Onderzoek Nederland and by Fundamentee Onder-20. See. for example K. F. Keton n Crystal Nucleation (1 9921 ibld 133, 29 j19931 zoek der Matere lFOM1 vith fnanca a d from Nedin Liqu~ds and Glasses H. Ehrenrech and D. Turn-24. J P. Hansen and L Verlet. Phys. Rev. 184, 151 erlandse Organsate voor Wetenschappejk Onderbull, Eds. !Acadernc Press Boston, 19911, vol. 45 ! I 9691 zoek. Computer t~m e vias prov~ded by Nat~onale pp. 75-177. As shown In t h s reference. the con-25. WethankE. J. Me~jerforcomputng padofthe phase Comp"er Faciteiten. stant of propo-ionaty IS system-specf~c. d~aqram In FIC. 1 B, and A, van Blaaderen. M. Doq-21. G. M Torre and J. P. Vaeau, Chem. Phys. Lett. 28 terom. J. ~r g n t h .The linear and nonlinear (X'2") optical responses of Langmuir monolayers of organically functionalized silver quantum dots were measured as a continuous function of interparticle separation under near-ambient conditions. As the distance between metal surfaces was decreased from 12 to -5 angstroms, both quantum and classical effects were observed in the optical signals. When the separation was less than 5 angstroms, the optical second-harmonic generation (SHG) response exhibited a sharp discontinuity, and the linear reflectance and absorbance began to resemble those of a thin metallic film, indicating that an insulator-to-metal transition occurred. This transition was reversible.

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The size dependence of the resonance Raman spectrum of CdS nanocrystals ranging in size from 10 to 70 Å radius has been studied. We find that while the lowest electronic excited state is coupled strongly to the lattice, this coupling decreases as the nanocrystal size is decreased. We demonstrate that the lifetime of the initially prepared excited state can influence the apparent strength of electron-vibration coupling. Absolute resonance Raman cross section measurements can be used to determine the value of the excited state lifetime, thus removing this parameter. The coupling to the lattice, while less in nanocrystals than in the bulk, is still greater than what is predicted assuming an infinite confining potential. The width of the observed LO mode broadens with decreasing size, indicating that the resonance Raman process is intrinsically multimode in its nature. The frequency of the observed longitudinal optic (LO) mode has a very weak dependence on size, in contrast to results obtained from multiple quantum well systems. The temperature dependence of the frequency and linewidth of the observed LO mode is similar to the bulk and indicates that the LO mode decays into acoustic vibrations in 2.5 ps.

Three different size distributions of Ge quantum dots (>200, 110, and 60 A) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 "C. The quantum dots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Colloidal suspensions of these quantum dots were prepared and their extinction spectra are measured with ultraviolet/visible (UVNis) and near infrared (IR) spectroscopy, in the regime from 0.6 to 5 eV. The optical spectra are correlated with a Mie theory extinction calculation utilizing bulk optical constants. This leads to an assignment of three optical features to the E(l), E( 0 '), and E( 2) direct band gap transitions. The E(0') transitions exhibit a strong size dependence. The near IR spectra of the largest dots is dominated by E(0) direct gap absorptions. For the smallest dots the near IR spectrum is dominated by the rz5-+L indirect transitions.

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