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

A novel fluorescent probe for NAD-consuming enzymes

A novel, fluorescent NAD derivative is processed as substrate by three different NAD-consuming enzymes. The new probe has been used to monitor enzymatic activity in a continuous format by changes in fluorescence and, in one case, to directly visualize alternative reaction pathways

Direct Evidence for the Participation of Oxygen Vacancies in the Oxidation of Carbon Monoxide over Ceria‐Supported Gold Catalysts by using Operando Raman Spectroscopy

Supported gold catalysts are highly active for a variety of reactions, including low‐temperature CO oxidation. It has been shown that reducible support materials, for example, ceria and titania, may significantly alter catalytic performance. However, the current understanding of the CO oxidation mechanism of such gold catalysts is still incomplete, as important details such as the activation of oxygen and the role of oxygen vacancies are unknown. To elucidate the role of the ceria support during room‐temperature CO oxidation, we employed operando Raman spectroscopy by simultaneously recording the Raman spectra of the catalyst and the gas‐phase FTIR spectra. Our results give first direct spectroscopic evidence for the participation of oxygen vacancies in the oxidation of CO over ceria‐supported gold, which thus underlines the crucial role of the support material for detailed understanding of the mode of operation of supported gold catalysts.Phantom of the opera‐ndo: By using operando Raman spectroscopy, direct spectroscopic evidence for the participation of oxygen vacancies in the oxidation of CO over ceria‐supported gold catalysts at room temperature is provided. The results underline the crucial role of the support material for detailed understanding of the mode of operation of supported gold catalysts.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137587/1/cctc201501129-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137587/2/cctc201501129.pd

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

Eski mektuplar

Ahmet Mithat'ın Tercüman-ı Hakikat'te tefrika edilen Eski Mektuplar adlı roman

Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport

Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tuneable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfil this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects towards the engineering of free-standing 2D carbon materials with tuneable properties on various solid substrates and on holey substrates as suspended membranes.Comment: 30 pages, 5 figure

The application of Graphene as a sample support in Transmission Electron Microscopy

Transmission electron microscopy has witnessed rampant development and surging point resolution over the past few years. The improved imaging performance of modern electron microscopes shifts the bottleneck for image contrast and resolution to sample preparation. Hence, it is increasingly being realized that the full potential of electron microscopy will only be realized with the optimization of current sample preparation techniques. Perhaps the most recognized issues are background signal and noise contributed by sample supports, sample charging and instability. Graphene provides supports of single atom thickness, extreme physical stability, periodic structure, and ballistic electrical conductivity. As an increasing number of applications adapting graphene to their benefit emerge, we discuss the unique capabilities afforded by the use of graphene as a sample support for electron microscopy.Comment: Review, to appear in solid state communication

Carbon nanosheets and their applications

Nottbohm CT. Carbon nanosheets and their applications. Bielefeld (Germany): Bielefeld University; 2009.Carbon nanosheets are a new kind of two-dimensional polymeric material that is fabricated by cross-linking aromatic self-assembled monolayers with electrons. Due to their uniform thickness of only about one nanometer, as well as their high chemical, mechanical, and thermal stability, such materials are of high interest for a wide variety of applications. Carbon nanosheets can be released from their original substrate and transferred to arbitrary new substrates. When placed on oxidized silicon wafers, nanosheets can be seen by the naked eye. Folds appear to be darker and blue shifted with respect to the substrate color. This effect was systematically studied for multilayer stacks of nanosheet by means of UV/Vis reflection spectroscopy and simulation. The oxide layer acts as an optical spacer, similar to an interferometer. Thus, the contrast was found to be dependent on the wavelength of light as well as the thickness of the oxide layer. On perforated substrates such as transmission electron microscopy (TEM) grids, large freestanding membranes with the thickness of one monolayer were obtained with an aspect ratio of up to 1:225000. This way, for the first time, the bottom side of the nanosheet became accessible for functionalization, and it was possible to selectively couple fluorescent dyes to the nanosheet; this highlights the versatility of the nanosheets. Freestanding nanosheets have been used as sample supports to image nanoparticles by TEM. Due to the thickness of the nanosheets, the nanoparticles can be observed in intricate detail, with only minimal background from the nanosheet as compared to conventional carbon films. Also, the individual layers of multi-walled carbon nanotubes (MWNT) were resolved. In scanning TEM even single gold atoms were observed. It is possible to deposit more than just nanoparticles onto freestanding nanosheets. The sheets can also be metalized in a variety of different ways either before transfer or afterwards; this was demonstrated by fabricating gold patterns onto freestanding nanosheets. As the nanosheet is stable under an electron beam, patterns can also be written by electron beam induced deposition (EBID). By annealing nanosheets in ultra high vacuum their conductivity can be adjusted flexibly. This is due to a gradual transition to a graphitic phase that reaches a sheet resistivity of only [almost equal to]100 k[Ohm]/sq at [almost equal to]1200 K. The structural changes that are associated with this transition are reflected in the TEM and Raman data. For multilayer stacks it was also possible to observe the transition to graphite by means of UV/Vis spectroscopy. Because of their stability and flexibility, carbon nanosheets will likely find a multitude of applications, including potential use as sensors, filtration membranes, sample supports, and even conductive coatings