A. P. Alivisatos

Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum

The homogeneous (single‐cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Resonance Raman studies of the ground and lowest electronic excited state in CdS nanocrystals

The size dependence of the resonance Raman spectrum of CdS nanocrystals ranging in size from 10 to 70 A 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.The size dependence of the resonance Raman spectrum of CdS nanocrystals ranging in size from 10 to 70 A 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 q...

Germanium quantum dots: Optical properties and synthesis

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 (UV/Vis) 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(1), 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 Gamma25-->L indirect transitions.

THRESHOLD FOR QUASICONTINUUM ABSORPTION AND REDUCED LUMINESCENCE EFFICIENCY IN CDSE NANOCRYSTALS

Luminescence excitation spectra are employed to study the electronic states of CdSe nanocrystals ranging in size from 9 to 26 A radius at 77 K. These studies show that all samples have, in addition to the discrete manifold of quantum confined electronic excitations, a threshold for continuum absorption. Absorption into this continuum results in substantially reduced luminescence efficiency.

Nonradiative damping of molecular electronic excited states by metal surfaces

Abstract In this review we discuss the interaction of a molecular excited state with a smooth substrate. Both theoretical and experimental work is treated. This discussion will concentrate on the classical treatment of the interaction because of its astounding success in comparison with experiment. We do however discuss the shortcomings of the classical treatment and some recent approaches to correcting these limitations. The experimental work is considered in detail but we focus on the region close to the substrate, less than 500 A away because the longer distance regime has been well reviewed. At the end of this article we briefly point out areas where future work is needed.

CdSe nanocrystals with a dipole moment in the first excited state

Stark effect modulation of the optical absorption spectrum of 40 A diam CdSe nanocrystals show the first excited state of these clusters has a dipole moment of 32±10 D.

Nonclassical behavior of energy transfer from molecules to metal surfaces: Biacetyl(3nπ*)/Ag(111)

The distance dependent lifetime of biacetyl separated from a Ag(111) crystal by NH3 spacer layers ranging in thickness from 28 to 457 A has been measured. We extended previous work, where the molecular emission was resonant with the silver interband/plasmon transition, to the case where the emission is below the interband transition. The modulation of the radiative rate is described inadequately by the classical theory for our experimental geometry. At short distances where nonradiative energy transfer to the metal is important, the classical prediction deviates from the data as well. These observations are consistent with a model in which energy is transferred to electrons localized at the metal surface but might also be explained by an inability of the classical theory to model the radiative rate properly.

Electric field modulation studies of optical absorption in CdSe nanocrystals: Dipolar character of the excited state

The Stark effect on the electronic absorption spectrum of CdSe nanocrystals has been studied for nanocrystals ranging in size from 80 to 20 A in diameter. For all but the smallest clusters, a second derivative line shape is observed, indicative of a dipole moment in the excited state. This result is independent of the surface modification and appears in both CdS and CdSe systems. The Δμ ranges from 15±10 D in the smallest clusters and up to 100±10 D in the largest; however, the increase is not monotonic, and in the very largest clusters studied (d≳70 A), the dipole moment decreases. The dipolar character is lost in clusters less than 25 A. These results can be explained by a model in which there is resonance of an interior state with a surface state at a particular size, with the mixing occurring on a preferred axis.

Electronic energy transfer at semiconductor interfaces. I. Energy transfer from two‐dimensional molecular films to Si(111)

The fluorescence decays from submonolayers of pyrene separated from Si(111) by Xe spacer layers are measured as a function of spacer thickness (17–200 A), pyrene coverage, and emission wavelength. The results are explained in terms of two decay channels: energy transfer and trapping among the molecules in the two‐dimensional pyrene overlayer, and excitation of electrons from the valence to the conduction band in the Si(111) by the dipole near field of the electronically excited pyrene molecule. The intralayer energy transfer is modeled using the Kohlrausch equation N(t)=N0u2009exp(−t/τ)α, in which α is related to the distribution of pyrene molecules in energy. Energy transfer from the molecule to the semiconductor is modeled using the classical image dipole theory. The classical model is used to calculate the energy transfer rates from a dipole to Si and GaAs as a function of dipole–semiconductor separation, and as a function of dipole emission wavelength.

Lattice reorganization in electronically excited semiconductor clusters

Variation de la diffusion Raman resonnante en fonction de la dimension (20-70 A˙) des particules de CdS. Interpretation par un recouvrement spatial de lelectron et du trou de lexciton, conduisant a un couplage de Frohlich reduit, et par un piegeage de letat excite electronique par des defauts (principalement superficiels) qui en reduisent la duree de vie