Stimuli-Responsive DNA-Linked Nanoparticle Arrays as Programmable Surfaces.

Self- and directed-assembly approaches have enabled precise control over the composition and geometry of 2D and 3D nanoparticle constructs. However, the resulting structures are typically static, providing only a single structural arrangement of the nanoparticle building blocks. In this work, the power of DNA-linked nanoparticle assembly is coupled to a grayscale patterning technique to create programmable surfaces for assembly and thermally activated reorganization of gold nanoparticle arrays. Direct grayscale patterning of DNA monolayers by electron-beam lithography (DNA-EBL) enables the production of surfaces with nanometer-scale control over the density of functional DNA. This enables tuning of the particle-surface interactions with single-nanoparticle resolution and without the need for a physical template as employed in most directed assembly methods. This technique is applied on suspended membrane structures to achieve high-resolution assembly of 2D nanoparticle arrays with highly mutable architectures. Gold nanorods assembled on grayscale-patterned surfaces exhibit temperature-dependent configurations and ordering behavior that result in tunable polarization-dependent optical properties. In addition, spherical gold particles assembled from a bimodal suspension produce arrays with temperature-dependent configurations of small and large particles. These results have important implications for the design and fabrication of reconfigurable nanoparticle arrays for application as structurally tunable optical metasurfaces.

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.

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.

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.

Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs.

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.

3D LITHOGRAPHY. Atomic gold-enabled three-dimensional lithography for silicon mesostructures.

Three-dimensional (3D) mesostructured semiconductors show promising properties and applications; however, to date, few methods exist to synthesize or fabricate such materials. Metal can diffuse along semiconductor surfaces, and even trace amounts can change the surface behavior. We exploited the phenomena for 3D mesoscale lithography, by showing one example where iterated deposition-diffusion-incorporation of gold over silicon nanowires forms etchant-resistant patterns. This process is facet-selective, producing mesostructured silicon spicules with skeletonlike morphology, 3D tectonic motifs, and reduced symmetries. Atom-probe tomography, coupled with other quantitative measurements, indicates the existence and the role of individual gold atoms in forming 3D lithographic resists. Compared to other more uniform silicon structures, the anisotropic spicule requires greater force for detachment from collagen hydrogels, suggesting enhanced interfacial interactions at the mesoscale.

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).