We use the discrete dipole approximation to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local electric field (E-field) enhancement near the particle surfaces. The results are used to determine what conditions are most favorable for producing enhancements large enough to observe single molecule surface enhanced Raman spectroscopy. Most of the calculations refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addition, for the dimer calculations we study the influence of dimer separation and orientation, especially for dimers that are separated by a few nanometers. We find that the largest /E/2 values for dimers are about a factor of 10 larger than those for all the monomers examined. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given /E/2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest /E/2 values we have calculated for spacings of 2 nm or more is approximately 10(5).

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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 condition. Similar ultrafast electron injection times were measured in TiO2 films sensitized by Ru(dcbpy)2(X)2 (X2 = 2CN- and dcbpy). Electron injection in these systems was found to compete with the vibrational energy relaxation process within the excited state of the molecules, leading to an injection yield that depends on the excited-state redox potential of the adsorbate. The injection rate from Ru(dcbpy)2(SCN)2 to different semiconductors was found to obey the trend TiO2 > SnO2 > ZnO, indicating a strong dependence on the nature of the semiconductor. To understand these observations, various factors, such as electronic coupling, density of states, and driving force, that control the interfacial ET rate were examined separately. The effect of electronic coupling on the ET rate was studied in TiO2 sensitized by three adsorbates, Re(L n )(CO)3Cl [L n is a modified dcbpy ligand with n (=0, 1, 3) CH2 units between the bipyridine and carboxylate groups]. We found that the ET rate decreased with increasing number of CH2 units (or decreasing electronic coupling strength). The effect of driving force was investigated in Ru(dcbpy)2X2 (X2 = 2SCN-, 2CN-, and dcbpy) sensitized SnO2 thin films. In this case, we observed that the ET rate increased with the excited-state redox potential of the adsorbates, agreeing qualitatively with the theoretical prediction for a nonadiabatic interfacial ET process.

We report the synthesis of new "branched" gold nanocrystals in high yield (over 90%) via a wet-chemical route. The branched nanocrystals exhibit a shape-dependent plasmon resonance that is red-shifted by 130−180 nm from the spherical particle wavelength. Discrete dipole approximation (DDA) calculations qualitatively replicate the observed optical extinction spectra of the nanocrystals, indicating that the surface plasmon resonance is mainly determined by in-plane dipole excitation associated with the sharp tips.

Electron injection rates from Ru(dcbpy) 2 (X) 2 [X ) 2NCS, 2CN, and dcbpy; dcbpy ) 2,2′-bipyridine-4,4′carboxylate] (called Ru535 or Ru N3, Ru505, and Ru470) to TiO 2 nanocrystalline thin films are examined as a function of adsorbate redox potential, pH of the solution, excitation wavelength, and solvent. For all three dyes, the injection kinetics are biphasic, consisting of a distinct ultrafast component (<100 fs) and slower components. Under different experimental conditions, the partitioning between these two components and the rate of the slow components change, but the rate of the fast component shows no noticeable variations within the ∼200 fs time resolution of the measurement. When Ru535, Ru505, and Ru470 were compared at the same pH, increasing amplitude and decreasing rate of slow component were observed, correlating with less negative excited-state redox potentials in these dyes. An analogous trend was seen for RuN3/TiO 2 by increasing the pH of the solution from pH ) 2 to 8 and changing from pH ) 2 aqueous solution to a (1:1) ethylene/propylene carbonate mixture. The injection dynamics are also dependent on excitation wavelength. The relative amplitude of the slow component increases when the excitation wavelength is changed from 400 to 630 nm. All data can be described by a two-state injection model, which attributes the fast (<100 fs) component to injection from a nonthermalized excited state and the slow component to injection from the thermalized excited state. The partitioning between these two components and the rate of the slow components depend on the relative energetics between dye excited states and the conduction band edge.