Christine M. Aikens

Electronic structure methods for studying surface-enhanced Raman scattering

This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering. Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references).

Chirality and Electronic Structure of the Thiolate-Protected Au38 Nanocluster

Structural, electronic, and optical properties of the thiolate-protected Au(38)(SR)(24) cluster are studied by density-functional theory computations (R = CH(3) and R = C(6)H(13)) and by powder X-ray crystallography (R = C(12)H(25)). A low-energy structure which can be written as Au(23)@(Au(SR)(2))(3)(Au(2)(SR)(3))(6) having a bi-icosahedral core and a chiral arrangement of the protecting gold-thiolate Au(x)(SR)(y) units yields an excellent match between the computed (for R = C(6)H(13)) and measured (for R = C(12)H(25)) powder X-ray diffraction function. We interpret in detail the electronic structure of the Au(23) core by using a particle-in-a-cylinder model. Although the alkane thiolate ligands are achiral, the chiral structure of the ligand layer yields strong circular dichroism (CD) in the excitations below 2.2 eV for Au(38)(SCH(3))(24). Our calculated CD spectrum is in quantitative agreement with the previously measured low-energy CD signal of glutathione-protected Au(38)(SG)(24). Our study demonstrates a new mechanism for the strong chiral response of thiolate-protected gold clusters with achiral metal cores and ligands.

Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms

We report reversible switching of paramagnetism in a well-defined gold nanoparticle system consisting of atomically monodisperse nanoparticles containing 25 gold atoms protected by 18 thiolates [abbreviated as Au(25)(SR)(18)]. The magnetism in these nanoparticles can be switched on or off by precisely controlling the charge state of the nanoparticle, that is, the magnetic state of the Au(25)(SR)(18) nanoparticles is charge-neutral while the nonmagnetic state is an anionic form of the particle. Electron paramagnetic resonance (EPR) spectroscopy measurements establish that the magnetic state of the Au(25)(SR)(18) nanoparticles possess one unpaired spin per particle. EPR studies also imply an unusual electronic structure of the Au(25)(SR)(18) nanoparticle. Density functional theory calculations coupled with the experiments successfully explain the origin of the observed magnetism in a Au(25)(SR)(18) nanoparticle as arising from one unpaired spin having distinct P-like character and delocalized among the icosahedral Au(13) core of the particle in the highest occupied molecular orbital. The results suggest that the Au(25)(SR)(18) nanoparticles are best considered as ligand-protected superatoms.

Electronic Structure of Ligand-Passivated Gold and Silver Nanoclusters

Gold and silver nanoclusters have unique molecule-like electronic structure and a nonzero HOMO-LUMO gap. Recent advances in X-ray crystal structure determination have led to a new understanding of the geometric structure of gold nanoparticles, with significant implications for electronic structure. The superatom model has been effectively employed to explain properties such as one- and two-photon optical absorption, circular dichroism, EPR spectra, and electronic effects introduced by nanoparticle doping. Future investigations may also lead to an understanding of nanoparticle luminescence, excited-state dynamics, and the metallic to molecular transition.

Silver Nanoparticles with Broad Multiband Linear Optical Absorption

Keywords: thiols ; cluster compounds ; luminescence ; quantum dots ; surface plasmon resonance Reference EPFL-ARTICLE-166610doi:10.1002/anie.200900298 Record created on 2011-06-06, modified on 2017-05-10

X-ray Crystal Structure and Theoretical Analysis of Au25-xAgx(SCH2CH2Ph)18(-) Alloy.

The atomic arrangement of Au and Ag atoms in Au25-xAgx(SR)18 was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [-SR-Au-SR-Au-SR-] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au18.3Ag6.7(SCH2CH2Ph)18. This composition is in reasonable agreement with the composition Au18.8Ag6.2(SCH2CH2Ph)18 measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au1@Au5.3Ag6.7@6×[-SR-Au-SR-Au-SR-]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster.

Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au25 Nanoparticle

Density functional theory calculations are employed to calculate geometries (R = H, CH(3), CH(2)CH(3), CH(2)CH(2)Ph) and excitation energies (R = H, CH(3), CH(2)CH(3)) for the Au(25)(SR)(18)(-) nanoparticle. The splitting between the first two peaks in the optical absorption spectrum is known to arise as a result of ligand-field splitting of superatom D orbitals, and the value of this splitting is found to be a very sensitive probe of gold-gold distances in the Au(25)(SH)(18)(-) nanoparticle core. LDA functionals such as Xalpha with a triple-zeta basis set are found to predict core geometries in good agreement with experiment, which suggests that this level of theory may be useful in future structural predictions. Asymptotically correct potentials SAOP and LB94 with triple-zeta basis sets yield excitation energies within 0.15-0.20 eV of experimental values; LB94 with a frozen-core basis set is found to be an inexpensive alternative to the preferred SAOP potential. The size of the ligand plays a minor role on the optical absorption spectrum and solvent effects on geometries and excitation energies are negligible, which demonstrates that the core geometric and electronic structure is primarily responsible for the discrete optical absorption exhibited by this nanoparticle.

Electronic structure and TDDFT optical absorption spectra of silver nanorods.

Density functional theory calculations are employed to determine optimized geometries and excitation spectra for small pentagonal silver nanorods Ag(n), with n = 13, 19, 25, 31, 37, 43, 49, 55, 61, and 67 in various charge states. The asymptotically correct SAOP functional is utilized in the excitation calculations. Silver nanorods exhibit a sharp longitudinal excitation that results from a mixture of orbital transitions; the wavelength for this excitation depends linearly on the length of the nanorod. The broad transverse excitation arises from multiple excited states. A particle-in-a-box model is employed to explain the linear dependence of the longitudinal excitation wavelength on nanorod length.

Theoretical analysis of the optical excitation spectra of silver and gold nanowires

The excitation spectra of linear atomic chains of silver and gold with various sizes have been calculated using time-dependent density functional theory. Silver chains show longitudinal and transverse peaks as well as a low-intensity d-band. The longitudinal peak, corresponding to the HOMO-LUMO transition (along the main axis of the chain), shifts linearly to the red as the length of the system increases, consistent with the particle-in-a-box model. The transverse peak remains at approximately constant energy for all systems studied and corresponds to ∑(m)→Π(m) transitions in the xy plane perpendicular to the chain. As the chain grows, transitions arising from d orbitals contribute to the transverse peak, which affects its oscillator strength. Contrary to silver, gold chains display a strong d-band that converges to a distinct pattern at a chain length of about twelve atoms. The transitions involved in the d-band originate from localized d-orbitals with a d(z(2)) character since they have the right symmetry to give transitions into the LUMO, LUMO + 1, …, which have ∑ symmetry. Transitions arising from these localized d-orbitals also affect the position of the longitudinal peak and generate a wide transverse band. Although the majority of the transitions involved in the transverse band have a d∑→Π or dΠ→∑ character, they are hidden by much stronger excitations of dΠ→Π character in gold nanowires.

The golden pathway to thiolate-stabilized nanoparticles: following the formation of gold(I) thiolate from gold(III) chloride.

Pathways for the formation of gold thiolate complexes from gold(III) chloride precursors AuCl(4)(-) and AuCl(3) are examined. This work demonstrates that two distinct reaction pathways are possible; which pathway is accessible in a given reaction may depend on factors such as the residue group R on the incoming thiol. Density functional theory calculations using the BP86 functional and a polarized triple-ζ basis set show that the pathway resulting in gold(III) reduction is favored for R = methyl. A two-to-one ratio of thiol or thiolate to gold can reduce Au(III) to Au(I), and a three-to-one ratio can lead to polymeric Au(SR) species, which was first suggested by Schaaff et al. J. Phys. Chem. B, 1997, 101, 7885 and later confirmed by Goulet and Lennox J. Am. Chem. Soc., 2010, 132, 9582. Most transition states in the pathways examined here have reasonable barrier heights around 0.3 eV; we find two barrier heights that differ substantially from this which suggest the potential for kinetic control in the first step of thiolate-protected gold nanoparticle growth.