Atomic Scale Mechanical and Chemical Properties of Surfaces

Funded by the DOE Office of Basic Energy Sciences
Miquel Salmeron,  Principal Investigator

The purpose of this project is to study physical and chemical processes at surfaces and interfaces. Adsorption, diffusion and reactions are studied at the molecular level using Scanning Tunneling and Atomic Force Microscopies. Mechanical properties of friction, adhesion and wear are also investigated at the atomic level to determine the mechanisms that govern the tribological properties. In particular we focus on the role of wetting films and lubricants in modifying these properties.  Experiments are carried out in the following areas:

a) structure, dynamics and reactions on surfaces with adsorbed atomic and molecular layers;
b) mechanism of energy transfer, via excitation of specific vibrational and electronic states in single atoms and molecules;
c) structure of liquids and molecular films to understand wetting at the molecular scale;
d) correlation between friction and structural properties of clean surfaces and of surfaces covered with lubricant layers.
e) development of novel instruments that facilitate obtaining microscopy and spectroscopy information about surfaces in equilibrium with gases at ambient pressures

CURRENT PROJECTS        

• Studies of water structure on metals (Pd, Ru and Cu) to determine growth structure and dissociation
• Manipulation of single molecules via vibrational and electronic excitations
• Surface chemistry of ammonia, hydrogen and CO on Pd and Ru single crystals
• Force Spectroscopy on single molecules using atomic force microscopy at liquid helium temperatures
• Studies of the molecular scale origin of energy dissipation in friction are carried out using model lubricant films, in the form of self-assembled monolayers. We use alkylamines, alcohols and fatty acids self-assembled on gold, mica and silica films on silicon wafers.
• Effect of water on the tribological and wetting properties of organic films. Adsorption of water is studied with in situ spectroscopies (XPS) as a function of end group chemistry (CH3, CH2OH, COOH, NH2).

We continue to develop novel instruments (for AFM, STM, various photon spectroscopies) to obtain microscopy and spectroscopy information on surfaces in equilibrium with gases at ambient  pressure.

Chemical and Biological Interactions at Environmental Interfaces
Funded by the DOE Office of Biological and Environmental Research
Miquel Salmeron,  Principal Investigator

This program seeks to provide a fundamental understanding of the chemical and physical processes occurring at solid-aqueous and solid-vapor interfaces of relevance to the Earth’s near-surface environment, more specifically determining the structure of water near the surfaces of natural systems including minerals and biological organisms, and the impact of environmental contaminants.  Most of the chemical reactions that are important for environmental processes take place at the solid-liquid interface between water or aqueous solutions and metals, minerals, soils, and rocks.    

We use scanning tunneling microscopy (STM), scanning polarization force microscopy (SPFM), non-contact atomic force microscopy (NC-AFM), in situ electron and photon spectrosocopies and the unique ambient-pressure photoemission spectroscopy (APPES) to investigate the adsorption of water on various types of environmentally relevant surfaces.     With these tools, we can measure the mobility of environmentally relevant ions, such as transition metal ions, on mineral surfaces.  We also investigate the wetting of mineral surfaces by water and aqueous solutions, as well as the solvation of these surfaces by liquid films.   We study the orientation of polar molecules at the surface, as well as chemical inhomogeneities of the liquid film.  Scanning tunneling microscopy allows imaging of electrically conductive surfaces with atomic resolution.  So we are able to study water adsorption on metal surfaces, closely examining the adsorption site and the orientation of single water molecules, their diffusion along the surface, and their agglomeration to dimers, mono, bi- and multilayers.  We
study ice and water surfaces in equilibrium with the vapor at ambient temperatures.  Of special interest are the thickness of the water layer as a function of humidity and the adsorption geometry of water molecules on well-ordered surfaces.

CURRENT PROJECTS

• Continuing studies of growth and structure of water films on oxide surfaces, extending previous work to include Fe2O3, MgO.
• Completing the study of water on TiO2, using NEXAFS to determine if possible the orientation and H-bonding structure of water in successive monolayers.
• Studing the influence of impurity species on the surface on molecular water structure. Solvation effects.
• Examining growth and structure of water on organic surfaces, starting with models such as self-assembled monolayers of alkyl- thiols, silanes and amines;  the influence of tail group (methyl, acid, alcohol); and ultimately films of membranes, proteins and humic substances.