Soft X-ray Studies of Cycloparaphenylene

Speaker: Dave Newby

When: February 25, 2013 (Mon), 04:00PM to 05:00PM (add to my calendar)
Location: PRB 365

This event is part of the Preliminary Oral Exam.

Examining Committee: Kevin Smith, Karl Ludwig, David Campbell, Rob Carey, Soumendra Basu

Abstract:

Carbon allotropes continue to be a fascinating and fertile ground for materials research. Cycloparaphenylenes (CPPs) are the shortest possible segment of armchair nanotubes, and are composed of an integer number [n] of benzene rings arranged in a ``hoop''. Recent interest in this molecule stems from its possible use as a building block for nanotubes. Non-[n]-selective synthesis of CPPs was first achieved in 2008 [1], and subsequent techniques continue to develop for selectively creating various [n] values in small quantities. Early UV-Vis studies of CPP in solution revealed a blue-shifted emission that increased with larger ring size[2]. This defies the typical trend for cyclic organic molecules, in which we expect the exact opposite trend. This quality alone has prompted many theoretical attempts to fully explain the optoelectronic behavior of the [n]-CPPs[3]. In 2012 the Jasti group at BU pioneered a technique for producing crystalline [8]- and [10]-CPP at the gram-scale[4]. These previously unattainable quantities are sufficient for molecular beam epitaxy (MBE) deposition on substrates, which lends itself perfectly to soft x-ray electronic structure analysis. Synchrotron radiation provides enough flux and monochromatic selectivity to study a number of unique electronic properties, and the X1B endstation at the NSLS hosts a number of experimental tools for exactly such analysis. We present a complement of results from various techniques, including x-ray absorption, emission, and photoelectron spectroscopies. The MBE deposition of CPP thin films will also be discussed, along with information on the orientation of CPP molecules on the substrate. The characterization provided by these techniques facilitates comparison with DFT, and we will elucidate our findings with DFT calculations. These measurements provide an unprecedented view of this new and exciting molecule, and may provide a stepping-stone to further refinement of a DFT-based understanding of its orbital structure.