Understanding Metal Nonoclusters through Ultrafast and Nonlinear Spectroscopy.

In the past 20 years, nanomaterials studies have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (> 2 nm) because of their potential applications, such as molecular electronics, image markers and catalysts. Moreover, the discovery of metal nanoclusters (< 2 nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties of nanomaterials are intriguing. Metal nanosystems in this size regime have electronic properties that are determined by both size and shape. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to study the metal core directly. Spectroscopic studies are carried out on two different metal nanosystems, gold and silver. Gold nanosystems are known for their high stability. Detailed characterization of gold nanosystems allows for modeling of the electronic and optical properties. Major optical and electronic differences between gold nanoparticles and nanoclusters can be observed around 2.2 nm, which was not known previously. Gold MPCs also exhibit emissions that are five orders of magnitude larger than bulk gold. Chemical dynamics such as electron-electron scattering and electron-phonon coupling can be used to explain the subtle differences between nanosystems. Silver and gold nanosystems are compared because of the similarity between their bulk properties. Silver MPCs exhibit similar optical properties as gold MPCs, but differ in key electronic transitions. The study of nanosystems aims to answer a few major questions. First, what is the effect of size on the electronic and optical properties of metal nanosystems? Second, what are the fundamental mechanisms that govern the electronic excitation? Can we take advantage of these new properties for optical and electronic applications? Finally, can we build better models to predict the properties of metal nanoclusters made and yet to be made? Nanosystem presents a new frontier in material science to be explored and exploited.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99761/1/sunghei_1.pd

Evolution of the Dynamics of As-Deposited and Annealed Lead Halide Perovskites

The rapid rise of organolead trihalide perovskites as solar photovoltaic materials has been followed by promising developments in light-emitting devices and lasers due to their unique and promising optical properties. Evolution of the photophysical properties in as-deposited or annealed CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ perovskite films processed through the interdiffusion method has been investigated. Absorption spectra showed broad band edge saturation in the as-deposited films in contrast to sharp excitonic absorption in the annealed films. Fluorescence emission of the perovskite films showed strong dependence on the halogen type with a very high quantum yield of ∼90% for the annealed CH₃NH₃PbBr₃ film. An explanation for this was provided based on its crystallinity and quantum confinement of the excitons. The emission showed weakly Stokes shifted bands. Time-resolved spectroscopic measurements were carried out to probe the ultrafast dynamics for the perovskites for the as-deposited or annealed films. We classified the evolution in the absorption features in the excited state of CH₃NH₃PbBr₃ perovskite films for the first time and compared them to CH₃NH₃PbI₃. We suggest a bleach feature below 400 nm as the charge transfer band, which results in the photoinduced absorption in the CH₃NH₃PbBr₃ perovskite film, a charge-separated band gap state, and the existence of intermediate excited-state species that regenerate the ground state

Synthesis of Ladder-Type Thienoacenes and Their Electronic and Optical Properties

A series of ladder-type thienoacenes based on benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) have been synthesized and characterized. They were shown to be p-type semiconductors with wide band gaps and able to support multiple stable cationic states. As the conjugation lengthens, these oligomers become more emissive, showing high quantum yields. They were shown to be good two-photon absorbers, exhibiting high two-photon absorption coefficients

Optical Properties and Structural Relationships of the Silver Nanoclusters Ag₃₂(SG)₁₉ and Ag₁₅(SG)₁₁

The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag₃₂ and Ag₁₅ (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag₃₂, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag₃₂(SG)₁₉ and Ag₁₅(SG)₁₁ have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metal–ligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of <i>K</i>_EET ≥ 9.7 × 10⁹ s^–1. A “dark state” competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag₁₅(SG)₁₁, with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag₃₂ recorded a possible metal core state at 530 nm, a metal–ligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag₃₂ are 100 nm blue-shifted in comparison to Au₂₅. The transient spectrum for Ag₁₅ shows a weak ground state bleach at ∼480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metal–ligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics

Erratum to: Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition) (Autophagy, 12, 1, 1-222, 10.1080/15548627.2015.1100356

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