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Thomas Taubner
PostDoc, Department of Materials Science and Engineering, Stanford University

Novel concepts in infrared imaging at nanoscale resolution

Within the recent years, various novel optical concepts have been invented to improve the diffraction-limited resolution of optical microscopy. Most of these concepts rely on tailoring the optical properties of materials by structuring them on a subwavelength-scale, e.g. to create resonant nanostructures that as act “optical antennas” for concentrating light into tiny volumes. I will present on the recent progress in nanoscale resolved imaging with infrared light that offers the capability of chemical sensing by directly probing molecular vibrations.

In the first part I will introduce a new imaging device called a superlens, based a thin film of Silicon Carbide (SiC) that uses phonon-polariton surface waves to create subwavelength-resolved images (1). I will show the latest results that have been obtained by phase-sensitive infrared near-field microscopy and provide new insight into the imaging process of such a device. Then, I will explain the basics near-field optical probing by the scattering of infrared light at a sharp metallic tip. The combination of the chemical sensitivity of infrared light with the spatial resolution of 10-30 nm allows for unambiguous nanoscale material characterization (2). In addition to molecular vibrations, the collective excitation of lattice vibrations („phonons“) in polar crystals (3) or charge carriers („plasmons“) in metals or doped semiconductors (4) are also accessible by infrared near-field microscopy. I will show applications in semiconductor imaging (5), material science (6), biology (7) and chemistry (2). Currently, the main limitation of this technique comprises of the low signals that demand tunable laser sources and restrict the spectral range of operation.

Consequently, I will introduce new concepts for increasing the sensitivity of infrared near-field spectroscopy, e.g. by using resonant nanostructures or infrared antennas: The near-field probing process can be enhanced by suitable substrates. Metallic AFM tips can be modified to act as “infrared antennas” that resonantly convert electromagnetic radiation into highly localized near-fields, enabling high-resolution near-field microscopy with increased sensitivity.

Barry C. Thompson
Department of Chemistry, University of California, Berkeley

Designing Conjugated Polymers for Photovoltaics

Composite solar cells based on conjugated polymers and fullerenes represent an attractive platform for organic photovoltaics based on the potential for low cost, solution processable, and flexible solar cells. Blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) represent the state-of-the-art, with efficiencies approaching 5%. While polymers with more optimal electronic structures than P3HT are known, enhanced performance is not observed due to the less than optimal blend morphology that is generated in fullerene-composites. We have made efforts to gain a deeper understanding of the polymer structure-function relationships in P3HT in order to establish fundamental structural principles for extension to polymers with more optimal electronic structures. Specifically we have focused on polymer regioregularity in P3HT as well as the alkyl substitution pattern in poly(3-alkylthiophene) copolymers. The synthesis of novel poly(alkylthiophenes) will be described along with the strong influence of alkyl substitution pattern and regioregularity on the thermal stability of the composite active layer, as well as the influence on the maximum achievable power conversion efficiency in these blend devices. Directions for application of structure-function relationships derived from polythiophenes will be described in the context of polymers with optimized electronic structures.

Richard D. Robinson
Lawrence Berkeley National Laboratory, UC Berkeley

Spontaneous Superlattice Formation in Nanorods through Partial Cation Exchange

Lattice mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth. In this talk I will discuss our recent results which demonstrate a strain-mediated colloidal route to synthesizing CdS-Ag2S nanorod superlattices through partial cation exchange [1].

Cation exchange provides a facile method for systematically varying the chemical composition within a colloidal nanocrystal. We have previously shown that cation exchange can be used to fully (and reversibly) convert CdSe, CdS, and CdTe nanocrystals to the corresponding silver chalcogenide nanocrystal by a complete replacement reaction of the Cd2+ cations for Ag+ cations [2]. The resultant material is the silver-anion analog of the starting material (i.e., Ag2Se, Ag2S, and Ag2Te). The reaction is rapid enough that complete exchange can be performed on non-equilibrium shaped nanocrystals -- such as rods, tetrapods, and hollow spheres -- without changing the shape of the crystal.

Here I investigate the step-wise evolution of heterostructures as the degree of cation exchange is gradually increased. A striped pattern on each nanorod is created spontaneously at a critical Ag+ concentration. Further examination of the pattern shows that it is periodic and equally spaced on each nanorod and thus a superlattice. We believe that three factors contribute to the superlattice self-organization: a positive value for the interfacial formation energy between the materials, the fast diffusion of the cations in the solids, and an epitaxial strain from the mismatched lattices. To study these forces we use ab initio calculations to determine material properties and interfacial energies, and a valence force field model (VFF) to determine the strain energies. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices. This work was supported by the U.S. Department of Energy under the contract number DE-AC02-05CH11231.

Sheng Meng
Department of Physics, Harvard University

Nano-bio materials for energy applications

The drive to meet humanity’s energy needs without adverse environmental effects, is creating wonderful opportunities for fundamental and applied research in materials. In particular, nano-structured materials (nanomaterials) research could lead to novel ways of solving energy-related problems. Every aspect of renewable energy applications can benefit from this research, including energy supply, storage and utilization. Combining nanomaterials with biomolecules to form hybrid nano-bio devices is a particularly promising avenue towards these goals. For example, natural photosynthesis has maximum photon capture efficiency approaching 100% and energy conversion efficiency [from light to O2 and primary products] reaching 60%; for comparison the best solar cell today has monochromatic efficiency <45%, using very expensive Si or GaAs semiconductors. The disappointing overall efficiency of <1% for sunlight-to-biomass conversion could be surpassed if hybrid nano-bio systems are engineered to utilize this energy directly.
In this talk, I will explore the theoretical prospects and some examples of the research projects we have been pursuing in the area of nanomaterials or nano-bio composites. I will discuss two broad themes: (a) how the theoretical rationale leads to the design of a new form of nanomaterials, metal-diboride nanotubes, for optimal hydrogen storage; (b) the challenges of attaching biomolecules to surfaces and nanomaterials and eventually using natural biomolecule as dyes to sensitize insulating nanowires for sunlight harvest. These examples illustrate the emerging and promising nano-bio approach to energy materials, and the predictive power of theoretical methods, such as the one we recently developed for treating electron-ion dynamics based on time-dependent density functional theory.

Matthew Tirrell
Departments of Chemical Engineering & Materials, University of California at Santa Barbara

Polyelectrolyte Complexes in Materials Science

Electrostatic interactions can be used to create new polymeric materials and assemblies. Layer-by-layer growth of oppositely charged polyelectrolytes is one of the best examples. Ability to exploit these possibilities depends on the understanding of long-ranged electrostatic interactions and their dependence on the ionic environment. End-tethered polyelectrolyte layers ("brushes") shrink monotonically in response to addition of mono-valent salt, which also produces corresponding monotonic changes in the range of the repulsive normal forces exerted by such brushes. High swelling and very low frictional forces have been reported under low salt concentrations. A new pattern of behavior is demonstrated here via surface force measurement on polyelectrolyte brushes in the presence of multi-valent ionic interactions, introduced via tri-valent aluminum or lanthanum cations (Al3+, La3+) or aggregates of cationic surfactants. Very low concentrations of added Al3+, La3+ or surfactant produce much stronger shrinkage of the brush than does mono-valent salt. Normal forces become strongly attractive under these circumstances. Multi-valent interactions enable tuning of polyelectrolyte brush structure and properties over a wide range, from compact, stiff and sticky to swollen, soft and repulsive.

Jennifer Dionne
Department of Applied Physics, California Institute of Technology

Between the looking-glasses: negative refraction and field-effect modulation in metal-insulator-metal waveguides

In recent years, plasmon components have rapidly evolved from discrete, passive structures toward integrated active devices that could comprise an all-optical networking technology. Such progress has been facilitated by opportunities to dispersion engineer metallodielectric systems. Surface plasmons provide access to an enormous phase space of refractive indices and propagation constants that can be readily tuned through variation of material, dimension, or geometry.

Opportunities for dispersion engineering are particularly pronounced in the metal-insulator-metal (MIM) waveguide. MIM waveguides can confine and guide light within optical volumes not significantly larger than the dielectric core, with reported modal volumes as small as one one-thousandth of a cubic wavelength. In this seminar, I will show how this basic plasmonic geometry can be used to develop a suite of passive and active plasmonic components, including a negative index metamaterial and a field effect modulator. In particular, I will discuss the wide range of positive, near-zero, and even negative indices accessible in MIM structures. Positive index modes are probed by near-field scanning microscopy, revealing surface plasmon wavelengths down to 105 nm for 640 nm excitation. Negative index modes are characterized through direct visualization of negative refraction. By fabricating prisms comprised of Au/Si3N4/Ag layers, we achieve the first experimental demonstration of a negative index material at visible frequencies, with potential applications for sub-diffraction-limited microscopy and electromagnetic cloaking.

We exploit this tunability of complex MIM mode indices to create a compact metal-oxide-Si (MOS) field effect plasmonic modulator. By transforming the MOS gate oxide into an optical channel, amplitude modulation depths of 11.2 dB are achieved in devices with channel areas as small as one one-hundredth of the optical wavelength squared. My talk will present both the theory and implementation of MIM-based metamaterials and devices, drawing on advances enabled by the field of plasmonics. Progress toward a three-dimensional negative index material and an all-optical Si modulator with gain will also be discussed.

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