Non-Covalent Synthesis as a New Strategy for Generating Supramolecular Layered Heterostructures

Article abstract of DOI:10.1021/acs.chemmater.7b03531

Noncovalent synthesis of stable heterostructures (graphene-BN, MoS₂-graphene) of layered materials has been accomplished by a ternary host-guest complex as a heterocomplementary supramolecular motif. Besides being reversible, this supramolecular strategy to generate heterostructures may find uses in many situations.

Full text of DOI:10.1021/acs.chemmater.7b03531

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Article
Non-Covalent Synthesis as a New Strategy for
Generating Supramolecular Layered Heterostructures
Ram Kumar, Krishnendu Jalani, Subi J. George, and C.N.R. Rao
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03531 • Publication Date (Web): 20 Oct 2017
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Chemistry of Materials
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Non-Covalent Synthesis as a New Strategy for Generating
Supramolecular Layered Heterostructures
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Ram Kumar, Krishnendu Jalani, Subi J. George,* and C. N. R. Rao*
Chemistry and Physics of Materials Unit, Sheikh Saqr Laboratory, International Centre for Materials Science
(ICMS), CSIR Centre of Excellence in Chemistry and New Chemistry Unit, Jawaharlal Nehru Centre for Advanced
Scientific Research (JNCASR), Jakkur, Bangalore, India-560064
ABSTRACT: Non-covalent synthesis of stable heterostructures (MoS2-BN, MoS2-graphene) of layered materials has been
accomplished by a ternary host-guest complex as a hetero-complimentary supramolecular motif. Besides being reversible,
this supramolecular strategy to generate heterostructures may find uses in many situations.
the use of a hetero-complimentary non-covalent interac-
tion to promote a A-B-A-B kind of assembly rather than
INTRODUCTION
Synthesis of heterostructures by the vertical stacking of
layered materials is actively being pursued to modulate
the functional properties of 2D materials.1-4 These novel
materials exhibit unique optical, electrical and thermal
properties due to synergistic effects and tunable composi-
tion of the individual components.5-13 Most 2D hetero-
structures are prepared by the mechanical assembling of
individual monolayers or few-layers of different materials,
the resultant materials being stabilized by weak van der
Waals forces.1 Although the van der Waals heterostruc-
tures exhibit a plethora of fascinating properties, synthe-
sis of extended stacks with precise sequence and composi-
tion of the components remains a challenge. Thus, in an
attempt to create stable, multi-component heterostruc-
tures, we have investigated a novel class of covalently
linked ladder-like materials.14 They have been synthesized
by covalently linking different 2D layered materials
through organic coupling reaction between surface func-
tional groups. This novel class of materials show interest-
ing properties15-18 and are different from the van der Waals
heterostructures.
self-sorted assemblies of individual components (-A-A-A-
or -B-B-B-). For this purpose, the host-guest chemistry of
cucurbit[8]uril (CB[8]) is used in the present manuscript,
which is known to form ternary complexes with appro-
priate donor and acceptor molecules with high associa-
tion constants.19-25 For example, the ternary complex of
CB[8], with naphthol donor (D) and viologen acceptor (A)
exhibits a high equilibrium association constant in the
order of 1012 M-2, which would definitely favor a hetero-
assembly. In our design strategy we have, therefore, func-
tionalized the electronically dissimilar 2D sheet structures
with naphthol and viologen moieties and used this re-
versible supramolecular motif to interlock the 2D sheets
structures to make supramolecular layered heterostruc-
tures (Scheme 1).
EXPERIMENTAL SECTION
Reagents and precursors: All the chemicals used in syn-
thesis were of high purity and obtained from commercial
sources. DMF was pre-dried before reaction. Carboxylate
functionalized graphene was obtained by microwave irra-
diation following standard procedure.26 Amine functional-
ized few-layer BN (≈1–4 layers) was prepared by mixing
boric acid and urea in 1:48 molar ratio and heating in high
purity ammonia atmosphere at 900 °C for 5 h.27-29
The synthetic heterostructures reported so far
represent exceptional examples in the family of 2D lay-
ered materials and advancement towards higher complex-
ity of heterostructures with multiple components and
precise sequences would have to be addressed to explore
novel applications. Such a non-covalent synthetic strategy
would yield supramolecular heterostructures with self-
organizing components and dynamic characteristics
which can be reversibly sequenced for various functions.
In this manuscript we describe the first successful non-
covalent synthesis of heterostructure assemblies of lay-
ered 2D sheets. This supramolecular approach facilitates
an unprecedented reversible, alternative stacking of lay-
ered materials by a solution state self-assembly process.
One of the key requisites for an efficient supra-
molecular strategy to construct such heterostructures is
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Synthetic procedures for G-VN:
diisopropylethylamine (DIPEA, 1.5 mL) was added and
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stirred for 5 min. Finally 160 mg of naphthol carboxylic
acid (4) which was synthesized by following standard
procedure,30 was added to that and stirred for 48 h. Nitro-
gen atmosphere was maintained during the reaction. Ob-
tained product was centrifuged and washed with ethanol
to remove DMF. The solid product was further dried at
100 °C under vacuum to give 140 mg final product.
Graphene
Graphene
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Synthetic procedures for 1T MoS2 and MoS2-Nap:
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Synthesis of G-VN: In a Schlenk flask, 100 mg of carbox-
ylated graphene was added, purged with N2, sealed using
septum and to that, 25 mL of dry DMF was added. Uni-
form dispersion was obtained by sonication for 1 h. To
MoS2
this
dispersion,
N-(3-dimethylaminopropyl)-N'-
MoS2-Nap
ethylcarbodiimide hydrochloride (EDC.HCl, 80 mg), 1-
hydroxybenzotriazole (HOBt, 90 mg), and N,N-
diisopropylethylamine (DIPEA, 1.2 mL) were added under
constant stirring. Then 110 mg of compound 2 (synthesis
shown below) in dry DMF was injected to the reaction
mixture and kept stirring for 24 h. Nitrogen atmosphere
was maintained during the reaction. The product was
isolated by centrifugation and washed with ethanol four
times. The product was dried under vacuum at 65 °C to
give 146 mg of the solid product (G-VN).
Synthesis of 1T MoS2: n-BuLi (3.0 mL, 1.6 M in hexane)
was added to a suspension of the bulk MoS2 (300 mg) in
dry hexane (10 mL) under an inert nitrogen atmosphere.
The reaction mixture was heated to reflux for 48 h and
then cooled to room temperature. The black precipitate
was filtered under a nitrogen atmosphere and washed
thoroughly with dry hexane to remove unreacted n-BuLi.
The black precipitate was immediately added to deion-
ized, degassed cold water (300 mL) and sonicated for 1 h
to facilitate exfoliation. The resulting aqueous dispersion
was centrifuged at 5000 rpm to remove the non-exfoliated
portion as sediments. The aqueous dispersion containing
exfoliated MoS2 was collected for the functionalization
reaction.
Synthesis of compound 2: 0.6 g (2 mmol) of compound 1
and 0.61 g (2.98 mmol) of 2-bromoethylamine hydrobro-
mide was mixed together in 40 mL of dry acetonitrile fol-
lowed by refluxing at 80 °C for 72 h under N2 atmosphere.
The product was precipitated out as orange solid which
was filtered and washed with acetonitrile to obtain the
Synthesis of MoS2-Nap: 60 ml of exfoliated 1T-MoS2 solu-
tion and 600 mg of 2-Bromo-naphthol were mixed with
20 ml of dry DMF in a round bottom flask and kept at
stirring for 2 days under N2 atmosphere. The product was
precipitated out as black solid which was filtered and
washed with ethanol. The obtained product was dried
under vacuum at 70 °C.
1
product in 42% yield. H NMR (400 MHz, D2O): δ = 9.27
(d, J = 6.8 Hz, 1H), 9.05 (d, J = 6.4 Hz, 2H), 8.68 (d, J = 6.8
Hz, 2H), 8.58 (d, J = 6.4 Hz, 2H), 5.16 (t, J = 6.8 Hz, 2H),
4.54 (s, 3H), 3.84 (t, J = 8 Hz, 2H). 13C NMR (100 MHz,
D2O): δ (ppm) 151.47, 149.47, 146.45, 146.21, 127.71, 126.89,
58.12, 48.53, 39.14. HRMS: m/z, calcd: [C13H17N3]: 215.1412,
found: 215.7940, [M]+.
RESULTS AND DISCUSSION
Synthetic procedures for BN-Nap:
In order to demonstrate this supramolecular design for
layered heterostructures, graphene-BN and graphene-
MoS2 hybrids were used. Graphene was functionalized
with the viologen acceptor and the BN and MoS2 sheets
with naphthol donors (Scheme 1). 2D sheets functional-
ized with different organic ligands were synthesized by
employing the EDC {1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide} coupling reaction (See experimental sec-
tion for details). Carboxylate functionalized graphene
was synthesized by microwave irradiation of graphene in
HNO3/H2SO4 mixture and subsequently reacted with
amine functionalized viologen to yield viologen function-
Boron nitride
Boron nitride
Synthesis of BN-Nap: 100 mg of BN-amine was added in
a Schlenk flask and sealed using septum followed by purg-
ing with N2. To that 30 mL of dry DMF was added and
sonicated for 1 h to obtain a uniform dispersion of solu-
alized
graphene (G-VN). BN-amine, synthesized by
heating boric acid and urea in an ammonia atmosphere,
was coupled with naphthol-carboxylic acid to produce
naphthol functionalized BN (BN-Nap). Functionalization
of MoS2 was achieved by surface modification strategies
tion.
Then
N-(3-dimethylaminopropyl)-N'-
ethylcarbodiimide hydrochloride (EDC.HCl, 80 mg), 1-
hydroxybenzotriazole (HOBt, 100 mg) and N,N-
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Chemistry of Materials
Furthermore, the absorption spectrum of 0.0075 wt%
that are reported to enhance its processability and chemi-
cal reactivity.31 Here 1T-MoS2 obtained by the lithium in-
tercalation of bulk MoS2 using n-BuLi, are shown to acts
as the nucleophile to form C-S bonds by virtue of the ex-
cess electrons on its surface. Hence nucleophilic substitu-
tion of exfoliated 1T-MoS2 with a 2-bromo-naphthol de-
rivative yielded the naphthol functionalized MoS2 (MoS2-
Nap). These chromophore functionalized 2D sheets and
their assemblies were characterized using spectroscopic,
microscopic and gravimetric techniques.
aqueous solution of G-VN shows a band at 260 nm corre-
sponding to the viologen chromophore (Figure S2a). The
percentage of viologen functionalization on graphene was
quantified to be ~15 wt%, from TGA analysis and UV-vis
spectrum of G-VN (Figures S2, S3). Viologen functionali-
zation stabilizes the exfoliated graphene sheets as evident
from the stable dispersions of G-VN in water and the
presence of isolated thin nanosheets in the TEM (Figure
S4). In the case of naphthol functionalized BN sheets
(BN-Nap), the UV-vis spectrum shows a broad absorption
(λmax = 270 nm) and blue emission (λmax = 358 nm) charac-
teristic of naphthol chromophores (Figure S5). The pres-
ence of naphthol chromophores on BN sheets is also evi-
dent from the corresponding excitation spectrum (Figure
S5). Further proof for naphthol functionalization was ob-
tained from the FTIR spectrum which clearly shows the
appearance of amide vibrational band at 1676 cm-1 in
comparison with the parent amine functionalized BN
(Figure 1a). TEM imaging of these samples (Figure S6),
shows exfoliated folded sheets of BN-Nap.
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The solid state C-MAS NMR spectrum of G-VN
showed the characteristic peaks of viologen (Figure S1)
wherein the signals at 30.18, 35.38, 62.08 ppm correspond
to the sp3 carbons whereas the signals in the range of 120-
180 ppm arise from the aromatic carbons.32 Viologen func-
tionalization on graphene was further confirmed by UV–
vis absorption and FTIR spectroscopy. The FTIR spectrum
of G-VN shows the Amide 1 vibrational frequency at 1656
cm-1, with decreased intensity of C=O stretching frequen-
cy (1726 cm-1) of the parent carboxylates groups, corrobo-
rating a successful EDC coupling reaction (Figure S2b).
Scheme 1. Design of supramolecular heterostructures: Schematic illustrations of the ternary supramolecular assembly approach
to make layered heterostructures: Acceptor (viologen) modified graphene (G-VN) forms heterostructures with donor (naphthol)
modified BN (BN-Nap) and MoS2 (MoS2-Nap) in presence of CB[8]. B, C, N, Mo, S in the sheet structures are represented by
red, pink, green, pale yellow and blue color spheres, respectively.
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Since the functionalization of MoS2 was achieved MoS2, in the presence of CB[8] (Scheme 1). We envisage
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by the nucleophilic substitution with bromo-naphthol
ligand, the presence of C-S vibrational stretching at 615
cm-1 in FTIR spectrum (Figure 1b) and absence of Br signal
in XPS measurements (Figure S8) confirms the presence
of naphthol chromophores on the MoS2-Nap sheets. Fur-
ther the TGA analysis of MoS2-Nap suggests 8 wt% of
naphthol functionalization on MoS2 (Figure S9). TEM
imaging of these functionalized nanosheets showed few-
layer sheets with interlayer distances ranging from 0.76
nm to 0.93 nm, reiterating the functionalization with
naphthol groups (Figure S10).
that the hetero-ternary supramolecular inclusion complex
of viologen and naphthol with CB[8] would drive the al-
ternative stacking of these 2D sheets to yield the supra-
molecular heterostructures. In a typical co-assembly pro-
cedure, the individual components were added to water
and sonicated for few hours to make well-dispersed aque-
ous solution of exfoliated sheets which would increase the
efficiency of ternary complexation. Ternary complex for-
mation was probed by changes in the spectroscopic prop-
erties, the resultant heterostructures being characterized
by X-ray diffraction and other microscopic techniques.
The UV-vis spectrum of the G-VN:BN-Nap hybrids in
presence of CB[8] showed a weak charge-transfer (CT)
band at 480 nm, characteristic of the viologen-naphthol
interaction, suggesting the formation of ternary complex
(Figure 2a). Quenching of naphthol emission in the co-
assembled solution was taken to indicate the participa-
tion of BN-Nap in ternary complex formation (Figure S11).
In the case of G-VN:MoS2-Nap hybrids, probing this CT
absorption was difficult due to the overlapping band orig-
inated from MoS2 (Figure 2b). However, a de-convoluted
absorption spectrum of the resulting hybrid by subtract-
ing the contributions from individual components reveal
a weak broad CT band absorption in the 470-500 nm re-
gion (Figure S12). Scattering in the UV-vis spectra after
co-assembly (Figure S11, S12) in both the cases suggests
the formation of an extended assembly in solution which
starts precipitating over time. This was also evident from
the photographs of the co-assembled solutions (insets of
Figure 2a and 2b) showing the formation of precipitates
(vial ii) within 2 hours of mixing the individual compo-
nents.
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BN
a)
B-N-B
out-of-plane
B-N in-plane transverse optical mode
Nap-Carb
C=O vibration
BN-Nap
Amide bond
vibration
4000
3200
2400
1600
800
Wavenumber (cm-1)
Nap-Br
b)
MoS -Nap
2
The supramolecular co-assembly process in solu-
tion could be probed via the zeta potential measure-
ments. G-VN sheets showed a negative zeta potential be-
cause of the presence of surface carboxylate anions in
presence of water.33 Small percentage of cationic viologen
groups on graphene surface is not sufficient to overcome
the negative charges of carboxylate anions and eventually
shows a negative zeta potential of -18 eV. On the other
hand, BN-Nap in water shows a negative zeta potential of
-25 eV, due to the presence of negatively charged oxygen-
containing groups (B-O-H, N-O-H) on BN surface. MoS2-
Nap having surface negative charge reveals a negative
zeta potential of -24 eV. Upon co-assembly with CB[8],
both G-VN:BN-Nap (-3 eV) and G-VN:MoS2 (-9.45 eV)
showed significant decrease in the zeta potential values
suggesting the formation of an extended layered hetero-
structures (Figure 2c, Figure S13).
C-S
C-Br
1400 1200 1000 800
600
400
Wavenumber (cm-1)
Figure 1. Comparison of the FTIR spectra of the precursors
boron nitride (BN), Naphthol-carboxylic acid (4) and the
naphthol functionalized BN (BN-Nap), showing the disap-
pearance of the C=O bond stretching vibration and the ap-
pearance of amide bond vibration, b) Proof of Naphthol
functionalization with MoS2. Infrared spectra of the Nap-Br
and MoS2-Nap showing the disappearance of C-Br bond
vibration and appearance of the C-S bond stretching vibra-
tion.
Decisive evidence of an extended assembly of su-
pramolecular heterostructures is provided by the Dynam-
ic Light Scattering (DLS) experiments, which show signif-
icant increase in the size of the co-assembled hybrids
compared to the individual components. The number
average size data depict larger assemblies for G-VN:BN-
Nap (1100 nm) and G-VN:MoS2-Nap (900 nm), compared
to the lower sizes for individual 2D sheets (Figure 2e,f).
After synthesizing and characterizing various
chromophore functionalized 2D sheets, we proceeded to
generate supramolecular 2D heterostructures. Thus, we
carried out the co-assembly of viologen appended gra-
phene sheets with the naphthol functionalized BN or
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Figure 2. a) Absorption spectrum showing the charge-transfer (CT) band at λmax = 480 nm after mixing BN-Nap and G-VN in
presence of CB[8]. b) Absorption spectrum of MoS2-Nap:G-VN:CB[8] ternary assembly and the de-convoluted spectrum to show
the CT band of the ternary complex. Inset of a) and b) shows the photographs of the vials with various components, i. BN-
Nap:G-VN, ii. BN-Nap:G-VN:CB[8], iii. BN-Nap:G-VN:CB[8]:ADA, with CB[8]:ADA ratio as 1:5. c) Bar diagram showing the zeta
potential changes for the reversible assembly of G-VN with BN-Nap and MoS2-Nap in presence of CB[8] and adamantyl amine
(ADA). d) Schematics for the reversible assembly of G-VN with BN-Nap and MoS2-Nap in presence of CB[8] and ADA. e) and f)
are the corresponding Dynamic Light Scattering (DLS) data showing the formation of extended heterostructures in presence of
CB[8] and its reversible assembly with ADA. ([BN-Nap] = 0.0075 wt%, [G-VN] = 0.0075 wt%, [MoS2-Nap] = 0.0075 wt%, CB[8] =
10-4 M, H2O).
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Thermogravimetric analysis (TGA) of the G-VN:BN-Nap
heterostructures in presence of CB[8] showed exceptional
stability as the weight loss is observed at higher tempera-
ture (400 °C) compared to the individual components, G-
VN (200 °C) and BN-Nap (150 °C), suggesting an exten-
sive crosslinking and chelation of the 2D sheets prevent-
ing the collapse of the scaffold (Figure S14).
(PXRD) patterns of the precipitates of G-VN:BN-
Nap:CB[8] and G-VN:MoS2-Nap:CB[8], dried under vac-
uum. The XRD data of
G-VN:BN-Nap assembly
showed the appearance of a new reflection at 2θ = 5.4° (d
= 1.63 nm) corresponding to the interlayer distance be-
tween the G-VN and BN-Nap sheets. A similar reflection
at low angle regions was observed for G-VN:MoS2-Nap
assembly at 2θ = 4.48° (d = 1.96 nm) corresponding to
the interlayer spacing between MoS2-Nap and G-VN. In-
terestingly, the broad reflection at 2θ ~ 25° corresponding
to a d spacing of 0.34 nm, observed in the BN-Nap and
G-VN XRD pattern, characteristic of van der Waals
layered structures, disappeared in the co-assembled mate-
rials suggesting an alternatively stacked heterostructure
(Figure 3).
A unique advantage of non-covalent interactions
is their reversible nature, which can be exploited to im-
part dynamic property to resultant supramolecular mate-
rials. We have further examined the reversibility of these
novel supramolecular heterostructures formed via ternary
host-guest complexation, by introducing a competitive
guest molecule into the system (Figure 2d). Adamentyla-
mine (ADA) was used as a competitive guest molecule, as
they are known to form a strong 1:1 inclusion complex
with CB[8] (Ka ~ 1011 M-1).34-35 Thus competitive replace-
ment of naphthol and viologen components by ADA
should result in the dis-assembly of the supramolecular
heterostructures. Interestingly both supramolecular het-
erostructures showed significant decrease in the size and
increase in the negative surface charge (Figure 2c-f), on
addition of 5 eq. ADA with respect to CB[8], thus support-
ing an unprecedented reversible stacking of 2D-
heterostructures. Formation of layered heterostructures
was also investigated using powder X-ray diffraction
Characterization of the heterostructures was car-
ried out with various microscopic techniques. TEM and
FESEM imaging of G-VN:BN-Nap heterostructures
showed extended structures formed by the multi-layer
stacking of 2D sheets (Figure S15 and S16). Detailed TEM
analysis of G-VN:MoS2-Nap heterostructures show an
interlayer spacing in the range of 1.5 to 2.0 nm, as evident
from the corresponding histogram profiles (insets, Figure
4a and 4b). This observation is consistent with the XRD
results and confirms the formation of alternate hetero-
structures.
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antibonding orbitals, associated with planar bonding and
sp2 hybridization of boron. The signals in the 200–216 eV
range are due to the 1s → σ* transition of boron. The peak
at 402.5 eV is assigned to 1s → π* transition of N, whereas
peaks at 409 and 416 eV are due to 1s → σ* transition of
BN. The C K shell ionization edge has bands at 286 and
298 eV corresponding to 1s → π* and 1s → σ* transitions
of graphene. The O K shell ionization is also observed at
543 eV due to oxygen containing functional groups in the
heterostructures. EDAX and EELS analyses of G-
VN:MoS2-Nap heterostructures in the presence of CB[8]
also reveal uniform distribution of Mo, S, C, O and N con-
firming the homogeneous nature of the heterostructures
(Figures S19-S20).
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Figure 3. PXRD spectra for the BN-Nap:G-VN:CB[8] and
MoS2-Nap:G-VN:CB[8] supramolecular heterostructures
along with their individual components (BN-Nap, G-VN,
MoS2-Nap). New peaks at 2θ = 5.4° (1.63 nm) for the BN-
Nap:G-VN hybrid and at 2θ = 4.48° (1.96 nm) for MoS2-
Nap:G-VN hybrid compared to BN-Nap, G-VN and MoS2-
Nap confirms the presence of alternately stacked hetero-
structures. The schematics adjacent to the each PXRD spec-
trum suggest the corresponding species.
Figure 5. a) FESEM image of the BN-Nap:G-VN supramo-
lecular heterostructures and the corresponding, b) elemental
mapping, B (yellow), C (red), N (green) and O (cyan). c) En-
ergy dispersive X-ray (EDAX) analysis of BN-Nap:G-VN hy-
brids showing the presence of B, C, N, O signals.
CONCLUSIONS
Figure 4. a), b) are the TEM images for the MoS2-Nap and
G-VN layered heterostructures showing the heterostructure
interlayer separation ranging from 1.5 nm to 2 nm. Insets
show the histogram profile corresponding to the interlayer
distance as marked by arrow heads in the image.
In conclusion, we have shown that a non-covalent
synthetic design as a new strategy for the reversible con-
struction of layered heterostructures of 2D materials. A
perfectly alternate stacking of 2D nano-sheets in these
novel supramolecular layered heterostructures have been
achieved by the use of a hetero-complementary host-
guest interaction with very high association constant.
This supramolecular strategy has been demonstrated with
the heterostructures of graphene with boron nitride and
MoS2, which were fully characterized by various tech-
niques. We believe that the supramolecular strategy pre-
sented here can be extended to the synthesis of novel het-
erostructures with multiple components and pro-
grammed sequences.
Elemental mapping of the G-VN:BN-Nap:CB[8]
assembly using energy-dispersive X-ray analysis (EDAX)
shows uniform distribution of B, N, C and O, confirming
the homogeneous nature of the heterostructures (Figure
5). Electron energy-loss spectra (EELS) of the G-VN:BN-
Nap heterostructures was recorded to identify the atomic
composition (Figure S17). The spectra show the character-
istic K-shell ionized edges of B, C, N, O at 193.3, 298,
409.7, 543 eV, respectively. The K-shell ionization edge at
193.3 eV is due to the transition of B 1s electron to the π*
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Chemistry of Materials
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This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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*Email: cnrrao@jncasr.ac.in; *Email: george@jncasr.ac.in
ACKNOWLEDGMENT
The authors thank the Department of Science and Technolo-
gy (DST), Jawaharlal Nehru Centre for Advanced Scientific
Research (JNCASR), India and Sheikh Saqr Laboratory (SSL),
JNCASR for the financial support.
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