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Bratislava, Slovakia

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.

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.

Gucmann F.,Slovak Academy of Sciences | Kudela R.,Slovak Academy of Sciences | Kordos P.,Slovak Academy of Sciences | Dobrocka E.,Slovak Academy of Sciences | And 5 more authors.
Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics | Year: 2015

III-As heterostructure field-effect transistors (HFETs) and metal-oxide-semiconductor HFETs with gate electrodes insulated by an amorphous layer of ex-situ-prepared mixture of Ga and As oxides are studied. Gate insulator was prepared by O2 plasma oxidation of undoped GaAs cap layer of epitaxially grown transistor structures in standard plasma unit commonly used for photoresist ashing. GaAs cap is gradually consumed by the oxidation turning it into Ga and As oxides and causing bottom surface of the oxide moving closer to the two-dimensional electron gas. Gate electrode "recessing" is a positive byproduct of the process. Expectedly, impact on HFETs' threshold voltage (Vth) was observed and shift from -2.17 to -1.15 V was achieved. X-ray reflectivity confirmed much higher oxidation tendency for N-type GaAs than for undoped GaAs with this oxidation technique. Strong Vth shift can be most likely attributed to negative oxide charge in plasma-grown oxide or its interface with GaAs. Excluding Vth shift, negligible impact of O2 plasma on electrical characteristics was observed. Negligible Fermi level (EF) pinning inherited from the oxidation process can be concluded. Such O2 plasma-grown oxides might serve as an efficient seeding layer for subsequent high-κ gate dielectric growth. The authors believe this method might help to create a high-quality interface reducing number of Fermi level-pinning traps induced by other ex-situ deposition techniques while providing fine control over Vth as well. © 2015 American Vacuum Society.

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.

Lobato B.,CSIC - National Coal Institute | Vretenar V.,R.O.S.A. | Vretenar V.,Center for Nanodiagnostics | Kotrusz P.,R.O.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.

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