The effects of chemisorption on the luminescence of CdSe quantum dots

We report on the effects of Lewis bases and other ligands on radiative recombination in CdSe quantum dots (QDs) in several solvents. Long-chain primary amines are found to be the most efficacious capping agents for CdSe QDs in nonpolar solvents. Primary alkylamines are superior to secondary and tertiary alkylamines. The kinetics of chemisorption and desorption in less polar solvents, such as hexane or chloroform, are temperature controlled and obey a Langmuir isotherm. Mercaptan adsorption also obeys a Langmuir isotherm, and alkylmercaptans rapidly displace amines, leading to luminescence quenching. In more polar solvents, such as toluene, ligands desorb, leading to luminescence quenching. It is proposed that surface Cd vacancies function as nonradiative recombination centers. The adsorption of a Lewis base to the QD raises the surface vacancy energy close to, or above, the conduction band edge and eliminates electron capture by the surface vacancies. Solvent polarity has a strong effect on luminescence since the solvent determines the extent of ligand adsorption to the QD surface.     

Subsecond luminescence intensity fluctuations of single CdSe quantum dots

Photoluminescence (PL) intermittency characteristics are examined for single quantum dots (QDs) in a CdSe QD sample synthesized at a slow rate at 75 degrees C. Although the PL quantum efficiency was relatively low ( approximately 0.25), we noticed that the PL intensity of single CdSe QDs fluctuated on a subsecond time scale with short-lived "on" and "off" states. The subsecond PL intensity fluctuations of CdSe QDs are different from "on" and "off" PL blinking generally observed for QDs fluctuating on a millisecond to minute time scale. We characterized single QDs by identifying polarized excitations, topographic imaging using atomic force microscopy (AFM), and transmission electron microscopy (TEM). From analysis of the PL intensity trajectories from >100 single CdSe QDs, the average intermittency time was 213 ms. From the PL quantum efficiency, slow growth of QDs, intensity trajectory analyses, and previous reports relating surface trap states and PL properties of QDs, we attribute the subsecond PL intensity fluctuations of single CdSe QDs and short-lived "on" and "off" states to a high-density distribution of homogeneous surface trap states.     

Functionalized CdSe quantum dots as selective silver ion chemodosimeter

CdSe quantum dots (QDs) have been prepared and modified with mercaptoacetic acid. They are water-soluble and biocompatible. To improve their fluorescence intensity and stability in water solution, bovine serum albumin (BSA) was absorbed onto their surface. Based on the quench of fluorescence signals of the functionalized CdSe QDs in the 543 nm wavelength and enhancement of them in the 570-700 nm wavelength range by Ag(I) ions at pH 5.0, a simple, rapid and specific method for Ag(I) determination was proposed. In comparison with single organic fluorophores, these nanoparticles are brighter, more stable against photobleaching, and do not suffer from blinking. Under the optimum conditions, the response is linearly proportional to the concentration of Ag(I) between 4.0 x 10(-7) and 1.5 x 10(-5) mol L(-1), and the limit of detection is 7.0 x 10(-8) mol L(-1). The mechanism of reaction is also discussed.     

Hydrophilization of CdSe quantum dots with surfactants

A procedure has been developed for hydrophilization of CdSe semiconductor quantum dots with anionic, cationic, and nonionic surfactants. Depending on the type of a surfactant, these particles may carry positive, negative, or no charges. In spite of the universality of the proposed procedure, the hydrophilization with specific surfactants must be carried out at specific concentrations of reagents and medium pH values. Under optimum conditions, the transfer of the quantum dots to water substantially increases the quantum yield of their luminescence. Conditions have been found under which the luminescence properties of the hydrophilic particles remain almost unchanged for a long time period.     

Upconversion luminescence from CdSe nanoparticles

Efficient upconversion luminescence has been observed from CdSe nanoparticles ranging in size from 2.5 to 6 nm. The upconversion luminescence exhibits a near-quadratic laser power dependence. Emissions from both excitons and trap states are observed in the upconversion and photoluminescence spectra, and in the upconversion luminescence the emission from the trap states is enhanced relative to the trap-state emission in the photoluminescence. The upconversion decay lifetimes are slightly longer than the photoluminescence decay lifetimes. Time-resolved spectral measurements indicate that this is due to the involvement of long decay components from surface or trap states. Both the photoluminescence and upconversion luminescence decrease in intensity with increasing temperature due mainly to thermal quenching. All the observations indicate that trap states work as emitters rather than as intermediate states for upconversion luminescence and that two-photon absorption is the likely excitation mechanism.     

The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution

The photoluminescence (PL) of CdSe quantum dots (QD) in aqueous media has been studied in the presence of gold nanoparticles (NP) with different shapes. The steady state PL intensity of CdSe QD (1.5-2 nm in size) is quenched in the presence of gold NP. Picosecond bleach recovery and nanosecond time-resolved luminescence measurements show a faster bleach recovery and decrease in the lifetime of the emitting states of CdSe QD in the presence of quenchers. Surfactant-capped gold nanorods (NR) with aspect ratio of 3 and surfactant-capped and citrate-capped nanospheres (NS) of 12 nm diameter were used as quenchers in order to study the effect of shape and surface charge on the quenching rates. The Stern-Volmer kinetics model is used to examine the observed quenching behavior as a function of the quencher concentration. It was found that the quenching rate of NR is more than 1000 times stronger than that of NS with the same capping material. We also found that the quenching rate decreases as the length of the NR decreases, although the overlap between the CdSe emission and the NR absorption increases. This suggests that the quenching is a result of electron transfer rather than long-range (Forster-type) energy transfer processes. The quenching was attributed to the transfer of electron with energies below the Fermi level of gold to the trap holes of CdSe QD. The observed large difference between NR and NS quenching efficiencies was attributed to the presence of the [110] facets only in the NR, which have higher surface energy.     

Computer-assisted study of silver absorption by porous silicon dioxide nanoparticles

The absorption of silver atoms by porous silicon dioxide particles is studied by the molecular dynamics method. Upon the absorption of silver atoms, (SiO₂)₅₀ nanoparticles do not increase their volume. A particle is divided into two unequal parts by an island shell formed from SiO₂ structural units, which causes anisotropy in the electrical and thermal conductivity of the nanocomposite. IR absorption and emission spectra, Raman spectra, as well as the number of electrons active with respect to IR radiation are calculated. The calculated absorption spectra show the mode corresponding to the stretching vibrations of Si-O surface groups. The addition of silver atoms to nanoparticles can enhance significantly the power of heat radiation emission.     

Arylthiolate-protected silver quantum dots

This paper describes a new, organic-soluble 4-tert-butylbenzyl mercaptan (BBT) monolayer-protected silver cluster (AgBBT MPC) as the first example of a dissolved silver nanoparticle that exhibits quantized one-electron double layer charging (QDL) voltammetry. Polydisperse AgBBT MPCs made by two different synthetic protocols, but with similar average core diameters (2.1 nm), exhibit sharply differing electrochemistry and optical absorbance spectra. A two-phase procedure (organic/aqueous, termed Prep A-AgBBT) produced MPCs exhibiting a 475 nm surface plasmon absorbance and QDL voltammetry. Neither property was seen for MPCs made by a single-phase procedure, termed Prep B-AgBBT. The difference is thought to reflect poor passivation to oxide formation in the latter Prep B procedure, which is supported by X-ray photoelectron spectroscopy results. Thermogravimetry, mass spectra, and electrochemistry results suggest an average stoichiometric formula of Ag140BBT53, but transmission electron microscopy shows that the products are also polydisperse and include polycrystalline aggregates. Dry, cast films of both Ag MPC preparations on interdigitated array electrodes exhibit low electron hopping conductivity, compared to Au MPCs.     

Probing cytotoxicity of CdSe and CdSe/CdS quantum dots

Toxicities of CdSe and CdSe/CdS quantum dots(QDs) synthesized by ultrasound-assisted methods were investigated in vitro and in vivo.Five human cell lines were used to assess the cytotoxicity of as-prepared CdSe and CdSe/CdS by assays of MTT viability,red blood cell hemolysis,flow cytometry,and fluorescence imaging.The results show that these QDs may be cytotoxic by their influence in S and G2 phases in cell cycles.The cytotoxicity of QDs depends on both the physicochemical properties and related to target cells.     

CdSe quantum dots in a columnar matrix

Cadmium selenide quantum dots (QDs) have been dispersed in a discotic liquid crystal columnar matrix. Thermophysical properties of these liquid crystal nanocomposites were investigated by UV-Vis spectroscopy, photoluminescence spectroscopy, differential scanning calorimetry, polarizing optical microscopy, DC conductivity and small angle X-ray diffraction.     

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.     

Effect of linker on the photosensitization of ZnO layers with CdSe quantum dots

In this work, zinc oxide (ZnO) nanoparticles (size <10 nm) were formed via precipitation in ethanolic solution. The zinc acetate and lithium hydroxide solutions in ethanol were mixed at 273 K temperatures under vigorous stirring. To study the effect of quantum dot (QD) coverage, we have prepared a colloidal suspension of capped CdSe QDs (size ~5 nm) by chemical route and anchored them to a nanoporous ZnO layer either by direct adsorption or through linker. Here a bifunctional molecule (mercaptopropionic acid, MPA, and thioglycolic acid, TGA) was previously adsorbed on the ZnO surface, which acted as a molecular cable. From TEM/SEM studies, it was observed that direct adsorption of CdSe QDs onto ZnO surface was not efficient. However, the bifunctional linker molecules particularly MPA facilitates binding of CdSe QDs to ZnO; and consequently, interparticle electron transfer is thus facilitated. The use of MPA linker despite of its long carbon chain also aids in the quenching of photoluminescence of CdSe on addition of ZnO in a more systematic manner indicating efficient charge transfer from CdSe into ZnO as compared with the without linker and with linker TGA case, respectively. Due to higher PL quenching and reduction in lifetime values, higher values of Stern–Volmer quenching constants were thus obtained for CdSe–ZnO composites with MPA as compared with TGA linker and without linker case, respectively. Nonlinear Stern–Volmer plots as observed for samples without linker case indicated heterogeneous quenching due to insufficient binding between CdSe QDs and ZnO. By means of spectroscopic (PL, UV–VIS, FTIR) and microscopic (TEM, SEM) techniques, we have demonstrated linker-dependent photosensitization mechanism of ZnO layers with CdSe QDs. Our data thus illustrate that interfacial-electron transfer kinetics in QD–linker–ZnO assemblies are almost independent of the length of alkyl-containing molecular linkers.     

Photoassisted synthesis of CdSe and core-shell CdSe/CdS quantum dots

This paper describes the synthesis of core-shell CdSe/CdS quantum dots (QDs) in aqueous solution by a simple photoassisted method. CdSe was prepared from cadmium nitrate and 1,1-dimethylselenourea precursors under illumination for up to 3 h using a pulsed Nd:YAG laser at 532 nm. The effects that the temperature and the laser irradiation process have on the synthesis of CdSe were monitored by a series of experiments using the precursors at a Cd:Se concentration ratio of 4. Upon increasing the temperature (80-140 degrees C), the size of the CdSe QDs increases and the time required for reaching a maximum photoluminescence (PL) is shortened. Although the as-prepared CdSe QDs possess greater quantum yields (up to 0.072%) compared to those obtained by microwave heating (0.016%), they still fluoresce only weakly. After passivation of CdSe (prepared at 80 degrees C) by CdS using thioacetamide as the S source (Se:S concentration ratio of 1) at 80 degrees C for 24 h, the quantum yield of the core-shell CdSe/CdS QDs at 603 nm is 2.4%. Under UV irradiation of CdSe/CdS for 24 h using a 100-W Hg-Xe lamp, the maximum quantum yield of the stable QDs is 60% at 589 nm. A small bandwidth (W1/2 < 35 nm) indicates the narrow size distribution of the as-prepared core-shell CdSe/CdS QDs. This simple photoassisted method also allows the preparation of differently sized (3.7-6.3-nm diameters) core-shell CdSe/CdS QDs that emit in a wide range (from green to red) when excited at 480 nm.     

Electrochemical properties of CdSe and CdTe quantum dots

Semiconductor nanocrystal quantum dots (QDs), owing to their unique opto-electronic properties determined by quantum confinement effects, have been the subject of extensive investigations in different areas of science and technology in the past two decades. The electrochemical behaviour of QDs, particularly for CdSe and CdTe nanocrystals, has also been explored, although to a lesser extent compared to the optical properties. Voltammetric measurements can be used to probe the redox levels available for the nanocrystals, which is an invaluable piece of information if these systems are involved in electron transfer processes. Electrochemical data can also foster the interpretation of the spectroscopic properties of QDs, and give insightful information on their chemical composition, dimension, and surface properties. Hence, electrochemical methods constitute in principle an effective tool to probe the quality of QD samples in terms of purity, size dispersion, and surface defects. The scope of this critical review is to discuss the results of electrochemical studies carried out on CdSe and CdTe core and core-shell semiconductor nanocrystals of spherical shape. Examples of emerging or potential applications that exploit electroactive quantum dot-based systems will also be illustrated.     

Thermostimulated formation of silver and gold nanoparticles in porous silicon dioxide matrices

Manifestations of thermostimulated formation and subsequent transformation of silver and gold nanoparticles in porous opal and Vycor glass matrices are studied using optical spectroscopy. Two temperature ranges for silver nanoparticles are revealed, where first-type particles transform into another type of particles. With gold nanoparticles in these matrices, a temperature range in which one type of particles transforms into another type is established. An effect of complete blackening of Vycor glass samples, caused by their annealing, is revealed, and a rationalization of this effect is given.     

Switching-on superparamagnetism in Mn/CdSe quantum dots

Mn ion doping of CdSe and other semimagnetic quantum dot (QDs) alloys has been an area of active speculation for over a decade. We report evidence of Mn(II) doping of CdSe grown from a cubic single source precursor that is superparamagnetic (SPM) with a blocking temperature of 40 K following thermal annealing. Prior to thermal annealing the 4 nm Mn/CdSe (1% Mn) QDs exhibit mainly paramagnetic behavior between 300 and 2 K, with a weak antiferromagnetic exchange. Following thermal annealing of the sample, high-temperature ferromagnetic exchange is observed in the magnetization data with the onset of an SPM phase at 40 K that exhibits a coercivity of 0.1 T at 2 K. The switching-on of SPM behavior is believed to be linked to ion migration with formation of (Se-Mn-Se-Mn-Se-Mn)n centers within the nanocrystal that exhibit coupled magnetic moments. Electron paramagnetic resonance (EPR) provides evidence of two distorted T(d) Mn core sites, a clustered site (dipolar broadened), and a localized Mn site (hyperfine-split). The ratio of the EPR signature for the dipolar broadened site increases following annealing and shows a hysteretic response around the blocking temperature. These observations suggest that thermal annealing results in enhanced cluster formation explaining the onset of the SPM phase in these nanoscale materials. Evidence of SPM behavior is evident in the field-dependent non-Langevin magnetization with a tangential loss in the ac-magnetic susceptibility and the Mydosh parameter (phi = 0.16).