Introduction

The p–n junction, is well established for its potential applications in photodiodes, photodetectors, photovoltaics, and solar cells based on silicon, germanium, and compound semiconductors1,2,3,4,5,6,7. Although silicon-based technologies remain the main material for current optoelectronic devices, the miniaturisation of devices with silicon as the active channel material has proven to be complicated by grain boundary constraints8,9,10. To resolve this, two-dimensional (2D) materials have been widely studied in the past two decades as alternatives owing to their superior mechanical and electronic properties at atomic scale thicknesses and lack of grain boundary constraints11,12,13.

Recent studies have focused on heterostructuring 2D materials, i.e. combining different 2D materials together to create ultrathin p–n junctions14,15,16. This approach is usually difficult to implement with the conventional bulk semiconductors, as the materials structure must be carefully aligned to avoid defects at the interface due to the crystal lattice mismatch17,18,19. Alternatively, heterostructuring 2D materials eliminates this issue, as the 2D materials are typically free from surface dangling bonds and are bound together by weak van der Waals forces, allowing for materials of different compositions, crystal lattices, and intrinsic properties to be heterostructured together20,21,22. The atomically thin nature of these semiconductor 2D materials can further facilitate the transformation from monolithic chips to appropriate flexible substrates, expanding their potential application by reducing their size, weight, and power requirements23,24,25,26.

To establish a suitable combination of p- and n-type materials for the desired application, band alignment engineering based on the band energy levels of individual materials are used to ascertain the type of band alignment of the resulting p–n junction27,28. Typically, Type-I (straggling) band alignment aids the recombination of charge carriers at the interface for electromagnetic wave emission29,30. In contrast, Type-II (staggered) band alignment is advantageous for separating charge carriers, making it suitable for photodetection and photovoltaic applications. Type-II van der Waals heterostructures hold significant promise for self-powered 2D excitonic photodetectors due to their ability to separate electrons and holes between the individual constituent 2D materials, reduced exciton binding energy, and potentially enhanced optical absorbance compared to their standalone components20,31. To date, several Type-II heterostructures of 2D materials are reported by carefully forming band alignment engineering and material selection for light-harvesting and self-powered applications32,33,34,35,36. However, current self-powered 2D photodetectors face critical challenges, including limited built-in electric fields leading to inefficient charge separation and material constraints due to symmetry37. Enhancing photocurrent and excitonic dynamics in heterojunctions remains complex, while ensuring wide-range light detection and stable performance over time. Moreover, integrating these devices onto flexible platforms, are significant barriers to their broader adoption38,39. All of this requires investigating different material heterostructures and understanding their interfacial dynamics.

Recent studies have indicated that heterostructuring of iron phosphorus trisulphide (FePS3) and molybdenum disulfide (MoS2) could provide an avenue to achieve tuneable photoluminescence and photocurrent generation based on the applied voltage40. The resulting heterostructure can form either a Type-I or Type-II band alignment based on the thickness-governed band gap of each material, which can be favourable for both enhanced emission or charge separation, respectively. However, due to the lack of consistent band gaps of FePS3 and proximity of band energy levels with MoS241,42,43,44, it is challenging to determine which type of band alignment is formed solely based on theoretical estimations alone45,46,47,48.

As multilayer FePS3 and MoS2 have similar band gaps and broadband optical absorption42,44, the resulting p-n junction is predicted to possess broadband photovoltaic capabilities in the visible regime of the electromagnetic spectrum. To date, most 2D-material-based p-n junction photovoltaic devices have been explored for single-wavelength photovoltaic applications49,50,51. This can be favourable for applications such as wavelength-selective self-driven photodetectors, however, they are not suitable for broadband photovoltaic detection, which has potential application in optical imaging. One study conducted by Long et al. reported extraordinary photovoltaic performance using MoS2-graphene-WSe2 heterostructures52, which exhibited high broadband current density of ~239 mA/cm2 to visible—mid-IR laser irradiation. However, this structure consisted of more than two layers of 2D materials, making the heterostructuring process quite complicated and thus limiting its translation to flexible platforms and scalability.

In this study, we present a simple heterostructure of FePS3 and MoS2 as the p- and n-type materials, respectively, and investigate its broadband optical response in the visible spectrum41,42,43,44, and demonstrate a broadband photovoltaic device. We have shown that heterostructuring multilayer MoS2 with FePS3 results in a Type-II band alignment, which improves the charge separation at the junction. The type of band alignment was verified with measurements of the band energy levels of each material, and the enhanced charge separation was also confirmed through excitonic lifetime measurements. These complement the significant reduction in response time of 60 and 99% to illuminations with 565 nm and 660 nm wavelengths observed at the junction, respectively. Furthermore, our heterostructure devices show a broadband photovoltaic response to visible light, where a short-circuit current (ISC) of up to 80 pA (current density of ~0.29 mA/cm2) is exhibited in response to visible wavelength illuminations without any electrical bias, which is comparatively higher than the reported two-materials based photovoltaic heterostructures. Our heterostructure devices can be translated onto flexible platforms as well, opening up opportunities for flexible broadband imaging applications and wearable devices with self-powering capabilities.

Results and discussion

Layout and fabrication of MoS2/FePS3 p–n heterojunctions

The p–n junction was realised by first heterostructuring mechanically-exfoliated MoS2 flakes (n-type) on top of FePS3 flakes (p-type) using a wet transfer method on a SiO2-coated p-doped silicon substrate with polycarbonate as a sacrificial layer, as depicted in Fig. 1a. This was followed by evaporation and patterning of gold (Au) contacts on the heterostructure (Fig. 1b), with nickel (Ni) as the adhesion layer (further fabrication details are outlined in the Methods section). Ni was selected as the adhesion layer for the device contacts as its work function is close to the conduction band of MoS2, making it more compatible53,54. Prior to studying the electrical properties of the heterojunction, the material properties of the structure were first analysed. Atomic force microscopy (AFM) analysis of the overlapping heterojunction reveals that the FePS3 and MoS2 flakes have thicknesses of 7.2 ± 0.9 nm and 13.4 ± 1.0 nm, respectively (Fig. 1c, d).

Fig. 1: Structure of heterostructure device.
figure 1

a A schematic illustration of p–n junction device on SiO2/Si substrate, where MoS2 was positioned on top of FePS3. b Optical micrograph of a representative FePS3-MoS2 heterostructure device fabricated with electrical contacts. Outline region in white indicates (c) the atomic force micrograph region of the heterostructure, where (d) the thicknesses of FePS3 and MoS2 were measured to be 7.2 ± 0.9 nm and 13.4 ± 1.0 nm, respectively.

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Spectroscopic characterisations

Raman spectroscopy was also performed on the individual materials with a 532 nm laser and 1800 g/mm grating (Fig. 2a). FePS3 exhibited multiple phonon modes at 155.9, 226.6, 246.0, 278.6, and 379.6 cm−1 (bottom panel, Fig. 2a), which is representative of multilayer monoclinic FePS355,56. MoS2 on the other hand exhibited E12g and A1g phonon modes at 380.6 and 405.1 cm−1, respectively, indicating that it is multilayer 2H-MoS2 (middle panel, Fig. 2a)57,58. The Raman spectrum of FePS3-MoS2 heterojunction (top panel, Fig. 2a) clearly shows A4g and A5g modes of FePS3, along with pronounced E12g and A1g phonon modes of MoS2. As such, indicating the formation of van der Waals heterojunction. In addition, ultraviolet–visible (UV–Vis) spectroscopy shows both MoS2 and FePS3 to have a broadband optical absorbance from UV to near-infra-red (IR), with peak optical absorbance at ~400 and ~310 nm wavelengths, respectively (Fig. 2b). The heterostructure also demonstrated a broadband optical absorbance of 250–1100 nm with peak absorbances at ~280, 450, 600 and 670 nm owing to a combination of optical absorbances from both individual materials (Fig. 2b). Photoluminescence (PL) characterisation with a 405 nm excitation laser on the individual materials found broad PL peaks centred at 675 and 515 nm, corresponding to MoS2 and FePS3, respectively (Fig. 2c). The observed PL emission from multilayer MoS2 flake may be attributed to the defect-mediated radiative recombination processes, where defect states in the flake act as localised centres for stimulated emission under the illumination of excitation laser59. Further, a broad peak around 500 nm present in all PL spectra, including spectrum from background i.e., SiO2/Si substrate, can be associated with the impurity-related transitions or surface states in SiO2 substrate60,61. However, it was observed that the PL intensity of the heterojunction (centred at 680 nm) has been quenched in comparison to the individual materials. The observed PL quenching in the heterojunction provides valuable insights into the band alignment between MoS2 and FePS3. This phenomenon is indicative of a Type-II (staggered) band alignment between the two materials. In a Type-II heterojunction, the conduction band minimum (CBM) and valence band maximum (VBM) of one material are both higher than those of the other material. This configuration promotes the spatial separation of photogenerated electrons and holes, with electrons transferring to the material with the lower CBM and holes to the material with the higher VBM. The resulting charge separation reduces the probability of radiative recombination, leading to the observed PL quenching62,63,64.

Fig. 2: Material characterisation of FePS3-MoS2 heterostructure.
figure 2

a Raman spectra of FePS3-MoS2 heterojunction (top panel), MoS2 (middle panel) and FePS3 (bottom panel) collected with 532 nm excitation laser and 1800 gr/mm grating. b Optical absorbance of FePS3, MoS2, and the heterojunction, where the junction encompasses the absorption range of both FePS3 and MoS2. c Photoluminescence (PL) spectra of FePS3 and MoS2 individually (with 405 nm excitation laser), as well as at the heterojunction, which showed quenching of PL.

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Broadband optoelectronic characterisations

Optoelectronic characterisations were performed on MoS2 and FePS3 individually, as well as across the heterojunction, where photodetection measurements were performed with a drain-source bias (VDS) of 2 V. The devices were illuminated with monochromatic light sources in visible to near-IR wavelengths (455–1050 nm) and a power intensity (PLED) of 3 mW/cm2. When probing the contacts across MoS2 independently, photocurrent generation was observed across a broadband range of light illumination (455–850 nm), with the highest change in photocurrent (i.e. ΔIDS = ILight − IDark) of ~8.1 µA occurring at 455 nm wavelength (Fig. 3a). However, the photocurrent measured under 850 nm is three orders of magnitude smaller (i.e., on the scale of nano amperes) than visible wavelengths (see Supplementary Fig. 1 in supplementary information). This can be associated with the reduced optical absorbance under the wavelengths higher than 850 nm, as shown by the absorbance profile of MoS2 in Fig. 2b. FePS3 on its own also exhibited broadband photoresponse to light excitation (455–1050 nm), with a relatively comparable photocurrent generation of ~0.05–0.08 nA across all illumination wavelengths (Fig. 3b). As such, the broadband optoelectronic behaviours and magnitude of photocurrent generated in both MoS2 and FePS3 correlate to the optical absorbance range as discussed above in Fig. 2b, where the higher photocurrent observed in MoS2 compared to FePS3 is attributed to its higher absorbance in the visible light.

Fig. 3: Photodetection characterisation of FePS3-MoS2 heterostructure.
figure 3

Photocurrent generated (ΔIDS) in (a) MoS2 under 455 nm to 850 nm wavelengths, b FePS3 under 455 nm to 1050 nm wavelengths, and c the heterojunction when illuminated with wavelengths in 455 nm to 660 nm spectral range. All measurements were performed with a drain-source bias (VDS) of 2 V and LED power intensity of 3 mW/cm2. d Illumination power-dependent photoresponsivity of FePS3-MoS2 heterostructure under 455, 565, and 660 nm wavelengths.

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Photoresponse measurements across the heterojunction showed photocurrent generation across a broadband spectral range as well, however, the detection range was narrowed to the visible regime (455–660 nm), with the highest photocurrent of 0.31 nA observed at 455 nm illumination (Fig. 3c). This observation also agrees well with the optical absorbance curve, where the light absorption range of the heterojunction is narrower compared to MoS2 or FePS3 individually. Responsivity (R) across different illumination wavelengths is also calculated (using equation \(R=\frac{\triangle {I}_{{DS}}}{{P}_{{LED}}\times A}\))65,66,67 to compare the photodetection properties between MoS2, FePS3 and the heterojunction, where ΔIDS is the difference in photocurrent generated (ILight – IDark), PLED is the power density (in W/cm2) of the illumination source, and A is the overlapping heterojunction area65,68 (calculated to be 27.354 µm2 using ImageJ software). Figure 3d shows illumination power-dependent photoresponsivity of FePS3-MoS2 heterojunction measured under 455, 565, and 660 nm. The wavelength-dependent responsivity of individual MoS2 and FePS3 is plotted in Supplementary information, Supplementary Fig. 2. Comparison of photoresponsivity between the devices shows that although the photoresponsivity across the heterojunction is lower than MoS2, it is comparable to the photoresponsivity of FePS3. This could be indicative that the photocurrent generation in the heterostructure was limited by the carrier mobility of FePS3 (Supplementary information, Supplementary Fig. 3), which inhibits the collection of photogenerated carriers at the electrical contacts69.

Broadband photovoltaic characterisations

In addition to a broadband photoresponse, the FePS3-MoS2 heterojunction also exhibited photovoltaic behaviour, where photocurrent generation was observed with light excitation without electrical bias (VDS = 0 V). ΔIDS–VDS curves across the heterojunction with illumination showed that the photocurrent does not pass through zero at zero bias (Fig. 4a). This observation was further confirmed by transient photocurrent generation observed at the heterojunction with VDS = 0 V across a broadband visible spectral range (455 – 660 nm), in which the highest photocurrent of 92 pA was generated at 660 nm (Fig. 4b). To determine the source of the photovoltaic behaviour across the heterojunction, photocurrent mapping was performed on the heterostructure region with a 635 nm laser excitation source. It was revealed that the photocurrent generation seen in the transient response originated from the overlapped heterojunction area (Fig. 4c), indicating that photogenerated carriers were effectively separated without any external bias owing to the presence of the p–n junction. Further confirmation of this phenomenon is shown in Supplementary Fig. 4 (Supplementary information), where photovoltaic behaviour was not observed when only individual materials were probed. Extraction of the short-circuit current (ISC) and open-circuit voltage (VOC) from the ΔIDS–VDS curves is also depicted in Fig. 4d, where VOC is comparable across the different illumination wavelengths, whilst the highest VOC of ~0.28 V and ISC of 80 pA was measured at 660 nm light excitation, which is equivalent to a current density (JSC) of ~0.29 mA/cm2. The heterojunction was also subjected to light excitation from a solar simulator (Supplementary Fig. 5), in which a photovoltaic response of ~157 pA (JSC = 1.72 mA/cm2) was observed under the simulated sunlight (1 sun, AM1.5G), indicating potential applicability in solar cell applications.

Fig. 4: Photovoltaic characterisation of FePS3-MoS2.
figure 4

a ΔIDS–VDS curves of the heterojunction with light excitation ranging from 455 to 660 nm. The inset shows an optical micrograph of the heterojunction area. b Photocurrent generated across the FePS3‑MoS2 junction when illuminated with illuminations of 455–660 nm and no applied bias (i.e. VDS = 0 V). c Photocurrent mapping of the heterostructure with 635 nm laser at 0 V bias, indicating the highest photoresponsivity at the overlap region of FePS3 and MoS2. d Extracted short‑circuit current (ISC) and open‑circuit voltage (VOC) of the heterostructure with respect to different illumination wavelengths based on the ΔIDS–VDS curves.

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Charge transport dynamics and type-II band alignment in MoS2–FePS3 heterojunctions

To understand the transport behaviour of photogenerated charge carriers across the heterojunction, the rise time (τrise), i.e. the time required for the generated photocurrent to increase from 10% to 90% in magnitude, between MoS2, FePS3, and across the heterojunction of the two materials was measured (see supplementary information, Supplementary Fig. 6). It was observed in Fig. 5a that when comparing the rise times across the different device configurations, the heterojunction had a much lower rise time (>2×) compared to the individual materials. This indicates that the photogenerated charges are being transported more efficiently to the contacts, allowing for steady-state to be achieved rapidly. It is important to mention that the response time of our individual materials might be comparatively slower than the reported literature which could be due to the several parameters such as thickness and quality of materials, excitation wavelength and power etc42,70,71,72. However, the primary purpose of comparing the response times of the heterojunction with those of the individual FePS3 and MoS2 devices (as presented in Fig. 5a) is not to claim superior speed but rather to substantiate the presence of a Type-II band alignment in the FePS3-MoS2 heterojunctions. To validate this observation, the band energy levels of MoS2 and FePS3 were measured using photoelectron spectroscopy in air (PESA) to identify the valence band maximum (EVB), whilst the conduction band minimum (ECB) was estimated based on the optical band gaps (EOpt) of the materials (see supplementary information, Supplementary Fig. 7). Based on the band energy levels extracted and averaged across multiple measurement values (Supplementary Table 1 in supplementary information), the MoS2–FePS3 heterojunction was found to form a Type-II band alignment (Fig. 5b). Existing literature has reported that heterostructures with Type-II band alignment show improved charge separation and reduced charge recombination27,32,73. Similarly, our heterostructure showed improved charge separation in the form of shorter response times (Fig. 5a), as well as reduced charge recombination based on quenching of PL observed in Fig. 2c. This is indicative that the electrons formed in FePS3 during photonic excitation are able to flow into the conduction band of MoS2, whilst the holes generated in MoS2 can transfer into the valence band of FePS3 to be collected at the respective electrical contacts with minimal barriers across the junction. This is further evident from the illumination power-dependent current–voltage characteristics of heterojunction under 455, 565 and 660 nm wavelengths, as shown in Supplementary Fig. 8. This photoresponse is consistent with the characteristics of photovoltaic effects, where the separation of photoexcited electron–hole pairs are driven by the interfacial built-in electric field at the p–n heterojunction.

Fig. 5: Excitonic lifetime measurements of FePS3–MoS2 heterostructure.
figure 5

a Comparison of rise time (τrise) between MoS2, FePS3, and across the heterojunction. b Expected Type-II band alignment of multilayered MoS2 and FePS3 heterostructure, where the valence band maximum (EVB) energy levels were extracted from PESA measurements, whilst the conduction band minimum (ECB) was estimated based on the optical band gaps. Excitonic kinetic comparison of MoS2, FePS3, and across the heterojunction at 545 nm (c) without (VDS = 0 V) and d with electrical bias (VDS = 2 V) respectively, where the structure was excited with a 480 nm laser and pump delay of 2.0–2.5 ps.

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In addition to measuring the photoresponse time and defining the band alignment experimentally, excitonic lifetime studies on MoS2, FePS3, and at heterojunction were performed using femtosecond pump-probe spectroscopy. It was revealed that with a 480 nm excitation laser and no electrical bias applied (Fig. 5c), MoS2 demonstrated a longer decay time for excited charge carriers to return to the ground state at a 545 nm probe wavelength than either the heterojunction or FePS3 individually (Table 1), with around 7% of the carriers relaxed in 2256 ps. For the heterojunction, 37% of the carriers relaxed in 658 ps, whereas in FePS3, around 29% of carriers relaxed in less than 13 ps. However, after applying a bias to the devices (VDS = 2 V) with the same excitation parameters, the relaxation time for 36% (A3) of the carriers was reduced to 629 ps (Table 2), while the relaxation time for FePS3 was increased to 83 ps for 43% (A2) of the carriers. It is important to note that after applying bias, only 1.3% (A3) of the carriers in the heterojunction have a relaxation time of 1612 ps, but the primary relaxation time (A1 and A2), which comprises the majority of the carriers, was drastically reduced, with 46.5% of the carriers relaxing in 0.36 ps and 52.2% relaxing in 0.78 ps (Fig. 5d). As a result of the perfect coupling between the MoS2 and FePS3 layers, the majority of carriers relax in a relatively short time frame. The short lifetime suggests a faster recombination rate and consequently a shorter response time, as also seen by the quenching of PL detected at the junction in Figs. 5a and 2d, respectively30,74,75.

Table 1 Kinetic lifetimes at 480 nm laser pump and VDS = 0 V.
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Table 2 Kinetic lifetimes at 480 nm laser pump and VDS = 2 V.
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Flexible platform integration and photovoltaic benchmarking

To investigate the translatability of the heterostructure onto the flexible platforms, FePS3-MoS2 devices were also fabricated on polyimide (PI) sheets with the same fabrication methods used for the rigid platform (Fig. 6a, b). Similar to the heterostructure on the SiO2/Si substrate, the junction exhibited a photovoltaic response to visible LED illumination at the same power intensity of 3 mW/cm2. A comparable photocurrent of up to ~87 pA was generated at the heterojunction with zero bias applied (Fig. 6c, d), indicating that the device has a similar photovoltaic performance regardless of whether the junction is on a rigid or flexible platform.

Fig. 6: FePS3-MoS2 heterostructure on flexible platforms.
figure 6

a Photograph of FePS3-MoS2 device on a polyimide (PI) sheet, and b a zoomed-in optical micrograph of the heterostructure. c Photovoltaic behaviour of heterojunction with visible light illumination at 3 mW/cm2 power intensity. d ΔIDS–VDS curves of the heterojunction on PI, indicating current generation at 0 V bias.

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Lastly, to understand the performance of the photovoltaic behaviour exhibited in our FePS3-MoS2 heterojunction, we compared them to existing 2D p-n junction-based heterostructures with broadband photovoltaic properties from the literature (Table 3). It was observed that under a low light intensity of 3 mW/cm2 (8.3 × 10−10 W) our heterostructure has a slightly lower short circuit current density (JSC of 0.29 mA/cm2) compared to some of the other p-n junctions. However, when the FePS3-MoS2 heterostructure is exposed to a higher light excitation of 50 mW/cm2 (1.8 × 10−8 W), it has a slightly higher JSC of 1.72 mA/cm2 compared to an existing FePS3-MoSe2 heterostructure with similar light excitation power (Supplementary Fig. 9). This is likely attributed to the slightly higher optical absorbance of MoS2 compared to MoSe2 in the visible light regime76. The responsivity of our FePS3-MoS2 heterojunction (480.9 mA/W) is comparable to other van der Waals heterostructures, particularly under low excitation power (8.3 × 10−10 W), see Table 3. Although the responsivity of our heterojunctions is lower than recently reported non-photovoltaic heterojunctions involving FePS3 or MoS2, such as β-In2Se3/MoS2 (616.7 A/W)77, FePS3/ReS2 (41.8 A/W)33, ReSe2/MoS2 (3.5 A/W)78, it outperforms systems such as FePS3-MoSe2 (52.0 mA/W)65, FePS3-WS2 (32.5 mA/W)79, and MoS2-WS2 (10.4 mA/W)80 etc. It is important to note that the performance of our heterostructure is still much lower than that of a three-layer p-gr-n heterojunction, where the graphene layer facilitates a more efficient charge transfer between the layers. However, this three-layer structure makes it harder for large-area fabrication in the future, and also increases the difficulty in translating the devices onto flexible platforms as well.

Table 3 Comparison of broadband photovoltaic performance with existing p-n junction heterostructures utilising 2D FePS3 and/or MoS2.
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In summary, we have successfully fabricated broadband optoelectronic devices based on 2D p-n junctions of FePS3 and MoS2. The fabricated heterojunctions exhibit broadband photoresponse to visible light illuminations of 455 – 660 nm, and also show 60 and 99% faster response under 565 and 660 nm illuminations, respectively, compared to when the individual materials are probed. The faster response time observed at the heterojunction is indicative of enhanced charge transfer, which is facilitated by the experimentally determined Type-II band alignment formed at the junction. In addition, quenching of PL and enhanced carrier relaxation at the junction studied through PL and excitonic lifetime measurements, respectively, also validate the improved charge separation across the heterojunction. The heterostructures can also be translated onto the flexible platforms such as polyimide and retain their broadband optoelectronic performance, demonstrating their potential for application in flexible self-driven optical imaging and smart wearable devices.

Methods

Fabrication of 2D heterostructures

FePS3-MoS2 heterostructure devices were fabricated by first transferring mechanically-exfoliated FePS3 flakes (from commercially purchased bulk crystal, 2D Semiconductors) onto 300 nm thermally oxidised SiO2-coated p-doped conductive silicon (Pure Wafer) using a gel film (Gel-Pak). MoS2 flakes (SPI Supplies) were then mechanically exfoliated onto polycarbonate (PC)-coated (HQ Graphene) glass slides, which act as a sacrificial layer in the wet transfer method. This was followed by alignment of the MoS2 flakes onto FePS3 using a transfer stage (HQ Graphene), with the stack then heated to 200 °C for 3 min to melt the PC film. The sacrificial PC layer was then removed by soaking it in chloroform (Sigma-Aldrich) for 2 min. To pattern the electrical contacts on the heterostructures, standard photolithography was performed using a maskless aligner (Heidelberg Instruments MLA150). Electron-beam evaporation (Kurt J. Lesker PVD75) of 10 nm nickel (Ni) and 100 nm gold (Au) was then performed, followed by a lift-off process in an acetone bath to realise electrical contact patterns on the FePS3-MoS2 heterostructures.

Material characterisation

AFM was performed on the heterostructures to identify the thickness of individual flakes using the tapping mode after fabrication of the electrical contacts (Bruker Dimension Icon). Raman and photoluminescence (PL) analysis of the structure was also performed using 532 nm and 405 nm lasers, respectively, at a laser power of 5 mW, 100× objective, and with 1800 gr/mm and 600 gr/mm gratings, respectively (Horiba LabRAM HR Evolution). UV–Vis spectroscopy was also performed on different locations of the heterostructure on a PDMS film with an incident light of 200 nm to 2000 nm and a 5 µm spot size (CRAIC Apollo Microspectrophometer).

Valence/conduction band energy level measurements

The VBM values of MoS2 and FePS3 were obtained via PESA measurements on an AC-2 photoelectron spectrometer (Riken-Keiki Co.). The ionisation energy is determined from the cross point of the photoelectron yield ratio and the UV energy applied (Supplementary Fig. 4). The conduction band minimum (CBM)/work function was measured using a Kelvin probe force microscope (KPFM) in air (Asylum Research Cypher ES). Mechanically exfoliated MoS2 and FePS3 flakes were transferred onto gold-coated silicon substrates, with the CBM values of each material referenced to the underlying gold film (Supplementary Fig. 10 in supplementary information).

Optoelectronic characterisation

Photoresponse characterisations were performed on the heterostructure device using a Keysight 2912B source measurement unit and a vibration-isolated probe station (Everbeing) at ambient conditions (in air at room temperature). All photoresponse characterisations (across MoS2, FePS3, and heterojunction) were performed with a drain-source bias (VDS) of 2 V. Photovoltaic measurements were performed at a VDS of 0 V. To measure the response of the devices to light, the devices were excited with uncollimated LEDs (Thorlabs Inc.) of various wavelengths (455 nm, 565 nm, 660 nm, 850 nm and 1050 nm) at a power intensity of 3 mW/cm2 (calibrated with a commercial photodiode, Thorlabs Inc. S120VC). Each cycle of photoresponse of the samples was performed with 10 s of illumination, followed by recovery of 10 s.

Excitonic lifetime measurements

Excitonic lifetime measurements were performed on the heterostructures with an ultrafast transient absorption spectroscopy (UFTS) setup, in which the setup comprises a Ti: sapphire-based mode-locked laser oscillator (Coherent Micra) with ~40 fs pulse width, an amplifier (Coherent Legend USP Elite), an optical parametric amplifier (Light Conversion Inc. TOPAS-C), and a spectrometer (Ultrafast Systems Helios). The output laser was passed through a beamsplitter, where the pump beam is pumped through the optical parametric amplifier, whilst the probe beam is passed through an 8 ns long delay stage to provide femtoseconds to nanoseconds temporal delay between the pump and probe.