Paul S. Weiss

Exploring and Controlling the Atomic-Scale World

We focus on gaining atomic-scale understanding and control of materials properties. We do this by exploring, probing, and manipulating interactions and dynamics at surfaces and interfaces. We use and extend scanning tunneling microscopy to explore the surface structures, motion, and perturbations due to adsorbates and surface features such as substrate steps and defects. We locate, study, and try to exploit the regimes in which our intuition (which is usually based on macroscopic measurements) breaks down. We are exploring the phenomena to be used, the ground rules, and the ultimate limits in nanometer-scale electronics, storage, and motors. Our microscopes not only serve as probes, but also allow us to manipulate matter on the atomic scale. We can thus interrogate the properties of uniquely configured atomic-scale structures. This has required the development of new tools with atomic-scale views of the surface.

Adsorbed atoms and molecules (and other surface features) perturb the electronic structure of the surrounding surface. Not only can we image electronic perturbations directly with the scanning tunneling microscope (STM), but we can tune the temperature of the experiments so as to allow the mobile molecules to probe, to decorate, and thus to highlight these surface sites. We relate the molecular positions to the modulated surface electronic structure (measured by STM spectroscopy) to elucidate the interactions of the adsorbate with the substrate. We have found important implications for these effects. In catalysis, reactants or intermediates can be guided into the correct configuration for reaction or can form complexes in the correct orientation for reaction. This greatly enhances the reaction rate. We are also attempting to use these effects to grow atomically precise structures on surfaces.

The complex and intertwined interactions of the molecules in monolayer films are more difficult to reduce to their component parts. We have determined relative interaction strengths by studying the phase behavior of mixed composition monolayers. We are able to control the motion of molecules between the film and solution in order to direct the assembly process. We control the defect density and type in the film to isolate single or bundled molecules for further study and to tailor the film properties.

We can place atoms where we want them on the surface using the STM tip as a tool. This allows us to assess the stability, dynamics, and properties of specifically constructed nanometer-scale structures. While we do not expect this to be a fabrication method for manufacturing, it allows us to target interesting structures synthetically and to search for novel and useful phenomena at a scale not otherwise accessible.

We are exploring the ultimate limits of logic, memory, and motors. We are testing organic molecules to see if they may be able to replace or to augment the functions of conventional microelectronics. This may be in the form of interconnects or in device components. We have measured the transconductance of single molecules. We are now developing the means to control this conductance en route to active devices. We determine the key electronic properties and couplings of molecules and work closely with synthetic groups to improve and to optimize these. We are also examining how we can use external fields to control the motion of molecules so as to turn them into molecular motors. Once again, our close collaborations with synthetic groups enable testing, optimization, and understanding of these molecular motors.

We have made significant advances in developing new tools that probe at the atomic scale. We have developed tools and methods for recording local optical, microwave, and other spectral signatures. Scanning probe microscopes are ideal for isolating single particles or molecules for study. What remains to be done is to develop interpretable spectroscopies. We compare our results to macroscopic measurements and to theory whenever possible. Each new capability that we have added has led to surprises at the atomic scale.

A further group effort involves trying to understand and to control biological membrane properties by controlling their local composition and structure. This work focuses on uptake, infection, transfection, and immune response by studying these processes at the single molecule and particle levels. We are developing the means to control and to probe biological and other fluid interfaces with the goal of controlling the resulting properties, in analogy to our capabilities on the flat surfaces of solids. We hold and orient cells or model membranes in solution with optical traps or with micromanipulators, then induce controlled collisions with cells, vesicles, model pathogens, or particles simultaneously held in another optical trap or driven by flow. We can control impact velocity and orientation, cell distortion, and solution environment. Our goal is to develop a comprehensive methodology for controlling membrane function through manipulation of the local membrane composition and structure.

Our group is made up of scientists and engineers from a range of different fields: chemistry, physics, biology, materials science, electrical engineering, and mechanical engineering. Likewise, the problems that we tackle straddle these and other disciplines. The cross-training required to do this work is an important part of the education that students in our group receive.