Chemistry and Structure of Graphene Oxide <i>via</i> Direct Imaging

Graphene oxide (GO) and reduced GO (rGO) are the only variants of graphene that can be manufactured at the kilogram scale, and yet the widely accepted model for their structure has largely relied on indirect evidence. Notably, existing high-resolution transmission electron microscopy (HRTEM) studies of graphene oxide report long-range order of sp² lattice with isolated defect clusters. Here, we present HRTEM evidence of a different structural form of GO, where nanocrystalline regions of sp² lattice are surrounded by regions of disorder. The presence of contaminants that adsorb to the surface of the material at room temperature normally prevents direct observation of the intrinsic atomic structure of this defective GO. To overcome this, we use an <i>in situ</i> heating holder within an aberration-corrected TEM (AC-TEM) to study the atomic structure of this nanocrystalline graphene oxide from room temperature to 700 °C. As the temperature increases to above 500 °C, the adsorbates detach from the GO and the underlying atomic structure is imaged to be small 2–4 nm crystalline domains within a polycrystalline GO film. By combining spectroscopic evidence with the AC-TEM data, we support the dynamic interpretation of the structural evolution of graphene oxide

Spatially Dependent Lattice Deformations for Dislocations at the Edges of Graphene

We show that dislocations located at the edge of graphene cause different lattice deformations to those located in the bulk lattice. When a dislocation is located near an edge, a decrease in the rippling and increase of the in-plane rotation occurs relative to the dislocations in the bulk. The increased in-plane rotation near the edge causes bond rotations at the edge of graphene to reduce the overall strain in the system. Dislocations were highly stable and remained fixed in their position even when located within a few lattice spacings from the edge of graphene. We study this behavior at the atomic level using aberration-corrected transmission electron microscopy. These results show detailed information about the behavior of dislocations in 2D materials and the strain properties that result

Atomic Structure and Dynamics of Defects in 2D MoS₂ Bilayers

We present a detailed atomic-level study of defects in bilayer MoS₂ using aberration-corrected transmission electron microscopy at an 80 kV accelerating voltage. Sulfur vacancies are found in both the top and bottom layers in 2H- and 3R-stacked MoS₂ bilayers. In 3R-stacked bilayers, sulfur vacancies can migrate between layers but more preferably reside in the (Mo–2S) column rather than the (2S) column, indicating more complex vacancy production and migration in the bilayer system. As the point vacancy number increases, aggregation into larger defect structures occurs, and this impacts the interlayer stacking. Competition between compression in one layer from the loss of S atoms and the van der Waals interlayer force causes much less structural deformations than those in the monolayer system. Sulfur vacancy lines neighboring in top and bottom layers introduce less strain compared to those staggered in the same layer. These results show how defect structures in multilayered two-dimensional materials differ from their monolayer form

<i>In Situ</i> Atomic Level Dynamics of Heterogeneous Nucleation and Growth of Graphene from Inorganic Nanoparticle Seeds

An <i>in situ</i> heating holder inside an aberration-corrected transmission electron microscope (AC-TEM) is used to investigate the real-time atomic level dynamics associated with heterogeneous nucleation and growth of graphene from Au nanoparticle seeds. Heating monolayer graphene to an elevated temperature of 800 °C removes the majority of amorphous carbon adsorbates and leaves a clean surface. The aggregation of Au impurity atoms into nanoparticle clusters that are bound to the surface of monolayer graphene causes nucleation of secondary graphene layers from carbon feedstock present within the microscope chamber. This enables the <i>in situ</i> study of heterogeneous nucleation and growth of graphene at the atomic level. We show that the growth mechanism consists of alternating C cluster attachment and indentation filling to maintain a uniform growth front of lowest energy. Back-folding of the graphene growth front is observed, followed by a process that involves flipping back and attaching to the surrounding region. We show how the highly polycrystalline graphene seed evolves with time into a higher order crystalline structure using a combination of AC-TEM and tight-binding molecular dynamics (TBMD) simulations. This helps understand the detailed lowest-energy step-by-step pathways associated with grain boundaries (GB) migration and crystallization processes. We find the motion of the GB is discontinuous and mediated by both bond rotation and atom evaporation, supported by density functional theory calculations and TBMD. These results provide insights into the formation of crystalline seed domains that are generated during bottom-up graphene synthesis

<i>In Situ</i> Atomic Level Dynamics of Heterogeneous Nucleation and Growth of Graphene from Inorganic Nanoparticle Seeds

An <i>in situ</i> heating holder inside an aberration-corrected transmission electron microscope (AC-TEM) is used to investigate the real-time atomic level dynamics associated with heterogeneous nucleation and growth of graphene from Au nanoparticle seeds. Heating monolayer graphene to an elevated temperature of 800 °C removes the majority of amorphous carbon adsorbates and leaves a clean surface. The aggregation of Au impurity atoms into nanoparticle clusters that are bound to the surface of monolayer graphene causes nucleation of secondary graphene layers from carbon feedstock present within the microscope chamber. This enables the <i>in situ</i> study of heterogeneous nucleation and growth of graphene at the atomic level. We show that the growth mechanism consists of alternating C cluster attachment and indentation filling to maintain a uniform growth front of lowest energy. Back-folding of the graphene growth front is observed, followed by a process that involves flipping back and attaching to the surrounding region. We show how the highly polycrystalline graphene seed evolves with time into a higher order crystalline structure using a combination of AC-TEM and tight-binding molecular dynamics (TBMD) simulations. This helps understand the detailed lowest-energy step-by-step pathways associated with grain boundaries (GB) migration and crystallization processes. We find the motion of the GB is discontinuous and mediated by both bond rotation and atom evaporation, supported by density functional theory calculations and TBMD. These results provide insights into the formation of crystalline seed domains that are generated during bottom-up graphene synthesis

Thermally Induced Dynamics of Dislocations in Graphene at Atomic Resolution

Thermally induced dislocation movements are important in understanding the effects of high temperature annealing on modifying the crystal structure. We use an <i>in situ</i> heating holder in an aberration corrected transmission electron microscopy to study the movement of dislocations in suspended monolayer graphene up to 800 °C. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. At room temperature, the dynamics of dislocation behavior is driven by the electron beam irradiation at 80 kV; however at higher temperatures, increased movement of the dislocation is observed and provides evidence for the influence of thermal energy to the system. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into annealing processes in graphene and the behavior of defects with increased heat

Europa und das Meer. Deutsches Historisches Museum, Berlin 13 June 2018 &#8211; 06 January 2019

The atomic structure of subnanometer pores in graphene, of interest due to graphene’s potential as a desalination and gas filtration membrane, is demonstrated by atomic resolution aberration corrected transmission electron microscopy. High temperatures of 500 °C and over are used to prevent self-healing of the pores, permitting the successful imaging of open pore geometries consisting of between −4 to −13 atoms, all exhibiting subnanometer diameters. Picometer resolution bond length measurements are used to confirm reconstruction of five-membered ring projections that often decorate the pore perimeter, knowledge which is used to explore the viability of completely self-passivated subnanometer pore structures; bonding configurations where the pore would not require external passivation by, for example, hydrogen to be chemically inert

<i>In Situ</i> Atomic Level Dynamics of Heterogeneous Nucleation and Growth of Graphene from Inorganic Nanoparticle Seeds

An <i>in situ</i> heating holder inside an aberration-corrected transmission electron microscope (AC-TEM) is used to investigate the real-time atomic level dynamics associated with heterogeneous nucleation and growth of graphene from Au nanoparticle seeds. Heating monolayer graphene to an elevated temperature of 800 °C removes the majority of amorphous carbon adsorbates and leaves a clean surface. The aggregation of Au impurity atoms into nanoparticle clusters that are bound to the surface of monolayer graphene causes nucleation of secondary graphene layers from carbon feedstock present within the microscope chamber. This enables the <i>in situ</i> study of heterogeneous nucleation and growth of graphene at the atomic level. We show that the growth mechanism consists of alternating C cluster attachment and indentation filling to maintain a uniform growth front of lowest energy. Back-folding of the graphene growth front is observed, followed by a process that involves flipping back and attaching to the surrounding region. We show how the highly polycrystalline graphene seed evolves with time into a higher order crystalline structure using a combination of AC-TEM and tight-binding molecular dynamics (TBMD) simulations. This helps understand the detailed lowest-energy step-by-step pathways associated with grain boundaries (GB) migration and crystallization processes. We find the motion of the GB is discontinuous and mediated by both bond rotation and atom evaporation, supported by density functional theory calculations and TBMD. These results provide insights into the formation of crystalline seed domains that are generated during bottom-up graphene synthesis

Elongated Silicon–Carbon Bonds at Graphene Edges

We study the bond lengths of silicon (Si) atoms attached to both armchair and zigzag edges using aberration corrected transmission electron microscopy with monochromation of the electron beam. An <i>in situ</i> heating holder is used to perform imaging of samples at 800 °C in order to reduce chemical etching effects that cause rapid structure changes of graphene edges at room temperature under the electron beam. We provide detailed bond length measurements for Si atoms both attached to edges and also as near edge substitutional dopants. Edge reconstruction is also involved with the addition of Si dopants. Si atoms bonded to the edge of graphene are compared to substitutional dopants in the bulk lattice and reveal reduced out-of-plane distortion and bond elongation. An extended linear array of Si atoms at the edge is found to be energy-favorable due to inter-Si interactions. These results provide detailed structural information about the Si–C bonds in graphene, which may have importance in future catalytic and electronic applications

Fabrication, Pressure Testing, and Nanopore Formation of Single-Layer Graphene Membranes

Single-layer graphene (SLG) membranes have great promise as ultrahigh flux, high selectivity membranes for gas mixture separations due to their single atom thickness. It remains a central question whether SLG membranes of a requisite area can exist under an imposed pressure drop and temperatures needed for industrial gas separation. An additional challenge is the development of techniques to perforate or otherwise control the porosity in graphene membranes to impart molecularly sized pores, the size regime predicted to produce high gas separation factors. Herein, we report fabrication, pressure testing, temperature cycling, and gas permeance measurements through free-standing, low defect density SLG membranes. Our measurements demonstrate the remarkable chemical and mechanical stability of these 5 μm diameter suspended SLG membranes, which remain intact over weeks of testing at pressure differentials of >0.5 bar, repeated temperature cycling from 25 to 200 °C, and exposure to 15 mol % ozone for up to 3 min. These membranes act as molecularly impermeable barriers, with very low or near negligible background permeance. We also demonstrate a 1077 °C temperature O₂ etching technique to create nanopores on the order of ∼1 nm diameter as imaged by scanning tunneling microscopy, although transport through such pores has not yet been successfully measured. Overall, these results represent an important advancement that will enable graphene gas separation membranes to be fabricated, tested, and modified <i>in situ</i> while maintaining remarkable mechanical and thermal stability