2D Heterostructures with Bi2S3 Nanowire and MoS2

Aug 29, 2016 - Herein, we demonstrate the one-step growth of a heterostructure on the basis of a 1D-Bi2S3 nanowire and a 2D-MoS2 monolayer through the...
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Co-nucleus 1D/2D Heterostructures with Bi2S3 Nanowire and MoS2 Monolayer: One-Step Growth and Defect-Induced Formation Mechanism Yongtao Li,† Le Huang,† Bo Li, Xiaoting Wang, Ziqi Zhou, Jingbo Li,* and Zhongming Wei* State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China S Supporting Information *

ABSTRACT: Heterostructures constructed by low-dimensional (such as 0D, 1D, and 2D) materials have opened up opportunities for exploring interesting physical properties and versatile (opto)electronics. Recently, 2D/2D heterostructures, in particular, atomically thin graphene and transition-metal dichalcogenides, including graphene/MoS2, WSe2/MoS2, and WS2/WSe2, were efficiently prepared (by transfer techniques, chemical vapor deposition (CVD) growth, etc.) and systematically studied. In contrast, investigation of 1D/2D heterostructures was still very challenging and rarely reported, and the understanding of such heterostructures was also not well established. Herein, we demonstrate the one-step growth of a heterostructure on the basis of a 1D-Bi2S3 nanowire and a 2D-MoS2 monolayer through the CVD method. Multimeans were employed, and the results proved the separated growth of a Bi2S3 nanowire and a MoS2 sheet in the heterostructure rather than forming a BixMo1−xSy alloy due to their large lattice mismatch. Defect-induced co-nucleus growth, which was an important growth mode in 1D/2D heterostructures, was also experimentally confirmed and systematically investigated in our research. Such 1D/2D heterostructures were further fabricated and utilized in (opto)electronic devices, such as field-effect transistors and photodetectors, and revealed their potential for multifunctional design in electrical properties. The direct growth of such nanostructures will help us to gain a better comprehension of these specific configurations and allow device functionalities in potential applications. KEYWORDS: heterostructure, 1D/2D, CVD growth, MoS2, Bi2S3

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approach, possesses its own advantages which can hardly be achieved by manual manipulation or even with the help of precise instruments. The transfer process for graphene/h-BN heterostructures introduced structural uncertainties due to the random stacking between graphene and the h-BN substrate.3 Precise alignment was observed for the epitaxial growth of single-domain graphene on h-BN.8,9 The lateral junction of WSe2/MoS2, where MoS2 was epitaxially grown along the front edge of the monolayer WSe2 flake with an atomically sharp transition in composition, was achieved using a two-step growth method.10 A one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane

eteroconnection in different two-dimensional materials has attracted a lot of attention for its interesting functions in nanoelectronics and optoelectronics. The common strategies to build heterostructures mainly involve “top-down” manual layer-by-layer stacking and “bottom-up” epitaxial growth by chemical vapor deposition (CVD) method.1,2 In the manual stacking approach, it provides us quite a large space to design and realize the contact between many different 2D materials, such as graphene/h-BN, MoS2/ WSe 2, and MoS2 /WS 2, in which some great physical phenomena have also been reported.3−5 For example, van der Waals stacking n-type MoS2 and p-type WSe2 revealed apparent rectifying characteristics due to their intrinsic type-II band alignment.6 Ultrafast charge transfer from MoS2 to WS2 and long lifetime of indirect excitons in MoSe2/WSe2 heterojunctions were also demonstrated.5,7 The direct CVD growth of the heterostructure, in comparison to the manual stacking © 2016 American Chemical Society

Received: July 25, 2016 Accepted: August 29, 2016 Published: August 29, 2016 8938

DOI: 10.1021/acsnano.6b04952 ACS Nano 2016, 10, 8938−8946

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Figure 1. Growth and morphologies of Bi2S3/MoS2 heterostructures. (a) Schematic illustration of the synthesis process for the heterostructures, in which a mixture of MoO3 and Bi2O3 served as a solid source and reacted with S vapor at high temperature. (b) Model of the heterostructures grown on the SiO2/Si substrate, where Bi2S3 nanowires locate above triangle MoS2 sheets. (c) SEM image of a typical 1DBi2S3/2D-MoS2 heterostructure on SiO2/Si substrate. (d) Optical image, SEM image, as well as its corresponding AFM height and phase images of the nanostructure. (e) Statistical diagram of the included angle between nanowires and edges of triangle sheets, and the ellipsis indicate the repeating statistic part exceeding a period of 30°.

than 200 cm2/(V·s) and an on/off ratio of 108.27 Based on this knowledge, we considered that the synthesis of a specific 1D/ 2D heterostructure with these two materials might lead to an interesting multifunctional design for electrical properties such as band alignment (Figure S1), rectification, photodetectors, etc. In broad terms, the direct growth of 1D/2D heterojunctions will also help us to gain a better comprehension of these specific configurations and allow device functionalities and properties in potential applications. Here, we demonstrate the one-step growth of a heterostructure from a 1D-Bi2S3 nanowire and a 2D-MoS2 monolayer by CVD. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed the fact that the Bi2S3 nanowire grew above the MoS2 monolayer. X-ray photoelectron spectroscopy (XPS) confirmed the existence of both MoS2 and Bi2S3, and microarea characterization by Auger electron spectroscopy (AES) further recognized the nanowires to be Bi2S3 and triangle sheets to be MoS2. Moreover, Raman spectra collected from the nanowire and sheet showed distinct signatures of Bi2S3 and MoS2, respectively, just in keeping with their individual growing products. High-resolution transmission electron microscopy (HR-TEM) presented a clear lattice morphology as well as diffraction patterns of MoS2 and Bi2S3, in which we found that Bi2S3 nanowires grew along the [001] direction (c-axis in an orthorhombic crystal system, space group of Pbnm, JCPDS No. 75-1306). Detailed AFM characteristics demonstrated that the nanowire and sheet contacted with each other only at the nucleation site of the triangle MoS2 single crystal, while they grew separately in other areas, implying the co-nucleus growth mode in the 1D-Bi2S3 nanowire/2D-MoS2 monolayer heterostructure. HR-TEM characteristics illustrated that multidefects and irregular morphology in the nucleation

interconnected heterostructures of WS2/MoS2 mainly via control of the growth temperature was also reported.11 Besides the bottom-up 2D/2D heterostructures of layered materials mentioned above, heterocontacted 2D/3D, 1D/3D, 1D/1D, and 0D/0D junctions were also synthesized with various approaches.12 For example, aligned monolayer MoS2 sheets were able to directly grow on gallium nitride substrates, forming the lattice-matched 2D/3D vertical heterostructures.13 One-dimensional nanowires or nanorods (such as InxGa1−xAs, InAsyP1−y, ZnO, etc.) were also obtained on bulk semiconductor substrates.14−16 Moreover, core−shell nanowires and nanoparticles, which are composed of different components in the core and in the shell respectively, formed the classical 1D/1D and 0D/0D appearances in the previous studies.17,18 In the case of the 1D/2D nanostructure, even though several groups have reported the InxGa1−xAs nanowires grown on transferred graphene in metal−organic chemical vapor deposition or molecular beam epitaxy systems,19−21 more heterostructures could only fabricated via manual stacking or hybridizing methods.22−24 Indeed, to date, the synchronous growth of 1D/2D nanostructures has rarely been reported in comparison to the other heterojunctions mentioned above, and the understanding of the forming mechanism and physical properties of such heterostructures was not well-established. The 1D semiconducting Bi2S3 nanowire was reported to possess distinct gate-tunable drain−source current behavior and excellent photosensitive performance with fast photoresponse speed,25,26 revealing its great potential in (opto)electronic devices. On the other hand, the 2D MoS2 monolayer, which consists of a layer of Mo atoms sandwiched between two layers of sulfur atoms, was also demonstrated to possess the prominent electrical properties with a high mobility of more 8939

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Figure 2. Elemental characteristics by XPS and microarea AES measurement. (a−c) XPS spectra of the samples on the SiO2/Si substrate, indicating the existence MoS2 and Bi2S3. (d−f) Microarea Auger mapping of S, Mo, and Bi, where we can see that the triangle sheet consists of Mo and S atoms while the nanowire consists of Bi and S. (g) Corresponding SEM image of the MoS2/Bi2S3 heterostructure in (d−f). (h) Detailed Auger spectrum corrected from the spot 1 (nanowire) and spot 2 (triangle sheet) in (g) and clear Bi signal appearing in the spectrum from the nanowire.

MoS2 sheets was counted to be 1−3 μm/min and 1−4 μm/ min, respectively, in which relative growth rate of the two materials depended on relative mass ratio of Bi2O3/MoO3 sources, and higher temperature would lead to their faster growth. Meanwhile, it is worth noting that the mass ratio of Bi2O3/MoO3 also plays an important role in the morphology of the heterostructures, where too low of a Bi2O3/MoO3 mass ratio can only grow MoS2 sheets, while too high of a ratio results in thicker wires in heterostructures, and the Bi2O3/ MoO3 mass ratio of 2−3 is a suitable range in our experiments (Figure S4). Moreover, our statistics from hundreds of heterostructures reveal that the nanowires may rotate with random angle with respect to the triangle sheet. While relatively more cases show that the nanowires tend to be parallel to (∼0°) or perpendicular to (∼30°) one edge of the triangle sheets (Figure 1e), these two prior results may be caused by their relatively lower forming energy in such special rotating configurations,13,28−30 and the reason for other random rotational angles will also be discussed in the following section with regard to defects. XPS was utilized to identify elemental composition in the synthetic samples. The measurement revealed the existence of Mo, S, and Bi elements on the substrate, in which prominent peaks at 232.97, 229.87, 227.32, 163.87, 162.82, and 158.57 eV were assigned to Mo (3d3/2), Mo (3d5/2), S (2s), S (2p1/2), S

site gave rise to the co-nucleus behavior in 1D/2D heterostructures, and the forming mechanism for such special performance was also investigated using first-principles calculations. Such 1D/2D heterostructures were also further fabricated and utilized in the (opto)electronic devices including field-effect transistors (FETs) and photodetectors, and their good potential in future applications was revealed.

RESULTS AND DISCUSSION The growing process was conducted in a CVD system, where a mixed powder of MoO3 and Bi2O3 reacted with sulfur vapor at a high temperature (650 °C) and synthesized the 1D-Bi2S3/2DMoS2 heterostructures on the SiO2/Si substrate (Figure 1a,b; the detailed growing process can be seen in the Methods section). Typical morphologies of the heterostructure are presented in Figures 1c and S2, where we can clearly see nanowires locate on triangle sheets from the distinct differences in brightness. AFM measurement was performed and confirmed that the thickness of the sheet was about 0.98 nm (Figure 1d), illustrating the monolayer nature of the sheets. Figure 1d also shows that length and diameter of the nanowire is 7.3 μm and 120 nm, respectively, and the aspect ratio is more than 60. Our experimental results reveal that the nanowires will grow longer rather than thicker for an extended growing time (Figure S3). Statistical growth rate of the Bi2S3 nanowires and 8940

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Figure 3. Raman and PL characterization of the Bi2S3/MoS2 heterostructure. (a) Black line Raman spectrum was corrected at the black spot in (d), and red line Raman spectrum was corrected from pure Bi2S3 nanowires grown by CVD method individually. (b) Green line Raman spectrum was corrected at the green spot in (d), and red line Raman spectrum was corrected from pure MoS2 monolayer grown by CVD method individually. (c) Photoluminescence spectrum taken at the green spot in (d). (d) Optical image of a Bi2S3/MoS2 heterostructure used for Raman characterization. (e,f) Raman intensity mapping collected at 404.4 and 236.0 cm−1, respectively. The measurements were performed with 532 nm laser excitation at room temperature.

morphologies of the MoS2 monolayer as well as the Bi2S3 nanowire in the heterostructure. Selected area electron diffraction (SAED) pattern taken from the sheet displayed a typical six-fold symmetry (Figure 4a), whose set of hexagonally arranged diffraction spots could be indexed to the (001) zone plane of MoS2 layered structures.34 HR-TEM image of the flake in Figure 4b also showed the lattice structure of the MoS2 monolayer where the bright spots corresponded to Mo atoms and S atoms were not displayed for their smaller atomic bulk.35,36 The bright spots of Mo illustrated distinct periodic arrangement, and distance between adjacent Mo atoms was measured to be 0.3158 nm, just in line with that of 0.316 nm in an optimized MoS2 lattice constant (Figure 4c). On the other hand, our measurement showed that nanowire in the heterostructure possessed a classical orthorhombic crystal lattice, where the SAED pattern in Figure 4d presents a periodical orthogonal feature. Careful analysis of these diffraction spots revealed (001) and (020) lattice plane spacings of 0.401 and 0.566 nm in the nanowire. Included angle between the (020) and (021) planes was 54.7°, and the (001) plane was perpendicular to the (020) plane. Such results were exactly in agreement with the lattice structure of Bi2S3 (Figure 4f).37−39 Besides, both SAED and HR-TEM demonstrated the [001] growing direction of the Bi2S 3 nanowire, whose lowmagnification TEM image is also shown in the inset of Figure 4e. Even though nanowires in heterostructures grew along the specific direction, we should also note that the different lattice plane of Bi2S3 could settle on the MoS2 sheets, such as the (100) plane in Figure 4e and the (−110) plane in Figure S6. The above characteristics provide us a path to gain more insight into the growth of Bi2S3/MoS2 heterostructures in the CVD system. Indeed, several products, such as BixMo1−xSy alloy,40−42 2D-layered Bi2S3/MoS2 stacking heterostructure,11,28 and 1D-Bi2S3/2D-MoS2 heterostructure, have been

(2p3/2), and Bi (4f7), respectively (Figure 2a−c). Moreover, microarea AES mapping (Figure 2d−f) was used to gain detailed element distribution in the 1D/2D heterostructure, where S and Mo were found to cover the whole triangle sheet while Bi appeared only in the position of the nanowire. As we can see in Figure 2h, the AES spectra corrected from spot 1 (in Figure 2g) showed an evident signal of Bi (at ∼105 eV), while no Bi signal was detected in spot 2. Such results provided strong evidence that the triangle sheet was MoS2, while the nanowire was made up of Bi2S3. Raman and PL spectroscopy were also employed to further characterize the 1D/2D heterostructures. As shown in Figure 3a, the black Raman spectrum collected from the black spot (in Figure 3d) revealed intensity peaks at about 187.1, 236.0, and 260.3 cm−1, which were in accordance with Raman peaks (red line in Figure 3a) of our CVD synthesized individual Bi2S3 nanowires (Figure S5a) and previous reports.31,32 Moreover, the Raman spectrum from the green spot (in Figure 3d) presented the classical E12g and A1g peaks of MoS2, which were also in keeping with the results from our CVD growing a pure MoS2 monolayer (red line in Figure 3b and sample image in Figure S5b).33 The PL emission from the green spot (in Figure 3d) possessed an intensity peak at 1.84 eV. To give more visual images, Raman mappings according to the intensities at 404.4 and 236.0 cm−1 were performed and exhibited the clear morphologies of the triangle sheet of MoS2 and the wire shape of Bi2S3. Indeed, the specific fingerprints of various materials attained from Raman spectroscopy provided us a definite way to identify uncertain substances. As a consequence, both elemental and Raman results illustrate a 1D-Bi2S3 nanowire and a 2D-MoS2 monolayer in the heterostructure, rather than a compound of BixMo1−xSy. We transferred the samples onto a Cu grid and performed HR-TEM and electron diffraction studies to gain the fine crystal 8941

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the MoS2 sheet prefers to combine with Mo atoms rather than Bi atoms to grow larger in the CVD chamber. Indeed, connection mode between a Bi2S3 nanowire and a MoS2 monolayer is another important issue in the 1D/2D heterostructures that has been rarely reported before. We wondered whether the Bi2S3 nanowire connects the MoS2 monolayer by line all over the area beneath the nanowire or just by one specific spot. It was fortunate for us to find the answer by means of scratching the nanowire away using a probe and exposing the underlying area so that we could obtain the exact information. Optical images of a typical Bi2S3/MoS2 heterostructure before and after being scratched are presented in Figure 5a, and AFM height and phase measurement were

Figure 4. Atomic structure of the MoS2 monolayer and Bi2S3 nanowire. (a,b) SAED pattern and HR-TEM image of the nanosheet in the heterostructure, where the clear honeycomb configuration matches the MoS2 lattice well. (c) Optimized lattice structure of monolayer MoS2. (d) SAED pattern of the Bi2S3 nanowire in the heterostructure. The exact corresponding lattice planes can be obtained on the basis of the measured spot distance as well as included angle, and (001), (021), and (020) lattice plane are demarcated. (e) High-resolution TEM image of the nanowire and its large-area TEM image are shown in the inset. Growing direction of the nanowire is demarcated as [001] in (d,e). (f) Schematic of the orthorhombic crystal of Bi2S3.

Figure 5. Contact and nucleation condition of the Bi2S3/MoS2 heterostructure characterized by AFM. (a) Optical image of a typical heterostructure before and after being scratched by the probe, where a part of the nanowire was scratched away so that the underlying as well as nucleation region was exposed. (b) AFM height image of the magnifying red frame area in (a). The nucleation area is marked by green dashed circular ring, and the blue dashed frame reveals the area being covered by a Bi2S3 nanowire. The bottom-right inset also presents the height profile along the black line, indicating no connection between the nanowire and the MoS2 monolayer, except for the nucleation site. (c) AFM phase image of the same area in (b); the area in the green dashed circle clearly shows the underlying SiO2 substrate after a Bi2S3 nanowire was scratched away with part of an adjacent MoS2 nanosheet for their connection in the nucleation site.

conjectured before we conducted the synthesis experiment using a mixture of Bi2O3/MoO3 as solid sources. The final outcomes revealed the fact that only 1D-Bi2S3/2D-MoS2 heterostructures could be obtained by this method. The underlying mechanism of this result is believed to be induced by the large difference in lattice forms these two materials, where MoS2 belongs to a hexagonal crystal system with lattice parameters of a = 0.316 nm, b = 0.316 nm, c = 1.820 nm, α = 90°, β = 90°, and γ = 120° (Figure 4c), whereas Bi2S3 belongs to an orthorhombic crystal system with lattice parameters of a = 1.112 nm, b = 1.125 nm, c = 0.397 nm, α = 90°, β = 90°, and γ = 90° (Figure 4f). The large lattice gap made them tend to grow into a MoS2 sheet and Bi2S3 nanowire individually, which has also been confirmed by AES as well as Raman characteristics in our experiments. On the other side, for the purpose of simulating an edge-synthetic situation for a largerarea MoS2 monolayer (Figure S7), our theoretical calculation also indicates the binding energy of 17.77 and 5.17 eV for Mo and Bi atom filling at the Mo vacancy in the MoS2 sheet, using the formula ΔE = Etotal − EMo(Bi) − EMo vacancy MoS2, where ΔE is the binding energy between the individual Mo (or Bi atom) and Mo vacancy MoS2, Etotal is the energy of the whole system, EMo(Bi) is the energy of individual Mo or Bi atoms, and EMo vacancy MoS2 is energy of the Mo vacancy of the MoS2 monolayer.43−45 Obviously, the calculation result reveals that

utilized to characterize the local region, as shown in Figure 5b,c, respectively. The blue dashed frame indicated the area covered by nanowire before, and the green dashed circle points out the nucleation region of the triangle MoS2 single crystal. It is clear in the AFM image that only the nucleation site of the MoS2 monolayer was torn away with the Bi2S3 nanowire to expose the underlying SiO2 substrate, while there is no damage in the other area beneath the nanowire. Inset at the bottom right of Figure 5b illustrates the AFM height profile of the black line passing through the edge of the MoS2 monolayer as well as the area under the nanowire, and no evident variation of height was found in the specific area, which was in accord with the result in the phase image where the phase of the region in the blue frame remained the same as in the other area of the MoS2 monolayer, meaning they have the same component. Besides, further evidence was also obtained when we grew Bi2S3 nanowires individually on the SiO2/Si substrate, where nanowires were 8942

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Figure 6. Nucleation spot of the heterostructure characterized by HR-TEM and the energy level of heterostructures obtained using firstprinciples calculation. (a) Optical image of typical MoS2 sheet transferred onto a TEM grid. (b) HR-TEM image of a MoS2 monolayer at red frame A in (a). The inset presents the FFT pattern of the TEM morphology. (c,d) HR-TEM morphology of the nucleation site at the red frame B in (a). (e) Schematic illustration of a MoS2 supercell with one S atom vacancy shown as a red circle. The yellow and gray balls denote S and Mo atoms, respectively. (f,g) Optimized model of a Bi atom located above a common sulfur atom away from the defect and a sulfur vacancy on the basis of supercell in (e). (h) Energy level comparison of (e−g) systems from first-principles calculations. (i) Schematic illustration of a MoS2 supercell with one Mo (blue circle) and two S (red circle) atoms (1Mo-2S) vacancies. (j,k) Optimized model of a Bi atom located above a sulfur atom away from the defect and around the 1Mo-2S vacancy on the basis of supercell in (i). (l) Energy level comparison of (i−k).

them. In a few cases, multidefects in the nucleation site can also lead to the multiple nanowires grown on top of a single MoS2 triangle (Figure S11). Meanwhile, for a better understanding about how the defects in the nucleation spot give rise to the co-nucleus behavior between MoS2 and Bi2S3 in such a 1D/2D heterostructure, two types of classical defects, including one sulfur atom vacancy (Figure 6e) and 1Mo-2S atom vacancy (Figure 6i), were theoretically calculated.49 The computational works were performed using a first-principles method, in which comparison of energy in the system could provide us an effective way to evaluate stability of a certain structural configuration. Comparison of a Bi atom located above a common sulfur atom away from the defect region or just above the sulfur vacancy in the defect spot was determined for the case in Figure 6e−g, and the computed result in Figure 6h clearly illustrates an energy level in the system in Figure 6f 1.57 eV lower than that in Figure 6e, and it was even 4.27 eV lower when a Bi atom was located on the defect via chemical bonds. Indeed, the lowerenergy level also meant a more stable existence in the system. It is similar to the case for a Bi atom associating with a 1Mo-2S defect MoS2 sheet in Figure 6i−l. Such a result reflects the knowledge that it is more favorable for Bi atoms to combine with defects rather than other common sulfur sites in MoS2, and high temperature in the furnace will also boost migration of Bi atoms toward the defect sites, which promotes the following nucleation of Bi2S3 nanowires and also results in the growth of co-nucleus Bi2S3/MoS2 heterostructures. As a matter of fact, for more cases in our experimental results, defect-induced conucleus growth of Bi2S3 and MoS2 might occur almost simultaneously at the initial stage due to the relatively high chemical reactivity between them (Figure S12). Different devices have also been fabricated, and measurement was systematically performed to evaluate the electrical transport

found on the surface of the substrate (Figure S5a) and some suspended nanowires could also be seen at the edge of the substrate (Figure S8). Such results provide a lesson that it is possible for a Bi2S3 nanowire and a MoS2 monolayer to grow individually, except for a co-nucleus spot, establishing the connection in the 1D/2D heterostructures. Further investigation was performed to gain a deeper insight into the co-nucleus behavior in 1D/2D heterostructures. The study above indicated that a Bi2S3 nanowire and a MoS2 monolayer would prefer to grow individually rather than turning into an alloy of BixMo1−xSy. However, previous reports have demonstrated that defects in 2D material possessed enhanced chemical reactivity with dangling bonds compared to their perfect crystal structures,35,46−48 which was easy to form a new hybrid structure with foreign material in the defect sites. In fact, in our synthesis experiments, most of the Bi2S3 nanowires were found to connect with the MoS2 sheets in the nucleation sites (Figure S9). Accordingly, a typical triangle MoS2 sheet without a Bi2S3 nanowire was transferred onto a Cu grid and characterized by HR-TEM in order to learn the nucleation situation in MoS2 sheets as well as heterostructures. Amplified TEM images of regions A and B (in Figure 6a) are presented in Figure 6b−d. An obvious difference can be seen from the two areas where the nucleation region appears to be multilayer MoS2 with disordered morphology, while the other areas are a homogeneous MoS2 monolayer. In general, it is natural for a crystal to nucleate at locations with higher chemical reactivity, and the disordered morphology as well as the concomitant defects (Figure S10) in the nucleation site of the MoS2 sheet offers such suitable surroundings to the Bi2S3 nanocluster for its initial nucleation and following growth into longer nanowires. Indeed, this forming mechanism also accounts for the various rotational angles between a Bi2S3 nanowire and a MoS2 triangle, where defects break the stable rotating configuration between 8943

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ACS Nano and photoresponse properties of the Bi2S3/MoS2 heterojunction. Nevertheless, the fact was that no valid electric signals could be probed for devices with a pair of Au electrodes contacting the Bi2S3 nanowire and MoS2 monolayer (Figure S13a,b). This result mainly originates from several reasons involving relatively inferior defect-induced connection and small point-contacted area between the nanowire and the sheet, which is illustrated in Figure 5 and Figure 6. Moreover, large resistance, obtained from an electrical resistance formula, R = ρ(l/A), also revealed the relatively poor conductivity in the Bi2S3 nanowire for its high ratio of l/A (as shown in Figure 1d). Bi2S3-on-MoS2 devices were also fabricated, and room temperature electrical properties of the devices are presented in Figure S14, where the variation of output behavior under different back-gate voltages indicated the distinct adjustment of our device by an electric field. The device showed a typical n-type channel in the Id−Vg plot, which was scanned from −40 to +40 V with Vd = 5 V (Figure S14c). Current on/off ratio of the FET reached 103 with an off current of about 7.8 × 10−11 A. Fieldeffect mobility of the device was extracted to be approximately 0.1 cm2/(V·s), which was comparable to the results in earlier reports of MoS2 sheets synthesized by CVD but much lower than the samples from mechanical exfoliation.27,34,50−52 Moreover, comparison of performance of individual Bi2S3 and MoS2 devices (in Figure S15) demonstrated that the MoS2 monolayer played the dominant role in the Bi2S3-on-MoS2 FET, while the Bi2S3 nanowire also contributed secondary electrical transport effects. This result was caused by the relatively larger conducting area of MoS2 compared to the area of Bi2S3 in the heterostructure device. The electrical response to illumination of the 532 nm laser is shown in Figure S14d, where power density of the light source was about 3.4 mW/ cm2. The photoconductivity has obviously increased under illumination to induce photogenerated electron−hole pairs, and the current of the device increased from 2 to 4.5 nA as the laser switched to the “on” state, giving a photoswitching ratio (Ilight/ Idark) of 2.25. These results demonstrate the good optoelectronic performance of the 1D/2D heterostructures and also confirmed their high quality.

upstream in a cooler zone, which was 12 cm away from the substrate. After that, a high flow of pure N2 blew across the quartz tube for about 5 min to drive away the air in the tube. During the growing process, the flow of N2 remained at 20 sccm, and temperature of the furnace was heated to 650 °C within 16 min and remained for 6 min to grow the Bi2S3/MoS2 heterostructures. The furnace was finally shut down and uncapped after the growth was finished. More details can be seen in Figure S16. The growth of an individual Bi2S3 nanowire and a MoS2 monolayer follows a similar process with only Bi2O3 and MoO3 sources, respectively. Transfer Process. The as-grown Bi2S3/MoS2 heterostructures were transferred by a wet-transfer method. First, poly(methyl methacrylate) (PMMA) was spin-coated onto the substrate and baked on a heating plate at 150 °C for 30 min. After that, the substrate with PMMA was soaked in NaOH solution until the PMMA film with heterostructures separated from the substrate and floated on the solution surface. The PMMA film was rinsed with deionized water several times and transferred onto a TEM grid and dried. Finally, the PMMA film was dissolved in acetone for 30 min. Fabrication of Devices. The devices were fabricated using the electron beam lithography method. Devices were first spin-coated a layer of 4% 950 K PMMA solution onto the SiO2/Si substrate at 4000 rpm. After being baked on a 200 °C heating plate for 4 min, the substrate was scanned and patterned by a 50 μC/cm2 electron beam in an EBL machine. A developing solution of methyl isobutyl ketone and isopropyl alcohol in a 1:2 (v/v) ratio was subsequently used to develop the features. A 60 nm gold layer was deposited onto the substrate. The final devices were obtained after being submersed in acetone to remove the remaining PMMA. The devices were then measured in air atmosphere at room temperature using an Agilent B2902A device. Density Functional Theory Calculations. The calculations were conducted using the projector-augmented wave method with the generalized gradient approximation of the Perdew−Burke−Ernzerhof exchange−correlation functional in VASP. The energy cutoff for planewave expansion was set at 450 eV. A vacuum layer thickness of greater than 12 Å was employed to prevent the correlation between adjacent layers. Brillouin zone sampling was processed with Monkhorst Pack special k-point meshes, and a k-point grid of 5 × 5 × 1 was chosen for the calculations. All the structures were fully relaxed using the conjugated gradient method until the Hellmann−Feynman force on each atom was less than 0.02 eV/Å.

CONCLUSIONS In summary, we have reported the one-step growth of the 1D/ 2D heterostructure from a Bi2S3 nanowire and a MoS2 monolayer by the CVD method. Different types of characteristic methods, such as AFM, Raman, TEM, etc., proved the separated synthesis of a Bi2S3 nanowire and a MoS2 sheet in the heterostructure. Moreover, co-nucleus growth, which is an important growth mode in 1D/2D heterostructures, was confirmed and investigated in our experiments and by theoretical calculation, which also provided us a way to gain more systematic understanding of the growth mechanism in these peculiar heterostructures. Multifunctional design for electrical properties, such as band alignment, rectification, photodetector, etc., might also be achieved in these 1D/2D heterostructures by using various low-dimensional nanowires, nanorods, and few-layered materials.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04952. Additional experimental details and Figures S1−S16 (PDF)

ASSOCIATED CONTENT S Supporting Information *

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

Y.L. and L.H. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge Dr. Lu Fangyuan for his experimental support with individual Bi2S3 nanowire device fabrication and measurement. This work was financially supported by the “Hundred Talents Program” of Chinese Academy of Sciences (CAS), the National Natural Science Foundation of China (Grant Nos. 61571415 and 51502283), and the CAS/SAFEA

METHODS Synthesis of the 1D-Bi2S3/2D-MoS2 Heterostructure. Typically, a mixture of Bi2O3 and MoO3 nanopowder was placed on a quartz boat in the center of a horizontal quartz tube furnace, with a clean SiO2/Si substrate placed inversely above the solid source. Another small quartz boat containing pure sulfur (0.5−1 g) was placed 8944

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ACS Nano

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DOI: 10.1021/acsnano.6b04952 ACS Nano 2016, 10, 8938−8946