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Revista Científic@ Universitas, Itajubá v.11, n.1, p. 68 – 76, 2024
ISSN Eletrônico: 2175-4020
Desenvolvimento de um sistema robusto para preparação de substrato e crescimento de
filmes finos de MoS2, pela técnica de CVD
Development of a robust system for substrate preparation and growth of MoS2 thin
films by CVD technique
Flavio Yuji Assahi1, João Ider2, Adhimar Flavio Oliveira3, Carla Rubinger4, Rero Marques Rubinger5
1 - Discente do Curso de Doutorado em Ciência e Engenharia de Materiais da Universidade Federal de Itajubá -
UNIFEI. E-mail.: flavioassahi@gmail.com
2 - Doutor em Ciência e Engenharia de Materiais da Universidade Federal de Itajubá - UNIFEI. E-mail.:
joaoider@gmail.com
3 - Docente da Universidade Federal de Itajubá - UNIFEI. E-mail.: adhimarflavio@unifei.edu.br
4 - Docente da Universidade Federal de Itajubá - UNIFEI. E-mail.: carlalr@unifei.edu.br
5 - Docente da Universidade Federal de Itajubá - UNIFEI. E-mail.: rero@unifei.edu.br
Recebido em: 22/02/24 Revisado em: 03/07/24 Aprovado em: 05/08/24
Resumo: O primeiro material bidimensional descoberto foi o grafeno em 2004. A partir de então, estudos foram
desenvolvidos com outros materiais e, um dos primeiros semicondutores a ser isolado foi o dissulfeto de
molibdênio, o MoS2. Ao contrário do grafeno, o MoS2 tem gap de energia, e na forma de monocamada o gap passa
de indireto para direto. Essa mudança resulta em uma fotoluminescência de alto brilho. Devido ao alto desempenho
elétrico e óptico, o MoS2 (2D) tem grande potencial de aplicação nos dispositivos eletrônicos e campos
fotoeletrônicos. Existem diversas técnicas para produzir este material, e uma delas é através do método de
Deposição Química de Vapor (CVD) que consiste na formação de cristais no substrato, pela deposição atômica ou
molecular, sendo o sólido oriundo de uma reação química onde os precursores estão na fase de vapor. O objetivo
deste trabalho será sintetizar e caracterizar o MoS2 pelo método de CVD, a partir de um sistema de equipamento
robusto construídos no laboratório. Para preparar os substratos foram submetidos a um tratamento de ultravioleta
para permitir que os materiais depositados resistam ao desgaste por contato. Então construiu-se uma câmara para
tratamento por radiação UV/ozônio, e, após a preparação, os substratos foram colocados em um forno com
temperatura controlada e com atmosfera inerte. Este forno tubular também foi construído com o intuito da
produção do MoS2. Após os ensaios foi possível obter a formação de MoS2 com a caracterização pelo MEV, EDS
e DRX para análise e identificação dos materiais formados no substrato.
Palavras-chave: semicondutor bidimensional, filme fino, dissulfeto de molibdênio (MoS2), deposição química
de vapor (CVD).
Abstract: The first two-dimensional material discovered was graphene in 2004. From then on, studies were
developed with other materials and one of the first semiconductors to be isolated was molybdenum disulfide,
MoS2. Unlike graphene, MoS2 has an energy gap, and in monolayer form the gap goes from indirect to direct. This
change results in high-brightness photoluminescence. Due to its high electrical and optical performance, MoS2
(2D) has great potential for application in electronic devices and photoelectronic fields. There are several
techniques to produce this material, and one of them is through the Chemical Vapor Deposition (CVD) method,
which consists of the formation of crystals on the substrate, through atomic or molecular deposition, with the solid
originating from a chemical reaction where the precursors are in the vapor phase. The objective of this work will
be to synthesize and characterize MoS2 using the CVD method, using a robust equipment system built in the
laboratory. They were subjected to an ultraviolet treatment to prepare the substrates to allow the deposited materials
to resist contact wear. A chamber was then built for UV/ozone radiation treatment, and, after preparation, the
substrates were placed in a temperature-controlled oven with an inert atmosphere. This tubular furnace was also
built with the aim of producing MoS2. After the tests, it was possible to obtain the formation of MoS2 with
characterization by SEM, EDS and XRD for analysis and identification of the materials formed on the substrate.
Keywords: two-dimensional semiconductor, thin film, molybdenum disulfide (MoS2), chemical vapor deposition
(CVD).
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Introduction
MoS2 has excellent electronic behavior
and mechanical properties. Research shows
that it is a photothermal material with higher
absorbance in the IR region than graphene
oxide and gold nanorods (Zhu, 2018). Thus, it
is possible to design suitable applications in
biomedical, such as cancer therapy or drug
delivery.
There are several methods of preparing the
MoS2 film. Common preparation methods
include micromechanical exfoliation, lithium-
ion intercalation, liquid-phase
ultrasonography, and the chemical vapor
deposition (CVD) method (Wu, 2013).
Among the different types of preparation,
there is the CVD technique, which allows the
growth of a thin film of nanostructured
material on a relatively small substrate, in the
order of centimeters, in addition to not having
external contamination during deposition and
there is the ease of cleaning of the materials
used (Song, 2012).
The CVD technique is widely used for a
variety of purposes. In industries, it is used
from the coating of parts to replace human
joints that are coated with a thin film making
them biocompatible, to the coating of parts for
engines (Pedersen, 2014). In the area of
electronics, advances are as significant as in
the construction of rechargeable batteries,
transistors, and solar cells, where thin films
must be uniform (Ohring, 2002).
Due to MoS2 being a promising two-
dimensional semiconductor, this work is
devoted to describing the construction of a
CVD furnace and a UV chamber for substrate
treatment and MoS2 film growth aiming at the
application of thermogenerators. Both the
construction of the chamber and the oven aim
at successful low-cost sample growth and at
the production of a higher-performing
semiconductor.
Material and Methods
Fused quartz glass discs with 99.99% level
purity were chosen as a substrate. For the disk
cleaning process, a digital ultrasound device
was used for the substrate cleaning process.
First, the discs were immersed in acetone and
placed in the ultrasound tank. Through the
sonication process, the substrates were left for
15 minutes to remove dirt such as dust. In
sequence, they passed through an isopropyl
bath and finally in deionized water. After this
step, the substrates were dried with a high-
purity nitrogen jet, thus properly sanitized for
the next step.
To produce high quality thin film MoS2 by
the CVD technique is necessary good
adhesion. For that, a surface treatment that
leaves open or dangling bonds is necessary
allowing the materials to resist wear due to
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contact (LI, 2014). This research is carried out
by exposing fused quartz glass discs to
UV/Ozone rays (Uvo).
UVO treatment has been used to
effectively remove surface contaminants from
various materials (Kohli, 2019). Compared to
other methods such as hydrofluoric solution
and high velocity air blasts, UVO treatment
generates little toxic or noxious gases and no
residual liquid during the cleaning process. It
can even produce nearly atomically clean
surfaces. In addition to surface cleaning, UVO
can also be used to modify the surface and
improve the adhesion of the material to be
deposited (Wang, 2017; Kimura, 2018; Wu,
2019).
The surface of quartz glass is mainly
composed of silanol groups (-SiOH), which
can oxidize in the presence of an ozone-rich
atmosphere and form siloxane groups (-Si-O-
Si-). The oxidation reaction can be seen in
Figure 1. Exposing quartz glass to UV rays
can cause the breaking of some chemical bonds
on the surface, resulting in the formation of
free radical groups. These free radicals can
then combine and form additional siloxane
groups on the surface, increasing its density.
These siloxane groups are naturally
hydrophobic and contribute to quartz’s low
surface energy and therefore to its
hydrophobicity (Zhuravlev, 2006; Owen,
2012; Ozçam, 2014; Schrader, 2018).
Figure 1 - Oxidation of substrate surface through UVO
treatment.
UV radiation/ozone treatment is often
used to improve the surface properties of
quartz glass. During this process, ozone acts as
a strong oxidant, accelerating the oxidation of
silanol groups and the formation of siloxane
groups on the surface. This contributes even
more to the increase in hydrophobicity and can
directly interfere with the surface quality of the
sample (Zhuravlev, 2006; Owen, 2012;
Ozçam, 2014; Schrader, 2018).
For the treatment of the substrates, a UVO
chamber was manufactured (Figure 2) to block
the rays emitted by the light source, adapted
with an air cooler for air circulation,
guaranteeing the elimination of the ozone
formed during exposure to UV rays.
Figure 2 - UVO chamber for UV radiation treatment of
substrates.
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After surface treatment, the substrates are
immediately inserted into the CVD oven for
MoS2 deposition. This oven is a tubular
chemical vapor deposition furnace adapted for
use as a CVD oven for the growth of MoS2 thin
films.
Basically, the chemical reactions that
occur in the CVD oven use MoO3 and sulfur
are the precursor. They are placed at a specific
position corresponding to temperature zones
that vaporize them prior to combination and
chemical reaction and subsequent drift towards
the substrate by an inert gas. For this, the oven
temperature is increased to 800 ºC, first, the
MoO3 vapor is partially transferred by the high
purity nitrogen carrier gas and the MoO3
molecules are reduced to metallic Mo prior to
binding to the substrate. In sequence, the sulfur
is also vaporized and part of it reacts with
oxygen, and part reaches the substrates and
reacts with metallic Mo. So, the result is the
formation of MoS2. The diagram of the MoS2
growth mechanism can be seen in Figure 3.
Figure 3 - Illustration of the MoS2 growth mechanism
on the substrate.
The oven reaction chamber is made up of
a quartz glass tube inside, supported by a
polygonal shape of stainless steel, see
Figure 4.
Figure 4 - Tubular oven used to prepare the MoS2
growth.
Heating is done by metallic resistances
mounted on the inner walls of the oven,
supported by refractory molds. The tube is held
in the oven by a fiberglass thermal tape. At
each end of the tube, stainless steel rings were
produced with openings for gas inlet and outlet
and 3 perforations to accommodate the
thermocouples. With these, it was possible to
map and define temperature variations in
relation to the length of the tube, see
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Figure 5. The software used to control the
ramp and temperature level was the same as
used for the unmodified oven, i.e. Flycon by
Flyeve.
Figure 5 - Mapping of oven temperature and position
distributions of each element.
In the experiment, the sulfur powder was
placed in the low temperature zone on the left
side of the tube furnace tube, the MoO3 powder
was placed in the high temperature zone in the
middle of the tube, and the substrate on the
right side of the tube. This scheme is
represented by
Figure 6. The sulfur source, in the low
temperature zone, was around 25 cm away
from the molybdenum source in the high
temperature zone, and the molybdenum source
was 15 cm away from the substrate. Pure
nitrogen gas flows from left to right in this
scheme.
Figure 6 - Sketch of MoS2 preparation by chemical
vapor deposition (CVD).
Immediately after evacuating the tube
with a mechanical pump, it is filled with high-
purity nitrogen gas and allowed to flow at a
constant rate for 5 min. The evacuation-fill
processes are repeated three times to vent air
and impurities from the tube furnace prior to
allowing the flowing. After that, high purity
nitrogen gas is introduced into the tube
furnace, and when the pressure reaches 1.2
atm, the exhaust valve is opened and adjusted
to make the pressure in the furnace greater than
1.0 atm. Then the oven is heated to a set
temperature and held for a certain time. In the
heating process, nitrogen gas is continuously
introduced into the furnace. As an inert to the
process gas, nitrogen gas is also the carrier
medium for both sulfur vapor from the low
zone to the high temperature zone and then to
the substrate. At high temperatures, the
molybdenum oxide vapor is reduced by the
sulfur vapor. The produced gas compound
reacted with the sulfur vapor to generate MoS2,
which is deposited on the substrate to form
MoS2 nanocrystals. As the volume of gas in the
tube shrinks with decreasing temperature
during the cooling process, nitrogen gas needs
to be supplied continuously until the
temperature is completely reduced to room
temperature to prevent outside air from
entering the tube oven.
Results and Discussion
The effect of UVO exposure time on substrate
surface performance was investigated by
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detecting hydrophobicity and surface
topography. Next, the film deposition
condition was investigated and optimized for
the control of parameters necessary for the
formation of high-quality MoS2 on the surface.
To investigate the hydrophobicity after the
UVO treatments, the contact angles of water
on the substrates were measured. Figure 7
shows the drops of water corresponding to the
different exposure times and in Figure 8 the
curves of the contact angles as a function of the
treatment time. The contact angle is measured
from the moment the drop remains constant.
The contact angle increased from 4º to 27º
when the UVO treatment time was up to 75
min. It is then observed that the hydrophobicity
is increased through the chemical modification
of the surface. It is believed that the siloxane
groups induced by UVO treatments are
hydrophobic, which increases the surface
tension of the dispersion of pure water
droplets.
Figure 7 - Drops of pure water deposited onto substrates
treated by UVO on substrates for (a) 0 min, (b) 15 min,
(c) 30 min, (d) 45 min, (e) 60 min, and (f) 75 min.
Figure 8 - Drop contact angles as a function of UVO
treatment time.
Since the contact angle stops to increase,
reaching 27º with the surface after an exposure
time of 60 minutes, it was adopted for the
treatment of the substrates. From this
treatment, OH groups were created, forming a
chemically active layer on the surface of the
substrate, favoring the adhesion of
subsequently deposited material.
Then, with the treatment finished, the
substrates were allocated in the tubular furnace
to allow a deposition of material on the surface
for the formation of MoS2. After
manufacturing the compound, Scanning
Electron Microscopy (SEM) analysis was
performed and the micrograph of Figure 9 was
obtained.
Figure 9 - Micrograph of MoS2 obtained by SEM.
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From Scanning Electron Microscope
(SEM) micrographs it is observed that different
types of structures are formed: by the letter A
in the form of plates in the part close to the
substrate with a dark tone, and by the letter B,
on top of smaller dispersed particles with a
lighter tone. To obtain the details of each type
of particle identified, the Energy Dispersive X-
ray Spectroscopy (EDS) analysis was carried
out allows identification of the chemical
elements present in the deposited samples. The
technique consists of analyzing the modified
wavelength emitted after the interaction
between the electromagnetic radiation emitted
by the equipment and the atomic structure of
the elements.
In Figure 10, it is observed in (b) the
micrograph of the MoS2 sample, that it was
possible to identify some plaques and small
particles above the substrate. Then the
selection of chosen points was subdivided into
3 groups: from 1 to 3 on top of the plates, from
4 to 6 on top of small particles, and from 7 to 9
on top of the substrate. It is observed in (a) the
results of the identification of chemical
elements in atomic percentage.
Figure 10 - Analysis of EDS obtained by SEM. In (a) the
atomic percentage table of each identified element and
in (b) the micrograph of the sample.
From the results obtained by EDS, for
groups 1 to 3, it is observed that the compound
formed has an atomic percentage equivalent to
5 parts of molybdenum to 1 of sulfur (Mo5xSx).
For groups 4 to 6, an atomic percentage of
approximately 1 part of molybdenum to 2 of
oxygen (MoxO2x). For the group from 7 to 9, it
is observed that it presents an atomic
percentage of approximately 1 part of
molybdenum for 3 of oxygen (MoxO3x).
Then, X-ray diffraction analysis (DRX)
was performed to identify the deposited
material. From the sets of peaks formed by the
diffraction of the beams, the diffractogram of
the analyzed material is obtained, Figure 11.
Figure 11 - X-ray diffractogram of the sample.
With the XRD results, it was possible to
determine the presence of 4 types of materials.
With the main component we have MoS2, in
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addition to identifying the oxides: MoO3,
MoO2 and Mo3O.
Compared with the work of (Alves, 2022),
who also used the CVD method, it generated
many intermediate compounds, resulting in the
formation of other types of materials: MoO2
and MoOS2. Then, they had to carry out
another resulfurization route to obtain the first
fine flakes of MoS2, but at a low density on the
substrate.
The formation of MoO2 occurs at
temperatures below 564 °C. Despite
introducing the substrates after this
temperature to avoid the formation of this
oxide, the presence of this compound in the
sample was observed, although in small
quantities. It is known that the decomposition
of excess MoO3(g) can form MoO2(s).
Therefore, the presence of MoO2 may have
occurred due to the high concentration of
MoO3(g) in the substrate region.
The presence of MoO3 is also observed,
which may have formed due to the
concentration of precursor gases not being
stoichiometrically balanced during the
chemical reaction in the substrate region. If the
evaporation rate of MoO3 is much higher than
that of S2, MoO3(g) may not be completely
reduced by S2(g) in time, favoring the formation
of MoO3 on the substrate.
Conclusions
The construction of the entire robust
system, even though it was built with
laboratory equipment and recycled castings,
enabled the growth of MoS2 using the
Chemical Vapor Deposition (CVD) method.
This method proved to be quite complex, as it
is necessary to have control of the various
parameters that influence the process.
The sample was characterized by SEM to
verify the different types and formats of
deposited substances. Qualitatively, the sample
underwent XRD analysis to identify the
materials present in the substrate. It was noted
that they generated many intermediate
compounds. However, MoS2 was found by
XRD to be one of the main components
formed.
So, the results were expected, with the
formation of promising MoS2 thin films that
could be applied to the construction of new
devices, such as thermogenerators.
Acknowledgments
The authors would like to thank the
FAPEMIG for its financial support.
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