High Formation Energy Research Articles

First-principles study of point defects in CePO4 monazite

CePO4 monazite is an important radiation-resistant material that may act as a potential minor actinides waste form. Here, we present the results of the calculations for the basic radiation defect modellings in CePO4 crystals, along with the examination of their defect formation energies and effect of the defect concentrations. This study focused on building a fully-relaxed CePO4 model with the step iterative optimization from the DFT-GGA calculations using the VASP and CASTEP databases. The results show that the Frenkel defect configuration resulting from the center interstitials has a lower energy when compared to two adjacent orthophosphate centers (the saddle point position). High formation energies were found for all the types of intrinsic Frenkel and vacancy defects. The formation energies conform to the following trend (given in the decreasing order of energy): Ce Frenkel (12.41 eV) > O Frenkel (11.02 eV) > Ce vacancy (9.09 eV) > O vacancy (6.69 eV). We observed almost no effect from the defect concentrations on the defect formation energies.

Hole-induced d0 ferromagnetism enhanced by Na-doping in GaN

The d0 ferromagnetism in wurtzite GaN is investigated by the first-principle calculations. It is found that spontaneous magnetization occurs if sufficient holes are injected in GaN. Both Ga vacancy and Na doping can introduce holes into GaN. However, Ga vacancy has a high formation energy, and is thus unlikely to occur in a significant concentration. In contrast, Na doping has relatively low formation energy. Under N-rich growth condition, Na doping with a sufficient concentration can be achieved, which can induce half-metallic ferromagnetism in GaN. Moreover, the estimated Curie temperature of Na-doped GaN is well above the room temperature.

Cadmium and lithium doping in silver orthophosphate: An ab initio study.

Using hybrid functional calculations, we investigate the effects of defects and defect complexes related with Cd, Li, and N impurities on the atomic and electronic properties of Ag3PO4. It was found that substitutional Cd on Ag lattice site (CdAg) contributes to the n-type conductivity of Ag3PO4. For substitutional Cd on P (or O) lattice site (CdP) (or CdO), it is not expected that Cd will incorporate into the P (or O) site due to the strong covalent interactions in the PO4 structural units. The interstitial Cd (Cdi) acts as a shallow donor, but its formation energy is relatively high compared with that of CdAg. For the (CdAg-2NO) complex, the formation of this inactive complex generates a fully occupied impurity band just above the valence band maximum of Ag3PO4, which significantly reduces the acceptor transition energy level. But the formation energy of the (CdAg-2NO) complex is even higher than that of the corresponding single point defect NO. Unlike LiP and LiO which has relatively high formation energy, interstitial Li (Lii or Lii(s)) with an appreciable solubility is likely to be the n-type dopant under O-poor condition.

Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO3 perovskites

Abstract

High photocatalytic performance of high concentration Al-doped ZnO nanoparticles

Abstract With increasing demands for environmental protection, photocatalyst has attracted great attentions, and ZnO based photocatalyst is an attractive system to study. Doping with other elements can significantly enhance the photocatalytic activity, conductivity and transmittance of ZnO. However, it is difficult to synthesize high doping concentration ZnO because of the high formation energy. In this work, high doping concentrations (up to 20 mol%) Al-doped ZnO (AZO) were prepared via sol-gel combustion method, which showed excellent photocatalytic activities, several times more efficient than previous reports. The nanoparticles containing 20% Al already showed unprecedented absorption capacity, leading to a great decrease of MO concentration from 200 to 2.7 mg/L, and further photodegrade MO thoroughly within 30 min irradiation. The mechanism of photocatalysis was also analyzed. XRD, TEM and EDX were used to demonstrate that Al has doped into ZnO crystal structure.

First principles study of magneto-optical properties of Fe-doped ZnO

Studies on optical band gaps and absorption spectra of Fe-doped ZnO have conflicting conclusions, such as contradictory redshifted and blueshifted spectra. To solve this contradiction, we constructed models of un-doped and Fe-doped ZnO using first-principles theory and optimized the geometry of the three models. Electronic structures and absorption spectra were also calculated using the GGA+U method. Higher doping content of Fe resulted in larger volume of doped system, and higher total energy resulted in lower stability. Higher formation energy also led to more difficult doping. Meanwhile, the band gaps broadened and the absorption spectra exhibited an evident blue shift. The calculations were in good agreement with the experimental results. Given the unipolar structure of ZnO, four possible magnetic coupling configurations for Zn14Fe2O16 were calculated to investigate the magnetic properties. Results suggest that Fe doping can improve ferromagnetism in the ZnO system and that ferromagnetic stabilization was mediated by p–d exchange interaction between Fe-3d and O-2p orbitals. Therefore, the doped system is expected to obtain high stability and high Curie temperature of diluted magnetic semiconductor material, which are useful as theoretical bases for the design and preparation of the Fe-doped ZnO system’s magneto-optical properties.

Ab initio study of vacancy formation in cubic LaMnO3 and SmCoO3 as cathode materials in solid oxide fuel cells.

Doped LaMnO3 and SmCoO3 are important solid oxide fuel cell cathode materials. The main difference between these two perovskites is that SmCoO3 has proven to be a more efficient cathode material than LaMnO3 at lower temperatures. In order to explain the difference in efficiency, we need to gain insight into the materials' properties at the atomic level. However, while LaMnO3 has been widely studied, ab initio studies on SmCoO3 are rare. Hence, in this paper, we perform a comparative DFT + U study of the structural, electronic, and magnetic properties of these two perovskites. To that end, we first determined a suitable Hubbard parameter for the Co d-electrons to obtain a proper description of SmCoO3 that fully agrees with the available experimental data. We next evaluated the impact of oxygen and cation vacancies on the geometry, electronic, and magnetic properties. Oxygen vacancies strongly alter the electronic and magnetic structures of SmCoO3, but barely affect LaMnO3. However, due to their high formation energy, their concentrations in the material are very low and need to be induced by doping. Studying the cation vacancy concentration showed that the formation of cation vacancies is less energetically favorable than oxygen vacancies and would thus not markedly influence the performance of the cathode.

Selective doping of nitrogen into carbon materials without catalysts

Selective doping of nitrogen into carbon materials in the absence of catalysts is a key for various applications of carbon materials. It is well known that most nitrogen-containing raw materials generate unnecessary functional groups during carbonization. Many researchers have noticed the significance of the selective nitrogen doping, whereas only a few works have reported the selective doping. In addition, those few works used catalysts to synthesize nitrogen-doped carbon materials, but the presence of catalysts limits the applications of nitrogen-doped carbon materials. This study found an unusual aromatic compound, imidazo[1,2-a]pyridine (IP), which maintained 88 % of the functional groups of as-received IP even after carbonization at 673 K in the absence of catalysts, and the functional groups were further maintained up to 773 K. The percentage of remaining functional groups was revealed using our state-of-the-art techniques of simulated X-ray photoelectron spectroscopy and Raman spectroscopy combined with transition state calculation using density functional theory. The low carbonization temperature as well as selective doping of nitrogen was achieved because of the low activation energy of dehydrogenation reaction among IP molecules compared to the high activation energy of radical formation for scission of C–N bonding.

Efficient expression of a heterologous gene in plants depends on the nucleotide composition of mRNA’s 5'-region

The contribution of nucleotide composition of mRNA 5'-region to the efficiency of expression at transcriptional and translational levels was studied in transgenic tobacco plants (Nicotiana tabacum L., cultivar Petit Havana) using a thermostable lichenase reporter gene. Synthetic sequence that contains CG-rich motifs, typical for 5'-region of plant genes, identified in silico, was constructed. Transgenic plant lines of N. tabacum were obtained; they contain thermostable lichenase reporter gene that is under control of the constitutive 35S RNA CaMV promoter and additional regulatory element: synthetic CG-rich sequence, which functions as the 5'-UTR (untranslated region) of the reporter gene mRNA or as 5'-region of the hybrid gene coding sequence, wherein the synthetic sequence is fused with the sequence of the reporter gene in its reading frame. Results of the comparative analysis of mRNA and protein levels in the obtained lines of transgenic plants showed that the synthetic CG-rich sequence significantly increases the level of transcription of the reporter gene and appear to have no negative effect on the efficiency of reporter mRNA translation, which may be due to the peculiarities of its nucleotide composition and structure, namely, due to the presence of motifs that are specific for 5'-regions of plant genes, as well as due to the properties of the secondary structure— the absence of hairpin structures with a high energy of formation. It was experimentally confirmed for the first time that the 5'-region of the genes with a high content of CpG dinucleotides can help to increase the transcription level of genes in plants.

Understanding hardness evolution of Zr–Si–N nanocomposite coatings via investigating their deformation behaviors

This work investigates microstructure, hardness and deformation behaviors of Zr–Si–N nanocomposite coatings with 0–20.9 at.% Si. Relationship between the hardness evolution and the deformation behaviors of the nanocomposite coatings was discussed as well. Dislocation glides together with microcrack initiation and propagation contribute to the plastic deformation of the nanocomposite coatings. The microcrack propagation along the columnar, yet weak bonded grain boundaries is found to be the primary factor inhibiting the hardness improvement of the Si-free ZrN coating. The Zr0.963Si0.037N0.916 coating exhibits the highest recorded hardness of 33.2±1.0GPa. The amorphous matrix exhibits a high energy of microcrack formation and propagation when the nanocomposite coating is exposed to indenter. The microcracks produced by nanoindentation in the Zr0.963Si0.037N0.916 coating preferentially propagate into the ZrN nanocrystals rather than along the grain boundaries. Both effects of grain size refinement and grain boundary strengthening result in the hardness improvement.

Ab Initio Study of He Migrations in Fcc Au-Ag Alloys

3Ag2 and AuAg). The results show that the migration mechanisms of He atoms mainly depend on the crystal structures of alloys, and their migration energy barriers are aected by the migration paths in Au‐Ag alloys. He interstitials preferentially occupied the most stable sites, but it is dicult for He interstitials to migrate to nearest most stable sites via second stable positions at room temperature. When He atom is at the tetrahedral position which has higher formation energy, it possibly migrates to nearest tetrahedral positions directly for AuAg alloy. In addition, comparing the migration of He defects in the two alloys, we found that the properties of migration energy and relative stability of He atoms probably slightly depend on the mass‐density of Au‐Ag alloys.

Defect energetics and magnetic properties of 3d-transition-metal-doped topological crystalline insulator SnTe

The introduction of magnetism in SnTe-class topological crystalline insulators is a challenging subject with great importance in the quantum device applications. Based on the first-principles calculations, we have studied the defect energetics and magnetic properties of 3d transition-metal (TM)-doped SnTe. We find that the doped TM atoms prefer to stay in the neutral states and have comparatively high formation energies, suggesting that the uniform TMdoping in SnTe with a higher concentration will be difficult unless clustering. In the dilute doping regime, all the magnetic TMatoms are in the high-spin states, indicating that the spin splitting energy of 3d TM is stronger than the crystal splitting energy of the SnTe ligand. Importantly, Mn-doped SnTe has relatively low defect formation energy, largest local magnetic moment, and no defect levels in the bulk gap, suggesting that Mn is a promising magnetic dopant to realize the magnetic order for the theoretically-proposed large-Chern-number quantum anomalous Hall effect (QAHE) in SnTe.

Structural, electronic, and magnetic properties of 3D metal trioxide and tetraoxide superhalogen cluster-doped monolayer BN

The structural, electronic, and magnetic properties of monolayer BN doped with 3D metal trioxide and tetraoxide superhalogen clusters are investigated using first-principle calculations. TMO3(4)-doped monolayer BN exhibits a low negative formation energy, whereas TM atoms embedded in monolayer BN show a high positive formation energy. TMO3(4) clusters are embedded more easily in monolayer BN than TM atoms. Compared with TMO3-doped structures, TMO4-doped structures have a higher structural stability because of their higher binding energies. Given their low negative formation energies, TMO4-doped structures are more favored for specific applications than TMO3-doped structures and TM atom-doped structures. Large magnetic moments per supercell and significant ferromagnetic couplings between a TM atom and neighboring B and N atoms on the BN layer were observed in all TMO4-doped structures, except for TiO4-doped structures.

Electronic and Magnetic Properties of Transition-Metal-Doped Monolayer Black Phosphorus by Defect Engineering

We investigate the structural, electronic, and magnetic properties of monolayer black phosphorus (M-BP) doped with 3d transition-metal elements by considering defects and defect complexes. The results show that pristine M-BP is nonmagnetic. P vacancy (VP) can be magnetic with a rather high formation energy, and the (VP + VP) complex is nonmagnetic as well. Doping with transition metals of V, Cr, Mn, Fe, or Ni can induce magnetism, but Co doping will not. The magnetism is originated from their d orbitals. The formation energies of Mn, Co, and Fe doping can be significantly reduced with the existence of P vacancy by the formation of a substitutional and P vacancy (TMP + VP) defect complex. In addition, (CoP + VP) has a magnetic moment of 0.10 μB, further demonstrating the important role of vacancy in the doping of transition-metal elements in M-BP.

Deprotonation effect of tetrahydrofuran‐2‐carbonitrile buffer gas dopant in ion mobility spectrometry

When dopants are introduced into the buffer gas of an ion mobility spectrometer, spectra are simplified due to charge competition. We used electrospray ionization to inject tetrahydrofuran-2-carbonitrile (F, 2-furonitrile or 2-furancarbonitrile) as a buffer gas dopant into an ion mobility spectrometer coupled to a quadrupole mass spectrometer. Density functional theory was used for theoretical calculations of dopant-ion interaction energies and proton affinities, using the hybrid functional X3LYP/6-311++(d,p) with the Gaussian 09 program that accounts for the basis set superposition error; analytes structures and theoretical calculations with Gaussian were used to explain the behavior of the analytes upon interaction with F. When F was used as a dopant at concentrations below 1.5mmolm(-3) in the buffer gas, ions were not observed for α-amino acids due to charge competition with the dopant; this deprotonation capability arises from the production of a dimer with a high formation energy that stabilized the positive charge and created steric hindrance that deterred the equilibrium with analyte ions. F could not completely strip other compounds of their charge because they either showed steric hindrance at the charge site that deterred the approach of the dopant (2,4-lutidine, and DTBP), formed intramolecular bonds that stabilized the positive charge (atenolol), had high proton affinity (2,4-lutidine, DTBP, valinol and atenolol), or were inherently ionic (tetraalkylammonium ions). This selective deprotonation suggests the use of F to simplify spectra of complex mixtures in ion mobility and mass spectrometry in metabolomics, proteomics and other studies that generate complex spectra with thousands of peaks. Copyright © 2016 John Wiley & Sons, Ltd.

Two-Dimensional Halide Perovskites: Tuning Electronic Activities of Defects.

Two-dimensional (2D) halide perovskites are emerging as promising candidates for nanoelectronics and optoelectronics. To realize their full potential, it is important to understand the role of those defects that can strongly impact material properties. In contrast to other popular 2D semiconductors (e.g., transition metal dichalcogenides MX2) for which defects typically induce harmful traps, we show that the electronic activities of defects in 2D perovskites are significantly tunable. For example, even with a fixed lattice orientation one can change the synthesis conditions to convert a line defect (edge or grain boundary) from electron acceptor to inactive site without deep gap states. We show that this difference originates from the enhanced ionic bonding in these perovskites compared with MX2. The donors tend to have high formation energies and the harmful defects are difficult to form at a low halide chemical potential. Thus, we unveil unique properties of defects in 2D perovskites and suggest practical routes to improve them.

Density-functional study on the structural and magnetic properties of N-doped graphene oxide

We have systematically investigated the structural and magnetic properties of N-doped graphene oxide by density-functional theory. Our results reveal that both the magnetic properties and the defect stability can be significantly affected by the bonding environment. Graphitic N defect has higher formation energy than both pyridinic and pyrrolic N defects. The pyrrolic N becomes more stable when its adjacent undercoordinated C atoms are bonded to functional groups. Weak spin-polarized or nonmagnetic state emerges when N defect couples to its nearest C atoms via the hybridization of π orbital. In contrast, strong spin-polarized state arises when the defect couples to its adjacent C atoms via the hybridization of σ orbital. Generally, ferromagnetic coupling occurs to those nearest coupled C atoms with dangling bonds. N defects do not incline to aggregate around the vacancies. Moderate N defects can prevent the undercoordinated C atoms from reconstruction. Nevertheless, excessive N defects bring about the undesired electron-doping, and consequently damage the ferromagnetism.

Optimized geometry, electronic structure and Ag adsorption property of nanosheet graphene with different symmetry shapes: a theoretical investigation

Optimized geometries and electronic structures of three different symmetry shapes of nanosheet graphenes with armchair and zig-zag edges were generated by using the generalized gradient approximation/Perdew–Burke–Ernzerhof (GGA/PBE) method of density function theory (DFT) with the double-zeta polarized (DZP) basis set. Based on the results, the calculated HOMO–LUMO energy gap (Eg = LUMO–HOMO) with different symmetry shapes, and nanosheet size for with zig-zag and armchair edges were also presented. Because the p π orbital was widely localized over the sheet surface, the calculated E g decreased with increasing the sheet size. Further, the quantum mechanics calculation was used to investigate the adsorption property of the Ag atom adsorbed to the nanosheet graphene (expressed by C90H30) surface. The calculations show that the Ag atom binds to the bridge site (B site) of triangular nanosheet graphene (C90H30) as it is the most stable adsorption site compared with others, and since it has higher formation energy (ΔE) with shorter distance between the Ag atom and the graphene surface. Above all, calculations suggest that the Ag-adsorbed nanosheet graphene is a good option as an adsorbent in environmental research.

Effect of Disorder on Electronic, Magnetic, and Optical Properties of Co2CrZ Heusler Alloys (Z = Al, Ga, Si, Ge)

This paper presents the effect of disorder on electronic, magnetic, and optical properties of Co2CrZ (Z = Al, Si, Ga, Ge) Heusler alloy using density functional theory. Binary mixing is the most common form of atomic disorder in these compounds. We have considered three types of disorders: DO 3, A2, and B2 disorder which corresponds to X-Y, X-Z, and Y-Z mixing, respectively. After structural optimization, we found that A2 disorder has high formation energy and is most unlikely to occur. The half-metallic nature of the alloy is destroyed in the presence of DO 3 and A2 disorder. The destruction of half-metallicity is due to reconstruction of energy states. B2 disorder retains the half-metallic nature of the alloy but spin-polarization value is reduced slightly as compared to the ordered alloy. In addition, the optical properties such as dielectric function, refractive index, absorption spectra, optical conductivity, reflectivity, and energy loss function of these alloys have also been investigated.

Formation of charged ferroelectric domain walls with controlled periodicity.

Charged domain walls in proper ferroelectrics were shown recently to possess metallic-like conductivity. Unlike conventional heterointerfaces, these walls can be displaced inside a dielectric by an electric field, which is of interest for future electronic circuitry. In addition, theory predicts that charged domain walls may influence the electromechanical response of ferroelectrics, with strong enhancement upon increased charged domain wall density. The existence of charged domain walls in proper ferroelectrics is disfavoured by their high formation energy and methods of their preparation in predefined patterns are unknown. Here we develop the theoretical background for the formation of charged domain walls in proper ferroelectrics using energy considerations and outline favourable conditions for their engineering. We experimentally demonstrate, in BaTiO3 single crystals the controlled build-up of high density charged domain wall patterns, down to a spacing of 7 μm with a predominant mixed electronic and ionic screening scenario, hinting to a possible exploitation of charged domain walls in agile electronics and sensing devices.