G r I I I I I I

Fig. 14. Plot of the A and B exciton energies of IF-MoS2 and IF-WS2 vs 1/L2Z, where Lz is the particle size and n on the upper scale is the number of MS2 layers. The ▲ and • symbols represent the data for the A and B excitons of IF-MoS2, respectively; the x and ▼ show the data for the A and B excitons of IF-WS2 (25 K). The ■ represent the A exciton of 2H-WS2 at 77 K [77,79]

4.3 Raman Spectroscopy

Raman and Resonance Raman (RR) measurements of fullerene-like particles ofMoS2 have been carried out recently (see Figs. 14, 15) [78,81]. Using 488 nm excitation from an Ar-ion laser light source, the two strongest Raman features in the Raman spectrum of the crystalline particles, at 383 and 408 cm-1, which correspond to the E2>g and A1g modes, respectively, (see Table 1), were also found to be dominant in IF-MoS2 and in MoS2 platelets of a very small size. A distinct broadening of these two features could be discerned as the size of the nanoparticles was reduced. In analogy to the models describing quantum confinement in electronic transitions, it was assumed that quantum confinement leads to contributions of modes from the edge of the Brillouin zone with a high density of phonon states to the Raman spectra. Taking account of the phonon dispersion curves near the zone edge and carrying out a lineshape analysis of the peaks led to the conclusion that the phonons are confined by coherent domains in IF nanoparticles of about 10 nm in size. Such domains could be associated with the faceting of the polyhedral IF structures.

RR spectra were obtained by using the 632.8 nm (1.96 eV) line of a He-Ne laser [81]. Figure 15 shows the RR spectra of a few MoS2 samples. Table 1 lists the peak positions and the assignments of the various peaks for the room temperature spectra. A few second-order Raman transitions were also identified. The intensity of the 226 cm-1 peak did not vary much by lowering the temperature, and therefore it cannot be assigned to a second-order transition. This peak was therefore attributed to a zone-boundary phonon, activated by

Fig. 15a,b. Resonance Raman (RR) spectra excited by the 632.8 nm (1.96 eV) laser line at room temperature (left) and 125 K (right), showing second-order Raman (SOR) bands for several MoS2 nanoparticle samples: IF-MoS2 200 A (curve a), IF-MoS2 800 A (curve b), PL-MoS2 50 x 300 A (curve c), PL-MoS2 5000 A (curve d), bulk 2H-MoS2 (curve e), where IF denotes inorganic fullerene-like particles and PL denotes platelets [81]

Fig. 15a,b. Resonance Raman (RR) spectra excited by the 632.8 nm (1.96 eV) laser line at room temperature (left) and 125 K (right), showing second-order Raman (SOR) bands for several MoS2 nanoparticle samples: IF-MoS2 200 A (curve a), IF-MoS2 800 A (curve b), PL-MoS2 50 x 300 A (curve c), PL-MoS2 5000 A (curve d), bulk 2H-MoS2 (curve e), where IF denotes inorganic fullerene-like particles and PL denotes platelets [81]

the relaxation of the q = 0 selection rule in the nanoparticles. Lineshape analysis of the intense 460 cm-1 mode revealed that it is a superposition of two peaks at 456 cm-1 and 465 cm-1. The lower frequency peak is assigned to a 2LA (M) process, while the higher energy peak is associated with the A2u mode, which is Raman inactive in crystalline MoS2, but is activated by the strong resonance Raman effect in the nanoparticles.

4.4 Mechanical Properties

The mechanical properties of the inorganic nanotubes have only been investigated to a small extent. An elastic continuum model, which takes into account the energy of bending, the dislocation energy and the surface energy, was used as a first approximation to describe the mechanical properties [82]. A first-order phase transition from an evenly curved (quasi-spherical) particle into a polyhedral structure was predicted for nested fullerenes with shell thicknesses larger than about 1/10 of the nanotube radius. Indeed, during the synthesis of IF-WS2 particles [83], it was observed that the nanoparticles were

Table 1. Raman peaks observed in the MoS2 nanoparticle spectra at room temperature and the corresponding symmetry assignments. All peak positions are in cm"1 [81]

Bulk

PL-MoS2

PL-MoS2

IF-MoS2

IF-M0S2

Symmetry

MoS2

5000 Â

50 x 30 â2

800 â

200 â

assignment

177

179

180

180

179

Alg (M) - LA (M)

226

227 248

226 248

LA (M)

-

-

-

-

283

Ei9 (r)

382

384

381

378

378

ei9 in

407

409

408

407

406

Alg (r)

421

419

-

weak

weak

-

465

460

455

452

452

2x LA (M)

-

-

498

495

496

Edge phonon

526

529

-

-

-

Elg (M) + LA (M)

-

-

545

545

543

-

572

572

~557

565

563

2x Elg (r)

599

601

595

591

593

Elg (M) + LA (M)

641

644

635

633

633

Alg (M) + LA (M)

transformed into a highly faceted structure, when the shell of the nanopar-ticles exceeded a few nm in thickness. Theoretical and experimental work is underway to elucidate the mechanical properties of inorganic nanotubes.

Along with unusual electronic and optical properties, inorganic nanotubes can also display dramatic mechanical properties [12]. Indeed, the axial Young's modulus of individual BN nanotubes has been measured using vibration reed techniques inside a TEM [12]. The elastic modulus is found to be of order 1 TPa, comparable to that of high quality carbon nanotubes. BN nanotubes thus have the highest elastic modulus of any known insulating fiber. Table 2 summarizes some of the predicted and measured properties of BœCyNz nanotubes. Less is known about other IF nanotubes. Clearly these fascinating materials deserve a great deal of further study.

Table 2. Summary of predicted and measured properties of BxCyNz nanotubes

Type of

Predicted Property

Experimental

Nanotube

Electrical"

£gap (eV) 1' (TPa)

Y (TPa) SWNT

Carbon

SC or M

0 to 1.5 1 to 7

1.35 yes

BN

SC

4 to 5.5 0.95 to 6.65

1.18 yes

BC2N (I)

SC or M

-

no

BC2N (II)

sc

1.28

no

BC3

M

-

no

a SemiConducting (SC) or Metallic (M)

a SemiConducting (SC) or Metallic (M)

0 0

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