Amy V. Walker

Structure and energetics of small gold nanoclusters and their positive ions

We performed density functional theory calculations at the PW91PW91/LANL2DZ, PW91PW91/Stuttgart 1997, PW91PW91/CRENBL, B3LYP/LANL2DZ, and SVWN5/LANL2DZ levels of theory to attain the minimum-energy structures of neutral and cationic gold clusters of up to nine gold atoms. We locate the 2D-to-3D (two-dimensional to three-dimensional) transition in cationic clusters as occurring between Au(8) (+) and Au(9) (+). We also demonstrate that we can correlate the 2D-to-3D transition in cationic clusters with a linear extrapolation of the energy differences of the lowest-lying 2D and 3D structures of cluster sizes below the transition. We then use the same approach to predict that the 2D-to-3D transition occurs in neutral clusters at Au(11); this is confirmed by locating 3D Au(11) structures that are lower in energy than the best 2D structures reported to date. We examine the effects of choice of basis sets and exchange-correlation functionals on the relative stabilities and other properties of the calculated structures. Finally we find that there is good agreement between calculated and experimental data for clusters with up to six constituent atoms. For clusters with more than seven atoms, there are significant differences observed between the calculated and experimental properties using SVWN5/LANL2DZ, but there is still good agreement for the other levels of theory used.

Interaction of vapor-deposited Ti and Au with molecular wires

We have investigated the interaction of vapor-deposited titanium and gold with a self-assembled monolayer (SAM) of 4-[4′-(phenylethynyl)-phenylethynyl]-benzenthiol, an unsubstituted oligo(phenylene-ethynylene), chemisorbed on a gold substrate, a typical SAM of interest for molecular electronics. Deposited titanium atoms are observed to react in a top-down fashion with the SAM molecules to form Ti–C bonds, destroying the monolayer structure. In contrast, deposited Au atoms undergo continuous penetration through the monolayer, even at high coverages, leaving the SAM “floating” on the Au substrate surface.

Benzaldehyde-functionalized polymer vesicles.

Polymer vesicles with diameters of ca. 100-600 nm and bearing benzaldehyde functionalities within the vesicular walls were constructed through self-assembly of an amphiphilic block copolymer PEO(45)-b-PVBA(26) in water. The reactivity of the benzaldehyde functionalities was verified by cross-linking the polymersomes and also by a one-pot cross-linking and functionalization approach to further render the vesicles fluorescent, each via reductive amination. In vitro studies found these labeled nanostructures to undergo cell association.

Why Is SIMS Underused in Chemical and Biological Analysis? Challenges and Opportunities

Improvements have led to many developments in SIMS, including better 2D MS imaging, the ability to perform molecular depth profiling, and the development of 3D MS imaging. (To listen to a podcast about this feature, please go to the Analytical Chemistry website at pubs.acs.org/ac.).

Prospects for imaging with TOF-SIMS using gold liquid metal ion sources

Abstract Gold liquid metal ion sources produce high quality TOF-SIMS spectra with excellent prospects for imaging using either Au++, Au+ or Au2+ primary ions. The beam is stable and exhibits a long lifetime when employing eutectic alloys of Si or Ge. In general, the yields are found to be considerably higher than when using Ga beams, but the increased yield associated with using dimer ions is also associated with an increase in surface damage. Finally, it appears that Au++ ion bombardment may yield improved spectra for certain types of compounds.

Building addressable libraries: a site-selective click-reaction strategy for rapidly assembling mass spectrometry cleavable linkers

A click-reaction was site-selectively carried out on 1000 and 12,000 microelectrode arrays and characterized using TOF-SIMS.

Site-selectively functionalizing microelectrode arrays: the use of Cu(I)-catalysts.

Site-selective Cu(I)-catalyzed reactions have been developed on microelectrode arrays. The reactions are confined to preselected electrodes on the arrays using oxygen as the confining agent. Conditions initially developed for the Cu(I)-catalyzed click reaction have proven general for the coupling of amine, alcohol, and sulfur nucleophiles to both vinyl and aryl iodides. Differences between reactions run on 1-K arrays and reactions run on 12-K arrays can be attributed to the 1-K array reactions being divided cell electrolyses and the 12-K array reactions being undivided cell electrolyses. Reactions on the 12-K arrays benefit from the use of a non-sugar-derived porous reaction layer for the attachment of substrates to the surface of the electrodes. The reactions are sensitive to the nature of the ligand used for the Cu catalyst.

Building Robust and Reliable Molecular Constructs: Patterning, Metallic Contacts, and Layer-by-Layer Assembly

We describe recent progress in our laboratories to build stable complex two- and three-dimensional molecular constructs. We have introduced a simple and robust method for constructing complex molecular devices using top-down and bottom-up techniques based on self-assembled monolayers (SAMs), lithography, and site-selective reactions. It has significant advantages over other methods; it is easily scaled up, affords precise nanoscale placement, and is extensible to many different materials. Several recent developments are discussed including the UV photopatterning and electron beam lithography of SAMs adsorbed on semiconductors, the site-selective deposition of metals using electroless deposition and low-temperature chemical vapor deposition, and layer-by-layer assembly using covalent coupling. Optimization and further development of these techniques requires a detailed understanding of the reaction pathways involved in the lithography of SAMs and of the interaction of SAMs with metals, organometallic compounds, ions, and other compounds.

Ionic liquid matrix-enhanced secondary ion mass spectrometry: the role of proton transfer.

AbstractRoom temperature ionic liquids (ILs) are effective matrices in secondary ion mass spectrometry (SIMS) and matrix assisted laser desorption ionization (MALDI). In this paper, we examine the role of proton transfer in the mechanism of secondary ion enhancement using IL matrices in SIMS. We employ hydrogenated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as analytes to investigate the origin of proton transfer. The data indicate that protons from the IL anion transfer to the analyte in solution leading to an increase in the secondary ion intensity of the protonated molecular ion. The chemical identity of the matrix cation also affects analyte signal intensities. Using deuterated DPPC we observe that protons (deuterium) from the DPPC tail group react with the cation of the IL liquid leading to an increase in (cation + D)+ ion intensities. Further, the data suggest that the transfer kinetics of deuterium (hydrogen) is correlated with the secondary ion enhancements observed. The highest secondary ion enhancements are observed for the least sterically hindered cation. Neither the proton affinity nor the pKa of the IL cation have a large effect on the analyte ion intensities, suggesting that steric factors are important in determining the efficacy of IL matrices for a given analyte.

Selective electroless deposition of copper on organic thin films with improved morphology.

We have investigated the selective electroless deposition (ELD) of Cu on functionalized self-assembled monolayers (SAMs). Previous studies have demonstrated that Cu deposits on -COOH and -CH(3) terminated SAMs using ELD. However, the deposited films were rough and contained irregular crystallites. Further, the copper penetrated through the film. In this Article, we demonstrate that copper can be selectively deposited on -COOH terminated SAMs with improved morphology and without penetration of copper through the organic layer. The method employs a Cu(II) seed layer and an additive, adenine or guanine. We demonstrate the efficacy of the technique on photopatterned -CH(3)/-COOH SAMs. Copper is observed to deposit only atop the -COOH terminated SAM area and not on the -CH(3) terminated SAM. The use of a Cu(II) seed layer increased the Cu ELD rate on both -COOH and -CH(3) terminated SAMs. The deposited copper layer strongly adheres to the -COOH terminated SAMs because the copper layer nucleates at Cu(2+)-carboxylate complexes. In contrast, the deposited copper layer can easily be removed from the -CH(3) terminated SAM surface because there is no specific copper-surface interaction. The additives adenine and guanine mediate the interaction of Cu(2+) and the deprotonated -COOH terminated SAMs via the formation of additive-carboxylate complexes. These complexes lead to significantly reduced copper penetration through the SAM. In the case of adenine, the diffusion of copper through the organic film was eliminated. This new technique for copper deposition will facilitate the development of inexpensive molecular electronics, sensors, and other nanotechological devices.