Introduction

Semiconductor nanoparticles have attracted much attention because of their interesting electrical and optical properties, originating from quantum confinement effect1. Among the various semiconductor nanomaterials, metal sulphides have received much attention due to their appropriate electronic band gap and bandposition2. So far as application point of view, metal sulfides based devices displayed efficient performances in several areas including fuel cell3, solar cell4, light-emitting diode5, gas sensor6, battery7, supercapacitor8, thermoelectric9, dielectric10 and memory11 applications.The performance of the devices predominantly depends on the nano architecture of the metal sulfides.

In this context, lead sulfide, a binary (IV–VI) semiconductor material, with moderately small bandgap and large exciton Bohr radius (18 nm), stand out as promising material that has been successfully used for different applications such as infrared sensors12, solar cells13, light-emitting diodes14, lasers15 and biological imaging16 due to its controllable size with variety of morphologies. Different morphologies, such as rod and cube17, spherical and dendritic18, wire19, and star20 shaped lead sulfide has been reported, at nanometer to micrometer scale, by several synthetic methods, such as, microwave, hydrothermal, solvothermal, and chemical or thermal decomposition, under different reaction conditions (precursor varities, temperature, time, solvent and stabillizer)17,21,22.

Nanocrystalline lead sulfide also exhibited the catalytic performance for the synthesis of amidoalkyl-naphthols under solvent-free conditions23 and for the chemical reduction of p-nitroaniline24. The narrow bandgap, high electron mobility and excellent chemical stability enable the lead sulfide as a promising photocatalytic material25. The lead sulfide nanoparticle initiated photocatalytic degradation of bromothymol blue has been reported due to electron–hole pair generation mechanism26.

Employing the band gap engineering, using different ligands, the electrical property of lead sulfide based field-effect transistor showed the transformation from ambipolar to strong n-type behaviour27. The colloidal lead sulfide quantum dots also contribute to the improvement of solar cell efficiencies through better carrier extraction28. The dopped lead sulfide nanocrystal experienced an improvement of optical properties due to charge injection29. Optical properties of lead sulfide, based on theoretical calculation, exhibited good reflection and absorption for ultraviolet electromagnetic waves, suggested a potential candidate for photoconductive devices in ultraviolet range30. Lead sulfide exhibited immense potential in microelectronics application. The lead sulfide nanocrystal, with cubic symmetry, exhibited low dielectric constant and dielectric loss over a wide range of temperature and frequency conditions31. Nanoparticles of lead sulfide grown within the pores of polyvinyl alcohol matrix exhibited the moderate value of dielectric constant at higher frequencies32. It has been reported that lead sulfide nanoparticles, doped with strontium33 or cerium34, displayed an improved dielectric and electrical characteristics, compared to the pure lead sulfide. Two dimensional cubic phase lead sulphide nanosheets, synthesized using solid state reaction method, exhibited high values of dielectric constant and have potential as capacitive storage device35.

Lead sulfide with cubic structure is common in literature, Table 1, however the formation of monoclinic lead sulfide is relatively rare36,37,38. In this study, a complexation mediated wet chemical route was applied for the synthesis of fluoroaniline stabilized monoclinic lead sulfide nanoparticles. The dielectric capacitance, energy density, AC-conductivity and field driven polarization performances of the synthesized material was investigated in form of a device under varying temperature and frequency conditions. A comparative study was also performed on the device based on aniline stabilized lead sulfide system. A mechanism based on the performances of the two devices is also discussed in this manuscript.

Table 1 The synthesis methodology, structure and physical properties of some of the reported lead sulfide based system.
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Result and discussion

A complexation mediated route was applied for the preparation of lead sulfide nanoparticles, where 4-fluoroaniline was served the role of a ligand and also as a stabilizer of the particles. The detail NMR characterization of the ligand and products are available in the supporting information (Table S1 and Figs. S1, S2). The TEM images, Fig. 1A,B, with different magnifications, of the fluoroaniline stabilized lead sulfide displayed the spherical shaped lead sulfide particles (dark spots) within the size range of 8–12 nm, presented using a histogram, Fig. 1A, inset. The selected area electron diffraction (SAED) pattern of lead sulfide displayed in Fig. 1B, inset, obtained from the groups of particles.

Figure 1
figure 1

The transmission electron microscope images of the fluoroaniline stabilized lead sulfide nanoparticles with different magnifications (A, B). The inset image in (A) represent the size distribution of the particles and the inset image in (B) is the selected area electron diffraction pattern of lead sulfide.

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The X-Ray diffraction pattern of lead sulfide nanoparticles was recorded within the range (2θ) from 20° to 80°, Fig. 2A. The overall diffraction pattern was matched according to the ICSD: 68712, which belongs to the monoclinic phase of lead sulfide with the space group of P1. The monoclinic structure, with the lattice constant values, a = 6.00 Å, b = 6.02 Å, c = 22.43 Å and α = γ = 90°, β = 95.4°, accompanied with a large unit cell volume of 808.54 Å3. Figure 2A, inset, represents the three dimensional unit cell structure of lead sulfide, elongated along the c-axis. In lead sulfide, with the atomic arrangement of lead and sulphur along the 8-asymmetrical lattice sites in the structure36,37,38. The details positional coordinates of lead sulfide system is mentioned in Table S2, supporting information.

Figure 2
figure 2

(A) X-ray diffraction pattern of lead sulfide (black line) was measured within the range of 20–80°. The diffraction pattern was indexed according to ICSD: 68712 (red colour bar) of the monoclinic structure. The inset figure shows the unit cell of lead sulfide projected along a-axis. (B) The deconvoluted Raman spectrum (green colour), within the range from 40 to 900 cm−1, with Raman active vibrational modes (ν1–ν9).

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The local structure of lead sulfide (PbS) nanoparticles was investigated using Raman spectroscopy technique. The Raman spectrum (Fig. 2B), was deconvoluted (green colour peaks) into nine Raman active vibrational modes, from ν1 to ν9, within the range from 40 to 900 cm−1. The spectrum consists of strong intensity vibrational modes (ν1 and ν2), diffuse modes (ν3, ν4, ν5 and ν8) and broad intensity modes (ν6, ν7 and ν9). Raman active modes positioned at ν1 ~ 81 cm−1 and ν2 ~ 137 cm−1 along with a shoulder like feature at ν3 ~ 184 cm−1, originated from the longitudinal optical phonon vibration of Pb–S phase39. The diffuse modes ν4 ~ 216 cm−1 and ν5 ~ 254 cm−1 are associated to the symmetrical vibration of nonlinear S-Pb–S chain and the radial vibration of the of (PbS)n cluster, (n = 1–9), respectively, in the monoclinic structure40. The broad peak at ν6 ~ 328 cm−1, combined with smaller peaks at ν7  ~ 410 cm−1 and at ν8  ~ 470 cm−1, related to the various stretching modes of S–S bond in the structure41. The vibrational mode observed at ν9  ~ 546 cm−1 correspond to the phononic replicas of Pb–S phase39.

Figure 3A,B show the distribution of local density of states (LDOS) of lead sulfide. The structure illustrated the s and p-orbital contribution in the density of states (Fig. 3A) and the atomic density of states of Pb and S (Fig. 3B). The effective distribution of the states concluded that the valance band edge has p-orbital like character and typically localized with the S-atom, whereas the Pb-atom contribute more towards conduction band42. The band structure of lead sulfide was obtained by density functional theory calculation (Fig. 3C), that supported the semiconductor nature of the material with a direct band gap of 2.27 eV. Figure 3D represents the deformation charge density of the lead sulfide structure. The blue part represents the gain of electrons (electron population), and the red part symbolizes the loss of electrons (electron reduction).

Figure 3
figure 3

The distribution of local density of states of the lead sulfide. (A) The s and p orbital density of states and (B) the atomic, Pb and S, density of states. (C) Band structure of lead sulfide. (D) Deformation charge density of lead sulfide. (The Cambridge Sequential Total Energy Package, CASTEP, was used to optimize the geometry and calculating local charge and electronic properties, Supplementry information).

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The qualitative analysis of the chemical states of the fluoroaniline stabilized lead sulfide nanocrystal was explored by X-ray photoelectron spectroscopy (XPS) technique. The survey spectrum (Fig. 4A), exhibited the presence of elemental peaks of C, N, O, F, S and Pb. The N (1 s) and O (1 s) peaks are positioned at 399.5 and 529.8 eV, respectively. The carbon, nitrogen and fluorine signals are originated from the organic matrix (fluoroaniline) and the O (1 s) signal is developed due to the surface oxidation of the sample. The binding energy peaks originated from the Pb 5d5/2, Pb 4f, Pb 4d5/2, Pb 4d3/2, and Pb 4p3/2 are also visible in the survey spectrum43. The high resolution Pb 4f spectrum (Fig. 4B) shows spin–orbit splitting of Pb (4f7/2) and Pb (4f5/2) with the peaks positioned at 139.1 eV and 143.8 eV, respectively, associated with the lead sulfide component44.

Figure 4
figure 4

(A) The XPS survey spectrum of fluoroaniline stabilized lead sulfide. High-resolution XPS spectrum of (B) Pb 4f, (C) S 2p, (D) C1s and (E) F1s.

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The deconvoluted of Pb 4f doublet peaks also display two additional small peaks positioned at 138.1 eV and 143.1 eV, originated from the surface oxidation of lead sulfide with the formation of lead sulfite (PbSO3) like species, under the exposure of aerial oxygen44. The high resolution S 2p core level spectrum, after deconvolution, Fig. 4C shows two peaks located at 161.5 eV and 162.7 eV, correspond to S (2p3//2) and S (2p1//2), for the sulfide ion of lead sulfide44. The formation of lead sulfite species, due to the surface oxidation of lead sulfide, was also evidenced in the spectrum, with the binding energy values positioned at 163.3 eV and 167.7 eV, also reported in other lead sulfide systems44. The values of the binding energy were calibrated utilizing C 1 s peak at 285.1 eV, as the internal standard, associated with the carbon from the aromatic ring (Fig. 4D). Other C 1 s peak with the binding energy value of 286.6 eV related to -C-F interaction form fluoroaniline45. Further, after deconvolution of the F1s spectra (Fig. 4E), two peaks are developed. The peak with the binding energy value of 689.6 eV is originated from the fluorine in fluoroaniline molecule and the other peak at 687.5 eV associated with lead-fluorine interaction from the neighbouring fluoroaniline molecule.

Figure 5A shows the temperature (from 30 to 120 °C) and frequency (from 100 Hz to 1 MHz) dependent dielectric constant (εʹ) of FALS based device. At 30 °C, the value of dielectric constant (εʹ) ~ 800 was achieved under the frequency condition of 100 Hz. With increasing the frequency, the dielectric constant decreased gradually and attained a stable value of εʹ ~ 360 at 1 MHz. At 120 °C, a substantial rise of the value of dielectric constant (εʹ) ~ 2000 was observed at the frequency of 100 Hz, which was also decreased gradually towards higher frequencies. The capacitive behaviour of the lead sulfide was associated to the dipolar type of polarization. The dipoles are formed due to the separation of bound charges in the form of Pb2+ cation and S2− anion, under the applied electric field. The structural analysis showed the deformation of charge density in the lead sulfide system and the charge transfer is directed towards the high electronegative sulphur atom. This kind of charge separation caused the formation of dipole moments. At lower frequency region, the induced dipoles are effectively followed with the applied field and attained a maximum value of dielectric constant. With increasing frequency, the dipoles are not able to respond with the field direction, which caused the decrease of dielectric constant values. The capacitive behaviour of the FALS based device was associated with the dipolar type of polarization (originated from the lead sulfide and highly polarised fluoroaniline molecule) and the space charge or interfacial polarization. The hydrogen atom of -NH2 group in fluoroaniline forms intermolecular hydrogen bond with the neighbouring fluorine of fluoroaniline, resulted high negative charge density (Scheme 1). This interface creates space charge within the system, which prompted further increase of dielectric constant value. In addition to that, the defect site of the nanoparticles and the organic–inorganic interface create the space charge within the system10,46,47. The space charge polarization is predominant towards the lower frequency range. The temperature effect on the dielectric constant of the device was associated with the loss factor of the material, as evidenced from the tan δ curve (Fig. 5A, inset). The rise in the tan δ curve with increasing the temperature implies that the space charges are thermally activated under the longer response time.

Figure 5
figure 5

(A) Dielectric constant (ε′) of FALS based device measured at 30, 60, 80, 100 and 120 °C under the frequency range from 100 Hz to 1 MHz. The variation of tan δ curves for selected temperatures, inset. (B) Graphical representation of capacitance as a function of frequency under applied voltage conditions. (C) 1/C2–V plot under selected frequency conditions. (D) and (C) represent the variation of carrier concentration (ND) and barrier potential (ϕB) as a function of frequency, for FALS based device.

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Scheme 1
scheme 1

The schematic diagram of aniline and fluoroaniline stabilized lead sulfide nanoparticles. Hydrogen bond formation in fluoroaniline stabilized lead sulfide system.

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The dielectric polarization of the device was further analysed in term of capacitance as a function of frequency, under a series of voltage conditions (from 20 to 100 V) (Fig. 5B). From the figure it is evident that at lower frequency and lower voltage the change of capacitance is more favourable. At low voltage condition the polarization of the dipoles become facile. With increasing external voltage, the dipoles of the lead sulfide are aligned along the field direction with a restriction of the reversible polarization that resulted the decrease of capacitance value. Figure 5C exhibited the graphical representation of the inverse of capacitance (1/C2) as a function of voltage (V) under the frequency condition of 500 Hz, 10 kHz, 100 kHz and 1 MHz. The slope of the plots suggests that the negative charge carriers are dominant in the lead sulfide system27,28. The variation of carrier density (ND) and barrier potential (ϕB) with respect to the frequency is plotted in the Fig. 5D,E, respectively. The increasing value of carrier density with frequency originated from the localized electronic state of the material with decreasing the value of barrier potential48,49. The dipolar contribution, originated from the bound charges of lead-sulfide system, is responsible for the localized deformation of electronic structure under applied frequency conditions.

The maximum energy storage density (Emax) of the FALS based device was calculated under different DC-bias condition using the equation, \({E}_{max}=\frac{1}{2} C{V}_{M}^{2}\) , where C represent the DC capacitance and VM is the applied voltage. The Emax value of ~ 0.10 mJ/g was achieved at 100 V (under 100 Hz frequency condition) and the Emax values decreased with increasing frequency (Fig. 6A). The variation of Emax is more prominent towards lower frequencies (below 1 kHz) under high voltage condition (Fig. 6B). The equivalent series resistance (ESR) of the device was extracted from AC-impedance measurement, which exposed an insignificant variation under different bias conditions (Fig. 6C) (in log–log scale). The circuit representation of the device Fig. 6D), can be sketched by assuming that the stored energy was discharged through a load resistance \({(R}_{load})\), connected in parallel with a capacitor (C) and a resistor (R). The product of capacitance and resistance is the time constant (t) and the ratio of energy density to time constant (RC) gives the power density, which is a gravimetric power density that determine the minimum time required to discharge the stored energy of the device. The maximum power \({(P}_{max})\) delivered to the load resistance can be expressed as \({P}_{max}={E}_{max}/2RC\). The calculated RC values were obtained 1.5 ms and 16 μs with the gravimetric power density values of 30 W/g and 340 W/g, at 100 Hz and 10 kHz, respectively, for the lead sulfide based device. A table of comparison based on the previously published results on gravimetric power density values for other organic–inorganic hybrid systems is mentioned in the supporting information, Table S3, as a ready reference.

Figure 6
figure 6

(A) Variation of energy density (Emax) with respect to frequency, under DC-voltage, (B) energy density (Emax) as a function of voltages under different frequency condition, (C) equivalent series resistance (ESR) as a function of frequency under different voltage condition (in log–log scale), (D) equivalent circuit representation for the energy and power transfer model at a certain voltage (VM), for the FALS based device.

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The mobility of the charge carrier of the FALS based device was analysed in terms of AC-conductivity as a function of frequency for 30, 80 and 120 °C (Fig. 7A). Figure shows that the AC-conductivity is directly proportional to the applied frequency and also with the temperature. At 100 Hz, the conductivity values are 2.1 × 10–4 S.m−1 and 2.3 × 10–3 S.m−1 at 30 and 120 °C, respectively. The conductivity curves, for each temperature, has two regions, designated as region (I) and region (II), marked in the Fig. 7A. All the conductivity curves are finally merged towards the region (II). The region (I) exhibits a slow progress of conductivity with increasing frequency, on the other hand, the conductivity increases sharply with increasing the frequency, in the region (II). The conductivity behaviour was analysed in term of Jonscher’s universal power law,\({\sigma }_{ac}= {\sigma }_{dc}+A\left(T\right){\omega }^{S},\) where, σdc is the DC-part of the conductivity, A is the temperature dependent coefficient of the system and \(S\) is the temperature dependent parameter that determine the conduction mechanism of the charge carriers. In the low frequency region (I), the slow progress of conductivity with frequency, indicates few trapped charges are available for the conduction process, whereas the region (II) specify that large number of trapped charges are available in the conduction process. Several models have been proposed to study the conduction mechanism of the charge carriers50. It is reported that the polaron and bipolaron types of charge carriers are responsible for conduction mechanism in hybrid material51. The hybrid system of metal sulfides and metal halides demonstrated the formation of polaronic charge species that control the hopping and tunnelling type of transport mechanism47,52. The conductivity curves were fitted using Jonscher’s power law for the regions (I) and (II) (Fig. 7A, red line), and the extracted S(I) and S(II) parameters were plotted in Fig. 7A, inset. The values of S(I) and S(II) are decrease with increasing temperature, suggested the hopping conduction of polaron was involved in the lead sulfide based device system. The variation of AC-conductivity (\({\sigma }_{AC}\)) with temperature (1/T), described by Arrhenius equation, \({\sigma }_{AC}\left(T\right)={\sigma }_{0}\mathrm{exp}\left(-{E}_{a}/{K}_{B}T\right),\) \({\sigma }_{0}\) is pre-exponential factor and \({E}_{a}\) is activation energy, was measured at the frequency of 10 kHz. The Ea value was obtained ~ 45 meV, extracted from the linear fit of the equation (Fig. 7B). The small Ea value recommended the polaron mediated conduction within the system53.

Figure 7
figure 7

(A) AC-conductivity behaviour of the FALS based device at 30, 80 and 120 °C, measured under the frequency range from 100 Hz to 1 MHz (fitting was done according to the Jonscher’s power law, red line). The inset figure shows the variation of S1 and S2 parameters, extracted from the region (I) and (II), respectively. (B) Arrhenius plot of AC-conductivity as a function of inverse temperature (1/T), at 10 kHz.

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Figure 8A shows the polarization–electric field (P–E) hysteresis loop for the FALS based device, measured under 1, 2 and 4 kV/mm, field conditions. The dipolar type of polarizability of lead sulfide was responsible for the formation of hysteresis loop. The unsaturated pattern of the loops was associated with the space charge contribution in the system54. A steady polarization hysteresis loop was observed for the device at ± 4 kV/mm (Fig. 8B), and maintained the maximum polarization value within the range from 1.1 to 0.99 μC/cm2 for 103 cycles (Fig. 8C). The polarizability of the covalent bond of the lead sulfide system was increased with increasing applied electric field and reached to the maximum polarization (Pmax) value of 1.6 μC/cm2 at 5 kV/mm (Fig. 8D). The dielectric breakdown strength of the device exhibited the tolerance up to 5.2 kV/mm of electric field, without any electrical breakdown and above that field the device experienced an electrical breakdown after 50 s (Fig. 8E).

Figure 8
figure 8

(A) Hysteresis loop, polarization vs electric field, of the FALS based device, measured at 100 Hz. (B) Hysteresis loop of the device under 4 kV/mm field condition, for 103 cycles. (C) Endurance behaviour of the device (in terms of maximum polarization, Pmax) for 1000 cycles, at 4 kV/mm. (D) Increment of maximum polarization (Pmax) values with respect to the applied electric field. (E) Dielectric breakdown strength (kV/mm) of the device as a function of time, measured at a scan rate of 200 V/s. Variation of (F) dielectric constant (ε′) and (G) polarization vs electric field, hysteresis loop of ALS and FALS based device.

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A comparative study was performed between aniline and fluoroaniline stabilized lead sulfide based devices to investigate the role of fluorine. A similar synthesis method, like FALS, was applied for the preparation of aniline stabilized lead sulfide nanoparticles (ALS).

The aniline stabilized lead sulfide (ALS) and fluoroaniline stabilized lead sulfide (FALS) exhibited the dielectric constant values (ε′)  ~  376 and ~ 800, respectively, at 30 °C under 100 Hz frequency condition (Fig. 8F). The graphical representation of polarization as a function of electric field exhibited the hysteresis loop with the maximum polarization values 0.29 and 0.46 μC/cm2 for ALS and FALS based devices, respectively, at 2 kV/mm (Fig. 8G). High dielectric constant value of FALS was associated with the dipolar type of polarization and space charge polarization. In FALS, the dipolar type of polarization was created from the lead sulfide and polarised fluoroaniline molecule, whereas in ALS only the dipoles of lead sulfide are responsible for polarization.

The hydrogen atom of the –NH2 group forms intermolecular hydrogen bonding with the fluorine of FALS (Scheme 1) resulted high negative charge density. This interface procedures additional space charge in the system, which prompts further enhancement of the dielectric constant value in FALS based device. The increased area of the hysteresis loop and maximum polarization value of the FALS based device as compared with ALS based device were originated due to the above reasons.

Lead sulfide, with different size, shape and morphology, have been extensively investigated because of their unique optical and electronic properties, which endows the various potential applications. The electrical and physical properties of some reported lead sulfide based systems are mentioned in Table 131,32,33,34,42,55,56,57,58,59,60,61.

Conclusion

A complexation mediated wet chemical route was applied for the preparation of fluoroaniline stabilized monoclinic lead sulfide nanoparticles. The sizes of the synthesized particles are in the range of 8–12 nm, as evidenced from the microscopy analysis. The Raman spectrum analysis showed the different vibrational modes associated with lead sulfide. The X-ray photoelectron spectroscopy analysis confirmed the formation of lead (II) sulfide. The fluoroaniline stabilized lead sulfide based device exhibited the dielectric constant value of ~ 2000 at 120 °C at 100 Hz. The capacitive behaviour was associated with the dipolar type of polarization and the space charge polarization. The AC-conductivity values of the device were obtained 2.1 × 10–4 S m−1 and 2.3 × 10–3 S m−1 at 30 and 120 °C, respectively, at 100 Hz and the conductivity mechanism was followed by the hopping conduction of the polarons. The gravimetric power density values of 30 W/g and 340 W/g, at 100 Hz and 10 kHz, respectively, were achieved for the device. The device showed the maximum polarization value of 1.6 μC/cm2 at 5 kV/mm. A quasi-stable maximum polarization value in the range of 1.1–0.99 μC/cm2 was noted at 4 kV/mm, for 103 cycles. Based on the above mentioned performances we could assume that the device has the potential as a pulse capacitor in microelectronic application.

Experimental section

Materials

Analytical grade chemicals (lead nitride, sodium sulfide and 4-fluoroaniline) were used in this study without further purification.

Synthesis of lead sulfide nanoparticles

In a typical experiment, 1.0 mL of 4-fluoroaniline was diluted in 10 mL of methanol in a 50 mL conical flask. An aqueous solution of lead nitrate (10 mL) with the concentration of 0.1 mol dm−3 was added to the diluted 4-fluoroaniline. A white precipitation of Pb(II)-fluoroaniline complex was formed immediately. To this precipitation, 5 mL of Na2S solution (0.1 mol dm−3) was added and instant change of colour of the precipitation in to black indicate the formation of fluoroaniline stabilized lead sulfide, FALS. The solid material was collected through filtration and dried under vacuum condition. The electrical properties of the synthesized materials were measured in the form of a device. The material characterization and device fabrication methods are available in the supporting information. The conditions set for computational analysis is also available in the supporting information.