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M. H. Köhler et al.
up to a few mm. In some cases, the MoO 3 is first deposited on the substrate and
then it is submitted to the sulfur vapor annealing. Some works have reported on the
deposition of Mo films that were later sulfurized to produce MoS 2 [43]. Modulating
the homogeneity and thickness of Mo and/or MoO 3 film determines the quality and
thickness of the MoS 2 film. In any case, this CVD approach has been shown to be
highly scalable.
In some cases, a single precursor which contains both elements has been used
[44]. (NH 4 ) 2 MoS 4 can be thermally decomposed on insulating substrates and, similarly to the process described above, can also yield large-area MoS 2 in the presence
of sulfur vapor. Moreover, transistors based on this CVD-prepared MoS 2 showed
good electronic properties with large on/off ratios [45]. Nevertheless, these values
are lower than those obtained from the mechanically exfoliated MoS 2 because
the crystal defects shown in the CVD-grown films can be deleterious to carrier
mobility [46]. Finally, recent works on a variation of CVD, the metal-organic
(MO)CVD using bis(tert-butylimido)-bis(dimenthylamido)molybdenum and dietyl
disulfide have produced MoS 2 layers from 1 to 25 nm thick at short deposition times
(90 s) and with great uniformity on 50 mm SiO 2 /Si wafers [47], making it very
promising for large-scale applications.
WS 2 posses similar electrical, mechanical, and optical properties as MoS 2 and
can be synthesized in a similar fashion. For instance, the band structure of WS 2 is
also dependent on the number of layers since bulk WS 2 is a semiconductor with an
indirect band gap of 1.4 eV, while monolayer WS 2 presents a direct band gap of
2.1 eV [48]. It is also possible to obtain WS 2 through micromechanical exfoliation
and CVD. Dry exfoliation is very similar as in the case of graphene and MoS 2 : a
tape is applied to a bulk WS 2 crystal removing from single to few layers of WS 2
which can be transferred on the desirable substrate. Superior crystal and electronic
properties are obtained using this method, but it lacks scalability. Chemical (wet)
exfoliation has also been reported [49].
The W-based precursors for CVD growth of WS 2 present higher melting points
than the ones used in the MoS 2 growth. For instance, W has a melting point of
3422
z C and WO 3 of 1473
z C [43]. This makes WS 2 deposition more challenging
than MoS 2 since the Mo-precursors present a lower melting point. Still, the basic
approach is similar for both materials: reaction of sulfur vapor with a W-precursor
at high temperatures, which is deposited on the substrate.
The most common precursors are WO 3 and sulfur powder. They are heated inside
a reactor under an inert gas flow, such as argon. The reactor temperature is usually in
the 850–950
z C range. Among the challenges also observed in the growth of MoS 2
(homogeneity, crystal quality, mechanical and electrical properties, layer number
control, etc.), one of the main issues for WS 2 growth is the high temperatures
employed in its process. In this manner, there is a limitation on the type of substrate
that can be used to grow WS 2 . Some works have reported on the use of alternative
W-precursors in order to lower growth temperature, such as the use of WCl 6 [50]. A
great effort must be made in order to grow WS 2 directly on several types of substrates
and materials.
M. H. Köhler et al.
up to a few mm. In some cases, the MoO 3 is first deposited on the substrate and
then it is submitted to the sulfur vapor annealing. Some works have reported on the
deposition of Mo films that were later sulfurized to produce MoS 2 [43]. Modulating
the homogeneity and thickness of Mo and/or MoO 3 film determines the quality and
thickness of the MoS 2 film. In any case, this CVD approach has been shown to be
highly scalable.
In some cases, a single precursor which contains both elements has been used
[44]. (NH 4 ) 2 MoS 4 can be thermally decomposed on insulating substrates and, similarly to the process described above, can also yield large-area MoS 2 in the presence
of sulfur vapor. Moreover, transistors based on this CVD-prepared MoS 2 showed
good electronic properties with large on/off ratios [45]. Nevertheless, these values
are lower than those obtained from the mechanically exfoliated MoS 2 because
the crystal defects shown in the CVD-grown films can be deleterious to carrier
mobility [46]. Finally, recent works on a variation of CVD, the metal-organic
(MO)CVD using bis(tert-butylimido)-bis(dimenthylamido)molybdenum and dietyl
disulfide have produced MoS 2 layers from 1 to 25 nm thick at short deposition times
(90 s) and with great uniformity on 50 mm SiO 2 /Si wafers [47], making it very
promising for large-scale applications.
WS 2 posses similar electrical, mechanical, and optical properties as MoS 2 and
can be synthesized in a similar fashion. For instance, the band structure of WS 2 is
also dependent on the number of layers since bulk WS 2 is a semiconductor with an
indirect band gap of 1.4 eV, while monolayer WS 2 presents a direct band gap of
2.1 eV [48]. It is also possible to obtain WS 2 through micromechanical exfoliation
and CVD. Dry exfoliation is very similar as in the case of graphene and MoS 2 : a
tape is applied to a bulk WS 2 crystal removing from single to few layers of WS 2
which can be transferred on the desirable substrate. Superior crystal and electronic
properties are obtained using this method, but it lacks scalability. Chemical (wet)
exfoliation has also been reported [49].
The W-based precursors for CVD growth of WS 2 present higher melting points
than the ones used in the MoS 2 growth. For instance, W has a melting point of
3422
z C and WO 3 of 1473
z C [43]. This makes WS 2 deposition more challenging
than MoS 2 since the Mo-precursors present a lower melting point. Still, the basic
approach is similar for both materials: reaction of sulfur vapor with a W-precursor
at high temperatures, which is deposited on the substrate.
The most common precursors are WO 3 and sulfur powder. They are heated inside
a reactor under an inert gas flow, such as argon. The reactor temperature is usually in
the 850–950
z C range. Among the challenges also observed in the growth of MoS 2
(homogeneity, crystal quality, mechanical and electrical properties, layer number
control, etc.), one of the main issues for WS 2 growth is the high temperatures
employed in its process. In this manner, there is a limitation on the type of substrate
that can be used to grow WS 2 . Some works have reported on the use of alternative
W-precursors in order to lower growth temperature, such as the use of WCl 6 [50]. A
great effort must be made in order to grow WS 2 directly on several types of substrates
and materials.