This document summarizes a study on producing oriented films of molybdenum disulfide (MoS2) flakes through a simple and inexpensive process. MoS2 flakes were ball milled and exfoliated in solution to reduce their size. Drops of the solution were deposited on a substrate to produce oriented films. X-ray diffraction results showed the films maintained the crystalline structure of MoS2 without additional phases. Simulations suggested the films oriented with molecular layers stacked parallel to the substrate, unlike randomly oriented flakes. This process could produce oriented mixed films with tailored properties for practical applications.

1. Orienting MoS2 flakes into ordered films
S. Appel • A. Volman • L. Houben •
Y. Gelbstein • M. Bar Sadan
Received: 9 May 2014 / Accepted: 14 July 2014 / Published online: 1 August 2014
Ó Springer Science+Business Media New York 2014
Abstract Layered transition metal di-chalcogenide
(TMD) materials exhibit a unique combination of structural
anisotropy combined with rich chemistry that confers
controllability over physical properties such as bandgap
and magnetism. Most research in this area is focused on
single layers that are technologically challenging to pro-
duce, especially when trying to dope and alloy the host
lattice. In this work, we use MoS2 flakes as a model system
for the production of deliberately oriented films for prac-
tical applications in which anisotropic materials are
required. The proposed production method combines ball
milling with exfoliation in solution of MoS2 flakes, fol-
lowed by their arrangement on a large centimeter-scale
substrate by a simple and non-expensive procedure. The
results show that the level of orientation achieved using the
proposed system is as good as that of materials that were
pressed and subjected to thermal treatment. The ball mill-
ing and exfoliation processes maintain the original crys-
talline structure of the MoS2 flakes, and the XRD results
show that additional crystallographic phases were not
produced. Lattice parameters are preserved, which verifies
that other species such as water molecules did not inter-
calate into the MoS2 molecules. The proposed method of
producing oriented films is universal, and as such, it is
useful both for pure materials and for mixtures of com-
pounds, the latter of which can be used to produce films
with specifically tailored physical properties.
Introduction
Transition metal di-chalcogenide (TMD) layered materials
have attracted extensive research in the last couple of years
because of their unique combination of atomically thin
layers with tunable properties [1–6]. TMD may be either
metallic (e.g., NbS2 [7–9]) or semi-conducting (e.g., MoS2
[9, 10] ) depending on the crystallographic phase and the
oxidation state of the metal. The layered structure is also
known to provide good tribological properties making it
suitable for dry lubrication applications [11].
The material properties of TMDs are strongly influenced
by their structures. In addition to quantum confinement,
interlayer coupling alters the band structure of TMD
semiconductors, thereby causing them to have thickness-
dependent symmetries. For example, the bandgap of pure
MoS2 shifts from a direct bandgap of 1.9 eV for a single
layer to indirect bandgaps of 1.6 eV for a bilayer and
1.3 eV for its bulk form [12–14]. The electronic and optical
Electronic supplementary material The online version of this
article (doi:10.1007/s10853-014-8471-1) contains supplementary
material, which is available to authorized users.
S. Appel Á A. Volman Á M. Bar Sadan (&)
Chemistry Department, Ben-Gurion University of the Negev,
Beer Sheva, Israel
e-mail: barsadan@bgu.ac.il
S. Appel
e-mail: apels@post.bgu.ac.il
A. Volman
e-mail: volman@post.bgu.ac.il
S. Appel Á A. Volman Á Y. Gelbstein
Materials Engineering Department, Ben-Gurion University of
the Negev, Beer Sheva, Israel
e-mail: yanivge@bgu.ac.il
L. Houben
Peter Gru¨nberg Institut 5 and Ernst Ruska Centre for Microscopy
and Spectroscopy with Electrons, Forschungszentrum Ju¨lich
GmbH, 52425 Ju¨lich, Germany
e-mail: l.houben@fz-juelich.de
123
J Mater Sci (2014) 49:7353–7359
DOI 10.1007/s10853-014-8471-1

2. properties of pure, single-layer MoS2 were also experi-
mentally explored by incorporating MoS2 in various
devices. Studies on field-effect transistors based on single-
layer MoS2 have shown its promising potential, with ION/
IOFF current ratios greater than 108
, electron mobility
comparable with that of graphene nanoribbons, and sub-
threshold swing of 74 mV per decade, in future nanoelec-
tronics applications [15]. This pioneering demonstration by
Yoon et al. [15] of MoS2 potential as a platform for low-
power transistors was followed by the implementation of
small-scale integrated circuits [16, 17] and small-signal
amplifiers [18]. The chemical durability of TMDs, their
general mechanical stability, well-defined structure, and
well-known production techniques such as CVD or exfo-
liation [19–24] facilitate the fabrication of such devices.
To date, research has focused mainly on pure TMD
compounds while failing to exploit their rich chemistries in
the study of thin layers. The lattice of layered TMDs
comprises tri-atomic slabs, in which metal atoms are
sandwiched between two chalcogen atom planes from the
top and the bottom. The multiple coordination numbers of
the atoms in such arrangements lead to polytypism, which
results in the same material possessing different properties
that could possibly be tuned by phase transformations.
From the chemistry point of view, the two-dimensional
structure is an ideal candidate for substitution reactions
[25]. The interlayer distances typical of TMDs create fast
diffusion paths into the depths of the layers, and in addi-
tion, the inner metal atomic plane is relatively easy to
access. In some cases, dopant atoms even contributed to the
stability of the low-dimensional structures, as was shown
theoretically for Nb-doped MoS2 nanotubes [26].
For the abovementioned phenomena to occur, however,
the TMDs must be manufactured using high-quality
materials on suitable substrates. To do otherwise could
compromise the performance of the materials. For exam-
ple, transport measurements have shown that for MoS2,
trap states within the crystal could mediate the mobility of
charge carriers [27]. The effects of grain boundaries within
the layers at the atomic level have lately attracted a lot of
attention in the community, as efforts are made to thor-
oughly explain the electronic performance of pure MoS2
devices [28–30]. Alternative methods of producing single
layers of MoS2 include harsh chemical exfoliation, which
may hamper utilization of the MoS2 layers in high-quality
electronic devices, and mechanical cleavage that inherently
produces many lattice defects.
Thus, although the properties of single-layer MoS2 are
attractive, a reliable procedure for manufacturing these
films at sufficiently high quality and on a large scale are
lacking, and therefore, current efforts in TMD-layered
materials concentrate on the mesoscale. One relevant field
of research that is steadily growing is focused on the inter-
slab coupling of different materials [31] and has as its goal
to combine materials with different magnetic, electronic,
and thermal properties using the rich chemistry of the
system. In practical terms, future commercial applications
require oriented films that can be produced on a larger scale
and with high uniformity and control over crystal orienta-
tion. Moreover, films in which the crystallites are arranged
with their basal planes parallel to the substrate surface
possess lower reactivities, i.e., good stabilities, and longer
endurance than films with randomly oriented crystallites
[32].
In this work, we used MoS2 flakes as a model system for
the production of oriented films. The thin MoS2 flakes were
homogenized by ball milling and sonication, and they were
deliberately oriented into thin films by a simple, non-
expensive procedure. Moreover, the proposed procedure
does not require a high-temperature treatment that could
induce diffusion and contamination from the substrate or
container material and increase manufacturing costs due to
the additional investments of energy and time that would
be required. We expect that the general exploitation of this
procedure will enable mixed films to be produced with
properties that are specifically tailored to different uses.
Materials and methods
Equal amounts of pure 2H-MoS2 powders (Sigma Aldrich
CAS 1317335) were ball milled in a Fritsch (Pulverisette 7)
ZrO2 grinder along with ZrO2 grinding balls (ø = 0.5 mm)
at 400 rpm for 2, 3, 12, 24, and 48 h. All samples were
milled in an N2 atmosphere to prevent oxidation and
hydration. After the milling process, the samples under-
went powder X-Ray Diffraction (XRD) analysis and
Scanning Electron Microscopy (SEM) imaging.
XRD data were collected on an Empyrean Powder
Diffractometer (PANalytical, NL) equipped with an
X’Celerator position sensitive detector using Cu Ka radia-
tion (l = 1.5405 A˚ ) and operated at 40 kV and 30 mA.
The usual Bragg–Brentano h/2h geometry was employed.
Scans of h/2h were run for 40 min in a 2h range of 10–110°
with the step equal to *0.0167°. SEM images were
acquired with a JEOL JSM-7400F high resolution cold
FEG-SEM with a point resolution of 1.5 nm at 3 kV.
To produce oriented films, the ball-milled MoS2 powder
was suspended in isopropanol (concentration of 200 mg/
ml). The suspension was pulse-sonicated for 30 min using
a Qsonica 125–W probe sonicator (on/off ratio of 8:2 s at
100 % amplitude) to exfoliate the MoS2 particles. Drops
from the solutions were deposited on Si/SiO2 wafers and
dried to produce the films. In addition, a suspension drop
was deposited on an ultrathin holey carbon grid for trans-
mission electron microscopy (TEM) imaging. TEM images
7354 J Mater Sci (2014) 49:7353–7359
123

3. were taken with a FEI Tecnai-12 TWIN instrument oper-
ated at an acceleration voltage of 120 kV.
Three equal amounts of the MoS2 powders were com-
pressed into three 5-mm pellets using a hydraulic press at a
pressure of 3.92 Mpa. The compressed pellets were inserted
into a furnace at a temperature of 300 °C for 24 and 72 h.
Results and discussion
Effect of ball milling on morphology, agglomeration,
and orientation
MoS2 nanocrystal morphologies before and after ball
milling for several milling times are displayed in the high-
resolution SEM images in Figs. 1 and 2. The typical
morphology of pristine MoS2 nanocrystallites was a sharp-
angled flake (see Fig. 1a) with micrometer scale lateral
dimensions (in this case *1 lm). Each flake comprised
several visible layers, each with thicknesses in the range of
10–20 nm (20-30 molecular layers) for overall flake
thicknesses of 100-600 nm (see Fig. 2a).
Once the pristine particles, which were thick with sharp
edges, underwent ball milling, their lateral diameters were
reduced and overall particle morphology appeared rounder
(see Fig. 1b–f). Moreover, particle thickness, initially
several hundred nm, was reduced to less than 100 nm (see
Fig. 2b). Small fragments that were present in the com-
mercial powder and that were also found in the 2-h ball-
milled samples were absent from the samples milled for
longer times. We ascribe this effect to cold welding pro-
cesses, which for milling times up to 12 h improved
homogeneity of particle sizes and shapes. At milling times
greater than 12 h, a slight decrease in particle lateral
dimensions was observed.
The experimental powder XRD patterns of the pristine
sample and of the samples ball milled for 2 and 12 h are
depicted in Fig. 3, while the full set of XRD data for all
samples is presented in the supplementary information
(Fig. S1). To compare between the different samples, all
patterns were normalized to the overall signal (the area
under the curve). The analysis of the lattice parameters of
the samples showed no significant changes that could have
been caused by the ball milling process. (See Table S1 for
lattice parameter calculations for all samples.) Therefore, it
is not likely that another phenomenon, such as the inter-
calation of water molecules between the MoS2 layers,
oxidation or the formation of other crystallographic struc-
tures (e.g., 3R-MoS2), took place during the milling pro-
cess, even after 48 h.
To characterize films morphologies and the stacking of
the MoS2 layers and flakes, we used simulations of three
Fig. 1 HR-SEM images of ball-milled MoS2 powders. a Commercial MoS2 as purchased and after milling times of b 2 h, c 3 h, d 12 h, e 24 h,
and f 48 h
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4. supercells with different MoS2 layers packing (See atom-
istic models in Fig. 4). Structure A is constructed from
narrow and rectangular small MoS2 layers stacked in a
long, thin design. Structure B has an aspect ratio close to 1
such that its stacked layers resemble a cube, and structure
C is a thin flake constructed from relatively wide, square-
shaped layers. For ease of comparison, the three structures
comprised similar numbers of atoms. See Table 1 for exact
supercell dimensions.
The powder XRD pattern of each supercell was calcu-
lated by the Debye scattering equation [33], which predicts
to first order the scattered intensity in powder diffraction
patterns from gases, liquids, and randomly-distributed
nano-clusters in the solid state. The Debye equation is
given by
I gð Þ ¼
XN
i¼1
XN
j¼1
fi gð Þfj gð Þ
sin 2pgrij
À Á
2pgrij
; ð1Þ
where g is the magnitude of the scattering vector in reci-
procal lattice distance units, N is the number of atoms,
fi(g) is the atomic scattering factor for atom i as a function
of g, and rij is the spatial distance between atom i and atom
j. The latter approach thus allows us to calculate the dif-
fraction pattern for a specific supercell, which is non-
periodic [34]. Each simulated particle contained about
59,000 atoms due to computational limitations, resulting in
particles with dimensions of a few dozen nm at most.
The simulated diffraction patterns are presented in
Fig. 5. The different diffraction patterns reflect the physical
dimensions of the nanostructure: according to the Debye–
Scherrer relationship, sharp spikes in the reflections that
constitute the diffraction pattern correspond to larger
dimensions in a certain direction, while broader and lower
reflection peaks correspond to smaller dimensions. The
same trend exists in the simulated patterns, where the
decline in the z dimension from structure A to B to C is
expressed by a corresponding attenuation in their 002
reflections (at 2h & 14°) relative to the intensities of the
other reflections, which correspond to the in-layer dimen-
sions. Thus, for structure A, the longer stacking of layers
relative to the in-plane dimensions is characterized by a
narrow, high 002 reflection while the other reflections are
broader and shorter.
Because the SEM images (Figs. 1, 2) show the indi-
vidual particles as thin flakes, their random orientation
within the film would result in a diffraction pattern similar
to that of structure C. However, the best qualitative fit to
the experimental results (Fig. 3) is of structure A, indi-
cating that the molecular layers within the film orient and
stack themselves to a considerable thickness relative to
their in-plane dimensions. Thus, better oriented films, i.e.,
films in which the molecular slabs are stacked parallel to
each other in the z direction, exhibit more pronounced 002
reflections that can be used to characterize the experimental
results. Note the significant difference between the simu-
lated and experimental patterns: the sharper (both higher
Fig. 2 HR-SEM images of ball-milled MoS2 powders. a Commercial
MoS2 as purchased (same sample as Fig 1a). The thickness of particle
1 is 193 nm. b Ball milled for 12 h (same sample as Fig. 1d). The
thicknesses of particles 1–3 is 85, 103 and 69 nm, respectively
Fig. 3 Experimental powder XRD patterns of pristine MoS2 (in blue)
and of MoS2 after 2 h (in black) and 12 h (in orange) of ball milling
(Color figure online)
7356 J Mater Sci (2014) 49:7353–7359
123

5. and narrower) reflection peaks of the experimental particles
demonstrate their larger sizes, by almost two orders of
magnitude, than the simulated supercells.
To enable the experimental diffraction patterns to be
directly compared (see Fig. 3), they were analyzed as fol-
lows: each XRD pattern was normalized to its overall
signal, and the area of each reflection was integrated. Since
the ratio of the 002 peak to the overall signal reflects the
degree of orientation of the flakes, the fraction of the
integrated intensity of the 002 peak to the overall signal
was calculated for all samples (see Table 2). In addition,
the ratio of the integrated intensities of each reflection was
compared to the integrated intensity of the 002 peak. The
results appear in Table 3.
Table 3 shows that during the ball milling process, the
relative areas of all reflections decreased until a minimal
value was achieved for 12 h of ball milling. The relative
intensity of all reflections was decreased by a factor of 2–3.
Accordingly, the 002 peak became more pronounced,
contributing 41.9 % of the overall signal (after 3 h of ball
milling) compared to 26.5 % in the pristine material
(Table 2). Therefore, the thinning and homogenization
produced by the ball milling process facilitated better
stacking of the thin flakes of MoS2 into agglomerates in
which the coherence length in the z direction is much larger
than particle lateral dimensions.
Orientating MoS2 on a Si substrate
The agglomeration properties of the ball-milled samples
were used to produce orientated films on a Si wafer sub-
strate. To further promote the formation of oriented
agglomerates, the MoS2 flakes were suspended in isopro-
panol using a probe sonicator. The suspension was
deposited on a Si wafer and the solvent was evaporated to
produce dry polycrystalline films that were 10–15 lm thick
and 1–2 cm in diameter. To compare the effects of soni-
cation and deposition on the substrate, powder samples of
both the pristine flakes and those that underwent ball
milling for 12 h at 400 rpm were deposited on a Si wafer.
Fig. 4 Atomistic models of
three MoS2 nanostructures.
Dimensions and atom numbers
are summarized in Table 1
Table 1 Geometrical dimensions and number of atoms for the su-
percells depicted in Fig. 4
a (nm) b (nm) c (nm) V (nm3
) N (atoms)
A 3.2 4.8 67.8 1022 59,400
B 10.7 9.4 10.2 1033 59,500
C 19.0 19.2 2.8 1015 59,220
Fig. 5 Simulated MoS2 powder XRD patterns for the three structures
described in Table 1 and depicted in Fig. 4 A (blue), B (green) and
C (red) (Color figure online)
Table 2 Ratio of the 002 reflection (integrated intensity) to the
overall signal
Sample I(002)/Itotal
No BM 0.265
BM 2 h 0.309
BM 3 h 0.419
BM 12 h 0.401
BM 24 h 0.400
BM 48 h 0.375
Exfoliation, No BM on Si wafer 0.434
Exfoliation, BM 12 h on Si wafer 0.623
Pressed pellet, No thermal treatment 0.439
Pressed pellet, Thermal treatment at 300 °C for 24 h 0.434
Pressed pellet, Thermal treatment at 300 °C for 72 h 0.592
Theoretical value for random powder 0.233
J Mater Sci (2014) 49:7353–7359 7357
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6. The resulting XRD patterns of the free powders and of the
powders deposited on the Si are presented in Figs. S3 and
S4, respectively. The analyzed relative peak ratios are
presented in Table 4. The ratio of the 002 peak to the
overall signal is presented in Table 2.
The peak ratios in Table 3 show that by itself, the ball
milling process improves MoS2 flake orientation by a
factor of three. Likewise, sonication followed by deposi-
tion on a Si substrate similarly improved flake orientation
(Table 4), a finding that is in accordance with the 002 peak
fraction values (Table 2) that were found to be similar for
ball milling and for exfoliation and deposition (around
41.9–43.4 %, respectively). The combination of ball mill-
ing, sonication, and deposition on a substrate further
improved flake orientation such that the 002 peak fraction
rose to 62.3 % (Table 2), relative to which the other
reflections were strongly attenuated (Table 4).
Because a common way to produce oriented materials
is by pressing them with a mold to produce dense pellets,
pristine MoS2 was pressed into a 5-mm pellets that were
annealed in an inert atmosphere for 24–72 h. It is well
known that the compression force tends to orient the
flakes while the subsequent thermal treatment promotes
the production of a dense, strongly oriented sample.
Therefore, the XRD pattern of the pellet was acquired,
and the data were analyzed. Pellet relative peak ratios are
also presented in Table 4. Results from the analyses of
the as-pressed samples are similar to those of the samples
that were ball milled for 3 h and to the exfoliated and
deposited films. Pellet thermal treatment of 24 h seems to
have had no significant effect, as witnessed by the similar
002 peak fractions of 43.9 % and 43.4 % for the as-
pressed samples versus those that underwent 24 h of
thermal treatment at 300 °C, respectively. However, after
72 h of thermal treatment at 300 °C, a marked change
was observed in the 002 peak fraction, which rose to
59.2 %.
Although the orientation of the ball milled, exfoliated,
and deposited films was only slightly higher than that of the
pressed pellet annealed for 72 h, an important difference
between the two is the size limitation of the compression
method. Similar experiments could not be done with a
30-mm pellet since the pressure (force/area) is much lower
in wider compression molds. The force required to produce
larger pellets is proportional to r2
, and therefore, standard
stainless steel molds may not be able to withstand the high
pressures that are required. In addition, for the same rea-
sons the mold shape must be circular shape and it can
produce very thick pellets one by one. While compression
produces a very dense pellet, it is not scalable. In contrast,
the ball milling process, the exfoliation, and the subsequent
deposition are all processes that can be scaled up. More-
over, the overall procedure is simpler and cheaper, since it
does not require several days of thermal treatment. Indeed,
thus far we have refrained from annealing the thin films
since it could cause contaminants from the Si wafer,
including oxygen from the native oxide layer, to diffuse
into the MoS2. Future efforts will consider alternative
substrates for which contamination is less likely. Alterna-
tively, the thermal treatment can be used in combination
with lift-off techniques to protect the films during the
process. The electronic properties of these films are beyond
the scope of the present paper and will be reported
elsewhere.
Table 3 Peak area of XRD
reflections (experimental and
simulated) of MoS2 samples
normalized to the area of the
002 reflection
Reflection Simulated
(random powder)
No BM BM
2 h
BM
3 h
BM
12 h
BM
24 h
BM
48 h
100 0.250 0.198 0.144 0.082 0.070 0.084 0.081
101 0.131 0.113 0.082 0.045 0.042 0.061 0.053
103 0.673 0.620 0.491 0.291 0.291 0.312 0.346
105 0.331 0.313 0.261 0.170 0.184 0.180 0.214
110 0.184 0.135 0.109 0.062 0.054 0.063 0.067
Table 4 Relative peak areas
from XRD patterns
(experimental and simulated) of
MoS2-oriented films and
pressed pellets
Reflection Exfoliation,
No BM on
Si wafer
Exfoliation,
BM 12 h on
Si wafer
Pressed pellet,
No thermal
treatment
Pressed pellet,
Thermal treatment at
300 °C for 24 h
Pressed pellet,
Thermal treatment at
300 °C for 72 h
100 0.034 0.008 0.012 0.013 0.004
101 0.021 0.006 0.008 0.009 0.003
103 0.110 0.026 0.075 0.074 0.031
105 0.061 0.021 0.095 0.093 0.040
110 0.007 0.002 0.016 0.018 0.010
7358 J Mater Sci (2014) 49:7353–7359
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7. Conclusions
In this paper, we propose a universal method for the
preparation of oriented films of layered materials, specifi-
cally, transition metal di-chalcogenides, using MoS2 as a
case study. The process involves a few steps operating in a
synergetic way: the sample was mechanically grinded by
ball milling for an optimized time of 3–12 h. Thereafter,
the flakes were further thinned using an exfoliation process
in a liquid solution followed by simple deposition and
drying on a substrate. The results show that the achieved
orientation is as good as that produced by the traditional
procedure of pellet pressing and thermal treatment for
3 days. The importance of such a method is rationalized by
the ability to exploit the diverse chemistry of layered
compounds and to produce mixtures of different com-
pounds for improved qualities.
Acknowledgements This research project was supported by Focal
Technology Area (FTA) project on Inorganic nanotubes (INT) from
nanomechanics to improved nanocomposites and by the GIF, the
German-Israeli Foundation for Scientific Research and Develop-
ment. M.B.S. appreciates the support from Dr. Dmitri Mogilyanski of
the Ilse Katz institute for nanoscience and nanotechnology.
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