Field of Expertise: Advanced Material Science

Advanced Materials Day 2012


Poster Session Thursday 21 June 2012      1

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Cluster Synthesis and Formation of Core-Shell Structures in Superfluid Helium Nanodroplets
Alexander Volk, Institute of Experimental Physics

Abstract: Chromium nanoclusters (CrN) were synthesised in superfluid nanodroplets and deposited on an amorphous carbon (a-C) surface. Transmission electron microscope (TEM) images shall be presented as well as microbalance measurements.
During the past decade, helium nanodroplets (HeN) turned out to offer a unique experimental environment, combining both a least perturbing superfluid quantum matrix at 0.4 K and a well defined local confinement, given by the droplet radius of a few nanometers [1]. Therefore these nanocryostats are a versatile tool for a wide field of spectroscopic investigations [2] and also well suited for the synthesis of metal nanoclusters.
The first surface deposition of clusters assembled in HeN has only recently been demonstrated with AgN [3]. Cluster synthesis in a superfluid environment may favour the formation of high spin species, as indicated in experiments with Alkali-metal oligomers [4] and Ag dimers [5]. Therefore the synthesis of tailored magnetic clusters may be possible in HeN, which is of particular interest for transition metal atoms with high magnetic moments such as Cr.
In a first step we were able to form CrN inside HeN [6] and to subsequently deposit the clusters on an a-C surface. Besides the TEM images and microbalance investigations, we seek to present more detailed chemical information, obtained from X-ray photoelectron spectroscopy (XPS) and electron energy loss spectrometry (EELS) measurements. The magnetic properties of the deposited clusters are planned to be investigated by means of magnetometric methods.
A further aim is to produce bimetallic core shell structures in HeN by sequential particle pickup. For fast oxidating materials like CrN, this can offer the possibility to passivate a cluster by enclosing it in a noble metal shell. Due to the formation process in the favourable superfluid environment, we expect a radial symmetric structure of these bimetallic nanostructures.

[1] J. P. Toennies and A. F. Vilesov; Angew. Chem. Int. Ed 43, 2622-2648 (2004)
[2] C. Callegari and W. E. Ernst; Handbook of High Resolution Spectroscopy, Eds. F. Merkt and M. Quack, 1st Edition, Vol. 3, p. 1551-1594
[3] E. Loginov, L. F. Gomez, and A. F. Vilesov; J. Phys. Chem. A 115, 7199-7204 (2011)
[4] M. Theisen, F. Lackner, and W. E. Ernst, J. Phys. Chem. A 115, 7005 (2011)
[5] A. Przystawik, P. Radcliffe, S. Göde, K. H. Meiwes-Broer and J. Tiggesbäumker; J. Phys. B: At. Mol. Opt. Phys. 39, 1183–1189 (2006)
[6] M. Ratschek, M. Koch, and W. E. Ernst; J. Chem. Phys. 136, 104201 (2012)

Proceeding contribution: Abstract_Volk_ISSPIC2012.pdf

Direct observation of ferroelectric domain wall motion under mechanical stress in the ESEM
Angelika Reichmann, Klaus Reichmann, Armin Zankel, P. Plt, Institute for Electron Microscopy

Abstract: A domain structure within single crystals, polycrystalline ceramics or thin films is a commonly observed feature of ferroic materials (ferroelectrics, ferromagnetics and ferroelastics) below a typical transition temperature usually referred to as Curie point. The formation of domains is a result of the minimization of electric or magnetic stray field energy and/or elastic energy. Ferroelectric materials respond to mechanical stress with a polarization. This polarization consists of an intrinsic part due to compression of the unit cell and an extrinsic part due to domain wall motion. Such a response can be observed in piezoelectric ceramics and is described as mechanical poling. The mechanical behaviour is called ferroelastic effect. The direct observation of the domain wall motion under mechanical stress provides a tool for evaluating the extrinsic contribution to the materials with respect to reversible and irreversible parts.
The samples were imaged by orientation contrast, an easy and quick method for investigating the microstructure of ceramic materials, enabling the observation of respective changes during in situ experiments. Crystal orientation contrast caused by orientation anisotropy of backscattered electrons can generate images in which grains of different orientations in polycrystalline material have different grey levels. Even ferroelectric domains can be observed because of their differing polarization axes giving rise to twin boundaries. As test sample Bariumtitanate BaTiO3, a ferroelectric material with a Curie-temperature of 127C was investigated. From such sintered pellets rectangular blocks of 775 mm3 were cut and one of the 77 mm2 faces was polished first with diamond paste. Due to the very low information depth of about 5-50 nm of the signal, a final polishing step using colloidal silica is necessary to get a fully distortion free surface.
In situ investigations were carried out in an ESEM equipped with a tensile stage using the compression mode. Domain wall motion was observed at a stress exceeding 33 MPa resulting in mechanical poling.

Microstructural Evolution of the Heat-affected Zone in 9%Cr Steels
Dilek Halici, Bernhard Sonderegger; Peter Mayr; Klaus Reichmann, Institut fr Werkstoffkunde und Schweitechnik

Abstract: 9-12% chromium steels with tempered martensitic microstructure are favored materials for creep exposed parts in fuel-fired thermal power plants.
Welded components, which are characterized by the welding fusion line and the heat-affected zone (HAZ) are used for the aforementioned facilities. The welding in turn causes an altering of the heat-affected zone, and this brings about a change in their mechanical properties. The objective of the present study therefore is to analyze the microstructural evolution of the HAZ in three 9%Cr steels during thermal treatments.

Poster: Advanced Materials Day2012_DilekHalici.pdf

Electrochemical Tuning of the Electrical Resistance of Nanoporous Platinum produced by Dealloying
E. Steyskal, R. Wrschum, Institut fr Materialphysik

Abstract: Owing to extremely high surface-to-volume ratios, nanostructured materials can exhibit tunable properties. Here, the electrical resistance R of nanoporous platinum produced by dealloying Cu75Pt25 is shown to be reversibly tunable by several percent upon electrochemical charging in an electrolyte. In the investigated potential range the charge coefficient (ΔR/R0)/ ΔQ exhibits a sign inversion. The positive sign behaviour, in accordance with previous results on other nanostructured metals, can be attributed to variations in the charge carrier density and interfacial scattering. In the negative sign regime, R exhibits a sluggish response to potential variations, indicating a slow ionic process. The influence of oxygen adsorption and surface oxidation will be discussed.

A novel structuring method to integrate organic field-effect transistor based sensors into microfluidic systems
Johannes Kofler, Institute of Solid State Physics

Abstract: Microfluidic systems gained tremendous importance over the last few
years. Key applications are, for example, clinical pathology and testing
of water for biochemical toxins and other dangerous pathogens. One
bottleneck for a broad commercial application is that sensitive and
cheap sensors are required. It is believed that organic sensing devices
meet these requirements and that their integration into microfluidic
systems has a high future potential. In this context, a novel structuring
method to fabricate microfluidic devices for integrated organic sensor
structures is investigated. This method is based on the common replica
molding process of a master using PDMS (polydimethylsiloxane). The
difference compared to equivalent methods is that the complex
functional parts (reservoir, mixer, etc.) of these masters are structured
with a high resolution (< 1 μm) by electron beam lithography whereas
the periphery is structured by an inkjet printer with a low resolution
(> 60 μm). Using this concept, resources and fabrication times are
minimized while high resolution and flexibility is retained. The
accordingly structured PDMS replicas are subsequently bonded onto an
organic sensor substrate using the stamp and stick bonding technique.

Kainz Theresa, Michael Naderer, Denis Schtz, Denis Orosel, Franz Mautner, Klaus Reichmann, Institut fr Chemische Technologie von Materialien

Abstract: The lead-free Bismuth sodium titanate bismuth potassium titanate solid solutions offer an interesting range of piezoelectric properties. For a successful production of high performing materials a better understanding of the formation reaction is necessary.
Various compositions of (100-x)BNT-xBKT (x=0, 10, 25, 50, 75, 90, 100) were prepared by the mixed oxide route. All samples were analyzed by thermogravimetry coupled with mass spectroscopy and differential scanning calorimetry. An additional mass loss was detected below 650C which can most likely be attributed to the volatilization of alkaline oxides. HT-XRD up to 770C indicated emerging and vanishing of phases during the calcination. Special attention will be paid to the differences and similarities in behavior between the individual composition.

Kerstin Schmoltner (a), K. Koren (a), J. Kofler (a), A. Klug (a) and E. J. W. List (a,b), (a) NanoTecCenter Weiz Forschungsgesellschaft mbH
(b) Institute of Solid State Physics, Graz University of Technology

Abstract: Organic field-effect transistors (OFETs) are highly promising candidates for chemical and biological sensing. Many organic compounds are solution-processable at low temperatures on a variety of substrates, which allows for cost-effective fabrication methods, leading to smart (disposable) sensor tags. Moreover, organic (semiconducting) materials can be tailored with respect to their chemical and physical properties, enhancing the sensitivity and selectivity towards a specific analyte. Concerning the detection of ions in aqueous solutions, a water-stable operation of OFET sensor elements is crucial. Thus low voltage operation is a prerequisite.
In this context, we investigate OFETs with reduced operating voltages by utilizing a thin high-k dielectric gate insulator. In particular, anodized aluminium oxide Al2O3 was studied thoroughly in capacitor structures, metal-insulator-semiconductor (MIS) diodes as well as bottom gate OFETs. These different architectures were used to compare bulk and interface properties of the constituting materials in detail. Low-voltage poly(3-hexylthiophene) (P3HT)-based top- and bottom-contact OFETs were investigated in terms of operational stability, field-effect mobility, switch-on voltages and on/off-current ratios. Moreover, information about doping, trapping processes and mobile charges were obtained by characterizing MIS diodes.

High-resolution laser dilatometry applied to relaxation phenomena in bulk metallic glasses
Martin Luckabauer1, Rainer Wunderlich2, Hans-Jrg Fecht2 and Wolfgang Sprengel1,
1Institute of Materials Physics, Graz University of Technology
2Institute of Micro and Nanomaterials, Ulm University

Abstract: Vacancy equilibration in some intermetallic compounds or relaxation in bulk metallic glasses are examples of volume related phenomena with very high time constants (up to 106 s). With common dilatometric measurement techniques it is difficult to realize stable measurement conditions in the ΔL/L ~ 10-7 range for such a long time scale. The aim of the high-resolution laser dilatometry project is to realize a measurement system for long-time, isothermal measurements. As an example results of the reversible formation and disappearance of free volume in the bulk metallic glass Zr56Al7Cu24Ni10Co3 are described in detail.

Strongly correlated quantum dots out of equilibrium
Martin Nuss, Institute of Theoretical and Computational Physics

Abstract: The theoretical understanding of the non-equilibrium behavior of strongly correlated quantum many body systems is a long standing challenge, which has become increasingly relevant with the progress made in the fields of molecular- and nano- electronics, spintronics or quantum optics and simulation. Besides seeding the fundamental concepts for promising future applications, developing a sound understanding of the plethora of previously unrecognized effects, arising in a non-equilibrium situation, is currently at the forefront of theoretical research. We present results for the non-linear response of a single quantum dot to an applied bias voltage. Non-equilibrium cluster perturbation theory, and its variational improvement, the non-equilibrium variational cluster approach, are used to obtain steady-state quantities like the full current-voltage characteristics of a single quantum dot. The non-equilibrium extensions of the well-established cluster perturbation theory and the variational cluster approach are based on the Keldysh Greens function method which allows, in this case, accessing single particle dynamic quantities on the whole complex plane. These flexible and versatile techniques can in principle be applied to any lattice Hamiltonian with local interactions, including multi-band and multi-impurity systems. Within this framework it is possible to work in the thermodynamic limit and therefore exchange particles with a bath and/or dissipate energy. We will highlight the importance of the self-consistently determined variational parameters, introduced in the non-equilibrium variational cluster approach. Furthermore we present data for the out of equilibrium electronic density of states. Results for the short time evolution after a sudden change in the system parameters were obtained by using Matrix Product States based methods. We compare our results in some limits with data from a real time evolution (Time Evolving Block Decimation), Quantum Monte Carlo, Functional Renormalization Group as well as Perturbation Theory and find good agreement.

The influence of Ti-nonstoichiometry in BNT
Michael Naderer, Theresa Kainz, Denis Schtz, Klaus Reichmann, Institute for Chemistry and Technology of Materials

Abstract: BNT is a promising candidate to replace PZT in certain applications. It is miscible with various other ferroelectric perovskites and is highly modifiable in its behavior by doping with iso- and aliovalent metal ions. In this work we show the influence of Ti-nonstoichiometry on material parameters and the microstructure. Nonstoichiometric BNT can maintain stable vacancy concentrations on both A- and B-site by evaporating Bi and Na or with the formation of a titanium-rich secondary phase. Unlike other lead-free materials like KNN, whose melting point changes dramatically with the stoichiometric ratio of alkaline ions to Nb, BNT has a kind of self-healing ability which makes processing more easy. The formation of an electrical inactive secondary phase has, besides induced mechanical stress during deformation, no impact on the behavior of the material.
The microstructure was investigated using XRD and SEM/EDX, electrical parameters include hysteresis loops, temperature dependent permittivity plots and small signal data at room temperature for permittivity, loss tangent and d33. Density values were obtained using XRD for theoretical density and Archimedes' method for disc samples.

Naveed Anjum

Defects in an iron-nickel meteorite examined with positrons
Peter Parz, Institut fr Materialphysik

Abstract: P.Parz, Matthias Leitner, Wolfgang Sprengel and Werner Puff

Graz University of Technology, Institute of Materials Physics, Petersgasse 16, 8010 Graz, AUSTRIA

A Gibeon meteorite (Fe-Ni meteorite) a typical iron-nickel meteorite with a composition of Ni 7.93 wt-%, Co 0.41 wt-%, P 0.04 wt-%, 91.62wt-% Fe and small amounts of carbon was investigates by positron annihilation methods. This meteorite was formed approximately 4 billion years ago. Upon the low cooling rates in space (~3510-6 K/a) the Widmansttten structure was formed. Positron lifetime measurements showed a high mean positron lifetime, which reveals open volume defects within this structure. Upon temperature treatment a change of the chemical environment of this open volume defects as well as their annealing could be monitored. It is concluded, that misfit dislocations between the Ni-rich (Taenite) and Ni-poor phase are present in the meteorite sample.

Proton implanted silicon wafers investigated by electron beam induced current measurements
S. Kirnstötter 1,2, P. Hadley1, W. Schustereder2, J. Laven2,3 H. J. Schulze², 1. Institute of Solid State Physics, TU Graz ,2. Infineon Technologies, 3. Universität Erlangen-Nürnberg

Abstract: Electron Beam Induced Current (EBIC) is an analysis method used in a Scanning Electron Microscope (SEM) to investigate buried junctions or defects in semiconductors. [1-3] During an EBIC measurement, the electron beam enters a semiconductor and generates electron-hole pairs. If the charge carriers diffuse into a region where there is a built-in electric field, such as a pn junction or a Schottky contact, charge separation will occur and a current will flow.
We have used EBIC to investigate proton implanted silicon wafers with implantation doses from 1013 p+/cm² to 1015 p+/cm² and with implantation energies from 500 keV to 5 MeV. [4] The implantation introduces vacancies, silicon interstitials, and hydrogen into the crystal. The sample is then annealed in the temperature range from 350-550°C and defect complexes form. The microscopic structure of these defect complexes is not completely understood. There is a class of oxygen-vacancy defect complexes called thermal donors that are known to act as donors in silicon. Beside EBIC we used voltage contrast imaging, Schottky contacts of tungsten tips and spreading resistance profiling (SRP) to investigate the electrical and materialistic properties of this wafers.

Poster: GADEST_2011.pdf

Sebastian Nau, Stefan Sax, Emil J. W. List, NanoTec Center Weiz Forschungsgesellschaft mbH

Abstract: One of the striking arguments for resistive switching based memory elements [1] is the potential integration in two-terminal crossbar structures without using any access transistor, which decreases the minimum cell size of an active storage element in a crossbar array drastically to 4F (where F is the minimum feature size, i.e. the electrode width in this case). Compared to conventional memory technologies such as dynamic RAM (cell size: 6F-8F) or static RAM (cell size: 140F), resistive switching-based RAM (RRAM) opens up a possibility for future high-density and inexpensive memory and storage media. Besides that, excellent access times, ON/OFF ratios, retention times and a wide variety of applicable materials and device architectures are reported. [2, 3]

However, the integration into a two-terminal array significantly complicates the read-out of a distinct memory element due to parasitic currents via adjacent elements in the low resistance state. In general, this issue is proposed to can be overcome by the introduction of a rectifying diode in series to each memory element.

In particular, we present recent results of different single hybrid organic/inorganic memory elements and single inorganic and organic rectifying diodes. Along these lines, the fundamental requirements for a successful integration of those two devices to a one diode one resistor crossbar array will be discussed.

[1] L. Ma, J. Liu, Y. Yang, Appl. Phys. Lett. 80, 2997 (2001)
[2] Y. Yang, J. Ouyang, L. Ma, R. Tseng, C.-W. Chu, Adv. Funct. Mater. 16, 1001 (2006).
[3] L. D. Bozano, B. W. Kean, M. Beinhoff, K. R. Carter, J. C. Scott, Adv. Funct. Mater. 15, 1933 (2005).

This work was supported by FP7-NMP-2010-SMALL-4 program, project number 263073.

SQUID magnetometry with in-situ cyclic voltammetry: Variation of the magnetic moment of γ-Fe₂O₃ nanoparticles
Stefan Topolovec, Heinz Krenn, Roland Wrschum , 1. Inst. fr Materialphysik, TU Graz, 2. Inst. fr Physik, Univ. Graz

Abstract: Electrochemical charging is one approach to achieve voltage-induced tuning of magnetic properties. For example the magnetic moment of γ-Fe₂O₃ nanoparticles can be tuned reversibly up to several percent by this method. To study the underlying electrochemical process in detail, we have designed a special electrochemical cell, allowing the recording of cyclic voltammograms in-situ in a SQUID magnetometer. From the combined measurements it became obvious, that the charging coefficient s=Δm/ΔQ significantly depend on the examined potential range.
Electrochemically induced reactions at the surface of the γ-Fe₂O₃ nanoparticles and charging induced variations of the magnetic surface anisotropy are considered as possible mechanism, which can explain the observed variations of the magnetic moment.

Numerische Modellierung und Validierung der Ausscheidungskinetik in hochentwickelten kriechbestndigen austenitischen Sthlen
Vujic Stojan, Werkstoffkunde und Schweitechnik

Abstract: Der steigende Energiebedarf fordert effiziente Kohlekraftwerke. Der weltweit durchschnittliche Wirkungsgrad von Kohlekraftwerken liegt bei etwa 30% (Deutschland 38%) und wird durch Dampftemperaturen von 600C und 285bar Dampfdruck erreicht. Eine Steigerung des Dampfzustandes auf 700C/350bar
fhrt zu einer Wirkungsgradsteigerung auf 50%. Werkstoffkandidaten fr diese Temperaturen sind austenitische berhitzer-Werkstoffe wie Sanicro25, HR3C
und DMV310. Fr die Bewertung dieser Werkstoffe in Bezug auf die Mikrostrukturentwicklung wurden umfangreiche experimentelle Untersuchungen (LIM, REM,Korngre, Hrte) und Berechnungen mit MatCalc durchgefhrt. Nachfolgend sind die Ergebnisse aus den Untersuchungen fr Sanicro25 dargestellt.

Poster: Poster Advanced Material Science 21.06.2012_Vujic Stojan.pdf