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Spectroscopy of single molecules in STM nanocavity
Doležal, Jiří ; Švec, Martin (advisor) ; Frank, Otakar (referee) ; Roslawska, Anna (referee)
Scanning tunneling microscopy-induced luminescence (STML) combined with high- resolution atomic force microscopy (AFM) is a powerful tool for studying the photophysics of individual molecular emitters on surfaces. However, the mechanism of energy conver- sion between tunneling electrons and photons in decoupled systems placed in a nanocavity of STM is not fully understood as it depends on many variables. This thesis presents a range of proof-of-concept experimental approaches. The vi- ability of CO-terminated tips for STML is demonstrated by performing subnanometer- resolved spectroscopy and mapping of photon intensity acquired over zinc phthalocyanine on NaCl/metal substrate. For the same molecule, time-resolved phase fluorometry is de- vised and is used to reveal the exciton and charge dynamics as a function of the applied bias voltage. Of more fundamental character, the role of the chromophore environment on its exciton emission and binding energy is studied. For the first time, we observed and explained the presence of molecular librations in molecules on the surface from a comb-like emission line resulting from the exciton-libron coupling and the chiral adsorp- tion geometry. Finally, exciton delocalization in molecular aggregates is mapped using the tip nanocavity capable of detecting the dark states,...
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In-situ Raman spectroelectrochemistry of graphene and related materials in concentrated aqueous electrolytes
Abbas, Ghulam ; Frank, Otakar (advisor) ; Kunc, Jan (referee) ; Voiry, Damien (referee)
The complex interplay between the material's properties and the electrochemical process often taking place at its ubiquitous edges makes it difficult to discriminate the role of single defects in the charge transfer processes in the material. Therefore, we have performed in-situ μ-droplet Raman spectroelectrochemistry (SEC) to identify the localized charge transfer processes through the basal plane and defects in a selected localized area of 10-20 μm2 of defect-free and defective monolayer graphene. It is noticed that two distinctive electron transfer processes of slower and faster rates exist side-by-side in the same sample but they are confined in the defect-free and defect-rich regions, respectively. Furthermore, in order to explore the electrochemical ion intercalation mechanism for rechargeable batteries, in-situ Raman SEC in a macro SEC cell was performed. It was observed that structural properties such as the lateral domain size (La), degree of graphitization (g), inter-defect distance (LD) and defect density (nD) have substantial influence on the electrochemical (de)intercalation of anion into natural and kish graphite during charge/discharge process. It was also revealed that ultrasound treatment of natural graphite reduces the La which enhances the reversibility of the anion...
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IMPOSING BIAXIAL STRAIN ON 2D LAYERED MATERIALS BY LIQUID-INDUCED SWELLING OF SUPPORTING POLYMER
Sampathkumar, Krishna ; Pekárek, J. ; Frank, Otakar
2D layered materials promise to revolutionize the field of electronics, photonics, optoelectronics, energy storage, and sensing, etc. 2D materials have exceptional mechanical properties, with critical elongation >10%. Employing the strain to manipulate the electronic structure of these 2D materials could lead to further improvement of their implementation in many aspects. The ease of manipulation of their electronic structure can be one of the critical factors for their utilization in photonic devices. Apart from the strain, which decreases (increases) the bandgap energy at the rate of similar to 100 meV under 1% of biaxial tension (compression), also the layer number causes bandgap energy change of, e.g., 0.5 eV between bulk (1.3 eV) and monolayer MoS2 (1.8 eV). In our work, we focus on using the swelling behavior of PMMA/SU8 polymer in methanol to impose the strain on 2D layered materials. In the first trials, we have shown that it is possible to reach a strain gradient from 0 to similar to 0.5% of biaxial strain via simple swelling of polymer substrates, both for graphene [1] and transition metal di-chalcogenides (TMDC) like MoS2. Raman spectroscopy was used to probe the lattice strain in the materials through measuring changes of vibrational frequencies, and photoluminescence was used to probe the strain-induced bandgap character and energy in TMDC at room temperature. The surface corrugation of the 2D material after the soaking was recorded with the help of atomic force microscope (AFM).
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Application of Raman spectroscopy for study of nitrogen containing compounds for astrobiological purposes
Culka, Adam ; Jehlička, Jan (advisor) ; Vlčková, Blanka (referee) ; Frank, Otakar (referee)
IV ABSTRACT This PhD thesis was focused on the evaluation of the application of Raman spectroscopy as an analytical method used specifically for research on selected nitrogen containing compounds in experiments, that are important in the astrobiological context. The results of experiments provided insight into the advantages as well as the limitations of the method in several applications that are expected to be encountered in the future planetary exploration missions with astrobiological context use, and where the Raman spectroscopy is proposed as an advantageous method. The Raman spectroscopy was tested in various experimental tasks. Several nitrogen containing minerals were analysed, where the features such as the indestructive analysis, small laser spot size, and selection of the excitation source enabled to acquire spectra of minerals even with miniature sample sizes, also the band assignment was published for the first time for some of the studied minerals. Artificially prepared samples of biomarkers mixed in mineral matrices were analysed to test the ability of the method to analyse complex mixtures: three biomarkers and two evaporitic minerals were analysed with a portable instrument, and all compounds were unambiguously detected. When the samples contained more of the similar compounds with many...
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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.
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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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