Christian Kübel

3D imaging of nanomaterials by discrete tomography

The field of discrete tomography focuses on the reconstruction of samples that consist of only a few different materials. Ideally, a three-dimensional (3D) reconstruction of such a sample should contain only one grey level for each of the compositions in the sample. By exploiting this property in the reconstruction algorithm, either the quality of the reconstruction can be improved significantly, or the number of required projection images can be reduced. The discrete reconstruction typically contains fewer artifacts and does not have to be segmented, as it already contains one grey level for each composition. Recently, a new algorithm, called discrete algebraic reconstruction technique (DART), has been proposed that can be used effectively on experimental electron tomography datasets. In this paper, we propose discrete tomography as a general reconstruction method for electron tomography in materials science. We describe the basic principles of DART and show that it can be applied successfully to three different types of samples, consisting of embedded ErSi(2) nanocrystals, a carbon nanotube grown from a catalyst particle and a single gold nanoparticle, respectively.

Synthesis and Self-Assembly of Functionalized Hexa-peri-hexabenzocoronenes

Monolayers of hexa-alkyl substituted derivatives of hexa-peri-hexabenzocoronene (HBC) 1b have previously been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is expected that different functional groups (electron donating or withdrawing) connected to the aromatic core will influence the packing pattern and possibly the current-voltage characteristics as well. In order to provide suitable model systems, a new synthetic approach to synthesize functionalized HBC derivatives has been developed. This was accomplished by [4 + 2]-cycloaddition of suitably bromo-substituted diphenylacetylenes and 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones followed by an oxidative cyclodehydrogenation with iron(III) chloride/nitromethane. Using this strategy three different substitution patterns were synthesized: 2-bromo-5,8,11.14,17-pentadodecylhexa-pecri-hexabenzocoronene (2a), 2,5-dibromo-8,11,14,17-pentadodecylhexa-peri-hexabenzocoronene (2b), and 2,11-dibromo5,8,14,17-pentadodecylhexa-peri-hexa-benzocoronene (2c). These bromo-substituted HBC derivatives were subjected to palladium catalyzed coupling reactions to give donor (alkoxy, amino) as well as acceptor (ester, cyano) substituted derivatives. The self-assembly of these new HBC derivatives was studied in the bulk as well as at an interface. DSC, optical microscopy, and X-ray diffraction revealed the existence of columnar mesophases. The bulk structure in the mesophase is largely insensitive to changes of the substitution pattern; however, in situ scanning tunneling microscopy at the solid-fluid interface between an organic solution of the HBC derivative and highly oriented pyrolytic graphite reveals different packing patterns of the first adsorbed monolayer.

Giant Polycyclic Aromatic Hydrocarbons

A new approach for the synthesis of extremely large PAHs in high yield is the oxidative cyclodehydrogenation of suitable oligophenylenes under Kovacic conditions. The synthesis of oligophenylene precursors and their cyclodehydrogenation (figure) are presented, and the supramolecular structures observed for large PAHs are discussed.

Multicolor Silicon Light-Emitting Diodes (SiLEDs)

We present highly efficient electroluminescent devices using size-separated silicon nanocrystals (ncSi) as light emitting material. The emission color can be tuned from the deep red down to the yellow-orange spectral region by using very monodisperse size-separated nanoparticles. High external quantum efficiencies up to 1.1% as well as low turn-on voltages are obtained for red emitters. In addition, we demonstrate that size-separation of ncSi leads to drastically improved lifetimes of the devices and much less sensitivity of the emission wavelength to the applied drive voltage.

Recent advances in electron tomography: TEM and HAADF-STEM tomography for materials science and semiconductor applications.

Electron tomography is a well-established technique for three-dimensional structure determination of (almost) amorphous specimens in life sciences applications. With the recent advances in nanotechnology and the semiconductor industry, there is also an increasing need for high-resolution three-dimensional (3D) structural information in physical sciences. In this article, we evaluate the capabilities and limitations of transmission electron microscopy (TEM) and high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) tomography for the 3D structural characterization of partially crystalline to highly crystalline materials. Our analysis of catalysts, a hydrogen storage material, and different semiconductor devices shows that features with a diameter as small as 1-2 nm can be resolved in three dimensions by electron tomography. For partially crystalline materials with small single crystalline domains, bright-field TEM tomography provides reliable 3D structural information. HAADF-STEM tomography is more versatile and can also be used for high-resolution 3D imaging of highly crystalline materials such as semiconductor devices.

Preparation of Monodisperse Silicon Nanocrystals Using Density Gradient Ultracentrifugation

We report the preparation of monodisperse silicon nanocrystals (ncSi) by size-separation of polydisperse alkyl-capped ncSi using organic density gradient ultracentrifugation. The ncSi were synthesized by thermal processing of trichlorosilane-derived sol-gel glasses followed by HF etching and surface passivation with alkyl chains and were subsequently fractionated by size using a self-generating density gradient of 40 wt % 2,4,6-tribromotoluene in chlorobenzene. The isolated monodisperse fractions were characterized by photoluminescence spectroscopy and high-angle annular dark-field scanning transmission electron microscopy and determined to have polydispersity index values between 1.04 and 1.06. The ability to isolate monodisperse ncSi will allow for the quantification of the size-dependent structural, optical, electrical, and biological properties of silicon, which will undoubtedly prove useful for tailoring property-specific optoelectronic and biomedical devices.

Investigations of voids in the aragonite platelets of nacre

We studied the structure of the aragonite platelets of Haliotis laevigata nacre, using conventional transmission electron microscopy, Z-contrast, electron tomography, energy-dispersive X-ray analysis and electron energy-loss spectroscopy. We observed faceted voids several nanometers wide within the aragonite platelets. The electron tomography investigations showed that the voids are distributed more or less randomly in the studied specimen and allowed an estimation of the order of magnitude of the width and the volumetric content of the voids. Further investigations of these voids revealed that they contain an increased amount of carbon, which suggests the existence of organic material within the voids.

Combination of in situ straining and ACOM TEM: a novel method for analysis of plastic deformation of nanocrystalline metals.

Nanocrystalline metals are expected to exhibit different deformation mechanisms when compared to their coarse grained counterparts because the dislocation storage capacity decreases and the grain boundary mediated processes become more pronounced with decreasing grain size. As a new approach to directly image and quantify the plastic deformation processes in nanocrystalline thin films, a combination of automated crystal orientation mapping in microprobe STEM mode with in situ straining inside a TEM was developed. ACOM-TEM closes the gap between EBSD and BF/DFTEM by providing full orientation maps with nanometer resolution. The novel combination with in situ straining provided for the first time the possibility to directly image and quantify the structural changes of all crystallites in the ensemble of a thin film at the nanometer scale during mechanical deformation. It was used to characterize the metallographic changes during tensile deformation of a nanocrystalline Au thin film prepared by magnetron sputtering. The investigation of the grain size, grain orientation and twinning on a global (grain average over a micron sized area) and local (assembly of selected grains) scale allowed for the development of an in depth picture of the deformation processes. Grain boundary motion and local grain rotation were two of the processes acting to dissipate the applied stress. Additionally, twinning/detwinning occurred simultaneously during straining. These processes, which occurred locally already in the micro-plastic regime, led to global grain growth starting at the transition to the macro-plastic deformation regime.

A ferrocene-based carbon–iron lithium fluoride nanocomposite as a stable electrode material in lithium batteries

A nanocomposite comprising carbon–iron LiF was prepared by pyrolysis of a mixture of ferrocene and LiF at 700 °C under an argon atmosphere for 2 h. The structure and morphology of the material was characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), BET analysis, and Mossbauer spectroscopy. The composite consists of multi-walled carbon nanotubes and onion-type graphite structures in which Fe and Fe3C nanoparticles are encapsulated, and LiF is dispersed within the carbon matrix. The sample contains both micro- (0.025 cm3 g−1) and mesopores (0.14 cm3 g−1), and has a total specific surface area of 82 m2 g−1. Its charge/discharge performances were studied in the potential range 0.5 V to 4.3 V at a current density of 20.83 mA g−1 at 25 °C. It exhibited an initial discharge capacity of 324 mAh g−1 with respect to the active mass of FeF3. After five cycles the capacity reached 280 mAh g−1 and is maintained at about 270 mAh g−1 over 200 cycles. A reversible specific capacity of about 170 mAh g−1 was realized when the potential range was between 1.3 and 4.3 V.

Nanosized polyphenylene dendrimers based upon pentaphenylbenzene units

A facile divergent synthesis of monodisperse polyphenylene dendrimers having diameters of 21–55 A is presented. These nanoparticles have been preparedvia a [2+4]cycloaddition–deprotection sequence using an excess of tetraphenylcyclopentadienone 1a as monomer and the tetraethynylbiphenyl 2 as core. Due to the dense packing of 22, 62 or 142 benzene rings in generations G 1 , G 2 and G 3 , respectively, the conformational freedom of the higher generations G 2 and G 3 is limited. Molecular mechanics calculations as well as molecular dynamics simulations are included in a discussion of the structure and the shape-persistence of G 2 . The calculations revealed that selected inner distances of the molecule varied only 5–10% during the molecular dynamics simulations, thus indicating that the overall shape of the molecule essentially did not change throughout the simulation time.