Correspondence can be addressed to Paul S. Weiss.

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Overview

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 adsorbed atoms and molecules and due to surface features such as substrate steps and defects. We locate, study, and try to exploit the regimes in which our intuition 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 and storage. Our microscopes serve not only 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. One new effort in our group looks at how we can bridge the gap between conventional optical microscopies and scanning probe microscopies.

A related effort involves trying to understand and control transport through membranes, adhesion to cells, infection, and immune response. This work focusses on developing models for anesthesia, uptake, transfection, and infection by studying these processes at the single molecule level. We are developing the means to control and to probe biological and other fluid interfaces in analogy to our capabilities on the flat surfaces of solids.

Nanometer-scale Electronics and Storage

We are exploring the ultimate limits of logic and memory. 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 conductance and switching of single and bundled molecules. 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 studying the nanometer-scale behavior of both inorganic and organic ferroelectric materials. We are developing new tools that take us to the nanometer scale and below. While there are predictions of interesting and useful behavior at this scale, there are no prior measurements. We seek to combine materials properties at this scale with our ability to pole (write) and to read back in order to enhance our resolution. We use single polymer molecules, oligomers and nanoparticles for this purpose. Once again, close coupling with synthetic groups positions us uniquely to explore this scale.

Surface Interactions

Adsorbed atoms and molecules (and other surface features) perturb the electronic structure of the surrounding surface. This leads to spatial oscillations in electronic structure that we can measure and that are felt by other adsorbates. We first saw this effect when we observed nucleophilic benzene molecules forming ordered structures, lining up at regions of high local density of empty states, even at coverages below 0.01 monolayers. 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 also relate the molecular positions to the modulated surface electronic structure (measured by STM spectroscopy) to elucidate the interactions of the adsorbate with the substrate. When the surface electrons are in dispersive states, the maximum electronic enhancements are at different positions for different electron energies. In our STM images, we find that the residence times of molecules are longer at these electronically favorable sites. We have determined the electron energies and thus the surface and molecular electronic states involved in bonding the molecules to the surface. 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 focussed on dynamics such as the newly discovered forms of motion discussed below. We have determined relative interaction strengths by studying the phase behavior of mixed composition monolayers. Varying intermolecular interactions not only lead to phase separation, but also affect the film structures by introducing edge tension at domain boundaries. In mixed composition films, surface defect densities are greatly reduced due to the more open film structures at domain boundaries during deposition. Edge tension also allows film motion via exchange. 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 in the film by composition and processing. We use this capability to isolate single or bundled molecules for further study and to tailor the film properties.

We are testing various means to "focus" chemical action to very small spots. These take advantage of interfacial or sequential reactions, or the uneven distribution of solute remaining from solutions dried under various conditions. We are also taking advantage of confined spaces such as channels smaller than the wavelength of light to probe, to separate, and even to collect single molecules. These new instruments have uses in ultrasensitive detection, in nanometer-scale patterning, and in the study of surfaces at the atomic scale.

Surface Motion, Dynamics, and Direct Manipulation

We look at three types of motion on surfaces -- diffusion, transient mobility, and induced motion. Real space diffusion measurements are typically hindered by the fact that measurements are much slower than the diffusion rates. We approach this problem in three ways. First, we can slow the diffusion rates down so as to be within the dynamic measurement range of our instruments by cooling the experiment. Second, we increase the effective dynamic range of our instrument by measuring adsorbate residence times at particular sites. For benzene on Cu{111} at 77K we monitored the atomic-scale dynamics at the interface between a 2D molecular solid (benzene tightly bound to substrate step edges) and a 2D molecular gas (benzene on substrate terraces). As for the surface of a 3D solid, diffusion along the 1D edge of the 2D solid was orders of magnitude faster than desorption back into the 2D gas. Because we analyze lattice sites independently, we were also able to show that there are special interface sites (such as substrate kinks) where diffusion is not possible and bonding is weakened to the point where desorption dominates over diffusion. Third, sufficiently increased attractions between adsorbates can slow down the rates of motion to be comparable to our data acquisition rates. Such was the case for our measurements of diffusion in self-assembled monolayers (SAMs) of alkanethiols on Au{111}. We discovered two heretofore unknown forms of surface motion. In one, the mobile species were complexes of the adsorbed alkanethiolates with Au atoms. Our proposed mechanism predicted that rates of motion would decrease with increasing alkyl chain length; this has recently been verified experimentally by Poirier et al. In the other, molecules within the densely packed layers exchange places, leading to coalescence and growth of phase-separated domains in mixed composition SAMs.

For lateral motion induced by surface processes such as adsorption or chemical reaction -- transient mobility -- we analyze the distributions of the final positions of adsorbates or reaction products in order to measure the distances covered and to elucidate the means by which energy is accommodated to the surface. We have made the first and only direct measurements showing nonzero accommodation lengths for molecules (benzene/Ni{110}) adsorbing onto surfaces from the gas phase as we had previously done for atoms (Xe/Pt{111}). By developing a 2D comb structure on the surface of Ni{110} using H-induced added row line defects and substrate steps, we created a surface for studying scattering in two dimensions. In the first such experiments we directed incident benzene molecules at these structures to show that molecules dissipated their energy and stuck to these line defects on their first collision rather than bouncing between the line defects.

We have the ability to place atoms where we would like them on the surface using the STM tip as a tool. This allows us to assess the stability 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 creating and operating the first synthetic molecular motors. These are different than all previous artificial motors in having no unknown internal interfaces. Thus, we are able to simulate their function all the way from quantum chemical calculations to mechanical engineering calculations.

Extending the Capabilities of the Scanning Tunneling Microscope

We have developed the first tunable ACSTMs in order to record images and spectra on insulator surfaces. While atomic force microscopes can record topographic information on insulating surfaces, very limited chemical/spectroscopic information is obtained. In the ACSTM, electrons are transferred back and forth between the ACSTM tunneling tip and the sample surface. In addition to allowing us to work with non-conductors, the energy and frequency dependences of this transfer process provide us with chemical information of individual molecular species. We probe the chemistry of insulator surfaces using microwaves reflected from the surface, microwave throughput attenuation, and harmonics of the microwaves generated by the tunnel junction. We have shown how harmonic amplitudes in nonlinear spectroscopy with the ACSTM can be interpreted in terms of molecular motions, charging, and electronic structure. We have used these nonlinearities to study the electronic energies of insulator surface states and are extending these experiments to measure the mobility of charge on silicate glass surfaces.

Conventional optical microscopies can be used to obtain images down to ca. half an optical wavelength (200 nm) resolution. Optical spectroscopies are well understood and many techniques for staining and molecular identification have been developed. This is not true for scanning probe microscopies. The closest relative to conventional microscopes -- the near-field optical microscope yields modest gains in resolution, but strongly perturbs the optical signals obtained from the features under study. We are attacking this problem on a number of fronts. We have built a scanning tunneling microscope in which we can measure photons emitted from the tunneling junction, excited by the tunneling electrons. This gives us tremendous gains in spatial resolution since the photons only come from the atoms or molecules through which electrons are tunneling.

Scanning probe microscopes are ideal for isolating single particles for study. What remains to be done is to develop interpretable spectroscopies. Our approaches are to perform local spectroscopies in the rotational, vibrational, and electronic ranges. We compare our results to macroscopic measurements and to theory whenever possible. We also study a wide range of materials form songle atoms to high aspect ratio heterostructured nanostructures. We collaborate with a number of synthetic and theoretical groups in these efforts.

Scanning Tunneling Microscopy and Spectroscopies of Nanometer-Scale Particles, G. S. McCarty, J. C. Love, J. G. Kushmerick, L. F. Charles, C. D. Keating, B. J. Toleno, M. E. Lyn, A. W. Castleman, Jr., M. J. Natan, and >P. S. Weiss, Journal of Nanoparticle Research 1, 459 (1999). (ABSTRACT)
Some of this research is done in collaboration with:
Will Castleman, Department of Chemistry, The Pennsylvania State University.
Thomas E. Mallouk, Department of Chemistry, The Pennsylvania State University.
Kazumi Matsushige and Hirofumi Yamada, Venture Business Laboratory, Department of Electronic Science and Engineering, Kyoto University.
James M. Tour, Department of Chemistry, Rice University.

Molecular-Scale Control & Measurement of Composition & Properties in Membranes

We are able to induce separation of components in membranes by manipulating their environment and applying forces to them. We measure these changes using fluorescent probes. By this method, we are attempting to control local properties such as adhesion, transport, infection, and immune response. We apply techniques including mechanical micromanipulation, multi-beam optical trapping, micropipette aspiration & injection, fluorescence microscopy, controlled collisions & interactions, and environmental control to these problems.

We have worked on model systems such as giant unilamellar phospholipid bilayer vesicles as well as on true biological systems. We are developing methods to perform these experiments by adapting and combining existing technologies with our own for the mechanical and optical manipulation of the membranes, as well as for imaging the results, and for measuring the resultant local properties. We relate membrane curvature to the local composition of multi-component lipid bilayers. Such variations within membranes are important biologically for such processes as including exocytosis, endocytosis, and adhesion.

We are also studying the first steps in respiratory infection by building realistic experimental models of lung surfactant membrane and studying the interactions of pathogens with these membranes. Once again, we use controlled collisions and fluorescent probes.

Future Prospects

We continue our efforts in all these areas. We are currently extending our understanding of STM images by recording homologous series of adsorbate species such as rare gases and substituted aromatics. Our ultrastable STM is capable of probing the same surface site for days at a time with neither drift nor contamination. We can therefore probe how images depend on the tunneling conditions over a wide range (i.e. five orders of magnitude in tunneling current and thus in electron density), on the local chemical environment, and on the adsorption site. We can then compare quantitatively to theoretical calculations to gain a predictive understanding of STM images that currently does not exist. We are developing our spectroscopic capabilities as well. We are attempting to record the vibrational spectra of single adsorbates via inelastic tunneling and the rotational spectra of single adsorbates via microwave spectroscopy. These spectroscopies will yield valuable information about intramolecular structure and bonding of individual molecules which we can relate to surface interactions by characterizing the surface chemical environment with the STM. We are conducting further experiments on interactions of surface adsorbates and on motion due to adsorption, reaction, and 2D scattering. In this way, a multifaceted atomic-scale view of the surface-adsorbate and adsorbate-adsorbate interactions is produced in terms of bonding, structure, and dynamics.

Collaborations

Jim Adair, Materials Research Laboratory, The Pennsylvania State University.
Dave L. Allara, Department of Chemistry, The Pennsylvania State University.
Will Castleman, Department of Chemistry, The Pennsylvania State University.
Nina Federoff, Biotechnology Institute, The Pennsylvania State University.
Robert H. Grubbs, Department of Chemistry, Cal Tech.
Naomi J. Halas, Department of Electrical and Computer Engineering, Rice University.
Rex Hjelm, Los Alamos National Laboratory.
James Hutchison, Department of Chemistry, University of Oregon.
Thomas N. Jackson, Department of Electrical and Computer Engineering, The Pennsylvania State University.
Chris Keating, Department of Chemistry, The Pennsylvania State University.
Thomas E. Mallouk, Department of Chemistry, The Pennsylvania State University.
Kazumi Matsushige and Hirofumi Yamada, Venture Business Laboratory, Department of Electronic Science and Engineering, Kyoto University.
Chris Murray, University of Pennsylvania.
Toshio Sakurai, Institute for Materials Research, Tohoku University.
James M. Tour, Department of Chemistry, Rice University.
Nick Winograd, Department of Chemistry, The Pennsylvania State University.

Return to Weiss Group home page

psw
Outdated page, sorry
6 November 2002, with some deletions on 9 April 2012