Imaging and Spectroscopy of Nanoscale Materials and Biological Systems

Figures 1 and 2

Figures 1 and 2. With a highly sensitive laser sample scanning confocal microscope the spectroscopy of individual molecules and nanoparticles can be studied (left). Optoelectronic materials will be studied at the nanoscale in fully functional single molecule/nanoparticle doped devices (right).

Nanoscale Optoelectronic Materials and Devices for Energy Conversion

Nanostructure of materials and interfaces is a key issue in achieving improved efficiencies for organic photovoltaic devices (OPV) and organic light emitting diodes (OLED). Through the development of molecular devices and single molecule/nanoparticle particle spectroscopic techniques for the study of organic optoelectronic materials we can address these issues. The research projects involve studying the spectroscopy of interesting optoelectronic materials such as conjugated polymers, nanoparticles and their hybrids at the single molecule/particle level, and using these single molecules/nanoparticles as probes to locally study interfaces and processes in fully assembled functioning devices at the nanoscale.

This multidisciplinary research program crosses the borders between materials science, engineering, physical chemistry, organic chemistry, and analytical chemistry and creates a bridge between fundamental research and technologically important applications.

Nanobiology: imaging and biophysical studies

The extreme sensitivity of single molecule laser scanning confocal microscopy allows us to detect the presence of a single molecule or nanoparticle. Combined with the excellent spatial resolution of this research tool we are developing the capability of tracking biological processes at the molecular level. We will quantitatively study biophysical processes at the single molecule level to unravel and understand the mechanism and kinetics of biologically important processes such as DNA and protein folding dynamics, and biochemical reactions involving enzymes.

Our work will also involve developing novel imaging and spectroscopic techniques for biological systems. This will include the tracking of individual nanoparticles to study processes inside living cells. This multidisciplinary research program will have a significant impact on the emerging fields of nanobiology and nanomedicine.

Most Notably...

Novel spectroscopic techniques have been developed that allow us to study the optical and opto-electronic properties of single molecules embedded in fully functional devices. Using these techniques we were able to study single MEH-PPV (a conjugated polymer) molecules inside a light-emitting-diode (LED) type device. LEDs function by recombination of charges inside the device to create excited states. These excited states are formed on materials such as MEH-PPV and can lead to light emission. The charges in the device interact with this excited state, and with impurties such as oxygen that are present in the device.

With our studies on single molecules embedded in such a device we discovered that the oxygen impurities in the device cause reversible photo-oxidation of the conjugated polymer molecules. This temporarily renders the molecule ineffective for light emission. The photo-oxidation could be reversed during the experiment by injecting an electron into the single molecule that was being studied. These experiments have lead to a detailed understanding of the photo-oxidation mechanism of conjugated polymers.

These studies have also revealed the importance of fundamental understanding of device processes in achieving efficient devices. A study of the singlet and triplet excited states of MEH-PPV formed on single molecules embedded in a device was performed. The experimental results demonstrate that: (i) triplet exciton pairs undergo efficient triplet-triplet annihilation on the << 30 time scale; (ii) triplet-triplet annihilation is the dominant mechanism for triplet decay at incident excitation powers = ?50 watts/cm2; (iii) singlet excitons are quenched by triplet excitons with an efficiency on the order of 1/2, and (iv) triplet excitons are efficiently quenched by hole polarons in conjugated polymers for hole polaron densities > 1016 charges/cm3, while singlet excitons are quenched with a much lower efficiency. These data give new fundamental insights into the complex interactions among excited and charged species that exist in these materials and the devices in which they are embedded. A significantly improved fundamental understanding of materials and devices is obviously still needed in order to develop efficient organic electronics.

1. Gesquiere, A. J.; Park, S. J.; Barbara, P. F. Hole-induced quenching of triplet and singlet excitons in conjugated polymers. Journal of the American Chemical Society 2005, 127, 9556.

2. Lee, Y. J.; Park, S.-J.; Gesquiere, A. J.; Barbara, P. F. Probing a molecular interface in a functioning organic diode. Applied Physics Letters 2005, 87, 051906.

3. Gesquiere, A. J.; Uwada, T.; Asahi, T.; Masuhara, H.; Barbara, P. F. Single molecule spectroscopy of organic dye nanoparticles. Nano Letters 2005, 5, 1321. (Highlighted in the Materials Research Society Bulletin, August 2005 issue (volume 30) on pages 575-576.)

4. Gesquiere, A. J.; Lee, Y. J.; Yu, J.; Barbara, P. F. Single molecule modulation spectroscopy of conjugated polymers. Journal of Physical Chemistry B 2005, 109, 12366.

5. Park, S. J.; Gesquiere, A. J.; Yu, J.; Barbara, P. F. Charge injection and photooxidation of single conjugated polymer molecules. Journal of the American Chemical Society 2004, 126, 4116.

6. Gesquiere, A. J.; Park, S. J.; Barbara, P. F. F-V/SMS: A new technique for studying the structure and dynamics of single molecules and nanoparticles. Journal of Physical Chemistry B 2004, 108, 10301.

For More Information
     Andre J. Gesquiere
     Department of Chemistry, CH 117
     University of Central Florida
     Orlando, FL 32816