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

Thin Solid Films 692 (2019) 137609

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

T

⁎

J.I. Contreras-Rascóna, J. Díaz-Reyesb, , S. Luna-Suárezb, R.C. Carrillo-Torresc, R. Sánchez-Zeferinoc a b c

Benemérita Universidad Autónoma de Puebla, Complejo Regional Centro Campus San José Chiapa, 2 Sur Ciudad Modelo, Puebla, 75010, México Centro de Investigación en Biotecnología Aplicada, Instituto Politécnico Nacional, Ex-Hacienda de San Juan Molino, Km. 1.5. Tepetitla, Tlaxcala, 90700, México Posgrado en Nanotecnología, Universidad de Sonora, Apdo. Postal 1626. Hermosillo, Sonora, 83000, México

A R T I C LE I N FO

A B S T R A C T

Keywords: Chemical bath deposition Lead sulphide Thin films Complexing agents X-ray photoelectron spectroscopy Raman scattering X-ray diffraction High-resolution transmission electron microscopy

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 PbSNitrate 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.

1. 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,

Corresponding author. E-mail address: [email protected] (J. Díaz-Reyes).

https://doi.org/10.1016/j.tsf.2019.137609 Received 6 March 2019; Received in revised form 24 September 2019; Accepted 3 October 2019 Available online 19 October 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

Thin Solid Films 692 (2019) 137609

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40 mA. Raman scattering experiments were performed at room temperature using the 6328 Å line of a He–Ne laser at normal incidence for excitation. The light was focused to a diameter of 6.0 µm at the sample using a 50 × (numerical aperture 0.9) microscope objective. The nominal laser power used in these measurements was 20 mW. Care was taken in order not to heat the sample to avoid heat induced crystallization during Raman measurements. Scattered light was analyzed using a Jobin–Yvon T64000 triple spectrometer, operating in the subtractive configuration, a holographic notch filter made by Kaiser Optical System, Inc. (model super Notch Plus), and a 256 × 1024-pixel CCD used as detector cooled to 140 K using liquid nitrogen and two interchangeable gratings (600 and 1800 g/mm). Typical spectrum acquisition times were limited to 60 s to minimize the sample heating effects mentioned above. Absolute spectral feature position calibration to better than 0.5 cm−1 was performed using the observed position of Si, which is shifted by 521.2 cm−1 from the excitation line. HRTEM studies were carried out in a JEOL JEM200 of 80–200 kV; the obtained image is recorded with a CCD camera in real time. The Gatan Digital Micrograph software was used for the analysis of HRTEM images. The transmittance measurement was performed using a Varian Cary 5000 UV–Vis-NIR spectrophotometer with a spectral range from 197 to 3300 nm at room-temperature. For the determination of the crystalline phase of the thin films the texture coefficient method was used [14-16], the normalised intensity I(hkl) of each reflection as a relation between the relative intensities of the analysed sample and the ones which correspond to the Ip(hkl) pattern using the relation, Eq. (1):

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), Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Raman spectroscopy, and transmittance.

In (hkl) =

I(hkl) Ip(hkl)

(1)

The texture coefficient (CT) [16] can be obtained from the crystalline phase analysis using the following equation:

CT(hkl) =

In (hkl) 1 m

∑m In (hkl)

(2)

3. Experimental results 3.1. 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 PbS-PEI sample has a size distribution, with an average thickness of 90.53 nm with a standard deviation of ± 3.55 nm of the same number of measured points. The analysis of the structural properties of CBD–PbS thin films in amorphous substrates obtained with different complexing agents with average thickness of ∼900 Å begins with the Fourier transform infrared spectroscopy that utilises the harmonic oscillator model for the determination of the oscillation wavenumber for the bi-atomic molecules present in the films. Fig. 2 shows the infrared spectra of Polyethyleneimine, Triethanolamine and ammonia nitrate CBD–PbS thin films, where their fundamental stretching frequencies were detected within the range between 2000 and 1550 cm−1. InTable 3 fundamental stretching frequencies are presented, detailed examination of shape and position of the absorption bands show significant changes in the FT–IR spectrum of the PbS-Nitrate film. The FT–IR spectra of the following complexing agents show similarities in shape and position of their respective absorption bands.

2. 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 system with a monochromated AlKα X-ray source (1486.6 eV) is employed to generate core excitations. The binding energy of all elements was calibrated using the CeC bonding in the C 1 s peak (284.8 eV). The films surface was etched with an Argon ion beam with an emission voltage of 3.0 kV during 12 s to remove the contaminants present on the surface. Afterwards, wide surveys scan of the films were taken with a pass energy of 72 eV in 5 sweeps. The crystalline phase and structure of the thin films were determined with a Bruker D8 Discover diffractometer, with parallel beam geometry and monochromator of gobel mirror, with a resolution of 0.002°, using the copper Kα radiation (λ=1.5406 Å) at 40 kV and 2

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Table 2 Deposit conditions in the synthesis of CBD-PbS thin films deposited at 70 ± 2 °C. Thin film

Complexing agent

pH proton concentration

Deposit time(min)

PbS-Nitrate PbS-TEA PbS-PEI

NH4NO3 N(CH2CH2OH)3 (C2H5N)n

9 12.5 11

12 9 7

Table 3 It presents the fundamental stretching frequencies of CBD-PbS-Nitrate, CBDPbS-Polyethyleneimine and CBD-PbS-thin films obtained by FT-IR. Additionally, their associated functional groups are shown. Thin film

PbSNitrate

Fig. 1. It shows the thin film thickness distribution of typical PbS–Nitrate sample.

PbSPolyethyleneimine

PbSTriethanolamine

Wavenumber(cm−1)

Associated functional group

Reference

3000–2800 1725–1690 1440–1200 1406 1725–1690 1400–1310 1440–1200 900–700 2565–2400 2150–1990 1230–1120 900–700

-CH2C=O -CO2H -CN -C=NH -SO2 -CO2H -S-O -S-H N=C=S -SO2 -SO2

[17] [17] [18] [18] [19] [19] [18] [17] [17] [17]

Table 4 Relative atomic concentrations of the elements obtained by XPS.

Table 1 Used precursors, their concentrations and volumes in the synthesis of the CBDPbS thin films. Concentration Molarity (M)

Volume (mL)

Pb(CH3COO)2 KOH NH2SCNH2 N(CH2CH2OH)3 NH4NO3 Precursor Formula * (C2H5N)n

0.1 0.1 0.1 0.1 0.1 Percentage (%) 10

39.5 2.5 9.5 58.5 58.5 Volume (mL) 58.5

Orbital

Relative concentration (%)

Pb S C O Na

4f7/2 2p 1s 1s 1s

PbS-Nitrate 28 23 39 10 —

PbS-Polyethyleneimine 3 4 61 21 10

PbS-Triethanolamine 19 11 15 50 4

Pb, S, as well as C from reference and O impurity. Oxygen in the samples is likely resulting from the adsorbed gaseous molecules since nanocrystalline materials exhibit a high surface-to-volume ratio [20]. The values of binding energy were adjusted to the measured value of 285 eV, against the value of adventitious C 1s of 284.8 eV, giving a 0.20 eV correction by charge effects. Fig. 3 illustrates the oxidation state of the used chemical elements, the most intense photoelectron transitions, S 2p (164 eV) and Pb 4f7/2 (137.35 eV) for the PbS-Nitrate are associated with the oxidation state of Pb(II) [21]. As is illustrated in the inset, the signal shifts to higher binding energies, Pb 4f7/2 (139.68 eV) for PbS–Polyethyleneimine and PbS–Triethanolamine, which shows the presence of PbO2 with oxidation state Pb(IV) in concordance with the chemical state identification algorithm of the PHI software and the XPS database SRD-20 of the National Institute for Standards and Technology (NIST).

Fig. 2. FT-IR spectra of (a) PbS–Nitrate, (b) PbS–Polyethyleneimine and (c) PbS–Triethanolamine thin films synthesised by chemical bath deposition technique.

Precursor Formula

Atom

3.3. X-Ray diffraction

* (C2H5N)n refers to the linear formula of Polyethyleneimine, its units are in column 2 in percentage and column 3 in millilitres, due to the fact that it is presented for sale in a transparent colloidal form.

The determination of the crystalline phases of the CBD–PbS thin films are obtained by X-ray diffraction and the texture coefficient is shown in Table 5. Fig. 4 illustrates the XRD diffractograms of the CBD–PbS thin films with different complexing agents deposited at Td=70 ± 2 °C, finding that they are of polycrystalline nature. Fig. 4a shows the PbS–Nitrate diffractogram, which is in concordance in angular position and intensity with the crystalline planes of the cubic phase of PbS –commercially known as Galena [JCPDS-ICDD X-ray diffraction card number 05-0592 for cubic phase PbS, n.d.]. The

3.2. X-ray photoelectron spectroscopy The relative atomic compositions of the CBD–PbS thin films were obtained by X-ray photoelectron spectroscopy and are shown in Table 4. The XPS survey spectra of the samples indicate the presence of 3

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Fig. 4. X-ray patterns of CBD-PbS thin films with different complexing agents: a) PbS–Nitrate, b) PbS–Polyethyleneimine and c) PbS–Triethanolamine.

Fig. 3. XPS survey spectra for CBD-PbS thin films using different complexing agents. Enlarged portions of the spectra between 160–130 eV are presented in the inset.

semiconductor compound [23]. In addition, today the assignation of vibrational bands in the Raman spectra of PbS are quite contradictory [24]. As is observed in the phonon dispersion curve for PbS calculated along the main directions in Brillouin zone [25,26], which is illustrated in Fig. 7, the Raman shift of the longitudinal optical (LO) mode depends on the direction in the Brillouin zone. It has been reported that the Raman scattering should yield only weak second- and higher order Raman lines from phonons at the critical points of the Brillouin zone with frequencies in the ranges of ∼200–215 and 400–450 cm–1 [27]. However, Smith et al. [25] assigned the Raman lines in the range of ∼205–210 cm–1 and its overtone, observed in the backscattering geometry in the Raman spectra of bulk PbS, to the LO phonon of the Brillouin zone centre (Γ). As is observed in Fig. 6, the features presented in the CBD–PbS Raman spectra depend strongly on the complexing agent used in the synthesis. The corresponding wavenumbers of the phononic modes were obtained by deconvolution using Lorentzian lines, which are illustrated in figure. The first Raman spectra show features near ∼185–188 cm–1 and in the range of 410–480 cm–1 that dominate in the Raman spectra of the CBD-PbS films; the latter feature has a pronounced doublet structure, the expansion of which using a Lorentzian yields two vibrational bands. With allowance for the phonon dispersion curves for PbS calculated along the main directions in Brillouin zone [25,28], the spectral bands can be assigned to the corresponding phonon modes of this material and their combined tones. The first two Raman spectra contain the first- and second order bands at 185–188 and 402–407 cm–1, respectively, which are due to the LO phonon at the Γ point of the Brillouin zone as was reported by Baranov et al. [24]. Additionally, the strong band at ∼458–463 cm–1 could correspond to the overtone of the LO phonon at the L point of the

diffractogram of Fig. 4b illustrates the polycrystallinity of the PbS–Polyethyleneimine thin film where the Galena phase was found and traces of the monoclinic phase of Lead-oxysulphate (Pb2(SO4)O)-commercially known as Lanarkite [JCPDS-ICDD X-ray diffraction card number 331486 for monoclinic phase Pb2(SO4)O, n.d.] determined by the shift in angular position in the most intense plane. Also, the polycrystallinity was determined by the texture coefficient of the three most intense crystalline planes of each phase [16]. In the same way, Fig. 4c illustrates the diffractogram of the PbS-Triethanolamine sample that determines the crystalline phases: Galena, Pb(SO4) -commercially known as Anglesite [JCPDS-ICDD X-ray diffraction card number 36-1461 for Orthorombic phase Pb(SO4), n.d.]- and Lead oxide (PbO1.57) [JCPDSICDD X-ray diffraction card number 26-0577 for monoclinic phase PbO1.57, n.d.]. Fig. 5 shows the images of the thin films obtained by high-resolution transmission electron microscopy for a) Ammonia Nitrate, b) Polyethyleneimine and c) Triethanolamine complexing agents, the insets are the result of image processing in the Fourier space. The micrographs showed spherical and distorted spherical structures with an average diameter of 5 nm, this behaviour has been reported previously [22]. 3.4. Raman spectroscopy Back scattering geometry has been used to record the Raman spectra of CBD-PbS nanofilms, which are shown in Fig. 6, which were grown with different complexing agents in the chemical synthesis. As is known, the lead sulphide has a NaCl-type crystal structure (space group Fm3m); therefore, first-order Raman scattering is forbidden in the

Table 5 Analysis of the crystalline phases found in CBD-PbS thin films using the texture coefficient method. Thin film sample

PbS-Nitrate PbS- Polyethyleneimine PbS- Triethanolamine

2θ (°)

30.074 25.963 43.058 30.074 25.963 43.058 30.074 25.963 43.058 47.097

Texture coefficient Galena TC(200)=0.96 TC(111)=0.82 TC(220)=0.53 TC(200)=0.74 TC(111)=0.36 TC(220)=0.96 TC(200)=0.80 TC(111)=0.76 TC(220)=0.46

4

Lanarkite

Anglesite

Lead oxide

— TC(11-2)=0.36 TC(310)=0.64 TC(11-3)=0.04 —

— — TC(121)=0.20 TC(021)=0.33 TC(212)=0.54

— — TC(−342)=0.99

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Fig. 5. HRTEM micrographs of CBD-PbS thin films with different complexing agents: a) PbS–Nitrate, b) PbS–Polyethyleneimine and c) PbS–Triethanolamine.

are recorded at room temperature measurements for the synthesised PbS nanofilms. It is evident from Fig. 7 that samples exhibit a strong absorption at wavelength range 1088–1830 nm suggesting blue shift w.r.t. the bulk PbS arising from quantum confinement effect in the nanoparticles. On the other hand, as the optical band gap (EG) of a semiconductor is related to the optical absorption coefficient (α) and the incident photon energy (hν). In order to quantify the optical band gap (EG) of the samples, the optical absorption coefficient (α) of nanofilms was obtained. The EG was then evaluated from the transmittance spectrum using the Tauc relation [25,32,33]: αhν = (EG − hν )n , where n depends on the type of optical transition that prevails. Specifically, n is 1/2 and 2 when the radiative transition is directly and indirectly allowed, respectively. The PbS is well-known to be a semiconductor with a direct allowed transition. The PbS average optical band gaps were estimated from the (αhν)2 vs hν graphic, which is

Brillouin zone, which is observed in the second-order Raman spectrum [25] or/and also to the transversal optical (TO) phonon of PbO (TOPbS) [29] because of the lead oxide is formed during the synthesis, as was verified by the results obtained by XPS. The vibrational feature at 323 or 370 cm−1 is associated at TO-PbO2 [30,31]. The corresponding wavenumbers of all the vibrational characteristics observed in the Raman spectra of the different samples are presented in Table 6 and their assignments. 3.5. Transmittance The effect of the different complexing agents used in the CBD-PbS thin films synthesis is reflected in the behaviour of the transmittance spectra between 197 and 3300 nm of the three samples that are shown in Fig. 8. The transmittance spectra in the ultraviolet and visible ranges 5

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Fig. 8. The transmittance spectra of the PbS nanofilms with different complexing agents deposited at Td=70 ± 2 °C, a) CBD–PbS–Nitrate, b) CBD–PbS–Polyethyleneimine and c) CBD–PbS–Triethanolamine.

Fig. 6. Raman spectra of the CBD-PbS thin films for the different complexing agents deposited at Td=70 ± 2 °C. Deconvolution of the measured Raman spectra for a) PbS–Nitrate, b) PbS–Polyethyleneimine and c) PbS–Triethanolamine.

Fig. 9. Tauc plots (showing the different band gap energies of the material) of the PbS nanolayers with different complexing agents deposited at Td=70 ± 2 °C: a) PbS–Nitrate, b) PbS–Polyethyleneimine and c) PbS–Triethanolamine. Fig. 7. Phonon Dispersion curves for PbS are illustrated along the main directions in the Brilloin Zone, which were obtained by Smith et al. [25].

expected to show a linear behaviour in the higher-energy region, which should correspond to a strong absorption near the absorption edge. Extrapolating the linear portion of this straight line to zero absorption edge gives the optical band gap energy of the films, which are shown in Fig. 9 for the three samples. The band gap value to the sample PbS–Nitrate, 0.91 eV was slightly higher than the value of bulk cubic phase PbS [0.4 eV] due to quantum confinement of galena nanocrystals. The result reveals that the PbS–Nitrate spectrum exhibits three direct absorption regions, indicated by linear relationships between (αhν)2 vs hν graphic. The respective band gap energies are for PbS–Nitrate are 0.77, 0.84 and 0.91 eV estimated by extrapolation (dashed lines in Fig. 9a). This transmittance spectrum is fundamentally different than that of a pure single crystal, which shows a direct absorption with the band gap of 0.91 eV, ascribed to the transition between valence and conduction bands due to the quantum confinement. This observed behaviour of three absorption regions, which has not been reported before for this material, suggests that the two bandgap energies around 0.77 and 0.84 eV belonged to the mid-trap state caused by –Pb dangling bond and S+2 levels [34]. The absorbance spectrum of

Table 6 Vibrational modes in Raman spectra of CBD-PbS thin films. Assigned phonon

CBDPbSNitrate

CBD-PbSPolyethyleneimine

CBD-PbSTriethanolamine

1LO-PbS 2LO(Γ)-PbS 1SO-PbS 2SO-PbS (1SO+2SO)-PbS 2LO(L) or/and TO-PbO TO-PbO2 Phononic replica Phononic replica

188 407 134 84 — 463 — — —

185 — 129 84 — 458 370 542 582

— 402 129 73 263 465 323 — —

6

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PbS–Polyethyleneimine thin film is fundamentally similar to that of galena nanoparticles, with absorption edges at 0.78, 0.88 and 0.91 eV. These slight shifts of the absorption bands were caused by the complexing agent, whose origin is similar to that previously discussed. For the PbS–Triethanolamine thin film, the main absorption band shifts towards the blue, 1.01 eV, due to the effect of the quantum confinement and for the other two the energies do not vary, therefore their origins are possibly the same as what was discussed above. Finally, the optical band gap energy of the PbS nanofilms is ranged from 0.91–1.10 eV, for the three different complexing agents, that are greater than the band gap in bulk PbS. This discrepancy is associated to the quantum confinement by average grain size [35,36]. In order to PbS–Triethanolamine nanofilm the perceptual concentration the Pb and S cores was measured and quantified gave an atomic ratio of Pb to S as 1.7, which suggests that the surface of the sample is quite rich in Pb, which suggests that the surface contains a high density of crystalline defects. Additionally, this nanofilm contains a high concentration of oxygen that reacts with the Pb forming PbOn, which passivates the surface and decreasing the size of the crystal resulting in high quantum confinement [20,37].

lattice constant value for PbS a = 5.901 Å, which are in concordance with the reported values [40,41]. The diffractogram in Fig. 3b of the PbS–Polyethyleneimine thin film exhibits the Galena phase again in the angular positions: 2θ=30.153, 26.646 and 43.024° corresponding to the (310), (11–2) and (11–3) planes indexed for the monoclinic Lanarkite phase [JCPDS-ICDD X-ray diffraction card number 33-1486 for monoclinic phase Pb2(SO4)O, n.d.]. The shift to higher angular positions at 2θ=26.646° is signal of a change in the band gap or the intercalation of traces of this new material; their values of texture coefficient are presented in Table 3. The X-ray patterns describe a symmetry group C2/m [16], whose lattice parameters were calculated with the DICVOL04 software too. The lattice constant values for Pb(SO4)2O are: a = 13.703 Å, b = 5.6033 Å, c = 7.112 Å, which are in concordance with the reported values [31,42]. The FTIR and XPS characterisations exhibit the -SO2 interaction and the percentage increase in relative concentration of oxygen in this film. For the PbS–Triethanolamine diffractogram of the thin film, the Galena phase was also found in the same angular positions described in Fig. 4a-b. There is a noticeable change of intensity in the angular positions at 2θ=26.712, 29.679 and 43.748° attributed to an intermediate compound formed in the chemical reaction, rich in -SO4 ions, that has been not dissociated in the reaction period, which is indexed as lead sulphate (Anglesite) Pb(SO4) at the angular position 2θ=47.097° A new preferential growth plane (−342) indexed for the Lead Oxide PbO1.57 compound [JCPDS-ICDD Xray diffraction card number 26–0577 for monoclinic phase PbO1.57, n.d.] that explains the shift to higher binding energies of the 4f orbital in XPS. The formation of Anglesite and lead oxides comes with the increase of the relative oxygen and sulphur concentrations in this film that have been determined by XPS and by the analysis of the texture coefficients shown in Table 3. The X-ray patterns describe a symmetry group Pbnm (62), whose lattice constant values for PbSO4 calculated by the DICVOL04 software are: a = 6.903 Å, b = 5.3723 Å and c = 8.4763 Å, which are in concordance with the reported values by Dove and Czank [43] and the obtained by JCPDS-ICDD X-ray diffraction card number 26–0577 for monoclinic phase PbO1.57, n.d. The monocrystalline nature of the undoped PbS thin film can be observed in the TEM micrograph shown in Fig. 5. The micrograph inset in Fig. 5a clearly shows the interplanar distances of the material with values of 0.2969, 0.3429 and 0.2099 nm corresponding to growth directions (200), (111) and (220) of the cubic phase of PbS. The inset in Fig. 5b illustrates the interplanar distances 0.3342 and 0.2969 nm associated with the growth directions (200) of the PbS cubic phase and (310) of the monoclinic phase of Pb2(SO4)O. Finally, in the inset of the micrograph of Fig. 4c an interplanar distance of 0.2969 nm of the cubic growth plane of PbS. The interplanar distance 0.3334 nm linked to the growth direction (021) of orthorhombic PbSO4 and 0.1911 nm of the (−104) monoclinic PbO1.57 were found, which correspond to the angular position 2θ=47.541° consequence of the dissociation of the intermediate compound Pb2(SO4)O. Raman spectrum of the PbS–Nitrate sample features an asymmetric fundamental stretch around 188 cm−1, which can be deconvoluted in two Lorentzian line shape signals as is shown in Fig. 6a. With allowance for the phonon dispersion curves for PbS (Fig. 7), the spectral bands can be assigned to the corresponding phonon modes of this material and their combined tones [25]. The PbS–Nitrate Raman spectrum presents vibrational bands that are observed at 84, 134, 188, 407 and 463 cm−1. The most prominent mode is found at 188 cm−1 near at the frequency of 1LO–PbS in cubic Galena phase [24]. In the same way, the Raman modes corresponding to the low-energy shoulders at 134 and 84 cm−1 originate optical surfaces phonon modes that correspond to the SO and 2SO modes of the PbS. The phononic band observed at 407 cm−1 is associated with the second longitudinal optical mode of the PbS (PbS2LO) [24]. From Fig. 6b can be noticed that the intensity of the PbS1LO-like and PbS-2LO-like bands decrease while 1SO-PbS mode intensity increases significantly, also it is notable the increase of the intensity the band at 452 cm−1 associated to the TO-PbO mode [29]

4. 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 non-saturated double bond S=O introduces the stretching frequency of SO2 in 1390 cm−1 that is strong for Polyethyleneimine and Triethanolamine but weak for Nitrate. The bands generated in the region 2979–3017 cm−1 of the normal vibration mode of the carboxylic group are associated to other vibrational mode in 1440 cm−1 for the ionized carboxylic group as a consequence of the dissociation of the lead acetate, which is the precursor of the lead ions [38]. This behaviour is observed in all films, these bands also can be associated to overtones of the bands in the frequencies 1539 and 1940 cm−1, respectively. The next band at 900 cm−1 shows very intense S-O bonds [38,39] as a consequence of the formation of sulphates and oxy-sulphates for the films synthesised with Polyethyleneimine and Triethanolamine complexing agents. In the last two Raman spectra the band at 1725–1690 cm−1 indexed to the group -C]NH substituted by the dissociation of the Polyethyleneimine [38]. According to molecular characterisation and compositional analysis by FT-IR, Fig. 3 shows the XPS diffractograms of the polycrystalline thin films, which allow identifying in the XPS diffractograms the surface species of the Lead, the Pb(II) cations present in the PbS films obtained with nitrate and Pb(IV) in the PbS films obtained with Polyethyleneimine and Triethanolamine, these oxidation states show a shift towards higher binding energies (see inset in figure) because they are linked to oxygen which is an atom more electronegative than sulphur, and that the partial concentration of oxygen is greater in the chemical reactions in these complexes causing a variation in the particle size according to that nanocrystalline materials have a high surface-volume ratio [18]. The diffractogram in Fig. 4a of the PbS nitrate film shows sharp and intense peaks what fit in the angular positions and intensities of the crystallographic card of the PbS cubic phase, known as galena [JCPDS-ICDD X-ray diffraction card number 05-0592 for cubic phase PbS, n.d.], which has a preferential growth in the (200) direction given by the angular position 2θ=30.074° In addition, there are important growths in the (111), (220) and (311) directions corresponding to the angular positions 2θ=25.963, 43.058, 50.976°, respectively. The X-ray patterns describe a symmetry group Fm-3m (225) whose lattice parameters were calculated using the DICVOL04 software, with an average 7

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coinciding with the Lead-Oxygen interaction of the compound identified by X-ray diffraction (XRD) as Lanarkite. The vibrational modes observed at 542, 582 and 957 cm−1 correspond to phononic replicas. Finally, the Raman spectrum of the PbS-Triethanolamine shown in Fig. 6c features an asymmetric fundamental Raman band around 128 cm−1 that corresponds to the 1SO-PbS mode. At high energy, the shoulder observed at 402 cm−1 is associated with the second longitudinal optical mode of the PbS (2LO-PbS) [24], and the vibrational modes observed at 323 and 465 cm−1 correspond to the TO-PbO2 mode [31] and TO-PbO from the formation of Anglesite (PbSO4) and Lead Oxide (PbO1.57), respectively. Finally, the transmittance analyses of the three nanofilms show that the visible spectral range of the PbS–Triethanolamine film varies about 10% (without considering the substrate contribution) when comparing against the Nitrate and Polyethyleneimine films. In thin films with small grain size, grain boundaries play the role of potential barriers for electrons [44]. Thus, reduction in the transmittance spectrum of the PbS–Triethanolamine thin film against the Nitrate and Polyethyleneimine films shown in Fig. 8 at the wavelengths lower than 1250 nm is given by an increase in the optical band gap, as shown in Fig. 9, which could be attributed to quantised levels in the conduction and valence bands product of the polycrystalline nature of the PbS–Triethanolamine film.

[8] [9]

[10] [11]

[12]

[13]

[14] [15] [16]

[17] [18]

5. Conclusions [19]

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, Polyethyleneimine and Triethanolamine as complexing agents; the crystalline structures identified by X-ray diffraction spectroscopy were pure polycrystalline PbS Galena for Ammonia Nitrate, a mix of Galena phase and a compound that cannot dissociate named Lanarkite, Pb2(SO4)O, for Polyethyleneimine. And finally, using the complexing agent Triethanolamine, PbS Galena, PbSO4 Anglesite and PbO1.57 Lead Oxide phases were found, being the last one a glass, and as such, insensitive to moisture and other contaminants, which agree with the HRTEM results. The lattice vibrations of the Pb-S and PbO interactions are successfully identified by Raman spectroscopy and reflect the effects of the complexing agents like the higher band gap in the PbS-Triethanolamine sample thanks to the crystallinity observed in its Raman spectrum. Thanks to the experimental control in the thin film synthesis, where only the complexing agent parameter is changed, it is safe to assure that this method is safe, economical, environmentallyfriendly and ready for high-scale production of thin films with molecular properties suitable for devices with optoelectronic and biomedical applications.

[20]

[21] [22] [23] [24]

[25] [26] [27]

[28]

[29]

[30]

[31]

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