During this digital era, a wide technological advancement in electronics is increasing, like the speed of light. This field of electronics is much more dependent on semiconductor materials because most of the applications used in these fields, such as IC, LCD/LED, BJT, MOSFET, transistor, resistor, photovoltaic solar powerare quite impossible to make without semiconductor materials [1-3]. It is also an essential fact that for the development of the national economy and national security, every country uses high-speed computing, large-capacity data communication, storage, processing, electronic devices, which also depend on a larger scale on semiconductors [4-6]. The conventional semiconductor materials used in electronics are changing every single second. For example, the quantum dots are replacing the conventional narrow and wide band gap semiconductor materials whose range is recorded by many researchers and scientists in between 1.8 to 4.0 eV [7,8]. Quantum dots are semiconductor particles having a few nanometer sizes. For example, SrTiO3, also known as strontium titanate, has less than 1.8 eV band gap and at room temperature and low electric field, a very large dielectric constant (300) [9,10]. Due to this a low band gap and a large dielectric constant value, it can store more charge, and that is why it has a wide range of uses, such as high-voltage capacitors, voltage-dependent resistors, optical instruments, jewelry stone, advanced ceramics, and substrates for superconductors [11,12].
However, the commercialization of DSSCs gets notably dragged due to the areal cost and bustling synthesis procedures of organic dyes and limited quantum efficiency. On the other hand, quantum dots (QDs) are considered excellent light harvesters to substitute organic dyes in these macroscopic solar cells due to their tunable band gaps, light-response ranges, scalable materials, and cost-effectiveness.
Narrow band gap semiconductors are a class of Quantum dots that have benefits like tunable band gaps due to the quantum confinement result , higher absorption coefficients than most organic dyes [14,15], and various exactions generation have more [16,17]. For macroscopic solar cells QDs have been analyzed to serve as light absorbers, such as CdS , CdSe, PbS, PbSe and integrated CdS/CdSe, CdS/PbS, CuInS2/CdS, CuInS2/In2S3, CdSe/CdTe, CdSe/GO, etc , inverter , Double Perovskites [19,20]. However, these materials are always restricted due to their intrinsic band gaps. Due to negligible toxicity, easy formulation, scalable materials, and performances, a class of lead-free perovskite-structured Quantum Dots has been highlighted nowadays [21,22]. The reduction of band gap using the doping process can make SrTiO3 more efficient quantum dots, and this investigation is based on the same principle.
In this investigation, the band gap was determined using computational tools, which supports less time and cost than experiments and compares the calculated band gap to the experiment value for its validation [23-30]. Additionally, to develop and enhance its activity, Ag was doped on SrTiO3 because Ag increases electron density, doping power, and decreases band gap by overlapping with the orbital of the materials and enhances quantum dots efficiency [24,31-34].
As quantum dots, there are not enormous studies on SrTi0.94Ag0.06O3. For this view, the main goal of this study is to investigate the effect of Ag-doped replacing Ti atom on the SrTiO3 compound, and its molecular formula becomes SrTi0.94Ag0.06O3 after 6% Ag doping. As a result, these properties, which have been derived from simulation after doping, have significant intensive attention for potential applications. For using the SrTiO3, this study supports to development of the theoretical concept using the first principle study and opens a new window for potential application by Ag doping in SrTiO3.
With instigating, before energy calculation, the geometrical optimization was acquired where the convergence criterion for the force between atoms was 3×10−6 eV/A˚, and the maximum displacement, the total energy, and the maximal stress were at 1×10−3A˚, 1×10−5 eV/atom and 5 × 10−2 GPa, respectively. After geometrical stability of the configuration, using the GGA with PBE functional from CASTEP code of the material studio version 8.0, the electronic band structure, the total density of state (TDOS), and partial density of state (PDOS) were computed .
The main reason is the most viable and acceptable method for computing the electronic structure and optical properties [31-33, 36, 37]. Using this condition and keeping the cut off at 500 eV, and k point at 2×2×2 with norm-conserving pseudopotentials, the band structure, density of state, and Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof (PBE)  functional of SrTiO3 and SrTi0.94Ag0.06O3 for the comparative study of band gaps were calculated and after that the optical properties like refractive index, reflectivity, absorption, conductivity, and loss function, were similarly simulated for computation.
Results and Discussions
Geometry of optimized structure
The lattice parameters value was computed for the crystals after the geometry optimization. Here, a = 5.629 Å, b = 5.629 Å and c = 13.886 Å while angles among them as α= 90.00°, β= 90.00° , and γ= 120.00° It is a monoclinic SrTiO3 crystal, and the space group is Hermanna Mauguin P63/mmc , triclinic crystal system, point group 6/mmm, hall-P 6c 2c, density 4.80 g/cm3 shown in Figure 1a. The Ag atom doping in optimized structures are accounted for in Figure 1b.
Electronic band structure
The Fermi energy level is situated at zero energy level to determine the electronic band structure of SrTiO3 and SrTi0.94Ag0.06O3. From Figure 2a for SrTiO3 quantum dots, the minimum of conduction (MCB) is obtained at the G symmetry point, whereas the maximum of valance bands (MVB) is also found at G symmetry points. A direct band gap is calculated with a value of 1.77 eV. The main key point of this research was noted that the band gap of SrTiO3 has been reduced, and it becomes more effective quantum dots when the Ag has doped by 6%. The reason is that Ag atom can enhance the electron density in both areas of valance band and conduction band besides d and p orbitals of Ag atom have quickly overlapped with the p and d orbitals with Sr and Ti metals for quickly transferring the hole electrons. That is why the minimum portion of Ag by 6% was doped in SrTiO3 and illustrated the effect on the band structure. The band gap was recorded at the G symmetry point for both MCB and MVB after doping, showing a direct band gap by 0.00 eV, which stands for that Ag atom has a high capacity on SrTiO3 to reduce the band gap and make it more efficient quantum dots.
The Density of states (DOS) and Partial density of state (PDOS)
The splitting of an orbital and the position of the electronic band structures are designated by a parameter known as the DOS. For calculating the DOS of Sr, Ti, Ag, and O atoms for SrTiO3 and SrTi0.94Ag0.06O3 crystals, the GGA with PBE method was executed. From the Figure 3b to 3c, the sum of DOS for SrTiO3 and SrTi0.94Ag0.06O3 crystals consists of 5s, 4p, and 3d orbitals for a Sr atom, 4s, 3p and 3d orbitals for Ti atom, and 5s, 4p, and 4d orbitals for Ag atom and 2s and 2p orbitals for O atom. Figure 3d to Figure 3f demonstrated that comparison s, p and d orbitals for SrTiO3 and SrTi0.94Ag0.06O3 and the comparison study focus that s, p, and d orbital of SrTi0.94Ag0.06O3 higher than SrTiO3.
Secondly, the DOS of SrTi0.94Ag0.06O3 in the conduction band is almost more significant than the DOS of SrTiO3 at 0.2 eV to 2.5 eV. The DOS of the valance band is found at -0.1 to -5.0 electron/eV, while the DOS of the conduction band is recorded at about 30 electron/eV. To compare the s, p, and d orbitals for both doping and undoped, the orbitals for SrTi0.94Ag0.06O3 are much higher than SrTiO3, and it can be said that the Ag doping on SrTiO3 has increased the DOS of any crystal showing in figures 3a, 3b, 3c, 3d, 3e, and 3f.
Reflectivity is the measurement of reflected light from the surface area of the material relative to the amount of light incident on the material. In this investigation, the reflectivity of SrTiO3 and SrTi0.94Ag0.06O3 is demonstrated in Figure 04. The initial reflectivity of SrTiO3 was recorded at 0.17, and with the increase of energy, it increases. Less reflectivity means more efficient quantum dots. After doping Ag, the initial reflectivity was recorded at 0.27, which decreased with the increase of energy and proved SrTi0.94Ag0.06O3 more efficient quantum dots.
The absorption spectrum of a material depends on the nature of the energy band gap, which pursues the indirect band gap usually absorbs more temperature than the direct band gap semiconductor. The absorption spectrum of SrTi0.94Ag0.06O3 is less than SrTiO3. With the increase of energy, the absorption of both materials SrTiO3 and SrTi0.94Ag0.06O3 increases. However, the SrTi0.94Ag0.06O3 revealed a better value of absorption than SrTiO3,, as demonstrated in Figure 5.
The optical and electronic properties of a material depend on physical parameters like refractive index and energy band gap. These properties describe the nature of the applications of semiconductors, electronic, optical, and optoelectronic devices. The band gap engineered structures for continuous and optimal absorption of broad band spectral sources are also dependent on these properties. The real part and imaginary part of the refractive index of the SrTiO3 and SrTi0.94Ag0.06O3 are shown in Figure 6, which is an inverse pattern.
The refractive index is higher for a real part at the primary point of photon energy at that time, the imaginary part showed a value almost close to 1 eV. Afterwards, they follow a constant pattern with slightly different refractive index values. It is the same for both undoped and doped.
The dielectric functions are calculated in the linear optical response regime within the electric dipole approximation. The required momentum matrix elements are obtained by using the calculated wave functions from our empirical pseudo potentials. Dielectric functions that describe the materials at the Nanoscale are needed, opening the way to interpret experimental data and design of the composites to obtain desired optical behavior. Despite the great interest in the research on NPs, there is presently no available information about the dielectric functions of nano-sized semiconductors that could be readily used for macroscopic modeling applications. The dielectric function is an essential tool to investigate their optical properties, which is related to adsorption properties as the following equation for solid.
ε = ε1 (ω) + iε2 (ω)
Here, ε1 (ω), and ε1 (ω) denotes the dielectric constant (real part) and the dielectric loss factor (imaginary part). The probability of photon absorption for the band structure of any material is closely related to the imaginary portion of the dielectric function. The real part of the dielectric constant maintained the energy storage potential in the electric field, while the imaginary segment indicates the opposite even for electric potential energy. From Figure 7, the actual portion is always higher than the imaginary part within the energy at 1.0 eV to 1.8 eV, but the energy range from 2 eV to 4 eV. The imaginary part shows a higher value than real portion for SrTiO3 and SrTi0.94Ag0.06O3.
The conductivity can be explained in terms of band gap. The electronic conduction is nothing but putting an electron in the conduction band by giving enough energy to electron and shining the material with light. This process can be accomplished. In a solid-state language, by creating a hole in the valence band the electron can go to conduction band with the help of photon energy. After that, an electrical conduction will occur due to free electrons and holes.
Figure 8 depicts the comparative study of the conductivity value of SrTiO3 and SrTi0.94Ag0.06O3 crystals. The conductivity values of both real and imaginary parts start from almost zero at 0.0 eV. The real part of conductivity increased with a similar trend for SrTiO3 and SrTi0.94Ag0.06O3 in the energy range from 0 eV to 5 eV and reached conductivity real peaked value 2.8 and 3.0. However, the conductivity value of SrTi0.94Ag0.06O3 within energy range 1 to3 eV is higher than SrTiO3. On the other hand, the imaginary part values of SrTiO3 and SrTi0.94Ag0.06O3 declined gradually after Fermi energy in the energy range from 3 eV and reached conductivity imaginary peaked values-2.1and -2.5.
The loss function is a crucial part of optical properties, which is composed of two regions of photon energy parts such as the lower photon energy part and higher photon energy part for crystal materials. The energy loss function is closely related to the dielectric function of the photocatalyst materials within the range of the dielectric theory validation. In the energy loss, function dielectric function reflects the response of a semiconductor to an external electromagnetic perturbation. The calculated exploration of loss function values for SrTiO3 and SrTi0.94Ag0.06O3 illustrates in figure 9. It can be seen that the loss function of SrTi0.94Ag0.06O3 is higher than SrTi0.94Ag0.06O3.
To sum up, the electronic, structural, and optical properties of hexagonal SrTiO3 and SrTi0.94Ag0.06O3 quantum dots based semiconductor was studied for the first time using the Density functional theory (DFT) by the first principle approach. First of all, the band gap of SrTiO3 has been recorded by 1.77 eV as supporting a semiconductor that has been filled up the lack of literacy about band gap and electronic structure for SrTiO3 of a theoretical result. The superior outcome of this study was obtained by Ag atom doping effect on electronic structure, especially band gap, DOS, PDOS, and optical properties. After Ag doping by 6% in SrTiO3, the band gap has shifted to 0.00 eV, acting as no resistance materials for optoelectronic devices, which has justified the PDOS and DOS diagram for delocalization of electrons. Secondly, the optical properties due to Ag doping have changed. The optical absorption, reflectivity, loss function, and conductivity of doped SrTiO3 are greater than undoped SrTiO3, but the real part of the dielectric function is lower than undoped. Finally, it can be said that the 6% Ag doping in SrTiO3 is almost acted as a superconductor for a wide band gap QDs materials.
I am thankful to Prof. Md. Wahab Khan, and Prof. Al-Nakib Chawdhury, Department of Chemistry, Bangladesh University of Engineering and Technology (BUET), Dhaka, for his advice and inspiration to carry on my research.
No potential conflict of interest was reported by the authors.
Ajoy Kumer : 0000-0001-5136-6166
Unesco Chakma : 0000-0003-1711-7216