Density functional theory study of different metal dopants influence on the structural and electronic properties of a tetragonal α-PbO

Lead is known to be one of the post-transition metals in the elements of the Periodic Table. It can be formed as lead sulfide, lead oxide, lead telluride, and lead selenide compounds when it combines with sulfur, oxygen, telluride, and selenide, respectively. Among these, lead oxide has attracted attention due to its good reactivity, cost effectiveness, experimental simplicity, and recyclability.¹ Lead oxide is a semiconductor metal oxide and can be found in various forms, including Pb₃O₄, PbO₂, Pb₂O₃ (α, β, and amorphous), and PbO (α and β).² PbO is type of lead oxide that consists of lead with +2 oxidation states. Recently, both α and β-PbO have attracted many researchers due to their phase changes by simple methods. α-PbO is red in color, has a tetragonal structure, and is also known as litharge. The β-PbO nanoparticle is yellow in color, has an orthorhombic structure, and is known as massicot. PbO has a wide range of applications, such as energy storage devices, luminescence materials, and gas sensors and as modifiers in glass, lead crystals, pigments, nanoelectronic devices, lead glazes, UV blockers, and pigments, and in decorative pottery.^3–7 Lead oxide is also used as a catalyst.⁸ Lead oxide also has potential application in nanodevices and other functionalized materials such as lithium rechargeable batteries, valve-regulated lead acid batteries, and lead acid batteries.⁹ Some reports also suggested that lead oxide is a useful photovoltaic material due to its high carrier mobility, strong absorption cross section, and conductivity. However, the low short circuit current limits its device performance.¹⁰ Despite all these advantages, including its availability and low cost, there is no detailed study on PbO materials. Lead oxide can be synthesized by physical, biological, and chemical techniques. Suryawanshi et al.¹¹ used the ultrasonic spray pyrolysis method to deposit Mn doped PbO thin films on a glass substrate. Noukelag et al.⁵ synthesized lead oxide nanoparticles via rosemary leaf extraction. Mythili and Arulmozhi used the chemical precipitation method to synthesize α-PbO nanoparticles.¹² 

The computational technique is also an effective technique to study the properties of materials. Density functional theory (DFT) is the most reliable method to study the properties of materials in the field of condensed matter of physics, materials science and engineering, chemistry, etc. In this work, we have used DFT by employing the Quantum ESPRESSO package to study the structural and electronic properties and effects of different metal dopants on an α-PbO semiconductor. Doping is one of the techniques to enhance the properties of semiconductors.¹³ Different dopants have been reported for α-PbO experimentally. For example, Srichai et al.¹⁴ used the chemical precipitation method for antimony (Sb³⁺) doped PbO nanoparticles. They argued that the tetragonal phase was obtained before and after Sb³⁺ doping. The energy bandgap of Sb³⁺-doped PbO nanoparticles decreased compared with that of undoped PbO nanoparticles. Azarang et al.¹⁵ also prepared Zn-doped PbO nanoparticles by depositing them on fluorine doped tin oxide with different Zn concentrations to investigate the photocurrent of SnSe nanoparticles, and the two phases of PbO with a majority of orthorhombic structures were revealed. Various scholars used computational methods to study the influence of metal/non-metal doping on physical properties of different metal oxides. For instance, Saini et al.¹³ reported different 3d transitional metal dopings of rutile TiO₂. According to their report, the 3d states of dopants provide impurity energy levels that can either modify both the valence and the conduction band or be revealed separately in the optical bandgap of rutile TiO₂. Depending on the shift in the impurity energy level either to the valence or the conduction band, they were classified as n- or p-type conductivity. Idrissi et al.¹⁶ used DFT calculations to characterize the doping effects of S on ZrO₂, and they observed the decrease in the bandgap due to the existence of impurity states of sulfur. Mbae and Muthui studied the effects of alkaline earth metal doping on the structural and electronic properties of the three phases of TiO₂.¹⁷ Tanko et al.¹⁸ studied the effects of fluorine doped MoO₂ on the structural stability and formation energy by DFT within the generalized gradient approximation (GGA) as implemented in Quantum ESPRESSO. Chetri et al.¹⁹ also reported the structural and optical properties of copper doped SnO₂ by using both experiments and the DFT technique. In Cu doping, they observed the variation in the bond length compared to the undoped one, which causes a change in the structural properties of the system. Vanpoucke et al.²⁰ also performed the DFT study on the influence of aliovalent dopants such as Mg, V, Co, Cu, Zn, Nb, Ba, La, Sm, Gd, Yb, and Bi and oxygen vacancies by selecting Cu, Zn, and Gd dopants on the structural (lattice parameters and crystal radii) and mechanical properties (bulk modulus and thermal expansion coefficient) of fluorite CeO₂. In another study, this group also applied group IV elements (C, Si, Ge, Sn, Pb, Ti, Zr, and Hf) as dopants in order to investigate the dopants’ influence on the properties of fluorite CeO₂ by using the DFT and DFT+U framework.²¹ The computational work has also been reported for different metal dopings of metal sulfides. Nabi et al.²² studied the electronic and magnetic properties of Cr and Mn and co-doping of Cr/Mn doped CdS via DFT. They identified the existence of electron hopping in Cr:CdS and Mn:Cr:CdS, where electron hopping was not allowed in Mn:CdS. Pimachev and Dahnovsky studied the optical and magnetic properties of manganese doped PbS quantum dots by using DFT.²³ Luo et al.²⁴ also reported the thermoelectric enhancement of Ga doped and Ga–In co-doped PbS by using DFT. According to their report, the introduction of Ga into PbS causes Fermi level pinning by creating a gap between valence and conduction bands while the Ga–In co-doping causes the formation of extra conduction bands. According to literature review, even though doping provides a flexible technique to alter the bulk properties of nanomaterials to open up new applications, a full computational research for α-PbO is still lacking. The current effort is motivated by this information gap. The structural and electrical properties of α-PbO doped with several metals are examined in this paper utilizing the DFT approach using the Quantum ESPRESSO package. We used different exchange correlations and four pseudopotentials to achieve the best bandgap approximation of undoped α-PbO. The Perdew–Zunger (PZ) local density approximation (LDA) with ultrasoft pseudopotentials (USPPs), the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) with PAW and USPPs, and the GGA of PBEsol with PAW potential have all been used. The GGA of PBE with USPPs provided the best approximation for the bandgap value. As a result, the GGA of the PBE exchange correlation with USPPs was used throughout the doping computation. For dopant calculations, we used different pseudopotentials to find the optimal optical bandgap of pure PbO, as well as kinetic energy cut-off and k-point convergence. Cell improvements for optimum electronic and structural properties were performed utilizing various pseudopotentials. Aliovalents and 3d transition metal multivalent dopants were used in this investigation. Thus, the oxidation number of lead (Pb) in the host compound is +2, and the +2 oxidation state is used for 3d transition metal dopants for charge balancing. For those d-block element dopants, a spin polarized calculation was performed to derive the magnetic moment, from which we can estimate the oxidation states, and to determine their spin polarizing influence on the electronic structure of α-PbO, which was not performed for other dopants. As a reference, we have also performed spin polarized calculation for pristine α-PbO. Their present oxidation number is directly applied to the aliovalent in order to investigate their influence on the structural and electrical features of the host tetragonal α-PbO.²⁵ 

In this work, the calculations were carried out by Density functional theory (DFT) tools under the implemented Quantum ESPRESSO package code.^23,24 The valence electron configurations of Pb is 5d¹⁰ 6s² 6p² and 2s² 2p² for O₂, which can be seen from the pseudopotentials used. Depending on the pseudopotentials chosen, the dopant configurations for valence electrons of Zn, Sn, Cd, In, Bi, Cu, Li, Ni, and Co are 4s² 4p^0.3 3d^9.7, 4d¹⁰5s² 5p², 5s² 5p^0.5 4d^9.5, 4d¹⁰5s² 5p¹, 4f¹⁴ 5d¹⁰6s² 6p³, 4s^1.53d^9.5, 2s¹, 4s² 3d⁸, and 4s²3d⁷, respectively. We performed our calculation for undoped bulk α-PbO with different exchange correlation functionals, such as Perdew–Zunger (PZ) within the local density approximation (LDA), the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE), and PBEsol. In the GGA, we have used the projector augmented wave (PAW) pseudopotential with Perdew–Burke–Ernzerhof (PBE) and PBEsol exchange-correlation functionals. We have also used ultrasoft pseudopotential (USPP) types for PZ and PBE exchange functionals. Convergence for kinetic energy cut-off and k-points was carried out. A kinetic energy cut-off of 48 Ry, which has been obtained from the convergence calculation, and a charge density cut-off of 392 Ry were used throughout the study. For the representation of Brillouin zone integration, the Monkhorst–Pack-k-point scheme with 6 × 6 × 4 k-points, which were obtained from the convergence calculations, has been used. For partial density of states (PDOS) investigation, a 22 × 22 × 12 k-point was used. For band structure calculations, the high symmetry k-point was used for convenience. For geometry optimization and to find the ground state energy, we have used the Broyden–Fletcher–Goldfarb–Shanno (BFGS) minimization method. For bulk structural relaxation, we used a 48 Ry plane wave cut-off and 6 × 6 × 4 k-point grid. The energy convergence criterion 1.0 × 10⁻⁸ eV for self-consistent relaxation was used.

Figure 1 depicts the crystal structure of pure bulk α-PbO. It has a tetragonal structure and belongs to the P4/nmm, 129 space group. The lead atoms are in positions (0, 0.5, 0.7756) and (0.5, 0, 0.2244), whereas the oxygen atoms are in positions (0, 0, 0) and (0.5, 0.5, 0). All the pseudopotentials employed in this study are to perform structural relaxation for geometry optimization. After optimization, the lead atom’s positions were modified to (0, 0.5, 0.7842) and (0.5, 0, 0.2158), whereas oxygen atoms remained at their original places (0, 0, 0) and (0.5, 0.5, 0) while employing the GGA of PBE within USPPs. Since the values of the lattice constants were altered during the calculation of vc-relax, the unit cell volume varied. In the case of PBE-USPPs, a modest increase in lattice constants is seen after vc-relaxation calculations. This value, however, agrees with the experimental value with just minor differences. Table I compares the estimated lattice constants a and b and lattice constant c for different pseudopotentials to other DFT studies and experimental data. The result determined by utilizing PBEsol agrees well with the experiments and other computational studies.²⁷ Furthermore, Figs. 2(a) and 2(b) show the total energy of pure α-PbO materials as a function of lattice constants a and c, respectively. As indicated in Fig. 2(a), the minimum energy was obtained at a = 3.997, whereas the minimum energy was obtained at 5.33 for lattice constant c, as shown in Fig. 2(b). In this scenario, the lattice constant c was held constant while the value of the lattice constant a was varied until the minimal energy was found. By altering lattice constant c, the value of the optimized lattice constant a was kept constant. The computed lattice constants closely match the experimental value with very minor deviations.

Table II shows the changes in lattice constants caused by dopants. Thus, the calculated lattice constants a and b for pristine α-PbO and those doped by Zn, Sn, In, Bi, Cd, Cu, Li Ni, and Co are given in Table II. These changes in lattice constants produce volume changes, which result in structural alterations in α-PbO. The lattice constant a changes slightly during doping. However, there is a significant change in lattice constant c when Li and Co are doped. As a result, structural characteristics may alter dramatically. In general, other dopants have little effect on structural properties. Because of the atomic size and electronegativity of the dopants, the bond lengths surrounding the dopants varied, resulting in structural property changes.¹⁸ This change in structural characteristics may enhance the system’s structural stability.

Figures 3–6 illustrate the electrical band structure and PDOS plot of pure tetragonal α-PbO using different pseudopotentials. It is critical to investigate the electronic structure of bulk α-PbO materials for application in optoelectronics technology. For the electrical properties of pure bulk α-PbO, we used several exchange correlations and pseudopotential types. The GGA of the PBEsol exchange functional with PAW potential, the GGA of the PBE exchange functional with USPP potential, and the LDA of the PZ exchange functional with USPP potentials were used and compared to the experimental results, as shown in Table I. This material is recognized to be an ionic bonding material. This ionic bonding can be explained by PDOS, as shown on the right side of Figs. 3(b)–6(b). The orbitals of 2p of O and 6p of Pb are different above the Fermi level. The PDOS has a bonding and anti-bonding character. According to the PDOS, the valence band maximum (VBM) is largely made up of oxygen (O) 2p orbitals combined with lead (Pb) 6s and 6p orbitals. The conduction band minimum (CBM) is mostly made up of Pb 6p orbitals. Thus, based on the PDOS calculations shown in Figs. 3(b)–6(b), Pb of 6p dominated the states above the Fermi level, while O 2p dominated the states below the Fermi level.

Different pieces of information can be extracted from the electrical band structure of pure bulk α-PbO. The presence of a narrow bandgap demonstrates that this is an oxide semiconductor. The valence band maximum (VBM) and conduction band minimum (CBM) are located at different high symmetry sites, confirming that this material is an indirect bandgap semiconductor. The CBM is located at the M-point, and the VBM is found at the Γ-point in the Brillouin zones, as seen in Figs. 3(a)–6(a). Except for the amount of the energy bandgap, the VBM and CBM positions are unchanged for all applied pseudopotentials. The reported experimental energy bandgap value of bulk α-PbO is 1.9 eV, while we calculated 1.08, 1.16, 1.30, and 1.75 eV using the LDA of PZ with USPP, the GGA of PBEsol with PAW, the PBE with PAW, and the PBE with USPP potentials, respectively. Grimes²⁷ employed the Garrity-Bennett-Rabe-Vanderbilt (GBRV)-type ultrasoft pseudopotentials to calculate the bandgap of bulk-PbO and obtained the same 1.16 eV that we did from the PBEsol PAW in Table I. In this case, we used exchange correlation and pseudopotentials to get the optimum bandgap approximation. Among all the results, the bandgap obtained by the GGA of PBE with USPP potential is quite close to the experimental value. As a result, we used the GGA of PBE with the USPP type throughout the doping, and the findings obtained by utilizing the GGA of PBE with USPP for pristine α-PbO are used as a reference to observe the dopant effects. In the plot of the band structure and PDOS, the highest occupied energy level (Fermi energy, E_f) position is indicated at zero values of energy E-E_f.

It is important to explore the band structure and density of states of α-PbO in order to develop applications in electronics technology. The effect of several metals on the electronic properties of α-PbO has been investigated in this section. We employed the same k-point mesh for the Brillouin zone sample and the same kinetic energy cut-off plane wave in doping after optimization and convergence calculations. The PBE pseudopotential is used through all dopings. The energy bandgap in pure bulk α-PbO narrows with respect to pristine α-PbO due to alteration of the valence and conduction bands or insertion of an impurity energy level. The variations in the energy bandgap due to dopants are given in Table III. The plots of the energy band structure and PDOS show that either the valence and conduction bands have been modified or an impurity energy level has been inserted. The appearance of the impurity energy level is caused by dopant state hybridization with O and Pb states. In transition metal doping, we employed dopants with 2+ oxidation states, and for convenience, we just performed spin-polarized calculations for magnetic moment estimation, which allows us to determine the oxidation states of these elements and to obtain the best electronic structures.

Group III–V Periodic Table elements include both metal and nonmetal elements. In this work, we only used the metal elements to incorporate into pristine α-PbO. When these metals are doped into pristine α-PbO, the structural, electrical, and optical properties of α-PbO are changed. Due to their advantages in improving the properties of intrinsic and oxide semiconductors, these groups of metal elements have been used as dopants in different semiconductors. In this work, we took one element from each group to incorporate into pristine α-PbO that may help incorporate the rest of the metals into α-PbO.

Tin (Sn) is a fourth group element that is employed as a covering material due to its low cost. When it is doped with metal oxides, it improves electron transport and extraction, which is useful in solar cells. In our case, Sn²⁺ is introduced into the host α-PbO for the benefits of charge balancing, which eliminates the formation of further impurity states. Figure 7(a) depicts the electronic band structure of Sn doped α-PbO. When compared to pure α-PbO, the location of the VBM and CBM does not change, resulting in an indirect bandgap. Sn doping in α-PbO causes impurity energy levels in both the valence and conduction bands, resulting in a reduction in the energy bandgap from 1.75 eV for pure α-PbO to 1.53 eV. As a result, both the valence and conduction bands have been changed, resulting in a smaller bandgap. As seen in Fig. 7(b), the Fermi level appears above the VBM in this instance, and electrons in the valence band can jump to the conduction band with a low energy demand compared to pure α-PbO. The reduction in the bandgap during doping may lead to red shifts in optical properties. Moreover, the contribution of the Sn dopant can also be observed from the PDOS shown in Fig. 7(b). The Sn 5p partial density of state contributes to the conduction band in addition to Pb 6p, while the Sn 5s and O 2p partial densities of state contribute to the valence band like pure α-PbO. The existence of Sn 5p states near the Fermi energy in the VBM confirms the incorporation of Sn into pure α-PbO.

Indium (In) is a third group element, which is suitable for doping into metal oxides due to its radius and valence. It is well-known for producing indium tin oxide (ITO), which is utilized in a variety of technologies, including solar cells, touch screen technology, and flat screen televisions. When it is doped into α-PbO, the conductivity properties improved due to valence band changes and the appearance of impurity states. The valence band is pushed down in this scenario, and an impurity state appears by overlapping with the Fermi energy level. As illustrated in Fig. 8(a), the emerging dopant state forms the VBM at Γ points and CBM at M points. The indirect bandgap property of α-PbO is retained in doped α-PbO, but the bandgap is lowered to 1.43 eV. The contribution of the partial density of state (orbital In 5s, 5p) overlap with the Fermi level is also visible in Fig. 8(b), which was not visible in undoped α-PbO. In the conduction band, the contribution of the In 5p partial density of state is significant, whereas the contribution of In 5p found near to the Fermi energy shows the incorporation of In into α-PbO and its correlation with the O 2p states (In–O).

Bismuth (Bi) is one of the fifth group elements that enhance the electronic conductivity when incorporated into metal oxides. The band structure of Bi doped α-PbO is presented in Fig. 9(a). In this case, the Fermi level penetrates into the conduction band with the appearance of states above the Fermi level. In this case, the position of the CBM is changed from the M-point to between Γ- and Z-points while the VBM appears at its position as in pristine α-PbO. By modification of the conduction band, the bandgap is reduced to 1.48 eV. From Fig. 9(b), the contribution of the Bi partial density of state orbital Bi 6p in the conduction band can be observed whereas Bi 6s is important in the valence band. The Bi 6p orbital state revealed the overlapping of the Fermi level with the conduction band. Thus, the conduction band states are mainly composed of Pb 6p and Bi 6p whereas the valence band states are composed of O 2p and Bi 6p and of Pb 6s and 6p orbitals. The formation of Bi 6p states in the conduction band and valence band shows the substitution of Pb by Bi and the correlation with O 2p states, respectively.

Alkali metals are found in group IA of the Periodic Table, and they are very reactive species; they can easily lose their single outermost valence electrons to form ionic compounds. Thus, they can easily react with nonmetal elements. The catalytic properties of materials can be improved when they are used as dopants. In addition to this, the structural, electrical, and optical properties of metal oxide can be improved when they are incorporated into them. From this group, we incorporated Li metal into PbO and investigated its effects on the properties of the hostmaterial.

Lithium (Li) is one of the least electronegative alkali metals, and when added to α-PbO, it increases electron transport in this material. As a result, when it is doped into α-PbO, it changes both the valence and conduction bands, which results in changing the electrical and optical properties of undoped α-PbO. Figure 10(a) depicts the electronic band structure of Li doped α-PbO. As can be seen in the band structure of Li doped α-PbO, an indirect bandgap similar to virgin α-PbO was produced. However, when compared to virgin α-PbO, the VBM and CBM are generated at distinct sites. VBM appears at the M-point, whereas CBM appears at the Z-point. In this case, the obtained bandgap is 1.57 eV, which is a small decrease when compared to the other dopants utilized in this work. This enlightened us. Doping Li into α-PbO has little effect on the system's optical and electrical properties. An additional set of conduction bands is revealed. Figure 10(b) depicts the contribution of the Li orbitals to density of states. By mixing with the Pb 6p orbital, the contribution of the Li partial density of state of orbital Li 2s is significantly detected in the conduction band, which confirms the Pb site substation. The Li 2s states found in the valence band near to the Fermi energy show its relation with O 2p states.

For the investigation of the electronic structure of transition metal doped α-PbO, we performed spin polarized calculations in order to include the influence of spin polarization of these dopants on the electronic properties of α-PbO. Transition metals have good electron transporting characteristics, and when doped into metal oxides, they improve the electronic properties. These elements have various valences and are partially or fully filled d orbitals, which can be classified as 3d, 4d, and 5d transition metals. Specially, in their oxide forms, they have potential technological applications in solar cells, photoelectrochemistry, energy conversion devices, and photocatalysis due to their low cost production and relative abundance.²⁸ Nickel (Ni), cobalt (Co), Copper (Cu), and Cadmium (Cd) are chosen from among the transition metals to examine their effects when integrated into α-PbO. In order to investigate the influence of these dopants on the electronic properties, we have performed spin polarized calculations for pristine α-PbO as a reference. Figures 11(a) and 11(b) show the electronic band structure diagram and PDOS of spin polarized pristine α-PbO. Compared to the non-polarized calculation, due to its non-magnetic properties, its electronic structure is almost the same. The energy bandgap value and contributions of orbital states also show the same properties as the non-polarized calculations.

Transition metals have noticeable high heat and electrical conductivity qualities. As a result of its incorporation into α-PbO, it improves its electrical characteristics. Because several of these transition metals have multivalents, only the analogous chemical state with a host substitution is used to avoid extra contaminants during charge balancing. A basic optimization of scf calculation was performed to determine their magnetic moment in order to estimate the oxidation state used. We can simply predict their oxidation states if we know their magnetic moment. In this case, the electronic valence configuration of Ni²⁺ is 3d⁸. As a result, Ni with a 2+ oxidation number was employed in the doping of α-PbO. In the case of Ni doped α-PbO, the valence and conduction bands are modified, and an impurity energy level is formed in the midst of the VBM and CBM, in contact with the CBM and separated from the VBM by a gap, as shown in Fig. 12(a). As a result, there is no gap between the impurity energy level and the conduction band; instead, the impurity energy level produces a bandgap with valence bands, resulting in a 1.36 eV bandgap. In actuality, this is a narrow bandgap for electron excitation from the valence band to the conduction band, which requires little energy to leap to the conduction band. The PDOS shown in Fig. 12(b) depicts the contribution of the Ni orbitals to the density of states. In the same conditions as other d-block doped α-PbO, the orbital Ni 3d states contribute more to the density of states below the Fermi level, whereas the orbital Ni 4p states contribute more to the density of states above the Fermi level. Compared to undoped α-PbO, the Ni-doped PDOS revealed many densities of states near to the Fermi level, besides the valence bands. The states found near the Fermi level are Ni 3d, Pb 6p, and O 2p and Ni 4p states of less density. The formation of Ni 3d states in this region shows the chemical reaction relations of Ni 3d with O 2p states (Ni–O).

According to the pseudopotentials utilized, Co has an electronic valence configuration of 3d⁷. As a result, cobalt with a 2+ oxidation number was employed in the doping of α-PbO. The valence band in Co²⁺ doped α-PbO has been modified by the introduction of the impurity energy level, and an extra impurity energy level has appeared between the VBM and CBM, as shown in Fig. 13(a). The upper impurity energy level that appears makes contact with the minimum of the conduction band and forms a minimum on the Z-point, and by overlapping with the lowest impurity level, the impurity level for valence band modification forms a maximum for the valence band at the A-point, reducing the energy bandgap to 1.31 eV. The decreased bandgap may be attributed to the strong correlation influence between the Co 3d states and the 6p of Pb.²⁹ As a result, Co-doped α-PbO possesses a half metallic property. It has p-type conductivity due to the relocation of the Fermi level position to the valence band. Except for the change in the position of the maximum and minimum of the valence and conduction bands, it retains its semiconducting indirect bandgap. Figure 13(b) shows the PDOS of Co-doped α-PbO. The orbital of Co 3d states and Pb 6p states dominates the density of states in the conduction band. The mixing of Co 3d, O 2p, Pb 6s, and Co 4s states results in the partial density of state in the valence band. An additional impurity energy level overlaps with the Fermi level, as shown in the band structure and PDOS. As a result, these impurity states serve as acceptors. In the same manner as Ni-doped α-PbO, the Co-doped α-PbO 3d states in the spin up and spin downs are asymmetrical, which may be attributed to a part of 3d state electrons transmitted to the conduction band via the Fermi level.³⁰ In addition to this, Co and Ni have unpaired electrons according to crystal field theory, from which we can estimate the magnetic moments.

Copper has a strong electrical conductivity quality, and when it is mixed into α-PbO, it increases the material’s electrical conductivity. Figure 14(a) depicts the plot of the Cu doped electronic band structure. The impurity energy level in Cu doped α-PbO appears in the midst of the VBM and CBM, extremely connected to the CBM, by establishing an indirect gap with the VBM at M- and A-points. As a result of pushing up the VBM and the contribution of Cu impurity, the bandgap is lowered to 1.30 eV. The donor level is the impurity condition that appears closest to the conduction band. In this situation, the Fermi level enters the valence band due to the development of some electron-filled states. The impurity level appeared above the valence band makes the electrons to get sufficient energy to jump from the valence band to the impurity level and then with less energy to the conduction band. As a result, some of the free electrons in the conduction band contribute to conductivity when it is in equilibrium. The PDOS data in Fig. 14(b) also confirm that the Fermi level penetrates the valence band. The orbital state contribution of dopants is also disclosed, demonstrating the contribution of partial density of states of orbitals Cu 3d and O 2p in the valence band. Other orbitals having partial densities of states in the conduction band include orbitals Cu 4p and Cu 4s, as well as the orbital Pb 6p. The contribution of Cu 3d states to the valence band shows its correlation with O 2p states and its strong correlation with Pb 6p in narrowing the bandgap.

Zn is a transition metal having a 2⁺ oxidation state that can be used to galvanize metals to prevent rusting. When integrated into tetragonal α-PbO, it improves the material’s electrical characteristics. Figure 15(a) depicts the band structure of Zn-doped α-PbO. As stated in Sec. III C, the band structure of pristine α-PbO confirms the bulk α-PbO’s indirect bandgap feature. The Zn-doped α-PbO band structure, on the other hand, exhibits the VBM at the R-point and the CBM at the M-point. In between these bandgaps, an impurity band from the dopant is emerged. The bandgap from the VBM to an impurity band is 0.81 eV whereas the gap between impurity bands to the CBM is 1.0 eV. This may show the good electrical and optical properties of this material as electrons can easily transferred from band to band by applying very low energy. The introduction of dopant states to the intrinsic semiconductor is one of the consequences of trapping the photogenerated carriers, which results in separation of the carriers. This nature is important in photocatalytic applications, which limit charge recombination by allowing charge carriers to diffuse to the surface.¹⁷ When Zn states are located above the valence band, they can operate as a trapping point for holes. Figure 15(b) illustrates the contribution of the orbitals of Zn, Pb, and O states to the density of states. The O 2p, Zn 3d, and Pb 6s mainly dominate the partial density of states below the Fermi level that is dominant in the valence band, while the contribution of the Pb 6p and Zn 4p partial densities of state dominates the conduction band. At the same time, the bandgap changes are attributable to Zn dopant orbital state hybridization with host metal oxide atoms.

Cd is a metal with extraordinarily high electrical conductivity that is used in batteries and electroplating. Doping it into α-PbO has the advantage of increasing electrical characteristics. The insertion dopant states in the bandgap are visible in the Cd-doped α-PbO electronic band structure, as shown in Fig. 16(a). As a result, an impurity energy state appears between the VBM and the CBM. The VBM is located at A-points, and the CBM is located at M-points. The bandgap between the VBM and an impurity band is 0.92 eV whereas the bandgap between the CBM and an impurity band is 0.46 eV. In addition, an extra conduction band is also seen on the band structure map. Figure 16(b) depicts the contribution of Cd dopant orbitals to the PDOS. The partial density of states above the Fermi level is mainly dominated by Pb 6p and Cd 4p states and with the small densities of Cd 4s and Pb 6s. Due to the incorporation of Cd, extra states emerged above the Fermi level between the valence band and conduction band compared to the undoped α-PbO. The contribution of O 2p and Cd 4d states is revealed for the valence band. The impurity states found between the VBM and the CBM are from the contribution of Cd 4p, Pb 6p, Cd 4s, and Pb 6s states, which are emerged due to Cd doping into α-PbO. In general, the half metal feature of Cd-doped α-PbO boosts its electrical conductivity.

When intrinsic semiconductors are doped with electrons, they become n-type semiconductors, whereas when doped with holes, they become p-type semiconductors. For intrinsic semiconductors, the energy level appears in the middle, shifting downward as hole doping concentration increases and upward as electron doping concentration increases. According to the plots of the electronic band structure, all dopants except Bi doped α-PbO have p-type conductivity, whereas Bi doped α-PbO has n-type conductivity. It meant that the n- or p-type conductivity characteristics of pure bulk α-PbO could be easily tuned for specific purposes by doping with different metal dopants. The energy level (Fermi level) can either penetrate the conduction or valence bands depending on the connections between carrier concentrations and density of states. Thus, if the carrier concentration is greater than the density of states during doping into intrinsic semiconductors, the energy level penetrates the conduction band, which is known as the Burstein–Moss effect.^26,27 This phenomenon is seen in Bi-doped α-PbO, but the energy level penetrates the valence band in In, Cu, Li, Ni, and Co doped α-PbO. The penetration of energy levels in the valence band may be attributed to a lack of electrons in the valence band, as opposed to the presence of surplus hole concentrations. The increasing concentration of holes in the valence band is known as the inverse Burstein–Moss effect.

Density functional theory was used to study the structural and electrical properties of doping of various metals on tetragonal α-PbO using the Quantum ESPRESSO package code within the GGA of the PBE exchange correlation with ultrasoft pseudopotential. For the best approximation of the bandgap of α-PbO, four distinct forms of pseudopotentials under the LDA and GGA exchange correlations are used. For the p-block metal elements and alkali metal dopants, we performed the non-polarized calculations whereas for d-block element dopants, we have performed spin polarized calculations to include their spin polarizing influence on electronic structures of α-PbO. The bandgap narrowing is disclosed as a result of various metal dopings caused by VBM and CBM changes. The obtained energy bandgaps for pristine α-PbO and Zn, Sn, Cd, In, Bi, Cu, Li, Ni, and Co doped α-PbO are 1.75, 1.35, 1.53, 1.38, 1.43, 1.48, 1.30, 1.57, 1.36, and 1.31 eV, respectively. The lattice constants have been changed compared to pristine α-PbO, which causes a change in the structural property of α-PbO. For Zn, Sn, Cd, In, Bi, Cu, Li, Ni, and Co doped α-PbO, lattice constants a and b are 4.071, 4.007, 3.955, 4.002, 4.215, 4.247, 3.929, 4.017, 3.778, and 3.759 Å, respectively; while lattice constant c is 5.507, 4.788, 5.300, 4.828, 4.915, 4.803, 4.268, 3.411, 4.245, and 3.951 Å, respectively. The shift in the Fermi level creates n-or p-type conductivity. Thus, α-PbO with different metal dopings can be utilized for the fabrication of semiconductor devices used as both n- and p-type semiconductors. Bi is the only dopant with n-conductivity, while the others have p-type conductivity. Because of the relatively tiny bandgap, Co-doping possesses a half metallic characteristic. These dopants may improve the structural properties of undoped α-PbO, which in turn may improve the optical and electrical properties of pure α-PbO by managing the bandgap. The PDOS figure shows the contribution of dopant orbitals to the partial density of state, as well as the position of the Fermi level and impurity energy level. Since the dopants in this study were chosen from various periods and groups of the Periodic Table, one can readily grasp the effects of these dopants in α-PbO and examine the impact of the elements in these periods and groups that were not used for doping into α-PbO in this study.

The authors have no conflicts to disclose.

Fikadu Takele Geldasa: Conceptualization (equal); Data curation (equal); Writing – original draft (equal); Writing – review & editing (equal). Mesfin Abayneh Kebede: Conceptualization (equal); Supervision (equal); Validation (equal). Megersa Wodajo Shura: Methodology (supporting); Resources (supporting). Fekadu Gashaw Hone: Conceptualization (equal); Supervision (equal); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.