Abstract

The creation of crystal phase heterostructures of transition metal chalcogenides, e.g., the 1T/2H heterostructures, has led to the formation of metal/semiconductor junctions with low potential barriers. Very differently, post-transition metal chalcogenides are semiconductors regardless of their phases. Herein, we report, based on experimental and simulation results, that alloying between 1T-SnS2 and 1T-WS2 induces a charge redistribution in Sn and W to realize metallic Sn0.5W0.5S2 nanosheets. These nanosheets are epitaxially deposited on surfaces of semiconducting SnS2 nanoplates to form vertical heterostructures. The ohmic-like contact formed at the Sn0.5W0.5S2/SnS2 heterointerface affords rapid transport of charge carriers, and allows for the fabrication of fast photodetectors. Such facile charge transfer, combined with a high surface affinity for acetone molecules, further enables their use as highly selective 100 ppb level acetone sensors. Our work suggests that combining compositional and structural control in solution-phase epitaxy holds promises for solution-processible thin-film optoelectronics and sensors.

Introduction

Heterostructures constructed from layered materials such as graphene, metal chalcogenides and black phosphorus (BP) have aroused particular interest due to their combined advantageous features and the emergence of unusual properties/functions1,2,3,4,5,6. As a group of the mostly studied layered materials, transition metal chalcogenides (TMCs) like MoS2 and WS2 exist in different crystal phases such as the 2H and 1T polytypes with distinct electronic properties. Their semiconductor-to-metal transition (i.e., 2H-to-1T) can be realized via Li-intercalation,7 strain engineering,8 e-beam/laser irradiation2 or doping/alloying2,7,8,9. This attractive feature allows for the formation of 2H/1T (semiconductor/metal) phase junctions that exhibit much reduced contact resistance compared to that of using noble metal contacts (e.g., Au) which normally interface with semiconducting TMCs with large Fermi level misalignment2,10. Besides TMCs, post-TMCs such as SnS2 and InSe are another important group of layered materials, exhibiting attractive electronic and optoelectronic properties for a wide range of applications, including transistors, photodetectors, and sensors11,12. Similar to many other semiconducting materials, the type of metal contact with post-TMCs plays a critical role in tuning their functional performance13. However, due to the less metallic nature of post-transition metals, post-TMCs are generally semiconductors regardless of their crystal phases, such as the 2H, 4H, and 1T polytypes14. Consequently, contacting post-TMCs with metallic layered materials to achieve low interfacial resistance remains a big challenge.

To date, much effort has been devoted to the preparation of various heterostructures based on layered materials, which display different geometric arrangements such as the lateral and vertical heterostructures1,3,15. In particular, to prepare vertical heterostructures in which dissimilar layered crystals are stacked one above the other in a pre-designed sequence, solid-state procedures, including dry transfer, chemical vapor deposition (CVD) and chemical vapor transport (CVT) method have mostly been applied3,16,17. This is because these methods allow good control over the spatial arrangement of the layers as well as the deposition sequence. Compared to these solid-state methods, solution-phased approaches are advantageous in terms of relatively easier procedures, low-cost setups, and most importantly, scalability18,19. However, direct wet-chemical growth of vertical heterostructures of layered metal chalcogenides has thus far been challenging.

In view of their potential applications, heterostructures/heterojunctions such as InSe/graphene, MoTe2/MoS2, MoS2/perovskite, and graphene/MoS2/graphene have recently shown promising performance in photodetectors, due to the improved charge separation/transport and enhanced light adsorption20,21,22,23. Meanwhile, development of gas sensors for detection of volatile organic compounds are important in applications such as environmental monitoring and non-invasive diagnosis of diseases based on breath analysis24,25. Chemiresistive sensors based on metal oxides/sulfides have been used for detection of volatile organic compounds, however, high operating temperatures (typically ≥150 °C) are usually required to achieve good sensitivity and selectivity24,26. Very recently, layered materials such as SnS2, WS2 and Ti3C2Tx have demonstrated great potential for room-temperature gas detection27,28,29. It is expected that creation of heterostructures may realize further improved sensing performance30,31,32.

In this contribution, nanoplates of SnS2, a typical n-type semiconductor, are used as synthesis templates for the surface deposition of layered Sn0.5W0.5S2 nanosheets, which show 83% metallic phase, leading to the formation of metal/semiconductor vertical heterostructures. Kelvin probe force microscope (KPFM) and tunneling atomic force microscopy (TUNA) analyses suggest the formation of ohmic-like contact at the Sn0.5W0.5S2/SnS2 interface. The resultant heterostructures are fabricated into chemiresistive sensors to detect acetone at room temperature and exhibit a good selectivity and a minimum detectable concentration down to 100 ppb. The good sensing performance could be attributed to the low charge transfer resistance at the Sn0.5W0.5S2/SnS2 interface that enables a much increased signal-to-noise ratio, and the alloying induced enhancement in surface gas adsorption.

Results

Synthesis and characterizations of Sn0.5W0.5S2/SnS2

As a representative post-TMC, SnS2 has been widely studied and applied in phototransistors and sensors for its favorable band structure and relatively high surface electronegativity12,33. Typically, SnS2 nanoplates were synthesized via a hydrothermal reaction with thiourea (CS(NH2)2) and tin tetrachloride hydrate (SnCl4•5H2O) as the precursors for S and Sn, respectively (see the Methods section for the detailed procedure)34. As shown in the scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscopy (AFM) images in Supplementary Figs. 1 and 2, hexagonal nanoplates with edge lengths ranging from 200 to 700 nm and an average thickness of 43 nm were obtained. As confirmed by the selected area electron diffraction (SAED) and X-ray diffraction (XRD) analyses, the nanoplates are α-SnS2 with a 1T structure (space group P\(\bar 3\)m1), where a = 3.65 and c = 5.90 (ICSD no. 42566)12 (Supplementary Figs. 1c and d). By adding (NH4)10H2(W2O7)6 to the precursors of the aforementioned synthesis solution, alloyed Sn1–xWxS2 nanosheets were in-situ synthesized and hybridized with SnS2 nanoplates as shown in Fig. 1a, b. These heterostructures show an average lateral size of 750 nm (Fig. 1a) and an average thickness of 60 nm (Fig. 1c and Supplementary Fig. 3). The side-view TEM image in Fig. 1d clearly shows a SnS2 nanoplate covered by Sn1–xWxS2 nanosheets on both its basal faces forming a vertical heterostructure. The deposited Sn1–xWxS2 nanosheets are typically 6–9 nm in thickness (Supplementary Fig. 4). Energy dispersive X-ray spectroscopy (EDX) mapping of a typical heterostructure reveals the distribution of Sn, W, and S elements (Fig. 1e), in which the center of the heterostructure shows a higher concentration of Sn compared to the edge. EDX line analysis of the cross-section of a typical heterostructure, which was prepared by cutting with focused ion beam (FIB), further indicates that the SnS2 nanoplate was covered by Sn1–xWxS2 nanosheets (Fig. 1f). EDX spot analyses on edges of Sn1–xWxS2 nanosheets suggest x ≈ 0.5, confirming Sn0.5W0.5S2 nanosheets were obtained (Supplementary Fig. 5).

Fig. 1
Fig. 1

Morphology and composition analyses of Sn0.5W0.5S2/SnS2 heterostructures. a SEM image of as-prepared Sn1–xWxS2/SnS2 heterostructures (scale bar, 1 μm). Inset: size distribution of Sn1–xWxS2/SnS2 heterstructures. b Top-view TEM image of typical Sn1–xWxS2/SnS2 heterostructures (scale bar, 200 nm). Inset: photograph of a solution of Sn1–xWxS2/SnS2 heterostructures showing the Tyndall effect. c AFM image and height analysis of a Sn1–xWxS2/SnS2 heterostructure (scale bar, 100 nm). Inset: thickness distribution of Sn1–xWxS2/SnS2 heterstructures, showing a mean value of 60 nm. d Side-view TEM image of a typical Sn1–xWxS2/SnS2 heterostructure, revealing Sn1–xWxS2 nanosheets grown on both the top and bottom basal faces of a SnS2 nanoplate (scale bar, 10 nm). e STEM image and EDX mapping on a typical Sn1–xWxS2/SnS2 heterostructure (scale bar, 100 nm). f Cross-sectional STEM image and EDX line analysis on a typical Sn1–xWxS2/SnS2 heterostructure (scale bar, 10 nm)

The structural properties of the heterostructures were investigated with XRD and high resolution TEM (HRTEM) analysis. In the XRD pattern in Fig. 2a, besides the characteristic peaks for 1T-SnS2, two relatively broader peaks observed at 8.9° and 17.8° could be attributed, respectively, to the (001) and (002) planes of Sn0.5W0.5S2 nanosheets with an enlarged interlayer spacing of about 1.0 nm. The enlarged spacing, which was also observed in side-view HRTEM images (Supplementary Fig. 6), may result from the use of CS(NH2)2 and (NH4)10H2(W2O7)6 in our synthesis, leading to the intercalation of NH4+ ions in between the adjacent layers35. This was also confirmed by X-ray photoelectron spectroscopy (XPS) analysis (Supplementary Fig. 7). The crystal structure of a typical heterostructure was further investigated by taking its SAED pattern. As shown in Fig. 2b, two sets of patterns both along the [001] zone axis show the epitaxial registry. The hexagonal pattern with six inner and six outer spots can be assigned to 1T-SnS2, and the measured (100) lattice spacing is 3.16 Å, in good agreement with the XRD result (Fig. 2a). The other set of pattern for Sn0.5W0.5S2 shows elongated spots, forming discontinued ring segments. This may result from the curled structure of the Sn0.5W0.5S2 nanosheets as well as possible misorientation. The measured lattice spacing for Sn0.5W0.5S2 (100) planes was 3.0 Å, corresponding to a lattice parameter of a = 3.46 Å. This reasonably falls in between that of WS2 (a = 3.16 Å) and SnS2 (a = 3.65 Å). Note that the mismatch between the (100) planes of SnS2 and Sn0.5W0.5S2 is 5%, which can be tolerated in van der Waals epitaxial growth of layered materials36. The top-view HRTEM image in Fig. 2c distinctly shows the relatively darker region for the SnS2 nanoplate covered with Sn0.5W0.5S2, and the brighter region for the periphery Sn0.5W0.5S2 nanosheet. The lattice fringes extend continuously from the SnS2/Sn0.5W0.5S2 center to the Sn0.5W0.5S2 edge, further confirming the epitaxial growth mode. Moiré patterns could be observed in some areas due to the overlap of Sn0.5W0.5S2 and SnS2 at small misorientation angles. For example, as shown in Fig. 2d, a Moiré pattern with a periodicity of 4.0 nm was observed when the Sn0.5W0.5S2 overlayer made a misorientation angle of 3° with SnS2 (see the detailed analysis in Supplementary Fig. 8). Note that the overlapping of two hexagonal lattice patterns normally produces a hexagonal Moiré pattern37, which was not observed in the present work. This may be due to the fact that the Sn0.5W0.5S2 nanosheets showed lattice distortion with varied interlayer spacings (0.6–1.0 nm, Supplementary Fig. 6), and thus were deviated from being perfectly flat on the SnS2 nanoplate. As a result, only short-range line-like Moiré patterns were observed38. High resolution scanning TEM (STEM) images of the edge area of a heterostructure show the 1T-phase-like atomic arrangement of Sn0.5W0.5S2 (Supplementary Fig. 9)39. It is interesting that zigzag lattice patterns for 1T’ or Td phases which have been previously observed in TMCs such as WS2 and WTe240,41 were not observed in Sn0.5W0.5S2. Based on our density functional theory (DFT) calculation results shown in Supplementary Table 1 and Supplementary Fig. 10, the W–S bonds tend to be shorter than the Sn–S bonds in the alloyed system, resulting in a distortion from the perfect in-plane symmetric 1T lattice. The distorted 1T structure was also reflected in the Raman spectrum in Supplementary Fig. 11, where in addition to the peaks at 310, 351, and 414 cm−1 that correspond to the SnS2-like A1g, WS2-like E2g and WS2-like A1g modes, respectively, the active modes observed at 171 cm−1 and 224 cm−1 in the lower frequency region could be attributed to distorted 1T-WS212,35. The dominant distorted 1T-phased W–S coordination was further confirmed by XPS analysis (Fig. 2e). It has been reported that the XPS band positions of a metal element are sensitive to their oxidation states, coordination geometries, and Fermi levels42,43. Normally in the Wf spectrum, the doublet peaks (32.1 and 34.2 eV) associated with W in the 1T-phased structure are downshifted by 0.6 eV relative to those associated with W in the 2H structure (32.7 and 34.8 eV). The deconvolution of the W 4f bands thus could enable the quantitative estimation of the 1T and 2H phase concentrations7. In our case, from the deconvoluted peak areas (Fig. 2e), the concentrations of the distorted 1T and 2H phases were calculated to be 83% and 17%, respectively. For the Sn 3d spectrum, the peak positions match well with the 1T-SnS2 structure (Fig. 2f)44.

Fig. 2
Fig. 2

Structural properties of Sn0.5W0.5S2/SnS2 heterostructures. a XRD pattern of Sn0.5W0.5S2/SnS2 heterostructures deposited on a glass slide. b SAED patterns of a Sn0.5W0.5S2/SnS2 heterostructure along the [001] zone axis (scale bar, 2 nm−1). c HRTEM image of a typical Sn0.5W0.5S2/SnS2 heterostructure lying flatly on a copper grid (scale bar, 2 nm). d A Moiré pattern with a periodicity of 4.0 nm was observed (scale bar, 5 nm), whose fast Fourier transform (FFT) diffraction pattern is shown as the inset. XPS (e) W 4f and f Sn 3d spectra of as-prepared Sn0.5W0.5S2/SnS2 heterostructures

The formation of 1T or distorted 1T structures have been observed previously in TMCs when they were intercalated with alkali metal ions (e.g., Li+, K+ etc.)39,45 or synthesized in the presence of ammonium containing precursors35 and hydrazine hydrate46. According to previous theoretical calculations, the presence of the positive counterions could cause an increase of the electron density of the d-orbital of the transition metals, leading to the stabilization of the 1T or distorted 1T phase47. Therefore, the realization of the distorted 1T-Sn0.5W0.5S2 in our present work might also be a result of the intercalated NH4+ cations from CS(NH2)2 and (NH4)10H2(W2O7)6 used in the synthesis solution.

Formation process of Sn0.5W0.5S2/SnS2

To investigate the formation process of the Sn0.5W0.5S2/SnS2 vertical heterostructures, intermediate products were collected at different reaction intervals for characterizations. At the beginning of the synthesis process, the precursors, i.e., (NH4)10H2(W2O7)6 and SnCl4·5H2O, reacted to produce Sn(HWO4)2•nH2O amorphous particles upon mixing at 80 °C (Supplementary Fig. 12)48, which were subsequently heated up to 220 °C under hydrothermal conditions. After the reaction had proceeded for 12 h at 220 °C, nanorods with lengths of 10–100 nm were observed together with SnS2 nanoplates in the solution (Fig. 3a, b). These nanorods are alloyed oxide of Sn and W with a formula of Sn0.17WO3 based on XRD pattern (ICSD No. 38043 [https://icsd.fiz-karlsruhe.de], Fig. 3c)49, EDX analysis (Supplementary Fig. 13a, b) and HRTEM imaging (Supplementary Fig. 13c). The reason why WS2 was not produced at this stage is that the bond energy of Sn–S was likely to be lower than that of W–S due to the larger ionic radius of Sn4+ compared to that of W4+50,51. As the reaction proceeded further, the amount of the Sn0.17WO3 nanorods decreased, and nanosheets started to form on the surfaces of the SnS2 nanoplates (Supplementary Fig. 14). Evidently, the evolution of the XRD patterns of the intermediate products indicates a decrease of the Sn0.17WO3 amount over time (Fig. 3c), which is accompanied with an increase of the Sn and W ion concentration in the solution based on the inductively coupled plasma mass spectrometry (ICP-MS) measurements (Supplementary Table 2). Based on our control experiments, Sn0.17WO3 nanorods could form at 180 °C and decompose at temperatures above 200 °C (Supplementary Fig. 15). This suggests that the Sn0.17WO3 nanorods forming at the beginning of the hydrothermal reaction gradually decomposed at 220 °C (step 1 in Fig. 3d), providing additional Sn and W ions with a high W/Sn ratio (>30) (Supplementary Table 2). Such a high W/Sn ratio could drive the growth of alloyed Sn0.5W0.5S2 nanosheets on the surface of SnS2 even though the formation of W–S bond is less favored compared to that of the Sn–S bond (step 2 in Fig. 3d). In addition, we also tried to extend the Sn–W binary system to Sn–Mo system, but found that, under certain conditions, Sn1–xMoxS2 nanosheets grew epitaxially on SnS2 mainly via the edge growth. This phenomenon might be due to the different synthesis energies required for basal growth or edge growth3, which requires our further investigation.

Fig. 3
Fig. 3

Formation process of Sn0.5W0.5S2/SnS2 heterostructures. a SEM image of the intermediate product obtained after the reaction proceeded for 12 h (scale bar, 200 nm). b TEM image of typical Sn0.17WO3 nanorods (scale bar, 100 nm). c XRD patterns of the intermediate products obtained at different reaction intervals. d Schematic illustration of the formation process of Sn0.5W0.5S2/SnS2 heterostructures. SnS2 nanoplates and Sn0.17WO3 nanorods formed together during the initial 12 h. After that the Sn0.17WO3 nanorods began to decompose (step 1), providing additional W and Sn ions for Sn0.5W0.5S2 nanosheets to grow on the surfaces of the SnS2 nanoplates (step 2)

Electronic properties of Sn0.5W0.5S2/SnS2

Alloying has been a powerful approach to tune the bandgaps of TMCs or to achieve their semiconductor-to-metal transitions9,52. However, alloying between layered TMC and post-TMC has not been reported so far, and thus the electronic properties of such alloys could be of great interest for potential applications. DFT calculations were performed to understand the electronic properties of the distorted 1T-Sn0.5W0.5S2 deposited on 1T-SnS2 with and without NH4+ intercalation. The detailed results of the optimized crystal structures, calculated Bader charges, band structures and density of states (DOS) are shown in Fig. 4, Supplementary Figs 16 and 17 and Supplementary Table 1. In contrast to a five-layer semiconducting 1T-SnS2 which shows no DOS at the Fermi level (Fig. 4b), a four-layer distorted 1T-Sn0.5W0.5S2 on a monolayer 1T-SnS2 exhibits intrinsic metallic behavior with observable DOS at the Fermi level, dominantly contributed from the W and S atoms and slightly from the Sn atoms (Fig. 4d). The calculated Bader charge of W atoms in distorted 1T-Sn0.5W0.5S2 is 0.20 e higher compared to that in 1T-WS2, whereas the Bader charge of Sn atoms in distorted 1T-Sn0.5W0.5S2 is 0.10 e lower compared to that in 1T-SnS2 (Fig. 4a, c and Supplementary Fig. 16). This suggests a charge redistribution in the Sn0.5W0.5S2 alloy by charge transfer from W to Sn atoms53. Similarly, the distorted 1T-Sn0.5W0.5S2 with intercalated NH4+ ions (20 mol%) also shows the metallic behavior (Supplementary Fig. 17). To experimentally verify the calculated results, the electronic properties of the SnS2 nanoplates and Sn0.5W0.5S2/SnS2 heterostructures were measured by fabrication of back-gated thin film field effect transistors (Supplementary Fig. 18). The drain current (Id) vs. drain-source voltage (Vds) curves at varied gate voltages reveal that the SnS2 nanoplates are typical n-type semiconductors (Supplementary Fig. 18a, b)12. In contrast, the IV curves of films prepared from Sn0.5W0.5S2/SnS2 heterostructures are almost insensitive to gate voltages (Supplementary Fig. 18c, d), suggesting the metallic charge transport through the Sn0.5W0.5S2 nanosheets, consistent with the theoretical predictions.

Fig. 4
Fig. 4

Calculated electronic structure of SnS2 and Sn0.5W0.5S2/SnS2 heterostructures. a Optimized crystal structure with calculated Bader charges. b Band structure and DOS of a 5-layer SnS2, showing an indirect band gap near the Fermi level. c Optimized crystal structure with calculated Bader charges. d Band structure and DOS of a four-layer 1T-Sn0.5W0.5S2 on a monolayer 1T-SnS2, showing the intrinsic metallic characteristic. The Fermi level is assigned at 0 eV

To exmamine the interface property between a SnS2 nanoplate and the surface deposited Sn0.5W0.5S2, its surface potential, which corelates to its work function, was analyzed by KPFM in air21. A PtIr tip was used as the probe, and a SiO2/Si substrate sputtered with Au/Cr with a theoretical work function (ɸAu) of 5.100 eV was used as the potential reference. The 2D potential image of a typical Sn0.5W0.5S2/SnS2 heterostructure is shown in Fig. 5a, where the color variation reflects the local surface potential difference (ΔV = ɸAu − ɸsample) (details on the calculation of the surface potentials are given in the Methods section). It can be seen that the surface potential of the Sn0.5W0.5S2/SnS2 heterostructure is 0.029 V lower than that of Au to give an estimated work function of 5.071 eV (Fig. 5a). On the other hand, an SnS2 nanoplate exhibits a work function of 5.110 eV (Fig. 5b). The decrease in surface potential after deposition of Sn0.5W0.5S2 on SnS2 suggests that the metallic Sn0.5W0.5S2 possesses a lower work function than that of the n-type SnS2. Although based on DFT calcuations, the work function of distorted 1T-Sn0.5W0.5S2 is 5.62 eV which is much higher than that of SnS2, it can be substantially lowered to 2.87 eV by introducing an NH4+ intercalation with a molar concentration of about 20% (Supplementary Table 1). Such work function modulation induced by doping or chemical absorbates has also been reported previously54. This also explains why the experimentally measured work function of Sn0.5W0.5S2 nanosheets with partial NH4+ intercalation was lower than that of SnS2. Therefore, an ohmic contact should form at the Sn0.5W0.5S2/SnS2 heterointerface (Fig. 5c), affording a low charge transfer resistance. This was further confirmed by measuring IV curves on individual SnS2 nanoplates or Sn0.5W0.5S2/SnS2 heterostructures deposited on highly oriented pyrolytic graphite (HOPG) with TUNA (Fig. 5d, e). The IV curve for a SnS2 nanoplate is highly asymmetric and the onset of the current rectification is at 3.50 V (Fig. 5d), indicating the presence of Schottky barrier at the PtIr tip/n-type SnS2 interface, provided that the work function for PtIr, HOPG, and SnS2 is 5.50, 4.60, and 5.11 eV, respectively55,56. In sharp contrast, the IV curve for Sn0.5W0.5S2/SnS2 is almost linear and symmetric with respect to 0 V, suggesting that the contact at SnS2/Sn0.5W0.5S2 interface is ohmic-like (Fig. 5e)57.

Fig. 5
Fig. 5

KPFM and TUNA analyses of SnS2 and Sn0.5W0.5S2/SnS2 heterostructures. 2D potential images of a a typical Sn0.5W0.5S2/SnS2 heterostructure (scale bar, 200 nm) and b a typical SnS2 nanoplate deposited on a SiO2/Si substrate coated with a thin film of Au/Cr (i.e., Au/Cr/SiO2/Si) (scale bar, 100 nm). c Schematic band alignment diagram for Sn0.5W0.5S2 and SnS2 before and after contact. EF, ECB, and EVB denote Fermi level, conduction band and valence band, respectively. IV curves measured with TUNA for d a SnS2 nanoplate and e a Sn0.5W0.5S2/SnS2 heterostructure, under a constant force and an applied bias voltage that was linearly ramped down

Sn0.5W0.5S2/SnS2 for photodetectors

The advantage of the facile charge transport across the ohmic-like heterointerfaces was demonstrated in fabrication of thin film photodetectors based on the Sn0.5W0.5S2/SnS2 heterostructures. Figure 6a shows the IV curves of the device under 405 nm laser illumination with power intensity varied from 0.45 to 1.05 mW. A clear rise of the photocurrent with increasing illumination intensity was observed, indicating the effective conversion of photon flux to photogenerated carriers. In addition, the Sn0.5W0.5S2/SnS2 photodetector showed symmetric and linear IV plots, which is in sharp contrast to the non-linear IV curves observed for SnS2-based device (Supplementary Fig. 19a). This further indicates the low resistance contact formed between the semiconducting and metallic components in Sn0.5W0.5S2/SnS2. The temporal photoresponse of the photodetectors was measured as well as shown in Fig. 6b, c and Supplementary Figs. 19b, c. The Sn0.5W0.5S2/SnS2 photodetector showed an abrupt rise of photocurrent with a fast response time of 42.1 ms (defined as the time required to increase 90% from the minimum to maximum current density), which is comparable and outperforms some previously reported TMC based photodetectors58,59. This value is also about 50 times shorter than that of the SnS2-based photodetector (2.10 s) (Supplementary Fig. 19c). Such markedly shortened response time suggests the rapid transport of charge carriers across the Sn0.5W0.5S2/SnS2 heterointerfaces60,61. Note that a relatively large dark current and thus a much reduced on/off ratio were observed for the Sn0.5W0.5S2/SnS2-based device as compared with the SnS2 device. This was due to the metallic nature of the Sn0.5W0.5S2 nanosheets. The similar phenomenon was reported previously in photodetectors based on graphene composites62,63.

Fig. 6
Fig. 6

Photodetector performance of Sn0.5W0.5S2/SnS2 heterostructures. a IV curves at different light intensity, b temporal photocurrent response and c a zoom-in view of the temporal photocurrent response of a photodetector based on Sn0.5W0.5S2/SnS2 heterostructures. The light source used for all measurements was a 405 nm laser

Sn0.5W0.5S2/SnS2 for gas sensing

The presence of a metallic component in a thin film channel may pose some limitation to its optoelectronic performance, such as the relatively large dark current observed in the aforementioned Sn0.5W0.5S2/SnS2 photodetector. However, the metallic structure might become an advantage for applications like sensors. As another proof of concept demonstration of the advantage of solution-processible functional materials, the Sn0.5W0.5S2/SnS2 heterostructures were deposited on Au interdigitated electrodes to fabricate chemiresistive gas sensors for detection of volatile organic compounds such as acetone, which is a potential biomarker for diabetes and lung cancer64. For comparison, SnS2-based gas sensors were also fabricated. The response–recovery curves of the gas sensors were measured under gas flows with increasing concentrations (typically 0.1–50 ppm) at room temperature (Fig. 7a, b and Supplementary Figs. 20 and 21). Taking sensing of acetone for example, the resistance of the Sn0.5W0.5S2/SnS2 sensor decreased upon exposure to acetone and the decrease in resistance (ΔR = Ra – R0) with increasing acetone concentration (Fig. 7a, b and Supplementary Fig. 20). A minimum detectable concentration of 0.1 ppm was achieved. This is 20 times lower compared to that of the sensor based on SnS2 which only afforded a minimum detectable concentration of 2 ppm (Supplementary Fig. 21), and to the best of our knowledge, outperforms other reported metal sulfide/oxide chemiresistive acetone sensors. More importantly, our sensor showed the best sensitivity (i.e., sensing response, ΔR/R0) at 100 ppb levels among all reported chemiresistive sensors operating at room temperature (Supplementary Table 3). Furthermore, as shown in Supplementary Fig. 22, a typical Sn0.5W0.5S2/SnS2 sensor was repeatedly exposed to 1 ppm acetone and then back to N2 gas for 10 cycles and showed an almost constant sensing response of 1.88 ± 0.07% (by taking standard deviation of the results from 10 cycles), indicating its good repeatability.

Fig. 7
Fig. 7

Gas sensing performance of Sn0.5W0.5S2/SnS2 heterostructures. a Response–recovery curves of a typical chemiresistive sensor fabricated from Sn0.5W0.5S2/SnS2 heterostructures in response to acetone gas with increasing concentrations. Inset: zoom-in response of the sensor towards 0.1 and 0.2 ppm acetone. b Normalized change of resistance of Sn0.5W0.5S2/SnS2 sensors at various acetone concentrations. Inset: zoom-in normalized change of resistance at low acetone concentrations. Each error bar indicates the standard deviation of the change of resistance for 5 experimental replicates. c Calculated adsorption energy, Ea (eV), of acetone on different sensing materials. d Side view of the fully relaxed structural model of Sn0.5W0.5S2 with surface adsorption of an acetone molecule. Cyan regions indicate charge accumulation, while pink regions represent charge depletion. e Comparison of the responses of the sensor towards different gases, including acetone, diethyl ether, propanal, toluene, and NO2

The improved sensitivity after deposition of Sn0.5W0.5S2 nanosheets on SnS2 nanoplates could be attributed to the following reasons. First, the formation of an ohmic-like contact between Sn0.5W0.5S2 and SnS2 that allowed rapid charge transfer across the metal–semiconductor interface partly led to a 35 times reduction in background noise and thus a much higher signal to noise (S/N) ratio (Supplementary Fig. 23)27. Second, it can be noted that, the response/recovery time of the Sn0.5W0.5S2/SnS2 gas sensor was longer than that of the SnS2 gas sensor (Fig. 7a and Supplementary Fig. 21), pointing to a chemical adsorption-related sensing pathway64,65. To confirm this, we calculated the adsorption energy of acetone on the different sensing materials (Fig. 7c). The adsorption energy of acetone on Sn0.5W0.5S2 is 2.37 eV is much larger compared to that on SnS2 (0.40 eV). This indicates that acetone molecules interact more strongly with Sn0.5W0.5S2. It is worth noting that the NH4+-intercalation did not significantly change the adsorption ability of Sn0.5W0.5S2 nanosheets towards acetone, with the adsorption energy slightly reduced by 0.09 eV. As further shown in Fig. 7d, there is an obvious charge accumulation on Sn0.5W0.5S2 due to charge transfer from the absorbed acetone molecule. Third, one of the advantages of using low-dimensional materials in gas sensing as compared with bulk crystals is their large specific surface areas, which are beneficial for providing large interfaces for channel–gas interaction25. Indeed, the deposition of wrinkled Sn0.5W0.5S2 nanosheets on SnS2 nanoplates increased the specific surface areas from typically 6.25 to 11.37 m²/g based on the Brunauer–Emmett–Teller (BET) measurements as shown in Supplementary Fig. 24. However, in spite of the beneficial effects from the large specific surface area of the wrinkled Sn0.5W0.5S2 nanosheets and their strong interaction with acetone, increasing the amount of Sn0.5W0.5S2 nanosheets on SnS2 did not further improve the sensing performance, but on the contrary, led to poorer sensitivity with a minimal detectable concentration of only 1 ppm (Supplementary Fig. 25). This suggests that the concentration of the metallic phase present in the hybrid sensing film should not be too high, otherwise, the gas-induced doping effect on the semiconducting SnS2 would be substantially weakened. Therefore, the presence of the semiconductor/metal heterostructures with a low charge transfer barrier, combined with sufficient active surfaces for strong gas adsorption are important in achieving low sensitivity in our thin film based gas sensors.

The selectivity of the Sn0.5W0.5S2/SnS2 sensor was also investigated by comparing its sensing response towards acetone with other electron donating gases like diethyl ether and propanal, neutral gas like toluene, and electron withdrawing gas like NO2 (Fig. 7e, Supplementary Fig. 26). The response of the Sn0.5W0.5S2/SnS2 sensor towards diethyl ether and propanal was 10 times lower compared to that towards acetone at 2 ppm. This is largely due to the weaker electron donating ability of diethyl ether and propanal compared to that of acetone (Fig. 7e). Under the exposure of toluene, the sensor showed no response. In sharp contrast, when responding to NO2, an electron withdrawing gas, the sensor showed increased resistance (Fig. 7e, Supplementary Fig. 26e), agreeing with the fact that SnS2 is an n-type semiconductor (Supplementary Fig. 18b).

Discussion

The epitaxial growth of metallic Sn0.5W0.5S2 nanosheets on the surfaces of SnS2 nanoplates were realized via a solution-phase epitaxial deposition process. Importantly, an alloyed metal oxide (i.e., Sn0.17WO3) was identified as an intermediate product which formed at 180 °C and decomposed subsequently at 220 °C to provide additional Sn and W ions with a high W/Sn ratio of about 30. We proposed that this high W/Sn concentration could drive the formation of alloyed Sn0.5W0.5S2 nanosheets despite the fact that the formation of Sn–S bond requires less energy compared to that of the W–S bond. Although 1T-SnS2 is a semiconductor, alloyed Sn0.5W0.5S2 showed 83% distorted 1T structure, which is metal-like as predicted by theoretical calculations. KPFM and TUNA measurements on a Sn0.5W0.5S2/SnS2 heterostructure suggested the formation of ohmic-like contact at the heterointerface, resulting in a low charge transfer resistance. The rapid charge transport at the Sn0.5W0.5S2/SnS2 heterointerface allowed for the fabrication of fast photodetector with a short response time of 42 ms. Additionally, when the Sn0.5W0.5S2/SnS2 heterostructures were fabricated into thin films for gas sensing, a much enhanced signal-to-noise ratio was achieved partly due to the presence of metallic Sn0.5W0.5S2 layers. Furthermore, Sn0.5W0.5S2 showed much enhanced surface adsorption of acetone than SnS2 based on theoretical calculations. As a consequence, selective and sensitive detection of acetone was achieved with an ultralow minimum detectable concentration of 100 ppb at room temperature. This, combined with large sensing responses at 100 ppb levels outperforms previously reported room-temperature chemiresistive sensors. Our use of semiconducting SnS2 nanoplates as synthesis templates for the solution-phase epitaxial growth of metallic Sn0.5W0.5S2 nanosheets demonstrates a promising way towards the facile, economic and high-yield preparation of functional hybrid nanomaterials. Synthesis of layered materials through alloying among elements with distinct electronic and chemical properties is expected to bring about more materials and unusual phenomena.

Methods

Materials

Ammonium tungstate hydrate ((NH4)10H2(W2O7)6, 99.99%), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 99.0%), thiourea (CS(NH2)2, 99.0%) and tin tetrachloride hydrate (SnCl4·5H2O, 99.9%) were purchased from Sigma-Aldrich (Shanghai, China). Ethanol (C2H5OH, ACS, 99.9%) was purchased from J&K chemical (Shanghai, China). The gaseous analytes (CH3COCH3, (CH3CH2)2O, CH3CH2CHO, C6H5CH3, and NO2) which were diluted with N2 gas at concentrations of 1000 ppm were purchased from Nanjing Teqi Co., Ltd. All chemicals were used as received without further purification. The deionized (DI) water was purified using Milli-Q3 System (Millipore, France).

Preparation of SnS2 nanoplates

In a typical process, 0.25 mmol of SnCl4·5H2O and 3.75 mmol of CS(NH2)2 were dissolved in 19.45 mL DI water and stirred for 2 h to form a homogeneous solution. This solution was transferred to a 25 mL Teflon-lined stainless steel autoclave, heated to 220 °C in an electrical oven and then maintained at this temperature for 12 h before being cooled down naturally to room temperature. The obtained product was centrifuged at 8000 rpm for 10 min, and the precipitate was washed with DI water for three times before further characterization.

Preparation of Sn0.5W0.5S2/SnS2 heterostructures

In a typical process, 0.25 mmol of (NH4)10H2(W2O7)6, 7.5 mmol of CS(NH2)2, and 0.5–0.625 mmol of SnCl4·5H2O were dissolved in 19.45 mL DI water and stirred at 80 °C for 2 h to form a homogeneous solution. This solution was then transferred to a 25 mL Teflon-lined stainless steel autoclave, heated to 220 °C in an electrical oven and then maintained at this temperature for 60 h before being cooled down naturally to room temperature. The obtained product was centrifuged at 8000 rpm for 10 min, and the precipitate was washed with DI water for three times before further characterization.

Characterizations

Scanning electron microscope (SEM, JEOL JSM-7800F, Japan), transmission electron microscope (TEM, JEOL 2100Plus, Japan) and high resolution transmission electron microscope (HRTEM, JEOL 2100 F, Japan) coupled with energy dispersive X-ray (EDX) spectroscope were used to investigate the compositional, morphological and structural features of the samples. X-ray diffraction (XRD, Rigaku SmartLab, Japan) was performed using CuKα radiation (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Japan) measurements were conducted on the different metal sulfide nanostructures, and the binding energies were corrected for specimen charging effects using the C 1 s level at 284.6 eV as the reference. Raman spectra (Horiba HR800, France) of the samples were collected with a 532 nm laser. Semiconductor parameter analyzer (Tektronix Keithley 4200, America) and probe station (Lake Shore TTPX, America) were used to investigate the semiconductor properties of the samples. A commercial atomic force microscope (AFM, Dimension ICON with Nanoscope V controller, Bruker) was used to investigate the electrical properties of the individual nanostructures in air. Inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700×, America) was used to measure the concentration of Sn and W ions in the synthesis solution. Brunauer–Emmett–Teller (BET, Micromeritics, ASAP2460, USA) measurements were carried out to determine the specific surface area and pore size distribution of various samples.

Semiconducting property characterization

After Au (50 nm)/Cr (30 nm) drain and source electrodes were deposited onto a SiO2 (285 nm)/Si substrate via thermal evaporation through a shadow mask, SnS2 nanoplates or Sn0.5W0.5S2/SnS2 heterostructures in water were drop-casted onto the substrate, acting as the channel to connect the drain and source electrodes with a channel length of 11 μm. The semiconducting properties of the channel materials were then characterized using a Keithley 4200 semiconductor characterization system operating at 77 K in vacuum (5 × 10−5 Torr).

KPFM and TUNA measurements

After Au (50 nm)/Cr (30 nm) was coated onto a SiO2 (285 nm)/Si substrate via thermal evaporation, SnS2 nanoplates (or Sn0.5W0.5S2/SnS2 heterostructures) in water were drop-casted onto the substrate. A KPFM (Dimension ICON with Nanoscope V controller, Bruker) was then used to characterize the surface potential of the SnS2 nanoplates (or Sn0.5W0.5S2/SnS2 heterostructures) at ambient conditions. The contact potential difference between the tip (PtIr) and the sample surface (VCPD), which is also referred to as the surface potential can be calculated by using the following equations:

$$V_{{\mathrm{CPD}}}{\mathrm{ = }}\frac{1}{e}\left( {\varphi _{\mathrm{t}} - \varphi _{\mathrm{f}}} \right)$$
(1)
$$\Delta V_{{\mathrm{CPD}}} = \Delta V_{{\mathrm{CPD}}}\left( {{\mathrm{film}}} \right) - \Delta V_{{\mathrm{CPD}}}\left( {{\mathrm{substrate}}} \right)$$
$$\Delta V_{{\mathrm{CPD}}}\,=\,\frac{1}{e}\left( {\varphi _{\mathrm{t}} - \varphi _{\mathrm{f}}} \right) - \frac{1}{e}\left( {\varphi _{\mathrm{t}} - \varphi _{\mathrm{s}}} \right) = \frac{1}{e}\left( {\varphi _{\mathrm{s}} - \varphi _{\mathrm{f}}} \right)$$
(2)

where φt, φs, and φf represent the work functions of the probe tip, the substrate, and the sample film, respectively.

PeakForce Tunneling atomic force microscopy (TUNA, Dimension ICON with Nanoscope V controller, Bruker) was used to investigate the current–voltage (IV) characteristics of individual SnS2 nanoplates or Sn0.5W0.5S2/SnS2 heterostructures. During the measurement, the PtIr tip was pressed against the sample with a constant force, feedback was switched to contact mode, and the voltage was linearly ramped up and down while the current signal was collected. Analysis of the IV curves was performed with the Nanoscope Analysis software.

Fabrication of photodetector and photoresponse measurements

Au (15 nm in thickness) interdigitated electrodes with a 10 nm spacing were deposited onto a SiO2 (300 nm)/Si substrate via magnetron sputtering through a shadow mask. After that, 0.5 µL of a concentrated dispersion (6.0 mg/mL) of the Sn0.5W0.5S2/SnS2 heterostructures or SnS2 nanoplates was drop-casted on the electrodes.

The current–voltage (IV) and the current–time (It) curves of the photodetectors were measured on a semiconductor characterization system (Keithley 4200, USA) in air at room temperature. A 405 nm laser was used for all the measurements. The actual power intensity was measured by a power meter (LP1, Sanwa Electric Instrument Co., Ltd., Japan).

Fabrication of chemiresistive sensors and gas sensing tests

Chemiresistive gas sensors were fabricated based on SnS2 nanoplates or Sn0.5W0.5S2/SnS2 heterostructures for sensing of various gases, including CH3COCH3, (CH3CH2)2O, CH3CH2CHO, C6H5CH3, and NO2. Typically, a drop of 100 μL aqueous solution containing 10 mM as-prepared SnS2 or Sn0.5W0.5S2/SnS2 was drop-casted onto an Au interdigitated electrode (with 0.1 mm spacing over a 2 × 1 cm2 area, Changchun Mega Borui Technology Co., Ltd) and then dried in oven at 60 °C. The gas sensing test was performed in an air-tight chamber with electrical feedthroughs at room temperature (25 °C). A constant current was applied to the sensor electrode, and the variation of the sensor resistance was monitored and recorded with the changes in the gas environment using a data acquisition system (34972A, Agilent) with a 20 channel multiplexer (34901A, Agilent). A typical sensing measurement cycle consisted of three sequential steps: (1) a dried N2 flow was introduced into the chamber to record a baseline resistance (R0); (2) a target gas, e.g., acetone, balanced in N2 was introduced, and the concentration increased (0.1–50 ppm) with progressive cycles; and (3) when the resistance of the sensor reached equilibrium in the target gas (Ra), the target gas was replaced by N2 to allow the resistance of the sensor to return to R0. All gas flows were set at 500 sccm, precisely controlled by using mass-flow controllers.

Computational methods

All the computations were performed with Vienna Ab initio simulation package (VASP) which is based on the density functional theory (DFT)66,67. The exchange-correlation interaction uses the general gradient approximation (GGA) formulated by Perdew–Burke–Ernzerhof (PBE)66. All electron interactions were described with projector augmented wave (PAW) pseudo potentials. Long-range dispersion corrections have been considered within the DFT-D2 method. The dispersion coefficient C6 and van der Waals radius R0 for H, C, N, O, S, Sn and W used in our DFT-D2 method were taken from previous reports68,69. The scale factor S6 was set to 0.75 because the GGA-PBE function was employed. An 11 × 11 × 1 k-point mesh was used for the interaction of the Brillouin-zone. The cutoff energy for the plane wave basis set was restricted to 400 eV, and a vacuum region of at least 12 Å was used in building the slab models. The convergence threshold was set as 10−4 eV in energy and 0.02 eV/Å in force, respectively. As shown in Fig. 4c, a four-layer 1T-Sn0.5W0.5S2 on a monolayer 1T-SnS2 was constructed with a 2 × 2 supercell, which contains 24 Sn, 16 W and 80 S atoms, respectively. Three NH4+ ions were introduced in the four-layer 1T-Sn0.5W0.5S2 on a monolayer 1T-SnS2 system (Supplementary Fig. 10). All atoms in the structure were fully relaxed to optimize without any restriction, and the convergence threshold was set as 10−4 eV in energy and 0.02 eV/Å in force, respectively. The optimized lattice constants and work functions (Φ) were summarized in Supplementary Table 1. The experimental lattice constant (a = 3.46 Å) was used in DFT calculations for the in-plane periodicity of the four-layer 1T-Sn0.5W0.5S2 on a monolayer 1T-SnS2 without and with intercalated NH4+ ions. To evaluate the stability of the adsorption of an acetone molecule on a three-layer 1T-SnS2 and three-layer 1T-Sn0.5W0.5S2 without and with intercalated NH4+ ions (each system contains a 4 × 4 supercell), the adsorption was defined by Ea=Eacetone + Esubstrate − Eacetone–substrate (where Eacetone–substrate is the total energy of the acetone/substrate compound systems, whereas Eacetone and Esubstrate are the energy of the isolated acetone molecule, and the total energies of upper two-layer relaxed and bottom-layer fixed 1T-SnS2 or distorted 1T-Sn0.5W0.5S2 without or with intercalated NH4+ ions systems, respectively. VESTA was used for preparation of the structure models70.

Data availability

The data that support the findings of this study are available from the corresponding author on request.

Additional information

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Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant no. 51322202 and 21373112), the Joint Research Fund for Overseas Chinese, Hong Kong and Macao Scholars (Grant no. 51528201) and the Young 1000 Talents Global Recruitment Program of China. Q.W. and X. Huang are grateful to the High Performance Computing Center of Nanjing Tech University for providing the computational resources. H.Z. thanks the support from MOE under AcRF Tier 2 (ARC 19/15, No. MOE2014-T2-2-093; MOE2015-T2-2-057; MOE2016-T2-2-103) and AcRF Tier 1 (2016-T1-001-147; 2016-T1-002-051), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. H.Z. also would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy facilities.

Author information

Author notes

  1. These authors contributed equally: Xiaoshan Wang, Zhiwei Wang.

Affiliations

  1. Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, 211816, China

    • Xiaoshan Wang
    • , Zhiwei Wang
    • , Jindong Zhang
    • , Xiang Wang
    • , Zhipeng Zhang
    • , Jialiang Wang
    • , Zhaohua Zhu
    • , Zhuoyao Li
    • , Yao Liu
    • , Ning Wang
    • , Jiaxu Yan
    • , Hai Li
    • , Xiao Huang
    •  & Wei Huang
  2. State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, 211816, China

    • Xuefeng Hu
    • , Junwen Qiu
    •  & Wei Zhang
  3. Cambridge Graphene Centre, University of Cambridge, Cambridge, CB3 0FA, UK

    • Guohua Hu
    •  & Tawfique Hasan
  4. Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore

    • Bo Chen
    • , Ning Wang
    • , Qiyuan He
    • , Junze Chen
    •  & Hua Zhang
  5. Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, 210023, China

    • Shaozhou Li
    •  & Wei Huang
  6. School of Chemistry and Molecular Engineering, Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, 211816, China

    • Qiang Wang
  7. Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an, 710072, China

    • Wei Huang

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Contributions

X. Huang and W.H. proposed the research direction and guided the project. Xiaoshan Wang and Z.W. designed and performed the experiments. J.Z., H.L., X. Hu, J.Q., G.H., and S.L. analyzed and discussed the experimental results. Xiang Wang, Z. Zhang, J.W., Z. Zhu, Z.L., Y.L., N.W., and B.C. performed some supporting experiments. Q.W. provided theoretical calculations and analyses. S.L., Q.H., J.C., J.Y., W.Z., T.H., and H.Z. contributed to the revision of the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding authors

Correspondence to Qiang Wang or Xiao Huang or Wei Huang.

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DOI

https://doi.org/10.1038/s41467-018-06053-z

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