界面电子转移对量子点荧光闪烁行为的影响

利用激光扫描共聚焦显微系统分别测量了CdSe/ZnS量子点在SiO2玻片表面、铟锡氧化物(ITO)纳米粒子表面和聚甲基丙烯酸甲酯(PMMA)薄膜表面上的荧光闪烁行为.研究发现,不同界面环境中量子点的亮态发光持续时间的概率密度都服从指数修正的幂律分布P(t)∝t-αexp(-t/μ).与处于SiO2玻片表面的情况相比,在ITO表面上的单量子点具有非常短暂的亮态发光持续时间,而在PMMA表面的单量子点亮态发光持续时间最长.这种荧光闪烁行为的不同主要归因于量子点与三种材料之间的界面电子转移特性.     

Statistics of photocounts recorded by a detector with variable quantum efficiency

We consider a problem of distribution of counts from an optical photodetector recording a non-polarized radiation field with properties of normal Markovian process. The well-known Mandel formula describing the probability P(n) of recording of n photocounts per observation interval (0, T) is generalized to the case of a detector with time-dependent quantum efficiency ε(t). Khar’kov State Polytechnical University, Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 8, pp. 999–1007, August, 1998.     

Model for the off-time distribution in blinking quantum dots

The understanding of blinking quantum dots (QDs) is an open problem since more than a decade. We have investigated the off-time distribution of a semiconductor QD on the basis of an Auger-induced release process of an electron deeply trapped in the QD shell. This release process has not yet been treated in the literature explicitly and starts with the optical generation of an additional electron-hole pair in the off-state of a QD, characterized by a valence band hole in the core and a trapped electron in the shell. This additional pair subsequently recombines and the recombination energy is transferred by an Auger process to the trapped electron. We discuss the efficiency of the release process as compared to the quenching process. For a deep trap occupation density (r₀ is the trap distance from the QD center) and a Förster-like release rate, we arrive at an off-time distribution with α=3/2 in agreement with experimental findings in many QDs.     

Sizes and fluorescence of cadmium sulfide quantum dots

Cadmium sulfide quantum dots have been synthesized by wet chemical deposition from an aqueous solution. The sizes of the quantum dots determined by dynamic light scattering directly in the colloidal solution and by intermittent-contact atomic force microscopy in the dry sediment agree with each other. It has been found that splitting of the fluorescence peaks of the quantum dots can be affected by the disorder of the atomic structure of cadmium sulfide quantum dots.     

Statistical model of fluorescence blinking

Blinking of single molecules and nanocrystals is modeled as a two-state renewal process with on (fluorescent) and off (non-fluorescent) states. The on and off-times may have power-law or exponential distributions. A fractional generalization of the exponential function is used to develop a unified treatment of the blinking statistics for both types of distributions. In the framework of the two-state model, an equation for the probability density p(t _on|t) of the total on-time is derived. As applied to power-law blinking, the equation contains derivatives of fractional orders α and β equal to the exponents of the on and off-time power-law distributions, respectively. In the limit case of α = β = 1, the distributions become exponential and the fractional differential equation reduces to an integer order differential equation. Solutions to these equations are expressed in terms of fractional stable distributions. The Poisson transform of p(t _on|t) is the photon number distribution that determines the photon counting statistics. It is shown that the long-time asymptotic behavior of Mandel’s Q parameter follows a power law: M(t) ∝ t ^γ . The function γ(α, β) is defined on the (α, β) plane. An analysis of the relative variance of the total on-time shows that it decays only when α = β = 1 or α < β. Otherwise, relative fluctuations either exhibit asymptotic power-law growth or approach a constant level. Analytical calculations are in good agreement with the results of Monte Carlo simulations.     

Millisecond fluorescence in InAs quantum dots embedded in AlAs

The temperature dependence of steady-state and time-resolved photoluminescence from self-assembled InAs quantum dots embedded in AlAs has been studied. Millisecond-long nonexponential photoluminescence decay is observed in the temperature range of 4.2–50 K. At higher temperatures, the decay time decreases to a few nanoseconds. The experimental results are interpreted using a model of singlet–triplet splitting of exciton levels in small dots in a dense quantum dot system with local carrier transfer between dots.     

Probing the dynamic fluorescence properties of single water-soluble quantum dots

The influence of surface modifications, including carboxyl-, amine-, and mercaptoundecanoicacid-functionalized chemistries, on the dynamic fluorescence properties of single quantum dots (QDs) are probed using a combined confocal fluorescence microscopy and spectroscopy system. Water-soluble QDs with carboxyl- or amine-functionalized surfaces prepared through a linker arm show significant fluorescence intermittency and appear statistically identical. In contrast, water-soluble QDs functionalized via mercaptoundecanoicacid, an electron-donating thiol group, demonstrate suppressed fluorescence intermittency suggesting the photostability of water-soluble QDs is strongly dependant upon the method of surface functionalization. In addition, the long-term photostability of the mercaptoundecanoicacid-functionalized QD demonstrates an excitation-time-dependent blue-shift in the fluorescence emission spectra. Our measurement and analysis techniques promise a wealth of insight in elucidating the photophysics of bio-conjugated QDs and assessing and designing bio-conjugated QDs for biomedical applications requiring long-term photostabililty.     

Zebrafish imaging and two-photon fluorescence imaging using ZnSe quantum dots

This study is to report a ZnSe quantum dot with a large two-photon absorption cross section and good biocompatibility,which can be used in bioimaging.Fluorescence emission at 410 nm is observed in the quantum dot under 760-nm laser excitation.These biocompatible quantum dots exhibit a two-photon cross-section of 9.1×10~5 GM(1 GM=10~(-50) cm~4·s/photon).Two-photon excited laser scanning microscopic images show that cells co-cultured with ZnSe quantum dots are found in the blue channel at a fluorescence intensity that is 14.5 times that of control cells not cocultured with quantum dots.After incubating zebrafish larvae with ZnSe quantum dots for 24 h,the fluorescence intensity of the yolk sac stimulated by ultraviolet light is 2.9 times that of the control group.The proposed material shows a great potential application in biological imaging.     

Resonance fluorescence in transport through quantum dots: noise properties

We study a two-level quantum dot embedded in a phonon bath and irradiated by a time-dependent ac field, and develop a method that allows us to extract simultaneously the full counting statistics of the electronic tunneling and relaxation (by phononic emission) events as well as their correlation. We find that the quantum noise of both the transmitted electrons and the emitted phonons can be controlled by the manipulation of external parameters such as the driving field intensity or the bias voltage.     

Concentration quenching of fluorescence of colloid quantum dots of cadmium sulfide

The influence of the concentration of cadmium sulfide nanoparticles in a colloid aqueous solution on its optical properties has been investigated. The band gap of cadmium sulfide nanoparticles is calculated based on the optical absorption spectra of solutions with different concentrations. The band gap coincides for all the samples irrespective of their concentration and is equal to 2.63 eV. It is found that the intensity of fluorescence substantially depends on the solution concentration. The fluorescence intensity considerably decreases in a wavelength range from 665 to 720 nm as the concentration increases above 3.25 mM. This concentration is the threshold of the concentration quenching for CdS nanoparticles.     

石墨烯及其掺杂量子点的荧光性能研究

采用基于密度泛函理论(DFT)的平面波超软赝势法,研究了石墨烯量子点的本征结构、边缘位置掺B、中间位置掺B、边缘位置掺N、中间位置掺N五种结构的电子结构和光学性质.结果表明,石墨烯量子点的激发能吸收主体是基质,杂质原子的掺入会导致禁带宽度变窄,费米能级升高,发射光谱红移,其中给电子体杂质会导致LUMO能级和HOMO能级升高,受电子体杂质会导致LUMO能级升高,HOMO能级降低.中间位置掺杂相对于边缘位置掺杂红移更明显.     

Resonance fluorescence from semiconductor quantum dots: beyond the Mollow triplet

We show that the resonance fluorescence spectrum of a quantum dot excited by a strong optical pulse contains multiple peaks beyond those of the Mollow triplet. We show that as the area of the optical pulse is increased, new side peaks split off the central peak and shift in frequency. A simple analytical theory has been derived, which quantitatively accounts for the appearance and position of the peaks. This theory explains the physics responsible for the multiple peaks. By considering the time-dependent spectrum we demonstrate a time ordering of the side peaks, which is further evidence for the suggested physical explanation.     

Cytotoxicity and fluorescence studies of silica-coated CdSe quantum dots for bioimaging applications

The toxicological effects of silica-coated CdSe quantum dots (QDs) were investigated systematically on human cervical cancer cell line. Trioctylphosphine oxide capped CdSe QDs were synthesized and rendered water soluble by overcoating with silica, using aminopropyl silane as silica precursor. The cytotoxicity studies were conducted by exposing cells to freshly synthesized QDs as a function of time (0–72 h) and concentration up to micromolar level by Lactate dehydrogenase assay, MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide] assay, Neutral red cell viability assay, Trypan blue dye exclusion method and morphological examination of cells using phase contrast microscope. The in vitro analysis results showed that the silica-coated CdSe QDs were nontoxic even at higher loadings. Subsequently the in vivo fluorescence was also demonstrated by intravenous administration of the QDs in Swiss albino mice. The fluorescence images in the cryosections of tissues depicted strong luminescence property of silica-coated QDs under biological conditions. These results confirmed the role of these luminescent materials in biological labeling and imaging applications.     

Mechanism of fluorescence energy transfer in aqueous solution cadmium sulfide quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) prepared by a convenient chemical method have been characterized using absorption, fluorescence, and photoluminescence excitation techniques. The photoluminescence excitation studies show that there is an electron transfer from the surface adsorbate (thiourea) to CdS QDs in aqueous solution. The excitation band with peak maximum at 5.8 eV is assigned to the electronic transitions in the chemisorbed thiourea, whereas the excitation band between 3.45 and 3.7 eV corresponds to the band-to-band transition within the nanocrystalline CdS host. The absorption spectroscopy of the CdS QD solutions shows a strong absorption peak which is generated from thiourea. The band-edge fluorescence of the CdS QDs has also been investigated. It is shown that the fluorescence property of the CdS QDs can be enhanced by adding cadmium chloride (CdCl₂) solution.     

Silicon quantum dots delivered phthalocyanine for fluorescence guided photodynamic therapy of tumor

Imaging-guided cancer therapy provides a simultaneous tumor imaging and treatment, which helps to eliminate the excessive toxicity to the healthy tissues. For this purpose, multifunctional probes capable of both imaging and curing are needed. In this work, we synthesize water-soluble silicon quantum dots(Si QDs) smaller than 5 nm. Such Si QDs are used for delivering the hydrophobic drug phthalocyanine(Pc). The as-prepared Si/Pc nanocomposite particles show efficient transmembrane delivery into cells and feasible biocompatibility. Moreover, these composite particles emit dualchannel fluorescence signals even after cellular internalization and demonstrate robust photostability in the Si channel.More interestingly, the Si/Pc composite particles show efficient photodynamic therapy effects against tumors both in vitro and in vivo.     

Correlation between fluorescence intermittency and spectral diffusion in single semiconductor quantum dots

We find a correlation between the dynamics of fluorescence intermittency and spectral diffusion in the spectroscopy of single CdSe nanocrystal quantum dots (QD). A statistical analysis of the data suggests two populations of blinking events: blinking followed by large spectral diffusion shifts and blinking with small or no spectral shifts. Although unexpected from earlier studies, the correlation between blinking and spectral shifting is consistent with a model of QD ionization as the mechanism for the blinking event, followed by a redistribution of local electric fields that results in spectral shifting.     

CdSe quantum dots decorated by mercaptosuccinic acid as fluorescence probe for Cu

Water-soluble CdSe quantum dots (QDs) were synthesized using mercaptosuccinic acid (MSA) as a stabilizer. The growth process and characterization of CdSe quantum dots were determined by transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, Ultraviolet-visible (UV-vis) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. Results demonstrated the MSA-capped CdSe QDs were highly crystalline and possessed good optical properties. Further, the resulting products could be used as fluorescent probes to detect Cu²⁺ ions in physiological buffer solution. The response was linearly proportional to the concentration of Cu²⁺ ion in the range 2×10⁻⁸- 3.5×10⁻⁷ mol L⁻¹ with a detection limit of 3.4 nmol L⁻¹.     

Investigation of ultrafast dynamics of CdTe quantum dots by femtosecond fluorescence up-conversion spectroscopy

The ultrafast carrier relaxation processes in CdTe quantum dots are investigated by femtosecond fluorescence upconversion spectroscopy.Photo-excited hole relaxing to the edge of the forbidden gap takes a maximal time of ～ 1.6 ps with exciting at 400 nm,depending on the state of the photo-excited hole.The shallow trapped states and deep trap states in the forbidden gap are confirmed for CdTe quantum dots.In addition,Auger relaxation of trapped carriers is observed to occur with a time constant of ～ 5 ps.A schematic model of photodynamics is established based on the results of the spectroscopy studies.Our work demonstrates that femtosecond fluorescence up-conversion spectroscopy is a suitable and effective tool in studying the transportation and conversion dynamics of photon energy in a nanosystem.