Probing biomolecules at surfaces by NEXAFS spectroscopy

Surface science has made great strides towards tailoring surface properties via self-assembly of nanoscale molecular adsorbates. It is now possible to functionalize surfaces with complex organic molecules and biomolecules. This leads a wide range of technological applications such as molecular electronic devices, biosensors, and bio-inspired photovoltaic devices. However, these macromolecules have complicated chemical sequences and internal structures and can assemble into multihierarchical, complex assemblies on surfaces. On the other hand, surface properties also play an important role because they determine the interaction of adsorbed molecules with the environment. Better understanding of the structural and electrical characteristics of surfaces and adsorbed molecules is therefore crucial and calls for a diversity of analysis methods.;This thesis shows how Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS), a synchrotron-based spectroscopic technique, can be used to characterize the assembly of molecules at surfaces in atom- and orbital-specific fashion. To illustrate the range of applications I begin with simple self-assembled monolayers (SAMs) with functional groups, proceed to polymer films, and finish with a small protein containing 124 amino acids (Ribonuclease A). NEXAFS provides element-specific and bond-specific information on the attachment of macromolecules to surfaces. In particular, the orientation of adsorbed molecules is revealed from the polarization dependence of the NEXAFS intensity, which can be explained with quantitative models. All these applications demonstrate that NEXAFS is particularly powerful in extracting orientational information in parallel with information on electronic structure of biomolecules at surfaces.