Comparative Study on the Growth of Silver Nanoplates on GaAs Substrates by Electron Microscopy, Synchrotron X-ray Diffraction, and Optical Spectroscopy

We have recently developed a simple and efficient approach involving the galvanic reaction between a pure aqueous solution of AgNO3 and GaAs wafers to directly grow high-quality Ag nanoplates with chemical clean surfaces on the GaAs wafers [Chem. Mater. 2007, 19, 5845; Small 2007, 3, 1964]. The capability to finely control the dimensions (i.e., size and thickness) of the Ag nanoplates and the time-dependent characterizations have not been explored yet. In this article, time-dependent evolutions of the Ag nanostructures grown on highly doped n-type GaAs wafers through the reactions with AgNO3 solutions, which have concentrations varying in the range of 1−10 M, for different times have been systematically investigated by employing various powerful techniques including electron microscopy, synchrotron X-ray diffraction, and optical microscopy. The results indicate that the sizes of Ag nanoplates can be tuned in the range from tens of nanometers to half a micrometer and their thicknesses can be varied from ∼20 to ∼160 nm by simultaneously controlling the concentration of AgNO3 solution and the growth time. The as-grown Ag nanoplates exhibit tunable strong extinction peaks in the ultraviolet−visible−near-infrared spectral regimes, where the GaAs substrates intensively interact with the light

Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO₂ Films: Site Specific Kinetics Revealed by Single-Particle Spectroscopy

The presenting surface of TiO₂ is one of the key factors that influence the photoinduced charge injection process from covalently bound chromophores. However, the dependence of electron transfer (ET) on TiO₂ surface properties (structure, defects, and facets) remains poorly understood due to the difficulties of deconvoluting the signal from a multitude of surface binding sites in highly heterogeneous ET systems. In an effort to correlate TiO₂ surface features with ET, we compare the photoinduced ET dynamics from single quantum dots (QDs) to polycrystalline TiO₂ thin films (pc-TiO₂) grown by atomic layer deposition (ALD) with that of porous TiO₂ nanoparticle films (np-TiO₂) by utilizing single-particle fluorescence spectroscopy. Unlike the broad distribution of ET rates (deduced from fluorescence lifetimes) on np-TiO₂, QDs on pc-TiO₂ exhibit two narrowly distributed ET rates that we attribute to reduced site heterogeneity. Variable temperature pc-TiO₂ annealing studies suggest that the double-peaked distribution of ET rates is related to TiO₂ surface defects, where QDs undergo more rapid ET. Further modification of pc-TiO₂ with a submonolayer of Al₂O₃ enables the selective exclusion of the more rapid ET pathway. More generally, this study provides insight into the role of surface defects in photoinduced ET into crystalline semiconductor oxides

Unusual Solvent Effects on Optical Properties of Bi-Icosahedral Au₂₅ Clusters

Temperature-dependent and time-resolved absorption measurements were carried out to understand the influence of solvent hydrogen bonding on the optical properties of bi-icosahedral [Au₂₅(PPh₃)₁₀(C₆S)₅Cl₂]²⁺ (bi-Au₂₅) clusters. Theoretical calculations have shown a low energy absorption maximum that is dominated by the coupling of the two Au₁₃ icosahedra, as well as a high energy absorption arising from the individual Au₁₃ icosahedra that make up the bi-Au₂₅ clusters. Temperature-dependent absorption measurements were carried out on bi-Au₂₅ in aprotic (toluene) and protic (ethanol and 2-butanol) solvents to elucidate the cluster–solvent hydrogen bonding interactions. In toluene, both the low and high energy absorption bands shift to higher energies consistent with electron–phonon interactions. However, in the protic solvents, the low energy absorption shows a zigzag trend with decreasing temperature. In contrast, the high energy absorption in protic solvents shifts monotonically to higher energy similar to that of toluene. Also at the temperature where the zigzag trend was observed, new absorption peaks emerged at higher energy region. The results are attributed to the hydrogen bonding of the solvent with Au–Cl leading to a disruption of the coupling of icosahedra, which is reflected in unusual trends at the low energy absorption. However, at the transition temperature, the hydrogen bonding solvents distort the icosahedrons so much so that the symmetry of Au₁₃ icosahedron is lifted leading to new absorption peaks at high energy. The transition happens at the dynamic crossover temperature where the solvent attains high density liquid status. Femtosecond time-resolved absorption measurements have shown similar dynamics for bi-Au₂₅ in ethanol and toluene with slower vibrational cooling in ethanol. However, the nanosecond transient measurements show significantly longer lifetime for bi-Au₂₅ in ethanol that suggest the solvent does have an influence on the exciton recombination

Triplet States with Unusual Spin Polarization Resulting from Radical Ion Pair Recombination at Short Distances

Triplet States with Unusual Spin Polarization Resulting from Radical Ion Pair Recombination at Short Distance

Ultrafast Charge Separation from Highly Reductive ZnTe/CdSe Type II Quantum Dots

The low electron affinity of ZnTe quantum dots (QDs) makes it of interest for critically important photocatalytic processes. However, studies of charge transfer from such materials to adsorbates have not been reported. Here, the ultrafast internal and external charge separation dynamics in ZnTe/CdSe core/shell type II QDs have been studied through femtosecond transient absorption spectroscopy. The internal electron transfer time from a ZnTe core to a CdSe shell was found to be 0.67 ps, while the external electron transfer time from QDs to adsorbed molecules was found to be <0.2 ps. Such a fast external charge separation time is due to the extraordinarily high conduction band energy potential of this QD composition. This study indicates the preservation of high photodriven reductive abilities in the ZnTe/CdSe core/shell type II heterostructures

Electroluminescence from Electrolyte-Gated Carbon Nanotube Field-Effect Transistors

We demonstrate near-infrared electroluminescence from ambipolar, electrolyte-gated arrays of highly aligned single-walled carbon nanotubes (SWNT). Using electrolytes instead of traditional oxide dielectrics in carbon nanotube field-effect transistors (FET) facilitates injection and accumulation of high densities of holes and electrons at very low gate voltages with minimal current hysteresis. We observe numerous emission spots, each corresponding to individual nanotubes in the array. The positions of these spots indicate the meeting point of the electron and hole accumulation zones determined by the applied gate and source−drain voltages. The movement of emission spots with gate voltage yields information about relative band gaps, contact resistance, defects, and interaction between carbon nanotubes within the array. Introducing thin layers of HfO2 and TiO2 provides a means to modify exciton screening without fundamentally changing the current−voltage characteristics or electroluminescence yield of these devices

Scattered Light Interference from a Single Metal Nanoparticle and Its Mirror Image

The spatial distribution of surface plasmon scattering from a single nanoparticle changes dramatically near a metal surface as a result of interference from the direct scattered light and indirect scattered light from the mirror reflection. The unique interference patterns have been reproduced by simulations based on Huygens−Fresnel wave propagation theory. The large spectral width of the surface plasmon scattering enables a vertical distance measurement with 10 nm resolution through this nonintrusive far field interferometry

Scattered Light Interference from a Single Metal Nanoparticle and Its Mirror Image

The spatial distribution of surface plasmon scattering from a single nanoparticle changes dramatically near a metal surface as a result of interference from the direct scattered light and indirect scattered light from the mirror reflection. The unique interference patterns have been reproduced by simulations based on Huygens−Fresnel wave propagation theory. The large spectral width of the surface plasmon scattering enables a vertical distance measurement with 10 nm resolution through this nonintrusive far field interferometry

Scattered Light Interference from a Single Metal Nanoparticle and Its Mirror Image

The spatial distribution of surface plasmon scattering from a single nanoparticle changes dramatically near a metal surface as a result of interference from the direct scattered light and indirect scattered light from the mirror reflection. The unique interference patterns have been reproduced by simulations based on Huygens−Fresnel wave propagation theory. The large spectral width of the surface plasmon scattering enables a vertical distance measurement with 10 nm resolution through this nonintrusive far field interferometry