Introduction

With the development of smart and Internet of Things (IoT) based technologies, great efforts have been made on wearable and flexible electronic devices which are mostly implemented on Polyethylene terephthalate (PET) and Polydimethylsiloxane (PDMS) substrates1. However, in addition to high manufacturing costs and lack of biodegradability, their fabrication processes require advanced laboratory equipment, which obstruct their possible practical applications2. Flexible paper-based electronic devices are a new class of high-performance devices that offer admirable properties with facile and low-cost fabrication processes, ending in lightweight and environmentally friendly devices that are very promising for the future of smart electronics3,4,5. Flexible photodetectors (PDs), as a member of this family, have received a great deal of attention, suitable for optoelectronic systems such as optical communications, environmental monitoring, and imaging6.

Two-dimensional (2D) materials, especially the chalcogenides family, have been increasingly used in optoelectronic devices in recent years due to their adjustable band gap, high electrical conductivity, and effective light-matter interactions7. Metal chalcogenide based PDs have shown high photoresponsivity, fast response time, and high quantum efficiency8. Interestingly, their paper-based PDs also demonstrates such a high performance9,10,11. Sahatiya et al. fabricated a MoS2-Carbon Quantum dot (CQDs) based PD on cellulose paper using a hydrothermal growth of MoS2 and casting of CQDs12. Their reported device showed a photoresponsivity of 18 mA/W in the visible region with a response time of ~ 0.57 s. Selamneni et al. hydrothermally grown MoS2 on cellulose paper followed by decorating with various metal nanoparticles (Au, Pt, and Pd)11. The photoresponsivities and response times of the PDs were measured to be around 45–100 mA/W and 0.9–1.2 s in the visible range, respectively. Cordeiro et al. introduced near-infrared PDs by growing MoS2 on cellulose paper using a hydrothermal technique. Their PD showed a photoresponsivity of about 200 mA/W and a response time of 3.7 s13. However, majority of methods used for the fabrication of paper-based PDs are time-consuming, complex, and consisting wet procedures. In contrast, rubbing of bulk crystal on ordinary paper can be a potential alternative to facile realization of paper-based PDs through a simple, fast and completely dry process. As an example, Mazaheri et al. transferred MoS2 flakes by rubbing the corresponding crystal on paper and reported a paper based PD with a photoresponsivity of 1.5 μA/W in a wavelength range of 365 to 940 nm14.

In this work, MoS2 and TiS3 bulk crystals were employed to fabricate paper-based PDs by transferring their corresponding flakes through finger-rubbing process. TiS3 is another class of 2D materials with MX3 structure, which its unique properties make it very suitable for optoelectronic applications15. TiS3 is an n-type semiconductor with a direct band gap of 1.1 eV independent of thickness with high reported electron mobility and photoresponsivity16. Unlike MX2, it has a chain-like structure that provides large aspect ratios with better electrical connectivity even in lower loading fractions16. In addition, the electronic band structure of TiS3 could be matched with MoS2 to provide a suitable heterostructure for optoelectronic applications. Accordingly, three types of MoS2, TiS3, and TiS3–MoS2 PDs were fabricated and their optoelectronic properties were carefully evaluated in the wavelength range of 405 to 810 nm. Based on the results, the highest photoresponsivity were measured for TiS3 PDs and the lowest for MoS2 PDs. In the case of TiS3–MoS2 (MoS2 film was placed on top of the TiS3 film and exposed to light), it exhibited a faster response time, and higher on–off ratio thanks to formation of band offset at semiconductor heterojunctions. Numerical simulations also confirmed the superiority of TiS3–MoS2 PDs over MoS2 PDs. Moreover, to evaluate the flexible performance of the PDs, the introduced devices were bent on both positive and negative curvatures and their Ids–Vds and photocurrent characteristics were measured under strain range of ± 0.27 to ± 0.54%. The results showed that MoS2 PDs had high flexibility, but TiS3 based PDs were associated with the decline of drain current and photodetection responsivity at applied strains larger than ± 0.33%.

Results and discussions

SEM image and XRD analysis of the grown TiS3 microcrystals are presented in Fig. S1. Before transferring the flakes, the surface of paper was investigated by SEM and EDX measurements. As can be seen in Fig. S2a, the paper is formed by stacked cellulose fibers and it presents deep gaps/voids between fibers. EDX analysis also confirms the presence of carbon (C), oxygen (O), calcium (Ca), silicon (Si), aluminum (Al), iron (Fe), sodium (Na), and manganese (Mg) in the raw paper where their atomic percentages are listed in the inset of Fig. S2b.

Figure 1 shows the characterization of the deposited flakes on paper substrates. The schematic illustration of the TiS3–MoS2 heterostructure and its corresponding photograph are also presented in panels (a) and (b) of Fig. 1, respectively. Figure 1c exhibits the SEM image of the MoS2 flakes on the paper. According to the figure, MoS2 flakes cover the fibers and gaps between fibers of the paper. The deposited TiS3 flakes are also presented in Fig. 1d in which the thickness of the film is measured to be ~ 10 μm. Moreover, the SEM image of the TiS3–MoS2 heterostructure is observed in Fig. 1e. Due to softness and flexibility of MoS2, it forms a smoother film than TiS3 on paper, which is visible in Fig. 1e. The EDX analysis of MoS2, TiS3, and TiS3–MoS2 samples on papers is presented in Fig. 1f,g. By rubbing MoS2 on the paper, the Mo and S elements becomes significant (Fig. 1f). The EDX analysis of TiS3 sample also proves the presence of S and Ti elements according to Fig. 1g. SEM–EDX analysis of transferred TiS3 flakes is also presented in Fig. S3. In the case of TiS3–MoS2 film, dominant elements are S, Mo, and Ti (Fig. 1h). The C, O and Ca elements come from raw paper. Table S1 summarizes the element contents of all samples with their weight and atomic percentages.

Figure 1
figure 1

(a) Schematic illustration of the TiS3-MoS2 film on the paper. (b) Photograph of the fabricated TiS3-MoS2 film on the paper. SEM image of the transferred (c) MoS2, (d) TiS3, (e) TiS3-MoS2 flakes on the paper. EDX analysis of the (f) MoS2, (g) TiS3, and (h) TiS3-MoS2 films on the paper.

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Figure 2a exhibits the magnified SEM image of the TiS3–MoS2 film where the MoS2 film is deposited on the TiS3 film. However, some TiS3 microcrystals may also appear on top of the film due to the rubbing process. The Raman spectra of MoS2, TiS3, and TiS3-MoS2 samples are also presented in Fig. 2b. For MoS2, E12g and A1g peaks are found at 379 and 403 cm−1, respectively17. In the case of TiS3, the 193, 385 and 627 cm−1 peaks refer to Ag and Eg modes, respectively18. There is also one additional peak located at 507 cm−1, which refers to B1g peak of TiO219. This oxide peak may have appeared during the growth of TiS3 microcrystals in the ampoule process. The Raman spectrum of TiS3–MoS2 sample contains all peaks of both structures. The corresponding Ag, E12g, and A1g peaks of TiS3 and MoS2 are also fitted in the Raman spectrum of the hybrid sample. Figure 2c shows the mapping analysis of the TiS3–MoS2 sample where the dominant elements are separately shown in Fig. 2d. Accordingly, O, C, and Ca elements are due to the raw paper, and elements of Ti, Mo, and S originate from the TiS3 and MoS2 films. It can be seen that all elements are uniformly distributed on the paper.

Figure 2
figure 2

Characterization of the TiS3-MoS2 film on the paper. (a) SEM image of the film. (b) Raman spectra of the MoS2, TiS3, and TiS3-MoS2 films. (c) Mapping analysis of the TiS3-MoS2 sample on the paper. d) Elemental mapping of six dominant elements in the TiS3-MoS2 sample.

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To investigate the optoelectronic properties of the films, MoS2, TiS3, and TiS3-MoS2 PDs were fabricated. Details of their fabrication are provided in the experimental section. The fabrication steps of MoS2 and TiS3 PDs are shown in Figs. S4 and S5, respectively. In the case of TiS3–MoS2 PD, the fabrication steps are presented in Fig. 3.

Figure 3
figure 3

Fabrication steps of the TiS3-MoS2 PD.

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Panels (a) to (c) of Fig. 4 display I–V characteristics of the MoS2, TiS3, and TiS3–MoS2 PDs at a bias range of − 10 to + 10 V in dark and under 532 nm laser illumination at different power intensities. In all three PDs, significant photocurrents were generated compared to the dark state. The incident power is normalized in terms of laser spot and PD’s channel areas. Accordingly, the effective incident power is calculated as:

$${\text{P}}_{{{\text{eff}}}} = {\text{ P}}_{{{\text{laser}}}} \times {\text{A}}_{{{\text{device}}}} /{\text{A}}_{{{\text{laser}}}}$$
(1)
Figure 4
figure 4

I-V characteristics of the (a) MoS2, (b) TiS3, and (c) TiS3-MoS2 PDs under 532 nm laser illumination at different power intensities. Photocurrent versus incident power intensity for (d) MoS2, (e) TiS3, and (f) TiS3-MoS2 PDs under 532 nm laser illumination at an applied voltage of 10 V. Photoresponsivity versus incident power intensity for (g) MoS2, (h) TiS3, and i) TiS3-MoS2 PDs under 532 nm laser illumination at a supply voltage of 10 V.

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where Plaser is the power of the laser, Adevice is the area of the PD’s channel, and Alaser is the area of the laser spot. The diameter of the 532 nm laser spot was 2.81 mm and its area (Alaser) was measured to be 6.22 mm2. As the laser intensity increases, the photocurrent also increases for all three PDs. In detail, the drain currents were measured in the range of ~ 0–4 nA, ~ 0–8 μA and ~ 0–450 nA for MoS2, TiS3, and TiS3–MoS2 PDs at a biasing voltage of 0–10 V, respectively. Interestingly, the Ids in TiS3–MoS2 is significantly higher than that of individual MoS2, which indicates the effective role of the band offset formation in increasing the photocurrent. The I–V curves of the all PDs in response to 405, 655, and 810 nm laser illuminations are also shown in Fig. S6 where all PDs respond to the laser irradiation by an increase in current. Figure 4d–i compare the photocurrent and photoresponsivity of all three PDs at different power intensities of 532 nm at an applied voltage of 10 V. According to Fig. 4d–f, with increasing effective incident power (from zero to a few mW), increase in the photocurrents are occurred. In the MoS2 PD, the photocurrent increases to about 1.6 nA. For TiS3, this value is more noticeable and a 1.0 μA increase of photocurrent is observed. In the case of TiS3-MoS2 PD, photocurrent increases up to 300 nA. The photoresponsivity of the PDs are calculated based on the following equation20:

$${\text{R}} = {\text{I}}_{{{\text{ph}}}} /{\text{P}}_{{{\text{eff}}}}$$
(2)

where Iph is the photocurrent of the PDs. In the MoS2 PD, increasing the effective power associates with decrease of the photoresponsivity. The same trends are also observed for TiS3, and TiS3–MoS2 PDs. As the incident power increases, more photocarriers are generated, which increases the recombination rate of photogenerated carriers or the possibility of being captured by the traps, leading to a decrease in R21,22. In general, the photoresponsivities are measured in the range of 0.4–1.2 μA/W, 0.4–1.6 mA/W, and 90–170 μA/W for MoS2, TiS3, and TiS3–MoS2 PDs, respectively. Accordingly, the photoresponsivity of the last two PDs is three and two orders of magnitude greater than that of MoS2 PD.

Figure 5a–c show the measured photocurrent in terms of the different effective powers of the 532 nm laser. Accordingly, an increase in the optical powers leads to an increase in photocurrent in all PDs. Panels (d) to (f) of Fig. 5 display the time trace response of the PDs to 532 nm laser illumination at a drain voltage of 10 V at an incident power of 10 mW. Rise time (τon) is defined as the time taken to increase the current from 10 to 90% of its baseline under laser illumination23. The fall time (τoff) is the time required to reduce the current from 90 to 10% of its baseline24. For MoS2, the rise and fall times are measured to be 2.17 and 9.99 s, respectively. In the TiS3 PD, the τon and τoff of 2.4 and 17.44 s are measured and in the TiS3–MoS2 PD, τon and τoff of about 0.96 and 8.42 s are calculated that it shows faster switching performance compared with two other PDs. In the hybrid PD, MoS2 is at the top and is more exposed to light, so the final behavior of the hybrid is more tended toward MoS2 than TiS3. For this reason, its photoresponsivity is associated with improvement compared with the MoS2 PD due to the formation of band offset and efficient charge separation. Moreover, the channel length of TiS3 is about 1 mm in the hybrid PD, which is half that of individual TiS3 PD. Hence, the needle-like TiS3 flake has lower series resistance and therefore shows a faster response time25,26. Furthermore, due to the growth process, TiS3 probably has more defects than MoS2, so it has a slower response time27. However, some part of the TiS3 is buried under (passivated by) MoS2 in the hybrid PD and is resulted in less ambient gas absorption, which speeds up the response time27.

Figure 5
figure 5

The measured Photocurrent as a function of laser intensities for (a) MoS2, (b) TiS3, and (c) TiS3-MoS2 PDs under 532 nm laser illumination at a constant voltage of 10 V. Time trace measurement of (d) MoS2, (e) TiS3, and (f) TiS3-MoS2 PDs under 532 nm laser illumination at an applied voltage of 10 V and laser intensity of 10 mW. The corresponding Peff was calculated to be 1.53 mW, 1.03 mW, and 1.39 mW for panels (d) to (f), respectively.

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Figure 6a compares the photoresponsivity of several fabricated PDs at an applied voltage of 10 V under a laser wavelength of 532 nm. It is observed that all three TiS3 PDs have the highest photoresponsivity, followed by TiS3–MoS2 and MoS2 samples, respectively. In general, the R are measured in the range of 0.67–1.56 mA/W for TiS3 PDs and 0.20–1.93 μA/W in the case of MoS2 PDs. For TiS3–MoS2 PDs, these values are measured in the range of 0.08 to 0.19 mA/W. Accordingly, MoS2 has the lowest photoresponsivity and TiS3 possess the highest photoresponsivity. However, both photoresponsivity and photocurrent of TiS3–MoS2 PDs are considerable compared to the MoS2 PD. Moreover, a similar trend is observed in the order of magnitude of all calculated R in these PDs. Figure 6b shows the photoresponsivity of all three devices in terms of different laser wavelengths at the same power of 15 mW and supply voltage of 10 V. All three PDs have a broad response within the range of 400 to 800 nm. The photoresponsivity of the TiS3 PDs are one order of magnitude greater than that of the TiS3–MoS2 PDs and two orders greater than that of the MoS2 PDs. Figure 6c compares the measured Ion/Ioff ratio and dark current of all three PDs under 532 nm laser irradiation. The error line shows the deviation of the values obtained for the same PDs. The on–off ratio (Ion/Ioff) is usually used to describe the signal-to-noise ratio28. According to Fig. 6c, the values of 1.35, 0.17, and 1.82 are obtained for MoS2, TiS3, and TiS3–MoS2 PDs, respectively. Based on them, the highest signal-to-noise ratio is related to the TiS3–MoS2 PDs. Interestingly, the dark currents are lowest in MoS2 (~ 1.2 nA) and highest in TiS3 (~ 6.0 μA). Since the light detection mechanism is based on the change of drain current in these PDs, less dark current provides better performance in light detection as will be discussed below29. Another important parameters are the rise and fall times which are presented in Fig. 6d. Accordingly, the TiS3-MoS2 PD has the fastest response time to laser radiation than the other two PDs. Figure 6e shows the energy band diagram of the TiS3–MoS2 heterostructure and its corresponding photodetection mechanism. TiS3 is an n-type semiconductor with an energy band gap of 1.1 eV and MoS2 is also an n-type semiconductor with an energy band gap in the range of 1.2–1.8 eV30,31. Here, the energy band gap of bulk MoS2 is considered because most layers are thick, although the deposited film can contain single and few layers of MoS2. The electron affinities of MoS2 and TiS3 are around 4 and 4.7 eV, respectively, and hence the conduction band of TiS3 is located of MoS2, forming an n–n+ heterostructure32,33. In the MoS2 film under laser irradiation, electron–hole pairs are generated and the photogenerated electrons enter into the TiS3 due to its lower conduction band which prevents electrons from recombination with the holes. Moreover, the valence band of MoS2 is located higher than that of TiS3 resulting in transferring of minority holes from TiS3 into MoS2. Hence, the recombination rate of the carriers is decreased in the heterostructure. Such charge separation in the heterostructure is responsible for faster response time, and improved photoresponsivity of the heterostructure compared with individual MoS2 PDs.

Figure 6
figure 6

Comparison of (a) photoresponsivity as a function of the incident power for several fabricated PDs at an applied voltage of 10 V under a laser wavelength of 532 nm, (b) the photoresponsivity of fabricated PDs under various laser wavelengths at an applied voltage of 10 V, (c) Ion/Ioff, and dark current, (d) rise and fall times of the MoS2, TiS3, TiS3-MoS2 PDs at a constant voltage of 10 V, and (e) Energy band diagram and detection mechanism in the TiS3-MoS2 heterojunction.

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In order to investigate the effect of thicknesses, the performance of the three hybrid PDs was evaluated with three different TiS3 thicknesses. In this regard, thicknesses of ~ 2, ~ 10 and ~ 20 µm were prepared where dark current and photoresponsivity of the PDs were measured as reported in the Fig. S7. Based on the result, as the thickness of TiS3 increases, the dark current increases but the photoresponsivity decreases. Less thickness of TiS3 leads to more changes in electrical current, which can be due to non-uniformity of the deposited film. Moreover, as the thickness increases, the upper layers are more involved in light absorption and the lower layers play a lesser role in the photogenerated carriers. Therefore, the ratio of photogenerated to non-photogenerated carriers decreases, which leads to a decrease in photoresponsivity. As can be seen, the thickness of ~ 10 µm generally shows better performance compared to less and more thicknesses.

To evaluate the air stability of the PDs, the performance of the fresh devices was also investigated after two weeks based on the measured photocurrent. Based on the Fig. S8, it can be seen that after two weeks of device life, the performance of the PDs is very close to the fresh states, which show their high environmental stability.

Since the substrate is made of paper, the performance of the introduced PDs was thoroughly investigated under applied strain. For this purpose, a homemade setup was fabricated to apply strain to the substrates as shown in Fig. S9. This setup includes a cage covered with aluminum foil to act as a Faraday cage. The sample is fixed to a motor shaft through a hook to provide upward and downward strains by moving in a clockwise or counter-clockwise direction. The strain applied to the PDs was calculated according to ε = t/2R equation, where t is the thickness of the substrate and r is the radius of bending curvature34. Figure S10 presents the applied strains in the PDs, which are performed in the range of − 0.54% to + 0.54%˚. As can be seen, at strains larger than ± 0.33%, a significant bending is occurred in the substrates.

Figure 7a and b show the schematic of applying the upward (uniaxial tensile) and downward (compression) strains in the PDs under exposing to 532 nm laser illuminations, respectively. Panels (c), (d), and (e) of Fig. 7 compare the Ids-V curves of the MoS2, TiS3, and TiS3–MoS2 PDs in the flat state and under negative and positive strains, respectively. For the MoS2 sample, it is observed that the drain current in the flat state almost remains the same as under different applied strains without any significant change. The flexible and soft structure of MoS2 is the main reason for this behavior35. Moreover, due to the polycrystalline structure of MoS2 flakes and their different thicknesses, the applied strain has a negligible effect on the channel current36. In the case of TiS3, it is found that the Ids decreases after applying downward strains, but a more significant decrease is observed in the upward strains. Moreover, after upward strain, by returning the TiS3 to its flat position, current does not return to its original value of before any strain. The structure of the needle-like TiS3 flake is more fragile than that of MoS2, and bending can break the contact between TiS3 and Ag paste, affecting the conductivity37. As a result, even after the sample has returned to the flat state, the current dramatically suffers from decline. In detail, by applying + 0.57% strain, a relative change of drain current |ΔI/Iflat| is measured to be ~ 88% compared to the flat state, which indicates the high fragility of the TiS3 flakes within the channel. A similar trend is observed in the TiS3–MoS2 PD, but with a more noticeable decrease after positive strains, which could be due to the separation of the two films at the interface and the reduction of the electric field in the channel.

Figure 7
figure 7

Schematic illustrations of (a) upward and (b) downward applied strains at the PDs under laser illuminations. Ids-V curves of the (c) MoS2, (d) TiS3, and (e) TiS3-MoS2 PDs under different applied strains. Photocurrent switching performance of the (f) MoS2, (g) TiS3, and (h) TiS3-MoS2 PDs in flat state and at applied strains of ± 0.33% under 532 nm laser illumination at a bias voltage of 10 V.

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Figure 7f–h show the photocurrent characteristics of all PDs under 532 nm laser irradiation at a drain voltage of 10 V and an incident power of 15 mW. For MoS2 PD, the photocurrent is well switched under negative and positive strains of 0.33% similar to the flat state. The lack of change in the photocurrent as well as the slight change in the drain current under the applied strain indicate that the tensile and compressive strains have no effect on the band gap, density of state of carriers, and barrier height at the contacts of the MoS2 PDs. This is probably due to the presence of MoS2 flakes of different thicknesses and their polycrystalline nature which minimize the effect of strain36. In the case of TiS3, the photocurrent is about 1.00 μA in the flat state, which is reduced to 0.50 and 0.25 μA in the applied strains of − 0.33 and + 0.33%. This reduction in photocurrent is not due to a change in the energy gap or piezoresistive effect, but rather to the breaking of the needle-like TiS3 flakes in the channel, which leads to their less contribution in the drain current and generation of photocarriers. For TiS3-MoS2, the current is about 250 nA in the flat state, which is reduced to ~ 70 and ~ 10 nA by applying positive and negative strains. As discussed, the brittle structure of TiS3 as well as the separation of the two films at the junction, especially at a strain of + 0.33% are the main factors in the TiS3–MoS2 PD performance drop under applied strains.

Numerical simulation is employed to present further insight about the physics of light-induced generation-recombination processes for charged carriers in these devices. We used the model to calculate the photocurrent of the devices in response to the incident light under various biasing conditions. Details of the simulation can be found in the supporting information section. Panels (a) to (c) of Fig. 8 depict the schematic of the devices consisting MoS2, TiS3 and heterostructure with stacked layers of both materials. Overlapped part in the heterostructure device was exposed to the incident light with the wavelength of 532 nm. Stray electromagnetic field due to unintentional exposure is also took into the account. Corresponding light-induced electric field can be seen in Fig. 8d for all devices due to laser exposure. Vertical electric field was established by applying a voltage difference between the two terminals at both ends, where contacts are located. Figure 8e–f show the electron and hole concentrations under this biasing condition in a logarithmic scale, respectively. As can be seen, the distribution of electrons and holes in the two structures of MoS2 and TiS3 is almost uniform, but in the TiS3–MoS2 device, the band offset accelerates the separation of electrons and holes and causes a non-uniform distribution of carriers at the two-layer boundary. These results indicate that the formed n–n + heterostructure can more efficiently separate the charge and confirms its superior optoelectronic performance compared with the individual MoS2 case.

Figure 8
figure 8

Schematic illustrations of device geometries for (a) MoS2 (b) TiS3 and (c) TiS3-MoS2 heterostructure, (d) exposed surface of the devices and light-induced electric field, (e) electron and (f) hole concentrations in a logarithmic scale for the three devices.

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At various light intensities, I–V characteristic curves has been simulated for MoS2, TiS3, and TiS3–MoS2 photodetectors. The obtained curves for drain currents are consistent with the measurements as illustrated in Fig. S11a–c. Relative differences between the amplitude of photocurrent is in all devices is in total agreement with the experiments where TiS3 and MoS2 PDs have higher and lower photocurrents, respect to the hybrid structure under identical biasing and light exposure conditions.

Table 1 compares the performance of the introduced PDs with some other reported paper-based PDs. According to it, most of them have a photoresponsivity in the range of a few μA/W. In the case of MoS2, it is observed that our introduced PD has a slightly higher photoresponsivity than the others. The TiS3 PD shows a much higher photoresponsivity (in the range of mA/W), which has a significant improvement in performance compared to other reported PDs. However, it shows a larger dark current and a smaller on/off ratio than MoS2 PDs which can limit its optical detection performance. Finally, the photoresponsivity of the TiS3–MoS2 heterostructure shows a significant improvement, which indicates its superiority over other works.

Table 1 Comparison of the performance of introduced paper-based PDs with other PDs based on different 2D materials.
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Conclusions

The two-dimensional (2D) layered family of transition metals chalcogenides shows high potential as photodetectors because they not only interact effectively with light but also provide high carrier transport properties. In this report, a very simple, fast and promising clean method is introduced to fabricate the paper-based photodetectors using MoS2, TiS3 and their integration. The all introduced photodetectors show remarkable photoresponsivities in the range of 405 to 810 nm. In the case of TiS3–MoS2 heterostructure, it associates with fast response time and large on–off ratio compared to individual MoS2 and TiS3 photodetectors. The numerical simulation results are consistent with the experimental results and confirm the superiority of the hybrid structure over the two other PDs. Moreover, the bending results of the photodetectors indicate that in applied strains smaller than ± 0.33%, these devices still show acceptable performance.

Methods

Material

Naturally occurring molybdenite crystal, from Wolfram Camp Mine, Queensland, Australia, were used. Silver paste was purchased from Dycotec Materials, UK DM-SIP-3060S. Ordinary papers (CopiMax, Thailand) were used as substrates.

Growth of TiS3 microcrystals

TiS3 microcrystals were synthesized by a solid–gas reaction of titanium powder through direct sulfurization. In detail, titanium powder (Goodfellow, > 99% purity) and sulfur powder (Merck, > 99.9% purity) were vacuum sealed in an ampule and heated up to 500 °C for 20 h. Then, the ampule was cooled down to room temperature and grown microcrystals were collected.

Fabrication of MoS2 (TiS3) paper-based photodetector

First, a piece of paper with 2 × 6 cm2 dimensions was cut and attached on a slide glass using adhesive tape. A shadow mask (a 1 × 6 mm2 rectangular window) was then opened in the center of the paper with adhesive tape. A piece of bulk MoS2 (or TiS3) was rubbed several times along the length and width of the open-window reign. After deposition of a homogeneous layer of MoS2 (TiS3), the mask was removed from the paper. Silver paste was then placed on the both sides of the deposited film, and two pieces of soft copper wire were mounted on the paste and kept at room temperature for 24 h for improving the adhesion. Then, the dried silver paste was covered with epoxy glue for strengthening and achieving higher mechanical stability. The photograph of the fabrication steps of MoS2 and TiS3 PDs are presented in the supporting file.

Fabrication of TiS3-MoS2 paper-based photodetector

First, a piece of paper with a dimension of 2 × 6 cm2 was cut and attached to a slide glass. A rectangular shadow mask (mask 1) was opened by the adhesive tape in the center of the paper. With the use of another mask (mask 2), this part was divided into two sections with dimensions of 1 × 4 mm2 (for TiS3 deposition) and 1 × 2 mm2 (for MoS2 deposition). A piece of bulk TiS3 was rubbed several times in the open-window region. Then, mask 2 was removed from the paper and MoS2 crystal was finger-rubbed several times on the 1 × 2 mm2 part of the bare paper and 1 × 1 mm2 part of the deposited TiS3. Thus, the center of the sample (device) with dimensions of 1 × 1 mm2 was a film composed of TiS3 and MoS2, in which TiS3 was placed beneath the MoS2 film. After removing mask 1 from the paper, a homogeneous film composed of TiS3, MoS2 and, TiS3–MoS2 with dimensions of 1 × 3, 1 × 1, and 1 × 1 mm2 can be achieved, respectively. Similar to previous PDs, copper wires were added to both sides of the film. The TiS3-MoS2 PD is ready for testing.

Characterizations

SEM images and EDX analyses were taken by TESCAN MIRA 3 and EDX (Energy-dispersive X-ray) systems. Crystalline structure was identified by using a Panalytical X`Pert Pro X-ray diffractometer at glancing angle configuration (incident angle of 1.7°) with CuKα radiation. Raman spectra of the transferred flakes were measured by Avantes (AVASPEC-ULS3648-RS) system under 532 nm laser illumination. Electrical measurements were carried out by a Keithley 2450 source meter. Optical measurements were done under different laser excitations including 405, 532, 655, and 810 nm at room temperature in air ambient. Bending tests were performed with a homemade setup capable of applying both upward and strains.