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Posted: May 27th, 2008
New tools to accelerate the development of molecular technologies
(Nanowerk Spotlight) One of the many fascinating concepts in nanotechnology is the vision of molecular electronics where tomorrow's engineers might use individual molecules to perform the functions in an electronic circuit that are performed by semiconductor devices today. This is just another example of scientists taking a cue from nature's playbook, where essentially all electronic processes, from photosynthesis to signal transduction, occur in molecular structures. The basic science on which molecular electronics technology would be built is now unfolding but researchers are still struggling with the most basic requirements for molecular electronics, for instance, how to precisely position individual molecules on a surface or how to reliably measure the resistance of a single molecule.
A tremendous amount of painstaking work goes into developing the kind of ultraprecise and ultrasensitive instruments that are required to develop electronics at the nanoscale. A recent example is a new device for measuring the conductance values of single-molecule junctions which are covalently bound to two electrodes.
"To a large degree, the continued advances in molecular electronics depend on a search for new molecular systems and new applications" Dr. Joshua Hihath explains to Nanowerk. "Each molecular system requires a great deal of time to measure and characterize, and often, the results are less promising once the system is understood. Researchers regularly carry out systematic measurements on families of related molecules in order to determine the underlying transport processes as well as the physical and chemical constraints on the molecular systems. However, most systematic studies require exhaustive efforts in data processing and experimental time to understand simple processes such as tunneling. More complicated transport phenomenon are sometimes not even possible to determine due to the number of samples needed to understand the conduction mechanism of the system."
Hihath, a post-doctoral researcher in Professor Nongjian Tao's research group in the Department of Electrical Engineering at Arizona State University, is first author of a recent paper in Nanotechnology in which he and Tao describe a novel conductance screening tool for molecules ("Rapid measurement of single-molecule conductance"). This device is capable of identifying signals due to a single molecule covalently bound between two electrodes, providing repeatable contacts to single molecules, and performing a statistical analysis of the results to take into account the affects of different contact geometries and molecular configurations.
"Our paper describes three major findings" says Hihath "First, it introduces a new tool we have developed that is inexpensive, easy to operate, and capable of quickly determining the electrical conductance of a single molecule bound to two electrodes. The second finding is that this type of experiment can be expanded to work in an array where different molecules are placed on a surface, and each one can be measured in sequence. These two findings open the door to faster more reliable measurements of the conductance of a single molecule. With these capabilities in hand, the device is proven to work in aqueous solutions where biologically relevant molecules such as amino acids or DNA can be studied."
Mechanical design of CSTM. (a) shows CAD drawing of the CSTM with tip holder movement in horizontal plane, and substrate movement in vertical axis. (b) Photograph demonstrating actual implementation of the completed device. (Reprinted with permission from IOP Publishing)
Tao and Hihath's Conductance Screening Tool for Molecules (CSTM) is based on two tools that are currently used for studying the electrical properties of a single molecule bound to two electrodes, the Scanning Tunneling Microscope (STM) break junction, and the Mechanically Controlled Break Junction (MCBJ).
In these tools, conductance measurements on a single-molecule junction are performed by having two atomic scale electrodes in the presence of molecules capable of binding to the electrodes through covalent linking. Generally, the electrodes are moved closer together under a fixed bias until the current reaches a sufficient level to saturate the current amplifier, when this level is reached the two electrodes are retracted until the current reaches zero.
The issues with each of these systems lie largely in low speed and high cost. One factor why the CSTM is inexpensive is that the system is designed to be a single computer system in which only one computer is used to control the mechanical actuation, and perform data acquisition.
"The CSTM's control system works in three primary modes: coarse movement, stable tunneling, and tapping" Hihath explains. "The feedback for all three modes is the current between the tip and the substrate. In fact, the only additional hardware needed to run the system in its basic configuration is a current preamplifier that provides a voltage signal proportional to the current through the junction."
Hihath says that their new tool combines some of the strengths of both the STM break junction and MCBJ. "The experiments conducted with the CSTM demonstrate that measurements can be carried out in an array, an important evolution in break junction experiments, and that molecules can be measured with asymmetric linkage to the electrodes. Furthermore, this is the first measurement of individual amino acid residues. Amino acids are the basic building blocks of proteins, and the study of these basic systems is important to understanding a variety of natural phenomenon ranging from oxidative damage in cells to photosynthesis."
Tao mentions that his group was motivated in this work by their ongoing efforts with other molecular systems such as DNA. "We have been using the STM break junction technique for measuring the conductance of molecular junctions, and have measured tens of molecules in recent years. However, such throughput is insufficient for conducting large scale studies of families of molecules, as such, a faster, more reliable tool was needed. Previously, we have done work on DNA molecules, but to truly understand charge transport in this complicated molecular system, large scale systematic studies need to be conducted. Therefore, a tool like our CSTM, that can measure many molecules per day is clearly needed to advance the studies of biologically relevant molecules."
Scientists will have to study and characterize large numbers of molecules in their quest for molecular biosensors and molecular electronics. A quick, inexpensive yet reliable conductance screening tool could provide a very useful first-pass for studying different molecular systems and allow researchers to focus more time on promising candidates. "Our new tool provides a clear opportunity to identify new molecular species that may provide interesting effects for molecular electronics based devices, and which molecules should be studied in greater detail" says Hihath
He also notes that work will continue in the field of molecular electronics in a couple of distinct lines. "One direction is the study of molecules that provide information about molecular systems for biological or sensing applications, another direction will be continued work on molecular electronics applications such as gate effects, switching, or negative differential resistance. And, an additional direction will be in creating and understanding more robust molecular systems that are stable and reliable at room temperature for extended periods of time."