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Fabrication And Properties Of Freestanding C60 Membranes
SCIENCE, no. 5103 (1993): 1887-1890
WOS SCIENCE
Abstract
Van der Waals forces that bind C60 molecular solids are found to be sufficiently strong to allow the reproducible fabrication of free-standing C60 membranes on (100) silicon wafers. Membranes, 2000 to 6000 angstroms thick, were fabricated by a modified silicon micromachining process and were found to be smooth, flat, and mechanically robu...More
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Introduction
- Fabrication and Properties of Free-Standing C, Membranes
- It is remarkable that a van der Waals molecular solid, in which all the molecules are independently rotating at room temperature, should be robust enough to hold together as a free-standing membrane.
- Characterization of the membranes by x-rays and bulge testing provides inforqa, tion about the structure and mechanical properties such as Young's modulus, internal stress, and fracture strength.
Highlights
- These remarkable properties alone provide ample motivation to consider novel ways to form and process C6, solids. In this revort. we describe the fabrication and characterization of free-standing C, membranes supported at the edges by [100] oriented Si frames
- It is remarkable that a van der Waals molecular solid, in which all the molecules are independently rotating at room temperature, should be robust enough to hold together as a free-standing membrane
- We describe our fabrica
- SCIENCE * VOL. 259 ' 26 MARCH 1993 haps most critical step is the use of CF, plasma etching to remove the Si3N4 film adjacent to the C, in the exposed areas of the wafers
- When combined with an estimate of the volume of igneous materials on Mars, this information suggests that the total amount of water outgassed since 3.9 billion years ago corresponds to global depths on the order of 200 meters
Results
- 259 ' 26 MARCH 1993 haps most critical step is the use of CF, plasma etching to remove the Si3N4 film adjacent to the C,, in the exposed areas of the wafers.
- The C,, films are etched by a sputtering process, and there is no chemical reaction between C6, and the excited species in the CF, plasma.
- Not to structural changes in the fcc lattice, which might be present near the surface of the etched film.
- -2 differencebetween the elastic constants of the two membranes reported here may be the result of processing variations and the use of different thickness substrates.
- Free-standing membranes are advantageous for physical characterization studies where the absence of a substrate greatly facilitates the measurement [such as electron energy loss, infrared absorption, and transmission electron mi-
- Because carbon with its six electrons has a low cross section to x-rays and high-energy electrons, C6, membranes might be considered as "transparent" substrates for TEM sample preparation and as masks in x-ray lithography.
- When combined with an estimate of the volume of igneous materials on Mars, this information suggests that the total amount of water outgassed since 3.9 billion years ago corresponds to global depths on the order of 200 meters.
- A recent estimate, based on the assumption that martian magmas have water contents similar to those of typical terrestrial basaltic magmas (1% by weight), corresponds to a global water depth of 150 m ( 5 ) .
Conclusion
- Atmospheric hydrogen, nitrogen, and argon isotopic abundances suggest that the amounts of outgassed water were limited, corresponding to global depths of 3.6, 8 to 133, and 6 to 10 m, respectively [6].
- These conflicting estimates for martian outgassed water can be reconciled in several u ways: The measured volume of igneous materials presumably represents volcanic activity since 3.9 billion years ago, the time at which stable crust began to form.
- The estimate of martian outgassing based on the volume of volcanic materials represents a critical lower limit for the planet's water inventory, one that can be quantified further by a consideration of
Funding
- There is ample space in the crystal structure (26% open volume for closepacked spheres) to accommodate guest atoms that, by donating electrons into the C,, SCIENCE VOL. 259 26 MARCH 1993 molecular bands, can induce conducting [5]
- A recent estimate, based on the assumption that martian magmas have water contents similar to those of typical terrestrial basaltic magmas (1% by weight), corresponds to a global water depth of 150 m ( 5 )
Reference
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- 17. We thank F. Wudl, C. Yonnani, R. Taylor, E. 21 December 1992; accepted 11 February 1993
- Van der Waals forces that bind C,, molecular solids are found to be sufficiently strong to allow the reproduciblefabrication of free-standing C,, membraneson (100) silicon wafers. Membranes, 2000 to 6000 angstroms thick, were fabricated by a modified silicon micromachining process and were found to be smooth, flat, and mechanically robust. An important aspect of the silicon-compatiblefabrication procedure is the demonstrationthat C,, films can be uniformly and nondestructivelythinned in a CF, plasma. Young's modulus and fracture strength measurementswere made on membranes with areas larger than 6 millimeters by 6 millimeters.%Imt ay be possible to use C,, membranes for physical property measurements and applications.
- AT&T Bell Laboratories, Murray Hill, NJ 07974 controlled temperatures. These insulating molecular solids have a spherical, closepacked, face-centered-cubic (fcc) structure with a lattice constant of 14.1 A (2) and van der Waals bonding between the molecules. There is ample space in the crystal structure (26% open volume for closepacked spheres) to accommodate guest atoms that, by donating electrons into the C,, SCIENCE VOL. 259 26 MARCH 1993 molecular bands, can induce conducting (5)
- 2000 to 6000 A and areas larger than 6 mm by 6 mm. At first sight, it is remarkable that a van der Waals molecular solid, in which all the molecules are independently rotating at room temperature, should be robust enough to hold together as a free-standing membrane. An estimate of the magnitude of the cohesive forces can be obtained from the heat of sublimation found with the
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- 21. We thank R. M. Fleming for x-ray characterization, T. A. Fulton for assistance with-mask generation, and R. H. Eick and J H. Marshall for technical assistance. We also appreciate useful discussions with J. M. Phillips.
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