Benjamin D. Myers

Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs

Borophene: Boron in two dimensions Although bulk allotropes of carbon and boron differ greatly, small clusters of these elements show remarkable similarities. Boron analogs of two-dimensional carbon allotropes such as graphene have been predicted. Now Mannix et al. report the formation of two-dimensional boron by depositing the elemental boron onto a silver surface under ultrahigh-vacuum conditions (see the Perspective by Sachdev). The graphene-like structure was buckled, weakly bonded to the substrate, and metallic. Science, this issue p. 1513; see also p. 1468 A two-dimensional boron allotrope forms after depositing its elemental vapor on a silver surface in vacuum. [Also see Perspective by Sachdev] At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.

Manganese oxide micro-supercapacitors with ultra-high areal capacitance

A symmetric micro-supercapacitor is constructed by electrochemically depositing manganese oxide onto micro-patterned current collectors. High surface-to-volume ratio of manganese oxide and short diffusion distance between electrodes give an ultra-high areal capacitance of 56.3 mF cm(-2) at a current density of 27.2 μA cm(-2).

Interfacial Self‐Assembly of Cell‐like Filamentous Microcapsules

We report herein the development of a self-assembly method to rapidly produce cell-like, filamentous microcapsules (MCs) that have high surface area and encapsulate liquids or gels. The fibrous surfaces and shell walls of the MCs can be biologically functionalized using bioactive peptide amphiphiles (PAs), and the cores can harbor biopolymers, proteins, and other macromolecules. This novel method combines the spray-based production of nebulized biopolymer microdroplets with the recently reported ultrafast self-assembly of oppositely charged, high-molecular-weight biopolymers and PAs. There are numerous techniques available for microcapsule formation such as interfacial coacervation or interfacial polycondensation, layer-by-layer (LbL) polyelectrolyte complexation and colloid-templated self-assembly, emulsification with polymer phase separation, spraydrying methods, and microfluidic emulsion droplet formation. The advantage of the method reported herein is the combination of a self-assembly process that leads to structural complexity with the very broad range of bioactivity offered by peptide amphiphiles. The bioactive filament-forming PAs are composed of a hydrophobic alkyl tail, and a b-sheet-forming peptide domain, followed by peptide sequences with charged amino acids or bioactive epitopes that can either bind to receptors or to specific proteins by design (Figure 1A). These molecules assemble into high-aspect-ratio filaments upon electrostatic screening of the charged amino acids and the formation of b sheets. Hydrophobic collapse of these filament-forming molecules under strong screening conditions leads to the display of a high density of biological signals on their surfaces (on the order of 10 signals per cm). In vivo and in vitro studies have shown that certain PA molecules that bear bioactive epitopes promote regeneration of spinal cord axons, angiogenesis, bone regeneration, cartilage repair, proliferation of bone marrow cells, and selective differentiation of neural progenitor cells into neurons. We previously demonstrated that solutions of PAs and oppositely charged biopolymers can self-assemble at the liquid–liquid interface to form hierarchically structured membranes that can be permeable to proteins to produce saclike structures on the macroscale with millisecond speeds (with size scales of millimeters). The shells of these sacs are highly structured and their surfaces are fully covered with nanoscale filaments. We have modified this approach for the production of filamentous MCs less than 100 mm in diameter (Figure 1C). These micrometer-scale objects could be created with highly bioactive properties, high surface area, and dimensions approaching those of cells. The first step in the MC formation requires generation of picoliter droplets of a biopolymer solution. We built a spraybased device that enables production of droplets with diameters (dMC) as small as 5 mm and an average production rate of 1! 10 microcapsules per second. The nebulizing device has three components: a) a pressure microinjector for the delivery of a biopolymer solution, b) a glass capillary (orifice diameter ca. 40 mm), and c) compressed gas (nitrogen or air; see the Supporting Information). We nebulized the stream of 0.25 wt% aqueous alginate (AL) solution using a high-velocity flow of nitrogen. The microdroplets of the biopolymer solution were directly ejected into a 0.1 wt% aqueous solution of C16V3A3K3 PA (Figure 1A) to induce the membrane-forming self-assembly process that occurs on the millisecond timescale. After allowing 15 min for dynamic selfassembly between the PA molecules and the biopolymer, Figure 1. A) Molecular structure of peptide amphiphile C16V3A3K3. B) Alginic acid. C) Schematic illustration of the cross-section of a PAalginate microcapsule.

Imaging and elemental mapping of biological specimens with a dual-EDS dedicated scanning transmission electron microscope

A dedicated analytical scanning transmission electron microscope (STEM) with dual energy dispersive spectroscopy (EDS) detectors has been designed for complementary high performance imaging as well as high sensitivity elemental analysis and mapping of biological structures. The performance of this new design, based on a Hitachi HD-2300A model, was evaluated using a variety of biological specimens. With three imaging detectors, both the surface and internal structure of cells can be examined simultaneously. The whole-cell elemental mapping, especially of heavier metal species that have low cross-section for electron energy loss spectroscopy (EELS), can be faithfully obtained. Optimization of STEM imaging conditions is applied to thick sections as well as thin sections of biological cells under low-dose conditions at room and cryogenic temperatures. Such multimodal capabilities applied to soft/biological structures usher a new era for analytical studies in biological systems.

Size-Selective Nanoparticle Assembly on Substrates by DNA Density Patterning.

The vision of nanoscale self-assembly research is the programmable synthesis of macroscale structures with controlled long and short-range order that exhibit a desired set of properties and functionality. However, strategies to reliably isolate and manipulate the nanoscale building blocks based on their size, shape, or chemistry are still in their infancy. Among the promising candidates, DNA-mediated self-assembly has enabled the programmable assembly of nanoparticles into complex architectures. In particular, two-dimensional assembly on substrates has potential for the development of integrated functional devices and analytical systems. Here, we combine the high-resolution patterning capabilities afforded by electron-beam lithography with the DNA-mediated assembly process to enable direct-write grayscale DNA density patterning. This method allows modulation of the functionally active DNA surface density to control the thermodynamics of interactions between nanoparticles and the substrate. We demonstrate that size-selective directed assembly of nanoparticle films from solutions containing a bimodal distribution of particles can be realized by exploiting the cooperativity of DNA binding in this system. To support this result, we study the temperature-dependence of nanoparticle assembly, analyze the DNA damage by X-ray photoelectron spectroscopy and fluorescence microscopy, and employ molecular dynamics simulations to explore the size-selection behavior.

Suppressing Electron Exposure Artifacts: An Electron Scanning Paradigm with Bayesian Machine Learning.

Electron microscopy of biological, polymeric, and other beam-sensitive structures is often hampered by deleterious electron beam interactions. In fact, imaging of such beam-sensitive materials is limited by the allowable radiation dosage rather that capabilities of the microscope itself, which has been compounded by the availability of high brightness electron sources. Reducing dwell times to overcome dose-related artifacts, such as radiolysis and electrostatic charging, is challenging due to the inherently low contrast in imaging of many such materials. These challenges are particularly exacerbated during dynamic time-resolved, fluidic cell imaging, or three-dimensional tomographic reconstruction-all of which undergo additional dosage. Thus, there is a pressing need for the development of techniques to produce high-quality images at ever lower electron doses. In this contribution, we demonstrate direct dose reduction and suppression of beam-induced artifacts through under-sampling pixels, by as much as 80% reduction in dosage, using a commercial scanning electron microscope with an electrostatic beam blanker and a dictionary learning in-painting algorithm. This allows for multiple sparse recoverable images to be acquired at the cost of one fully sampled image. We believe this approach may open new ways to conduct imaging, which otherwise require compromising beam current and/or exposure conditions.

Stage-Rocked Electron Channeling for Crystal Orientation Mapping

Microstructural analysis by crystal orientation mapping of bulk functional materials is an essential and routine operation in the engineering of material properties. Far and away the most successfully employed technique, Electron Backscattered Diffraction (EBSD), provides high spatial resolution information at the cost of limited angular resolution and a distorted imaging condition. In this work, we demonstrate a stage-rocked electron channeling approach as a low-cost orientation mapping alternative to EBSD. This is accomplished by automated electron channeling contrast imaging (ECCI) as the microscope stage physically tilts/rotates a sample through a reduced hemisphere of orientations followed by computational reconstruction of electron channeling patterns (ECP). Referred to as Orientation Mapping by Electron Channeling (OMEC), our method offers advantages in terms of local defect analysis, as it combines the advantages of selected area ECP (SACP) and ECCI. We also illustrate dynamic or “adaptive” sampling schemes to increase the throughput of the technique. Finally, we discuss the implications for sample analysis in which large 3D maps of ECCI images can be routinely constructed of challenging crystalline samples. As an electron channeling-based approach to orientation mapping, OMEC may open new routes to characterize crystalline materials with high angular and spatial resolution.

Reducing Electron Dose and Sample Damage with Bayesian Machine Learning and Self-Organizing Neural Networks

There is a pressing need in the electron microscopy community to identify new methods to strongly reduce the sensitivity of observed specimens to the strong electron currents modern microscopes are capable of. For many nontraditional materials, such as biological or hybrid polymeric structures, the limiting factor is rarely the power of the actual microscope, but instead the sample’s structural integrity to the intense and finely focused electron probe. These specimens are particularly sensitive to effects of radiolysis and electrostatic charging, which often prevent more than a one-shot capture of the area of interest. New studies involving in-situ fluidic or gaseous environments are also suffering from significant electron medium interactions, which can often preclude quantitative measurements in such environments. These studies are complicated by the need to capture dynamic phenomena with multiple exposures, which can significantly increase the number of electrons injected per unit area.

Temperature-Controlled Fluidic-Cell Scanning Electron Microscopy

Fluidic-cell electron microscopy has generated a great deal of interest due to the potential for real-space and real-time imaging of nanometer-scale objects in a fluidic environment [1]. While the possibilities of this technique have motivated a number of interesting studies from nanoparticle growth to electrochemical processes, there remain a number of significant challenges to achieving true in situ microscopy on technologically relevant samples [2, 3]. One of these challenges is the lack of temperature control in commercially available systems for fluidic-cell transmission electron microscopy (TEM). Even in newer TEM fluidic-cell systems with integrated resistive heating and temperature sensing elements, there are significant unresolved questions of temperature control and uniformity. Clearly, achieving temperature control inside the geometrically restricted environment of the TEM sample holder is a significant challenge. However, the scanning electron microscope (SEM) chamber does not have the same restrictions on sample size. Here, we demonstrate the use of a custom built MEMS-based SEM fluidic cell with temperature control to address this limitation.

Directed assembly in epitaxial zinc oxide films on focused ion beam modified sapphire substrates

A new method for directed self-assembly using focused ion beam (FIB) and physical vapor deposition is presented. The high resolution and site-specific patterning capabilities of FIB are coupled with the self-assembly process in heteroepitaxial thin film growth. An efficient FIB-induced damage mechanism is exploited to pattern amorphous regions in sapphire substrates which direct the subsequent assembly of a sputter-deposited zinc oxide film. This novel approach allows for the fabrication of in-plane nano- to microscale heterostructures comprising epitaxial regions with high strain and defect density that are separated by regions of randomly oriented (in-plane) grains with much lower strain and defect density.A new method for directed self-assembly using focused ion beam (FIB) and physical vapor deposition is presented. The high resolution and site-specific patterning capabilities of FIB are coupled with the self-assembly process in heteroepitaxial thin film growth. An efficient FIB-induced damage mechanism is exploited to pattern amorphous regions in sapphire substrates which direct the subsequent assembly of a sputter-deposited zinc oxide film. This novel approach allows for the fabrication of in-plane nano- to microscale heterostructures comprising epitaxial regions with high strain and defect density that are separated by regions of randomly oriented (in-plane) grains with much lower strain and defect density.