Pb 6p Orbitals Research Articles

Theoretical study on the structural, electronic, mechanical, vibrational, thermodynamical, and optical properties of the two-dimensional PbC nanomaterials

Two-dimensional structures have attracted attention for application in nanoelectronics and optical devices; then, in this work, we are reporting the predicted physical properties (from first-principles calculations) for the two-dimensional PbC systems. Those physical properties reveal that the PbC monolayers (M-PbCs) in crystallographic planes (111) and (100); moreover, the PbC2 structures (paramagnetic and anisotropic compounds) are thermodynamical, structural, and mechanically stable but energetically and dynamically unstable at T = 0 K. However, the PbC2 non-magnetic (NM) is the most stable system at high temperatures. The M-PbCs exhibit sp 2 hybridization while the PbC2 NM shows sp 3 d 2 hybridization, forming a hexagonal lattice; meanwhile, the strong interaction at the C’s double bond in the PbC2 ferro and antiferromagnetic configurations (MAG) generates a rectangular lattice. These systems are ductile materials; however, the PbC2 (with metallic bonds) is more ductile than the M-PbCs due to the pronounced participation of the Pb 6p-orbitals. The M-PbCs have associated greater values for the hardness (than those for the PbC2 systems), but at high temperatures, the PbC2 MAG exhibits the highest mechanical resistance. The calculated optical data show that the M-PbCs and the PbC2 NM are promising as refractory materials. At the same time, the PbC2 MAG could be helpful in optical and optoelectronic devices capable of operating in the low frequencies of the UV region and in the infrared and visible regions.

First-principles study of the effect of pressure on the physical properties of PbC

Silicon carbide has been used as a cutting material and as a semiconductor in lighting and power electronics. Results from some studies, carried out on IV-IV group carbides like GeC and SnC, allow to identify potential technological applications of these carbides in extreme environments, opening the possibility to find new carbides for similar applications. For this work, the PbC was studied under hydrostatic pressure in the framework of the Density Functional Theory, obtaining relevant information on its structural, electronic, mechanical, vibrational, thermodynamical, and optical properties. The optimized lattice parameter and volume, and electronic bands structures type agree with the available theoretical data at zero GPa. The calculated enthalpy values show a phase transition, from the B3 structure (CsCl-type) to the B1 structure (rocksalt or NaCl-type), at 23.5 GPa. The PbC is energetically, mechanically, and dynamically stable for all the pressure values in the studied range; it is a metallic, anisotropic, and brittle material with paramagnetic ionic-covalent bonds and good hardness (the highest mechanical resistance was found above T = 370 K). As the pressure increases, it was noted: (i) the increase of the electronic cloud around the C and Pb atoms, (ii) the DOS spread, (iii) the change to be a ductile material with a tendency to the metallic bonds and (iv) an increase of the hardness and the Young modulus, due to C 2p and Pb 6p-orbitals. Our results show that the PbC is a promising material for applications in the development of optical and optoelectronic devices, and to be used as a protective coating against the low frequencies in the UV and infrared and visible regions.

A DFT study of the Pb ion adsorption on smithsonite (1 0 1) surface in aqueous system

Understanding the adsorption mechanism of Pb ion on smithsonite surface is crucial for the sulphidation xanthate flotation of smithsonite. In this paper, the adsorption mechanism of Pb ion on smithsonite (101) surface in aqueous system was first investigated by DFT simulation. We further considered the effect of water solvent and analysed the structures of Pb ion hydrates in aqueous system, and finally the [PbOH(H2O)4]+ and Pb(OH)2(H2O)2 were employed as the main solvation components to study the Pb ion adsorption. Then, the adsorption behaviour and bonding mechanism of which on smithsonite surface were studied by analysing the adsorption configurations and electronic properties. We found that the hollow site with an eight-membered ring was the main adsorption site. In addition, H-bond played an important role in the stability of the adsorption configurations of the Pb ion hydrates on smithsonite surface. Moreover, we found that the bonding of Pb-O was mainly attributed to the hybridization of Pb 6s and Pb 6p orbitals with O 2p orbital and hybridization of Pb 5d orbital with O 2s orbital, and the formed PbO bond exhibited some ionic bond characteristic. The obtained adsorption structures corresponded well with the previous experiment.

Extensive investigation of structural, electronic, optical, and thermoelectric properties of hybrid perovskite ( CH 3 NH 3 PbBr 3 ) with mechanical stability constants

Recently, Organometallic halide based perovskites have emanated as an auspicious candidate as a solar cell absorber layer. In this article, we have explored the fundamental properties such as structural, electronic, optical, elastic, and thermoelectric parameters of CH3NH3PbBr3 through first-principles calculations, because it has accomplished the entire criterion to use in photovoltaic and thermoelectric applications. We have used full-potential linearized augmented plane wave method (FP-LAPW) within DFT and implemented in Wien2k. The generalized gradient approximation (GGA) parameterized by Wu-Cohen (WC) has been used to optimize the lattice parameter, while for band gap calculations different exchange-correlation potentials (LDA/GGA) have been used. The band gap up to 2.26 eV has been achieved by doing some appropriate changes in the parameter of TB-mBJ exchange-correlation potential. The nature of band gap is direct and exist at R (0.5 0.5 0.5) symmetry point of the Brillouin zone. All the optical spectral response between 2 and 5 eV is due to the transition of Br 5p with little contribution Pb 5s orbital electrons of VBM to Pb 6p orbitals in CBM and a minor contribution of second band gap components also incorporate. As well as, a high absorption coefficient shows that it may be strongly applied in photovoltaic devices. The orientation of organic cations (CH3NH3)+ has no considerable impact on the band structure formation. To render a solid foundation about the application in the thermoelectric device up to the high-temperature region, the thermoelectric parameters have been discussed at optimal carrier concentration and definite temperature range. The measurement of elastic constants, B/G and Poisson's ratio indicates the ductile nature of CH3NH3PbBr3. To the best of my knowledge, most of the investigations have been discussed first time for this material.

Light Absorption Properties and Electronic Band Structures of Lead-Vanadium Oxyhalide Apatites Pb5 (VO4 )3 X (X=F, Cl, Br, I).

The Pb-V oxyhalide apatite compounds Pb5 (VO4 )3 X (X=F, Cl, Br, I) were successfully synthesized using a facile solution method and studied with respect to their structural/optical characteristics and electronic band structures. UV-visible diffuse reflectance spectroscopy, electrochemical analysis and first-principles calculations showed that the synthesized apatites behaved as n-type semiconductors, with absorption bands in the UV-visible region that could be assigned to electron transitions from the valence band to a conduction band formed by hybridized V 3d and Pb 6p orbitals. Among the apatites examined, Pb5 (VO4 )3 I had the smallest band gap of 2.7 eV, due to an obvious contribution of I 5p orbitals to the valence band maximum. Based on its visible light absorption capability, Pb5 (VO4 )3 I generated a continuous anodic photocurrent under visible light (λ>420 nm) in a solution of 0.1 m NaI in acetonitrile.

Interfacial Bonding and Electronic Structure between Copper Thiocyanate and Hybrid Organohalide Lead Perovskites for Photovoltaic Application.

Interface engineering of halide perovskite solar-cell device layers has been showing potential for the enhancement of efficiency and stability. The replacement of spiro-OMeTAD with cheap hole extraction layer CuSCN has emerged as one effective method for the long-term large-scale application. To clarify the interfacial mechanism between the halide perovskite layer and the CuSCN layer, first-principles calculations were performed. An interface-induced electrostatic potential increase and electron-hole excitation were observed at the interfaces. A Cu-I bond and a Pb-N bond were formed at the interface. The MAI interface exhibited a larger ionic displacement and a higher interface-induced potential than the PbI interface. Strong hybridization among Cu 4d, I 5p, N 2p, and Pb 6p orbitals at the interface was found to be the origin of the electron-hole excitation, interface reconstruction, electron charge redistribution, and stability enhancement.

Ba2M(C3N3O3)2 (M = Sr, Pb): Band Engineering from p-π Interaction via Homovalent Substitution in Metal Cyanurates Containing Planar π-Conjugated Groups.

The two new metal cyanurates Ba2M(C3N3O3)2 (M = Sr, Pb) were successfully grown by a solid-state cyclotrimerization reaction. The electronic energy bands of Ba2M(C3N3O3)2 are totally divergent in spite of their same structures and similar interlayer distances. Theoretical calculations show the narrowing band gap of Ba2Pb(C3N3O3)2 stems from the strong interaction between Pb 6p orbitals and anti-π orbitals in (C3N3O3)3- groups.

The structural, electronic, and optical properties of organic–inorganic mixed halide perovskites CH3NH3Pb(I1−yXy)3 (X = Cl, Br)**Project supported by the Financial Support from the “12th Five-Year” Science and Technology Research Project of the Education Department of Jilin Province (Grant No. [2016] 494) and the National Natural Science Foundation of China (Grant No. 21173035).

Methylammmonium lead iodide perovskites (CH3NH3PbI3) have received wide attention due to their superior optoelectronic properties. We performed first-principles calculations to investigate the structural, electronic, and optical properties of mixed halide perovskites CH3NH3Pb(I1−yXy)3 (X = Cl, Br; y = 0, 0.33, 0.67). Our results reveal the reduction of the lattice constants and dielectric constants and enhancement of band gaps with increasing doping concentration of Cl−/Br− at I−. Electronic structure calculations indicate that the valance band maximum (VBM) is mainly governed by the halide p orbitals and Pb 6s orbitals, Pb 6p orbitals contribute the conduction band minimum (CBM) and doping does not change the direct semiconductor material. The organic cation [CH3NH3]+ does not take part in the formation of the band and only one electron donates to the considered materials. The increasing trends of the band gap with Cl content from y = 0 (0.793 eV) to y = 0.33 (0.953 eV) then to y = 0.67 (1.126 eV). The optical absorption of the considered structures in the visible spectrum range is decreased but after doping the stability of the material is improving.

Theoretical study on the stability and photoelectric properties of APbI3 perovskite

The rapid development of organic-inorganic hybrid perovskite solar cells has recently attracted the worldwide attention because their power conversion efficiency has risen from 4% to higher than 20% within just six years. It is well known that the perovskite materials with APbI3 crystal structure have a 3D framework of corner-sharing PbI6 octahedra, in which each Pb atom bonds with six I atoms, and the A cations fill in the octahedral interstices. At present, a lot of researches have focused on the synthesis and doping modification of perovskite materials. However, it is hard to detect directly the weak interactions between A cations and PbI6 skeleton in the APbI3 crystal structure through experiments, which have effect on the structural stability and electronic properties. To provide a full understanding of the interplay among size, structure, and organic/inorganic interactions, the stability, electronic structures and optical properties of APbI3 (A denotes Cs+, NH4+, MA+, FA+) were investigated by the plane-wave ultra soft pseudo potentials. Two dispersion corrections were taken into account in the weak interactions between A cations and PbI6 skeleton in the APbI3 crystal structure, respectively. The results show that the type and size of cations affect the distortion of PbI framework, indicating that the larger the radius of the A cation is, the stronger the interaction between the A cation and the PbI framework is. Further, it is identified that after geometry relaxation, the orientation of A cations (A denotes NH4+, MA+, FA+) is easy to change, and the PbI frameworks present structural distortion. CsPbI3 is more stable energetically than other three kinds of perovskite materials. For the PbI6 octahedra, the large dipole moments of 0.23D and 0.32D for the generalized-gradient approximation method or 0.28D and 0.29D for the local-density approximation method are also present in MAPbI3 and FAPbI3, respectively. In addition, the energy band structures, which affect the generation and migration of photon-generated carriers and optical properties, will alter with the structural distortion of PbI frameworks. By analyzing the energy band structures and corresponding density of states, we find that four systems have similar band structures near the Fermi energy, namely, the top of valance band is mainly contributed by I 5p orbitals, while the bottom of conduction band is dominated by Pb 6p orbitals and partly contributed by I 5p orbitals. A little difference of their electronic structures and optical absorption spectra originates from the distortion of PbI6 octahedra in APbI3 crystal structures. It is noted that the contribution of the ions Cs+ and FA+ on the top of valance band is slightly larger than that of the ions NH4+ and MA+. Compared with other three kinds of perovskite materials, CsPbI3 presents the narrowest direct band gap, the lowest effective carrier mass and excellent visible-light and infrared absorption. The results may provide some theoretical guidance for further research on perovskite materials in the application of solar cells.

Band alignment of MAPb(I1–xBrx)3 thin films by vacuum deposition

In this work, the mixed bromide iodide lead perovskites CH3NH3Pb(I1–xBrx)3 (0 ≤ x ≤ 0.67) thin films were prepared by co-evaporation of CH3NH3I, PbI2, and PbBr2. The electronic properties of CH3NH3Pb(I1–xBrx)3 thin films were investigated by X-ray and ultraviolet photoelectron spectroscopy in-situ. The results of core level binding energy show that there is no chemical shift of the C1s, N1s, Br3d5, and I3d5 when the Br composition changes, while there is an approximately linear chemical shift of Pb4f7 to higher binding energy as the Br composition increases. The density functional theory calculation reveals that there is more charge transfer from Pb to Br than I, which results in the chemical shift of Pb4f states. On the other hand, the valence band maximum increases as the Br composition increases, while the work function shows no obvious change, because the conduction band is dominated by Pb 6p orbitals while the valence band is dominated by halide p orbitals. Our work demonstrates the adjustability of the energy level alignment of MAPb(I1–xBrx)3 by the Br composition.

Exploring the electronic structure of Pb2+ ions containing material Pb16(OH)16(NO3)16

A theoretical band structure calculation for lead nitrate hydroxide Pb16(OH)16(NO3)16 single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70eV in close agreement to the measured one (3.78eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbOn ployhedra and N–O of [NO3]− triangle. The calculated bond lengths and angles show good agreement with the experimental data.

Study on the correlations between the structure and photoelectric properties of CH3NH3PbI3 perovskite light-harvesting material

The [PbI6] inorganic framework in the CH3NH3PbI3 could be affected by the oxidization effect of I− ions in atmosphere. The [PbI6] octahedral torsion angle increased ∼4.94° after the newly prepared CH3NH3PbI3 powders were stored in dry atmosphere for 30 days at 20 °C according to the results of XRD Rietveld refinement and X-ray Absorption Fine Structure. Analyses based on density of states indicated that UV–vis adsorption edge at ∼850 cm–1 corresponded to electron transition from I-5p orbitals to empty Pb-6p orbitals. The increased [PbI6] octahedral torsion angle made the band gap energy of CH3NH3PbI3 increased according to First-principle calculation and UV–vis absorption spectra. As a result, the band gap energy of CH3NH3PbI3 were strongly related to the structure of [PbI6] inorganic framework because the electrons diffused along the [PbI6] octahedral chains in CH3NH3PbI3 perovskite.

Overall Splitting of Water by RuO2-Loaded PbWO4 Photocatalyst with d10s2-d0 Configuration

Tetragonal lead tungstate, PbWO4, consisting of a WO4 tetrahedron showed high and stable photocatalytic activity for the overall splitting of water, producing a stoichiometric quantity of H2 and O2 under UV irradiation when loading RuO2 on the metal oxide. The activity was found to strongly depend on the preparation temperature of PbWO4. In the temperature range 773−1273 K, a maximum appeared at around 1023 and 973 K when prepared in air and under a N2 atmosphere, respectively. X-ray diffraction analysis and SEM observation showed that high activity was achieved with a combination of crystallized PbWO4 with a high dispersion of RuO2 particles. The DFT calculation for PbWO4 with the d10s2-d0 electronic configuration showed that the top of the valence band (HOMO) was composed of hybridized O 2p + Pb 6s orbitals, while the bottom of the conduction band (LUMO) consisted of hybridized W 5d + O 2p + Pb 6p orbitals. Considerable dispersion was observed for both the conduction and valence bands, which differed fr...

Electronic structure of Pb1−xLaxTiO3 ferroelectric materials from Ti 2p and O 1s soft x-ray absorption spectroscopy

The electronic structure of Pb1−xLaxTiO3 (PLT) compounds for x ranging from 0to30at.% of La is investigated by means of soft x-ray absorption near edge structure (XANES) at the Ti L3,2 and O K edges. The greatest modification in the structure of the Ti 2p XANES spectra of the PLT compounds is observed in the region of the high energy peak of the L3 edge (eg states), which exhibits a splitting in the undoped sample. As the amount of lanthanum increases, this splitting becomes less pronounced. This modification is interpreted as a decrease in the degree of disorder of titanium atoms, which is correlated to the substitution of Pb by La atoms. The structural changes observed at the low energy peaks of the O K-edge XANES spectra of the PLT compounds may be interpreted in terms of hybridization between O 2p, Ti 3d, and Pb 6p orbitals. A decrease in the degree of hybridization observed as Pb atoms are replaced by La atoms may be related to the differences in the ferroelectric properties observed between x=0.0 and x=0.30 compounds.