The creation of a single electron-hole pair (i.e., exciton) per incident photon is a fundamental limitation for current optoelectronic devices including photodetectors and photovoltaic cells. The prospect of multiple exciton generation per incident photon is of great interest to fundamental science and the improvement of solar cell technology. Multiple exciton generation is known to occur in semiconductor nanostructures with increased efficiency and reduced threshold energy compared to their bulk counterparts. Here we report a significant enhancement of multiple exciton generation in PbSe quasi-one-dimensional semiconductors (nanorods) over zero-dimensional nanostructures (nanocrystals), characterized by a 2-fold increase in efficiency and reduction of the threshold energy to (2.23 ± 0.03)E(g), which approaches the theoretical limit of 2E(g). Photovoltaic cells based on PbSe nanorods are capable of improved power conversion efficiencies, in particular when operated in conjunction with solar concentrators.

A series of gold nanoclusters stabilized by ligands containing short ethylene oxide oligomers of fixed length were prepared and characterized. The thiols CH3(OCH2CH2) n SH (where n = 2, 3, and 4) were substituted onto the surface of 1.8-nm hexanethiol-capped gold clusters by a thiol-exchange reaction, and the resulting nanoclusters were characterized by NMR, FTIR, and UV/vis spectroscopies; TGA; and TEM analysis. A degree of ligand exchange greater than 99% was achieved, and the gold core diameter remained unchanged in the final material. Of particular interest, the cluster with n = 2 was water-insoluble, whereas those with n = 3 or 4 were water-soluble. The thin-film electrical conductivities of these clusters were compared with those of alkanethiol-capped clusters of similar chain lengths and found to be roughly 1 order of magnitude greater. In a chemical vapor sensor configuration, this series of clusters displayed strong electrical responses that showed a correlation between the length of the ethylene oxide ligand and the polarity of the vapor.

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination. This Voc of 692 ± 7 mV for 1.4 eV PbS QDs is a result of improved passivation of the defective QD surface, demonstrating as a function of the QD bandgap (Eg). Comparing experimental Voc variation with the theoretical upper-limit obtained from one diode modeling of the cells with different Eg, these results clearly demonstrate that there is a tremendous opportunity for improvement of Voc to values greater than 1 V by using smaller QDs in QD solar cells.

A series of primary, secondary, and tertiary amines have been examined as cosolvents with trioctylphosphine oxide (TOPO) for the synthesis of CdSe nanoclusters. Syntheses were conducted in 66 mol % hexadecylamine (HDA), dodecylamine (DDA), dioctylamine (DOA), or trioctylamine (TOA) in TOPO and the growth rates and size distributions of the resulting products compared with those obtained from the same reaction conducted in pure TOPO. DOA was found to advantageously slow the growth rate of the nanoclusters and produce material with a narrow size distribution and moderate fluorescence quantum yield. Thermal gravimetric analysis (TGA) of the products has been used to quantify the ratio of TOPO to amine on the nanocluster surface and shows that the primary amines attain the highest packing densities. STM imaging of materials synthesized in the presence of HDA or DOA and then self-assembled on nonanedithiol SAMs shows improved surface stability for the DOA synthesized products.

Over the past decade, there has been a dramatic increase of interest in both the preparation and properties of nanocrystalline materials. This interest has been fueled by the unique properties that such materials possess when compared to bulk phases, 1 as well as the potential they hold for such varied applications as electronics, 2 catalysis, 3 and biological labeling. 4 The majority of the work in this field has focused on transition metal and semiconductor particles, with particular emphasis on gold 5 and II-VI compounds such as CdSe. 6 Although the formation of colloidal silicon 7 and germanium 8 nanoclusters has been studied, little work has been directed toward examining the preparation and fundamental properties of nanocrystalline main-group metals.Recent theoretical studies suggest that bismuth materials of reduced dimensions may exhibit enhanced thermoelectric properties at room temperature. 9 Quantum confinement has already been exploited to increase the thermoelectric figure of merit, ZT, for PbTe 10 quantum well superlattices and an even larger thermoelectric effect might be achieved with bismuth under such dimensionally restricted conditions. 9a The focus thus far has been on making these measurements on bismuth nanowires, 11 which possess diameters of 13-110 nm and lengths on the order of 10 µm.Despite the enormous potential of this material for thermoelectric applications, little work has been directed toward the solution synthesis of nanocrystalline bismuth clusters. To the best of our knowledge, only one report exists of such material being prepared as a stable colloid, 12 and these clusters were prepared at very low concentrations in an aqueous polymer, making the isolation of significant quantities of such material difficult. In addition, the size of the nanocrystals was determined to be 8 or 12 nm, sizes which are large when compared to the smallest diameters of noble metal nanoclusters. 5 Very recently, the synthesis of nanocrystalline bismuth using an in situ polymerization process was reported; 13 however, the resulting particles were even larger (20 nm) and were also suspended in a polymer matrix. Although this polymer coating protects the particles from oxidation, it once again makes the further manipulation and characterization of the product difficult. The focus of our effort has been the isolation of macroscopic quantities of bismuth nanocrystals with diameters below 10 nm that could be obtained in a more easily manipulated form.A variety of chemical methods have been reported in the literature for the preparation of noble metal nanoparticles, such as sonochemical reduction, 14 reduction in the presence of capping agents, 15 and reduction in inverse micelles. 16 We have recently examined a variety of reactions, in an attempt to prepare nanocrystalline bismuth clusters, 17 and have found difficulty in identifying a suitable capping agent. Traditional choices such as alkanethiols and TOPO have failed to provide proper control over both the nucleation and growth in our experiments with ...

We report the first synthesis and characterization of nanocrystalline Bi2Te3 with particle sizes less than 10 nm. The reaction of BiOClO4·xH2O with (Me3Si)2Te in a reverse micelle formed by sodium dioctylsulfosuccinate and water in hexane has produced good yields (71%) of a black powder that has been characterized by elemental analysis, X-ray diffraction, and transmission electron microscopy.

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We investigate the fine structure of band-edge excitons in PbSe nanocrystals and nanorods using circularly polarized magnetophotoluminescence and optically detected magnetic resonance and, based on the results, propose a singlet-triplet model of exciton photoluminescence from nondegenerate conduction and valence bands. From the data and model we extract g-factors for electrons and holes of +1.2 and +0.8, respectively. The splitting of the triplet ground state, which is responsible for the low-temperature photoluminescence, is 88 eV for nanorods, and less than 20 eV for nanocrystals. The intervalley splitting of the electron and hole levels in the nanocrystals is much larger than the electron-hole exchange interaction.