Stress, structure, and stoichiometry effects on the properties of nanomaterials IV

Resume : Despite of great amount of research on the properties of multiferroic BiFeO3 (BFO), one of the most fundamental aspects, the lattice vibration, and the assignment of the phonons symmetry is still controversial. Strain engineering has recently become a powerful method to vary physical properties of multifunctional oxides, as BFO, opening a new direction of investigation. Tetragonally distorted BFO recently attracted a lot of attention because of the larger spontaneous polarization as compared to its rhombohedral counterpart. We present results on highly strained (when grown on LaAlO3 substrates) and nearly pseudomorphic BFO films characterised using Raman spectroscopy [1]. In order to rule out the possible influence of extrinsic factors, such as growth conditions, crystalline quality of substrates, or film thickness, (which are present when comparing different films on different substrates) we used the method of piezoelectrically induced strain. In this case the strain can be effectively transferred to the films by an elastic coupling at the interface substrate/film. This method allowed a quantitative correlation between strain and the shift of the Raman-active phonons as well as the determination of the Grüneisen parameters for specific phonon modes. [2] By using azimuthal rotation dependent Raman measurements and based on the strong intensity variation of the polar Raman modes, we have shown that Raman spectroscopy is a powerful tool for the investigation of ferroelastic domains in BFO crystals and thin films [3]. [1] Appl. Phys. Lett. 106, 012908 (2015) [2] Appl. Phys. Lett. 108, 042902 (2016) [3] J. Raman Spectrosc. 46, 1245 (2015)

Resume : The study of novel physical properties appearing when two materials are interfaced has become one of the major fields of research in solid-state physics over the last decade. As the materials involved in those new physical phenomena are often complex oxides, many factors (such as strain, oxygen stoichiometry, cation intermixing, electronic reconstructions) have to be considered when discussing their origin. If the two interfaced materials have different octahedral tilt systems, one has to adapt to the other. Electron microscopy imaging (HAADF, ABF) and EELS spectroscopy are key tools for their study. I will be reporting recent results enlightening the effect of oxygen octahedral coupling on the change of the magnetic easy-axis in some ferromagnetic manganite films. [1] I will show how this change of magnetism can be related to a change in hybridization between the metal and the oxygen orbitals [2]. Another example of hybridization effects on physical properties will be presented in RNiO3 (R=Sm, Nd)superlattices will be also presented. N.G. acknowledges funding from the Geconcentreerde Onderzoekacties (GOA) project “So-larpaint” of the University of Antwerp and the Research Foundation Flanders (FWO, Belgium) project 42/FA070100/6088 “nieuwe eigenschappen in complexe Oxides”. References: 1. Z. Liao, M. Huijben, Z. Zhong, N. Gauquelin, et al. Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling Nat. Mater. 15 (2016), 425-431. DOI: 10.1038/nmat4579 2. Z. Liao, N. Gauquelin, et al. Thickness Dependent Properties in Oxide Heterostructures Driven by Structurally Induced Metal–Oxygen Hybridization Variations Adv. Funct. Mater. 27 (2017) 1606717. DOI: 10.1002/adfm.201606717

Resume : The organization of nanowires on surfaces remains a major obstacle towards their large scale integration into functional devices. In order to overcome these issues, aligned arrays of heterostructured horizontal planar core-shell ZnSe@ZnTe nanowires were grown exploiting the epitaxial relations with the substrate in a guided growth approach to form well organized assemblies. We exploit the directional control of the guided growth for the parallel production of multiple radial p-n heterojunctions. The formed arrays exhibit great optoelectronic properties, with dark currents below the detection limit and upon illumination a rectifying behavior with photovoltaic characteristics. By the use of atomic resolution (S)TEM together with Geometric Phase Analysis (GPA), a deep understanding of the strain fields on the different nanostructures can be obtained. In that framework, we perform a study of the relaxation mechanisms taking place in the structure and how are they affected by the core morphology and substrate orientation with the aim of being able to exploit the strain on them to optimize the electronic behavior of the nanostructures.

Resume : Titanium nitride (TiN) films were grown by a pulsed laser deposition technique using a variety of deposition parameters such as substrate temperature, ambient gas pressure, target- substrate distance, substrate materials, etc. The TiN thin films fabricated at temperatures in the range of 500-800⁰C in vacuum ambient are found to be epitaxial with (111) orientation. Low-temperature transport properties were systemically in TiN films with different room temperature resistivities (100-500 _ohm-cm) under an applied magnetic field from 0 to 5.0 T. The temperature dependence of resistivity shows a generally minimum behavior at low temperatures (T<40 K) under various applied fields. Best settings were made by considering both the electron-electron (e-e) interactions in terms of T1=2 dependence and the Kondo-like spin dependent scattering in terms of ln T dependence. The Hall measurements and data analysis have shown that the charge carriers are electron in metallic TiN films. For example, the Hall coefficient and electron density at 300 K were found to be -6.4*10 -5 cm 3/C and 9.7*1022/cm 3, respectively. The authors acknowledge the financial support of the University of Nebraska-Lincoln MRSEC Center via grant number (NSF-DMR 1420645) and NSF-MPS-DMR (Grant No. 0820521).

Resume : Pulsed laser deposition or PLD is known as a technique by which complex materials can be stoichiometrically transferred from a target to a substrate, providing that the ablation threshold is exceeded. For a multi-target component, it frequently happens that there is loss of the lightest and the most volatile component in the film. A very well studied case in the one of oxides, for which the O2 or N2O background gases can reduce the loss of oxygen in the growing films. A much less studied case is the one of sulfides or selenides, such as the solar cell absorber layers of CIGS (Cu(Ga,In)Se2) and CZTS (Cu2ZnSnS4). While the former material has been studied comprehensively during the last thirty year as absorber layer, the latter is relatively new, promising material, which recently has reached a solar cell efficiency slightly below 10 %. Films of CZTS have turned out to be difficult to produce by PLD because the mass transfer from target to films is significantly incongruent. The films were produced by PLD at a fluence from 0.2 J/cm2 to 2 J/cm2 at room temperature with nanosecond lasers with wavelengths of 248 nm or 355 nm in vacuum. The resulting film composition was deficient in sulfur in general, but the most surprising feature was a strong decrease in the copper content of the films with decreasing fluence. There was a clear decrease of the number of droplets on the films with decreasing fluence as well. A similar trend was observed for Cu2SnS3 or Cu2ZnSn. [A. Cazzaniga et al., Solar Energy Mat. Solar Cells, 166, 91 (2017)]

Resume : Layer-structured materials are advantageous for supercapacitor applications owing to their ability to host a variety of atoms or ions, large ionic conductivity and high surface area. In particular, ternary or higher-order layered materials provide a unique opportunity to develop stable supercapacitor devices with high specific capacitance values by offering additional redox sites combined with the flexibility of tuning the interlayer distance by substitution. CuSbS2 is a layered ternary chalcogenide material that is composed of earth-abundant and low toxicity elements. We have developed a simple colloidal method for the synthesis of CuSbSexS2-x mesocrystals over the whole composition range (0 ≤ x ≤ 2) by substitution of S with Se. Our approach yields mesocrystals with belt-like morphology for all the compositions. Structural studies indicate that substitution of sulfur with selenium in CuSbS2 enables tuning the width of the interlayer gap between the layers. To investigate the suitability of CuSbSexS2-x mesocrystals for supercapacitor applications, we have carried out electrochemical measurements by cyclic voltammetry and galvanostatic charge-discharge measurements in KOH, NaOH and LiOH electrolytes. Our investigations reveal that the mesocrystals exhibit promising specific capacitance values with excellent cyclic stability. The work was done in collaboration with K. Ramasamy, R. Gupta, H. Sims, S. Ivanov, and S. Palchoudhury.

Resume : The atomic scale structure of semiconductor thin films, interfaces, and nanoparticles crucially depends on the strengths of the interatomic bonds present in the material. However, detailed knowledge of the force constants is missing even for technologically relevant mixed III-V semiconductor alloys. The force constants used in structural modelling of random alloys are therefore often simply adopted from the respective binary parent materials. To overcome this unsatisfying situation, we studied the composition-dependent element-specific bond-stretching force constants in (In,Ga)As using temperature-dependent x-ray absorption fine structure spectroscopy. Measurements in the temperature range between 35K and 300K were performed at the Ga-K and In-K edge, yielding the temperature evolution of the variance of the first nearest neighbour distance distribution which is linked to the interatomic force constants via a correlated Einstein model. The results show a remarkable nontrivial composition-dependence of the Ga-As and In-As bond-stretching force constants. While the Ga-As bond in GaAs is significantly stiffer than the In-As bond in InAs, an inversion of the bond strengths is visible for (In,Ga)As with the Ga-As bond being softer than the In-As bond in this ternary alloy. As a consequence, in order to accurately model nanostructures of random semiconductor alloys the bond stretching force constants of these materials should not simply be adopted from the binary parent materials.

Resume : Harvesting energy by direct conversion of sunlight to electricity using photovoltaic (PV) technology is emerging as a leading contender for next-generation green power production. Thin film solar cells based on chalcopyrite Cu(In,Ga)Se2 (CIGS) are considered as potential candidates due to many promising features. High performance at laboratory scale (20.8% efficiency) and remarkable stability of CIGS-based solar cells are the key features that compete directly with (poly)crystalline silicon cells. The quaternary CIGS compound is a direct-band semiconductor with superior absorption characteristics and is considered to be the most suitable absorber material for thin film applications. Many deposition methods are being utilized to grow CIGS thin films; however, only few works on the pulsed laser deposition (PLD) technique have been reported. In this presentation, the growth of CuIn0.7Ga0.3Se2 on soda-lime glass substrates using PLD will be presented. The influence of the growth conditions of CIGS films will be investigated through a complete characterization of structure, composition and morphology. Also, electrical and optical measurements of CIGS films will be performed in order to study the transport and optical properties of the films. In this presentation, the optimum PLD growth conditions for CIGS films will be presented, based on an overall assessment of the film charcateristics.

Resume : Nanoheteroepitaxy [1,2], i.e. the growth of thin films on nanoscale patterned surfaces, holds the promise of allowing for a defect reduction in layers in lattice mismatched materials systems by partitioning misfits strains among the deposited film and the substrate. Usually, the growth of thin films on cylindrical nanorods is considered to this end. Different models have been proposed to predict the diameter of such nanorods below which the formation of misfit dislocations is energetically less favourable compared to straining both the deposit and the rod-shaped substrate. Continuum mechanics and molecular statics calculations predict favourable rod diameters far below 100 nm, depending on the misfit between deposit and substrate and on elastic materials properties. Nanorods can be created using both, top-down and bottom-up techniques, the former allowing for an accurate control of nanorod shape and orientation, the latter for large-area pattern repetition. Examples will be presented where the reduction of rod diameters leads to a significant decrease of the density of extended defects in small deposits of MBE grown III-V semiconductors. [1] R. Kemper, D.J. As, J.K.N. Lindner: in: “Silicon-based Nanomaterials”, eds. H. Li et al., Springer Series in Mat. Sci. 187 (2013) 381 [2] Th. Riedl and J.K.N. Lindner, Heteroepitaxy of III–V Zinc Blende Semiconductors on Nanopatterned Substrates, in: Nanoscaled Films and Layers, L. Nanai (Ed.), InTech (2017), DOI: 10.5772/67572.

Resume : We report a novel method for synthesis and processing of pure and NV-doped nanodiamonds with sharp NV0 and NV- transitions at ambient temperatures and pressures in air. Carbon films are melted by nanosecond lasers in super undercooled state and quenched rapidly. We can form single-crystal nanodiamonds, microdiamonds, nanoneedles and microneedles, and large-area films. Substitutional nitrogen atoms and vacancies are incorporated into diamond during rapid liquid-phase growth, where dopant concentrations can far exceed thermodynamic solubility limits through solute trapping. These nanodiamonds can be placed deterministically and the transitions between NV- and NV0 can be controlled electrically and optically by laser [1]. [1] J. Narayan and A. Bhaumik, Materials Res. Letters 2016, http://dx.doi.org/10.1080/21663931.2015.1126865; Materials Res. Letters, 2016 http://dx.doi.org/10.1080/21663831.2016.1249805

Resume : Electrocatalytical activity of graphene is strongly linked to its electron density of states (DOS), which is very sensitive to both the carbon honeycomb lattice deformations as well as the impurities. Thus, stress in combination with impurities is a natural route for tuning graphene properties towards fabrication of effective graphene based electrochemical devices. To confirm this supposition, we have undertaken research into the effect of electronic properties of uniaxially stressed graphene with covalently bonded moieties, vacancies and charged impurities on heterogeneous electron transfer kinetics. DOS calculations show that band gap in perfect graphene is very sensitive to direction of the tensile strength. For a band gap opening, the zigzag direction of deformation is preferred. Moieties and vacancies in zigzag strained graphene suppress the band gap value. Introduction of charged adatoms or substitutional impurities leads to the smearing of the band gap and creation of pseudo-gap. Cathodic reaction rate calculated for ferrocyanide/ferricyanide redox couple is very sensitive to both the strain as well as the presence of impurities. Stretching along zigzag by 10% leads to the increase of the standard rate constant by 50%. While vacancies and covalently bonded moieties forming defect band within the band gap do not influence strongly the standard electron transfer rate constant, its value is very sensitive to the presence of charged adatoms and substituitional impurities.

Resume : Carbon nanofibers amongst other one dimensional nanostructures such as carbon nanotubes,nanowires have been used in wide potential applications such as sensors,high strength materials,super capacitors and dye sensitized solar cells. CNF are prepared by electrospinning PAN followed by stabilization and carbonization. We have carried out a detailed study on the electrical transport and hall effect measurements on carbon nano fibers annealed at different temperatures (700 C - 1400 C). It has been observed that ration of D to G peaks in Raman spectroscopy and the full width half maximum(FWHM) of 100% peak (26 degree) in the x-ray diffraction spectra decreases as the annealing temperature increases from 700 C-1400 C, which suggests that the crystallinity of the carbon nanofibers increases as the temperature increases. Also the diameter of the carbon nanofibers decreases from 51.17 nm to 15.89 nm as the annealing temperature increases from 700 C to 1400 C. These nanofibers have been found to exhibit semiconducting behavior in the temperature range 350K-10K with room temperature resistivity varying from 10-50 mOhm.cm. The hall effect measurements show that the hall coefficient value was -5.6*10^-10 cm3/C at 300K for the carbon nanofibers that was annealed at 1200 C. The negative sign of the hall coefficient suggests that the charge carrier is electron. The electron carrier concentration and their mobility were calculated to be 1.1*10^20/cm3 and 25.8 cm2/Vs, respectively. The value of electronic charge carrier concentration is almost an order of magnitude higher than the values reported for the carbon nanofibers synthesized under similar conditions. Due to semiconducting nature of the carbon nanofibers and superior charge carrier concentration and carrier mobility carbon nanofibers have the potential to be used in non-silicon based integrated circuits.

Resume : ZnO is one of important materials for various applications owing to unique properties like as high n-type conductivity, large exciton binding energy, active surface, and variety of nanostructures. ZnO has crystallographic polarity along c-axis because of lacking center of inversion symmetry, and the effects of polarity has been reported on single crystals. In this paper, we report the effects of polarity on polycrystalline ZnO thin films deposited on glass substrates by magnetron sputtering utilizing polarity controlled templates. The electrical, optical and structural properties showed significant differences between Zn-polar and O-polar ZnO even under the same deposition conditions. The ZnO films on Zn-polar face showed higher Hall mobility, and dominant near band edge emission in PL. On the other hand, ZnO films on O-polar face showed lower Hall mobility, and intense deep level emission. Both films showed c-axis orientation perpendicular to substrates. While, the O-polar ZnO showed larger crystallite size. It was suggested that the different properties are possibly due to formation of native defects depending on the polarity. The electrical properties of Ga-doped ZnO films were also strongly influenced by the polarity [1]. We reported higher conductivity on Zn-polar face compared to the O-polar face. Using the Zn-polar ZnO template, we observed enhancement of Hall mobility for extremely thin Ga-doped ZnO films. [1] Phys. Status Solidi-Rapid Res. Lett., 10, 535 (2016).

Resume : Molecular orientation in solid thin films is needed for their applications to devices because of drastic enhancements of their performances in a direction such as carrier mobility or nonlinear optical properties. Hence, the oriented growth on aligned polytetrafluoroethylene(PTFE) also has been studied since its discovery in 1991[1], orientating a remarkably wide range of deposited materials such as linear molecules, discotic molecules, macromolecules, crystals, nanoparticles, nanotubes, nanocomposites, and J-aggregates[3]. However, the driving force of this remarkable effect is still an open question. The first author proposed previously that the atomic grooves between adjacent PTFE chains account for the effect through a good correlation between an experiment and an MD simulation, where a deposited molecule orients along the grooves[3]. We will demonstrate here the further results that convince us of this atomic groove effect; progress in computing ability have enabled us to simulate the difference in the orientation by tiny changes in a molecule end, showing the possibility of a computational molecular design. This work is supported by JSPS-KAKENHI(JP17K4996). [1] J. C. Wittmann, et.al., Nature, 352, 414 (1991); [2] T. Tanaka, et al., Langmuir, 32, 4710 (2016); [3] T. Tanaka, et.al., J. Phys. Chem. B, 106, 564 (2002).

Resume : Nanosized metal ferrite particles are promising materials for contrast-enhanced magnetic resonance imaging. The grain size, magnetic properties and state of the surface are the key features of the ferrites for this application. The mentioned parameters of oxide materials are known to depend strongly on the methods of synthesis. It this work, an inorganic modification of sol-gel method was chosen due to its advances important for medical use. Namely, it makes possible room temperature preparation of multi-component oxide materials with a size down to 5 nm and chemically active surface. It allows controlling the phase structure of materials and the homogeneity of the components distribution. Thus, the sol-gel method provides either multi-phase composites or single-phase solid solutions depending on the application. In this work, Mn-doped magnetite nanoparticles have been prepared by sol-gel method via inorganic metal precursors. The particles were stabilized in a form of sol with polyelectrolyte shell. The prepared ferrites were studied by XRD, TEM, SEM, and magnetometry. It was shown that the prepared materials are MnxFe3-xO4 single-phased solid solutions with spinel structure for x value from 0 up to 0,7. The average particle size was estimated to be 10 nm. The saturation magnetization measured for the powdered samples grows up from 49 to 59 emu/g with increasing Mn content within the indicated x range.

Resume : This work reports on theoretical investigation of the temperature and hydrostatic pressure effects on the confined donor impurity in a GaAs hollow cylindrical shell quantum dot. The charges are assumed to be completely confined to the interior of the shell with rigid walls. Within the framework of the effective-mass approximation and by using a simple variational approach, we have computed the donor binding energies as a function of the shell size in order to study the behavior of the electron-impurity attraction for a very small thickness under the influence of both temperature and hydrostatic pressure. Our results show that the temperature and hydrostatic pressure have a remarkable influence on the impurity binding energy for large shell QDs, and has a significant influence on the electron-donor recombination rate for small QDs. The binding energy is more pronounced with increasing pressure and decreasing temperature for any impurity position and QD size. The opposing effects cause by temperature and pressure reveal a big practical interest and offer an alternative way to the tuning of correlated electron-impurity transitions in optoelectronic devices.

Resume : New physical effects in an influence of nanoparticles interaction with Cu-Al-Mn matrix on phase transformations and on mechanical properties were observed. Ferromagnetic nanoparticles formed under 473 K for 3 h aging in 1.5 kOe with different orientation provides stress reduction, arising during martensite transformations and superelasticity increasing. Precipitated nanoparticles oriented growth in field direction and their volume fraction increasing favor of induced martensitic transformation reversibility were investigated. Established effect mechanism has been proposed. Internal stresses under martensitic transformations arise as a result of austenite phase shear deformation. From other side, austenite aging before martensite transformation results in nanoparticles Cu2AlMn coherently connected with a matrix precipitation, that reduces internal stresses during transformation. This leads to thermal hysteresis of martensitic transformation narrowing and superelasticity increasing. The competition between described processes driving forces on bulk-, micro- and nanolevels is discussed. The role of ferromagnetic nanoparticles in Cu-Al-Mn alloy phase transformations illustrates bridging scales as a key tendency in modern physical nanomaterial science.

Resume : The appearance of tetragonal phase in nanoscale V and V/Ag thin films was investigated by applying synchrotron irradiation ( λ=0.108 nm) and high energy electron diffraction (100 keV). Films were deposited by PVD electron-beam sequential evaporation in 10-7 Pa vacuum onto SiO2 (001) substrates. According to in-situ electron diffraction data the tetragonal distortion of vanadium bcc phase (β-phase) takes place during films annealing at 400°C in 10-3 Pa vacuum. It is suggested that it is caused by formation of oxygen solid solution in vanadium. The vanadium monoxide VO is formed from this phase at temperature of 600°C. Vanadium bcc phase transformation into body-centered tetragonal β-phase within temperature range 400–600°С was analyzed in SPring-8 synchrotron center (beamline BL44B2). The tetragonality in vanadium bcc phase is manifested in splitting of (110) and (200) reflections in (101-110) and (200-002) doublets. The degree of tetragonality depends on annealing temperature and concentrations of O and Ag in V film. The influence of additional Ag layer on nanostructure of V thin films and physical (electrical) properties was studied as well. Visualization models of structure and phase composition formation in V(25 nm) and V(25 nm)/Ag(25 nm) thin films during annealing up to 600°C in vacuum 10-3 Pa and 10-7 Pa have been proposed based on the obtained data as well.

Resume : Electrospinning (ES) is a simple and versatile technique to produce submicron and micron polymer fibers. ES has the potential for low cost/energy manufacturing with a high material utilization ratio and high positional accuracy. Because electrospun polymer fibers have cylindrical geometry, they are promising building blocks for small and soft photonic devices including waveguides and sensors. However, propagation losses in electrospun fibers are still high; in the range of several tenths of dB/cm. Recently, we have reported that one of the main reasons for the high propagation loss in electrospun fibers is excess light scattering caused by density inhomogeneity [Y. ishii et al. APL Materials, 2, 066104, 2014]. In this study, we have firstly investigated propagation loss in electrospun fibers before and after heating with the aim of decreasing their density inhomogeneity. Electrospun fibers of amorphous poly(DL- lactic acid) (PDLLA) with a mean diameter of 1.47 μm were fabricated and their propagation loss was measured before and after heating. The heating temperature was above the glass transition temperature of PDLLA (53.6℃). After cooling to room temperature, the propagation losses of three single fibers were evaluated at 533 nm wavelength. All the fibers showed smaller propagation losses after heating, decreasing from 17 to 8.1 dB/cm. This result confirmed the feasibility of lowering the propagation loss in electrospun fibers by heating.

Resume : Bulk ferroelectric ceramics obtained by doping KNbO3 with BiFeO3 (KBNFO) show interesting photovoltaic properties, due to the capability of tailoring the bandgap widths by means of the doping content (ranging from ≈3,22 eV, for pure KNbO3, to ≈2,25 eV, when the doping is 25%). This change in the bandgap is achieved without modifying the crystal structure, and with very little changes in the unit cell parameters. Similar effects in bandgap and cell parameters have been observed when doping KNbO3 with other oxides like BiMnO3 (KBNMO) and BiCoO3 (KBNCO). Despite these promising features, at the best of our knowledge, studies of these ceramics in thin film form are scarce. In this work we used pulsed laser deposition technique to grow thin films of K0,75Bi0,25Nb0,75Fe0,25O3, K0,85Bi0,15Nb0,85Mn0,15O3 and K0,85Bi0,15Nb0,85Co0,15O3 onto STO and MgO single crystal substrates. Crystal structure characterizations were performed by means of θ/2θ scans and reciprocal space maps. Results revealed that the crystallinity of KBNFO films growth onto STO was better than those of films grown onto MgO. Moreover, in both substrates, films were epitaxial, with a cube-on-cube relationship. In addition to KBNFO crystallites, the presence of non textured KBi2Nb5O16 was also detected.

Resume : The family of metallophthalocyanine (MePc) thin films appears as promising material for miniaturised electronic devices already applied in e.g. organic light emitting diodes, organic thin film field effect transistors, solar cells and gas sensors. Application of organic layers is strongly related to the properties of their surface and bulk structure, which in turn depend on the technological conditions during deposition process. In work, the effect of surface morphological ordering of 50 nm-thick copper phthalocyanine (CuPc) layers on the layer's susceptibility to ambience-induced degradation processes was examined. The distinction between structures were caused by different deposition rates: 0.01 nm/s (r1) and 0.02 nm/s (r2) during physical vapor deposition of layers. The surface morphology was diagnosed by atomic force and scanning electron microscopies exhibiting compact, ordered topography for layer with r1 while randomly distributed bigger crystallites on rougher and more expanded area for layer with r2. X-ray diffraction revealed mean grain size in bulk slightly larger for r2. Energy dispersive X-ray spectroscopy demonstrated C/Cu and N/Cu ratios more pronounced for layer with r2. Morphological features and traces of CuPc-air interaction were mirrored in the Raman spectra. Increased peak intensity of oxidation products and peak shift attributed to surface disorder recorded for layers with r2 proved their stronger inclination to air-originated degradation processes.

Resume : Graphene can be used as a top contact in the solar cell, because it has good values of conductivity, optical transparency and mechanical properties. Thus, the study of graphene influence on the structure and properties of materials surfaces is interesting. In our previous work, we have studied the "bcc Fe / Graphene" system. In this work we performed the research of the "hcp Ti / Graphene" system. Modified embedded atom method (MEAM) describes well the relaxation processes, even the anomalous. MEAM was used to study of relaxation surfaces (0001), (1-100), (11-20) Ti before and after the graphene coating for two temperatures: 300K and 400K. Interplanar distances, atoms distribution along a normal to the surface, radial distribution function for surface layers were calculated. Their analysis is presented in this report. The form of the relaxation changes after the graphene coating. Reconstruction of the surface layer was detected in the system (11-20) Ti/graphene. The relief of the structure was demonstrated for different surfaces. Total and axial stresses were calculated for all systems and for the systems' components separately. Their analysis is presented also. Comparison of the results for the "bcc Fe / Graphene" and "hcp Ti / Graphene" systems were conducted. Analysis of data allows making conclusions about time-temperature stability of the system and its potential use in solar energy.

Resume : Post annealing of Fe/Pt multilayers is a possible way for the production of hard magnetic L10 FePt thin films for high-density magnetic recording. The aim was to investigate the influence of annealing atmosphere and thickness of layers on L10 FePt phase formation. [Pt/Fe]4 and [Pt/Fe]8 films with total thikness of 30 nm were deposited by magnetron sputtering on SiO2(100 nm)/Si(001) substrate at RT. Annealing of the films was carried out in vacuum and in N2(RTA) in the range of 500 - 900ºC for 30 s. The films were investigated by XRD, SQUID and AFM. The as-deposited [Pt/Fe]4 sample exhibits satellite reflections around the solid-solution peak due to layered structure. The increase in number of layers at reducing of layer thickness promotes in activation of diffusion processes on the interfaces and leads to shift satellite and solid-solution reflections in side of smaller and larger angles, accordingly. A1 → L10-FePt phase transformation in annealed in vacuum films takes place at 700°C. The ordering temperature can be reduced by RTA to 500C in both films. RTA due to tensile in-plane strain promotes the grain growth in [001] direction. The greater amount of grains oriented perpendicular to substrate is formed in [Pt/Fe]4 films annealed as in vacuum as in N2 atmosphere. Additional number of interfaces prevents the grain growth. In films after RTA the magnetic properties are improved and film's roughness is decreased.

Resume : Tin sulfides are of great interest for future optoelectronic applications due to their physical properties, high chemical stability and earth abundance of elements [1,2]. Additionally, the electrical and optical properties of SnS, SnS2 or Sn2S3 can be easily tailored by modifying the growth conditions or doping with different dopants without significantly disturbing their crystal structure. Tin sulfides nanosize powder is obtained in mill grinding process. Structural and optical properties of the samples are studied by XRD, Raman spectroscopy and luminescence. Influence of magnetic dopants (Fe, Mn, Co, Cr, Ni) on structural and optical properties will be discussed. [1] N. Koteeswara Reddy, M. Devika, E. S. R. Gopal, Review on Tin (II) Sulfide (SnS) Material: Synthesis, Properties, and Applications, Critical Reviews in Solid State and Materials Sciences Vol. 40 , Iss. 6, 2015. [2] J. M. Skelton, L. A. Burton, F. Oba, A. Walsh, Chemical and Lattice Stability of the Tin Sulfides, The Journal of Physical Chemistry C 121 (12), 6446-6454, 2017.

Resume : Simultaneous influences of hydrostatic pressure and temperature combined to the size effect on the behavior of the exciton in 2D AlAs/GaAs/AlAs quantum ultra thin disk are investigated. Our approach is performed in the framework of effective mass theory and adiabatic approximation by using a variational method with a robust trial wave function and taking into account the dependence of the size, the dielectric constant and the effective masses on the pressure and temperature. Variations of the excitonic binding energy, photoluminescence energy and oscillator strength are determined according to hydrostatic pressure and temperature for different confinement regimes. The results of our numerical calculations show that the applied pressure favors the electron-hole attraction while the temperature tends to decrease the exciton binding energy. Another interesting result is the possibility of transforming a thin quantum disk into a large-gap material by strain effect. The opposing effects caused by temperature and pressure reveal a big practical interest and offer an alternative way to the tuning of the excitonic transition in optronic devices, instead of modifying the size of the quantum dots or searching for new materials.

Resume : In the last decades, theoretical predictions (see papers by J. Emmerlich et al.) supported by subsequent experimental works showed that ductile ceramic materials with intrinsically layered structures are possible with the X2BC structure, where X= Mo, Ta or W. These materials hence joined the group of prospective candidates for protective hard coatings employed in cutting and forming applications as they exhibit a high stiffness in combination with moderate ductility. In this work we report on our continued research of nanostructured Mo-B-C coatings prepared on WC-Co hard metal substrates by magnetron sputtering. The microstructure of deposited thin layers was studied after additional heat treatment by means of scanning and transmission electron microscopy (SEM and TEM) on cross sections prepared using a focused ion beam in SEM. Both undisturbed layers and the volume under indentation prints were observed. TEM observations showed that elements redistribution due to annealing and namely Co diffusion from the substrate lead to the formation of a new continuous Mo-Co-B interfacial layer. These observations were correlated with elastic modulus, indentation hardness and fracture resistance of the coatings obtained from nanoindentation experiments in both static and dynamic loading regime using a Berkovich indenter. Crack resistance was tested using indentation experiments with cube-corner indenter. The research was supported by the Czech Science Foundation (Project 15-17875S).

Resume : X-ray spectrometry based on radiometrically calibrated instrumentation, which ensures the physical traceability of quantification to the SI units, is a unique feature of PTB. For X-ray spectrometry, various beamlines are available at PTB’s laboratory at BESSY II in the spectral ranges of soft and hard X-rays (78 eV to 10.5 keV), as well as the “BAMline” for photon energies up to 60 keV [1–3]. With X-ray spectrometry, surfaces, solids, liquids, nanolayers and nanostructures can be characterized with respect to their physical and chemical properties – such as chemical composition, elemental depth profiles, layer thicknesses, and species and coordination fractions. The functionality of nano-scaled materials with designed physical and chemical properties can provide new functionalities for applications in areas such as health, energy, transport, and climate protection. Such materials have to be developed increasingly fast, which requires reliable analytical characterization methods for a timely correlation of the functionalities with the underlying material properties. However, only few reference materials are available in the different nanotechnology related categories whereas several dozens of new materials are being created every month. This is becoming increasingly problematic as a reliable chemical traceability of numerous characterization technologies can thus not be guranteed. This circumstance – which limits materials research and development – can be counteracted by reference measurement procedures such as, for example, X-ray spectrometry. References: [1] F. Senf et al., J. Synchrotron Rad. 5, 780 (1998) [2] M. Krumrey et al., Nucl. Instrum. Meth.A 467–468, 1175 (2001) [3] W. Görner et al., Nucl. Instrum. Meth. A 467–468, 703 (2001)

Resume : Bulk ferroelectric ceramics obtained by doping KNbO3 with BiFeO3 (KBNFO) show interesting photovoltaic properties, due to the capability of tailoring the bandgap widths by means of the doping content (ranging from ≈3,22 eV, for pure KNbO3, to ≈2,25 eV, when the doping is 25%). This change in the bandgap is achieved without modifying the crystal structure, and with very little changes in the unit cell parameters. Similar effects in bandgap and cell parameters have been observed when doping KNbO3 with other oxides like BiMnO3 (KBNMO) and BiCoO3 (KBNCO). Despite these promising features, at the best of our knowledge, studies of these ceramics in thin film form are scarce. In this work we used pulsed laser deposition technique to grow thin films of K0,75Bi0,25Nb0,75Fe0,25O3, K0,85Bi0,15Nb0,85Mn0,15O3 and K0,85Bi0,15Nb0,85Co0,15O3 onto STO and MgO single crystal substrates. Crystal structure characterizations were performed by means of θ/2θ scans and reciprocal space maps. Results revealed that the crystallinity of KBNFO films growth onto STO was better than those of films grown onto MgO. Moreover, in both substrates, films were epitaxial, with a cube-on-cube relationship. In addition to KBNFO crystallites, the presence of non textured KBi2Nb5O16 was also detected.

Resume : Ga-doped ZnO (GZO) is promising candidates for transparent electrodes in optoelectrical applications, and plasmonic materials for near-infrared applications. It is essential to increase carrier mobility as well as to control carrier concentration for high performance of GZO films. In highly doped semiconductors, the mobility is limited by ionized impurity scattering. However, it was reported that the mobility enhanced in extremely thin GZO films deposited on epitaxial ZnO layer, and it was explained by a very high Debye-tail mobility at the interface [1]. In this paper, we explore such the enhancement of the Hall mobility in GZO/ZnO heterostructures fabricated on glass substrates. The GZO films deposited on Zn-polar ZnO showed higher conductivity compared to those deposited on O-polar ZnO [2]. Accordingly, the 200-nm-thick ZnO layer was firstly deposited on Zn-polar ZnO template by RF magnetron sputtering followed by deposition of a GZO layer by DC magnetron sputtering in a same deposition system. The thickness of GZO layer were varied between 2 nm and 100 nm. The Hall mobility of 100-nm thick GZO was 25 cm^2/Vs. It slightly decreased with decreasing the thickness, however, it abruptly enhanced below 20 nm. The Hall mobility as high as 34 cm^2/Vs was obtained in the 2-nm-thick GZO films. [1] D. Look et al, Appl. Phys. Lett. 106 (2015) 152102. [2] L. Nulhakim et al, Phys. Status Solidi-Rapid Res. Lett., 10, 535 (2016).

Resume : The aim of the work reported here is the growth of TiO2 films and nanostructures starting from a Ti6Al4V target on a (100)-oriented silicon substrate using the pulsed laser deposition (PLD) technique. The depositions were carried out in different atmospheres (vacuum, Ar or O2) and at room temperature (RT) or 500 °C. To promote a nanostructuring of these films, a post deposition hydrothermal treatment at different temperatures in the range (150-600 °C) has been applied. From structural point of view the obtained films were investigated by grazing and symmetrical incidence X-ray diffraction (GIXRD and XRD). The surface morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Also, the chemical composition of the deposited films was investigated using X-ray energy dispersive (EDX) and X-ray photoelectron spectroscopy (XPS). The electrical characteristics of the deposited films were measured by a four point probe technique. The results showed that the films deposited under high oxygen pressures (10 mbar) are oxidised and develop nanostructures that could lead to the formation of nanotubes when they are further oxidized under water vapour. Such surface morphologies are beneficial for implants osseointegration.

Resume : Indium-gallium-zinc oxide (IGZO) has generated substantial interest in the last decade for use as a channel layer for transparent thin film transistors (TTFT). IGZO TTFTs with top gate – bottom contacts geometry were fabricated using the pulsed laser deposition (PLD). Amorphous indium zinc oxide (IZO) thin films were grown for the source-drain and gate contacts. A study concerning the dependence of these thin films transport properties on the oxygen gas pressure and laser fluence has been performed. Various IZO films with In/(In+Zn) values of 50%, 70% and 90% and IGZO (In:Ga:Zn ratios 1:1:1 or 1:1:2) target stoichiometries were used. HfO2 thin films, also deposited using PLD technique were used as gate insulator. The leakage currents and breakdown field of the gate insulator films were investigated for various PLD deposition conditions. The source-drain and gate contacts were patterned using metallic shadow masks produced by electro-erosion technique. The study presented here concerns the influence of the PLD working conditions (gas type and pressure, the target-substrate distance and the laser pulse energy) on the films interfaces quality and their electrical properties in order to obtain the optimal performance in a TTFT.

Resume : Amorphous and nanocrystalline indium zinc oxide (IZO) films have excellent opto-electronic properties that have been exploited in transparent thin film transistors and display devices. Pulsed laser deposition (PLD) is one laboratory technique that can be easily used to grow thin films having various structures and compositions to investigate their properties. We used the PLD technique to grow thin films of IZO with In/(In+Zn) values from 0.1 to 0.9 on Si and glass substrates from room temperature up to 100 °C. The grown films and devices were irradiated by gamma radiation to investigate the effects of radiation on their structure and properties. The deposited films were investigated by atomic force microscopy, X-ray reflectivity (XRR) and X-ray diffuse scattering. In addition, we also used optical reflectometry and photoluminescence (PL) to characterize the changes of the optical properties caused by irradiation. The results showed that these amorphous transparent oxide films could withstand a high level of gamma radiation without adverse effects upon their structure, stoichiometry or optical and electrical properties.

Resume : Thin ZrC and ZrN films have important applications in advanced nuclear reactors, fusion installations or space exploration, where there are strong radiation fields. It has been recently reported that thin films, which are nanostructured, having grain sizes usually smaller than 30 nm, behaved differently under irradiation than polycrystalline or single crystal films and materials. To investigate in detail the radiation effects on properties and structure of such materials we used the Pulsed Laser Deposition (PLD) technique to grow nanocrystalline or amorphous thin films from inexpensive targets. By simply changing the deposition parameters, films possessing different chemical compositions and/or structures could be readily obtained. The effects of 800 keV Ar and 1.0 MeV and 1.5 MeV Au ions on the microstructure of nanocrystalline ZrC and ZrN thin films were investigated using high resolution transmission electron microscopy and X-ray diffraction investigations. The results confirmed that nanocrystalline films could withstand high irradiation fluences without degrading their crystalline structure, while the Si substrate was completely amorphized on a depth of 700 nm. XRD investigations found that there is grain growth in such films as an effect of ion irradiation. The results are compared to those reported on polycrystalline or single crystal materials.

Resume : The aim of the work reported here is the growth of TiO2 films and nanostructures starting from a Ti6Al4V target on a (100)-oriented silicon substrate using the pulsed laser deposition (PLD) technique. The depositions were carried out in different atmospheres (vacuum, Ar or O2) and at room temperature (RT) or 500 °C. To promote a nanostructuring of these films, a post deposition hydrothermal treatment at different temperatures in the range (150-600 °C) has been applied. From structural point of view the obtained films were investigated by grazing and symmetrical incidence X-ray diffraction (GIXRD and XRD). The surface morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Also, the chemical composition of the deposited films was investigated using X-ray energy dispersive (EDX) and X-ray photoelectron spectroscopy (XPS). The electrical characteristics of the deposited films were measured by a four point probe technique. The results showed that the films deposited under high oxygen pressures (10 mbar) are oxidised and develop nanostructures that could lead to the formation of nanotubes when they are further oxidized under water vapour. Such surface morphologies are beneficial for implants osseointegration.

Resume : Thin (<1 µm) and very thin (<100 nm) film materials are key ingredients for many high tech devices. Challenging applica-tions for these films are under development in the fields of energy storage, thermoelectrics, photovoltaics, nanoelectronics, hard coatings and smart biomaterials. Inexpensive and fast techniques are required for the precise and accurate measure-ments of the structure and the elemental composition of thin films. Very few techniques such as Rutherford backscattering and transmission electron microscopy are recognized to enable accurate compositional analyses of thin films. Unfortu-nately, both are quite complex and expensive and require high vacuum. Other techniques that are well established for ele-mental analysis of bulk materials, suffer from difficulties of measurement calibration for thin films. Therefore, calibration-free laser-induced breakdown spectroscopy appears as very promising new solution to overcome these major difficulties. Here we report on LIBS analyses of thin films based on plasma modeling. The measurement concern relative simple bi-elemental films and complex multielemental coatings produced by pulsed laser deposition. The results are compared to those obtained with standard-less complementary measurements using energy-dispersive X-ray spectroscopy (low accura-cy) and Rutherford backscattering spectrometry (high accuracy). The influences of the experimental conditions and the accuracy of available spectroscopic data on the analytical results are discussed.

Resume : Ferroelectric tunnel junctions (FTJs) have emerged as promising candidates for novel memory devices. The success of this approach relies on the ability to build ferroelectric tunnel junctions with large electroresistance (ER) response. In ferroelectric tunnel junctions, ER can be understood on the basis of the change of the tunnel barrier height upon polarization reversal in presence of asymmetric electrodes. However, other mechanism may also contribute to the electroresistance, such as space-charge effects or ionic motions, their combination determining the magnitude and sign of the electroresistance (ER=(Rup-Rdown)/Rdown, where Rup and Rdown are the resistance values for polarizations with opposite sign). Understanding the thickness, time-dependent, and temperature response of ER in FTJs is crucial to fully understand the origin of ER with different sign and magnitude. Here, we report on the systematic investigation of ER dependence on ferroelectric sample thickness and writing time (tw) of Pt/ BaTiO3/La0.7Sr0.3MnO3/ SrTiO3 Ferroelectric Tunnel Junctions. Our results reveal that the different conductance states of the barrier have a radically different sign, magnitude and dependence on the writing time (for writing times longer than 1s) and temperature depending on sample thickness. Having all the samples the same top and bottom electrodes and consequently similar ferroelectric/metal interfaces, we can conclude that ER results from a combination of a fast FE polarization switching and a slower process, which origin will be discussed. We believe that the presented results will help to the better understanding of the ER phenomena in thin ferroelectric films.

Resume : Previous studies have shown that the usage of nanoscale growth areas allows to reduce the density of extended defects in heteroepitaxial semiconductor layers grown on mismatched substrates [1]. While these studies employed a top-down approach for creating the nanopatterns on the substrate surface, in the present contribution we exploit an inexpensive and scalable bottom-up approach, i.e. nanosphere lithography combined with plasma assisted reactive ion etching, for fabricating nanopillar arrays on GaAs(111)A surfaces. Regular arrays of both, nanopillars and nanocones can be fabricated by this technique. The dimensions and morphologies of these nanostructures are characterized in terms of the fabrication conditions by using scanning electron microscopy. The GaAs nanostructures were overgrown with InAs by molecular beam epitaxy. Transmission electron microscopy investigations of these nanostructures were performed to understand the formation of misfit-related defects as a function of the GaAs nanopillar dimensions and array periodicity. [1] R. Kemper, D.J. As, J.K.N. Lindner: in: “Silicon-based Nanomaterials”, eds. H. Li et al., Springer Series in Materials Science 187 (2013) 381

Resume : The achievement of high-quality transparent conductive oxides (TCO), based on indium tin oxide (ITO) is of great interest for terahertz devices, thin film solar cells or gas sensors. We studied TCO thin films prepared by radio frequency magnetron sputtering technique onto glass substrates from a target of ITO (In2O3:Sn) having a molar ratio of 90:10. A comparative analysis was conducted in measuring the thickness of the corresponding oxide thin films, using SEM, interferential microscopy and stylus profilometry methods. The structural and optical properties of these samples were investigated and it was found that the respective films show typical TCO properties. After deposition, the temperature dependences of the structural and optical properties were investigated using thin-film samples subjected to rapid thermal annealing (RTA) in air at temperatures up to 723 K. The influences of post deposition thermal treatment on structural properties of these oxides based on XRD, XRR and XPS results were discussed. Surface morphology of the rapid thermally annealed samples appeared as granular and polycrystalline with high optical transparency and good electrical conductivity. Transmission spectra in the spectral range 190 – 3000 nm evidenced that direct bandgaps ranged between 3.35 eV and 3.65 eV from these ITO films. The results are discussed in correlation with the optical properties of the thin films and the role of annealing treatment in oxide thin films.

Resume : Localized cooling in micro- and nano-electronics as well as energy autonomy in applications such as wireless sensor networks and wearable electronics could be well served by thin film thermoelectric devices fabricated on solid and/or flexible substrates. Bi0.5Sb1.5Te3 is considered to be a state-of-the-art p-type thermoelectric material, at temperatures near room temperature, due to its high power factor and ZT value. Nevertheless, the deposition of Bi0.5Sb1.5Te3 thin films with bulk-like thermoelectric properties remains a great challenge. We have grown p-type Bi0.5Sb1.5Te3 thin films onto different types of substrates such as fused silica and Kapton using pulsed laser deposition and home-made targets. The films were grown at room temperature and then were subjected to a post-deposition ex-situ annealing process. In this poster presentation, we will present our recent results on Seebeck coefficient, electrical resistivity and Hall carrier concentration as a function of temperature (200-390K). We will discuss how the thermoelectric properties of the obtained films are affected by the substrate type and stoichiometry. Also, we will address the effect of post-annealing treatment on the structural and thermoelectric properties. The power factor values of our post-annealed films are similar to those of the best bulk materials.

Resume : Transition metal oxides (TMOs) exhibit a rich variety of properties which can be exploited for a wide range of applications including ultra-high-density magnetic data storage, spintronics, quantum computing and thermal management applications. The discovery of high thermal conductivity in cuprates has increased the interest for low-dimensional Heisenberg magnetic systems. It has been demonstrated that, in addition to electrons and phonons, heat can also be transported by magnetic excitations (magnons or spinons) in quasi low-dimensional electrically-insulating quantum magnets. The magnetic heat conduction is highly anisotropic, outweighs the lattice contribution and has mainly been studied in novel complex transition metal oxides with quasi 1D spin structures such as the “two-leg ladder” compound (La,Sr,Ca)14Cu24O41. The room-temperature magnetic thermal conductivity (of the order of 100 Wm−1K−1) is as high as in metals. Nevertheless, the fabrication of such compounds has proven to be challenging due to the complexity of the structure. The influence of the deposition temperature and layer thickness of the La5Ca9Cu24O42 films grown by pulsed laser deposition (PLD) on (100) and (110) SrTiO3 substrates is investigated in detail. In this work, we have focused on the influence of the strain nature on the single-crystal charater of the grown films. Single-domain La5Ca9Cu24O42 films based on HRXRD and HRTEM studies were obtained only on (100) SrTiO3 under certain conditions.

Resume : Owing to its resemblance to the major inorganic constituent of bone and tooth, hydroxyapatite (HA) is recognized as one of the most biocompatible materials and is widely used in bone replacement and regeneration. HA nano-fibers are recognized as useful toughening agents of other biomaterials. However, it has been reported that the degradation rate of HA in vivo is below 10% each year, which is far slower than the growth rate of newly formed bone. Therefore, it is of general interest to improve the degradation rate of this bioactive material. Ample researches have been conducted to improve the degradation rate of HA, such as increasing its porosity, decreasing the Ca/P ratios, decreasing the grain size or crystallinity and introducing some cations into the apatite lattice, etc. For instance, numerous structural defects detected in the carbonate-containing apatites result in its increasing dissolution values. Based on those previous researches, we could draw the conclusion that the crystal defects in the apatite play important roles upon their reactivity and solubility. Nevertheless, there appears to have no efficient strategy provided to substantially improve the HA degradation property.