Professor Kerry W. Hipps of Washington State University is supported by the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry to develop a temperature-controlled sample chamber which facilitates variable-temperature STM measurements at the solid liquid interface. The newly developed STM facility will be used to study several basic surface science problems, including the temperature dependence of the structures and formation kinetics of bimolecular physical adsorbates, the temperature dependence of phase transformations in covalently attached monolayers, the mechanisms and kinetics of chemical transformations of preformed organic adlayers, and the role of hydrogen bonding in weakly bound self-assembled structures.
The ability to see the ordering and re-ordering of molecules on a surface at different temperatures will significantly impact our understanding of these elementary chemical processes, which are of great importance in many fields, including chemical catalysis, sensor development, solar cells and self-assembly in biological systems. Students will be trained in cutting edge research methods, and WSU outreach programs will be exploited to help Hispanic and Native American students become acquainted with science and math.
Understanding and predicting the chemistry that occurs at the solution-solid interface is of critical importance for a wide range of modern scientific and technological problems. Molecular self-assembly from solution onto surfaces is widely embraced as a strategy for creating adlayers with desirable electronic, photonic, and chemical properties. Catalysis and battery development also are intimately dependent on the chemistry that occurs at the solution solid interface. If we are to develop a rational method for predicting the surface structures that yield optimal processes, we must understand the fundamentals about how adlayers are formed and react at the solution-solid interface. Throughout the last two centuries, understanding chemical processes has included the ability to measure kinetics (rates of reaction) and to measure and/or predict the thermodynamically stable product. This competition between kinetics and thermodynamics is a fundamental conflict that underlies all of chemistry. Thus, a qualitative and quantitative understanding of the kinetics and thermodynamics that occur at the solution-solid interface is an essential component to achieving the desirable goal of predicting surface structures and their chemical and electronic properties. Unfortunately, there is very little known quantitatively about kinetics and thermodynamics at the solution-solid interface, and only somewhat more is know qualitatively. Thus, an important and rapidly advancing frontier is the quantitative understanding of the relative roles of kinetics and thermodynamics at the solid solution interface. And these critical data require variable temperature and composition measurements. This great importance of these temperature and composition dependents data is counter balanced by a very limited selection of compatible tools for studying the solution-solid interface at the sub molecular – or even molecular – level. Scanning probe microscopy, and especially scanning tunneling microscopy (STM) is one of the very few techniques that offer the ability to perform these studies in various solution environments, at varying temperature and pressure, and with changing solute composition. Thus, STM will become the primary tool for analyzing structure, monitoring the time dependence of processes, and extracting thermodynamic data from systems where the critical action occurs at the nanoscale. In our studies supported by NSF we have made the first thermodynamic measurements of a reacting system using time and temperature dependent STM imaging. We have also, for the first time, made definitive measurements of desorption rates and the energy required to return an adsorbed molecule to the solution phase. To date, most variable temperature and composition STM studies at the solid-solution interface have been performed with instruments using heated samples (only). This instrument configuration dramatically limits the scope of problems that can be studied. With this NSF funding we have developed anew STM wherein both the sample and all moving parts of the microscope are at the same temperature and pressure. This design was published in the Review of Scientific Instruments and will generate data that significantly advances several areas of science and technology.
