Single crystal

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Monocrystalline silicon ( ingot ) for wafer production

A single crystal or monocrystal is a macroscopic crystal whose components ( atoms , ions or molecules ) form a continuous, uniform, homogeneous crystal lattice . This distinguishes single crystals from polycrystalline aggregates, twinned crystals or amorphous substances ( glasses ).

Chemical classification

Analogous to the classification of all chemical substances , single crystals can be divided into two fundamental groups according to their chemical structure:

Single crystals for jewelry purposes, here brilliant cut diamonds
The carbon modification diamond is often used as a single crystal, not only in the form of diamonds as jewelry, but also when used as a cutting tool, for example in some glass cutters .

Application for analysis

The crystal structure analysis to elucidate crystal or molecular structures is now a standard method in chemistry and biochemistry. However, crystallization is a prerequisite for this, which can be very difficult, especially with biological molecules. Ideally, the investigation is carried out on a single crystal. Sometimes this is impossible because there are not enough single crystals of a substance available. Nowadays it is possible to evaluate the diffraction pattern of crystal powders within the framework of a crystal structure analysis, but information is lost here due to the superimposition of diffraction maxima, so that the results are of lower quality. But even single crystals that have been grown in a complex manner still have lattice defects .

To determine the structure of chemical compounds using X-rays (which can also be generated by means of a synchrotron ) or neutron beams , single crystals are required in order to determine, among other things, the exact bond lengths and the arrangement of the atoms in a molecule. The crystals used for this are mostly smaller than a millimeter. For macromolecules , macrocyclic compounds and natural products , including proteins , DNA and RNA , single crystal structure analysis can be used to determine the three-dimensional structures down to atomic resolution, if single crystals can be obtained.

Mechanical-technical application

Single crystals are used in technology because of their reproducible properties. Since they have almost no grain boundaries or other structural defects, the mechanical strength of the material increases, for example. So z. B. turbine blades made of a single crystal nickel-based superalloy . These single crystals have a uniform alignment of the lattice structure, but can definitely have several phases .

Single crystalline surfaces and two-dimensional crystals

The surfaces of inorganic single crystals are also single crystalline. They can be understood as two-dimensional single crystals if one only looks at the top layer and are the subject of research in the field of surface chemistry and physics. Single crystal surfaces with a low index are e.g. B. Si (111), Ag (100) or Au (110). On these surfaces, the atoms are arranged on flat terraces, which are interrupted by mostly monoatomic steps. Adsorbates show a different behavior at these stages than on atomically smooth areas. If a single layer of organic molecules is applied to monocrystalline surfaces, self-organizing monolayers are usually obtained with low coverage . These organic layers, which are only one molecular layer high, can be referred to as two-dimensional single crystals, analogous to inorganic single-crystalline surfaces. As with single crystals made up of atoms, the molecules are also highly ordered here. Graphene , a free-standing layer of carbon atoms , lacks long-range crystalline order as expected without a flat base and forms a wavy defect structure.


Individual evidence

  1. Jacqueline Wahl, Ken Harris: New single crystal superalloys - overview and update . In: MATEC Web of Conferences . tape 14 , 2014, ISSN  2261-236X , p. 17002 , doi : 10.1051 / matecconf / 20141417002 ( [accessed July 26, 2020]).
  2. Thomas Waldmann, Christina Nenon, Katrin Tonigold, Harry E. Hoster, Axel Groß, R. Jürgen Behm: The role of surface defects in large organic molecule adsorption: substrate configuration effects. In: Physical Chemistry Chemical Physics. 14, No. 30, 2012, p. 10726, doi : 10.1039 / c2cp40800g .