Our group investigates chemistry at the nanometer length scale, working at the intersection between solid state chemistry and advanced characterization. The goal of our group is to combine the design and synthesis of new nanostructured materials, with characterization using advanced transmission electron microscopy techniques such as atomic resolution imaging, spectroscopic mapping, and in situ microscopy. Microscopy is performed both on-site and at Oak Ridge National Lab, with opportunities for group members to work in both locations.
In situ TEM
The driving forces and mechanisms of crystal structure growth and transformation in solid state chemistry are often inferred from ex situ studies. For this reason, the mechanistic descriptions may not be well understood. A complete understanding of the mechanism of crystal growth or transformation is necessary, however, to be able to control material synthesis to engineer desired structures, morphologies, and properties. In our group we try to gain understanding via an in situ approach in which we observe solid state chemistry processes in real-time.
An example of one of these experiments is shown in the movie. Here we observe the etching of a tin dioxide nanowire catalyzed by the gold nanoparticle at its tip, in which the process is driven by in situ heating in a transmission electron microscope (TEM).
This process is the essentially the opposite of the metal nanoparticle-catalyzed vapor-liquid-solid (VLS) growth mechanism, and has been dubbed SLV. Using compositional analysis, we track the Sn content of the Au catalyst particle as the nanowire dissolves at the solid-liquid interface, and evaporates at the liquid droplet surface. This method provides an experimental platform to explore the VLS nanowire growth mechanism. The video shows an excerpt of data published in ACS Nano.
We are interested in synthesizing nanostructures in which functional interfaces occur as a result of the synthetic process, removing the need for step-by-step fabrication. The focus of our research is to make materials for energy applications, such as photovoltaic materials, plasmonic resonators, Li-ion conductors and electrode materials for batteries.
Inspired by our previous in situ TEM work to use the SLV mechanism to dissolve nanowires, we achieved the synthesis of "negative nanowire" arrays within single-crystalline zinc oxide and tin(IV) oxide substrates, demonstrating control over shape, size, alignment, and growth direction. This work was published in Chemistry of Materials.
The SEM image shows zinc oxide nanowires grown using a low-temperature solution phase synthesis, resulting in aligned nanowires, which each contain a single, axial, p-n homojunction, as confirmed by transport and cathodoluminescence measurements. This work was published in Nanoscale.
Shown in the image is a Z-contrast image of a silver nanorod taken in the STEM. Maps showing the spatial distribution of the two lowest energy plasmon modes were collected using electron energy loss spectroscopy (EELS), and compared to theoretical simulations. This work was published in Nano Letters. See also our other publications in plasmonics and spectroscopic mapping.
Microscopy can be used not only to image atomic structure and composition profiles, but also to produce maps of properties such as oxidation state, plasmonic resonance response, band-gap variation, and energy conversion efficiency. In our group, the more exotic of these mapping techniques are achieved using the ultra-high-resolution, aberration-corrected, high spectral resolution, scanning transmission electron microscopes (STEMs) at ORNL.