/pdf/studies-in-surface-science-and-catalysis-2ogdlcr44y.pdf

Studies in Surface Science and Catalysis

This is the report of a DOE-sponsored workshop organized to discuss the status of our understanding of charge-transfer processes on the nanoscale and to identify research and other needs for progress in nanoscience and nanotechnology. The current status of basic electron-transfer research, both theoretical and experimental, is addressed, with emphasis on the distance-dependent measurements, and we have attempted to integrate terminology and notation of solution electron-transfer kinetics with that of conductance analysis. The interface between molecules or nanoparticles and bulk metals is examined, and new research tools that advance description and understanding of the interface are presented. The present state-of-the-art in molecular electronics efforts is summarized along with future research needs. Finally, novel strategies that exploit nanoscale architectures are presented for enhancing the efficiences of energy conversion based on photochemistry, catalysis, and electrocatalysis principles.

Charge Transfer on the Nanoscale: Current Status

Electron transmission through molecules and molecular interfaces has been a subject of intensive research due to recent interest in electron-transfer phenomena underlying the operation of the scanning-tunneling microscope on one hand, and in the transmission properties of molecular bridges between conducting leads on the other. In these processes, the traditional molecular view of electron transfer between donor and acceptor species gives rise to a novel view of the molecule as a current-carrying conductor, and observables such as electron-transfer rates and yields are replaced by the conductivities, or more generally by current-voltage relationships, in molecular junctions. Such investigations of electrical junctions, in which single molecules or small molecular assemblies operate as conductors, constitute a major part of the active field of molecular electronics. In this article I review the current knowledge and understanding of this field, with particular emphasis on theoretical issues. Different approaches to computing the conduction properties of molecules and molecular assemblies are reviewed, and the relationships between them are discussed. Following a detailed discussion of static-junctions models, a review of our current understanding of the role played by inelastic processes, dephasing and thermal-relaxation effects is provided. The most important molecular environment for electron transfer and transmission is water, and our current theoretical understanding of electron transmission through water layers is reviewed. Finally, a brief discussion of overbarrier transmission, exemplified by photoemission through adsorbed molecular layers or low-energy electron transmission through such layers, is provided. Similarities and differences between the different systems studied are discussed.

http://atto.tau.ac.il/~nitzan/anrev-published.pdf

Electron transmission through molecules and molecular interfaces

Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)2(SCN)2 (dcbpy = 2,2‘-bipyridine-4,4‘-dicarboxylate) sensitized TiO2 thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO2. The injection process was characterized by a fast component, with a time constant of <100 fs, and a slower component that is sensitive to sample con...

Ultrafast Electron Transfer Dynamics from Molecular Adsorbates to Semiconductor Nanocrystalline Thin Films

Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals

▪ Abstract Two-photon photoemission is a promising new technique that has been developed for the study of electron dynamics at interfaces. A femtosecond laser is used to both create an excited electronic distribution at the surface and eject the distribution for subsequent energy analysis. Time- and momentum-resolved two-photon photoemission spectra as a function of layer thickness fully determine the conduction band dynamics at the interface. Earlier clean surface studies showed how excited electron lifetimes are affected by the crystal band structure and vacuum image potential. Recent studies of various insulator/metal interfaces show that the dynamics of excess electrons are largely determined by the electron affinity of the adsorbate. In general, electron dynamics at the interface are influenced by the substrate and adlayer band structures, dielectric screening, and polaron formation in the two-dimensional overlayer lattice.

http://www.cchem.berkeley.edu/cbhgrp/papers/711.pdf

Femtosecond dynamics of electrons on surfaces and at interfaces.

Femtosecond time-resolved two-photon photoemission spectrocopy (TPPE) has been used to measure the lifetimes of image potential electrons at alkane mono- and bilayers on Ag(111). The n = 1 lifetimes for zero, one, and two layers of n-heptane on Ag(111) are 32 ± 10 fs, 155 ± 20 fs, and 1580 ± 200 fs, respectively. This approximately exponential increase in lifetime is consistent with a tunneling picture in which the adlayer forms a barrier that slows the decay of an image potential electron back into the metal. The existence of the tunneling barrier is consistent with the repulsive electron affinity of the longer chain n-alkanes in the condensed phase. The lifetimes of the higher quantum states indicate that the presence of the monolayer significantly reduces coupling of the image states to the bulk band structure, so that further changes in lifetime are determined by the adlayer barrier and an attempt rate related to the classical oscillation time in the modified image potential well. These results are compared with quantitative predictions of a model by Cole which considers the tunneling barrier presented by the layer and the effect of the layer on the attempt rate. These results are considered in the context of previous TPPE studies of metal-insulator interfaces.

Femtosecond studies of electron tunneling at metal-dielectric interfaces

The energies and dispersions of the image states and quantum well electronic states in layers of Xe and Kr on a Ag(111) substrate were determined by angle‐resolved two‐photon photoemission (ARTPPE). For Xe, we measured binding energies of unoccupied electronic states for 1–9 layers and their parallel dispersion out to 4 layers. We measured the binding energies for a monolayer of Kr and dispersions for one and two layers. The n=2 and n=3 image states of the bare metal evolve into quantum well states of the layer (states of the Xe conduction band discretized by the boundary conditions of a 2‐D slab) at higher Xe thicknesses, where the n=2,3 states exhibit both a perpendicular and parallel dispersion similar to that of the bulk Xe conduction band. The n=1 state appears to evolve with coverage as an image state screened by the Xe layer, with appreciable electron density in the vacuum. A continuum dielectric model (modified image state picture) reproduces the gross trends in the data, while an explicit quantum...

http://www.cchem.berkeley.edu/cbhgrp/papers/JCP03883.pdf

Interfacial quantum well states of Xe and Kr adsorbed on Ag(111)

The dynamics of excited electrons in insulator quantum well states on a metal substrate were determined by femtosecond two-photon photoemission for the first time. Lifetimes are reported for the first three excited states for 1{endash}6 atomic layers of Xe on Ag(111). As the image states evolve into quantum well states with increasing coverage, the lifetimes undergo an oscillation as the layer boundary crosses each node of the wave function. The lifetime data are modeled by extending the two-band nearly free-electron approximation to account for the presence of a dielectric layer. The lifetimes are shown to depend on the spatial distribution of the interfacial electron. {copyright} {ital 1997} {ital The American Physical Society}

Dynamics and Spatial Distribution of Electrons in Quantum Wells at Interfaces Determined by Femtosecond Photoemission Spectroscopy

Evidence for electron localization in 2D at metal-dielectric interfaces is reported. A nondispersive peak, derived from the image-potential state and observed in angle-resolved two-photon photoemission, is assigned to localized excess electrons. The image electron on alkane monolayers is a free electron. Bilayers of chain or ring alkanes show both localized and delocalized states; electrons are localized on trilayer n-pentane. Bilayers and trilayers of neopentane show no localization. Our results link the geometry of adlayer molecules with localization of excess electrons at interfaces.

R. L. Lingle

Papers

Femtosecond dynamics of electrons on surfaces and at interfaces.

Femtosecond studies of electron tunneling at metal-dielectric interfaces

Interfacial quantum well states of Xe and Kr adsorbed on Ag(111)

Dynamics and Spatial Distribution of Electrons in Quantum Wells at Interfaces Determined by Femtosecond Photoemission Spectroscopy

Two-dimensional localization of electrons at interfaces.