Graphene: Synthesis, Functionalization and Properties - Graphene and Its Fascinating Attributes - page 25

The bulk sample gives bands at 406.5 and 381.2 cm -1 due to the
A 1g and E 2g modes with the full-width half maximum (FWHM) of 2.7
and 3.1 cm -1 respectively. Interestingly, few-layered MoS 2 prepared
by lithium intercalation exhibits the corresponding bands at 404.7 and
379.7 cm -1 . The sample obtained by Method 2 show these bands at
404.7 and 377.4 cm -1 . There is a clear softening of the A 1g and E 2g modes
in the graphene analogues of MoS 2 . Furthermore, the FWHM values
are larger in the graphene-like samples, the values varying from 10 to
16 cm -1 compared to ~3 cm -1 in the bulk sample. The broadening of the
Raman bands is considered to be due to the phonon confinement. The
broadening also suggests that the lateral dimensions of these layers are in
the nano regime.
Graphene analogues of WS 2 have been prepared by two methods as
mentioned earlier. XRD patterns of these samples show the absence of
the (002) reflection as shown in the Fig. 15(b). EDAX analysis confirms
the stoichiometry of the products to be WS 2 . The TEM images in
Figs. 16(d) and 16(e) reveal that WS 2 obtained from both methods 1 and
2 mostly consists of bilayers and single layers. The separation between
the WS 2 layers in the bilayers sample is in the 0.65-0.70 nm range. WS 2
layers obtained by the thiourea method show an interlayer spacing of
~ 0.9 nm.
AFM images and height profiles of the WS 2 obtained by method 1
confirms the existence of 2-3 layers of WS 2 with an average thickness of
~1.3 nm. Presence of bilayers of WS 2 in the sample obtained by the
thiourea method is also confirmed by AFM studies. Raman spectra of
WS 2 obtained by both the methods show softening of the bands due to
the A1g mode (see Fig. 17(b)). Compared to the narrow bands at 351
(E 2g ) and 420 cm -1 (A 1g ) of bulk WS 2 with FWHM values around 7.8 and
2.4 cm -1 respectively, the spectrum of WS 2 obtained from lithium
intercalation shows bands at 350 and 415 cm -1 with FWHM values of
13.7 and 8.4 cm -1 . The Raman spectrum of WS 2 layers synthesized by
Method 2 also shows similar softening of the Raman bands and increase
in the FWHM [81].
Single- and few-layer graphene analogues of BN flakes were prepared
by reacting boric acid with different proportions of urea at 900°C [82].
The number of BN layers decreases with the increase in urea content in