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Mechanical and electrical properties of molecules in self-assembled films

Atomic scale imaging, manipulation, and spectroscopy Mechanical and electrical properties of molecules in self-assembled films Ambient pressure photoelectron spectroscopy for environnemental sciences Studies of friction, adhesion, and wear at the nanometer scale Electronic, mechanical, and chemical properties of nanoclusters Structure of thin liquid films and wetting Nanoscale material imaging and manipulation (Molecular Foundry) Catalytic and chemical properties of surfaces

Updated by Franck, May 27 2008

Structural, Mechanical and Electrical Properties of Molecular Films

POSTDOC: Bas Hendriksen
STUDENTS: Yabing Qi, Florent Martin

FORMER RESEARCHERS AND GUESTS: Imma Ratera, Jinyu Chen, Jeong Park, Daniel Esteban (Universitat Politècnica de Catalunya), Kyal Wright (Norfolk State University), Gilbert Amani (Dorsey High School, Los Angeles), Violeta Navarro (Universidad Complutense de Madrid)

The detail of the instrument can be found here ?

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 oligo(phenylene vinylene) 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 Fréchet group at UC Berkeley.

Related publications

-Mechanical and charge transport properties of alkanethiol self-assembled monolayers on Au(111) surface: The Role of Molecular Tilt
Yabing Qi, Imma Ratera, Jeong Y. Park, Paul D. Ashby, Su Ying Quek, J. B. Neaton, Miquel Salmeron
Langmuir 24, 2219 (2008) LBNL-69E

-Atomic Force Microscopy Nanotribology Study of Oligothiophene Self-Assembled Films
I. Ratera, J. Chen, A. Murphy, D.F. Ogletree, J.M.J. Frechet, and M. Salmeron
Nanotechnology. 16, S235-S239 (2005). LBNL-58931

-AFM study of ß-Substituted-7T Oligothiophene films on Mica: Mechanical Properties and humidity dependent phases
J. Chen, I. Ratera, D.F. Ogletree, M. Salmeron, A.R. Murphy and J.M.J. Frechet
Langmuir. 21, 1080-1085 (2005). LBNL-55455

-Preparation and Nanoscale Mechanical Properties of Self-Assembled Carboxylic Acid Functionalized Pentathiophene on Mica
Jinyu Chen, Amanda R. Murphy, Joan Esteve, D. F. Ogletree, Miquel Salmeron, and Jean M. J. Fréchet
Langmuir. 20,7703 (2004)

Previous projects

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.

Self-assembled alkanethiol molecule islands on Au(111)	Filtered lateral force image of the molecular lattice