Molybdenum Disulfide and Tungsten Disulfide as Novel …
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plane on both sides. They usually present a hexagonal crystal structure, where the
wide range of possible compositions leads to several distinct optical, electrical, and
mechanical properties. For instance, TMDs can present metallic (NbTe 2 , TaTe 2 ) and
superconducting [35] (NbS 2 , TaS 2 ) or semiconducting [36] (MoS 2 , WS 2 ) electronic
properties. Interestingly, TMD properties have some degree of reliance upon the
number of layers, where the chalcogen–metal–chalcogen structure is considered a
monolayer. One of the most common properties dependent on the number of layers
is the TMDs’ band structure. Many of them present a transition from an indirect
(bulk material) to a direct (monolayer) band gap, allowing for optoelectronic device
application [37]. One example is the MoS 2 transition from bulk with a 1.3 eV indirect
band gap to a 1.8 eV direct band gap in monolayer form [38]. A similar effect is
also observed in graphene when it presents a band gap different from zero [39] or
superconductivity [40] in a bilayer structure.
The reproducible synthesis of 2D materials is mandatory for characterizing these
layer properties, as well as providing a path for their integration into a variety of applications. It is possible to distinguish between the two most widely used methods to
synthesize single- and few-layer 2D materials: the top-down and bottom-up methods.
The first is based on the exfoliation of layered bulk van der Walls crystals, while
the latter is obtained by CVD growth on different substrates. The micromechanical
exfoliation has been used routinely to obtain 2D flakes, which may contain several
crystal layers to a single monolayer. Van der Walls crystalline solids are usually
employed as the base material for exfoliation. The crystal structure is composed of
atom layers strongly bonded covalently or ionically in-plane, and these planes are
stacked together by weak van der Walls forces. These van der Walls energies are in
the range of a few of meV, which enable the easy exfoliation of the atomic layers.
This was the approach used to obtain the first graphene layers [2] and it is still used
for a wide range of materials, predominantly using a tape as the (dry) exfoliation
method. 2D materials such as MoS 2 , black phosphorus [41], and h-BN [42] have
been obtained using this procedure. The obtained 2D monolayers present the same
crystal structure as their bulk counterparts and can remain thermodynamically stable
under ambient conditions up until weeks. This method of isolation of individual and
few layers still is one of the most widely used to study their properties and for the
fabrication of electronic devices, since it can produce monolayers with high quality.
Nevertheless, it is mostly limited to crystal sizes in the order of some µm, making it
unsuitable for large-area applications.
Conversely, CVD growth is able to produce high-quality 2D materials with
controlled size, number of layers, and superior electronic properties. Specifically,
this method has been used to produce large-area MoS 2 , WS 2 , graphene, and h-BN,
which can be integrated into the fabrication of nanoelectronic devices.
MoS 2 growth can be obtained using two different precursors, one that contains Mo
and another that contains sulfur. The process involves typically MoO 3 and S powders,
where the substrate can be a dielectric (such as SiO 2 ) or conductor (graphene), among
others. It consists of heating MoO 3 to become volatile as some sub-oxide (MoO x )
and then reacts with the sulfur vapor to produce MoS 2 of the desired substrate.
There are many variations of this process that can lead to MoS 2 layers with sizes
201
plane on both sides. They usually present a hexagonal crystal structure, where the
wide range of possible compositions leads to several distinct optical, electrical, and
mechanical properties. For instance, TMDs can present metallic (NbTe 2 , TaTe 2 ) and
superconducting [35] (NbS 2 , TaS 2 ) or semiconducting [36] (MoS 2 , WS 2 ) electronic
properties. Interestingly, TMD properties have some degree of reliance upon the
number of layers, where the chalcogen–metal–chalcogen structure is considered a
monolayer. One of the most common properties dependent on the number of layers
is the TMDs’ band structure. Many of them present a transition from an indirect
(bulk material) to a direct (monolayer) band gap, allowing for optoelectronic device
application [37]. One example is the MoS 2 transition from bulk with a 1.3 eV indirect
band gap to a 1.8 eV direct band gap in monolayer form [38]. A similar effect is
also observed in graphene when it presents a band gap different from zero [39] or
superconductivity [40] in a bilayer structure.
The reproducible synthesis of 2D materials is mandatory for characterizing these
layer properties, as well as providing a path for their integration into a variety of applications. It is possible to distinguish between the two most widely used methods to
synthesize single- and few-layer 2D materials: the top-down and bottom-up methods.
The first is based on the exfoliation of layered bulk van der Walls crystals, while
the latter is obtained by CVD growth on different substrates. The micromechanical
exfoliation has been used routinely to obtain 2D flakes, which may contain several
crystal layers to a single monolayer. Van der Walls crystalline solids are usually
employed as the base material for exfoliation. The crystal structure is composed of
atom layers strongly bonded covalently or ionically in-plane, and these planes are
stacked together by weak van der Walls forces. These van der Walls energies are in
the range of a few of meV, which enable the easy exfoliation of the atomic layers.
This was the approach used to obtain the first graphene layers [2] and it is still used
for a wide range of materials, predominantly using a tape as the (dry) exfoliation
method. 2D materials such as MoS 2 , black phosphorus [41], and h-BN [42] have
been obtained using this procedure. The obtained 2D monolayers present the same
crystal structure as their bulk counterparts and can remain thermodynamically stable
under ambient conditions up until weeks. This method of isolation of individual and
few layers still is one of the most widely used to study their properties and for the
fabrication of electronic devices, since it can produce monolayers with high quality.
Nevertheless, it is mostly limited to crystal sizes in the order of some µm, making it
unsuitable for large-area applications.
Conversely, CVD growth is able to produce high-quality 2D materials with
controlled size, number of layers, and superior electronic properties. Specifically,
this method has been used to produce large-area MoS 2 , WS 2 , graphene, and h-BN,
which can be integrated into the fabrication of nanoelectronic devices.
MoS 2 growth can be obtained using two different precursors, one that contains Mo
and another that contains sulfur. The process involves typically MoO 3 and S powders,
where the substrate can be a dielectric (such as SiO 2 ) or conductor (graphene), among
others. It consists of heating MoO 3 to become volatile as some sub-oxide (MoO x )
and then reacts with the sulfur vapor to produce MoS 2 of the desired substrate.
There are many variations of this process that can lead to MoS 2 layers with sizes