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Chemical Crystallography and Electron Microscopy

The scope of the service is the characterization of materials and the structure determination of chemical substances with crystallographic and microscopic methods. This includes single crystal structure determination, transmission and scanning electron microscopy in connection with different methods for sample preparation. Our research areas are electron density determination, polymorphism and crystal-engineering studies.

Christian W. Lehmann

Prof. Dr. Christian W. Lehmann

since 2009
Honorarprofessor for Inorganic Chemistry at the Bergische Universität Wuppertal
since 2008
Head of department Electron Microscopy at the Max-Planck-Institut für Kohlenforschung
since 1998
Head of department Chemical Crystallography at the Max-Planck-Institut für Kohlenforschung
1997
Visiting Professor at the University of LaPlata (Argentina)
1993-1997
Senior Research Officer, Durham University (Great Britain)
1991-1992
Post-Doc Queen-Mary-College London and University of Cardiff (M. B. Hursthouse)
1990
Doctorate in Crystallography (P. Luger, FU-Berlin)
1988
Sabbatical at the University of Pittsburgh (G. A. Jeffrey)
1984-1988
Studies for a degree in Chemistry at the FU-Berlin
1964
Born in Berlin
 

Research Topics

Experimental electron density determination
Experimental electron density determination

Experimental electron density determination

The electron density of a molecule is the key for the understanding of chemical interactions. On one hand, the electron density can be calculated directly from the wave function. On the other hand, the electron density - in contrast to the wave function - can be observed experimentally. Of special interest for us are transition metal catalysts, co-ordination polymers and organic molecules with special photonic properties. The optical properties of these so-called push-pull chromophores change sometimes drastically between being in solution or forming a crystalline solid. Since the crystalline state usually forms the basis for applications, crystallographic methods are especially useful for these investigation.

Polymorphism and Crystal-Engineering
Polymorphism and Crystal-Engineering

Polymorphism and Crystal-Engineering

Polymorphism describes the ability of a chemical substance to form more than one crystalline state. These different crystalline states have different properties with respect to solubility, colour, or melting point. This phenomenon is known for inorganic and organic molecules. Polymorphism is especially interesting in cases of pigments and pharmaceuticals. X-ray powder diffraction is exceptionally suited to distinguish between different polymorphs. The high transmission of the X-rays allows investigation even of formulated and packaged tablets and can be used to detect counterfeits.

 
 

Instrumentation

Single crystal structure determination
Single crystal structure determination

Single crystal structure determination

Currently, there are three single crystal diffracometers in use: (Bruker AXS Mach3) with area detectors (Apex II, KappaCCD, Proteum). The X-rays are generated by rotating anodes (Bruker AXS FR591) with Molybdenum and Copper anodes, respectively.

  • Determination of the exact three –dimensional arrangement of all atoms contained in a crystalline chemical substance
  • Information about constitution, conformation, exact bond lengths and angles as well as the three-dimensional packing in the solid state
  • Measurements of organic and metal organic substances, Screening of protein crystals
  • Especially substances sensitive to air, temperature and moisture at temperatures down to -173 °C
  • Measurements of extremely small crystals using focussing X-ray mirrors
  • Determination of the absolute configuration of chiral substances
Powder diffractometry
Powder diffractometry

Powder diffractometry

For powder diffraction there is one Stoe STADI P diffractometer, equipped with a primary monochromator and Cu-Kα1 radiation in Debye-Scherer-Geometry, plus a position sensitive image plate detector.

  • studies of polymorphism
  • Co-Crystal studies and reactive ball milling
  • Rietveld refinement and ab-initio structure solution of organic molecules
Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM)

Transmission electron microscopy of nano particles and nano structured materials (zeolites and mesoporous silicates and their replica) reveals structural details in the sub nanometer range.

  • High resolution imaging and chemical analysis using a 200 kV electron microscope with cold field emitter, CCD-camera and EDS (Hitachi HF-2000)
  • Cryo microscopy (100 K) and thermo microscopy (Gatan sample holder)
  • Determination of particle size distributions and optimization of reaction conditions (100 kV, Hitachi H-7100 and H-7500)
Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM)

Scanning electron microscopy is employed to visualize details of the sample surface on the nano meter scale.

  • Ultra high resolution scanning electron microscopy with a point resolution of up to 4 Å at 30 kV (Hitachi S-5500)
  • Combination with bright field and dark field scanning transmission electron microscopy (STEM)
  • Elemental analysis using EDS achieving nano meter resolution
  • SEM of bulk samples in combination with EDS and under variable pressure (Hitachi S-3500N)
Sample preparation for Electron microscopy
Sample preparation for Electron microscopy

Sample preparation for Electron microscopy

The sample preparation for electron microscopy is of utmost importance. Delicate structures must not be destroyed or changed; the sample should be representative for the whole material.

  • Preparation of carbon support films with a thickness of a few nano meters; tungsten coating using our in-house designed coating apparatus
  • Preparation of ultra thin sections using an ultra microtome (Reichert Ultracut)
  • Gold coating of SEM samples for the deposition of a conducting layer using a sputtering process (Balzers MED010)
  • Preparation of cross-section of extremely fragile micro- and mesoporous nano structures (Hitachi E-3500)
 

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