In 1992 Tenne and coworkers showed that nanoparticles of the layered compound such as WS2 are unstable in the platelet form and they spontaneously form closed cage structures similar to carbon fullerenes and carbon nanotubes. This instability was attributed to the highly reactive dangling bonds of both sulfur and tungsten atoms, which appear at the periphery of the nanoparticles.
From an application point of view, fullerene-type MoS2 has superb self-lubricating properties, which opens application possibilities in the car, aerospace, micro-electronics and numerous other industries (http://www.apnano.com/ ). Other proposed applications for chalcogenide nanotubes include nanocomposites, ultrastrong tips for scanning probe microscopy, Li-batteries, catalysis and electrochemical hydrogen storage.
We are interested in the development of synthetic procedures for the synthesis and systematic chemical modification of MQ2 metal chalcogenide nanotubes (M = early transition metal, Q = chalcogen), e.g. by electrochemical intercalation of alkali metals, substitution of the metal M. Intercalation reactions allow to adjust the valence electron concentration of the chalcogenide nanotubes within a given composition window. The systematic variation of the electronic properties by means of intercalation is studied spectroscopically in order to established structure/property relationships for this group of materials and to compare the properties of the nanoscopic structures with those of the corresponding bulk compounds.
VS2 nanotubes and U/I diagram for the electrointercalation of Cu in NT-VS2.
Angew. Chem. Int. Ed. Engl. 2005 44, 262-265.
Many questions remain to be addressed. One challenging problem concerns the question how the nanoparticles of such layered compounds fold and form closed cages. As early as 1993 Tenne proposed (Nature, 365, 113 (1993)) that rhombi and triangles may be formed. This idea was verified by Heben et.al. (Nature, 397, 114 (1999)), who used laser ablation of MoS2 targets to obtain MoS2 nanooctahedra, having six rhombi in their corners. In parallel we could show that curved structures are obtained from MoS2 nanoparticles below a critical size (Ø ~ 10 nm) by heating (J. Mater. Chem. 1998, 8, 241). Single wall MoS2 nanotubes with a diameter less than one nm were reported by Remskar et al. (Science, 292, 479 (2001)).
Equally relevant is the question how unique the properties of these nanoparticles are in comparison to the macroscopic materials. Using DFT clculations, Seifert et.al., (Phys. Rev. Lett., 85, 146, (2000)) have shown that the bandgap of MoS2 and W2 nanotubes decreases with shrinking nanotube diameter. This result is counterintuitive in comparison to semiconductor quantum dots (and semiconductive carbon nanotubes), in which the bandgap expands with shrinking diameter. Indeed using optical measurements, and more recently scanning probe microscopy of IF-WS2, we have reached similar conclusions.
Photoemission spectra of individual selected MoS2 nanotubes on a Si subtrate with femtosecond laser excitation and the associated computed density of states.