Shining Light on Growth-Dependent Surface Chemistry of Organic Crystals: A Polarized Raman Spectroscopic and Computational Study of Aspirin

Understanding
and controlling crystal surfaces is a critical problem
in contemporary materials science. Organic crystal surfaces present
the additional complexity of multiple terminations with different
functional groups. These alternate terminations influence the surface
chemistry and exert control over many material properties. While established
tools for surface characterization exist, few provide the chemical
information required to unambiguously identify functional groups.
Polarized Raman spectroscopy is a versatile tool that can provide
detailed chemical information on molecular materials, and, when used
in a microscope configuration, can be used to map substrates on a
micron scale. In this work, we demonstrate the use of polarized Raman
to study the surface chemistry of aspirin. By analyzing crystals grown
under a variety of conditions, we relate the growth solvent to the
surface termination and reconcile the conflicting results in the literature
on the nature of the <100> surface. Our results are supported
by
detailed first-principles modeling of the surfaces and their vibrational
spectra. This study establishes the potential of polarized Raman microscopy
as a tool for organic surface science that, when combined with predictive
modeling, provides a powerful means to understand and ultimately control
surface chemistry.