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 properties of a material depend on the type of motion its electrons can execute, which depends on the space available for them (i.e., on the degree of their spatial confinement). Thus, the properties of each material are characterized by a specific length scale, usually on the nanometer dimension. If the physical size of the material is reduced below this length scale, its properties change and become sensitive to its size and shape. In this Account we describe some of the observed new chemical, optical, and thermal properties of metallic nanocrystals when their size is confined to the nanometer length scale and their dynamical processes are observed on the femto- to picosecond time scale.

Some interesting properties of metals confined in time and nanometer space of different shapes.

Noble metal particles have long fascinated scientists because of their intense color, which led to their application in stained glass windows as early as the Middle Ages. The recent resurrection of colloidal and cluster chemistry has brought about the strive for new materials that allow a bottoms-up approach of building improved and new devices with nanoparticles or artificial atoms. In this review, we discuss some of the properties of individual and some assembled metallic nanoparticles with a focus on their interaction with cw and pulsed laser light of different energies. The potential application of the plasmon resonance as sensors is discussed.

Optical properties and ultrafast dynamics of metallic nanocrystals.

Optical Studies of Dynamics in Noble Metal Nanostructures

The femtosecond optical response of noble metal nanoparticles and its connection to the ultrafast electron dynamics are discussed in light of the results of high-sensitivity femtosecond pump−probe experiments. The physical origins of the nonlinear responses in the vicinity of the surface plasmon resonance and interband transition threshold are analyzed using extension of the theoretical models used in the bulk materials. These responses contain information on the electron interaction processes (electron−electron and electron−phonon scattering) that can thus be directly investigated in the time domain. Their size and environment dependences are discussed, and the results are compared to the ones in the bulk materials. Time-resolved techniques also permit direct study of the vibrational modes of metal nanoparticles and, in particular, the determination of their damping, which is a sensitive probe of the nature of the surrounding matrix and of the interface quality.

Ultrafast Electron Dynamics and Optical Nonlinearities in Metal Nanoparticles

Electron-phonon interaction and hot electrons in small metal islands

In small metallic particles of certain dimensions, the well known ‘bulk’ electron-phonon energy exchange may become ineffective and, as a result, the governing effect proves to be that of interaction between electrons and surface modes. The paper presents a microscopic theory of such electron-surface energy exchange based upon a quantum-kinetic treatment.

Surface electron-phonon energy exchange in small metallic particles

The effect of the complete disappearance of electron-phonon energy exchange attributed to Cherenkov emission generation in island metal films (the island size being less than critical) is investigated. The observed heating of electrons in island metal films exposed to electromagnetic radiation is thus explained.

Electron—lattice exchange in metal island films

Many phenomena observed in island metallic films (IMFs) under laser irradiation (Borsjak et al. 1965, Tomchuk and Fedorovich 1966 a, b, Pagnia and Sotnik 1988) or the passage of an electric current (Benditskii et al. 1986, 1988, Tomchuk 1988) can be understood and consistently interpreted in terms of hot electrons. The energy fed to the IMF is assumed to heat the electron gas while the crystal lattice remains cold. The current heating of electrons leads to large values of Q because of the strong homogeneity of the current inside the IMF. The current flows along some fixed current lines and its density inside these lines is very high. The main factors are discussed determining the possibility of steady-state (stationary) electron heating in IMFs which cannot occur in bulk metal.

E. D. Belotskii

Papers

Electron-phonon interaction and hot electrons in small metal islands

Surface electron-phonon energy exchange in small metallic particles

Electron—lattice exchange in metal island films

Hot electrons in island metallic films

Theory of hot electrons in island metal films