1 um We seek to understand the electrical and optical properties of single organic semiconducting molecules contacted on either end by metal electrodes. One of our approaches is to attach DNA as a template to active electronic and optical molecules at each end to produces a triblock structure (DNA-fluorophore-DNA, or “DFD”) shown on the Scheme 1. This DFD supramolecule is synthesized by the amide-coupling reaction, followed by DNA hybridization. In addition, the supramolecule can be characterized by scanning force and single-molecule fluorescence microscopy (Fig. 1). Both synthesis and characterization can potentially provide a method to prepare and investigate single molecule electronics through the DNA metallization process. Fig. 1. shows a direct evidence for the successful synthesis of DFD supramolecule. Single-molecule fluorescence of DFD supramolecule and fluorophore-long DNA (undesired product) appears as white spots; white square indicates the area of the scanning force microscopy (SFM) image (Fig. 1, A). SFM is introduced to locate the fluorescent spot within DFD supramolecule; the relative distance among fluorescent spots was calculated and measured, and a green contour is placed adjacent to each DNA strand for ease in visualization (Fig. 1, B). J.K. Lee, F. Jäckel, W. E. Moerner, Z. Bao, Small 2009, In press. Fig. 1. Characterization of DFD supramolecule using single-molecule fluorescence microscopy (A) and scanning force microscopy (B). White scale bars indicate 1 um. Scheme 1. Schematic representation of fluorophore-bis(micron-sized DNA) supramolecule Towards Single-Molecule Electronics: Synthesis and Characterization of Fluorophore-Bis(long DNA) Zhenan Bao, Stanford University, DMR

Large Single-Molecule Fluorescence Enhancements using Gold Bowtie Nanoantennas Zhenan Bao, Stanford University, DMR Gold bowtie nanoantennas are useful in the near infrared for confining local electromagnetic fields far beyond the diffraction limit, as well as enhancing the optical intensity by a factor of 1,000 in a (20 nm) 3 region. We studied the effect of these extremely confined, localized fields on a nanoscale emitter, a single molecule of TPQDI. Fluorescence from a molecule can be enhanced or quenched by the presence of a nearby metal depending on its location and orientation. Discrete photobleaching or blinking steps in the time trace reveal that, even though the total signal collected is from ~200 molecules, in some cases, 1 molecule can be responsible for half of the total signal. By comparing the step of a highly enhanced molecule near a bowtie to the normal photobleaching step from a molecule away from the bowtie, enhancements in collected fluorescence have been measured up to ~1300x! SEM 100nm 2  m Top: Schematic and SEM image of gold bowtie nanoantenna coated with fluorescent TPQDI dye molecules. Bottom: Confocal fluorescence image on 4x4 array of TPQDI coated bowtie nanoantennas with fluorescence time trace of one nanoantenna exhibiting single-molecule blinking and then photobleaching, which reveals the size of the emission from one molecule. Fluorescence Counts A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Muellen, and W.E. Moerner

Lithographic Positioning of Fluorescent Molecules on High-Q Photonic Crystal Cavities Zhenan Bao, Stanford University, DMR Photonic crystal cavities can confine light into volumes smaller than a cubic optical wavelength with extremely high quality factor, producing a strong interaction between light and emitters located in or near the cavity. Experiments are limited by the precision with which cavity and emitters can be spatially aligned and by the spectral range of the emitters. Photonic crystal cavities were fabricated in GaP, a material that yields cavity resonances in the near-IR from 735nm-860nm with quality factors up to 12,000. Photonic crystals were coated with a thin poly(methyl methacrylate) (PMMA) layer that was doped with randomly oriented photoluminescent molecules (DNQDI). Since the molecules are doped into PMMA, an E-beam lithography resist, a simple e-beam lithography step can expose all the molecule- doped resist that is not on the cavity and leave behind only molecule-doped PMMA on the cavity, effectively aligning the molecules to the cavity region in a lithographic, and therefore scalable, step. Before Lithography After Lithography 500nm Top: Experimental schematic of lithography to define molecules above photonic crystal cavity. Bottom: AFM image of localized dye-doped PMMA over a photonic crystal cavity with fluorescence spectra exhibiting the high-Q of the photonic crystal cavity mode. K. Rivoire, A. Kinkhabwala, F. Hatami, W. T. Masselink, Y. Avlasevich, K. Muellen, W.E. Moerner, and J. Vuckovic

Education: Four graduate students (one female), nine undergraduate students (four female), three community college students (one female and one Hispanic), four high school students, and two high school teachers, and two postdoctoral fellows (one female) have worked on research supported by this NSF Award. Societal Impact: This research relates to the problem of understanding and controlling individual molecules, a key component in eventual molecular scale devices for computing, telecommunications, medicine, and ultrasensitive detection. Reliable electrical contacts to a single molecule must be realized so that we can gain fundamental understanding of charge transport through single molecules. High school teacher with a graduate student A community college student working on testing Towards Single-Molecule Electronics: Synthesis and Characterization of Fluorophore-Bis(long DNA) Zhenan Bao, Stanford University, DMR