First-principles Calculations Research Articles

Structural superlubricity at homogenous interface of penta-graphene

Two-dimensional (2D) van der Waals layered materials have been widely used as lubricant. Penta-graphene (PG), a 2D carbon allotrope exclusively composed of irregular carbon pentagons has recently been predicted to have superlubricating property. In the present study, by combining the molecular dynamics simulation and first-principles calculations, we investigated the frictional property of PG in both commensurate and incommensurate contacts. Our calculations show the ultra-low friction at the interface of relatively rotated bilayer PG with twist angles of more than 10° away from the commensurate configuration. Meanwhile, our calculations demonstrate the isotropy of the ultra-low friction at the interface of incommensurate contact, in contrast to the anisotropic of the commensurate contacting interface. Additionally, the evolution of friction force and the fluctuation of potential energy along sliding path correlate closely with the interface’s structure. The energetics and charge density explain the difference between the friction at the interfaces of the commensurate and incommensurate contacts. Not only that, we found the correlation between the intrinsic structural feature and interlayer binding energy. Importantly, our findings on the retainment of the ultra-low friction under work conditions indicates that the superlubricating state of PG has good practical adaptability.

A zero-dimensional perovskite with high-efficiency luminescence and transient response for advanced information encryption

Due to the increasingly complex network attacks and advancements in computational technology, finding new and efficient encryption techniques is an urging task. As a promising material, the application of perovskite luminescent materials in information encryption is limited to pre-planned specific scenarios or applications, lacking the ability to respond to temporary demands. In this work, we have designed a novel cadmium-based zero-dimensional perovskite, TEA2CdCl4, which presents blue luminescence centered at 413 nm originating from organic cations. Upon Sb3+ doping, it not only exhibits intense red emission at 623 nm with a high photoluminescence quantum efficiency up to 100 %, but also brings outstanding resistance to thermal quenching. Especially, an ultrafast response speed could be achieved during decryption process which has been verified by first-principles calculations. Based on the advantages, a large-area optical anti-counterfeiting paper for temporary information encryption transmission is designed, which not only verifies the potential for secure information transmission and protection, but also introduces a new concept for the application of organic–inorganic hybrid perovskites in the field of information encryption.

Insights into Antisite Defect Complex Induced High Ferro-Piezoelectric Properties in KNbO3 Perovskite: First-Principles Study.

Improving ferro-piezoelectric properties of niobate-based perovskites is highly desirable for developing eco-friendly high-performance sensors and actuators. Although electro-strain coupling is usually obtained by constructing multiphase boundaries via complex chemical compositions, defect engineering can also create opportunities for novel property and functionality advancements. In this work, a representative tetragonal niobate-based perovskite, i.e., KNbO3, is studied by using first-principles calculations. Two intrinsic types of Nb antisite defect complexes are selected to mimic alkali-deficiency induced excess Nb antisites in experiments. The formation energy, electronic profiles, polarization, and piezoelectric constants are systematically analyzed. It is shown that the structural distortion and chemical heterogeneity around the energetically favorable antisite pair defects, i.e., (NbK4·+KNb4'), lower the crystal symmetry of KNbO3 from tetragonal to triclinic phase, and facilitate polarization emergence and reorientation to substantially enhance intrinsic ferro-piezoelectricity (i.e., spontaneous polarization Ps of 68.2 μC/cm2 and piezoelectric strain constant d33 of 228.3 pC/N) without complicated doping and alloying.

Modulation of Photocatalytic CO2 Reduction by n-p Codoping Engineering of Single-Atom Catalysts.

Transition metal (TM) single-atom catalysts (SACs) have been widely applied in photocatalytic CO2 reduction. In this work, n-p codoping engineering is introduced to account for the modulation of photocatalytic CO2 reduction on a two-dimensional (2D) bismuth-oxyhalide-based cathode by using first-principles calculation. n-p codoping is established via the Coulomb interactions between the negatively charged TM SACs and the positively charged Cl vacancy (VCl) in the dopant-defect pairs. Based on the formation energy of charged defects, neutral dopant-defect pairs for the Fe, Co, and Ni SACs (PTM0) and the -1e charge state of the Cu SAC-based pair (PCu-1) are stable. The electrostatic attraction of the n-p codoping strengthens the stability and solubility of TM SACs by neutralizing the oppositely charged VCl defect and TM dopant. The n-p codoping stabilizes the electron accumulation around the TM SACs. Accumulated electrons modify the d-orbital alignment and shift the d-band center toward the Fermi level, enhancing the reducing capacity of TM SACs based on the d-band theory. Besides the electrostatic attraction of the n-p codoping, the PCu-1 also accumulates additional electrons surrounding Cu SACs and forms a half-occupied dx2-y2 state, which further upshifts the d-band center and improves photocatalytic CO2 reduction. The metastability of Cl multivacancies limits the concentration of the n-p pairs with Cl multivacancies (PTM@nCl (n > 1)). Positively charged centers around the PTM@nCl (n > 1) hinders the CO2 reduction by shielding the charge transfer to the CO2 molecule.

Atomistic structures and stabilities of MgAl2O4 surfaces under processing conditions: A first-principles thermodynamics study

Surfaces, in contacting ambient environments, are of particular importance for controlling sintering, thin-film growth, and catalytic activity of oxides. Magnesium aluminate spinel (MgAl2O4), an essential oxide for both functional and structural ceramics, has attracted considerable attentions across diverse scientific fields. Nonetheless, a central challenge in rationally manipulating the properties of MgAl2O4 lies in the atomistic understanding of its surface structures under processing conditions. This study presents comprehensive first-principles calculations of MgAl2O4 (110) and (111) polar surfaces with/without off-stoichiometries. We predict from a thermodynamic model the dependence of surface stabilities on critical processing parameters, namely the oxygen partial pressure (PO2) and temperature (T). The findings indicate that at typical sintering temperatures, the non-stoichiometric (110) surface transits from Al/Mg-rich to O-rich with increasing PO2. In contrast, the non-stoichiometric (111) surface stably terminates in a Mg-rich configuration at low PO2, gradually shifting to Al-rich and then to Mg/O-rich as PO2 increases. The correlation between surface off-stoichiometry and stability is elucidated through electronic structure analysis. The investigation offers fundamental insights into surface structures and physicochemical behavior of MgAl2O4, thereby facilitating the rational processing control of complex oxides for applications in catalysis and electronics.

Theoretical regulating the T-site composition of Janus MXenes achieving efficient bifunctional ORR/OER catalysts

The development of metal-air batteries relies heavily on efficient bifunctional catalysts for oxygen reduction/evolution reactions (ORR/OER). Herein, we employ first-principles calculations to investigate the structure-activity relationship between the composition of terminal atoms and the catalytic activity of Pt-doped VCl-Ti2CO2-xClx single-atom catalysts (SACs). Firstly, the results demonstrate clear linear correlations between the composition and the work function, interlayer average transfer charges (Δq), and the integrated crystal orbital Hamilton population (ICOHP) of the Pt-O bond that adsorbs OH. Besides, the difference in the average transferred charge between the Ti and the C layers (Δq5–3) and the ICOHP of Pt-O bond adsorbing OH can be used as effective descriptors of bifunctional performance of Pt-doped VCl-Ti2CO2-xClx. Furthermore, the bonding and charge transfer between the active site Pt and adsorbed intermediate OH are elucidated, revealing the bridging role of dxz and dz2 orbitals of Pt during the activation process. Based on the above understanding, we reported a catalyst of Pt-VCl-Ti2CO0.22Cl1.78 with a low bifunctional overpotential (ηBi = 1.00 V), effectively reducing the overpotential of Pt-doped Ti2CO2. This study extensively investigates the impact of terminal composition control on the ORR/OER activity, providing robust support for the design of highly active Janus MXene-based SACs.

First-principles calculation to investigate phase transition and associated changes in electronic, excitonic, and optical behavior of RbMgH3 perovskite polymorphs

Motivated by recent experimental and theoretical research on the ability to store and release hydrogen from a hydride perovskite with a large ionic character called RbMgH3, we conducted a study using density functional theory (DFT) to investigate the optoelectronic behavior of RbMgH3 in both Cubic and Hexagonal structures, as well as its phase transition.Our findings reveal that RbMgH3 exhibits an indirect band gap of 2.41 eV in the Hexagonal phase and a direct band gap of 2.15 eV in the Cubic phase. Additionally, we calculated the Gibbs energy to predict the occurrence of the phase transition. It was determined that RbMgH3 undergoes a structural transformation from Cubic to Hexagonal at a pressure of 1.78 GPa. This transition has the potential to enhance the storage kinetics of hydrogen, making RbMgH3 suitable for various applications.

Harnessing BN co-doping for superior thermal transport in phagraphene monolayer

In this study, we thoroughly explored the thermal transport and thermoelectric properties of BN-co-doped phagraphene structures in the context of first-principles computations combined with machine learning interatomic potential (MLIP) techniques. Our results demonstrate that doping positions can critically tune the thermal properties, offering potential advantages for both thermoelectric and heat transfer applications. Notably, enhanced thermal conductivity has been obtained for phagraphene with 10 % co-doping. Additionally, the rectangular structural symmetry of phagraphene plays an important role for targeted thermal transport. Further, we observe negative Grüneisen parameter in these structures, suggesting negative thermal expansion. This will serves as an unique mechanism for controlling the thermal conductivity at different frequencies. Interestingly, n-type behavior of the structures is indicative of its negative Seebeck coefficient within the temperature range of 300–900 K. Moreover, these structures display significantly larger electrical conductivity compared to other two-dimensional (2D) materials. Apart from that, the calculated figure of merit of the structures under a constant relaxation time at 300 K shows considerably better response for some specific doped structures. We believe this study will play an important role in understanding the importance of structural modifications in tailoring the thermal properties of carbon-based 2D systems.

Anharmonic and quantum effects in Pm3̄ AlM(M = Hf, Zr)H6 under high pressure: A first-principles study.

First-principles calculations were employed to investigate the impact of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of Pm3̄ AlM(M = Hf, Zr)H6 at pressures of 0.3-21.2GPa (AlHfH6) and 4.7-39.5GPa (AlZrH6) within the stochastic self-consistent harmonic approximation. A correction is predicted for the crystal lattice parameters, phonon spectra, and superconducting critical temperatures, previously estimated without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for lattice dynamics. The findings suggest that quantum ionic fluctuations have a significant impact on the crystal lattice parameters, phonon spectra, and superconducting critical temperatures. Based on our anharmonic phonon spectra, the structures will be dynamically stable at 0.3GPa for AlHfH6 and 6.2GPa for AlZrH6, ∼6 and 7GPa lower than pressures given by the harmonic approximation, respectively. Due to the anharmonic correction of their frequencies, the electron-phonon coupling constants (λ) are suppressed by 28% at 11GPa for AlHfH6 and 22% at 30GPa for AlZrH6, respectively. The decrease in λ causes Tc to be overestimated by ∼12K at 11GPa for AlHfH6 and 30GPa for AlZrH6. Even if the anharmonic and quantum effects are not as strong as those of Pm3̄n-AlH3, our results also indicate that metal hydrides with hydrogen atoms in interstitial sites are subject to anharmonic effects. Our results will inevitably stimulate future high-pressure experiments on synthesis, structural, and conductivity measurements.

Theoretical prediction on Irida-graphene monolayer as promising anode material for lithium-ion batteries

With the growing interest in energy storage technologies, the demand for alternative rechargeable batteries has become increasingly urgent. We evaluate the feasibility of a two-dimensional (2D) material, Irida-graphene (IG) monolayer, as an anode for lithium-ion batteries (LIBs) through first-principles calculations. IG monolayer, after Li adsorption, exhibits metallic property, suggesting the good electrical conductivity. Additionally, in comparison with IG bilayer, IG monolayer possesses low diffusion barriers (0.19–0.24 eV), high theoretical capacity (1115.8mA h/g), and low average open circuit voltage (0.317 V). Furthermore, under the influence of solvent effects, the adsorption performance becomes even more stable, and the theoretical capacity increases. These intriguing findings make IG monolayer great potentials for the anode of LIBs.

Understanding Pressure Effects on Structural, Optical, and Magnetic Properties of CsMnF4 and Other 3dn Compounds.

The pressure dependence of structural, optical, and magnetic properties of the layered compound CsMnF4 are explored through first-principles calculations. The structure at ambient pressure does not arise from a Jahn-Teller effect but from an orthorhombic instability on MnF63- units in the tetragonal parent phase, while there is a P4/n → P4 structural phase transition at P = 40 GPa discarding a spin crossover transition from S = 2 to S = 1. The present results reasonably explain the evolution of spin-allowed d-d transitions under pressure, showing that the first transition undergoes a red-shift under pressure following the orthorhombic distortion in the layer plane. The energy of such a transition at zero pressure is nearly twice that observed in Na3MnF6 due to the internal electric field and the orthorhombic distortion also involved in K2CuF4. The reasons for the lack of orthorhombic distortion in K2MF4 (M = Ni, Mn) or CsFeF4 are also discussed in detail. The present calculations confirm the ferromagnetic ordering of layers in CsMnF4 at zero pressure and predict a shift to an antiferromagnetic phase for pressures above 15 GPa consistent with the reduction of the orthorhombicity of the MnF63- units. This study underlines the usefulness of first-principles calculations for a right interpretation of experimental findings.

Exploring the physical properties of novel ACu3S3 (A = Sc, Y) semiconductors via first-principles calculation

Some of the distinguishing features of direct band gap ternary-type semiconductors include their adjustable optoelectronic properties and high thermal stability. The state-of-the-art density functional theory is employed to investigate the complex interplay in novel ACu3S3 (A = Sc, Y) ternary chalcogenides' electronic, optical, thermodynamic, and thermoelectric features. Based on the results, direct energy gaps nature in both these materials were observed as a result of the S-p and the Sc/Y-d orbital hybridizations. The YCu3S3 possesses a greater energy gap at the (Γ) point, resulting in higher effective mass for holes and lower effective mass for electrons as compared to ScCu3S3. The prime distinctive peaks in the ε1(ω) are observed in the visible range which then drops to zero at 8.5 eV and 8.0 eV for ScCu3S3 and YCu3S3, respectively. Both ScCu3S3 and YCu3S3 exhibited strong absorption over the whole visible and ultraviolet regions. From the noticed peaks in the reflectivity spectra, these materials can be employed efficiently as ultra-violet reflectors. The drop in the Grüneisen parameter with rising temperature shows that some vibrational modes relaxed as the temperature rises. The ScCu3S3 and YCu3S3 show a p-type conduction nature due to the positive value of the Seebeck coefficient over the entire temperature range.

Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr3: A Change Point Analysis of Widefield Imaging Data.

Photoluminescence intermittency remains one of the biggest challenges in realizing perovskite quantum dots (QDs) as scalable single photon emitters. We compare CsPbBr3 QDs capped with different ligands, lecithin, and a combination of oleic acid and oleylamine, to elucidate the role of surface chemistry on photoluminescence intermittency. We employ widefield photoluminescence microscopy to sample the blinking behavior of hundreds of QDs. Using change point analysis, we achieve the robust classification of blinking trajectories, and we analyze representative distributions from large numbers of QDs (Nlecithin = 1308, Noleicacid/oleylamine = 1317). We find that lecithin suppresses blinking in CsPbBr3 QDs compared with oleic acid/oleylamine. Under common experimental conditions, lecithin-capped QDs are 7.5 times more likely to be nonblinking and spend 2.5 times longer in their most emissive state, despite both QDs having nearly identical solution photoluminescence quantum yields. We measure photoluminescence as a function of dilution and show that the differences between lecithin and oleic acid/oleylamine capping emerge at low concentrations during preparation for single particle experiments. From experiment and first-principles calculations, we attribute the differences in lecithin and oleic acid/oleylamine performance to differences in their ligand binding equilibria. Consistent with our experimental data, density functional theory calculations suggest a stronger binding affinity of lecithin to the QD surface compared to oleic acid/oleylamine, implying a reduced likelihood of ligand desorption during dilution. These results suggest that using more tightly binding ligands is a necessity for surface passivation and, consequently, blinking reduction in perovskite QDs used for single particle and quantum light experiments.

Development of a DFT-driven thermodynamic framework for ferrous alloy electrochemistry in acidic media

The electrochemical behavior of different phases (ferrite, austenite, martensite, cementite) and effect of alloying addition in ferrous alloys was examined in acidic media. A comprehensive thermodynamic model, incorporating first-principles calculations, was constructed to predict cathodic and anodic behavior in acidic environments. Cementite emerged as the most corrosion resistant phase, followed by austenite, while ferrite displayed the least resistance. Interestingly, carbon additions improved corrosion resistance in ferrite and austenite. Alloying with Cr, Mn, and Cu was observed to hinder anodic dissolution, reducing electrochemical activity. Finally, the thermodynamic framework provides a reliable and efficient screening tool that can accelerate the development of alloys and coatings.

Prediction of Novel Trigonal Chloride Superionic Conductors as Promising Solid Electrolytes for All-Solid-State Lithium Batteries.

Recently emerging lithium ternary chlorides have attracted increasing attention for solid-state electrolytes (SSEs) due to their favorable combination between ionic conductivity and electrochemical stability. However, a noticeable discrepancy in Li-ion conductivity persists between chloride SSEs and organic liquid electrolytes, underscoring the need for designing novel chloride SSEs with enhanced Li-ion conductivity. Herein, an intriguing trigonal structure (i.e., Li3SmCl6 with space group P3112) is identified using the global structure searching method in conjunction with first-principles calculations, and its potential for SSEs is systematically evaluated. Importantly, the structure of Li3SmCl6 exhibits a high ionic conductivity of 15.46 mS cm-1 at room temperature due to the 3D lithium percolation framework distinct from previous proposals, associated with the unique in-plane cation ordering and stacking sequences. Furthermore, it is unveiled that Li3SmCl6 possesses a wide electrochemical window of 0.73-4.30V vs Li+/Li and excellent chemical interface stability with high-voltage cathodes. Several other Li3MCl6 (M = Er, and In) materials with isomorphic structures to Li3SmCl6 are also found to be potential chloride SSEs, suggesting the broader applicability of this structure. This work reveals a new class of ternary chloride SSEs and sheds light on strategy for structure searching in the design of high-performance SSEs.

Theoretical insights into the structural and optoelectronic properties of the ternary cesium tetrafluoridobromate semiconductor CsBrF4

This work deals with the first theoretical study of structural, electronic, and optical properties of CsBrF4 ternary compound by using first-principle calculations. The calculated structural parameters and atomic positions are in accordance with the experimental ones declared in the literature. The obtained electronic properties signalize that this compound is a semiconductor material with a direct energy band gap. According to the investigated optical properties, the vast optical absorption range in the ultraviolet region indicates that CsBrF4 semiconductor could be useful for specific applications in ultraviolet optoelectronic devices; consequently, this theoretical study is the potential to support future experimental research.

First-principles study of the structure, magnetism, and electronic properties of the all-Heusler alloy Co2MnGe/CoTiMnGe(100) heterojunction.

Based on first-principles calculations in the density functional theory, we systematically investigated the possible interface structure, magnetism, and electronic properties of the all-Heusler alloy Co2MnGe/CoTiMnGe(100) heterojunction. The calculation indicated that the Co2MnGe Heusler alloy is a half-metal with a magnetic moment of 4.97μB. CoTiMnGe is a narrow-band gap semiconductor and may act as an ultra-sensitive photocatalyst. We cannot find an "ideal" spin-polarization of 100% in CoCo termination and MnGe termination. Due to the interface interaction, the direct magnetic hybridization or indirect RKKY exchange will be weakened, leading to an increase in the atomic magnetic moment of the interfacial layer. For eight possible heterojunction structures, the half-metallic gaps in the Co2MnGe bulk have been destroyed by the inevitable interface states. The spin-polarization value of 94.31% in the CoCo-TiGe-B heterojunction revealed that it is the most stable structure. It is feasible to search for high-performance magnetic tunnel junction by artificially constructing suitable all-Heusler alloy heterojunctions.

Piezoelectricity in chalcogenide perovskites

Piezoelectric materials show potential to harvest the ubiquitous, abundant, and renewable energy associated with mechanical vibrations. However, the best performing piezoelectric materials typically contain lead which is a carcinogen. Such lead-containing materials are hazardous and are being increasingly curtailed by environmental regulations. In this study, we report that the lead-free chalcogenide perovskite family of materials exhibits piezoelectricity. First-principles calculations indicate that even though these materials are centrosymmetric, they are readily polarizable when deformed. The reason for this is shown to be a loosely packed unit cell, containing a significant volume of vacant space. This allows for an extended displacement of the ions, enabling symmetry reduction, and resulting in an enhanced displacement-mediated dipole moment. Piezoresponse force microscopy performed on BaZrS3 confirmed that the material is piezoelectric. Composites of BaZrS3 particles dispersed in polycaprolactone were developed to harvest energy from human body motion for the purposes of powering electrochemical and electronic devices.

Band alignment and charge transfer mechanisms in the prediction of advanced gate dielectrics for carbon nanotube field-effect transistors

The search for novel gate dielectrics to mitigate interface state density is a pressing concern in the fabrication of high-performance carbon nanotube field-effect transistors (CNT-FETs). In this study, we investigated the feasibility of employing strontium titanate (STO) with an ultra-high dielectric constant as the gate dielectric in CNT-FETs. By combining X-ray photoelectron spectroscopy measurements with first-principles calculations, we determined the band offsets between CNT and STO, as well as between CNT and conventional gate dielectrics such as HfO2 and SiO2. The valence/conduction band offset (VBO/CBO) at the STO/CNT interface is found to be 1.1 eV/1.5 eV, which is the smallest among all studied interfaces, followed by those at HfO2/CNT interface (VBO = 1.3 eV/CBO = 3.4 eV) and SiO2/CNT interface (VBO = 3.6 eV/CBO = 4.5 eV). Moreover, charge transfer at the STO/CNT interface exhibits two orders of magnitude lower values compared to that at HfO2/CNT interface and is also significantly smaller than that at SiO2/CNT interface. This phenomenon can be ascribed to the combined effects of van der Waals forces at the STO/CNT interface and fewer dangling bonds on the STO surface. These findings suggest exceptional compatibility between STO and CNT, making STO highly suitable for high-performance CNT-FETs.

The thermal conductivity of iron under the Earth's inner core pressure

Abstract The thermal conductivity of ε-iron at high pressure and high temperature is a key parameter to constrain the dynamics and thermal evolution of the Earth’s core. In this work, we use first-principles calculations to study the Hugoniot sound velocity and the thermal transport properties of ε-iron. The total thermal conductivities considering lattice vibration is 200 W/mK at the Earth’s inner core conditions. The suppressed anharmonic interactions can significantly enhance the lattice thermal conductivity under high pressure, and the contribution of the lattice thermal conductivity should not be ignored under Earth's core conditions.