499 research outputs found
Mechanically stacked 1 nm thick carbon nanosheets: Ultrathin layered materials with tunable optical, chemical and electrical properties
Carbon nanosheets are mechanically stable free-standing two-dimensional
materials with a thickness of ~1 nm and well defined physical and chemical
properties. They are made by radiation induced cross-linking of aromatic
self-assembled monolayers. Here we present a route to the scalable fabrication
of multilayer nanosheets with tunable electrical, optical and chemical
properties on insulating substrates. Stacks up to five nanosheets with sizes of
~1 cm^2 on oxidized silicon were studied. Their optical characteristics were
investigated by visual inspection, optical microscopy, UV/Vis reflection
spectroscopy and model calculations. Their chemical composition was studied by
X-ray photoelectron spectroscopy. The multilayer samples were then annealed in
ultra high vacuum at various temperatures up to 1100 K. A subsequent
investigation by Raman, X-ray photoelectron and UV/Vis reflection spectroscopy
as well as by electrical four-point probe measurements demonstrates that the
layered nanosheets transform into nanocrystalline graphene. This structural and
chemical transformation is accompanied by changes in the optical properties and
electrical conductivity and opens up a new path for the fabrication of
ultrathin functional conductive coatings.Comment: 36 pages, 7 Figure
Energy-filtered transmission electron microscopy of biological samples on highly transparent carbon nanomembranes
Ultrathin carbon nanomembranes (CNM) comprising crosslinked biphenyl
precursors have been tested as support films for energy-filtered transmission
electron microscopy (EFTEM) of biological specimens. Due to their high
transparency CNM are ideal substrates for electron energy loss spectroscopy
(EELS) and electron spectroscopic imaging (ESI) of stained and unstained
biological samples. Virtually background-free elemental maps of tobacco mosaic
virus (TMV) and ferritin have been obtained from samples supported by ~ 1 nm
thin CNM. Furthermore, we have tested conductive carbon nanomembranes (cCNM)
comprising nanocrystalline graphene, obtained by thermal treatment of CNM, as
supports for cryoEM of ice-embedded biological samples. We imaged ice-embedded
TMV on cCNM and compared the results with images of ice-embedded TMV on
conventional carbon film (CC), thus analyzing the gain in contrast for TMV on
cCNM in a quantitative manner. In addition we have developed a method for the
preparation of vitrified specimens, suspended over the holes of a conventional
holey carbon film, while backed by ultrathin cCNM
Nanostructuring Graphene by Dense Electronic Excitation
The ability to manufacture tailored graphene nanostructures is a key factor
to fully exploit its enormous technological potential. We have investigated
nanostructures created in graphene by swift heavy ion induced folding. For our
experiments, single layers of graphene exfoliated on various substrates and
freestanding graphene have been irradiated and analyzed by atomic force and
high resolution transmission electron microscopy as well as Raman spectroscopy.
We show that the dense electronic excitation in the wake of the traversing ion
yields characteristic nanostructures each of which may be fabricated by
choosing the proper irradiation conditions. These nanostructures include unique
morphologies such as closed bilayer edges with a given chirality or nanopores
within supported as well as freestanding graphene. The length and orientation
of the nanopore, and thus of the associated closed bilayer edge, may be simply
controlled by the direction of the incoming ion beam. In freestanding graphene,
swift heavy ion irradiation induces extremely small openings, offering the
possibility to perforate graphene membranes in a controlled way.Comment: 16 pages, 5 figures, submitted to Nanotechnolog
Investigation of the thermophysical properties of high-melting materials with the aid of a complex of instruments
The evaporation rate, vapor pressure, heats of evaporation reaction (sublimation, dissociation), enthalpy, electrical resistance, heat capacity, emissivity, and heat conductivity of various carbides, borides, sulfides, nitrides, selenides, and phosphides were investigated. A set of high temperature high vacuum devices, calorimeters (designed for operation at 400 to 1300 K and from 1200 K), and mass spectrometers, most of which were specially developed for these studies, is described
Single-walled carbon nanotubes and nanocrystalline graphene reduce beam-induced movements in high-resolution electron cryo-microscopy of ice-embedded biological samples
For single particle electron cryo-microscopy (cryoEM), contrast loss due to
beam-induced charging and specimen movement is a serious problem, as the thin
films of vitreous ice spanning the holes of a holey carbon film are
particularly susceptible to beam-induced movement. We demonstrate that the
problem is at least partially solved by carbon nanotechnology. Doping
ice-embedded samples with single-walled carbon nanotubes (SWNT) in aqueous
suspension or adding nanocrystalline graphene supports, obtained by thermal
conversion of cross-linked self-assembled biphenyl precursors, significantly
reduces contrast loss in high-resolution cryoEM due to the excellent electrical
and mechanical properties of SWNTs and graphene
Selective ion sieving through arrays of sub-nanometer nanopores in chemically tunable 2D carbon membranes
Two-dimensional (2D) membranes featuring arrays of sub-nanometer pores have applications in purification, solvent separation and water desalination. Compared to channels in bulk membranes, 2D nanopores have lower resistance to transmembrane transport, leading to faster passage of ions. However, the formation of nanopores in 2D membranes requires expensive post-treatment using plasma or ion bombardment. Here, we study bottom-up synthesized porous carbon nanomembranes (CNMs) of biphenyl thiol (BPT) precursors. Sub-nanometer pores arise intrinsically during the BPT-CNM synthesis with a density of 2 ± 1 pore per 100 nm2. We employ BPT-CNM based pore arrays as efficient ion sieving channels, and demonstrate selectivity of the membrane towards ion transport when exposed to a range of concentration gradients of KCl, CsCl and MgCl2. The selectivity of the membrane towards K+ over Cl− ions is found be 16.6 mV at a 10 : 1 concentration ratio, which amounts to ∼30% efficiency relative to the Nernst potential for complete ion rejection. The pore arrays in the BPT-CNM show similar transport and selectivity properties to graphene and carbon nanotubes, whilst the fabrication method via self-assembly offers a facile means to control the chemical and physical properties of the membrane, such as surface charge, chemical nature and pore density. CNMs synthesized from self-assembled monolayers open the way towards the rational design of 2D membranes for selective ion sieving
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Nanoscale friction on MoS2/graphene heterostructures
Stacked hetero-structures of two-dimensional materials allow for a design of interactions with corresponding electronic and mechanical properties. We report structure, work function, and frictional properties of 1 to 4 layers of MoS2 grown by chemical vapor deposition on epitaxial graphene on SiC(0001). Experiments were performed by atomic force microscopy in ultra-high vacuum. Friction is dominated by adhesion which is mediated by a deformation of the layers to adapt the shape of the tip apex. Friction decreases with increasing number of MoS2 layers as the bending rigidity leads to less deformation. The dependence of friction on applied load and bias voltage can be attributed to variations in the atomic potential corrugation of the interface, which is enhanced by both load and applied bias. Minimal friction is obtained when work function differences are compensated
First-principles investigation of electron-induced cross-linking of aromatic self-assembled monolayers on Au(111)
We have performed a density functional theory study of the possible layered
geometries occurring after dehydrogenation of a self-assembled monolayer (SAM)
of biphenyl-thiol molecules (BPTs) adsorbed on a Au(111), as it has been
experimentally observed for low energy electron irradiated SAMs of
4'-nitro-1,1'-biphenyl-thiol adsorbed on a Au(111) surface. [Eck, W. et al.,
Advanced Materials 2000, 12, 805] Cross-link formation between the BPT
molecules has been analyzed using different models with different degrees of
complexity. We start by analyzing the bonding between biphenyl (BP) molecules
in a lineal dimer and their characteristic vibration frequencies. Next, we
consider the most stable cross-linked structures formed in an extended
free-standing monolayer of fully dehydrogenated BP molecules. Finally, we
analyze a more realistic model where the role of the Au(111) substrate and
sulphur head groups is explicitly taken into account. In this more complex
model, the dehydrogenated BPT molecules are found to interact covalently to
spontaneously form "graphene-like" nanoflakes. We propose that these
nanographenes provide plausible building-blocks for the structure of the carbon
layers formed by electron irradiation of BPT-SAMs. In particular, it is quite
tempting to visualize those structures as the result of the cross-link and
entanglement of such graphene nanoflakes.Comment: 9 pages, 5 figure
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