MoS₂ and CdMoS₄ nanostructure-based UV light photodetectors

Abstract

We have developed MoS₂ nanosheets and CdMoS₄ hierarchical nanostructures based on a UV light photodetector. The surface morphologies of the as-prepared samples were investigated via field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The performance parameters for the present photodetectors are investigated under the illumination of UV light having a wavelength of ∼385 nm. Upon the illumination of UV light, the CdMoS₄-based photodetector device showed a better response to UV light compared to the MoS₂ device in terms of photoresponsivity, response time (∼72 s) and recovery time (∼94 s). Our results reveal that CdMoS₄ hierarchical nanostructures are practical for enhancing the device performance.

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Introduction

Photodetectors based on nanostructured materials are the significant components in nanoelectronic and optoelectronic devices.^1–4 Photodetectors with quick photoresponse and high photoresponsivity are in much demand for real-world applications such as optical imaging and communication.^2,5 To date, photodetectors from numerous nanomaterials (0D, 1D, and 2D) with remarkable response time and high responsivity have been reported to have excitation wavelengths ranging from ultraviolet to near-infrared region.^6–12 For the development of high-performance photodetectors, semiconducting materials with an appropriate bandgap and nanostructured morphology are highly desirable and important. Although two-dimensional (2D) graphene has attracted significant interest in numerous nanoelectronic systems due to its outstanding electronic, thermal, and mechanical properties, further development has been restricted in optoelectronic devices due to its zero bandgap or semimetallic nature.¹³ In previous reports, researchers have developed a strategy to create a bandgap in graphene and utilized it for broadband photodetector application.¹⁴ However, the presence of favourable bandgap in other 2D inorganic layered materials such as MoS₂, WS₂, MoSe₂, SnSe₂ and black phosphorous suggests great promise in the fabrication of large scale photodetector devices.^15–23 These 2D materials also possess a layer-dependent tunable bandgap, i.e., direct bandgap in monolayer and indirect bandgap in the bulk form. It makes these materials a potential candidate in numerous applications such as field effect transistors, solar cells, gas sensors, and energy storage devices.^24–28 Due to the strong light absorption in the visible to near-infrared region, MoS₂ is widely used in optoelectronic systems. In 2012, Zhang et al. reported a single-layer MoS₂ phototransistor for the first time and obtained maximum photoresponsivity of ∼7.5 mA W⁻¹ at an applied gate voltage (V_g, 50 V).¹² Jason et al. reported near-infrared photodetection using bilayer MoS₂, where they have injected hot electrons into MoS₂ in order to originate sub-bandgap photocurrent results into photogain of the order of ∼10⁵. This photogain leads to a photoresponsivity of 5.2 A W⁻¹ at 1070 nm, which is much higher than that of Si-based photodetectors.²⁹ Also, another report from Zhai et al. on monolayer MoS₂ coupled with an organic molecule showed a fast response time of 8 ms and maximum photoresponsivity of ∼430 A W⁻¹ after Al₂O₃ passivation.³⁰ In addition, reports on MoS₂-based photodetectors show that it reduces the recombination rate of charge carriers along with photoabsorber, thus leading to an increase in the photocurrent.^31–33 Herein, we report the fabrication of 2D MoS₂ nanosheets and 3D CdMoS₄ nanoflowers based on UV light photodetectors. The solvothermal route was implied for the synthesis of these structures, followed by structural and morphological investigation. We also demonstrated the device performance of these samples under UV light illumination and their cyclic response.

Experimental method

Synthesis of MoS₂ nanosheets

Initially, we dissolved 2 mmol of ammonium molybdate in (40–50 ml) methanol with the help of a stirrer. To this mixture, we added dissolved thiourea in methanol dropwise, followed by stirring for 15 min. The whole solution was then transferred into a Teflon-lined stainless steel autoclave at 150 °C for 48 h. The precipitate was obtained using a Whatman filter paper, followed by washing with ethanol several times and then heated in an oven at 80 °C for 4 h. The MoS₂ powder sample was then annealed at 400 °C for 4 h in an N₂ atmosphere.

Synthesis of CdMoS₄ nanoflowers

For the CdMoS₄ nanoflowers synthesis, we have taken 2 mmol of cadmium nitrate and dissolved it in 40–50 ml methanol. Also, we prepared a 20 ml ammonium molybdate solution by dissolving a ammonium molybdate precursor in methanol with the help of a stirrer and continued for 10 min. To this solution, we added a dissolved cadmium nitrate solution dropwise under constant stirring for 10 min. Next, dissolved thiourea was added dropwise to it, and the solution mixture was stirred further for 15 min. The solution was then transferred into a Teflon-lined stainless steel autoclave at 150 °C for 48 h. After completion of the reaction, the reactor was allowed to cool down to room temperature naturally, followed by washing the product with distilled water and filtered using a Whatman filter paper. The precipitate obtained was washed with ethanol several times and then heated in an oven at 80 °C for 4 h. The CdMoS₄ powder sample was then annealed at 400 °C for 4 h in an N₂ atmosphere.

Both the products were further analysed using numerous microscopy and spectroscopy techniques. The details on the synthesis of MoS₂ nanosheets and CdMoS₄ nanoflowers were reported previously.³⁴

Material characterizations

The structural investigations were carried out via X-ray powder diffraction technique (XRD-D8, Advance, Bruker-AXS) and Raman spectrometry (HR-800, Horiba Jobin Yvon, France) at an excitation laser wavelength of 632.8 nm with a power density of 6.37 × 10⁷ W cm⁻¹.² The surface morphologies of the as-prepared samples were characterized using a field emission scanning electron microscope (FESEM, Hitachi, S-4800) and transmission electron microscope (TEM, JEOL, 2010F Instrument).

Device fabrication and electrical measurements

The devices were fabricated on a indium tin oxide (ITO)-coated glass substrate with a source-drain separation of 500 µm. The central region of the conducting surface was etched with the zinc dust and concentrated HCl treatment. Then, we drop-casted our prepared material in this nonconducting region of the ITO substrate. Further, we annealed the device at 200 °C for 8 h in vacuum in order to obtain good adhesion of the as-prepared material with the glass substrate. The electrical characterizations of the fabricated devices for MoS₂ and CdMoS₄ samples were carried out using a Keithley 2612A source meter, which was connected to a computer through a GPIB 288A interface. All the measurements were done at room temperature.

Results and discussion

XRD of MoS₂ and CdMoS₄

The phase formation of the as-synthesized MoS₂ and CdMoS₄ samples were confirmed by XRD and depicted in Fig. 1(a and b). The XRD patterns of the MoS₂ samples synthesized at 400 °C for 4 h are shown in Fig. 1(a), which match with the JCPDS data card no. 00-009-0312, confirming the formation of a pure hexagonal phase of layered MoS₂ and match well with the earlier report.³⁵Fig. 1(b) shows the XRD pattern for the CdMoS₄ sample annealed at 400 °C for 4 h. Indexing of the annealed CdMoS₄ sample was carried out using the Powder 4-DICVOLE software, which validated the formation of the monoclinic structure of CdMoS₄.

Fig. 1 XRD pattern of (a) MoS₂ and (b) CdMoS₄ samples annealed at 400 °C, Raman spectra of (c) MoS₂ and (d) CdMoS₄ samples annealed at 400 °C.

Raman spectroscopy of MoS₂ and CdMoS₄

In view of this, the as-prepared samples were characterized via Raman spectroscopy and are shown in Fig. 1(c) and (d). The two Raman peaks assigned to the MoS₂ sample correspond to the E¹_2g (in-plane) and A_1g (out-of- plane) vibration mode, as shown in Fig. 1(c).

These two Raman modes E¹_2g and A_1g appear at 382.5 and 406 cm⁻¹, respectively. The separation of 27.56 cm⁻¹ between these two peaks specified the presence of the few-layer nature of MoS₂. The intense Raman peaks at 125, 150, 182, 238, 285, 307, 398, 664, 754, 819, 860 and 890 cm⁻¹ were observed for annealed CdMoS₄ sample shown in Fig. 1(d).

FESEM and TEM of MoS₂

In order to investigate the surface morphology of the as-synthesized MoS₂ and CdMoS₄ samples, FESEM analysis was carried out and the results are depicted in Fig. 2 and 3, respectively. We observed some brake petals of MoS₂ sheets of different sizes located on the long sheet shown in Fig. 2(a and b). The long honeycomb-like sheets measured up to 10 µm and the thickness was found to be ∼200 nm (Fig. 2(a and b)). The pore size of the honeycomb sheet is 40–50 nm, and the thickness of the single cell is ∼10 nm. The TEM image depicted in Fig. 2(c and d) represents the sheet-like structure of MoS₂ and matches well with the FESEM results. The MoS₂ samples were entirely honeycomb-like sheets without aggregation.

Fig. 2 (a and b) FESEM images, (c and d) TEM images and (e and f) deconvoluted XPS spectra for Mo 3d and S 2p elements of the MoS₂ samples annealed at 400 °C.

Fig. 3 (a and b) FESEM images, (c) TEM image and (d–f) deconvoluted XPS spectra for Cd 3d, Mo 3d, and S 2p of the CdMoS₄ nanoflowers annealed at 400 °C.

XPS of MoS₂

The X-ray photoelectron spectroscopy (XPS) was carried out for MoS₂ sheets, and core-level XPS spectra for Mo 3d and S 2p are shown in Fig. 2(e and f). Fig. 2(e) displays few characteristic peaks corresponding to the orbital of Mo²⁺, Mo⁴⁺ and Mo⁶⁺ observed at 232.29/229.05, 234.86/231.57 and 236.07/233.03 eV, respectively. In addition, one peak for S 2s is observed at 226.4 eV, which specifies the presence of bridging S²⁻.³⁶ The two peaks at binding energies of 161.8 eV and 163 eV are detected for S 2p, which can be attributed to S 2p_3/2 and 2p_1/2, respectively, as shown in Fig. 2(f). One minor peak for S at a higher binding energy of 168.52 eV appeared due to surface oxidation of MoS₂ sheets.³⁷

FESEM and TEM of CdMoS₄

The surface morphology of CdMoS₄ samples is shown in Fig. 3(a–c). Fig. 3(a and b) shows the FESEM images that clearly show the formation of a porous nanoflower-like surface morphology of the CdMoS₄ sample. The average size of the nanoflower was observed to be ∼3 µm and thickness ∼50 nm. This kind of porous nano flower-like structure of CdMoS₄ provides a fair amount of surface area, thus enabling easy transportation of generated electron–hole to the surface and enhancing the photodetector activity. Fig. 3(c) shows the TEM image for the as-prepared CdMoS₄ sample. An original CdMoS₄ nanoflower in Fig. 3(c) shows a marigold flower-like microstructure of ∼2 µm. The uniform hierarchical nanostructures consist of well-organized independent nano petals with a length of 20–30 nm (Fig. 3(c)).

XPS of CdMoS₄

Fig. 3(d–f) present the XPS spectra for Cd 3d, Mo 3d and S 2p elements of CdMoS₄ annealed at 400 °C. The Cd 3d doublet peaks at 413.4 eV and 406.6 eV are observed for Cd 3d_3/2 and 3d_5/2, respectively, as shown in Fig. 3(d). Fig. 3(e) displays two prominent peaks at 232.24 eV and 229.1 eV for Mo 3d, indicating the presence of a +4 oxidation state for Mo. The S 2p spectra in Fig. 3(f) also show two peaks at 163.1 eV and 161.8 eV, assigned to S 2p_1/2 and S 2p_3/2, respectively. Our XPS results match well with the earlier reports and further confirm the existence of Cd, Mo and S in CdMoS₄ nanoflowers. It is apparent that the surface of CdMoS₄ is made up of comparatively smooth sheets.

The UV-visible spectroscopy has been carried out for MoS₂ and CdMoS₄ annealed samples to investigate the optical properties presented (see in ESI Fig. S1†). The red plot shows the broad absorption for CdMoS₄ nanoflowers in the UV region compared to MoS₂ (black color).

Photoresponse study of MoS₂

The as-synthesized MoS₂ and CdMoS₄ samples were further used for UV light photodetection. The experimental setup used for the measurement of UV light photodetection is shown in ESI Fig. S2.†Fig. 4(a) shows the I–V characteristics of the MoS₂ nanosheet sensor device for numerous power densities under UV light ranging from 0–200 mW cm⁻². We observed that the current increases with the increase in the power density of UV light. Photocurrents as a function of the power density plot are shown in Fig. 4(b), which indicates that the photocurrent increases with an increase in the power density. The photocurrent is calculated using the formula shown in eqn (1).

I_p = I_illumination − I_dark (1)

Fig. 4 MoS₂ nanosheet-based photodetector device. (a) current–voltage (I–V) characteristics, (b) photocurrent as a function of power density, (c) photoresponsivity vs. power density and (d) current–time (I–t) plot.

Fig. 4(c) shows the photoresponsivity vs. power density plot, whereby photoresponsivity is defined as the ratio of photocurrent to power density. We observed that the photoresponsivity increases with the increase in the power density. The photocurrent response of the MoS₂ nanosheet photodetector is shown in Fig. 4(d), which is measured under light illumination and dark conditions at an applied bias voltage of 0.5 V. The response time and recovery time with the MoS₂ nanosheet-based sensor were ∼118 s and ∼123 s, respectively.

Photoresponse study of CdMoS₄

Similarly, we have performed the UV light photodetector measurements for the CdMoS₄ device shown in Fig. 5. Fig. 5(a) shows the I–V characteristics of the CdMoS₄ nanosheet sensor device at various power densities of UV light ranging from 0–200 mW cm⁻². We observed the increment in the current value with the increase in power density. The photocurrent as a function of power density is shown in Fig. 5(b). Upon increasing the power density, the photocurrent also increases. The photoresponsivity vs. power density plot is depicted in Fig. 5(c). The obtained maximum photoresponsivity was ∼4 µA cm² W⁻¹ at a power density of 200 mW cm⁻², which is much higher than that of the MoS₂-based device. The response time and recovery time for the CdMoS₄-based device are calculated from the I–t plot shown in Fig. 5(d). The values are found to be ∼74 s and ∼94 s, respectively, which are lower than those of the device fabricated for the MoS₂ nanosheet sample.

Fig. 5 CdMoS₄ nanoflower-based photodetector device (a) current–voltage (I–V) characteristics, (b) photocurrent as a function of power density, (c) photoresponsivity vs. power density and (d) current–time (I–t) plot.

The proposed mechanism details under biasing and light illuminations using ITO electrodes, as shown in ESI Fig. S3(a) and (b).† The role of Cd was to produce impurity energy levels into the material, which shifts the absorption edges toward a longer wavelength, resulting in the enhancement in the properties of the photodetector. The enhanced photoexcited carrier density of the CdMoS₄ sample suppressing the electron–hole recombination helps in improving the performance of the photodetector. In addition, the self-assembled nanoflowers from thin petals and the high surface area required for the absorption of light results in a higher number of charge carriers. The electron transfer becomes faster because of the thin petal-like morphology of the CdMoS₄ sample, which also inhibits charge recombination. The comparative photodetector performance of MoS₂ and CdMoS₄ with previously reported Mo-based devices is presented in ESI Table 1.† The cyclic photoresponse study for the annealed MoS₂ and CdMoS₄ samples was carried out for more than 2000 s, and the various cycles have been recorded in light and dark conditions. The current–time (I–t) plots for the MoS₂ and CdMoS₄-based devices are shown in ESI Fig. S4(a) and (b),† respectively. Both the devices displays good photoresponse stability and reproducibility under UV light illumination. The results reveal that the CdMoS₄ device showed good cyclic stability without showing any change in the photocurrent value compared to the MoS₂ device for a long periods.

Conclusion

In conclusion, the structural and morphological investigation of as-synthesized MoS₂ nanosheets and CdMoS₄ nanoflowers were investigated via spectroscopy and microscopy techniques. The UV photodetector devices were fabricated on ITO (indium tin oxide)-coated glass substrates. The CdMoS₄ nanoflowers device showed a better response to UV light compared to pristine MoS₂ nanosheets in terms of photoresponsivity, whereby the response time was 74 s and the recovery time was 94 s. Our results reveal that the device performance of CdMoS₄ materials can be improved by making composite or heterostructures using other 2D materials or by functionalization.

Author contributions

MSP has carried out the experimental work on device fabrication, analysis and first writing of the draft; SRK synthesized the materials and performed the XPS measurements; BBK and DJL supervised the overall work with comments on the first draft, writing, reviewing and editing.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

M. S. Pawar acknowledges CSIR-Delhi [File No: (31/11(1084)/2019-EMR-I)]. Authors thank Director-CSIR-National Chemical laboratory, Pune (India) for the research facilities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1na00326g

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