Introduction

During the past decades, two-dimensional (2D) materials have attracted enormous attention for photodetection applications due to their layered structure, mechanical flexibility, layer-dependent electronic band structures and easily constructed heterostructures [1,2,3,4,5]. Generally, the ideal detectors should have fast respond, high sensitivity and air stability, which are rare to meet at the same time in one material [6,7,8,9,10,11,12]. As the most extensively studied 2D material, graphene shows a relatively low optical absorption coefficient due to the lack of a band gap [13,14,15]. Though MoS2 has shown good properties in optoelectronic applications, the corresponding photodetectors only respond to the visible region, leading to limitation of their application in the infrared region [16,17,18]. As an analog to graphene, black phosphorus with a direct bandgap also shows promising applications in photodetectors, while it is not stable in the air [19,20,21]. Due to the availability of 2D materials with various bandgaps and work functions, bandgap engineering of heterostructures can be realized through heterogeneous stacks built by different 2D materials, providing solutions for multifunctional hybrid photodetectors [22,23,24,25].

As a new type of 2D semiconductor, Bi2O2Se has been reported to possess high mobility and superior air stability [26]. Bi2O2Se consists alternative compensating cations ((Bi2O2) 2 n+ n ) and anions (Se 2 n− n ). The layers are held together by weak electrostatic forces with an interlayer spacing of about 0.608 nm (Fig. 1a) [27]. However, the high dark current and small on/off ratio limit the performance of the photodetector based on Bi2O2Se on f-mica substrate [28, 29]. It has been reported that the Type-II heterojunction can reduce the dark current, modulate the interlayer transition energy and induce the charge spatial separation [30,31,32,33,34]. The Bi2O2Se typically exhibits an indirect band gap of ~ 1.14 eV and the monolayer MoSe2 exhibit a direct bandgap of ~ 1.51 eV [35,36,37]. Therefore, 2D MoSe2 can form a type-II van der Waals heterostructure with Bi2O2Se, which is quite promising for the performance improvement in Bi2O2Se-based photodetectors [38,39,40].

Figure 1
figure 1

Characterization of Bi2O2Se nanoplates. a Schematic illustration of the crystal structure of Bi2O2Se with tetragonal structure. b Schematic illustration of the experimental setup for the synthesis of Bi2O2Se nanoplates. c Typical OM image of the as-synthesized 2D Bi2O2Se nanoplates on mica. d HRTEM image of the Bi2O2Se nanoplate. The inset shows a SAED pattern of the Bi2O2Se nanoplate. e XRD spectra of Bi2O2Se nanoplates grown on mica (red) and a reference mica substrate (black). f The AFM image of a Bi2O2Se nanoplate. The inset shows the thickness of this Bi2O2Se nanoplate is 4.5 nm from the AFM cross-sectional profile along the white line

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Here, we constructed a type-II Bi2O2Se/MoSe2 van der Waals heterojunction and investigated the photodetection performance. Compared with the 2D Bi2O2Se photodetector, the dark current was significantly reduced and the on/off ratio was greatly improved in the Bi2O2Se–MoSe2 heterostructure. Moreover, the Bi2O2Se–MoSe2 based photodetector showed a broadband photoresponsivity from visible (405 nm) to near-infrared (808 nm) light illumination. Compared with the 2D Bi2O2Se photodetector, the photocurrent response and recovery time were greatly reduced in the heterojunction photodetector. Under the illumination of 780 nm laser, the heterojunction showed a responsivity of 413.1 mA W−1 and a detectivity of 3.7 × 1011 Jones at 2 V voltage.

Experimental section

Synthesis of 2D Bi2O2Se

The 2D Bi2O2Se crystals were synthesized through chemical vapor deposition method (CVD) in a dual zone split tube furnace (OTF-1200X, Hefei Kejing Material Technology Company Ltd., China) equipped with a 40-mm-diameter quartz tube. Typically, the source materials of Bi2O3 powder (aladdin, 99.99%) and Bi2Se3 power (aladdin, 99.99%) were placed in the center and upstream by 5 cm of high-temperature zone, respectively. The freshly cleaved fluorophlogopite mica ([KMg3(AlSi3O10)F2]) was placed in the center of low-temperature zone as the target growth substrate (Fig. 1b). High-purity Ar gas was used as the carrier gas with a constant flow rate of 300 sccm and the pressure was kept at 0.3 atm. The whole reaction process was carried out under the furnace temperature of 640 °C and 540 °C and the growth was maintained for 30-60 min. Then, the furnace was naturally cooled down to the room temperature.

Characterization

The morphology of the as-grown Bi2O2Se was characterized by optical microscopy (OM, Olympus BX51 M microscope). The X-ray diffraction (XRD, X’Pert Powder PANalytical B.V.) was used to confirm the lattice structure of the 2D Bi2O2Se. The atomic force microscopy (AFM, AIST-NT) was used to investigate the thickness of 2D Bi2O2Se crystals. The element valence state of 2D Bi2O2Se crystals was characterized by X-ray photoelectron spectroscopy (XPS, AXIS Supra). Raman spectra were collected with a confocal Raman spectrometer (LabRAM HR Evolution) using a He–Ne laser (633 nm). The as-synthesized samples were transferred onto the copper grid supported lacey carbon film by using a HF-etched transfer method and the crystal structure was characterized by transmission electron microscopy (TEM, JEOL 2100) [26, 41].

Device preparation

The Bi2O2Se/MoSe2 heterojunctions were achieved through a polymethyl methacrylate (PMMA)-mediated transfer method. Firstly, the PMMA (450 K, Suzhou Research Semiconductor Company Ltd., China.) was spin-coated over 2D MoSe2 grown on SiO2/Si substrates (Shenzhen 6Carbon Technology Company Ltd.) and baked at 80–90 °C for 15 min to facilitate intimate adhesion of the PMMA layer with MoSe2. After that, the SiO2/Si substrate was etched in 2.5 mol L−1 NaOH solution to separate the PMMA/MoSe2 with the Si substrate. After several washes in the deionized (DI) water, the PMMA-MoSe2 film was then transferred onto the mica substrate with grown 2D Bi2O2Se. After baking at 110 °C for 30 min, the PMMA was finally removed with acetone and the mica substrate was cleaned with alcohol and DI water. The metallic contacts for the Bi2O2Se/MoSe2 heterojunction were fabricated by standard electron beam lithography, thermal deposition of Cr/Au (10 nm/50 nm), and lift-off processes.

Optoelectronic measurements

The electrical measurements were performed using Lake Shore CPX probe station and Keithley 4200 semiconductor characterization system. The excitation laser wavelengths used in this paper were 405 nm, 515 nm, 660 nm, 780 nm and 808 nm.

Results and discussion

Single crystalline 2D Bi2O2Se samples were synthesized through a low-pressure CVD method. According to the OM image (Fig. 1c), Bi2O2Se nanoplates up to 40 μm with square morphology were obtained, indicating the tetragonal structure of Bi2O2Se. The corresponding high-resolution transmission electron microscopy (HRTEM) image in Fig. 1d indicates the single crystallinity and tetragonal structure of our synthesized Bi2O2Se nanoplates, which is also confirmed by the selected area electron diffraction (SAED) pattern (insert in Fig. 1d). As indicated in the HRTEM, d-spacing of 0.27 nm and 0.19 nm correspond, respectively, to the (200) and (110) planes with angle of 45°. The XRD spectrum (Fig. 1e) peaks of 14.39°, 29.15°, 44.44° correspond to the (002), (004) and (006) diffractions of tetragonal Bi2O2Se. According to the AFM image shown in Fig. 1f, the surface of the as-grown sample is homogeneous and the thickness is about 4.5 nm, which corresponds to a seven-layer sample.

The CVD synthesized 2D MoSe2 (Shenzhen 6Carbon Technology Company Ltd.) generally showed a thickness of 1.3 nm (Fig. S1(a)). The HRTEM image and corresponding SAED pattern (Fig. S1(b)) showed the single crystallinity of the synthesized MoSe2. With PMMA-assisted transfer method, the Bi2O2Se/MoSe2 heterojunctions were constructed by transferring MoSe2 directly onto the as-grown Bi2O2Se on mica (Fig. 2a). Due to the inertness of f-mica and strong electrostatic interaction between the Bi2O2Se with the substrate, hydrofluoric acid (HF) becomes the only f-mica etchant which can help the transfer of Bi2O2Se/MoSe2 from the f-mica to the TEM grid. However, as a component of typical buffered oxide etchant, Bi2O2Se can be inevitably etched by HF during the transfer process. Therefore, the Raman spectroscopy was performed to further verify the Bi2O2Se/MoSe2 heterojunction. Figure 2b shows the corresponding Raman spectra for MoSe2 and Bi2O2Se obtained from point 1 and point 2 in Fig. 2a, where the A1g (240 cm−1), \( E^{1}_{{2{\text{g}}}} \) (280 cm−1) mode of MoSe2 and A1g (160 cm−1) mode of Bi2O2Se can be observed. The Raman intensity mappings of the white framed area in Fig. 2a with the A1g mode of MoSe2 at 240 cm−1 (Fig. 2c) and A1g mode of Bi2O2Se at 160 cm−1 (Fig. 2d) show clearly the formation of the Bi2O2Se/MoSe2 heterojunction (Fig. 2e).

Figure 2
figure 2

Raman spectroscopy analysis of the Bi2O2Se/MoSe2 heterojunction. a The OM image of a Bi2O2Se/MoSe2 heterojunction. b The Raman spectra taken from points 1 and 2 in a, showing pristine MoSe2 and Bi2O2Se areas, respectively. The Raman intensity mappings at c 240 cm−1 (A1g mode of MoSe2) and d 160 cm−1 (A1g mode of Bi2O2Se). e The superposition of c and d

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In order to study the photoresponse performance of the Bi2O2Se/MoSe2 heterojunction, two-terminal devices were fabricated and the schematic illustration of this device is presented in Fig. 3a. Figure 3b shows the I − V curves of the photodetector in dark and under light illuminations with various wavelengths. According to our results, it can be seen that the photodetector experienced a remarkable current increase under light illuminations and the Ion/Ioff ratios at 2 V bias voltage are 17.89, 746.45, 434.58, 218.01 and 60.25 when illuminated by lasers with wavelength of 405 nm (0.2 mW cm−2), 515 nm (40.9 mW cm−2), 660 nm (114.1 mW cm−2), 780 nm (1197.1 mW cm−2) and 808 nm (32.47 mW cm−2). When compared with the performance of our synthesized pure Bi2O2Se (Fig. S2 in the supporting information), the dark current was significantly lowered (2.1 pA vs. 150 pA) and the Ion/Ioff ratio was greatly improved (746.45 vs. 20.28 at 515 nm) through the construction of Bi2O2Se/MoSe2 heterojunction.

Figure 3
figure 3

a Schematic illustration of the Bi2O2Se/MoSe2 heterojunction photodetector. bI − V characteristics of the photodetector in dark and under illumination of lasers with wavelength of 405, 515, 660, 780, and 808 nm, respectively. cI − V characteristics of the Bi2O2Se/MoSe2 heterojunction photodetector under illumination of 780 nm laser with various laser intensities. d Responsivity and detectivity of the Bi2O2Se/MoSe2 heterojunction photodetector under illumination of 780 nm laser with different power at 2 V bias

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To quantify the effect of light intensity on the device performance, the I − V characteristics of the Bi2O2Se/MoSe2 heterojunction photodetector were measured under illumination of 780 nm laser with power intensities ranging from dark to 1191.7 mW cm−2. The responsivity (R) and specific detectivity (D*) of this photodetector can be obtained based on the following equations [42,43,44,45,46]:

$$ R({\text{A}}\;{\text{W}}^{ - 1} ) = \frac{{I_{\text{ph}} }}{{P_{\text{opt}} \cdot S}} $$
$$ D^{*}({\text{Jones}}) = \frac{\sqrt A \cdot R}{{\sqrt {2eI_{\text{D}} } }} $$

where Popt, S, A, e, and ID are light intensity, irradiation area, device area, unit charge, and dark current, respectively. The responsivity and detectivity as a function of light intensities (780 nm) are presented in Fig. 3d. Both the responsivity and the detectivity decrease with the increasing light intensities, and reach 413.1 mA W−1 and 3.7 × 1011 Jones (1 Jones = 1 cm Hz1/2 W−1) under the intensity of 0.09 mW cm−2.

The I − V curves under light illumination with different wavelengths revealed that the Bi2O2Se/MoSe2 heterojunction photodetector has a broad photoresponse from 405 to 808 nm (Fig. 3b). Therefore, the photoresponse properties of Bi2O2Se/MoSe2 heterojunction photodetector to incident light with different wavelengths were further investigated. As shown in Fig. 4a–e, the photodetector exhibited stable and repeatable photoresponse to lasers with wavelengths of 405 nm (0.2 mW cm−2), 515 nm (40.9 mW cm−2), 660 nm (114.1 mW cm−2), 780 nm (1197.1 mW cm−2) and 808 nm (32.47 mW cm−2) under bias of 1.5 V, indicating that Bi2O2Se/MoSe2 heterojunction photodetector can response to light signals with a wide spectrum range from visible to NIR.

Figure 4
figure 4

a–e Time-dependent photoresponse of Bi2O2Se/MoSe2 heterojunction photodetector at 1.5 V voltage under illumination of 405, 515, 660, 780, and 808 nm laser, respectively. f Schematic illustration of the energy band diagrams for the Bi2O2Se/MoSe2 heterojunction

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The response speed of the Bi2O2Se/MoSe2 heterojunction photodetector was evaluated by analyzing the rising and falling edges of individual response cycle (Fig. S4). The response time was calculated to be 0.79/0.49 s under illumination of 515 nm with power intensity of 40.9 mW cm−2 at 1.5 V bias. Compared with the 2D Bi2O2Se photodetector (Fig. S3), the photocurrent response and recovery time was greatly reduced in the heterojunction photodetector. The device performances of the Bi2O2Se/MoSe2 heterojunction photodetector and some recently reported 2D material-based photodetectors are summarized in Table 1. According to the summarized date, our reported Bi2O2Se/MoSe2 heterojunction photodetector show higher detectivity and the measured rise/decay time is closely comparable to the values reported for many 2D materials-based photodetectors [47,48,49,50,51,52,53,54,55,56,57,58]. Moreover, the Bi2O2Se/MoSe2 heterojunction show higher responsivity than other 2D material-based heterojunctions like CuO/MoS2, MoS2/MoTe2, MoS2/graphene etc. [47,48,49,50,51] It has been reported that the longer rise and decay time observed in similar systems can be attributed to unavoidable intrinsic and/or extrinsic charge traps, e.g., surface states and atmospheric contamination [59, 60]. In our devices, the heterojunction was fabricated by transferring MoSe2 onto Bi2O2Se through a PMMA-assisted method, which can inevitably introduce undesired contamination at the interface between Bi2O2Se and MoSe2 [36]. The chemical residues left on the surface of active materials during the removal of PMMA will form undesired charge impurities [22]. Moreover, the etching process with NaOH to separate the PMMA/MoSe2 with the Si substrate may also introduce some chemical degradations on the MoSe2 layers, resulting in a relatively longer rise and decay time. Since the direct growth of 2D heterojunction will help us get a high-quality clean interface, growing Bi2O2Se/MoSe2 heterostructures without a transferring method can help to further improve the corresponding photodetection performance of the Bi2O2Se/MoSe2 system [61].

Table 1 2D material-based photodetectors and performance comparison with our results
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Figure 4f illustrates the alignment of electronic bands of Bi2O2Se and MoSe2. It shows that the electron affinities of Bi2O2Se and MoSe2 are 4.54 eV and 3.96 eV, the bandgaps are 1.14 eV and 1.51 eV, respectively. Consequently, the Bi2O2Se/MoSe2 heterostructure forms a type-II heterojunction, with the conduction band minimum residing in Bi2O2Se and the valence band maximum in MoSe2 [26, 35, 36]. In type-II heterojunctions, the conduction band minimum and valence band maximum reside in two separate materials. Photoexcited electrons and holes therefore prefer to stay at separate locations. As demonstrated in Fig. 4f, electron-hole pairs exist under the light illumination, and the electrons on the conduction band transfer from MoSe2 to Bi2O2Se, while the holes on the valence band transfer from Bi2O2Se to MoSe2, resulting in the efficient charge separation [62].

Conclusion

In conclusion, the 2D layered Bi2O2Se samples have been synthesized with low-pressure CVD method and the Bi2O2Se/MoSe2 heterojunction with type-II band alignment was constructed for photodetection. Our results indicate that this heterojunction photodetector showed broadband detection ranging from visible (405 nm) to near infrared (808 nm) with a responsivity of 413.1 mA W−1, detectivity of 3.7 × 1011 Jones (at 780 nm). Compared with the 2D Bi2O2Se photodetector, the dark current was significantly reduced and the Ion/Ioff ratio was greatly improved. Importantly, the rise/decay time of the Bi2O2Se/MoSe2 heterojunction photodetector was reduced from 1.92/1.31 to 0.79/0.49 s under the illumination of 515 nm (40.9 mW cm−2 at 1.5 V). Our results showed that 2D Bi2O2Se/MoSe2 heterojunction has promising applications in the field of broadband and fast photodetection.