The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

The field of nanoparticle research has drawn much attention in the past decade as a result of the search for new materials. Size confinement results in new electronic and optical properties, possibly suitable for many electronic and optoelectronic applications. A characteristic feature of noble metal nanoparticles is the strong color of their colloidal solutions, which is caused by the surface plasmon absorption. This article describes our studies of the properties of the surface plasmon absorption in metal nanoparticles that range in size between 10 and 100 nm. The effects of size, shape, and composition on the plasmon absorption maximum and its bandwidth are discussed. Furthermore, the optical response of the surface plasmon absorption due to excitation with femtosecond laser pulses allowed us to follow the electron dynamics (electron−electron and electron−phonon scattering) in these metal nanoparticles. It is found that the electron−phonon relaxation processes in nanoparticles, which are smaller than t...

Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods

Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

/pdf/nanoscale-thermal-transport-1frv881kzm.pdf

Nanoscale thermal transport

The thermal boundary resistance present at interfaces between helium and solids (Kapitza resistance) and the thermal boundary resistance at interfaces between two solids are discussed for temperatures above 0.1 K. The apparent qualitative differences in the behavior of the boundary resistance at these two types of interfaces can be understood within the context of two limiting models of the boundary resistance, the acoustic mismatch model, which assumes no scattering, and the diffuse mismatch model, which assumes that all phonons incident on the interface will scatter. If the acoustic impedances of the two media in contact are very different, as is the case for helium (liquid or solid) in contact with a solid, then phonon scattering at the interface will reduce the boundary resistance. In the limiting case of diffuse mismatch, this reduction is typically over 2 orders of magnitude. Phonons are very sensitive to surface defects, and therefore the Kapitza resistance is very sensitive to the condition of the interface. For typical solid-solid interfaces, at which the acoustic impedances are less different, the influence of diffuse scattering is relatively small; even for the two limiting cases of acoustic mismatch and diffuse mismatch the predicted boundary resistances differ by very little (\ensuremath{\lesssim} 30%). Consequently, the experimentally determined values are expected to be rather insensitive to the condition of the interface, in agreement with recent observations. Subsurface (bulk) disorder and imperfect physical contact between the solids play far more important roles and led to the irreproducibilities observed in the early measurements of the solid-solid thermal boundary resistance.

Thermal boundary resistance

The relaxation processes of electrons and spins systems following the absorption of femtosecond optical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probe techniques. The magnetization of the film drops rapidly during the first picosecond, but different electron and spin dynamics are observed for delays in the range 0--5 ps. The experimental results are adequately described by a model including three interacting reservoirs (electron, spin, and lattice).

Ultrafast spin dynamics in ferromagnetic nickel.

Background and Objective

Photodisruption in ocular media with high power pulsed lasers working at non-absorbing frequencies have become a well established surgical tool since the late seventies Shock waves and cavitation bubbles generated by the optical breakdown may strongly influence the surgical effect of photodisruptive lasers We have investigated the shock wave and cavitation bubble effects of femtosecond laser pulses generated during photodisruption in corneal tissue and water The results are compared to those obtained with longer laser pulses



Study Design/Materials and Methods

Laser pulses with 150 fs duration at ∼620 nm wavelength have been focused into corneal tissue and water to create optical breakdown Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles



Results

A rapid decay of the shock waves is observed in both materials with similar temporal characteristics and with a spatial range considerably smaller than that of shock waves induced by picosecond (or nanosecond) optical breakdown Cavitation bubbles are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses In corneal tissue, single intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses



Conclusions

The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers This indicates a potential benefit from the applications of femtosecond laser technology to intraocular microsurgery © 1996 Wiley-Liss, Inc

Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water

Nonequilibrium Phonon Dynamics

Vibronic structure observed in the emission and absorption spectra of alkali halides containing either ${\mathrm{Sm}}^{2+}$, ${\mathrm{Eu}}^{2+}$, or ${\mathrm{Yb}}^{2+}$ is reported, and shown to result from pseudolocalized vibrations occurring at the rare-earth defect. A model for the defect is defined, and an analysis of its vibrational frequencies is given for the sample case of a RbCl host lattice. A Green's function formalism is used in the analysis. The calculation, in which a number of approximations are made, yields the qualitative features of the experimental results.

Rare-Earth Ions in the Alkali Halides. II. Pseudolocalized Vibrational Frequencies

Measurements have been made of the crystal-field splitting of the $^{7}F_{J}$ energy levels of the $4{f}^{6}$ configuration of the ${\mathrm{Sm}}^{2+}$ ion which is dissolved in a number of alkali-halide lattices. Crystal-field term assignments were made from the results of polarized-emission measurements. Observed crystal-field splittings are compared to those predicted from point-charge models. Contributions to the total crystal field are considered to include that due to the presence of a nearest-neighbor cation vacancy as well as that due to displacements, from normal lattice positions, of the nearest-neighbor halide ions and the ${\mathrm{Sm}}^{2+}$ ion. The splitting is shown to arise in part from a displacement of the ${\mathrm{Sm}}^{2+}$ and the cation vacancy toward each other. The extent of the validity of the use of a point-charge model for this case is discussed.

Rare-Earth Ions in the Alkali Halides. I. Emission Spectra of Sm 2+ -Vacancy Complex

Electron-lattice interaction is discussed for the case of rare-earth ions in the alkali halides. A selection rule is derived to show that for the present defect electronic transitions can be coupled only to vibrational modes of ${A}_{1}$ symmetry. Under the assumption of electrostatic coupling between the electron and the lattice vibrations, a coupling function is derived which is proportional to the projection of the field of the electron under-going a transition on to the eigenvectors of the vibrations. It is further shown that this coupling function can be determined from the details of the structural form of the vibronic spectra. A number of other features of the vibronic spectra are accounted for through the properties of Franck-Condon integrals.

Rare-earth ions in the alkali halides. iii. electron-lattice coupling and the details of the vibronic spectra

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