Polarization‐Sensitive and Self‐Driven Pyro‐Phototronic Photodetectors Based on MoS2‐Water Heterojunctions

Polarization‐sensitive and self‐driven pyroelectric‐based photodetectors have recently gained interest due to their potential application in artificial electronic eyes, biomedical imaging, and optical switches. Here, a photodetector based on light modulation‐induced polarization and depolarization of water molecules on the surface of a 2D MoS2 crystal is reported. The MoS2‐water heterostructure photodetector serves as a self‐driven pyro‐phototronic device that converts light‐induced thermal energy to electrical signals, leading to a transient photoresponsivity as high as 24.6 mA W−1 and a specific detectivity of 2.85 × 108 Jones under 470 nm wavelength at zero bias. Due to the formation of a built‐in electric field at the MoS2‐water interface, this structure also has a high steady–state responsivity of 3.62 A W−1 and detectivity of 9.18 × 108 Jones at 3 V bias, along with a fast response time of ≈0.74 ms. Moreover, due to the rearrangement of the hydrogen bond network in the liquid water upon visible light illumination, the MoS2‐water photodetector is light polarization‐sensitive. The simple fabrication process, low cost, polarization sensitivity, and high performance of the MoS2‐water structure make it an excellent candidate for liquid‐compatible photodetectors.


Introduction
Photodiodes are widely utilized across various industries including telecommunications, [1] imaging, [2] medical treatment, [3] and environmental sensing. [4]Among various materials, 2D transition metal dichalcogenides (TMDs), such as MoS 2 , have DOI: 10.1002/adom.202302651Conventional photodetectors require an external voltage bias which can limit their applications due to energy consumption and extra cost. [11]p-n junction photodiodes are commonly used to obtain photocurrent at zero bias but are difficult to achieve in atomically thin TMDs due to the challenges of doping 2D materials or requiring transfer techniques to form heterojunctions. [12][13][14] Photodetectors based on the pyroelectric effect are a new type of photodetector that can detect light signals by converting lightinduced thermal energy to electrical energy, which can enhance the transient response of the device. [11]In addition, polarization sensitivity is a crucial feature for photodetectors due to their applications in various fields such as navigation, [17] biomedical imaging, [18] and optical switches. [11]ne way to create polarization-sensitive photodetectors is by using materials with anisotropic crystal structures such as black phosphorus, [18] TiS 3 , [19] and WS 2 . [20]However, 2D MoS 2 crystals are not responsive to light polarization, making it necessary to find new methods to develop MoS 2 -based polarization-sensitive photodetectors. [21]For example, polarization sensitivity can be achieved through the use of heterostructures between MoS 2 and anisotropic materials such as GeSe, [18] ReSe 2 , [22] and Te, [23] or by using the rolling-up technique. [21]Despite the potential benefits, the fabrication of such structures is challenging due to the complexity of the fabrication process and 2D material transfer methods. [13] For instance, we previously reported that a silicon-water heterostructure can be used for self-driven and polarization-sensitive photodetectors based on the interfacial pyroelectric effect. [11][32][33] However, silicon has an indirect bandgap and is rigid and brittle, which can limit its use in flexible wearables and biomedical optoelectronic applications.Here, by using the ultrahigh visible light absorption of MoS 2 , [34] and the formation of a heterojunction between MoS 2 and water, a self-driven and polarization-sensitive pyro-phototronic MoS 2water photodetector is reported.This heterostructure photodetector shows a transient photoresponsivity as high as 24.6 mA W −1 and a specific detectivity of 2.85 × 10 8 Jones under 470 nm wavelength at zero bias.It also shows a high steady-state responsivity of 3.62 A W −1 with a detectivity of 9.18 × 10 8 Jones at 3 V bias, along with the fast rise/fall times of ≈0.741/2.83ms.The combination of the simple fabrication process and high performance makes the MoS 2 -water heterojunction an attractive candidate for dual steady-state and transient type photodetectors.

Results and Discussion
The MoS 2 -water heterojunction consists of two Cr/Au electrodes, one of which overlaps a mechanically exfoliated multilayer MoS 2 crystal.A thin film of Al 2 O 3 is used to electrically passivate one electrode, and a window is created to expose MoS 2 to air.The fabrication process is illustrated in Figures S1 and S2 (Supporting Information).Figure 1a presents the optical image of the fabri-cated MoS 2 -based structure from the top view, which displays a portion of the MoS 2 flake and one of the Cr/Au electrodes exposed to air.The Raman spectroscopy of the MoS 2 crystal shows two sharp peaks, the in-plane mode (E 2g ) at ≈384 cm −1 and the out-of-plane mode (A 1g ) at 408 cm −1 (Figure 1b).This confirms that the MoS 2 flake is in a semiconducting phase (2H-phase) as previously reported. [34]Finally, 50 μL deionized (DI) water is placed on the active area so that one Cr/Au electrode is connected to DI water and the other one is connected to the MoS 2 crystal, forming a MoS 2 -water heterostructure device, as schematically shown in Figure 1c. Figure 1d shows the linear scale currentvoltage (I-V) characteristics of the MoS 2 -water device under dark conditions, demonstrating a current rectification ratio, defined as the current ratio at ±1 V, of ≈22 without a gate voltage.This rectification behavior is due to an internal electric field formed by an energy band bending at the interface between MoS 2 and water.The inset of Figure 1d shows the actual MoS 2 -water device connected to two probes in a chamber during the measurement.Figure 1e shows the I-V characteristics of the MoS 2 -water heterostructure under dark and light irradiation.The energy band diagrams of water and MoS 2 when they are separate and in contact are illustrated in Figure S3 (Supporting Information).The energy band structure of water has previously been reported in triboelectric studies at the silicon-water interfaces. [28,35]To determine the band bending at the MoS 2 -water interface, we also examined the optoelectronic and electronic behavior of n-type and p-type silicon-water heterojunctions.The Fermi level of n-type silicon is higher than that of water due to the high electron concentration which results in the transfer of electrons to the water upon physical contact, leading to the band bending at the interface [28] (Figure S4a,b, Supporting Information).This band bending results in a diode-like rectifying behavior in the IV characteristics of siliconwater heterojunction (Figure S4c, Supporting Information).On the other hand, the electrons transfer from water to p-type silicon, resulting in a different band bending and IV characteristics [35] (i.e., opposite polarity) (Figure S4d-f, Supporting Information).Although MoS 2 is typically an n-type semiconductor, the IV characteristics and band bending of MoS 2 -water heterojunction are similar to those of p-type silicon-water probably due to the low electron concentration of MoS 2 . [36]pon white light illumination (solar spectrum, AM1.5G) at an intensity of 100 mW cm −2 , the concentration of photogenerated carriers in the MoS 2 increases, leading to an increase in the photogenerated current density of the MoS 2 -water device and an open circuit voltage of ≈0.52 V, demonstrating the photovoltaic effect [37] (Figure 1e).To study the pyroelectric behavior of the MoS 2 -water junction, a series of heat pulses at temperatures of ≈40, 60, 80, and 100 °C were applied to the backside of the MoS 2 -water device (Figure 1f).When a heat pulse is applied to the device, a short-duration positive current peak is generated due to the depolarization of water dipoles on the MoS 2 surface.A built-in electric field at the MoS 2 -water interface facilitates the collection of pyroelectric charges at zero bias, leading to a sharp spike in current response due to the incident light turning on and off.During steady-state heating, the positive peak quickly declines to near zero due to a decrease in temperature gradient over time.When the heat pulse is removed, a negative current peak is generated that similarly dissipates quickly (Figure 1f).Therefore, heating and cooling generate pyroelectric currents in opposite directions.
Due to the lack of net orientation, water on its own has minimal or no pyroelectricity. [32]However, it has been shown that the alignment of polarized water molecules can exhibit crystalline properties and pyroelectric behavior.For example, pyroelectric behavior has been observed in electrically biased water ice [32] and silicon-water heterojunctions. [11]Similar to pyroelectric materials such as BaTiO 3 (BTO), [38] there is a linear relationship between the transient current and the change in temperature (ΔT) for the MoS 2 -water heterojunction device (Figure S5, Supporting Information).
The MoS 2 -water heterojunction exhibits a light-induced pyroelectric effect in the configuration illustrated in Figure 2a, which can be used for photodetection.When a light pulse is applied, the photodetector response measured between the two Au contacts consists of transient peaks generated due to the pyroelectric effect in addition to a steady-state current generated by the photovoltaic effect. [39]The self-driven transient photocurrent of the MoS 2 -water device under 470 nm wavelength light at different intensities of 1.5-6.8mW cm −2 are shown in Figure 2b, showing distinguishable transient responses for different light intensities due to polarization and depolarization of water molecules on the surface of MoS 2 crystal as the incident light is turned on and off.The amplitude of the positive transient peaks is plotted as a function of light intensity in Figure 2c.Under zero-bias conditions, the steady-state photocurrent is low compared to the transient photocurrent.To further assess the photodetection per-formance of MoS 2 -water photodetector, two critical parameters, responsivity (R), and detectivity (D), are calculated using the following equations: [40] where I ph = |I light |-|I dark | is the photocurrent amplitude, P is the incident light intensity, I d is the dark current, e is the charge of an electron and S is the effective surface area of the MoS 2 -water interface, which is ≈904 μm 2 (Figure S6, Supporting Information).The detectivity of a photodetector measures its ability to detect weak light signals and the responsivity measures the photocurrent generated per input optical power.Figure 2d shows the transient responsivity and detectivity of the MoS 2 -water heterojunction where the photocurrent (I ph ) is the amplitude of the positive transient peak.The highest transient responsivity and detectivity of 24.6 mA W −1 and 2.85 × 10 8 Jones are achieved at the lowest light intensity of 1.5 mW cm −2 , respectively.Both the transient responsivity and detectivity decrease as a function of increasing the light intensity, which may be attributed to electron trap saturation and an increase in electron-hole recombination. [41]The photoresponse rise and fall times of the MoS 2 -water device at zero bias were also measured to be ≈3.2 and ≈150 ms, respectively (Figure 2e).The transmission spectrum of water on a glass substrate reveals a high transmission of ≈95% at a wavelength of 470 nm (Figure S7, Supporting Information).
We have also previously reported that the water-MoS 2 device demonstrates a reflectance of ≈3.5% at the same wavelength. [36]herefore, the absorption of water is minimal at this specific wavelength.MoS 2 on its own is not a polarization-sensitive material due to its isotropic structure.In fact, two factors are likely responsible for the polarization sensitivity of MoS 2 -water photodetector.First, light reflection at the water surface is polarization-dependent, as shown in the reflectance spectrum on the surface of waterglass (Figure S8, Supporting Information).Second, the terahertz frequency range, including the visible spectrum, can rearrange the hydrogen bonds, stimulate water molecules, and generate anisotropy, [26][27]42] leading to polarization sensitivity. The hotoresponse of the MoS 2 -water photodetector at different polarization angles is demonstrated in Figure 2f.The polarization angle was controlled by passing natural light with a wavelength of 470 nm through a rotation-controllable crystal polarizer (Inset of Figure 2f).Figure 2g shows the polar plot of the MoS 2 -water device for different polarization angles of 0-360°, showing distinguishable transient photocurrents at different angles.The polarization sensitivity of the MoS 2 -water device makes it suitable for many applications including artificial electronic eyes, [11] biomedical imaging, [18] and optical switches.[11] Compared to previously reported self-powered pyrophototronic photodetectors, listed in Table 1, the MoS 2 -water heterojunction offers a simple fabrication process, high responsivity, and polarization sensitivity.Additionally, the MoS 2 -water heterojunctions can be used for the development of flexible and lightweight photodetectors that can be integrated into various devices and systems due to the excellent mechanical flexibility of 2D MoS 2 .[7] Although the steady-state photocurrent amplitude is negligible compared to the transient photocurrent at zero bias, it can increase significantly at larger bias voltages due to the enhanced collection of the photogenerated electron/holes.[54] The schematic of the MoS 2 -water photodetector under bias voltages is shown in Figure 3a. Figure 3b-f shows the photoresponse of the photodetector to incident light ( = 470 nm, light intensity = 6.8 mW cm −2 ) under different bias voltage conditions of 0, 0.5, 1, 2, and 3 V, respectively.At each bias voltage, the incident light was turned on and off three times to show the repeatability of the photoresponse.By increasing the bias voltage, the amplitude of the transient current peak decreases, which may be due to the weakening of the pyroelectric effect due to joule heating induced by the applied bias voltage.[39] The steady-state photocurrent increases from ≈97 pA to ≈223 nA when increasing the bias voltage from 0 to 3 V (Figure 3g), resulting in an enhancement of the steady-state responsivity from 1.58 to 3628 mA W −1 (Figure 3h).The steady-state detectivity also enhances from 1.86 × 10 7 to 9.18 × 10 8 Jones with increasing bias voltage (Figure 3h).The  2. The MoS 2 -water photodetector device benefits from high responsivity, a simple fabrication process, and compatibility for applications that involve an aqueous environment.
To examine the effect of water volume on the photoresponse of the MoS 2 -water device, different amounts of DI water droplets (5, 20, 40, and 80 μL) were used and the photoresponses were recorded (Figure S9, Supporting Information).Both steady-state and transient photocurrents decrease with increasing the size of the water droplet, which could be due to the water curvaturedependent reflection of light on the water surface.We also observed that the photoresponse is highly dependent on the incident light angle (Figure S10, Supporting Information).The maximum photocurrent is achieved when the incident light is normal to the sensor surface (incident light angle of 90°), which could be due to the incident angle-dependent reflection at the water surface and change in light intensity.Therefore, the steady-state and transient photocurrents highly depend on the curvature of a water droplet and incident light angle.
The physical mechanism of the pyro-phototronic effect at the MoS 2 -water interface is described using a four-stage photoresponse process illustrated in Figure 4.In stage 1, the light is turned on, inducing both pyroelectric and photovoltaic effects (Figure 4a).In this stage, the photogenerated electrons in the MoS 2 flow from MoS 2 to water due to band bending at the MoS 2water interface (Figure 4b).De-polarization of water molecules on the surface of the MoS 2 crystal and the presence of a built-in electric field at the MoS 2 -water interface result in a large transient pyroelectric current peak when the light is turned on (Figure 4c).In stage 2, when the incident light has been on for a period of time, the pyroelectric current disappears due to the device reaching an equilibrium temperature and only the photocurrent due to the photovoltaic effect remains (Figure 4d). Figure 4e shows the band-diagram during this stage and the flow of photo-generated electrons from MoS 2 to water due to the built-in electric field at the MoS 2 -water interface, resulting in the generation of a steadystate photocurrent (Figure 4f).Note that this steady-state photocurrent enhances significantly when a non-zero bias voltage is applied across the MoS 2 -water device (Figure 3g).In stage 3 g) The steady-state photocurrent of the MoS 2 -water device under different bias voltages of 0-3 V at the same light intensity of 6.8 mW cm −2 .h) The steady-state responsivity and detectivity of the MoS 2 -water device under different bias voltages of 0-3 V at the same light intensity of 6.8 mW cm −2 .i) The photoresponse rise/fall times of the MoS 2 -water device at a bias voltage of 3 V.
(Figure 4g-i), the incident light is turned off, and a transient pyroelectric current peak with negative polarity, opposite to that of stage 1, is generated due to the device cooling down and fast polarization of water molecules on the MoS 2 (Figure 4g).The band diagram illustrating a flow of electrons from water to MoS 2 during this stage is shown in Figure 4h.In the absence of incident light, the photocurrent is generated only by the pyroelectric effect and has no steady-steady photovoltaic component (Figure 4i).After a period of time without incident light, the pyroelectric effect disappears, and the current returns to about zero, as shown in stage 4 (Figure 4j-l).The photoresponse characteristics and energy band structure of the MoS 2 -water heterojunction in relation to the n-type and p-type silicon-water heterostructures are illustrated in Figure S11 (Supporting Information).The photoresponse in MoS 2 -water structure is similar to that of p-type silicon-water due to the similar orientation of water molecules at their surfaces.
DI water was used as the liquid in this study and H 2 O being a polar molecule is critical for the generation of pyroelectric currents at the MoS 2 -liquid interface.Replacing DI water with toluene, a non-polar solvent, results in no pyroelectric effect and therefore no transient current upon light illumination (Figure S12, Supporting Information).The equilibrium state of ordered water molecules on the MoS 2 substrate is schematically illustrated in Figure 5a.When a change in temperature is induced by light, dipoles are excited, raising their energy levels, causing a deviation from the mean equilibrium ordered position and a reduction in the electrical dipole moment (Figure 5b).This results in the release of electrons/holes that move to the external circuit, generating a transient current response.Immediately thereafter,   the device has enough time for complete capacitive charging and therefore both transient and steady-state responses are visible.As the frequency increases and the time interval between two consecutive pulses is lower than the rise time of the device, the transient response diminishes, resulting in semi-square wave photocurrents.This square-shaped response is maintained up to a frequency of ˜10 kHz and the maximum detectable frequency is ˜100 kHz.The distinguishable photoresponses over a wide range of frequencies make the MoS 2 -water photodetectors suitable for high-frequency optoelectronic applications.
In this work, to fabricate MoS 2 -water heterojunction devices, we used mechanically exfoliated MoS 2 flakes, and the DI water droplet was drop-casted on the structure as a proof-of-concept photodetector device.A major disadvantage of using exfoliated flakes is that we do not have control over the dimension, thickness, and shape of the flakes.Furthermore, water droplet shape, curvature, and amount cannot be accurately controlled in this method, resulting in varying photoresponses from device to device.To expand this proof-of-concept to an array of MoS 2 -water photodetectors on a single chip, large-area 2D MoS 2 can be produced through chemical vapor deposition (CVD) [58] or wet chemical processes, [59] etched in rectangular shapes, and transparent polymers (e.g., PDMS) or conductive 2D materials (e.g., graphene) can be deployed to encapsulate the water. [60]The simple fabrication process, self-driven capability, fast response, and polarization sensitivity make the MoS 2 -water structure an excellent candidate for optoelectronic applications.

Conclusion
Here, the MoS 2 -water heterostructure has been demonstrated for self-driven and polarization-sensitive photodetection applications.The pyroelectric response is based on the polarization and depolarization of water molecules on the MoS 2 surface.
The MoS 2 -water photodetector illustrates a transient photoresponsivity as high as 24.6 mA W −1 and a specific detectivity of 2.85 × 10 8 Jones under 470 nm at zero bias.It also shows a high steady-state responsivity of 3.62 A W −1 with a high detectivity of 9.18 × 10 8 Jones at 3 V bias, along with the rise/fall times of ≈0.741/2.83ms.The simplicity, low cost, and high performance of the MoS 2 -water structure make it an excellent candidate for liquid-compatible photodetectors.

Experimental Section
Device Preparation: MoS 2 flakes were mechanically exfoliated from a bulk MoS 2 crystal (SPI Supplies) using Nitto SPV224 tape and transferred onto a p-type (100) Si wafer coated with thermally oxidized 300 nm SiO 2 .Cr/Au (10/60 nm) electrodes were patterned by photolithography in a way that one electrode is on the MoS 2 crystal, followed by the thermal evaporation and the lift-off process in an acetone bath.Al 2 O 3 (40 nm) was deposited on the device using an atomic layer deposition (ALD) system.Then, a window was opened on the device in a way that part of the MoS 2 crystal and the second electrode was exposed to air using photolithography and wet etching by buffered oxide etchant (BOE) 6:1 solution.The substrate was then cleaned using acetone, IPA, and DI water.Finally, a 50 μL water droplet was placed on the exposed MoS 2 flake.The fabrication process is shown in Figures S1 and S2 (Supporting Information).
Material Characterization: The structure of the MoS 2 device was characterized by Optical Microscopy.The Raman spectra of the MoS 2 device were recorded on a Renishaw inVia a confocal Raman microscope equipped with a 514 nm continuous-wave excitation laser.
Electrical/Optical Characterization: Electrical measurements were performed using a Keithley 2400 source meter at room temperature.The Si substrate was not contacted and left electrically floating.For the photodetection measurements, an LED with an adjustable intensity and a wavelength of 470 nm was utilized.For the rise/fall time and frequencydependent photoresponse measurements, the MoS 2 -water photodetector was illuminated with a 470 nm LED powered by a function generator (Tektronix AFG3151C).The MoS 2 -water device was connected in series to a resistor (15 MΩ), and an oscilloscope (Tektronix MDO3104) was used to measure the voltage across the resistor.All measurements were conducted at atmospheric pressure and room temperature.

Figure 1 .
Figure 1.Structural, electrical, optical, and thermal characterization of the MoS 2 -water heterostructure device.a) The optical image of the fabricated structure before placing the water droplet on it.b) Raman spectroscopy of the MoS 2 crystal, indicating the semiconducting 2H phase.c) Schematic of the MoS 2 -water heterostructure device.d) The I-V characteristics of the MoS 2 -water heterostructure device at dark conditions.The inset shows the actual MoS 2 -water device in a chamber during the measurement.Note that the MoS 2 crystal is electrically grounded.e) The I-V characteristics of the MoS 2 -water device at dark and light conditions (Solar spectrum AM1.5G, 100 mW cm −2 ), showing the photovoltaic effect.f) The pyroelectric current of the MoS 2 -water device when multiple heat pulses are applied to the back of MoS 2 -water structure using a heat source.

Figure 2 .
Figure 2. Self-driven photodetection characterization of the MoS 2 -water heterostructure device.a) Schematic illustration of the MoS 2 -water heterostructure photodetector.Note that the electrode connected to MoS 2 is electrically grounded.b) Photoresponse of the MoS 2 -water device at multiple light intensities of 1.5, 2.85, 4.06, 5.17, 6.16, and 6.8 mW cm −2 at zero bias.The wavelength of light is 470 nm.c) The transient photocurrent of the MoS 2water device as a function of light intensity (1.5-6.8 mW cm −2 ).d) Transient responsivity and detectivity of the MoS 2 -water device as a function of light intensity (1.5-6.8 mW cm −2 ).e) The photoresponse rise/fall times of the MoS 2 -water device at zero bias.f) The photoresponse of the MoS 2 -water device at different polarization angles.g) Polar plot of the MoS 2 -water device under a natural light ( = 470 nm) as a function of polarization angle, showing polarization sensitivity.

Figure 3 .
Figure 3.The effect of bias voltage on the photoresponse.a) The schematic of the MoS 2 -water photodetector under different bias voltages.The photoresponse of the MoS 2 -water device under different bias voltages of b) 0, c) 0.5, d) 1, e) 2, and f) 3 V with the same light intensity of 6.8 mW cm −2 .g)The steady-state photocurrent of the MoS 2 -water device under different bias voltages of 0-3 V at the same light intensity of 6.8 mW cm −2 .h) The steady-state responsivity and detectivity of the MoS 2 -water device under different bias voltages of 0-3 V at the same light intensity of 6.8 mW cm −2 .i) The photoresponse rise/fall times of the MoS 2 -water device at a bias voltage of 3 V.

Figure 4 .
Figure 4. Working mechanism of the MoS 2 -water pyro-phototronic photodetector.a-c) Stage 1: when the incident light is turned on, both pyroelectric and photovoltaic currents are present, which results in a large transient response.d-f) Stage 2:when the incident light is kept on for some time: the pyroelectric current disappears due to the lack of temperature gradient and there is only steady-state photovoltaic current.g-i) Stage 3: when the incident light is turned off, a pyroelectric current in the opposite direction is generated due to the temperature gradient.j-l) Stage 4: after the light remains off for a moment, both pyroelectric and photovoltaic currents disappear and the current returns to zero.

Figure 5 .
Figure 5. Working mechanism of the MoS 2 -water photodetector.a) The equilibrium position state of ordered water molecules on the MoS 2 .b) The excitation of dipoles under light illumination and reduction in the electrical dipole moment.

Figure 6 .
Figure 6.The light frequency-dependent photoresponse of the MoS 2 -water device.a) The setup schematic for measuring the frequency-dependent photoresponse.b) Frequency-dependent photoresponse of the MoS 2 -water device tested in the air with light pulse frequencies from 1 Hz to 100 kHz and V bias = 0 ( = 470 nm).

Table 1 .
Comparison between the MoS2-water photodetector with other self-powered pyro-phototronic photodetectors in terms of light polarization sensitivity, wavelength (), light intensity, response time, and responsivity.

Table 2 .
Comparison between the MoS 2 -water photodetector with other polarization-sensitive MoS 2 -based heterostructure photodetectors in terms of mechanism, wavelength (), applied bias voltage, response time, and responsivity.