Introduction

For years, PbI2 have been extensively studied for radiation detector in all forms like: crystal, films, and nanostructure (Dmitriev et al. 2008; Bhavsar and Saraf 2003; Wang et al. 2017; Tan et al. 2017; Lan et al. 2017). PbI2 has impressive optoelectrical properties such as wide energy bandgap (2.3–2.5) eV, which enable device to be operated at room temperature, the excellent light absorption coefficient makes it possible to fabricate an ultrathin device, high resistivity of it suppress leakage current, including high chemical stability and well-organized lamellar structure. The most important feature of PbI2 is its large atomic number (ZPb = 82, ZI = 53), which make it fast and highly sensitive detector (Wang et al. 2017; Liu et al. 2016; Zhong et al. 2016; Zheng et al. 2016). PbI2 is also a vital material for the active matrix flat panel imagers (AMFPIs) that can be used as detectors for X-ray digital radiography using the direct conversion method (Condeles and Mulato 2011; Street et al. 2002; Mulato et al. 2001; Ahmad et al. 2009; Woo and Kim 2012).

Doping has been extensively used to tune the properties of nanomaterial according to the device requirements (Khan and Almohammedi 2017; Babkair et al. 2015; Khan et al. 2011). Usually, dopants incorporate into the crystal lattice and modify the electronic structure of the host materials. Commonly, Cu, Ag (Rybak 2014; Bhavsar 2012), Fe, Ni (Rybak et al. 2005), Al (Mousa and Al-rubaie 2011), Zn (Bhavsar and Saraf 2003), Cs (Shkir et al. 2017), etc. have been used as dopants in PbI2. Recently, considerable interest in the rare-earth (RE) ion-doped wide bandgap semiconductors has emerged due to their success in the technologies of lasers, optical fibers, fluorescent lamps, light-emitting displays, bioscience, and imaging (Shkir et al. 2017; Gu et al. 2003; Bünzli et al. 2007; Kenyon 2002). Among the REs metals, trivalent Neodymium (Nd3+) is one of the extensively used element for high power laser applications (Yu et al. 2007). The electronic configuration of Nd () provides a number of important lasing transition including 4F3/2 → 4I9/2 (900 nm), 4F3/2 → 4I11/2 (1.06 µm) and 4F3/2 → 4I13/2 (1.35 µm laser channel) (Liu et al. 2009). Usually, \(4f \leftrightarrow 4f\)intra-configurational transition is forbidden in REs ion, but when doped into the wider bandgap semiconductors, these forbidden transitions become partially allowed. Furthermore, these ions act as luminescent centers in the bandgap of semiconductors due to their intra-configurational transitions and abundant energy levels (Kumar et al. 2017; Eliseeva and Bünzli 2010; Auzel 2004; Bünzli and Piguet 2005).

In recent years, PbI2 gains a new interest as an important precursor material for perovskite solar cells and the performance of these solar cell is competing with the established thin-film photovoltaic technologies such as mc-Si solar cells, CdTe and CIGS (Ono et al. 2018; Mesquita et al. 2017; Saliba 2018; Saliba et al. 2018; Salado et al. 2018). It is expected that the Nd-doping can further improve the performance of perovskite solar cells using down-conversion process (Chen et al. 2012; Lian et al. 2013; Zhang and Huang 2010). Nd induces near-infrared emission about 900 nm; therefore, when a high-energy UV photons absorbed by solar cells, Nd can convert it in to near-infrared lower energy photons, which can be absorbed by the cell and hence improve the device performance.

Previously, a lot of studies have been reported on Nd-doped ZnO and SnO2, focused on the luminescence, magnetic, electrical, and photo catalytic properties (Bouras et al. 2016; Kumar and Sahare 2012; Rinnert et al. 2012; Shide et al. 2010; Rani et al. 2015; Satpal and Athawale 2018), but the reports on the effect of Nd-doping in PbI2 thin films are absent. Recently, we have synthesized nanorods and single-crystal nanosheets of pure and Nd-doped PbI2 using microwave-assisted method and studied their structural, optical, and dielectric properties (Shkir and AlFaify 2017). These preliminary results indicate the Nd-doped PbI2 as potential material for future photovoltaic devices. Usually, photovoltaic and photodetector devices are made in thin-film configuration. In addition, thin films are having benefit of low temperature crystallization and device size compare to bulk and are easy to integrate in microelectronics. Owing to such tremendous usage of films, it is essential to investigate the optical, electrical, and dielectric properties of Nd-doped PbI2 material in thin-film configuration.

In this paper, we are reporting the preparation of pure and Nd-doped thin films of PbI2 using cost-effective spin-coating method. We have used various concentrations of Nd dopants in PbI2 and investigated the effect of doping concentration on the structural, optical, dielectric, and electrical properties of PbI2 thin films.

Experimental procedure

About 0.3 gm material from previously synthesized pure nanorods (NRs) and 1, 3, and 5% Nd-doped PbI2 nanosheets (NSs) by our group (Shkir and AlFaify 2017) was liquefy in 5 ml N,N-Dimethylformamide (DMF) at 60 °C with continuous stirring for 30 m. Highly transparent solutions with yellow color were attained and subjected to film deposition by spin-coating technique at a constant rate 3000 rpm for 30 s on well-cleaned glass substrates in single layer. Thickness of deposited films was measured to be ~ 100 nm. The current–voltage (I–V) characteristics were recorded in the device configuration shown in the schematic below. For device fabrication, first, we have spin casted the thin film of pure and Nd-doped PbI2 on pre-cleaned FTO substrates using above method, and then, gold (Au) electrodes were deposited by sputtering method.

All-deposited films on glass substrates were tested by Shimadzu LabX XRD-6000 diffractometer, Japan for structural analysis. FT-Raman spectra were measured for vibrational study and SEM (JSM 6360 LA, Japan) was employed to analyze the surface morphology of the prepared films. The more surface studies were also performed and details are provided as supplementary information.

Absorbance (A), transmittance (T), and reflectance (R) were traced using JASCO V-570, UV–Vis–NIR spectrophotometer over 200–2500 nm wavelength region. Optical limiting behavior of films towards two CW lasers of wavelength, i.e., 532 nm and 632.8 nm, has been studied. I–V characteristics of all the prepared devices were recorded on Keithley 4200-SCS system at room temperature.

Results and discussion

Structural and vibrational studies

Figure 1a displays the indexed XRD patterns for pure, 1%, 3%, and 5% Nd-doped PbI2 films. All films are prefrentially grown along (001) plane and belongs to hexagonal phase of 2H-PbI2 Polytypes (JCPDS# 7-0235). The measured data for all films were subjected to refinement process to POWDERX software to get their lattice constants and are provided in Table 1. All parameters are well matched with standard values JCPDS# 7-0235. Tabulated lattice constants are observed to be increased with increase in doping concentration signify the incorporation of Nd in PbI2 lattice. Similar type of observations was reported previously (Shkir and AlFaify 2017). For evaluating the crystallite size (D), density of dislocation (δ), and strain (ε) values, we need full-width at half-maxima (β) and angular positions (2θ) of peaks which are obtained from XRD-600 software. Using these values, D was calculated from Scherrer’s formula (Shkir et al. 2018; AlFaify and Shkir 2019) \(\left[ {D\,=\,\frac{{0.9\lambda }}{{\beta \cos \theta }}} \right]\) and found in range of 21–31 nm. Furthermore, δ and ε values were estimated from (Mohd et al. 2018, 51; Shkir et al. 2017a, b, c) \(\delta \,=\,1/{D^2}\) and \(\varepsilon = \frac{{{{\upbeta }}\cos {{\theta }}}}{4}\), respectively, and found in range of 2.6215–4.1621 (× 10−3) nm−2 and 1.5695–2.0097, respectively. The value of \(\delta\) is reducing with increase of crystallite size indicates lessening of defects in grown films. Texture coefficient (TC) was estimated (Shkir et al. 2018) and is tabulated in Table 1, which specify that the TC is large for (001) direction, and hence, films are oriented along (001) plane and 3% Nd:PbI2 posses larger TC value compare to other films.

Fig. 1
figure 1

a XRD pattern and (b) FT-Raman spectra for Nd:PbI2 films

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Table 1 Estimated lattice constants, D, δ, ε, and TC values for all Nd:PbI2 films
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The measured FT-Raman spectra of all films are displayed in Fig. 1b which further confirmed their same Polytypic nature and phase in accord to XRD analysis. Raman modes in all films are positioned at ~ 68 ± 2, 91 ± 2, 107 ± 2, 160 ± 2, and 212 ± 2 cm−1, and these modes are showing shift towards lower wavenumber compare to pure as well as reported for bulk crystal (Shkir and AlFaify 2017; Sears et al. 1979; Kasi et al. 2007; Wangyang et al. 2016). As 2H-PbI2 ought to 3 atoms/unit cell and hence involve 9 degree of vibrations. Out of them two key modes for symmetric stretching, A1g at ~ 97 cm−1 and twice degenerated, Eg at 74 cm−1 for shearing motion of two I layers (Kasi et al. 2007). However, in the presently grown films, these are positioned at 91 ± 2 and 68 ± 2 cm−1. It can be noticed from these values that they have slight shift owing to transform to nano-dimension crystallites’ development in films. The other modes are twice in energy and assigned as overtones to basics modes (Sears et al. 1979).

EDX/SEM mapping, morphological, and roughness analyses

Figure 2 reveals the EDX spectrum and SEM-elemental mapping image for 3 wt.% Nd:PbI2 film. The existence of Pb, I, and Nd is confirmed in EDX spectrum; moreover, the homogeneous distribution/doping of Nd in PbI2 is exposed in SEM mapping image. Elemental mapping is one of the good tool to confirm the homogeneity of dopant. Figure 3a–d displays the SEM micrographs for all films which will reveal the effect of Nd doping on surface morphology of films and grain size. SMILE View software was employed to measure the grain size formed in films. Morphology of the pure PbI2 film is shown in Fig. 3a, includes nanoparticles of spherical and irregular shapes as well as nanosheets with average grain size of about 71 nm. The spherical nanoparticles along with few nanorods like morphology are revealed in Fig. 3b for 1 wt.% Nd:PbI2 film and the average grain is reduced to 60 nm. Furthermore, when PbI2 films were fabricated with 3 and 5 wt.% Nd doping, the morphology reveals spherical nanoparticles of low dimension viz. ~ 55.5 nm and 54.8 nm, respectively. The grains size attained from SEM images is higher compared to crystallite size (Table 1). The possible reason might be that single grain possess a number of domains of varied orientations that eventually add to bigger size acquire by SEM than XRD. Furthermore, the roughness of films was noticed to be very low (see supplementary data) which is quite comparable with and even less than the previous reports on pure and doped ZnO films (Madhi et al. 2016; Haarindraprasad et al. 2015).

Fig. 2
figure 2

EDX/SEM mapping image for 3 wt.% Nd:PbI2 film

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Fig. 3
figure 3

SEM images for (a) 0.0 wt.%, (b) 1.0 wt.%, (c) 3.0 wt.%, and (d) 5.0 wt.% Nd:PbI2 films

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Optical properties

The measured optical A, T, and R spectra are given in Fig. 4a, b; from it, we can notice that the films are showing low absorption and high transmittance values. The T value for all films is found in range of 85–90% in visible to NIR region, and such high transmittance indicates that these films can be used as a down shifting converter in solar cells. The absorption edge is noticed to be shifted towards lower wavelength [see inset of Fig. 4a] tends to increase in energy-gap value of films due to Nd doping.

Fig. 4
figure 4

Spectra for (a) Abs. and (b) T & R for all Nd:PbI2 films

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Further, the energy gap (Eg) of all films was evaluated with the help of Tauc’s relation: \({(\alpha h\upsilon )^{1/n}}=A(h\upsilon - {E_{\text{g}}})\); here, \(\alpha\) is coefficient of absorption, A is constant, h is Plank invariable, and n is associated with transition progression and taken 1/2 for direct bandgap, Eg.direct. For evaluating Eg.direct from the above relation, we have first calculated \(\alpha\) from: \(\alpha =2.303\frac{A}{t}\); here, t is thickness. Hence, Tauc’s plot is depicted in Fig. 5a as \({(\alpha h\upsilon )^2}\)\(~{({\text{eV}}.{\text{c}}{{\text{m}}^{ - 1}})^2}\) vs. \(h\upsilon ~({\text{eV}})\). In this figure, a line has been drawn to \(h\upsilon ~({\text{eV}})\) axis, where \({(\alpha h\upsilon )^2}=0\). and Eg.direct have been noted for pure, 1, 3, and 5% Nd-doped PbI2 films ~ 2.45, 2.47, 2.50, and 2.58 eV, respectively. The calculated Eg.direct values are increased by Nd doping and are higher compared to bulk PbI2 crystals, i.e., 2.27 eV (Zhu et al. 2007). In addition, these bandgaps are much smaller than we have reported for nanostructured (Shkir and AlFaify 2017) and are in close harmony to the prior reports for thin film (Bhavsar and Saraf 2003; Kasi et al. 2007; Caldeira Filho and Mulato 2011; Ghosh et al. 2008). However, there is another sub-energy gap Eg. which shows reduction from 2.30 eV for pure PbI2 to 2.24 eV for 5% Nd-doped PbI2.

Fig. 5
figure 5

Plots of (a) energy gap, (b) k, and (c) n for all Nd:PbI2 films

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There are another two fundamental parameters which played important role in photonics devices known as: absorption (k) and refractive (n) indices. These parameters were estimated using the equations: \(k=\frac{{\alpha \lambda }}{{4\pi }}\)and \(n=\frac{{(1+R)}}{{(1 - R)}}+\sqrt {\frac{{4R}}{{{{\left( {1 - R} \right)}^2}}} - {k^2}} ~\) (Shkir et al. 2017) and are revealed in Fig. 5b, c as a function of λ, correspondingly. The k values are showing reduction up to 600 nm, and after that, they becomes almost constant up to NIR region, however, varied with Nd-doping concentrations. The n values are also reducing up to 1200 nm and after that becomes stable. The stable value of n is ~ 2 for all films with minute variation and in accord with the previous one (Buckman et al. 1975).

Dielectric analysis

By keeping in mind about the applications of films as capacitive storage, CMOS, ICs, DRAM, etc. (Panda and Tseng 2013; Ren et al. 2000; Moazzami et al. 1992; Chaneliere et al. 1998), we also investigated the dielectric properties of prepared thin films. The real and imaginary parts of dielectric constant, (\(\varepsilon ^{\prime}\) & \(\varepsilon ^{\prime\prime}\)) were evaluated for all films through: \(\varepsilon =\varepsilon '+i\varepsilon ''\), these parameters gives the idea regarding capacitive and resistivity activities in thin films (Usha et al. 2013). Here, \(\varepsilon ^{\prime}\) and \(\varepsilon ^{\prime\prime}\) were assessed using n and k values through (Kim et al. 2012): \(\varepsilon ^{\prime}={n^2} - {k^2}\) and \(\varepsilon ^{\prime\prime}=2nk\), respectively. The estimated respective values of \(\varepsilon ^{\prime}\) and \(\varepsilon ^{\prime\prime}\) vs. hʋ (eV) are plotted in Fig. 6a, b. Figure 6a provides us the value of \(\varepsilon ^{\prime}\), ~ 4 at low energy, however, with increase of energy, its value increasing, and at 2.5 eV, its value is ~ 23, while at 3.6 eV, it becomes ~ 83 for pure PbI2 thin film. However, these values reduced to 10 (at 2.5 eV) and 15 (at 3.6 eV) for 1% and 11 (at 2.5 eV) and 21 (at 3.6 eV) for 5% Nd-doped thin films. Moreover, in 3% Nd:PbI2, these values are higher compared to all other films which are 28 (at 2.5 eV) and 193 (at 3.6 eV). Such variation may be occurred due to various reasons like: films quality, size of grains, disorderness etc. Figure 6b tells the story about \(\varepsilon ^{\prime\prime}\) behavior with hʋ (eV), and it is noticeable from figure that \(\varepsilon ^{\prime\prime}\) has similar behavior to \(\varepsilon ^{\prime}\) and its value is in range of 0.1–1. The \(\varepsilon ^{\prime\prime}\) values are showing the increment with increase of energy for all films. The tanδ values are also revealed in Fig. 6c, which is found in range of 0.01–0.06. Its value is lowest for 3% Nd:PbI2 films and highest for 1% Nd:PbI2 films. The low values of tanδ specify that the films are free from foremost defects.

Fig. 6
figure 6

Plots for (a) \(\varepsilon ^{\prime}\), (b) \(\varepsilon ^{\prime\prime}\), (c) tanδ, and (d) optical limiting of all Nd:PbI2 films

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Optical limiting (OL) study

Optical limiters are highly applicable in protecting the optical sensors which are used in link with lasers and arc welders from damaging the light levels (Tutt and Kost 1992; Wood et al. 1989). Owing to such tremendous usage of optical limiters, we have decided to investigate OL behavior of all films using two laser sources of wavelengths viz. 532 nm and 632.8 nm. On incidence of 532 nm (power = 18.5 mW) and 632.8 nm (power = 368.9 µW), the output power from all films was measured through a power meter and is mentioned in Table 2 and also depicted in Fig. 6d. From figure, it is revealed that the saturated output power from both the lasers is weaken with increase of Nd-doping in PbI2. Such behavior signifies that Nd is taking part in alteration of OL conduct of PbI2 films, owing to change in films quality and crystallinity doped films contains great figure of molecules than pure PbI2 (Holmen and Haakestad 2016; Poornesh et al. 2010). Therefore, the fabricated films of PbI2 with Nd-doping will be useful as optical limiters to protect sensor devices from strong lights.

Table 2 Optical limiting parameters of Nd:PbI2 films
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I–V electrical studies

Figure 7 shows the forward and reverse I–V characteristics of pure and Nd-doped PbI2 thin films in the device configuration shown in Scheme 1. It is observed from figure that the current in forward bias is slightly lower than the reverse bias. This is due to higher work functions of anode (Au, 5.0 eV) compared to cathode (FTO, 4.4 eV), indicating that the electron injection is more convenient from FTO than Au. Moreover, the slope of I–V curves is 1.2, 1.3, 1.3, and 1.6 for the Nd-doping concentration of 0%, 1%, 3%, and 5 wt.%, respectively. As the slope of I–V curve of pure PbI2, thin film is close to 1; thus, ohmic conduction is the dominant charge transport mechanism in these films. The sub-linear relation indicates the presence of traps in the bandgap of semiconductor. Traps are the sub-energy states in the bandgap of semiconductor that limits the motion of charge carriers. Traps are originated from the dislocation or defect present in the crystal structure and also due to the presence of impurities in the materials. The increase of slope with doping concentration is directly related to the increase of trap density in the bandgap of semiconductor. Due to increase of trap density, the charge carriers, who were usually travel to collecting electrodes in pure PbI2, get trapped to these additional trap states and do not participate in the current and result in decrease of current and increase of resistivity of film. The resistivity of the prepared thin films was calculated from the linear part of the I–V curve (0–1 V) through the relation: \(\rho =\frac{{VA}}{{Id}}\), where A is active area and d is film thickness. The calculated resistivity of prepared thin films are: 5.14 × 108 Ω-cm, 9.52 × 108 Ω-cm, 7.97 × 108 Ω-cm, and 1.18 × 109 Ω-cm for doping concentration of 0%, 1%, 3%, and 5%, respectively. The resistivity of pure film is much smaller than the resistivity of the bulk crystal (1013 Ω cm) (Bünzli et al. 2007), and it is comparable to previous value for thin film (Caldeira Filho and Mulato 2011); however, it is about three orders of magnitude smaller than the values obtained by other authors of Refs. (Tan et al. 2017; Kenyon 2002; Yu et al. 2007).

Scheme 1
scheme 1

Schematic of device for I–V measurement

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Fig. 7
figure 7

I–V characteristics of pure and Nd-doped thin films

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Conclusion

In conclusions, we have prepared the thin films of pure and Nd-doped PbI2 and investigated their structural, optical, dielectric, and electrical properties. Good crystalline nature and single-phase confirmation for all films was done by XRD and FT-Raman spectra. Crystallite and grain size are of low dimension for all prepared films and also this signify less roughness of films. The prepared thin films show good optical transparency in range of 85–90% which is quite high suggests their applications as a down shifting converter in solar cells. The values of direct bandgap are from 2.45 to 2.58 eV owing to Nd doping. Moreover, additional bandgap observed which shows reduction from 2.30 to 2.24 eV with the increase of Nd-doping concentration. The stable value of n is found ~ 2 and ε’ value is observed in range of 4–27 and is dependent on energy. The prepared thin films show good optical limiting behavior and can be used to protect the sensor devices from intense light. The resistivity of thin films increases from 5.14 × 108 to 1.18 × 109 Ω-cm for 5% Nd:PbI2 films, and this might be due to increase of traps in doped PbI2 thin films.