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EXPERIMENTAL STUDY OF PIB-BASED CVD GRAPHENE TRANSFER EFFICIENCY
Bouša, Milan ; Kalbáč, Martin ; Jirka, Ivan ; Kavan, Ladislav ; Frank, Otakar
The transfer of graphene prepared by Chemical Vapor Deposition (CVD) from metal catalyst to target substrate is an important step in preparing desirable nanoscale structures in various fields of science, and thus searching for fast, cheap and clean method attracts great interest. Investigation of mechanical properties of graphene, which are crucial for applications in flexible electronics, performed on bendable synthetic materials, requires a transfer technique using polymers soluble in aliphatic solvents harmless for target polymer substrates. In this study we explore a dry technique using polydimethylsiloxane (PDMS) as stamping polymer and polyisobutylene (PIB) layer as graphene-support polymer. After the transfer PDMS is peeled off and PIB is dissolved in hexane, hence this method fulfils the above mentioned prerequisite. The effectiveness of this transfer was examined by scanning electron microscopy, optical microscopy and Raman microspectroscopy including micro-mapping, and finally by X-ray photoelectron spectroscopy. With all methods carried out, it was found that this sort of stamp-technique is suitable for a high precision transfer of small grains of CVD graphene onto polymer substrates with large yields and similar purity compared to poly(methylmethacrylate) (PMMA)based transfer methods. However, it introduces substantial quantity of surface discontinuities, and therefore this is not a proper method for large scale applications.
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STRAIN ENGINEERING OF THE ELECTRONIC STRUCTURE OF 2D MATERIALS
del Corro, Elena ; Peňa-Alvarez, M. ; Morales-García, A. ; Bouša, Milan ; Řáhová, Jaroslava ; Kavan, Ladislav ; Kalbáč, Martin ; Frank, Otakar
The research on graphene has attracted much attention since its first successful preparation in 2004. It possesses many unique properties, such as an extreme stiffness and strength, high electron mobility, ballistic transport even at room temperature, superior thermal conductivity and many others. The affection for graphene was followed swiftly by a keen interest in other two dimensional materials like transition metal dichalcogenides. As has been predicted and in part proven experimentally, the electronic properties of these materials can be modified by various means. The most common ones include covalent or non-covalent chemistry, electrochemical, gate or atomic doping, or quantum confinement. None of these methods has proven universal enough in terms of the devices' characteristics or scalability. However, another approach is known mechanical strain/stress, but experiments in that direction are scarce, in spite of their high promises.\nThe primary challenge consists in the understanding of the mechanical properties of 2D materials and in the ability to quantify the lattice deformation. Several techniques can be then used to apply strain to the specimens and thus to induce changes in their electronic structure. We will review their basic concepts and some of the examples so far documented experimentally and/or theoretically.
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GRAPHENE UNDER UNIAXIAL DEFORMATION: A RAMAN STUDY
Frank, Otakar ; Tsoukleri, G. ; Parthenios, J. ; Papagelis, K. ; Riaz, I. ; Jalil, R. ; Novoselov, K. S. ; Kalbáč, Martin ; Kavan, Ladislav ; Galiotis, C.
The presented work summarizes various aspects of uniaxial deformation in monolayer graphene studied by means of Raman spectroscopy. Graphene flakes were subjected to tension - compression uniaxial loading using the cantilever beam technique. The evolution of the Raman single-resonance (G) and double-resonance (2D) bands was monitored at strain levels < 1%. The position of all peaks redshifts under tension and blueshifts under compression. The G peak splitting into two sub-bands (G(-) and G(+)) which is caused by symmetry lowering, is observed in both strain directions. The sub-bands' intensities are used to calculate the crystal lattice orientation of the measured graphene flakes with respect to the strain axis. The nature and splitting of the 2D band even in the unstrained flakes, when excited by the 785 nm (1.58 eV) laser line, is interpreted as the interplay between two distinct double resonance scattering processes.
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