Národní úložiště šedé literatury Nalezeno 8 záznamů.  Hledání trvalo 0.00 vteřin. 
Introducing Functionality by Graphene Covalent Grafting
Kovaříček, Petr ; Kalbáč, Martin
Graphene is a material of great potential in a broad range of applications, for each of which specific tuning of the material’s properties is required. This can be achieved, for example, by covalent functionalization. We have exploited protocols for surface grafting by diazonium salts, by nucleophilic and electrophilic substitution to perform graphene covalent modification of graphene on substrates. The painstaking analysis problem of monolayered materials was addressed using Raman spectroscopy, SERS, SEIRA, MS, AFM, XPS and SEM/EDX. The covalent grafting was shown to tolerate the transfer process, thus allowing ex post transfer from copper to other substrates. Functional devices often require combination of several materials with specific functions, such as graphene-polymer hybrid heterostructures. We have used the developed methodology of chemical functionalization for preparation of PEDOT:Graphene bilayers by selective in situ polymerization of EDOT on covalently functionalized graphene. The polymerization proceeds exclusively on the grafted graphene, and patterned structures with high spatial resolution down to 3 μm could have been prepared. The composite exhibits enhanced efficiency of electrochemical doping compared to pristine graphene, unsymmetrical transport characteristic with very good hole-transporting properties and efficient electronic communication between the two materials. The covalent functionalization of graphene thus introduces advanced functionality to the material, broadening its scope of applications.
Příprava a studium vrstev sulfidu molybdeničitého
Jurková, Kateřina ; Kalbáč, Martin (vedoucí práce) ; Nižňanský, Daniel (oponent)
Tato práce se zabývá přípravou a studiem sandwichových heterostruktur MoS2 a grafenu. Vrstvy MoS2 připravené mechanickou exfoliací byly detekovány pomocí optické mikroskopie a charakterizovány pomocí Ramanovy spektroskopie a mikroskopie atomárních sil (AFM). Dále byly sestaveny spektroelektrochemické cely pro sandwichové heterostruktury MoS2/grafen a studována Ramanova a fotoluminiscenční spektra v zá- vislosti na vloženém elektrickém napětí. Byla také provedena spektroelektrochemická měření na mono- až čtyřvrstvě MoS2 v mikrokapce. Byly pozorovány změny v parametrech pásů E1 2g (E') a A1g (A1') v Ramanových spektrech a pásů přímých i nepřímých fotoluminiscenčních přechodů ve fotoluminiscen- čních spektrech MoS2 v závislosti na vloženém napětí. Při spektroelektrochemickém měření v mikrokapce bylo pozorováno rozdělení nepřímého přechodu do dvou složek I- a I+ . Klíčová slova: MoS2, grafen, Raman, spektroskopie, spektroelektrochemie 1
Spektroskopie a spektroelektrochemie MoS2
Jurková, Kateřina ; Kalbáč, Martin (vedoucí práce) ; Matulková, Irena (oponent)
Tato práce se zabývá studiem vrstev MoS2 připravených mechanickou exfoliací. Vrstvy byly detekovány pomocí optické mikroskopie a pomocí AFM byl určen jejich počet. Vrstvy byly charakterizovány pomocí Ramanovy spektroskopie s různými excitačními vlnovými délkami. Dále byla sestrojena spektro-elektrochemická cela pro vzorek tvořený vrstvou MoS2 a dvěma vrstvami grafenu o různém izotopovém složení a provedeno měření Ramanových spekter při napětí 0,0 V až 0,9 V. Ve spektru grafenu byl pozorován větší posun G módu na vrstvách s MoS2 než na vrstvách grafenu. Klíčová slova: MoS2, grafen, Raman, spektroskopie, spektroelektrochemie
Experimental and Theoretical Comparative Study of Monolayer and Bulk MoS2 under Compression
del Corro, Elena ; Morales-García, A. ; Peňa-Alvarez, M. ; Kavan, Ladislav ; Kalbáč, Martin ; Frank, Otakar
Recently, a new family of 2D materials with exceptional optoelectronic properties has stormed into the scene of nanotechnology, the transition metal dichalcogenides (e.g., MoS2). In contrast with graphene, which is a zero band gap semiconductor, many of the single layered materials from this family show a direct band-gap in the visible range. This band-gap can be tuned by several factors, including the thickness of the sample; the transition from a direct to indirect semiconductor state takes place in MoS2 when increasing the number of layers from 1 towards the bulk. Applying strain/stress has been revealed as another tool for promoting changes in the electronic structure of these materials; however, only a few experimental works exist for MoS2. In this work we present a comparative study of single layered and bulk MoS2 subjected to direct out-of-plane compression, using high pressure anvil cells and monitoring with non-resonant Raman spectroscopy; accompanying the results with theoretical DFT studies. In the case of monolayer MoS2 we observe transitions from direct to indirect band-gap semiconductor and to semimetal, analogous to the transitions observed under hydrostatic pressure, but promoted at more accessible pressure ranges (similar to 25 times lower pressure). For bulk MoS2, both regimes, hydrostatic and uniaxial, lead to the semimetallization at similar pressure values, around 30 GPa. Our calculations reveal different driving forces for the metallization in bulk and monolayer samples.
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.
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.
Graphene Field-Effect Transistor as a Probe of Doping by Adsorbed Oxygen Molecules
Blechta, Václav ; Mergl, Martin ; Drogowska, Karolina ; Kučera, Lukáš ; Valeš, Václav ; Červenka, Jiří ; Kalbáč, Martin
Graphene has high potential in chemical sensing, thus understanding adsorption and charge transfer between graphene and adsorbed molecules is essential. We show that graphene field-effect transistor exhibits a moderate sensoric response towards oxygen at temperature of 150 °C. Field-effect transistors serve as a tool to probe electronic properties of graphene. We demonstrate that adsorption of oxygen molecules onto graphene leads to an upshift of Dirac point and light changes in mobility of charge carriers.
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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