*2.2. 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 SnS2 and Sn0.97Zn0.03S2 nanoflakes was inferred through X-ray di ffractometer (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 Sn0.97Zn0.03S2 was obtained using X-ray photoelectron spectroscopy (K-Alpha<sup>+</sup>, ThermoFisher Scientific, Waltham, MA, USA). In order to avoid charging e ffect, during the measurement, charge neutralization was performed with an electron flood gun (K-Alpha<sup>+</sup>, 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).

#### *2.3. Device Fabrication*

Initially, 2 mg of samples SnS2 and Sn0.97Zn0.03S2 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.

#### **3. 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–c, it is clear that pristine SnS2 possess typical nanoflakes like structures with hexagonal stacking. Similarly the Sn0.97Zn0.03S2 nanoflakes (Figure 2d–f) also possess indistinguishable hexagonal morphology of pristine SnS2. The inset of Figure 2c,f displays the selected area electron diffraction (SAED) pattern revealing polycrystalline structure of the obtained samples. Energy dispersive spectroscopy (EDS) analysis was further employed in TEM mode to study the homogeneous distribution of Zn element in Sn0.97Zn0.03S2 nanoflakes. Figure 3a–d displays the TEM image and TEM-EDS mapping of Sn0.97Zn0.03S2 nanoflakes. As seen from Figure 3d, Zn element is distributed evenly throughout the whole structure of Sn0.97Zn0.03S2 nanoflakes.

**Figure 1.** Morphological and structural characterization of SnS2 and Sn0.97Zn0.03S2 nanoflakes. (**<sup>a</sup>**–**<sup>c</sup>**) low magnification and high magnification scanning electron microscopy (SEM) image of SnS2; (**d**–**f**) low magnification and high magnification SEM image of Sn0.97Zn0.03S2 nanoflakes showing their hexagonal structure.

**Figure 2.** (**<sup>a</sup>**–**<sup>c</sup>**) Transmission electron microscopy (TEM) images of SnS2 and inset in Figure 2c shows selected area electron diffraction (SAED) pattern of SnS2 nanoflakes; (**d**–**f**) TEM images of a typical Sn0.97Zn0.03S2 nanoflakes with SAED pattern in inset of Figure 2f, revealing polycrystalline structure.

**Figure 3.** (**a**) TEM image of Sn0.97Zn0.03S2 nanoflakes and Energy dispersive spectroscopy (EDS) elemental mapping of Sn (**b**), S (**c**) and Zn (**d**) from selected area for 2D Sn0.97Zn0.03S2.

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<sup>−</sup>1, which is related to A1g phonon vibration mode of SnS2 [38–40].

**Figure 4.** Structure properties of SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes. (**a**) X-ray diffraction pattern of SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes; (**b**) Raman spectrum of SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes at excitation wavelength of 532 nm.

To elucidate the chemical composition of pristine and Sn0.97Zn0.03S2 nanoflakes, XPS measurements have been carried out and shown in Figure 5a. XPS full survey spectrum (Figure 5a) confirms the presence of Zn doping in SnS2. 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 Zn2<sup>+</sup> bonding state [43], confirming Zn2<sup>+</sup> ions have been incorporated into the SnS2.

**Figure 5.** (**a**) Full survey spectra of SnS2 and Sn0.97Zn0.03S2 sample. (**b**) X-ray photoelectron spectroscopy (XPS) core level Sn 3d spectra of SnS2 and Sn0.97Zn0.03S2 nanoflakes. (**c**) S 2p core level spectra of SnS2 and Sn0.97Zn0.03S2 nanoflakes. (**d**) Zn 2p core level spectra of Sn0.97Zn0.03S2.

Figure 6a shows UV–visible absorption spectrum of SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 in the range of 300–750 nm. SnS2 displays a strong absorption in visible part of the solar spectrum. However, in contrast the samples Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 displayed a broad light absorption in 300 to

750 nm, which indicates that doping Zn ion can result in extending of absorption edge of SnS2. 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.

**Figure 6.** Properties of SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes. (**a**) UV−vis absorption spectrum of the SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 nanoflakes. (**b**) Tauc's plot extracted from the absorption spectrum revealing their direct band gap.

Mott–Schottky (M–S) analysis was made to study the electrical properties of pristine SnS2, Sn0.99Zn0.01S2 and Sn0.97Zn0.03S2 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 [46–48]

$$\mathbf{1}/\mathbb{C}^2 = (\mathbf{2}/\mathbf{e}\varepsilon\,\varepsilon\_o\mathcal{N}\_\mathrm{d})[(V\_{\mathrm{fb}} - V) - \mathbf{k}\_\mathrm{B}T/\mathrm{e}] \tag{1}$$

where e is the electronic charge, ε is the dielectric constant of SnS2, ε0 is the relative permittivity, *N*d dopant density, *V* the applied potential, *C* the specific capacitance, kB the Boltzmann constant and *V*fb 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 *V*fb 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 (*N*d). The values of carrier density was estimated from the Equation (1) to be about 1.46 × 10<sup>19</sup> and 0.47 × 10<sup>19</sup> and in case of SnS2 and Sn0.97Zn0.03S2 nanoflakes.

**Figure 7.** Mott–Schottky plots of (**a**) SnS2 and (**b**) Sn0.97Zn0.03S2 nanoflakes.

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 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 (*I*light/*I*dark) 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 8.** (**a**) Schematic representation of the photoelectronic device. (**b**) I-V characteristics of SnS2 device under different illumination intensities (Inset shows the I-V characteristics under dark and illumination intensity 84.0 mW/cm2). (**c**) I-V characteristics of Sn0.97Zn0.03S2 device under illumination conditions. (**d**) I-V characteristics of Sn0.97Zn0.03S2 device under different light intensities (55, 61.8, 74.0, 84.0 mW/cm2).

Figure 9a shows light intensity-dependent photocurrent values of pristine SnS2 and Sn0.97Zn0.03S2 device. The observed photocurrent value to illumination intensities sugges<sup>t</sup> 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 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 o ff 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 e ffective 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 di fferent 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 e ffective 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.

**Figure 9.** (**a**) Light intensity-dependent photocurrent values of pristine SnS2 and Sn0.97Zn0.03S2 device. Time-dependent photocurrent response of (**b**) SnS2 device and (**c**) Sn0.97Zn0.03S2. (**d**) Time-dependent photocurrent response of Sn0.97Zn0.03S2 device under di fferent illumination intensities.

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 di fferent, 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. As a result, the photoexcited charge carriers are swept towards ITO electrodes, involving in enhancement of photocurrent (Figure 10b).

**Figure 10.** Energy diagram of the Sn0.97Zn0.03S2/ITO Schottky junction under (**a**) dark and (**b**) illumination conditions.
