Cryo-crinkling: what happens to carbon films on copper grids at low temperature

A study of the surface flatness of carbon films on copper grids used for cryo-electron microscopy has been carried out using a Hitachi S-900 low-voltage SEM. Dramatic changes in flatness were observed after cooling from room temperature to -170 degrees C. The changes were similar both for carbon films that had been floated from a mica surface and for those initially deposited on the surface of plastic films. Results demonstrate that films prepared on copper grids that appear flat at room temperature become extensively, but reversibly, puckered at -170 degrees C. The linear thermal expansion coefficient (alpha) for copper is 16.2 x 10(-6)/degrees C and the puckering can be explained by assuming that the coefficient for amorphous carbon is substantially less. Measurements on grids made of titanium, molybdenum and tungsten (coefficients 8.5, 5 and 4.5 x 10(-6)/degrees C, respectively) showed significantly less puckering.