Article

Flow Rate-Driven Morphology Evolution of Chemical Vapor

Deposited WS2 at Varying Temperatures
Himal Pokhrel , Sanjay Mishra and Shawn Pollard *

Department of Physics and Materials Science, University of Memphis, Memphis, TN 38152, USA;
hpokhrel@memphis.edu (H.P.); srmishra@memphis.edu (S.M.)
* Correspondence: shawn.pollard@memphis.edu

Abstract: Due to its unique electronic and optical properties, tungsten disulfide (WS2 ) is a promising
material for various device applications. However, achieving an efficient and cost-effective method
for synthesizing large-area uniform WS2 is still challenging. In this work, we demonstrate the
synthesis of few-layer WS2 crystallites by NaCl-assisted low-pressure chemical vapor deposition and
study the effect of temperature and the carrier gas flow rate on the morphology, structure, and optical
properties of the as-grown WS2 films. We observe transitions between regular triangular to strongly
disordered structures with sizes up to 50 µm through temperature and carrier gas flow rate tuning.
As-grown samples were characterized by Raman spectroscopy, scanning electron microscopy, and
X-ray photoelectron spectroscopy. The result of this work provides a path toward the optimization of
growth conditions for obtaining WS2 with desired morphologies for various applications.

Keywords: 2D materials; chemical vapor deposition; transition metal dichalcogenides; thin film growth

1. Introduction
Two-dimensional transition metal dichalcogenides (2D-TMDs) have garnered signifi-
cant interest due to their remarkable electrical, optical, and chemical properties. Among
various 2D-TMDs, WS2 has demonstrated high carrier mobility [1], strong spin-orbit cou-
Citation: Pokhrel, H.; Mishra, S.; pling [2–4], and strong light–matter interactions [5,6] which have led to its proposed use
Pollard, S. Flow Rate-Driven in a variety of optoelectronic devices [7]. Further, due to its direct band gap of ~2.1 eV in
Morphology Evolution of Chemical
monolayer form and indirect band gap of ~1.3 eV in bulk form, as well as its high stability,
Vapor Deposited WS2 at Varying
it is one of the most studied TMDs [8–10]. Along with the layer-dependent band gap,
Temperatures. Solids 2024, 5, 510–519.
it exhibits significant spin orbit-induced band splitting and spin valley coupling [11,12].
https://doi.org/10.3390/
This material is a promising candidate for application in semiconductor devices such as
solids5040034
transistors and solar cells [13–15]. Due to their range of potential applications, a wide range
Academic Editor: Stefano Agnoli of work has focused on the ability to control the morphology of TMDs, including WS2 , with
chemical vapor deposition (CVD) being the most widely studied synthesis method owing
Received: 11 September 2024
Revised: 4 October 2024
to its scalability, cost-effectiveness, and the wide range of methods available in which to
Accepted: 15 October 2024
transfer as-grown material to other substrates for incorporation into devices [16].
Published: 17 October 2024 However, CVD is further complicated by the wide range of experimental parameters
that influence growth, including temperature, precursor material quantities, carrier gas
type and flow rate, growth time, the inclusion of promotor materials, and growth geom-
etry, among others [17]. For example, the morphology may be tuned from triangular to
Copyright: © 2024 by the authors. hexagonal by controlling the relative transition metal and chalcogen termination growth
Licensee MDPI, Basel, Switzerland. rates, which may be influenced by modulating the availability of each respective gas
This article is an open access article phase precursor. Previous works have demonstrated this through the tuning of precursor
distributed under the terms and
quantities [18], temperature [19], source substrate distances [20,21], or even through the
conditions of the Creative Commons
introduction of an Ar plasma during growth [22]. More recently, various works have
Attribution (CC BY) license (https://
focused on the growth of irregular 2D-TMDs, including fractal and dendritic growth. The
creativecommons.org/licenses/by/
transition from regular triangular or hexagonal structures is driven by competition between
4.0/).

Solids 2024, 5, 510–519. https://doi.org/10.3390/solids5040034 https://www.mdpi.com/journal/solids

Solids 2024, 5 511

attachment rates of atoms to nucleated structures, detachment, and boundary diffusion.

In the model of diffusion-limited aggregation, fractal or dendritic growth occurs once the
attachment rate is too rapid and newly attached adatoms cannot diffuse to low-energy sites.
This may occur in growth regimes with limited diffusion rates, i.e., at low temperatures
or with a high metal-to-chalcogen ratio [23]. Other works have shown the ability to tune
growth from triangular to disordered via increasing precursor mass transfer by increasing
carrier gas flow rates [20]. Due to the higher fraction of edge sites that are catalytically
active for hydrogen evolution reaction in these disordered structures, these structures have
been proposed for use in electrocatalysis [24,25]. Of interest in our work is the role of
temperature and carrier gas flow rate on the morphology transformation of ultrathin WS2
grown via halide-assisted LP-CVD [26].
The use of NaCl as a promotor material to enhance the growth of TMDs from solid
source transition metal oxide precursors has been previously established owing to its ability
to lower the evaporation temperature of transition metal oxides and form volatile interme-
diate products, which enhances the formation rates of TMDs from precursor compounds.
Previously, we have demonstrated the ability to tune growth from triangular to dendritic
WS2 through varying the amount of a NaCl promotor during growth, which allows the
metal flux to be modulated [27]. Precursor flux may be further controlled by various
other means, including increasing the temperature and carrier gas flow rates. Specifically,
increasing the temperature may result in increased generation of gas-phase WO3 and other
volatile precursors, as well as increased chemical reaction rates driving the decomposition
of WO3 and the formation of WS2 , while increasing flow rates has been previously shown
to increase mass transfer due to the improved transport of precursors to the substrate re-
gion [20,28,29]. Here, we demonstrate the ability to generate highly disordered WS2 flakes
with sizes above 50 µm by tuning both flow rates and temperature during the low-pressure
chemical vapor deposition (LP-CVD) growth of WS2 with a fixed quantity of NaCl.

2. Materials and Methods

WS2 crystals were grown on SiO2 /Si substrates with a 300 nm thick thermal oxide
layer by LP-CVD in a single heating zone furnace. Prior to growth, the nominally 5 cm
long, 1.9 cm wide substrates were cleaned through a series of 15 min ultrasonication steps
in acetone, isopropanol, and de-ionized water, respectively. Sulfur and tungsten oxide
powders were precursor materials, with NaCl as a promotor material. A total of 500 mg
of sulfur (99.98% purity, Sigma-Aldrich, St. Louis, MO, USA), and a mixture of 50 mg
of WO3 (99.9% purity, Sigma-Aldrich) were placed in two different alumina combustion
boats of 10 mm in height and 6 cm in length. Subsequently, 2.5 mg of NaCl (99.5% purity,
Sigma-Aldrich) was mixed into the boat containing WO3 . The WO3 /NaCl mixture was
placed near the leading edge of the boat. The boats were loaded in a quartz tube with an
inner diameter of 44 mm, with the substrate was placed on top of the combustion boat
containing the mixture of WO3 and NaCl, with the leading edge of the substrate aligned
with the center of the mixture. A schematic of the LPCVD set-up used for the growth of
WS2 films is shown in Figure 1a. The boat was placed in the central heating zone while the
sulfur was placed at the edge of the heating zone to control the vapor flow and prevent the
early evaporation of sulfur.
After reaching a base pressure of nominally 400 mTorr, the tube was purged with
500 SCCM of argon gas. The tube was then heated to 300 ◦ C in 15 min and allowed to sit
at 300 ◦ C under Ar flow for 10 min. The temperature of the furnace was then increased
to the growth temperature at the rate of 10 ◦ C per minute. The growth of the sample
was carried out at temperatures of 700 ◦ C, 800 ◦ C, and 900 ◦ C for 15 min. During the
growth, the temperature of the crucible containing sulfur was fixed at nominally 180 ◦ C.
At each temperature, the flow rate of Ar was chosen to be either 25, 50, or 75 SCCM,
allowing for the role of both temperature and flow rate on the resultant morphology to be
observed. Following growth, the sample was rapidly cooled down to room temperature.
The temperature profile for growth at 900 ◦ C is shown in Figure 1c and is similar for
 Solids 2024, 5 512

Solids 2024, 5, FOR PEER REVIEW 3

all temperatures studied. The process pressure for growth at 25, 50, and 75 SCCM was
approximately 1.2, 1.5, and 1.8 Torr, respectively.

Figure 1. (a) Schematic of the CVD set-up and (b) a diagram depicting a typical zonal growth pattern,
Figure 1. (a) Schematic of the CVD set-up and (b) a diagram depicting a typical zonal growth pat-
with different
tern, with regions
different (a–d)
regions corresponding
(a–d) correspondingto bulk, large
to bulk, flakes,
large transitioning
flakes, to smaller,
transitioning isolated
to smaller, iso-
growth, and no growth regions, respectively. (c) The temperature profile used during heating,
lated growth, and no growth regions, respectively. (c) The temperature profile used during heating, growth,
and cooling.
growth, and cooling.

Following
After reaching growth,
a basethe sample’s
pressure chemical composition
of nominally 400 mTorr, was examined
the tube by X-ray
was purged photoe-
with 500
SCCM of argon gas. The tube was then heated to 300 °C in 15 min and allowed to sitXPS
mission spectroscopy (XPS) using a Thermo Scientific (Waltham, MA, USA) K-Alpha at
system
300 withAr
°C under a 0.1
floweVfor
step size. The
10 min. The temperature
presence of WS 2 was
of the also confirmed
furnace by Raman
was then increased to spec-
the
troscopy
growth in a standard
temperature at thebackscattering geometry
rate of 10 °C per minute.andThea growth
50x objective
of the at room was
sample temperature
carried
using a Thermo Scientific DXR Raman Microscope with a 532 nm laser.
out at temperatures of 700 °C, 800 °C, and 900 °C for 15 min. During the growth, the An 1800 line/mm
tem-
grating of
perature andthea crucible
50 µm slit aperturesulfur
containing were was
used.fixed
Measurements
at nominallywere carried
180 °C. outtemper-
At each near the
region’s
ature, centerrate
the flow at various
of Ar was distances
chosen tofrom the upstream
be either edge
25, 50, or of the substrate.
75 SCCM, allowing forRaman data
the role
were smoothed using a Savitzky–Golay filter. The variation in the morphology
of both temperature and flow rate on the resultant morphology to be observed. Following was studied
using a Hitachi-4700 (Tokyo, Japan) field emission scanning electron microscope (FESEM)
growth, the sample was rapidly cooled down to room temperature. The temperature pro-
at an accelerating voltage of 10 kV and an emission current of 10 µA.
file for growth at 900 °C is shown in Figure 1c and is similar for all temperatures studied.
The process pressure for growth at 25, 50, and 75 SCCM was approximately 1.2, 1.5, and
3. Results
1.8 Torr, respectively.
Figure 2 shows the typical zonal growth pattern [30,31] for samples grown under
Following growth, the sample’s chemical composition was examined by X-ray pho-
varying temperatures and flow rates. Increasing the temperature or the flow rate leads to a
toemission spectroscopy (XPS) using a Thermo Scientific (Waltham, MA, USA) K-Alpha
more extensive growth region, as expected. Along the leading edge of the substrate, which
XPS system with a 0.1 eV step size. The presence of WS2 was also confirmed by Raman
is close to the WO3 precursor, we observe mostly bulk growth, with growth transitioning
spectroscopy in a standard backscattering geometry and a 50x objective at room temper-
to thin, isolated crystals, with size and density decreasing further from the leading edge.
ature using a Thermo Scientific DXR Raman Microscope with a 532 nm laser. An 1800
The extent of the growth and width of each region grows with both increasing temperature
line/mm grating and a 50 µm slit aperture were used. Measurements were carried out near
and flow rate, which is expected due to the increased precursor quantity and reaction rates
the region’s center at various distances from the upstream edge of the substrate. Raman
resulting from both.
data were smoothed using a Savitzky–Golay filter. The variation in the morphology was
studied using a Hitachi-4700 (Tokyo, Japan) field emission scanning electron microscope
(FESEM) at an accelerating voltage of 10 kV and an emission current of 10 µA.

3. Results
Figure 2 shows the typical zonal growth pattern [30,31] for samples grown under
varying temperatures and flow rates. Increasing the temperature or the flow rate leads to
a more extensive growth region, as expected. Along the leading edge of the substrate,
which is close to the WO3 precursor, we observe mostly bulk growth, with growth transi-
tioning to thin, isolated crystals, with size and density decreasing further from the leading
edge. The extent of the growth and width of each region grows with both increasing tem-
perature and flow rate, which is expected due to the increased precursor quantity and
reaction rates resulting from both.
Solids 2024, 5, FOR PEER REVIEW 4
Solids 2024, 5 513

Figure 2. (a) Optical images showing the zonal growth pattern and relative extent of growth for
Figure 2. (a) Optical images showing the zonal growth pattern and relative extent of growth for (top)
(top) varying temperatures at a flow rate of 50 SCCM and (bottom) at varying flow rates at ◦800 °C,
varying temperatures at a flow rate of 50 SCCM and (bottom) at varying flow rates at 800 C, and
and representative SEM images in the region of (b) 0–1 cm, (c) 1–2 cm, and (d) 2–3 cm along the
representative
midpoint of theSEM images
growth in The
zone. the region of (b)
scale bar is 10–1
cmcm, (c) 1–2
for (a) andcm,
100and
µm (d)
for 2–3 cm along the midpoint
(b–d).
of the growth zone. The scale bar is 1 cm for (a) and 100 µm for (b–d).
Raman spectroscopy, shown in Figure 3, confirmed that this growth corresponds to
Raman spectroscopy, shown in Figure 3, confirmed that this growth corresponds to
WS2. Raman measurements were taken in the central region 2 cm downstream from the
WS2 . Raman measurements were taken in the central region 2 cm downstream from the
leading edge of the substrate, as shown in Figure 4. Three primary peaks were observed
leading edge of the substrate, as shown in Figure 4. Three primary peaks were observed
for all samples at nominally 323, 352, and 420 cm−1, in good agreement with that of me-
for all samples at nominally 323, 352, and 420 cm−1 , in good agreement with that of
chanically exfoliated WS2 and suggesting a lack of significant doping of halide or alkali
mechanically exfoliated WS2 and suggesting a lack of significant doping of halide or alkali
metal atoms [26]. The peak at 352 cm−−11is a convolution of the in-plane vibrational E112g
metal atoms [26]. The peak at 352 cm is a convolution of the in-plane vibrational E 2g
mode
mode and
and aa second-order
second-order mode mode of of the
the longitudinal
longitudinal acoustic
acoustic phonon
phonon 2LA(M)
2LA(M) peak.
peak. The
The
peak near 420 cm −1 corresponds to the out-of-plane vibrational A1g mode, while the peak
− 1
peak near 420 cm corresponds to the out-of-plane vibrational A1g mode, while the
around 323 cm−1 has − been previously reported to result from a 2LA(M)-E22g hybrid mode
peak around 323 cm 1 has been previously reported to result from a 2LA(M)-E2 2g hybrid
[32,33]. As the peak separation between E 2g and A1g1 increases with increasing layer num-
1
mode [32,33]. As the peak separation between E 2g and A1g increases with increasing
ber, the difference between these peaks from sample to sample may be used to identify
layer number, the difference between these peaks from sample to sample may be used
increased growth thickness [34]. At a fixed temperature of 800 °C, the peak◦ separation
to identify increased growth thickness [34]. At a fixed temperature of 800 C, the peak
changes from 67.2 cm−1 at 25 SCCM to 68.2 cm−1 at 50 SCCM and 68.9 cm−1 at 75 SCCM,
separation changes from 67.2 cm−1 at 25 SCCM to 68.2 cm−1 at 50 SCCM and 68.9 cm−1 at
indicating that increased flow rates resulted in thicker growth at equal distances down-
75 SCCM, indicating that increased flow rates resulted in thicker growth at equal distances
stream, which is consistent with the greater growth observed in optical images shown in
downstream, which is consistent with the greater growth observed in optical images shown
Figure 2a.2a.
in Figure Further,
Further,increasing
increasing flow
flowrates
ratesresults
resultsininananincreasing
increasing A
A1g /E112g intensity ratio,
1g /E 2g intensity ratio,
which
which is also associated
is also associatedwith withincreasing
increasinggrowth
growthatat a given
a given downstream
downstream distance.
distance. Simi-
Similarly,
larly, peak separations of 66.3, 68.3, and 69.6
− 1 cm −1 were found for growth at a constant 50
peak separations of 66.3, 68.3, and 69.6 cm were found for growth at a constant 50 SCCM
SCCM flow
flow rate andrate and temperatures
temperatures of 700,
of 700, 800, and800,900 and 900 °C, respectively,
◦ C, respectively, consistent consistent with
with expected
expected
increasing increasing
thicknesses thicknesses
at a fixedatposition
a fixed for position for increasing
increasing temperatures.
temperatures. For all
For all samples,
samples, Raman spectra indicate growths of between
Raman spectra indicate growths of between 1 and 4 layers [11]. 1 and 4 layers [11].
Solids 2024, 5, FOR PEER REVIEW 5
Solids 2024, 5 514

Figure 3. Raman
Figure 3. Raman spectra for for
spectra samples
samplesgrown
grownat (a)
at (a) °C ◦and
800800 varying
C and flow
varying rates,
flow andand
rates, (b)(b)
varying
varying
temperatures and a fixed 50 SCCM flow rate. Measurements were taken 2 cm from
temperatures and a fixed 50 SCCM flow rate. Measurements were taken 2 cm from the leading the leading edge edge
of the substrate. Note that measurements taken at 50 SCCM were taken from different
of the substrate. Note that measurements taken at 50 SCCM were taken from different samples. samples.
However,
However,thisthis
does notnot
does result in changes
result to the
in changes Raman
to the Raman spectra. Raman
spectra. Ramanmodes
modesareare
marked
marked in in
(a).(a).

SEM SEMimages
images at 25at SCCM
25 SCCM are are
shownshown in Figure 4a, with
in Figure representative
4a, with representativeimages takentaken
images at
given substrate locations for varying flow rates to understand
at given substrate locations for varying flow rates to understand changes in morphology changes in morphology
driven
drivenby by
flow rate
flow andand
rate temperature.
temperature. TheThe total growth
total growth region
regionincreases
increasesas the temperature
as the temperature
increases,
increases,with more
with more growth
growth downstream.
downstream. At At
700700°C,◦thin triangular
C, thin triangularcrystals
crystals areare
observed
observed
both
bothat the substrate
at the substrate leading
leading edgeedgeandand 1 cm downstream.
1 cm downstream. Increasing
Increasing temperatures
temperatures leadleadto to
growth
growth upupto to
4 cm
4 cmand andalsoalsodrastic
drasticmorphology
morphology changes,
changes, which
whichareare
especially
especially apparent
apparent
at at
900900°C,◦ C,
wherein
wherein thethetriangular
triangular structure
structure becomes
becomes disordered,
disordered, with
withsimilar
similar structures
structures
referred
referredtoto as asdendritic,
dendritic, fractal,
fractal, or semi-compact
or semi-compact growth in growth
previous inworks
previous works
[24,25,27,35,36].
[24,25,27,35,36].
However, increasing However, increasing the
the downstream downstream
distance decreases distance decreases
the overall structurethe size
overall
while
structure size while
the ordered the ordered
triangular crystaltriangular
structure is crystal structure
recovered. Thisisrecovery
recovered. of This recovery
triangular of
growth
occurs at
triangular greater
growth distances
occurs for increased
at greater distances for temperatures or flow rates.or At
increased temperatures flow50rates.
SCCM, At a
50 similar
SCCM, transition is apparent,
a similar transition as shownasinshown
is apparent, Figurein4b. However,
Figure the disordered
4b. However, transition
the disordered
occurs at the lower temperature of 800 ◦ C, with a significantly larger disorder apparent at
transition occurs at the lower temperature of 800 °C, with a significantly larger disorder
900 ◦ C at
apparent and900disordered growth extending
°C and disordered further downstream.
growth extending At 75 SCCM,
further downstream. At 75 as SCCM,
shown in
◦
as Figure
shown4c, in disordered growth appears
Figure 4c, disordered growth even at 700 even
appears C, andat the
700disordered
°C, and the region at higher
disordered
temperatures
region at higher extends to evenextends
temperatures more distant
to even regions
more of the substrate.
distant regions of It the
is apparent
substrate. thatIt both
is
increased
apparent thatflow
bothrate and temperature
increased flow rate and significantly
temperature enhance this transition
significantly enhance to disordered
this transi-
growth.
tion However,
to disordered all structures
growth. However, retain a semi-compact
all structures retain triangular form. triangular
a semi-compact Even at theform. lowest
temperatures, increasing flow rates result in disordered edges.
Even at the lowest temperatures, increasing flow rates result in disordered edges. For all For all samples, increasing
the distance
samples, fromthe
increasing thedistance
source results
from the in source
lower density
results ingrowth and the growth
lower density decreased andsize the of
individual elements, as expected owing to the gradient
decreased size of individual elements, as expected owing to the gradient in metal precur-in metal precursor availability
sorfurther from further
availability the substrate
from the leading edge.leading edge.
substrate
XPS measurements, shown in Figure 5, were taken 1 cm downstream from samples
grown at 800 ◦ C and 25 and 75 SCCM flow rates. Survey spectra indicate only small
amounts of NaCl are present following growth, as indicated by the barely noticeable Na
1s peak. W 4f and S 2p signals are shown in Figure 5b,c, with fitting to Voigt profiles for
each peak performed using the method described in [37]. The W 4f signal is comprised
of W 4f5/2 and W 4f7/2 doublets belonging to W6+ and W4+ oxidation states, as well as
a contribution from the W 5p3/2 state at a slightly higher binding energy. Fits assumed
an area ratio between corresponding doublets of 4:3 per the multiplicity of corresponding
levels, as well as equal line widths. The W4+ peaks are centered at binding energies of 35.1
and 33.0 eV for W 4f5/2 and W 4f7/2 , respectively, which are characteristic of WS2 , with no
Solids 2024, 5 515

noticeable shift for both samples. Additional peaks corresponding to W6+ , characteristic of
WO3 , are found at 37.8 and 35.9 eV. Binding energies for the S 2p doublet corresponding
to S 2p1/2 and S 2p3/2 states are found at 163.8 and 162.5 eV for both samples. As no
significant differences in intensity ratios or peak locations for W4+ and W6+ species are
observed between samples, the transformation is not associated with changes to the local
Solids 2024, 5, FOR PEER REVIEW 6
chemical environment and instead is related to the kinetics of crystal growth of similar
species at the surface.

Figure
Figure 4. SEMimages
4. SEM imagesatatvarious
variousdistances
distances along
along thethe leading
leading edge
edge of the
of the substrate
substrate for varying
for varying tem-
temperatures
peratures at aatconstant
a constantflowflow
raterate of (a)
of (a) 25 25 SCCM,
SCCM, (b)(b)
50 50 SCCM,
SCCM, and
and (c)(c)
75 75 SCCM.
SCCM. TheThe scale
scale barbar is
is 10
10
µmµmforfor
allall imagesinin(a)
images (a)and
and100
100µm µminin(b)
(b)and
and(c).
(c).The
Themagnified
magnified image
image on the inset (b)
(b) has
has aa scale
scale
barof
bar of10 10µm
µmand andindicates
indicatessmall-scale
small-scaletriangular
triangulargrowth.
growth.

XPS measurements, shown in Figure 5, were taken 1 cm downstream from samples

grown at 800 °C and 25 and 75 SCCM flow rates. Survey spectra indicate only small
amounts of NaCl are present following growth, as indicated by the barely noticeable Na
1s peak. W 4f and S 2p signals are shown in Figure 5b,c, with fitting to Voigt profiles for
each peak performed using the method described in [37]. The W 4f signal is comprised of
 to S 2p1/2 and S 2p3/2 states are found at 163.8 and 162.5 eV for both samples. As no signif-
icant differences in intensity ratios or peak locations for W4+ and W6+ species are observed
between samples, the transformation is not associated with changes to the local chemical
environment and instead is related to the kinetics of crystal growth of similar species at
Solids 2024, 5 516
the surface.

XPS spectra correspond with samples grown at and

800 the◦ C and the indicated flow rates.
FigureFigure
5. XPS5.spectra correspond with samples grown at 800 °C indicated flow rates. (a)
Survey (a)spectra
Surveyshowing the Na 1s,the
spectra showing O KLL,
Na 1s,WO4s, O 1s,
KLL, WW 4s,4P , WW4p
O1/21s, 4P , C, 1s,
3/21/2 W 4pW3/2
4d,, C
S 2s,
1s, SW2p,
4d,SiS2p,
2s, S 2p,
and WSi4f states
2p, and in
W the orderin
4f states ofthe
decreasing
order of binding energy.
decreasing (b,c)energy.
binding show high-resolution scans corre- scans
(b,c) show high-resolution
sponding to the W 4f to
corresponding and S2p
the Wstates.
4f andThe
S2pdifferent rowsdifferent
states. The in (b) and (c) represent
rows in (b) andflow rates indicated
(c) represent flow rates
in (b).indicated in (b).

4. Discussion
The transition from regular triangular to dendritic growth is explained by the competi-
tion between attachment rates to the nucleated structures, detachment, and edge diffusion
rates. Adatoms can diffuse along the edge for slow growth to form the most stable con-
figuration, resulting in compact domains. However, if the attachment rate is too high or
the diffusion rate too low, adatoms can no longer sufficiently diffuse to the lowest energy
sites [25,35,38,39]. Kinetic Monte Carlo (KMC) studies have shown that the degree of edge
disorder and transition to dendritic growth at a given temperature is then related to the total
flux and flux ratio of chalcogen and transition metals (C/M ratio) [36]. For example, at a low
metal flux, growth was shown to produce compact triangles with no sensitivity to the C/M
ratio. However, increasing the metal flux while maintaining a low C/M ratio leads to the
growth of semi-compact, disordered triangular structures similar to those observed in our
works. It is clear that increasing temperature at the growth site and the WO3 /NaCl mixture
will result in an increased metal flux while maintaining the same flux of S to the substrate
and, therefore, can drive the transition from triangular to semi-compact structures seen in
this work, as this will result in maintaining a low C/M ratio while enhancing the metal
 ple, at a low metal flux, growth was shown to produce compact triangles with no sensi-
tivity to the C/M ratio. However, increasing the metal flux while maintaining a low C/M
ratio leads to the growth of semi-compact, disordered triangular structures similar to
those observed in our works. It is clear that increasing temperature at the growth site and
Solids 2024, 5
the WO3/NaCl mixture will result in an increased metal flux while maintaining the same
517
flux of S to the substrate and, therefore, can drive the transition from triangular to semi-
compact structures seen in this work, as this will result in maintaining a low C/M ratio
while enhancing the metal flux. Similarly, the recovery of triangular growth further down-
flux. Similarly, the recovery of triangular growth further downstream may be explained by
stream may be explained by decreasing tungsten precursor concentrations with distance
decreasing tungsten precursor concentrations with distance from the source material. The
from the source material. The role of flow rate is somewhat more complex. Increasing flow
role of flow rate is somewhat more complex. Increasing flow rates can enhance the mass
rates can enhance the mass transfer process, transforming growth from thermodynamic
transfer process, transforming growth from thermodynamic to kinetic [20]. While both
to kinetic [20]. While both chalcogen and metal flux increase in this scenario, the ratio may
chalcogen and metal flux increase in this scenario, the ratio may not significantly change.
not significantly change. As a result, flux may play a similar role to temperature in deter-
As a result, flux may play a similar role to temperature in determining the morphology
through the
mining morphology
primarily modulatingthrough primarily
the metal modulating
flux while having the metal
a lesser fluxonwhile
effect having
the C/M a
ratio.
lesseriseffect
This furtheronsupported
the C/M ratio. This
by the is further supported
observation by the
of a transition from observation
straight toofcurved-edge
a transition
from straight to curved-edge triangular growth, as shown in Figure
triangular growth, as shown in Figure 6, wherein this describes how well the edge conforms 6, wherein this de-
to a straight line connecting triangle vertices. For all semi-compact domains, a curveFor
scribes how well the edge conforms to a straight line connecting triangle vertices. awayall
semi-compact domains, a curve away from a straight line is clearly visible,
from a straight line is clearly visible, which reverses midway through the given edge. This which reverses
midway through
structure has beenthe given previously
reported edge. This to structure has an
result from been reportedofpreviously
imbalance to result
the C/M ratio and
frombean
can imbalance
driven by eitherof the
too C/M
high ratio and of
measures canchalcogen
be drivenorby either
metal too [20].
fluxes highThe
measures
recoveryof
chalcogen or metal fluxes [20]. The recovery of the straight edge with
of the straight edge with increasing distance downstream indicates that this is the result increasing distance
downstream
of a C/M ratio indicates thatlow,
that is too this is the result
wherein theofratio
a C/M ratio that
increases is too downstream.
further low, wherein the ratio
Similar
increases further downstream. Similar deviations from straight-line
deviations from straight-line edges were also demonstrated for semi-compact growth edges were also
in
demonstrated
KMC models [36]. for semi-compact growth in KMC models [36].

Figure 6.
Figure 6. SEM
SEMimages
imagesshow
showthetheemergence
emergenceofof deviations
deviations from
from straight-edge
straight-edge triangular
triangular growth
growth as
as the
the flow rate is increased from (a) 25 SCCM to (b) 75 SCCM. Otherwise, images are taken
flow rate is increased from (a) 25 SCCM to (b) 75 SCCM. Otherwise, images are taken under identical under
identicalconditions
growth growth conditions
at 700 ◦ C at
and700 °C and
1 cm 1 cm downstream.
downstream.

5. Conclusions
We have
have grown
grown WS WS22 crystals with low-pressure CVD on thermally oxidized oxidized Si sub-
strates. We observe a transition from small, smooth triangles to large, semi-compact
strates. We observe a transition from small, smooth triangles to large, semi-compact growth
with zagged edges by systematically varying the temperature and carrier gas
growth with zagged edges by systematically varying the temperature and carrier gas flow flow rate.
The
rate.physical and chemical
The physical properties
and chemical of synthesized
properties samplessamples
of synthesized have been analyzed
have with the
been analyzed
help
with of
theoptical
help ofmicroscopy, Raman spectroscopy,
optical microscopy, X-ray photoelectron
Raman spectroscopy, spectroscopy
X-ray photoelectron (XPS),
spectros-
and
copySEM. In and
(XPS), line SEM.
with previous reports,
In line with increasing
previous metal
reports, flux andmetal
increasing decreasing thedecreasing
flux and C/M ratio
transforms
the C/M ratioa sample from atraditional
transforms compact
sample from triangular
traditional growth
compact to moregrowth
triangular complex,to semi-
more
compact growth with significant disorder while maintaining a roughly triangular structure.
Temperature enhances metal flux via the increased evaporation and decomposition of
the WO3 precursor itself. Meanwhile, flow rate modifications play a similar, albeit more
complicated, role by enhancing both metal and sulfur flux. However, our results indicate
that metal flux enhancements play a dominant role, resulting in changes in morphology
similar to those caused by enhancing temperature. The ability to control the morphology
of the 2D-TMDC by controlling the process parameters has important applications in opto-
electronics and electrochemistry, allowing for the enhancement or suppression of disorder
while also allowing for the tunability of the number of active edge sites.

Author Contributions: Conceptualization, H.P. and S.P.; methodology, H.P. and S.P.; formal analysis,
H.P., S.P. and S.M.; investigation, H.P.; resources, S.P and S.M.; data curation, H.P.; writing—original
draft preparation, S.P. and H.P.; writing—review and editing, H.P., S.P. and S.M.; visualization,
H.P. and S.P.; supervision, S.P.; project administration, S.P. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Solids 2024, 5 518

Data Availability Statement: Data supporting the conclusions of this article will be made available
by the authors on request.
Acknowledgments: Support for XPS measurements from Felio Perez at the University of Memphis
Integrated Microscopy Center for XPS measurement is greatly appreciated.
Conflicts of Interest: The authors declare no conflicts of interest.

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