High performance photodetectors based on In₂S₃, In₂S_1.5Se_1.5 and In₂Se₃ nanostructures

Abstract

One-dimensional (1D) nanostructures, including nanofibers, nanocubes, and nanoplates of In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ semiconductors, were synthesized using a hydrothermal technique for studying their photodetector behaviour. Photocurrent measurements demonstrate that semiconducting In₂Se₃ 1D nanofibers display an exceptional response to white light, with a photo responsivity of 185.8 mA W⁻¹ and a detectivity of 22.6 × 10⁹ Jones, which is four times greater than that of In₂S₃ and In₂S_1.5Se_1.5 nanostructures. The response times of the In₂Se₃ photodetectors are impressively short, measuring only 8.8 seconds. The superior responsivity detectivity and faster response time of the 1D In₂Se₃ nanofibers can be attributed to their large surface area, which boosts light absorption and facilitates efficient charge transfer, thereby enhancing the overall device performance.

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1. Introduction

Layered transition metal dichalcogenides (TMDCs), graphene and black phosphorus have attracted research due to their new and different physical properties, exceptional structure and promising applications.^1–8 In the field of photovoltaic development, the substitute of lethal materials with eco-friendly materials is a crucial requirement. In thin film solar cell production, using CdS as a buffer layer led to the highest efficiency of 21.7%.^9,10 Nevertheless, in large-scale manufacturing, cadmium sulfide causes ecological damage owing to significant toxicity. Ergo, in these times it is essential to replace CdS with eco-friendly materials. As an alternative to potentially hazardous materials (CdS, PbS, CdTe, etc.), compound semiconductors like In₂S₃, In₂Se₃, ZnO, ZnSe, SnS, SnS₂, etc. are abundant.^11–15 Among all compounds, In₂S₃ and In₂Se₃ could be the best choice for optoelectronic device fabrication owing to their photoconductive character, non-toxicity, elevated optical transmittance in the visible range (70–80%), wider band gap (2.1 to 2.7 eV) and low production cost.^16,17 Indium selenide (In₂Se₃) and indium sulphide (In₂S₃) are n-type semiconductors from the III–VIA group and have recently gained popularity due to their exceptional charge transport properties, superior mechanical flexibility, and strong light–matter interactions.^18–21 In₂S₃ and In₂Se₃ have frequently shown different valence-varying structures with different crystalline phases, such as α-In₂Se₃,²² β-In₂Se₃,²³ γ-In₂Se₃,²⁴ κ-In₂Se₃,²⁵ α-In₂S₃, β-In₂S₃, and γ-In₂S₃.²⁶ The materials In₂S₃ and In₂Se₃ are currently under discussion as potential possibilities for a wide range of applications including photodetectors,²⁷ solar cells,²⁸ photoelectrocatalytic water splitting,²⁹ gas sensors,³⁰ electromechanical devices and piezotronic sensors,³¹ electronic skin strain sensors,³² electroresistance switching in ferroresistive memory junctions,³³etc. Furthermore, the extraordinary properties of In₂S₃ and In₂Se₃ make them more advantageous in various applications than metal oxides such as CuO, ZnO, and Fe₂O₃.³⁴

Indium sulphide occurs in three different crystal forms in nature. The yellow α-In₂S₃ having a cubic structure is stable at temperatures above 422 °C. The spinel structure material β-In₂S₃ (red) is stable at room temperature. The layer structure of γ-In₂S₃ is unaffected by temperatures exceeding 775 °C. Among these three, α-In₂S₃ is given the most consideration since it crystallises in a spinel shape and is stable at room temperature despite its high vacancy density.^35,36

Indium selenide exists in six crystallographic forms in nature, i.e. the hexagonal and rhombohedral structure of α-In₂Se₃, hexagonal and rhombohedral structure of β-In₂Se₃, hexagonal structure of γ-In₂Se₃ and triclinic structure of δ-In₂Se₃. Temperature-dependent phase transitions for bulk In₂Se₃ consist of the α-In₂Se₃ to β-In₂Se₃ phase transition at 200 °C, the β-In₂Se₃ to γ-In₂Se₃ phase transition at 520 °C, and the γ-In₂Se₃ to δ-In₂Se₃ transition at 730 °C.^37,38 Individual sheets or layers of α and β phases have five sporadic sub-layers of indium and selenium atoms. van der Waals interaction allows bordering layers to combine. Vacancies are arranged in a screw pattern on every third In site, making the γ-phase a defective wurtzite structure. Different configurations of the vacancy screws allow it to crystallise in one of many different space groups, including P6₁, P6₅, P6₁22, or P6₅22.^39,40

There are plenty of forms of Indium sulfide and Indium selenide available, such as self-assembled porous 3D flowerlike β-In₂S₃,⁴¹ nanoparticles of In₂S₃,⁴² β-In₂S₃ nanoplates,⁴³ 1D β-In₂S₃/In₂O₃ micro-tubes,⁴⁴ In₂S₃ thin film,⁴⁵ In₂S₃/BiOI composites,⁴⁶ Ni-doped In₂S₃ powders,⁴⁷ nanosheets of In₂S₃/S-C₃N₄-dots,⁴⁸ In₂S₃ quantum dots,^49,50 In₂S₃ single crystal,^51,52 γ-In₂S₃ and β-In₂S₃ colloidal nanoplatelets,²⁶ hierarchical Bi₂S₃/In₂S₃ core/shell microspheres,⁵³ In₂Se₃ nanowires,⁵⁴ monolayer β-In₂Se₃ nanosheets,⁵⁵etc.

In this work, a high-yield hydrothermal technique was utilized to synthesize nanostructures and 1D nanofibers of In₂S₃, In₂S_1.5Se_1.5 and In₂Se₃. The use of sonic waves in the sonochemical technique prevented aggregation of the nanostructures and 1D nanofibers during the growth process, resulting in high-quality products. The nanostructures and 1D nanofibers were systematically characterized for their chemical composition, structure, morphology, etc. Photodetectors based on the as-grown product were fabricated and investigated systematically. The results of photocurrent measurements indicate that 1D nanofibers made of semiconducting In₂Se₃ exhibit an outstanding response to white light, showing a photo-responsivity of 180.5 mA W⁻¹. This value is four times higher than the photo-responsivity of In₂S₃ and In₂S_1.5Se_1.5 nanostructures.

2. Experimental

2.1. Chemicals

Indium(III) chloride tetrahydrate (InCl₃·4H₂O) [∼99.99%, Sisco Research Laboratories (SRL) Pvt. Ltd, India], thioacetamide (C₂H₅NS) [∼99.00%, Sisco Research Laboratories (SRL) Pvt. Ltd, India], selenium dioxide (SeO₂) [∼98.00%, HiMedia Laboratories Pvt. Ltd, Mumbai, India], N-cetyl-N,N,N-trimethylammonium bromide (CTAB) [∼99.00%, HiMedia Laboratories Pvt. Ltd, Mumbai, India], and hydrazine monohydrate (NH₂NH₂·H₂O) [∼98.00%, Sigma-Aldrich, United States].

2.2. Synthesis of In₂S₃ nanostructures

Initially, 20 mL of 0.5 M InCl₃·4H₂O (2.06 g) is mixed with 20 mL of 0.5 M C₂H₅NS (0.22 g) under constant stirring for 30 minutes. The aforementioned solution is then augmented with 1.08 g of 0.2 M surfactant CTAB. The solution is then placed in a double-walled stainless steel vertical autoclave (operating at 383 K for 12 hours, with a pressure range of 10–12 bar, 230 V, 50–60 Hz, and 2 kW). The final solution takes on a yellow hue. The precipitates are then filtered via Grade-5 filter paper. Multiple washes are used to remove contaminants from the yield before it is dried in an oven for 10 hours at 318 K.

2.3. Synthesis of In₂S_1.5Se_1.5 nanostructures

At first, 20 mL of 0.5 M InCl₃·4H₂O (1.70 g) is mixed with 20 mL of 0.5 M SeO₂ (0.33 g) and stirred for 30 minutes. After 30 minutes of stirring, 20 mL of 0.5 M NH₂NH₂·H₂O (0.26 g) is introduced to the solution. After that 20 mL of 0.5 M C₂H₅NS (0.09 g) is added to the solution under stirring for 30 minutes. Finally, 0.54 g of surfactant CTAB (0.2 M) is added to the aforesaid mixture. The prepared solution is placed into the vertical autoclave under the same circumstances as previously stated. The final product has a brownish-yellow hue. The filtration and drying conditions remain constant.

2.4. Synthesis of In₂Se₃ nanostructures

Initially, 20 mL of 0.5 M InCl₃·4H₂O (1.44 g) is mixed with 20 mL of 0.5 M SeO₂ (0.56 g) under steady stirring for 30 minutes. After 30 minutes of stirring, 20 mL of 0.5 M NH₂NH₂·H₂O (0.51 g) is introduced to the solution. The aforementioned solution is then augmented with 0.18 g of surfactant CTAB at a concentration of 0.2 M. Under the same conditions, it is then transferred to the vertical autoclave. A brownish hue develops in the final solution. The process of filtration and drying does not change.

2.5. Reaction mechanism

The reaction mechanism of indium sulfide (In₂S₃), indium selenide (In₂Se₃), and mixed indium sulfoselenide (In₂S_1.5Se_1.5) compounds can be described as follows as shown in Fig. 1. The formation mechanism of InS and InSe can be described through pathway I and pathway II, respectively. In the case of In₂S_1.5Se_1.5, the reaction mechanism combines the pathways for In₂S₃ and In₂Se₃. The sulfur and selenium atoms simultaneously react with indium atoms, leading to the formation of indium sulfoselenide, as depicted in pathway III.

Fig. 1 The reaction pathway for the synthesis of In₂S₃, In₂Se₃ and In₂S_1.5Se_1.5 nanostructures.

3. Results and discussion

3.1. X-ray photoelectron spectroscopy (XPS)

The electronic states and chemical composition of the surface of the In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ nanostructures were studied using XPS and exhibited in Fig. 2. Fig. 2(a and b) displays an XPS spectrum of In₂S₃ nanostructures. The survey spectra exhibit the presence of In, S, and C elements, as shown in Fig. S1(a) (ESI†). The XPS spectra of the In 3d core level are further decomposed into two peaks at the binding energy of 445 eV and 452.6 eV corresponding to In 3d_5/2 and In 3d_3/2,⁵⁶ as depicted in Fig. 2(a). The S 2p spectrum (Fig. 2b) shows two peaks at 161.3 eV and 162.5 eV, corresponding to S 2p_3/2 and S 2p_1/2. On the other hand, Fig. 2(c–e) demonstrates the In₂S_1.5Se_1.5 nanostructure XPS spectra. The survey spectrum of In₂S_1.5Se_1.5 showing the presence of In, S, Se, and C elements is depicted in Fig. S1 (b) (ESI†). In the XPS spectrum of In 3d, the main peaks observed at 445 eV and 452.6 eV correspond to In 3d_5/2 and In 3d_3/2 core levels, as shown in Fig. 2(c). Moreover, spectrum S 2p, as depicted in Fig. 2(d), reveals four peaks at 161.1 eV and 162.3 eV, corresponding to S 2p_3/2 and S 2p_1/2. The peaks at 163.7 eV and 164.9 eV are attributed to S2p_3/2 and S 2p_1/2 of S in C–S, respectively. Also, Fig. 2(e) shows that the Se 3d peaks are located at 54.8 eV and 55.7 eV, respectively.⁵⁷ Additionally, Se–O bonding structures at 59.1 eV confirm the oxidation of Se species (SeO₂) on the surface.⁵⁷ Furthermore, Fig. 2(f and g) demonstrates pristine In₂Se₃ nanostructure XPS spectra. The survey spectra of In, Se and C elements are shown in Fig. S1(c) (ESI†). In the high-resolution In 3d spectrum, the binding energies at 445 eV and 552.6 eV are attributed to the In 3d_5/2 and In 3d_3/2, as depicted in Fig. 2(f). In the spectrum of Se 3d, two small peaks for binding energies at 54.8 eV and 55.7 eV are observed for Se–Se bonds. Also, the 59.1 eV peak is consistent with SeO_x, as shown in Fig. 2(g). The SeO_x peak shows a good signal compared to the Se–Se bonds, indicating that the Se–Se bonds are fragile. However, there is a very tiny chemical shift, which means that the In, S, and Se valence states are still stable for In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ nanostructures.^57,58

Fig. 2 (a) and (b) The core level spectra of indium (In 3d) and sulfur (S 2p) in In₂S₃ nanostructures. (c)–(e) The core level spectra of indium (In 3d), sulfur (S 2p), and selenium (Se 3d) in In₂S_1.5Se_1.5 nanostructures. (f) and (g) The core level spectra of indium (In 3d) and selenium (Se 3d) in In₂Se₃ nanostructures.

3.2. X-ray diffraction (XRD)

The XRD profile of the nanostructures is shown in Fig. 3. The patterns are recorded in a range of 2θ from 10° to 80°. The observed peaks are sharp at the top and have broad width at half maxima indicating that the nanoforms are of good polycrystallinity having fine grains. From the analysis, In₂S₃ nanostructures possess a hexagonal structure of the unit cell with lattice parameters a = b = 7.6231 Å, c = 32.35 Å, α = β = 90°, and γ = 120° which agrees well with JCPDS card no. 00-032-0456. In₂S_1.5Se_1.5 nanostructures possess a cubic structure of the unit cell with lattice parameters a = b = c = 10.8 Å, and α = β = γ = 90° which agrees well with JCPDS card no. 00-032-0456. In₂Se₃ nanostructures possess a hexagonal structure of the unit cell with lattice parameters a = b = 7.05 Å, c = 19.88 Å, α = β = 90°, and γ = 120°, which agrees well with JCPDS card no. 00-032-0456.

Fig. 3 The XRD patterns of (a) In₂S₃, (b) In₂S_1.5Se_1.5, and (c) In₂Se₃ nanostructures.

3.3. Scanning electron microscopy (SEM)

The morphology of the In₂S₃, In₂S_1.5Se_1.5 and In₂Se₃ nanostructures is depicted in Fig. 4. Fig. 4(a) shows the FESEM image of In₂S₃, revealing the presence of nanostructures with an average size ranging from 700 nm to 2500 nm. The size distribution of the nanostructures is displayed in Fig. 4(b). Some of the nanostructures exhibit cubic shapes, as shown in the inset of Fig. 4(a). On the other hand, Fig. 4(c) displays the FESEM image of In₂S₁.₅Se_1.5, indicating the presence of nanostructures and nanoplates. The formation of In₂S_1.5Se_1.5 nanostructures and nanoplates may be attributed to the combination of In₂S₃ and In₂Se₃ nanostructures, which is supported by the mixed phase observed in the XRD profile of In₂S_1.5Se_1.5. The width of some nanoplates is around 1250 nm, as shown in the inset of Fig. 4(c). The size distribution of the nanoplates is depicted in Fig. 4(d). Furthermore, EDS mapping of In₂S₃ and In₂S_1.5Se_1.5 nanostructures is shown in Fig. S2 (ESI†). In contrast, the FESEM image of In₂Se₃ shows a nanofiber morphology as shown in Fig. 4(e). The length of a single nanofiber is approximately 3660 nm, and the width is around 139 nm. The average width distribution of the nanofibers is presented in Fig. 4(f). These nanofiber morphologies enhance the performance of the photodetector due to their large surface area. Additionally, the EDS mapping of the In₂Se₃ nanofibers confirms the successful presence of indium and selenium elements within the nanofiber structure, as shown in Fig. 4(g).

Fig. 4 Surface morphology and size distribution analysis of (a) and (b) In₂S₃ nanostructures, (c) and (d) In₂S_1.5Se_1.5 nanostructures and nanoplates, and (e) and (f) In₂Se₃ nanofibers, and (g) EDS mapping of In₂Se₃ nanofibers.

3.4. Energy dispersive spectroscopy (EDS)

EDS is a reliable and effective technique for identifying elemental composition in nanostructures.^59–62 The EDS spectra of the as-synthesized nanostructures are shown in Fig. S3 (ESI†). Compared to the In₂S_1.5Se_1.5 and In₂Se₃ nanostructures, the amount of In in the as-synthesized In₂S₃ is slightly higher, according to the data analysis. In general, they are nearly stoichiometric. The absence of any other peaks in the spectra is strong evidence that the synthesised nanostructures are chemically pure.

3.5. Photo detection application

To study the photo-detection applications of In₂S₃, In₂S_1.5Se_1.5 and In₂Se₃ nanostructures separately, a glass substrate is used and the channel is prepared by making thin and straight marks using a glass cutter. Then, the suspensions of In₂S₃, In₂S_1.5Se_1.5 and In₂Se₃ nanomaterials are drop-casted multiple times on a channel with continuous heating at 100 °C to remove the dispersing medium (distilled water). Two copper wires are bonded on the prepared samples using Ag paste. The prepared devices were further annealed at 120 °C for 5 h and used for photo-detection experiments. The various temporal detecting experiments are performed using a Keithley-4205 SMU.

First, as shown in Fig. 5(a), the current–voltage characteristics of the devices were measured. The I–V curves for the devices are linear, indicating that the connections between the nanostructures and the Ag electrodes are ohmic. The current is found to increase as Se replaces S. In order to examine the transient photoresponse of the prepared devices, the light is turned on and off at regular intervals. At a bias voltage of 1 mV and under white light with a power intensity of 10 mW cm⁻², the device's current is plotted as a function of time in Fig. 5(b). The devices had a consistent and reliable response to white light. Time constants are calculated for both the current rise (τ_rise - the time needed for the current to increase by 90%) and decay (τ_decay - the time needed for the current to decrease by 10%). In addition to these, the photocurrent (I_ph = I_illI_dark, where I_ill is the current under illumination and I_dark is the current in the dark) appears to be amplified from 2 μA for pure In₂S₃ to 70 μA (35 times) for the In₂Se₃ sample. The photoresponse of the In₂Se₃ nanofibers under different illumination intensity (10, 20, 50 and 100 mW cm⁻²) is shown in Fig. 5(c). A detailed photodetection study of In₂Se₃ device was carried out with different illumination sources. Fig. 5(d) shows the wavelength dependent pulse-photo response, examined at 485 nm, 560 nm and 670 nm light sources at intensity 5 mW cm⁻² and bias voltage 1 mV. The good response at 670 nm illumination is due to the maximum optical absorbance for the In₂Se₃ nanofiber device.⁶³ In contrast, nanofibers have a larger surface area-to-volume ratio than nanostructures, which can enhance the absorption of incident light and increase the number of charge carriers generated. This can lead to a higher photocurrent and sensitivity in photodetection. The detector parameters utilised in the quantitative study, including photo-responsivity, detectivity, EQE (%), rise time, and decay time at specific conditions are listed in Table 1. They are evaluated using standard equations reported by Zhou J et al.⁵⁰ A comparison of the reported photodetection parameters is provided in Table 1.

Fig. 5 The (a) I–V characteristics, (b) pulse photoresponse of In₂S₃, In₂S_1.5Se_1.5, and In₂S₃ nanostructures, (c) pulse photoresponse of In₂Se₃ nanofibers at different illumination, and (d) pulse photoresponse of In₂Se₃ nanofibers at different intensity.

Table 1 The typical photoresponse parameters of the In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ nanostructures at 1 mV bias voltage and comparison with reported data

Sample	Illumination source	Intensity (mW cm^−2)	Photo current (μA)	Responsivity (mA W^−1)	Detectivity ×10^9 (Jones)	EQE (%)	Rise time (s)	Decay time (s)
In2S3	White LED	10	2	5.3	1.45	—	5.9	6.7
In2S1.5Se1.5	White LED	10	18	47.7	9.44	—	5.0	6.2
In2Se3	White LED	10	70	185.8	23.3	—	8.8	9.5
	White LED	20	101	146.0	18.3	—	8.8	9.5
	White LED	50	308	163.5	20.5	—	8.8	9.5
	White LED	100	680	180.5	22.6	—	8.8	9.5
	485 nm LED	5	6.8	36.0	2.36	9.22	8.8	9.5
	560 nm LED	5	8.0	42.4	2.74	9.41	8.8	9.5
	670 nm LED	5	10.1	53.1	3.46	9.85	8.8	9.5
Reported In2S3^64	White light Bias 5 V	100	100.5	0.59	0.10	—	3.7	3.7
Reported few layered InSe^65	633 nm LASER Bias 10 V	0.28	—	—	54.7	—	—	—

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1D In₂Se₃ nanofibers are likely to have a higher photodetector response compared to In₂S₃ and In₂S_1.5Se_1.5 nanostructures due to their unique properties. One possible reason for this higher response is the morphology of the nanofibers. Nanofibers have a high surface area to volume ratio, which can increase the interaction between the material and the incident light, leading to a higher photodetector response. Another factor is the electronic properties of In₂Se₃. In₂Se₃ has a narrow bandgap, which means that it can absorb a wide range of wavelengths of light, making it a good candidate for photodetection. Additionally, In₂Se₃ has high carrier mobility and a long carrier lifetime, which can enhance the efficiency of charge separation and transport, leading to a higher photodetector response.

The photodetector properties of 1D In₂Se₃ nanofibers are studied through detectivity and responsivity. The results show that the nanofibers exhibit a detectivity of 22.6 × 10⁹ Jones. These detectivities are higher than those of other Se-based materials such as 2D layered Ta₂NiSe₅,⁶⁶ PdSe₂ flakes,⁶⁷ SnS₂/InSe heterostructures,⁶⁸ ZrSe₃⁶⁹ and PANI/ZnO.⁷⁰ These findings suggest that 1D In₂Se₃ holds promise as a material for utilization in photodetector devices.

4. Conclusion

The hydrothermal method was successfully employed to deposit nanomaterials of In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ onto glass substrates. A study was conducted to investigate the effects of semiconductors In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ on their structural, morphological, compositional, electrical and photodetector properties. The presence of constituent elements in the In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ was confirmed through EDS and XPS studies. The responsivity values of the In₂S₃, In₂S_1.5Se_1.5, and In₂Se₃ photodetectors were approximately 5.31, 47.7, and 185.8 mA W⁻¹, respectively. The highest detectivity of 23.3 × 10⁹ Jones was obtained in the In₂Se₃ sample. The photodetectors exhibited very short response times, ranging from 5 to 8.8 seconds. The higher photodetector response of the 1D In₂Se₃ nanofibers compared to In₂S₃ and In₂S_1.5Se_1.5 nanostructures may be due to their larger surface area-to-volume ratio, crystal structure, and composition, and the dimensions of the nanofibers. These findings suggest that In₂Se₃ nanofibers have potential applications in visible-light selective photo-sensing devices.

Data availability

Data will be provided upon request.

Conflicts of interest

There is no conflict of interest for any author.

Acknowledgements

The authors wish to acknowledge the National Dong Hwa University, Department of Physics, Hualien 97401, Taiwan, for generously offering the necessary facilities and resources for conducting this study. Furthermore, the authors would like to extend their appreciation to Sardar Patel University P.G, Department of Physics, Vallabh Vidyanagar - 388120, Gujarat, India, for providing the essential facilities and resources for the successful completion of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ma00808h

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