International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 3.5, p. 176

Section 3.5.3.2. Sublimed films

N. J. Tighe,a J. R. Fryerb and H. M. Flowerc

a42 Lema Lane, Palm Coast, FL 32137-2417, USA,bDepartment of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, and cDepartment of Metallurgy, Imperial College, London SW7, England

3.5.3.2. Sublimed films

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Most organic compounds can be sublimed under vacuum to give an epitaxic layer on a suitable substrate. Specimens of compounds ranging from paraffins to polynuclear hydrocarbons (Fryer & Smith, 1982[link]; Fryer & Ewins, 1992[link]) and porphyrins (Fryer, 1994[link]) have been prepared in this way. A small amount of material is placed in a molybdenum boat with a perforated cover and sublimed under high vacuum onto a heated substrate. Potassium chloride crystals cleaved in air (100) provide successful substrates. Crystal size and order increase with substrate temperature, however, and a high temperature leads to re-evaporation of the compound. Sometimes, the temperature difference between film deposition and re-evaporation is as small as 30 K. An empirical guide to the optimum substrate is one-third of the boiling-point temperature of the compound. Normally, the crystalline film produced is 10–15 nm thick and is discontinuous.

Following the compound sublimation, carbon is evaporated onto the compound and the film is floated off the KCl substrate onto a water surface. The carbon film can prevent disintegration of the organic compound. Specimens of the film are then picked up on grids.

Organic crystals easily undergo phase changes, so that the crystal modification of the evaporated epitaxic film may not be that of the bulk material. The structure may also vary between preparations on different substrates or between different temperatures on the same substrate.

References

Fryer, J. R. (1994). Electron microscopy of polymeric phthalocyanines. MSA Bull. 24, 521–526.
Fryer, J. R. & Ewins, C. (1992). Epitaxial growth of thin films of perylene. Philos. Mag. A66, 889–898.
Fryer, J. R. & Smith, D. J. (1982). High resolution electron microscopy of molecular crystals. I. Quaterrylene. Proc. R. Soc London Ser. A, 381, 225–240.








































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