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The method used to obtain these cubes was the same as that for synthesizing carbon nanocapsules filled with rare-earth1,2 and iron-group metals3: that is, evaporating a metal-loaded graphite rod (anode) in a helium atmosphere by a direct-current arc discharge. The metal-loaded anode was prepared by packing a hole (measuring 30 mm deep by 3.2 mm in diameter) drilled in a graphite rod (50 mm long by 6 mm diameter) with small pieces of calcium (purity 99%) or strontium (purity 99%). The graphite used for the rod was of 99.998% purity, as was that for the 13-mm cathode. The helium (purity 99.999%) introduced into the arc chamber had a pressure of typically 100 and 600 torr. Discharge current and voltage were 70 amp and about 25 V respectively.

After arc evaporations using both calcium and strontium, we found, in the soot deposited on the cathode surfaces, abundant cubic cages of graphitic carbon (Fig. 1a). The cages range in size from 20 to 100 nm and consist of multiwalled graphitic carbon with a spacing of 0.34 nm (Fig. 1b). Adjacent graphitic layers were out of register, or turbostratic. At some of the cubes' corners, extrusion of graphitic layers can be seen, while breakages or smooth folding occurs at other corners (arrows in Fig. 1b). Most of the hollow cubes contain smaller cubes, which are also hollow.

Figure 1: Transmission electron microscope images of hollow rectangular parallelepiped graphitic cages formed by the arc evaporation of a carbon/calcium composite.
figure 1

a, High-density aggregation of hollow rectangular parallelepipeds (low magnification). b, Aggregation of multiwalled graphitic carbon with a spacing of 0.34 nm (high resolution). At some corners, extrusion (arrows A, B) and breakage (arrow C) of graphitic layers can be seen, and sometimes graphitic layers are folded continuously (arrow D).

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Some other shapes of nano-cage — fullerenes4 and nanotube tips5 — are closed by the introduction of 12 pentagons into the hexagon network. But in the case of a rectangular parallelepiped cage with eight vertices, pentagons or even four-membered rings cannot effect closure. Therefore, the rectangular corners suffer from geometrical frustrations, which may be relaxed by either extruding or discontinuing (breakage) graphitic layers at the corners.

The cubic shape is probably produced by folding graphitic sheets into a rectangular form, a process catalysed by alkaline-earth metals. But questions remain over the detailed structures at the corners and the role of metallic atoms.

Although the hollow cubic cages predominated, we also found square cages filled with foreign materials. Most of the filling crystallites were carbides (CaC2 and SrC2), but sometimes metallic strontium was encapsulated. Although these carbides and metal are hygroscopic, the crystallites nesting in the graphite cages did not degrade even when they were exposed to air. The closed graphitic cages effectively shield the inner materials from moisture, in a similar way to the carbon nanocapsules with polyhedral1,2 and spherical shapes3 that have been reported previously.

Graphitic carbon is found in forms and shapes as various as single-walled6,7 and multiwalled tubules8, cones9, polyhedra1,2 and spheres3,10. The topology of graphite networks governs the physical properties of nanometre-sized graphitic materials, so our success in producing a new form of graphitic cages should provide further opportunities for exploration of their exotic and unique properties.