Atomic Structure of ABC Rhombohedral Stacked Trilayer Graphene

We distinguish between Bernal and rhombohedral stacked trilayer graphene using aberration-corrected high-resolution transmission electron microscopy. By using a monochromator to reduce chromatic aberration effects, angstrom resolution can be achieved at an accelerating voltage of 80 kV, which enables the atomic structure of ABC rhombohedral trilayer graphene to be unambiguously resolved. Our images of ABC rhombohedral trilayer graphene provide a clear signature for its identification. Few-layer graphene interfaces with ABC:BC:BCAB structure have also been studied, and we have determined the stacking sequence of each graphene layer and consequently the 3D structure. These results confirm that CVD-grown few-layer graphene can adopt an ABC rhombohedral stacking

Inflating Graphene with Atomic Scale Blisters

Using 80 kV electron beam irradiation we have created graphene blister defects of additional carbon atoms incorporated into a graphene lattice. These structures are the antithesis of the vacancy defect with blister defects observed to contain up to six additional carbon atoms. We present aberration-corrected transmission electron microscopy data demonstrating the formation of a blister from an existing divacancy, together with further examples that undergo reconfiguration and annihilation under the electron beam. The relative stability of the observed variations of blister are discussed and considered in the context of previous calculations. It is shown that the blister defect is seldom found in isolation and is more commonly coupled with dislocations where it can act as an intermediate state, permitting dislocation core climb without the atom ejection from the graphene lattice required for nonconservative motion

Formation of Klein Edge Doublets from Graphene Monolayers

With increasing possibilities for applications of graphene, it is essential to fully characterize the rich topological variations in graphene edge structures. Using aberration-corrected transmission electron microscopy, dangling carbon doublets at the edge of monolayer graphene crystals have been observed. Unlike the single-atom Klein edge often found at zigzag edges, these carbon dimers were observed in various edge structure environments, but most frequently on the more stable armchair edges. Observation of this Klein edge doublet over time reveals that its existence enhances the stability of armchair edges and is a route to atom abstraction on zigzag edges

Bond Length and Charge Density Variations within Extended Arm Chair Defects in Graphene

Extended linear arm chair defects are intentionally fabricated in suspended monolayer graphene using controlled focused electron beam irradiation. The atomic structure is accurately determined using aberration-corrected transmission electron microscopy with monochromation of the electron source to achieve ∼80 pm spatial resolution at an accelerating voltage of 80 kV. We show that the introduction of atomic vacancies in graphene disrupts the uniformity of C–C bond lengths immediately surrounding linear arm chair defects in graphene. The measured changes in C–C bond lengths are related to density functional theory (DFT) calculations of charge density variation and corresponding DFT calculated structural models. We show good correlation between the DFT predicted localized charge depletion and structural models with HRTEM measured bond elongation within the carbon tetragon structure of graphene. Further evidence of bond elongation within graphene defects is obtained from imaging a pair of 5-8-5 divacancies

PbTe Nanocrystal Arrays on Graphene and the Structural Influence of Capping Ligands

The effective utilization of graphene in optoelectronic devices requires blending with other semiconductor materials, such as lead chalcogenide nanocrystals, and an understanding of the organic–inorganic interface at the atomic level. In this paper, we present the first report of the successful self-assembly of close-packed monolayer arrays of PbTe nanocrystals on monolayer graphene. By using ultraclean chemical vapor deposition grown graphene and aberration corrected transmission electron microscopy the unambiguous delineation of a ∼2 nm soft ligand nanocrystal shell is achieved, which is compared to subsequent studies after ligand exchange is performed on the same nanocrystal-graphene hybrid. Ligand exchange was shown to remove the soft ligand shell surrounding the nanocrystals and to disassemble the close-packed monolayers into amorphous nanocrystal aggregate film on the graphene substrate. Limited oriented attachment between nanocrystals in the anisotropic film was observed, and this was not significantly affected by extended vacuum annealing

Preferential Pt Nanocluster Seeding at Grain Boundary Dislocations in Polycrystalline Monolayer MoS₂

We show that Pt nanoclusters preferentially nucleate along the grain boundaries (GBs) in polycrystalline MoS₂ monolayer films, with dislocations acting as the seed site. Atomic resolution studies by aberration-corrected annular dark-field scanning transmission electron microscopy reveal periodic spacing of Pt nanoclusters with dependence on GB tilt angles and random spacings for the antiphase boundaries (<i>i.e.</i>, 60°). Individual Pt atoms are imaged within the dislocation core sections of the GB region, with various positions observed, including both the substitutional sites of Mo and the hollow center of the octahedral ring. The evolution from single atoms or small few atom clusters to nanosized particles of Pt is examined at the atomic level to gain a deep understanding of the pathways of Pt seed nucleation and growth at the GB. Density functional theory calculations confirm the energetic advantage of trapping Pt at dislocations on both the antiphase boundary and the small-angle GB rather than on the pristine lattice. The selective decoration of GBs by Pt nanoparticles also has a beneficial use to easily identify GB areas during microscopic-scale observations and track long-range nanoscale variances of GBs with spatial detail not easy to achieve using other methods. We show that GBs have nanoscale meandering across micron-scale distances with no strong preference for specific lattice directions across macroscopic ranges

One-Pot Synthesis of Lithium-Rich Cathode Material with Hierarchical Morphology

Lithium-rich transition metal oxides, Li_1+<i>x</i>TM_1–<i>x</i>O₂ (TM, transition metal), have attracted much attention as potential candidate cathode materials for next generation lithium ion batteries because their high theoretical capacity. Here we present the synthesis of Li­[Li_0.2Ni_0.2Mn_0.6]­O₂ using a facile one-pot resorcinol–formaldehyde method. Structural characterization indicates that the material adopts a hierarchical porous morphology consisting of uniformly distributed small pores and disordered large pore structures. The material exhibits excellent electrochemical cycling stability and a good retention of capacity at high rates. The material has been shown to be both advantageous in terms of gravimetric and volumetric capacities over state of the art commercial cathode materials

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

Structural Reconstruction of the Graphene Monovacancy

Two distinct configurations of the monovacancy in graphene have been observed using aberration-corrected transmission electron microscopy (AC-TEM) at 80 kV. The predicted lower energy asymmetric monovacancy (MV), exhibiting a Jahn–Teller reconstruction (r-MV), has been observed, but in addition, we have imaged instances of a symmetric monovacancy (s-MV). We have used geometric phase analysis (GPA) to quantitatively determine the strain in the lattice surrounding these two defect configurations and show that the Jahn–Teller reconstruction generates significant extra strain compared to the symmetric MV case. Density functional theory calculations demonstrate that our experimental images of the two different monovacancies show good agreement with both the low energy r-MV and the metastable structures