Bratislava, Slovakia
Bratislava, Slovakia

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Varga M.,ASCR Institute of Physics Prague | Izak T.,ASCR Institute of Physics Prague | Vretenar V.,R.Ø.S.A. | Vretenar V.,Center for Nanodiagnostics | And 10 more authors.
Carbon | Year: 2017

As long as the carbon family materials are pristine, powerful methods for their individual characterization have been established and well understood. However, as soon as these materials contain a mixture of sp2 and sp3 hybridized carbons, the spectroscopic characterization becomes challenging. In this paper we present on some aspects of multiply spectroscopic characterizations of sp2-sp3 carbon heterosystems, which can play an important role for their correct structural assignment. We study a composite of single-wall carbon nanotubes (SWCNT) with diamond nanoparticles and a SWCNT paper coated with nanocrystalline diamond films. The SWCNT paper serves here as a reference. We evaluate the surface free energy measured by contact angle technique, morphology imaged by scanning electron microscopy and the chemical composition determined by X-ray photoelectron spectroscopy, Raman spectroscopy at different excitation wavelengths and by attenuated total reflection (ATR) Fourier Transform Infrared spectroscopy (FTIR) using different ZnSe ATR prisms. We demonstrate that, because of the resonant nature of Raman process in SWCNTs, the particular conditions at Raman measurement (laser excitation wavelength, laser intensity) strongly influence the measured spectra of the sp2-sp3 hybridized heterosystems. We also show how the dimensions of the ATR prism affect the results of the FTIR measurements of the SWCNT papers. © 2016 Elsevier Ltd


Ivanco J.,Slovak Academy of Sciences | Halahovets Y.,Slovak Academy of Sciences | Vegso K.,Slovak Academy of Sciences | Klackova I.,Slovak University of Technology in Bratislava | And 7 more authors.
Thin Solid Films | Year: 2016

The strain-resistance transduction of commercially available In2O3/Au/Ag layered films supported by polyethylene terephthalate (PET) foil was studied. A colossal gauge factor as high as about 60,000 was observed upon a strain of 1%. Such ultra-high response is presumably owing to conductivity derived from the tunnelling current across cracks formed in the strained In2O3 film; the cracks – as visualized by scanning electron microscopy – are oriented perpendicularly to the strain vector and are noted for their uniform width across the sample. It is further demonstrated that the standardly defined gauge factor is incommensurate for strain sensors with transduction based on the tunnelling current and a redefined formula for the gauge factor was proposed. © 2016 Elsevier B.V.


Ivanco J.,Slovak Academy of Sciences | Vegso K.,Slovak Academy of Sciences | Siffalovic P.,Slovak Academy of Sciences | Kostiuk D.,Slovak Academy of Sciences | And 5 more authors.
Key Engineering Materials | Year: 2015

To examine perspectives of nanoparticle films in the role of active elements in strain sensors, morphological and electrical properties of self-assembled Au nanoparticle monolayer prepared by modified Langmuir-Schaefer technique onto supporting Mylar foil were studied under elongation. Along the probing of electrical response (characterized by the gauge factor of about 60), the small-angle x-ray scattering (SAXS) characterization assessed an average interparticle distance change, which was shown to vary proportionally to the substrate elongation. The approach allowed to unambiguously address the mechanism of the deformation-resistivity transduction. © 2015 Trans Tech Publications, Switzerland.


Vegso K.,Slovak Academy of Sciences | Jergel M.,Slovak Academy of Sciences | Siffalovic P.,Slovak Academy of Sciences | Kotlar M.,Slovak Academy of Sciences | And 5 more authors.
Sensors and Actuators, A: Physical | Year: 2016

A strain gauge based on a monolayer of colloidal gold nanoparticles deposited on a flexible Mylar foil by a modified Langmuir-Schaefer method was tested in situ under external uniaxial stress by the small-angle X-ray scattering (SAXS) technique. Simultaneously, the stress-strain curve of the foil was measured. A high-flux laboratory X-ray source allowed fast data collection with 10 s temporal resolution. The monolayer exhibits a linear response in terms of the interparticle distances up to the 13% substrate strain while keeping its full integrity. This result indicates the dominant role of the pair potential function between the nanoparticle surfactant molecules and molecules of the substrate and compares well with previous quasi-static measurements of a similar strain gauge, suggesting none or negligible effect of the transient strain phenomena longer than the sampling interval on the gauge response. Keeping a constant strain on finishing the stretching, fast transient effects with characteristic times down to the limit imposed by the X-ray detector time resolution were not observed either during the SAXS pattern collection. A different stress behavior of the interparticle distance in the direction perpendicular to stretching comparing with a monolayer of colloidal iron-oxide nanoparticles studied previously reveals the surfactant effect on the gauge response controlled by the interparticle pair potential function. The results obtained suggest that the colloidal gold-nanoparticle monolayer on a flexible substrate is a prospective strain gauge with a very fast linear response in a broad strain range. © 2016 Elsevier B.V. All rights reserved.


Stehlik S.,ASCR Institute of Physics Prague | Varga M.,ASCR Institute of Physics Prague | Ledinsky M.,ASCR Institute of Physics Prague | Jirasek V.,ASCR Institute of Physics Prague | And 12 more authors.
Journal of Physical Chemistry C | Year: 2015

High-pressure high-temperature (HPHT) nanodiamonds originate from grinding of diamond microcrystals obtained by HPHT synthesis. Here we report on a simple two-step approach to obtain as small as 1.1 nm HPHT nanodiamonds of excellent purity and crystallinity, which are among the smallest artificially prepared nanodiamonds ever shown and characterized. Moreover we provide experimental evidence of diamond stability down to 1 nm. Controlled annealing at 450 °C in air leads to efficient purification from the nondiamond carbon (shells and dots), as evidenced by X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and scanning transmission electron microscopy. Annealing at 500 °C promotes, besides of purification, also size reduction of nanodiamonds down to ∼1 nm. Comparably short (1 h) centrifugation of the nanodiamonds aqueous colloidal solution ensures separation of the sub-10 nm fraction. Calculations show that an asymmetry of Raman diamond peak of sub-10 nm HPHT nanodiamonds can be well explained by modified phonon confinement model when the actual particle size distribution is taken into account. In contrast, larger Raman peak asymmetry commonly observed in Raman spectra of detonation nanodiamonds is mainly attributed to defects rather than to the phonon confinement. Thus, the obtained characteristics reflect high material quality including nanoscale effects in sub-10 nm HPHT nanodiamonds prepared by the presented method. © 2015 American Chemical Society.


Schmiedova V.,Brno University of Technology | Pospisil J.,Brno University of Technology | Zmeskal O.,Brno University of Technology | Vretenar V.,Center for nanodiagnostics
Materials Science Forum | Year: 2016

The paper deals with the study of the optical properties of graphene oxide (GO) by inkjet printing. Defined structure of GO can be obtained by reduction of prepared layers either by heating or by UV radiation (rGO). The dispersion function for the refractive index and extinction coefficient of GO and both rGO thin films were measured by spectroscopic ellipsometry in the wavelength range of 200 – 850 nm. Spectroscopic ellipsometry (SE) was used to characterize the optical response of a layer of GO reduced by UV and thermal reduction GO in the visible range. © 2016 Trans Tech Publications, Switzerland.


Lobato B.,CSIC - National Coal Institute | Vretenar V.,R.Ø.S.A. | Vretenar V.,Center for Nanodiagnostics | Kotrusz P.,R.Ø.S.A. | And 3 more authors.
Journal of Colloid and Interface Science | Year: 2015

The current energy needs have put the focus on highly efficient energy storage systems such as supercapacitors. At present, much attention focuses on graphene-like materials as promising supercapacitor electrodes.Here we show that reduced graphite oxide offers a very interesting potential. Materials obtained by oxidation of natural graphite and subsequent sonication and reduction by hydrazine achieve specific capacitances as high as 170F/g in H2SO4 and 84F/g in (C2H5)4NBF4/acetonitrile. Although the particle size of the raw graphite has no significant effect on the physico-chemical characteristics of the reduced materials, that exfoliated from smaller particles (<75μm) result more advantageous for the release of the stored electrical energy. This effect is particularly evident in the aqueous electrolyte.Graphene-like materials may suffer from a drop in their specific surface area upon fabrication of electrodes with features of the existing commercial devices. This should be taken into account for a reliable interpretation of their performance in supercapacitors. © 2015 Elsevier Inc.


Sojkova M.,Slovak Academy of Sciences | Strbik V.,Slovak Academy of Sciences | Chromik S.,Slovak Academy of Sciences | Liday J.,Slovak University of Technology in Bratislava | And 5 more authors.
Vacuum | Year: 2015

Abstract Thin films of thallium based superconductors were prepared successfully using sputtering target fabricated as a mixture of barium fluoride, calcium fluoride and copper oxide. Such a target is stable, does not degrade and does not involve any special handling (vacuum or argon atmosphere storage). Precursor films sputtered from this target contained less than 5 at.% of fluorine so no fluorine content reduction is needed. Depending on the substrate, film thickness and synthesis conditions, Tl-2212 phase or a mixture of Tl-2212 and Tl-2223 phases was prepared. Although Tl-2212 phase has epitaxial character, Tl-2223 phase, growing on the top of Tl-2212, is only c-axis oriented. The highest values of TC0 were obtained using 300 nm thick precursor films thallinated at 860 C. After the thallination the films contained a mixture of Tl-2212 and Tl-2223 phase with a critical temperature of TC0 = 106 K and a critical current density (at 77 K) up to 6 × 104 A/cm2. Lower TC0 and JC values suggest that further optimization of the film synthesis is needed; however, the first results show that the stable sputtering target prepared from fluorides is a good candidate for high quality Tl-based thin film fabrication. © 2015 Elsevier Ltd.


Ivanco J.,Slovak Academy of Sciences | Halahovets Y.,Slovak Academy of Sciences | Vegso K.,Slovak Academy of Sciences | Klackova I.,Slovak University of Technology in Bratislava | And 6 more authors.
IOP Conference Series: Materials Science and Engineering | Year: 2016

The resistance of indium-oxide covered polyethylene terephthalate foils (IO-PET) shows an extreme sensitivity to tensile strain. In terms of the deformation-resistance transduction, the gauge factor as high as about 60 000 was recorded upon the relative elongation up to 1%. Except the onset of deformation, the nearly exponential dependence of the resistance on strain suggests that the conductivity of the strained films is governed by tunnelling mechanism; this notion is supported by the formation of scattered cracks in the IO- PET film. The cracks are oriented perpendicularly to the strain vector and are characterized by a rather similar and uniform width. Appropriateness of the standard definition of the gauge factor for strain sensors, which are governed by tunnelling conductance, is critically discussed. © Published under licence by IOP Publishing Ltd.


Kim Y.,Pohang University of Science and Technology | Kim Y.,Max Planck Institute For Festkoperforschung | Lee D.S.,Korea Institute of Science and Technology | Jung S.,Korea Research Institute of Standards and Science | And 6 more authors.
Nano Letters | Year: 2015

We have investigated fractional quantum Hall (QH) states in Bernal-stacked bilayer graphene using transconductance fluctuation measurements. A variety of odd-denominator fractional QH states with νQH → νQH + 2 symmetry, as previously reported, are observed. However, surprising is that also particle-hole symmetric states are clearly resolved in the same measurement set. We attribute their emergence to the reversal of orbital states in the octet level scheme induced by a strong local charge imbalance, which can be captured by the transconductance fluctuations. Also the even-denominator fractional QH state at filling -1/2 is observed. However, contrary to a previous study on a suspended graphene layer [ Ki et al. Nano Lett. 2014, 14, 2135 ], the particle-hole symmetric state at filling 1/2 is detected as well. These observations suggest that the stability of both odd and even denominator fractional QH states is very sensitive to local transverse electric fields in bilayer graphene. © 2015 American Chemical Society.

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