Graphene analogues of layered metal selenides

Article abstract of DOI:10.1039/c1dt10652j

Graphene analogues of MoSe₂ and WSe₂ have been prepared by three different chemical methods and characterized by electron microscopy and other methods. Graphene analogues of these diselenides as well as of GaSe have also been obtained by liquid-phase exfoliation. Raman spectra of the graphene analogues show significant changes relative to those of the bulk samples.

Full text of DOI:10.1039/c1dt10652j

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COMMUNICATION
Graphene analogues of layered metal selenides†
H. S. S. Ramakrishna Matte, Blake Plowman, Ranjan Datta and C. N. R. Rao*
Received 13th April 2011, Accepted 27th June 2011
DOI: 10.1039/c1dt10652j
Graphene analogues of MoSe2 and WSe2 have been prepared
by three different chemical methods and characterized by
electron microscopy and other methods. Graphene analogues
of these diselenides as well as of GaSe have also been obtained
by liquid-phase exfoliation. Raman spectra of the graphene
analogues show significant changes relative to those of the
bulk samples.
or tungstic acid was reacted with excess selenourea in an N2
atmosphere at 773 K. Products of these reactions were character-
ized by transmission electron microscopy (TEM), field emission
scanning electron microscopy (FESEM), atomic force microscopy
(AFM) and Raman spectroscopy in addition to X-ray diffraction
(XRD).
Molybdenum selenide prepared by the three chemical methods
do not show the (002) reflection in the XRD patterns as can
be seen from Fig. 1(a). The absence of the (002) reflection in
the XRD patterns indicates that the products formed in the
reactions contain only a few layers. Similar XRD patterns were
also obtained in the case of WSe2 prepared by these reactions
as shown in Fig. 1(b). Energy dispersive X-ray analysis (EDAX)
showed the products to be stoichiometric MoSe2 and WSe2.
Graphene analogues of selenides prepared by different chemical
methods were characterized by FESEM (see Fig. S-1†). In Fig. 2,
we present TEM and AFM images of MoSe2 showing the presence
of one to three layers. Fig. 2(a) and (b) show graphene-like layers
obtained from methods 2 and 3 with a layer separation of 0.65–
0.70 nm. AFM images and the corresponding height profiles of
0.9 nm to 1.9 nm in Fig. 2(c) and (d) confirm the presence of one
Inorganic analogues of fullerenes as well as of carbon nan-
otubes comprising layered metal sulfides and other inorganic
materials have been prepared and characterized adequately.1–4
These inorganic nanomaterials constitute an important family
of nanomaterials. Recent research on graphene5–7 has created
interest in preparing and characterizing graphene analogues of
layered inorganic materials.8,9 Some progress has been made in
the synthesis of inorganic graphenes in the view of exciting
possibilities and potential applications.10,11 Exfoliated MoS2 forms
aqueous suspensions of single layers wherein sulphur atoms
are bonded with molybdenum in an octahedral arrangement
with the 2a0 superlattice.12 A structural transition from the
hexagonal to the trigonal prismatic structure has been indicated
in lithium- intercalated MoS2.13 Exfoliation has been employed
to prepare graphene analogues of metal chalcogenides, following
the procedure used for graphene.14,15 We have been exploring the
synthesis of graphene-like inorganic materials, especially those
formed by layered metal selenides by different chemical methods.
In this communication, we report the successful results obtained
in the case of MoSe2 and WSe2. We have also obtained graphene
analogues of these chalcogenides and of GaSe by exfoliation in N-
methylpyrrolidone.
In order to prepare graphene-like MoSe2 and WSe2 three
different chemical methods were employed.‡ Method 1 involved
the intercalation of n-butyllithium in bulk MoSe2 and WSe2
followed by exfoliation in water. In the process of exfoliation,
lithium-intercalated MoSe2 and WSe2 react with water forming
lithium hydroxide and hydrogen gas, causing the separation of
layers. In method 2, molybdic acid was reacted with selenium
metal under hydrothermal conditions. In Method 3, molybdic
Chemistry and Physics of Materials Unit, CSIR Centre of excellence and
International Centre for Materials Science, Jawaharlal Nehru Centre for
Advanced Scientific Research, Jakkur P. O., Bangalore, 560064, India.
E-mail: cnrrao@jncasr.ac.in; Fax: + 91-80-22082760; Tel: + 91-80-
23653075
Fig. 1 XRD patterns of (a) bulk MoSe2 and graphene-like MoSe2 layers
obtained by chemical methods and (b) of bulk WSe2 and graphene- like
WSe2 layers obtained by chemical methods.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1dt10652j
10322 | Dalton Trans., 2011, 40, 10322–10325
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The Royal Society of Chemistry 2011
©

Fresnel’s contrast16 before performing HREM. Fresnel contrast is
formed under defocus conditions due to abrupt change in inner
potential. Fourier filtered HREM images obtained from the same
area where one can separate out single-layer MoSe2 near the edge
and the second layer away from the edge (which is slightly rotated
with respect to the first layer) are shown in Fig. 3(b) and (c)
respectively. The images and the associated Fourier transforms
show hexagonal symmetry.
Fig. 4 shows TEM and AFM images of WSe2 prepared by two
chemical methods. The TEM image in the Fig. 4(a) corresponds to
WSe2 prepared by method 1 showing sheet-like morphology. It has
lateral dimensions of several micrometres. WSe2 prepared by the
selenourea method contains predominantly 2 layers (Fig. 4(b)). A
TEM image showing 4–5 layers is given in the inset of this Figure.
Fig. 4(c) and 4 (d) show AFM height profiles of 0.7–1.9 nm
supporting the TEM data. Fig. 3(d) presents a HREM image
of single-layer WSe2 prepared by method 3. The corresponding
Fourier filtered image is shown in Fig. 3(e), where the bright dots
correspond to the heavy W atoms.
Fig. 2 TEM image of MoSe2 layers obtained by (a) method 2 and (b)
method 3, AFM images and height profiles of MoSe2 layers from (c)
method 2 and (d) method 3.
to three layers with the lateral dimensions varying from 300 nm to
few micrometres. We have provided the distribution of layers from
the AFM data for method 3 (in Figure S-2†).
In Fig. 3 we show high-resolution transmission electron micro-
scope (HREM) images of MoSe2 and WSe2 layers obtained from
method 3 recorded with a FEI TITAN3TM aberration-corrected
microscope with a negative Cs ~ 30 mm and a positive defocus Df
~ 8 nm. Here, the atoms appear bright in a dark background.
Fig. 3(a) shows graphene-like MoSe2 with single and bi-layer
structures. We have verified the region as single layer using the
Fig. 4 TEM image of WSe2 layers obtained by (a) methods 1 and (b)
method 3, AFM images and height profiles of MoSe2 layers from (c)
method 1 and (d) method 3.
We have obtained few-layer WSe2 by liquid-phase exfoliation
of bulk WSe2 in N-methylpyrrolidone (NMP). The dispersion
formed after 10 min of sonication, characterized by microscopic
techniques revealed the formation of few-layer WSe2.
Raman spectra of graphene-like MoSe2 prepared by different
methods were compared with the spectrum of bulk MoSe2. The
bulk sample gives Raman bands at 168.8 and 242 cm-1 due to the
E1g and A1g modes with the full-width half maxima (FWHM) of
4.1 and 3.7 cm-1 respectively.
Interestingly, few-layered MoSe2 prepared by method 1 exhibits
the corresponding bands at 168.1 and 239.4 cm-1. The sample
obtained by method 3 shows these bands at 168 and 239.9 cm-1 (See
Fig. 5). There is a clear softening of the A1g and E1g modes in the
graphene analogues of MoSe2. The softening of modes observed
Fig. 3 (a) High-resolution TEM image of layered MoSe2 from method
3. Fourier transformed images of single and bilayers are shown in (b) and
(c). (d) High resolution TEM image of WSe2 and (e) the Fourier transform
image of single layer WSe2.
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The Royal Society of Chemistry 2011
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able to fabricate transistors with graphene-like molybdenum
chalcogenides.
Notes and references
‡ Synthesis:
Method 1: MoSe2 and WSe2 were intercalated and exfoliated in two
steps. In the first step, 100 mg MoSe2/WSe2 were weighed and soaked
in 10 ml of n-butyllithium18 in 20 ml of hexane and refluxed in a nitrogen
atmosphere for 48 h at 373 K. The intercalated samples were washed with
hexane several times to remove any excess n-butyllithium. In the second
step exfoliation of the Li-intercalated samples was carried out in distilled
water, followed by ultra-sonication for 10 min. In the process, evolution
of H2 gas occurred with the formation of an opaque suspension of the
layered selenide. The suspension was centrifuged and the solid product
was collected for further characterization. Method 2: MoSe2 layers were
prepared using a hydrothermal method wherein 1 mmol of molybdic acid,
2 mmol of selenium metal and 30 mmol of NaBH4 were taken in 10 ml of
deionised water in a 23 ml capacity of Teflon-coated autoclave and heated at
453 K for 24 h to yield few-layer MoSe2. The product obtained was washed
with ethanol and dried at 278 K and characterized further. (yield ~75%)
Method 3: Layered MoSe2 and WSe2 were prepared using selenourea as
the selenium source. In a typical synthesis, molybdic acid or tungstic acid
was ground with excess of selenourea (molybdic acid (0.032 g)/tungstic
acid (0.049 g) : thiourea (1.18 g) ∫ 1 : 48) and heated at 773 K for
Fig. 5 Raman spectra of bulk MoSe2 and graphene-like MoSe2 layers
obtained by chemical methods and inset is bulk WSe2 and graphene-like
WSe2 layers obtained by method 1.
here is similar that reported in the case of graphene analogues
of MoS2.9 The FWHM values are larger in the graphene-like
samples, the values varying from 6–8 cm-1 compared to ~4 cm-1
in bulk MoSe2. Raman spectra of graphene-like WSe2 obtained
by both the methods show softening of the bands due to the
A1g and E1g modes. Compared to the narrow bands at 175 cm-1
(E1g) and 255 cm-1 (A1g) of bulk WSe2 with FWHM values
around 3.1 and 5.1 cm-1 respectively, (see inset of Fig. 5) the
spectrum of WSe2 obtained from method 1 shows bands at 171
and 249.7 cm-1 with FWHM values of 24 and 43 cm-1. The Raman
spectrum of WSe2 layers synthesized by method 3 shows similar
softening of the Raman bands and increase in the FWHM values.
The broadening of the Raman bands is considered to be due to
phonon confinement.17
GaS and GaSe are both layered materials having mica-like
morphology. We could exfoliate bulk GaS and GaSe in N-
methylpyrrolidone (NMP) to form stable dispersions of the
materials mainly containing one to three layers. Fig. 6(a) shows
the sheet-like morphology of GaSe while Fig. 6(b) gives the AFM
image with a height profile of 1.3 nm confirming the formation of
few-layered GaSe.
◦
3 h under nitrogen atmosphere with heating rate of 20 C min-1. The
product was cooled to room temperature in a nitrogen atmosphere (yield
~60%).
Characterization: The products of the above reactions were characterized
by various techniques. X-Ray diffraction (XRD) patterns were recorded
using Cu-Ka radiation on a Rich-Siefert XRD-3000-TT diffractometer.
FESEM images were obtained using a FEI NOVA NANOSEM 600.
EDAX was carried out with the same system. TEM images and atomic
arrangement of hexagonal MoSe2 and WSe2 in Fig. 3 (no reconstruction)
were obtained from the FEI TITAN (cube) 80–300 kV aberration corrected
transmission electron microscope with a negative spherical aberration
coefficient (Cs) of ~-30 mm and a positive defocus about +8 nm, where
atomic potentials appear with bright contrast in a dark background.
AFM measurements were carried on Vecco digital instruments, di Innova.
Samples for AFM measurements were prepared by spin coating the MoSe2
and WSe2 solutions on Si substrate. Raman spectra of samples were
recoreded with a 515.5 nm Ar laser using JobinYvon Lab Ram HR
spevtrometre.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10322–10325 | 10325
©