Hamers Group Research Projects
Using "designer chemistry" to understand and control the electronic properties of molecules and molecular interfaces
While the microelectronics industry has long revolved around inorganic materials such as silicon and gallium arsenide, there is now great interest in the use of organic molecules as electronic devices. Organic molecules such as pentacene are organic semiconductors that can, in principle, replace silicon. It is even possible to use individual molecules as active devices like diodes and transistors, leading to the emerging area of "molecular" electronics.
Whether one is dealing with a large ensemble of molecules or a single molecule, the use of molecules in microelectronics has a number of important challenges. One of the big challenges is that it is necessary to control molecules assemble into more complex structures. A second challenge is that while the electronic properties of individual, isolated molecules may be understood, in order to use molecules as electronic elements it is necessary to make electrical connections to them. Chemists usually think of electrical connections between atoms as chemical bonds. Since molecular devices will ultimately be fabricated on some kind of substrate (such as silicon), it is necessary to understand how the electrical contact between the molecules and the underlying substrate (and eventually, some type of "top contact" ) depend on the detailed chemical and physical structure at the interface.
We are researching the fabrication and characterization of well-defined interfaces between silicon and organic molecules. Much of this research focuses on understanding how to control the chemistry to achieve high selectivity in how molecules with extended pi-conjugated systems bond to surfaces. We are characterizing the interface chemistry and the electrical properties. A particularly important topic is simply to identify what kind of "atomic" contact provides the best electrical contact between an organic molecule and the underlying substrate. By varying the chemical identity of the single-atom "tether" linking molecules to surfaces and then characterizing how this changes the electrical properties, we are obtaining insights into how to design molecular interfaces with specific types of electrical behavior.
As a second step, we are investigating how to fabricate a "top contact" onto organic structures. Because organic molecules typically have very low surface energies, deposition of metals onto organic layers typically does not produce well-defined metal layer, as the metal prefers to "ball up" on the surface in order to minimize the total surface energy. We are investigated new strategies for controlling the chemical structure (and hence, the surface energy) of molecular interfaces, and understanding how this affects the electrical transport properties.
This research involves a combination of methods. Scanning tunneling microscopy is used to examine molecular structures on silicon surfaces, providing an atomic-resolution picture of the interface. Electrical properties are measured on a "single-molecular" level using scanning tunneling microscopy/spectroscopy methods, and are also performed on larger bulk junctions using more conventional electrical characterization methods. Valence-band photoemission spectroscopy is used to determine the electronic energy levels of the molecules and the substrate. The electrical characteristis of bulk junctions are also measured using a conventional electrical impedance-measuring system.
While much of this research uses ultrahigh vacuum surface-science methods for preparation and characterization, we are also interested in extending this work to solution-phase methods of attachment. This aspect of the research borrows from the wet-chemical methods developed for biological attachment of molecules (described in the other research projects).