Understanding the chemical vapor deposition (CVD) kinetics of graphene growth is important for advancing graphene processing and achieving better control of graphene thickness and properties. In the perspective of improving large area graphene quality, we have investigated in real-time the CVD kinetics using CH(4)-H(2) precursors on both polycrystalline copper and nickel. We highlighted the role of hydrogen in differentiating the growth kinetics and thickness of graphene on copper and nickel. Specifically, the growth kinetics and mechanism is framed in the competitive dissociative chemisorption of H(2) and dehydrogenating chemisorption of CH(4), and in the competition of the in-diffusion of carbon and hydrogen, being hydrogen in-diffusion faster in copper than nickel, while carbon diffusion is faster in nickel than copper. It is shown that hydrogen acts as an inhibitor for the CH(4) dehydrogenation on copper, contributing to suppress deposition onto the copper substrate, and degrades quality of graphene. Additionally, the evidence of the role of hydrogen in forming C-H out of plane defects in CVD graphene on Cu is also provided. Conversely, resurfacing recombination of hydrogen aids CH(4) decomposition in the case of Ni. Understanding better and providing other elements to the kinetics of graphene growth is helpful to define the optimal CH(4)/H(2) ratio, which ultimately can contribute to improve graphene layer thickness uniformity even on polycrystalline substrates.
For pt.III see ibid., vol.16, p.2005 (1983). Measured values of the electrical conductivity, sigma , and electron spin density (g=2.0057) of microcrystalline silicon can be essentially determined by the extent of the contamination of the samples by oxygen unless special precautions are taken as regards the sample preparation and/or handling. For samples deposited at a floating potential, two kinds of oxygen incorporation are identified: irreversible formation of Si-O bonds on the grain boundaries (and on the sample surface) and a reversible absorption which is probably associated with a nondissociative O2
delta - (ads) state. The latter results in a decrease of sigma RT by up to five orders of magnitude, an increase of the activation energy, epsilon a, and of the preexponential factor, sigma 0, as well as in an increase of the electron spin density. A reversible desorption of oxygen leads to an increase of sigma RT up to not less than about 10-2 Omega -1 cm-1 and a decrease of the EPR signal below the detection limit of less than 1016 cm-3. In order to avoid such effects a negative bias has to be applied to the substrate during deposition. Samples of undoped mu c-Si deposited in this way show neither the incorporation of oxygen into the bulk nor significant changes in the dark conductivity even after long-term exposure to air.
Self-assembled monolayers (SAMs) derived of 4-methoxy-terphenyl-3'',5''-dimethanethiol (TPDMT) and 4-methoxyterphenyl-4''-methanethiol (TPMT) have been prepared by chemisorption from solution onto gold thin films and nanoparticles. The SAMs have been characterized by spectroscopic ellipsometry, Raman spectroscopy and atomic force microscopy to determine their optical properties, namely the refractive index and extinction coefficient, in an extended spectral range of 0.75-6.5 eV. From the analysis of the optical data, information on SAMs structural organization has been inferred. Comparison of SAMs generated from the above aromatic thiols to well-known SAMs generated from the alkanethiol dodecanethiol revealed that the former aromatic SAMs are densely packed and highly vertically oriented, with a slightly higher packing density and a absence of molecular inclination in TPMT/Au. The thermal behavior of SAMs has also been monitored using ellipsometry in the temperature range 25-500 degrees C. Gold nanoparticles functionalized by the same aromatic thiols have also been discussed for surface enhanced Raman spectroscopy applications. This study represents a step forward tailoring the optical and thermal behavior of surfaces as well as nanoparticles.
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