Organic Molecular Thin Films on Device-Relevant Substrates

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Groce, Michelle Anne
Einstein, Theodore L
Organic thin films are central to many cutting-edge electronic devices. Improving our understanding of the characteristics of thin films is important not only to the development of condensed matter physics but also to our ability to engineer specialized devices that we demand be ever smaller, less expensive, and more efficient. This thesis applies the experimental techniques of scanning tunneling microscopy and spectroscopy to the task of characterizing submonolayer thin films of two types: the organic semiconductor C<sub>60</sub> on silicon oxide, and self-assembling porous networks of trimesic acid on graphite. Capture zone analysis of the initial nucleation regime for C<sub>60</sub> on ultrathin silicon oxide is reported. The critical nucleus size, reflecting the largest unstable cluster of particles on a surface, is found to have a parabolic dependence on temperature rather than a monotonically increasing one. Between stages of stable monomers (<italic>i</italic>=0$) at < 300 K and > 480 K, a peak corresponding to <italic>i<italic>=1 is found at 386&plusmn;3 K. This unique temperature dependence is attributed to defect-like variation in the silicon oxide surface. The first successful room-temperature UHV STM of trimesic acid on graphite is also presented here. These exploratory studies indicate the potential for a variety of porous hexagonal networks of trimesic acid to exist on a graphitic surface at room temperature. Significant electronic effects on graphite from trimesic acid lattices are shown via scanning tunneling spectroscopy, including an electronic state at -0.14 V that appears in networks whose pores are filled with excess TMA guest molecules. Ultimately, if the growth of TMA films could be extended to graphene, then the periodicity of electronegative oxygen atoms in molecules physisorbed on the graphene surface is predicted to provide a slight energy shift between the degenerate sublattices, opening a band gap. Promising directions for future research in these areas are also discussed.