New methods for controlling molecular motion on surfaces

Tufts University, Medford MA

Investigators

Abstract

In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program, Charles Sykes of Tufts University is investigating ways to direct and control the motion of molecules on surfaces. This motion occurs in two ways: molecules can travel across the surface in straight or curved lines or they can spin in place like a top. Currently, it is not well-understood how energy flowing through molecules causes them to move. This is preventing the advancement of new technologies. New methods for separating or purifying molecules require that these two types of movement be controlled, yet this can be very difficult to do. Unique new applications, including tiny molecular-sized pumps, sensors, optoelectronics, and assemblies, could become possible if a way can be found to control these motions. In this research, a sophisticated imaging system known as a scanning tunneling microscope is being used to visualize the motion of individual molecules on surfaces. Electrical pulses can be applied to the molecules in order to gain a better understanding of how energy flow leads to molecular motion. The ultimate goal of the work is the discovery of valuable design principles for the construction of molecular machines and molecular-sized devices. The work is having a broader impact through presentations the researchers are making at local high schools. Newly developed science demonstrations, "meet a scientist" days, and the inclusion of high school students in the research are bringing the work to a broad audience. The wider public is also engaged in this research through the production of YouTube videos featuring this cutting-edge research. The research is aimed at developing new methods for the controlled rotation of molecules and their unidirectional translation across surfaces by addressing several key questions. First, the relationship between a molecule's intrinsic chirality, its adsorption configuration, and its potential energy landscape on the surface must be understood. Next, molecular functionality that enables cargo to be bound and released is investigated. Finally, the electrically excited hopping of molecules with ratchet-like energy landscapes is being studied to better control unidirectional motion, providing more general approaches to the directional transport of molecules on surfaces that do not require the molecules to be a specific shape/functionality. While the electronic or vibrational states of the molecules in these experiments is to be excited with electrons from a scanning tunneling microscope tip, this method for globally inducing directed motion of all the molecules on a surface is possible by coupling to the same modes either with a macroscopic electron or light source. Discovery of novel microscopic mechanisms for directed molecular motion on surfaces will provide important proof of principle demonstrations that are generalizable in other systems and fields. This type of enabling technology is crucial for the rational design of new approaches for mass transport, separations and enantiopurifications.