Because of the great technological interest of applications in nanoscience, many research works both by experimentation and simulation have been conducted to elucidate the mechanism of understanding the trajectory of electrons and photons revealing the electronic and optical properties of certains nanomaterials doped thin oxide such as titanium oxide (TiO₂) and magnesium monoxide (MgO). TiO₂ is one of the most widely used nanomaterials in a wide range of applications, including as a white pigment for the pharmaceutical and food industry, skin sunscreen, photo-catalysts [1] [2] , water treatment, hydrogen production, ethylene recovery and antimicrobial activity [3] [4] [5] [6] [7] . One of the major problems with TiO₂ is that the extent of electron hole recombination is higher than that of some other promising materials [8] . In an attempt to overcome this problem, researchers are implementing many ways to improve the solar conversion efficiency of TiO₂ while using doping studies [9] . Metal oxides are recognized as the best TiO₂ dopants. The most common that has been synthesized into a range of nanostructure morphologies is MgO. It is known as an inert material with a high melting point, as a typical wide band-gap insulator. And its substrate has been widely used for high-Tc superconductor (HTSC) thin-film coating applications worldwide. When it is used as a substrate for nanoparticle catalysts, the main physical properties need to be better understood for the transition from solid state to molecule scale [10] . Impressive results in medical biotechnology indicated that the TiO₂ nano-thin film coating stimulated the adhesion and proliferation of coronary arterial endothelial cells with additional characteristics acting as a protective barrier [11] . The data revealed that surface morphology and surface hydrophilia contributed to the success of the atomic layer deposition nanoscale coating, which also acted as a protective layer inhibiting the release of harmful degradation products from the magnesium-zinc stent [11] . Based on the studies of Luis Anaya et al., mixed oxide nanoparticles (MONs, TiO₂-ZnO-MgO) obtained by the sol-gel method were characterized by transmission electron microscopy and thermogravimetric analysis. In addition, the effect of MON on the microbial growth of Escherichia coli, Salmonella paratyphi, Staphylococcus aureus, and Listeria monocytogenes, as well as the toxicity against Artemia salina by the lethal concentration test was evaluated [12] . Several research teams have also printed thin films to study the experimental physical properties of materials [13] . This study can be carried out thanks to the Monte Carlo (MC) simulation method which is used because of its high accuracy such as Gamma ray transport [14] . MC was used to reduce the complexity of mathematical expressions in which Fourier random characteristics are applied to approach the Exponentiated Quadratic kernel [15] . It is possible to simulate certains phenomena such as the cascade of particles in materials of Novel lead oxide-based flexible dosimeters for electron therapy [16] [17] . Monte Carlo simulation is a crucial tool for specifying the possible density of dispersed particles, the generation of secondary radiation, and the transmission ratio of primary particles from the material structure. Various simulation software, including Geant4, MCNPX, and PyPENELOPE, were composed to simulate the interactions between the particles and the designed structure. Most of these simulation programs depend on multiple scattering theories for electron transport to reduce computation time [18] . PENELOPE means Penetration and energy loss of positrons and electrons in matter. It uses the Monte Carlo method for the simulation of numerous apparatuses as dosimeter and material structure. It will therefore be a tool of choice to simulate MgO/TiO₂. PENELOPE combines numerical and analytical total and differential cross sections (DCS) to describe the different interaction mechanisms. These cross-sections are the result of approximate physical models and, therefore, are affected by systematic uncertainties. The interaction mechanisms considered in PENELOPE, and the corresponding DCSs, are as follows: Elastic diffusion of electrons and positrons where these cross sections were calculated using the program ELSEPA [19] , Inelastic collisions of electrons and positrons [20] [21] , Electron impact ionization [22] , Bremsstrahlung emission by electrons and positrons [23] [24] [25] , Positron annihilation, Coherent dispersion (Rayleigh) of photons, Incoherent dispersion (Compton) of photons [26] , Photoelectric absorption of photons [27] , Production of electron-positron pairs [28] . Buse et al. conducted a study on the structural, morphological and optical properties and performance of gas sensors of titanium dioxide (TiO₂) doped thin films with magnesium oxide (MgO). The material was therefore manufactured in different thicknesses namely 125 nm, 140 nm and 190 nm. The best results obtained with the material of thickness 125 nm. They showed that the dopant is able to improve the electrical performance and properties of the sensor [29] . In our work, we will attempt to provide additional information on particle properties in order to explain the microscopic electron propagation in MgO/TiO₂.

This work was performed with Monte Carlo simulation code using the PyPENELOPE software which is a version of PENELOPE executed by the Python program as was the case in several works [30] [31] [32] [33] [34] . It is one of the best tools to evaluate the transport of the particle in the material describing the different types of interaction. The material MgO/TiO₂ studied by Buse et al. which presented certain physical properties but without giving the characteristics of particle propagation related to the absorption, backscattering and transmission properties of electrons and photons in matter. In this work, we simulated a primary electron incident beam and sent perpendicularly on the MgO/TiO₂ material in the optimal conditions in order to study the propagation effects. Our beam source was at the initial coordinate position (0; 0; 0) under a polar angle range of zero to 180˚. The number of electrons was a range from 7.5 × 10⁶ to 26.3 × 10⁶. The diameter of the beam was set to 10 µm. Multilayer geometry was used for the material consisting of MgO density 3.35 g/cm³ and TiO₂ density 4.23 g/cm³. The default interaction forcing was configured and used during the simulation. These simulations were performed for different values of incident irradiation of range 0.5 KeV to 20 KeV. The work of Buse et al. showed the 125 nm thickness material is promising for the sensor fabricator [29] . It will therefore be the subject of our study. Each molecule composing the material was half the total thickness i.e., 62.5 nm for MgO and 62.5 nm for TiO₂. A laptop computer performed the simulation with i5 2.4 GHz CPU and 8 GB RAM under Windows operating systems. The PyPENELOPE software simulation parameters were defined as in Table 1 [35] .

Many research teams studied the MgO/TiO₂ material to understand its geometrical structure, electronic structure, and other general interfacial properties, which are not yet clear from a microscopic perspective. In this work, the MgO/TiO₂ thickness proposed by Buse et al. was 125 nm. Table 2 below presents the results of simulations relating to the fraction of particles absorbed, backscattered or transmitted in the material according to the different beam energies.

The fractions of absorbed, backscattered and transmitted radiation within MgO/TiO₂ structure were obtained after simulating a number of electrons ranging from 7.5 × 10⁶ to 26.3 × 10⁶ and their distributions were estimated via the pyPENELOPE Monte Carlo code. Our study is focused on the propagation effect of a range of 0.1 KeV to 20 KeV of incident primary irradiation on 125 nm as thickness of MgO/TiO₂ materials. Absorbed fraction for primary irradiation spread on a range of 1.484 × 10⁻⁵ to 0.8133 on all the energy. It decreases to the smallest value except for secondary electron which increases to a peak at 5 KeV before decreasing to 10 KeV (Figure 1(a)). Regarding backscattered fraction, Primary irradiation fraction is high for small incident beam energy. Figure 1(b) show the plots decrease when the energy increase. Regarding the transmitted fraction, almost all incident primary beam was transmitted when the energies increase. Figure 1(c) shows from 0.5 to 10 KeV a growing right and after 10 KeV up to 20 KeV, a plateau whose constituent points vary very little keeping a transmission fraction ranging from 93 to 99.9% within material. Uncertainty is low for all calculation (Table 2). The transmission fraction is acceptable because its agreement with Senol’s works [16] . The highest energy gives therefore a better transmitted fraction.

The probability density of backscattered and transmitted particles distributed according to the energy in the material of 125 nm, is illustrated in the following Figure 1 and Figure 2.

For incident primary irradiation of 20 KeV, the energy distribution of the backscattered electrons extends over a range from zero to 20 KeV. A peak of the probability density just after 0 KeV is observed and decreases immediately before increasing again for another peak around 18 KeV. After this peak, the probability again reaches another peak at 20 KeV. This shows that the primary electrons propagate in cascade by penetrating in the structure of the material thus creating interactions with the electrons present on the transitions of the

atomic elements constituting the material. Thus, a low probability density of electrons and photons is evidenced. The backscattered photons weakly emit two signals in peak before 1 KeV, a peak at about 1.2 KeV and a peak between 4 and 5 KeV (Figure 2). A similar phenomenon is also observed for transmitted photons (Figure 3). Concerning transmitted electrons, a density of probability almost zero during the simulation and begins to increase from 17 KeV and reaches a peak around 19 KeV (Figure 3).

The polar angle changes from 0 to 180˚ during the simulation. The probability density started to increase to 120˚ for emerging electrons and reach a peak at 180˚. Emerging photon probability density is stable during simulation from 0 to 180˚ but drop only in 90˚ (Figure 4).

The peaks observed then of emission and backscattering of the particles also appear on Figure 5(a) of the photon spectrum.The curve described by the photon spectrum is Bremsstrahlung phenomenon. It occurred on all energy range (Figure 5).

Figure 6 showed that the characteristic energy of titanium (Z = 22) electron transitions is between 4 and 5 KeV. These lines are very different, we can quote KL3, KL2, KM3 and KM2 sub-shell occupied under certain incident energy like 10 and 20 KeV. Typically, Kβ line of Ti is 4.931 KeV [36] . Kα line of Mg (1.253 KeV) [37] is higher than 1 KeV, it is therefore not shown in Figure 6(c). The particle occupation of these titanium and magnesium shell and sub-shell are responsible for the better transmission fraction rate of primary electrons in the material and cause the phenomena such as the emission of characteristics,

fluorescence X-rays and bremsstrahlung. At low incidence energy (1 KeV), the particle occupancy of these shell and sub-shell is low and non-existent in magnesium (Figure 6(c)). Indeed, these high-energy electrons must release energy to fill the lower energy gaps in the atom. These generated photons can be classified as radiation contamination for sensor application. Therefore, potential radiation contamination consists of generated X-rays, including the continuum (Bremsstrahlung), X-ray characteristics and fluorescence [38] .

At the end of this study, it was demonstrated the good quality of the Monte Carlo simulation method by the PyPENELOPE software which was revealed to be a tool of choice in the calculation of the absorption, backscatter and transmission fraction of particles through the material. Under an incident energy of 20 KeV of primary electrons, more than 99.9% of these particles were transmitted in the material MgO/TiO₂ thanks to several radiation interaction matter which governs this particle propagation namely the characteristic line, fluorescence and bremsstrahlung. These interactions were produced on KL3, KL2, KM3, and KM2 shell and sub-shell of Mg and Ti. These properties may therefore improve the performance of this material in the manufacture of future sensors.

The study was performed by Medical Physics research unit in Côte d’Ivoire, ICTP, Trieste University in Italy. The authors acknowledge with gratitude the support from the unit.

The authors declare no conflicts of interest regarding the publication of this paper.

Allangba, K.N.P.G., Fagnidi, Y.K.H., N’guessan, H., Traoré, Z. and Kamenan, K.A. (2024) Evaluation of Particle Properties of MgO/TiO₂ Material by Monte Carlo Simulation Method. Journal of Materials Science and Chemical Engineering, 12, 49-60. https://doi.org/10.4236/msce.2024.122004