key: cord-0709617-5alotpf3
authors: Gunasekeran, S.; Thangaraju, D.; Marnadu, R.; Chandrasekaran, J.; Shkir, Mohd.; Durairaja, A.; Valente, M.A.; Elang, M.
title: Photosensitive activity of fabricated core-shell composite nanostructured p-CuO@CuS/n-Si diode for photodetection applications
date: 2020-10-14
journal: Sens Actuators A Phys
DOI: 10.1016/j.sna.2020.112373
sha: 22db0658c7a924993de9ae8f855fd5b1378a258b
doc_id: 709617
cord_uid: 5alotpf3

Development of photo detectors based on different semiconducting materials with high performance has been in progress in recent past, however, there is a lot of difficulties in developing the more effective photo detectors-based devices with high responsivity, detectivity and quantum efficiency. Hence, we have synthesized pure CuS and CuO@CuS core-shell heterostructure based photo detectors with high performance by simple and cost-effective two-step chemical co-precipitation method. The phase purity of CuS and CuO@CuS composite was observed by XRD analysis and the result were verified with Raman spectroscopy studies. Sphere like morphology of pure CuS and core-shell structure formation of CuO@CuS are observed with scanning and transmission electron microscopes. The presence of expected elements has been confirmed with EDX elemental mapping. Light harvesting photodiodes were fabricated by using n-type silicon substrate through drop cost method. Photo sensitive parameters of fabricated diodes were analyzed by I–V characteristics. The p-CuO@CuS (1:1)/n-Si diode owned a maximum photosensitivity (Ps) ∼ 7.76 [Formula: see text] 10(4) %, photoresponsivity (R) ∼ 798.61 mA/W, external quantum efficiency ([Formula: see text])∼309.66 % and specific detectivity (D*) ∼ 8.19 × 10(11) Jones when compared to p-CuS/n-Si diode. The obtained results revealed that the core/shell heterostructure of CuO@CuS is the most appropriate for photo detection.

Incorporating novel structure in photo detector is an important task to enhance the optoelectrical performance of the devices. Recently, hybrid nanomaterials and its structural, morphological, and electrical properties are drawn much attention in fabrication of devices such as solar cell, photo detectors, and super capacitors [1] . Many transition metal oxide (TMO) and transition metal chalcogenide (TMC) nanomaterials are well-known semiconducting materials, which are widely used for construction of optoelectronic devices due to its potential light absorbing nature. The generation of charge carriers and the charge carrier transport mechanism of TMO and TMC nanomaterials are most favorable for photo sensitive device fabrication [2] . In previous reports TMO (MoO3, WO3, and CuO) and TMC (ZnS, MoS2, WS2

and CuS) based photo detectors and are widely studied in terms of effect of morphology, concentration and temperature [3] [4] [5] [6] .

The heterostructure in optoelectronic application will improve the light abortion rate of the junction and it has strong current carrying nature. The heterojunction with different material with same conducting nature will improve the photo sensing nature of the device. Particular interest, the development of core/shell heterostructure are attractive towards the photo detectors because of their surface to volume ratio, which enable high photo response and charge transport mechanism without any loss of charge carriers [7] . Photo sensitive parameters were effectively influenced by incorporating the core/shell based sensitive layer which reduce the width of the depletion layer, barrier height, and increase the free carrier charge transport [8] .

ZnO/graphene core-shell structure-based photo detector was reported by Shao et al. [13] . In UV photodetector and noticed the enhancement [15] , Tian reported the hybrid CuO@In2O3 with R = 2.24 × 10 4 A/W [29] . These studies are showing very high values compare to current results; however, the currently developed photodetector is the first-time report to the literature and further work is in progress to achieve higher photodetection properties by varying the several preparation parameters. As the core/shell combinations of TMO and TMC are expected to enhance the photocurrent and responsivity of the photo detector. Most of the core/shell heterojunction-based photodiodes was reported on ptype or n-type material, still efficiency in Photo response is lagging, to achieve better photo response both material for the heterojunction employed as p-type semiconducting material [30] .

These reports indicate that the core-shell system based on CuO & CuS both possess highly improved photodetection properties which makes it more useful in nanoelectronics devices with fascinating functions. Also, among several materials CuO & CuS are a versatile p-type semiconducting material with extensive properties such as earth abundance, mechanical stability, narrow band gab, cost effective, are increasing attention in the fabrication of photo detector [31, 32] . Till date the CuS with CuO as core/shell heterojunction is not documented.

So, in this report, synthesis of pure CuS and CuS@CuO core/shell nanocomposite was done by the two-step co-precipitation method. The photo response properties of fabricated photodiodes such as p-CuS/n-Si and p-CuO@CuS-n/Si are explored in detail.

Copper (II) chloride dihydrate (98.5%, CuCl2.2H2O), Thiourea (99%, CH4N2S), Ammonia Solution (assay 25%, NH3OH) were procured from Merck Life Sciences. All the chemicals used in this work are used as purchased.

The synthesis of CuS was carried out through co-precipitation method. CuCl2.2H2O (5 mmol) was dissolved in 80 ml of distilled water and CH4N2S (10 mmole) was added. The solution was subjected to vigorous stirring about 30 minutes at room temperature. The solution was adjusted to pH 10 using NaOH solution at 60 ºC. The above solution was stirred for 2 h and formed precipitate was centrifuged (3000 rpm for 10 minutes) and collected. Obtained precipitate was washed about three times using distilled water and ethanol. The precipitate was dried at 50 ºC in hot air oven and stored for further characterization.

Pure CuO nanoparticles were co-precipitationally achieved. In this method, 0.2 mole of CuCl2.2H2O dissolved in 100 mL distilled water and stirred for 30 minutes to get homogenous green solution. The pH of above solution was adjusted to pH 9 using NH4OH solution. The color of the solution turned in to black, when the solution attained pH 9. The collection of the precipitate was done after centrifuging at 3000 rpm for 5 minutes and washing with ethanol and distilled water.

The core-shell CuO@CuS (1:0.5) prepared through by two-step chemical co-precipitation method. Previously prepared 0.440 g of CuO was completely dispersed in 80 ml of distilled water and stirred for 10 min after that 2.5mmole of CuCl2 2H2Owas added as a copper source and stirred about 20 minutes for homogeneous dispersion. Thiourea (5 mmole) is added as a sulfur source after a continuous stirring about 30 minutes the solution changed from greenish to navy blue color. pH of the Solution was adjusted to 9 at 60 ºC and stirred for 2 hours. Formed precipitate was centrifuged at 3000 rpm and washed several times with distilled water and ethanol. The procedure was repeated for CuO@CuS (1:1) as mentioned above except Cu (5 mmole) and sulfur source (10 mmole).

The photodiodes are constructed by using one side polished n-Si substrate (1×1). Before starts the coating process the substrate was well cleaned to remove dust particle, oil and grease, organic/inorganic impurities, and native oxide layer over the surface of Si substrate. Substrates are immersed in 2 mL of iso-propanol and ultrasonicate for 5 minutes to remove the dust particle, then it transferred to another beaker which contains acetone in order to remove the oil and grease on the substrate. Next step in the cleaning process, substrate was cleaned using piranha solution (H2SO4-H2O2) (2:1) to eliminate inorganic/organic residues on Si substrate.

Finally, substrates were immersed in (HF: H2O) solution (1:10 ratio) for 10 minutes after that washed with DI water to eradicate the native SiO2 layer [33] . As synthesized (50 mg) nanoparticle were dispersed in solution of cyclohexane (1 mL) and oleylamine (20 µL) to form ink type coating solution. Prepared pure CuS, CuO@CuS(1:0.5) and CuO@CuS(1:1) ink was coated on the n-Si substrate by drop cost method. The n-Si substrate was dried in room temperature about 1 hour further annealed at 220 °C for one hour under N2 atmosphere after coating a desired layer. An adhesive silver paste was applied on both sides to make the better contact and dried at ambient temperature about 5 hours. Schematic diagram of diode fabrication was presented in Figure 1 .

Xpert pro powder X-ray diffractometer equipped with CuKα radiation source of 1.54 Å. Raman spectra of the synthesized samples were recorded by Jobin Yvon HR 800 spectrometer with 532 nm laser sources. Morphology and elemental combination were analyzed using S-3400N

Hitachi field emission scanning electron microscope (FESEM). The morphology of CuO@CuS 

The XRD pattern obtained for pure CuO, CuO@ CuS (1:0.5) and CuO@CuS (1:1) are compared in Figure 2 . Obtained XRD pattern of CuS nanoparticle (Figure 2 The broad and sharp reflection patterns were observed for CuS and CuO, respectively, which indicates the major size difference between core and shell particles [1, 34] . No impurities from the other phases incorporated in the specimen are observed. 

The Raman spectrum of the CuS, core-shell CuO@ CuS (1:0.5) and CuO@CuS (1:1) nanoparticle were compared in Figure 3 (a-c). An intense peak observed around ~ 474 cm -1 and a tiny peak ~ 266 cm -1 in Figure 3 [36] [37] [38] . A tiny shoulder peak observed in the spectra at ~266 cm -1 is arising from the A1g vibration bond of Cu-S [39] . Figure 3(b) shows the Raman spectra of core shell CuO@CuS (1:0.5) nanocomposite exhibits both the characteristic Raman vibrational modes of CuO and CuS respectively. These results are good agreement with XRD results. In this spectrum two shorten peaks at 295, 333 cm -1 are corresponding to Ag mode and Bg mode of CuO formation respectively [40] . Similarly, high intensity Raman characteristic peaks are observed for CuO@CuS(1:1) nanoparticle corresponding to CuS (Figure 3(c) ), which explores that the high concentration of CuS particle over the surface of CuO of the core/shell formation. Characteristic peak at 473 cm -1 for both CuO@CuS (1:0.5) and CuO@CuS (1:1) samples, which is clearly red shifted when compared to bare CuS sample. The reason for the shifted peak may be raised at the junction of core-shell structure, because low ionic radii oxygen atoms replaced some of the high ionic radii sulphur site of CuS.

The surface morphology of the synthesized pure CuS (Figure 4) , CuO@CuS (1:0.5) ( Figure 5) and CuO@CuS (1:1) ( Figure 6 ) was investigated by FE-SEM. In Figure 4 (a-c), CuS particles were appeared as sphere like morphology and particles agglomeration may be due to physisorption of individual particles. It is clearly found that the size of particles is ~20 nanometre. EDX and elemental imaging of synthesised CuS were presented in Figure 4 (d-g), which evident the presence of Cu and S elements. Figure 5 Remarkably, the plate like shapes was found because of CuO particles and CuS were randomly dispersed over plates as illustrated in Figure 5 and 6. The reason could be the incorporation of different concentration of CuS to CuO gives a plate like shapes of metal oxide and metal sulphide spheres, respectively.

TEM analysis was done for as synthesised CuO@CuS (1:1) Core@shell particles to understand the core/shell formation in detail and captured pictures were shown in Figure 7 (a-f). The micrographs Figure 7 (a-c) clearly indicate that ~100 nm CuO sheets were surrounded by CuS particles, this approved the core/shell system formation. Figure 7 (d-e) shows the HRTEM pictures of core@shell CuO@CuS. The crystal space distance was measured at 0.13 nm agreement with the CuO of (002) [41] and the adjacent inter-planar spacing (0.191 nm) were measured in agreement with the CuS plane of (110) [42] . Interplanar spacing results confirms that the core is CuO and shell is CuS particles. The SAED pattern of CuO@CuS (1:1)

Core@shell is shown in Figure 7 (f) and confirms that the CuO@CuS nanoparticle appeared crystalline in nature. (1)

where n is Ideality factor, KB is Boltzmann constant, T is temperature in kelvin, Rs is series resistance, Io is reverse saturation current and V is applied voltage. The reverse saturation current (I o ) calculated from the following equation [44] .

here A is area of contact and A * is the Richardson constant, ϕ B is zero bias barrier height. The dark current and photo current were found to be varied from 1. 

The experimental values of n reduced from 11.49 to 2.94 with CuS level. For ideal diode, the n value should be unity (n=1). Here, the n value of p-CuO@CuS/n-Si diode is greater than one which suggest the non-ideal behaviour of the fabricated diodes. The non-ideal behaviour of the diodes is mostly due to in homogeneity in the surface state, native oxide layer, diffusion current [47, 48] . The decrease in ideality factor will enable the interface state to reduce the recombination of charge carriers in the junction [49] . for the fabricated device can be calculated by the following equation [51] .

where A * is Richardson constant. The estimated value of barrier height was summarized in Table 1 

The estimated values of Ps, R, QE and D* for different systems are listed in Table 1 . From Table 1 The quantum efficiency is an additional factor used to analyse the performance of the device. The QE can be defined as the fraction of incident photons which contribute to the external photocurrent [56] [57] [58] .

Rhc qλ (7) here h is plank constant, c is light speed, R is responsivity, q is electron charge and λ is used light wavelength. The variations of QE with forward voltage and CuS levels are shown in Figure 10 . can be calculated from the following equation [59] [60] [61] [62] .

The optimum signal detecting nature of detector is interrupted by noise produced in photodiode which is due to the thermal motion of charge carriers and electron hole pair recombination [63, 64] . The detectivity is relatively low for the p-CuS/n-Si diode due to the large dark current and Table 2 [12, [65] [66] [67] .

In summary the CuS and core/shell CuO/CuS heterostructure were synthesized through chemical co-precipitation method. The XRD patterns were conforms the phase purity of samples which confirmed 

 The manuscript has not been previously published,  is not currently submitted for review to any other journal,  and will not be submitted elsewhere before a decision is made by this journal

Authors declares that there is no conflict of interest in current article

Authors have no conflict to declare in current work Table 1 : Photodiode parameter of (a) p-CuS/n-Si,(b) p-CuO@CuS/n-Si (1:0.5),(c) p-CuO@CuS/n-Si (1:1) based diode such as Idealityfactor (n), Barrier height (ФB), Photosensitivity (PS), Photoresponsivity (R),External Quantum efficiency (EQE)%, Specific detectivity (D*). 

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University of Aveiro as an Assistant Professor. His research focuses on dielectric, ferroelectric, piezoelectric, and multiferroic properties of glass, glass ceramics, nanomaterials, single crystals and thin films

The Author (D. Thangaraju