40 research outputs found
Graphene edge structures: Folding, scrolling, tubing, rippling and twisting
Conventional three-dimensional crystal lattices are terminated by surfaces,
which can demonstrate complex rebonding and rehybridisation, localised strain
and dislocation formation. Two dimensional crystal lattices, of which graphene
is the archetype, are terminated by lines. The additional available dimension
at such interfaces opens up a range of new topological interface possibilities.
We show that graphene sheet edges can adopt a range of topological distortions
depending on their nature. Rehybridisation, local bond reordering, chemical
functionalisation with bulky, charged, or multi-functional groups can lead to
edge buckling to relieve strain, folding, rolling and even tube formation. We
discuss the topological possibilities at a 2D graphene edge, and under what
circumstances we expect different edge topologies to occur. Density functional
calculations are used to explore in more depth different graphene edge types.Comment: Additional figure in published versio
Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition
Cross-sectional area and volume become difficult to define as material
dimensions approach the atomic scale. This limits the transferability of
macroscopic concepts such as Young's modulus. We propose a new volume
definition where the enclosed nanosheet or nanotube average electron density
matches that of the parent layered bulk material. We calculate the Young's
moduli for various nanosheets (including graphene, BN and MoS2) and nanotubes.
Further implications of this new volume definition such as a Fermi level
dependent Young's modulus and out-of-plane Poisson's ratio are shown
Predicting experimentally stable allotropes: Instability of penta-graphene
International audienceIn recent years, a plethora of theoretical carbon allotropes have been proposed, none of which has been experimentally isolated. We discuss here criteria that should be met for a new phase to be potentially experimentally viable. We take as examples Haeckelites, 2D networks of sp2-carbon–containing pentagons and heptagons, and “penta-graphene,” consisting of a layer of pentagons constructed from a mixture of sp2- and sp3-coordinated carbon atoms. In 2D projection appearing as the “Cairo pattern,” penta-graphene is elegant and aesthetically pleasing. However, we dispute the author’s claims of its potential stability and experimental relevanc
Pyrene coating transition metal disulfides as protection from photooxidation and environmental aging
This article belongs to the Section Nanocomposite Materials.Environmental degradation of transition metal disulfides (TMDs) is a key stumbling block in a range of applications. We show that a simple one-pot non-covalent pyrene coating process protects TMDs from both photoinduced oxidation and environmental aging. Pyrene is immobilized non-covalently on the basal plane of exfoliated MoS2 and WS2. The optical properties of TMD/pyrene are assessed via electronic absorption and fluorescence emission spectroscopy. High-resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy confirms extensive pyrene surface coverage, with density functional theory calculations suggesting a strongly bound stable parallel-stacked pyrene coverage of ~2–3 layers on the TMD surfaces. Raman spectroscopy of exfoliated TMDs while irradiating at 0.9 mW/4 μm2 under ambient conditions shows new and strong Raman bands due to oxidized states of Mo and W. Yet remarkably, under the same exposure conditions TMD/pyrene remain unperturbed. The current findings demonstrate that pyrene physisorbed on MoS2 and WS2 acts as an environmental barrier, preventing oxidative surface reactions in the TMDs catalyzed by moisture, air, and assisted by laser irradiation. Raman spectroscopy confirms that the hybrid materials stored under ambient conditions for two years remained structurally unaltered, corroborating the beneficial role of pyrene for not only hindering oxidation but also inhibiting aging.This research was funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 642742, under the “Graphene Flagship” project grant agreement No 785219 and under the ESTEEM-3 project grant agreement No 823717. This research was also partially funded by the project “Advanced Materials and Devices” (MIS 5002409), which is implemented under the “Action for the Strategic Development on the Research and Technological Sector” funded by the Operational Program “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020) and co-financed by Greece and the European Union (European Regional Development Fund). This work was supported by the COST Action CA15107 MultiComp. This research was also supported by the Spanish Ministerio de Economia y Competitividad (MAT2016-79776-P), from the Government of Aragon and the European Social Fund under the project “Construyendo Europa desde Aragon” 2014–2020 (grant number E13_17R).Peer reviewe
Ripple edge engineering of graphene nanoribbons
It is now possible to produce graphene nanoribbons (GNRs) with atomically
defined widths. GNRs offer many opportunities for electronic devices and
composites, if it is possible to establish the link between edge structure and
functionalisation, and resultant GNR properties. Switching hydrogen edge
termination to larger more complex functional groups such as hydroxyls or
thiols induces strain at the ribbon edge. However we show that this strain is
then relieved via the formation of static out-of-plane ripples. The resultant
ribbons have a significantly reduced Young's Modulus which varies as a function
of ribbon width, modified band gaps, as well as heterogeneous chemical
reactivity along the edge. Rather than being the exception, such static edge
ripples are likely on the majority of functionalized graphene ribbon edges.Comment: Supplementary Materials availabl
Stable hydrogenated graphene edge types: Normal and reconstructed Klein edges
Hydrogenated graphene edges are assumed to be either armchair, zigzag or a
combination of the two. We show that the zigzag is not the most stable fully
hydrogenated structure along the direction. Instead hydrogenated Klein
and reconstructed Klein based edges are found to be energetically more
favourable, with stabilities approaching that of armchair edges. These new
structures "unify" graphene edge topology, the most stable flat hydrogenated
graphene edges always consisting of pairwise bonded C2H4 edge groups,
irrespective the edge orientation. When edge rippling is included, CH3 edge
groups are most stable. These new fundamental hydrogen terminated edges have
important implications for graphene edge imaging and spectroscopy, as well as
mechanisms for graphene growth, nanotube cutting, and nanoribbon formation and
behaviour.Fundação para a Ciência e a Tecnologia (FCT
Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition
Abstract Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS 2 ) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown
Pyrene-functionalized tungsten disulfide as stable resistive photosensor
Pyrene carrying an 1,2-dithiolane linker was employed to functionalize exfoliated WS2 and the resulting material was used in a proof-of-concept application as a photoresistor type sensor. The WS2–pyrene hybrid material was comprehensively characterized by spectroscopic, thermal and microscopy techniques, coupled to density functional theory modelling. The high solubility of the WS2–pyrene hybrid material allows easy manipulation in wet media, making it suitable for device fabrication. Thus, a two-terminal resistive photosensor was developed and tested for photodetection. The photosensitivity of WS2 was improved by the presence of covalently attached pyrene by a factor of 2–3, the response linearly dependent on light intensity. Device reaction time was also improved, and critically the photosensor stability was significantly enhanced. Functionalization of exfoliated WS2 material heals vacancies, oxidation and other damage sites liable to impede photoelectric response. This proof-of-concept study opens the way for incorporation of diverse chromophores active in the visible and/or NIR region of the electromagnetic spectrum to WS2 in order to stabilise it and broaden its photoresistive sensing applicability
Bromine polycondensation in pristine and fluorinated graphitic carbons
Despite decades of study the precise behavior of bromine in graphitic carbons remains unclear. In this report, using Raman spectroscopy, we reveal two types of bromine structure in graphitic carbon materials. Between fluorinated graphene layers with a composition close to C2F, Br2 molecules are intercalated in a form similar to liquid bromine. Bromination of pristine and low-fluorinated graphitic carbons behaves very differently with distinct Br-related Raman spectra. With the guidance of density functional theory (DFT) calculations, all Raman features are assigned to normal vibration modes of specific bromine species over graphene and fluorinated graphene. When intercalated between extended non-fluorinated sp2-hybridized carbon regions, physisorbed Br2 molecules move freely across the non-functionalized region toward the CF border. Multiple Br2 molecules then combine spontaneously into Br3-based chains, whose coupling activates otherwise Raman inactive modes. Significant charge transfer to bromine species occurs in this case. DFT calculated frequencies match precisely the experimental Br-related Raman bands observed in the intercalation carbon compounds. The fluorine-catalyzed bromine chain-formation process shown here is general and should also operate with edges and other defect species
Laser-deposited carbon aerogel derived from graphene oxide enables NO2-selective parts-per-billion gas sensing
Laser-deposited carbon aerogel is a low-density porous network of carbon clusters synthesized using a laser process. A one-step synthesis, involving deposition and annealing, results in the formation of a thin porous conductive film which can be applied as a chemiresistor. This material is sensitive to NO2 compared to ammonia and other volatile organic compounds and is able to detect ultra-low concentrations down to at least 10 parts-per-billion. The sensing mechanism, based on the solubility of NO2 in the water layer adsorbed on the aerogel, increases the usability of the sensor in practically-relevant ambient environments. A heating step, achieved in tandem with a microheater, allows the recovery to the baseline making it operable in real world environments. The operability at room temperature, its low cost and scalable production makes it promising for Internet-of-Things air quality monitoring