research_projects

Polyfluorenes (PF) have emerged as attractive alternatives to other conjugated polymers due to their strong blue emission, high charge mobility, excellent chemical and thermal stability, and thus great prospects for device applications.

PF derivatives utilize solubilizing side chain substituents anchored at the bridging carbon atom as shown in the figure above. Side chain substitution, in addition to improving the solution processing and film forming characteristics, confers new functionality. These side chains give rise to a rich array of mesomorphic behavior with the appearance of the nematic liquid crystalline phase. In the PF family, di-octyl substituted PF (PF8) and ethyl-hexyl substituted PF (PF2/6) have received a lot of attention mainly due to their mesomorphism and multitude of crystalline phases.

There are many ambiguities regarding chain conformations and crystalline phases in PFs. Vibrational frequencies and intensities determined by Raman spectroscopy are strongly influenced by variations in the chain planarity as well as side chain conformations. Application of Raman scattering is particularly useful in PFs for discerning the families of chain conformers and structural phases.

We are also involved with detailed Raman spectra calculations of F8 and F2/6 oligomers using density-functional theory. Our recent work shows that the beta phase in PF8, which is known to originate from regions from chain planarity, is incompatible with the overall 3-D crystalline phase. This is a direct consequence of the conformation of the side chains. The figure belowshows the Raman spectra of PF2/6 and PF8 before and after annealing. After annealing they adopt a crystalline phase. PF8 is most notably characterized by the appearace of two new peaks at 290 and 370 cm-1 after annealing.. The latter Raman peak corresponds to the LAM (longitudinal accordion motion) mode of alkane chains (shown in the animation).

Recent Work:
M. Arif, C. Volz, and S. Guha, Phys. Rev. Lett. 96, 025503 (2006).
B. Tanto, S. Guha, C.M. Martin, U. Scherf, and M.J. Winokur, Macromolecules 37, 9438 (2004).

2. Electrical characterization of organic LEDs and MIS structures

Organic light-emitting diodes (LEDs) have attracted considerable attention since the early 90s when long-chain polymer based-LEDs were first realized. The preferred design for a commercial OLED is a two-layer structure as shown in the animated figure below. The hole-transporting layer (HTL) is laminated over a transparent indium tin oxide (ITO) anode structure, over which the emitting layer (ETL) film is applied. A cathode with a low work function is deposited to provide electron injection into the conduction band of the ETL.

A major focus of our work is in organic optoelectronics device fabrication using PFs to understand the process of charge injection and transport using both electrical and optical techniques.

(a) Charge injection and transport

Charge injection in blue-emitting polymers is a topic of heavy debate. Our current-voltage (I-V) characteristics of PF-based diodes agree very well with a shallow trap space-charge-limited conduction model. Comparison of I-V characteristics from PF2/6 and PF8 based devices shed new light on the mechanism of charge injection and transport.

(b) Capacitance-voltage characterizations
Our recent work involves fabrication of MIS (metal-insulator-semiconductor) structures with PF2/6 using Al2O3 as insulating oxide and p+ Si as the metal layer. The goal of this work is to combine high k dielectrics with PFs for future applications in complementary metal-oxide-semiconductor devices.

The figure shows the capacitance-voltage (C-V) curves of a PF2/6-based MIS diode for different frequencies. Counterclockwise loop of the hysteresis in the C-V curve of PF2/6 MIS diode indicates a positive carrier injection into the PF2/6 layer with subsequent trapping. From the hysteresis in C-V measurements, the trapped charges in PF2/6 can be estimated. Further, parallel conductance as a function of frequency from these devices fit very well with a single time constant model, corroborating our I-V characteristics.

Recent Work:
M. Yun, R. Ravindran, M. Hossain, S. Gangopadhyay, U. Scherf, T. Bünnagel, F. Galbrecht, M. Arif, and S. Guha, Appl. Phys. Lett. 89 , 013506 (2006).

3. Surface-enhanced Raman scattering of biomolecules

Nanoparticle research is at the leading edge of a rapidly growing technology in nanoscale, medical, and biological sciences. Advances in the preparation of gold and silver nanoparticles have rekindled interest in surface-enhanced Raman scattering (SERS) as a means to detect single molecules and to identify their chemical structure. SERS now provides a potential for label-free detection of biomolecules.

Our work involves SERS studies of DNA nucleosides using biologically benign agarose-stabilized gold nanoparticles. We find the SERS activity to be an order of magnitude higher with these particles compared to citrate-based gold nanoparticles. The higher SERS activity is explained in terms of the agarose matrix, which provides pathways for the gold nanoparticles to have distinct arrangements that result in stronger internal plasmon resonances. This further opens up the possibility of using biologically benign SERS substrate for biosensing applications.

Recent Work:
V. Kattumuri, S. Guha , M. Chandrasekhar, K. Raghuraman, K. V. Katti , K. Ghosh and R.J. Patel, Appl. Phys. Lett.88, 153114 (2006).