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In-plane Mosaic Potential Growth of Large-Area 2D Layered Semiconductors MoS2-MoSe2 Lateral Heterostructures and Photodetector Application Xiaoshuang Chen, Yunfeng Qiu, Huihui Yang, Guangbo Liu, Wei Zheng, Wei Feng, Wenwu Cao, Wenping Hu, and PingAn Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13379 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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ACS Applied Materials & Interfaces

In-plane Mosaic Potential Growth of Large−Area 2D Layered Semiconductors MoS2-MoSe2 Lateral Heterostructures and Photodetector Application Xiaoshuang Chen,†,‡ Yunfeng Qiu,† Huihui Yang,† Guangbo Liu,† Wei Zheng,† Wei Feng,† Wenwu Cao,‡ Wenping Hu,*,‡,§ and PingAn Hu*,† †

Key Lab of Microsystem and Microstructure of Ministry of Education, Harbin Institute of

Technology, Harbin 150080, China ‡

Department of Physics, Harbin Institute of Technology, Harbin 150080, China

§

Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,

Beijing 100190, China KEYWORDS: mosaic, lateral heterostructure, MoS2/MoSe2, 2D material, photodetector

ABSTRACT: Considering the unique layered structure and novel optoelectronic properties of individual MoS2 and MoSe2, as well as the quantum coherence or donor-acceptor coupling effects between these two components, rational design and artificial growth of in-plane mosaic MoS2/MoSe2 lateral heterojunctions film on conventional amorphous SiO2/Si substrate are highly demanded. In this paper, large−area, uniform, and high−quality mosaic MoS2/MoSe2

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lateral heterojunctions film was successfully grown on SiO2/Si substrate for the first time by chemical vapor deposition (CVD) technique. MoSe2 film was grown along MoS2 triangle edges, and occupied the blanks of the substrate, finally leading to the formation of mosaic MoS2/MoSe2 lateral heterojunctions film. The composition and microstructure of mosaic MoS2/MoSe2 lateral heterojunctions film were characterized by various analytic techniques. Photodetectors based on mosaic MoS2/MoSe2 lateral heterojunctions film, triangular MoS2 monolayer and multilayer MoSe2 film are systematically investigated. The mosaic MoS2/MoSe2 lateral heterojunctions film photodetector exhibited optimal photoresponse performance, giving rise to the responsivity, detectivity and external quantum efficiency (EQE) up to 1.3 A W-1, 2.6 × 1011 Jones and 263.1 % under the bias voltage of 5 V with 0.29 mW cm-2 (610 nm), respectively, possibly due to the matched band alignment of MoS2 and MoSe2 and strong donor-acceptor delocalization effect between them. Taking into account the similar edge conditions of TMDCs, such facile and reliable approach might open up a unique route for preparing other 2D mosaic lateral heterojunctions film in a manipulative manner. Furthermore, the mosaic lateral heterojunctions film like MoS2/MoSe2 in present work will be a promising candidate for optoelectronic fields.

1. INTRODUCTION Atomically thick 2D layered nanomaterials such as metal chalcogenides, transition metal dichalcogenides (TMDCs), transition metal oxides, and other 2D compounds have attracted tremendous attention due to their exotic physical, chemical, electronic and optical properties that are essential for transistors, batteries, photodetectors, sensors and HER applications.1-5 TMDCs (WS2, WSe2, MoS2, MoSe2) have a 2D layered architecture similar to graphene, which play an


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important role in film growth and physical properties modulation. 2D hexagonal lattice layers composed of transition metal atoms sandwiched between two sheets of chalcogen atoms by covalent bonds. The interactions between 2D layers are van der Waals forces.6-8 Due to extensive applications of monolayer and few−Layer TMDCs, many fruitful synthesis techniques have been reported, for example, perylene-3,4,9,10-tetracarboxylic acid tetra-potassium salt (PTAS) as the seed on SiO2/Si substrate,9 H2S plasma sulfurization of sputtered WO3,10 iodine as the transport agent,11 or vapor phase reaction of Se and WO3 powders.12 Despite the direct band gap of TMDCs can be obtained via thinning bulk materials down to monolayer, the development of photodetectors is still greatly hindered due to the unsuitable band gap values, the relatively lower light response, and sluggish electron transfer pathways. As is known, suppressing photo-induced electron-hole recombination is an effective way to enhance the photocurrent, leading to the enhancement of photoresponsivity, detectivity, and external quantum efficiency (EQE), which are vital parameters for the practical applications of photodetector. Very recently, great efforts have recently been devoted to the development of lateral or vertical heterostructures for 2D TMDCs materials with adjustable compositions. On the one hand, lateral or vertical heterostructures of TMDCs will open up thrilling fresh devices. On the other hand, such combination of different components with appropriate band diagram is effective to improve the photodetector performance due the presence of multi electron or hole transfer pathways. Zhang et al. have prepared vertical heterostructures of layered metal dichalcogenides (MoS2−SnS2, WS2−SnS2, WSe2−SnS2) under mild CVD reaction conditions by van der Waals epitaxy.13 Large−area 2D MoS2 and WS2 heterostructures have been synthesized by using conventional polydimethylsiloxane (PDMS) stamping combined with high−pressure CVD technique grown monolayers.14 Shim et al. have reported MoSe2/graphene heterostructures


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by thermal CVD method with the conventional transfer technique.15 Some special phenomenons including additional photoluminescence peaks and strong quenching of characteristic photoluminescence are observed in these heterostructures, providing the opportunities for modulating their optoelectronic effects. However, one may find that transfer technique has been widely applied in previous fabrication approaches of heterostructures with some issues, such as atomically thick films with several fissures and wrinkles, 2D crystal layers folding and chemical degradability at random locations.15,16 Meanwhile, the chemical residues left on the surface of active materials during the removal of polymethylmethacrylate (PMMA) will form the undesired charge impurities, which are responsible for the degradation of device performance. Therefore, growing 2D TMDCs heterostructures without transferring method is crucial for obtaining ideal active materials for the fabrication of photodetectors. Some successfully technical routes have been employed to obtain vertical or in-plane 2D layered semiconductor heterojunctions as a crucial procedure towards opening up novel fields in materials science, functionally electronic and optoelectronic device physics and engineering. For example, vertical and in-plane WS2/MoS2 heterostructures with high-quality vertical stack and in-plane interconnection have been resoundingly grown via control of the growth temperature combined with CVD technique.2 Duan et al. have also triumphantly prepared MoS2–MoSe2 and WS2–WSe2 lateral heterostructures with seamless high-quality in-plane junction through in situ adjustment of the vapour-phase precursors during the growth of these 2D materials.17 Huang et al. have reported 2D MoSe2–WSe2 lateral heterostructures using physical vapour transport, and well-defined MoSe2–WSe2 lateral heterostructures displayed clear and perfect high-quality in-plane link.18 CVD technique has been manifested to be a triumphant approach to prepare various 2D layered materials, such as graphene, MoS2, MoSe2, h−BN, WS2 and WSe2.19-26 MoS2 and MoSe2


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are representative layered semiconducting TMDCs materials with similar crystal framework. Moreover, the reported theoretical calculation results demonstrate that quantum coherence and donor-acceptor delocalization can accelerate fast charge transfer at the MoS2/MoSe2 heterojunctions interface. The electron−hole recombination is obviously much slower when passing through the MoS2/MoSe2 heterojunctions interface than in individual MoS2 and MoSe2, encouraging prolonged charge separation to obtain effective light-to-electricity transformation.27 In this work, considering matching band alignment for electron or hole transfer between MoS2 and MoSe2, we report the growth of mosaic MoS2/MoSe2 lateral heterojunctions film for the first time using CVD approach in situ synthesis without transferring process under atmospheric pressure. Triangular monolayer MoS2 was firstly grown on SiO2/Si substrate by CVD method. Subsequently, large−area MoSe2 membrane was grown along MoS2 triangle edges, and occupied the blank space of the substrate. Eventually, MoS2 triangles were embedded into the large−area MoSe2 film to form mosaic MoS2/MoSe2 lateral heterojunctions film. The large−area and high−quality mosaic MoS2/MoSe2 lateral heterojunctions film are characterized by diverse analytic testing techniques. The photodetectors based on mosaic MoS2/MoSe2 lateral heterojunctions film, triangular MoS2 monolayer and multilayer MoSe2 film were constructed and the former photodetector exhibited optimal photodetector performance, probably due to the favorable suppression of photo-induced electron-hole recombination process and long charge carrier lifetime. The responsivity, detectivity and EQE of this photodetector are up to 1.3 A W-1, 2.6 × 1011 Jones and 263.1 % at Vds = 5 V with light wavelength of 610 nm under 0.29 mW cm-2, respectively. Our work demonstrates that mosaic MoS2/MoSe2 lateral heterojunctions film is a promising material for optoelectronic device. Moreover, we believe that present work will be a


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facile yet reliable route to fabricate other ultrathin mosaic 2D TMDCs lateral heterojunctions on various substrates and encourage extensive applications. 2. EXPERIMENTAL SECTION 2.1. Growth of Triangular MoS2 Monolayers. Triangular MoS2 monolayers were grown on SiO2/Si substrate. Growth substrate was treated in H2SO4/H2O2 (3:1) solution at 83 °C for 1 hour and washed with isopropyl alcohol, acetone, ethanol and deionized water for 10 minutes, respectively. And then the substrate was dealt with O2 plasma for 5 minutes. The growth process was carried out in a two−zone horizontal CVD system using MoO3 (99.9%, Aladdin) and S (99.95%, Aladdin) powders as precursors. MoO3 powder and the substrate were put on a quartz slice in high temperature zone (~670 °C). The distance between MoO3 and the substrate was 2~4 cm. Additionally, we put S powder on another alumina boat and located at the center of low temperature zone (~250 °C), which is at the upper stream position. During the reaction, 10~20 sccm high−purity Ar gas was used as the gas carrier. The growth process was kept for 20 minutes under atmospheric pressure. The CVD furnace was naturally cooled down to room temperature. 2.2. Growth of Mosaic MoS2/MoSe2 Lateral Heterojunctions Film. MoSe2 film was grown along MoS2 triangle edges, and occupied the blanks of the substrate with MoS2 triangles to form large−area mosaic MoS2/MoSe2 lateral heterojunctions film in the end. The growth process was similar to the growth of triangular MoS2 monolayers. MoO3 (99.9%, Aladdin) and Se (99.999%, Aladdin) powders were used as precursors. MoO3 powder and the substrate were placed in on an alumina boat located in high temperature zone center of the furnace (~750 °C). The substrate with triangular MoS2 monolayers was near to MoO3 precursor. The Se powder was placed in a separate alumina boat at the upstream side of the CVD furnace and the temperature was


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maintained at ~350 °C. The mixed Ar/H2 (6:1) gas was used as the gas carrier and reducing atmosphere during all the process. The reaction time was kept for 5 minutes at atmospheric pressure. Quartz tube is promptly moved into and removed from the high temperature zone of the furnace during and after the growth of the MoSe2 film. 2.3. Growth of MoSe2 Film. The growth of MoSe2 film is the same as the mosaic MoS2/MoSe2 lateral heterojunctions film except that the growth substrate is completely blank without MoS2 triangles. 2.4. Preparation of TEM Sample. TEM sample is obtained from the SiO2/Si substrate through a polymethylmethacrylate (PMMA) assisted technique. In the first place, a PMMA thin film was spin-coated at 3,000 r.p.m for 30 s on the surface of the nanolayers/SiO2/Si substrate. Subsequently, the SiO2 layer was etched in 2 M KOH solution and the PMMA/nanolayers layer would float on the liquid surface. The PMMA/nanolayers was washed by deionized water for some times, and then transferred onto a TEM grid and dried. In the end, the PMMA thin film was removed by acetone. 2.5. Characterizations. The obtained samples were characterized by optical microscopy (Leica DM4500P), atomic force microscopy (AFM, Nanoscope IIIa Vecco), transmission electron microscopy (TEM, Tacnai−G2 F30, accelerating voltage of 300 kV), Raman and photoluminescence (PL) spectra (LabRAM XploRA, incident power of 1 mW, excitement wavelength 532 nm). Cr/Au electrodes were fabricated using film−plating machines (ZHD−300) by a shadow mask. Photoelectrical measurements of the device were performed using a semiconductor analyzer (Keithley 4200 SCS) combined with a Lakeshore probe station at room temperature. 3. RESULTS AND DISCUSSION


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Large−area, uniform, and high−quality mosaic MoS2/MoSe2 lateral heterojunctions film on SiO2/Si substrate was successfully fabricated through a two−step CVD procedure. Figure 1 schematically illustrates the two−step CVD route for the preparation of mosaic MoS2/MoSe2 lateral heterojunctions film. First, triangular MoS2 monolayers were grown on substrate via the first CVD procedure. Subsequently, large−area MoSe2 film is merged together with triangular MoS2 monolayers, generating a continuous mosaic MoS2/MoSe2 lateral heterojunctions membrane by adjusting some technical parameters of CVD procedure. H2 plays a crucial role as reducing agent in the formation of MoSe2 film, consistent with the work reported by Wang et al..16,23,28 The experimental details are described in Experimental Section.

Figure 1. Schematic illustration of the two−step CVD technique for the preparation of mosaic MoS2/MoSe2 lateral heterojunctions film on SiO2/Si substrate. Figure 2 displays structures and morphologies of as−grown triangular MoS2 monolayers and mosaic MoS2/MoSe2 lateral heterojunctions film. Schematic illustration and optical image of triangular MoS2 monolayers on SiO2/Si substrate were shown in Figure 2a and b, which are brown color under the optical microscope. Growth at ∼670 °C for 20 min formed a high coverage of triangular MoS2 monolayers (Figure 2b) with almost homogeneous edge dimensions of ~10 µm. The optical image clearly shows that all MoS2 monolayers possess uniform surface, which are proved by the homogeneity in color displayed in optical image. Schematic illustration and optical image of as−grown mosaic MoS2/MoSe2 lateral heterojunctions film on SiO2/Si substrate is shown in Figure 2c and d, displaying clear boundary between MoS2 and MoSe2


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interface under optical microscope via optical contrast difference, while triangular MoS2 monolayers show a relatively light brown color and MoSe2 film as deep reddish brown color. MoSe2 film was grown along MoS2 triangle edges at ~750 °C for 5 min. Finally, MoSe2 film occupied the blank regions of the substrates with MoS2 triangles to form large−area mosaic MoS2/MoSe2 lateral heterojunctions film. Raman and PL spectroscopies of mosaic MoS2/MoSe2 lateral heterojunctions film were measured at room temperature using a laser wavelength of 532 nm. Figure 2e shows Raman spectra measured from triangular MoS2 monolayer, MoSe2 film and MoS2/MoSe2 boundary region, respectively. Two main characteristic peaks of MoS2 centered at 382.1 and 401.5 cm-1 appear in MoS2 Raman spectrum (blue line). These characteristic peaks were attributed to the in−plane E12g vibration and out of plane A1g mode of MoS2 with a frequency difference of 19.4 cm-1, demonstrating that the obtained MoS2 is monolayer. Raman characteristic peaks appear at 238.7 cm-1 and 285.4 cm-1 in Raman spectrum (red line), ascribable to the out of plane A1g vibration and in−plane E2g mode of MoSe2. At MoS2/MoSe2 boundary, four characteristic peaks were obtained. PL spectra of these different regions are also acquired in Figure 2f, suggesting disparate PL peaks for each region, as well as the characteristic peaks of MoS2 and MoSe2 simultaneously appeared at the interface. PL peak at 1.83 eV was attributed to the direct intraband recombination of the photogenerated electron hole pairs of monolayer MoS2, suggesting high crystallinity of triangular MoS2 monolayer.29 It is also in agreement with the previous report for MoSe2 PL peak at 1.54 eV.16,30 Raman intensity mapping (Figure 3c) for A1g mode at 401.5 cm-1 of a MoS2 triangle marked in square frame of optical image (Figure 3a) shows uniform red color, which indicates that triangular MoS2 district has uniform vibrational modes to further illustrate obtained high−quality MoS2. Optical image of mosaic MoS2/MoSe2 lateral heterojunctions and corresponding MoS2


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Raman intensity mapping for A1g mode at 401.5 cm-1 and MoSe2 Raman intensity mapping for A1g mode at 238.7 cm-1 are shown in Figure 3b, d and e, respectively. Raman intensity mapping images for MoS2 (Figure 3d) and MoSe2 (Figure 3e) in square frame of Figure 3b reveal that MoS2 semaphore is situated in the central triangle region, and MoSe2 signal is merely appear in the exoteric outskirts of triangular MoS2 district. These results display that MoSe2 membrane was grown along triangular MoS2 edges and occupied the blanks of the substrate to form mosaic MoS2/MoSe2 lateral heterojunctions film.

Figure 2. Structure and characterization of triangular MoS2 monolayers and mosaic MoS2/MoSe2 lateral heterojunctions film. (a,b) Schematic illustration and optical image of as−grown triangular MoS2 monolayers on SiO2/Si substrate. (c,d) Schematic illustration and optical image of as−grown mosaic MoS2/MoSe2 lateral heterojunctions film on SiO2/Si substrate. Raman (e) and PL (f) spectra of mosaic MoS2/MoSe2 lateral heterojunctions film on MoS2, MoSe2, and interface district.


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Figure 3. Optical image (a) of triangular MoS2 monolayers and corresponding Raman intensity mapping (c) of A1g mode at 401.5 cm-1. Optical image (b) of mosaic MoS2/MoSe2 lateral heterojunctions film and corresponding MoS2 Raman intensity mapping (d) of A1g mode at 401.5 cm-1 and MoSe2 Raman intensity mapping (e) of A1g mode at 238.7 cm-1. Atomic force microscope (AFM) was used to identify the thickness of thin layer products. Figure 4b shows AFM image of a single MoS2 triangle obtained from optical image (Figure 4a). A distinct height of ∼0.8 nm is monitored by AFM cross section analysis (Figure 4c) thread the triangle in Figure 4b, indicating the monolayer structure of MoS2. Figure 4d presents 3D AFM image of this MoS2 triangle. The result reveals that MoS2 triangle was bulgy on the surface of SiO2/Si substrate. AFM image (Figure 4f) from mosaic MoS2/MoSe2 lateral heterojunctions in optical image (Figure 4e) indicates that MoSe2 (light color) film is existed in outward periphery of triangular MoS2 (deep color) by different color contrast. Height profile (Figure 4g) along the dark line in Figure 4f, showing the height diversity is ∼1.5 nm as approaching from the MoSe2 film to the MoS2 triangle. The outward periphery (MoSe2 film) is higher than triangular MoS2, which demonstrate that MoSe2 film along MoS2 triangle grow to form mosaic MoS2/MoSe2 lateral heterojunctions film, consistent with Raman, PL spectra and Raman mappings. Figure 4h exhibits 3D AFM image of this mosaic MoS2/MoSe2 lateral heterojunctions, indicating that


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MoS2 triangle was cupped. In other words, MoSe2 film is higher than MoS2 triangle. However, some thick MoS2 triangles can also be observed in optical images (Figure 4a and e). The height of MoS2 triangles in this case is higher than MoSe2 film. AFM image and the corresponding height profile of the mosaic MoS2/MoSe2 lateral heterojunctions film with the thick MoS2 at the centre are shown in Figure S1. The thick MoS2 triangle is 4 nm higher than MoSe2 film.

Figure 4. (a) Optical image of triangular MoS2 monolayers. (b) 2D AFM image obtained from the optical image in (a) and its corresponding height profile (c). (d) 3D AFM image of the MoS2 triangle. (e) Optical image of mosaic MoS2/MoSe2 lateral heterojunctions film. (f) 2D AFM image captured from the optical image in (e) and its corresponding height profile (g). (h) 3D AFM image of the mosaic MoS2/MoSe2 lateral heterojunctions film. In order to give a whole and distinct analysis of the interface, the small and thick heterostructure was selected for TEM research. The low−resolution TEM image (Figure 5a) of a single mosaic MoS2/MoSe2 lateral heterojunction presents an obvious boundary by overtly differentiable comparison in heterostructure location, with the thick and gloomy interior triangle corresponding to MoS2, and outer brighter district for MoSe2 film with slight wrinkles caused by


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transfer process. The thinner heterostructure was also shown in Figure S2, a filmy triangle exists in the middle of a large film. SAED pattern in MoS2 triangle (Figure 5b) apparently exhibits one single set of hexagonally arranged diffraction spots that can be refered to six−fold symmetry of the [001] zone axis of MoS2 lattice structure. SAED pattern of MoSe2 film (Figure 5c) displays a polycrystalline crystal structure and diffraction points of (100) and (110) lattice planes. HRTEM image (Figure 5d) at the interface further confirmed the joint of single crystalline MoS2 and polycrystalline MoSe2. The lattice plane spacing of (100) plane in MoS2 region is 0.27 nm, in agreement with the previous MoS2 reports.19,31,32 However, slight fluctuation and roughness exist in MoS2/MoSe2 interface, which derive from the growth of foreign MoSe2 along with the MoS2 fringe.

Figure 5. TEM characterization of mosaic MoS2/MoSe2 lateral heterojunctions film. (a) Low−magnification TEM image. (b,c) SAED patterns taken on the area of triangular MoS2 and MoSe2 film regions, respectively. (d) HRTEM image of MoS2/MoSe2 interface. To enhance light-to-electricity conversion efficiency, electron−hole pairs are needed to be separated into free charges as far as possible. The inset in Figure 6d illustrates the conduction (C) and valence (V) bands alignment in MoS2/MoSe2 heterojunctions, forming typical type-Ⅱ band


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alignment.33,34 The lowest C and the highest V bands exist in MoS2 and MoSe2, respectively. Such alignment urges effective separation of photoinduced electrons and holes, not only lead to long photoexcited charge carrier longevity, but also suppress the interfacial electron−hole recombination. It will produce efficient p-doping in MoSe2 and n-doping in MoS2 nanolayers to increase the number of the carriers in the circuit for improve photodetector performance.27,35,36 Transfer curves (Isd-Vbg) of back-gated MoS2 and MoSe2 devices have been shown in Figure S3 to exhibit their n-type semiconductor characteristic. Therefore, electron is their principal carrier in MoS2 and MoSe2 nanolayers. They both possess stream of electrons during the testing process. Moreover, Long et al. have reported quantum coherence and donor−acceptor delocalization can encourage quick charge transfer at the MoS2/MoSe2 heterojunctions interface by using nonadiabatic molecular dynamics and time-domain density functional theory simulation methods.27 The simulation results display that quantum coherence at the MoS2/MoSe2 heterojunctions interface, encouraged by crucial delocalization of photoinduced states between the donor and acceptor substances, supports to hinder the electron−hole pair interattraction and generates valid charge separation.27 Therefore, the electron−hole recombination is distinctly much slower through the MoS2/MoSe2 heterojunctions interface than that in independent MoS2 and MoSe2, promoting effective charge separation to improve optoelectronic performance. The long photogenerated charge carrier lifetime at the MoS2/MoSe2 heterojunctions interface suggests that heterojunctions can be applied to build highly efficient optoelectronic devices. For this purpose, the mosaic MoS2/MoSe2 lateral heterojunctions photodetector was fabricated on SiO2/Si substrate using a shadow mask combined with metal Cr/Au thermal evaporation contacts. Figure 6a displays the 3D schematic view of mosaic MoS2/MoSe2 lateral heterojunctions photodetector. The corresponding optical image of practical photodetector device is shown in


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Figure 6b. Optoelectronic behaviors are detected under ambient environment at room temperature. The photodetector was measured under diverse light wavelengths and light intensities (Figure 6c-e). The photodetector displays an extensively spectral response from ultraviolet to visible illumination (Figure 6c and d). With increased bias voltage Vds, higher photocurrent was observed (Figure 6c) in dark and different light wavelengths with intensity of 0.29 mW/cm2 by reason of the augment in carrier drift speed and the associative reduce of carrier Tt (transit time) under high Vds. Figure 6d exhibits the plot of the photocurrent vs. light wavelength at the light intensity of 0.29 mW/cm2 with Vds = 5 V, which is drawn from Figure 6c. The photocurrent displays a good improvement with depressed light wavelength, because of large excitation energy was supplied by high photon energies of low light illumination wavelength to generate more excitons. The photocurrent dependent light intensity is investigated in Figure 6e at a light illumination wavelength of 610 nm. It demonstrates that the photocurrent increases with light intensity from 0.29 to 0.77 mW/cm2 at Vds = 5 V. In addition, Figure 6e shows a better linear relationship between photocurrent and light intensity, suggesting that the photocurrent relies on the number of photogenerated carriers. The photoresponsivity (R= I/PS, I is generated photocurrent, P is incident light intensity, S is effective illuminated area), detectivity (D = RA1/2/(2eId)1/2, A is effective illuminated area, e is electron charge, Id is dark current) and EQE (EQE = hcR/eλ, h is Plank constant, c is light velocity, e is electron charge, λ is incident light wavelength) are pivotal parameters to assess the performance of the photodetector, which are shown in Figure 6f and S6. The effective illumination area of present device is ~22×49 µm. The photoresponsivity, detectivity and EQE of mosaic MoS2/MoSe2 lateral heterojunctions photodetector are calculated as 1.3 A W-1, 2.6 × 1011 Jones and 263.1 % under the light illumination wavelength of 610 nm at the light intensity of 0.29 mW cm-2 with the bias voltage


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of 5 V, respectively, comparable to the other 2D TMDCs−based photodetector.32,37 In order to make a comparison, triangular MoS2 monolayer and multilayer MoSe2 film photodetectors are also fabricated and measured in Figure S4-6. The results exhibit that mosaic MoS2/MoSe2 lateral heterojunctions film photodetector possesses superior performance under the same conditions to the control groups. The responsivity, detectivity and EQE of triangular MoS2 monolayer and multilayer MoSe2 film photodetector are 0.4 and 0.7 A W-1, 1.0 × 1011 and 1.9 × 1011 Jones, 89.1 and 150.9 %, respectively.

Figure 6. Mosaic MoS2/MoSe2 lateral heterojunctions film photodetector. (a) Schematic view of the photodetector. (b) Optical image of the photodetector device. (c) Ids-Vds characteristic of the photodetector in dark and under various light wavelength with light intensity of 0.29 mW/cm2. (d) Photocurrent as a function of light wavelength at Vds = 5 V with light intensity of 0.29 mW/cm2, Inset: schematic of the conduction (C) and valence (V) bands alignment in heterostructures photodetector under illumination. (e) Photocurrent as a function of light intensity with light


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wavelength of 610 nm at Vds = 5 V. (f) Responsitivity and detectivity of the photodetector with different light intensity, light wavelength of 610 nm at Vds = 5 V. Furthermore, to evaluate the stability and reproducibility of photoswitching in photodetector, the photoswitching on and off curves of the three photodetectors as a function of time for six cycles of sequential measurements with light illumination wavelength of 610 nm and light intensity of 0.29 mW/cm2 at Vds = 5 V are presented in Figure 7a. All three photodetectors exhibit good response and recovery characteristics during six cycles, which manifests that these three photodetectors have good stability and repeatability. The response and recovery time of the photodetector are defined as the time gap for the response to increase and decline from 10% to 90% or 90% to 10% of its maximum value.38,39 As exhibited in Figure 7b, response and recovery times are obtained to be 6.3 and 4.1 s for MoS2, 0.4 and 0.6 s for MoSe2, 0.6 and 0.5 s for MoS2/MoSe2 lateral heterojunctions, respectively. The results show that these three photodetectors possess fine response and recovery speed.

Figure 7. (a) Stability of mosaic MoS2/MoSe2 lateral heterojunctions film, triangular MoS2 monolayer and multilayer MoSe2 film photodetector with light wavelength of 610 nm, light intensity of 0.29 mW/cm2 at Vds = 5 V. (b) The corresponding single cycle photoresponse of laser on and off. 4. CONCLUSIONS


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In summary, we present a novel method to grow large−area, uniform, and high−quality mosaic MoS2/MoSe2 lateral heterojunctions film on SiO2/Si substrate by CVD approach without transferring process. This facile and reliable method opens up a unique route for preparing other mosaic TMDCs lateral heterojunctions film in a manipulative manner. The mosaic MoS2/MoSe2 lateral heterojunctions film is characterized by different analytic instruments such as optical microscope, Raman, PL, AFM, and TEM. In addition, photodetectors based on mosaic MoS2/MoSe2 lateral heterojunctions film, triangular MoS2 monolayer and multilayer MoSe2 film were fabricated and investigated. The mosaic MoS2/MoSe2 lateral heterojunctions film photodetector displayed optimal photoresponse performance in comparison with individual component. The matched band alignment between MoS2 and MoSe2 are possibly responsible for the effective suppression of photo-excited electron-hole recombination, leading to the favorable n-doping of MoS2 and p-doping of MoSe2, eventually resulting to the improved photodetector performance. The responsivity, detectivity and EQE of mosaic MoS2/MoSe2 lateral heterojunctions film photodetector increased to 1.3 A W-1, 2.6 × 1011 Jones and 263.1 % under the bias voltage of 5 V at 0.29 mW cm-2 with the light illumination wavelength of 610 nm, respectively. These results demonstrate that mosaic MoS2/MoSe2 lateral heterojunction film is a promising material for optoelectronic device. We believe that this simple and reliable CVD technique can be scaled up to prepare other ultrathin mosaic 2D TMDCs lateral heterojunctions on different substrates and improve comprehensive applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXXXX.


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Optical images, AFM image, TEM image, transfer characteristics, Ids-Vds characteristics, responsitivity, detectivity and EQE curves (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel: +86 451 86403583. Fax: +86 451 86403583 (P. A. Hu). *E-mail: [email protected] (W. P. Hu). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, No.61172001, 21373068, 21303030), the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900. REFERENCES (1) Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-Like Two-Dimensional Materials. Chem. Rev. 2013, 113, 3766-3798. (2) Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and In-plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135-1142. (3) Li, H.; Duan, X.; Wu, X.; Zhuang, X.; Zhou, H.; Zhang, Q.; Zhu, X.; Hu, W.; Ren, P.; Guo, P.; Ma, L.; Fan, X.; Wang, X.; Xu, J.; Pan, A.; Duan, X. Growth of Alloy MoS2xSe2(1-x)


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