|
 |
I have taught and done research at Haverford since 1992. I received a B.A. (physics) from Wesleyan in 1981, and a Ph.D. (physics) from Harvard in 1989. From 1989 to 1992, I was a postdoctoral researcher at the University of Texas at Austin. My wife, Marian McKenzie, is an elementary school librarian. We have three children, ages 8 to 13.
In fall '08, I'm teaching Fundamental Physics I (Physics 105a), Electronic Instrumentation and Computers (Physics 316a), and Research in Nanoscale Physics (Physics 415a).
| Working in collaboration with my wife, Marian McKenzie, I write songs about physics for use in the classroom. These serve as powerful teaching tools. In just one or two minutes, a song can dramatically transform the classroom atmosphere, creating a learning environment in which even students who are unsure of themselves are willing to speak up and ask questions. They engage additional areas of the students' brains; when a song is performed in class, students' retention of material for the entire class session is improved. Songs enhance the approachability of the professor, so that students are more willing to ask for help outside of class time. Of course, songs can also serve as effective memory aids, though this is much less important than the effects on classroom atmosphere and the relationship between the students and the professor. |  |
||
Songs have been sung at social gatherings of physicists for almost a century. I have brought this tradition to the March Meeting of the American Physical Society (APS), the world's largest annual gathering of physicists. For the last three years, I have hosted an evening sing-along during this conference, with sponsorship provided by the APS. The sing-alongs provide a great way for physicists to make new acquaintances. I contributed significantly to the article in the July 2005 issue of "Physics Today" about physics songs. I suggested and provided the lyrics for eight of the eleven songs included in the article. One of these was co-written by my wife and me; the other seven were written by Tom Lehrer, Prof. Arthur Roberts, Prof. Gilbert Stead, and Dr. James Livingston. |
 |
||
| I also run the world's premiere website devoted to collecting and organizing all songs about physics: PhysicsSongs.org This serves as a resource for students and teachers, and also functions as an archive for this important niche of physics history. The site includes many of the songs written by my wife and me, dozens of other songs submitted by other teachers and students, and significant historical collections, such as the recordings of Tom Lehrer's "Physical Revue", put on in 1951. (These are posted with the permission of Mr. Lehrer, Prof. Norman Ramsay who made the recordings, and all the surviving members of the cast.) The site also includes the complete songs of Prof. Arthur Roberts (posted with the permission of his family), which were written from 1946 to 1985. |
|
| Porphyrin-peptide complexes | ||
There are two routes to the creation of nanoscale objects: top-down and bottom-up. In top-down manufacture, techniques such as electron beam lithography are used to create desired patterns by carving them out of thin films of resist. In the bottom-up approach, one instead chooses or designs molecules which spontaneously connect together to make nanostructures. The top-down approach allows one to create very complex structures in well defined locations. For the bottom-up approach, there can be a great challenge in synthesizing the molecules, and in determining the conditions under which they assemble into the desired structures. However, once these tasks have been accomplished, making additional samples requires only simple mixing of reagents and heating. Further, the final nanostructures are energy-minimized and so are stable, unlike nanostructures made by top-down processing which can sometimes be destroyed by thermal diffusion. The goal of the larger field is to create electronic circuits that assemble themselves out of solution, and perhaps have capabilities not available from circuits made by traditional top-down approaches. The field is still very new, at the stage where much of the basic physics is not yet understood. The research in my group is meant to improve our understanding of the rules that govern self-assembly and of the basic physics that explains the photoelectronic properties of the resulting nanostructures.
Return to Research Interests Summary
There are two projects underway in this area:
We showed in 20031 that when the pH of a solution of the porphyrin tetrakis(4-sulfonatophenyl)porphine is lowered, the molecules self-assemble into long, straight nanowires of uniform cross-section, as shown in the Atomic Force Microscope (AFM) images to the right. 1 - Schwab, A.D.; Smith, D.E.; Rich, C.S.; Young, E.R.; Smith, W.F.; de Paula, J.C. , J. Phys. Chem. B 2003, 107, 11339-11345.
|
 |
Our experiments2 on the electrical conductivity of the rods show some very exciting behaviors. In the dark, the rods do not conduct. When light is applied, the conductivity jumps suddenly, then grows further (as shown in region i) over a period of several minutes. If the light is turned off, the system "remembers" this slow growth period for up to about 15 minutes! If the light is left on, but zero voltage is applied, the sample generates electrical current (as shown in region iii), with a trainable polarity. These effects are shown in the figure to the right. This work is done in collaboration with Prof. A. T. Charlie Johnson and his research group at the University of Pennsylvania; all of the synthesis, Atomic Force Microscopy, and photoelectronic measurements are done at Haverford, while electron beam lithography to create the nanoscale contact pads we need for these measurements is done at Penn. This research is supported by the National Science Foundation. 2 - Schwab, A.D.; Smith, D.E.; Bond-Watts, B.; Johnston, D.E.; Johnson, A.T.; Hone, J.; de Paula, J.C.; Smith, W.F. Nano Letters 2004, 4, 1261-1265. |
 |
In more recent experiments, we have found that when samples are illuminated over a period of several hours, the photoconductivity continues to grow, and also changes from the "non-persistent" photoconductivity shown above to a partially persistent character. In other words, after long illumination, the conductivity does not all drop to zero immediately when the illumination is removed, but instead there is a sudden drop in conductivity, and the rest decays away slowly. |
 |
| We have developed a qualitative model to explain all these behaviors. In our model, the persistent photoconductivity is associated with excitation of the porphyrin molecules into the triplet state.
Since it is well known that oxygen quenches this triplet excitation, our model predicts that adding atmospheric oxygen should reduce the persistent photoconductivity. This prediction is borne out by our experiments. Ordinarily, our experiments are conducted under dry nitrogen gas. We observe that when even very low levels of oxygen (less than 1%) are added to this gas, the photoconductivity decreases dramatically. Both the persistent and non-persistent photoconductivity decrease, but the persistent channel is affected much more, so that the fraction of photoconductivity that is persistent plummets. When the oxygen is removed, the photoconductivity (both persistent and non-persistent) recovers over a period of several hours. |
 |
| By applying a voltage to an electrode which is close to the sample but not in electrical contact with it (a “gate voltage”), one can gain significant insights into the quantum states responsible for electrical conduction. For an ordinary n-type conductor (i.e. a conductor for which the mobile charges have a negative effective charge), applying a positive gate voltage increases the conductivity (essentially because more mobile charges are attracted into the sample), while applying a negative gate voltage decreases the conductivity. However, our samples display a different behavior which, as far as we are aware, has never been observed in any system. When a positive gate voltage is applied, the photoconductivity increases (as would be expected for an n-type conductor), but then, even though the gate voltage is held constant, about 2/3 of this initial increase decays away. If instead a negative gate voltage is applied, the photoconductivity decreases (again as expected for an n-type conductor), but again about 2/3 of this initial change decays away. |  Each black line corresponds to the current obtained for a given bias voltage (red numbers on the left). With a step-up in gate voltage (blue), there is a corresponding increase in conductivity, much of which decays away. Compare the results with an expected (red) behavior of typical n-type conduction. |
Return to Research Interests Summary
| By combining porphyrins with peptides, we hope to gain greater control over the shape and functionality of self-assembled nanostructures than is possible with porphyrins alone. Our understanding of the way the amino acid sequence determines the properties of a peptide is relatively good. Professors Rob Fairman and Karin Akerfeldt are applying this understanding to create porphyrin-peptide systems that self-assemble into nanowires. In the Smith group, we are measuring the photoelectronic properties of these nanowires. |
|
last updated 10-30-08
Department of Physics
