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 © 2015 American Chemical Society
Received: January 8, 2015 Revised: April 11, 2015 Published: May 1, 2015 3703
DOI: 10.1021/acs.nanolett.5b00079 Nano Lett. 2015, 15, 3703−3708
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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. 3704
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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 3705
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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 3706
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Nano Letters 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.
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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.
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ACKNOWLEDGMENTS This work was supported by Institute for Basic Science (IBS), Korea, under the Project Code (IBS-R014-G1). REFERENCES
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