Structural, Mechanical and Electrical Properties of Molecular Films
Molecular Electronics and Organic Electronics use single molecules and molecular films as electronic materials and functional components. Understanding the mechanisms responsible for electrical conduction in organic molecules and organic molecular layers is not only important for molecular/organic electronics, but also for other systems in which charge transport through organic molecules plays a role. In this project we focus on the relation between the structural, mechanical, and the electrical properties of monolayers of organic molecules supported on solid substrates. We use an atomic force microscope (AFM) in combination with electrical current measurements. We try to understand how electrons (or holes) flow through molecular monolayers, how the charge transport depends on the structure of the monolayer (i.e the lattice structure, molecular orientation, defects, domain boundaries), and how the charge transport responds to mechanical forces and chemical inputs.
In our experiments we use the tip of an AFM to image the structure of the molecular monolayer, to mechanically manipulate the molecular monolayer, and to measure the current between the tip and a conducting substrate perpendicularly through the monolayer by means of IV spectroscopy and current mapping. In collaboration with the Nanofabrication Facility at The Molecular Foundry we are developing ultraflat, coplanar nanoelectrodes, which will allow us to also determine the transport properties laterally through the monolayer. Synchrotron X-ray absorption and photoelectron emission spectroscopy, performed at the Advance Light Source, complement our AFM measurements and give information of the electronic structure and the orientation of the molecules of the monolayers.
Molecular monolayers of oligothiophene and perylene based molecules, used in our experiments, are prepared either by self-assembly or the Langmuir-Blodgett technique. These tailor-made conjugated molecules were synthesized by the Organic Nanostructures Facility at The Molecular Foundry and the Schore group at UC Davis.
B.L.M. Hendriksen, F. Martin, Y. Qi, C. Mauldin, N. Vukmirovic, J. Ren, H. Wormeester, A.J. Katan, V. Altoe, S. Aloni, J.M.J. Fréchet, L.-W. Wang, and M. Salmeron
Electrical transport properties of oligothiophene-based molecular films studied by current sensing atomic force microscopy
Nano letters, 2011. 11(10): p. 4107-12. http://www.ncbi.nlm.nih.gov/pubmed/21848283
F. Martin, B. Hendriksen, A. Katan, I. Ratera, Y. Qi, B. Harteneck, J.A. Liddle, and M. Salmeron
Ultra-flat coplanar electrodes for controlled electrical contact of molecular films
Review of Scientific Instruments, 2011. 82(12): p. 123901-123901. http://link.aip.org/link/RSINAK/v82/i12/p123901/s1&Agg=doi
G. Koshkakaryan, P. Jiang, V. Altoe, D. Cao, L.M. Klivansky, Y. Zhang, S. Chung, A. Katan, F. Martin, M. Salmeron, B. Ma, S. Aloni, and Y. Liu
Multilayered nanofibers from stacks of single-molecular thick nanosheets of hexakis(alkoxy)triphenylenes
Chemical communications (Cambridge, England), 2010. 46(45): p. 8579-81. http://www.ncbi.nlm.nih.gov/pubmed/20972497
The research is to be conducted with an atomic force microscope (AFM ) to study organic molecule SAM's. AFM is used to measure normal and lateral (thus frictional) forces between sharp micromachined Si tips and Au substrates (fully or partially) covered with organic molecules. At present the organic molecule system I mainly focus on is alkanethiols. Because of its structural simplicity and well defined ordering, alkanethiol SAMs have been extensively studied. The structural information such as the arrangement and tilting of the molecules has been complementarily obtained by interferometry, IRAS, FTIR, Raman spectroscopy, XPS, and other techniques. Besides forming a full monolayer, sometimes it's desirable to form a partial nanolayer (islands of molecules), because the height of standing molecules can be directly measured. Previous studies indicate that under normal loads, the alkanethiol chains undergo stepwise discrete changes in the island height due to neighboring molecules interlocking with each other.
Currently, we are modifying our AFM in order to map simultaneously the current, the topography and the friction of such molecular SAMs. In this way we will be able to explore the correlation between the mechanical properties of molecules with electronic transport phenomena. As a first order approximation, the molecular junction can be divided into 3 parts, two metallic electrodes on each side, and the molecules are sandwiched between them. The charge transport through a molecular junction depends not only on the molecular orbitals, also their alignment relative to the electrode Fermi levels. An example is that when the molecular levels are lined up with the electrode Fermi levels, the resonance tunneling may give rise to a significant increase in conductance. In collaboration with Alexander Liddle of the Nanofabrication lab at LBNL, we are going to built planar electrodes for 3-terminal measurements, which will allow us to study the transport anisotropy, and to manipulate the molecular levels relative to the electrode Fermi levels.
In the future, the research will be expanded to various types of molecules with highly delocalized orbitals such as thiolated stilbenes and oligothiophenes.
Here are contact mode images of the alkanethiol molecules self-assembled on Au substrate.
left: Self-assembled alkanethiol molecule islands on Au(111)
right: Filtered lateral force image of the molecular lattice