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In this study, the aggregation dynamics of magic-sized CdTe quantum dots is investigated. The experiments show that the growth kinetics of the quantum dots is very sensitive to the Cd/Te ratio. The crossover from tellurium-rich to cadmium-rich conditions produces a very different aggregation pattern, which can be explained by the lack of formation of magic-sized nanoparticles during the reaction conditions. A simple simulation that includes both monomer-induced growth and aggregation growth is presented to reproduce the experimentally observed aggregation patterns. The simulation results strongly suggest that the experimental aggregation pattern can be reproduced if initially a double magic-sized distribution is assumed. The numerical data clearly show that the aggregation is enhanced by the dipole−dipole interaction. Simulations also suggest that the neck formation of the CdTe quantum dot aggregates is unlikely under the experimental conditions. The absence of neck formation is in agreement with the expectations during high-temperature synthesis. [ABSTRACT FROM AUTHOR]

This work aims to experimentally determine the polarizability of confined electron in CdSe quantum dots (QD). The dielectric response of uncharged and charged CdSe quantum dots (3.2 and 6.3 nm) has been measured using terahertz time-domain spectroscopy in the frequency range of 2.0−7.0 THz. A strong coupling between the surface plasmon and surface phonons appears upon charging the QDs. The absolute polarizability of an electron in 3.2 and 6.3 nm charged QDs are experimentally determined to be 0.5 ± 0.1 × 103Å3and 14.6 ± 0.3 × 103Å3, respectively, and the values agree reasonably well with theory and the previous experiment. The observed plasmon−phonon coupling is expected to play an important role in electron relaxation in absence of a hole in CdSe QDs. [ABSTRACT FROM AUTHOR]

The reaction C2H + O2 → CH(A2Δ) + CO2 is investigated using Fourier transform visible emission spectroscopy. C2H radicals, produced by 193 nm photolysis of C2H2, react with O2 molecules at low total pressures to produce electronically excited CH(A2Δ). Observation of the CH(A2Δ−X2Π) electronic emission to infer nascent rotational and vibrational CH(A2Δ) distributions provides information about energy partitioning in the CH(A2Δ) fragment during the reaction. The rotational and vibrational populations of the CH(A2Δ) product are determined by fitting the rotationally resolved experimental spectra with simulated spectra. The CH(A2Δ) product is found to be rotationally and vibrationally excited with Trot ≅ 1150 K and Tvib ≅ 1900 K. The mechanism for this reaction proceeds through one of two five-atom intermediates and requires a crossing between electronic potential surfaces. The rotational excitation suggests a bent geometry for the final intermediate of this reaction before dissociation to products, and the vibrational excitation involves an elongation of the C−H bond from the compressed transition state to the final CH(A) state. [ABSTRACT FROM AUTHOR]

Fourier transform visible spectroscopy, in conjunction with VUV photons produced by a synchrotron, is employed to investigate the photodissociation of CH3CN. Emission is observed from both the CN(B2−X2) and CH(A2−X2) transitions; only the former is observed in spectra recorded at 10.2 and 11.5 eV, whereas both are detected in the 16 eV spectrum. The rotational and vibrational temperatures of both the CN(B2) and CH(A2) radical products are derived using a combination of spectral simulations and Boltzmann plots. The CN(B2) fragment displays a bimodal rotational distribution in all cases. Trot(CN(B2)) ranges from 375 to 600 K at lower K‘ and from 1840 to 7700 K at higher K‘ depending on the photon energy used. Surprisal analyses indicate clear bimodal rotational distributions, suggesting CN(B2) is formed via either linear or bent transition states, respectively, depending on the extent of rotational excitation in this fragment. CH(A2) has a single rotational distribution when produced at 16 eV, which results in Trot(CH(A2)) 4895 ± 140 K in v‘ 0 and 2590 ± 110 K in v‘ 1. From thermodynamic calculations, it is evident that CH(A2) is produced along with CN(X2) H2. These products can be formed by a two step mechanism (via excited CH3and ground state CN(X2)) or a process similar to the “roaming” atom mechanism; the data obtained here are insufficient to definitively conclude whether either pathway occurs. A comparison of the CH(A2) and CN(B2) rotational distributions produced by 16 eV photons allows the ratio between the two excited fragments at this energy to be determined. An expression that considers the rovibrational populations of both band systems results in a CH(A2):CN(B2) ratio of (1.2 ± 0.1):1 at 16 eV, thereby indicating that production of CH(A2) is significant at 16 eV. [ABSTRACT FROM AUTHOR]

In this study, a synthetic method to produce water-soluble iron−gold (Fe−Au) alloy nanoparticles is described. The diameter of the alloy nanoparticles is 4.9 ± 1.0 and 3.8 ± 1.0 nm for two different precursors of iron, ferrous sulfate heptahydrate (Fe 2+) and iron pentacarbonyl (Fe 0). The X-ray powder diffraction of the alloyed nanoparticles shows an appreciable shift in 2θ peak positions relative to pure gold or iron. Using Vegard’s law, we estimated the particle’s iron content to be 14.8 ± 4.7 mol %. The lattice constant of the alloy nanoparticles is found to be 4.0449 ± 0.0375 Å. The high-resolution transmission electron microscopy images show the presence of icosahedral structures in agreement with the previous high temperature synthesis of the Fe−Au alloy nanoparticles. The optical absorption of the alloy nanoparticles is distinctive from pure gold nanoparticles and shows a relatively narrow absorption band in the range of 642−662 nm depending upon the amount of gold precursor used. [ABSTRACT FROM AUTHOR]

In this study, the interactions of two gold nanoparticles of different sizes (average diameters of 3.7 ± 2.6 and 17 ± 3 nm) with octameric mycobacterial porin A from Mycobacterium smegmatis(MspA) and a mutant of MspA featuring a cysteine mutation in position 126 (Q126C) are investigated. From the observation of enhanced photoluminescence quenching, it is inferred that the presence of eight cysteines in the MspA Q126C mutant significantly enhances the binding of selected small gold nanoparticles within the inner pore of MspA. The large gold nanoparticle/porin complex shows photoluminescence enhancement, which is expected since the larger nanoparticles cannot dock within the homopore of MspA due to size exclusion. In addition to the fluorescence experiments, observation of energy transfer from the small gold nanoparticles to the MspA shows the close proximity of the small gold nanoparticles with the porin. Interestingly, the energy transfer of the large nanoparticle/MspA complex is completely missing. From high-performance liquid chromatography data, the estimated binding constants for small Au@MspA, large Au@MspA, small Au@MspA cys, and large Au@MspA cysare 1.3 × 10 9, 2.22 × 10 10, > 10 12(irreversible), and 1.7 × 10 10, respectively. [ABSTRACT FROM AUTHOR]