Nanoscale Studies of Molecules, Metals and Alloys.
Abstract: The advent of scanning tunneling microscopy (STM) changed the way
the world perceived surfaces at the nanoscale. STM is a technique capable of imaging
surfaces at the atomic scale, allowing researchers to probe surface processes with
previously unavailable resolution. While STM is a powerful tool used in many nanoscale
applications, this thesis examines three overarching themes: the ... read moreinvestigation of adsorbate
interactions, the elucidation of surface structures, and the study of catalytically active
sites in alloys. STM maps the topographic and electronic structures of surfaces and
adsorbates. STM can therefore be used to image the atomic structure of a surface as well as
the shape and positions of individual molecular adsorbates. We have used this capability to
study the effect that adsorbates have on a surface and each other, and to measure the
processes that molecules undergo on a surface. In particular, the following chapters
outline how STM studies reveal an inert substrate restructuring in the presence of a weakly
adsorbed molecule (Chapter 3), dipole-driven ferroelectric ordering on a metal surface
(Chapter 4), extended hydrogen-bonded networks and their structural dependence on coverage
(Chapter 5), and the rotational dynamics of individual surface-bound molecules (Chapter 6).
Capturing interactions and dynamics on a molecular level allows us to reveal the driving
forces behind these processes. The high resolution of STM at low temperatures allows us to
image the structure of a static lattice of surface atoms. Using STM, we were able to
elucidate the atomic structure of chiral sites at the step edges of an intrinsically chiral
surface (Chapter 7), which has previously been shown to enantiospecifically interact with
adsorbates. Identifying the surface structure allows us to better predict how adsorbates or
reactants will interact with such metals. Additionally, we characterized the temperature
dependence of the geometric structure of a catalytically relevant Pd/Au bimetallic alloy
(Chapter 8). Because STM measures the geometric and electronic structure of surfaces, we
were able to measure the charge transfer of individual Pd atoms when alloyed into an inert
Au surface. These results demonstrated the first simultaneous geometric and electronic
measurements of individual atoms in a Pd/Au alloy. Finally, using STM we identified the
subtle structural differences between two bimetallic alloys (Pd/Cu and Pd/Au) and studied
their interaction with hydrogen (Chapter 9). Hydrogen was used to probe the active Pd sites
of the alloys. Although dissociation and spillover of hydrogen occurred over individual Pd
atoms in Cu, H uptake was only observed on Au when Pd was present as large particles on the
surface. The strong Pd-H interaction on Au resulted in Pd segregation on top of the Au
surface. These results show how the surface affects the adsorbates and how adsorbates can
influence the surface structure. Each of these examples will show how STM can be used to
study systems unlike any other instrument and deepen our understanding of atomic-scale
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Chemistry.
Advisor: Charles Sykes.
Committee: Arthur Utz, Mary Shultz, and Cynthia Friend.
Keywords: Chemistry, and Physical Chemistry.read less