Synthesis CdSe(x)S(1-x) core/shell type quantum dots via one injection method

The photoluminescence quantum yield (PL-QY) of ternary colloidal CdSe(x)S(1-x) quantum dots (QDs), which were prepared by a one-injection method, enhances with increasing S content. The possible enhancement mechanism was explored by structural analysis via X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Both found that the enhancement of PL-QY of ternary CdSe(x)S(1-x) QDs strongly correlated with self-formed core/shell conformation in the non-coordination solution.     

Luminescence quenching in self-assembled adducts of [Ru(dpp)3]2+ complexes and CdTe nanocrystals

We have investigated chloroform solutions containing tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) and CdTe nanocrystal quantum dots (5.6 nm diameter). The electronic levels of these two components are such that the Ru complex can act as an energy donor towards the quantum dot, which can thus behave as an energy acceptor. Steady-state and time-resolved spectroscopic experiments indicate that the Ru complexes and the CdTe nanocrystals self-assemble to give stable 1?:?1 adducts, in which the luminescence of the former units is strongly quenched. Such a quenching can be ascribed to either energy transfer to the CdTe quantum dot, or to electron transfer from the CdTe valence band to the excited Ru complex. However, no supporting evidence for the occurrence of photoinduced energy transfer in the adduct could be found. The CdTe luminescence is also slightly quenched in the presence of the ruthenium complex. The strong association of the metal complexes with the nanocrystals suggests that self-assembly strategies may be effectively employed to achieve surface functionalization of semiconductor quantum dots with molecular units.     

Electronic absorption spectroscopy of cobalt ions in diluted magnetic semiconductor quantum dots: demonstration of an isocrystalline core/shell synthetic method.

This paper reports the application of ligand-field electronic absorption spectroscopy to probe Co(2+) dopant ions in diluted magnetic semiconductor quantum dots. It is found that standard inverted micelle coprecipitation methods for preparing Co(2+)-doped CdS (Co(2+):CdS) quantum dots yield dopant ions predominantly bound to the nanocrystal surfaces. These Co(2+):CdS nanocrystals are unstable with respect to solvation of surface-bound Co(2+), and time-dependent absorption measurements allow identification of two transient surface-bound intermediates involving solvent-cobalt coordination. Comparison with Co(2+):ZnS quantum dots prepared by the same methods, which show nearly isotropic dopant distribution, indicates that the large mismatch between the ionic radii of Co(2+) (0.74 A) and Cd(2+) (0.97 A) is responsible for exclusion of Co(2+) ions during CdS nanocrystal growth. An isocrystalline core/shell preparative method is developed that allows synthesis of internally doped Co(2+):CdS quantum dots through encapsulation of surface-bound ions beneath additional layers of CdS.     

Optical properties of Ag/CdTe nanocomposite self-organized by electrostatic interaction

Ag/CdTe nanocomposite was prepared via self-organization process by electrostatic interaction between positively charged CdTe quantum dots and negatively charged Ag nanoparticles and examined with respect to their optical properties. The positively charged CdTe quantum dots and negatively charged Ag nanoparticles were synthesized separately by modifying nanoparticles surface with cationic and anionic thiol compounds, respectively. The result showed that the mixing ratio of Ag nanoparticles to CdTe quantum dots is an important parameter for controlling resulting composites. The resulting solution is optically transparent if one component is in excess. Photoluminescence of CdTe quantum dots undergoes considerably quenching if CdTe nanocrystals are in excess and SERS spectra of BVPP absorbed on Ag colloid became stronger if Ag nanoparticles are in excess. Nevertheless, while the ratio is approximately 1, micrometer-sized solid composite is obtained with the elapse of 1h after mixing. SERS spectra for solid composite only exhibit the signals of the CdS nanocrystal which reflected that prolonged refluxing during the synthesis leads to a partial hydrolysis of the thiols and to the incorporation of the sulfur from the thiol molecules into the the growing nanoparticles to form mixed CdTe(S) nanocrystal, similar to CdTe/CdS core/shell structure. From the results, we conclude that optical properties of Ag/CdTe are dependent on the mixing ratio of both nanoparticles.     

Colloidal CdSe quantum dot electroluminescence: ligands and light-emitting diodes

We examine the effects of surface ligand exchange on the performance of hybrid organic/inorganic light emitting diodes (LEDs) that use colloidal nanocrystal quantum dots as emissive centers. Using a series of primary alkylamines with different alkane chain lengths, we exchange the native surface ligands on a series of CdSe/CdZnS/ZnS core/shell/shell nanocrystal quantum dots and compare the differences in photoluminescence and electroluminescence efficiency of the emissive quantum dot layer. We fabricate LEDs made with octadecylamine-, octylamine-, and butylamine-exchanged quantum dots. We find that the differences in electroluminescence efficiency of the devices are not always proportional to the photoluminescence quantum efficiency of the quantum dots. We discuss this trend both in terms of the competing needs of high photoluminescence efficiency and good charge injection and energy transfer, and also in terms of the different processability and film morphology arising from the use of nanoparticles passivated with shorter ligands. Correspondence: David S. Ginger, Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA     

Large scale synthesis of stable tricolor Zn(1-x)Cd(x)Se core/multishell nanocrystals via a facile phosphine-free colloidal method

Here we report a new "green" method to synthesize Zn(1-x)Cd(x)Se (x = 0-1) and stable red-green-blue tricolor Zn(1-x)Cd(x)Se core/shell nanocrystals using only low cost, phosphine-free and environmentally friendly reagents. The first excitonic absorption peak and photoluminescence (PL) position of the Zn(1-x)Cd(x)Se nanocrystals (the value of x is in the range 0.005-0.2) can be fixed to any position in the range 456-540 nm. There is no red or blue shift in the entire reaction process. Three similar sizes of alloyed Zn(1-x)Cd(x)Se nanocrystals with blue, green, and yellow emissions were successfully selected as cores to synthesize high quality blue, green, and red core/shell nanocrystal emitters. For the synthesis of core/shell nanocrystals with a high quantum yield (QY) and stability, the selection of shell materials has been proven to be very important. Therefore, alternative protocols have been used to optimize thick shell growth. ZnSe/ZnSe(x)S(1-x) and CdS/Zn(1-x)Cd(x)S have been found as an excellent middle multishell to overcoat between the alloyed Zn(1-x)Cd(x)Se core and ZnS outshell. The QYs of the as-synthesized core/shell alloyed Zn(1-x)Cd(x)Se nanocrystals can reach 40-75%. The Cd content is reduced to less than 0.1% for Zn(1 -x)Cd(x)Se core/shell nanocrystals with emissions in the range 456-540 nm. More than 15 g of high quality Zn(1-x)Cd(x)Se core/shell nanocrystals were prepared successfully in a large scale, one-pot reaction. Importantly, the emissions of such thick multishell nanocrystals are not susceptible to ligand loss and stability in various physiological conditions.     

A facile approach to synthesize high-quality Zn(x)Cu(y)InS(1.5+x+0.5y) nanocrystal emitters

A facile approach to synthesize Zn(x)Cu(y)InS(1.5+x+0.5y) nanocrystal emitters was presented. The compositions of these nanocrystals were precisely controlled, and the relative PL quantum yields were up to 40%, with tunable emissions in 450-640 nm.     

Magnetic fluorescent delivery vehicle using uniform mesoporous silica spheres embedded with monodisperse magnetic and semiconductor nanocrystals

We synthesized uniform pore-sized mesoporous silica spheres embedded with magnetite nanocrystal and quantum dots. The magnetic separation, luminescent detection, and controlled release of drugs were demonstrated using the uniform mesoporous silica spheres embedded with monodisperse nanocrystals.     

Designer variable repeat length polypeptides as scaffolds for surface immobilization of quantum dots

We demonstrate the use of a series of engineered, variable-length de novo polypeptides to discretely immobilize luminescent semiconductor nanocrystals or quantum dots (QDs) onto functional surfaces. The polypeptides express N-terminal dicysteine and C-terminal hexahistidine residues that flank a variable number (1, 3, 5, 7, 14, 21, 28, or 35) of core beta-strand repeats, with tyrosine, glutamic acid, histidine, and lysine residues located at the turns. Polypeptides have molecular weights ranging from 4 to 83 kDa and retain a rigid structure based on the antiparallel beta-sheet motif. We first use a series of dye-labeled polypeptides to test and characterize their self-assembly onto hydrophilic CdSe-ZnS QDs using fluorescence resonance energy transfer (FRET). Results indicate that peptides maintain their beta-sheet conformation after self-assembly onto the QD surfaces, regardless of their length. We then immobilize biotinylated derivatives of these polypeptides on a NeutrAvidin-functionalized substrate and use them to capture QDs via specific interactions between the peptides' polyhistidine residues and the nanocrystal surface. We found that each of the polypeptides was able to efficiently capture QDs, with a clear correlation between the density of the surface-tethered peptide and the capacity for nanocrystal capture. The versatility of this capture strategy is highlighted by the creation of a variety of one- and two-dimensional polypeptide-QD structures as well as a self-assembled surface-immobilized FRET-based nutrient sensor.     

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.     

Type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures

Type-II band engineered quantum dots (CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures) are described. The optical properties of these type-II quantum dots are studied in parallel with their type-I counterparts. We demonstrate that the spatial distribution of carriers can be controlled within the type-II quantum dots, which makes their properties strongly governed by the band offset of the comprising materials. This allows access to optical transition energies that are not restricted to band gap energies. The type-II quantum dots reported here can emit at lower energies than the band gaps of comprising materials. The type-II emission can be tailored by the shell thickness as well as the core size. The enhanced control over carrier distribution afforded by these type-II materials may prove useful for many applications, such as photovoltaics and photoconduction devices.     

Self Assembly and Electronic Structure of ZnO Nanocrystals

In this paper, we report the synthesis and self assembly of various sizes of ZnO nanocrystals. While the crystal structure and the quantum confinement of nanocrystals were mainly characterized using XRD and UV absorption spectra, the self assembly and long range ordering were studied using scanning tunneling microscopy after spin casting the nanocrystal film on the highly oriented pyrolytic graphite surface. We observe self assembly of these nanocrystals over large areas making them ideal candidates for various potential applications. Further, the electronic structure of the individual dots is obtained from the current–voltage characteristics of the dots using scanning tunneling spectroscopy and compared with the density of states obtained from the tight binding calculations. We observe an excellent agreement with the experimentally obtained local density of states and the theoretically calculated density of states. We dedicate this work to Professor C. N. R. Rao on the occasion of his 75th birthday.     

Crystalline superlattices from single-sized quantum dots

Despite the recent progress toward the synthesis of monodisperse semiconducting nanocrystals, it remains a challenge to prepare quantum dot structures with a precise number of atoms. Here, we report synthesis, crystal structure, and optical properties of a family of cadmium sulfide nanocrystal superlattices assembled through single-sized semiconducting clusters. Clusters of various sizes have been made. The largest cluster determined from single-crystal analysis has a total of 138 metal-chalcogen sites. It is the largest known single-sized II-VI quantum dot and is also the first one with more than 100 metal-chalcogen sites. X-ray powder diffraction (XRD) and optical absorption studies indicate the presence of even larger single-sized quantum dots (>200 metal-chalcogen sites). These clusters consist of cubic zinc blende-type core and hexagonal wurtzite-type corners and can exist in up to five isomeric forms that differ only in the position of the hexagonal-cubic interface.     

Exciton interactions in CdS nanocrystal aggregates in reverse micelle

Here we report the formation and spectroscopic properties of cadmium sulfide (CdS) nanocrystal systems: individual nanocrystal and CdS aggregates. The optical absorption and luminescence spectra of the aggregated CdS nanocrystals and individual nanocrystal show exciton aggregate and individual exciton characteristics. Although it is not Bose-Einstein condensation, such aggregated quantum dots (QDs) seem to supply us opportunity to study the interactions and condensation of excitons in multi-QDs system, not in the separated QDs system.     

Synthesis and size control of luminescent ZnSe nanocrystals by a microemulsion-gas contacting technique

A scalable method for controlled synthesis of luminescent compound semiconductor nanocrystals (quantum dots) using microemulsion-gas contacting at room temperature is reported. The technique exploits the dispersed phase of a microemulsion to form numerous identical nanoreactors. ZnSe quantum dots were synthesized by reacting hydrogen selenide gas with diethylzinc dissolved in the heptane nanodroplets of a microemulsion formed by self-assembly of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer in formamide. A single nanocrystal is grown in each nanodroplet, thus allowing good control of particle size by manipulation of the initial diethylzinc concentration in the heptane. The ZnSe nanocrystals exhibit size-dependent luminescence and excellent photostability.     

Cellulose nanocrystal submonolayers by spin coating

Dilute concentrations of cellulose nanocrystal solutions were spin coated onto different substrates to investigate the effect of the substrate on the nanocrystal submonolayers. Three substrates were probed: silica, titania, and amorphous cellulose. According to atomic force microscopy (AFM) images, anionic cellulose nanocrystals formed small aggregates on the anionic silica substrate, whereas a uniform two-dimensional distribution of nanocrystals was achieved on the cationic titania substrate. The uniform distribution of cellulose nanocrystal submonolayers on titania is an important factor when dimensional analysis of the nanocrystals is desired. Furthermore, the amount of nanocrystals deposited on titania was multifold in comparison to the amounts on silica, as revealed by AFM image analysis and X-ray photoelectron spectroscopy. Amorphous cellulose, the third substrate, resulted in a somewhat homogeneous distribution of the nanocrystal submonolayers, but the amounts were as low as those on the silica substrate. These differences in the cellulose nanocrystal deposition were attributed to electrostatic effects: anionic cellulose nanocrystals are adsorbed on cationic titania in addition to the normal spin coating deposition. The anionic silica surface, on the other hand, causes aggregation of the weakly anionic cellulose nanocrystals which are forced on the repulsive substrate by spin coating. The electrostatically driven adsorption also influences the film thickness of continuous ultrathin films of cellulose nanocrystals. The thicker films of charged nanocrystals on a substrate of opposite charge means that the film thickness is not independent of the substrate when spin coating cellulose nanocrystals in the ultrathin regime (<100 nm).     

Size and ligand effects on the electrochemical and spectroelectrochemical responses of CdSe nanocrystals

The electrochemical properties of CdSe quantum dots with electrochemically inactive surface ligands (TOPO) have been investigated in comparison with the analogous nanocrystals containing electrochemically active oligoaniline ligands. The TOPO-capped nanocrystals have been studied in a wide size range (from 3 to 6.5 nm) with the goal to amplify the influence of the quantum confinement effect on the electrochemical response. The determined HOMO and LUMO levels have been found in good agreement with the ones obtained from photoluminescence studies and those predicted theoretically. Ligand exchange with aniline tetramer significantly influences the voltammetric peaks associated with the HOMO oxidation and the LUMO reduction of the quantum dots, which are shifted to higher and lower potentials, respectively. These shifts are interpreted in terms of the positive ligand charging which precedes the oxidation of the nanocrystals and the insulating nature of the ligand in the case of the nanocrystal reduction. The ligand-nanocrystal interactions have also been studied by UV-Vis-NIR and Raman spectroelectrochemistry in comparison with a specially prepared model compound which, apart from the anchoring function is identical to the grafted oligoaniline ligand. Both spectroelectrochemical techniques clearly indicate the same nature of the oxidation/reduction pathway for both the model compound and the grafted ligand. The influence of the grafting is manifested by a shift in the onset of the ligand oxidation as compared to the case of the "free" model compound. Since both components (ligands and nanocrystals) mutually influence their electrochemical and spectroelectrochemical properties, the newly developed system can be considered as a true molecular hybrid. Such hybrids are of interest because the potential zone of the ligand electroactivity is well separated from that of the nanocrystals and, as a result, the organic part can be electrochemically switched between the semiconducting and the conducting states with no change in the oxidation state of the nanocrystal. The newly developed system offers therefore the possibility of an electrical addressing of individual nanocrystals via the conducting ligands.     

Solar cells from colloidal nanocrystals: Fundamentals,materials, devices,and economics

Recent advances in colloidal science are having a dramatic impact on the development of next generation low-cost and/or high-efficiency solar cells. Simple and safe solution phase syntheses that yield monodisperse, passivated, non-aggregated semiconductor nanocrystals of high optoelectronic quality have opened the door to several routes to new photovoltaic devices which are currently being explored. In one route, colloidal semiconductor nanocrystal “inks” are used primarily to lower the fabrication cost of the photoabsorbing layer of the solar cell. Nanocrystals are cast onto a substrate to form either an electronically coupled nanocrystal array or are sintered to form a bulk semiconductor layer such that the bandgap of either is optimized for the solar spectrum (1.0–1.6 eV if the photon to carrier quantum yields less than 100%). The sintered devices (and without special efforts, the nanocrystal array devices as well) are limited to power conversion efficiencies less than the Shockley–Queisser limit of 33.7% but may possibly be produced at a fraction of the manufacturing cost of an equivalent process that uses vacuum-based deposition for the absorber layer. However, some quantum confined nanocrystals display an electron-hole pair generation phenomena with greater than 100% quantum yield, called “multiple exciton generation” (MEG) or “carrier multiplication” (CM). These quantum dots are being used to develop solar cells that theoretically may exceed the Shockley–Queisser limit. The optimum bandgap for such photoabsorbers shifts to smaller energy (0.6–1.1 eV), and thus colloidal quantum dots of low bandgap materials such as PbS and PbSe have been the focus of research efforts, although multiple exciton generation has also been observed in several other systems including InAs and Si. This review focuses on the fundamental physics and chemistry of nanocrystal solar cells and on the device development efforts to utilize colloidal nanocrystals as the key component of the absorber layer in next generation solar cells. Development efforts are put into context on a quantitative and up-to-date map of solar cell cost and efficiency to clarify efforts and identify potential opportunities in light of technical limitations and recent advances in existing technology. Key nanocrystal/material selection issues are discussed, and finally, we present four grand challenges that must be addressed along the path to developing low-cost high-efficiency nanocrystal based solar cells.