Metastable and Nanostructured Materials

Selected publications

 Theses are listed here, and a complete list of all publications can be found there.

Improved hydrogen storage properties of LiBH4 via nanoconfinement in micro- and mesoporous aerogel-like carbon

LiBH4, a complex hydride, contains the highest amount of hydrogen among the solid hydrogen storage materials and is thus a promising material for mobile applications. Unfortunately, the hydrogen release occurs only at unfavorably high temperatures. Recently, it was shown that both the thermodynamics and the reaction kinetics of complex hydrides can be effectively influenced by confining them in scaffolds with nanoscopic pores. We have therefore studied the effect of nanoconfining LiBH4 in aerogel-like nitrogen-doped carbon scaffolds prepared by salt templating. They offer large pore volumes which allows for high LiBH4 loadings of 40 - 50 wt.%. The hydrogen desorption temperature, which is above 400°C for the bulk hydride, is reduced to 310°C upon this nanoconfinement with an onset temperature as low as 200°C. Partial rehydrogenation can be achieved under moderate conditions (60 bar hydrogen, 300°C). In-situ STEM EELS measurements at temperatures up to 400°C show, that the boron and LiH remains within the carbon scaffold also in the de-hydrogenated state. The relatively small degree of rehydrogenation is due to a partial oxidation of the amorphous boron as revealed by solid state 11B NMR.

A. Surrey et al., International Journal of Hydrogen Energy 41 (2016) 5540-5548 URL

Segregation phenomena in Nd–Fe–B nanoparticles

Rare earth transition metal compounds like Nd2Fe14B among the magnets with the highest energy product and coercive field. Nonetheless, there is still demand further to improve the magnetic properties of these alloys. The present study reveals the investigation on the formation and phase stability of Nd2Fe14B nanoparticles from the gas phase which could serve as a model system for their bulk counterparts. Particular attention is paid to the question, if the intermetallic Nd2Fe14B phase form in particles with only a few nanometers in size, which grow without contact to any solid or liquid matrix in a low pressure Ar atmosphere. It also addresses the possibility of segregation that goes along with the phase formation and how this possibly affect the magnetic properties. Aberration-corrected transmission electron microscopy was used in combination with spectroscopic methods to determine the local atomic structure and the chemical composition, and it was found that, Nd segregates towards the particle surface. The magnetic properties of Nd-Fe-B nanoparticles ensembles as determined from VSM measurements are correlated with the resulting core-shell structure of these Nd2Fe14B particles.

F. Schmidt et al., J. Nanopart. Res. 17 (2015) 170 URL

Modification of the structural and magnetic properties of granular FePt films by seed layer conditioning

The steadily increasing amount of digital information necessitates the availability of reliable high capacity magnetic data storage. Here, future hard disk drives with extended areal storage densities beyond 1.0 Tb/in2 are envisioned by using high anisotropy granular and chemically L10-ordered FePt (002) perpendicular media within a heat-assisted magnetic recording scheme. Perpendicular texturing of the [001] easy axes of the individual grains can be achieved by using MgO seed layers. It is therefore investigated, if and how an Ar+ ion irradiation of the MgO seed layer prior to the deposition of the magnetic material influences the MgO surface properties and hereby the FePt [001] texture. Structural investigations reveal a flattening of the seed layer surface accompanied by a change in the morphology of the FePt grains. Moreover, the fraction of small second layer particles and the degree of coalescence of the primarily deposited FePt grains strongly increases. As for the magnetic performance, this results in a reduced coercivity along the magnetic easy axis (out of plane) and in enhanced hard axis (in-plane) remanence values. The irradiation induced changes in the magnetic properties of the granular FePt-C films are traced back to the accordingly modified atomic structure of the FePt-MgO interface region.

S. Wicht et al., J. Appl. Phys. 117 (2015) 013907 URL

Electron vortex beams prepared by a spiral aperture with the goal to measure EMCD on ferromagnetic films via STEM

X-ray magnetic circular dichroism is a well established method to study element specific magnetic properties of a material, while electron magnetic circular dichroism (EMCD), which is the electron wave analogue to XMCD, is scarcely used today. Recently discovered electron vortex beams, that carry a discrete orbital angular momentum (OAM) L, are also predicted to reveal dichroic signals. Since electron beams can be easily focused down to sub-nanometer diameters, this novel technique promises the possibility to quantitatively determine local magnetic properties with unrivalled lateral resolution. As the spiralling wave front of the electron vortex beam has an azimutally growing phase shift of up to 2π and a phase singularity in its axial center, specially designed apertures are needed to generate such non-planar electron waves. We report on the preparation and successful implementation of spiral apertures into the condenser lens system of an aberration-corrected FEI Titan3 80-300 transmission electron microscope (TEM). This setup allows to perform scanning TEM (STEM) with vortex beams carrying user-selected OAM. First experiments on the interaction of the vortex beam with a poly-crystalline sample are presented. Within the achieved signal to noise ratio no EMCD signal has been detected. This finding is supported by simulations of inelastic scattering of a beam generated by spiral aperture.

D. Pohl et al., Ultramicroscopy 150 (2015) 16-22 URL

Silicon carbide embedded in carbon nanofibres: structure and band gap determination

Materials drastically alter their electronic properties when being reduced to the nanoscale due to quantum effects. Concerning semiconductors, the band gap is expected to broaden as a result of the quantum confinement. In this study we report on the successful synthesis of wide bandgap SiC nanowires (with great potential for applications) and the local determination of their band gap. Their value was found to be higher with respect to bulk SiC. The nanowires are grown as a heterostructure, i.e. encapsulated in carbon nanofibres via dc hot-filament Plasma-Enhanced Chemical Vapour Deposition on the Si/SiO2 substrate. The structure of the as-produced carbon nanofibres was characterized by means of aberration-corrected high-resolution transmission electron microscopy. Two different pure SiC polytypes, namely the 3C (cubic) and the 6H (hexagonal) as well as distorted structures are observed. The SiC nanowires have diameters in the range of 10–15 nm and lengths of several hundred nanometers. The formation of the SiC is a result of the substrate etching during the growth of the CNFs and a subsequent simultaneous diffusion of not only carbon, but also silicon through the catalyst particle.

A. Bonatto Minella et al., Phys. Chem. Chem. Phys. 16 (2014) 24437 URL

Near-Surface Strain in Icosahedra of Binary Metallic Alloys: Segregational versus Intrinsic Effects

A systematic structural analysis of FePt, CuAu, and Au icosahedral nanoparticles is presented. The uncovered particles are prepared by inert gas condensation and thermally equilibrated through in-flight optical annealing. Aberration-corrected high-resolution transmission electron microscopy reveals that the crystal lattice is significantly expanded near the particle surface. These experimental findings are corroborated by molecular statics simulations that show that this near-surface strain originates from both intrinsic strain due to the icosahedral structure and a partial segregation of the larger of the two alloy constituents to the particle surface.

D. Pohl et al., Nano Lett. 14 (2014) 1776 URL

Investigation of early cell–surface interactions of human mesenchymal stem cells on nanopatterned b-type titanium–niobium alloy surfaces

Multi-potent adult mesenchymal stem cells (MSCs) derived from bone marrow have therapeutic potential for bone diseases and regenerative medicine. However, an intrinsic heterogeneity in their phenotype, which in turn results in various differentiation potentials, makes it difficult to predict the response of these cells. The aim of this study is to investigate initial cell–surface interactions of human MSCs on modified titanium alloys. Gold nanoparticles deposited on b-type Ti–40Nb alloys by block copolymer micelle nanolithography served as nanotopographical cues as well as specific binding sites for the immobilization of thiolated peptides present in several extracellular matrix proteins. MSC heterogeneity persists on polished and nanopatterned Ti–40Nb samples. However, cell heterogeneity and donor variability decreased upon functionalization of the gold nanoparticles with cyclic RGD peptides. In particular, the number of large cells significantly decreased after 24 h owing to the arrangement of cell anchorage sites, rather than peptide specificity. However, the size and number of integrin-mediated adhesion clusters increased in the presence of the integrin-binding peptide (cRGDfK) compared with the control peptide (cRADfK). These results suggest that the use of integrin ligands in defined patterns could improve MSCmaterial interactions, not only by regulating cell adhesion locally, but also by reducing population heterogeneity.

R. Medda et al., Interface Focus 4 (2014) 185501 URL

Atomic surface diffusion on Pt nanoparticles quantified by high-resolution transmission electron microscopy

Aberration-corrected high-resolution transmission electron microscopy allows for the delocalization-free observation of atomic motions on metallic surfaces and thus enables measurements of the diffusion of single atoms on the surfaces of nanoscopic objects such as nanoparticles. Using this recently introduced method, the diffusion coefficient for surface self-diffusion of Pt nanoparticles is determined through the fluctuating occupation of the particle's atomic columns. This diffusion coefficient is determined to lie in the range D ∼ (10−17 … 10−16) cm2/s.

S. Schneider et al., Micron 63 (2014) 52-56 URL

Atomic resolution structure–property relation in highly anisotropic granular FePt-C films with near-Stoner-Wohlfarth behaviour

Chemically ordered and highly textured L1FePt-C granular films are potential media for future heat-assisted magnetic recording. Vibrating sample magnetometry of such films in fields up to 14 T reveals a perpendicular coercivity of up to µ0HC = 4.92 T and an anisotropy field of µ0HA = 9.2 T, which translates to an (uni-axial) anisotropy constant as high as KU = 5.3 MJ/m³. An analysis of the remanent magnetization and demagnetization curves shows that the spatially separated FePt nanoparticles act as a Stoner-Wohlfarth ensemble of uni-axial nanomagnets with negligible dipolar inter-particle coupling. The magnetic texture spread of 23° as determined from an analysis of the hard axis magnetization curve is found to be clearly larger than the structural texture width of roughly 3°. Aberration corrected high-resolution transmission electron microscopy reveals that the latter is due to the remaining roughness of the seed layer that causes the particle growth to nucleate at step edges of this layer.

S. Wicht et al., J. Appl. Phys. 114 (2013) 063906 URL


The impact of oxygen on the morphology of gas-phase prepared Au nanoparticles

We present an easy procedure for the synthesis of single crystalline gold nanoparticles with a mean diameter of 4 nm using a DC-sputtering in an argon-oxygen gas mixture. Morphology population statistics have been determined to quantify the influence of oxygen. It is found that the particles undergo a structural transition from predominantly icosahedral to single crystalline particles with increasing amount of oxygen. Aberration-corrected high-resolution transmission electron microscopy investigation proves that likewise prepared single crystalline nanoparticles are defect and oxygen free. In contrast, the icosahedral particles prepared with pure argon show the presence of edge dislocations pointing to an energetic disfavoring already at these relatively small particle sizes. This morphology control of clean and uncovered Au nanoparticles provides a high application potential, e.g., for studying the influence of the particle morphology on plasmonic and catalytic properties.

D. Pohl et al., Appl. Phys. Lett. 101  (2012) 263105 URL

Quantitative measurement of the surface self-diffusion on Au nanoparticles by aberration-corrected transmission electron microscopy

We present a method that allows for a quantitative measurement of the surface self-diffusion on nanostructures, such as nanoparticles, at the atomic scale using aberration-corrected high-resolution transmission electron microscopy (HRTEM). The diffusion coefficient can be estimated by measuring the fluctuation of the atom column occupation at the surface of Au nanoparticles, which is directly observable in temporal sequences of HRTEM images. Both a Au icosahedron and a truncated Au octahedron are investigated, and their diffusion coefficients are found to be in the same order of magnitude, D = 10–17 to 10–16 cm2/s. It is to be assumed that the measured surface diffusion is affected by the imaging electron beam. This assumption is supported by the observed instability of a (5 × 1) surface reconstruction on a {100} Au facet.

A. Surrey et al., Nano Lett. 12 (2012) 6071 URL

Understanding the Metal-Carbon Interface in FePt Catalyzed Carbon Nanotubes

Any tip functionalization of carbon nanotubes, for which the relative orientation between their (metallic) catalyst particle and the nanotube axis is essential, requires a detailed knowledge of the nature of the internal interface between the particle and the outgrown tube. In the present work, this interface is characterized with atomic precision using state-of-the-art low-voltage aberration-corrected transmission electron microscopy in combination with molecular dynamics simulations for the case of hard-magnetically terminated carbon nanotubes. Our results indicate that the physical principle based upon which the interfacial metal facet is chosen is a reduction of the desorption energy for carbon.

D. Pohl et al., Phys. Rev. Lett. 107 (2011) 185501 URL

The effect of oxidation on the surface-near lattice relaxation in FeNi nanoparticles

The near-surface oxidation-induced lattice relaxation and compositional changes of FeNi alloy nano-particles are investigated. Using a newly developed transfer system, the particle structure was characterised by means of aberration-corrected HR-TEM prior to exposing the particles to ambient air. This allows for a comparison of oxidised and un-oxidised particles, respectively. Independent of the oxidation, the surface-near and/or interface-near metal lattice was found to be expanded by up to 3%. EELS profiles clearly reveal an enrichment of Fe at the particle surfaces. MD simulations in combination with HR-TEM contrast simulations were conducted to investigate the effect of the Fe enrichment on the structural relaxation. The results show that a surface-near over-stoichiometric enrichment of Fe indeed causes a dilation that counteracts a compression of the lattice at the particle surface as obtained for homogeneously alloyed particles. Besides, the large lattice mismatch between the metallic cores and the NiFe2O4 shells causes the formation of step dislocations in the close vicinities of the interface. In essence, the surface-near lattice relaxation in oxide free particles is found to be due to a segregation of Fe to the surface, whereas in the case of shell-core particles, no systematic influence of the oxide on the lattice relaxation was found.

B. Bieniek et al., J. Nanopart. Res. 13, p. 5935 (2011) URL

Carbon nanotubes terminated with hard magnetic FePt nanomagnets

The advancement in carbon nanotube (CNT) technology includes significant interest in their functionalization to modify their chemical and physical properties. In particular, the selective functionalization of the CNT ends opens exciting opportunities to design nanoscale architectures and networks. The realization of hard-magnetically terminated CNT via plasma enhanced chemical vapor deposition from Fe-Pt thin films is reported. Although FePt is rarely used as a catalyst for CNT synthesis the said binary catalyst affords attractive hard magnetic properties when present in the chemically ordered L10 phase.

F. Schäffel et al., Appl. Phys. Lett. 94 (2009) 193107 URL



Department Info

Dr. Bernd Rellinghaus

Phone: +49-351-4659-754
Fax: +49-351-4659-9754
Email: b.rellinghaus(@t)