Facility Director, Imaging and Manipulation of Nanostructures
Stanford University, Postdoctoral Fellow, Department of Chemistry, Advisor: W. E. Moerner, 2003-2006
Yale University, Ph.D., Department of Applied Physics, Advisor: R. D. Grober, Dissertation titled "Three-Dimensional Imaging Spectroscopy of the III-Nitride Material System", 2003
Yale University, M.S., Department of Applied Physics, Advisor: R. D. Grober, 1998
U.C. Berkeley, B.A., Department of Physics, Research Advisor: R. W. Falcone, 1997
My research is focused on understanding the nano- and meso-scale interactions between localized states in materials (i.e. studying the competition between localized and delocalized states), and relating these properties with material and device functionality. We do this by correlating spatially-dependent physical properties (e.g. electronic structure) with chemical information (e.g. molecular composition, reaction rates and dynamics) and morphological structure.
This work is enabled by new multimodal and multidimensional spectroscopic methods that we are continuously developing, which provide unique access to physiochemical behavior at relevant length scales in real environments encountered in energy applications. These are typically grounded in (nano)optical and scan-probe techniques, exploiting the chemical information and high spatial, spectral and temporal resolution afforded by them.
The recent invention of single metallic optical nanoantennas has greatly improved the mismatch between light and nanometer-scale objects. An ultimate design goal is to place a nanoantenna on a scanning probe, thus yielding an extremely intense near-field optical light source with high local contrast that has applications as diverse as ultrasensitive biological detection, nanolithography, high-density data storage and high resolution optical microscopy and spectroscopy.
Manipulating Nanoscale Light Fields with the Plasmonic Color Nanosorter
A central goal of plasmonics is complete control over optical signals at deeply sub-wavelength scales. The recent invention of optical nanoantennas has led to a number of device designs that provide confinement of optical fields at nanometer length scales. For photonic applications, however, the effectiveness of these structures would be significantly improved by the added ability to spatially sort the optical signals based on a physically accessible parameter such as energy/color. Read the full research paper. (Access required)
Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals
While single-molecule imaging has become increasingly routine in biology, no probes with ideal single-molecule properties have been described. Significant progress has been made in the synthesis and certain applications of upconverted nanoparticles (UCNPs), but their optical characterization has been limited to ensemble-averaged measurements. We show here that individual UCNPs are bright enough to be imaged with a modest-power CW laser, they exhibit no blinking, are exceptionally photostable, and they display no spectral overlap with celluar autofluorescence. We also find that UCNPs may be rendered water-soluble by wrapping with low molecular weight amphiphilic polymers, resulting in well-dispersed aqueous nanoparticles with undiminished photophysical characteristics. Polymer-wrapped UCNPs are endocytosed by murine fibroblasts and show strong upconverted luminescence, with no measurable anti-Stokes background autofluorescence. These findings suggest that UCNPs are ideally suited for single-molecule imaging experiments. Read the full research paper.
Our research targets the plant cell wall because this will be the primary source of biomass for the conversion of cellulosic material to biofuels, such as ethanol. A critical step in this conversion is the breakdown of the biomass to a form that is amenable to enzymatic or microbial generation of the constituent sugar units from the long-chain carbohydrate polymers in the cell wall. This biomass breakdown uses chemical, heat, or pressure treatments that remain expensive and limit the economic viability of biofuels. The detailed knowledge of the plant cell wall that we propose to generate will allow researchers to visualize the chemical and physical obstacles to breakdown. This will be vital in developing better biomass breakdown procedures, and identifying plant material that may naturally be better suited to use in such processes.
Label-free in situ imaging of lignification in the cell wall of low lignin transgenicPopulus trichocarpa
Chemical imaging by confocal Raman microscopy was used for the visualization of the cellulose and lignin distribution in wood cell walls. Lignin reduction in wood can be achieved by, for example, transgenic suppression of a monolignol biosynthesis gene encoding 4-coumarate-CoA ligase (4CL). Here we have used confocal Raman microscopy to compare lignification in wild type and lignin-reduced 4CL transgenic Populus trichocarpa stem wood with spatial resolution that is considerably sub-mm. Analyzing the lignin Raman bands in the spectral region between 1600 and 1700 cm-1, differences in lignin content and localization are mapped in situ. Transgenic reduction of lignin is particularly pronounced in the S2 wall layer of fibers, suggesting that such transgenic approach may help overcome cell wall recalcitrance to wood saccharification. Spatial heterogeneity in the lignin composition, in particular with regard to ethylenic residues, is observed in both samples. Read the full research paper[access required].
Application of plasmonic devices in spectroscopy has been limited by the lack of methods for integrating specifically-engineered plasmonic nanostructures into actual devices; (for example, fabricating plasmonic antennae on preexisting AFM cantilevers for next-generation near-field optical probes). At the Molecular Foundry, we have a dedicated effort concentrating on fabricating reproducible nanoantenna-based scanning probes that strongly enhance and confine light fields. These engineered probes are expected to significantly advance the technique of near-field optical microscopy, opening a wide range of previously inaccessible applications.
Functional plasmonic antenna Scanning Probes fabricated by Induced Deposition Mask Lithography
We have fabricated plasmonic bow-tie antennae on the apex of silicon atomic-force microscope cantilever tips that enhance the local silicon Raman scattering intensity by ~ 4*104 when excited near the antenna resonance. The antennae were fabricated using a novel method, Induced Deposition Mask Lithography (IDML), capable of creating high-purity metallic nanostructures on non-planar, non-conducting substrates with high repeatability. IDML involves electron-beam induced deposition of a W or SiOx hard-mask on the material to be pattered, in our case a 20 nm Au film, followed by Ar ion etching at ~ 800 eV to remove the mask and the unmasked gold, leaving a chemically pure Au bowtie antenna. Antenna function and reproducibility was confirmed by comparing Raman spectra for excitation polarized parallel and perpendicular to the antenna axis, as well as by dark-field spectroscopic characterization of resonant modes. The field enhancement of these plasmonic AFM antennae-tips was comparable that of antennas produced by electron-beam lithography on flat substrates.