Investigation of samarium-doped PbS thin films fabricated using nebulizer spray technique for photosensing applications

Highlights

• Samarium-doped PbS thin films fabricated via. Nebulizer spray pyrolysis.

• Role of samarium doping on the physical properties of the PbS lattice studied.

• Photocurrent enhancement caused by samarium doping with PbS investigated.

Abstract

Lead sulfide (PbS) and samarium-doped (1, 3, and 5 wt%) PbS thin film layers were coated facilely on glass slides using the nebulizer spray procedure. To investigate the doping effect on the crystal structure, morphology, light absorption, and emission features of the deposited films X-ray diffraction, Raman, Scanning electron microscope, UV–Visible absorption, and photoluminescence spectroscopic analyses were carried out. A Keithley source meter was used to study the electrical characteristics of the thin film coatings. All the prepared films reveal the fcc lattice structure of PbS. Additional diffraction peaks related to the Sm₂O₃ phase are observed when 5 wt% of Sm was added to PbS. From Raman analysis, the peaks observed at 192, 235, and 465 cm⁻¹ confirm the presence of the PbS phase. The scanning electron micrograph of the PbS thin film reveals that it has tightly packed grains of spherical shape. In the case of Sm-doped PbS films, the mean grain size increases with the Sm doping concentration. The energy dispersive X-ray analysis shows the existence of Pb, S, and Sm which authenticates the presence of the Sm element in the PbS matrix. The optical studies reveal that the 5 wt% Sm-doped PbS thin film has lower transmittance and higher absorption value. Moreover, the optical band gap value is decreased from 2.15 to 1.58 eV when Sm doping concentration increases from 0 to 5 wt%. The highest photocurrent is observed for the 3 wt% Sm-doped PbS thin film sample. The photocurrent enhancement due to the samarium doping with PbS makes it a potential candidate for the photosensor applications.

Introduction

Semiconducting materials are playing a significant role in electro-optical devices. Their applications extend to energy conversion, photocatalysis, optoelectronics, and nonlinear optics, due to their unique properties [[1], [2], [3]]. PbS is a IV-VI semiconducting chalcogenide having direct bandgap energy of 0.4 eV. PbS has been used as an infrared sensor and solar cell owing to its narrow bandgap [[4], [5], [6], [7]]. PbS nanostructure has a bulk exciton Bohr radius of about 18 nm, so it possesses strong quantum confinement in the nanosized structure. According to the effective mass model the bandgap can be controlled by modifying the particle size. The PbS material has many applications such as photocatalysts, new-generation solar cells, thermoelectric materials, infrared detectors, biological imaging [[8], [9], [10], [11], [12]]. S. Wageh et al. [13] fabricated Al/PbS/p-Si/Al photodiode with a high dark rectification ratio of 5.85 × 10⁴ at an applied voltage of ±4 V. G.N. Ankah et al. [14] developed PbS quantum dot based photodetectors for sensing X-rays. They optimized the value of layer thickness for detectors employed in medical imaging. T. Ghomian et al. [15] studied the role of isopropylamine capping on the PbS quantum dots for enhanced photosensing and photovoltaic applications. M. Shkir et al. [16] investigated improved photosensitivity of PbS nanosheets with the increase of current intensity. Recently, L. Mochalov et al. demonstrated the fabrication of nanostructured polycrystalline PbS thin films using the low-temperature non-equilibrium radio frequency plasma deposition method [17].

There are various methods available to prepare thin films, viz: pulsed electro-deposition, dc magnetron sputtering, thermal evaporation, rf-magnetron sputtering, spray pyrolysis, chemical bath deposition, and nebulizer spray pyrolysis method [[18], [19], [20], [21], [22], [23]]. The spray pyrolysis method is relatively easier and economical amongst the above-mentioned methods and using this method one can deposit thin films of large area and control the deposition parameters. The particle size and the thickness can be controlled in the nanometer range using a unique method of nebulizer spray pyrolysis (NSP) technique [24]. Besides, the nebulizer spray process possesses several merits compared to the conventional ones. We can achieve a good crystalline film by varying experimental parameters like reducing droplet size and temperature of the substrate. Thus, nebulizer spray pyrolysis can be effectively used to deposit PbS films. Doping is an effective technique to modify the morphology of surface and structure; to regulate particle size. Further, doping is helpful in improving the physicochemical features of films [25]. Rare earth doping (e.g. Ce, Eu, Sm, Er, etc.) improves the optical properties of the nanostructured materials due to their incomplete 4f electron configurations [[26], [27], [28], [29]]. M. Faraz et al. demonstrated an improved photocatalytic activity of Sm-doped ZnO nanoparticles against Malachite Green (MG) dye [30]. W. Naffouti et al. reported the effect of samarium doping on the physical properties of TiO₂ thin films and they observed enhanced photoluminescence properties of TiO₂ thin films on samarium doping [29]. L. Saravanan et al. observed a high specific area about 140 m²/g in the case of Sm-doped CdS nanocrystals obtained from co-precipitation method [26].

In this study, we prepared PbS and samarium-doped PbS thins films and studied the role of samarium on the structure, light absorption and emission, morphology, and photocurrent properties of the PbS thin film coatings.