A broad outline of research interests

Summary of research interests

Our group explores light-matter interactions in organic optical materials.

In particular, we are interested in:

  1. understanding basic physics of exciton and photogenerated charge carrier dynamics in organic semiconductors and inorganic-organic polymer nano-composites
  2. utilizing photophysical and electronic properties of individual molecules in studies of complex environments such as polymers and biological systems
  3. developing novel high-performance optical materials for electronic and photonic applications
  4. exploring applications of organic molecules in nanoscale electronic and all-optical devices

Experimental set-ups that are currently under development in our lab utilize ultrafast time-resolved spectroscopy, ultrafast and cw photoconductivity techniques, luminescence spectroscopy, and single-molecule fluorescence microscopy.

Motivation

Organic optical materials have been extensively studied as an alternative to inorganic materials due to their low cost, easy fabrication, and tunable properties. Applications of organic optical materials include xerography, thin-film transistors, light-emitting diodes, solar cells, photorefractive devices, and many others [1-4]. By slight synthetic modifications or doping, it is possible to vary optical properties (such as absorption and fluorescence spectra), thermal and structural properties (such as phase transition temperatures and a type of packing in a crystallographic unit cell), and electronic properties (such as charge carrier mobility) of organic materials and therefore, tailor them for specific applications [5-7]. In spite of many demonstrated and even commercialized applications of organic materials, a number of issues, both fundamental and applied, remain. For example, basic physics of light-induced charge carrier generation and subsequent transport, the processes that lay foundation for most of the applications of organic optical materials, is not understood [8,9]. On the applied side, it is often challenging to make a series of organic thin films with exactly reproducible properties. Indeed, the dependence of the thin film structure on the fabrication methods and conditions and the relationship between the structure and optical and electronic properties of the film are not straightforward. Therefore, systematic comprehensive studies are needed to reveal the physical nature of all processes contributing to the device performance and understand structure-property relationships [5,6,7,10,11].

As the technology develops towards nanoscales, organic molecules attract more and more attention as potential nano-devices. Studies that utilize single-molecule fluorescence spectroscopy revealed that individual organic molecules are promising candidates for nanoscale optical switches, probes of nanoenvironment, nanomotors, and so forth [12-16]. Both the physical description of the stochastic behavior of single-molecule devices and the development of the actual nanoscale electronic and all-optical devices are at their initial stages – they represent exciting opportunities for persistent and creative individuals !

References
[1] S. R. Forrest, “The path to ubiquitous and low cost organic electronic appliances on plastic,” Nature 428, 911-918 (2004).
[2] T. W. Kelley, P. F. Baude, C. Gerlach, D. E. Ender, D. Muyres, M. A. Haase, D. E. Vogel, and S. D. Theiss, "Recent progress in organic electronics: materials, devices and processes," Chemistry of Materials 16, 4413-4422 (2004).
[3] C. D. Dimitrakopoulos and P. R. L. Malenfant, "Organic thin film transistors for large area electronics," Advanced Materials 14(2), 99+ (2002).
[4] O. Ostroverkhova and W. E. Moerner, "Organic photorefractives: mechanisms, materials and applications," Chemical Reviews 104(7), 3267-3314 (2004).
[5] O. Ostroverkhova, U. Gubler, D. Wright, W. E. Moerner, M. He, and R. Twieg, "Recent Advances in Understanding and Development of Photorefractive Polymers and Glasses," Advanced Functional Materials 12(9), 621-629 (2002).
[6] O. Ostroverkhova, W. E. Moerner, M. He, and R. J. Twieg, "Role of temperature in controlling performance of PR organic glasses," Chemphyschem 4(7), 732-744 (2003).
[7] K. A. Willets, O. Ostroverkhova, M. He, R. J. Twieg, and W. E. Moerner, "Novel fluorophores for single-molecule imaging," Journal of the American Chemical Society 125(5), 1174-1175 (2003).
[8] N. S. Sariciftci, ed., Primary Photoexcitations in Conjugated Polymers: Molecular Exciton versus Semiconductor Band Model (World Scientific, Singapore, 1997).
[9] F. A. Hegmann, O. Ostroverkhova, and D. G. Cooke, "Probing Organic Semiconductors with Terahertz Pulses" in "Photophysics of Molecular Materials" ed. by G. Lanzani (Wiley-VCH, Weinheim), in press (2005).
[10] O. Ostroverkhova, D. G. Cooke, S. Shcherbyna, R. F. Egerton, F. A. Hegmann, R. R. Tykwinski, and J. E. Anthony, "Band-like transport in pentacene and functionalized pentacene thin films revealed by transient photoconductivity," Physical Review B 71, 035204 (2005).
[11] O. Ostroverkhova, S. Shcherbyna, D. G. Cooke, R. Egerton, R. R. Tykwinski, S. R. Parkin, J. E. Anthony, and F. A. Hegmann, "Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology", Journal of Applied Physics 98, 033701 (2005).
[12] W.E. Moerner, "A dozen years of single-molecule spectroscopy in physics, chemistry, and biophysics", Journal of Physical Chemistry B 106 (5), 910-927 (2002).
[13] W.E. Moerner, D. P. Fromm, "Methods of single-molecule fluorescence spectroscopy and microscopy", Review of Scientific Instruments 74 (8), 3597-3619 (2003).
[14] K. A. Willets, S. Y. Nishimura, P. J. Schuck, R. J. Twieg, and W. E. Moerner, “Nonlinear optical chromophores as nanoscale emitters for single-molecule spectroscopy”, Accounts of Chemical Research 38 (7), 549-556 (2005).
[15] K. A. Willets, O. Ostroverkhova, S. Hess, M. He, R. J. Twieg, and W. E. Moerner, "Novel fluorophores for single-molecule imaging," Proc. of SPIE 5222, 150-157 (2003).
[16] K. A. Willets, P. R. Callis, and W. E. Moerner, "Experimental and theoretical investigation of environmentally sensitive single-molecule fluorophores," Journal of Physical Chemistry B 108(29), 10465-10473 (2004).