Flexible near-infrared (NIR) photodetector (PD) is of substantial interest owing to its wide application in telecommunication, remote sensing, medical diagnostics, biological imaging and optical communication, etc.^[1−5]. Developing NIR PD with a high light-to-dark ratio, detectivity, fast response time, flexibility and stability represents a key in realizing image sensing application^[6−9]. Many efforts have been devoted towards high-performance NIR PD, including modification of photosensitive materials with quantum dot, construction of heterojunction^[10−12]. Two-dimension (2D) materials based van der Waals heterostructures that possess excellent mechanical strength, fast electron mobility, low power dissipation, tunable photoelectrical properties and superior structural stability offer an effective pathway to improve the optoelectrical performance of the NIR PD compared to heterostructures with different dimensionalities such as 0D-1D, 1D-2D, 0D-2D heterostructures^[13−19]. Researchers have developed a variety of two-dimensional van der Waals heterojunction to construct NIR PD, for instance, Chen et al.^[20] realized the application of PD in near infrared region by integrating GeSe/MoS₂ van der Waals heterojunction and poly (vinylidene fluoride-trifluoroethylene)-based ferroelectric polymer. Kim’s group proposed a NIR PD based on serial nano-bridge MoS₂ multi-heterojunctions^[21], which exhibited a photoresponsivity of 1.65 × 10⁴ A/W at 1064 nm and fast response/recovery time of 1.5/2.5 ms.

For the originator of two-dimensional materials, graphene has a short carrier lifetime because of its zero band gap and low absorption luminosity^[22], the mobility of transition metal dichalcogenides (TMDs) are affected by temperature^[23−25], which limits their application in optoelectronic devices. Compared with other two-dimensional materials, MXenes (2D transition-metal carbides and nitrides with the general formula M_n+1X_nT_x, where M is an early transition metal, X represents carbon and/or nitrogen, T_x stands for surface termination, n = 1−4) have ultra-high conductivity, hydrophilicity and tunable electrical properties while overcoming these problems, especially they have abundant surface functional groups, which makes them possible to form van der Waals heterostructure with other semiconductor materials^[26−28]. Thus, it is the main candidate for constructing high-performance infrared PDs. The heterojunction PD combined with Ti₂CT_x and InSe by Yang et al.^[29] shows avalanche carrier multiplication effect, which makes the device have high responsivity and detectivity of 1 × 10⁵ A/W and 7.3 × 10¹² Jones. Hu et al.^[7] realized a Ti₃C₂T_x MXene-based heterojunction NIR PD prepared by modifying Ti₃C₂T_x with phenylsulfonic acid groups, which has a microampere level high current without amplification. Therefore, it still be highly desirable to develop MXene based 2D van der Waals heterojunction for high performance NIR PDs. The other 2D semiconductor side is considered suitable for opting the Ⅱ−Ⅴ group compounds with a narrow bandgap. ZnSb nanoplate that has a unique orthorhombic and abundant covalent bond, through different calculation methods, the bandgap of which are calculated to be 1.004, 0.46, and 0.714 eV, belongs to an ideal NIR photosensitive materials^{[30, 31]}. Combining the most studied 2D Ti₃C₂T_x MXene materials and 2D ZnSb nanoplates to build 2D materials based van der Waals heterostructures will be a new way to obtain high-performance NIR PDs.

In this work, we provided a facile chemical vapor deposition method to synthesize the 2D ZnSb nanoplate, through the calculation of density functional theory (DFT), the 2D ZnSb had a bandgap of 1.022 eV, which was then placed one by one on the patterned Ti₃C₂T_x MXene microelectrode array using flexible polyethylene terephthalate (PET) substrate via typical photolithography technology to form 2D van der Waals heterostructures. At 1342 nm laser illumination, the fabricated ZnSb/Ti₃C₂T_x photodetectors showed high light-to-dark current ratio of 4.98 and fast response/recovery time (2.5/1.3 s) (bias voltage is 0.1 V), excellent photoresponsivity of 143.49 μA/W and satisfactory specific detection rate of 2.39 × 10⁸ Jones (bias voltage is 1 V). Moreover, the ZnSb/Ti₃C₂T_x devices exhibited superior flexibility with no apparent performance degradation under increased bending angles varying from 0^o to 150^o. Even after 5000 bending cycles, the devices can work normally. The ZnSb/Ti₃C₂T_x NIR PD devices were further integrated into a 26 × 5 array, which is utilized to sense an image with the word "MXene". The successful fabrication of 2D ZnSb/Ti₃C₂T_x van der Waals heterostructures opens up a new avenue for novel NIR PD arrays.

Ti₃C₂T_x MXene was synthesized via the mixed acid method, followed by the intercalation process with LiCl. The schematic diagram of the Ti₃C₂T_x in water solution is displayed in Fig. 1(a), the transmission electron microscope (TEM) image of Ti₃C₂T_x nanosheets is shown in Fig. S1. Fig. 1(b) depicts the photolithography technology to prepare patterned Ti₃C₂T_x microelectrodes. The inset shows the digital image of the Ti₃C₂T_x MXene suspension. After the photolithography procedure, the photoresist was peeled off, leaving a patterned photoresist. Ti₃C₂T_x MXene was then coated on the photoresist, which was put in acetone after fully drying. The patterned Ti₃C₂T_x MXene can be obtained. Fig. 1(c) illustrates the digital photography of the prepared Ti₃C₂T_x MXene microelectrodes. The surface and thickness of the Ti₃C₂T_x MXene were also measured, as shown in Figs. 1(d) and 1(e). From Fig. 1(d) we can see, the thickness of Ti₃C₂T_x MXene nanoflakes is about 2.37 nm (approximately two layers thick). Fig. 1(e) demonstrates the lateral size of the Ti₃C₂T_x MXene nanoflakes exceeds 10 μm. The ZnSb synthesized via the CVD method was also characterized by SEM and TEM, as displayed in Figs. 1(f)−1(j). Fig. S2 shows the SEM images of the ZnSb nanoplate synthesized on the mica substrate at different magnifications, which indicates the average length of the ZnSb nanoplates is over 20 μm. The SEM image of the single ZnSb nanoflakes is provided in Fig. 1(f). The hexagonal shape can be clearly seen.

Figs. 1(g) and 1(h) exhibit the TEM images and corresponding EDS mapping of an individual ZnSb nanoplate. The elemental images of the Zn and Sb keep well with the framework of the ZnSb nanoplate, which confirms the pure phase of the synthesized ZnSb samples. Fig. 1(i) shows the high-resolution TEM (HRTEM) image of the as-synthesized ZnSb nanoplates. The lattice distance observed from HRTEM were 0.208 and 0.311 nm, which are assigned to the (222) and (200) planes of the ZnSb nanoplate with orthorhombic structures (PDF no. 05-0714)^[31]. Because of the low production of the ZnSb on the mica substrate, the XRD samples of the ZnSb are hard to collect. To further demonstrate the pure orthorhombic crystal structure of the ZnSb nanoplates. The elemental spectrum of the prepared samples is presented in Fig. 1(j), which indicates the existence of the Zn and Sb element, the atomic percentage ratio of the Zn and Sb element was calculated to be 1 : 1, confirming the successful synthesis of the pure ZnSb nanoplates. The XRD pattern of the Ti₃C₂T_x MXene is provided in Fig. 1(k), the characteristic peak (002) for the Ti₃C₂T_x MXene nanoflakes is located at ~7.5^o, confirming the fully delamination of the Ti₃C₂T_x nanoflakes^[32].

The microscope images patterned Ti₃C₂T_x MXene microelectrode array is provided in Fig. S3. The gap between the two electrodes is 10 μm, which is suitable for the ZnSb nanoplates with an average lateral size of 20 μm. When the ZnSb nanoplates were placed on the Ti₃C₂T_x MXene microelectrode, the fabrication process was fully completed. Fig. 2(a) shows the schematical illustration of the structure of the ZnSb/Ti₃C₂T_x van der Waals heterojunction based flexible NIR PDs. The photoelectric performances of the fabricated device were then systematically measured at room temperature. With the bias voltage of 1 V, the photocurrent generated by irradiating the device with lasers with different wavelengths is as shown in Fig. 2(b), in which a current of 9.7 pA is generated by irradiating with 1342 nm. Fig. 2(c) depicts the I−V curves of ZnSb/Ti₃C₂T_x NIR PD under dark and various NIR wavelengths including 915, 1064, 1342, 1550 and 2200 nm at bias voltage ranging from −5 to 5 V, indicating the device has a broad response range of wavelength from 915 to 2200 nm. The I−T characteristics of the ZnSb/Ti₃C₂T_x NIR PD measured at 0.1 V with laser illumination of different wavelengths are provided in Fig. 2(d). The dynamic cyclic current changes while turn on and turn off laser periodically shows a stable response to different wavelengths in the NIR region. The calculated on/off ratio at different wavelengths of 915, 1064, 1342, 1550, and 2200 nm corresponds to the 2.81, 3.19, 4.98, 2.46, and 3.82, which is two folds higher than the previously reported ZnSb based NIR PDs, indicating the high-performance of the fabricated ZnSb/Ti₃C₂T_x NIR PD.

We choose the laser with the best performance of 1342 nm to continue to study the effects of different optical power densities on the devices. Figs. 2(e)−2(g) are all test results of ZnSb/Ti₃C₂T_x NIR PD at optical power densities of 15.9, 36, 74.8, 132, and 178 mW/cm². As shown in Figs. 2(e) and 2(f), the photocurrent and the on/off ratio of the device add with the increase of optical power density under the bias voltage of 1 V, reaching the maximum at 178 mW/cm², which are 9.7 pA and 12.76 respectively. Fig 2(g) illustrates the I−V profiles of the ZnSb/Ti₃C₂T_x NIR PD in the dark and irradiating to 1324 nm laser with different light intensities. It can be observed that the photocurrent at the same wavelength increases as the light intensities increase because the photogenerated charge carriers are proportionate to the increased light intensities. It’s worth to mentioning the photocurrent of the ZnSb/Ti₃C₂T_x under the same conditional shows obvious decrease in comparison to ZnSb PD using Au as electrodes^[31], which provide a lower power dissipation. Moreover, the linear I−V curves suggested the Ohmic contact between ZnSb nanoplates and Ti₃C₂T_x electrodes, but the I−V curves do not pass through the origin when at the bias voltage of 0 V, revealing the existence of the Schottky barrier in the construction of ZnSb/Ti₃C₂T_x heterojunction.

Fig. 3(a) shows the microscope image of the fabricated ZnSb/Ti₃C₂T_x MXene NIR PD devices. The ZnSb nanoplate tightly coated on the patterned Ti₃C₂T_x MXene electrodes, and the lateral size of ZnSb was large enough to cross the gap between two finger electrodes. The crystal structure of ZnSb nanoplates with orthorhombic structures is presented in Fig. 3(b). Fig. 3(c) and Table S1 display the photoresponsivity and detectivity curves of the PD devices to laser irradiation of different wavelengths varying from 915 to 2200 nm. The responsivities (detectivity) of the fabricated devices show a value of 47.3, 40.1, 74.4, 37.4, and 69.4 μA/W (8.7 × 10⁷, 7.4 × 10⁷, 12.4 × 10⁷, 7.6 × 10⁷, and 14.2 × 10⁷ Jones), which is assigned to the 915, 1064, 1342, 1550, and 2200 nm laser. The EQE value of the fabricated devices under various NIR wavelengths is provided in Fig S4. The highest EQE value of 6.88 × 10⁻³% was obtained under 1342 nm NIR illumination. Furthermore, the responsivity, detectivity, and EQE value of the ZnSb/Ti₃C₂T_x MXene NIR PD devices under 1342 nm NIR laser illumination with different light intensities are displayed in Figs. 3(d) and 3(e), the data are summarized in Table S2 and Table S3. The high responsivity of 143.5 μA/W, the detectivity of 2.39 × 10⁸ Jones and EQE of 13.26 × 10⁻³% were obtained when the light density was kept at 15.9 mW/cm². The response and recovery times that reflect the speed of a PD to sense a target light are also an important parameter to evaluate the performance of a PD device. In general, the response/recovery times can be defined as the required time when photocurrent rise/decay from 10%/90% to 90%/10%^[33]. Fig. 3(f) exhibits the response and recovery time of the fabricated devices, which shows 2.5 and 1.3 s, respectively. According to the atomic ratio of Zn : Sb = 1 : 1, we calculated the bandgap width of ZnSb used in the experiment by first-principle calculations, in which the hybrid density functional (HSE06) and GGA-PBE functional with the non-conserving pseudopotential were used to describe the electronic exchange and correlation effects, and the Methfessel−Paxton electronic smearing were adopted for the integration in the Brillouin zone for geometric optimization. Fig. 3(g) displays the molecular model of ZnSb, and the bandgap and density of states of it are shown in Fig. 3(h). Through the simulation of density functional theory (DFT), the bandgap of ZnSb is 1.022 eV. The energy band diagram of ZnSb/Ti₃C₂T_x heterojunction is shown in Fig. 3(i). According to previous work^[7], the work function (W_F) of Ti₃C₂T_x MXene with metalloid characteristics is 4.45 eV. When Ti₃C₂T_x MXene contacts ZnSb with semiconductor characteristics, electrons accumulate in the conduction band and form Ohmic contact.

The flexibility properties could ensure the devices normally work under bending states, which might meet the demand for portable and wearable electronics^{[33, 34]}. The 2D materials based van der Waals heterostructures have demonstrated excellent mechanical stability. Here, the ZnSb/Ti₃C₂T_x MXene based NIR PD devices were also tested under bending states. Fig. 4(a) depicts the digital photography of the assembled NIR PD devices. Fig. 4(b) shows the on/off ratio curves of the flexible ZnSb/Ti₃C₂T_x MXene NIR PD devices under different bending angles ranging from 0^o to 150^o at a constant bias voltage of 0.1 V. It can be observed that there was no change to the on/off ratio when the device was bent, indicating the excellent flexibility of the fabricated NIR PD devices. The cyclic fatigue measurement was also carried out, as shown in Fig. 4(c). The device was dynamically tested for 5000 bending cycles. One bending cycle of the device was recorded from 0^o to 150^o and then back to 0^o. It can be seen, even after 5000 bending cycles, the ZnSb/Ti₃C₂T_x MXene NIR PD shows excellent cycling stability with no obvious current attenuation, revealing superior flexibility and mechanical stability of the fabricated PD devices.

To explore the potential application of the assembled NIR PD devices, the PD devices were integrated into 26 × 5 device arrays to realize image sensing function^{[35, 36]}. Fig. 4(d) displays the schematic illustrations of the measurement method of the PD devices to sense the words "MXene". Under normal imaging conditions, a mask plate with a pattern should be placed above the device, and the light passes through the unobstructed part to present the pattern on the device array, but because the experiment uses a transparent but not hollow mask, the probe cannot penetrate, so in order to provide light with the "MXene" image, the mask with the "MXene" pattern was placed on the back of the devices, which was irradiated from the backside by the 1342 nm laser with the light intensity of 132 mW/cm². Due to the excellent photoresponse properties and flexibility, the 26 × 5 PDs device arrays can serve as 26 × 5 pixel flexible image sensors to detect the target image. The photocurrent and dark current of each pixel image sensor were measured, then all the on/off ratios of the ZnSb/Ti₃C₂T_x MXene NIR PD arrays were calculated. The image with the word "MXene" can be observed in Fig. 4(e), which demonstrates the feasibility of the ZnSb/Ti₃C₂T_x MXene PD for NIR image sensing application.

Materials: Ti₃AlC₂ MAX was prepared by Carbon-Ukraine. Sb powers (99.999%), ZnSb (99.99%), hydrofluoric acid (HF, 40%), hydrochloric acid (HCl, 9M), and lithium chloride (LiCl, 99%) were purchased from Aladdin Industrial Inc. Mica substrate was procured by Himedia Laboratories Pvt. Ltd. Double distilled (DI) water was utilized for all the experiments. All chemicals were used directly without further purification.

Preparing ZnSb nanoplates: In a typical chemical vapor deposition (CVD) process^[31], 0.08 g of Sb powders and 0.05 g of ZnSb were mixed. The ball milling process was carried out to ensure the powders were mixed well, which then were placed into the small quarts tube. The mica substrate with a size of 1 × 1 cm² was fixed in the tube nozzle. After that, the whole small quarts tube was set in the normal quarts tube. The growth process was performed at 825 ℃ with a heating rate of 20 ℃/min and kept at 825 ℃ for 2 h in an Ar atmosphere. When the reaction was finished, the mica substrate with deposited ZnSb nanoplates was collected.

Synthesize of the Ti₃C₂T_xnanoflakes: A mixture solution containing 1.5 g of Ti₃AlC₂ MAX phase 13.5 mL deionized (DI) water, 13.5 mL HCl, and 3 mL HF were stirred at room temperature for 24 h^[37]. The sediment was washed with DI water several times until the pH value reached neutral, which was then added into 15 mL DI water containing 1.5 g of LiCl under vigorous stirring at room temperature for 12 h. The monolayer Ti₃C₂T_x suspension was washed with DI water several times and collected by centrifugation at 3500 rpm for 5 min.

Fabrication of ZnSb/Ti₃C₂T_x van der Waals heterojunction based flexible PD array: Firstly, the Ti₃C₂T_x MXene microelectrode array was prepared via a typical photolithography technology on the flexible PET substrate using the spin coater^[26]. Second, the ZnSb nanoplates were placed on the Ti₃C₂T_x electrode with the help of the thin optical fiber one by one. Finally, the flexible PD arrays were obtained.

Figure of merits: Responsivity (R) can be expressed as the following equation^[38]:

1R=(Ion−Ioff)/(P⋅S).

Here I_on is the photocurrent. I_off represents the dark current. P is the optical power density. S stands for the effective illumination area.

The specific detection rate (D*) can be defined by the formula:

2D*=[ΔI/(P⋅S)]/(2eIoff/S)1/2=RS1/2/(2eIoff)1/2.

Here e is the amount of electron charge.

External quantum efficiency (EQE) can be written as:

3EQE=(hc/eλ)R.

Here h is the Planck constant, c is the light velocity and λ represents the excitation wavelength.

Characterization: The surface morphology of the ZnSb was characterized using the scanning electron microscopy (SEM) system (Zeiss Supra55) and transmission electron microscopy (HRTEM; JEOLJEM-2010HT). The thickness, surface morphology and crystallinity of the prepared Ti₃C₂T_x were conducted by atomic force microscopy (Bruker Dimension Icon) and powder X-ray diffraction (Rigaku D/Max-2550). The electrical properties of the photodetector were measured by a Keithley 1500-SCS semiconductor characterization system linked with a probe station. All tests were carried out at room temperature.

In conclusion, a 2D ZnSb/Ti₃C₂T_x heterojunction based NIR PD was successfully fabricated by the typical photolithography technology. Owing to the tight connection between 2D ZnSb nanoplates and Ti₃C₂T_x nanoflakes, the fabricated ZnSb/Ti₃C₂T_x photodetectors showed a high light-to-dark current ratio of 4.98 and fast response/recovery time (2.5/1.3 s) (bias voltage is 0.1 V), excellent photoresponsivity of 143.49 μA/W and satisfactory specific detection rate of 2.39 × 10⁸ Jones (bias voltage is 1 V) at 1342 nm laser illumination. Moreover, the ZnSb/Ti₃C₂T_x devices exhibited superior mechanical stability with no obvious performance degradation even after 5000 bending cycles. The PD devices array consists of 26 × 5 PDs were prepared and used to sense an image with the word "MXene", demonstrating the potential application of the fabricated ZnSb/Ti₃C₂T_x photodetectors in the image sensing field.