Atomic-scale observation of rotational misorientation in suspended few-layer graphene sheets

STEM multiplication nano-moiré method with large field of view and high sensitivity

Strain is one of the important factors that determine the photoelectric and mechanical properties of semiconductor materials and devices. In this paper, the scanning transmission electron microscopy multiplication nano-moiré method is proposed to increase the measurement range and sensitivity for strain field. The formation principle, condition, and measurement range of positive and negative multiplication moiré fringes (PMMFs and NMMFs) are analysed in detail here. PMMF generally refers to the multiplication of field of view, NMMF generally refers to the multiplication of displacement measurement sensitivity. Based on the principle of multiplication nano-moiré, Theoretical formulas of the fringe spacing and strain field are derived. Compared with geometric phase analysis of deformation measurements based on high-resolution atom images, both the range of field of view and the sensitivity of displacement measurements of the multiplication moiré method are significantly improved. Most importantly, the area of field of view of the PMMF method is increased by about two orders of magnitude, which is close to micrometre-scale with strain measurement sensitivity of 2 × 10^-5. In addition, In order to improve the quality of moiré fringe and the accuracy of strain measurement, the secondary moiré method is developed.The strain laws at the interface of the InP/InGaAs superlattice materials are characterised using the developed method.

TEM nano-Moiré evaluation for an invisible lattice structure near the grain interface

Moiré technique is a powerful, important and effective tool for scientific research, from the nano-scale to the macro-scale, which is essentially the interference between two or more periodic structures with a similar frequency. In this study, an inverse transmission electron microscopy (TEM) nano-Moiré method has been proposed, for the first time, to reconstruct an invisible lattice structure near the grain interface, where only one kind of lattice structure and Moiré fringe were visible in a high resolution TEM (HRTEM) image simultaneously. The inversion process was performed in detail. Three rules were put forward to ensure the uniqueness of the inversion result. The HRTEM image of a top-coat/thermally grown oxide interface in a thermal barrier coating (TBC) structure was observed with coexisting visible lattice and Moiré fringes. Using the inverse TEM nano-Moiré method, the invisible lower layer lattice was inversed and a 3-dimensional structure near the interface was also reconstructed to some degree. The real strain field of oriented invisible and visible lattices and the relative strain field of the Moiré fringe in the grain and near the grain boundary were obtained simultaneously through the subset geometric phase analysis method. The possible failure mechanism and position of the TBC spallation from the nano-scale to the micro-scale were discussed.

The Nature of Metastable AA' Graphite: Low Dimensional Nano- and Single-Crystalline Forms

Over the history of carbon, it is generally acknowledged that Bernal AB stacking of the sp² carbon layers is the unique crystalline form of graphite. The universal graphite structure is synthesized at 2,600~3,000 °C and exhibits a micro-polycrystalline feature. In this paper, we provide evidence for a metastable form of graphite with an AA' structure. The non-Bernal AA' allotrope of graphite is synthesized by the thermal- and plasma-treatment of graphene nanopowders at ~1,500 °C. The formation of AA' bilayer graphene nuclei facilitates the preferred texture growth and results in single-crystal AA' graphite in the form of nanoribbons (1D) or microplates (2D) of a few nm in thickness. Kinetically controlled AA' graphite exhibits unique nano- and single-crystalline feature and shows quasi-linear behavior near the K-point of the electronic band structure resulting in anomalous optical and acoustic phonon behavior.

Evolution of Moiré Profiles from van der Waals Superstructures of Boron Nitride Nanosheets

Two-dimensional (2D) van der Waals (vdW) superstructures, or vdW solids, are formed by the precise restacking of 2D nanosheet lattices, which can lead to unique physical and electronic properties that are not available in the parent nanosheets. Moiré patterns formed by the crystalline mismatch between adjacent nanosheets are the most direct features for vdW superstructures under microscopic imaging. In this article, transmission electron microscopy (TEM) observation of hexagonal Moiré patterns with unusually large micrometer-sized lateral areas (up to ~1 μm²) and periodicities (up to ~50 nm) from restacking of liquid exfoliated hexagonal boron nitride nanosheets (BNNSs) is reported. This observation was attributed to the long range crystallinity and the contaminant-free surfaces of these chemically inert nanosheets. Parallel-line-like Moiré fringes with similarly large periodicities were also observed. The simulations and experiments unambiguously revealed that the hexagonal patterns and the parallel fringes originated from the same rotationally mismatched vdW stacking of BNNSs and can be inter-converted by simply tilting the TEM specimen following designated directions. This finding may pave the way for further structural decoding of other 2D vdW superstructure systems with more complex Moiré images.

The Hide-and-Seek of Grain Boundaries from Moiré Pattern Fringe of Two-Dimensional Graphene

Grain boundaries (GBs) commonly exist in crystalline materials and affect various properties of materials. The facile identification of GBs is one of the significant requirements for systematical study of polycrystalline materials including recently emerging two-dimensional materials. Previous observations of GBs have been performed by various tools including high resolution transmission electron microscopy. However, a method to easily identify GBs, especially in the case of low-angle GBs, has not yet been well established. In this paper, we choose graphene bilayers with a GB as a model system and investigate the effects of interlayer rotations to the identification of GBs. We provide a critical condition between adjacent moiré fringe spacings, which determines the possibility of GB recognition. In addition, for monolayer graphene with a grain boundary, we demonstrate that low-angle GBs can be distinguished easily by inducing moiré patterns deliberately with an artificial reference overlay.

Breakdown into nanoscale of graphene oxide: confined hot spot atomic reduction and fragmentation

Nano-graphene oxide (nano-GO) is a new class of carbon based materials being proposed for biomedical applications due to its small size, intrinsic optical properties, large specific surface area, and easy to functionalize. To fully exploit nano-GO properties, a reproducible method for its production is of utmost importance. Herein we report, the study of the sequential fracture of GO sheets onto nano-GO with controllable lateral width, by a simple, and reproducible method based on a mechanism that we describe as a confined hot spot atomic fragmentation/reduction of GO promoted by ultrasonication. The chemical and structural changes on GO structure during the breakage were monitored by XPS, FTIR, Raman and HRTEM. We found that GO sheets starts breaking from the defects region and in a second phase through the disruption of carbon bonds while still maintaining crystalline carbon domains. The breaking of GO is accompanied by its own reduction, essentially by the elimination of carboxylic and carbonyl functional groups. Photoluminescence and photothermal studies using this nano-GO are also presented highlighting the potential of this nanomaterial as a unique imaging/therapy platform.

Formation of secondary Moiré patterns for characterization of nanoporous alumina structures in multiple domains with different orientations

We first report the formation of secondary Moiré patterns from electron Moiré fringes to characterize nanostructures in multiple domains with different orientations. The pitches and the orientations of the nanoporous alumina arrays in several domains are simultaneously measured using only one electron Moiré image.

Determination of the stacking order of curved few-layered graphene systems

We report a facile method to efficiently visualize the atomic carbon network of curved few-layered graphitic systems including folded bi-layer graphene, nanoribbon edges and multi-walled carbon nanotubes (straight and bent), via the processing of aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) images. This technique is also able to atomically resolve the structure of overlapping graphene layers with different orientations, thus enabling us to determine the stacking order of multiple graphene layers. To the best of our knowledge, we are the first to identify the stacking order of a misoriented 4-layer closed-edge graphene and a metal-semiconductor double-walled carbon nanotube junction.

Band gap opening in methane intercalated graphene

Recent experimental work has demonstrated production of quasi-free-standing graphene by methane intercalation. The intercalation weakens the coupling of adjacent graphene layers and yields Dirac fermion behaviour of monolayer graphene. We have investigated the electronic characteristics of a methane intercepted graphene bilayer under a perpendicularly applied electric field. Evolution of the band structure of intercalated graphene as a function of the bias is studied by means of density-functional theory including interlayer van der Waals interactions. The implications of controllable band gap opening in methane-intercalated graphene for future device applications are discussed.

Direct imaging of atomic-scale ripples in few-layer graphene

Graphene has been touted as the prototypical two-dimensional solid of extraordinary stability and strength. However, its very existence relies on out-of-plane ripples as predicted by theory and confirmed by experiments. Evidence of the intrinsic ripples has been reported in the form of broadened diffraction spots in reciprocal space, in which all spatial information is lost. Here we show direct real-space images of the ripples in a few-layer graphene (FLG) membrane resolved at the atomic scale using monochromated aberration-corrected transmission electron microscopy (TEM). The thickness of FLG amplifies the weak local effects of the ripples, resulting in spatially varying TEM contrast that is unique up to inversion symmetry. We compare the characteristic TEM contrast with simulated images based on accurate first-principles calculations of the scattering potential. Our results characterize the ripples in real space and suggest that such features are likely common in ultrathin materials, even in the nanometer-thickness range.

Tunable bands in biased multilayer epitaxial graphene

We have studied the electronic characteristics of multilayer epitaxial graphene under a perpendicularly applied electric bias. Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electronic density-of-states in valence bands near the Fermi level. Evolution of the electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using first-principles density-functional theory including interlayer van der Waals interactions. The experimental and theoretical results demonstrate that the tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed.

Nanoscale charge distribution and energy band modification in defect-patterned graphene

Defects were introduced precisely to exfoliated graphene (G) sheets on a SiO(2)/n(+) Si substrate to modulate the local energy band structure and the electron pathway using solution-phase oxidation followed by thermal reduction. The resulting nanoscale charge distribution and band gap modification were investigated by electrostatic force microscopy and spectroscopy. A transition phase with coexisting submicron-sized metallic and insulating regions in the moderately oxidized monolayer graphene were visualized and measured directly. It was determined that the delocalization of electrons/holes in a graphene "island" is confined by the surrounding defective C-O matrix, which acts as an energy barrier for mobile charge carriers. In contrast to the irreversible structural variations caused by the oxidation process, the electrical properties of graphene can be restored by annealing. The defect-patterned graphene and graphene oxide heterojunctions were further characterized by electrical transport measurement.

Amine-modified graphene: thrombo-protective safer alternative to graphene oxide for biomedical applications

Graphene and its derivatives have attracted significant research interest based on their application potential in different fields including biomedicine. However, recent reports from our laboratory and elsewhere have pointed to serious toxic effects of this nanomaterial on cells and organisms. Graphene oxide (GO) was found to be highly thrombogenic in mouse and evoked strong aggregatory response in human platelets. As platelets play a central role in hemostasis and thrombus formation, thrombotoxicity of GO potentially limits its biomedical applications. Surface chemistry of nanomaterials is a critical determinant of biocompatibility, and thus differentially functionalized nanomaterials exhibit varied cellular toxicity. Amine-modified carbon nanotubes have recently been shown to possess cytoprotective action, which was not exhibited by their relatively toxic carboxylated counterparts. We, therefore, evaluated the effect of amine modification of graphene on platelet reactivity. Remarkably, our results revealed for the first time that amine-modified graphene (G-NH(2)) had absolutely no stimulatory effect on human platelets nor did it induce pulmonary thromboembolism in mice following intravenous administration. Further, it did not evoke lysis of erythrocytes, another major cellular component in blood. These findings contrasted strikingly the observations with GO and reduced GO (RGO). We conclude that G-NH(2) is not endowed with thrombotoxic property unlike other commonly investigated graphene derivatives and is thus potentially safe for in vivo biomedical applications.

Thrombus inducing property of atomically thin graphene oxide sheets

Graphene oxide (GO), the new two-dimensional carbon nanomaterial, is extensively investigated for potential biomedical applications. Thus, it is pertinent to critically evaluate its untoward effects on physiology of tissue systems including blood platelets, the cells responsible for maintenance of hemostasis and thrombus formation. Here we report for the first time that atomically thin GO sheets elicited strong aggregatory response in platelets through activation of Src kinases and release of calcium from intracellular stores. Compounding this, intravenous administration of GO was found to induce extensive pulmonary thromboembolism in mice. Prothrombotic character of GO was dependent on surface charge distribution as reduced GO (RGO) was significantly less effective in aggregating platelets. Our findings raise a concern on putative biomedical applications of GO in the form of diagnostic and therapeutic tools where its prothrombotic property should be carefully investigated.

Facile one-step transfer process of graphene

Chemical vapour deposition (CVD) is emerging as a popular method for growing large-area graphene on metal substrates. For transferring graphene to other substrates the technique generally used involves deposition of a polymer support with subsequent etching of the metal substrate. Here we report a simpler one-step transfer process. Few-layer graphene (FLG) grown on a Cu substrate were transferred to a silanized wafer by just pressing them together. Hydrogen bonding between the hydroxyl group on FLG and the amine group on silane molecules facilitate the transfer.

Made-to-order nanocarbons through deterministic plasma nanotechnology

Through a combinatorial approach involving experimental measurement and plasma modelling, it is shown that a high degree of control over diamond-like nanocarbon film sp3/sp2 ratio (and hence film properties) may be exercised, starting at the level of electrons (through modification of the plasma electron energy distribution function). Hydrogenated amorphous carbon nanoparticle films with high percentages of diamond-like bonds are grown using a middle-frequency (2 MHz) inductively coupled Ar+CH4 plasma. The sp3 fractions measured by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy in the thin films are explained qualitatively using sp3/sp2 ratios 1) derived from calculated sp3 and sp2 hybridized precursor species densities in a global plasma discharge model and 2) measured experimentally. It is shown that at high discharge power and lower CH4 concentrations, the sp3/sp2 fraction is higher. Our results suggest that a combination of predictive modeling and experimental studies is instrumental to achieve deterministically grown made-to-order diamond-like nanocarbons suitable for a variety of applications spanning from nano-magnetic resonance imaging to spin-flip quantum information devices. This deterministic approach can be extended to graphene, carbon nanotips, nanodiamond and other nanocarbon materials for a variety of applications.