6.2.2. Definition of Effective Properties 3064 6.3. Response Properties to Magnetic Fields 3066 6.3.1. Nuclear Shielding 3066 6.3.2. Indirect Spin−Spin Coupling 3067 6.3.3. EPR Parameters 3068 6.4. Properties of Chiral Systems 3069 6.4.1. Electronic Circular Dichroism (ECD) 3069 6.4.2. Optical Rotation (OR) 3069 6.4.3. VCD and VROA 3070 7. Continuum and Discrete Models 3071 7.1. Continuum Methods within MD and MC Simulations 3072

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Quantum mechanical continuum solvation models.

Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

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.

We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 A. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the...

(CdSe)ZnS Core-Shell Quantum Dots - Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites

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Table Of Integrals Series And Products

CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

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Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions

Time- and frequency-dependent measurements of optical phase conjugation in gold colloids are reported. It is shown that the nonlinear response is fast on a 5-psec time scale and can be assigned to the electrons of the small metal spheres. The frequency-dependent measurements were performed in the neighborhood of the surface-plasma resonance and give experimental evidence of the validity of our resonant enhancement model. We also report the first model calculation of the third-order Kerr susceptibility of small metal particles. The dominant terms of this electric-dipole contribution are emphasized and lead to an expression for χ(3) that varies roughly as the inverse third power of the radius of the particles. This model accounts for the observed anisotropy of this susceptibility and provides an estimate of its magnitude, in agreement with the measured values. The limited size effect on the linear susceptibility is also discussed.

Optical nonlinearities of small metal particles: surface-mediated resonance and quantum size effects

We first consider theoretically the various mechanisms contributing to the Kerr nonlinearity in small gold particles. The major ones are the conduction electron intraband contribution, the saturation of direct interband transitions and the change in dielectric constant due to hot photoexcited electrons. We present experimental results obtained using optical phase conjugation in gold-doped glasses. By varying several parameters, we were able to ascertain the origin of the nonlinear response: the main contributions are the hotelectron and the interband contributions. All experimental results, including saturation behaviour, are fully understood.

The optical kerr effect in small metal particles and metal colloids: The case of gold

We study electron-phonon coupling in the case of Fr\"ohlich or polar interaction, with special emphasis on the size dependence of the coupling strength for semiconductor nanospheres exhibiting quantum confinement of the carriers. We first derive the expression of the vibrational LO (longitudinal optic) and SO (surface optic) eigenfunctions for a sphere in the continuum approximation. After having quantized the vibrational eigenmodes, we give the electron-phonon interaction Hamiltonian. Using a model electronic charge distribution, we then show that the coupling strength is size independent when the typical dimensions of the electron charge distribution scale as the sphere radius. These theoretical considerations are then compared with experimental results obtained using resonant Raman scattering by CdSe-doped glasses with particles of various sizes. The experiments confirm the size independence of the coupling strength and also show the existence of surface modes.

Size dependence of electron-phonon coupling in semiconductor nanospheres: The case of CdSe.

We present experimental data that show that the greatly improved performance of a new class of photorefractive polymers [see Donckers , Opt. Lett.18, 1044 ( 1993)] is too large to be explained by the simple electro-optic photorefractive effect alone. In these materials a photoconducting polymer host is doped with a small concentration of a sensitizer and a large concentration of a nonlinear optical chromophore that has orientational mobility at ambient temperatures. We present a theoretical model for a new orientational enhancement mechanism in which both the birefringence of the sample and the electro-optic coefficient are periodically modulated by the space-charge field itself. The predictions of this model for the size of the enhancement (which is greater than an order of magnitude in diffraction efficiency), the polarization anisotropy between p-polarized and s-polarized readout, and the presence of index modulation at twice the grating wave vector are in good agreement with the measured properties. This orientational enhancement mechanism should be important in any system in which the nonlinear optical chromophores have sufficient orientational mobility and dipole moment so as to be oriented by the space-charge field itself.

François Hache

Papers

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions

Optical nonlinearities of small metal particles: surface-mediated resonance and quantum size effects

The optical kerr effect in small metal particles and metal colloids: The case of gold

Size dependence of electron-phonon coupling in semiconductor nanospheres: The case of CdSe.

Orientationally enhanced photorefractive effect in polymers