|
|
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.
|
|
| |
|
|
Electrochemical Characterization of TiO2 Blocking Layers for Dye-Sensitized Solar Cells
Kavan, Ladislav
Thin compact layers of TiO2 are grown by thermal oxidation of Ti, by spray pyrolysis, by electrochemical deposition, and by atomic layer deposition. These layers are used in dye-sensitized solar cells to prevent recombination of electrons from the substrate (FTO or Ti) with the hole-conducting medium at this interface. The quality of blocking is evaluated electrochemically by methylviologen, ferro/ferricyanide, and spiro-OMeTAD as the model redox probes. Two types of pinholes in the blocking layers are classified, and their effective area is quantified. Frequency-independent Mott–Schottky plots are fitted from electrochemical impedance spectroscopy. Certain films of the thicknesses of several nanometers allow distinguishing the depletion layer formation both in the TiO2 film and in the FTO substrate underneath the titania film. The excellent blocking function of thermally oxidized Ti, electrodeposited film (60 nm), and atomic-layer-deposited films (>6 nm) is documented by the relative pinhole area of less than 1%. However, the blocking behavior of electrodeposited and atomic-layer-deposited films is strongly reduced upon calcination at 500 °C. The blocking function of spray-pyrolyzed films is less good but also less sensitive to calcination. The thermally oxidized Ti is well blocking and insensitive to calcination.
|
|
| |
|
|
DYE SENSITISED SOLAR CELLS: NEW CHALLENGES FROM GRAPHENE CATHODE AND IODINE-FREE ELECTROLYTE
Kavan, Ladislav
Graphene nanoplatelets (GNP) in the form of thin semitransparent film on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for Co(L)2; where L is 6-(lH-pyrazol-l-yl)- 2,2‘-bipyridine. GNP films with optical transmission below 88% are outperforming the activity of platinized FTO for the Co2+/3+(L)2 redox reaction. Dye-sensitized solar cells with Y123 dye adsorbed on TiO2 photoanode achieved energy conversion efficiencies between 8 to 10 % for both GNP and Pt-based cathodes. However, the cell with GNP cathode is superior to that with Pt-FTO cathode in fill factors and in the efficiency at higher illumination intensities.
|
|
|
Electrochemical and spectroelectrochemical study of nanocrystalline TiO2 anatase with exposed (001) faces
Lásková, Barbora ; Zukalová, Markéta ; Bouša, Milan ; Kavan, Ladislav ; Liska, P. ; Grätzel, M.
The electrochemical and spectroelectrochemical behavior ot TiO2 anatase with a predominant (001) face was studied and compared to a reference anatase material with dominating (101) face. The electrochemical measurements indicate that Li-ion insertion/extraction is facilitated for TiO2 anatase with (001) faces as compared to (101) one. The performance of both different crystal morphologies as a photoanode material in DSC (dye sensitized solar cell) was tested too. The (001) face adsorbed smaller amount of the used dye sensitizer (C101) but provides larger open circuit voltage (Uoc) of the solar cell. The different band energetics of both particular morphologies was suggested to be the reason for the voltage enhancement of the device employing (001) anatase. To prove our hypothesis and determine their flatband potential, a spectroelectrochemical study of transparent films of nanocrystalline (001) and (101) anatase on coaducting glass was carried out. UV/Vis spectra were measured at potentials between 0 and -1.4V vs SCE and confirmed the negative shift of flatband potential for (001) anatase as compared to (101) one.
|
|
|
Modification of TiO2 electronic properties by means of chemical treatment (doping)
Zukalová, Markéta ; Lásková, Barbora ; Procházka, Jan ; Bastl, Zdeněk ; Havlíček, D. ; Kavan, Ladislav
The product of the reaction of nanofibrous anatase with ammonia in gas phase is nanofibrous cubic titanium oxynitride, c-TiO2N3. In contrary to the preparation of cubic titaniurn oxynitride from microcrystalline TiO2, requiring temperatures above 800°C, the conversion of nanofibrous TiO2 to TiO2N3 is completed at about 500°C due to its enhanced chemical reactivity. This conversion is fully reversible; the product of the back reaction is again nanofibrous Ti02. By the thermal treatment of nanofibrous anatase in H2 flow at 800°C for 2 hours the mixture of black disordered anatase, rutile and titanium suboxides was prepared. Evidently, particular phases can be synthesized simply by tuning the temperature of the treatment, which is a subject of the further research.
|
|
|
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.
|
|
|
16th International Symposium on Intercalation Compound - ISIC16. Abstracts Book
Zima, Vítězslav ; Kavan, Ladislav ; Čejka, Jiří ; Lang, Kamil ; Vlček, Milan ; Beneš, L. ; Svoboda, Jan ; Melánová, Klára ; Frumarová, Božena ; Knotek, P. ; Jůza, Josef ; Látalová, Petra
The topic of the papers published in this proceedings is chemistry, properties and applications of intercalation compounds and related materials.
|
|
| |
host ::
RSS.