A ternary SnS1.26Se0.76 alloy for flexible broadband photodetectors

Layered two-dimensional (2D) materials often display unique functionalities for flexible 2D optoelectronic device applications involving natural flexibility and tunable bandgap by bandgap engineering. Composition manipulation by alloying of these 2D materials represents an effective way in fulfilling bandgap engineering, which is particularly true for SnS2xSe2(1−x) alloys showing a continuous bandgap modulation from 2.1 eV for SnS2 to 1.0 eV for SnSe2. Here, we report that a ternary SnS1.26Se0.76 alloy nanosheet can serve as an efficient flexible photodetector, possessing excellent mechanical durability, reproducibility, and high photosensitivity. The photodetectors show a broad spectrum detection ranging from visible to near infrared (NIR) light. These findings demonstrate that the ternary SnS1.26Se0.76 alloy can act as a promising 2D material for flexible and wearable optoelectronic devices.


Introduction
Flexible optoelectronic devices that can be bent, stretched, twisted and folded have triggered wide interest due to their greater superiorities in exibility, lightweight and transparency, 1-3 and thus have potential applications in exible solar cells, 4 video imaging, 5 optical ber communication. 6musclelike transducers, 7 machine vision, 8 and so forth.Layered 2D materials are naturally suitable for various thin-lm (opto) electronic devices due to their ultrathin thickness and exibility. 9,10Although graphene has excellent mechanical 11 and (opto)electronic 12 properties and can offer a series of new possibilities at reduced dimensionality, it is limited to application in photoelectronic devices due to its gapless nature. 13As an alternative, several 2D materials with sizable bandgaps and superior (opto)electronic performances [14][15][16] have been considered as potential candidates for next-generation ultrathin and exible photoelectronics. 17For example, signicant attention has been devoted to monolayer MoS 2 , which exhibits a photoresponsivity as high as 5.75 Â 10 3 A W À1 at a bias voltage of 5 V. 18 The carrier mobility of MoS 2 at extremely low temperature can be as high as 34 000 cm 2 V À1 s À1 , 19 and could be increased by strain engineering. 20In addition to the choice of materials, bandgap engineering 21,22 enables to the modulation of electronic structure of 2D materials (e.g.indirect-to-direct transition), thus manipulating their optical and electronic properties.To date, alloying 2D materials has been demonstrated to be an effective way in fullling bandgap engineering for many optoelectronic applications. 23,24s environmental-friendly, reserve-abundant and low-cost materials, layered tin dichalcogenides (SnS 2 , SnSe 2 ) are desirable for sustainable development of exible optoelectronic devices.Alloying of SnS 2 and SnSe 2 is oen effective for bandgap engineering, in which a continuously tunable bandgap of SnS 2x Se 2(1Àx) (0 < x < 1) from 2.1 eV (SnS 2 ) to 1.0 eV (SnSe 2 ) could be achieved. 25The layered SnS 2 has a rapid photoresponsive feature with response time as short as 5 ms, 26 and 1T atomic layered SnSe 2 with a very close bandgap to silicon (1.1 eV) displays a good responsivity of 0.5 A W À1 and a fast response time of 2.2 AE 0.3 ms. 27Previous reports indicated that physical properties of a ternary compound SnS 2x Se 2(1Àx) can be tunable by varying the atomic composition x, giving rise to remarkable (opto)electronic properties that may not appear in either of its binary compounds. 28,29As a member of SnS 2x Se 2(1Àx) alloy, few layered SnS 1.26 Se 0.76 (x $ 0.6) with a hexagonal CdI 2 -type structure in space group P 3m1 is easily obtained by mechanical exfoliation from a crystal due to weak van der Waals force among layers.This low-dimensional semiconductor nanosheets with applicable bandgap represents a typical candidate for exible optoelectronic devices due to its excellent optoelectronic properties possibly for broad spectra detection.However, the reports so far on the SnS 1.26 Se 0.76 nanosheet-based exible optoelectronic devices are very scarce, especially in the aspects of its mechanical durability, reproducibility, and high photosensitivity.In this work, we investigate exible photodetectors based on ternary SnS 1.26 Se 0.76 nanosheets, and demonstrate that the photodetectors exhibit a high photoresponsivity of $262 A W À1 , a high detectivity of 1.98 Â 10 11 jones and a fast response time of $10 ms to 532 nm light.Interestingly, the devices display a broad spectra response ranging from visible to NIR light and an ultrasensitive, reversible, and mechanical durable photoresponse even aer being bent for 100 times.The ndings suggest that SnS 1.26 Se 0.76 can act as a promising 2D material for exible and wearable optoelectronic devices.

Experimental
2.1 Growth of SnS 1.26 Se 0.76 crystal SnS 1.26 Se 0.76 crystal was synthesized by reacting the pure elements in quartz ampoules via chemical vapor transport using iodine as transport agent. 30,31Specic stoichiometric uniform mixtures of Sn powder (Alfa Aesar, 99.98%), sublimed S powder (Alfa Aesar, 99.5%), and Se powder (Alfa Aesar, 99+%) were mixed in air and put into quartz tubes, and then sealed into the furnace to sinter for getting SnS 1.26 Se 0.76 polycrystal.Subsequently, the SnS 1.26 Se 0.76 polycrystal was mixed with iodine (density: 5 mg cm À3 ), followed by seal in the silica tubes and heating for 10-20 days in a horizontal two-zone furnace.The as-grown SnS 1.26 Se 0.76 single crystal was transported from the hot to cold zone.Finally, the SnS 1.26 Se 0.76 crystal was obtained by natural cooling to room temperature.

Structural characterization of as-synthesized SnS 1.26 Se 0.76 crystal
The crystallographic structures of bulk SnS 1.26 Se 0.76 were examined by X-ray diffraction (XRD, D/MAX-TTRIII (CBO)) technique.Morphologies were characterized by eld emission scanning electron microscopy (FESEM, S4800,Tokyo, Japan).Specimens for transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) observations were prepared by transferring SnS 1.26 Se 0.76 nanosheets to the micro TEM grid.The TEM, high resolution TEM (HRTEM), selected area electron diffraction (SAED) and high-angle annular dark-eld (HAADF) and bright-eld (BF) STEM images were acquired by double aberration-corrected scanning transmission electron microscope (FEI G3 Cubed Themis 60-300 kV Monochromated X-FEG S/TEM).The elemental analysis were carried out by energy dispersive X-ray spectroscopy on the STEM (EDS, Super-X EDX System: four windowless Silicon Dri Detector).OLS4000) and an atomic force microscope (AFM, Multimode 8, Bruker).The Raman mapping and spectrum were taken using a spectrometer with an excitation wavelength of 514 nm (Renishaw inVia), aimed to characterize the composition-dependent vibration modes of the synthesized SnS 1.26 Se 0.76 alloy crystals.Microphotoluminescence (PL) spectrum was measured with the same instrument (Renishaw inVia).Theoretical calculations of the optical bandgap and electronic band structure for the bilayer and multilayer SnS 1.26 Se 0.76 nanosheets were carried out based upon density functional theory (DFT).

Fabrication and measurements of photodetector
For the device fabrication, SnS 1.26 Se 0.76 nanosheets were obtained by mechanically exfoliating of bulk SnS 1.26 Se 0.76 crystals onto a exible PET substrate.TEM copper grid as a shadow mask with a typical gap of 15 mm was mounted on surface of exible PET substrate covered by SnS 1.26 Se 0.76 nanosheets.10 nm Cr and 60 nm Au were subsequently evaporated as the electrodes of two-terminal photodetector, and the devices were prepared aer removing the copper grid.All the electrical measurements were performed with a semiconductor test system (Agilent-B2902) at room temperature in the ambient.The 450, 532, 638 and 808 nm lasers were employed to perform photoresponse experiments.AFM topography (Fig. 2(b)) reveals that the area of the palest blue color in the orange square in Fig. 2(a) has a thickness of 2.36 nm (i.e.2-3 layer).To characterize the composition dependent vibration modes of the SnS 1.26 Se 0.76 crystal, we present in Fig. 2(c) Raman mapping image for the intensity of A 1g(Sn-Se) vibration peak (200-205 cm À1 ).The intensity of bilayer SnS 1.26 Se 0.76 sample is much weaker than that of few-layer one, as is also reected in the thickness-dependent Raman spectrum (Fig. 2(d)).All the samples have characteristic peaks at about 204 and 304 cm À1 , which are assigned to the A 1g(Sn-Se) and A 1g(Sn-S) vibration modes, 35,36 while the weak peaks located at 136 cm À1 is assigned to in-plane mode E g(Sn-Se) .The Raman mode of E g(Sn-S) is not observed possibly resulting from the weak electron-phonon interaction in thin layer samples.For the akes with different thicknesses, the positions of these observed peaks almost remain unchanged but their intensities increase with the thickness of SnS 1.26 Se 0.76 nanosheets.The Raman peak positions are almost the same for the samples with thicknesses ranging from 2.36 to 12.13 nm, indicating that the layer thickness has minor effect on Raman behavior.

Results and discussion
Fig. 1(e) shows PL spectrum, where the intense peak position is located at about 1.55 eV, indicating that the optical bandgap is 1.55 eV.The intensity of PL is weaker for thicker sample due to the indirect bandgap nature of SnS 1.26 Se 0.76 , which can also be proven by calculated electronic band structure of bilayer and bulk SnS 1.26 Se 0.76 (Fig. 2(f) and S2 †).From Fig. 2(f), the bilayer SnS 1.26 Se 0.76 is calculated to be an indirect band gap semiconductor with conduction band minimum (CBM) located at G point and valence band maximum (VBM) at M point.This differs from the SnS 1.26 Se 0.76 bulk case showing an indirect band gap CBM at G point and VBM at a general point along M-G line (Fig. S2 †).The bandgaps for the bilayer and bulk SnS 1.26 Se 0.76 are estimated as 0.88 eV and 0.94 eV, respectively, which are smaller than the experimental data due to the wellknown underestimation of bandgap in the hybrid functional theory (HSE06). 37The narrower bandgap for the bilayer with respect to its bulk implies a broad spectra detection from visible to NIR light for optoelectronic device applications.Fig. 3(b-e) exhibit high photoresponse to 532 nm light with various illumination.Fig. 3(b) shows the typical I-V curves of a device (inset of Fig. 3(b)) in the dark and under different irradiances at 532 nm.The observed linear plots of I-V indicate an ohmic contact between the SnS 1.26 Se 0.76 lm and Au/Cr electrodes.Upon decreasing the light irradiance, the photocurrent decreases, while the photoresponsivity increases (Fig. 3(c)).Here, photocurrent (I ph ) is dened as the differences between I illuminated and I dark (I ph ¼ I illuminated À I dark ) and photoresponsivity (R l ) is dened as the photocurrent generated per unit power of incident light on the effective area (S) (R l ¼ I ph / PS).In addition, the detectivity (D*) reects the photodetector's sensitivity, and can be calculated by D* ¼ R l S 1/2 /(2eI dark ) 1/2 , where e is the electronic charge.At a bias voltage of 2 V and a light irradiance of 3.4 mW cm À2 , the device shows a photoresponsivity of 262 A W À1 and a detectivity of 1.98 Â 10 11 jones under the illumination of 532 nm laser.The efficient irradiation area S is 120 mm 2 .The corresponding light intensity dependence of the photocurrent can be expressed by a power law function of I ph $ P a , which indicates increased photons generate more electrons.A power dependence of I ph $ P 0.53 is estimated for our photodetector by tting the experimental data, and the deviation from the ideal slope may be attributed to the long-channel in the SnS 1.26 Se 0.76 photodetector.
To shed light on the photoresponse with the laser on and off, we measure time-dependent photoresponse of SnS 1.26 Se 0.76 nanosheet-based devices at various light intensity at 2 V bias (532 nm light) as shown in Fig. 3(d).With the on/off switching of incident laser, a highly stable and reversible photo-switching behavior between two states is observed and the sharp rise and drop of current with light on and off conrm that the device is ultrasensitive to light.The response time (s on , dened as he time needed to reach 90% of the maximum photocurrent) and recovery time (s off , dened as the time needed to drop to 10% of the maximum photocurrent) are estimated to be $0.01 s and $0.32 s, respectively, from an enlarged single circle in Fig. 3(e), which are superior to the values reported for most of layered materials. 38,391][42] As can be seen from Table 1, the response time in our work is very fast, but the decay time is longer than most of the previous report.The decay time of the device contains three procedures: the rst stage is the recombination of excess carrier, followed by the empty of shallow and deep traps, respectively. 43According to the atom ratio measured by EDS, we can conrm that there are small amount of Sn vacancies appearing in SnS 1.26 Se 0.76 crystal during the synthesis process.Therefore, the observed long decay time of the device was caused by the slow release of electrons trapped in the Sn vacanciesinduced deep traps 44,45 because the de-trap time of carriers from a deep trap can be prolonged by several orders of magnitude as compared to shallow traps, resulting in additional decay of the device. 46,47In addition, the SnS 1.26 Se 0.76 shows pronounced photoresponse under a voltage bias of 2 V for 450, 532, 638, and 808 nm laser (Fig. 3(f)), and the devices show a high stability under different irradiances for different laser wavelengths (Fig. 3(d, g-i)).The SnS 1.26 Se 0.76 photodetector exhibits the highest R l to 532 nm light ($262 A W À1 ) and a relatively low R l to NIR light ($120 mA W À1 ).The related performance parameters are listed in Table 2 and detailed experimental data are given in Fig. S4-S6.† As summarized in Tables 1 and 2, our photodetector shows a good photoresponse, indicating that the SnS 1.26 Se 0.76 nanosheets can serve as an ideal 2D material as highperformance photodetectors.Further work we will focus on the study of increasing photocurrent by decreasing membrane thickness or fabricating heterojunction structure and improve experimental method to study the response to weaker light. 48,494 Durability of SnS 1.26 Se 0.76 exible photodetector Apart from the stability and sensitivity, mechanical exibility is also an important parameter to evaluate for the exible photodetector.In order to assess the durability of the SnS 1.26 Se 0.76 exible optoelectronic devices, the devices were bent repeatedly for 100 times with a bending radius of 5.5 mm, as shown in Fig. 4(a) and (b).Fig. 4(c) shows the I-V characteristics of the photodetector under a dark and 532 nm laser irradiance of 3.4 mW cm À2 and Fig. 4(d) under a dark and 808 nm laser irradiance of 59.9 mW cm À2 aer bending for 100 times.The I-V plots differ slightly for both dark current and photocurrent compared with unbending case, which can be attributed to the destructive contact barrier between SnS 1.26 Se 0.76 nanosheet and electrodes aer plenty of continuous mechanical bending.Although the photocurrent has a slight decrease aer bending, a sharp increase in photocurrent is observed once the devices is illuminated by either 532 nm laser or 808 nm laser, as shown in Fig. 4(e) and (f).Similar time trace of photoresponse is also observed under illumination with 450 nm and 638 nm laser, as shown in Fig. S7.† From the time-dependent photocurrent, we can see that when the laser switches from "OFF" to "ON" state, the current rapidly reaches a relatively large value and then slowly increases to a saturation value.The photocurrent decreases from $1.08 mA to $1.03 mA under 532 nm laser illumination of 3.4 mW cm À2 at a bias of 2 V, which leads to a minor decrease in photoresponsivity from 262 A W À1 to 250 A W À1 .However, the response time and recovery time remain almost unchanged aer sustaining 100 times of bending.The photoresponse parameters of s on , s off and R l are summarized in Table 2   a These devices are measured under illumination of the 532 nm, 450 nm, 638 nm and 808 nm laser with a light intensity of 3.4 mW cm À2 , 8.0 mW cm À2 , 60.3 mW cm À2 and 59.9 mW cm À2 , respectively.The corresponding voltage bias are all 2 V. b These PET devices are measured aer bending 100 times with a bending radius of 5.5 mm.
and aer bending 100 times with different illumination wavelength.These ndings show that SnS 1.26 Se 0.76 nanosheet-based photodetector maintains excellent mechanical durability, reproducibility, and high photo-sensitivity, demonstrating great potential for advanced exible and wearable optoelectronic device applications.

Conclusions
We perform a systematic study on exible photodetectors based on high-quality single crystalline ternary SnS 1.26 Se 0.76 nanosheets.The photodetectors exhibit a good broad spectra photoresponse in the region of visible (450 nm) to NIR (808 nm) light, high photoresponsivity of $262 A W À1 , and fast response time of $10 ms to 532 nm light.Furthermore, such photodetector can maintain excellent mechanical durability, reproducibility, and high photo-sensitivity even aer being bent for 100 times with a bending radius of 5.5 mm, showing great potential in the application of advanced exible and wearable optoelectronic devices.The ndings pay a new way in employing other layered alloy materials in exible optoelectronic devices based on bandgap engineering.

3. 1
Structural characterization of as-synthesized SnS 1.26 Se 0.76 crystal Bulk SnS 1.26 Se 0.76 single crystals were synthesized by the chemical vapor transport (CVT) method with iodine as a transport agent (see Experimental).Fig. 1(a) presents the atomic structure of SnS 1.26 Se 0.76 viewed from side and top, showing that it has a hexagonal CdI 2 -type structure with periodic S(Se)-Sn-S(Se) stacking along the c-axis and connecting with each layer through van der Waals forces.X-ray diffraction (XRD) of the synthesized SnS 1.26 Se 0.76 single crystals (Fig. 1(b)) reveals ve sharp peaks which correspond to (001), (002), (003), (004), and (005) peak derived from c-axis of the bulk, consolidating its single crystalline nature.The inset of Fig. 1(b) shows a FESEM image of SnS 1.26 Se 0.76 single crystal exhibiting its layered structure.Further EDS analysis (Fig. 1(c)) reveals that the Sn/S/ Se atomic ratio in the sample is roughly estimated to be 1 : 1.26 : 0.76, matching well with the nominal formula of SnS 2x Se 2(1Àx) (x $ 0.6).Fig. 1(d)-(g) shows a typical TEM image of a SnS 1.26 Se 0.76 few-layer ake and the corresponding EDS element mapping for Sn, S and Se.The EDS mappings conrm a highly uniform distribution of Sn, S and Se elements.

Fig. 1
Fig. 1(h) shows HRTEM image of the SnS 1.26 Se 0.76 sample, revealing a hexagonal structured lattice with an interplanar distance of 0.34 nm, consistent with that of the (100) plane of SnS 1.26 Se 0.76 .The corresponding SAED pattern shown in Fig. 1(i) displays the lattice arrangement, indicating that the SnS 1.26 Se 0.76 sample is stacked along [001] direction with good crystallinity.To further extract atomic information, we show in Fig. 1(j) and (k) atomic-resolution HAADF and BF STEM images of the SnS 1.26 Se 0.76 ake viewed from the [001] axis.Since intensity of an atomic column in a HAADF STEM imaging mode is proportional to Z 1.7 (Z is atomic number), 32-34 the brightest spots (corresponding to the highest intensity line prole in Fig. S1(c) †) represent the Sn atomic columns (Z ¼ 50), while the medium-brightness and lowest-brightness spots (corresponding to the medium-high and lowest intensity line prole in Fig. S1(c), † respectively) represent the Se-rich (Z ¼ 34) and S-rich (Z ¼ 16) atomic columns, respectively, as also sketched in Fig. S1(d).† Interestingly, the Se and S atoms are partially ordered and periodically distributed in the SnS 1.26 Se 0.76 single crystal.The obtained atomic-resolution HAADF and BF STEM images (Fig. 1(k) and (j)) agree well with the atomic model (Fig. 1(a)), showing a symmetrical hexagonal lattice fringe due to the equivalent locations of Se and S in the atomic structure of SnS 1.26 Se 0.76 .

Fig. 1
Fig. 1 Structural characterization of as-synthesized SnS 1.26 Se 0.76 crystal.(a) Atomic model of SnS 1.26 Se 0.76 from a side and top view (along the [001] direction).(b) Single-crystal XRD spectrum.Inset: SEM image shows the layered structure of SnS 1.26 Se 0.76 .(c) Energy-dispersive spectroscopy (EDS) result.(d) Typical TEM image of a SnS 1.26 Se 0.76 flake and corresponding EDS element mapping image for (e) Sn (f) S and (g) Se.(h) High-resolution TEM (HRTEM) image of SnS 1.26 Se 0.76 viewed along [001] direction.(i) The corresponding selected area electron diffraction (SAED) pattern.(j and k) Atomic scale HAADF and BF STEM image of SnS 1.26 Se 0.76 along [001] direction.

3. 3
Photoresponse of exible optoelectronic devices based on SnS 1.26 Se 0.76 nanosheets To probe the broad spectrum optoelectronic characteristic of SnS 1.26 Se 0.76 , we fabricate exible photodetectors based on SnS 1.26 Se 0.76 nanopsheets on PET substrate by traditional device fabrication techniques and measure the photoresponse from visible to NIR light.Fig. 3(a) sketches the structure of a SnS 1.26 Se 0.76 nanosheets-based two-terminal exible photodetector (inset signies the optical image of electrodes array).

Fig. 2
Fig. 2 Optical properties of SnS 1.26 Se 0.76 nanoplates deposited on SiO 2 /Si substrate.(a) Optical and (b) AFM images with different color showing different thickness.The thickness of 2.36 nm measured by AFM is about 2-3 layers.(c) Raman (200-205 cm À1 ) mapping image about the intensity of A 1g(Sn-Se) vibration mode measured in the same SnS 1.26 Se 0.76 sample.(d) Raman spectrum dependent on thickness.(e) Photoluminescence spectrum and (f) calculated electronic band structure of bilayer SnS 1.26 Se 0.76 indicating bilayer SnS 1.26 Se 0.76 is an indirect bandgap semiconductor.The calculation also demonstrates bulk SnS 1.26 Se 0.76 is indirect bandgap semiconductor.

Fig. 3
Fig. 3 Photoresponse of flexible optoelectronic devices based on SnS 1.26 Se 0.76 nanoplates fabricated on PET substrate.(a) Schematic illustration of the structure of a SnS 1.26 Se 0.76 nanosheets-based two-terminal optoelectronic device; inset is the optical image of electrodes array.(b) I-V curves in the dark and under different irradiances with laser wavelength of 532 nm; inset is the optical image of the device and thickness is $60 nm.(c) Plots of the photocurrent and photoresponsivity against irradiance at 2 V bias (532 nm light).(d) Time-dependent photoresponse of SnS 1.26 Se 0.76 nanoplate device at various light intensity at 2 V bias (532 nm light).(e) A separated temporal photocurrent response and reset cycle; inset is an enlarged view of the temporal photocurrent response.(f) I-V curves in the dark and in the presence of 375, 473, and 632 nm laser of a single SnS 1.26 Se 0.76 nanoplate photodetector at a fixed irradiance of 60 mW cm À2 ; inset is the optical image of the device.(g)-(i) are timedependent photoresponse of SnS 1.26 Se 0.76 nanoplate device under different irradiances with laser wavelength of 450 nm, 638 nm and 808 nm, respectively.The corresponding voltage bias is 2 V.
2.3 Optical spectroscopy measurements of SnS 1.26 Se 0.76 nanoplates SnS 1.26 Se 0.76 nanosheets with different thickness were mechanically exfoliated from bulk SnS 1.26 Se 0.76 crystals onto a SiO 2 /Si substrate.Morphology and thickness of the SnS 1.26 Se 0.76 nanosheets were obtained by a laser scanning confocal microscope (LSCM, Olympus, LEXT-

Table 1
before Comparison of outstanding figures-of-merit for photodetectors reported based on tin dichalcogenides (SnS 2 , SnSe 2 ) a Flexible photodetector.

Table 2
Summary of the performance parameters for SnS 1.26 Se 0.76 flexible photodetector a in this work Wavelength [nm] Rise time [s] Recovery time [s] R l [mA W À1 ]