RESUMO
Core/shell quantum dots (QDs) have been extensively studied, yet their optical properties widely vary among studies. Such variation may arise from the variation in interfacial structures induced by the subtle difference in each synthetic procedure. Here, we studied the interfacial structures of CdSe/ZnS QDs using the time-of-flight medium energy ion-scattering spectroscopy (TOF-MEIS), which offers the radial elemental distributions as well as the overall elemental compositions of QDs. The TOF-MEIS spectra provided strong evidence for the existence of an alloyed layer at the interface between CdSe and ZnS in typical CdSe/ZnS QDs. On the basis of the emission and absorption spectra of QDs sampled during the synthesis, we conclude that such interfacial alloying is caused by the dissolution of CdSe seeds during the synthesis steps. Such a dissolution mechanism is further corroborated by the observation that the ligand environment of solvent (X or L type) leads to different shapes of interfaces.
RESUMO
The C-C coupling reactions of aliphatic alcohols to aromatics and larger-mass compounds have large endothermicities and activation energies, calling for catalysts operating at high temperatures. Here, we demonstrate that plasmon-excited nanoparticles catalyze the C-C coupling of aliphatic alcohols at room temperature to produce polyaromatic hydrocarbons and graphene oxide. The conversion is quenched by radical and electron scavengers and by the surface passivation of metals, suggesting that the reaction proceeds through alkoxy, peroxyl, hydroxyalkyl, and alkyl radical intermediates created by the metal to molecule transfer of plasmonic hot carriers. Besides being the first realization of C-C coupling of aliphatic alcohols at room temperature, the result constitutes a rare example of an endothermic plasmon-induced reaction producing new bonds and a new method for photogenerating graphene derivatives. More importantly, the result demonstrates the facile generation of organic radicals directly from alcohols, which may be used as precursors for radical-based organic reactions.
RESUMO
Luminescent hexagonal dominant copper indium sulphide (h-dominant CIS) quantum dots (QDs) by precursor-injection of mixed metal-dialkyldithiocarbamate precursors. Owing to the different reactivity of the precursors, this method allowed the CIS QDs to grow while retaining the crystallinity of the hexagonal nucleus. The photoluminescence (PL) spectra exhibited dual emission (600-700 nm red emission and 700-800 nm NIR emission) resulting from the combined contributions of the hexagonal (wurtzite) h-CIS and tetragonal (chalcopyrite) t-CIS QDs, i.e. the NIR and red emissions were due to the h-CIS QDs and coexisting t-CIS QDs (weight ratio of h-CIS/t-CIS ~ 10), respectively. The PL intensities of the h-CIS as well as t-CIS QDs were enhanced by post-synthetic heat treatment; the t-CIS QDs were particularly sensitive to the heat treatment. By separating h-CIS and t-CIS successfully, it was demonstrated that this phenomenon was not affected by size and composition but by the donor-acceptor pair states and defect concentration originating from their crystal structure. The h-dominant CIS QDs in this work provide a new technique to control the optical property of Cu-In-S ternary NCs.
RESUMO
The initial observations of surface-enhanced Raman scattering (SERS) from individual molecules (single-molecule SERS, SMSERS) have triggered ever more detailed mechanistic studies on the SERS process. The studies not only reveal the existence of extremely enhanced and confined fields at the gaps of Ag or Au nanoparticles but also reveal that the spatial, spectral, and temporal behaviors of the SMSERS signal critically depend on many factors, including plasmon resonances of nanostructures, diffusion (lateral and orientational) of molecules, molecular electronic resonances, and metal-molecule charge transfers. SMSERS spectra, with their molecular vibrational fingerprints, should in principle provide molecule-specific information on individual molecules in a way that any other existing single-molecule detection method (such as the ones based on fluorescence, mechanical forces, or electrical currents) cannot. Therefore, by following the spectro-temporal evolution of SMSERS signals of reacting molecules, one should be able to follow chemical reaction events of individual molecules without any additional labels. Despite such potential, however, real applications of SMSERS for single-molecule chemistry and analytical chemistry are scarce. In this Account, we discuss whether and how we can use SMSERS to monitor single-molecule chemical kinetics. The central problem lies in the experimental challenges of separately characterizing and controlling various sources of fluctuations and spatial variations in such a way that we can extract only the chemically relevant information from time-varying SMSERS signals. This Account is organized as follows. First, we outline the standard theory of SMSERS, providing an essential guide for identifying sources of spatial heterogeneity and temporal fluctuations in SMSERS signals. Second, we show how single-molecule reaction events of surface-immobilized reactants manifest themselves in experimental SMSERS trajectories. Comparison of the reactive SMSERS data (magnitudes and frequencies of discrete transitions) and the predictions of SMSERS models also allow us to assess how faithfully the SMSERS models represent reality. Third, we show how SMSERS spectral features can be used to discover new reaction intermediates and to interrogate metal-molecule electronic interactions. Finally, we propose possible improvements in experimental design (including nanogap structures and molecular systems) to make SMSERS applicable to a broader range of chemical reactions occurring under ambient conditions. The specific examples discussed in this Account are centered around the single-molecule photochemistry of 4-nitrobenzenethiol on metals, but the conclusions drawn from each example are generally applicable to any reaction system involving small organic molecules.
