The structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed.

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Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects

Recovery and reuse of expensive catalysts after catalytic reactions are important factors for sustainable process management. The aim of this Review is to highlight the progress in the formation and catalytic applications of magnetic nanoparticles and magnetic nanocomposites. Directed functionalization of the surfaces of nanosized magnetic materials is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.

Magnetically Separable Nanocatalysts: Bridges between Homogeneous and Heterogeneous Catalysis

Introduction Sp3-hybridized semiconductors (including InP, GaAs, CdSe, and Si) are remarkable from the perspective of physical chemistry. A single electron created by HOMOLUMO promotion moves rapidly in response to an applied electric field, because there is little lattice distortion (i.e., small Franck-Condon factors) accompanying its creation. According to Marcus-Hush electron transfer theory, electron motion is resonant in the limit of vanishing reorganization energy. Franck-Condon factors are also small around an electron-hole pair, which is an electronically excited state. As a consequence, radiationless internal conversion (unimolecular decay converting electronic energy into heat) is extremely slow. Excited states decay radiatively in a defect-free, direct gap semiconductor such as CdSe. These simple spectroscopic facts have important practical consequences. Semiconductor lightemitting diodes have narrow emission bands and can show near 30% efficiency in converting electrical power into light. Semiconductor lasers and diodes also show excellent long-term stability against photochemical and current-induced degradation, when compared with many organic materials. All these properties reflect the strong chemical bonding, and the extremely delocalized nature of the electronic wave functions. Semiconductor nanocrystals lie between the traditional regimes of chemistry and solid-state physics.1 Nanocrystal research was initially motivated by an effort to understand the evolution of bulk structural and electronic properties from the molecular scale.2 Presently, technological interest in nanocrystals stems from the prospect of creating novel materials with distinct physical properties. Nanocrystals act like molecules as they interact with light via their electronic transition dipoles. Yet, their delocalized solid-state parentage causes them to display unusual photophysics relative to molecules. In many molecules vibronic interaction in the excited state is strong as the wave function is localized on just one or a few bonds. The molecular excited state has a different structure which promotes fast nonradiative deactivation into the ground state. Emission quantum yields can be low, and often sensitive to quenching by the local environment. The situation is different in nanocrystals. In a 23 A diameter nanocrystal, for example, the wave function is delocalized over ∼100 unit cells with little probability density at the surface. This suggests that, in the absence of defects, internal or surface, a nanocrystal should exhibit near unity fluorescence quantum yield, and partial protection from quenching. The emission spectrum should be sharp as the Franck-Condon factors are small. At room temperature nanocrystals can be better photoemitters than bulk semiconductors because in nanocrystals the electron and hole remain superimposed due to quantum confinement. Nanocrystals have the potential to serve as ideal chromophores if their surface chemistry can be understood and controlled. In this Account we describe nanocrystal photophysics and make comparisons between inorganic solid-state materials and organic dye molecules.

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Luminescence Photophysics in Semiconductor Nanocrystals

https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

▪ Abstract Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observ...

/pdf/organic-molecules-in-the-interstellar-medium-comets-and-14nzw41xt5.pdf

Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth

We present results on the photoluminescence (PL) properties of silicon nanocrystals as a function of their size. The nanocrystals are synthesized by laser pyrolysis of silane in a gas flow reactor and deposited at low energy on a substrate after a mechanical velocity and size selection. Both the photoluminescence spectroscopy and yield have been studied as well as the effect of aging of the samples in air. The measurements show that the PL of the silicon nanocrystallites follows the quantum confinement model very closely. The apparent PL yields are rather high (up to 18%). From evaluation of the size distribution obtained by atomic force microscopy it is concluded that the intrinsic PL yield of the nanocrystals can reach almost 100%. These results enabled us to develop a simple theoretical model to describe the PL of silicon nanocrystals. This model can also explain the changes of PL with aging of the sample, just by invoking a decrease of the size of the crystalline core as a result of oxidation.

/pdf/photoluminescence-properties-of-silicon-nanocrystals-as-a-2odriqe5b4.pdf

Photoluminescence properties of silicon nanocrystals as a function of their size

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

Silicon clusters and nanocrystals have been generated by CO 2-laser-induced decomposition of SiH 4 in a flow reactor. By introducing a conical nozzle into the reaction zone, the clusters are extracted into a molecularbeam machine and analyzed with a time-of-flight mass spectrometer ~TOFMS!. Since the clusters have a size-dependent velocity, a mechanical velocity selector is used to further narrow their size distribution and to select a specific mean size. Employing this technique, silicon clusters with different preselected mean sizes have been deposited at low energy on various substrates. Photoluminescence ~PL! and resonant Raman spectra of the resulting films are presented. The crystallite sizes deduced from the Raman spectra confirm the TOFMS measurements. The PL spectra are shifted with decreasing cluster size to smaller wavelengths. Our results agree very well with theoretical predictions for silicon quantum dots. @S0163-1829~97!08035-1#

Photoluminescence and resonant raman spectra of silicon films produced by size-selected cluster beam deposition

The infrared spectroscopic behavior of nano-sized carbon grains produced by laser-driven pyrolysis of acetylene (C2H2) is presented with respect to the internal structure of the particles investigated by electron energy loss spectroscopy and high-resolution transmission electron microscopy. Carbon grain samples were synthesized at different condensation conditions, and the effect of the pyrolysis temperature and pyrolysis mode (pulsed versus continuous wave) on the carbon structure was investigated. The size distribution of the carbon clusters synthesized in the flow reactor was determined by means of time-of-flight mass spectroscopy. Despite the CH absorption features attributed to saturated aliphatic hydrocarbons adsorbed by the grain surfaces, the infrared spectra of the neat carbon grains show only weak CH features. The investigations show that the formation and growth of polycyclic aromatic structural units are involved in the carbon grain condensation process. We conclude that the low feature-to-continuum ratio in the IR spectra of the grains is a typical property of carbonaceous dust formed by the pyrolysis of acetylene. The lack of observational evidence for hydrocarbon dust in the outflows of carbon-rich asymptotic giant branch (AGB) stars can be rationalized by our spectroscopic results. An evolutionary sequence of the circumstellar carbonaceous material during the AGB to planetary nebula (PN) phase transition can be deduced from our results by comparison with the IR spectral behavior of carbonaceous grain materials synthesized in other condensation systems.

Infrared Spectroscopy of Nano-sized Carbon Grains Produced by Laser Pyrolysis of Acetylene: Analog Materials for Interstellar Grains

We used laser-induced decomposition of silane for the fabrication of nanosized Si particles and studied in detail their structural characteristics by conventional and high resolution electron microscopy. The silane gas flow reactor incorporated in a molecular beam apparatus was operated without size selection to achieve a broad size distribution. Deposition at low energy on carbon substrates yielded single crystalline, spherical Si particles almost completely free of planar lattice defects. The particles, covered by thin amorphous oxide shells, are not agglomerated into larger aggregates. The lattice of diamond cubic type exhibits deviations from the bulk spacing which vary from distinct contraction to dilatation as with decreasing particle size the oxide shell thickness is reduced. This effect is discussed in terms of the strong Si/oxide interfacial interaction and compressive stresses arising upon oxidation. A negative interface stress, as determined from the size dependence of the lattice spacing, limits the curvature of the interface, i.e., at small sizes Si oxidation must be considered as a self-limiting process.

http://www-old.mpi-halle.mpg.de/mpi/publi/pdf/516_99.pdf

B. Kohn

Papers

Photoluminescence properties of silicon nanocrystals as a function of their size

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

Photoluminescence and resonant raman spectra of silicon films produced by size-selected cluster beam deposition

Infrared Spectroscopy of Nano-sized Carbon Grains Produced by Laser Pyrolysis of Acetylene: Analog Materials for Interstellar Grains

Lattice contraction in nanosized silicon particles produced by laser pyrolysis of silane