Elsevier

Thin Solid Films

Volume 692, 31 December 2019, 137609
Thin Solid Films

Characterisation of chemical bath deposition PbS nanofilms using polyethyleneimine, triethanolamine and ammonium nitrate as complexing agents

https://doi.org/10.1016/j.tsf.2019.137609Get rights and content

Highlights

  • Thin films of high crystalline quality were obtained by chemical bath deposition.

  • Polycrystalline thin films obtained using ammonia nitrate as complexing agent.

  • Galena and Lanarkite phases were obtained using polyethyleneimine complexing agent.

  • The thin films of chemical-bath-deposition lead sulphide present quantum confinement.

Abstract

This work presents the structural characterisation of PbS nanofilms deposited by the chemical bath deposition technique at 70±2°C using Polyethyleneimine, Triethanolamine and Ammonium nitrate as complexing agents, which allow a controlled and constant ion by ion reaction in aqueous medium whose chemical bath reactions take place in basic solutions with typical pH values 9–12, distinguishing the complexes obtained by their thermodynamic stability and kinetic stability. The PbS fundamental stretching frequencies were determined by Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy gives the relative atomic composition and identification of the most intense photoelectron transitions S2p (164 eV) and Pb4f 7/2 (137.34 eV) for the PbS-Nitrate film, which are associated with the Pb (II) oxidation state. The shift to higher binding energies, Pb4f7/2 (139.01 eV) for PbS-Polyethyleneimine and PbS-Triethanolamine show the presence of PbO2 with oxidation state Pb (IV). X-ray diffraction analysis and Raman spectroscopy reveal that PbS deposited nanofilms had pure cubic galena crystalline phase when ammonium nitrate was used as complexing agent, with the Polyethyleneimine complexing agent, the formation of cubic PbS in cubic phase with monoclinic Lanarkite Pb2(SO4)2 traces were observed. Finally, using Triethanolamine as complexing agent, cubic phase PbS with orthorhombic Anglesite and lead oxide (x∼1.57) traces were found. The surface morphology of the samples was obtained by High Resolution Transmission Electron Microscopy. The thin films show three direct band gaps, around 0.77–0.78 and 0.84–0.88 eV belonged to the mid-trap state caused by –Pb dangling bond and S+2 levels and the band gap energy at 0.91–1.10 eV was attributed to the quantum confinement associated to grain size, which were obtained by transmittance.

Introduction

Combinations of group IV-VI semiconductor materials are becoming subject of interest, given their optical and electronic properties. One of these materials is lead sulphide (PbS), given its applications in electroluminescent devices, photovoltaic cells, gas sensors and other optoelectronic devices [1,2]. The PbS band gap energy is in the range of 0.39–0.41 eV at room temperature and it is used widely in infrared detectors [3], lead as a tetravalent atom, commonly forms a p-type semiconductor when it is combined with sulphur atoms [4]. Its application as a binary, in three terminal devices, allows to behave as a resistance or capacitor, which enables the fabrication of circuits [5]. The effect of multiple exciton generation was discovered in PbS nanostructures, which is very promising for using in photovoltaic cells [6], in optical emissions of semiconductor nanostructures of photonic crystal cavities at room temperature [7], temperature sensors with transfer functions proportional to the temperature in K, °C or°F [8,9]. Its application in hybrid heterojunction solar cells in which control the size and morphology by chemical methods confirm the commitment of the scientific community in this type of lead chalcogenides [9]. Crystallinity particularly allows the desirable characteristics of PbS as thin film, preparation of PbS thin films by different techniques provide answers today to the type of property and special applications that are currently required. The different synthesis methods for PbS thin films have their own morphology and properties. In order to obtain PbS thin films with suitable properties for the specific application, understanding of the synthesis methods is required, but even more, knowledge of the kind of precursors that allows controllable growth of this type of films. Different techniques are used to obtain thin films [10], among which ones, chemical bath deposition (CBD) is a low toxicity technique that complies with proper concentration and volume control of its precursors for growing PbS thin films. The properties of these films are dependent of the growth conditions, where the crystal structure is the main contributor to the electrical and optical properties. One of the first successful syntheses using this technique was reported by Pop et al. [11] in 1997, and it is the most currently used for the synthesis of thin film chalcogenides compounds. Therefore, this work presents a review of the kind of complexing agents used in chemical bath deposition, and a detailed discussion about their characteristics for a contribution in the research for a safe, environmentally friendly, and ready to large-scale production of high purity and crystallinity PbS thin films, complexing agent. By choosing an appropriate complexing agent, the concentration of the metal ions is controlled by the concentration of the complexing agent. Complexing agents act as a link between the substrate and the solid phase. In this particular study focus on the influence of a complexing agent on PbS. The kinetics of growth of a thin film in this process is determined by the ion-by-ion deposition of the chalcogenide on nucleating sites on the immersed surfaces. Initially, the film growth rate is negligible because an incubation period is required for the formation of critical nuclei from a homogeneous system onto a clean surface. Once nucleation occurs, the rate rises rapidly until the rate of deposition equals the rate of dissolution. Consequently the film attains a terminal thickness. The metal (M2+) ion concentration decreases with increasing concentration of the complexing ions. Consequently, the rate of reaction and hence precipitation is reduced leading to a larger terminal thickness of the film [12]. Some of the most common complexing agents are ammonia, Ethylene-diamine (ED) and ethylene-diamine-tetra-acetic-acid (EDTA) [13]. Nevertheless, the use of ammonia has decreased due to its toxicity and volatility [13].

In this work reports the synthesis and characterisation of PbS thin films obtained by chemical bath deposition (CBD) technique at 70±2°C using Polyethyleneimine, Triethanolamine and Ammonium nitrate as complexing agents. Its main goal is to demarcate the role of complexing agents in the CBD grown PbS thin films. As complexing agents are one of the crucial chemical additives in preparation of thin films. The effects of the complexing agent on chemical composition and optical and structural properties of the PbS thin films were studied systematically by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Raman spectroscopy, and transmittance.

Section snippets

Materials and methods

Metrology of the thin films was performed with a Philips PZ2000 200 mm laser ellipsometer. The FT-IR spectra were recorded using a Perkin Elmer spectrophotometer with a deuterated triglycine sulphate (DTGS) detector, in the 400–4000 cm−1 wavelength region, with ±4 cm−1 error rate. The chemical analysis was performed by X-Ray Photoelectron Spectroscopy (XPS) with a Thermo Scientific K-Alpha system (Thermo Fisher Scientific, UK) in ultrahigh vacuum condition (Pressure of 1 × 10−9 Torr). The XPS

Infrared spectroscopy

The measurement of the thickness by ellipsometry of the samples ensures a good control of the superficial growth of the film. The thickness distribution PbS-Nitrate sample shown in Fig. 1 has a mean thickness of 92.71 nm, with a standard deviation of ±7.68 nm of 100 points measured from the surface. The thickness distribution PbS-TEA sample has a size distribution, with an average thickness of 89.55 nm with a standard deviation of ±2.61 nm of the same number of measured points. Similarly for

Discussion

In this work, the effect of various complexing agents in the synthesis of CBD-PbS thin films was studied. The Fourier-transform infrared spectroscopy shows the fundamental stretching frequency range of the non-saturated region of the double bond of S=O from 2000 to 1550 cm−1, which appears in the PbS thin films synthesised with Polyethyleneimine and Triethanolamine shown in Fig. 2b-c that does not appear in the PbS thin film obtained with nitrate [17,38], see Fig. 2a. The presence of a

Conclusions

The results of the study of the Fourier-transform infrared spectroscopy give a brief certainty of the intermediate compounds and possible products generated by the physical properties of the complexing agents. The study of X-ray photoelectron spectroscopy identified the oxidation state Pb(II) of the PbS compound for the film obtained with Ammonia Nitrate complexing agent; the (II) and (IV) oxidation states of Lead found in PbS, Pb(SO4), PbSO2 and Pb1.57. For the films that used Ammonia Nitrate,

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