Soon after the first systematic description of the structure of carbon nanotubes (CNTs) by Iijima (1991) researchers have recognized the enormous potential CNTs have as materials for the construction of nanoscale electronic, optical, mechanical or biosensoric and biomimetic devices of high functional density. Particularly single- and double-walled CNTs have attracted attention because their structure is completely defined by a few structural parameters facilitating a clear description of their metallic or semiconducting properties. Multiwalled CNTs (MWCNTs), on the other hand, are far more complex by nature but imply the advantages of higher mechanical and chemical stability, and higher electric power density. While the recent discovery of flat, isolated graphene (two-dimensional carbon) with zero-gap semiconducting properties by Novoselov et al. (2004) and graphene nanoribbons (quasi one-dimensional carbon) which show a size-dependent energy gap have provided enormous new stimuli to carbon nanoresearch, the potentials of scrolled graphene (also termed carbon nanoscrolls, CNSs) have not yet entirely been realized. We are interested in (i) the mechanisms of CVD and catalyst-assisted growth of CNSs and CNTs; (ii) the chirality of the nanotubes; (iii) their behavior during doping, intercalation and functionalization with foreign elements or compounds; (iv) a comparison of the electrical and mechanical properties of CNTs and CNSs.

Schaper, A.K., Wang, M.S., Xu, Z., Bando, Y., Golberg, D.: Comparative studies on the electrical and mechanical behavior of catalytically grown multiwalled carbon nanotubes and scrolled graphene. Nano Lett. 11, 3295-3300 (2011).

Catalytic carbon canotubes with metallic nanowire encapsulations

Multi-walled carbon nanotubes comprise a large variety of different shapes, dimensions, and structures, and may occur as empty tubes, doped or intercalated with foreign elements or compounds, or with complete metallic nanowire encapsulations. Very attractive property phenomena are provided by self-organization and parallel alignment during catalytic growth of the tubes. Flat films consisiting of arrays of highly ordered metal-filled tubes potentially show high density magnetic data storage capacity and exceptional field-emission capabilities. We have been analysing carbon nanotubes with encapsulated Fe-nanowires using transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), energy filtering TEM, ⁵⁷Fe Mössbauer spectroscopy, and SQUID magnetometry.

Schaper, A.K., Hou, H., Treutmann, W., Phillipp, F.: Multi-walled carbon nanotubes without and with metal filling. J. Metastable & Nanocryst. Mater. 23, 301-304 (2005).

Electron microscopy and diffraction of radiation-sensitive nano-sized organic systems

Any structure characterization of polymeric organic materials using electron microscopy requires special care as to the high sensitivity of these materials to high energy electrons. We use transmission electron diffraction and microscopy techniques under low-dose conditions with special focus on cryo-protection of the objects at liquid nitrogen or even helium temperature. For improving the generally weak image contrast selective chemical staining and energy-filtering microscopy as well as incoherent Z-contrast microscopy are applied. The studies provide information on the structure and structure/property relationships of natural and man-made nanofibers and nanotubes, on the molecular orientation and phase transitional behaviour in thin liquid crystalline films, and on the ordering and alignment of the mesophase in discotic liquid crystal-containing template-grown polymer composites.

Schaper, A.K., Kurata, H., Yoshioka, T., Tsuji, M.: Composite Structure of Liquid Crystal/Polymer Nanotubes Revealed by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy. Microsc. & Microanal. 13, 336-341 (2007).

Structure/property behaviour of nanofibers prepared from regenerated silk fibroin

This recent project deals with the molecular and supermolecular structure of regenerated Bombyx mori silk and with structure effects onto its mechanical properties. Regenerated silk fibroin is prepared from raw silk material and electrospun into nanofbers, ribbons, and fiber bundles. Variation of the conditions of spinning, fixation, and post-treatment allows to lend the different but well-defined structures to the products which we study by the aid of electron and x-ray diffraction. Detailed mechanical tests and deformation analyses as well as in situ-diffraction studies using the electron synchrotron source DESY (Hamburg), and neutron-scattering with deuterated samples at FRM-II (Garching) help to elucidate the complex mechanical behaviour of the regenerated silk. We have observed a pure reversible contraction in dependence on the meta-stable ordering oft the highly oriented polypeptide chains, but even the phenomenon of irreversible supercontraction. Combined with their excellent biocompatibilty, silk fibroin-based materials are highly promising for diverse biomimetic applications as, e.g., the development of artificial muscles and tendons, of scaffold structures for cell growth or wound healing, or of nanoscopic sensor devices. Those perspectives appear realistic in view oft first successful attempts towards fabrication of silk-analog recombinant proteins through synthetic gene expression within microbial systems. With that, silk material of very specific properties should become available.

Yoshioka, T., Kawahara, Y., Schaper, A.K.: Cyclic or permanent? Structure control of the contraction behavior of regenerated Bombyx mori silk nanofibers. Macromolecules 44, 7713-7718 (2011).

Incommensurability in crystals and incommensurate-commensurate phase transitions

Most crystalline compounds with the general formula X₂T¹T²₂O ₇ show, at room temperature, a misfit between the tetrahedral layers and the X-cation intermediate layers, which gives rise to incommensurate modulation of the structure. Those crystals are described within the "superspace" formalism. The structure response to the lattice misfit consists in atomic displacements accompanied by deformation and rotation of the T¹- and T²-tetrahedra as well as by displacive modulation of the X-cations. It is suggested that their oxygen coordination becomes reduced with increasing lattice distortion, and that there is a tendency of the low coordination number being associated with varying degrees of characteristic short-range ordering. In certain systems it is possible to approach a low-temperature commensurate lock-in phase. Determination of the crystallography, and of the morphologies of nano- and micrometre domains associated with the structure modulation, is of general interest for the understanding of relevant properties, such as temperature- and composition-dependent phase changes, the ability to lasing of doped melilites, and their magnetic behaviour. In situ heating and cooling experiments within the TEM are favourable tools to gain insight into these issues.

Jia, Z.H., Schaper, A.K., Massa, W., Treutmann, W., Rager, H.: Structure and phase transitions in Ca₂CoSi₂O₇ - Ca₂ZnSi₂O₇ solid solution crystals. Acta Cryst. B62, 547-555 (2006).

Carbon and carbon/metal hybrid nanostructures

Carbon nanomaterials comprise a large variety of different shapes, dimensions, and structures and may provide confinements for metallic or molecular encapsulations. Some property phenomena are particularly attractive in combination with their capability to self-assembling into highly ordered arrays. Accordingly, the possible fields of application are diverse making use, e.g., of the electrochemical energy storage capability, their electrical, optical or magnetic properties, or their chemical reactivity. Spherical core/shell-structures with the shell being composed of multiple graphene layers, can act as nanoscale drug carriers or markers in diagnostic and therapeutic medicine, or even as reactor chambers for specific physico-chemical reactions and phase transitions.

Schaper, A.K., Phillipp, F., Hou, H.: Melting behavior of copper nanocrystals encapsulated in onion-like carbon cages. J. Mater. Res. 20, 1844-1850 (2005).

Perovskite structures and superlattice ordering

ABO₃-type mixed oxides are presently extensively studied owing to their unusual magnetic, electronic and transport properties. Along the compositional join SrSnO₃ - SrFeO_2.5 experimental findings using high-resolution TEM along with image simulation, x-ray diffraction, and Mössbauer spectroscopy suggest a complex phase transitional behaviour that comprises the cubic perovskite structure built up by succcessive octahedral layers, and a brownmillerite-like structure which can be considered a superstructure of the cubic perovskite formally deduced by subtracting one of two oxygens, so that half of the octahedral sites get transformed into tetrahedral sites. The resulting structure consists of alternating octahedral and tetrahedral layers with a sequence OTOT’OTOT, in the other superstructures one out of three, and one out of four octahedral layers are converted into tetrahedral layers, leading to the stacking sequences OOTOOT’ and OOOTOOOT’. Nonstoichiometric SrFeO_y has three modifications with different amounts of oxygen vacancy concentrations: cubic for 2.97 > y > 3.0, tetragonal for y = 2.875, and orthorhombic for y = 2.75. In contrast to SrSnO₃, there are two different B-positions present in equal amounts: one is occupied by Fe⁴⁺ in octahedral coordination the other being Fe³⁺ in a tetragonal pyramidal site.

Schott, S., Jia, Z.H., Schaper, A.K., Thangadurai, V., Schmid-Beurmann, P., Weppner, W.: Superlattice ordering in SrFeO3-δ: Electron microscopy and diffraction study. phys. stat. sol. (a) 202, 2330-2335 (2005).

Crystallization and ordering in liquid crystals and liquid crystalline polymers

Usually, dilatometric and calorimetric methods are used to investigate the kinetics of crystallization. X-ray methods offer another possibility to obtain information on the crystallization process either by measuring the intensity variations of the diffuse scattering at very small angles, or by monitoring the integral intensity of a selected crystalline reflection. Due to a much smaller probe size, electron diffraction provides additional information on the local molecular organization within a single liquid crystal domain. From certain liquid crystal compounds it is known that they pass through an intermediate hexatic state during transition from the crystalline smectic to a liquid-like smectic phase. This hexatic state is characterized by long-range bond-orientational order over macroscopic dimensions, but only short-range positional order propagating through domains a few tens of nanometers in size. The hexatic order parameter is represented by a two-component vector field corresponding to the superfluid helium XY universality class. The bond-orientational order correlations extent over three-dimensional stacks of hexatic layers which exhibit only weak interlayer coupling. Direct proof of the corresponding sixfold intensity modulation is provided by in situ low-dose selected area electron diffraction observations.

Schaper, A.K., Zhao, S.R., Kutoglu, A.: Bond-orientational ordering in cyclohexyl-biphenyl type liquid crystals. Mater. Sci. Forum 539-543, 3485-3490 (2007).

Deposition of catheter-delivered nanoparticles in the animal arterial vessel wall

In recent years, stenting has provided an important breakthrough in standard angioplasty procedures. However, restenosis remains a major critical factor that limits the outcome of any catheter-based interventional angioplasty. The most crucial process is the neo-intimal hyperplasia at the site of the primary lesion during first months after stent implantation. Currently, different routes of drug administration in restenosis therapy are under investigation. Among those, local delivery and sustained regional release of a pharmacologic agent encapsulated in a nanoparticle carrier system offers a promising therapeutic approach to reducing restenosis and minimizing undesirable systemic side effects. However, little is known about the migration, the cellular uptake and the release behavior of the particles in the vessel wall. Using confocal laser scanning microscopy in combination with transmission electron microscopy, we observed a size-dependent penetration of nanoparticles into the aorta abdominalis of New Zealand White rabbits.

Westedt, U., Barbu-Tudoran, L., Schaper, A.K., Kalinowski, M., Alfke, H., Kissel, T.: Size-dependent nanoparticle deposition after porous balloon delivery into normal arterial vessels: Nanoparticle localization by confocal laser scanning microscopy and transmission electron microscopy. AAPS Pharm. Sci. 4, article 41 (2002).

Cryo-scanning electron microscopy of biological nanostructures

Reports on the so-called „Lotus-Effect“ (Barthlott, 1996) have triggered enormous interest in the diverse discoveries by nature of unprecedented materials and materials properties because of their potentials for wide-ranging man-made practical applications. The hydrophobic and self-cleaning behavior of the sacred lotus leaves from Nelumbo nucifera is considered a nice example of the fundamental effects of surface structuring on the nanoscale. There is a pronounced morphological hierarchy dominated by a few microns wide papillae arising from single epidermal cells, and being covered by a dense layer of epicuticular wax crystals of nanotubular shape. The chemical composition of the layer contributes to the structure-induced particular wetting and repelling characteristics but proves also responsible for its sensitivity to high temperature and electron irradiation. In those cases, cryo-protection at liquid nitrogen temperature, often used in transmission electron microscopy, is indispensable in achieving high-quality and high-resolution imaging also in scanning electron microscopy.

Schaper, A.K., Yoshioka, T., Ogawa, T., Tsuji, M.: Electron microscopy and diffraction of radiation sensitive nanostructured materials. J. Microscopy 223, 88-95 (2006).