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the sequence of deposition and etching steps used to fabricate free-standing C60 membranes [9.32]. First, low-stress, 1000 Â Si3N4 films are deposited on both the top and bottom of the central region of the 100-|U,m-thick Si (100) wafer. After a photolithography step, which allows the removal of the Si3N4 film from the central region of the top side of the substrate, an anisotropic KOH etching solution is used to remove the Si in the central region of the wafer, leaving the lower Si3N4 film intact. A thin (2000-6000 Â) C60 film is then deposited over the lower Si3N4 film by conventional sublimation techniques, and the Si3N4 film backing of the C60 film is then removed by CF4 reactive plasma etching. What remains after this process is a C60 membrane supported by the surrounding Si wafer which functions as a frame. The C60 membrane was further thinned by continuing the last CF4 reactive plasma etch without apparent damage to the film, as demonstrated in Fig. 9.13, which shows the evolution of the ultraviolet (UV)-visible optical spectrum of a membrane with increased exposure to the CF4 plasma etch for 8, 14,

5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 29 (Cu radiation)

Fig. 9.11. X-ray diffraction patterns of three C60 films with indicated thickness grown on a hydrophobic Si (100) substrates. Data were collected in the 8 — 28 scan mode and at a fixed monitor count. For covenience in plotting, the three traces are displaced from one another vertically [9.31].

5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 29 (Cu radiation)

Fig. 9.11. X-ray diffraction patterns of three C60 films with indicated thickness grown on a hydrophobic Si (100) substrates. Data were collected in the 8 — 28 scan mode and at a fixed monitor count. For covenience in plotting, the three traces are displaced from one another vertically [9.31].

and 18 min. The spectra are shown in comparison to that prior to etching (labeled 0 min). The spectra exhibit an optical absorption edge in the red, as well as the prominent, dipole-allowed absorption bands involving transitions between molecular states near the Fermi energy (see §13.3.2). As the membrane is thinned, the optical absorption decreases, as expected, and the absorption bands and absorption edge retain their shape, indicating that the C60 molecular shells, as well as the fee crystal structure, remain intact.

9.3. Synthesis of Doped C60 Crystals and Films

In this section, we review general experimental techniques for the preparation of doped fullerene-based solids. Details concerning the preparation and purification of the fullerene starting material appear elsewhere (see

(A) Low-stress Si3N4 deposition

Two-inch Si witer (100 ^m thick)

(B) Photolithography

(C) Si3N4 etching by CF4 plasma

CFj plasma

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