Joseph M. Grogan

The Nanoaquarium: A Platform for In Situ Transmission Electron Microscopy in Liquid Media

Transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) are powerful tools for imaging on the nanoscale. These microscopes cannot be typically used to image processes taking place in liquid media because liquid simply evaporates in the high-vacuum environment of the microscope. In order to view a liquid sample, it is thus necessary to confine the liquid in a sealed vessel to prevent evaporation. Additionally, the liquid layer must be very thin to minimize electron scattering by the suspending medium. To address these issues, we have developed a flow cell with a height of tens of nanometers, sandwiched between two thin silicon nitride membranes. The cell is equipped with electrodes for actuation and sensing. The cell is thin enough to allow the transmission of electrons and the real-time imaging of nanoparticles suspended in liquid. This paper details the fabrication process, which relies on plasma-activated wafer bonding. Some of the advantages of our nanoaquarium include the thinnest observation chamber of any reported in situ TEM/STEM device, integrated electrodes for sensing and actuation, and wafer-scale processing that allows bulk device production. Device performance was demonstrated by STEM imaging of gold and polystyrene nanoparticles suspended in water with excellent resolution. Potential applications of the device include imaging of colloidal crystal formation, aggregation, nanowire growth, electrochemical deposition, and biological interactions.

Polymeric microbead arrays for microfluidic applications

Microbeads offer a convenient and efficient means of immobilizing biomolecules and capturing target molecules of interest in microfluidic immunoassay devices. In this study, hot embossing is used to form wells enabling the direct incorporation of a microbead array in a plastic substrate. We demonstrate two techniques to populate the well array with beads. In the first case, encoded beads with various functionalizations are distributed randomly among the wells and their position is registered by reading their encoding. Alternatively, beads are controllably placed at predetermined positions and decoding is not required. The random placement technique is demonstrated with two functionalized bead types that are distributed among the wells and then decoded to register their locations. The alternative, deliberate placement technique is demonstrated by controllably placing magnetic beads at selected locations in the array using a magnetic probe. As a proof of concept to illustrate the biosensing capability of the randomly assembled array, an on-chip, bead-based immunoassay is employed to detect the inflammatory protein Interleukin-8. The principle of the assay, however, can be extended to detect multiple targets simultaneously. Our method eliminates the need to interface silicon components with plastic devices to form microarrays containing individually addressable beads. This has the potential to reduce the cost and complexity of lab-on-chip devices for medical diagnosis, food and water quality inspection, and environmental monitoring.

Control of Electron Beam-Induced Au Nanocrystal Growth Kinetics through Solution Chemistry.

Measurements of solution-phase crystal growth provide mechanistic information that is helpful in designing and synthesizing nanostructures. Here, we examine the model system of individual Au nanocrystal formation within a defined liquid geometry during electron beam irradiation of gold chloride solution, where radiolytically formed hydrated electrons reduce Au ions to solid Au. By selecting conditions that favor the growth of well-faceted Au nanoprisms, we measure growth rates of individual crystals. The volume of each crystal increases linearly with irradiation time at a rate unaffected by its shape or proximity to neighboring crystals, implying a growth process that is controlled by the arrival of atoms from solution. Furthermore, growth requires a threshold dose rate, suggesting competition between reduction and oxidation processes in the solution. Above this threshold, the growth rate follows a power law with dose rate. To explain the observed dose rate dependence, we demonstrate that a reaction-diffusion model is required that explicitly accounts for the species H(+) and Cl(-). The model highlights the necessity of considering all species present when interpreting kinetic data obtained from beam-induced processes, and suggest conditions under which growth rates can be controlled with higher precision.

Visualization of Active and Passive Control of Morphology during Electrodeposition

Morphological instability, particularly dendrite formation, can cause potentially catastrophic failure in rechargeable batteries. Morphological instabilities can lower the quality of electroplated coatings, yet may also be useful in forming porous deposits. Thus, it is important to develop strategies to control these instabilities. Liquid cell electron microscopy allows us to image in real time and with nanoscale resolution the evolution of the solid-liquid interface during electrochemical deposition as a function of process conditions [1-3]. This allows us to obtain insights into the mechanisms leading to instabilities and to investigate strategies for controlling electrodeposited morphology.

Radiolysis during Liquid Cell Electron Microscopy

Recent advances in liquid cell electron microscopy [1, 2] have enabled real time imaging of objects suspended in liquids and processes taking place in liquids with the nanometer resolution of the electron microscope. As ionizing radiation passes through the suspending medium, energy is transferred from the fast-moving electrons to the irradiated medium. This energy excites and dislodges orbital electrons, which results in the generation of radical and molecular species such as H2, O2, H2O2, and hydrated electrons [3-5]. The hydrated electrons, oxidizing agents, and gaseous species can cause, respectively, reduction and precipitation of cations from solution, dissolution of metals, and nucleation and growth of bubbles [6-10]. A quantitative understanding of electron beam-induced effects is critical to assess whether the electron beam significantly affects the imaged phenomenon, to correctly interpret experiments carried out with liquid cells, mitigate unwanted effects, and take advantage of beam effects.

In Situ Liquid Cell TEM/STEM with the Nanoaquarium

The last few years have seen a flare of efforts to develop devices that allow real-time, in situ imaging of dynamical, nanoscale processes in fluid media with the resolution of a TEM or STEM (scanning TEM). Liquid cell TEM/STEM devices confine a thin slice of liquid sample in a sealed chamber sandwiched between two electron-transparent membranes, thus preventing evaporation while allowing the electron beam to pass through the sample to produce an image. The liquid slice must be hermetically sealed from the vacuum chamber of the electron microscope and sufficiently thin to minimize electron scattering by the suspending medium.