Organic-inorganic lead-halide perovskites have been the subject of recent intense interest due to their unusually strong photovoltaic performance. A new addition to the perovskite family is all-inorganic Cs-Pb-halide perovskite nanocrystals, or quantum dots, fabricated via a moderate-temperature colloidal synthesis. While being only recently introduced to the research community, these nanomaterials have already shown promise for a range of applications from color-converting phosphors and light-emitting diodes to lasers, and even room-temperature single-photon sources. Knowledge of the optical properties of perovskite quantum dots still remains vastly incomplete. Here we apply various time-resolved spectroscopic techniques to conduct a comprehensive study of spectral and dynamical characteristics of single- and multiexciton states in CsPbX3 nanocrystals with X being either Br, I, or their mixture. Specifically, we measure exciton radiative lifetimes, absorption cross-sections, and derive the degeneracies of the band-edge electron and hole states. We also characterize the rates of intraband cooling and nonradiative Auger recombination and evaluate the strength of exciton-exciton coupling. The overall conclusion of this work is that spectroscopic properties of Cs-Pb-halide quantum dots are largely similar to those of quantum dots of more traditional semiconductors such as CdSe and PbSe. At the same time, we observe some distinctions including, for example, an appreciable effect of the halide identity on radiative lifetimes, considerably shorter biexciton Auger lifetimes, and apparent deviation of their size dependence from the "universal volume scaling" previously observed for many traditional nanocrystal systems. The high efficiency of Auger decay in perovskite quantum dots is detrimental to their prospective applications in light-emitting devices and lasers. This points toward the need for the development of approaches for effective suppression of Auger recombination in these nanomaterials, using perhaps insights gained from previous studies of II-VI nanocrystals.

The use of semiconductor nanocrystal quantum dots (QDs) in optoelectronic devices typically requires postsynthetic chemical surface treatments to enhance electronic coupling between QDs and allow for efficient charge transport in QD films. Despite their importance in solar cells and infrared (IR) light-emitting diodes and photodetectors, advances in these chemical treatments for lead chalcogenide (PbE; E = S, Se, Te) QDs have lagged behind those of, for instance, II-VI semiconductor QDs. Here, we introduce a method for fast and effective ligand exchange for PbE QDs in solution, resulting in QDs completely passivated by a wide range of small anionic ligands. Due to electrostatic stabilization, these QDs are readily dispersible in polar solvents, in which they form highly concentrated solutions that remain stable for months. QDs of all three Pb chalcogenides retain their photoluminescence, allowing for a detailed study of the effect of the surface ionic double layer on electronic passivation of QD surfaces, which we find can be explained using the hard/soft acid-base theory. Importantly, we prepare highly conductive films of PbS, PbSe, and PbTe QDs by directly casting from solution without further chemical treatment, as determined by field-effect transistor measurements. This method allows for precise control over the surface chemistry, and therefore the transport properties of deposited films. It also permits single-step deposition of films of unprecedented thickness via continuous processing techniques, as we demonstrate by preparing a dense, smooth, 5.3-μm-thick PbSe QD film via doctor-blading. As such, it offers important advantages over laborious layer-by-layer methods for solar cells and photodetectors, while opening the door to new possibilities in ionizing-radiation detectors.

Understanding the mechanism of water adsorption on silica is important in many fields of science and technology, such as geo- and atmospheric chemistry. Vibrational IR-visible sum-frequency generation (SFG) spectroscopy of hydroxyls (~3100-3800 cm(-1)) at the amorphous SiO2 surface in contact with air of varying relative humidity provides information about the adsorption sites and orientation of water molecules. The similar magnitudes of the resonant and nonresonant contributions to the interfacial second-order susceptibility, χ((2)), allow the phases of the various hydroxyls (SiOH; HOH), and thus their orientations with respect to the surface, to be determined. The surface silanols (SiOH) appear to interact weakly with adsorbed water as indicated by the persistence of the narrow surface silanol (SiOH) peak at ~3750 cm(-1) as the relative humidity of ambient air increases from <5% to >95%. Adsorbed water molecules are represented by two oppositely oriented hydroxyl modes, at ~3350-3400 and ~3650 cm(-1), respectively. The weakly hydrogen-bonded water hydroxyls (~3650 cm(-1)) are oriented toward the silica substrate and are assigned to water molecules that aggregate over the hydrophobic silica areas with exposed siloxane bridges. We believe that this is the first experimental identification of water molecules in contact with siloxane network whose hydrophobic nature has been predicted by molecular dynamics simulations for tetrahedral (SiO4) surface of kaolinite. The SFG data suggest that, at the molecular level, hydroxylated amorphous fused silica has hydrophobic character.

We describe an ultrabroadband IR-visible sum-frequency (SF) setup that allows simultaneous acquisition of the entire vibrational spectrum of water molecules at mineral surfaces in the OH stretching region without ever tuning the IR laser pulses. Our newly developed 800-nm pumped noncollinear optical parametric amplifier (NOPA) generates broadband mid-IR pulses (~1800-3500 nm, or ~2900 - 6000 cm-1) with bandwidths >600 cm-1 at half-maximum at near 3500 cm-1. Using the ultra-broadband IR NOPA, we constructed a sum-frequency vibrational spectrometer that allowed the acquisition of spectra of the OH stretches of water at hydrophilic and hydrophobic silica surfaces, over the frequency range ~2900 - 3800 cm-1, within 60 s, much shorter than with scanning SFG spectrometers. The ultra-broadband SFG spectrometer reported here can be potentially applied to time-resolved measurements of kinetics at interfaces.

Vibrational sum-frequency generation (SFG) spectroscopy of interfacial water at mineral/aqueous interfaces is extended to the near-IR range containing the low cross section stretch + bend combination bands (νcomb = νOH + δHOH) of liquid water at silica surfaces near 5000-5300 cm(-1), for the first time. The assignments of SFG spectra are supported by FTIR and Raman spectroscopic measurements of the bulk water νcomb modes. The SFG spectra contain significant contributions from two combinations, [νs + δ] ≈ 5060 cm(-1) and [νas + δ] ≈ 5300 cm(-1). These measurements provide the first, to our knowledge, reported probe of the bending mode of water at buried interfaces. The data suggest that the interfacial water bending mode is blue-shifted from that of bulk water.