Paul S. Weiss
Professor of Chemistry and Physics
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http://stm1.chem.psu.edu/
Paul Weiss is interested in understanding and controlling molecules at the smallest imaginable scale. Single molecules, molecules interacting on surfaces, molecules in monolayers, molecular rulers, molecular motors, controlled patterning in nanolithography, and membranes and their biological properties - these are just a few examples of the territory his group covers. The addition of biomaterials to his repertoire started after his first sabbatical 7 years ago when he had his research team start looking at how theories they had developed about solid surfaces could be ported over to biology.
One of the things his team has become extremely proficient at is measuring molecule function. They've developed tools to measure single molecules, a daunting task as it is difficult to measure molecules one at a time. His group has learned how to make 10s to 100s of thousands of measurements on like, single molecules and do it in an automated way that isn't time intensive. With an automated collection and analysis process, 100 single molecules (single molecule switches, for example) can be interrogated simultaneously, covering a dynamic range of as much as 11 orders of magnitude. This is an advance his group has pioneered - getting enough measurements to maintain objectivity without losing the insight gained from measuring individual molecules. All data is kept for reanalysis and reevaluation, and a molecular design might be tailored to test what has been learned. "It's easy to get an image of something you're looking for - it's one of the problems in this field, so you want some way to quantify it that takes one's innate mode of pattern recognition out of the picture," says Professor Weiss. "We've learned how to control molecules - from atoms moving to conductance; switching to other changes; diffusion; interacting molecules, and the like - it's opened up all kinds of possibilities for us."
Perspective views of several molecules inserted into a dodecanethiolate monolayer. The lower image reveals that several of the molecules have switched OFF. [Imaging conditions: 470 ° x 470 °; 1.0 pA; Vtip = -1.5 V.]
Molecular Switches
Another breakthrough has been in determining the mechanism for molecular switches. His group collaborates with many other groups on campus and elsewhere to design and create molecules to test various hypotheses. For example, there are a number of suggestions for how a particular family of molecules could "switch" in a device. Pieces that were thought to be an important part of the switching mechanism were added to (or taken out of) a particular molecule and tested. Once his group and their collaborators had an idea about how the switching mechanism actually worked and how they could control it, researchers elicited changes, or movement, by applying an electric field that turned the polarity of the molecule upside down (inverting its dipole moment) to get switching behavior. Whereas some have argued that one particular chemical side group was responsible for the switching function, the Weiss group deleted the side group and showed that the molecules still switched, but randomly. Their interpretation: the side group provided a handle to tilt the molecule. With these same molecules, it was possible to select environmental interactions to stabilize them in one state or another.
A Diverse Work Group
Professor Weiss likes to have many different fields represented in his group, especially since each field has a very different approach to collecting and interpreting data. Over the years he has found this to be crucial in developing new methods and equipment for "pushing the envelope." Group member backgrounds include chemistry, physics, biochemistry, biology, computer science, electrical engineering, mechanical engineering, engineering science and mechanics, and biophysics. "A tremendously important part of our lab is that we regularly have discussions for determining approaches and what the impact will be. The work that we do straddles many fields and we have to be able to talk to many audiences, understand their questions and concerns, and hopefully, exploit the various approaches to advantage."
The group collaborates widely, especially with the synthetic chemists who make molecules and who often ask for assistance in measuring an interesting property. One of the group's strongest collaborations is with Dave Allara (Chem. and MatSE), one of the worlds best infrared spectroscopists, and one of the discoverers of self-assembled monolayers that are the basis of a lot of the group's work. "Our ability to functionalize surfaces selectively has led to all kinds of things at Penn State and has been motivated by Dave Allara's original work at Bell Labs more than 20 years ago."
Ripples in the electron density of a stepped gold surface taken using scanning tunneling microscopy at 4 Kelvin. The central triangle is a single atom high terrace containing ~ 650 gold atoms. The "billiard balls" are maxima in the electron density at the Fermi level and are caused by constructive interference of the electron waves as they bounce off the edges of the triangular gold box.
Development of Measurement Tools
"One of the great things about the scanning tunneling microscope [STM] is that you can look at a surface the way a molecule sees it. You can "see" where electrons are (and where they are not) across a surface, but you can also look at the image as a function of energy - what the electronic landscape looks like." An example: The billiard design image (see image below) was created by choosing the energy to get that specific pattern. It's actually a series of wave interferences - different wavelengths produced different numbers of dots in different positions. The "protrusions" (the "billiard balls") in the electron density are constructive interference. Where electrons are at a particular energy can be mapped, then, by placing molecules on the surface and seeing where they go - Weiss and his group determined the energy of the electrons that form the interaction between the molecules and the surface. Looking at how electronic structure and chemistry were coupled was one of the goals Professor Weiss had as an undergrad. Now that his group is doing it every day - something he thought was going to take a lot longer to achieve - he's looking for new worlds to conquer.
Currently, the group is expanding the capabilities for probe microscopes and developing new nanoscale analysis tools. This is a very large effort and takes a significant amount of group time and effort. In a few specific instances, STM capabilities have been extended into local spectroscopies.
Extended Collaboration Network
The Weiss group has connections to companies and people in different fields and they work very closely with IBM (Yorktown Heights) making nanoparticles. One of the group's visiting scientists, Dr. Tomo Takami, is the chief scientist for his company in Japan and is halfway into a 3-year stint studying single molecule motors. The Weiss group also is collaborating with a former student, Dr. Stephan Stranick (now at NIST); a former post doc, Dr. Kevin Kelly (now an assistant professor of electrical and computer engineering at Rice University); a professor at the University of Rochester; and the leading producer of research STMs to develop new and better research tools.
Integrating Teaching and Research
Professor Weiss is involved in training scientists at every level. He teaches honors freshman chemistry and welcomes visiting scientists, post docs, grad students, and undergrads into his group. He has a major role in teaching in the lab and sorting students into the leading labs on campus in their first year to give them research opportunities as soon as possible.
Even with a rigorous teaching, research, and speaking schedule, Paul tries to keep up with his group, the rest of the outside world, and is always looking toward the future. His rule of thumb for keeping ahead in research: you should assume that anything you can do has been done and then figure out what you can do next. That way, you're not worried about what others are doing and you can go off exploring on your own where you're likely to find things that are more interesting and that no one else is doing. Most of the accomplishments out of his lab have come from speculation and observation that first appeared odd. While only a fraction of these have borne fruit, exploring in this way has been a successful strategy. However, now that molecules can be designed to test an idea, and the measurement capabilities exist, a whole new era has opened for the group. In many ways their work has stayed "out there" - where other people aren't working - and that's the way Paul Weiss likes it.
Bio
Professor Weiss received his SB and SM in Chemistry (1980) from MIT and his PhD in Chemistry (1986) from the University of California at Berkeley. He was a Post-doctoral Member of the Technical Staff at AT&T Bell Laboratories (1986-1988) and a Visiting Scientist at IBM (Almaden Research Center, 1988-1989) before he started at Penn State in 1989 as an assistant professor. Recently, he has been a visiting professor at The University of Washington (Department of Molecular Biotechnology, 1996-1998) and at Kyoto University (Department of Electronic Science and Engineering, 1998). Currently, he is the Director of the Center for Molecular Nanofabrication and Devices and Associate Director of the Center for Nanoscale Science. A few noteworthy awards and honors: Scanning Microscopy International Presidential Scholarship (1994); B. F. Goodrich Collegiate Inventors Award (1994); American Chemical Society Nobel Laureate Signature Award for Graduate Education in Chemistry (1996); National Science Foundation Creativity Award (1997-1999); American Association for the Advancement of Science, Fellow (2000); and American Physical Society, Fellow (2002). Paul Weiss also actively participates in numerous research and service organizations.
