Effective Modulation of Optical and Photoelectrical Properties of SnS2 Hexagonal Nanoflakes via Zn Incorporation

Tin sulfides are promising materials in the fields of photoelectronics and photovoltaics because of their appropriate energy bands. However, doping in SnS2 can improve the stability and robustness of this material in potential applications. Herein, we report the synthesis of SnS2 nanoflakes with Zn doping via simple hydrothermal route. The effect of doping Zn was found to display a huge influence in the structural and crystalline order of as synthesized SnS2. Their optical properties attest Zn doping of SnS2 results in reduction of the band gap which benefits strong visible-light absorption. Significantly, enhanced photoresponse was observed with respect to pristine SnS2. Such enhancement could result in improved electronic conductivity and sensitivity due to Zn doping at appropriate concentration. These excellent performances show that Sn1−xZnxS2 nanoflakes could offer huge potential for nanoelectronics and optoelectronics device applications.

Meanwhile, SnS 2 is considered as one of the promising layered materials with excellent visible light absorption and electrical properties. It possesses band gap (2.1-2.3 eV), n-type characteristics, high sensitivity and high surface activity for applications in Li-ion batteries [25], photovoltaic devices [26] and photodetector [27,28]. Variety of nanostructures such as nanoflakes, nanosheets and nanoplates through physical and chemical techniques including chemical vapor deposition, solvothermal and hydrothermal methods have been reported by several groups [29,30]. Among them, nanoflakes preparation via hydrothermal method have attracted considerable interest due to its low cost and large-scale production at low temperatures. Similarly, many efforts have also been made in controlling morphology and enhancing the photoelectrical, chemical and physical properties for improving the device performance. Moreover, dopants in semiconductor could lead to reduction in particle size,

Synthesis of Sn 1−x Zn x S 2 Nanoflakes
SnS 2 and Sn 1−x Zn x S 2 nanoflakes were prepared via low cost hydrothermal route reported previously [35]. In brief, 0.1753 g SnCl 4 ·5H 2 O (Tin (IV) chloride pentahydrate) and 0.15 g thioacetamide (TAA) were dissolved in 80 mL distilled water, stirred for 1 h to result in homogeneous solution. The prepared solution was transferred to 100 mL Teflon-line autoclave, sealed and heated up to 160 • C for 12 h and finally cooled to room temperature. The prepared SnS 2 nanoflakes were then washed with ethanol and deionized water repeatedly and finally dried at 60 • C for 12 h in electric oven. For the synthesis of Sn 1−x Zn x S 2 nanoflakes, 1 and 3 mmol% of Zinc chloride was added to the precursor solution.

Characterization
The morphological evolution of the sample was examined using field-emission scanning electron microscopy (FESEM, Philips, Model: XL-30, Amsterdam, The Netherland) and field-emission transmission electron microscopy (FE-TEM, JEM-2100F HR, Tokyo, Japan). The phase purity and crystal structure of SnS 2 and Sn 0.97 Zn 0.03 S 2 nanoflakes was inferred through X-ray diffractometer (SmartLab, Rigaku Corporation, Tokyo, Japan). The Raman measurements were performed in a micro-Raman spectrometer (DawoolAttonics, Model: Micro Raman System, Seongnam, Korea) using an excitation wavelength of 532 nm. The chemical composition of Sn 0.97 Zn 0.03 S 2 was obtained using X-ray photoelectron spectroscopy (K-Alpha+, ThermoFisher Scientific, Waltham, MA, USA). In order to avoid charging effect, during the measurement, charge neutralization was performed with an electron flood gun (K-Alpha+, ThermoFisher Scientific, USA). The absorbance spectrum was recorded using a UV/VIS spectrophotometer (K LAB, Model: Optizen POP, Daejeon, Korea). A Keithley 617 semiconductor parameter analyzer (Tektronix, Beaverton, OR, USA; Model: Keithley 617) was employed to study the photo-response of the device under solar simulator (Newport, OR, USA; AM1.5) (SERIC, Model: XIL-01B50KP).

Device Fabrication
Initially, 2 mg of samples SnS 2 and Sn 0.97 Zn 0.03 S 2 were added in 10 mL methoxy-ethanol solvent separately and magnetic stirred for 30 min followed by sonication of about 30 min to form colloidal suspension. The resulting suspension was then spin casted on cleaned and patterned ITO/glass substrate at 1000 rpm and dried at 100 • C for 5 min. Several cycles of spin casting process was repeated to obtain a continuous film.

Results and Discussions
The morphological features of SnS 2, Sn 0.99 Zn 0.01 S 2 ( Figure S1) and Sn 0.97 Zn 0.03 S 2 products were examined with the aid of field-emission scanning electron microscope (FESEM) technique. The image seen from Figure 1a-c confirms hexagonal nanoflakes with smooth surface and homogeneous distribution in case of pristine SnS 2 . However, on doping with Zinc the morphology appears to be similar with that of pristine nanoflakes with some random aggregates on the surface of SnS 2 (Figure 1d,e). Additionally, transmission electron microscope (TEM) was employed to further investigate the detailed morphological information of SnS 2 and Sn 0.97 Zn 0.03 S 2 products. Figure 2 shows TEM images of pristine SnS 2 and Sn 0.97 Zn 0.03 S 2 nanoflakes with different magnifications. From the Figure 2a substrate at 1000 rpm and dried at 100 °C for 5 min. Several cycles of spin casting process was repeated to obtain a continuous film.

Results and Discussions
The morphological features of SnS2, Sn0.99Zn0.01S2 ( Figure S1) and Sn0.97Zn0.03S2 products were examined with the aid of field-emission scanning electron microscope (FESEM) technique. The image seen from Figure 1a-c confirms hexagonal nanoflakes with smooth surface and homogeneous distribution in case of pristine SnS2. However, on doping with Zinc the morphology appears to be similar with that of pristine nanoflakes with some random aggregates on the surface of SnS2 ( Figure  1d,e). Additionally, transmission electron microscope (TEM) was employed to further investigate the detailed morphological information of SnS2 and Sn0.97Zn0.03S2 products. Figure 2 shows TEM images of pristine SnS2 and Sn0.97Zn0.03S2 nanoflakes with different magnifications. From the Figure  2a      The crystallographic pattern of as synthesized SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes are investigated by XRD analysis and presented in Figure 4a. Here, the strong diffraction peak observed at 2θ = 14.92° belongs to (001) diffraction, is an indication of the hexagonal structure of SnS2 [37]. However, the diffraction peak (001) tends to shift towards smaller angle on Zn doping. This shifting indicates that Zn ions replace Sn sites in the SnS2 crystal matrix. Furthermore, no peaks related to other compounds namely, ZnS and ZnSnS3 are observed in the XRD pattern. Additionally, Raman measurement was further analyzed to study detailed information about the structural properties of Zn doped SnS2 nanoflakes. Raman spectrum for sample SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes are displayed in Figure 4b. Here, in case of pristine SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes, a strong signal was observed at 312 cm −1 , which is related to A1g phonon vibration mode of SnS2 [38][39][40].   The crystallographic pattern of as synthesized SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes are investigated by XRD analysis and presented in Figure 4a. Here, the strong diffraction peak observed at 2θ = 14.92° belongs to (001) diffraction, is an indication of the hexagonal structure of SnS2 [37]. However, the diffraction peak (001) tends to shift towards smaller angle on Zn doping. This shifting indicates that Zn ions replace Sn sites in the SnS2 crystal matrix. Furthermore, no peaks related to other compounds namely, ZnS and ZnSnS3 are observed in the XRD pattern. Additionally, Raman measurement was further analyzed to study detailed information about the structural properties of Zn doped SnS2 nanoflakes. Raman spectrum for sample SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes are displayed in Figure 4b. Here, in case of pristine SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes, a strong signal was observed at 312 cm −1 , which is related to A1g phonon vibration mode of SnS2 [38][39][40]. The crystallographic pattern of as synthesized SnS 2 , Sn 0.99 Zn 0.01 S 2 and Sn 0.97 Zn 0.03 S 2 nanoflakes are investigated by XRD analysis and presented in Figure 4a. Here, the strong diffraction peak observed at 2θ = 14.92 • belongs to (001) diffraction, is an indication of the hexagonal structure of SnS 2 [37]. However, the diffraction peak (001) tends to shift towards smaller angle on Zn doping. This shifting indicates that Zn ions replace Sn sites in the SnS 2 crystal matrix. Furthermore, no peaks related to other compounds namely, ZnS and ZnSnS 3 are observed in the XRD pattern. Additionally, Raman measurement was further analyzed to study detailed information about the structural properties of Zn doped SnS 2 nanoflakes. Raman spectrum for sample SnS 2, Sn 0.99 Zn 0.01 S 2 and Sn 0.97 Zn 0.03 S 2 nanoflakes are displayed in Figure 4b. Here, in case of pristine SnS 2 , Sn 0.99 Zn 0.01 S 2 and Sn 0.97 Zn 0.03 S 2 nanoflakes, a strong signal was observed at 312 cm −1 , which is related to A 1g phonon vibration mode of SnS 2 [38][39][40]. Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 12  Figure 5b,c displays the XPS spectra of Sn 3d and S 2p peaks for Sn0.97Zn0.03S2 nanoflakes. As observed in Figure 5b,c, the peaks of Sn 3d at 486.33 and 494.4 eV of Sn 3d is ascribed to Sn3d3/2 and Sn3d5/2 and peaks at 161.2 and 163.3 eV correspond to S 2p peaks of SnS2. These results are consistent with those reported for SnS2 [41,42]. The binding energies of Sn 3d5/2 peak corresponding to pristine SnS2 was observed at 486.47 eV. Subsequently doping with Zn on SnS2, peaks of Sn 3d5/2 shifts to lower energy position to 486.33 eV. The shifting in the binding energy value of Sn 3d5/2 peak was about 0.14 eV compared to pristine SnS2. This shift might be due to Zn ion replace Sn sites in the SnS2 crystal lattice. Figure 5d shows the XPS spectrum for Zn in SnS2 nanoflakes. Besides, the Zn 2p3/2 peak appeared at 1021.3 eV is attributed to Zn 2+ bonding state [43], confirming Zn 2+ ions have been incorporated into the SnS2.    Figure 5b,c displays the XPS spectra of Sn 3d and S 2p peaks for Sn 0.97 Zn 0.03 S 2 nanoflakes. As observed in Figure 5b,c, the peaks of Sn 3d at 486.33 and 494.4 eV of Sn 3d is ascribed to Sn3d 3/2 and Sn3d 5/2 and peaks at 161.2 and 163.3 eV correspond to S 2p peaks of SnS 2 . These results are consistent with those reported for SnS 2 [41,42]. The binding energies of Sn 3d 5/2 peak corresponding to pristine SnS 2 was observed at 486.47 eV. Subsequently doping with Zn on SnS 2 , peaks of Sn 3d 5/2 shifts to lower energy position to 486.33 eV. The shifting in the binding energy value of Sn 3d 5/2 peak was about 0.14 eV compared to pristine SnS 2 . This shift might be due to Zn ion replace Sn sites in the SnS 2 crystal lattice. Figure 5d shows the XPS spectrum for Zn in SnS 2 nanoflakes. Besides, the Zn 2p 3/2 peak appeared at 1021.3 eV is attributed to Zn 2+ bonding state [43], confirming Zn 2+ ions have been incorporated into the SnS 2 .   Figure 5b,c displays the XPS spectra of Sn 3d and S 2p peaks for Sn0.97Zn0.03S2 nanoflakes. As observed in Figure 5b,c, the peaks of Sn 3d at 486.33 and 494.4 eV of Sn 3d is ascribed to Sn3d3/2 and Sn3d5/2 and peaks at 161.2 and 163.3 eV correspond to S 2p peaks of SnS2. These results are consistent with those reported for SnS2 [41,42]. The binding energies of Sn 3d5/2 peak corresponding to pristine SnS2 was observed at 486.47 eV. Subsequently doping with Zn on SnS2, peaks of Sn 3d5/2 shifts to lower energy position to 486.33 eV. The shifting in the binding energy value of Sn 3d5/2 peak was about 0.14 eV compared to pristine SnS2. This shift might be due to Zn ion replace Sn sites in the SnS2 crystal lattice. Figure 5d shows the XPS spectrum for Zn in SnS2 nanoflakes. Besides, the Zn 2p3/2 peak appeared at 1021.3 eV is attributed to Zn 2+ bonding state [43], confirming Zn 2+ ions have been incorporated into the SnS2.    This results suggests that samples Sn 0.97 Zn 0.03 S 2 possess greater potential than that of pristine sample SnS 2 to drive photo excited charge carriers under the light irradiation. The values estimated was found to be 2.24 eV for sample SnS 2 which is consistent with our previous result (Figure 6b). However, the values was found to be 2.19 and 2.09 eV for sample Sn 0.99 Zn 0.01 S 2 and Sn 0.97 Zn 0.03 S 2 . It shows band gap becomes narrower than pristine SnS 2 as the Zn content increases [44,45]. This reduction in the band gap might be due to modification in the electronic structures of SnS 2 due to Zn doping, which results in creating energy levels in the band gap. This band gap could result in better absorption in visible region and can increase photo excited charge carriers under illumination. This results suggests that samples Sn0.97Zn0.03S2 possess greater potential than that of pristine sample SnS2 to drive photo excited charge carriers under the light irradiation. The values estimated was found to be 2.24 eV for sample SnS2 which is consistent with our previous result (Figure 6b). However, the values was found to be 2.19 and 2.09 eV for sample Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2. It shows band gap becomes narrower than pristine SnS2 as the Zn content increases [44,45]. This reduction in the band gap might be due to modification in the electronic structures of SnS2 due to Zn doping, which results in creating energy levels in the band gap. This band gap could result in better absorption in visible region and can increase photo excited charge carriers under illumination.
where e is the electronic charge, ε is the dielectric constant of SnS2, ε0 is the relative permittivity, Nd dopant density, V the applied potential, C the specific capacitance, kB the Boltzmann constant and Vfb the flat band potential. The M-S plots of pristine SnS2, Sn0.99Zn0.01S2 ( Figure S2) and Sn0.97Zn0.03S2 nanoflakes are displayed in Figure 7. Here Vfb was determined from intercept between the extrapolated linear plot of the curve and was estimated to be ~0.67 V for pristine SnS2 and 0.64 V for Sn0.97Zn0.03S2 nanoflakes. Additionally the difference in the slope reflects the variation in the carrier density (Nd). The values of carrier density was estimated from the Equation (1)   A photoelectronic device was constructed on samples SnS2 and Sn0.97Zn0.03S2 to study its potential for optoelectronics applications (Figure 8a), (for the details of fabrication process refer Mott-Schottky (M-S) analysis was made to study the electrical properties of pristine SnS 2 , Sn 0.99 Zn 0.01 S 2 and Sn 0.97 Zn 0.03 S 2 nanoflakes. Generally, Mott-Schottky plot was employed to determine the donor density (N d ) and flat band potential (V fb ) of the materials. M-S analysis are generally expressed by [46][47][48] 1 where e is the electronic charge, ε is the dielectric constant of SnS 2 , ε 0 is the relative permittivity, N d dopant density, V the applied potential, C the specific capacitance, k B the Boltzmann constant and V fb the flat band potential. The M-S plots of pristine SnS 2 , Sn 0.99 Zn 0.01 S 2 ( Figure S2) and Sn 0.97 Zn 0.03 S 2 nanoflakes are displayed in Figure 7. Here V fb was determined from intercept between the extrapolated linear plot of the curve and was estimated to be~0.67 V for pristine SnS 2 and 0.64 V for Sn 0.97 Zn 0.03 S 2 nanoflakes. Additionally the difference in the slope reflects the variation in the carrier density (N d ).
The values of carrier density was estimated from the Equation (1)  This results suggests that samples Sn0.97Zn0.03S2 possess greater potential than that of pristine sample SnS2 to drive photo excited charge carriers under the light irradiation. The values estimated was found to be 2.24 eV for sample SnS2 which is consistent with our previous result (Figure 6b). However, the values was found to be 2.19 and 2.09 eV for sample Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2. It shows band gap becomes narrower than pristine SnS2 as the Zn content increases [44,45]. This reduction in the band gap might be due to modification in the electronic structures of SnS2 due to Zn doping, which results in creating energy levels in the band gap. This band gap could result in better absorption in visible region and can increase photo excited charge carriers under illumination.
where e is the electronic charge, ε is the dielectric constant of SnS2, ε0 is the relative permittivity, Nd dopant density, V the applied potential, C the specific capacitance, kB the Boltzmann constant and Vfb the flat band potential. The M-S plots of pristine SnS2, Sn0.99Zn0.01S2 ( Figure S2) and Sn0.97Zn0.03S2 nanoflakes are displayed in Figure 7. Here Vfb was determined from intercept between the extrapolated linear plot of the curve and was estimated to be ~0.67 V for pristine SnS2 and 0.64 V for Sn0.97Zn0.03S2 nanoflakes. Additionally the difference in the slope reflects the variation in the carrier density (Nd). The values of carrier density was estimated from the Equation (1)   A photoelectronic device was constructed on samples SnS2 and Sn0.97Zn0.03S2 to study its potential for optoelectronics applications (Figure 8a), (for the details of fabrication process refer A photoelectronic device was constructed on samples SnS 2 and Sn 0.97 Zn 0.03 S 2 to study its potential for optoelectronics applications (Figure 8a), (for the details of fabrication process refer Expt. sections). I-V curves of pristine SnS 2 nanoflakes at various illumination intensities and dark condition is displayed in Figure 8b. Inset shows I-V curves of the pristine SnS 2 nanoflakes under dark and illumination. Here, the I-V curve shows a roughly symmetric behavior indicating Schottky-like junction established at ITO and SnS 2 contacts. The dark current was noted to be 0.29 µA at a bias of 3 V. In contrast, the enhancement of current was measured and the value reaches to 0.98 µA under illumination, demonstrating excellent photosensitivity of the SnS 2 samples. I-V curves of Sn 0.97 Zn 0.03 S 2 nanoflakes device under illumination and dark is displayed in Figure 8c. Here, the value of dark current was found to increase than that of pristine SnS 2 , which suggests reduction in resistance of SnS 2 after Zn doping. However, a notable enhancement in photocurrent under illumination was noted compared to that of dark current at same bias voltage in Sn 0.97 Zn 0.03 S 2 nanoflakes device, indicating their excellent sensitivity. Moreover, photo to dark current (I light /I dark ) ratio for Sn 0.97 Zn 0.03 S 2 device (~10.1) tends to increase compared to pristine SnS 2 (~3.37). The high sensitivity and enhancement in photocurrent of Sn 0.97 Zn 0.03 S 2 nanoflakes reveal the effective separation of photoexcited carriers in samples, which are actually promoted after Zn-doping. Figure 8d shows I-V curves of the Sn 0.97 Zn 0.03 S 2 device measured at room temperature under different light intensities. The photocurrent increases with increasing light intensities revealing strong and clear photon-induced currents phenomena, indicating excellent photoresponse ability of the device. Under illumination, photoexcited charge carriers are mainly generated in Sn 0.97 Zn 0.03 S 2 . Then the charge carriers are quickly separated and driven towards the nearby electrodes due to built-in electric field created at the interface, resulting in photocurrent generation. Expt. sections). I-V curves of pristine SnS2 nanoflakes at various illumination intensities and dark condition is displayed in Figure 8b. Inset shows I-V curves of the pristine SnS2 nanoflakes under dark and illumination. Here, the I-V curve shows a roughly symmetric behavior indicating Schottky-like junction established at ITO and SnS2 contacts. The dark current was noted to be 0.29 μA at a bias of 3 V. In contrast, the enhancement of current was measured and the value reaches to 0.98 μA under illumination, demonstrating excellent photosensitivity of the SnS2 samples. I-V curves of Sn0.97Zn0.03S2 nanoflakes device under illumination and dark is displayed in Figure 8c. Here, the value of dark current was found to increase than that of pristine SnS2, which suggests reduction in resistance of SnS2 after Zn doping. However, a notable enhancement in photocurrent under illumination was noted compared to that of dark current at same bias voltage in Sn0.97Zn0.03S2 nanoflakes device, indicating their excellent sensitivity. Moreover, photo to dark current (Ilight/Idark) ratio for Sn0.97Zn0.03S2 device (~10.1) tends to increase compared to pristine SnS2 (~3.37). The high sensitivity and enhancement in photocurrent of Sn0.97Zn0.03S2 nanoflakes reveal the effective separation of photoexcited carriers in samples, which are actually promoted after Zn-doping. Figure 8d shows I-V curves of the Sn0.97Zn0.03S2 device measured at room temperature under different light intensities. The photocurrent increases with increasing light intensities revealing strong and clear photon-induced currents phenomena, indicating excellent photoresponse ability of the device. Under illumination, photoexcited charge carriers are mainly generated in Sn0.97Zn0.03S2. Then the charge carriers are quickly separated and driven towards the nearby electrodes due to built-in electric field created at the interface, resulting in photocurrent generation.  Figure 9a shows light intensity-dependent photocurrent values of pristine SnS2 and Sn0.97Zn0.03S2 device. The observed photocurrent value to illumination intensities suggest that the charge carrier photo-generation efficiency is proportional to the number of photons absorbed by the pristine SnS2 and Sn0.97Zn0.03S2 nanoflakes. Reliable response speed and stability to illumination conditions are crucial for the photoelectronic device. To address this concern, time related photoresponse of pristine SnS2 and Sn0.97Zn0.03S2 device was measured with turning light on/off condition for a period  Sn 0.97 Zn 0.03 S 2 nanoflakes. Reliable response speed and stability to illumination conditions are crucial for the photoelectronic device. To address this concern, time related photoresponse of pristine SnS 2 and Sn 0.97 Zn 0.03 S 2 device was measured with turning light on/off condition for a period of 10 seconds for multiple cycles. Figure 9b,c shows time related photoresponse of the pristine and Sn 0.97 Zn 0.03 S 2 device under several switch on and switch off conditions. Here, the photocurrent of pristine SnS 2 was found to be 0.8 µA. Interestingly the photocurrent is improved by two fold in case of Sn 0.97 Zn 0.03 S 2 nanoflakes (1.75 µA) compared to pristine SnS 2 (Figure 9c). The photoresponse enhancement could be related to Zn ions which acts as an effective dopant and enhance charge separation taking place at the interface. The rise/decay time was measured to be 0.2 and 0.2 s. The reason for the relative longer response speed in our case is probably related to the formation of interface states between the Sn 0.97 Zn 0.03 S 2 nanoflakes and ITO substrate, which can block the photo-generated carriers, resulting in long life time of the photo-generated carriers. Meanwhile, the device shows no fluctuation under illumination for several repetitive cycles, inferring the excellent stability of the Sn 0.97 Zn 0.03 S 2 device. The time related response of the Sn 0.97 Zn 0.03 S 2 device under varied light intensities are displayed in Figure 9d. Here, the photocurrent value varies with different light intensities demonstrating excellent reproducibility of Sn 0.97 Zn 0.03 S 2 based device. Such high and stable photoresponse behavior may come from the fact that Zn ions act as an effective dopant and result in increased light absorption, which enhances photogenerated charge carriers and leads to an enhanced photocurrent of the device. Thus, photoelectrical studies on Sn 0.97 Zn 0.03 S 2 nanoflakes illustrates that Zn doping in SnS 2 results in significant enhancement of their optoelectronic properties, which leads to improved conductivity and sensitivity. Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 12 of 10 seconds for multiple cycles. Figure 9b,c shows time related photoresponse of the pristine and Sn0.97Zn0.03S2 device under several switch on and switch off conditions. Here, the photocurrent of pristine SnS2 was found to be 0.8 μA. Interestingly the photocurrent is improved by two fold in case of Sn0.97Zn0.03S2 nanoflakes (1.75 μA) compared to pristine SnS2 (Figure 9c). The photoresponse enhancement could be related to Zn ions which acts as an effective dopant and enhance charge separation taking place at the interface. The rise/decay time was measured to be 0.2 and 0.2 s. The reason for the relative longer response speed in our case is probably related to the formation of interface states between the Sn0.97Zn0.03S2 nanoflakes and ITO substrate, which can block the photo-generated carriers, resulting in long life time of the photo-generated carriers. Meanwhile, the device shows no fluctuation under illumination for several repetitive cycles, inferring the excellent stability of the Sn0.97Zn0.03S2 device. The time related response of the Sn0.97Zn0.03S2 device under varied light intensities are displayed in Figure 9d. Here, the photocurrent value varies with different light intensities demonstrating excellent reproducibility of Sn0.97Zn0.03S2 based device. Such high and stable photoresponse behavior may come from the fact that Zn ions act as an effective dopant and result in increased light absorption, which enhances photogenerated charge carriers and leads to an enhanced photocurrent of the device. Thus, photoelectrical studies on Sn0.97Zn0.03S2 nanoflakes illustrates that Zn doping in SnS2 results in significant enhancement of their optoelectronic properties, which leads to improved conductivity and sensitivity. The mechanism involved in the enhanced photoresponse of Sn0.97Zn0.03S2/ITO structure was explained through energy band diagram in Figure 10. Since the work function between ITO and Sn0.97Zn0.03S2 is different, a Schottky-type behavior is established at Sn0.97Zn0.03S2/ITO interface ( Figure  10). Due to this behavior, an electric field was established at the Sn0.97Zn0.03S2/ITO interface. This electric field then accelerates the separation of the photoexcited charge carriers without the application of any applied bias. When illuminated, photoexcited charge carriers produced in Sn0.97Zn0.03S2 are then separated at the Sn0.97Zn0.03S2/ITO interface. This charge carriers separation which was induced due to the electric field results in band bending at the Sn0.97Zn0.03S2/ITO interface. The mechanism involved in the enhanced photoresponse of Sn 0.97 Zn 0.03 S 2 /ITO structure was explained through energy band diagram in Figure 10. Since the work function between ITO and Sn 0.97 Zn 0.03 S 2 is different, a Schottky-type behavior is established at Sn 0.97 Zn 0.03 S 2 /ITO interface ( Figure 10). Due to this behavior, an electric field was established at the Sn 0.97 Zn 0.03 S 2 /ITO interface. This electric field then accelerates the separation of the photoexcited charge carriers without the application of any applied bias. When illuminated, photoexcited charge carriers produced in Sn 0.97 Zn 0.03 S 2 are then separated at the Sn 0.97 Zn 0.03 S 2 /ITO interface. This charge carriers separation which was induced due to the electric field results in band bending at the Sn 0.97 Zn 0.03 S 2 /ITO interface. As a result, the photoexcited charge carriers are swept towards ITO electrodes, involving in enhancement of photocurrent (Figure 10b). Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 12 As a result, the photoexcited charge carriers are swept towards ITO electrodes, involving in enhancement of photocurrent (Figure 10b).

Conclusions
In summary, Sn0.97Zn0.03S2 nanoflakes were prepared via low temperature hydrothermal synthesis. The modulation of the structural and photoelectrical properties in SnS2 via doping Zinc have been discussed in detail. A shift in XPS peak of Sn 3d5/2 and S 2p3/2 has been observed in Sn0.97Zn0.03S2 nanoflakes due to Zn ion replaced Sn sites in the SnS2 crystal lattice. Optical properties studies show that Sn0.97Zn0.03S2 nanoflakes possess higher visible-light absorption than that of pristine SnS2. Photoelectrical properties based on Sn0.97Zn0.03S2 nanoflakes reveal that Zn doping leads to significant improvement in conductivity and sensitivity to illuminations compared to pristine SnS2. Such an excellent performance of Sn0.97Zn0.03S2 nanoflakes may endow it as a potential candidate for emerging 2D materials in optoelectronic applications.

Conclusions
In summary, Sn 0.97 Zn 0.03 S 2 nanoflakes were prepared via low temperature hydrothermal synthesis. The modulation of the structural and photoelectrical properties in SnS 2 via doping Zinc have been discussed in detail. A shift in XPS peak of Sn 3d 5/2 and S 2p 3/2 has been observed in Sn 0.97 Zn 0.03 S 2 nanoflakes due to Zn ion replaced Sn sites in the SnS 2 crystal lattice. Optical properties studies show that Sn 0.97 Zn 0.03 S 2 nanoflakes possess higher visible-light absorption than that of pristine SnS 2 . Photoelectrical properties based on Sn 0.97 Zn 0.03 S 2 nanoflakes reveal that Zn doping leads to significant improvement in conductivity and sensitivity to illuminations compared to pristine SnS 2 . Such an excellent performance of Sn 0.97 Zn 0.03 S 2 nanoflakes may endow it as a potential candidate for emerging 2D materials in optoelectronic applications.