Diamond films are attractive for use as optical windows and in other applications requiring strength, transparency and chemical stability. In particular, nanocrystalline diamond films are smooth and can be electrically conductive -- attributes that could be very useful. Such nanocrystalline diamond films in the past, however, have been, at most, a few microns thick. When researchers tried to grow thicker films, high stresses in the diamond hampered the process.Bright- and dark-field cross-sectional transmission electron microscopy images reveal that the diamond films are composed of relatively short grains.Now scientists from Lawrence Livermore National Laboratory in Livermore, Calif., and from the University of Ulm in Germany have devised a way to produce freestanding nanostructured diamond films that are about 80 µm thick and of relatively low stress. According to the investigators, the films are smoother by a factor of 10 than standard diamond films of the same thickness.Using commercial deposition equipment from CemeCon AG of Aachen, Germany, the researchers produce the diamond films on silicon substrates that are treated to enhance nucleation. A mixture of methane and hydrogen serves as a feedstock, along with a low concentration of oxygen to allow nanocrystalline growth. Lawrence Livermore physicist and research team member Sergei O. Kucheyev credited the development of the deposition formula to a colleague from Ulm, Kai Brühne.In a demonstration of the technique, the researchers freed a deposited film from the substrate and characterized the thin layer of diamond. They found it to be composed of grains shorter than 1500 nm. This is in contrast to the roughly 25-µm-long grains found in microcrystalline diamond films. The difference in grain size is one reason why films produced using the new method are so much smoother than those created previously.They found that the method produces films that are hard and that incorporate few contaminants. Optically, the nanocrystalline films are highly transparent, achieving the same transmission in the infrared as had been measured before in films only 4 µm thick.Although the new technique works, it cannot be performed quickly. To grow an 80-µm-thick film took the investigators 11 runs and a total of nearly 500 hours of deposition time.