Advances in the Creation of Filled Nanotubes and Novel Nanowires
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MRS BULLETIN/AUGUST 1999
materials (e.g., Si, SiOv, SiC, MoS 2 , WS 2 , etc.) are able to form novel nanowires and fullerene-like morphologies. This review discusses methods for generating these fascinating structures and evaluates their possible applications in mate rials science and engineering.
Chemical/Wetting Routes Carbon nanotubes consist of concentric, hexagon-rich cylinders made up of sp2 hybridized carbon, as in graphite, and are terminated by end caps arising from the presence of 12 pentagons, gen erally six per end (see Figure 2). The strain introduced by the pentagonal rings renders the caps chemically reactive. Consequently, it is possible to oxidize the caps to
produce open-ended nanotubes, which can then be filled with various substances. A chemical route for opening multi walled carbon nanotubes (2-10 nm OD, < 2 microns in length) is achieved by boiling suspensions of nanotubes in aqueous nitric acid for several hours at 150°C.8 This method results in the retention of 98% of total mass, with 80-90% of the tubes having open end caps.8 When the oxidation (4-5 h at 140°C) is carried out in the presence of metal nitrates, 60-70% of the tubes contain metal oxide nanocrystals (3-6 nm OD, 10-30 nm in length).8 Using this wet-chemical technique, it is possible to insert oxides of Ni, 8 Co, 8 Fe,8 U,8 Mo, 9 Sn, u ) ' u Nd, 12 Sm, 12 Eu, 12 La,' 2 Ce,12 Pr,12 Y,12 Zr,12 and Cd.12 Pure metals, such as Pd,13 Ag,14 and Au;15 proteins or enzymes (e.g., Zn 2 Cd 5 -metallothionein, cytochrome c(3), /3-lactamase 1, and DNA complexes);1516 and Au2S3,12 AuCl,14 and CdS 12 can also be introduced. (See Fig ure 3 and References 11 and 12.) Recently, single-walled carbon nanotubes (1.4-5 nm OD) have also been filled, for example, with Ru metal.17 Unfortunately, only 5% of the tubes contained Ru. Recent research hasshown thatthefillingof single-walled carbon nanotubes is inefficient,18 and instead, wetting of the externa 1 surfaces w i t h low surface-tension materials (
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