Role of structure of C-terminated 4H-SiC(000) surface in growth of graphene layers - transmission electron microscopy and density functional theory studies

Principal structural defects in graphene layers, synthesized on a carbon-terminated face, i.e. the SiC(000) face of a 4H-SiC substrate, are investigated using microscopic methods. Results of high-resolution transmission electron microscopy (HRTEM) reveal their atomic arrangement. Mechanism of such defects creation, directly related to the underlying crystallographic structure of the SiC substrate, is elucidated. The connection between the 4H-SiC(000) surface morphology, including the presence of the single atomic steps, the sequences of atomic steps, and also the macrosteps, and the corresponding emergence of planar defective structure (discontinuities of carbon layers and wrinkles) is revealed. It is shown that disappearance of the multistep island leads to the creation of wrinkles in the graphene layers. The density functional theory (DFT) calculation results show that the diffusion of both silicon and carbon atoms is possible on a Si-terminated SiC surface at a high temperature close to 1600{\deg}C. The creation of buffer layer at the Si-terminated surface effectively blocks horizontal diffusion, preventing growth of thick graphene layer at this face. At the carbon terminated SiC surface, the buffer layer is absent leaving space for effective horizontal diffusion of both silicon and carbon atoms. DFT results show that excess carbon atoms converts a topmost carbon layer to sp2 bonded configuration, liberating Si atoms in barrierless process. The silicon atoms escape through the channels created at the bending layers defects, while the carbon atoms are incorporated into the growing graphene layers. These results explain growth of thick graphene underneath existing graphene cover and also the creation of the principal defects at the C-terminated SiC(0001) surfaceComment: 20 pages,11 figure

Graphene growth on Ge(100)/Si(100) substrates by CVD method

The successful integration of graphene into microelectronic devices is strongly dependent on the availability of direct deposition processes, which can provide uniform, large area and high quality graphene on nonmetallic substrates. As of today the dominant technology is based on Si and obtaining graphene with Si is treated as the most advantageous solution. However, the formation of carbide during the growth process makes manufacturing graphene on Si wafers extremely challenging. To overcome these difficulties and reach the set goals, we proposed growth of high quality graphene layers by the CVD method on Ge(100)/Si(100) wafers. In addition, a stochastic model was applied in order to describe the graphene growth process on the Ge(100)/Si(100) substrate and to determine the direction of further processes. As a result, high quality graphene was grown, which was proved by Raman spectroscopy results, showing uniform monolayer films with FWHM of the 2D band of 32 cm−1

Study of Implantation Defects in CVD Graphene by Optical and Electrical Methods

A Chemical Vapor Deposition graphene monolayer grown on 6H⁻SiC (0001) substrates was used for implantation experiments. The graphene samples were irradiated by He+ and N+ ions. The Raman spectra and electrical transport parameters were measured as a function of increasing implantation fluence. The defect concentration was determined from intensity ratio of the Raman D and G peaks, while the carrier’s concentration was determined from the relations between G and 2D Raman modes energies. It was found that the number of defects generated by one ion is 0.0025 and 0.045 and the mean defect radius about 1.5 and 1.34 nm for He+ and N+, respectively. Hole concentration and mobility were determined from van der Pauw measurements. It was found that mobility decreases nearly by three orders of magnitude with increase of defect concentration. The inverse of mobility versus defect concentration is a linear function, which indicates that the main scattering mechanism is related to defects generated by ion implantation. The slope of inverse mobility versus defect concentration provides the value of defect radius responsible for scattering carriers at about 0.75 nm. This estimated defect radius indicates that the scattering centres most likely consist of reconstructed divacancies or larger vacancy complexes

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Abstract The paper describes experimental studies of Plasma-Focus (PF) discharges carried out within the modernized PF-360 facility, which was operated with an additional D 2 -gas puffing into the region of the collapsing current sheath and PF pinch formation, i.e. into space in front of the electrode outlet. The main aim of these studies was to increase a neutron yield from PF discharges by using fast deuteron beams, which are usually emitted from a pinch column and which can interact with additional D 2 -gas target

Formation of GeO2 under Graphene on Ge(001)/Si(001) Substrates Using Water Vapor

The problem of graphene protection of Ge surfaces against oxidation is investigated. Raman, X-Ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements of graphene epitaxially grown on Ge(001)/Si(001) substrates are presented. It is shown that the penetration of water vapor through graphene defects on Gr/Ge(001)/Si(001) samples leads to the oxidation of germanium, forming GeO2. The presence of trigonal GeO2 under graphene was identified by Raman and XRD measurements. The oxidation of Ge leads to the formation of blisters under the graphene layer. It is suggested that oxidation of Ge is connected with the dissociation of water molecules and penetration of OH molecules or O to the Ge surface. It has also been found that the formation of blisters of GeO2 leads to a dramatic increase in the intensity of the graphene Raman spectrum. The increase in the Raman signal intensity is most likely due to the screening of graphene by GeO2 from the Ge(001) surface

Fluorophosphate Nucleotide Analogs and Their Characterization as Tools for 19F NMR Studies

To broaden the scope of existing methods based on 19F nucleotide labeling, we developed a new method for the synthesis of fluorophosphate (oligo)nucleotide analogues containing an O to F substitution at the terminal position of the(oligo)phosphate moiety and evaluated them as tools for 19F NMR studies. Using three efficient and comprehensive synthetic approaches based on phosphorimidazolide chemistry and tetra-n-butylammonium fluoride, fluoromonophosphate, or fluorophosphate imidazolide as fluorine sources, we prepared over 30 fluorophosphate-containing nucleotides, varying in nucleobase type (A, G, C, U, m7G), phosphate chain length (from mono to tetra), and presence of additional phosphate modifications (thio, borano, imido, methylene). Using fluorophosphate imidazolide as fluorophosphorylating reagent for 5′-phosphorylated oligos we also synthesized oligonucleotide 5′-(2-fluorodiphosphates), which are potentially useful as 19F NMR hybridization probes. The compounds were characterized by 19F NMR and evaluated as 19F NMR molecular probes. We found that fluorophosphate nucleotide analogues can be used to monitor activity of enzymes with various specificities and metal ion requirements, including human DcpS enzyme, a therapeutic target for spinal muscular atrophy. The compounds can also serve as reporter ligands for protein binding studies, as exemplified by studying interaction of fluorophosphate mRNA cap analogues with eukaryotic translation initiation factor (eIF4E)

Organic functionalization of Epitaxial Graphene on SiC

Trabajo presentado en GraphITA, celebrado en Bolonia (Italia) del 14 al 18 de septiemnbre de 2015.A truly developed carbon-based nanoelectronics requires, among other features, low price, large production scale, large domain area and high crystalline quality. Epitaxially grown graphene on silicon carbide (SiC) can meet all of these requirements[1]. Moreover, functionalization of graphene is expected to be an important step for development of graphene-based materials with tailored features, due to the possible control of optical and electronic properties[2,3]. However, the high chemical inertness of graphene makes it difficult a covalent and controlled functionalization with organics. Most of the works performed until now involve either controlled adsorption of low-interacting molecular structures or unruly functionalization. In this work, we target to develop new strategies for controlled covalent anchoring of the organic molecules to the graphene lattice that can be used either to modify their properties or as a link for anchoring larger nanostructures. In our experiment, high quality graphene was epitaxially grown on a 4H-SiC(0001) substrate with n-type doping character by chemical vapor deposition (CVD) at 1600oC under an argon (Ar) laminar flow in a hot-wall Aixtron VP508 reactor. The graphene produced by this method is much less sensitive to SiC surface defects, resulting in higher electron mobility than those grown by Si sublimation process. For epitaxial CVD growth, we formed an Ar boundary layer thick enough to prevent Si sublimation, but allowing for the diffusion of propane gas that was led into the reactor as the precursor which enables the controlled synthesis of a precisely determined number of graphene layers. The functionalization protocols have been performed in ultra high vacuum, and we have chosen p-aminophenol organic molecules because they include two functional groups: amine and hydroxyl, allowing us to determine which one is more prone to make a bond with graphene. Prior and upon adsorption, the surfaces have been studied in-situ using several techniques, as low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Moreover, several aspects of the system have been theoretically investigated by first-principles density functional theory calculations (DFT). Making use of atomic-resolved STM images (see figure), XPS spectra, LEED analysis and DFT calculations, we have demonstrated that we can form chemical bonding of this molecule in two different configurations. In the first the amine group loose one of the H atoms and bond by the nitrogen to the carbon. In the second the hydroxyl group dehydrogenates and reacts with the surface. We show that about 67% of the adsorbed p-aminophenol molecules were covalently bound by the nitrogen, while the other 33% by the oxygen atom.N

Synthesis of Fluorophosphate Nucleotide Analogues and Their Characterization as Tools for ¹⁹F NMR Studies

To broaden the scope of existing methods based on ¹⁹F nucleotide labeling, we developed a new method for the synthesis of fluorophosphate (oligo)­nucleotide analogues containing an O to F substitution at the terminal position of the (oligo)­phosphate moiety and evaluated them as tools for ¹⁹F NMR studies. Using three efficient and comprehensive synthetic approaches based on phosphorimidazolide chemistry and tetra-<i>n</i>-butylammonium fluoride, fluoromonophosphate, or fluorophosphate imidazolide as fluorine sources, we prepared over 30 fluorophosphate-containing nucleotides, varying in nucleobase type (A, G, C, U, m⁷G), phosphate chain length (from mono to tetra), and presence of additional phosphate modifications (thio, borano, imido, methylene). Using fluorophosphate imidazolide as fluorophosphorylating reagent for 5′-phosphorylated oligos we also synthesized oligonucleotide 5′-(2-fluorodiphosphates), which are potentially useful as ¹⁹F NMR hybridization probes. The compounds were characterized by ¹⁹F NMR and evaluated as ¹⁹F NMR molecular probes. We found that fluorophosphate nucleotide analogues can be used to monitor activity of enzymes with various specificities and metal ion requirements, including human DcpS enzyme, a therapeutic target for spinal muscular atrophy. The compounds can also serve as reporter ligands for protein binding studies, as exemplified by studying interaction of fluorophosphate mRNA cap analogues with eukaryotic translation initiation factor (eIF4E)