Optical Properties of MS 2 (M = Mo, W) Inorganic Fullerenelike and Nanotube Material Optical Absorption and Resonance Ra
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Optical properties of MS2 (M 5 Mo, W) inorganic fullerenelike and nanotube material optical absorption and resonance Raman measurements G. L. Frey and R. Tenne Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, Israel
M. J. Matthews, M. S. Dresselhaus, and G. Dresselhaus Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 6 February 1998; accepted 7 May 1998)
The optical properties of inorganic fullerene-like and nanotube MS2 (M Mo, W) material are studied through absorption and resonance Raman, and compared to those of the corresponding bulk material. The absorption measurements show that the semiconductivity is preserved. Nevertheless, the positions of the excitons are altered in comparison to the bulk. The Raman spectra of the nanoparticles show a close correspondence to that of the bulk. However, the first-order peaks are broadened and, under resonance conditions, new peaks are observed. The new peaks are assigned to disorder-induced zone edge phonons.
I. INTRODUCTION
Carbon fullerenes and nanotubes have been investigated intensively in the last few years. The driving force for their formation is the instability of nanoscopic planar graphene sheets. In analogy to that, it was suggested that nanoscopic planar sheets of layered dichalcogenides and other layer compounds are unstable and would transform, under appropriate conditions, into caged structures.1 This concept extends the scope of fullerene-like materials into inorganic solid state compounds. Consequently, an abundance of other fullerene-like or nanotube materials have been produced and studied in recent years. The first inorganic compounds to exhibit fullerene-like structures were the layered metal dichalcogenides MX2 (M Mo, W; X S, Se), generically called inorganic fullerenelike material-IF1,2 (see Fig. 1). Subsequently, the largescale synthesis of IF-MS2 (M Mo, W) powder has been reported.3–5 Furthermore, these IF metal dichalcogenides exhibit a variety of sizes and shapes and have been investigated by x-ray powder diffraction (XRD),3 scanning tunneling microscopy (STM),6 high resolution transmission electron microscopy (TEM),3–5 optical absorption spectroscopy,7 and Raman scattering.8 The XRD analysis revealed that the bulk 2H structure of the unit cell is locally preserved within an IF particle. Nonetheless, a shift of the XRD (0002) peak of the IF phase indicates a lattice expansion of ca. 2% between two adjacent MS2 slabs along the c-axis compared to the bulk 2H polytype.4 This expansion is attributed to the strain in the bent layers and to discommensuration between layers of different radii, i.e., different number of atoms. In this paper we present a comprehensive study of the optical absorption and resonance Raman (RR) 2412
J. Mater. Res., Vol. 13, No. 9, Sep 1998
spectra of the MS2 fullerene-like and nanotube materials, and compare them to those of the analogous 2H bulk. Although the optical properties of the cage
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