We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.

The two tautomeric forms of 2-hydroxypyridine (2-HP) have been studied in a supersonic jet expansion. Time-of-flight mass spectroscopy (TOFMS) and emission spectroscopy of the lactim and lactam tautomers have been studied and are reported here. The lactim spectrum is similar to an earlier TOFMS spectrum and has its origin at 36 136 cm'1 11. Evidence of mixing of the * and * electronic states is seen in the lactim spectrum. The mixing is removed in the disolvate water cluster but not in monosolvate clusters of ammonia or water. The lactam is shown to be nonplanar giving rise to two origins in the excitation spectrum at 29 832 and 29 935 cm'1. The ammonia and water lactim cluster spectra show significant shifts to the red while the cluster spectra of the lactam show large shifts to the blue. Experimental evidence for strong hydrogen bonding in these clusters is discussed. Intramolecular and intermolecular proton transfer in 2-HP and its clusters is discussed in the context of these data.

In this paper we critically examine the literature and provide new data on fundamental optical properties of II–VI quantum dots (QDs). Specifically, we examine the integrated extinction coefficients and radiative lifetimes of different sizes of CdSe and CdTe QDs and different shell thicknesses in CdTe/CdSe core/shell QDs. We have synthesized particles having very high quantum yields and find that the measured radiative lifetimes are considerably longer and have a very different size dependence than what has been previously reported. In a simple two-level system the integrated extinction coefficients (or oscillator strengths) are related to the radiative lifetimes through the Einstein relations. The situation is more complicated in the case of II–VI QDs because of the thermal accessibility of dark states resulting from the valence band fine structure. There are significant but not equal populations in both bright and dark sublevels of the 1Se-1S3/2 exciton and in the dark 1Se-1P3/2 exciton. These Boltzmann populations depend on the QD size and shape. We find that in all three cases, quantitative or semiquantitative agreement between the measured radiative lifetimes and values calculated from the integrated extinction coefficients is obtained only if Boltzmann populations in all of the thermally accessible bright and dark states are considered. We also find that the shell thickness dependence of the radiative lifetimes of the CdTe/CdSe core/shell particles can be quantitatively understood in terms of overlap of calculated electron and hole wave functions. The results and analyses presented here clarify several discrepancies in the literature.

The spectroscopy and excited state proton transfer (ESPT) dynamics of 4′-N,N-dimethylamino-3HF (I) and 4′-N,N-diethylamino-3HF (II) have been studied in acetonitrile/benzene solvent mixtures. Solvent composition-dependent spectral shifts are observed and can be understood in terms of an Onsager cavity model. Analysis of these spectral shifts accurately predicts solvent composition-dependent excited state equilibrium constants, which are also experimentally determined. The ESPT rates are analyzed within the framework of a transition state theory treatment of solvent polarization-mediated proton transfer. This treatment is analogous to electron transfer theory. In this treatment, the energetics of the transition state are largely determined by known solvent properties and the solvent-dependent spectroscopy. This analysis yields solvent-dependent ESPT activation energies. The corresponding calculated ESPT rates are in excellent agreement with the experimentally determined rates.

The dynamics of the CdSe nanorod synthesis reaction have been studied, giving attention to the kinetics of magic-sized clusters (MSCs) that form as intermediates in the overall reaction. The MSCs have a distinct absorption peak, and the kinetics of this peak give insight into the overall reaction mechanism. In these studies, the reaction mixture consists primarily of Cd(phosphonate)(2) and trioctyl phosphine selenium in a solution of trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO). We find that the rate at which precursors react to form CdSe monomers and the rates at which monomers react to form nanoparticles can be varied by changing the chemistry of the reaction mixture. Decreasing the TOP concentration decreases the extent to which selenium is bound, both in the precursors and on the particles' surfaces, and thereby increases both the precursor to monomer and monomer to particle reaction rates. Decreasing the phosphonate concentration decreases the extent to which phosphonate binds cadmium in the precursors and on the surface of the nanoparticles, also increasing the rates of both reactions. This is also accomplished by the addition of inorganic acids which protonate the phosphonates. The presence of inorganic acids (impurities) is the primary reason that the overall synthesis reaction is faster in solutions made with technical grade rather than purified TOPO. The TOP and phosphonic acid concentrations are coupled because excess phosphonic acids react with TOP, forming TOPO and less strongly binding species, specifically phosphinic acids, phosphine oxides, and phosphines.

The synthesis and characterization of 3.5, 4.5, and 8 nm diameter MoS 2 nanoclusters are described. Each size is characterized by TEM images, electron diffraction, and optical spectroscopy. These nanoclusters can be synthesized directly or may be synthesized from samples of polydisperse nanoclusters. Obtaining larger or smaller discrete sizes from polydisperse samples indicates that these nanocluster sizes are thermodynamically favored, that is, these sizes correspond to free-energy minima. Electron diffraction and static spectroscopic results indicate that the particles are three atoms thick, specifically, single S-Mo-S trilayer disks. Static and time-resolved absorption and emission spectroscopy were used to determine the polarizations of the lowest energy allowed transitions in each case. We find that the 8 nm particles have a lowest energy absorption maximum at 473 nm that has mixed xy and z polarizations, 4.5 nm particles have absorption maxima at 400 and 440 nm that are xy and z polarized, respectively, and 3.5 nm particles have an absorption maximum at 362 nm that is z polarized. These results indicate that the polarization of the lowest observed transition is size-dependent and that the ordering of the lowest excited states changes as the particle size decreases.

The origin of the ubiquitous low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature is usually assigned as a "surface optical" (SO) phonon, but it is only slightly affected by modifying the surface through exchanging ligands or adding a semiconductor shell. Here we present excitation profile data showing that the lowfrequency shoulder loses intensity as the excitation is tuned to longer wavelengths, closer to resonance with the lowest-energy 1S e À1S 3/2 excitonic transition. Calculations of the resonance Raman spectra are carried out using a fully atomistic model with an empirical force field to calculate the phonon modes and the standard effective mass approximation envelope function model to calculate the electron and hole wave functions.When a force field of the Tersoff type is used, the calculated spectra closely resemble the experimental ones in showing mainly the higher-frequency LO phonon with 1S e À1S 3/2 resonance but showing intensity in lower-frequency features with 1P e À1P 3/2 resonance. These calculations indicate that the main LO phonon peak involves largely motion of the interior atoms, while the low-frequency shoulder is more equally distributed throughout the crystal but not surface-localized. Interestingly, very different results are obtained with the widely used Coulomb plus Lennard-Jones force field developed by Rabani, which predicts far more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.

Transient absorption spectroscopy has been used to study the rates of electron transfer (ET) from CdSe and CdSe/ZnS core/shell nanorods to adsorbed methyl viologen, MV 2þ . The nanorods are excited with 387 nm light, producing electrons 7700 cm -1 above the conduction band edge. Kinetics are measured in particles without adsorbed MV 2þ , giving electron cooling and electron-hole recombination times. The kinetics obtained with and without adsorbed MV 2þ are compared to infer the ET rates. The results indicate that electron cooling occurs on the 0.7-1.8 ps time scale, with the fastest cooling occurring from the highest energy states. Hot electron transfer from the highest energy levels competes with electron cooling, occurring on the 0.5 ps time scale. Bare particle (relaxed) electron transfer occurs on the time scale of less than or about 4 ps. This is faster than biexciton Auger recombination which occurs on the 50 ps time scale. The energy dependence of the ET times can be semiquantitatively understood in terms of penetration of the conduction band wave function past the particle surface and overlap with the adsorbed MV 2þ . In CdSe/ZnS particles, ET to adsorbed MV 2þ is slower than electron cooling, and hot electron transfer does not occur. For a 1.0 nm thick ZnS shell, the ET from the bottom of the conduction band occurs on a range of time scales, with the fastest component of about 45 ps.