| AVS 57th International Symposium & Exhibition | |
| Surface Science | Wednesday Sessions |
| Session SS-WeA |
| Session: | Chemisorption and Surface Reactions |
| Presenter: | X. Wang, University of Wisconsin-Madison |
| Authors: | X. Wang, University of Wisconsin-Madison R.J. Hamers, University of Wisconsin-Madison R. Ruther, University of Wisconsin-Madison |
| Correspondent: | Click to Email |
The grafting of organic molecules on semiconductor surfaces initiated by UV light has become an efficient means to tailor the chemical and physical properties of surfaces of materials, enabling their integration with various applications of the devices. The mechanism of photochemical grafting of alkenes to group IV semiconductors (diamond, silicon, germanium, etc) has remained poorly understood. We have demonstrated that a previously unrecognized process—photoelectron emission from semiconductors to reactant liquid—is a nearly universal mechanism for initiating grafting of alkenes to surfaces and is broadly applicable to a wide range of semiconductors.
The charge transfer processes that occur during the photochemical grafting to diamond surfaces were investigated by spectrally resolved photoelectron yield experiments. X-ray and ultraviolet photoelectron spectroscopy measurements (XPS, UPS) establish a clear correlation between the photoelectron yield, the grafting efficiency at different wavelengths, and the valence electronic structure of the substrate and of the reactant molecule.
While our initial work focused on detailed studies on diamond, more recently we have shown that this mechanism is also responsible for initiating UV-induced grafting onto other semiconductors, most notably both silicon and germanium. By intentionally reducing the bulk carrier lifetime in Si (by doping with Au) and comparing the grafting efficiency, we showed that the rate of UV-induced grafting is independent of the bulk carrier lifetime. This observation is important as it allows us to immediately rule out the bulk exciton mechanism as the primary pathway. Our results also showed that the rate of grafting was directly connected to the electron affinity of the reactant molecules. These results are important because they show that photoemission can also dominate as an initiation process with smaller bandgap semiconductors, such as silicon and germanium, where photoemission and exciton processes can both take place. We have hypothesized that the reason why the photoemission can be dominant even on silicon is that photoemission is an irreversible process, while in an exciton process the concentration of holes is reduced by recombination processes. Our studies provide new insights into the nature of photochemical functionalization on the surfaces of semiconductors and a fundamental understanding of the mechanism will facilitate the design and synthesis of well defined functional interfaces.