Wavelength dependent UV-Vis photodetectors from SnS₂ flakes

Abstract

2D layered materials have attracted increasing interest, owing to their unique properties and large potential for versatile applications. As one of the 2D layered semiconductors, tin disulfide (SnS₂) is rarely reported compared with other 2D materials like molybdenum disulfide (MoS₂). Herein, high quality SnS₂ flakes were grown by a facile and low-cost path, and photodetectors based on thin SnS₂ flakes were fabricated and characterized. These flakes are of high quality according to the results of XRD, Raman and TEM measurements, and present hexagonal and half-hexagonal forms with an average diameter of 100 μm. The devices based on these SnS₂ flakes showed wavelength dependent photo-responsive characteristics as the illuminating wavelength varied in the UV-Vis range (from 100 to 800 nm). They also showed excellent photo-responsive characteristics under monochromic illumination using three different wavelengths (533, 405 and 255 nm) with high photo-responsivity and high external quantum efficiency (EQE). The experimental results agree well with the first-principles calculated band structure and optical absorption coefficient curve.

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Introduction

Two-dimensional (2D) materials such as graphene and metal dichalcogenides have unique properties and extensive potential applications, and have attracted much attention in recent years.^1,2 As a representative 2D material, graphene has a lack of a band gap. And graphene has fatal obstacles for electronic and photonic applications.^3,4 Thus, metal dichalcogenides with a natural band gap came into the view of researchers, such as molybdenum disulfide (MoS₂),^5–7 tungsten disulfide (WS₂),⁸ molybdenum diselenide (MoSe₂),^9–11 tungsten diselenide (WSe₂)¹² and tin disulfide (SnS₂).¹³ Most of these have similar layered sandwich structures. Due to their special characteristics, such as theoretically high carrier mobility, indirect-to-direct band gap transformation and large surface area, they have flourished in their potential applications in transistors,^12,14,15 chemical gas sensors^16–18 and photodetectors.^19,20 In several metal dichalcogenides, there is an increase in band gap energy with a decrease in the layer number, which is observed through changes in photoconductivity, absorption spectra and photoluminescence. And the photoluminescence quantum yield in the bulk and single layer forms differs by several orders of magnitude. For MoS₂ the bulk indirect band gap of 1.3 eV increases to a direct band gap of 1.8 eV in the single layer form. The quantum yield is about 10⁻⁵ to 10⁻⁶ for the bulk, and is about 10⁻³ for the single layer.^21,22 Metal dichalcogenides have huge potential for use in tunable wavelength photodetectors, including in UV-Vis photodetectors used in multiple fields such as in artificial retinas, fire-warning systems and environmental pollution monitoring. Moreover their flexibility and transparency make them promising optoelectronic materials for application in wearable electronics.

As a 2D material, SnS₂ is abundant in the earth and environmentally friendly. In the layered sandwich structure of SnS₂ crystals, each layer of Sn atoms is sandwiched between two layers of S atoms and the layers of S–Sn–S interact with each other through van der Waals (VDW) interactions (Fig. 1a), which allow adjacent layers to slide. However, SnS₂ has not yet attracted as much attention as other 2D materials. It is an indirect semiconductor material with a band gap of 2.1 eV,^23–25 which would show more potential for modulating energy levels by doping and other methods. But SnS₂ easily decomposes into tin and sulfur at 650 °C, which is an obstacle in the synthesis of SnS₂ at high temperatures. SnS₂ was successfully prepared using some chemical methods, such as hydrothermal methods^26,27 and chemical transport deposition.²³ Also, recently Huang et al. synthesized high-quality hexagonal nanosheets on carbon cloth via chemical vapour deposition (CVD), and they have huge potential in highly sensitive phototransistors.²⁸ In this work, we grew SnS₂ flakes via a facile and low-cost method. Furthermore, photodetectors based on the prepared SnS₂ flakes were fabricated, and their photoresponsive characteristics under UV-Vis illumination at different wavelengths were investigated in detail.

Fig. 1 (a) The crystal structure of SnS₂. (b) Illustration of the growth process; all details of the experiments are presented in the ESI.† (c) Typical OM photograph of the as-prepared SnS₂ flakes. (d, e and f) XRD pattern of SnS₂ and Raman spectra of a flake in the circle marked in (c). (g and h) A HR-TEM image and corresponding SEAD pattern of SnS₂.

Results and discussion

Fig. 1c represents the morphology of the as-grown SnS₂ thick flakes. The flakes show obvious hexagonal and half-hexagonal shapes, and are randomly dispersed on the substrate with a mean diameter of about 100 μm. The flakes have golden and gray colors owing to different heights, and the pink parts represent the SiO₂/Si substrate.

To confirm the composition and crystal structure of the SnS₂ flakes, Raman spectra and XRD patterns were both detected and are shown in Fig. 1d–f. Fig. 1d shows the XRD pattern of the as-grown SnS₂ flakes, which is assigned to a 2T-type layered structure (JCPDS: 23-677). The plane indexes according to the crystalline planes of SnS₂ are labelled in the pattern, and there are no other peaks belonging to SnS, SnO or SnO₂.^29,30

Fig. 1e represents a Raman spectrum of the hexagonal flake in the marked circle in Fig. 1c. In the Raman spectrum, two strong Raman peaks are observed obviously in the range from 100 to 800 cm⁻¹. The steep one is located at 313.2 cm⁻¹, which is related to the A_1g vibration mode of SnS₂. The other broad one is located at around 159 cm⁻¹, and it can be divided into an intergradation of four peaks using Lorentz fitting, which are located at 137.2, 159.1, 187.1 and 205.1 cm⁻¹, as shown in Fig. 1f. The peak at 205.1 cm⁻¹ is assigned to the E_g vibration mode of SnS₂, and the other three peaks are attributed to the second-order vibration modes of SnS₂.³¹ The Raman spectra were in accord with previous studies on SnS₂ Raman spectra^32,33, and in conclusion the results of Raman and XRD measurements confirmed that the obtained samples are pure SnS₂.

The quality of the SnS₂ flakes is further investigated using transmission electron microscopy (TEM). A typical high resolution TEM image and the related selected area-electron-diffraction (SAED) pattern are shown in Fig. 1g and h. In the HR-TEM image two lattices were marked, one corresponds to the (100) plane of SnS₂, with an inter-plane distance of 0.31 nm. The other lattice corresponds to the (110) plane, with an inter-plane distance of 0.18 nm. The diffraction points are clear and regularly arranged in the SAED pattern. Both the HRTEM and SAED images clearly demonstrate the high crystallinity of the as-grown SnS₂ flakes.

To further understand the growth mechanism of the SnS₂ flakes, as-grown SnS₂ flakes with different growth times from 2 to 6 min were studied. According to the images of the as-grown SnS₂ flakes (Fig. S1, ESI†) the whole growth process can be mainly divided into four stages, and the related representative images are shown in Fig. 2a. The reaction times are 1, 2, 3 and 3.8 min, respectively. In the first stage, SnO powder was heated to SnO vapor. Then the SnO vapor made contact with the substrate and deposited on it. A Raman measurement was performed, and a Raman spectrum of a dot in the first stage is shown in Fig. 2b. There are three peaks located at 110, 206 and 516 cm⁻¹. The peak at 516 cm⁻¹ belongs to silicon, and the peaks at 110 and 206 cm⁻¹ are assigned to the A_1g and B_1g vibration modes of SnO. SnO was confirmed in this stage according to the results of the Raman measurements. In the second stage, SnO particles began to react with S and small SnS₂ particles formed. The whole reaction can be described as:

2SnO + 5S → 2SnS₂ + SO₂

Fig. 2 The growth mechanism of SnS₂ flakes. (a) The four stages of growth as summarized, with the corresponding OM photographs below. (b) Raman spectrum of dots in the first stage. (c) Raman spectrum of samples in the second stage.

SnO dots from the first stage became SnS₂ crystal nuclei after this reaction. The strongest peak at 313 cm⁻¹ in the Raman spectrum (Fig. 2c) is related to the A_1g vibration mode of SnS₂. The peak located at 520 cm⁻¹ belongs to silicon. As the reaction continued, SnS₂ grew larger based on the crystal nuclei, as shown in the third and fourth stages. According to the Bravais rule, the important crystal planes with the highest reticular densities and the greatest interlayer distances govern the crystal morphology during the growth process. The interlayer distance of the (001) plane is larger than other crystal plane distances for SnS₂, and the (001) plane has a higher reticular density than other planes. The distance between atoms in this plane is smallest and the binding between them is strongest. Also this plane has the lowest surface energy, as in the previous literature.^34,35 Atoms in the plane easily absorb other different atoms. When SnS₂ nuclei formed, they were irregular and randomly located on the substrate with various directions. In the third stage, SnS₂ flakes were irregular due to their nuclei. Finally, SnS₂ flakes with different directions formed after the process of deposition–growth–nucleation–regrowth.

Due to the rough surface and difficulty in controlling SnS₂ flakes, it is difficult to fabricate electrodes in situ for device fabrication. Thus, adhesive tape was used to mechanically exfoliate SnS₂ flakes, and then the SnS₂ flakes were transferred onto a SiO₂/Si substrate. Devices were fabricated using photo-lithography using 300 nm Au and 50 nm Cr as the electrodes. Five devices were made and the transport and photoresponsive properties were measured. Fig. 3a and c represent an OM photograph and AFM image of a typical device. The size of the gap between the two electrodes is 4 × 20 μm and the size of the SnS₂ flake in the gap is 4 × 7 μm. Before photo-lithography, Raman measurements were taken for the flake. The Raman spectrum is shown Fig. 3b, and there are two peaks which have different intensities. Peak B at 520 cm⁻¹ belongs to silicon and peak A at 313 cm⁻¹ is related to the A_1g vibration mode of SnS₂. Compared to the Raman spectrum in Fig. 1e, the existence of peak B means that the flake shown in Fig. 3a is thinner. Fig. 3c depicts an AFM image of a thin SnS₂ flake, with black dashed lines surrounding it. The thickness of the flake was also detected, and the height of the SnS₂ flake is about 111 nm as shown in Fig. 3d.

Fig. 3 OM (a) and AFM (c) images of a typical device based on a thin SnS₂ flake. (b) The Raman spectrum of the thin SnS₂ flake; (d) height of the SnS₂ flake measured using AFM; (e) the current curve under illumination with wavelengths varying from 100 to 800 nm continuously.

The photo-responsive characteristics of the devices under a continuously changing wavelength of incident light at room temperature and under normal air conditions were investigated. Fig. 3e shows the current curve under illumination with a wavelength varying from 100 to 800 nm continuously. During the process of measurement, the power density of the incident light remained almost the same. The current had its highest value in the range from 100 to 200 nm, then the current began to decrease slightly. Then when the wavelength changed from 250 to 360 nm, the current decreased steeply at first. At last in the range of 360 to 800 nm, the current decreased smoothly. The devices showed different photo-responsive characteristics under different illuminating wavelengths in the UV-Vis range.

Moreover, the photo-responsive characteristics of the devices were further investigated under three types of monochromatic illumination: green illumination (533 nm), blue illumination (405 nm) and ultraviolet illumination (255 nm). All the results of the PR measurements under the three different illuminations are shown in Fig. 4. Fig. 4a–c depict IV curves under green, blue and ultraviolet illumination. All the IV curves are almost linear, which demonstrates that the contact of the device is ohmic. The range of bias voltage is from 5 to −5 V. IV curves were measured after 1 minute of illumination. When the absolute value of the bias voltage increased, electrical current also increased.

Fig. 4 The photo-detection of a prototypical device based on a thin SnS₂ flake. (a, b and c) I vs. V curves for the device under illumination from three different wavelengths: 533, 405 and 255 nm. (d, e and f) I vs. T curves for the device in response to light switching on/off once. (g, h, and i) Time dependent current curves when the device was alternately illuminated using illumination at three different wavelengths.

At a bias voltage of 1 V, current–time (I–T) curves were measured when switching lights on and off repetitively. The illumination time period was 10 seconds. When the illumination was turned on, the electrical current increased rapidly, and then increased smoothly to reach saturation, and this state was defined as “ON”. After 10 seconds the illumination was turned off and immediately the electrical current reduced, finally recovering to the original state, which was defined as “OFF”. The shapes of the I–T curves under three different illuminating lights are different. The shape of the I–T curves under green illumination is similar to that under blue illumination, and is much different to that under ultraviolet illumination. The times to reach saturation after the illumination was turned on were 42 ms (533 nm), 42 ms (405 nm) and 2.01 s (255 nm). And the decaying times when the illumination was turned off were 40 ms (533 nm), 44 ms (405 nm) and 8.40 s (255 nm).

With repetitively switching the illumination on and off, the electrical current also was switched between “ON” and “OFF”, which demonstrated that the photo-responsive characteristic of the device is stable and reversible. The interval time between two adjacent “ON” states was 30 s, and the time period of the “ON” state was 10 s. To make observation more obvious, contour lines were added in Fig. 4g–i. With different wavelengths of incident light, the electrical current of the “ON” and “OFF” states showed different values. And the on/off ratio (I_on/I_off)^5,36,37 also showed a wavelength dependent phenomenon. All the experimental results are summarized in Table 1.

Table 1 The summary of photo-response characteristics for the SnS₂ devices

Wavelength (nm)	On/off ratio	Saturation/decaying time (ms)	Responsivity (A W^−1)	EQE (%)
255	3.5	2010/8400	10.88	5290.6
405	102.6	42/44	1.568	480.1
533	12.5	42/40	0.305	71

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The photo-responsivity R_λ and the external quantum efficiency (EQE) are both critical parameters for evaluating the sensitivity of photodetectors. The photo-responsivity R_λ is defined as the photocurrent generated per unit power of incident light on the effective area of a photodetector. The EQE is defined as the number of electrons detected per incident photon. The R_λ and EQE can be calculated using the following equations:³⁸

R_λ = ΔI_λ/(P_λS)

EQE = hcR_λ/(eλ)

where ΔI_λ is the photocurrent, P_λ is the incident light power density, S is the effective illuminated area, about 28 μm² in this work, h is Planck’s constant, c is the velocity of velocity, e is the electronic charge, and λ is the incident light wavelength. For 533 nm illumination, ΔI_λ = 4.725 nA, P_λ = 55.4 mW cm⁻², S ≈ 28 μm², and the R_λ and EQE can be estimated to be about 0.305 A W⁻¹ and 71%, respectively. For 405 nm illumination, ΔI_λ = 136.06 nA, P_λ = 310 mW cm⁻², and the R_λ and EQE can be estimated to be about 1.568 A W⁻¹ and 480.1%, respectively. And for 255 nm illumination, ΔI_λ = 2.08 nA, P_λ = 0.68 mW cm⁻², and the R_λ and EQE can be estimated to be about 10.88 A W⁻¹ and 5290.6%, respectively. The fabricated devices showed different photo-responsive characteristics under different laser lights, which are also summarized in Table 1. Although the photo-responsivity at 255 nm is highest, the saturation time and decaying time are the longest.

To further understand the band–property relationship of the photo-responsive characteristics, first-principles calculations were performed using the Vienna ab initio simulation package (VASP)^39,40 on the basis of density-functional theory (DFT).^41,42 The band structure of bulk SnS₂ is shown in Fig. 5b. The result reveals that bulk SnS₂ is an indirect semiconductor, and the minimum indirect band gap is 2.27 eV. According to Planck’s formula E = hω = 2πhc/λ (h is Planck’s constant, c is the velocity of light and λ is the incident light wavelength), the energy of 533, 405 and 255 nm light is about 2.32, 3.06 and 4.86 eV, respectively. The three energies are in accord with the direct transmission at the Γ, M′ and Γ′ points in Fig. 5b. They are higher than the indirect band gap energy of bulk SnS₂. When the illumination was turned on, electrons in the valence band absorbed the energy of photons and transmitted to the matching conduction band directly without the assistance of phonons as illustrated in Fig. 5a. Compared to green illumination, when blue illumination was turned on, electrons in the valence band would get more energy, and could transmit to a higher conduction band. When ultraviolet illumination was turned on, electrons absorbed photons with even higher energies, bringing photon-generated carriers with more energy, large enough to stimulate more additional carriers through impact ionization. However, the electrons excited by the ultraviolet photons interact with the crystal lattice strongly, and partial energy is emitted or absorbed in the form of phonons from the excited electrons, making them change position in the conduction band. As a result, the carrier mobility and recombination are hindered. Thus the saturation time and decaying time under ultraviolet illumination are longer than those under green and blue illumination.

Fig. 5 The results of first-principles calculations. (a) An illustration of photodetectors based on SnS₂. (b) The calculated band structure of bulk SnS₂. (c) The calculated optical absorption coefficient curve as a function of photon energy (from 0 to 12 eV). (d) A zoomed-in data image of absorption coefficient vs. photon energy (from 0 to 6 eV).

The optical absorption coefficient of the vertical plane direction as a function of photon energy is illustrated in Fig. 5c and d. In the low energy range (below about 2.5 eV), the value of the absorption coefficient is very small, then it begins to increase suddenly until the energy reaches 4.5 eV, which means that the photons in the region of 2.5 to 4.7 eV can be effectively absorbed. After that the coefficient exhibits a monotonously rapid increase.

Conclusions

In conclusion, we have grown SnS₂ flakes via a facile and low-cost method. These flakes show hexagonal and half-hexagonal structural forms with a mean diameter of about 100 μm. Results of XRD, Raman and TEM measurements demonstrate that the SnS₂ flakes were of high quality. The whole process can be summarized as a deposition–growth–nucleation–regrowth process. Photodetector devices were fabricated based on SnS₂ flakes through a further mechanical exfoliation method. The photo-responsive characteristics under several types of illumination with different wavelengths were systematically investigated. Photo-responsive characteristics under a continuously changing wavelength of incident illumination were also investigated. The device showed wavelength dependent performance. The wavelengths of illumination used were 533, 405 and 255 nm. The device has the highest responsivity of 10.88 A W⁻¹ for the UV light of 255 nm, but a fast response to visible illumination at wavelengths of 533 and 405 nm (with saturation and decaying times of less than 50 ms). Such photo-responsive properties were further confirmed using first-principles calculations. The experimental results agree well with the calculated band structure and optical absorption coefficient of the vertical plane direction as a function of photon energy. With the potential for photo-detection, this work is intended to spark research groups and researchers’ passion for SnS₂, and promote the development of 2D materials.

Acknowledgements

This work was financially supported by the “Hundred Talents Program” of the Chinese Academy of Sciences (CAS), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experiment details; OM images of samples with different reaction times. See DOI: 10.1039/c5ra24905h

‡ These authors contributed equally to this work.

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