Development of morphology tuned SnS hierarchical structures for enhanced photosensitive photodiode fabrication

Hierarchical structure transformation and surface modification of solvothermal method synthesized SnS with oleic acid in ethylene glycol solvent were discussed in detail. The structural, optical, and morphology of as prepared SnS samples were examined by X-ray diffraction (XRD), Raman Spectroscopy, and Field Emission Scanning Electron Microscopy (FE-SEM). XRD verifies the orthorhombic crystal structure of the SnS phase for all synthesized samples. Single-phase nature of synthesized particles was confirmed with Raman characterization. Morphology evolution of SnS from regular to hierarchical structures upon adding oleic acid is performed through FE-SEM analysis. Junction diodes p-SnS/n-Si fabricated with different oleic acid concentrations (0.5, 1.5, and 2.5 mL) synthesized SnS particles show better photo-response, which can be used in photodiode applications. results confirm the phase purity of SnS with well-identified Raman


Introduction
Single-phase metal chalcogenide synthesis using the hydrothermal method is still challenging because of their growth behaviour at high temperatures. Maintain stoichiometric in a harsh environment while growing metal chalcogenide is complicated and always ended in multiple phases. Suitable solvent and combination of surfactant can be the solution for controlling multiple phases in metal chalcogenide [1].
Non-oxide semiconducting materials such as metal chalcogenides and nitrides are the most attractive materials due to their potential properties such as good light-sensor, visiblelight-driven catalysis, and suitable band gap for high-end sensing applications [2].
Semiconducting materials with low and narrow bandgap with layered chalcogenides materials attract researchers to develop various energy application fields like supercapacitors, lithiumion batteries, and electrical and optoelectrical devices [3][4][5]. Due to its high durability and cost-effectiveness, abundant in nature, and the synthesis of nontoxic semiconducting materials at the nano range is very easy [6]. From metal chalcogenides, SnS is one of the potential materials with a narrow bandgap, excellent absorption, eco friendly, cost effective, earth-abundant are the properties to make the SnS nanomaterial to construct useful optoelectronic device application [7-10]. Generally, SnS possess an orthorhombic crystal structure with double-layered formation, and bonding between Sn and S is light because of Van der Waals forces [11][12]. Tin sulfide has different phases like SnS, Sn2S3, SnS2, Sn3S4, and Sn4S5 with a different stoichiometric ratio of tin and sulfur [13]. These different phases of SnS are possibly prepared with several methods like wet-chemical, co-precipitation and hydrothermal [14][15][16][17][18][19]. Among them, the hydrothermal method is preferred to synthesis single-phase SnS nanostructure with different particle morphology. SnS capable of possessing both p-type and n-type semiconducting nature corresponds to the element of tin and changing the synthesis conditions of SnS. These materials have day light photocatalytic nature for applying dyes degradation applications due to their mobility of charge carriers upon photon strike [20]. Various structure of SnS material widely in various applications such as holographic recording, photovoltaic material, catalyst for hydrogen production, anode material in Li-Ion batteries, photocatalyst, photodetector, photoelectrochemical cells, Drug delivery, and dye synthesized solar cell [21][22][23][24][25][26][27][28][29]. In recent years, chalcogenides,

Synthesis of SnS particles
The solvothermal method has been adopted for the synthesis of SnS hierarchical structures. SnCl2.2H2O (1 mmole) and CH3CSNH2 (1 mmole) were dissolved in 30 mL (A) and 10 mL (B) ethylene glycol, respectively. Both A and B solutions were ultrasonicated for complete precursor dissolution and further stirred for 30 min. The final solution was loaded into 100 mL Teflon lined autoclave, and the temperature was raised to 175 ℃. The fixed temperature was maintained for 6hours to facilitate the reaction. Then, obtained solution cool down to ambient temperature further centrifuge for separation of the synthesized sample then the collected precipitate were rinsed with ethanol twice for eliminating the impurities. The above procedure was followed for oleic acid, included synthesis as mentioned, and different oleic acid concentrations (0.5, 1.5, and 2.5 mL) are added to the mixed solution.

Diode fabrication
The silicon (Si) substrate with n-type nature with the dimension of (1x1 cm) with ~260 (±30) μm thickness and resistivity of ~40−50(±5) Ω/cm are the specification of the substrate used in the diode construction. Before coating, the substrate was washed with a piranha solution (H2O2:H2SO4::2:1) for eliminating the organic, metallic impurities and oxidized layer on n-Si wafer surface. The above-washed substrate was ultrasonicated with distilled water as a sequenced process. Synthesized pure and oleic acid assisted SnS (40 mg) was dispersed in cyclohexane (1 mL) with a drop of oleic acid. By drop-casting technique, the formed SnS-oleic acid solution was coated on the n-Si. The fabricated layer was annealed at 220 ºC for sixty minutes to remove excess oleic acid. Silver glue (ELTECK) was applied on either sides to make the good ohmic contact for diode performance characterization. The schematic diagram of SnS + oleic acid based diode is presented in Fig.1.

Characterization techniques
Powder X-Ray diffraction (XRD) was performed to investigate structural properties of synthesized pure and oleic acid-assisted SnS sample (RINT-2200 X-ray diffractometer

Structural analysis
The comparative diffraction patterns of pure SnS and SnS particle synthesized with different oleic acid concentrations (0.5, 1.5, and 2.5 mL) were shown in  Fig. 3., which reveals the impact of surfactant concentration on the SnS material.

Raman analysis
Raman analysis carried for pure SnS and particles synthesized with different proportions of oleic acid, shown in Fig. 4. The characteristics of Raman peaks confirm the formation of the SnS material. The observed Raman mode at 76 cm -1 is attributed to the B1g mode, which is purely belongs to SnS phase [33]. The peak at 98 cm -1 is presented for the Ag mode which arises due to the Sn and S atoms' inter atomic vibration in SnS lattice. Another peak found at 158 cm -1 is assigned to the B2g mode of the SnS. The observed broad peak at 187 cm -1 and a tiny bump near 212 cm -1 are assigned for Ag mode of SnS nanoparticle, and such results confirm the formation of SnS [34]. The observed result from Raman analysis is consistent with the XRD pattern.

Surface morphology
The particle morphology of synthesized pure and different oleic acid included SnS was analyzed by FESEM and recorded micrographs are compared in Fig. 5(a-h). When increasing the concentration of oleic acid, the SnS is changed from average to dense hierarchical patterns. The evolution of the hierarchical flower-like structure of SnS concerning the increased volume of oleic acid in ethylene glycol is presented in Fig. 5(a-b), which reveals that there are no hierarchical patterns formed in the particle without oleic acid in the synthesis. A small amount of oleic acid (0.5 mL) changes the morphology of SnS into a hierarchical flower. The flat flakes composed of flower structure of SnS with 0.5 mL oleic acid are shown in Fig. 5(c-d). Further increase of oleic acid concentration as 1.5 mL, the spherical hierarchical flower-like structure of SnS was formed and is shown in Fig. 5(e-f). At 2.5 mL oleic acid, the spherical hierarchical structure of SnS is comprised of primary and secondary particles and is shown in Fig. 5(g-h). Different morphology of the particles may be a reason for the enhancing photo-sensing nature of the sensing layer.

I-V characterization
The current-voltage studies of the constructed diodes are carried under light and dark conditions. The forward and reverse current measurement varied from -3 to +3 V was shown in Fig. 6 Fig. 7. Based on thermionic emission theory, the diode's current transport mechanism is explained by the following equation [35]. Here, The Ideality factor (n) is calculated from the following equation

Photodetector analysis
Photosensitive parameters such as photoresponsivity (PS), responsivity (R), external quantum efficiency (EQE), and specific detectivity (D*) of fabricated devise are compared in  given by equation (7) and maximum EQE value obtained for pristine SnS based photodiode resulted in 28.5 %, which shows the efficiency of gain in the detector. The specific detectivity is an important parameter that characterizes the detector's sensitivity nature and measures the signal-to-noise ratio given by expression (8). The photodetection performance of the diode depends on the spectral detectivity value. The value of the specific detectivity changed from 1.01×10 11 to 1.78×10 11 jones, which infers that the increase in the surfactant with SnS influences the value of specific detectivity. The p-SnS + Oleic acid (1.5mL) based diode has a higher value of D* compared with other diodes. The obtained result is higher than the previously reported value of SnS based photodiode [40,41]. From the result, SnS + oleic acid are a suitable material for optoelectronic device application.   Table 1: Photodiode parameter of p-SnS/n-Si, p-SnS + Oleic acid (0.5 mL)/n-Si), p-SnS + Oleic acid (1.5 mL)/n-Si), p-SnS + Oleic acid (2.5 mL)/n-Si) based diode such as Ideality factor (n), Barrier height (ФB), Photosensitivity (PS), Photoresponsivity (R), Quantum efficiency (QE)%, Specific detectivity (D*).