Introduction

Two-dimensional layered metal dichalcogenides (LMDs) materials including molybdenum disulfide (MoS2), tungsten disulfide (WS2) and black phosphorus have attracted a huge of interests in the last decade due to their graphene-like structures, various energy band gaps, excellent mechanical properties, high transparency and unique optoelectronic properties [1]. By taking advantage of these features, new type of electronic and optoelectronic devices has appeared and been developed, including tunnel transistor, flexible electronics, photovoltaics and light-emitting devices. As an emerging material of LMDs, tin disulfide (SnS2) is an n-type indirect bandgap semiconductor with a band gap ranging from 2.0 to 2.6 eV [2, 3]. It has a layered sandwich structure, and every two adjacent layers of S–Sn–S interacted with each other by Van der Waals forces. In addition, its component elements Sn and S are earth abundant and environment friendly [4]. Sn element has good chemical activity, and nanomaterials based on Sn have various applications [5,6,7,8]. And the electronic, physical and chemical properties of SnS2 have been extensively studied, and SnS2 has promising uses in solar cell [9], lithium battery [10], flexible devices [11] and optoelectronic devices [12]. Moreover, photodetectors and photovoltaic devices are important potential applications of SnS2. Nevertheless, researches on the photosensitive properties of SnS2 nanoflakes are less performed.

In this work, we synthesized SnS2 nanoflakes (NFs) via a hydrothermal method using stannic chloride pentahydrate (SnCl4·5H2O) and thiourea (CH4N2S) as precursors [13]. Furthermore, we fabricated simple prototype photodetectors based on the as-synthesized SnS2 NFs to explore the photoresponsive characteristic of the SnS2. Impressively, SnS2 NFs-based photodetectors exhibited high photodetection performance with a fast response time of 7.3 ms and high detectivity of 1.53 × 1010 jones. Thus, the excellent optical response characteristics of the device, such as stability and repeatability, as well as fast response time, cater well to the requirements for the next-generation photovoltaic devices, making the SnS2 NFs a promising component of high-performance optoelectronic devices.

Experiment section

Synthesis of SnS2 nanoflakes

SnS2 was synthesized in a 50-ml stainless steel autoclave by a facile hydrothermal method. Analytical grade stannic chloride pentahydrate (SnCl4·5H2O) and thiourea (CH4N2S) were used as precursor reagents to make a suitable amount of stoichiometric mixture in autoclave. Thirty milliliter distilled water was added into the autoclave and stirred for 1 h. Then, the autoclave was sealed and maintained at 180 °C for 24 h, and the furnace cooled to room temperature naturally. The products were collected by centrifugation and well washed with absolute ethanol and distilled water twice before drying in a vacuum box at 50 °C for 12 h [14].

Characterization

Morphology and phase structure are characterized by a scanning electron microscopy (SEM, FEI NanoSEM450) and an X-ray diffraction (XRD, Rigaku, Smartlab) using Cu Ka radiation (λ = 0.15406 nm). The elemental composition was investigated by an X-ray photoelectrons Spectroscopy (XPS, Thermo Escalab 250Xi) and an energy-dispersive spectroscopy (EDS, AMETEK Octane Plus). And the vibrational modes were studied by a micro-Raman spectrometer (Renishaw inVia, 532 nm laser excitation).

Device fabrication and measurement

Devices based on the as-synthesized SnS2 NFs were fabricated to investigate their photoresponsive characteristics. Firstly, an ITO glass was separated by a non-conducting gap with a width of ~ 50 µm. Secondly the SnS2 NFs were dispersed in ethanol and then deposited on the glass. Finally, the gap was fulfilled with the SnS2 NFs. For these obtained devices, all electrical measurements were taken using a semiconductor device analyzer (Keysight, B1500A).

Results and discussion

Figure 1a shows a panoramic SEM image of the SnS2 NFs obtained at 180 °C for 24 h, and a 4 times zoom-in image is presented in Fig. 1b. It can be seen that the as-synthesized products have distinct hexagonal flake-like structures, with plane surfaces, a mean diameter of ~ 500 nm and an average thickness of ~ 40 nm. The NFs aggregate together with random directions. EDS measurements were also taken, and the result shows the atomic ratio of Sn to S is probably 1:2.1. XRD measurements were taken, and Fig. 1c depicts the obtained XRD pattern of the as-synthesized SnS2 NFs. The XRD patterns were well indexed to the pure hexagonal phase of H-SnS2 (JCPDS No. 23-0677) with the lattice constants a = 3.648 Å and c = 5.899 Å and the space group P-3m1, and the corresponding Miller indices were also marked. Sharp diffraction peaks demonstrate high crystallinity of the as-synthesized SnS2. No other phases or impurities such as SnCl4, S and SnS were detected [15]. Raman spectroscopy was used to study vibrational modes of the SnS2 NFs. As illustrated in Fig. 1d, there is only one peak located at 315 cm−1, and it is assigned to the A1g phonon mode of SnS2, which is consistent with a previous report [16, 17].

Figure 1
figure 1

a, b Panoramic SEM image of the as-synthesized products grown at 180 °C for 24 h and 4 times enlarged image. The inset is the EDS spectrum of the SnS2 NFs. c XRD pattern, d Raman spectrum and e XPS spectrum of the SnS2 NFs

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The chemical composition of the SnS2 NFs was determined by XPS, and the results are illustrated in Fig. 1d. The peak at 162.1 eV corresponds to the binding energy of S2− 2p, and the corresponding binding energy of Sn4+ 3d3/2 and Sn4+ 3d5/2 is 495 and 486.6 eV, respectively. The atomic ratio of the as-synthesized SnS2 is defined as \( A_{\text{Sn:S}} = \frac{{I_{1} /S_{1} }}{{I_{2} /S_{2} }} \), where \( I_{1} \) and \( I_{2} \) are relative areas under the Sn and S peaks, respectively. \( S_{1} \) and \( S_{2} \) are the corresponding sensitivity factors, which are 3.2 and 0.35, respectively. By calculation, the ratio of the SnS2 NFs is 1:2.1, which is consistent with the EDS analysis. According to the results above, it is confirmed that the hydrothermal synthesized SnS2 NFs have high crystallinity and purity.

In order to explore the growth process and influence parameters during hydrothermal synthesis of the as-synthesized SnS2, samples under different conditions were prepared. Figure 2a–c shows SEM images of the products grown at different reaction times from 8 to 16 h. And Fig. 2d-f shows SEM images of the products synthesized at different temperatures from 120 to 180 °C. As the reaction time increased, SnS2 became bigger and more regular. Meanwhile, their differences became smaller and smaller, while morphology depicted in Fig. 2f is much different from the others. The products synthesized at 120 °C are gel-like and probably amorphous. Morphologies of the products synthesized at 140 and 160 °C are similar. The products obtained at higher reaction temperature are larger and more regular, which indicates better crystallinity.

Figure 2
figure 2

SEM images of the products synthesized for different reaction times (a–c) and different reaction temperatures (d–f). g XRD patterns of the products synthesized at different reaction temperatures ranging from 120 to 180 °C. h XRD patterns of the products synthesized for different reaction times ranging from 8 to 24 h

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XRD patterns of the products obtained under different conditions are also shown in Fig. 2g and h. In Fig. 2g, the intensity of these diffraction peaks increase with temperature, which indicates SnS2 with better crystallinity can be obtained at a higher reaction temperature. Moreover, there only some smooth peak envelopes can be found in the XRD pattern of the products grown at 120 °C, indicating the products should be amorphous. XRD patterns are similar in Fig. 2h, except that the intensity of some diffraction peaks is slightly different.

The whole synthesis process can be divided into two reactions process, which can be expressed as below:

$$ {\text{SC}}({\text{H}}_{2} {\text{N}})_{2} + {\text{H}}_{2} {\text{O}} \to {\text{CO}}_{2} + {\text{NH}}_{3} + {\text{H}}_{2} {\text{S}} $$
(1)
$$ {\text{SnCl}}_{4} + {\text{NH}}_{3} + {\text{H}}_{2} {\text{S}} \to {\text{NH}}_{4} {\text{Cl}} + {\text{SnS}}_{2} $$
(2)

At the first step, thiourea was hydrolyzed, and released S2− ions at high temperature. Then stannic chloride pentahydrate reacted with S2− ions, and precipitates were synthesized at the second step. The whole reaction was both effected by temperature and reaction time. However, the hydrolyzation of the thiourea was more effected by temperature than reaction time. Consequently, it is concluded by comparison that the reaction temperature is a more important parameter than the reaction time for the synthesis of SnS2 by hydrothermal method.

The schematic diagram of a prototype SnS2 photodetector is illustrated in Fig. 3a. The gap between two electrodes is ~ 50 um. And the as-synthesized SnS2 filled in the gap. Figure 3b and c shows the current (I)–voltage (V) characteristics of the as-synthesized SnS2-based photodetector under illumination of a green laser and blue laser with different light power intensities. The wavelength of the green laser and the blue laser is 532 nm and 405 nm, separately. The I–V curves with the applied voltage ranging from − 3 to 3 V indicate good ohmic contacts. The photocurrent \( I_{\text{ph}} \) is defined as \( I_{\text{ph}} = I_{\text{illuminated}} { - }I_{\text{dark}} \), and \( I_{\text{ph}} \) increased as light power intensities increased [18]. The relationship between light power intensity and photocurrent was studied, and \( I_{\text{ph}} \) as a function of light power intensity is shown in the inset of Fig. 3. By fitting the data, \( I_{\text{ph}} \) can be expressed by the equation of \( I_{\text{ph}} = aP^{\alpha } \), where a and α are 5.67 × 10−12 and 0.68 for the green laser, 3.78 × 10−11 and 0.48 for the blue laser, respectively. The deviation of the ideal index of α = 1 means that the light energy converted from the external light energy to the current is lost, which is related to the complicated process of photon absorption, electron hole separation, and carrier transport.

Figure 3
figure 3

a Schematic illustration of the single SnS2 NFs-based photodetector device configuration for photocurrent measurements. b, c I–V characteristics in logarithmic coordinates of the device in the dark and in the presence of the green laser (b, 532 nm) and blue laser (c, 405 nm) illumination with different light power intensities. Inset in b, c shows the light power intensity dependence of the photocurrent under the bias voltage of 10 V. d Energy band diagrams of a SnS2 NFs-based device

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Three important parameters for characterizing the performance of the photodetector are the photoresponsivity (R), external quantum efficiency (EQE) and detectivity (\( D^{ * } \)) [19]. The R can be defined as \( R = I_{\text{ph}} /PS \), where \( I_{\text{ph}} \) is the photocurrent, P is the light power intensity, and S is the effective exposure area of the photodetector. For the device under illumination of the green laser, R is calculated to be 4.64 mA W−1. The EQE can be expressed by the equation of \( {\text{EQE}} = hcR/e\lambda \), where h is Planck’s constant, c is the light velocity, e is the electron charge, and \( \lambda \) is the incident light wavelength. Based on these data, the EQE is about 1.08% for the green laser. In addition, photosensitivity reveals the sensitivity of photodetector, which can be defined as \( D^{ * } = \frac{{RS^{1/2} }}{{(2eI_{\text{dark}} )^{1/2} }} \), where \( I_{\text{dark}} \) is the dark current. The calculated \( D^{ * } \) for the green laser is 1.2 × 109 jones. Similarly for the blue laser, the R calculated is about 58.5 mA W−1, the EQE is about 17.9%, and the \( D^{ * } \) is about 1.53 × 1010 jones, which is comparable to MoSe2, graphene and other materials [20]. The relatively low R and EQE compared with other SnS2 monolayer and multilayers are probably due to two reasons [21]. One is the as-synthesized SnS2 was aggregated NFs, whose scale and thickness limited the performance of the photodetector. The other is the prototype photodetector was simple, and the existence of crystal boundaries hampered the transport of free electrons.

The photoresponsive processes of SnS2-based photodetectors were further studied. And the corresponding mechanism is proposed in Fig. 3d. Under the bias voltage and without illumination, there is a small current, which is called as the “OFF” state. While the laser is switched on and shined on the devices, the absorbed photons transit electrons located in valence band directly to conduction band. And the free carriers increase due to photon absorption, leading to the reduction in the semiconductor resistance. The newly photogenerated free electrons and holes shift in opposite directions under bias voltage and result in photocurrent [22, 23]. The state is called the “ON” state. When the laser is switched off, excited electrons in conduction band decrease rapidly and return to valence through radiative recombination transition. And the devices recover to original “OFF” state. With the intermittent illumination of lasers, the photodetector transforms between “ON” and “OFF” states and forms a stable and periodic process.

The time-resolved photoresponse of the device was also measured under illumination of lasers with different wavelengths (green, 532 nm; blue, 405 nm) and different light power intensities (0.317, 0.41 and 0.54 mW/cm2 for the green laser; 0.031, 0.058 and 0.098 mW/cm2 for the blue laser), and the results are depicted in Fig. 4a and b. The external bias voltage was set as 5 V. The curves of the device exhibit good stability and repeatability. The response and decay time are also critical parameters of evaluating the performance of the photodetector. Figure 4c–f shows the response and decay curves under illuminations of the green and blue lasers. In addition, in order to calculate the response time (\( \tau_{\text{r}} \)) and decay time (\( \tau_{\text{f}} \)) more accurately, we have fitted the curve in Fig. 4c–f, and the two fitting formulas are followed [24]:

Figure 4
figure 4

Time-resolved photoresponse of the device at the bias voltage of 5 V, under illumination of the a green and b blue lasers. c, d The response time and decay time of the photocurrent under illumination of the green laser at Vds = 5 V. e, f The response and decay time of the photocurrent under illumination of the blue laser at Vds = 5 V

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$$ I_{\text{ds}} = I - I_{0} \times \exp [ - (t - t_{0} )/\tau_{\text{r}} ] $$
(3)
$$ I_{\text{ds}} = I + I_{0} \times \exp [ - (t - t_{0} )/\tau_{\text{f}} ] $$
(4)

Under illumination of the green laser, the fitted response and decay time are both 45 ms. Impressively, under illumination of the blue laser, the SnS2 NFs-based photodetector shows a fast response time ~ 7.3 ms and a decay time ~ 16 ms. Importantly, the response speed of the SnS2 photodetector under illumination of the blue laser is faster than those of reported SnS2-based photodetectors [25, 26]. It may be due to two reasons: one is high crystallinity of the SnS2 NFs, and the other is SnS2 is more sensitive to incident with shorter wavelength [3].

Conclusion

In conclusion, we successfully synthesized SnS2 NFs with high crystallinity and purity via a facile hydrothermal method. The synthesis process was also discussed, and it revealed that the reaction temperature is a more important parameter than the reaction time. Furthermore, through the in-depth study of the photoresponsive of SnS2 NFs, it is concluded that SnS2 has reversible and sensitive optical response characteristics. Impressively, the photodetector based on SnS2 NFs exhibits an excellent performance with a fast response time of 7.3 ms and high detectivity of 1.53 × 1010 Jones. In this work the synthesis process, devices fabrication and responsive characterization of nano-SnS2 were systematically studied and it may pave the way for the synthesis of LMDs and expand their utilizations for advanced optoelectronics devices.