David L. Allara

Chemistry at Interfaces

The unique molecular and atomic features of the interfaces between materials often control the useful functions of both synthetic and naturally occurring structures. Examples include the rate and specificity of electrochemical processes, the adhesive strength and conductivity of thin metal-film coatings on polymer or ceramic substrates in an electronic circuit component, the compatibility of a biological implant, the efficiency of a semiconductor transistor with a chemically modified interface, and the corrosion of a structural metal part induced by its working environment.

The major objectives of Professor Allara's research program are the development of a fundamental understanding of the chemical structures and processes that occur at these interfaces, particularly for interfaces where one of the adjoining phases is organic, and the utilization of this information for the development of practical applications including advanced microelectronic devices, chromatography and biomedical implants. The approach is very interdisciplinary and includes analytical, physical, physical organic and materials chemistry, as well as physics, materials science and biology.

An interface is a complex boundary region that can be viewed as a slice of material, often as thin as one or two molecules. Extreme demands are placed on the chemical and structural probes needed to study these regions. Professor Allara researches both the development of informative model chemical structures and the development of sensitive molecular-structure probes.

One type of model consists of a supported film of monolayer dimensions such that all the molecular groups examined will be part of the interface. One system of great utility is an organized monolayer assembly of multifunctional organosulfur compounds on a gold surface. Attachment to the gold occurs via a bivalent sulfur atom. Other groups such as amino, hydroxyl carboxylate and derives esters, methyl, and fluoroalkyl arrange themselves in the interior or at the ambient interface as dictated by thermodynamics and molecular structure. These model structures have provided details about the molecular basis of the wetting of liquids by an organic surface, transport of electrons, ions and metal atoms through monolayer coatings, adhesive chemical-bonding interactions in polymer coatings and interaction mechanisms of biological entities, e.g., proteins and cells, with organic surfaces. One of the fascinating aspects of these systems is the alteration of chemical-reaction mechanisms of organic groups because of the "two-dimensional" nature of the assembly. Recent efforts have focused on the assembly of highly oriented monolayers of fully conjugated molecules on metal electrode substrates. This work is now leading to the development of ultra-small electronic devices that will be used to assemble a computer based on molecular components.

Characterization of these material structures is performed in the Allara lab, as well as in collaboration with other scientists, primarily by infrared vibrational spectroscopy, optical wavelength ellipsometry, X-ray photoelectron spectroscopy, electrochemistry, scanning tip microscopy, quartz crystal microgravimetry, and time-of-flight secondary ion mass spectrometry. In the case of vibrational spectroscopy, it has been necessary to develop new types of experimental and theoretical approaches in order to provide quantitative characterization of structural features such as surface orientation, group conformations, and intermolecular interactions. In addition to using state-of-the-art instruments, a combination of molecular vibration analysis, classical electromagnetic theory and quantum chemical calculations have proved useful for theoretical interpretations. Other promising characterization techniques are continually being evaluated. Of recent interest is grazing incidence x-ray scattering (at synchrotron facilities). Current work also includes characterization of the electrical transport properties of single molecules through the use of nanofabricated device structures.