Seznam abstraktů 2022: POSTEROVÁ SEKCE A
Datum: 20.10.2022
Nowadays, 2D materials are one of the most studied classes of materials. In addition to the most famous graphene, progress has been achieved in the study and use of the fundamental properties of transition metal dichalcogenides (TMD). Complementary, diamond as a representative of 3D materials has gained a reputation as an extremely versatile material due to its extraordinary combination of physical/chemical/electrical/optical properties. Besides these particular forms of 2D and 3D materials, their heterostructures become very attractive due to new phenomena and functions (bandgap engineering, enhanced charge transport, optical interaction, etc.). Therefore, it is not surprising that intensive technological efforts have logically reached a mature stage, and their combinations have expanded dramatically. However, individual technological procedures are still minimally investigated and described. Here, we will demonstrate a proof-of-concept for the preparation of TMD/diamond heterostructures, where two different strategies were investigated: a) growth of TMD layers (specifically MoS2 and PtSe2) on diamond films, and b) growth of diamond films on TMD layers. Considering the growth conditions for TMD and diamond materials, heterostructures based on TMDs on diamond can be prepared more easily. Regardless of the morphology of the continuous diamond film, the grown TMD layer mostly copies the diamond surface. However, the heterostructure diamond on TMD is a highly challenging task. It was found experimentally that the combination of deposition temperature and aggressive chemical-plasma environment during diamond growth places high demands on the resistance and stability of TMD layers. ACKNOWLEDGEMENTS: This work was supported by the MoRePro project No. 19MRP0010.
The main purpose of this work is to explore the possibilities of preparing conducting composite containing graphite or graphene from anilinium/montmorillonite intercalates. The use of only a precursor (without polymerization) is a fundamental difference compared to earlier research devoted to the preparation of graphite/graphene-silicate composites from polyaniline/montmorillonite intercalates. Therefore, the preparation procedure tested in this study represents another way to obtain an electrically conductive material, the electrically conductive component of which is formed in situ. Four different concentrations of anilinium sulfate solution were used to prepare these intercalates (0.2M; 0.4M; 0.6M; 0.8M). Montmorillonite was added to these solutions, and after 7 days of intercalation, the solutions were filtered and the resulting intercalates were dried. From each sample, one tablet was pressed, and the electrical conductivity of these tablets was measured before and after their calcination in argon atmosphere at 1300 °C. Electrical conductivity of the calcined samples crushed to powder was also measured. For the analysis of uncalcined samples, X-ray diffraction analysis was used. Calcined samples were analyzed using X-ray diffraction analysis and Raman spectroscopy. These analyses in combination with electrical conductivity measurements revealed that conducting composite can be prepared using the method described above. ACKNOWLEDGEMENTS: This work was supported by the Ministry of Education, Youth and Sports of Czech Republic project SP2022/61.
Magnetic nanoparticles are attractive nanomaterials for many applications. They are used in many areas, such as catalysts, drug nanocarriers, magnetic-resonant imaging, magnetic recording, and magnetic data storage [1]. One of the most topical magnetic nanoparticle research fields is on mixed metal-lanthanide oxides, especially Ce-doped iron oxide nanoparticles. The applications of these systems utilize the iron oxide magnetic properties combined with the unique redox activity of cerium [2]. Therefore, the Ce-doped iron oxide nanoparticles could be used as removable catalysts using an external magnetic field. In addition, biocompatible iron oxides (magnetite/maghemite) and cerium oxides (CeO2) have significant medical applications [2], and their composite structure can bring new application opportunities in this field. In general, a nanoparticle's physical and chemical properties are determined by the nanoparticle size, shape, and surface quality, as well as by a phase and morphological composition. These structural characteristics can be modified by optimizing the used synthesis method. We prepared a series of Ce-doped iron oxide nanoparticle systems using thermal decomposition via tuning synthesis parameters, such as the temperature of degassing and reflux, nucleation time, used surfactants, and the mole ratio of the used precursors - cerium and iron acetylacetonates. Synthesized nanoparticles had cuboid and spherical shapes of the Fe-Ce-O composition and were characterized using the TEM, EDX, and XRD methods. REFERENCES: [1] Y. Tian, et al. Mater. Chem. 21 (2010), 2476.; [2] M.M.T. Jansman, et al., Catalysts 9 (2019), 691. ACKNOWLEDGEMENTS: The work was supported by the Czech Science Foundation, Project No. 21-31852J.
Cerium oxide nanoparticles (nanoceria) are currently one of the most investigated nanomaterials because of their attractive properties used in biomedical applications, catalysis, fuel cells, and many others. These attractive properties are connected with the Ce3+ and Ce4+ valency state ratio. In the nanoparticle form, cerium oxides contain a mixture of Ce3+ and Ce4+ on the nanoparticle surfaces. Switching between these two states requires oxygen vacancies. Therefore, nanoceria's inherent ability to act as an antioxidant in an environmentally-dependent manner and a “redox switch” to confer auto-regenerating capabilities by automatically shifting between Ce4+ and Ce3+ oxidation states is significantly affected by surface morphology. Regarding this demanded behavior, we aimed to characterize synthesized nanoparticle surface quality and its influence on the cerium oxidation states. The received results were used to evaluate the synthesis method's suitability for suggested utilization.
We used nanoparticles prepared by electron beam evaporation. This unique physical method includes nanoparticle creation through the fast cooling process followed by breaking radiation damaging nanoparticle surfaces to create surface off-stoichiometry. We prepared a sample containing clusters of a mixture of ultra-small nanoparticles and approximately 100 nm particles. X-ray diffraction confirmed the CeO2 phase in both components. To extract the finest component, we used centrifugal size fractionation. We received 200 nm clusters of 2-10 nm nanoparticles. Nanoparticle shapes and facet types were analyzed using transmission electron microscopy methods. We found out most nanoparticles were formed with truncated octahedrons containing {1,1,1} and {1,0,0} facet types and truncated cuboctahedrons containing {1,1,1}, {1,0,0}, and additional {1,1,0} facets. No octahedron (without truncation) containing only {1,1,1} facets was observed. Nanoparticle shapes containing {1,1,0} and {1,0,0} are suitable for redox activity. Some amount of irregular shapes, beneficial for redox activity, was also observed. Spectroscopy methods confirmed Ce3+ content.
ACKNOWLEDGEMENTS: The research has been supported by the Czech Science Foundation (Project 21-31852J) and by Russian Foundation for Basic Research (Project 20-58-26002). We acknowledge CzechNanoLab Research Infrastructure supported by MEYS CR (LM2018110).
Doping is one of the most suitable ways of tuning the electronic properties of topological insulators and a promising means of a band gap opening. We combine theoretical (ab initio) and experimental (XPS, ARPES) methods to analyze the electronic properties and chemical states of atoms and defects in the substitutional or interstitial position in the topological insulator Bi2Se3 doped with VIIIB and VIB columns elements that can be achieved using the free melt crystallization method of the sample growth. The relationship between the position of Dirac cone and valence band maximum is shown and discussed.
Increasing the absorption of light that can be converted into electrical current in III-V semiconductor nanowires (NWs) for photovoltaic solar cells (PVSCs) is crucial for enhancing the overall efficiency and reducing cost. We theoretically investigate the effects of the surrounding media on the optical absorptance of a periodic square array of GaAs NWs as a candidate for PVSCs. The filling media considered in the present work are: PMMA, Polycarbonate, Polystyrene and PVP, commonly used in the science and technology of semiconductor PVSCs. The simulated system consists of a top layer of ITO, an intermediate layer of GaAs NWs surrounded by an embedding medium and a substrate layer of Si. The embedded GaAs NWs are assimilated to homogeneous thin film by means of an effective medium approximation, whose dielectric function is evaluated by the Maxwell-Garnett theory [1].
Optical properties such as: optical absorptance, reflectance and transmittance of the entire system are modelled through the well-known transfer matrix formalism [2]. The obtained results are discussed in terms of embedding medium refractive indices and sizes. For s- and p-polarization, we observe an oscillating behavior in the absorptance for the four media and different sizes investigated. The presence of a medium with permittivity smaller than that of GaAs NWs yields a different effective optical path, affecting interference phenomena and thus reflectance, transmission and absorptance modulations [3].
REFERENCES: [1] J.C. Maxwell Garnett, Trans. of Royal Society (1904) v. CCIII 385.; [2] N.M. Ali and N.H. Rafat, Renewable and Sustainable Energy Reviews 68 (2017) 212.; [3] F. Floris et al., Nanomaterials 7 (2017) 400.
Fast charging/discharging capability is one of the critical parameters for next-generation energy storage devices. Li-ion capacitors have been proposed as an alternative approach to overcome the low energy density of conventional electrochemical double layer capacitors (EDLCs). A higher energy density could be achieved by replacing one activated carbon electrode with a Li-ion insertion/deinsertion electrode, which has a much higher specific capacity. The selection of this type of electrodes for Li-ion capacitors is the key to achieving desired performances. Nb2O5 is one of the promising high-power anode materials. The Nb2O5-based Li-ion capacitors working in the safe voltage range are free of the potential safety problems associated with the electrolyte decomposition, resulting in stable cycle life. However, further progress of Nb2O5 anodes for practical applications faces challenges due to their intrinsic poor electric conductivity (~3×10-6 S cm−1).
This study used different carbons to build a gradient electrically conductive network for Nb2O5. This conductive network was composed of a thin carbon layer on the Nb2O5 surface, carbon black for joining Nb2O5 particles, and carbon nanotubes for long-distance electron transfer. Characterizations were conducted such as scanning electron microscope and X-ray diffraction. The electrochemical performance was tested in coin cell 2032 with the lithium anode. As a result, the Gradient conductive network enhances the rate capability of Nb2O5.
ACKNOWLEDGEMENTS: This work is supported by the Ministry of Education, Youth, and Sports, Czech Republic (INTER-EXCELLENCE program under the grant agreement No. LTT20005).
Perovskites are the most studied photovoltaic material due to adjustable band gap, high carrier mobility, and high tolerance to defects. In this work, we used microspectroscopy to study the degradation and aging of the perovskites after two years in storage. Perovskites solar cells (PSC) with different ratios of methylammonium / lead Iodide (MAI/PbI2 = 0.9, MAI/PbI2 = 1.0) and the two types of the charge transporting materials were studied. SnO2 was used as the electron-transporting layer (ETL), and Spiro-OMeTAD was used as the hole-transporting layer (HTL). PSC morphology was characterized by Atomic Force Microscopy (AFM). The spectra and maps were recorded by the Confocal Photoluminescence micro-spectroscopy (PL). The time-resolved photovoltage (TRPV) was measured by the Kelvin probe (KP) in the dark and under illumination. Two light sources were used for sample illumination, Solar Simulator and a Schott halogen lamp. After two years of storage, the perovskites changed from silver-black to yellow, a sign of degradation and decomposition to PbI2. The colour change is most pronounced for samples with HTL. PL maps correlated with morphology. PL spectra with HTL were left-shifted, probably related to changes in colour/composition. PSC with ETL showed the most significant change in the PV. PV was negative, opposite to the PV measured two years ago. PV values were higher for samples with CTL and lowered without CTL under illumination by a Solar simulator. The influence of the substrate on the optoelectronic properties was minimal. ACKNOWLEDGEMENTS: The work was supported by the project CZ.02.1.01/0.0/15_003/0000464 - Centre of Advanced Photovoltaics.
The nanocrystalline diamond (NCD) film reveals a unique combination of physical, chemical, and optoelectronic properties, which makes it a promising material for various sensing applications (gases, biomolecules, etc.). Further, to enhance gas sensor response, selectivity, or reproducibility, its surface is often engineered with specific terminations, functional groups or (bio) molecules, thin film, etc. Here, we investigated the influence of preparation technology (two types of CVD systems), b) top-surface termination (oxygen vs. hydrogen), and c) Au nanoparticles on the gas sensing properties of NCD films. The thin layer production, analysis (by Scanning Electron Microscopy and Raman spectroscopy), and electrical properties measurement are described. The electrical properties (resistance changes) are measured for oxidizing gas NO2, reducing gas NH3, and neutral synthetic air at temperatures varied from room temperature to 125°C. The measured parameters of the tested diamond sensors are compared and subsequently evaluated. The surface morphology is significant for gas sensors, as a larger surface area (higher roughness) increases the response to different gases. While the NCD termination with hydrogen is principally required for the gas sensor functionality, the Au nanoparticles further enhanced the sensor dynamic response and response magnitude.
In this research, we present the design and preparation of a platform for a gas sensor application based on the ink printed of silver nanoparticles on flexible substrates. In addition, we summarize defects and problems that may occur when using UV curable or flexible materials (such as cracks or pinholes), methods of curing and sintering, and different plasma treatments applied to achieve suitable wettability. Inkjet printing allows localized deposition of ink at low temperatures on a large area and can be used for the deposition of both conductive and dielectric materials. The printed platform consists of four interdigital electrodes (IDE), and connector pads printed with silver ink and sintered with intense pulse light. The dielectric ink printed around the IDE structures protects the connection pathways and defines the sensing area. The multilayer structure was printed on PET and polyimide substrates. Before printing, plasma treatment of the surface of the polyimide is necessary. Three combinations of electrodes with different widths and gaps were designed and the structures were printed with three different resolutions to verify the printability on the used combination of substrate and ink. Bending tests that show the flexibility of printed structures were performed as well.
Lasers represent a breakthrough that have changed the modern world. Great scientific effort has been dedicated to increase the lasers’ power and to expand their operating wavelengths from telecommunication range 1.2-1.7 μm to longer wavelengths. This expansion suffers from a lack of properties of common materials. Silicate glasses are not transparent above 2.6 μm and fluoride or chalcogenide glasses exhibit low thermal stability that prevents high-power generation. Rare earth-doped Y2Ti2O7 ←← represents a suitable alternative for its high thermal stability low-phonon energy activating the luminescence transition up to 3 μm.
We present a CO2 laser-assisted preparation of nanocrystalline Ho-doped Y2Ti2O7 waveguides. The rib-planar waveguides were prepared by selective laser sintering of powders or by laser-induced crystallization of amorphous coatings. We studied the processing parameters, such a scanning speed, pulse repetition and average laser power, allowing to tailor the nanocrystals’ size within a range of 20 nm to 160 nm. The transparency and the luminescence properties of prepared waveguides were correlated with the structural properties of the nanocrystals. The processing parameters were modified to minimize the fiber attenuation caused by the Rayleigh scattering on the nanocrystals and to improve the luminescence properties above 2.5 μm.
The results present fundamental information about the laser processing of nanocrystalline rare earth-doped titanates. The promising application of prepared nanocrystals and prepared microstructures in the field of planar optical amplifiers and transparent ceramic optical fibers is widely discussed in the contribution. ACKNOWLEDGEMENTS: This work was supported by the Czech Science Foundation, contract No 22-17604S.
Open-source software, devices, and systems allow other researchers and investigators to replicate, develop and enhance the reported systems in the literature. This paper introduces an open-source system for optical fiber grinding and polishing, which facilitates and speeds up the polishing and grinding process. The polishing system is based on an Arduino microcontroller board, an open-source development board that utilizes an easy programming language enabling almost everyone to develop and write their codes and projects. In addition, the reported device targets low-cost fabrication allowing more researchers to fabricate their grinding-polishing machine. The device consists of the control circuits, stepper motor, breadboard, and a 3D printed desk where the polishing and grinding films should be attached. The device is controlled via 3 buttons: start/shut down button, speed up button and speed down button.
Focus of this paper is to build and investigate an optical fiber sensor based on silica nanofibers prepared by a reliable and low cost electrospinning technique to detect water content in DOT-4 brake fluid. For the best of our knowledge, this is a novelty study of optical fiber sensor to detect water content in an aqueous substance using electrospinning nanofibers. The proposed sensor has a sensitivity of 5.9 nW/1% of water change.
Self-made metal oxide MnO2 nanowires (NWs) were hydrothermally synthesized and electrically characterized. The nanowires were from ~ 3 to ~ 10 μm long and from ~ 20 to ~ 100 nm in diameter. The nanowires were suspended in water and deposited on gold interdigitated electrode (IDE) chip using dielectrophoresis (DEP) to align them perpendicularly across the electrodes. Screening of the MnO2 material properties, such as semiconductor type, vacancies concentration and relative permittivity was performed by means of impedance and Mott-Schottky analyses. The conductivity was measured both in synthetic air and in nitrogen ambient. The tests consisted in measuring resistivity of the NWs in relation to temperature of the bottom-placed heater under the IDE chip. The temperature went from room temperature up to 300°C. The resistivity changes were observed accounting for oxygen reduction on the NWs surface as the electrons were moving from the NWs to the oxygen. The resistivity was explored at a constant current arrangement test. Overall, we observed changes in the electrical properties of the MnO2 nanowires upon oxygen adsorption, such as activation energy. Therefore, in future experiments, we might also explore the effect of other gases on the NW’s electrical properties, e.g., ethanol, H2, NO2.
Pulse anodization of aluminum is a promising method to produce porous photonic crystals (PCs) with tunable photonic stopbands (PSBs) by a fully programmable change of electrochemical conditions. The spectral position of the PSBs is mainly determined by thickness and porosity of alternate layers forming the periodic structure. Temperature is one of the most important factors influencing the growth of porous anodic alumina (PAA). It alters the rate of PAA formation and the porosity of PAA layers. In this work, the effect of the anodizing temperature on the characteristic PSBs of graded-index PCs produced by sinusoidal pulse anodization (SPA) is analyzed. Two different anodization periods (tp) of the sinusoidal function were used whereas the amplitude and the number of cycles were kept constant. It was revealed that upon increasing the temperature from 5 to 30 °C the ion transport from electrolyte reservoir to the pore base becomes progressively hindered by increasing total PAA thickness which was manifested in the delayed current recovery to the initial values. As an effect, the periodic structures with inhomogeneous layers' thickness were formed providing broad and irregular PSBs in optical spectra. The quality of PSBs can be, however, significantly improved by post-processing pore broadening. Well-resolved and intensive peaks, which shift towards red part of the spectrum with increasing temperature and tp, could be then obtained. After a proper modification of the pulse sequence (e.g. application of the current density-controlled mode) the PCs can be used in optical sensing platforms operating from UV to NIR spectral range.
Graphitic carbon nitride (g-C3N4) is a 2D semiconducting nanomaterial with interesting thermal, mechanical, chemical and photochemical properties. It absorbs visible light due to the band gap energy of 2.7 eV. However, there are some problems associated with g-C3N4, such as fast recombination of photoinduced electrons and holes and low specific surface area. The problems can be solved by its doping with metal and non-metal elements. Especially, its doping with environmental-friendly non-metals, such as S, O, P, N, and C is preferred.
Doping of g-C3N4 with oxygen [1] and phosphorus [2] were studied recently. In this work, the carbon-rich g-C3N4 was prepared by calcination of the mixtures of i) melamine and hydroxyethyl cellulose and ii) melamine and glucose. The prepared materials were characterized by common methods and their photocatalytic activity was tested by the decomposition of Ofloxacin.
ACKNWLEDGEMENTS: This work was supported by Visegrad Fund (project No. 8F21007), EU structural funding in Operational Program Research, Development and Education (project No. CZ.02.1.01/0.0/0.0/16_019/0000853 „IET-ER“) and by VŠB-TU Ostrava (project No. SP 2022/61). The authors also thank the Large Research Infrastructure ENREGAT (project No. LM2018098).
REFERENCES: [1] Praus, P., Smýkalová, A., Foniok, K., Matějka, V., Kormunda, M., Smetana, B., Cvejn, D., The presence and effect of oxygen in graphitic carbon nitride synthetized in air and nitrogen atmosphere. Appl. Surf. Sci., 2020, 529, 147086. https://doi.org/10.1016/j.apsusc.2020.147086.; [2] Matějka, V., Škuta, R., Foniok, K., Novák, V., Cvejn, D., Martaus, A., Michalska, M., Praus, P., The role of the g-C3N4 precursor on the P doping using HCCP as a source of phosphorus. J.Mater. Res. Technol., 2022, 18, 3319-3335. https://doi.org/10.1016/j.jmrt.2022.04.019.
The mixed metal oxide Ti2Nb10O29 (TNO) is considered as a potential anode material for high-performance Li-ion storage devices owing to its excellent stability and safety. On the other hand, the lower ionic conductivity limited its practical applicability in energy storage devices. Here in this work, we systematically enhanced the conductivity and cyclability of TNO by the addition of glucose-derived carbon to the structure. This TNO@C nanoarchitecture enables high Li+ ion transportation and improved electrochemical performance. The nanosized TNO@C with interconnected pores on the structure reduce the Li+ diffusion distance and enhances the reaction kinetics and promises the participation of the inner active sites during the energy storage. Besides, the coated carbon layers on the surface of TNO help to increase the electric conductivity, which supports fast Li+-ion transfer and improves the rate performance. Consequently, the TNO@C anode possesses an excellent reversible capacity and long-term cycle stability. Moreover, here the optimized strategy to combine the carbon layer on the TNO structure paves a new insight into the structural modification of TNO-based anode.
Carbon materials constitute the major group of materials used for decades in heterogeneous catalysis as catalyst supports. The variability of the nanostructure of carbon materials results in a uniquely wide range of their physical and chemical properties. In this work we study how the nanostructure affects thermal stability of 2D carbon materials (graphene, oxidized graphene and multilayer graphene), 3D graphene analogues (fullerene, nanotubes and zeolite-templated carbon), ordered 3D carbons (CMK-3, 3DOmm) and traditional carbon materials (amorphous carbon and graphite). The combination of structural, thermogravimetric and calorimetric analysis under identical conditions for all carbon nanomaterials showed how thermal stability increases with the regularity of the graphene structure and layering of graphene sheets and decreases with the number of oxygen-containing functional groups and the unsaturated carbon atoms at the defects and the edges. The relationships between the type of carbon structure and stability provide information for targeting the nanostructure of carbon materials with respect to their stability in catalysis.
ACKNOWLEDGEMENTS: This work was supported by the Czech Science Foundation under Project No. 21-07753S. The authors acknowledge the assistance provided by the Research Infrastructure NanoEnviCz (Project No. LM2018124), supported by the Ministry of Education, Youth and Sports of the Czech Republic and The European Union - European Structural and Investments Funds in the frame of Operational Programme Research Development and Education.
The semiconductor nanorod-based structures have attracted significant research interest as prospective building blocks of novel nano-electronic devices. In this study, we investigated the electrical properties of the interface between individual ZnO nanorods and conductive electrodes. We demonstrate that a stable ohmic contact can be prepared between ZnO nanorods and graphene by eliminating polymer residues on the graphene surface by local Joule heating. To characterize the interface between individual ZnO nanorods and graphene, we developed a technique using a nanomanipulator in the chamber of the scanning electron microscope. This technique allows us (a) to pick a nanorod of choice from the array, (b) to rotate it so that the junction is formed by the top facet of the as-grown nanorod, and (c) to select the area of graphene without defects. In contrast to conventional approaches, our method eliminates the extensive processing chemistry and allows us to observe structural changes in-situ when performing the electrical characterization.
In order to boost specific quality and organoleptic features, food colorants have been extensively employed in the food processing industry, medicine, and cosmetics manufacture as one of the most significant food additives. However, their intense usage and consumption has been linked to a variety of adverse consequences, including: short-, medium- and long-term toxicity, allergic responses, respiratory affections, behavioural, and neurocognitive impacts on children, as well as their possible carcinogenic effects. In this general context, enhancing food safety and quality is essential for human welfare, and the intensive use of Sunset Yellow in food sector is correlated with the need for developing trustworthy, accurate, sensitive, and selective analytical tools for its identification and quantification. Carbon-based materials are currently at the forefront of materials research due to their outstanding physical, mechanical and electrical properties and exceptional catalytic/electrocatalytic activity. The ability to be dispersed in various polymer matrix leads to a new class of polymer nanocomposites with a wide range of applicability (e.g. food packaging, biosensors, water treatment or drug delivery). The main goal of this study was to provide a facile, rapid, inexpensive way for the green, one-step and large-scale preparation of chitosan / carbon base nanomaterial, trough electrochemical exfoliation of graphite rods, without the use of any organic solvent. The obtained nanocomposite was characterized from morphological and structural point of view. Furthermore, the applicability in environmental analysis of chitosan/carbon based-glassy carbon modified electrodes for accurate detection and quantification of Sunset Yellow was also reported.
Constructing vertical 2D heterostructured materials has attracted the attention of scientists due to their properties, attractive for electronic and photonic applications. Interfaces and interactions between the heterostructured layers have a critical influence on the performance of the applications. Graphene and h-BN, thanks to their good lattice match, are ideal candidates for such structures. The scope of this study is the preparation of Graphene layers through the segregation of carbon from the Pt substrate after synthesizing the h-BN overlayer.
The first step is introducing C2H4 gas inside the ultrahigh vacuum (UHV) SEM chamber, where the sample is heated up to 900˚C. The gas decomposes and forms the Graphene layer on the Pt wire substrate. Next, the temperature of the sample is increased up to 1000˚C, which results in dissolving the entire Graphene layer inside the platinum bulk. To synthesize the h-BN layer on the surface, the Ammonia Borane powder is heated to 90˚C in a separate container and introduced into the chamber. The evaporation products form a 2D hexagonal Boron Nitride layer over the platinum with dissolved carbon. After the completion of the h-BN layer on the surface, the sample’s temperature is decreased slowly down to 850˚C, resulting in carbon segregation to form the Graphene layer(s), and then the sample is quenched rapidly. We will present this process in a series of movies, which is possible thanks to the SEM observation during the growth and segregation.
Coal can be used as a natural source of carbon nanoparticles. Isolation of nanoparticles from coal is usually considered as a more economic and greener way than synthetic approaches. In this contribution we report our first results on using the lignite mined in the region of South Moravia (Czech Republic) for isolation of carbon nanoparticles. This lignite, or young brown coal, is rather specific, belongs to the class of ortho-lignite and contains relatively high amount of organic matter. Lignite was disintegrated by a combination of physical and chemical processes - by the action of ultrasound, by oxidative treatment using hydrogen peroxide, and heating. Isolated particles were characterized for their size, chemical composition using a set of spectroscopic techniques, and by electron microscopy. South Moravia lignite can be used as a source of carbon nanoparticles, but further research is necessary and should be focused primarily on purification of the isolated material and on increasing its yield.
Unlike graphene, its fully fluorinated derivative, fluorographene, is a wide-gap semiconductor/insulator [1], making the material possibly useful for power electronics, high-temperature electronics, and short-wavelength optical applications. Despite the promising applications of FG, the fundamental issue of the size of its bandgap has not been resolved yet due to the striking contrast between experimental observations and numerous ab-initio band structure calculations [2]. Here, we resolve this long-standing issue by demonstrating a direct optical band gap at 5.75 eV through diffuse reflectance spectroscopy measurements of graphite fluoride in the UV-VIS range. This coincided with ab-initio calculations‘ results based on the finite momentum Bethe-Salpeter equation by showing an excitonic peak at 5.65 eV of a Frenkel exciton bound to a single atom. The position of the first exciton peak was found to be independent of the number of fluorographene layers [3]. We also found that the absorption bands at lower energies arise from fluorine vacancies present in the material. The higher the content of such fluorine vacancies, the narrower the band gap. Since more intense UV-VIS absorption bands were recorded at lower energies ranging between 2.8 and 4.8 eV in material with lower fluorine content, we thus explain the former discrepancies between the experimental findings and theory on the fluorographene bandgap.
REFERENCES: [1] Small 2010, 6, No. 24, 2877-2884.; [2] Appl. Mat. Today 9 (2017) 60-70.; [3] Ann. Phys. 526, No. 9-10, 408-414 (2014).
Diamond is recognized as one of the most promising wide bandgap materials for advanced electronic applications. However, for many practical uses, hybrid diamond growth combining metal electrodes is often demanded. Here, we present the influence of thin metal (Ni, Ir, Au) and metal oxide (NiO, IrO) layers on diamond growth by microwave plasma chemical vapor deposition (MWCVD) employing two different concepts. In the first concept, a flat substrate (GaN or Si) was initially coated with a thin metal layer, then exposed to the diamond MWCVD process. In the second concept, the thin diamond film was firstly formed, then it was overcoated with the metal layer and finally, once again exposed to the diamond MWCVD. It should be mentioned that this concept allows the “implementation” of the metal electrode into the diamond bulk. It was confirmed that the Ni or NiO thin films (15 nm) hinder the formation of diamond crystals resulting in the formation of an amorphous carbon layer. Contrary to this finding, Ir and IrO layers resulted in a successful overgrowth by the fully closed diamond film. However, by employing concept 2 (i.e. hybrid diamond/metal/diamond composite), the thin Ir layer was found to be unstable and transferred into the isolated clusters, which were overgrown by the diamond film. Using the Au/Ir (30/15 nm) bilayer system stabilized the metallization and no diamond growth was observed on the metal layer. We believe that the presented results will contribute to the further development of built-in and/or buried electrodes in diamond-based radiation detectors.
MXenes are synthesized by etching MAX phase precursors to produce multilayer MXenes, where individual 2D sheets are held together by van der Waals forces. Single 2D flakes of MXene are produced by chemical intercalation to delaminate multilayer MXenes, affecting the properties of MXenes.This work demonstrates an alternative approach, shearing multilayer MXenes with a three-roll mill to produce single-layer and few-layer Ti3C2Tx flakes without chemical intercalant and its use in supercapacitors.
Recently, a simple and effective method of the n-type doping of ZnO thin films using methane as the dopant precursor for RF-magnetron sputtering deposition has been suggested. It has been demonstrated that the main doping agent was hydrogen released by methane decomposition, but the doping efficiency of methane appeared to be significantly higher than that of pure molecular hydrogen. It has also been shown that the electrical conductivity of the ZnO film depends on its thickness. However, the reasons for the thickness dependence have not been discussed. Therefore, in this contribution, we present a detailed analysis of the correlation of the thickness-dependent morphological peculiarities of the ZnO films and their electronic properties. The analysis has focused on the nanoscale characterization of the growth morphology and of the grain boundaries by scanning electron microscopy (SEM), scanning probe microscopy (AFM, KFFM, SSRM, SCM), and optical spectroscopies (spectral ellipsometry, Raman scattering spectroscopy). Two types of inhomogeneity of electronic properties have been identified: (a) inhomogeneity over the film thickness and (b) inhomogeneity related to the grain boundaries on the film surface. The doping decreased with increasing film thickness and was enhanced at the grain boundaries. Both these effects are preliminarily ascribed to the inhomogeneous concentration of hydrogen, which is related to its diffusion limits.
Light-emitting negatively-charged silicon vacancy (SiV) centers can be formed at high densities in diamond thin films using the chemical vapor deposition process. Diamond thin films can be relatively cheaply fabricated on a large scale, which might be applicable in photonics or biosensing. However, the practical usage of the films is limited due to their polycrystalline nature causing the strain in the material and also the unwanted background photoluminescence (PL). To suppress the strain and the background PL, we investigated the influence of deposition process parameters on the optical properties of diamond thin films grown on silica substrates. Namely, we systematically increased the amount of CH4 from 1 to 15 % to H2 and studied how it influences the chemical, structural and photoluminescence properties of the grown films. Raman spectra showed that by increasing the amount of CH4 gas, the diamond Raman peak of the films shifts towards the values typical for the monocrystalline diamond, which suggests the strain relaxation in the material. This relaxation effect corroborate with PL where the redshift of the SiV zero-phonon-line towards the values measured in the monocrystalline diamond. The highest (5-fold) value for the ratio of the PL intensity from SiV centers and the background PL is found for the samples grown at 4% CH4.
Diamond, due to its wide range of outstanding properties, is recognized as a promising material for numerous applications. However, the diamond film deposition usually has to be tailored to specific needs. For example, in the case of optically transparent and thermally stable quartz glass or fused silica, the risk of the thick diamond layer delamination due to the difference in thermal expansion coefficients and the residual stresses occurrence must be considered.
In this work, we investigate the low temperature (< 500 °C) diamond film deposition on fused silica realized in two different focused microwave plasma systems, i.e. a multimode clamshell cavity (MCC) and an ellipsoidal cavity reactor. During the experiments, the methane to hydrogen ratio in the process gas mixture varied from 1 % to 15 %.
For both deposition systems, the methane increase led to an increase in the diamond growth rate, e.g. it raised by a factor 3 for the MCC system. Moreover, the MCC reactor resulted in the growth of well-shaped diamond crystals at a methane to hydrogen ratio as high as 15%. This finding is in good agreement with Raman shift measurements, which also revealed a low content of non-diamond phases in the films. The enhanced diamond growth rate at low temperatures is advantageous for overcoating not only fused silica but also for thermally sensitive substrates, e.g. optical elements, photonic crystals, sensors, etc.
The progress in many fields is associated with an existence of high-power lasers operating in the infrared range 2 -3 μm. This spectral range is used for an eye-safe laser machining, navigating and tracking systems, etc. However, widely used silica glass materials exhibit limited spectral transparency in this spectral region while chalcogenide glasses with good transparency in infrared exhibit low thermal stability which limits the laser emitted power. Transparent ceramics represent a promising alternative to the common laser glasses for high thermal stability and high transparency in infrared. The research on new ceramic phosphors emitting in the infrared region is a challenge that we address in this contribution.
We present a versatile approach to nanocrystalline coatings of Eu or Ho-doped La2Ti2O7 crystallizing in a pyrochlore crystal structure. Amorphous transparent coatings were prepared by sol-gel method followed by thermal treatment to induce the formation of nanocrystals with tailored structural properties. We studied the effect of the nanocrystal morphology on the optical properties with special scope on the luminescence at 2.1 and 2.9 μm. The methodical approach was optimized to minimize the scattering optical losses and to improve the luminescence properties which correlates to the crystallinity of the coatings. We tested selected coatings as the active medium in a solid-state laser setup and we evaluated the laser characteristics.
The proposed materials represent the thermally and chemically stable alternative to conventional silica or chalcogenide glasses allowing preparation of planar solid-state lasers operating in harsh environment.
We investigated the effect of spin coating parameters on the electrical properties of Ti3C2 MXene thin films deposited from non-aqueous suspensions in N,N-dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP) on gold interdigitated electrodes (IDE). The electrical properties of DMF-MXenes and NMP-MXenes films are characterized by impedance spectroscopy (4 Hz - 8 MHz at 1 V) using gold IDE with 25 m gap. The electrical conductivity of MXene films decreases with increasing spin coating speed from 300 to 900 rpm. The series resistance (Rs) and double layer capacitance remain similar (Cdl). In all cases, MXenes deposited from DMF have five orders of magnitude higher electrical conductivity (lower Rct) than MXene films deposited from NMP. It is correlated with the thin film morphology obtained by scanning electron microscopy (SEM). These findings can be useful for possible application of MXenes as charge transport layers in hybrid photovoltaic devices.