Boron-doped diamond exhibits a characteristic S-shaped I-V curve at room temperature with two electrical conductivity states, i.e., low and high conductivity, at high electric fields (50 - 250 kV.cm-1) due to the carrier freeze-out and impurity impact ionization avalanche effect [1]. To our knowledge, the carrier multiplication during the change of the conductivity state has not been studied yet. In this article, we investigate theoretically the effect of acceptor concentration and compensation level on the carrier multiplication coefficient at room temperature to determine the optimal dopants concentration of maximum carrier multiplication. The hole concentration of boron-doped diamond has been calculated for various acceptor concentration and compensation ratio by solving numerically the charge neutrality equation within the Boltzmann approximation of the Fermi-Dirac statistic. These values were used to determine theoretical carrier concentration multiplication coefficients at room temperature as a function of the acceptor concentration and compensation ratio. The calculated multiplication coefficient is maximum for an acceptor concentration of ca. 2 1018 cm-3 and it increases significantly with the compensation above 0.2 % to values well above 1000. Determined theoretical values are consistent with the ratio of the carrier concentration at room temperature and the acceptor concentration available in the literature as well as the current multiplication observed in boron-doped diamond due to impurity impact ionization avalanche [1].
ACKNOWLEDGEMENTS: The financial support from the following sources is gratefully acknowledged: the Czech Science Foundation grant ID: GACR 17-05259 S and J. E. Purkyne fellowship awarded to V. Mortet by Academy of Sciences of the Czech Republic.
REFERENCE: [1] V. Mortet, D. Trémouilles, J. Bulíř, P. Hubík, L. Heller, E. Bedel-Pereira, A. Soltani, Peculiarities of high electric field conduction in p-type diamond, Applied Physics Letters. 108 (2016) 152106. doi:10.1063/1.4946853.
Silicon nitride find its application in quality of high-temperature dielectrics. Different properties and features of microstructure of pressureless sintered silicon nitride have been described a lot. The effect of various oxide additives and sintering regimes has been also shown and discussed often. Obviously, the microstructure is one of the key factors affecting the mechanical properties of silicon nitride. In particular, β-silicon nitride is characterized by a higher fracture toughness, due to the ability to stop the propagation of cracks by elongated grains. Thus, it is obvious that regulation of the rate of phase transformation, investigation of mechanical properties and microstructure, depending on the type of additives are very important at the moment. Rare earth oxides are usually used as additives. But the high cost is the main disadvantage of rare earth oxides. Influence of such oxides as La2O3 and Gd2O3 has recently been of great interest. In particular, the separate and combined effect of La2O3 and Gd2O3 with Al2O3 on microhardness, oxidation resistance, microstructure and phase composition of silicon nitride have already been analyzed [1]. In particular, it has been shown that Gd2O3 leads to an intensification of the phase transformation and formation of elongated grains of silicon nitride, an increase in crack resistance, while La2O3 led to an increase in microhardness [1]. In our work morphology of the starting powder, particle size of powders both silicon nitride and such oxide additives as La2O3,Y2o3, Al2O3, MgO and Gd2O3, are demonstrated and characterized.
REFERENCES: [1] C. C. Guedes-Silva, F. Machado de Souza Carvalho, J. Carlos Bressiani "Effect of rare-earth oxides on properties of silicon nitride obtained by normal sintering and sinter-HIP", Journal of rare earths, vol. 30, 2012, pp. 1177-1183.
In this work, we present photoluminescence (PL) properties of Si-V colour centres in diamond and zinc oxide, which are perspective for life science and photonic applications. The nanocrystalline diamond films were grown by microwave plasma enhanced CVD system, while ZnO nanocrystals were grown by magnetron sputtering. Temperature dependent steady-state PL of Si-V centres was studied within the range 11-300 K. PL properties are correlated with deposition process conditions, namely substrate material, and the substrate temperature (350-1100°C). Quartz or Si substrates and substrate temperature of 800°C were found to be optimal for achievement of the highest yield of Si-V centre photoluminescence. For all the diamond samples, the temperature dependent PL spectra exhibited the blue shift in zero-phonon line (ZPL) position with decreasing temperature. This effect is discussed from the point of view of temperature behaviour of Si-V electronic transition energy. For selected samples, ZPL narrowing were analyzed as well. Temperature development of PL integral intensity driven by Boltzmann activated process indicates the contribution of other centres in luminescence mechanism. Finally, comparison and outlooks of PL properties of diamond and ZnO thin films will be presented. This work was supported by the CSF project 16-10429J.
The antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure is a promising material for the AFM spintronics. The resistivity measurements indicate the presence of defects
about whose types and concentrations is more speculated as known. We confirmed vacancies on Mn or Cu sublattices and Mn on Cu and Cu on Mn antisites as most probable defects in CuMnAs by ab-initio total energy calculations.
We have estimated resistivities of possible defect types as well as resistivities of samples for which the X-ray structural analysis is available. In the latter case we have found that samples with Cu- and Mn-vacancies with low formation energies have also resistivities which agree well with the experiment. Finally, we have also calculated exchange interactions and estimated the Néel temperatures by using the Monte Carlo approach. A good agreement with experiment was obtained.
Mechanical alloying is a powder metallurgy processing technique involving cold welding, fracturing, and rewilding of powder particle in a high energy. It has been used to obtain nano-crystalline alloy. Fe 53%atm. Si
28%atm B19% atm. alloys were synthesized using a planetary ball mill (Retsch PM400). X-ray diffraction was used to identify and characterize various phase during the milling process. We used the hyper frequence and the electromagnetic methods, to characterize the variation of structure and their influence. The shape of hyperfine field distributions used is discussed. It is shown that the FeSi solid solution was formed after 4 hours milling. The study state grain size is about 17 nm.
The first-principles Projector Augmented Wave method (PAW) is used to investigate the electronic, phonon band structure and dielectric properties of four bulk silicon carbide (SiC) polytypes. We employ PAW pseudopotential density functional theory with PBE and hybrid HSE06 approximations of the exchange-correlation functional. Many body effects are incorporated using the GW approximation of the self-interaction to study SiC properties. GW method in its single-shot variant, and which is based on the many body perturbation theory (MBPT), is used to calculate the quasi-particle (QP) energies of the band structure and the dielectric properties for different polytypes. The electronic band structure determination within GW method uses the Wannier procedure where a basis set of maximally localized Wannier functions (MLWF) is constructed to interpolate the QP energies of few regular mesh k-points to the high symmetry lines in Brillouin zone. As a consequence of QP correction to the Kohn-Sham energies band gap is increased by up to 3 eV in case of 4H-SiC, as compared to PBE band gap. GW results are comparable to those of hybrid functionals and are in good agreement with experimental results. The optical properties are then studied within PBE, HSE06 and including many body effects. In addition, phonon band structure has been investigated within HSE06 and compared to previous PBE results. We found good agreement with theoretical and experimental available data.
The formation process of BaTiO3 nanocrystals in a water vapor medium (under subcritical conditions) and in supercritical water has been studied. The synthesis of BaTiO3 was implemented using BaO and TiO2 as the initial substances at a temperature from a range of 130-400°C and a pressure of 0.27-26.00 MPa for 0 to 20 hours. From the kinetic data obtained with XRD analysis of the synthesized samples and also from the study of the crystal structure by TEM, the conversion of starting oxide mixture in a water vapor atmosphere at 130-150°C and 0.27-0.48 MPa occurred via a solid-phase mechanism. The process included dissociative adsorption of H2O molecules on the TiO2 surface accompanied by breakage of Ti-O bonds and by the increase of solid-state mobility. This facilitated the diffusion of Ba2+ ions into TiO2 matrix and its restructuring to perovskite. Higher chemical activity of H2O molecules in the supercritical region (400°C, 26.00 MPa) allowed the complete conversion of the equimolar mixture of reagents into BaTiO3, whereas in the subcritical medium this is possible at a Ba:Ti>1 molar ratio. The considered process synthesis underlies the energy-saving and environmentally friendly way of obtaining BaTiO3 nanopowder for ceramic production in the microelectronic industry [Patent № RU2637907].
ACKNOWLEDGEMENTS: The work was carried out with the financial support of The Ministry of Education and Science of the Russian Federation within the framework of the State Task № 10.2729.2017 from May, 31 2017 (Number for publications: 10.2729.2017/4.6.).
Graphene is unarguably very promising material for countless applications including also gas sensing. For this particular application, it is very beneficial to understand the nature and strength of interaction of graphene with the adsorbed molecules. Molecular dynamics (MD) simulations matured to a very useful technique providing insights into complex molecular systems on the atomistic scale. However, the prediction potential of classical MD is given by quality of used parameters, including parameters for Lennard-Jones (LJ) term describing dispersion and repulsion part of intermolecular interaction potential. So far, LJ parameters used for graphene have not been thoroughly tested. Using MD we quantified the adsorption enthalpies of several organic volatile molecules (dichloromethane, nitromethane, ethanol, acetone, acetonitrile, ethyl acetate, hexane, cyclohexane, dioxane, benzene, and toluene) to graphene surface. The calculated adsorption enthalpies were compared with those acquired experimentally using an inverse gas chromatography technique. For each molecule, seven set of simulations were performed with altered LJ parameters and the accuracy of derived adsorption enthalpies was assessed. The average error of Δ⟨HFF⟩ in respect to ΔHexp suggests that OPLS-AA parameters delivered the best agreement with experiments. Needless to say that all tested LJ parameters were suitable for semi-quantitative estimates of the interaction energies of the molecules with the graphene. This implies that MD simulation can provide correct order of adsorption enthalpies of adsorbates and OPLS-AA in addition provides reliable quantitative estimates.
Since its first isolation, graphene have attracted a particular attention due to its extraordinary physical properties. Possible aplications in electronic and spintronic devices are limited by the missing band gap and the zero density of states at the Fermi level and by diamagnetic nature of graphene.Several strategies have been proposed and reported to develop magnetically active graphene-based material with spin-polarized conductive behavior including presence of vacancies or line defects, covalent functionalization, spatial restrictions and doping by heteroatoms. Cutting graphene sheet to create graphene nanoplatelets or graphene nanoribbons can lead to evolution of magnetic ordering due to the edges. Usually two fundamental types of edges are considered, i.e. zigzag and armchair edges, leading to zigzag and armchair nanoribbons. While armchair edges are nonmagnetic, zigzag edges have localized magnetic moments. The inter-edge interaction can be either ferromagnetic or antiferromagnetic and it is governed by the width of nanoribbons. Combination of doping and quantum confinement effect, leeds to interesting electronic and magnetic properties, i.e. half-metallicity, single-edge conductivity and ferromagnetic ordering. Here we present complex study of the efect of doping by nitrogen and boron atoms, with different levels of concentrations contained in both types of nanoribbons, on magnetic and electronic properties. Besides that the influence of dopation of nanoribbons on the Curie temperatures is discussed.
The development of various flexible and stretchable materials has attracted interest for promising application such as wearable electronic. Presented work is focused mainly on pressure sensors. One of target application of wearable sensors is to detect human motion and to monitor physical activity. Second aim is analysed reached date from mentioned pressure sensors. Data can be used for improving athletic performance and for delivering better physical performance.
This study aims to design a multifunctional, wearable, nanocoposite-based sensing system. Carbon nanotube (CNT) based composite were fabricated by ultrasound assisted mixing. Thin films were integrated on stretchable rubber base material to form of active layer with specific morphology. Active layer was integrated on flexible insole. Furthermore, the sensors were demonstrated to detect human motion style for example running, jumping and march. Quantification and qualification of human motion were successfully achieved. Presented sensors were characterized by advantages such as flexibility, ease of fabrication, lightweight, low-cost, and user comfort.
ACKNOWLEDGEMENT: This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic - Program NPU I (LO1504) and with the support of the Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF) and the national budget of the Czech Republic, within the framework of the project CPS-strengthening research capacity (reg. number: CZ.1.05/2.1.00/19.0409). And we are also thank to Internal Grant Agecy of Tomas Bata University in Zlin (IGA/CPS/2018/005). Cooperation with company HD Geo s.r.o. with team of Miomove (Daniel Majc) is also thanked.
Very recent work using scanning probe microscopy imaging techniques has demonstrated that N-doping of graphene can even tune the physicochemical properties of molecules adsorbed on its surface.1 Moreover, N-doped graphene offered a multitude of applications in the fields like spintronics, energy generation/storage and nanoelectronics.2-5 Here we investigate the possibility of tuning the electronic and magnetic properties of iron(II) phthalocyanine (FePc) molecules via non-covalent interaction by its positioning on N-doped graphene.
We show the unique capability of N-doped graphene to behave as a non-covalent tuner of the electronic and spin properties of molecules.6 The STM images of FePc molecules adsorbed at either graphene or N-dopants showed a different contrast and number of lobes. High-resolution atomic force microscopy imaging with a CO-functionalized tip showed that the transition can be characterized by a distinct sub-molecular contrast over molecules located on pristine graphene and near to the N-dopant for the triplet and singlet spin states, respectively. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules.
REFERENCES: [1] V. D. Pham, J. Lagoute, O. Mouhoub, F. Joucken, V. Repain, C. Chacon, A. Bellec, Y. Girard, S. Rousset, ACS Nano 8, 9403 (2014). ; [2] Z. Yang, J. Ren, Z. Zhang, X. Chen, G. Guan, L. Qiu, Y. Zhang, H. Peng, Chem. Rev. 115, 5159 (2015).; [3] D. W. Boukhvalov, M. I. Katsnelson, Nano Lett. 8, 4373 (2008).; [4] W. Han, R. K. Kawakami, M. Gmitra, J. Fabian, Nat. Nanotechnol. 9, 794 (2014).; [5] P. Błoński, J. Tuček, Z. Sofer, V. Mazánek, M. Petr, M. Pumera, M. Otyepka, R. Zbořil, J. Am. Chem. Soc. 139, 3171 (2017).; [6] B. D. L. Torre, M. Švec, P. Hapala, J. Redondo, O. Krejčí, R. Lo, D. Manna, A. Sarmah, D. Nachtigallová, J. Tucek, P. Blonski, M. Otyepka, R. Zboril, P. Hobza, P. Jelinek, Nat. Commun. 9, 2831 (2018).
The method of carbon nanotubes (CNT) addition to a friction stir welded dissimilar joint has been developed. The CNT agglomerates are produced in shape of paper sheets. CNT sheets added to the gap of butt joint during fixture before friction stir welding. Use of CNT sheets provide ability to measure the weight ratio of CNT in welds. Experimental addition of CNT in Al to Cu dissimilar joint for obtaining 7% CNT in weld was done. XRD study of weld metal shows no formation of carbides as CNT degradation products. SEM study of different weld join parts shows evidence of carbon presence. EDX mapping of carbon in the joint showed that both Al and Cu weld matrix parts have carbon inside.
Carbon nanostructures with high surface area and flake like morphology promote fast ion-transport kinetics, making them an ideal choice for multifarious applications in energy storage, environmental remediation, and catalysis. The porous few-layer graphene sheets (PFLG) of large surface area with abundant micro and mesopores were synthesized from renewable natural polysaccharides, which are plenty available from various sources such as botanical (Guar gum), tree exudates (Gum Karaya, Gum Arabic) and bacteria (xanthan gum), are presented. The preparation of PFLG was carried out via carbonization of the bio-based materials under nitrogen atmosphere and then followed by physical exfoliation via probe sonicator. The PFLG particles were characterized by SEM, FTIR, Raman spectroscopy, Nitrogen adsorption-desorption analysis to examine the surface area by Brunauer Emmett Teller (BET) method. The synthesized PFLGs possess a large specific surface area in the range of 1200-1800 m2g-1 with large pore volumes as confirmed by nonlocal density functional theory (NLDFT) method. The sheet-like morphology and the porous structure of the PFLGs were determined by SEM, while the Raman spectroscopy confirmed the graphitization and formation of graphene-like structures. This study provides a potential eco-friendly route for large scale synthesis of high quality bio-based graphene and carbon structures from economical and renewable materials. The synthesized PFLGs could be applied in supercapacitors, environmental remediation and biomedical fields.
Recently 2D nanomaterials became a subject of interest due to their specific properties that differ from those of their 3D counterparts. 2D transition metal carbides and nitrides called MXenes exhibit complex, layered structure providing a large number of compositional alternatives that make them materials of choice for a variety of applications. In our study, we focused on investigation of electrochemical performance of Ti3C2TX (MXene, where T: =O, -OH, -F) in an aqueous solution for promising sensing applications. Ti3C2TX was examined for its ability to detect significant analytes, such as O2, H2O2 and NADH, for potential future construction of biosensors. The Ti3C2TX MXene was proved as an excellent catalyst for reduction of H2O2 down to nM level with a response time of ∼10 s. Nevertheless, we revealed that pristine Ti3C2TX MXene is stable in a cathodic and an anodic potential window until +0.2 V. In an effort to enhance stability in an anodic potential window we also studied further modification of Ti3C2TX sheets with Pt nanoparticles. The H2O2 sensor composed of Ti3C2TX/PtNP layer deposited a on glassy carbon electrode (GCE) offered LOD of 448 nM at 0 V (vs. Ag/AgCl) in comparison to value of 883 μM observed for GCE modified with pristine Ti3C2TX. Furthermore, redox performance of small organic molecules (acetaminophen, ascorbic acid, dopamine, uric acid) was investigated with Ti3C2TX/PtNP sensor. These molecules could be detected at much higher potential compared to H2O2 with LODs down to nM level.
ACKNOWLEDGEMENTS: The financial support from ERC (311532), VEGA 2/0137/18 and APVV 15-0227 is acknowledged.
The vision of wearable electronic devices technology assume that future electronic systems will be an integral part of our everyday wear. Such electronic devices have to meet very specific load requirements such as flexibility and durability, able to measure for example strain in large extends or they should be able to be resistant in many of washing cycles. Wearable systems will be characterized by the ability to automatically analysed the activity and behaviour of their user and use this information to modify the configuration and functionality of the system. Wearable electronics is undoubtedly the phenomenon of today's times. It is prepared on a light and flexible substrates, it has a broad assumption and great potential for portable devices in wearable technology. In the last decade, was found that the properties and application of carbon nanotubes (CNTs) and their polymeric nanocomposites have become to be a very active area of this kind of research.
In our contribution we demonstrate application of flexible carbon nanotube strain sensor. This composite was prepared by novel technique using thermoplastic filtering membrane prepared by technology of electrospinning. Aqueous dispersion of MWCNT was filtered through membrane, dried and then welded on the surface of polyester strip and encapsulated by thin silicone layer. Sensitivity to applied strain of breath was further increased using Potassium permanganate (KMnO4) oxidised tubes.
This procedure leads to a composite based strain sensor which is flexible and stretchable, very sensitive to applied strain, reversible and durable in many applied cycles. To demonstrate its practical applicability a monitoring of human breathing was selected. The sensor was placed on the test T-shirt in the chest section and dress up with volunteer to monitor his breathing. Several breathing characteristics were measured and analysed. It demonstrate wave characteristics of deep breathing (full exhale), and breathing with lasting breath. Breath monitoring is a response to the change of electrical resistance of CNT sensor fixed on T-shirt by its deformation.
ACKNOWLEDGEMENT: This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic - Program NPU I (LO1504) and with the support of the Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF) and the national budget of the Czech Republic, within the framework of the project CPS-strengthening research capacity (reg. number: CZ.1.05/2.1.00/19.0409). Cooperation with company HD Geo s.r.o. with team of Miomove (Daniel Majc) is also thanked.
In this work, ethanol decomposition in microwave plasma torch at atmospheric pressure and liquid exfoliation of graphite by modified Hummers method were used for preparation of graphene nanosheets. Both methods produce similar but distinctively different graphene nanostructures. Size and topography of as prepared nanostructures were obtained by electron microscopy with varying accelerating voltage using secondary and backscattered electron detectors. The Wavelength dispersive spectroscopy was used to asses the nanosheet contamination by foreign elements during the preparation process. Further information about crystallography and sp2/sp3 content were determined by X-ray diffraction analysis, X-ray photoelectron spectroscopy and Raman spectroscopy. Main characteristics such as size, thickness and amount of defects are determined for both types of graphene nanosheets.
The environmental contamination by heavy metals is one of the global problems that disturb the ecological balance. A platform for the simultaneous electrochemical detection of trace heavy metal ions has been developed based on the gold nanoparticles-cysteine-multiwall carbon nanotube (AuNPs-Cys-MWCNTs) modified carbon paste electrode using square wave anodic stripping voltammetry. The voltammetric method was optimized with respect to accumulation time, accumulation potential and other parameters. The synthesis different sizes of AuNPs-Cys-MWCNTs composite were described and characterization using Dynamic light scattering and Scanning electron microscopy was performed. The AuNPs-Cys-MWCNTs composite significantly increases the electrode sensitivity for Cd2+, Pb2+, Cu2+ and Zn2+. The AuNPs-Cys-MWCNTs composite has promising significance for future using, for example as a modification of the screen printed electrodes, which would be used for measurement in situ.
We theoretically study the nonequilibrium Kondo effect in a quantum dot attached to monolayer graphene leads with an asymmetric bias voltage and apply a magnetic field on the dot. We employ the pseudogap Anderson Hamiltonian [1] in order to describe the system and use the equation-of-motion technique to determine the retarded Green's function of the dot. The Coulomb interaction between localized electrons in the quantum dot is assumed to be infinite. In absence of a magnetic field, at low temperatures and out of equilibrium, Kondo resonances appear in the density of states at the chemical potentials and their amplitudes decrease with increasing temperature and at high temperatures disappear completely. By applying a magnetic field, the Kondo peaks split into two peaks, with smaller amplitudes, shifted from the chemical potentials by the Zeeman energy. In our graphene-based system, contrary to a quantum dot with metallic electrodes [2-3], the Kondo peak does not rise up in the differential conductance at zero bias voltage due to the unique properties of graphene. The Kondo resonance appears at the non-value of fixed chemical potential that split into two smaller peaks by applying a magnetic field.
REFERENCES: [1] S.T. Rodriguez, et al., Physica E 96, 1 (2018); [2] N.S. Wingreen, Y. Meir, Phys. Rev. B 49, 11040 (1994); [3] Y. Meir, et al., Phys. Rev. Lett. 70, 2601 (1993).
The existence of contaminations is particularly dire in the case of graphene and other tow-dimensional materials (2D), as they consist essentially only surfaces. Using High-resolution scanning transmission electron microscopy (HRSTEM) technique, we have found that there can be a significant amount of silica rich impurities on the surface of graphene oxide and atomically thin MoS2. These silica rich impurities can mask most of the surface area and are exceptionally hard to remove. With the assistance of direct observation of the contaminants the rout toward the production of clean materials is outlined here.
Many promising technological applications of graphene need high-quality large-scale sheets to be synthesized. There are various methods for large area graphene synthesis; however, each has some limitation in terms of size, quality, or device integration. Fabrication of graphene from diamond is thus nowadays being explored as a promising alternative technology using thermal annealing without or with assistance of catalysts [Nat. Commun. 7 (2016) 12099].
In our work we used X-ray photoelectron spectroscopy for in-situ analysis of the temperature dependence of graphitization process of undoped nanocrystalline diamond thin films (65 nm thickness, 50-80 nm grain size) on Si wafer substrates [J. Phys. Chem. C 122 (2018) 6629]. We investigate the role of Ni as a catalyst facilitating the process. The annealing consisted of a step-like ramp (10 °C/min average) and XPS measurement at the steady temperature for 20 min. Thus the vacuum annealing is much slower than commonly applied RTA annealing in forming gas. Three major stages of diamond film transformation are determined from XPS analysis of C1s peak during the thermal annealing in the temperature range from 300 °C to 800 °C.
Formation of the disordered carbon phase is observed by XPS at 400 °C. With increasing temperature the disordered carbon progressively transforms to graphitic phase whereas the diamond phase disappears from the surface (i.e. from XPS probe depth of about 3 nm). A disordered carbon is still observed. Raman spectroscopy shows that in the temperature range 600-700 °C, the nanocrystalline diamond thin film is preserved in the sub-surface while graphitic phase forms on the surface. A multiphase graphitic carbon / disordered carbon / nanocrystalline diamond hetero-structure is thus formed. Lateral and transversal electrical conductivity of such structures is analyzed. Prospects for controlled growth of wafer-scale graphene are discussed. The work has been supported by GACR and FWF bilateral project (16-34856L).
Recently the interaction of metallic nanostructures and organic molecules provided exciton transition in hybrid systems has practical interest. The study of such structures improves the understanding of the interaction of light with matter and the development of novel sensors, solar cells and other photonic devices. The correlation of the plasmon and exciton resonances has importance for the performance characteristics of such devices. There are many studies of plasmon-exciton structures in colloidal solutions, which are simple and good models for basic research. The dry solid samples, such as thin films, with thermal and photostability at room conditions are needed for practical application. There has been little attention paid to the size of the molecule, which has influence on the formation of exciton transition in J-aggregates and interaction with plasmon nanoparticles, with than paid little attention
In our work, we present results of the overlapping exciton resonances of J-aggregates of the cyanine dyes and plasmon of the inhomogeneous ensemble of metallic nanoparticles. The arrangement of molecules near the plasmon nanoparticles in hybrid films leads to a changes in the optical properties, for example, to an increase in absorption. The interaction in strong coupling regime caused to a variation in the optical properties, due to the formation of a spectral dip.
We have developed the technology of the nominally undoped ZnO nano-crystalline thin films deposited by DC reactive magnetron sputtering of Zn target in the gas mixture of argon and oxygen plasma serving as a seeding layer for hydrothermal growth of the ZnO nanocolumns. We have shown that lack of the seeding layer leads to the random growth of low quality nanocrystals. Our recent results showed a significant increase in the length of the ZnO nanocolumns grown under UV irradiation, as well as the improvement in their uniformity. An enhancement of the optical and opto-electrical quality of the UV irradiated samples was also reported. Here we present the latest results on the enhancement of the photoluminescence in ZnO nano-columns after plasma hydrogenation. The photoluminescence in near UV region (peak at 380 nm) has been enhanced by several orders of magnitude whereas the deep defect related yellow photoluminescence (broad band 550-600 nm) has been significantly decreased. We explain the observed phenomena by passivation of defects at grain boundaries that not only decrease the defect related photoluminescence but it also significantly prolongs the lifetime of excitons and thus increasing the UV photoluminescence. The defects were also detected by the optical absorption spectroscopy and the electron paramagnetic resonance.
ACKNOWLEDGEMENTS: This work was supported by the CSF project 16-10429J, and the MEYS project LTC17029 INTER-COST Action MP1406 and by Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 - CZ.02.1.01/0.0/0.0/16_019/0000760).
DC reactive magnetron sputtering of Zn metal target in the gas mixture of argon and oxygen plasma has been applied to deposit nominally undoped nanocrystalline ZnO thin films [1-2]. The changes of electrical conductivity, free carrier concentration and localized defect density have been observed in ZnO thin films after hydrogen plasma treatment. To clarify these changes, the mobility and carrier concentration were measured by the temperature dependent electrical resistivity and Hall effect using the van der Pauw method. We show how the plasma treatment is related to the electrical properties and optical absorption, photoluminescence and Raman spectra. ACKNOWLEDGEMENTS: This work was supported by the CSF project 16-10429J and MEYS project LTC17029 INTER-COST Action MP1406.
On the quartz substrates the thin films of amorphous hydrogenated silicon (a-Si:H) were deposited by PECVD method. This deposition process was 5 times interrupted. During this time the vacuum chamber was pumped up to 10-5 Pa and on the surface the Germanium or Tin films were evaporated in the thickness of few nm. The materials form isolated nanoparticles (NPs) on the a-Si:H surface. Then the deposited NPs were covered and stabilized by a-Si:H layer by PECVD. Those two deposition processes were alternated 5 times. The a-Si:H thin films with integrated Ge/Sn NPs were characterized optically by PDS method, and microscopically by atomic force microscopy. Optical and microscopic properties of the structures are correlated and discussed considering their application in photovoltaics.
ACKNOWLEDGEMENTS: This work was supported by the CSF project 16-10429J, and by the Czech Science Foundation, project no. P108/12/G108.
The aim of this work is to understand the early stages in the growth mechanism of invar (FeXN100-X) alloys and also to study the influence of potential on the evolution of their crystalline structures.The used chemical solutions [0.1M] were freshly prepared according to the electrolytic composition previously defined. The current transients obtained from the chronoamperometry (CA) experiments were performed with the single negative potential steps. The growth Kinetics of three-and two dimensional (3D and 2D) clusters have been examined by many authors [9,13] and under different experimental conditions accounting for various limitation factors. Two limiting cases can be observed: instantaneous nucleation, were all nuclei are formed immediately after the potential step; and progressive nucleation, where the number of nuclei increases during the whole deposition process. Because Ni-Fe electrodeposition is controlled by diffusion the Scharifker and Hills (S-H) model [10] was used to deal with the present current transients. By comparing the experimental data with theoretical current transients in a non dimensional form by plotting (i/imax)2 versus (t/tmax), the nucleation process with 3D growth under diffusion control can be revealed.
One of the disadvantages of more extensive utilization of magnetorheological fluids in broad applications is their kinetic instability over the time. Non-stabilized MR fluids showed strong destabilization in both centrifugal and gravitational field.
Magnetorheological fluids containing magnetic particles (5 - 44 micrometers) have been prepared in suitable carrier fluids (oil-based) and surface active agents (non-ionic tensides) with nano-sized dimension have been used as stabilizers. The kinetic stability of magnetorheological fluids has been studied by analytical centrifugation. Iron particles chemically treated by non-ionic tenside (10 wt. %) showed higher stability over the time in comparison with non-treated iron particles.
We report on the properties of Eu-doped ZnO nanorods prepared by chemical bath deposition from aqueous solutions in an autoclave. The autoclave enables to grow the nanorods above the boiling point of water. The nanorods were characterized by scanning electron microscopy, secondary ion mass spectroscopy, and low temperature photoluminescence spectroscopy. Under the UV light excitation, the nanorods show a strong blue emission at 450 nm, which is associated with the 4f65d-4f4 transitions of Eu2+ ions. We discuss the mechanisms responsible for the blue luminescence. We further clarify how the growth parameters affect the morphology and the optical properties of the nanorods. Growth temperatures above the boiling point of water significantly influence the intensity ratio of the near band edge emission to the deep level emission and greatly enhance optical quality of the nanorods.
The importance of ZnO is related to its broad application potential in electronics, light-emitting devices, nanogenerators, sensors, piezoelectronics, solar cells, medicine, etc. A variety of ZnO nanostructures can be prepared from both the vapor phase and from chemical solutions. To achieve epitaxial growth of ZnO nanostructures, a substrate with the same crystal lattice and similar lattice parameters is required. Bulk substrates of ZnO are expensive and only available in small dimensions. GaN has similar properties to ZnO with only 1.8 % difference in the lattice constant. While GaN bulk substrates show similar weaknesses as ZnO substrates, GaN epitaxial layers on sapphire or Si are available in large dimensions. Nevertheless, these epitaxial layers show larger dislocation densities and surface inhomogeneities. Therefore, to uniformly nucleate the ZnO nanostructures on GaN epitaxial layers, we use focused ion beam (FIB) lithography.
We employ FIB to create nucleation centers on GaN epitaxial templates for the growth of ZnO nanorods, which is subsequently performed by hydrothermal solution method. We show that by this technique, periodic arrays of ZnO nanorods with different dimensions and spacing can be fabricated. These periodic arrays help us identify the growth mechanisms of ZnO nanorods. The nanorod arrays are further characterized by transmission electron microscopy to study the interface of the nanorods with the substrate and the crystallinity of the nanorods. Moreover, we perform in-situ electrical measurements of as-grown nanorods with the nanomanipulator tip in the scanning electron microscope to investigate the influence of the ion dose on the electronic transport in GaN/ZnO nanoscale interfaces.
Due to their soft operational capacity and magnetic properties, Iron Nickel alloys are of great commercial interest. A simple and inexpensive technique for the production of Nickel-Iron thin films is electrodeposition. A lot of physical and chemical parameters (substrates, concentration, current density, potential, temperature, pH, agents of addition......) can significantly influence the physical properties, such as homogeneity, bright, structure and morphology of the Ni-Fe deposits. This paper presents a study into some characteristics of Ni-Fe deposits on three different substrates. All the electrochemical experiments were performed in a three electrode cell in which the volume of the bath was 150ml. Electrodeposition of Ni-Fe was carried out potensiostatically from a Brenner type electrolytic bath in 0.1 aqueous solutions of Ni-Fe. The applied potential is-1.20V and the deposition time varies from 10 min to 30 min for all experiments.
The applications of the surface-enhanced Raman scattering (SERS) are greatly expanding. Although still not fully understood, the prevailing explanation takes into account a combination of electromagnetic and chemical enhanced contributions to Raman scattering process. Two main requirements for high-quality SERS substrates include large enhancement factors (mainly in tiny hot spots regions) and homogeneity of substrates. In this contribution, we have concentrated on the substrates formed with various types of gold nanodumbbell (AuND) structures. The numerical simulations were performed using the finite difference time domain (FDTD) method (Lumerical FDTD Solutions). Single AuNDs were considered as two spheres or ellipsoids (either oblate or prolate) interconnected with a rod. AuNDs were embedded in a uniform surrounding medium and tested for various polarized light excitations. The relevant key parameters of AuNDs were extracted from the corresponding experimental characterization, mainly SEM images. Next, we have considered AuND dimer structures, with the interparticle distance of several nanometers. Finally, we prepared AuNDs by the recently established seed-mediated growth method of AuNRs in the presence of iodide ions. ACKNOWLEDGEMENTS: This work was supported by the Czech Science Foundation 18-10897S project and the student project.
Magnetorheological fluids (MR fluid) are a kind of smart material, composed of magnetic particles dispersed in carrier fluid, which can change their viscosity in millisecond under the application of external magnetic field . The magnetic particles form chain like structure under the applied magnetic field, thus enhancing their viscosity to a solid like state, and on removing the field they return back to the initial state. In this work silicon oil based MR fluid are prepared using Fe3O4 nano-hollowspheres (NHSs) of two different diameters 250nm and 700nm and Fe3O4 nano particles (NPs) of 100 nm diameters. Ideally larger particles are expected to show higher shear stress. In our present work the composite effect of particle size and the morphology on shear stress is investigated. The magnetic NPs and NHSs are synthesized via solvothermal technique. All the samples are characterized using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and Vibrating Sample Magnetometer (VSM). Same concentration (50% by weight) of each sample is used to prepare the MR fluids to exactly know the effect of particle size and morphology on MR effect. Shear stress of all the MR Fluid samples is measured using Anton Paar rheometer MCR-301. The NHSs based MR fluid showed higher MR effect and better stability over NPs based one.
Spectroscopy based on surface enhanced Raman scattering (SERS) is a powerful technique which can be used for detection of chemical substances at very low concentration levels or study of molecular structure taking advantage of specificity of rotational-vibrational spectrum for different molecules. A key requirement for successful use of this technique is the preparation of high-quality reproducible SERS-active substrates. In this work we focus on preparation of substrates based on monolayer colloidal crystals (MCCs) consisting of spherical SiO2 nanoparticles covered by thin layer of magnetron sputtered gold. These substrates can be prepared in a simple, cheap and scalable approach by self-assembly of hydrophobized nanoparticles on the surface of water followed by transfer on silicon wafer, annealing in air and metallization. Among main advantages of SERS-active substrates prepared in this way are high thermal stability and low intensity of background signal thanks to absence of organic materials which can be source of unwanted spectral interference (as for substrates based on polymeric nanoparticles). The method described can be optimized for the preparation of layers consisting of particles with broad range of types and diameters. For example, we have successfully achieved formation of MCCs using SiO2 particles with diameters ranging from 50 nm to several hundred nanometers or even composite core-shell nanoparticles with 21 nm gold cores coated with 19 nm thick SiO2 shells. SERS activity, excellent spatial reproducibility and low background signal of prepared substrates have been confirmed by measurement of SERS spectra of 4-aminothiophenol.
Iron oxides, especially the Fe3O4 and γ-Fe2O3 phases, play a significant role in potential applications of nanomaterials due to their suitable features such as biocompatibility and magnetic and optoelectronic properties. Moreover, it is convenient to use Fe3O4 for some applications, such as Li-ion batteries, because of its combined valence states of iron. Therefore, it is highly desired to distinguish the two, otherwise similar, iron oxide phases and to evaluate the degree of magnetite’s stoichiometry. Genuinely, a proper characterization of these compounds can be problematic due to their similar structures and the common occurrence of non-stoichiometric iron oxide forms and their mixtures in the nanosystems. In this respect, Mössbauer spectroscopy is regarded as a powerful tool for the identification of different forms of iron oxides as well as for distinguishing between core-shell structures and mixtures. The present contribution is focused on theoretical Mössbauer spectra of Fe3O4, γ-Fe2O3 and their different degrees of stoichiometry, with regards to the correct evaluation of the spectra. Mössbauer spectra of two distinct levels of non-stoichiometric maghemite and magnetite were modelled and discussed. Furthermore, theoretical examples of Mössbauer spectra of Fe3O4/γ-Fe2O3 mixture of 1/1 particle ratio and a core-shell structure, where magnetite core takes up 75% of the particle’s volume, are presented.
We present a technique to fabricate in glass substrates sets of microchannels with silver nanoislands grown on the bottom of the channels for microfluidics applications. The channels were selectively etched in a soda-lime silicate glass substrate through a mask formed in the same glass using thermal poling with a structured anodic electrode. Self-arranged silver nanoislands were grown on the bottom of the channels by means of silver out-diffusion in hydrogen atmosphere from the glass preliminary subjected to silver-sodium ion exchange. Growth of silver nanoislands on the glass surface outside the channels was suppressed by the poling. The depth of the channels, bottom surface roughness, morphology and characteristics of the silver nanoisland film are characterized with atomic force and scanning electron microscopy, profilometry and optical spectroscopy. Obtained results were used to optimize the consequence and conditions of thermal poling, ion exchange, chemical etching by polishing NH4F.8H2O etchant, and annealing in hydrogen procedures. The microchannels with silver nanoislands were tested in Surface Enhanced Raman Scattering (SERS) measurements with the use of BPE (1,2-Di(4-pyridyl)ethylene) as the analyte. Essential enhancement of Raman signal by the nanoislands allowed for the registration of a monomolecular layer of the analyte. Finally, the best SERS performance was demonstrated by the channel with silver nanoislands formed in the consequence of 1) ion exchange, 2) poling, 3) etching, 4) hydrogen annealing. In this case, nanoislands of 15 nm average radius occupied maximal percentage, ~37%, of the channel bottom; the channel had maximal depth, 1.3 μm, and minimal bottom roughness.
This work describes the use of inkjet printing to design fractal antenna based on Sierpinski arrowhead curve architecture. The antenna is printed using silver nanoparticles on a flexible plastic substrate PET (Polyethylene terephthalate). Silver nanoparticles were synthetized by the solvothermal precipitation technique. The diameter of the prepared silver nanoparticles ranges from 20 to 200 nm measured with the help of the SEM analysis. Using a combination of printing technology and fractal geometry to achieve very advanced parameters in three frequency bands 650 MHz, 1,5 GHz and 2,5 GHz. This study is also focusing on the simulation of the main parameters of the prepared antenna.
Sierpinski arrowhead curve architecture antenna on polymer substrate may find use in many portable devices like mobile phones, where it would be possible to implement the antenna on the cover in the form of printing, thus compliant important benefits with respect to size and weight (0.221g).
ACKNOWLEDGEMENT: This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic - Program NPU I (LO1504) and with the support of the Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF) and the national budget of the Czech Republic, within the framework of the project CPS-strengthening research capacity (reg. number: CZ.1.05/2.1.00/19.0409).
2D Transition metal dichalcogenides (TMDC) - Efficient layered semiconducting materials scopes wide to enhance semiconducting technologies in range of fields such as solar energy conversion, energy storage, transistors, gas sensors, photo-detectors, waste water management and many more. Tremendous research work has been carried out in these recent years which already justified the importance of TMDC as an effective substitute of Graphene which commits so much versatility unlike Graphene. Nonetheless, synthesis of such TMDC material with control, scalability and homogeneity is a limit with current day technology as well and hence we tried to work with this aspect. We here present an approach in which the bulk and shiny WS2 crystal charge was synthesized by direct vapor transport (DVT) method and then crystalline shiny powder was used to make the nanosheets of the WS2. Nanosheets were fabricated by application of ultrasonication of bulk WS2 in N-Methyl-2-pyrrolidone (NMP) for 6 hours (with 2 sec on and off time). Collected supernatants from the solution were put through various characterizations like XRD, RAMAN and TEM to understand the structure and morphology of the prepared samples. To continue, the prepared sample give the hexagonally structured nano-crystalline sheets of ~ 15 μm width and showed a Raman shift of ~ 13 cm-1 which confirms the sample identity in nanometric range. Furthermore, the UV-Vis spectroscopic analysis resulted with the direct band gap of ~1.7 eV which is clear indication of size dependent transition in the band gap of WS2 nanosheets. To sum up, the yield of WS2 nanosheets is scalable with shape, size and optical tuning in properties allows the employment in optoelectronic devices.
We use the formerly derived explicit analytical expressions for the conductivity of nanostructured superconductors supercooled below the critical temperature in electric field. Computer simulations reveal that the negative differential conductivity region of current-voltage characteristic leads to excitation of electric oscillations. We simulate a circuit with distributed elements. This gives a hint that a hybrid device of nanostructured superconductors will work in terahertz frequencies. If the projected setup is successful, we consider the possibility for it to be put in a nanosatellite (such as a CubeSat). A study of high temperature superconductor layers in space vacuum and radiation would be an important technological task.
Iron(II) phthalocyanine (FePc), an important member of the phthalocyanines family has potential applications in the various fields, such as electrocatalysis,1 magnetic switching,2 electrochemical sensing,3 and phototheranostics.4 Inspite of the importance of electronic properties of FePc in such wide applications, a reliable determination of its ground-state is still challenging. Here we present combined state of the art computational methods and experimental approaches i.e. Mӧssbauer spectroscopy and SQUID magnetic measurements to identify the ground-state of FePc.5 Multi-reference calculations clearly proved that the ground-state of the isolated FePc is quintet showing its larger stability with respect to the triplet ground-state previously favorized by DFT calculations. These conclusions perfectly correlate with experimental results based on Mӧssbauer spectroscopy and magnetic measurements of FePc frozen in chlorobenzene. In these conditions Fe(II)Pc behaves like in the gas-phase as verified by magnetic data.
REFERENCES: [1] R. Cao, R. Thapa, H. Kim, X. Xu, M. G. Kim, Q. Li, N. Park, M. Liu, J. Cho, Nat. Commun. 2013, 4, 2076.; [2] J. Girovsky, J. Nowakowski, M. E. Ali, M. Baljozovic, H. R. Rossmann, T. Nijs, E. A. Ae-by, S. Nowakowska, D. Siewert, G. Srivastava, C. Wäckerlin, J. Dreiser, S. Decurtins, S. -X. Liu, P. M. Oppeneer, T. A. Jung, N. Ballav, Nat. Commun. 2017, 8, 15388.; [3] H. Xu, C. Liao, Y. Liu, B.-Ce Ye,B. Liu, Anal. Chem., 2018, 90, 4438.; [4] . Jia, J. Ge, W. Liu, X. Zheng, M. Wang, H. Zhang, P. Wang, ACS Appl. Mater. Interfaces, 2017, 9, 21124.; [5] D. Nachtigallová, A. Antalík, R. Lo, R. Sedlák, D. Manna, J. Tuček, J. Ugolotti, L. Veis, Ö. Legeza, J. Pittner, R. Zbořil, P. Hobza, Chem. Eur.J. 2018, 24, 13413.