Date: Wednesday, October 28, 2009

Time: 4:00pm

Location:   Interschool Lab Room 750 Schapiro/CEPSR

Abstract: Fabricating circuits and solar cells by solution-processing readily available organic compounds is a potentially revolutionary concept. Given the meager understanding of organic semiconductor materials, however, development of organic devices proceeds largely by trial and error. While tremendous effort is being expended to mass-produce organic devices, almost no attention has been devoted to developing a microscopic understanding of fundamental processes in organic semiconductors. To accelerate development, we need a better microscopic understanding. 

We use electric force microscopy to make local measurements of electrostatic potential and capacitance in working organic devices.  Measurements take place with devices in vacuum or in nitrogen, and in some cases over a large temperature range and under variable-wavelength irradiation.  The resulting data has allowed us to address long-standing puzzles related to ion motion, metal-to-organic charge injection, and charge trapping in organic semiconductor materials.  Case studies involving both small-molecule and polymeric semiconductors will be presented.

 The ability to characterize organic material at nanometer spatial resolution with chemical specificity lags far behind our ability to synthesize new materials and make devices.  I will present my group's work to develop scanned-probe approaches to detecting magnetic resonance and electric field fluctuations at nanoscale resolution.  We have recently succeeded in developing attonewton-sensitivity cantilevers with overhanging, integrated 70-nm diameter nickel nanorod tips.  We have developed a new, force-gradient approach to detecting magnetic resonance that extends mechanical detection of spin magnetization to any sample with a spin-lattice relaxation time longer than ~0.2 ms.  We have used the new approach to observe electron spin resonance, at ~400-spin sensitivity, from a nitroxide spin label widely used in structural biology.  We have invented and demonstrated new schemes for evading surface- and detector-noise in scanned probe microscopy.  These developments open up exciting possibilities for achieving routine single-electron imaging and nanometer-scale nuclear magnetic resonance imaging in biomacromolecules and other interesting materials.

This EFRC Seminar is hosted by Prof. Kymissis

EFRC Seminar: Exciton Energies, Decay Dynamics and Localization in Individual Single-Walled Carbon Nanotubes

Speaker: Professor Achim Hartschuh

Ludwig-Maximilians University

Date: Tuesday, September 22, 2009

Time: 4:00pm

Location: Interschool Lab, Room 750 Schapiro/CEPSR

Abstract: Optical excitation of semiconducting nanotubes creates excitons that determine nearly all light-based applications. Confocal elastic scattering and photoluminescence (PL) microscopy of single small-diameter nanotubes on substrates was used to determine the energies of the two lowest excitonic transitions. The decay dynamics was studied by time-correlated single photon counting. Mono- or bi-exponential decay in the picosecond to nanosecond time domain is observed depending on the nanotube material. Antenna-enhanced near-field PL and Raman imaging with a spatial resolution better than 15 nm was used to probe the spectroscopic properties of excitonic states along single nanotubes. We first show that a metal antenna can enhance both excitation and radiative rate and we give an estimate of the different contributions. Towards the nanotube ends the PL intensity was found to decrease on a length scale of few tens of nanometers probably caused by diffusional exciton transport to quenching end states. Very bright and strongly confined PL observed for some nanotubes is attributed to exciton localization caused by spatial energy fluctuations.

This EFRC Seminar is hosted by: Prof. Heinz.

Dielectric Optical Antenna Emitters and Metamaterials

Speaker: Dr. Jon Schuller

Stanford University

Date: Wednesday, Sept. 2, 2009

Time: 11 AM

Location: Room 415 Schapiro/CEPSR

Abstract: Optical antennas are critical components in nanophotonics research due to their unparalleled ability to concentrate electromagnetic energy into nanoscale volumes. Researchers typically construct such antennas from wavelength-size metallic structures. However, recent research has begun to exploit the scattering resonances of high-permittivity particles to realize all-dielectric optical antennas, emitters, and metamaterials. In this talk, we experimentally and theoretically characterize the resonant modes of subwavelength rod-shaped dielectric particles and demonstrate their use in negative index metamaterials and novel infrared light emitters. At mid-infrared frequencies, Silicon Carbide (SiC) is an ideal system for studying the behavior of dielectric optical antennas. At frequencies below the TO phonon resonance, SiC behaves like a dielectric with very large refractive index. Using infrared spectroscopy and analytical Mie calculations we show that individual rod-shaped SiC particles exhibit a multitude of resonant modes. Detailed investigations of these SiC optical antennas reveal a wealth of new physics and applications. We discuss the distinct electromagnetic field profile for

each mode, and demonstrate that two of the ielectric-type Mie resonances can be combined in a particle array to form a negative index metamaterial [1]. We further show that these particles can serve as "broadcasting" antennas. Using a custom-built thermal emission microscope we collect emissivity spectra from single SiC particles at elevated temperatures, highlighting their use as subwavelength resonant light emitters [2].

Finally, we derive and verify a variety of general analytical results applicable to all cylindrical dielectric antennas [3] and discuss extensions of the demonstrated concepts to different materials systems and

frequency regimes.

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[1] J.A. Schuller et al. "Dielectric Metamaterials Based on Electric and Magnetic Resonances of Silicon Carbide Particles," Phys. Rev. Lett. v.99 p.107401 (2007).

[2] J.A. Schuller et al. "Optical Antenna Thermal Emitters," accepted for publication in Nature Photon.

[3] J.A. Schuller et al. "General Properties of Dielectric Optical Antennas," in preparation.