Investigation of gamma induced effects on the properties of gamma irradiated Ce2S3 thin films

Nanostructure cerium sulphide (Ce2S3) thin films were prepared using successive ionic layer adsorption and reaction (SILAR). The properties of the prepared samples were investigated as a function of gamma rays’ doses of 0, 250, 500, and 1000Gy, respectively. The X-ray diffraction (XRD) results suggest an orthorhombic phase structure for Ce2S3 thin films and the crystallinity is enhanced with increasing gamma-ray doses. The irradiated thin films exhibit a variation in the energy band gap associated with the quantum confinement effect with larger grain size. This simple strategy of modifying properties of Ce2S3 thin films by the incident gamma rays can be an attractive way to investigate this material for dosimetry and radiation detection.


Introduction
Cerium is a most abundant element in the lanthanides series of elements and also called the rare earth elements.These elements have unique metal color pigments and unique luminescence characteristics [1][2][3].Furthermore, the rare earth metal sulfide has attracted extraordinary attention due to the arrangement of sulfur with metal, which is predictable to generate surprising properties due to numerous oxidation states of sulfur [4].Cerium sulfide (Ce2S3), is featured unusual properties due to remarkable thermal stability, electronic, and mechanical properties accompanied by superior electrical conductivity and optical characteristics [5][6][7].It presents three type of structural phases, namely the orthorhombic α-Ce2S3, the metabolic β-Ce2S3 and the hexagonal γ-Ce2S3 with different structural, optical and electrical properties [8][9][10].
Several investigations have been directed to study the induced modification of high energy  rays on the material's nanostructure, examine the phenomena of interaction of radiation with nanomaterials and explore the role of radiation dose.Not long ago, nanostructure thin films have been used in devices for detecting gamma rays.Irradiating thin films by high ionization energy radiation generates the structural imperfection or lattice defects in the form of deficiencies, as well as defect clusters forming due to the heat transformation and dislocation atoms near the interface.A frequent report has been published to detailly study the impacts of gamma ray irradiation on the physical characteristics of thin films [11][12][13][14].In this study, the characterization and analysis of gamma induced modification of SILAR deposited Ce2S3 thin films have been comprehensively investigated.The main motivation of this research is to assess the degree of the radiation induced modification of the prepared Ce2S3 thin films properties in order to promote this material for radiation sensing and dosimatric applications.In order to do that, the structural, optical and electrical properties of Ce2S3 thin films have been experimentally measured at different gamma ray's doses.

Preparation of Cerium sulfide thin film
For the first time, glass substrates were used in a SILAR deposition process to deposit Ce2S3 thin films.The solutions concentration, PH values, immersion and rinsing times were optimized to deposited the homogeneous Ce2S3 thin film.All the chemicals were used of analytical grade (Sigma Aldrich).For the preparation of Ce2S3 thin films, the 0.025 M cesium nitrate Ce(NO3)2 cationic solution with pH∼8 and 0.025 M and sodium sulfide (Na2S) anionic solution of pH∼11 were used.A glass substrate was rinsed and then dipped into a Ce(NO3)2 solution for 2 minutes to produce cerium ions, which were then adsorbed to form Ce2S3 on the substrate.The cerium ion adsorbed substrate was washed by deionized water for 30 s to prevent unattached ions and impurities.In next step, same substrate was dipped in Na2S solution for 2 min, sulfide ions countered with the adsorbed cerium ions and washed again by deionized water for 30 s.For this case, one SILAR cycle of Ce2S3 thin film has been completed.To get the appropriate thickness of thin films 35 times SILAR cycles were repeated.

Gamma irradiation
The Cerium sulfide thin films were irradiated by a gamma ray source (Co-60), which has half-life of 5.2714 years.The source irradiation rate, which was used to monitor the received radiation dose, was found to be 927.299Gy/h (Nordion, model GC-220).Three thin films of Cerium sulfide were exposed to different level of radiation doses: 250, 500, and 1000 Gy.The exposure time was used to determine the required dose.

Characterization
An X-ray diffractometer (Panalytical X'Pert3) equipped with a Cu-K of wavelength 1.5406 as the X-ray source was used to perform a structural analysis of both the prestine and gamma irradiated thin films.Gravimetric analysis was used to quantify the thickness of the samples.The samples' transmission and photoluminescence spectra were measured using a JASCO-V 670 UV-Vis spectrophotometer for the optical studies.

Experimental results and discussion
The XRD experimental results of the prestine and  rays exposed Ce2S3 samples with different gamma doses, ranged between (0-1000 Gy), is exhibited in Figure 1.Non-irradiated and gamma irradiated Ce2S3 samples are featured a crystalline structure with an orthorhombic phase, which presents a space group Pnam-62 [JCPDS No. JCPDS-21-0189].The obtained 2θ values, which are 25.45, 35.97, 36.97, and 39.93° correspond to the (111), ( 051), ( 231) and (330), respectively.The intensity of XRD peaks reduced of all samples as the gamma rays' doses increased.This may be due to the reduction in the crystallinity of Ce2S3 structures in (111) direction.Moreover, slightly broadening of XRD main peak of irradiated samples have also been noticed as shown in the inset of Figure 1.The presented results show a good agreement with the reported effects obtained by Aldawood et al. [15].The structural parameters of Ce2S3 samples pre and post  rays exposed with variation of gamma ray doses value can be computed by (111) orientation using the Debye Scherrer formula (Eq.( 1)) [16], The crystallite size reduced with increasing the gamma dose, as given in table 1.This may be due to the deep gamma rays penetration into the Ce2S3 leading to host thin films alteration by pushing the basis from their regular lattice and excruciating the molecules into small particles.In this phenomena, the gamma photons lose their energy that may generate or heal the defects in the structure [17].This distortion in the crystal structure by gamma irradiation can be endorsed due to either the changes in lattice vacancies in host materials or the augmented in the reduction of the micro-strain in gamma exposed samples, which also be appeared in the peak broadening [18,19].
The lattice specification values of the orthorhombic phase structure are measured by XRD diffraction planes using the following formula: where the inter-planar spacing is represented by 'd', the lattice parameters are represented by 'a, b and c' and the Millar indices of the plane are '(h, k, l)'.The slightly deviation in lattice parameters of Ce2S3 thin films by means of gamma dose can be attributed to the modifications in lattice parameters.These changes can be due to the lattice vacancies alteration in the thin films, induced by gamma rays [20].Figure 2 (a) demonstrates the alteration in the proportion optical transmission in the range of 250-800 nm wavelength of the as prepared and  rays exposed Ce2S3 samples with numerous gamma dose (250-1000 Gy).It is experiential that the transmission in visible region has been reduced considerably with gamma ray exposure dose values.This could be due to the increases in the defects density with increasing the high power transformation to the host material structure, which provides the additional states of photon absorption in the thin films [14].In the region of wavelength near the range up to 700 nm, the blue shift in the band edge is observed with increasing the gamma dose suggesting the decreased in the optical band gap.
(5) where 'T' represents the optical transmission, 't' is the width of the Ce2S3 film, 'ν' is the electromagnetic waves frequency, 'h' is the plank's constant, 'A' is an arbitrary constant, and 'n = ½ for direct electron (a) 1 ln 100 transition [22].The expected optical band gap of the Ce2S3 films was calculated from the extrapolation of linear portion of the (αhν) 2 vs. (hν) graph to the horizontal axis as indicated in Figure 2(b).The estimated optical band gap of Ce2S3 thin films before and after gamma are calculated using the above equations.The increase in the optical band gap with increasing gamma ray dose from 0 to 1000 Gy is shown in a sub-figure inside Figure 2 (b).The deviations in the optical band gap were indorsed to radiation induced imperfections and induced traps concentration, that could change the absorption edge [22].After gamma exposure, the traps concentration in conduction and valence bands of the thin films are dependent on the irradiation dose level.Hence, we can control that gamma ray modifications meaningfully and change the electrical properties of the interacting thin films [22].Aldawood et al. described similar consequences for gamma exposed PbI thin films [15].

Conclusion
The method of sequential ionic layer adsorption and reaction (SILAR) was used to create Ce2S3 thin films.Structure and optical characteristics of the produced thin films were investigated as a function of gamma radiation with dosage variation.Ce2S3 crystallite size decreased from 84.39 to 53.36 nm as gamma dosage was increased, as seen by the XRD pattern.UV transmission spectra were used to determine the energy band gaps of both pre-and post-irradiated samples.As gamma doses were raised, a slight blue shift occur in the optical band gap, from 1.88 eV to 1.99 eV.

Figure 1 :
Figure 1: (a) XRD patterns of un radiated and gamma exposed Ce2S3 samples.The structural parameters of Ce2S3 samples pre and post  rays exposed with variation of gamma ray doses value can be computed by (111) orientation using the Debye Scherrer formula (Eq.(1))[16],

Figure 2 (
Figure 2 (a) Transmittance spectra, (b) Tauc's plot for band gap calculation Ce2S3 thin films at different gamma irradiation doses and (inset) variation of band gap with gamma irradiation ofCe2S3 thin films.The optical energy band gap (Eg) is an imperious factor for different advanced applications of nano structural scale thin films.The UV-Vis transmission spectra has been used to compute the band gap of as prepared and gamma ray exposed Ce2S3 thin films.To analyze the band gap value (Eg), the absorption coefficient (α) values is obtained using formula 4 and 5, respectively[21]: