Lindsey R. Madison

Plasmonic nanopipets, which are functionalized with plasmonic nanomaterials, are widely used in near-field spectroscopy to enhance the optical processes. The fabrication and screening processes of these nanopipets are critical for their intended applications. Here, an easy-to-implement fabrication scheme based on high-vacuum sputtered gold island films was reported. Additionally, the process of using surface-enhanced Raman spectroscopy (SERS) to evaluate the plasmonic enhancement of nanopipets was described for near-infrared laser excitation, and it could be readily applied to other optical windows. These results indicated that the reported plasmonic nanopipets can be used as Tip-SERS probes. Finally, the scanning ion-conductance microscopic tip-enhanced Raman spectroscopy was demonstrated with the optimized plasmonic nanopipet. Together with density functional theory simulation, the observed near-field Raman spectra were analyzed and assigned. These plasmonic nanopipets can be used to investigate a range of surface science problems in heterogeneous electrochemical interfaces, as well as in soft and biomimetic materials.

An approach for evaluating spectra from ground state probability amplitudes (GSPA) obtained from diffusion Monte Carlo (DMC) simulations is extended to improve the description of excited state energies and allow for coupling among vibrational excited states. This approach is applied to studies of the protonated water trimer and tetramer, and their deuterated analogs. These ions provide models for solvated hydronium, and analysis of these spectra provides insights into spectral signatures of proton transfer in aqueous environments. In this approach, we obtain a separable set of internal coordinates from the DMC ground state probability amplitude. A basis is then developed from products of the DMC ground state wave function and low-order polynomials in these internal coordinates. This approach provides a compact basis in which the Hamiltonian and dipole moment matrix are evaluated and used to obtain the spectrum. The resulting spectra are in good agreement with experiment and in many cases provide comparable agreement to the results obtained using much larger basis sets. In addition, the compact basis allows for interpretation of the spectral features and how they evolve with cluster size and deuteration.

Approaches for using diffusion Monte Carlo (DMC) to evaluate matrix elements involving two vibrational wave functions are explored. In the first part of this study, overlaps between a wave function obtained using DMC and one that can be calculated analytically are evaluated. In this case, the analytical wave function is used as the guiding function for an importance sampled DMC simulation. The accuracy of the calculated overlaps is found to depend strongly on the accuracy of the calculated descendant weights, which are obtained in the DMC simulation. While a single evaluation of the descendant weights is sufficient for obtaining projected probability amplitudes or expectation values of multiplicative operators, averages of multiple independent evaluations of the descendant weights are required to obtain accurate matrix elements. This approach is investigated for one-dimensional model systems as well as H₂CO, H₂D⁺ and D₂H⁺. The approach is extended to the evaluation of matrix elements of the dipole moment operator between the ground state and states with one quantum of excitation in one of the OH stretching vibrations in H₃O₂}^-. For these calculations, the wave functions for both the ground and excited states are evaluated using DMC. The described methodology opens the possibility of evaluating matrix elements involving two different states, both of which are evaluated using DMC.

The behavior of CO₂ and N₂, both as single components and as binary mixtures, in two cage-type silica zeolites was studied using atomistic simulations. The zeolites considered, ITQ-3 and paradigm cage-type zeolite ZK4 (the all-silica analog of LTA), were chosen so that the principles illustrated can be generalized to other adsorbent/adsorbate systems with similar topology and types of interactions. N₂ was chosen both because of the potential uses of N₂/CO₂ separations and because it differs from CO₂ most significantly in the magnitude of its Coulombic interactions with zeolites. Despite similarities between N₂ and CO₂ diffusion in other materials, we show here that the diffusion of CO₂ within cage-type zeolites is dominated by an energy barrier to diffusion located at the entrance to the narrow channels connecting larger cages. This barrier originates in Coulombic interactions between zeolites and CO₂’s quadrupole and results in well-defined orientations for the diffusing molecules. Furthermore, CO₂’s favorable electrostatic interactions with the zeolite framework result in preferential binding in the windows between cages. N₂’s behavior, in contrast, is more consistent with that of molecules previously studied. Our analysis suggests that CO₂’s behavior might be common for adsorbates with quadrupoles that interact strongly with a material that has narrow windows between cages.