Programmable Sub-nanometer Sculpting of Graphene with Electron Beams

Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without <i>C</i>_<i>s</i> correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains

Programmable Sub-nanometer Sculpting of Graphene with Electron Beams

Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without <i>C</i>_<i>s</i> correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains

Liquid Metal: An Innovative Solution to Uniform Graphene Films

The self-limited chemical vapor deposition of uniform single-layer graphene on Cu foils generated significant interest when it was initially discovered. Soon after, the fabrication of real uniform graphene was found to need extremely precise control of the growth conditions. Slight deviations terminate the self-limiting homogeneous growth, inevitably leading to multilayer graphene formation. Here we propose an innovative way to utilize liquid metals to resolve this thorny problem. In stark contrast to the low carbon solubility found in solid metals (e.g., Cu), catalytically decomposed carbon atoms are embedded in liquid metals. During cooling, the homogeneous solidified surface forms from the quasi-atomic smooth liquid surface, and carbon precipitation is blocked by the frozen metal lattices, which are insoluble to carbon. The underlying liquid bulk acts as a container to buffer the excess carbon supply, which normally would lead to the formation of multilayer graphene in the conventional CVD process. As a result, the growth of graphene becomes governed by a self-limiting surface catalytic process and is robust to variations in growth conditions. With simplicity, scalability, and a large growth window, the use of liquid metals provides an attractive solution to obtain uniform graphene

Programmable Sub-nanometer Sculpting of Graphene with Electron Beams

Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without <i>C</i>_<i>s</i> correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains

Electrical Properties of Hybrid Nanomembrane/Nanoparticle Heterojunctions: The Role of Inhomogeneous Arrays

Investigation of charge transport mechanisms across inhomogeneous nanoparticle (NP) layers in heterojunctions is one of the key technological challenges nowadays for developing novel hybrid nanostructured functional elements. Here, we successfully demonstrate for the first time the fabrication and characterization of a novel hybrid organic/inorganic heterojunction, which combines free-standing metallic nanomembranes with self-assembled mono- and sub-bilayers of commercially available colloidal NPs with no more than ∼10⁵ NPs. The low-temperature conductance–voltage spectra exhibit Coulomb features that correlate with various interface’s configurations, including the presence of inhomogeneities at the nano- and micrometer scale owing to the NP size, the micrometer-sized voids, and the thickness of the layers. The charge transport features observed can be explained by a superposition of conductance characteristics of each individual type of NPs. The procedure adopted to fabricate the heterojunctions as well as the theoretical approach employed to study the charge transport mechanisms across the NP layers may be of interest for investigating different types of NPs and commonly obtained inhomogeneous layers. In addition, the combination of metallic nanomembranes with self-assembled layers of NPs makes such a hybrid organic/inorganic heterostructure an interesting platform and building block for future nanoelectronics, especially after intentional tuning of its electronic behavior

Lattice Expansion in Seamless Bilayer Graphene Constrictions at High Bias

Our understanding of sp² carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bilayer graphene at high bias, little to nothing is known. To this end, we investigated bilayer graphene constrictions with closed edges (seamless) at high bias using <i>in situ</i> atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bilayer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 × 10⁹ A cm^–2 and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5% and an isotropic component showing an expansion exceeding 1%. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states, thus weakening bonds. The altered character of C–C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bilayer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor

On the Role of Vapor Trapping for Chemical Vapor Deposition (CVD) Grown Graphene over Copper

The role of sample chamber configuration for the chemical vapor deposition of graphene over copper was investigated in detail. A configuration in which the gas flow is unrestricted was shown to lead to graphene with an inhomogeneous number of layers (between 1 and 3). An alternative configuration in which one end of the inner tube (in which the sample is placed) is closed so as to restrict the gas flow leads a homogeneous graphene layer number. Depending on the sample placement, either homogeneous monolayer or bilayer graphene is obtained. Under our growth conditions, the data show local conditions play a role on layer homogeneity such that under quasi-static equilibrium gas conditions not only is the layer number stabilized, but the quality of the graphene improves. In short, our data suggests vapor trapping can trap Cu species leading to higher carbon concentrations, which determines layer number and improved decomposition of the carbon feedstock (CH₄), which leads to higher quality graphene

Universal Substrate-Trapping Strategy To Grow Strictly Monolayer Transition Metal Dichalcogenides Crystals

Monolayer transition metal dichalcogenides (TMDs) possess great potential in the electronic and optoelectronic devices on account of their unique electronic structure as well as outstanding characteristics. However, presented growth approaches are hardly to avoid multilayers and the root cause of this thermodynamic growth process lies on the overflowing transition metal sources. Here, a novel substrate-trapping strategy (STS) is developed to achieve monolayer TMDs crystals over the whole substrate surface. A designed substrate with appropriate reaction activity to fix the extra precursors is the key, for which the dominance of the dynamics will be established, thus leading to strict self-limited monolayer growth behavior. The high-quality nature of the synthesized monolayer MoS₂ crystals is also clarified by transmission electron microscopy characterizations and field-effect transistors performance. Excellent tolerance to variations in growth parameters of STS is therefore exhibited and, moreover, it is also verified in achieving strictly monolayer WS₂ crystals, thus demonstrating the universality of this approach. The facile strategy opens up a new avenue in growth of monolayer TMDs and may facilitate their industrial applications

<i>In Situ</i> Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes

ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the <i>in situ</i> formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures

Nanosized Carbon Black Combined with Ni₂O₃ as “Universal” Catalysts for Synergistically Catalyzing Carbonization of Polyolefin Wastes to Synthesize Carbon Nanotubes and Application for Supercapacitors

The catalytic carbonization of polyolefin materials to synthesize carbon nanotubes (CNTs) is a promising strategy for the processing and recycling of plastic wastes, but this approach is generally limited due to the selectivity of catalysts and the difficulties in separating the polyolefin mixture. In this study, the influence of nanosized carbon black (CB) and Ni₂O₃ as a novel combined catalyst system on catalyzing carbonization of polypropylene (PP), polyethylene (PE), polystyrene (PS) and their blends was investigated. We showed that this combination was efficient to promote the carbonization of these polymers to produce CNTs with high yields and of good quality. Catalytic pyrolysis and model carbonization experiments indicated that the carbonization mechanism was attributed to the synergistic effect of the combined catalysts rendered by CB and Ni₂O₃: CB catalyzed the degradation of PP, PE, and PS to selectively produce more aromatic compounds, which were subsequently dehydrogenated and reassembled into CNTs via the catalytic action of CB together with Ni particles. Moreover, the performance of the synthesized CNTs as the electrode of supercapacitor was investigated. The supercapacitor displayed a high specific capacitance as compared to supercapacitors using commercial CNTs and CB. This difference was attributed to the relatively larger specific surface areas of our synthetic CNTs and their more oxygen-containing groups