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 the electron mean free path, are independent of their size or shape. Intense laser heating of the electrons in these particles is also found to cause a shape transformation (photoisomerization of the rods into spheres or fragmentation), which depends on the laser pulse energy and pulse width.

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The size and temperature dependence of the plasmon absorption is studied for 9, 15, 22, 48, and 99 nm gold nanoparticles in aqueous solution. The plasmon bandwidth is found to follow the predicted behavior as it increases with decreasing size in the intrinsic size region (mean diameter smaller than 25 nm), and also increases with increasing size in the extrinsic size region (mean diameter larger than 25 nm). Because of this pronounced size effect a homogeneous size distribution and therefore a homogeneous broadening of the plasmon band is concluded for all the prepared gold nanoparticle samples. By applying a simple two-level model the dephasing time of the coherent plasmon oscillation is calculated and found to be less than 5 fs. Furthermore, the temperature dependence of the plasmon absorption is examined. A small temperature effect is observed. This is consistent with the fact that the dominant electronic dephasing mechanism involves electron-electron interactions rather than electron-phonon coupling.

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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.

Gold-silver alloy nanoparticles with varying mole fractions are prepared in aqueous solution by the coreduction of chlorauric acid HAuCl 4 and silver nitrate AgNO 3 with sodium citrate. As the optical absorption spectra of their solutions show only one plasmon absorption it is concluded that mixing of gold and silver leads to a homogeneous formation of alloy nanoparticles. The maximum of the plasmon band blue-shifts linearly with increasing silver content. This fact cannot be explained by a simple linear combination of the dielectric constants of gold and silver within the Mie theory. On the other hand, the extinction coefficient is found to decrease exponentially rather than linearly with increasing gold mole fraction x Au . Furthermore, the size distribution of the alloy nanoparticles is examined using transmission electron microscopy (TEM). Highresolution TEM (HRTEM) also confirms the formation of homogeneous gold-silver alloy nanocrystals. IntroductionThe intense research in the field of nanoparticles by chemists, physicists, and materials scientists is motivated by the search for new materials in order to further miniaturize electronic devices as well as by the fundamental question of how molecular electronic properties evolve with increasing size in this intermediate region between molecular and solid-state physics. [1][2][3][4][5][6][7] Possible future applications include the areas of ultrafast data communication and optical data storage. [4][5][6] Semiconductor nanoparticles are also used in building solar cells 7 and metal nanoparticles are very important as catalysts because of their high surface-to-volume ratios. 4 Metal nanoparticles have mainly been studied because of their unique optical properties as especially nanoparticles of the alkali metals and the noble metals copper, silver, and gold have a broad absorption band in the visible region of the electromagnetic spectrum. [8][9][10][11][12][13][14][15] Solutions of these metal nanoparticles show a very intense color, which is absent for the bulk material as well as for the atoms. Their origin is attributed to the collective oscillation of the free conduction electrons induced by an interacting electromagnetic field. These resonances are also denoted as surface plasmons.Mie 16 was the first to explain this phenomenon by applying classical electrodynamics to spherical particles and solving Maxwell's equations for the appropriate boundary conditions. The total extinction cross section composed of absorption and scattering is given as a summation over all electric and magnetic multipole oscillations. The Mie theory has the advantage of being conceptually simple and has found wide applicability in explaining experimental results. 8,9,14,15 However, all of the material properties are represented by a complex dielectric function of the absorbing metal nanoparticles thus obscuring somehow the underlying microscopic events, such as the possible decay mechanisms of the coherent motion of the free electrons.

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