Support films can be prepared from many different materials, the more commonly used being plastics, evaporated carbon or silicon monoxide. However the final choice of material depends on the nature of the specimen to be examined. Plastic films are inexpensive and convenient but can be decomposed by the electron beam and therefore are liable to move slightly when first irradiated. This makes them unsuitable for high resolution work. Carbon and silicon monoxide films have greater mechanical strength and stability under the beam when compared to plastic films. Ready-made film-coated substrates are also available (see pages 27 - 36). For TEM applications up to 100 kV, we offer Pioloform ® and Formvar ® coated grids without a carbon layer. A range of high quality Formvar/carbon films, which are cast thin films of Formvar strengthened by a thin evaporated carbon layer, are highly electron transparent and have fine grain and low contrast to augment specimen structure images. Also available is a range of lacey carbon and holey carbon films, ideal for looking at acicular crystals and biological/materials specimens. Silicon monoxide, silicon dioxide or silicon nitride films can be used where a non-carbon support film is required. Pure silicon films provide a stable support and the greatest transparency for TEM applications. LUXFilm ™ TEM supports have been developed for demanding TEM imaging of thick sections in the 80 - 300 kV range. Metallic substrates are also offered to meet a wide range of applications, providing higher surface energy compared to carbon support films. 2 - Support films and substrates Film-forming solutions A variety of film-forming solutions are available to prepare support films and replicas of smooth surfaces. Formvar ® (polyvinyl formal) is normally used as a 0.25 % solution in chloroform. The thickness and stability of the film can be altered by adjusting the concentration between 0.25 % and 0.5 %. Pioloform ® is polyvinyl butyral dissolved in chloroform (0.3 - 1.5 %). The solution should be stored in a refrigerator in an amber glass bottle, and brought up to room temperature before use. Butvar ® B98 is a polyvinyl butyral resin containing 20 % polyvinyl alcohol, and is normally used as a 0.25 % solution in chloroform. It is an alternative support film to Formvar as reported by Handley and Olsen, Ultramicroscopy 4, p479 - 480, 1979. Films made from Butvar are hydrophilic, and are therefore suitable for negative staining methods, having good mechanical stability, high electron transparency and minimum intrinsic structure. Butvar B98 can also be applied to grids as an adhesive. R1202 Formvar. 25 g R1201 Formvar. 100 g R1275 Pioloform. 10 g R1276 Butvar B98. 25 g R1277 Silicon monoxide. 10g High purity optical grade single crystals of sodium chloride, potassium bromide and potassium chloride are excellent substrates for growing epitaxial films. These crystals are ideal for thin film research and are excellent substrates on which to evaporate metals. For most applications it is advisable to use only freshly cleaved substrates. Pieces of 1 - 2 mm thickness can be cleaved to be used as substrates. The cleaved surfaces produce atomically flat areas however cleavage steps over different atomic planes should be expected. The material is supplied as 10 mm square crystals of (100) orientation. G3900 Potassium bromide crystal, 10 x 10 x 50 mm G3941 Sodium chloride crystal, 10 x 10 x 50 mm G3979 Potassium chloride crystals, 10 x 10 x 10 mm. Pack of 5 Film-forming materials 23 Crystal substrates Other replica making materials See Materials science specimen preparation, pages 292 - 293.