Research interest and activities

Molecular Electronics - Single molecule conductance measurement

From a fundamental view, this topic is essentially a continuation of several decades of electron transfer research of donor-molecule-acceptor system. From an application view, it's of great interest to explore the capabilities of molecules as building blocks in future nano-electronics. A major challenge of molecular electronics is to fabricate nanodevices based on molecules that can replace, or at least augment, present silicon-based semiconductor technology to function as interconnects, switches, transistors or more complex devices.

Many techniques have been developed to probe the conductance of single molecules, either using a fixed gap between two electrodes (fabricated by lithography techniques) or using a flexible electrode gap (such as Mechanical break-junction, Conductive AFM and STM break junction method developed at ASU). The main technique we currently used for single molecule conductance measurement is STM break junction method. Briefly, a break-junction is formed repeatedly by pushing a gold probe into the Au (111) substrate (covering molecules with sticky groups at both ends) and recording the current vs. time as the probe is withdrawn. Steps in the trace of current vs. time correspond to integer numbers of molecules trapped in the gap. Histograms are built based on thousands of such traces. The peaks in the histograms give the single molecule conductance value. Photochromatic molecule, different molecular wires are studied by this method. Recently, I mainly studied the charge transport of Au-redox molecule-Au junctions immersed in an electrolyte using STM.

Fig.1 STM break junction method

Porphyrin Molecule-Organic Photovoltaic

Porphyrins and their derivatives, such as chlorophylls, hemoglobin, and cytochromes, are essential in biocatalytic reactions. All of these molecules share in common the porphyrin macrocyclic substructure, consisting of a conjugated ring of four pyrrole subunits which form a planar, tetradentate unit. Porphyrin molecules with anchor groups for metal electrode are synthesized by Gust and Moor's group at ASU Chemistry department. The charge transport properties of porphyrin molecules (also the response of conductance to light) between solid electrodes are studied and are of great interest for the future application of porphyrin in organic photovoltaic.

Porphyrin electrochemistry is the most significant biological process carried out by plants and animals (for example, photosynthesis and respiration, respectively). In the systems which carry out the processes of photosynthesis or respiration, at least one metal ion is coordinated to a pophyrin macrocycle. Note that the metal ion has its axial coordination sites open and so may take on one or two additional monodentate ligands.

My efforts are focused on study how the iron redox potential affected by the subtle change of axial ligation at single molecule level. Cyclic voltammetry is a usual way to explore the change of reduction potential of the iron center for a monolayer of molecule on the metal surface. We found conductance-electrode potential curve (GE) is an excellent method for exploring the same effect but at single molecule level. Huge current enhancement is observed when the charge transfer between local STM tip and global metal surface is mediated by the iron redox state of individual porphyrin molecule.

Fig. 2 Probing individual molecule redox property by STM

Nanodevice fabrication

Optical and E-beam Lithography, Focused Ion Beam (FIB) and electrochemical deposition methods are used to fabricate nanostructures. One of the applications of these nanodevices is to probe the charge transport of long molecule wires (bigger than 10 nm). Fixed nanogap devices do not generally give reliable or reproducible results with metal electrodes at separations of 1-5 nm, because the gap geometry is poorly characterized and variable with time, due to electromigration of metal atoms(especially at room temperature and in liquid). However, nanogap with bigger range is relatively easy for fabrication and calibration. Long molecule wires (such as conducting polymers, biopolymers) are good candidate for nano fabrication device.

Fig.3 nanogap fabricated by Electrochemical method (left) and EBL (right)

Biomolecule imaging

We are developing novel method to image DNA in their natural environment (water) by STM through tunneling current. It has potential to be applied in DNA sequencing. The method can be expanded to other biomolecules.