Surbhi Lal

Solar Vapor Generation Enabled by Nanoparticles

Solar illumination of broadly absorbing metal or carbon nanoparticles dispersed in a liquid produces vapor without the requirement of heating the fluid volume. When particles are dispersed in water at ambient temperature, energy is directed primarily to vaporization of water into steam, with a much smaller fraction resulting in heating of the fluid. Sunlight-illuminated particles can also drive H(2)O-ethanol distillation, yielding fractions significantly richer in ethanol content than simple thermal distillation. These phenomena can also enable important compact solar applications such as sterilization of waste and surgical instruments in resource-poor locations.

Tailoring plasmonic substrates for surface enhanced spectroscopies

Our understanding of how the geometry of metallic nanostructures controls the properties of their surface plasmons, based on plasmon hybridization, is useful for developing high-performance substrates for surface enhanced spectroscopies. In this tutorial review, we outline the design of metallic nanostructures tailored specifically for providing electromagnetic enhancements for surface enhanced Raman scattering (SERS). The concepts developed for nanoshell-based substrates can be generalized to other nanoparticle geometries and scaled to other spectroscopies, such as surface enhanced infrared absorption spectroscopy (SEIRA).

Noble Metal Nanowires: From Plasmon Waveguides to Passive and Active Devices

Using chemical synthesis, researchers can produce noble metal nanowires with highly regular, crystalline properties unachievable by alternative, top-down nanofabrication methods. Sitting at the intersection of nanochemistry and nanooptics, noble metal nanowires have generated intense and growing research interest. These nanostructures combine subwavelength transverse dimensions (50-100 nm) and longitudinal dimensions that can reach tens of micrometers or more, which makes them an ideal platform to launch surface plasmon waves by direct illumination of one end of the structure. Because of this property, researchers are using noble metal nanowires as a tool for fundamental studies of subwavelength plasmon-based optics and the properties of surface plasmon guided wave propagation in highly confined geometries below the classical optical diffraction limit. In this Account, we review some of the recent developments in plasmonic nanowire fabrication, nanowire plasmon imaging, and nanowire optical components and devices. The addition of an adjacent nanowire, substrate, or other symmetry-breaking defect can enable the direct coupling of light to and from free space to the guided waves on a nanowire structure. Such structures lead to more complex nanowire-based geometries with multiple optical inputs and outputs. Additional nanowire imaging methods are also possible: plasmon propagation on nanowires produces intense near-field diffraction, which can induce fluorescence in nearby quantum dots or photobleach adjacent molecules. When the nanowire is deposited on a dielectric substrate, the plasmon propagation along chemically synthesized nanowires exceeds 10 μm, which makes these structures useful in nonlocal applications such as remote surface-enhanced Raman spectroscopy (SERS) sensing. Nanowires can be used as passive optical devices, which include, for example, polarization manipulators, linear polarization rotators, or even broadband linear-to-circular polarization converters, an optical function not yet achievable with conventional diffraction-limited optical components. Nanowires can also serve as highly directional broadband optical antennas. When assembled into networks, plasmonic nanowires can be used to create optical devices, such as interferometric logic gates. Individual nanowires function as multiple input and output terminals in branched network geometries, where light incident on one wire can turn the emission from one or more output wires on or off. Nanowire-based devices that could exploit this effect include nanoscale routers and multiplexers, light modulators, and a complete set of Boolean logic functions.

Direct Optical Detection of Aptamer Conformational Changes Induced by Target Molecules

Aptamers are single-stranded DNA/RNA oligomers that fold into three-dimensional conformations in the presence of specific molecular targets. Surface-enhanced Raman spectroscopy (SERS) of thiol-bound DNA aptamer self-assembled monolayers on Au nanoshell surfaces provides a direct, label-free detection method for the interaction of DNA aptamers with target molecules. A spectral cross-correlation function, Gamma, is shown to be a useful metric to quantify complex changes in the SERS spectra resulting from conformational changes in the aptamer induced by target analytes. While the pristine, unexposed anti-PDGF (PDGF = platelet-derived growth factor) aptamer yields highly reproducible spectra with Gamma = 0.91 +/- 0.01, following incubation with PDGF, the reproducibility of the SERS spectra is dramatically reduced, yielding Gamma = 0.67 +/- 0.02. This approach also allows us to discriminate the response of a cocaine aptamer to its target from its weaker response to nonspecific analyte molecules.

A Plethora of Plasmonics from the Laboratory for Nanophotonics at Rice University

The study of the surface plasmons of noble metals has emerged as one of the most rapidly growing and dynamic topics in nanoscience. Key advances in the synthesis of noble metal nanoparticles and nanostructures have resulted in a broad variety of structures whose geometries can be controlled systematically at the nanoscale. Arising from these efforts is a new level of insight and understanding regarding the fundamental properties of localized plasmons supported by these structures, and, in particular, the properties of interacting plasmon systems. This additional insight has led to the design of plasmonic systems that support coherent phenomena, such as Fano resonances. A broad range of applications are emerging for these structures: single- nanoparticle and nanogap chemical sensors, low-loss plasmon waveguides, and active plasmonic devices and detectors. Applications in biomedicine that exploit the strong photothermal response of nanoparticle plasmons have developed and advanced into clinical trials. The Laboratory for Nanophotonics at Rice has been home to many of these advances. Here, we showcase a variety of functional plasmonic materials and nanodevices emerging from our individual and collaborative efforts.

Comparison of infrared, Raman, photoluminescence, and x-ray photoelectron spectroscopy for characterizing arc-jet-deposited diamond films

Impurities and growth-related defect structures are mainly responsible for low thermal conductivity of chemical vapor deposited diamond films. Different quality arc-jet-deposited, free-standing diamond samples were obtained from industry. Fourier transform infrared (FTIR), Raman, and x-ray photoelectron spectroscopy (XPS) were used to determine the quality of these samples. The nondiamond carbon was estimated from the 1560 cm−1 broad peak intensity, the CHx integrated peak absorbance, and the C1s plasmon loss features for Raman, FTIR, and XPS studies, respectively. The diamond quality was also determined from the Raman diamond peak full width at half maximum (FWHM) and XPS valence band spectra. It was observed that the higher the hydrogen content (determined by FTIR), the darker the color of the film, the larger the nondiamond 1560 cm−1 peak intensity, and the larger the FWHM of the Raman diamond peak at 1332 cm−1. Negligible difference in the C1s diamond bulk plasmon loss peak was observed for films of w...

Effect of diamond film quality on tungsten related photoluminescence peaks

We have studied a series of polycrystalline diamond films of various qualities grown by arc jet chemical vapor deposition method. We have used continuous wave photoluminescence (PL) to study the intensity, width and emission energy of defects, and study the correlation of these parameters to the quality of the diamond films. Recently, it has been proposed that impurities are more easily incorporated into highly defective films, and thus the defect PL may be used to determine impurity concentration in the films. We find that the intensity of the PL from the 1.735 eV center is easily affected by the quality and may be quenched in poor quality samples, and thus it is not an appropriate marker for defect concentration. We find the center with an emission line at 1.805 eV to be a better candidate for being used as a marker for defect concentration. This center is observed in different quality samples, and its line width and emission energy are not affected by film quality.

The electronic structure of tungsten impurities in diamond films

A model for the electronic structure of the tungsten impurity in diamond is presented that explains recent photoluminescence results. The model is based on the Ludwig and Woodbury model for interstitial transition-metal impurities in silicon and includes Jahn-Teller coupling, which almost entirely quenches the orbital angular momentum.

Plasmon-plasmon interaction between gold nanoshells and gold surfaces

Summary form only given. Gold nanoshells are colloidal particles with a dielectric core covered by a gold shell. The plasmon resonance of the nanoshells can be tuned by varying the ratio of the core/shell radii. An enhancement in the electromagnetic energy can be found, at resonance, in the region close to the nanoshell known as the near field. It has been shown theoretically and experimentally that if the evanescent near fields of a surface plasmon polariton and a particle plasmon overlap, an efficient exchange of energy from the freely propagating electromagnetic waves into surface plasmons can be achieved. Previous experiments used particles that lacked the extraordinary tunability of nanoshells. We give our sample geometry. After evaporating a layer of gold onto a glass slide, we deposit self-assembled monolayers (SAMs) of a polymer (PDDA) and Hectorite (a synthetic clay), to control the spacing between the gold surface and the nanoshells.