VOLUME 82, NUMBER 7 PHYSICAL REVIEW LETTERS 15 FEBRUARY 1999 “Bucky Shuttle” Memory Device: Synthetic Approach and Molecular Dynamics Simulations Young-Kyun Kwon, 1 David Tománek, 1 and Sumio Iijima 2 1 Department of Physics and Astronomy, and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824-1116 2 NEC Corporation, Fundamental Research Laboratories, 34 Miyukigaoka, Tsukuba, Ibaraki 3058501, Japan (Received 23 October 1998) Thermal treatment is reported to convert finely dispersed diamond powder to multiwall carbon nanocapsules containing fullerenes such as C 60 . We investigate the internal dynamics of a related model system, consisting of a K@C 1 60 endohedral complex enclosed in a C 480 nanocapsule. We show this to be a tunable two-level system, where transitions between the two states can be induced by applying an electric field between the C 480 end caps, and discuss its potential application as a nonvolatile memory element. [S0031-9007(99)08484-7] PACS numbers: 61.48. + c, 61.50.Ah, 73.61.Wp, 81.10.Aj Carbon nanotubes [1,2], consisting of seamless and atomically perfect graphitic cylinders a few nanometers in diameter, have been synthesized in bulk quantities [3,4]. The unusual combination of their molecular nature and micrometer-size length [5,6] gives rise to uncommon elec- tronic properties of these systems. Electrical transport measurements for individual nanotubes indicate that these systems may behave as genuine quantum wires [7], non- linear electronic elements [8], or transistors [9]. Potential use of nanotube-based two-level systems for permanent data storage, discussed here, would significantly extend their range of application. Here, we present evidence that unusual multiwall nano- tube structures, such as the “bucky shuttle” [10], self- assemble from elemental carbon under specific conditions. Our molecular dynamics simulations indicate that the bucky shuttle shows an unusual dynamical behavior that suggests its use as a nanometer-sized memory element. We show that a nanotube memory would combine high switching speed, high packing density, and stability with nonvolatility of the stored data. The system described in this study was produced by thermally annealing diamond powder of an average di- ameter of 4–6 nm which was prepared by the detonation method (Cluster Diamond, Toron Company Ltd.). The powder was heated in a graphite crucible in inert ar- gon atmosphere at 1800 ± C for 1 hour. This treatment transforms the diamond powder into graphitic nanostruc- tures presented in transmission electron microscope im- ages shown in Fig. 1. A large portion of this material consists of multiwall capsules with few layers, the small- est structures being fullerenes with a diameter close to that of C 60 . In several cases depicted in Fig. 1, the enclosed fullerenes may move rather freely inside the outer capsule, like a bucky shuttle. An enlargement of one of such structures in Fig. 1 is displayed in Fig. 2(a). Figure 2(b) illustrates a cor- responding model, consisting of a C 60 encapsulated in a C 480 capsule. The energetics of the C 60 @C 480 system is shown in Fig. 2(c). The ends of the outer capsule are halves of the C 240 fullerene, the optimum structures to hold a C 60 molecule at an interwall distance of 3.4 Å. These end caps connect seamlessly to the cylindrical por- tion of the capsule, a 1.5 nm long segment of the 10, 10 nanotube [3]. The interaction between the unmodified C 60 molecule and the enclosing capsule is similar to that found in C 60 crystals and nanotube bundles [3]; it is dominated by a van der Waals and a weak covalent interwall in- teraction that is proportional to the contact area between the constituents. An additional image charge interaction, FIG. 1. Transmission electron microscope images depicting multiwall carbon structures that self-assemble during the ther- mal annealing of nanodiamond powder under the conditions described in this report. The smallest spherical structures are C 60 molecules that are always found near the end of the cap- sule, where the attractive interwall interaction is strongest. 1470 0031-9007 99  82(7)  1470(4)$15.00 © 1999 The American Physical Society