1. science
2. physics

Letter
pubs.acs.org/NanoLett
Deterministic Two-Dimensional Polymorphism Growth of Hexagonal
n‑Type SnS2 and Orthorhombic p‑Type SnS Crystals
Ji-Hoon Ahn,†,∥ Myoung-Jae Lee,†,∥ Hoseok Heo,†,‡ Ji Ho Sung,†,‡ Kyungwook Kim,†,§ Hyein Hwang,†,§
and Moon-Ho Jo*,†,‡,§
†
Center for Artificial Low-Dimensional Electronic Systems, Institute for Basic Science (IBS), ‡Division of Advanced Materials Science,
and §Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), 77
Cheongam-Ro, Pohang 790-784, Korea
S Supporting Information
*
ABSTRACT: van der Waals layered materials have large crystal
anisotropy and crystallize spontaneously into two-dimensional
(2D) morphologies. Two-dimensional materials with hexagonal
lattices are emerging 2D confined electronic systems at the limit
of one or three atom thickness. Often these 2D lattices also
form orthorhombic symmetries, but these materials have not
been extensively investigated, mainly due to thermodynamic
instability during crystal growth. Here, we show controlled polymorphic growth of 2D tin-sulfide crystals of either hexagonal
SnS2 or orthorhombic SnS. Addition of H2 during the growth
reaction enables selective determination of either n-type SnS2
or p-type SnS 2D crystal of dissimilar energy band gap of
2.77 eV (SnS2) or 1.26 eV (SnS) as a final product. Based on
this synthetic 2D polymorphism of p−n crystals, we also demonstrate p−n heterojunctions for rectifiers and photovoltaic cells,
and complementary inverters.
KEYWORDS: van der Waals layered materials, two-dimensional materials, tin disulfides, tin monosulfides, vapor transport synthesis,
polymorphism
I
comprises zigzag double planes of the Sn and chalcogen atoms
separated by a van der Waals gap (Figure 1b). In bulk crystal
forms, hexagonal SnS2 and orthorhombic SnS exhibit n-type
and p-type semiconductor characteristics, respectively.17,18
Growth of SnS2 crystals is thermodynamically stable in ambient
conditions, and its 2D crystal absorbs visible light effectively.19 In
contrast, 2D SnS crystals have a narrow Eg and are therefore
expected to be optically active in the near-infrared spectral
range.19 The fact that the SnS2 and SnS states exhibit n-type and
p-type characters and dissimilar Eg and are thus optically active in
complementary broad spectral ranges suggests that a synthetic
polymorphism of 2D p−n components may have various
electronic and optical applications. Here, we report deterministic
polymorphism growth of 2D hexagonal SnS2 and orthorhombic
SnS crystals by tuning the amount of H2 added during gas-phase
synthesis.
In our study, the 2D Sn-sulfide crystals were synthesized on
SiO2/Si substrates by a vapor transport method from pure SnO2
and S powder precursors in a 12-in. hot-wall quartz-tube.20 Before
synthesis, we performed simple thermodynamic calculations
of the gas-phase reactions from SnO2 and S precursors. In the
n hexagonal van der Waals layered crystals, such as
graphene,1,2 h-BN,3,4 and hexagonal transition-metal dichalcogenides,5−9 the constituent atoms within the monolayer plane
are covalently bonded with a large bonding energy of 200−
6000 meV, whereas the individual monolayer are vertically
joined by weak van der Waals interactions with energies of
40−70 meV.10,11 Typically, these substances spontaneously form
two-dimensional (2D) crystals, and thereby establish unit-cell
confined electronic systems in a hexagonal momentum space. In
this regard, Sn-sulfides are particularly interesting class of the 2D
semiconductors with layered crystal structures because these
sulfides exist in diverse crystal phases, such as hexagonal and
orthorhombic, due to the versatile oxidation characteristics
of Sn and chalcogen elements. Notably, Sn-dichalcogenides,
such as SnS2 and SnSe2, crystallize two-dimensionally into
hexagonal unit cells with the Sn oxidation state of +4, to form
semiconductors with a large band gap Eg12−14 in which the Sn
ions are coordinated to six chalcogen ions in the octahedral sites
with space group P3m
̅ 1 within a monolayer, which is stacked on
top of another monolayer by van der Waals interaction without
translational displacements (Figure 1a). However, Sn-chalcogenides
can also crystallize in orthorhombic unit cells to form 2D
Sn-monochalcogenides15,16 in which Sn ions with oxidation
state of +2 are coordinated to three chalcogen ions to form an
orthorhombic unit cell with the space group of Pnma, which
© XXXX American Chemical Society
Received: January 8, 2015
Revised: April 11, 2015
A
DOI: 10.1021/acs.nanolett.5b00079
Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Crystal structures of (a) hexagonal SnS2 and (b) orthorhombic SnS crystals in different sectional views. The Sn and sulfur atoms are colored in
blue and orange, respectively. (c) Change in Gibbs free energy ΔG° during formation of SnS2 and SnS compounds from SnO2 and S in N2 atmosphere
and in N2−H2 atmosphere. Gray line, SnS2 and SnS in N2; blue line, SnS in N2; red line, SnS in N2−H2.
Detailed growth procedures are depicted in the Figure S1 of
Supporting Information. Under pure N2 gas at 620−680 °C,
large-area 2D crystals (more than several tens of microns in
width) with hexagonal or triangular facets were obtained
(Figure 2a); i.e., reaction of SnO2 with S vapor under the inert
condition yields hexagonal SnS2 crystals with release of SO2 gas
as a byproduct. The thickness of the 2D crystals decreased
as the gas-flow rate decreased and could be as thin (∼1.1 nm) as a
unit lattice of hexagonal SnS2 (Figure 2c, inset). Atomic force
microscopy (AFM) line profiles confirmed that the facets were of
uniform thickness except at the crystal centers which may be
nuclei for the 2D growth.7
Raman spectra (Figure 2c) were obtained for crystals of
various thickness. The A1g phonon mode at 317 cm−1 is assigned
to the 2D SnS2 crystals,22−24 and the Eg phonon mode at 208 cm−1
corresponds to thick SnS2 crystals.25 As the thickness decreased
to the nanometer scale, the Eg peak disappeared, presumably due
to the reduction in the scattering centers for in-plane scattering,26
and these match well with previously reported spectra of
nanostructured SnS2.23,24 When the carrier gas included H2, the
growth products were typically transformed to rectangular 2D
facets (Figure 2b, inset; Figure S2b). Addition of H2 to the gas
encouraged growth of orthorhombic SnS 2D crystals. Typically,
these crystals were favored at H2/N2 > 0.4. In this case, SnO2
reacts with S vapor by H2 addition, then orthorhombic SnS crystals grow with release of SO2 and H2S gas byproducts (Figure 2b).
The minimum thickness was ∼12.1 nm, which corresponds to
10 or 11 unit cells. Below the H2/N2 ratio of 0.4, the final
products start to form irregular facets, and near the H2/N2 ratio
∼0, the hexagonal facet is stabilized (see Figure S2 for more
details). The fact that the minimum achievable thickness of SnS is
thicker than that of the SnS2 may be due to the relatively larger
van der Waals energy in the orthorhombic cell with its zigzag
arrangement of Sn and S atoms compared to the hexagonal cell.
The Raman spectra (Figure 2d) typically showed four major
peaks; those at 94, 188, and 217 cm−1 can be assigned to the Ag
phonon modes, and that at 160 cm−1 corresponds to the B3g
mode of orthorhombic SnS.27,28
High-resolution transmission electron microscopy investigations revealed the polymorphic phases of the 2D SnS2 and SnS
crystals. Diffraction patterns constructed by fast Fourier
transform of each image (insets in Figure 2e,f) corroborate the
phase index in each growth condition. The measured interplanar
distances were 0.317 (Figure 2e) and 0.293 nm (Figure 2f),
standard condition, the reaction can be predicted by the change
ΔG°rxn in the standard Gibbs free energy, which is a function of
temperature as follows:
ΔG°rxn = ΔH °f,rxn − T ΔS°f,rxn
⎛
= ⎜ΔH °298,rxn +
⎝
⎞
T
∫298 ΔCp,rxn dT ⎟⎠
⎛
− Trxn⎜⎜ΔS°298,rxn +
⎝
T
∫298
ΔCp ,rxn
T
⎞
dT ⎟
⎠
(1)
where ΔXrxn = Σ(# of moles)Xproducts − Σ(# of moles)Xreactants,
ΔH°f and S°f are the standard enthalpy and entropy of formation,
ΔH°298 and S°298 are the standard enthalpy and entropy of
formation at 298 K, T is the reaction temperature [K], and Cp is
the specific isobar heat capacity. When the SnO2 powders are
heated to react with S2 gas in an inert atmosphere, reaction forms
SnS or SnS2 by
SnO2 (s) + S2 (g) → SnS(s) + SO2 (g)
(Reaction 1)
SnO2 (s) + 1.5S2 (g) → SnS2 (s) + SO2 (g)
(Reaction 2)
Using eq 1 and the thermodynamic data (Table S1),21
calculated values of ΔG°rxn of each reaction in the pertinent
temperature range 500−700 °C were positive for SnS growth but
negative for SnS2 growth (the red curve in Figure 1c); i.e., SnS
growth from SnO2 and S precursors is not spontaneous in an
inert atmosphere. Nonetheless, because the Sn oxidation state in
SnS is +2, which is less than the +4 in SnS2, SnS growth can be
promoted in a reducing atmosphere. Therefore, we considered a
reaction with the addition of H2:
SnO2 (s) + 1.5S2(g) + H 2(g)
→ SnS(s) + SO2 (g) + H 2S(g)
(Reaction 3)
ΔG°rxn for SnS growth with H2 addition is negative and is also
lower than that of SnS2 in the temperature range of interest
(Figure 1c). This result indicates that we can control the final
product by adding H2 to influence ΔG°rxn during growth.
Inspired by the result of this thermodynamic calculation, we
designed a series of reactions to grow 2D SnS2 and SnS crystals
(Figure S1). To emulate the standard conditions of thermodynamics, we established an inert (N2) or a reducing (N2/H2)
ambient (total pressure 700−800 Torr) during crystal growth.
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DOI: 10.1021/acs.nanolett.5b00079
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Figure 2. Growth schematics and representative optical microscope images of (a) hexagonal 2D SnS2 in N2 and (b) orthorhombic 2D SnS in N2−H2.
Raman spectra of (c) SnS2 2D crystals and (d) SnS 2D crystals of various thickness. Insets: atomic force microscope images. High resolution
transmission electron microscope images of (e) a SnS2 crystal and (f) a SnS crystal. The corresponding FFT-diffraction patterns of the insets clearly
show the hexagonal and orthorhombic lattices.
thickness dependence of spectral photocurrent (Figure 3a). The
range of photoresponse of SnS2 crystals was expanded to the
lower energy of ∼2.1 eV, and the corresponding absorption edge
shows a red-shift with increasing thickness. The optical band gap
notably increased as t decreased toward the monolayer regime
(Figure 3b), and the extracted Eg of 2.77 eV in our 2D SnS2 is
significantly higher than those of the bulk SnS2 of 1.82−2.2 eV
across the indirect Eg,12,32,33 which values match well with our
SnS2 above 10 nm in thickness. Thereby, this observation suggests that our 2D crystals exhibit optical confinement effects, by
which the absorption edge progressively increases as crystal
thickness decreases.34 Our observations are consistent with
predictions by density-functional tight-bonding calculation that
indirect band gap size increases ∼2.81 eV for SnS2 monolayers.35
In contrast, the 2D SnS crystals had an absorption edge of
1.26 eV (Figure 3c), which is within the range of values reported
previously (0.9−1.27 eV), suggesting absence of the size-effect in
this thickness regime.36,37
The dark electrical conductivity was 2.86 × 103 S/m for the 2D
SnS crystal and 2.17 S/m for the 2D SnS2, and the photoconductivity was 3.85 × 103 and 85.23 S/m at 2.33 and 3.06 eV,
respectively (Figure S3c,d). The higher conductivities of 2D SnS
crystals can be attributed to the narrower Eg compared to 2D
SnS2.16,32 The gate voltage (Vg)-dependent transport characteristics at Vds = 1.0 V show the typical n-type and p-type characters
for the SnS2 and SnS field-effect transistors (FETs), respectively
(Figure 4a). The SnS2 n-FET had the on/off current ratio of
2 × 104, and the field effective electron mobility μe = 2.16 cm2 V−1 s−1
at room temperature; this is larger than μe ≈ 1 cm2 V−1 s−1 reported
which are consistent with the (100) plane of the hexagonal SnS2
and the (101) plane of the orthorhombic SnS, respectively. The
interaxial angles of 120° and 94.9°/85.1° are also consistently
assigned to SnS2 and SnS, respectively.
To characterize the specific semiconductor properties of 2D
SnS2 and SnS crystals, we fabricated simple back-gated transistors
that incorporate individual crystals on SiO2/degenerate Si substrates. Metal contacts were fabricated with Ti/Au and Ni/Au
as the source/drain electrodes for SnS2 (3 nm thick) and SnS
(30 nm thick) crystals, respectively. First, the spectral photocurrent Iph responses were measured while illuminating the
devices with a supercontinuum laser equipped with a
monochromator. We confirmed that the major photoresponses
are spatially from the channels, not from the contact barriers, by
scanning Iph mapping (Figure S3a,b), thus confirming the
effectively intrinsic photoresponses of the crystals. The
absorption edge can be determined by converting from measured
spectral Iph to effective absorption coefficient α by the relation29
α = −1/t(1 −(Iph/(1 − R))(hυ/eηP)), where hυ is the photon
energy, t is the thickness of the 2D crystal, P is the incident optical
power, R is the reflectance, and η is the photon-to-carrier conversion efficiency. We assume that η = 1, and that the voltage
application has negligible effect on R.30 The relation between hυ
and α is expressed by (αhυ)m = B(hυ − Eg), where B is a constant,
and with m = 2 for direct band gap transition and m = 1/2 for
indirect band gap transition.31 Because both SnS2 and SnS
semiconductors in the bulk forms to show indirect band gap
transitions, the optical band gap can be extracted by extrapolating
the linear region of a (αhυ)1/2 vs hυ plot. We investigated the
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Figure 3. Thickness dependence of (a) spectral responsivity (Iph/hυ) and (b) optical bandgap of SnS2 crystal extracted by extrapolating the linear region
of inset of (αhυ)1/2 vs hυ plot. (c) Spectral responsivity of 30 nm-thick SnS field-effect transistors. Insets: plots of (αhυ)1/2 vs hυ for determination of
band gap.
Figure 4. (a) Transfer characteristics of SnS2 and SnS back-gate transistors. Blue line, SnS2 transistor showing typical n-type characteristics; red line, SnS
transistor with p-type characteristics. (b) Gate tunable output characteristics of SnS2−SnS vertical heterojunctions; inset, device schematics. (c) Dark
I−V curve and Iph−V curve under 3.06 eV light excitation with a power of 3.2 μW; (left inset, photovoltaic I−V curves, showing the open-circuit voltage
of 0.21 V and the short-circuit current of 1.69 nA; right inset, corresponding band diagram of the SnS2−SnS vertical heterojunctions at Vb = 0 V,
illustrating the photovoltaic effect).
from an exfoliated 2D SnS2.17 The SnS p-FET had on/off ratio
of only ∼1.5, but the field effective hole mobility was μh =
10.55 cm2 V−1 s−1. Having established the synthetic Sn-sulfide
polymorphism for the 2D p−n components, we built vertical and
lateral devices by incorporating individual 2D SnS2 and SnS
crystals. We first stacked the two 2D crystals by manual transfer
to construct the vertical p−n heterojunctions for rectifiers and
photovoltaic cells. The Vg-dependent output characteristics of
the 2D SnS2−SnS vertical heterojunction showed a rectifying
diode behavior (Figure 4b), which is effectively modulated by the
applied electric field Vg.38 The output current of the diode is
largely governed by the higher resistivity of the n-type SnS2 than
of the p-type SnS in the p−n series resistor and thus increases as
Vg increases. The rectification ratio = (forward current)/(reverse
current) at bias voltage Vb = ±2 V increased from 9.4 to 33.7 as Vg
increased from −40 to 40 V. The diode parameters from the
Shockley diode equation with a series resistance Rs, which is
related to the metal/SnSx contacts were deduced; extracted
values were saturation current Is = 0.04 nA, Rs = 0.52 GΩ, and
ideality factor n = 6.7. These values of Rs and n differ greatly from
the ideal values. However, different from a conventional p−n
diode, the 2D p−n junction diodes do not allow a depletion
region across the two adjacent layers, so the classical exponential
characteristics may not be representative. They can be better
approximated by interlayer recombination processes between
two majority carriers across the abrupt potential discontinuity,
such as by Langevin recombination or Schokley−Read−Hall
recombination mediated by the interlayer defect states, as
suggested by the work of an MoS2/WSe2 monolayer p−n stack.39
Investigations of these unique 2D phenomena will be a focus of
our future work. Our polymorphic 2D p−n stack can operate
as an ultrathin photovoltaic cell.40 Under 405 nm illumination
with a power of 3.2 μW, the photoresponsivity for forward bias
as a photoconductor was 4.56 mA/W and reverse bias as a
photodiode was 27.09 mA/W (Figure 4c), which are moderate
values with other 2D-based photodetectors.41 Our p−n junction
shows a photovoltaic effect with an open-circuit voltage of
∼0.21 V and a short-circuit current of ∼1.69 nA (inset, Figure 4c).
The corresponding external quantum efficiency was calculated
to be ∼0.13%, which is comparable to those of other highperformance monolayer semiconductor 2D p−n junctions.42,43
On the basis of the extracted band gap and electron affinity
available in the literature of 4.2 eV for SnS2 and 3.14 eV for
SnS,44,45 the band diagram of the heterojunction at Vb = 0 V can
be illustrated (right inset, Figure 4c). We assume a type-II flat
band alignment for simplicity, as discussed above. Our photovoltaic cell is reminiscent of organic heterojunction cells in that
charge separation arises from discontinuous energy alignments at
D
DOI: 10.1021/acs.nanolett.5b00079
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the heterointerfaces, in this case the band offsets of ∼0.2 eV
between the SnS2 conduction (EC,SnS2) and SnS valence (EV,SnS)
band. The maximum open-circuit voltage of our photovoltaic
cells of ∼0.2 V under 405 nm illumination (inset, Figure 4c) is
consistent with this band-offset approximation.41,44 We ensure
that the observed photovoltaic responses only pertain to the
junction from scanning Iph mapping, where the short circuit
current is localized at the junction between n-type SnS2 and
p-type SnS (Figure S5). As another demonstration of 2D p−n
polymorphism, we constructed a complementary metal−oxide−
semiconductor (CMOS) inverter (Figure S6); the observed general
CMOS inverter features qualitatively suggest a possibility of 2D
logic operations based on the synthetic 2D p−n polymorphism.
In summary, we successfully synthesized 2D tin sulfide crystals
of either hexagonal SnS2 or orthorhombic SnS and determined
that the type of crystal formed can be controlled by adding H2 to
the feed gas to control thermodynamics during growth. Our 2D
polymorphic crystals show n-type (SnS2) and p-type (SnS)
semiconductor characteristics, and we demonstrated the
feasibility of using the crystals as polymorphic 2D heterostructure device for rectifiers, photovoltaic cells, and complementary
inverters. Our methods may guide development of synthetic
polymorphism of other 2D materials for the 2D electronic and
optoelectronic heterostructures.
■
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ASSOCIATED CONTENT
S Supporting Information
*
Experimental details of polymorphism growth, photocurrent
mapping and Raman characterization, and demonstration of
CMOS inverter. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.nanolett.5b00079.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Author Contributions
∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This work was supported by Institute for Basic Science (IBS),
Korea, under the Project Code (IBS-R014-G1).
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