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Research Summary for the Marder Research Group

The Marder group is interested in localization and delocalization in conjugated molecules, the influence of structure on these properties, and the implications of these properties for photonic and electronic applications. Over the past several years Prof. Marder and Dr. Steve Barlow have been working closely with a diverse group of undergraduate and students, postdoctoral researchers, and research scientists on fundamental studies of charge delocalization in mixed-valence systems and cyanines, including both purely organic and metal-containing systems. We use the insight gained from these studies to guide research programs in organic electronics and photonics; aspects of these are briefly described below. The projects provide our group members with opportunities to receive an interdisciplinary training and with opportunities to collaborate closely with many other research groups at Georgia Tech and around the world.

Organic Electronics

There is currently much interest in organic electronics; i.e. the fabrication and study of electronic and optoelectronic devices such as light-emitting diodes, photovoltaics, field-effect transistors, and phototransistors based on readily-processed, low-cost organic (or metal-organic) molecular or polymeric materials. A key requirement for the further development of this field, including stimulating the development of new types of device, is the availability of materials that are readily processible from solution and that have enhanced charge-carrier mobilities (tendency for charge carriers to migrate through the material under the influence of an electric field) relative to those presently available, approaching that of amorphous silicon. We are undertaking an interdisciplinary collaborative research effort involving chemical synthesis and characterization of new amorphous, liquid-crystalline, crystalline molecular and polymeric electron- and hole-transport materials that could be used in organic electronic and optoelectronic applications. We work very closely with the Kippelen and Brédas groups on all aspects of these projects. Materials are characterized in the Kippelen group by a variety of techniques to determine their charge-carrier mobilites and their utility in a variety of device configurations; in addition the Bredas group perform computational studies to better understand molecular parameters related to the charge-transport process such as reorganization energies and intermolecular interactions.

Currently we focusing mainly on understanding charge transport in: » transition-metal organometallic compounds, » transition-metal coordination compounds, » photocrosslinkable hole- and electron-transport polymers, » discotic columnar liquid-crystalline materials, » ligand-functionalized nanoparticles and composites.

We have synthesized perylene and coronene diimides as materials for charge transport applications. These materials exhibit liquid crystalline phases, which can be tuned by changing the core as well as the mesogenic groups. We have incorporated tested the charge mobilities of these materials and have obtained values as high as 2 cm²/Vs.

Polarized Optical Microscopy Images of Various Derivatives in their Liquid Crystalline Phases

Nonlinear Optics

We are interested in dyes that simultaneously absorb two photons to create excited states that lie above the ground state by an energy equal to the sum of that of the two absorbed photons. Two-photon absorption allow one to excite molecules with very high spatial selectivity only at the focus of a laser beam. We have found that molecules with donor-acceptor-donor (D-A-D), and acceptor-donor-acceptor (A-D-A) structural motifs exhibit exceptionally large two-photon absorptivities. We are now designing and synthesizing two-photon absorbing molecules and materials with additional chemical and optical functionality that can generate radicals or acids, that can be used as photo-deprotecting groups, and that can be used in biological imaging and sensing. At the same time, we are developing material systems for 3D optical microfabrication of structures that are of interests for MEMS, photonics, microfluidics and tissue engineering applications. Here we are working closely with the Perry group – which is developing materials and processes for efficient 3D microfabrication, as well as characterizing two-photon cross-sections – and with the Brédas group – which uses theoretical methods to help us predict the properties of two-photon absorbing chromophores. In addition we have initiated a collaboration with the groups of Eric van Stryland and David Hagen at the University of Central Florida on the characterization of two-photon-absorbing properties across a broad spectral range.

Beverina, L.; Fu, J.; Leclercq, A.; Zojer, E.; Pacher, P.; Barlow, S.; Van Stryland, E. W.; Hagan, D. J.; Bredas, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 7282-7283.

We are also interested in non-resonant third-order nonlinear optical effects including third-harmonic generation and four-wave-mixing-derived effects. Third-order nonlinear optical materials can be use for applications in signal processing, optical computing and eye and sensor protection. Current materials are locked into regions of structure-property space where adequate optical nonlinearities cannot be achieved because the molecular structures do not allow for the optimal electronic distribution and delocalization. We are designing extended conjugated materials with optimized dipolar and quadrupolar polarization and delocalization for such applications. In addition we are investigating the enhancement of third-order nonlinearities of conjugated organic materials through resonance effects, including those associated with local-field enhancements in metal nanoparticle composites and fractal structures, and with resonant multi-level conjugated systems; these offer potential for several-orders-of-magnitude enhancement of third-order nonlinearity relative to current materials. In this area we are also working closely with the Perry, Kippelen and Brédas groups.

Surface Modification of Materials for Photonics and Electronics

The efficacy of devices made out of the materials described above has a great dependence on the overall ability of the organic layer to interact with the metal oxide layer upon which it is deposited. To that end, we are currently examining methods that allow for the alteration of surface properties of the metal oxide/organic interface in order to elicit control of surface properties including surface dipole (which impacts the Fermi energy level and can enhance charge injection), wetting properties, and dielectric permittivity.

Representative Phosphonic Acids of Interest

In particular, phosphonic acids are being examined in order to accomplish such surface modifications and device enhancements. Recently, we have shown that phosphonic acids can readily modify barium titanate and related nanoparticles in order to increase the permittivity of the material. Current methods involve the use of high temperature processing in order to achieve such permittivity values, but such measures can lead to high levels of dielectric loss or even low dielectric strength. Consequently, it is highly desirable to have a method for chemical modification that can readily modify the metal oxide surface leading to high permittivity values and a high dielectric strength material. Similarly, using chemical methods allows for the modification of functionality within the molecule, which can lead to finely tuned surface properties. The SEM images below of spin coated nanocomposite thin films of barium titanate truly demonstrate the influence of phosphonic acids on the metal oxide.

Surface and cross-sectional SEM images of spin-coated nanocomposite thin films

As can be seen on the left, the thin films consisting entirely of unmodified barium titanate nanoparticles have an inhomogeneous structure consisting of fissures, chasms, and aggregation of material. Yet, in the phosphonic acid modified barium titanate nanocomposites (as seen on the right) there is an overall homogeneity to the structure without the amassment of any aggregation and the minimization of defects within the surface. Also, the effective permittivity of the material increased for the phosphonic acid modified nanocomposites when compared with their unmodified counterparts. Such results are highly encouraging and our collaborative efforts in this area are continuing.