Conserved atomic bonding sequences and strain organization of graphene grain boundaries.

The bulk properties of polycrystalline materials are directly influenced by the atomic structure at the grain boundaries that join neighboring crystallites. In this work, we show that graphene grain boundaries are comprised of structural building blocks of conserved atomic bonding sequences using aberration corrected high-resolution transmission electron microscopy. These sequences appear as stretches of identically arranged periodic or aperiodic regions of dislocations. Atomic scale strain and lattice rotation of these interfaces is derived by mapping the exact positions of every carbon atom at the boundary with ultrahigh precision. Strain fields are organized into local tensile and compressive dipoles in both periodic and aperiodic dislocation regions. Using molecular dynamics tension simulations, we find that experimental grain boundary structures maintain strengths that are comparable to idealized periodic boundaries despite the presence of local aperiodic dislocation sequences.

Performance enhancement of a graphene-zinc phosphide solar cell using the electric field-effect.

The optical transparency and high electron mobility of graphene make it an attractive material for photovoltaics. We present a field-effect solar cell using graphene to form a tunable junction barrier with an Earth-abundant and low cost zinc phosphide (Zn3P2) thin-film light absorber. Adding a semitransparent top electrostatic gate allows for tuning of the graphene Fermi level and hence the energy barrier at the graphene-Zn3P2 junction, going from an ohmic contact at negative gate voltages to a rectifying barrier at positive gate voltages. We perform current and capacitance measurements at different gate voltages in order to demonstrate the control of the energy barrier and depletion width in the zinc phosphide. Our photovoltaic measurements show that the efficiency conversion is increased 2-fold when we increase the gate voltage and the junction barrier to maximize the photovoltaic response. At an optimal gate voltage of +2 V, we obtain an open-circuit voltage of V oc = 0.53 V and an efficiency of 1.9% under AM 1.5 1-sun solar illumination. This work demonstrates that the field effect can be used to modulate and optimize the response of photovoltaic devices incorporating graphene.

3D motion of DNA-Au nanoconjugates in graphene liquid cell electron microscopy.

Liquid-phase transmission electron microscopy (TEM) can probe and visualize dynamic events with structural or functional details at the nanoscale in a liquid medium. Earlier efforts have focused on the growth and transformation kinetics of hard material systems, relying on their stability under electron beam. Our recently developed graphene liquid cell technique pushed the spatial resolution of such imaging to the atomic scale but still focused on growth trajectories of metallic nanocrystals. Here, we adopt this technique to imaging three-dimensional (3D) dynamics of soft materials instead, double strand (dsDNA) connecting Au nanocrystals as one example, at nanometer resolution. We demonstrate first that a graphene liquid cell can seal an aqueous sample solution of a lower vapor pressure than previously investigated well against the high vacuum in TEM. Then, from quantitative analysis of real time nanocrystal trajectories, we show that the status and configuration of dsDNA dictate the motions of linked nanocrystals throughout the imaging time of minutes. This sustained connecting ability of dsDNA enables this unprecedented continuous imaging of its dynamics via TEM. Furthermore, the inert graphene surface minimizes sample-substrate interaction and allows the whole nanostructure to rotate freely in the liquid environment; we thus develop and implement the reconstruction of 3D configuration and motions of the nanostructure from the series of 2D projected TEM images captured while it rotates. In addition to further proving the nanoconjugate structural stability, this reconstruction demonstrates 3D dynamic imaging by TEM beyond its conventional use in seeing a flattened and dry sample. Altogether, we foresee the new and exciting use of graphene liquid cell TEM in imaging 3D biomolecular transformations or interaction dynamics at nanometer resolution.

Continuity of graphene on polycrystalline copper.

The atomic structure of graphene on polycrystalline copper substrates has been studied using scanning tunneling microscopy. The graphene overlayer maintains a continuous pristine atomic structure over atomically flat planes, monatomic steps, edges, and vertices of the copper surface. We find that facets of different identities are overgrown with graphene's perfect carbon honeycomb lattice. Our observations suggest that growth models including a stagnant catalytic surface do not apply to graphene growth on copper. Contrary to current expectations, these results reveal that the growth of macroscopic pristine graphene is not limited by the underlying copper structure.

Atomic-scale characterization of graphene grown on copper (100) single crystals.

Growth of graphene on copper (100) single crystals by chemical vapor deposition has been accomplished. The atomic structure of the graphene overlayer was studied using scanning tunneling microscopy. A detailed analysis of moiré superstructures present in the graphene topography reveals that growth occurs in a variety of orientations over the square atomic lattice of the copper surface. Transmission electron microscopy was used to elucidate the crystallinity of the grown graphene. Pristine, defect-free graphene was observed over copper steps, corners, and screw dislocations. Distinct protrusions, known as "flower" structures, were observed on flat terraces, which are attributed to carbon structures that depart from the characteristic honeycomb lattice. Continuous graphene growth also occurs over copper adatoms and atomic vacancies present at the single-crystal surface. The copper atom mobility within vacancy islands covered with suspended graphene sheets reveals a weak graphene-substrate interaction. The observed continuity and room-temperature vacancy motion indicates that copper mobility likely plays a significant role in the mechanism of sheet extension on copper substrates. Lastly, these results suggest that the quality of graphene grown on copper substrates is ultimately limited by nucleation at the surface of the metal catalyst.

Preparation and properties of metallic, superhard rhenium diboride crystals.

Single crystals of ReB(2) have been prepared from an aluminum flux under inert gas flow. The crystals are typically 1-3 mm in diameter and 500 microm thick, growing along the [002] direction with a distinct hexagonal morphology. Vickers microhardness and nanoindentation testing indicate that the (002) plane possesses the highest hardness with measured values of 40.5 and 36.4 GPa, respectively. The elastic anisotropy was examined and the indentation moduli of the basal plane and an (hk0) plane of unknown indices are 675 and 510 GPa, respectively. Four-probe electrical resistivity measurements demonstrate that ReB(2) is the hardest material known to exhibit metallic behavior. Thermogravimetric analysis indicates that the crystals are stable in air up to 1000 degrees C due to the formation of a protective boron oxide coating.

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples.

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a ß-Si3N4 wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and lattice fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.

Molecular Arrangement and Charge Transfer in C60/Graphene Heterostructures.

Charge transfer at the interface between dissimilar materials is at the heart of electronics and photovoltaics. Here we study the molecular orientation, electronic structure, and local charge transfer at the interface region of C60 deposited on graphene, with and without supporting substrates such as hexagonal boron nitride. We employ ab initio density functional theory with van der Waals interactions and experimentally characterize interface devices using high-resolution transmission electron microscopy and electronic transport. Charge transfer between C60 and the graphene is found to be sensitive to the nature of the underlying supporting substrate and to the crystallinity and local orientation of the C60. Even at room temperature, C60 molecules interfaced to graphene are orientationally locked into position. High electron and hole mobilities are preserved in graphene with crystalline C60 overlayers, which has ramifications for organic high-mobility field-effect devices.

Atomic Defects in Two Dimensional Materials.

Atomic defects in crystalline structures have pronounced affects on their bulk properties. Aberration-corrected transmission electron microscopy has proved to be a powerful characterization tool for understanding the bonding structure of defects in materials. In this article, recent results on the characterization of defect structures in two dimensional materials are discussed. The dynamic behavior of defects in graphene shows the stability of zigzag edges of the material and gives insights into the dislocation motion. Polycrystalline graphene is characterized using advanced electron microscopy techniques, revealing the global crystal structure of the material, as well as atomic-resolution observation of the carbon atom positions between neighboring crystal grains. Studies of hexagonal boron nitride (hBN) are also visited, highlighting the interlayer bonding, which occurs upon defect formation, and characterization of grain boundary structures. Lastly, defect structures in monolayer polycrystalline transition metal dichalcogenides grown by CVD are discussed.

Studies of the dynamics of biological macromolecules using Au nanoparticle-DNA artificial molecules.

The recent development of graphene liquid cells, a nanoscale version of liquid bubble wrap, is a breakthrough for in situ liquid phase electron microscopy (EM). Using ultrathin graphene sheets as the liquid sample container, graphene liquid cells have allowed the unprecedented atomic resolution observation of solution phase growth and dynamics of nanocrystals. Here we explore the potential of this technique to probe nanoscale structure and dynamics of biomolecules in situ, using artificial Au nanoparticle-DNA artificial molecules as model systems. The interactions of electrons with both the artificial molecules and the liquid environment have been demonstrated and discussed, revealing both the opportunities and challenges of using graphene liquid cell EM as a new method of bio-imaging.

A low noise all-fiber interferometer for high resolution frequency modulated atomic force microscopy imaging in liquids.

We have developed a low noise all-fiber interferometer for use as the deflection sensor in liquid environment frequency modulated atomic force microscopy (FM-AFM). A detailed description and rationale for the choice of the critical components are provided along with the design of a simple alignment assembly. The optimization of the deflection sensor toward achieving the highest possible sensitivity and lowest deflection noise density is discussed in the context of an ideal interference cavity. Based on the provided analysis we have achieved deflection noise densities of 2 fm/square root(Hz) on commercially available cantilevers in both ambient and liquid environments. The low noise interferometer works without the need for differential detection, special focusing lenses, or polarization sensitive optics, dramatically simplifying measurements. True atomic resolution imaging of muscovite mica by FM-AFM in water is demonstrated using the developed deflection sensor.

Structural-mechanical characterization of nanoparticle exosomes in human saliva, using correlative AFM, FESEM, and force spectroscopy.

All living systems contain naturally occurring nanoparticles with unique structural, biochemical, and mechanical characteristics. Specifically, human saliva exosomes secreted by normal cells into saliva via exocytosis are novel biomarkers showing tumor-antigen enrichment during oral cancer. Here we show the substructure of single human saliva exosomes, using a new ultrasensitive low force atomic force microscopy (AFM) exhibiting substructural organization unresolvable in electron microscopy. We correlate the data with field emission scanning electron microscopy (FESEM) and AFM images to interpret the nanoscale structures of exosomes under varying forces. Single exosomes reveal reversible mechanical deformation displaying distinct elastic, 70-100 nm trilobed membrane with substructures carrying specific transmembrane receptors. Further, we imaged and investigated, using force spectroscopy with antiCD63 IgG functionalized AFM tips, highly specific and sensitive detection of antigenCD63, potentially useful cancer markers on individual exosomes. The quantitative nanoscale morphological, biomechanical, and surface biomolecular properties of single saliva exosomes are critical for the applications of exosomes for cancer diagnosis and as a model for developing new cell delivery systems.