Growth and Characterizations of Rhenium Disulfide (ReS2) Single Crystals

Article abstract of DOI:10.1002/pssa.202000687

Rhenium disulfide (ReS₂) single crystals are grown by chemical vapor transport technique. Powder X-ray diffraction analysis of the single crystals shows them to possess ReS₂ phase with triclinic unit cell structure. Energy dispersive analysis of X-rays shows the crystals to be slightly rich in sulfur and deficient in rhenium. The optical bandgap obtained of the as-grown single crystals is 1.35 eV. The surface morphology study done by scanning electron microscopy shows that the crystal surface to be flat with layer edges, such observation states that the growth mechanism of crystal have happened by mechanism of sheet spreading. The transmission and diffraction mode electron microscopy shows the single crystals to be layered and crystalline, respectively. The Raman peaks are well assigned to the ReS₂. Thermogravimetric and differential thermogravimetric analysis shows the single crystals to disintegrate by two steps. The differential thermal analysis shows that the ReS₂ possesses initial endothermic followed by exothermic nature for fast heating rates. In case of a slow heating rate of 10 K min^?1, other than endothermic followed by exothermic, the end temperature range shows endothermic nature. The kinetic parameters determined by Kissinger relation shows the single-crystal samples to disintegrate at a higher temperature range.

Full text of DOI:10.1002/pssa.202000687

ORIGINAL PAPER
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Growth and Characterizations of Rhenium Disulfide (ReS2)
Single Crystals
Atriy Ghetiya,* Sunil H. Chaki,* Ankurkumar J. Khimani, Anilkumar B. Hirpara,
Rohitkumar M. Kannaujiya, Shivam Patel, and Milind P. Deshpande
between the atoms is of strong covalent type
Rhenium disulfide (ReS2) single crystals are grown by chemical vapor transport
technique. Powder X-ray diffraction analysis of the single crystals shows them to
possess ReS2 phase with triclinic unit cell structure. Energy dispersive analysis of
X-rays shows the crystals to be slightly rich in sulfur and deficient in rhenium. The
optical bandgap obtained of the as-grown single crystals is 1.35 eV. The surface
morphology study done by scanning electron microscopy shows that the crystal
surface to be flat with layer edges, such observation states that the growth
mechanism of crystal have happened by mechanism of sheet spreading. The
transmission and diffraction mode electron microscopy shows the single crystals
to be layered and crystalline, respectively. The Raman peaks are well assigned to
the ReS2. Thermogravimetric and differential thermogravimetric analysis shows
the single crystals to disintegrate by two steps. The differential thermal analysis
shows that the ReS2 possesses initial endothermic followed by exothermic nature
for fast heating rates. In case of a slow heating rate of 10 K minꢀ1, other than
endothermic followed by exothermic, the end temperature range shows endo-
thermic nature. The kinetic parameters determined by Kissinger relation shows
the single-crystal samples to disintegrate at a higher temperature range.
and between the interlayers is of very weak
van der Waals type.[8] Thus, the TMDCs can
be exfoliated from multilayer 3D to mono-
layer 2D, thus providing immense opportu-
nity to be of worth in present day device
miniaturization.[9] The 3D bulk TMDCs
exhibit indirect optical bandgap which trans-
forms to direct optical bandgap when
trimmed down to 2D monolayer.[10] The var-
ied TMDC optical bandgap values fall within
the range of solar spectrum of 1–2.5 eV as
well as their electrical transport properties
are in complement with high-quality semi-
conductor properties.[3] These favorable
basic optical and electrical properties make
TMDCs a potential material for optoelec-
tronic devices. The 2D monolayer TMDCs
have isotropic transport properties, and
due to presence of weak van der Waals
bonding between stacked layers, the 3D
TMDCs possess anisotropic properties.
The transition from indirect to direct optical
bandgap happens when the dimension of a sample decreases from
3-dimensions to 2-dimensions in the case of TMDCs materials.
The potential basic properties of TMDCs and their swift variation
during trimming from multi to monodimension attracted
researchers to perform studies on them in this age of compactness
and efficiency. The most studied TMDCs are the elemental com-
bination formed of group elements from IVB (Ti, Zr), VB (V, Nb,
Ta), VIB (Cr, Mo, W) metal atoms and group VIA (S, Se, Te) chal-
cogen atoms. Recently, other than the conventional TMDCs, a
TMDC formed of group VIIB element Re with chalcogens has
received much interest. This is due to its anisotropic properties
even in monolayer, the converse of other TMDCs,[11] whereas
the optical bandgap variation from indirect to direct with decrease
in layers observed in other TMDCs is not observed in Re chalco-
genides. The Re chalcogenide shows a direct optical bandgap in all
dimensions. These complementary properties of Re chalcogenides
compared to other TMDCs have attracted many researchers to
study the compounds in different forms ranging from nano to
thin films and crystals.[12–14] The authors got interested due to
these properties of the Re chalcogenides and did comprehensive
study on ReS2 in its single-crystal form.
1. Introduction
The transition metal dichalcogenides in short expressed as
TMDCs are compound semiconductors with advanced properties,
and have thus found to surmount the erstwhile silicon technol-
ogy.[1–5] The TMDCs possess lamellar layered structure with each
layer formed of a combination of transition metals and chalcogens
with formula MX2 (M: transition metals; X: chalcogens) arranged
to form a thin layer. The layers get stacked one on another to form
the bulk. A single layer of TMDCs is formed of metal ions sand-
wiched between two chalcogens.[6,7] The intralayer bonding
A. Ghetiya, Dr. S. H. Chaki, Dr. A. J. Khimani, A. B. Hirpara,
R. M. Kannaujiya, Dr. S. Patel, Dr. M. P. Deshpande
P. G. Department of Physics
Sardar Patel University
Vallabh Vidhyanagar, Gujarat 388120, India
E-mail: atriyghetiya@gmail.com; sunilchaki@yahoo.co.in
Dr. S. H. Chaki
Department of Applied and Interdisciplinary Sciences
Sardar Patel University
Vallabh Vidyanagar, Gujarat 388120, India
ReS2 does not possess conventional TMDC 2H structure but
possesses a twisted 1T phase.[15] The lesser symmetry causes
anisotropy in electronic[16] and optical properties[17,18] in the
monolayer dimension. The experimental study of ReS2 optical
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssa.202000687.
DOI: 10.1002/pssa.202000687
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absorbance and theoretical study by density functional theory
(DFT) showed it to possess a direct optical bandgap in the range
1.3–1.6 eV for both the bulk and monolayer.[19–22] Thus, the opti-
cal bandgap of ReS2 is isotropic in nature. In contrast, the trans-
port properties are anisotropic in nature; they vary with the
dimensions of the layers.[16] The isotropic optical bandgap of
ReS2 matches with the solar range. Thus the isotropic bandgap
and the anisotropic transport properties make ReS2 a potential
applicant in photocatalysis, solar cells, photodetectors, superca-
pacitor, etc.[20] The other applications of ReS2 are in nitrogen
reduction reaction and hydrogen evolution reaction.[14,23]
These applications arise due to the weak interlayer coupling
in ReS2, thus proving it to be a lead material for future green
chemistry at ambient condition.[14,23] The varied electrical trans-
port nature of ReS2 makes it interesting for a variety of energy
conversion and storage base applications.[24,25] In this study the
authors grew single crystals of ReS2 by chemical vapor transport
(CVT) technique in a closed tube schematic. The grown single
crystals were completely characterized for their varied basic prop-
erties before performing thorough thermal study.
Figure 1. Photograph of CVT as-grown ReS2 single crystals.
condition, it was removed from the furnace muffle and carefully
broken at the growth zone region to retrieve the grown single
crystals. A photograph of average large size CVT as-grown
ReS2 single crystals is shown in Figure 1. The ReS2 single crystals
were shinning brownish black in color with average large size
dimensions of 8 mm ꢁ 5.5 mm ꢁ 2 mm.
2. Experimental Section
The ReS2 single crystals were grown by CVT using iodine as the
transporting agent. Rhenium metal powder (SRL, India, purity
99.9999%) and sulfur powder (Alfa Aesar, USA, purity
99.99%) were loaded in stoichiometric proportion in an HCl,
HF washed and double distilled water rinsed fully dried quartz
ampoule. Along with precursor elements, iodine was also put
into the ampoule as a transporting agent. The total weight of
the stoichiometric elemental composition loaded into the quartz
ampoule was not more than 8 gm. The transporting agent iodine
amount was taken at the rate of 4 mg ccꢀ1 of ampoule dimen-
sion. The quartz ampoule dimension was 2 cm (I.D.) ꢁ 20 cm
(length). The precursor element–loaded quartz ampoule was
evacuated to ꢂ10ꢀ5 torr and vacuum sealed. The individual ele-
ments were well mixed by vigorous physical shaking of the
ampoule. The complete elemental mix is kept at one end of
the ampoule known as the charge zone and the empty end of
the ampoule was the growth zone. The vacuum-sealed quartz
ampoule having the elemental mix at the charge zone end
and the other growth zone end empty was placed in a horizontal
dual-zone cylindrical furnace. The temperature of the furnace
was slowly raised at a rate of 16 K minꢀ1 such that the elemental
mix–loaded quartz ampoule’s charge zone was at 1093 K and
growth zone was at 1043 K. The ampoule was maintained at
these intermediate temperatures for 50 h. This process of main-
taining the elemental mixture–filled ampoule for a finite time
period at intermediate temperature was to allow the precursor
elements to react with each other to form the ReS2 compound.
Next the ampoule temperature was further raised to single-
crystal growth temperatures such that the charge zone was at
1393 K and the growth zone was at 1343 K. The ampoule was
kept in this growth temperature range for 120 h. On completion
of the growth run, the furnace temperature was slowly cooled
3. Characterization
The CVT as-grown ReS2 single crystals were fully characterized for
crystal structure, stoichiometry, morphology, etc. before
performing in-depth thermal analysis. A Philips X’PERT MPD
powder X-ray diffractometer was used to analyze the phase and
unit cell structure of the sample. The stoichiometry of the sin-
gle-crystal sample was determined by dispersive energy analysis
of X-rays (EDAX) technique attached to a NOVA-450 electron
microscope. The optical absorption study was conducted with
the help of a double beam double monochromator ratio recording,
Perkin Elmer Lambda-19 UV–vis–NIR spectrophotometer. The
electron microscope used for EDAX analysis in scanning (SEM)
mode was used to study the surface morphology of the as-grown
single crystal sample. A JEOL JEM-2100 HR electron microscope
was used in high-resolution transmission (HRTEM) and diffrac-
tion (SAED) modes to characterize the as-grown single-crystal
sample. The Raman spectroscopy on the as-grown ReS2 single
crystals was done by a micro-Raman model STR 500. The thermo-
curves of the single-crystal samples were recorded using a Seiko
SII-EXSTAR TG/ DTA-7200 thermal analyzer.
4. Results and Discussion
4.1. Structure
The X-ray diffraction (XRD) of the as-grown ReS2 single crystal is
shown in Figure 2. The analysis of the XRD pattern was done by
PowderX software. All the peaks are indexed as that of ReS2
possessing triclinic unit cell structure having lattice parameters
a ¼ 0.641 nm, b ¼ 0.634 nm, and c ¼ 0.645 nm and angles
α ¼ 91.60ꢃ, β ¼ 105.04ꢃ, and γ ¼ 118.97ꢃ. The obtained lattice
parameters and angles matched with standard reported data[26,27]
and JCPDS File No. 052-0818. The XRD shows the (001) plane to
be the most prominent compared to other observed planes. This
down to ambient condition at slow cooling rate of 30 K minꢀ1
.
Slow cooling was preferred to avoid thermal cracks in the grown
single crystals. After the ampoule was cooled to ambient
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4.3. Optical Properties
The recorded optical absorption spectrum in the wavelength
range of 300–1500 nm for the as-grown ReS2 single crystals is
shown in Figure 4a. The spectrum showed that the as-grown
ReS2 single crystals have absorbance in the wavelength range
of ꢂ560–720 nm. The optical bandgap is determined using
n
the relation ðαhνÞ ¼ Aðhν ꢀ EgÞ, where n represents the transi-
tions, A is the optical transition–dependent constant, Eg is the
optical energy bandgap, ν is the frequency of the incident beam,
h is Planck’s constant, and α is the absorption coefficient.
The value of the optical energy bandgap Eg obtained from the
graph of (αhν)2 versus hν, Figure 4b, taking the intercept on
the hν axis came out to be 1.35 eV. This direct optical bandgap
value lies within the reported value of 1.3–1.5 eV.[19-22] The obser-
vation shows that the obtained direct optical bandgap value is in
good agreement with the reported value.
Figure 2. The XRD of as-grown ReS2 single crystals.
4.4. Field Emission Scanning Electron Microscopy (FESEM)
confirms that the growth of the ReS2 single crystal have happen
along the direction of (001) plane. The unit cell volume deter-
mined from the observed lattice parameters is 54.75 A3. The
determined unit cell volume matches the reported unit cell vol-
ume of 54.75 A3.[28]
The common features observed on the as-grown CVT ReS2
single-crystal surfaces under FESEM are shown in Figure 5a,b.
The photographs of fresh single crystals were taken just after
retrieval from the ampoule after the crystal growth run. The analy-
sis of the photographs (Figure 5a) clearly shows the crystal surface
to be flat. A few flakes of the crystals are observed on the flat surface
(Figure 5b). Other than this, iodine patches are also observed on
the surface. The iodine patches are formed from the cooling of the
transporting agent on the smooth crystal surface during crystal
growth. The flat surfaces of the crystal faces confirm the crystal
growth to have happened by the sheet-spreading mechanism.
During crystal growth the charge powder evaporates from the
high-temperature end of the charge zone to settle to the lower tem-
perature growth zone end to form crystals at the nucleation sites.
The transportation is done by the transporting agent iodine. The
transported charge powder grows to a crystal by the sheet spread
mechanism, thus forming a flat surface. No other dislocations or
defects are observed on the surfaces, showing the present growth
condition to be optimum for the growth of ReS2 single crystals by
the sheet spread mechanism.
4.2. Elemental Composition
The EDAX spectrum of the CVT as-grown ReS2 single crystal is
shown in Figure 3.
The spectrum shows the presence of only two peaks corre-
sponding to Re and S, showing no impurity in the as-grown
single crystals. The observed EDAX data along with standard data
are tabulated in Table 1.
Comparison of both the observed and standard data shows
the as-grown single crystals to be of ReS2 stoichiometry. The
as-grown single crystals are slightly Re deficient and S rich.
4.5. High-Resolution Transmission Electron Microscopy
(HRTEM)
The recorded HRTEM image and the SAED pattern of the CVT
as-grown ReS2 single crystals is shown in Figure 6a,b, respec-
tively. The samples for the electron transmission and diffraction
analysis were prepared by exfoliating the ReS2 single crystals.
The as-grown single crystals were dispersed in N-methyl-2-
pyrrolidone (NMP) and ultrasonicated for 3 h. After 3 h ultraso-
nication a drop was taken on a carbon film present on a copper
grid sample holder for HRTEM analysis. The excess NMP was
absorbed by filter paper and allowed to dry in the open. The
HRTEM image of the taken ReS2 exfoliated sample, shown in
Figure 6a, clearly shows overlap of multiple layers of the ReS2
sample. The overlap of multiple layers shows the sheet spread
mechanism to be the means of crystal growth. This result of
HRTEM substantiates the FESEM observation. The SAED
Figure 3. The EDAX spectrum of ReS2 single crystals.
Table 1. The EDAX data of CVT as-grown ReS2 single crystals.
Element
Weight%
Atomic%
Observed
Standard
Observed
Standard
Re
S
74.27
25.73
74.38
25.62
65.84
34.16
66.66
33.33
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Figure 4. The a) absorbance spectrum and b) plot of (αhυ)2 versus hυ of ReS2 single crystals.
Figure 5. FESEM images of CVT as-grown ReS2 single crystals at two different magnifications.
Figure 6. a) HRTEM image and b) SAED pattern of CVT as-grown ReS2 single crystals.
image, shown in Figure 6b, shows the presence of a well-
organized spot pattern confirming the sample to be single crystal.
Raman data are also given in the table to compare with the
observed data.[29]
The observed initial Raman peaks lying in the range 145.1
and 285.6 cmꢀ1 are the primary ones which match with the
standard reported peaks of exfoliated ReS2.[30,31] Minor dispar-
ities in the peak positions are observed; these arise due to the
different crystallite sizes and plane orientations of the present
crystals. Also, the variation arises due to different growth con-
ditions in this case. The peaks lying in the higher wavenumber
region 312.8–420.8 cmꢀ1 are comparatively weak. They match
4.6. Raman Spectroscopy
The recorded Raman spectrum on CVT as-grown ReS2 single
crystal is shown in Figure 7.
All the observed Raman peaks along with frequency
positions are tabulated in Table 2. The reported standard
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4.7. Thermal Analysis
The recorded thermogravimetric (TG), differential thermogravi-
metric (DTG), and different thermal analysis (DTA) curves of
CVT as-grown ReS2 single crystals are shown in Figure 8a–c.
The thermocurves are recorded in the temperature range of
ambient and 1233 K in inert nitrogen (N2) atmosphere.
The curves are recorded simultaneously for three heating
rates of 10, 15, and 20 K minꢀ1. The starting weights of
single-crystal samples taken for all thermal analyses are between
11 and 13 mg.
The analysis of TG curves (Figure 8a) shows the single-crystal
samples to have minor weight loss in the initial temperature
range followed by continuous weight loss till the measured tem-
perature of 1233 K. The nature of the curves is near-similar for all
three heating rates. The corresponding DTG curves display two
peaks, showing that the single-crystal samples decompose in two
steps. The temperature ranges for both the regions having minor
and major weight loss in percentage data are tabulated in Table 3.
The temperature range of the TGs is determined from the DTG
peaks’ positions. The end point of the derivative of the weight
loss is considered to be the finish point of the first temperature
range, 509 K.
Figure 7. The Raman spectrum of CVT as-grown ReS2 single crystals.
Table 2. The Raman data of as-grown ReS2 single crystals along with
standard data.
Observed Standard Symmetry
Origin of phonon mode
Raman
Raman
mode
The thermal data in Table 3 show that the ReS2 single crystals
had negligible weight loss in the temperature range of step I,
ambient to ꢂ509 K for all three heating rates. The weight losses
for the three heating rates are below 1%. The minor initial weight
losses are due to loss of moisture from the hydrous samples as
well as removal of lightly attached molecules from the samples.
The EDAX showed the single crystals to be slightly sulfur rich,
which gets removed in the initial temperature rise. The cause is
that the vaporization of the sulfur element is far below that of the
rhenium element. This observation shows the CVT as-grown
ReS2 single crystals to be stable in the temperature range of
ambient to 509 K.
In the temperature range of step II, from 510 to 1233 K the
samples showed major weight loss, showing the disintegration
of the single-crystal samples. Detailed observation of the curves,
as shown in Figure 8a, shows that as the heating rate increases
the weight loss increases. The percentage weight loss in case of a
heating rate of 10 K minꢀ1 is ꢂ18.56%, whereas it increases to
35.02% in case of 15 K minꢀ1 heating rate and for the heating
rate of 20 K minꢀ1 the weight loss is nearly 70.31%. The obser-
vation affirms that when the heating rate increases, the sample
decomposition temperature shifts toward lower values, meaning
frequency frequency
[cmꢀ1
]
[cmꢀ1
]
145.1
154.9
217.4
238.8
285.6
145.9
153.1
217.2
237.1
284.2
Ag
Eg
Eg
Eg
Cp
Out-of-plane vibrations of Re atoms
In-plane vibrations of Re atoms
In-plane vibrations of Re atoms
In-plane vibrations of Re atoms
In-plane and out-of-plane vibration
of Re and S atoms
312.8
311.0
Eg
In-plane and out-of-plane vibration
of Re and S atoms
322.2
349.8
408.2
420.8
324.9
348.8
407.3
418.7
Cp
Cp
Cp
Ag
In-plane and out-of-plane vibration of S atoms
In-plane and out-of-plane vibration of S atoms
In-plane and out-of-plane vibration of S atoms
Out-of-plane vibrations of S atoms
well with the reported peaks[31,32] and are the second order
Raman modes. The symmetry and phonon modes assigned
to the observed Raman peak and reported data are also tabulated
in Table 2.
Figure 8. The a) TG, b) DTG, and c) DTA curves of CVT as-grown ReS2 single crystals.
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Table 3. The thermal data of CVT as-grown ReS2 single crystals.
4.8. Kissinger Relation
Heating
rates
Weight loss [%]
Total weight
loss [%]
DTG peak
position Tm [K]
The important thermodynamic parameters of the CVT as-grown
ReS2 single crystals are determined using the Kissinger relation.
In these calculations the thermal parameters obtained from the
DTG curves are used. The mathematical expression of the
Kissinger relation is as shown.[29]
Temperature range
[K minꢀ1
]
Step I
Step II
303–1233 K
1st peak
2nd peak
303–509 K
510–1233 K
10
15
20
0.85
0.94
0.84
17.71
34.08
69.47
18.56
35.02
70.31
492.01
493.89
497.32
797.54
800.81
803.04
ꢀ
ꢁ
β
Tm2
AR
Ea
dff ðαÞg
dα
Ea
RTm
ln
¼ ln
þ ln
ꢀ
(1)
where α is the fraction of weight loss and f(α) is the function that
relies on the value of α. This equation is strictly constrained to a
first order reaction value as per the kinetic model, i.e.,
d[f(α)]/dα ¼ ꢀ1, and so its value should be independent of the
heating rate. Otherwise, the value of the initial term on
the right-hand side of Equation (1) would be changed, and the
activation energy value obtained consist of some error, and
the Kissinger plot would be out of its linearity.[29] Therefore,
the d[f(α)]/dα term is eliminated and further simplification is
deduced as[31]
that at a higher heating rate, the sample gets heated to a temper-
ature in a short span and thus the decomposition temperature
shifts to a lower value. The impetuous temperature rise due
to high heating rates enhances the sample decomposition, result-
ing in high weight loss.[33] Thus, with increased heating rates the
weight loss increases.
The corresponding DTG curves, displayed in Figure 8b, clearly
show the presence of two peaks. This further substantiates the
observation of TG curves stating two-step decomposition of
the CVT as-grown ReS2 single crystals. The DTG peak positions
Tm are tabulated in Table 3. The Tm values for both the peaks
clearly shows that as the heating rate increases the magnitude
value increases. This happens due to the prevalence of heat trans-
fer constraints[34,35] at high heating rates. The simultaneously
recorded DTA curves show that for heating rates of 15 and
20 K minꢀ1 (Figure 8c) there is an initial endothermic nature fol-
lowed by exothermic nature. But for the heating rate of
10 K minꢀ1 (Figure 8c) there is an initial endothermic nature
followed by exothermic and at last endothermic. The initial
endothermic nature for all three heating rates are due to
vaporization of the moisture from the hydrous sample as well
as release of lightly attached molecules on the sample surfaces.
In the present sample the lightly attached molecules are the
slightly excess sulfur as confirmed from the EDAX data.
The outgoing moisture as well as the lightly attached molecules
takes away the heat, leading to the endothermic nature. The loss
of moisture, as well as lightly attached molecules, is confirmed by
the minor weight loss observed in the same temperature range,
step I, of TG curves. The exothermic nature after the step I tem-
perature range shows disintegration of the ReS2 sample with
temperature rise. The sample disintegration releases energy in
the form of heat, showing the exothermic nature. In the case
of the 10 K minꢀ1 heating rate, an endothermic nature is
observed at the end range of the analyzed temperature, 980
and 1233 K. This appears due to slow heating. At a slow heating
rate the applied heat gets fully dissipated amongst the sample,
leading to complete disintegration of the sample before melting.
In case of fast heating rates of 15 and 20 K minꢀ1 the endother-
mic nature gets shifted to a high-temperature range due to heat
transfer constraints.[34,35]
β
Tm2
AR
Ea
ln
¼ ln
ꢀ
Ea RTm
(2)
Here, β denotes the heating rate, Tm is the DTG peak posi-
tions, and R is the gas constant. The Arrhenius plot between
β
1
Tm
ln T2 and
for CVT as-grown ReS2 single crystals is shown
m
in Figure 9a,b. The two curves are for two peaks.
The thermodynamic parameters activation energy (Ea), activa-
tion enthalpy difference (ΔH), activation entropy difference (ΔS),
and Gibb’s free energy change (ΔG) for CVT as-grown ReS2 sin-
gle crystals are calculated from the slopes and intercepts obtained
from the Kissinger plots, (Figure 9a,b). The equations used for
the calculations of the thermodynamic parameters are as follows
ΔH ¼ Ea–RTm
ΔS ¼ 2.303R log
ΔG ¼ ΔH–TΔS
(3)
(4)
(5)
ꢂ
ꢃ
Ah
kTm
The determined kinetic parameters of the CVT as-grown ReS2
single-crystal samples are tabulated in Table 4.
The thermal activation energy Ea values obtained for the CVT
as-grown ReS2 single crystals show that the magnitude value of
Ea is large for step II compared to step I. The reason is the weight
loss is highest in the temperature range of step II. This is due to
disintegration of the single-crystal samples at high temperature.
The large value of Ea shows the sample disintegration due to high
temperature. The determined Arrhenius constant A values are
large in step II compared to step I, further supporting the
observation of the large Ea value in step II.
The determined ΔS values show that it is negative in step I and
step II. The negative ΔS value in step I states ordering to happen
with rise in temperature. The step I temperature between
ambient and 509 K removes moisture as well as the lightly
attached molecules along with the minor defects, dislocations,
stress, etc. The negative value of ΔS substantiate that the
In all the DTA curves a small kink is observed at around
the temperature of 500 K. The kink is seen to shift to higher
temperature for faster heating rates. The experimental
limitations constrain the understanding of the kink in the
DTA curves.
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Figure 9. The Kissinger plots of as-grown ReS2 single crystals.
1.35 eV. The microstructure study by FESEM showed flat surfa-
ces having layer edges confirming crystal growth by the sheet
spread mechanism. The layered nature of the as-grown crystals
is confirmed by HRTEM analysis and the spot SAED pattern con-
firmed the crystalline character. The Raman spectroscopy of the
crystals showed all the peaks are of ReS2 only. The TG analysis
showed the CVT as-grown ReS2 single-crystal samples to disinte-
grate by two steps. The two-step disintegration is supported by the
presence of two peaks in the corresponding DTG curves. The
corresponding DTA showed an initial endothermic and later exo-
thermic nature for faster heating rates of 15 and 20 K minꢀ1. The
slow heating rate of 10 K minꢀ1 showed the initial endothermic
followed by exothermic and at later high temperature an endother-
mic nature. The thermodynamic parameters determined using
Kissinger relations showed the thermal activation energy value
to be high in the high-temperature range compared to the low-
temperature range. This indicates decomposition of the sample
at high temperature. The entropy change of the sample showed
negative value in both steps. The negative value in the initial tem-
perature range shows ordering to occur with temperature rise. The
enthalpy change is positive in both the steps, indicating heat
absorption by the sample. The Gibbs free energy change values
are positive for both the steps, showing prompt nonspontaneous
absorption of heat as the temperature rises. The thermodynamic
parameters showed the single-crystal samples disintegrate at high
temperature, leading to weight loss being observed in the TG
curves. This study showed good-quality single crystals with least
defects as observed from the FESEM can be grown by CVT.
The direct optical bandgap of the as-grown CVT ReS2 lying in
the visible range with good thermal stability upto ꢂ509 K makes
it an impending aspirant for optoelectric applications.
Table 4. The thermodynamic parameters of CVT as-grown ReS2 single
crystal.
DTG
Peak
Ea
A
ΔS
ΔH
ΔG
[J moleꢀ1
]
[minꢀ1
]
[J · Kꢀ1 moleꢀ1
]
[J moleꢀ1
]
[kJ moleꢀ1
]
Step I
5379.08
2.28
ꢀ557.95
ꢀ522.37
821.60
500.008
481.188
Step II
20 492.65
23.67
13 837.23
ordering occurs in the crystal by removal of defects, dislocations,
stress, etc. The removal of moisture and lightly attached mole-
cules gives negligible weight loss in step I. However, in step
II, the material gets ordered before commencing of structure
degradation. The determined ΔH values are positive for both
the steps, showing absorption of heat. The ΔH value is more
in step II compared to step I, thus showing absorption of more
heat in step II. The large heat absorption leads to disintegration
of ReS2 single crystals. The disintegration is corroborated by the
maximum weight loss in the temperature range of step II. Step I
also has ΔH positive but the magnitude value is small, showing
no sample disintegration. The weight loss in step I is also small
and is due to moisture evaporation and loss of lightly attached
molecules. The ΔG values are positive for both the steps.
The positive ΔG values show the process of disintegration to
be nonspontaneous. All the parameters calculated earlier are rep-
resenting DTG peak positions shown in Table 3. The overall
inference drawn from the determined thermodynamic parame-
ters are the CVT as-grown ReS2 single crystals are stable in the
initial temperature range of ambient to 509 K. Above this tem-
perature range, 510–1233 K, the samples disintegrate and lead
to weight loss.
Acknowledgements
5. Conclusion
A.G. is thankful to the Scheme of Developing High Quality Research
(SHODH) by KCG (Knowledge Consortium of Gujarat)—Government
of Gujarat for providing financial support in research and analysis.
A.B.H. is thankful to the Department of Science and Technology (DST),
Technology Bhawan, New Mehrauli Road, New Delhi-110016, for the
award of the INSPIRE Fellowship (IF180246) to conduct this research
work. All the authors are thankful to the Sophisticated Instrumentation
Centre for Applied Research & Testing (SICART), Vallabh Vidyanagar,
Gujarat, India, for XRD, EDAX, FESEM, UV–vis–NIR spectroscopy analy-
sis, and DST- SAIF, Cochin, for HRTEM of the samples.
Single crystals of ReS2 are successfully grown by CVT technique
using iodine as the transporting agent. The powder XRD of
as-grown crystals showed them to be single-phase ReS2 having
triclinic unit cell structure in good match with the reported data.
The EDAX of the crystals showed them to be free of any impurity
and slightly rhenium deficient and sulfur rich. The optical analy-
sis in the wavelength range of 300 and 1500 nm showed that the
as-grown ReS2 single crystals possess a direct optical bandgap of
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[14] F. Lai, N. Chen, X. Ye, G. He, W. Zong, K. B. Holt, B. Pan, I. P. Parkin,
T. Liu, R. Chen, Adv. Funct. Mater. 2020, 30, 1.
[15] T. Fujita, Y. Ito, Y. Tan, H. Yamaguchi, D. Hojo, A. Hirata, D. Voiry,
M. Chhowalla, M. Chen, Nanoscale 2014, 6, 12458.
[16] Y. C. Lin, H. P. Komsa, C. H. Yeh, T. Björkman, Z. Y. Liang, C. H. Ho,
Y. S. Huang, P. W. Chiu, A. V. Krasheninnikov, K. Suenaga, ACS Nano
2015, 9, 11249.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
Research data are not shared.
[17] O. B. Aslan, D. A. Chenet, A. M. van der Zande, J. C. Hone,
T. F. Heinz, ACS Photonics 2016, 3, 96.
[18] F. Liu, S. Zheng, X. He, A. Chaturvedi, J. He, W. L. Chow, T. R. Mion,
X. Wang, J. Zhou, Q. Fu, H. J. Fan, B. K. Tay, L. Song, R. H. He,
C. Kloc, P. M. Ajayan, Z. Liu, Adv. Funct. Mater. 2016, 26, 1169.
[19] Z. G. Yu, Y. Cai, Y. W. Zhang, Sci. Rep. 2015, 5, 1.
[20] D. Ghoshal, A. Yoshimura, T. Gupta, A. House, S. Basu, Y. Chen,
T. Wang, Y. Yang, W. Shou, J. A. Hachtel, J. C. Idrobo, T. M. Lu,
S. Basuray, V. Meunier, S. F. Shi, N. Koratkar, Adv. Funct. Mater.
2018, 28, 1.
Keywords
characterization, chemical vapor transport, ReS2, single crystals, thermal
analysis
Received: November 3, 2020
Revised: March 9, 2021
Published online:
[21] X. Su, B. Zhang, Y. Wang, G. He, G. Li, N. Lin, K. Yang, J. He, S. Liu,
Photonics Res. 2018, 6, 498.
[22] K. Dileep, R. Sahu, S. Sarkar, S. C. Peter, R. Datta, J. Appl. Phys. 2016,
119, 114309.
[23] Q. Sun, B. Zhang, L. Diao, B. Chen, K. Song, L. Ma, L. Ma, F. He,
F. He, J. Mater. Chem. A 2020, 8, 11607.
[24] K. Ghosh, S. Ng, C. Iffelsberger, M. Pumera, ACS Appl. Energy Mater.
2020, 3, 10261.
[25] P. Pazhamalai, K. Krishnamoorthy, V. K. Mariappan, A. Sathyaseelan,
S. J. Kim, Mater. Chem. Front. 2020, 4, 3290.
[1] S. Yu, X. Wu, Y. Wang, X. Guo, L. Tong, Adv. Mater. 2017, 29,
1606128.
[2] H. Taghinejad, A. Eftekhar, A. Adibi, Opt. Mater. Express. 2019,
9, 1590.
[3] M. Taghinejad, Z. Xu, H. Wang, H. Taghinejad, K. T. Lee,
S. P. Rodrigues, A. Adibi, X. Qian, T. Lian, W. Cai, Small 2020,
16, 1.
[4] S. H. Chaki, G. K. Solanki, A. J. Patel, S. G. Patel, High Press. Res. 2008,
28, 133.
[26] S. Kim, H. K. Yu, S. Yoon, N.-S. Lee, M. H. Kim, CrystEngComm. 2017,
19, 5341.
[5] S. Chaki, A. Agarwal, Synth. React. Inorganic, Met. Nano-Metal Chem.
2008, 38, 267.
[6] C. W. Monolayers, Large-Area Synthesis of Highly, American Chemical
Society, Taipei, Taiwan 2013.
[7] J. P. Tailor, D. S. Trivedi, S. H. Chaki, M. D. Chaudhary,
M. P. Deshpande, Mater. Sci. Semicond. Process. 2017, 61, 11.
[8] W. S. Yun, S. W. Han, S. C. Hong, I. G. Kim, J. D. Lee, Phys. Rev. B –
Condens. Matter Mater. Phys. 2012, 85, 1.
[9] N. D. Hien, C. V. Nguyen, N. N. Hieu, S. S. Kubakaddi, C. A. Duque,
M. E. Mora-Ramos, L. Dinh, T. N. Bich, H. V. Phuc, Phys. Rev. B. 2020,
101, 1.
[27] B. Jariwala, D. Voiry, A. Jindal, B. A. Chalke, R. Bapat, A. Thamizhavel,
M. Chhowalla, M. Deshmukh, A. Bhattacharya, Chem. Mater. 2016,
28, 3352.
[28] D. Hou, Y. Ma, J. Du, J. Yan, C. Ji, H. Zhu, J. Phys. Chem. Solids 2010,
71, 1571.
[29] M. Rahman, K. Davey, S. Z. Qiao, Adv. Funct. Mater. 2017, 27,
1606129.
[30] U. Res, C. M. Corbet, C. Mcclellan, A. Rai, S. S. Sonde, E. Tutuc,
S. K. Banerjee, C. E. T. Al, ACS Nano 2015, 9, 363.
[31] S. Cao, Y. Xing, J. Han, X. Luo, W. Lv, W. Lv, B. Zhang, Z. Zeng,
Nanoscale 2018, 10, 16805.
[10] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, M. S. Strano,
Nat. Nanotechnol. 2012, 7, 699.
[32] M. Hafeez, L. Gan, H. Li, Y. Ma, T. Zhai, Adv. Funct. Mater. 2016, 26,
4551.
[11] C. H. Ho, Y. S. Huang, K. K. Tiong, J. Alloys Compd. 2001, 317–318,
222.
[33] M. Heydari, M. Rahman, R. Gupta, Int. J. Chem. Eng. 2015, 2015,
481739.
[12] X.-G. Gao, X.-K. Li, W. Xin, X.-D. Chen, Z.-B. Liu, J.-G. Tian,
Nanophotonics 2020, 9, 1717.
[34] S. Patel, S. H. Chaki, P. C. Vinodkumar, Thermochim. Acta 2020,
689, 178614.
[13] A. Varghese, D. Saha, K. Thakar, V. Jindal, S. Ghosh, N. V. Medhekar,
S. Ghosh, S. Lodha, Nano Lett. 2020, 20, 1707.
[35] R. M. Kannaujiya, A. J. Khimani, S. H. Chaki, S. M. Chauhan,
A. B. Hirpara, M. P. Deshpande, Eur. Phys. J. Plus. 2020, 135, 1.
Phys. Status Solidi A 2021, 2000687
2000687 (8 of 8)
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