Exploring the weak visible-near-infrared and NO2 detection capabilities of PbS/Sb2O5 heterostructures with DFT interpretations

The need for photosensors and gas sensors arises from their pivotal roles in various technological applications, ensuring enhanced efficiency, safety, and functionality in diverse fields. In this paper, interlinked PbS/Sb2O5 thin film has been synthesized by a magnetron sputtering method. We control the temperature to form the nanocomposite by using their different nucleation temperature during the sulfonation process. A nanostructured PbS/Sb2O5 with cross-linked morphology was synthesized by using this fast and efficient method. This method has also been used to grow a uniform thin film of nanocomposite. The photo-sensing and gas-sensing properties related to the PbS/Sb2O5 compared with those of other nanomaterials have also been investigated. The experimental and theoretical calculations reveal that the PbS/Sb2O5 exhibits extraordinarily superior photo-sensing and gas-sensing properties in terms of providing a pathway for electron transport to the electrode. The attractive highly sensitive photo and gas sensing properties of PbS/Sb2O5 make them applicable for many different kinds of applications. The responsivity and detectivity of PbS/Sb2O5 are 0.28 S/mWcm−2 and 1.68 × 1011 Jones respectively. The sensor response towards NO2 gas was found to be 0.98 at 10 ppb with an limit of detection (LOD) of 0.083 ppb. The PbS/Sb2O5 exhibits high selectivity towards the NO2 gas. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to analyze the geometries, electronic structure, and electronic absorption spectra of a light sensor fabricated by PbS/Sb2O5. The results are very analogous to the experimental results. Both photosensors and gas sensors are indispensable tools that contribute significantly to the evolution of technology and the improvement of various aspects of modern life.


Introduction
The scientific community and the Industrial Revolution are projected to be greatly impacted by nanotechnology, which has been hailed as a breakthrough technology.This is mostly because of the unique features of nanomaterials brought by size reduction, which may allow for the development of a new Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. generation of devices that have ultra-high detection capability at room temperature [1][2][3][4].The integration of silicon-based photonics with microprocessor chips has made great strides in recent years [5][6][7].Researchers are becoming increasingly interested in low-power, small-scale germanium photodetectors that can be integrated with silicon transistor technology as a crucial part of optical interconnection, as predicted by Moore's law in photonics.Image sensing, optical communication, and nighttime surveillance are just a few of the many potential uses for broadband photodetectors [8].To create high-performance infrared photodetectors, many different materials were investigated.Lead chalcogenide quantum dots (QDs), such as PbS, have also received a lot of attention because of their high infrared absorbance, tunable band gap via size modification, and excellent compatibility with low-cost solution-based processing methods [9][10][11].Some theoretical model have also been developed to analyze the electronic properties of the nanomaterials [12][13][14][15].PbS QDs typically have poor performance because of their low charge carrier mobility.Therefore, combining PbS QDs with a high-mobility material is a common strategy to improve device performance.
PbS/Sb 2 O 5 is a promising material system for NO 2 gas sensing applications due to its exceptional chemical sensitivity and electrical properties.Lead sulfide (PbS) is a semiconductor with a narrow bandgap, making it highly sensitive to changes in ambient conditions.When combined with antimony pentoxide (Sb 2 O 5 ), PbS/Sb 2 O 5 heterostructures exhibit improved gas sensing capabilities, particularly for nitrogen dioxide (NO 2 ) detection [16][17][18].The unique properties of PbS/Sb 2 O 5 , including a large surface area, high electron mobility, and strong chemical interactions with NO 2 molecules, allow for efficient adsorption and detection of NO 2 .This material system can be employed in gas sensors to monitor and quantify NO 2 concentrations in various environmental and industrial settings, making it a valuable tool for air quality assessment and safety applications.
Although antimony oxides and sulfides showed promise as thin film capacitors, research into these materials was limited.Most of the limited research on Sb 2 O 5 and Sb 2 O 3 was performed with antimony electrodes that had been passivated in various electrolytes [19].The majority of these studies focused on determining the parameters of film growth and the dissolution kinetics of the anodic oxide film that was formed on antimony in various electrolytes.Based on photo-polarization and photo-resistance measurements, the semiconducting parameters and photoelectrochemical properties of oxide films formed galvanostatically on antimony up to high polarization potentials were studied [20,21].
Sb 2 O 5 (antimony pentoxide) is not commonly used as a standalone material for photodetectors.However, it can be incorporated into composite structures or used in combination with other materials to form heterojunctions or thin films for photodetection applications.Antimony pentoxide has a wide bandgap of around 2.2-2.7 eV, depending on the crystal structure and synthesis method [22,23].This wide bandgap makes it transparent to visible light but sensitive to ultraviolet (UV) light.When Sb 2 O 5 absorbs UV photons, electron-hole pairs (excitons) are generated in the material.To improve the performance of Sb 2 O 5 -based photodetectors, researchers often combine it with other materials to enhance their light absorption and charge transport properties.For example, Sb 2 O 5 can be used as an active layer in a heterojunction structure, where it is paired with materials like TiO 2 or ZnO.These heterojunctions can provide efficient charge separation and transport, leading to enhanced photodetection performance.In addition, Sb 2 O 5 can also be used in thin-film form by depositing it onto a substrate using techniques like physical vapor deposition (PVD) or chemical vapor deposition (CVD) [24][25][26][27].Thin films offer advantages such as large surface area and flexibility, which can be beneficial for certain photodetection applications.Overall, while Sb 2 O 5 alone may not be commonly used as a photodetector material, it can be combined with other materials or incorporated into composite structures to create efficient photodetection devices, particularly for UV light detection.
In this paper, PbS incorporated with Sb 2 O 5 has been synthesized by using a simple chemical method.After combining the matrix of the PbS and Sb 2 O 5 the photodetection capability was increased.Furthermore, by utilizing PbS and Sb 2 O 5 as the components, the PbS/Sb 2 O 5 heterostructure was successfully fabricated on the interdigital gold electrode.These findings show that PbS/Sb 2 O 5 photodetectors are extremely attractive candidates for future applications requiring large-area, high-sensitivity, high-speed, and broadspectrum photoresponses.Furthermore, a theoretical model has been developed by using the crystal structure of the PbS/Sb 2 O 5 .The results show that the ionization enthalpy and electron affinity increase after introducing Sb 2 O 5 in PbS.

Materials and method
Sulfur powder with a purity >99.5% has been purchased from Alfa Aesar.Acetone and ethanol having >99.8% purity has been purchased from ECHO.
PbS/Sb 2 O 5 heterostructures has been prepared by the combination of sputtering and sulfonation process.The PbSb gun was used as a source for the deposition.In a typical process, a highly cleaned glass substrate was placed in the center of the sputtering machine.PbSb film of thickness 125 nm has been deposited by using a magnetron sputtering method as shown in figure S1.After that, oxygen was introduced inside the chamber and annealed for 10 min the PbSb was converted into PbSbO 6 .Furthermore, the film undergoes the sulfonation process.In which, the thin film has been placed in a closed chamber and pure 1 g sulfur powder has been introduced into the chamber.Then the chamber temperature and pressure increase to 430 °C and 10 TPa respectively for 1 h.During this process, the sulfur particles get decomposed and get attached to the nucleation sites of the PbSb film as shown in figures S1 and S2.Furthermore, the film was characterized and used as a photodetector.

Fabrication and measurement of photodetection
In this process, the fabricated thin film was annealed at 100 °C for 2 h.Furthermore, the film has been placed in the RF sputtering chanmbed in which the gold electrode has been deposited on the corner of the thin film as shown in figure S5.Moreover, the thin film was placed in the photo-sensing chamber for further measurement.The photo sensing measurement has been done by using our homemade photo sensing setup as shown in figure S6.The setup contains a Keithley source meter 2400, an LED light source, a power supply and a monitoring screen for recording the data.The change in the resistance and IV measurement at the different light intensities with a variation of power has been measured by using Keithley electrometer 2400.All the photodetection measurements were taken at room temperature (300 K).During the sensing measurement, the temperature was held at 27 ± 2 °C and relative humidity was maintained at 65%.As a measure of a photosensor's sensitivity to light, 'photoresponsivity' (R) is calculated using the formula: R = I / PA, where I is the photocurrent, P is the power of incident light, and A is the effective area of the photosensor.The detectivity of the photosensor has been calculated by using the formula: Similarly, the linear dynamic range (LDR) and noiseequivalent power (NEP) have been calculated by using equations ( 5) and (6) respectively.
Furthermore, The gas sensor response has been calculated by using the ratio of resistance in air with the resistance in gas.
Where R g is the resistance in the presence of gas and R a is the resistance of thin film in air.

Computational method
The entire calculations for the molecule were performed by density functional theory (DFT) with B3LYP/LANL2MB method.All the molecules were designed by using the Gaussian 16 software package [28].Gaussview 6.0 program [29] was used to get a graphical representation and the pictorial visualization of the calculated data.The density of the state has been plotted by using Gaussum 3.0 software [30].We have computed TD-B3LYP/LANL2MB level of theory because we designed the photosensor by using PbS/Sb 2 O 5 .

Characterization details
The synthesized PbS/Sb 2 O 5 heterostructures has been analyzed using HR-XRD diffractometer using CuKα radiation in the range of 20-80°.High-resolution tunneling electron microscopy (HRTEM) has been used to analyze the surface morphology of the sample.The lattice fringes and the crystallite peak have been analyzed by using HRTEM and x-ray photoelectron microscopy (XPS).

Result and discussion
In a typical synthesis process, after the sulfonation process, the PbSb has been converted into the PbS/Sb 2 O 5 as shown in supporting information.The synthesized product was first analyzed by using HRTEM, in which the image was taken by the various stages of crystal formation.Figures 2(a), (b) shows the as-grown PbSb film by using the RF sputtering method.The film further undergoes the sulfonation process in which the PbS/Sb 2 O 5 has been formed.Figures 2(c), (d) shows the lattice fringes of PbS having an interplanar spacing of 0.297 nm and 0.342 nm corresponding to (200) and (111) planes respectively.Furthermore, the (101) and (202) planes correspond to Sb 2 O 5 respectively as shown in figure 2(f).Furthermore for analyzing the elemental composition the EDX has been preformed and the data has been illustrated in figure S7.
The crystallite plane of the thin film has been confirmed by the XRD analysis and depicted in figure 3(a).The results reveal that the crystallinity of the material increased after the annealing process.The peaks at 24.8, 31.variation in the photocurrent density of the thin film at various wavelengths of light has been depicted in figure 5(a).The LED of different wavelengths has been used as an optical source in this experiment.The thin film-based device reacted to all wavelengths of light by producing a higher current than in the dark.In the thin film, light with photon energy greater than the tiny direct bandgap of PbS/Sb 2 O 5 may efficiently create hole-electron pairs.A basic metal-semiconductormetal structure made up of two Schottky barrier contracts placed back-to-back may be presented as the structure of this PbS/Sb 2 O 5 thin-film device.The pairs in the PbS/Sb 2 O 5 thin film split apart under an electric field, producing photocurrent.The rise and decay time of PbS and PbS/Sb 2 O 5 has been calculated by using the exponential fitting and illustrated in table 2.
A dark current (I dark ) may flow through a photo sensor in the absence of light and with a bias applied.As the photosensor is exposed to light, photon absorption creates more free charge carriers, which decreases the electrical resistance of the semiconductor and forms electron-hole (e-h) pairs.The photo-generated e-h couples are split by the applied voltage, which causes them to drift in opposing directions toward the metal leads, resulting in a net increase in current (I photo ).The generation of photocurrent is caused by the separation of e-h couples.3.
Upon conducting an in-depth analysis of the photo-sensing properties of the thin film, it was subsequently subjected to gas-sensing measurements.The collected sensing data revealed a notable and rapid variation in the resistance of the thin film upon exposure to NO 2 gas, as visually depicted in figure 6(a).Particularly, the most significant alteration in resistance was observed when the concentration of NO 2 gas reached 500 parts per billion (ppb).To assess the reliability of the PbS/Sb 2 O 5 sensor, its repeatability was analyzed at a fixed NO 2 concentration of 10 ppb, as indicated in figure 6(b).The sensor demonstrated remarkable consistency and repeatability under these conditions.
Furthermore, the sensor exhibited remarkable selectivity, primarily responding to NO 2 gas while showing minimal interference from other gases, as illustrated in figure 6(c).The limit of detection (LOD) for the PbS/Sb 2 O 5 sensor was determined by fitting a linear curve to the gas sensing response data, resulting in an LOD of 0.083 ppb, as portrayed in figure 6(d).Figure 6(e) presents the response and recovery times at various NO 2 concentrations.It was observed that the response time decreased as the gas concentration increased.This phenomenon can be attributed to the fact that at lower concentrations, gas molecules primarily interact with the heterojunction.However, at higher gas concentrations, interactions occur not only at the heterojunction but also on the material's surface, leading to a faster response time.Conversely, at higher gas concentrations, the gas molecules adhere more strongly to the surface, necessitating a longer time for desorption, resulting in an increased recovery time.In figure 6(f), the band diagram of the PbS/Sb 2 O 5 system is presented, offering a visual representation of the energy levels and charge carrier behavior within the material.
It is generally acknowledged that the performance of a photodetector is always correlated with the absorption and desorption of oxygen molecules under dark and light circumstances.In a dark state, oxygen molecules are adsorbed onto the Sb 2 O 5 film surface, where they quickly form negatively charged electron trap sites, resulting in a larger depletion layer or a depletion layer with poor conductivity.Furthermore, when visible light strikes the surface of PbS/Sb 2 O 5 heterostructures so the electron-hole pairs are generated.The thickness of the depletion layer then decreases as a consequence of the holes migrating to the Sb 2 O 5 surface and releasing oxygen ions.Therefore, electrons get excited from the valance band to the conduction band.The photocurrent is increased as a result of the produced carriers being directed toward the gold electrodes by the action of the external electric field.Less photocurrent occurs when light is out because oxygen molecules are reabsorbed.
Heterostructures play a pivotal role in augmenting photosensing capabilities by leveraging the unique electronic and optical properties inherent in the interface between different semiconductor materials.The controlled integration of distinct materials within a heterostructure enables efficient charge separation, enhanced carrier mobility, and extended light absorption spectra, collectively contributing to improved photoresponse.This synergy allows for the creation of tailored band structures and energy alignment, facilitating the generation and transport of charge carriers with higher efficiency.Additionally, the reduction of recombination losses at the heterojunction interfaces further amplifies the sensitivity and responsiveness of the photosensing device.In essence, heterostructures serve as strategic platforms for optimizing the performance of photosensors, offering a pathway to advanced functionalities and heightened performance in light detection applications.
The sensing mechanism of PbS/Sb 2 O 5 heterostructures has been shown in figure 7(a).In which, when the light strike  on the surface then due to photoexcitation the distribution of polarized charge along the z-axis of Sb 2 O 5 creates a positive charge along the junction and a negative along the gold electrode.The generated charges on the electrodes would be redistributed together with the increase in photons to balance the pyroelectric polarization potential, causing electron flow in the external circuit from the Au electrode to another side.These enhancements in the electrons are responsible for the generation of photocurrents.Similarly, in dark conditions, when an electric potential is applied across the electrodes the Sb 2 O 5 molecules get polarized and generate some dark current in the devices.A band diagram of illuminated and dark conditions has been depicted in figure 7(a).In this case, a built-in field is formed in the device from the positive to the negative electrode, indicating the band structure of the device when a potential drift is applied and under dark circumstances.Due to the adsorption of certain oxygen species ) on the material surface and the electrodematerial junction, a depletion region arises between oxygenated vacancies and the material surface under ambient air conditions.When light is supplied to the material, these oxygenation vacancies are removed from the surface and e-h couples are produced.Under sufficient potential drift, the electrodes collect light-induced e-h pairs, increasing photocurrent relative to the dark level.Upon illumination, e-h couples transfer from the valence band to the conduction band, however in nanohybrid materials, this transition may also occur from defect states to the conduction band.Since photon energy cannot directly excite an electron from the valence band to the conduction band, the majority of photocurrent in nanohybrid materials is generated through the valence band to conduction band transitions.Conduction band electrons and holes recombine via recombination sites or band-to-band annihilation when the light is turned off.The gas sensing mechanism of PbS/Sb 2 O 5 heterostructures for NO 2 detection depends on the generation of oxygen species at the material's surface.This gas-sensing process is schematically depicted in figure 7(b).Initially, the PbS/Sb 2 O 5 thin film is exposed to ambient air, leading to its interaction with oxygen in the air and the subsequent formation of oxygen species on its surface.These oxygen species serve as highly reactive entities that play a critical role in the subsequent interaction with NO 2 molecules.Upon exposure to NO 2 gas, the NO 2 molecules interact with the surfacebound oxygen species.This interaction leads to the chemical conversion of NO 2 into nitrogen monoxide (NO) and molecular oxygen (O 2 ).Importantly, this chemical reaction results in the release of free electrons into the PbS/Sb 2 O 5 heterostructures.As a result, the conductivity of the thin film increases, and a noticeable reduction in electrical resistance is observed, as confirmed through experimental analysis.This change in electrical properties serves as the basis for the sensitive and selective detection of NO 2 gas in PbS/Sb 2 O 5 gas sensors.From this diagram, it has been observed that the electron density in PbS/Sb 2 O 5 increases in comparison to the PbS.The enhancement in the electron density of PbS/Sb 2 O 5 heterostructures provides free carriers to the material and generates more photo-electrons.Therefore, this material is very sensitive to the weak signal of the photon.Furthermore, some more parameters such as ionization potential, electron affinity, HOMO-LUMO gap and charge transfer for PbS and PbS/Sb 2 O 5 by using the HOMO-LUMO energy levels as reported in our previously published articles [40,41]   From table 4, it has been observed that the HOMO-LUMO gap decreases after heterojunction formation and it is also responsible for an increase in the charge transfer.A higher value of charge transfer makes them compatible with the detection of very weak light signals.Furthermore, due to having less electronegativity most of the free carriers are formed on the surface and react with the oxygen and also generate photoelectrons.Moreover, Upon a thorough examination of alterations in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap pre-and post-interaction with various gas species, a discernible trend has emerged.Notably, it has been observed that the HOMO-LUMO gap experiences a reduction in the presence of oxidizing gases, while it exhibits an increase when exposed to reducing gases.Importantly, these findings align closely with the experimental observations, reinforcing the consistent correlation between gas interactions and changes in the HOMO-LUMO gap.
Furthermore, for analyzing the optical properties of the material the first excitation energy of the material has been calculated as shown in figures 9(a), (b).From this figure, we observed the p-p transition from orbital 10 to orbital 11 in PbS crystal.However, in PbS/Sb 2 O 5 there are three possible p-p transitions are possible from orbital 29 to orbital 31, orbital 30-31 and orbital 31 to orbital 30.In which the transition from orbital 30-31 is the most active in this material.However, if we compare the first excitation energy of the PbS and PbS/Sb 2 O 5 heterostructures then it has been found the excitation energy of PbS/Sb 2 O 5 heterostructures less than the PbS therefore the photon having less energy is also capable of exciting the electrons.Due to having such less transition energy the material is capable of detecting very weak signals as well the signal for higher wavelengths.system occupies.The enhancement in the energy level and contraction in HOMO-LUMO levels confirms the increment in electron-hole pairs after the PbS/Sb 2 O 5 formation.

Conclusion
In summary, PbS/Sb 2 O 5 heterostructure has been synthesized by the combination of sputtering and sulfonation processes.The characterization results reveal that sputtered PbS/Sb 2 O 5 were successfully converted into PbS/Sb 2 O5 after the sulfonation process.The photo-sensing results reveal that the PbS/Sb 2 O 5 has higher detection capability in comparison to the pure PbS nanomaterial.The responsivity and detectivity of PbS/Sb 2 O 5 are 0.28 S/mWcm −2 and 1.68 × 10 11 Jones respectively.The sensor response towards NO 2 gas was found to be 0.98 at 10 ppb with an LOD of 0.083 ppb.The PbS/Sb 2 O 5 exhibits high selectivity towards the NO 2 gas.Furthermore, DFT was used to analyze the molecular properties of the material.The theoretical results confirm that the charge transfer increases after the heterojunction formation.The theoretical results are very analogous to the experimental results.


After the sulfonation process, it has been observed the band gap of the material decreased as shown in figures S4(a)-(d).The reaction mechanism for the conversion of PbSb 2 O 6 to the PbS/Sb 2 O 5 is depicted in figure 1.From this figure in the first step when the sulfur reacts with PbSb 2 O 6 in an intermediate state the relative energy change was found to be −93.1 Kcal mol −1 .After that at a high temperature (430 °C) the sulfur atom gets attached and forms PbS/Sb 2 O 5 with a relative energy change is −1.13 Kcal mol -1 .Therefore, the proposed mechanism has been verified by theoretical calculations.
2, 38.1, 53.8 and 54.9 corresponds to the (001), (200) (220) (311) (222) plane of PbS and peak at 25.3, 43.8, 52.2 and 63.15 corresponds to (101) (110) (202) (301) planes of Sb 2 O 5 respectively.Furthermore, the thin film has been analyzed by an XPS spectrophotometer.To confirm the formation of PbS/Sb 2 O 5 and to simplify the XPS spectra, all peaks were deconvoluted using Gaussian-Lorentzian peak fitting and the results are depicted in figures 3(c)-(f).Furthermore, a summary of the binding energy of Pb 4f 7/2, Sb 3d 3/2 , O 1s and C 1s has been shown in table 1.The peak of the oxygen (O 1s) binding energy overlaps the peak of the Sb 3d 5/2 component binding energy.Deconvolution of the spectrum shows the binding energy value at 531.5 eV corresponds to Sb 2 O 5 .Similarly, the peak belongs to the 530.4 eV, 539.8 eV and 34.4 eV belong to Sb 3d 5/2 , Sb 3d 3/2 and Sb 4d respectively.Deconvolution of the Pb component provided two different values of binding energy 137.5 eV and 142.4 eV corresponding to Pb 4f 7/2 and Pb 4f 5/2 respectively.It is interesting to note that during the sulfonation and oxidation process, the oxygen and sulfur atoms get attached to the PbSb and it causes a small shift in the peak of Sb 2 O 5 .To investigate the electrical and photoelectrical capabilities of a PbS/Sb 2 O 5 thin film, the device was vertically illuminated with light of varying intensities, and the associated photoelectric behavior was recorded at room temperature.The response of the device has been measured by using different light intensities varying from 5 to 40 mW cm −2 and wavelength from 450 to 940 nm.The variation in the conductance of the PbS thin film by using light of different wavelengths has been depicted in figures S8(a)-(f).Due to its high light-absorption coefficient, it is anticipated that the PbS/Sb 2 O 5 thin film would demonstrate strong responsiveness to light illumination as shown in figures 4(a)-(f).The optoelectronic characteristics of each thin film were then studied at room temperature in the air.The IV diagram of PbS and PbS/Sb 2 O 5 at different wavelengths with different illumination intensities is shown in figures S9 and S10.The

Figure 5 (
photodetector with previously published articles has been shown in table3.Upon conducting an in-depth analysis of the photo-sensing properties of the thin film, it was subsequently subjected to gas-sensing measurements.The collected sensing data revealed a notable and rapid variation in the resistance of the thin film upon exposure to NO 2 gas, as visually depicted in figure6(a).Particularly, the most significant alteration in resistance was observed when the concentration of NO 2 gas reached 500 parts per billion (ppb).To assess the reliability of the PbS/Sb 2 O 5 sensor, its repeatability was analyzed at a fixed NO 2 concentration of 10 ppb, as indicated in figure6(b).The sensor demonstrated remarkable consistency and repeatability under these conditions.Furthermore, the sensor exhibited remarkable selectivity, primarily responding to NO 2 gas while showing minimal interference from other gases, as illustrated in figure 6(c).The limit of detection (LOD) for the PbS/Sb 2 O 5 sensor was determined by fitting a linear curve to the gas sensing response data, resulting in an LOD of 0.083 ppb, as portrayed in figure6(d).Figure6(e) presents the response and recovery times at various NO 2 concentrations.It was observed that the response time decreased as the gas concentration increased.This phenomenon can be attributed to the fact that at lower concentrations, gas molecules primarily interact with the heterojunction.However, at higher gas concentrations, interactions occur not only at the heterojunction but also on the material's surface, leading to a faster response time.Conversely, at higher gas concentrations, the gas molecules adhere more strongly to the surface, necessitating a longer time for desorption, resulting in an increased recovery time.In figure 6(f), the band diagram of the PbS/Sb 2 O 5 system is

Figure 5 .
Figure 5. (a) Variation in photocurrent density by changing voltage of PbS/Sb 2 O 5 (b) comparison in the responsivity at different wavelengths of PbS and PbS/Sb 2 O 5 (c) variation in the detectivity and NEP of PbS/Sb 2 O 5 heterostructure at a different wavelength (d) variation in the LDR and I p /I d of PbS/Sb 2 O 5 heterostructure at a different wavelength (e) variation in the photocurrent of PbS/Sb 2 O 5 heterostructure at a different wavelength (f) comparison of photocurrent density in IR region for PbS and PbS/Sb 2 O 5 heterostructures.

Figure 6 .
Figure 6.(a) Variation in the resistance of PbS/Sb 2 O 5 thin film at different concentrations of NO 2 (b) repeatability test of PbS/Sb 2 O 5 sensor at 10 ppb (c) selectivity test of PbS/Sb 2 O 5 (d) response versus gas concentrations (e) response and recovery time at various concentration of NO 2 (f) band diagram of PbS/Sb 2 O 5 heterostructures.
In order to find the photodetection sensing mechanism of PbS and PbS/Sb 2 O 5 a theoretical model has been developed by using the XRD data of the nanomaterials.The theoretical results reveal that after incorporating Sb 2 O 5 in the matrix of PbS the charge-carrying density around the outermost layer increases and which is responsible for the generation of photocurrent.The electron localized function (ELF) of PbS and PbS/Sb 2 O 5 has been shown in figures 8(a) and (b) respectively.

Figures 10 (
a), (b) shows the density of the state plot of PbS, PbS/Sb 2 O 5 and PbS/Sb 2 O 5 with NO 2 , O 3 , NO and CO.The red line represents the vacant level and the green line represents the occupied levels of the PbS/Sb 2 O 5. The number of states per interval of energy at each energy level that is open to being occupied is described by a system's density of states (DOS) in quantum mechanics (QM).At a given energy level, high DOS indicates a large number of states that are open for occupancy.A DOS of zero indicates that at that energy level, no states may be inhabited.In general, DOS represents an average throughout the time and space that the

Figure 9 .
Figure 9. First excitation of the electrons from different orbitals of (a) PbS (b) PbS/Sb 2 O 5 heterostructures.

Table 2 .
Comparison of rise and decay time of PbS and PbS/Sb 2 O 5 hetrostructure.

Table 3 .
Comparison of various parameters of PbS/Sb 2 O 5 photodetector with previously published articles.

Table 4 .
Different theoretical parameters of PbS and PbS/Sb 2 O 5 by using simulations.Theoretical parameters PbS PbS/Sb 2 O 5 With NO 2 With O 3 With CO With NO