Carbon nanomembranes (CNMs) supported by polymer: mechanics and gas permeation.

Gas permeation characteristics of carbon nanomembranes (CNMs) from self-assembled monolayers are reported for the first time. The assembly of CNMs onto polydimethylsiloxane (PDMS) support membranes allows mechanical measurements under compression as well as determination of gas permeation characteristics. The results suggest that molecular-sized channels in CNMs dominate the permeation properties of the 1 nm thin CNMs.

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. Herein, a route is presented to the scalable fabrication of multilayer nanosheets with tunable electrical, optical, and chemical properties on insulating substrates. Stacks of up to five nanosheets with sizes of ≈1 cm(2) on oxidized silicon are studied. Their optical characteristics are investigated by visual inspection, optical microscopy, UV-vis reflection spectroscopy, and model calculations. Their chemical composition is studied by X-ray photoelectron spectroscopy. The multilayer samples are then annealed in an ultrahigh 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.

Holey nanosheets by patterning with UV/ozone.

Self-assembled monolayers (SAMs) of 1,1'-biphenyl-4-thiol are patterned by using UV/ozone treatment through a shadow mask. After subsequent irradiation with low-energy electrons, the SAM is cross-linked to form a two-dimensional, holey nanosheet with a thickness of approximately 1 nm. This nanosheet is mechanically stable and can be released from the original gold substrate and transferred to new substrates, including transmission electron microscopy (TEM) grids. The process is monitored by complementary microscopic techniques as well as X-ray photoelectron spectroscopy (XPS).

Fluorescently labeled 1 nm thin nanomembranes.

Fluorescent labeling of self-assembled monolayers (SAMs) has a great potential for chemical and biotechnological sensing. However, its use is limited by the quenching of the fluorescence in the proximity of the conducting substrates. We show that this quenching can be overcome by the labeling of a cross-linked aromatic SAM (nanosheet) and its subsequent transfer onto a non-conducting substrate. We demonstrate the successful labeling of nanosheets with a fluorophore (tetramethylrhodamine) and its subsequent transfer to oxidized silicon, where they are detected by optical as well as fluorescence microscopy. Fluorescently labeled freestanding nanosheets, i.e. nanomembranes were obtained by a similar transfer of the nanosheets to TEM grids.

Fully cross-linked and chemically patterned self-assembled monolayers.

Mechanically stable monolayers with chemically functional patterns are fabricated by the combination of spatially resolved chemical lithography with complete cross-linking of aromatic self-assembled monolayers (SAMs). The process starts with a local electron exposure of a SAM of 4'-nitro-1,1'-biphenyl-4-thiol that converts the nitro into amino groups and, additionally, generates a pattern of cross-linked and non cross-linked regions. In the next step, molecules in the non cross-linked regions are exchanged for 1,1'-biphenyl-4-thiol. A subsequent electron exposure cross-links these regions, yielding a fully cross-linked, chemically patterned SAM. The reverse process that generates chemically complementary patterns is also demonstrated. For both processes, X-ray photoelectron spectroscopy and atomic force microscopy are used to monitor the fabrication steps and to determine the kinetics of the thiol exchange. The functionality of the fully cross-linked, chemically patterned monolayer is tested by the site selective derivatisation with pentanoic acid chloride.

Novel carbon nanosheets as support for ultrahigh-resolution structural analysis of nanoparticles.

The resolution in transmission electron microscopy (TEM) has reached values as low as 0.08 nm. However, these values are not accessible for very small objects in the size range of a few nanometers or lower, as they have to be placed on some support, which contributes to the overall electron-scattering signal, thereby blurring the contrast. Here, we report on the use of nanosheets made from cross-linked aromatic self-assembled monolayers as TEM sample supports. When transferred onto a copper grid, a single 1.6-nm-thick nanosheet can cover the grid and is free standing within the micron-sized openings. Despite its thinness, the sheet is stable under the impact of the electron beam. Micrographs taken from nanoclusters onto these nanosheets show highly increased contrast in comparison to the images taken from amorphous carbon supports. In scanning transmission electron microscopy with nanosheet support, a size analysis of sub-nanometer Au clusters was performed and single Au atoms were resolved.