metallic WSe2 nanosheet heterostructure films

Materials Science in Semiconductor Processing 107 (2020) 104851

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Excellent photoresponse performances of graphene/metallic WSe2 nanosheet heterostructure films H.-Y. He *, Z. He, Q. Shen School of Material Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, 710021, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Metallic WSe2 Nanosheet Graphene Photodetection Interface effect

The metallic (1T) structure of transition metal dichalcogenides is of many peculiar chemical and physical properties such as higher carrier mobility and narrower bandgap compared to 2H structure and so maybe beneficial in their photodetection application. Currently, efficient assembly of 1T-WSe2 in the one-step process is still an enormous challenge. Here we report a modified facile hydrothermal approach assembling 1T-WSe2 nanosheets and 1T-WSe2/reduced graphene oxide (RGO) hybrids. The photodetector based on the assembled 1TWSe2 nanosheets showed very high photocurrent and extremely intense and fast light responses. The RGO incorporation introduced a remarkable charge transfer at the heterojunction interface and further dramatically heightened the photocurrent and light responsivity.

1. Introduction Nanostructured photodetectors operating from visible to ultraviolet (UV) are have attracting much attention due to their appealing property for various applications. Photodetectors based on low-dimensional nanostructured materials have become one of the most attractive photoelectronic devices that can be made using individual or assemblies of nanostructures. Transition metal dichalcogenides (TMD) and gra­ phene are all two-dimensional materials. Recently, TMD and RGO/TMD have been studied for photodetection applications because of high car­ rier mobilities and internal quantum efficiency [1–6]. The 1T structure of TMD material is very important for photo-detection application because of its high carrier mobilities and narrow bandgap compared to the 2H structure. However, the TMD in the reported photodetectors were all 2H structure. The WSe2 synthesized with many chemical pro­ cesses such as chemical vapor deposition [7,8] and solvothermal [9] usually were 2H structure. Currently, 1T-TMD is mainly obtained by some exfoliation [10–17]. But these exfoliation methods were partially inefficient to obtain 1T-WSe2 nanosheets [11,15–17]. The RGO is an ideal template to induce the formation of two-dimension materials. Up to date, the effect of some assemble methods and template effect of graphene on the phase and performances of WSe2 are insufficiently explored. Besides, RGO was efficiently utilized to construct hybrid products with some semiconductors to achieve outstanding

performances by interface effect that results in efficient separation of photogenerated charges and high photocurrent [18,19]. This work presents the fabrication and highly efficient photo­ detection performance of 1T-WSe2 and RGO/1T-WSe2 utilizing a onestep hydrothermal route. 2. Material and method The raw analytical reagents directly purchased without further pu­ rification. The samples in this work were prepared by the following routes. First, 10 ml W4þ precursor solution with a concentration of 0.20 mol/l was prepared with ammonium paratungstate ((NH4)10(H2W12O42)⋅4H2O) and 10 ml deionized water. At the same time, another 10 ml W4þ precursor solution with a concentration of 0.2 mol/l was prepared by replacing in 10 ml deionized water with 10 ml GO aqueous solution (1 mg/ml) to achieve a GO to Mo mass ratio of 0.05. ~2.5 ml HCl aqueous solution (37%) was added into the two W4þ solutions to reduce the GO to the RGO and W6þ to W4þ. Citrate acid (C6H8O7) with a molar ratio of citric acid/W4þ ¼ 1.5 as a chelating agent was also added. 5 ml Se2 aqueous solutions with a concentration of 0.8 mol/l were prepared with selenium powder and reducing agent potassium borohydride (KH4B). Afterword, the W4þ solutions and Se2 aqueous solutions were mixed in 25 ml hydrothermal kettle. Meanwhile, polyethylene naphtholate (PEN) substrates were ultrasonically cleaned

* Corresponding author. E-mail address: [email protected] (H.-Y. He). https://doi.org/10.1016/j.mssp.2019.104851 Received 28 August 2019; Received in revised form 12 November 2019; Accepted 14 November 2019 Available online 17 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.

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3. Results and discussion The TEM micrograph (Fig. 1) shows that the nanostructures are of nanosheet morphology with a curved edge and the wide size of ~40–150 nm. The XRD analysis (Fig. 2a) indicated that the nano­ structures are hexagonal WSe2 (JCPDS: 87–2418, a ¼ b ¼ 3.286 Å, and c ¼ 12.98 Å). The Raman spectra (Fig. 2b) showed three peaks centered at 135, 189, 260 cm 1. These peaks are similar with the 150.7 (J2), 211.4 (J3), 289 (E12g) cm 1 modes of 1T-MoSe2 [20]. The generally reported mode at ~245 cm 1 for 2H–WSe2 (E12g) [8,21] does not appear. The 135 cm 1 mode has been reported for 1–5 L mechanical exfoliated WSe2 that was identified to be 1T-phase [21]. Sokolikova et al. [22] reported that there is currently no information on the vibrational modes of the 1T0 -phase of WSe2, although overall lower symmetry of the 1T0 -phase suggests a wider set of the Raman active modes. They gave the experimental Raman modes at 104.5, 149, 177, 218, 236.3 and 258 cm 1 for the 1T0 - phase of WSe2. The reported mode at 258 cm 1 is closed to the mode at 260 cm 1 in this work. The reported modes at 177 and 218 cm 1 are closed to the wide mode at 189 cm 1 in this work. The 1T- and 1T0 -phases have similar lattice structures and thereby having similar Raman modes. It can be believed that the Raman mode can slightly shift because of the crystallinity that determine the lattice spacing and interatomic interaction. Thus, we identify all the samples to be pure 1T-phase. Moreover, the weak mode at 325 cm 1 corresponding to multilayer structure [21,23] appears for the WSe2 nanostructures, but not for the RGO/WSe2 nanostructures. This implies that RGO results in the transfer from multilayer to a single layer. This work suggests a new and efficient method for assembling 1TWSe2. The citric acid and low hydrothermal temperature could play a critical role in the formation of 1T structure. The citric acid slowed down the reaction rate between W4þ and Se2 , which is favorable for the formation of few-layers nanosheets and so the 1T structure. The lowtemperature is a necessary condition for the formation of 1T structure because 1T-structure is thermodynamically stable only at low-

Fig. 1. TEM micrographs of the (a) WSe2 and (b) WSe2/rGO nanostructures.

and then vertically placed into the kettles. After stirring for 5 min, the kettle was sealed and undergone a hydrothermal treatment at 160 � C for 24 h. When naturally cooled for 12 h, the films on the PEN substrate were rinsed with deionized water and the powders were collected by washing with deionized water for five times. Final products were dried at 50 � C for 10 h. The films on the PEN substrate were further con­ structed to be a photodetection device by depositing Au electrodes. The powder samples were used for some characterizations. The samples were characterized with D/Max-2200PC X-Ray diffractometer (XRD, CuKα1, λ ¼ 0.15406 nm), G2 F20 S-TWING2 F20 STWIN transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Renishaw-invia Raman spectrophotometer. The samples were also characterised with FluoroMax-4P fluorescence spec­ trometer, Cary 5000 spectrophotometer, and Axis Ultradld X-ray photoelectron spectroscopy (XPS). The current-voltage (I–V) spectra were determined with YGCS multichannel electrochemical workstation. The spectral response characteristics were determined with an Oriel Apex monochromator sources.

Fig. 2. (a) XRD patterns, (b) Raman spectra, (c) photoluminescence spectra, and (d) UV–visible absorbance spectra of the WSe2 and RGO/WSe2 nanostructures. 2

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Fig. 3. (a) XPS survey spectra and XPS spectra of (b) W 4d3/2, (c) Se 3d, and (d) C 1s of the WSe2 and RGO/WSe2 nanostructures.

temperature. Similarly, we have synthesized the 1T-MoS2 and 1T-MoSe2 by low-temperature chemical solution reaction and hydrothermal pro­ cesses [24–26], where citric acid and RGO all played an important role to varying degrees. Fig. 2b shows D-band at ~1350 cm 1 and G-band at ~1599 cm 1 for RGO. The ratio (ID/IG) of band intensity was estimated to be 1.625 for the MoSe2/rGO nanostructures. This ratio value is larger than that of the graphene oxide (0.89), implying a good reduction of the GO. Fig. 2c showed one strong emission at ~766 nm as excited at 445 nm. This emission corresponds to a bandgap of 1.62 eV. Currently, the bandgap of 1T-WSe2 is rarely reported although the bandgap of 2H–WSe2 have been determined by photoluminescence spectra analyses (1.62–1.63 eV [27,28]), ultraviolet photo-electron spectroscopic mea­ surements (2.08 eV [28]), normalized wavelength dependence of the photocurrent signals (1.68 eV [29]), and a theoretical calculation (~1.55 eV [30]). 1T-WSe2 has a narrower bandgap than 2H–WSe2. The photoluminescence spectra analysis in this work gives a bandgap that is

only near or slightly narrower than the reported value of 2H–WSe2. This could originated from the well known quantum refinement effect because of nano-scale size of the 1T-WSe2. The RGO incorporation resulted in the enhanced emission intensity. This implies the carrier transfer from RGO to WSe2. Similar photoemission enhancement has also been reported for MoS2 [31,32] and WSe2 [33] by coupling plasmons. Diffused-reflection absorbance spectra as shown in Fig. 2d reveals strong absorptions in UV to near-infrared region. In ~700–1500 nm, RGO/WSe2 hybrids show a stronger absorption than the WSe2. This could come from the higher absorbance and localized surface plasmons of RGO in this region. The stronger absorption could be also ascribed to the thinner WSe2 nanosheets resulted from the template effect of RGO that increases the site density of light response in higher wavelength region. The element chemical state and charge transfer between the RGO and WSe2 were investigated by XPS analyses. The survey spectrum of

Fig. 4. (a) Schematic illustration of the photodetector and (b) band alignment of RGO/WSe2. 3

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Fig. 5. (a) I–V curves in the dark and under 540 nm illumination with 0.2 μW/cm2, (b) Dynamic photoresponse measured in response to 540 nm light on and off with different incident light powers, (c) Photocurrent as a function of the incident light intensity, (d) The enlarged figure of the dynamic photocurrent at 1.0 μW, (e) Photoresponsivity as a function of the incident light intensity, and (f) spectral photoresponsivity of the WSe2 and RGO/WSe2 photodetector.

WSe2 and RGO/WSe2 nanostructures (Fig. 3a) illustrates the peaks corresponded to W, Se, and C. The W peak at 283.0 eV is ascribed to the W 4d3/2 spin-orbital splitting photo-electrons in the WSe2 nano­ structures (Fig. 3b). This peak is shifted to lower binding energy region (282.4 eV) after the incorporation of RGO. The peak at 53.0 eV corre­ sponded to Se 3d also shifts to lower binding energy (52.9 eV) after the incorporation of RGO (Fig. 3c). Whereas the C 1s peak at 283.1 eV up­ shifts to 283.2 eV after the incorporation of RGO (Fig. 3d). The shift of binding energies of W4þ 4d3/2, Se 3d, and C 1s reveals an obvious charge transformation from RGO to WSe2. This is consistent with the photoluminescence analysis. Fig. 4a illustrates the schematic diagram of the photodetector. The deposited WSe2 and RGO/WSe2 films on the PEN substrate are smooth and dense. To a great extent, the charge transfer at heterojunction interface related to their band energy level alignment. Using reported

methods [34,35], the absolute work function (χ) of an intrinsic semi­ conductor can be calculated. For the WSe2, χ was calculated to be 5.34 eV. The density functional theory calculation gave χ of 4.25 eV for single-layer RGO [36]. Therefore, the Ec (6.15 eV) and Ev (4.53eV) of WSe2 were calculated on basis of the relations (Ec ¼ χ þEg/2 and Ev ¼ χ -Eg/2) and the Eg (~1.62 eV) of the WSe2 as obtained from the photoluminescence spectra. Because the Ec of the WSe2 was lower than that of RGO, the photogenerated electrons can efficiently transfer from the RGO to the WSe2 (Fig. 4b). This is consistent with the photo­ luminescence and XPS spectra analyses. The I–V curves of the WSe2 and RGO/WSe2 photodetectors were determined in the dark and under 540 nm irradiation with the intensity of 0.2 mW/cm2 and showed in Fig, 5a. The photocurrent (Iph ¼ Ilight Idark) enlarged as the increase of applied bias. When the applied bias is 6 V, the photocurrents of the WSe2 and RGO/WSe2 4

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photodetectors are ~45 and 73 μA, respectively, larger values than 8.34 μA of 2H–MoS2/graphene photodetector [1]. These higher photo­ responses of WSe2 and photodetector could be mainly originated from the high conductivity of 1T-structures. When the RGO incorporation, the RGO/WSe2 showed higher photoresponse than the WSe2. This could be due to the RGO-introduced interface effect resulting in higher conduc­ tivity. The photocurrents under different light powers illustrations were measured at 5 V bias voltage. The results indicated that the WSe2 and RGO/WSe2 photodetectors had high photocurrents under different incident light power (Fig. 5b) and the photocurrent enlarged as the in­ crease of the incident light power (Fig. 5c). The time-resolved photo­ current responding to “on” and “off” of the light irradiation (Fig. 5d) indicates that the times of rising and decay were 0.001 and 0.002 s, respectively. These responses are 20 times faster than that (0.02 and 0.03 s) of the graphene-MoS2 heterostructure [1] and even 280–750 times faster than that (0.28 and 1.5 s) of the other graphene-MoS2 het­ erostructures [2,3]. This faster response can be ascribed to the higher conductivity of 1T-structure of WSe2. The responsivity (R) also is an important performance index of a photodetector. The R was calculated with the relation R¼(Ilight Idark)/ Pph (where Pph is the power of incident light). The responsivity decreases with increasing Pph (Fig. 5e). This predictable tendency agrees with the result of the MoS2/graphene photodetectors [2,3]. At the minimal Pph of 0.2 μW/cm2, the RGO results in a maximal R increase from ~89 to ~105 A/W. These R values are as high as ~2 magnitude orders of R of the high-crystallinity graphene (~0.835 A/W) [1], five magnitude orders of R of the exfoliated MoS2 (~0.42 mA/W) [4], and four magnitude orders of R of graphene photodetector (~6.1 mA/W) [3]. The photoresponsivity of the WSe2 and RGO/WSe2 photodetector measured in the range of 300–1000 nm at 5 V bias voltage and showed in Fig. 5f. The photodetectors show higher photoresponsivity in the UV–visible region. Low photoresponsivity in the near infrared region could be related to the low light absorbance. The wavelength depen­ dence of the photoresponsivity shows a band edge at ~770 nm. This agrees with the bandgap of WSe2 (~1.53 eV) estimated from the UV–visible spectra. This provides the possibility of wavelength-selective photodetection in comparison with the pure graphene photodetectors [37].

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4. Conclusion RGO/1T-WSe2 nanostructures were synthesized by a modified hy­ drothermal process at a low temperature. The hydrothermal process is an efficient method for depositing smooth and dense WSe2 in the RGO/ WSe2 nanostructure films on soft polyethylene naphtholate substrate. The synthesized WSe2 and the WSe2 in the RGO/WSe2 nanohybrids showed 1T nanosheet structure. The 1T-structure led to a remarkably enhanced photocurrent because of high carrier mobility. At the same time, RGO incorporation introduced an efficient charge transfer at het­ erojunction interface, and thereby remarkably enhancing the photo­ current. The WSe2 nanostructure photodetectors showed extremely intense light responses. The RGO incorporation further enhanced light responses. The photoresponsivity of the WSe2 and RGO/WSe2 reached ~89 and ~105 A/W. The times of rising and decay of the light response were as short as 0.001 and 0.002 s, respectively. The excellent light response performances are ascribed to the high electrical conduction resulted from the 1T-structure of WSe2 and the interface-induced effect between the WSe2 and RGO. Moreover, the photodetectors showed selectively intenser spectra responses in the UV–visible range of 300–700 nm than in longer wavelength, the band edge appeared at ~770 nm. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 5

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