observed even in the presence of high background concentrations of interfering negatively or positively charged proteins, suggesting that aptamer-capped QDs could be useful for label-free protein assays.     

Time- and polarization-resolved optical spectroscopy of colloidal CdSe nanocrystal quantum dots in high magnetic fields

In an effort to elucidate the spin (rather than charge) degrees of freedom in colloidal semiconductor nanocrystal quantum dots, we report on a series of static and time-resolved photoluminescence measurements of colloidal CdSe quantum dots in ultrahigh magnetic fields up to 45 T. At low temperatures (1.5-40 K), the steady-state photoluminescence (PL) develops a high degree of circular polarization with applied magnetic field, indicating the presence of spin-polarized excitons. Time-resolved PL studies reveal a marked decrease in radiative exciton lifetime with increasing magnetic field and temperature. Except for an initial burst of unpolarized PL immediately following photoexcitation, high-field time-resolved PL measurements reveal a constant degree of circular polarization throughout the entire exciton lifetime, even in the presence of pronounced exciton transfer via F?rster energy transfer processes.     

Air-stable PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystal quantum dots and their applications

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.     

The interaction between some diamines and CdSe quantum dots

The interaction of some diamines (ethylenediamine (EDA), 1,6-hexanediamine (HDA), o-phenylenediamine (OPD)) with CdSe quantum dots (QDs) is reported. With increasing concentration of EDA from 0 to 2.0 x 10(-6) mol l(-1), slight fluorescence enhancement is observed. However, the CdSe QDs fluorescence quenching is seen at relatively higher concentration of EDA. There is a red-shift of 0-7 nm in fluorescence emission spectra of CdSe QDs when the concentration of EDA is changed from 2.0 x 10(-6) to 8.0 x 10(-6) mol l(-1). The resonance light scattering (RLS) spectra of CdSe QDs have little change when the concentration of EDA is less than 5.0 x 10(-6) mol l(-1). It indicates there are little large particles formed in the solution. However, a significant increase of the RLS is observed in the 300-500 nm wavelength range after adding higher concentration than 5.0 x 10(-6) mol l(-1) EDA, which could be attributed to the large particles formed. The interaction between HDA and CdSe QDs is similar to that of EDA. However, with the OPD, it is found that the interaction is much different from those of EDA, HDA, and that the quenching, even at low concentration, is effective for CdSe QDs emission. The quenching phenomenon could be explained by a surface bound complexation equilibrium model.     

Effect of the thiol-thiolate equilibrium on the photophysical properties of aqueous CdSe/ZnS nanocrystal quantum dots

We study the effect of thiols on the emission efficiency of aqueous CdSe/ZnS core/shell nanocrystal quantum dots (NQDs). We observe that the impact of thiol addition on emission quantum yields (QYs) is time-, concentration-, and pH-dependent. Further, we use a combination of time-resolved spectroscopic methods to determine the mechanism by which thiol addition can cause either increases or decreases in QYs. Specifically, through transient absorption measurements, we show that thiol addition can improve passivation of electron traps, increasing QYs at low thiol concentrations. Further, using nanosecond photoluminescence (PL), we observe that at higher thiol concentrations, hole traps are introduced that reduce PL QYs. Last, through a combination of pH-dependence and control studies (e.g., addition of 2-methyl thioethanol to an aqueous NQD solution and addition of dodecanethiol to a hexane NQD solution), we demonstrate that it is the ability of thiols in aqueous solution to generate thiolate that is the source of both favorable and adverse QY changes. Our results contribute to the understanding of the role of surface ligands, which is critical to the design of stable, high-quantum-yield, nonblinking NQDs.     

Study on the interaction between CdSe quantum dots and hemoglobin

The interaction between CdSe quantum dots (QDs) and hemoglobin (Hb) was investigated by ultraviolet and visible (UV-vis) absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and fluorescence (FL) spectroscopy. The intensity of UV-vis absorption spectrum of a mixture of CdSe QDs and Hb was obviously changed at the wavelength of 406nm at pH 7.0, indicating that CdSe QDs could bind with Hb. The influences of some factors on the interactions between CdSe QDs and Hb were studied in detail. The binding molar ratio of CdSe QDs and Hb was 12:1 by a mole-ratio method. The mechanism of the interaction between CdSe QDs and Hb was also discussed.     

Computation of ionization and various excited state energies ofhelium and helium‐like quantum dots

In the effective mass approximation, we calculated the wave functions and some energy states of helium and helium‐like quantum dots (QDs) with impurity charges Z = 0, 1, 2, 3, and 4. In addition, we carried out the ionization energies of these QDs as a function of dot radius, and we investigated the influence of impurity on the ionization energy. We utilized the method that is a combination of quantum genetic algorithm (QGA) and Hartree‐Fock Roothaan (HFR). The results are in a good agreement with literature results. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Energy transfer between CdS quantum dots and Au nanoparticles in photoelectrochemical detection

This work is the first report on the energy transfer between CdS quantum dots and Au nanoparticles for photoelectrochemical biosensing applications.     

CdTe荧光量子点与苯胺之间相互作用的研究

以巯基乙酸为稳定剂在水相中合成了CdTe荧光量子点,并使之与苯胺(Aniline)耦联.通过红外光谱分析,证实CdTe荧光量子点同Aniline的耦合主要是通过量子点周围巯基乙酸的—COOH与苯胺的—NH2形成的氢键实现的.将CdTe-苯胺、CdTe-丙氨酸(Alanine)耦合物荧光光谱的强度进行对比,发现苯环的存在可以极大地增强耦合物的荧光强度.     

Coupled and decoupled dual quantum systems in one semiconductor nanocrystal

Dual quantum systems, 0-dimensional quantum dot, and 2-dimensional quantum wells were constructed in one II-VI semiconductor nanocrystal by the epitaxial growth of a barrier (ZnS) layer between the systems in solution. By alteration of the thickness of the barrier layer, the two quantum systems were controlled to either electronically coupled or decoupled. Evidence of optical coupling between the two band gap emissions was also observed. The position and relative intensity of the two emissions can be independently tuned by reaction conditions. Total photoluminescence quantum efficiency of the dual emitting bands reached as high as 30% at room temperature under synthetic conditions not optimized for high emission.     

Quantum confinement effects on charge-transfer between PbS quantum dots and 4-mercaptopyridine

We obtain the surface enhanced Raman spectra of 4-mercaptopyridine on lead sulfide (PbS) quantum dots as a function of nanoparticle size and excitation wavelength. The nanoparticle radii are selected to be less than the exciton Bohr radius of PbS, enabling the observation of quantum confinement effects on the spectrum. We utilize the variation of nontotally symmetric modes of both b(1) and b(2) symmetry as compared to the totally symmetric a(1) modes to measure the degree of charge-transfer between the molecule and quantum dot. We find both size dependent and wavelength dependent resonances in the range of these measurements, and attribute them to charge-transfer resonances which are responsible for the Raman enhancement.     

Spectroscopic studies on the interaction between human hemoglobin and CdS quantum dots

The interaction between human adult hemoglobin (Hb) and bare CdS quantum dots (QDs) was investigated by fluorescence, synchronous fluorescence, circular dichroism (CD), and Raman spectroscopic techniques under physiological pH 7.43. The intrinsic fluorescence of Hb is statically quenched by CdS QDs. The quenching obeys the Stern-Volmer equation, with an order of magnitude of binding constant (K) of 10(7). The electrostatic adsorption of Hb on the cationic CdS QDs surface is energetically favorable (DeltaS(0)=70.22 Jmol(-1)K(-1), DeltaH(0)=-23.11 kJmol(-1)). The red shift of synchronous fluorescence spectra revealed that the microenvironments of tryptophan and tyrosine residues at the alpha(1)beta(2) interface of Hb are disturbed by CdS QDs, which are induced from hydrophobic cavities to a more exposed or hydrophilic surrounding. The secondary structure of the adsorbed Hb has a loose or extended conformation for which the content of alpha-helix has decreased from 72.5 to 60.8%. Moreover, Raman spectra results indicated that the sulfur atoms of the cysteine residues form direct chemical bonds on the surface of the CdS QDs. The binding does not significantly affect the spin state of the heme iron, and deoxidation is not expected to take place on the coated oxyhemoglobin. The change of orientation of heme vinyl groups was also detected.