Department of Chemistry and Biochemistry

Austin, TX 78712-1167

Phone: (512)-471-2053; FAX (512)-471-3889 e-mail: p.barbara@mail.utexas.edu

My research program is focussed on the development and application of new optical scanning probe techniques for the analysis of organic, biological and hybrid thin-film materials. Such materials are opening the door for powerful new electronic and photonic devices with unique capabilities and extraordinarily low cost. A typical device includes two or more layers of organic and inorganic materials that are often patterned on the nanometer distance scale. Devices under investigation include organic light emitting diode displays, organic solar cells, and RNA modified surfaces. It is increasingly apparent that one of the greatest obstacles to developing fundamental and practical advances in nanostructured organic thin film devices has been the absence of established experimental techniques that allow for the spatial measurement of device operation and function on the nanometer scale.

Toward this goal graduate students in Professor Barbara’s group are developing new optical scanning probe techniques for organic materials analysis, including the first scanning probe technique for imaging charge injection. The apparatus is a modified fluorescence near field scanning optical microscope that uses an aluminum coated, "sharpened" optical fiber tip with an electrical bias to simultaneously optically excite the thin film sample under investigation, and induce charge injection between layers in the thin film device. The charge injection is monitored indirectly by either the modulation of the fluorescence intensity of the sample or the modulation of the tip/sample interaction force, which is measured in an analogous fashion to atomic force microscopy.

Techniques are also being developed to image electronic energy migration in organic thin-films. This a critical process in solar cells, where energy migration is necessary to transfer the energy from the site of optical absorption to the site of charge separation. Graduate students in Prof. Barbara’s group are studying energy migration with a new imaging technique that employs near-field excitation to create a small, @ 50 nm initially excited region of the sample and far-field imaging to observe the energy migration. The technique is being applied to measurements of energy migration lengths in nanostrucutured materials. The impact of the morphology on the energy migration process is the central issue of interest.

A final focus of the group is the development of new analysis techniques based on single molecule spectroscopy (sms) for the analysis of complex polymeric samples with large molecular weight distributions. In particular, sms has been used to investigate fluorescent conjugated polymers Such materials are being extensively utilized in organic light emitting diode devices, optical and chemical sensors, and many other electronic and photonic applications. Students in Prof. Barbara’s group have used sms to avoid the obscuring effects of molecular heterogeneity in polymeric samples. The data reveal many unexpected features. For example the sms distribution of fluorescence maxima show two peaks due to two different conformations. Interestingly, the two conformations are unresolvable by standard spectroscopic methods that record spectra of the ensemble (i.e. many molecules at a time, not one at a time as in sms).

(1) English, D. S. Furube, A., and Barbara, P. F. "Single Molecule Spectroscopy in Oxygen Depleted Films" Chem. Phys. Lett., 324, 15-19, (2000)

(2) Adams, D. M,. Kerimo, J., Liu, C.-Y., Bard, A. J., and Barbara, P. F. "Electric Field Modulated Near Field Photoluminescence of Organic Thin Films" J. Phys. Chem. B, 104, 6728 -6736, (2000)

(3) Dehong Hu, Ji Yu, Kim Wong, Biman Bagchi, Peter J. Rossky & Paul F. Barbara " Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations " Nature, 405(6790), 1030-1033, (2000)

(4) Sandra R. Whaley, D. S. English, Evelyn L. Hu, Paul F. Barbara & Angela M. Belcher Nature, 405(6787), 665-668, (2000)

(5) Jason D. McNeill, Donald B. O'Connor, and Paul F. Barbara "Imaging Organic Device Function With Near Field Scanning Optical Microscopy"J. Chem. Phys., 112, 7811, (2000)

(6) David M. Adams, Josef Kerimo, Donald B. O'Connor, and Paul F. Barbara" Spatial imaging of singlet energy migration in perylene bis(phenethylimide) thin films" J. Phys. Chem. A, 103, 10138-10143, (1999)

(7) Chan, B., K. Weidemaier, W.-T. Yip, P. F. Barbara and K. Musier-Forsyth "Intra-tRNA distance measurements for nucleocapsid protein-dependent tRNA unwinding during initiation of HIV reverse transcription." Proceedings of the National Academy of Sciences, 96, 459-464 (1999).

(8)Adams, D. M.; Kerimo, J.; Olson, E. J. C.; Zaban, A.; Gregg, B. A.; Barbara, P. F.," Spatially Resolving Nanoscopic Structure and Excitonic-Charge-Transfer Quenching in Molecular Semiconductor Heterojunctions" J. Am. Chem. Soc. 1997, 119, 10608.

(9)Alivisatos, A. P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J.; Stupp, S. I.; Thompson, M. E.," From Molecules to Materials: Current Trends and Future Directions", Advanced Materials 1998, 10, 1297-1336.

(10) Barbara, P. F.; Adams, D. M.; O'Connor, D. B.," Characterization of Organic Thin Film Materials with Near Field Scanning Optical Microscopy (NSOM)", Annual Review of Material Science 1999, 29, 433-469.