Graphene-templated synthesis of C -axis oriented Sb2Te3 nanoplates by the microwave-assisted solvothermal method

Article abstract of DOI:10.1021/cm502749y

The successful mechanical exfoliation and chemical synthesis of graphene has attracted considerable attention for the synthesis of other two-dimensional materials on graphene template. Chalcogenide materials such as Sb₂Te₃ are of interest due to the rhombohedral lattice structure with two-dimensional hexagonally closed-packed atomic layers along the c-axis. Here we synthesized c-axis-oriented Sb₂Te₃ nanoplates (NPs) on graphene substrates by the microwave-assisted solvothermal method. The microwave irradiation resulted in a higher temperature of graphene, compared with the synthesis solution, which was revealed by the single-mode microwave experiments and an analytical model based on energy balance and convective heat transfer. Besides, the lattice mismatch between c-axis-oriented Sb₂Te₃ and bridge sites of graphene was only 4%, which is also favorable for the graphene-templated Sb₂Te₃ synthesis. c-Axis-oriented single-crystalline Sb₂Te₃ NPs as large as 7 μm could be successfully synthesized on graphene with negligible damage of the graphene template. Larger surface coverage could be obtained by merging Sb₂Te₃ NPs. The merged Sb₂Te₃ NPs were polycrystalline with rotated grain boundaries. This work provides a facile, rapid, and low-cost synthesis route of c-axis-oriented Sb₂Te₃ NPs on graphene templates, which may be extended for the synthesis of various two-dimensional materials with hexagonally closed-packed atomic layers along the c-axis.

Full text of DOI:10.1021/cm502749y

Article
pubs.acs.org/cm
Graphene-Templated Synthesis of c‑Axis Oriented Sb Te Nanoplates
2
3
by the Microwave-Assisted Solvothermal Method
†
‡,⊥
†
†
§
†
Swati Singh, Seunghyun Hong, Wonjae Jeon, Dongmok Lee, Jae-Yeol Hwang, Seulky Lim,
†
†
†
†,§
,§,∥
Gi Duk Kwon, Didier Pribat, Hyunjung Shin, Sung Wng Kim, and Seunghyun Baik*
†
‡
§
Department of Energy Science, SKKU Advanced Institute of Nanotechnology (SAINT), Center for Integrated Nanostructure
∥
Physics, Institute for Basic Science (IBS), and School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746,
Republic of Korea
*
S Supporting Information
ABSTRACT: The successful mechanical exfoliation and chemical synthesis of
graphene has attracted considerable attention for the synthesis of other two-
dimensional materials on graphene template. Chalcogenide materials such as
Sb Te are of interest due to the rhombohedral lattice structure with two-
2
3
dimensional hexagonally closed-packed atomic layers along the c-axis. Here we
synthesized c-axis-oriented Sb Te nanoplates (NPs) on graphene substrates by the
2
3
microwave-assisted solvothermal method. The microwave irradiation resulted in a
higher temperature of graphene, compared with the synthesis solution, which was
revealed by the single-mode microwave experiments and an analytical model based on energy balance and convective heat
transfer. Besides, the lattice mismatch between c-axis-oriented Sb Te and bridge sites of graphene was only 4%, which is also
2
3
favorable for the graphene-templated Sb Te synthesis. c-Axis-oriented single-crystalline Sb Te NPs as large as 7 μm could be
2
3
2
3
successfully synthesized on graphene with negligible damage of the graphene template. Larger surface coverage could be obtained
by merging Sb Te NPs. The merged Sb Te NPs were polycrystalline with rotated grain boundaries. This work provides a facile,
2
3
2
3
rapid, and low-cost synthesis route of c-axis-oriented Sb Te NPs on graphene templates, which may be extended for the
2
3
synthesis of various two-dimensional materials with hexagonally closed-packed atomic layers along the c-axis.
INTRODUCTION
process due to the momentum of depositing atoms (∼10
19,24
■
eV).
Here we synthesized c-axis-oriented Sb Te nanoplates
The successful mechanical exfoliation and chemical synthesis of
graphene, single atomic layer of sp -hybridized carbon atoms,
2
2
3
(
NPs) on graphene templates by the microwave-assisted
has initiated tremendous research activities in two-dimensional
1
,2
solvothermal method. The microwave-induced temperature
rise of the synthesis solution containing the graphene substrate
was faster than the solution without graphene, which was
investigated using a single-mode microwave reactor. An
analytical model based on the energy balance and convective
heat transfer between graphene and solution revealed a surface
temperature of graphene 110 °C greater than that of the
solution at a microwave power of 50 W. The lattice structure
investigation also revealed that the lattice constant mismatch
between c-axis-oriented Sb Te and bridge sites of graphene
(
2D) materials. The layered materials such as boron nitrides,
transition-metal dichalcogenides, and chalcogenides exhibited
unique mechanical, electrical, and optical properties. Among
them, chalcogenide materials (Bi Te , Sb Te , Bi Se , etc.) have
3
,4
2
3
2
3
2
3
attracted considerable attention as near room temperature
thermoelectric materials and topological insulators comprising
5
,6
only one Dirac cone.
High-quality chalcogenide thin films were typically synthe-
sized by high-vacuum technologies such as molecular beam
epitaxy (MBE), metal organic chemical vapor deposition,
pulsed laser deposition, thermal evaporation, and sputter-
2
3
was only 4%. These are favorable for the solution-based
7
−18
ing.
Recently, chalcogenide thin films were also constructed
synthesis of Sb Te on graphene substrates. c-Axis-oriented
2
3
on chemically inert graphene substrates using MBE as well as
single-crystalline Sb Te NPs as large as 7 μm could be
19−22
2
3
sputtering methods.
Especially, the simple sputtering
successfully synthesized on graphene templates. The Sb Te
2
3
method enabled the preferential c-axis orientation of Bi Te3
2
thickness on graphene was tens of nanometers. The single-
crystalline Sb Te NPs could be merged during the synthesis,
and Sb Te thin films on graphene templates possibly due to
2
3
2
3
the hexagonal lattice structure, although the precise mechanism
19
resulting in larger polycrystalline NPs with rotated grain
boundaries. This solution-based synthesis of c-axis-oriented
was not revealed. Sb Te is a V VI pnictogen chalcogenide
2
3
2
3
with a small band gap and belongs to the R3
rhombohedral structure) with 2D hexagonally closed-packed
atomic layers along the c-axis. One of the hurdles of these
high-vacuum technologies is the scale-up synthesis at low cost.
Besides, graphene was heavily damaged during the sputtering
̅
m space group
(
2
3
Received: July 25, 2014
Revised: February 25, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/cm502749y
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Chemistry of Materials
Article
Sb Te NPs on graphene may enable scale-up synthesis at low
cost with negligible damage of the graphene template.
RESULTS AND DISCUSSION
As shown in Figure 1a, the optical transmittance of graphene
on a glass substrate was ∼97.7% at 550 nm. There was a
2
3
■
EXPERIMENTAL METHODS
■
Synthesis and Transfer of Graphene. Graphene was
synthesized on a Cu substrate by the thermal chemical vapor
deposition method similar to the previously published
1
,19,25,26
protocol.
electropolished Cu substrate (The Nilaco Corporation, Tokyo,
Briefly, graphene was synthesized on an
26
Japan) in a 2 in. quartz tube. First, the copper substrate was
heated to 1000 °C and kept constant for 30 min under H flow.
2
In the next step, H and CH were introduced at 1000 °C for
2
4
10 min to synthesize graphene. Finally, the Cu substrate was
rapidly cooled to room temperature without any gas flow. The
synthesized graphene film was transferred onto glass or SiO2
(
(
300 nm)/Si substrates by the poly(methyl methacrylate)
PMMA) coating (Sigma-Aldrich, St. Louis, No. 182265),
copper etching (Transene, Danvers, MA, CE-100), and PMMA
1
,19,25,26
removal (acetone) process.
Microwave-Assisted Solvothermal Synthesis of
Sb Te . The microwave-assisted solvothermal synthesis of
2
3
27−30
Sb Te NPs was published previously.
2
Here we modified
3
the procedure to synthesize Sb Te NPs on graphene. The
2
3
Sb Te3 NP synthesis was carried out using a multimode
2
microwave reactor (CEM Co., Matthews, NC; MARS6). First,
tellurium (Te, Sigma-Aldrich, 266418, 1 g) was dissolved in
trioctyl phosphine (TOP, Sigma-Aldrich, 117854, 10 mL) in a
microwave-heated autoclave (CEM Co.). The temperature was
set at 220 °C for 2 min at a microwave power of 600 W
followed by cooling to room temperature in 20 min. This
resulted in yellow-colored ligated complex solution of Te-
2
9
TOP. Separately, SbCl (Sigma-Aldrich, 117854, 1 g) was
3
dissolved in 1,5-pentanediol (Sigma-Aldrich, P7703, 20 mL) by
ultrasonication (490 W, 3 min). Finally, the two solutions were
mixed and heated in a Teflon autoclave with and without the
graphene on SiO /Si wafer to synthesize Sb Te NPs. The
2
2
3
reaction temperature was ramped to 220 °C in 45 s and kept
constant for 10 s at a microwave power of 600 W. The mixture
Figure 1. (a) Raman spectrum of graphene on a SiO /Si substrate at
2
was cooled down to room temperature in 20 min. The Sb Te3
2
an excitation wavelength of 532 nm. The transmittance of graphene on
a glass substrate and an optical image are also shown. (b) Microwave-
induced heating of the synthesis solution with and without the
graphene substrate. The temperature of graphene estimated by an
analytical model is also shown.
NPs synthesized on graphene was gently rinsed with acetone
followed by drying at ambient air conditions. The Sb Te NPs
2
3
synthesized in solution was vacuum-filtered, washed with
acetone and isopropanol, and dried at ambient air conditions.
Characterizations. Raman measurements were carried out
using a Renishaw spectrometer. The excitation wavelength was
negligible D mode with an intensity ratio of G-to-2D modes
smaller than 0.5, indicating successful synthesis of high-quality
532 nm. The X-ray diffraction (XRD) data were obtained by a
1,31
monolayer graphene.
The microwave-induced heating of
Rigaku X-ray diffractometer (Rigaku Smart Lab, Rigaku
Corporation, Tokyo, Japan). All measurements were carried
out under the identical parallel beam mode (Cu Kα radiation at
graphene was investigated using a single-mode microwave
reactor (Korea Microwave Instrument Co., Seoul, Korea, 2.45
GHz, WR-340 waveguide), and a schematic of experimental
4
5 kV and 200 mA). The joint committee for powder
diffraction standards card number used for peak indexing was
1-393. The scanning electron microscopy (SEM, JEOL,
32
setup is provided in Figure S1 of the Supporting Information.
The microwave power was set at 50 W since the solution
temperature increased rapidly in a short period of time at a
higher power. The single-mode microwave system generates
periodic electromagnetic fields whereas a multimode microwave
7
Tokyo, Japan, JSM7600F), high-resolution transmission
electron microscopy (HRTEM, JEOL, JEM2100F), and atomic
force microscopy (AFM, Seiko Instruments, Chiba, Japan,
SPA400) analyses were also carried out. For HRTEM
measurements, Sb Te NPs grown on graphene were trans-
3
2,33
system produces random nonuniform field distribution.
As
shown in Figure 1b, the microwave-induced temperature
2
3
increase of the synthesis solution containing the transferred
3
ferred to a lacey carbon grid using a drop of diluted
hydrofluoric acid.
graphene on a SiO /Si substrate (10 × 10 × 0.5 mm ) was
2
investigated as a function of time. As a control, the temperature
B
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Article
of synthesis solution with a bare SiO /Si substrate was also
monitored. Surprisingly, the temperature increase of the
larger microwave absorption cross section of graphene
compared with the synthesis solution. The high temperature
of graphene is favorable for the solution-based synthesis of
Sb Te . However, the temperature of graphene decreased in
Regime 2 even with the continuous microwave irradiation. This
could be due to the heat loss to the solution container. The
heat loss to the container is no longer negligible as the solution
temperature increased beyond 74 °C, leading to under-
estimation of graphene temperature.
2
solution with graphene (T ) was greater than that of the
w
solution without graphene (Tw/o). The difference between Tw
and Tw/o was due to the greater microwave-induced heating of
graphene compared with the synthesis solution.
An analytical model based on energy balance was developed
to estimate the microwave-heated temperature of graphene
2
3
(
T ). The energy difference between two solutions at T and
g
w
Tw/o was equated with the heat transfer from graphene to the
surrounding solution. A uniform temperature of the solution
and negligible heat loss to the outside of the solution (i.e.,
solution container) was assumed.
The Sb Te NP synthesis study was carried out at a greater
microwave power (600 W) using a multimode microwave-
assisted solvothermal reactor (MARS6) as shown in Figure 2a.
2
3
∑
C m (T − T ) = hA (T − T )Δt
p
i
w
w/o
s
g
w
(1)
i
where C is the specific heat capacity [J/g·K], m is the mass [g],
p
h is the heat-transfer coefficient between graphene and the
surrounding medium, A is the heat-transfer surface area, and
s
Δt is the heat-transfer duration. The subscript i designates each
component in solution (1: 1,5-pentanediol; 2: SiO /Si
2
substrate; 3: tellurium; 4: SbCl ), and material properties are
3
summarized in Table S1 of the Supporting Information. The
liquid property was approximated using that of the major
solvent 1,5-pentanediol. The mass of graphene was neglected
since it was significantly smaller than other components.
The natural convection heat-transfer coefficient of a
horizontal plate under a laminar flow regime was employed
34
for the calculation.
1
/4
3
⎛
⎜
gβ(T − T )L
c
⎞
g
w
0.54k
Pr
⎟
2
⎝
v
⎠
h =
Lc
(2)
where k is the thermal conductivity of the fluid [W/m·K], g is
2
the local acceleration due to gravity [m/s ], β is the volume
expansion coefficient [1/K], L is the characteristic length [m],
c
2
v is the kinematic viscosity of the fluid [m /s], and Pr is the
Prandtl number. L was obtained by the area-to-periphery ratio
c
of graphene. Finally, eqs 3 and 4 were obtained by combining
eqs 1 and 2.
1
/4
3
gβL
v
c
0.54k(
Pr)
2
Figure 2. (a) Schematic of the synthesis process. (b) SEM image of
5
/4
∑
C m (T − Tw/o) =
A (T − T ) Δt
p
i
w
s
g
w
Sb Te NPs synthesized in solution. (c) SEM image of Sb Te NPs
i
2
3
2
3
Lc
synthesized on graphene. An SEM image of a bare SiO /Si wafer after
2
(3)
syntheis of Sb Te NPs is provided in the inset. (d) Raman spectra of
2
3
pure Sb Te NPs and Sb Te NPs on graphene. (e) Comparison of the
2
3
2
3
Tg = T
w
D-mode of graphene after synthesis of Sb Te3 by sputtering or
2
4
/5
microwave-assisted solvothermal methods. The spectrum of the
⎡
⎢
⎢
⎢
1/4
⎤
⎥
⎥
⎥
3
gβL
v
Sb Te −graphene bilayer synthesized by sputtering was reproduced
c
2
3
0.54k(
Pr)
2
19
from a literature for comparison.
+
∑ C m (T − Tw/o)/
AsΔt
p
i
w
i
Lc
⎢
⎣
⎥
⎦
The synthesis temperature was optimized at 220 °C (see Figure
S3 of the Supporting Information). A greater synthesis
temperature produced more Sb Te NPs. However, it was
(
4)
The calculated T using eq 4 is shown in Figure 1b. The
g
2
3
experimentally measured solution temperatures were first fitted
using third-order polynomial equations as shown in Figure S2
of the Supporting Information. The fitted data were used for
the model since a small fluctuation in the solution temperature
difficult to investigate c-axis-aligned Sb Te NPs on graphene
2 3
due to the vigorous out-of-plane growth of Sb Te .
2
3
Figure 2b shows an SEM image of Sb Te NPs
2
3
homogeneously nucleated in solution. Hexagonal and trun-
cated-triangular NPs could be observed similar to the particles
previously synthesized by the microwave-assisted solvothermal
resulted in a large variation in T . The graphene temperature
g
increased rapidly in Regime 1 as the microwave irradiation time
increased. The temperature difference between graphene and
solution was as large as 110 °C, demonstrating a significantly
27−30
method.
Energy-dispersed X-ray analysis confirmed the
near-stoichiometry of Sb Te NPs as shown in Figure S4 of the
2
3
C
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Chemistry of Materials
Article
37
Supporting Information. The Te-to-Sb ratio was 1.485. Figure
(Figure 3d). The lattice mismatch between the lattice
2
c shows Sb Te NPs synthesized on graphene. Most NPs were
constant of Sb Te (S = 4.260 Å) and the distance between
2
3
2
3
flat whereas some NPs were grown in the out-of-plane direction
of graphene. The greater surface temperature as well as
hexagonal lattice structure of graphene contributed to the
synthesis of Sb Te NPs on graphene as will be discussed
bridge sites of graphene (G = 4.440 Å) was only ∼4%. The
small mismatch in lattice constants is favorable for the synthesis
of c-axis-aligned Sb Te NPs on graphene. The Sb Te NPs
2
3
2
3
could not be synthesized on the bare SiO /Si substrate by the
2
3
2
shortly. As a control, a bare SiO /Si wafer was inserted in the
microwave-assisted solvothermal method probably because of
the abscence of lattice similarity (Figure 2c inset).
2
solution, and Sb Te synthesis was carried out using an
2
3
identical condition. However, no Sb Te NPs could be
Some Sb Te NPs on graphene were grown in the out-of-
2
3
2
3
observed on the surface of the SiO /Si wafer as shown in the
plane direction of graphene as shown in Figure 2c. These NPs
could be removed by controlled sonication (500 W, 5 min) as
shown in Figure 4a. Only flat Sb Te NPs grown in the in-plane
2
inset of Figure 2c. The Sb Te NPs could not also be observed
2
3
by an optical image taken at a lower magnification (see Figure
S5 of the Supporting Information). Figure 2d shows Raman
spectra of pure Sb Te NPs and Sb Te NPs synthesized on
2
3
2
3
2
3
−1
graphene. The peak at 117 cm corresponds to the in-plane
vibrational mode (E (2)) of rhombohedral Sb Te . The
35
g
2
3
sample with Sb Te NPs synthesized on graphene shows peaks
2
3
3
1,35
related to both Sb Te and graphene.
The D, G, and 2D
2
3
modes of graphene are magnified in Figure 2e. The increase in
−1
D mode around 1300 cm was very small, indicating minimal
damage of graphene substrate during the microwave-assisted
solvothermal synthesis. We previously investigated Sb Te −
2
3
graphene bilayers by the sputtering method. There was a large
D mode, indicating severe damage of graphene due to the
1
9,24
momentum of depositing atoms (∼10 eV).
In contrast, the
2
pristine sp -hybridized carbon structure of graphene could be
conserved during the microwave-assisted solvothermal syn-
thesis process.
Figure 3a,b shows schematics of the lattice structures of
rhombohedral Sb Te NPs on (0 0 1), (0 1 0), and (1 0 0)
2
3
Figure 4. (a) SEM images of Sb Te3 NPs on graphene after
2
sonication. Selected area electron diffraction analysis of a single-
crystalline Sb Te NP is also shown. (b) XRD analysis of randomly
2
3
oriented pure Sb Te powders, Sb Te NPs on graphene, and Sb Te
2
3
2
3
2
3
NPs on graphene after sonication.
Figure 3. Blue atoms, Sb; pink atoms, Te. (a) (0 0 1) plane of Sb Te .
2
3
The unit cell is shown by the red line and shaded region. (b) (0 1 0)
and (1 0 0) planes of Sb Te . (c, d) The Sb Te unit cell on the (0 0
2
3
2
3
direction of graphene could be observed after sonication. The
size of single hexagonal NPs was as large as 7 μm. The selected
area electron diffraction (SAED) analysis revealed the single-
1) plane was overlaid on the bridge sites of graphene. The lattice
constant of Sb Te (S) was 4.260 Å. The distance between bridge sites
2
3
of graphene (G) was 4.440 Å, resulting in a lattice mismatch of ∼4%.
crystalline structure of Sb Te3 NPs. Symmetric hexagonal
2
diffraction spots could be observed with a calculated interplanar
distance of 0.214 nm, which corresponds to the reflection from
planes. The lattice structures of distinct major planes of Sb Te3
2
crystal ((1 1 0), (1 0 1 0), (0 1 5), and (2 0 5) planes) are also
provided in Figure S6 of the Supporting Information. The
hexagonal lattice structure could be observed only on the (0 0
the (1120) plane of rhombohedral Sb Te with a [0001] zone
̅
2 3
29,30
axis. The X-ray diffraction (XRD) data of pure randomly
oriented Sb Te powders, Sb Te NPs on graphene, and Sb Te
3
2
3
2
3
2
23
l) plane with a lattice constant of 4.260 Å. The Sb Te unit
NPs on graphene after sonication are compared in Figure 4b.
2
3
cell on the (0 0 1) plane was overlaid on the lattice of graphene
as shown in Figure 3c,d. The first-principle band calculation
based on density functional theory revealed that the most stable
adsorption site for nonmetallic elements is the bridge site of
graphene lattice. The distance between bridge sites of
graphene was 4.440 Å, as shown in the magnified diagram
The pure Sb Te powders showed characteristic peaks of
2
3
randomly oriented Sb Te indexed to the rhombohedral Sb Te
2
3
2
3
crystal structure. Surprisingly, as-grown Sb Te NPs on
2
3
graphene exhibited strong preferential (0 0 l) orientation
with a small (0 1 5) peak. The (0 1 5) was the strongest mode
of randomly oriented Sb Te powders which was also observed
3
6
2
3
D
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on the sputtered Sb Te film on graphene. This (0 1 5) peak
Article
19
2
3
could come from the small number of Sb Te NPs synthesized
2
3
in the out-of-plane direction of graphene as shown in Figure 2c.
As shown in Figure S7 of the Supporting Information, the
intensity of (0 1 5) and non-(0 0 l) peaks increased for as-
grown Sb Te NPs on graphene as the synthesis duration
2
3
increased from 45 to 150 s at 220 °C. More randomly oriented
Sb Te NPs could be observed on graphene from SEM and
2
3
XRD analysis. As shown in Figure 4b, only (0 0 l) peaks could
be observed for Sb Te NPs on graphene after sonication,
2
3
demonstrating a complete c-axis orientation. This is also
consistent with the SEM observation (Figure 4a).
Figure 5a,b shows SEM and HRTEM images of merged
Sb Te NPs on graphene. As shown in the magnified HRTEM
2
3
Figure 6. (a) SEM image of randomly oriented Sb Te NPs
2
3
synthesized in solution. (b, c) AFM image of an Sb Te NP and
2
3
merged Sb Te NPs synthesized on graphene. The scale bar is 500 nm.
2
3
(
d) Histogram of thickness of randomly oriented Sb Te NPs
2 3
synthesized in solution (average ∼100 nm) and Sb Te NPs
2
3
synthesized on graphene (average ∼40 nm). The scanned area on
2
graphene was 20 × 20 μm .
smaller than that of the randomly oriented Sb Te NPs
2
3
synthesized in solution (∼100 nm). It is possible that the mass
transport of reactants was limited due to the graphene substrate
compared with the Sb Te NPs synthesized in solution, which
2
3
Figure 5. SEM (a) and HRTEM (b) images of merged Sb Te NPs on
have the reactant supply from both sides of NPs.
Graphene is an excellent substrate to synthesize layered
materials with similar crystal structures since the strong
bonding of carbon atoms provides chemical inertness. This
results in an atomically sharp interface between Sb Te and
2
3
graphene. (c) Magnified HRTEM image of the square area shown in
b). (d) SAED analysis of the square area shown in (c).
(
2
1
image, dislocation could be observed at the seam of merged
NPs (Figure 5c). The spacing between adjacent lattice fringes
2
3
graphene by suppressing interfacial chemical reaction. The
unique properties achieved in this study are discussed below.
First, the solvothermal synthesis method constructed c-axis-
aligned Sb Te NPs on graphene due to the lattice similarity
turned out to be 0.214 nm, which corresponds to the (112
̅
0)
plane of Sb Te The SAED analysis performed on the seam
2
3.
revealed azimuthally rotated symmetric hexagonal diffraction
spots (Figure 5d). This indicates the merging of two single-
crystalline Sb Te NPs with rotated grain boundaries,
2
3
(Figure 4a). The spin coating or drop casting of solution
synthesized Sb Te NPs on graphene resulted in randomly
2
3
2
3
demonstrating a polycrystalline nature. The calculated inter-
planar distance was 0.214 nm from SAED analysis, which is
consistent with the spacing between the lattice fringes shown in
Figure 5c. The diffraction spots correspond to the [0001] zone
oriented aggregates of Sb Te NPs on graphene (see Figure S9
2 3
of the Supporting Information). Graphene has highly
19,38
anisotropic thermal and electronic transport properties.
Therefore, the preferred c-axis orientation of Sb Te NPs is
2
3
2
9,30
axis projection of reciprocal lattice of Sb Te .
important for studying anisotropic thermal and electric
transport properties of two-dimensional graphene/Sb Te
2
3
Figure 6a shows an SEM image of randomly oriented Sb Te3
2
2
3
NPs synthesized in solution. Figure 6b shows an AFM image of
a single-crystalline Sb Te NP on graphene with a thickness of
heterostructures.
Second, Ohmic contact was realized between the graphene
and Sb Te NPs synthesized on graphene as shown in Figure 7.
2
3
5
0 nm. The AFM image of merged Sb Te NPs on graphene is
2 3
2
3
shown in Figure 6c. The height of each NP was different,
resulting in a step. The height profile and AFM image of other
merged NPs are also provided in Figure S8 of the Supporting
Information. The step height between merged NPs could be
The applied voltage was normalized with the thickness of
Sb Te NPs to compare the induced current at the identical
2
3
electric field strength (voltage/thickness). The current linearly
increased with the applied voltage, quickly reaching an
instrument compliance (100 mA). The Sb Te NPs with
clearly observed. This indicated that single-crystalline Sb Te3
2
2
3
NPs were first separately synthesized on graphene followed by
merging of NPs as the growth continued. Although a large
surface coverage could be obtained by merging Sb Te NPs,
different thickness resulted in similar current-electric field
behavior. The current−voltage characteristics were obtained in
ambient atmosphere by contact mode (2 nN loading force)
conductive atomic force microscopy (C-AFM). The platinum-
coated silicon cantilever with a force constant of 0.2 N/m was
2
3
single crystallinity of the merged NPs could not be obtained
Figure 5d). As shown in Figure 6d, the average thickness of
Sb Te NPs synthesized on graphene was ∼40 nm. This was
(
39−41
used for the scan, and graphene was grounded.
Most
2
3
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DOI: 10.1021/cm502749y
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
transfer. Besides, the small lattice mismatch (4%) between c-
axis-oriented Sb Te NPs and the distance between bridge sites
2
3
of graphene facilitated the synthesis of Sb Te NPs on
2
3
graphene. The synthesized Sb Te NPs on graphene were
2
3
aligned along the c-axis. Each Sb Te NP was single-crystalline
2
3
whereas merged NPs were polycrystalline with rotated grain
boundaries. The size of single-crystalline Sb Te NP was as
2
3
large as 7 μm. This graphene-templated solvothermal method
may be extended for the c-axis-oriented synthesis of various 2D
materials with rhombohedral structures for the study of Dirac
Fermions and thermoelectrics in the future.
ASSOCIATED CONTENT
■
*
S
Supporting Information
Schematic of the single-mode microwave reactor, material
properties, microwave-induced heating of graphene, micro-
wave-assisted solvothermal synthesis conditions, energy-dis-
persive X-ray analysis, optical microscopy, lattice analysis, XRD
analysis, AFM, and SEM analysis. This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 7. Current-electric field characteristics measured by C-AFM.
Two Sb Te NPs synthesized on graphene (thickness = 32 and 54
2
3
nm) are compared with two Sb Te NPs spin-deposited on graphene
2
3
(
thickness = 60 and 133 nm). Representative AFM images are
provided, and the scale bar is 500 nm. The current-electric field
characteristics of spin-deposited Sb Te3 NPs on graphene are
magnified in the inset plot.
2
AUTHOR INFORMATION
■
Corresponding Author
E-mail: sbaik@me.skku.ac.kr.
*
Sb Te NPs deposited by spin coating were aggregated with
random orientation (Figure S9 of the Supporting Information).
Present Address
Department of Physics and Graphene Research Centre,
National University of Singapore, 117542, Singapore.
2
3
⊥
Only a few c-axis-aligned Sb Te NPs were observed from the
2
3
2
entire graphene substrate (1 × 1 cm ) after the spin deposition.
However, solid electrical contact with graphene was not
realized, resulting in negligible current (Figure 7). The
difference in thickness between the synthesized and spin-
deposited Sb Te NPs was as large as 4.15 times. However, the
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
2
3
This research was supported by Fundamental Technology
Research Program (2014M3A7B4052200) through the Na-
tional Research Foundation of Korea (NRF) grant funded by
the Korean government (MSIP), IBS-R011-D1, and the
Human Resources Development program (No.
difference in current was much greater due to the poor contact
between spin-deposited Sb Te NPs and graphene.
2
3
The significantly increased carrier mobility and electrical
conductivity of graphene/Sb Te bilayer films were previously
2
3
reported, compared with those of the pure Sb Te film, because
2
3
3
2
−1 −1
19
2
0124010203270) of the KETEP grant funded by the Korean
of the high mobility (2.34 × 10 cm V s ) and electrical
conductivity (23 164 S cm ) of graphene. Such graphene/
Sb Te bilayers were previously constructed using MBE or
−1
government Ministry of Knowledge Economy.
2
3
1
9,20,22
sputtering methods.
The MBE method requires ultra-
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DOI: 10.1021/cm502749y
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