Atomic scale fabrication and characterization of novel one-dimensional organosilicon nanostructures: A scanning tunneling microscopy study.

摘要

The electronic and chemical properties of organic functionalized silicon surfaces have received significant attention not only from a fundamental perspective, but also due to the technological relevance for electronic and biological and chemical sensing applications. A particular class of organic molecules, which self-assemble into one-dimensional chains covalently bound on Si(100)-2x1:H, reveal interesting binding chemistries and electronic properties depending on the terminal group of the molecule and the reaction sites at the surface. An ultra-high vacuum (UHV) scanning tunneling microscope (STM) was used to discern the atomic scale chemical binding and electronic properties of several molecules which form one-dimensional nanostructures on Si(100)-2x1:H. In order to demonstrate that replacing deuterium with hydrogen would allow for non-STM studies of the binding configurations of chain growing molecules, deuterium was used in place of hydrogen on either the molecule or the silicon to investigate the reaction mechanism for forward and reverse molecular chain growth. UHV STM studies showed that replacing deuterium with hydrogen on either the molecule or the surface does not inhibit the formation of the molecular nanostructures on the passivated Si(100)-2x1 surface. The o-phthalaldehyde (OP) nanostructure growth mechanism is dependent on the reaction site the molecule binds to at the surface. At a single dangling bond site, OP forms chains attached via a single Si-O covalent bond, while at a dangling bond pair site, the chain is attached via two Si-O covalent bonds on a single silicon dimer. UHV-STM measurements of both the topography and electronic structure reveal distinctive binding configurations of the OP chains. Though chain growth has primarily been observed with 1-alkene and 1-aldehyde molecules, 1-alkyne molecules, namely phenylacetylene (PA), also exhibit chain growth behavior. While some 1-alkynes undergo a two-step reaction with the silicon surface, thus eliminating the double bond attachment, the UHV-STM observed the PA molecules form chains with the C=C double bond intact which leads to a fully conjugated molecular nanostructure. Heteromolecular nanostructures of PA and styrene were fabricated and a difference in the electronic structure was observed over a one volt range. This distinction is attributed to the difference in the electronic properties of the two molecules due to enhanced electronic transport through the conjugated PA nanostructure. By varying the functionality of the PA molecule, additional techniques, such as x-ray photoelectron spectroscopy and sum frequency generation, were used to verify the retention of the double bond in the molecular nanostructures. These studies reveal the important role the binding chemistry plays on the electronic properties and growth kinetics for a variety of molecules, as well as demonstrates the power of the UHV STM as a tool to probe the atomic scale properties of molecular nanostructures.