First-Principles Investigation of Size Effects on Cohesive Energies of Transition-Metal Nanoclusters.

The cohesive energy of transition-metal nanoparticles is crucial to understanding their stability and fundamental properties, which are essential for developing new technologies and applications in fields such as catalysis, electronics, energy storage, and biomedical engineering. In this study, we systematically investigate the size-dependent cohesive energies of all the 3d, 4d, and 5d transition-metal nanoclusters (small nanoparticles) based on a plane-wave-based method within general gradient approximation using first-principles density functional theory calculations. Our results show that the cohesive energies of nanoclusters decrease with decreasing size due to the increased surface-to-volume ratio and quantum confinement effects. A comparison of nanoclusters with different geometries reveals that the cohesive energy decreases as the number of nanocluster layers decreases. Notably, monolayer nanoclusters exhibit the lowest cohesive energies. We also find that the size-dependent cohesive energy trends are different for different transition metals, with some metals exhibiting stronger size effects than others. Our findings provide insights into the fundamental properties of transition-metal nanoclusters and have potential implications for their applications in various fields, such as catalysis, electronics, and biomedical engineering.

Interface Engineering at Sc2C MXene and Germanium Iodine Perovskite Interface: First-Principles Insights.

In recent years, Ge-based halide perovskite has gained increasing attention due to its potential in the development of lead-free perovskite solar cells. Here, through first-principles calculations, we explored the possibilities to enhance the optoelectronic properties of Ge-based perovskites via interfacial engineering between germanium iodine perovskite and 2D scandium-carbide MXene with various termination groups including F, O, and OH. We first evaluated the relative stability of the material interfaces and found that MAI-terminated interfaces are energetically more favorable than the GeI2-terminated interfaces. The MAI/F interface exhibits a type-II band alignment that can promote the photogenerated electron-hole separation. Moreover, the work function of the heterostructures can be tuned from 2.60 to 4.45 eV via using various termination groups. Additionally, 2D Sc2C MXene can also significantly enhance the light absorption. These results indicate that the 2D MXene serves as one promising candidate for optimizing the properties of perovskite solar cells via interface engineering.

Perovskite superlattices with efficient carrier dynamics.

Compared with their three-dimensional (3D) counterparts, low-dimensional metal halide perovskites (2D and quasi-2D; B2An-1MnX3n+1, such as B = R-NH3+, A = HC(NH2)2+, Cs+; M = Pb2+, Sn2+; X = Cl-, Br-, I-) with periodic inorganic-organic structures have shown promising stability and hysteresis-free electrical performance1-6. However, their unique multiple-quantum-well structure limits the device efficiencies because of the grain boundaries and randomly oriented quantum wells in polycrystals7. In single crystals, the carrier transport through the thickness direction is hindered by the layered insulating organic spacers8. Furthermore, the strong quantum confinement from the organic spacers limits the generation and transport of free carriers9,10. Also, lead-free metal halide perovskites have been developed but their device performance is limited by their low crystallinity and structural instability11. Here we report a low-dimensional metal halide perovskite BA2MAn-1SnnI3n+1 (BA, butylammonium; MA, methylammonium; n = 1, 3, 5) superlattice by chemical epitaxy. The inorganic slabs are aligned vertical to the substrate and interconnected in a criss-cross 2D network parallel to the substrate, leading to efficient carrier transport in three dimensions. A lattice-mismatched substrate compresses the organic spacers, which weakens the quantum confinement. The performance of a superlattice solar cell has been certified under the quasi-steady state, showing a stable 12.36% photoelectric conversion efficiency. Moreover, an intraband exciton relaxation process may have yielded an unusually high open-circuit voltage (VOC).

Elucidate interfacial disorder effects on the perpendicular magnetic anisotropy at Fe/MgO heterostructure from first-principles calculations.

The interfacial perpendicular magnetic anisotropy (PMA) plays a key role in spintronic applications such as memory recording and computational devices. Despite robust PMA being reported at the Fe/MgO interface, there are still inconsistencies in the disorder effects on the interfacial magnetic anisotropy. Here we reported a comprehensive study of the influence of the interfacial disorder, including the underoxidization, overoxidization, and oxygen migration, on the PMA of the Fe/MgO interface using first-principles calculations. Compared to the pristine Fe/MgO interface, the underoxidation at the Fe/MgO interface keeps the interfacial PMA but reduces the interfacial anisotropy constant (Ki). The overoxidization and oxygen migration at the interface both reduce theKiand even switch the easy magnetization axis from the out-of-plane to in-plane direction at high oxygen percentage. In all the cases, theKiwas found strongly correlated to the difference of the orbital magnetic moment along the in-plane and out-of-plane direction. Calculated layer-resolved and orbital-resolvedKirevealed that the orbital coupling between thedxyanddx2-y2states of the interfacial Fe layer plays a key role in determining the interfacial magnetic anisotropy. This work provides deep insights into the oxidation effects on the interfacial magnetic anisotropy of Fe/MgO system and a possible avenue to tune theKivia interfacial engineering.

High-Throughput Design of Interfacial Perpendicular Magnetic Anisotropy at Heusler/MgO Heterostructures.

The perpendicular magnetic anisotropy (PMA) at ferromagnet/insulator interfaces has important technological applications, such as in the fields of magnetic recording and sensing devices. The perpendicular magnetic tunnel junctions (p-MTJs) with strong PMA have recently attracted increasing interest because they offer high stability and device performance toward low energy consumption. Heusler alloys are a large family of compounds that offer promising magnetic properties for developing p-MTJs. However, it is challenging to select appropriate combinations of Heusler ferromagnets and insulators with the desired interfacial properties. Here, we report a systematic high-throughput screening approach to search for candidate Heusler/MgO material interfaces with strong PMA and other desired material properties for spintronic technologies. On the basis of the open quantum material repositories, we developed a series of material descriptors, including formation energy, convex hull distance, magnetic ordering, lattice misfit, magnetic anisotropy constant, cleavage energy, and tunnel magnetoresistance, to filter candidate Heusler/MgO interfaces among the possible 40 000 ternary Heusler compounds. After a comprehensive screening, five full-Heusler compounds, including Co2CrAl, Co2FeAl, Co2HfSn, Fe2IrGa, and Mn2IrGe, and two half-Heusler compounds, PtCrSb and PtMnAs, were found to be promising for designing p-MTJs. This work demonstrates a new way for the high-throughput design of functional material interfaces for spintronic applications via exploiting the open quantum material repositories and developing effective material descriptors along with the large-scale ab initio calculations for material interfaces.

Nonreciprocal Transport in a Bilayer of MnBi2Te4 and Pt.

MnBi2Te4 (MBT) is the first intrinsic magnetic topological insulator with the interaction of spin-momentum locked surface electrons and intrinsic magnetism, and it exhibits novel magnetic and topological phenomena. Recent studies suggested that the interaction of electrons and magnetism can be affected by the Mn-doped Bi2Te3 phase at the surface due to inevitable structural defects. Here, we report an observation of nonreciprocal transport, that is, current-direction-dependent resistance, in a bilayer composed of antiferromagnetic MBT and nonmagnetic Pt. The emergence of the nonreciprocal response below the Néel temperature confirms a correlation between nonreciprocity and intrinsic magnetism in the surface state of MBT. The angular dependence of the nonreciprocal transport indicates that nonreciprocal response originates from the asymmetry scattering of electrons at the surface of MBT mediated by magnon. Our work provides an insight into nonreciprocity arising from the correlation between magnetism and Dirac surface electrons in intrinsic magnetic topological insulators.

Two-Dimensional Electron Gas at the Spinel/Perovskite Interface: Suppression of Polar Catastrophe by an Ultrathin Layer of Interfacial Defects.

Two-dimensional electron gas (2DEG) at the interface between two insulating perovskite oxides has attracted much interest for both fundamental physics and potential applications. Here, we report the discovery of a new 2DEG formed at the interface between spinel MgAl2O4 and perovskite SrTiO3. Transport measurements, electron microscopy imaging, and first-principles calculations reveal that the interfacial 2DEG is closely related to the symmetry breaking at the MgAl2O4/SrTiO3 interface. The critical film thickness for the insulator-to-metal transition is approximately 32 Å, which is twice as thick as that reported on the widely studied LaAlO3/SrTiO3 system. Scanning transmission electron microscopy imaging indicates the formation of interfacial Ti-Al antisite defects with a thickness of ∼4 Å. First-principles density functional theory calculations indicate that the coexistence of the antisite defects and surface oxygen vacancies may explain the formation of interfacial 2DEG as well as the observed critical film thickness. The discovery of 2DEG at the spinel/perovskite interface introduces a new material platform for designing oxide interfaces with desired characteristics.

A fabrication process for flexible single-crystal perovskite devices.

Organic-inorganic hybrid perovskites have electronic and optoelectronic properties that make them appealing in many device applications1-4. Although many approaches focus on polycrystalline materials5-7, single-crystal hybrid perovskites show improved carrier transport and enhanced stability over their polycrystalline counterparts, due to their orientation-dependent transport behaviour8-10 and lower defect concentrations11,12. However, the fabrication of single-crystal hybrid perovskites, and controlling their morphology and composition, are challenging12. Here we report a solution-based lithography-assisted epitaxial-growth-and-transfer method for fabricating single-crystal hybrid perovskites on arbitrary substrates, with precise control of their thickness (from about 600 nanometres to about 100 micrometres), area (continuous thin films up to about 5.5 centimetres by 5.5 centimetres), and composition gradient in the thickness direction (for example, from methylammonium lead iodide, MAPbI3, to MAPb0.5Sn0.5I3). The transferred single-crystal hybrid perovskites are of comparable quality to those directly grown on epitaxial substrates, and are mechanically flexible depending on the thickness. Lead-tin gradient alloying allows the formation of a graded electronic bandgap, which increases the carrier mobility and impedes carrier recombination. Devices based on these single-crystal hybrid perovskites show not only high stability against various degradation factors but also good performance (for example, solar cells based on lead-tin-gradient structures with an average efficiency of 18.77 per cent).

Designing an All-Solid-State Sodium-Carbon Dioxide Battery Enabled by Nitrogen-Doped Nanocarbon.

All-solid-state sodium-carbon dioxide (Na-CO2) battery is an emerging technology that effectively utilizes the greenhouse gas, CO2, for energy storage with the virtues of minimized electrolyte leakage and suppressed Na dendrite growth for the Na metal anode. However, the sluggish reduction/evolution reactions of CO2 on the solid electrolyte/CO2 cathode interface have caused premature battery failure. Herein, nitrogen (N)-doped nanocarbon derived from metal-organic frameworks is designed as a cathode catalyst to solve this challenge. The porous and highly conductive N-doped nanocarbon possesses superior uptake and binding capability with CO2, which significantly accelerates the CO2 electroreduction and promotes the formation of thin sheetlike discharged products (200 nm in thickness) that can be easily decomposed upon charging. Accordingly, reduced discharge/charge overpotential, high discharge capacity (>10 000 mAh g-1), long cycle life, and high energy density (180 Wh kg-1 in pouch cells) are achieved at 50 °C.

Stability diagrams, defect tolerance, and absorption coefficients of hybrid halide semiconductors: High-throughput first-principles characterization.

On the basis of the screened 29 hybrid halide compounds from our prior study [Y. Li and K. Yang, Energy Environ. Sci. 12, 2233-2243 (2019)], here, we reported a systematic computational study of the stability diagram, defect tolerance, and optical absorption coefficients for these candidate materials using high-throughput first-principles calculations. We took two exemplary compounds, MA2SnI4 and MA3Sb2I9, as examples to show the computational process, and they are discussed in detail. This work is expected to provide a detailed guide for further experimental synthesis and characterization, with the potential to develop novel lead-free optoelectronic devices.

Strain engineering and epitaxial stabilization of halide perovskites.

Strain engineering is a powerful tool with which to enhance semiconductor device performance1,2. Halide perovskites have shown great promise in device applications owing to their remarkable electronic and optoelectronic properties3-5. Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization6-8, electrostriction9, annealing10-12, van der Waals force13, thermal expansion mismatch14, and heat-induced substrate phase transition15, the controllable and device-compatible strain engineering of halide perovskites by chemical epitaxy remains a challenge, owing to the absence of suitable lattice-mismatched epitaxial substrates. Here we report the strained epitaxial growth of halide perovskite single-crystal thin films on lattice-mismatched halide perovskite substrates. We investigated strain engineering of α-formamidinium lead iodide (α-FAPbI3) using both experimental techniques and theoretical calculations. By tailoring the substrate composition-and therefore its lattice parameter-a compressive strain as high as 2.4 per cent is applied to the epitaxial α-FAPbI3 thin film. We demonstrate that this strain effectively changes the crystal structure, reduces the bandgap and increases the hole mobility of α-FAPbI3. Strained epitaxy is also shown to have a substantial stabilization effect on the α-FAPbI3 phase owing to the synergistic effects of epitaxial stabilization and strain neutralization. As an example, strain engineering is applied to enhance the performance of an α-FAPbI3-based photodetector.

First-principles studies of polar perovskite KTaO3 surfaces: structural reconstruction, charge compensation, and stability diagram.

Polar perovskite oxides are of considerable interest for developing advanced functional materials with exceptional electronic properties for their unique polar characters. A cleavage of polar perovskite oxides along the charged layers leads to an electrostatic instability on the cleaved surfaces, and a charge compensation is required to stabilize these surfaces. In this work, we have systemically studied 25 types of surface models of polar KTaO3 perovskite oxide, including (001), (110), and (111) surfaces with various types of surface terminations, using first-principles electronic structure calculations. The surface structural reconstruction, electronic structures, and thermodynamic properties including cleavage energy and surface energy are investigated. The phase stability diagrams of the (001), (110), and (111) surfaces are constructed with respect to the chemical potentials of component elements. The KO(001), O(110), and KO2(111) terminations are more likely to be formed than other types of terminations in corresponding surfaces, consistent with experimental observations on KTaO3(001) surfaces. This work provides useful guidance for accurate control of surface morphology for tailing functional properties of polar KTaO3 perovskite oxide.

Tuning Interfacial Magnetic Ordering via Polarization Control in Ferroelectric SrTiO3/PbTiO3 Heterostructure.

The electromagnetic properties at the interface of heterostructure are sensitive to the interfacial crystal structure and external field. For example, the two-dimensional magnetic states at the interface of LaAlO3/SrTiO3 are discovered and can further be controlled by electric field. Here, we study two types of heterostructures, TiO2/PbTiO3 and SrTiO3/PbTiO3, using first-principle electronic structure calculations. We find that the ferroelectric polarization discontinuity at the interface leads to partially occupied Ti 3d states and the magnetic moments. The magnitude of the magnetic moments and the ground-state magnetic coupling are sensitive to the polarization intensity of PbTiO3. As the ferroelectric polarization of PbTiO3 increases, the two heterostructures show different magnetic ordering that strongly depends on the electron occupation of the Ti t2g orbitals. For the TiO2/PbTiO3 interface, the magnetic moments are mostly contributed by degenerated d yz/d xz orbitals of interfacial Ti atoms and the neighboring interfacial Ti atoms form ferromagnetic coupling. For SrTiO3/PbTiO3 interface, the interfacial magnetic moments are mainly contributed by occupied d xy orbital because of the increased polarization intensity, and as the electron occupation increases, there exists a transition of the magnetic coupling between neighboring Ti atoms from ferromagnetism to antiferromagnetism via the superexchange interaction. Our study suggests that manipulating the polarization intensity is one effective way to control interfacial magnetic ordering in the perovskite oxide heterostructures.

Size effects and odd-even effects in MoS2 nanosheets: first-principles studies.

Molybdenum disulfide (MoS2) nanostructures have been widely used as catalysts in the petroleum refinery industry for the hydrodesulfurization process, in which sulfur vacancies play a critical role in determining the catalytic activity. Here we report size effects and odd-even effects on the formation of sulfur vacancies in the triangular MoS2 nanosheets using first-principles calculations. By modeling four types of edge structures of MoS2 nanosheets, S-terminated edges are found to be energetically more favorable than Mo-terminated edges, and are then selected for studying energetics of sulfur vacancies. Two types of sulfur dimer vacancies at the center (VS@Cen) and at the corner (VS@Cnr) of the edges of S-terminated MoS2 nanosheets are modeled, respectively. Our results reveal a strong odd-even effect on the formation of sulfur dimer vacancies, particularly for small MoS2 nanosheets, in terms of the size of nanosheets that is defined by the number of Mo atoms on the edge. The VS@Cen dimer vacancy has a low formation energy at an even-number but a high formation energy at an odd-number, while the VS@Cnr dimer vacancy exhibits a complete opposite trend. These results indicate that small MoS2 nanosheets can exhibit unique material properties for catalytic applications.

Comparison Studies of Interfacial Electronic and Energetic Properties of LaAlO3/TiO2 and TiO2/LaAlO3 Heterostructures from First-Principles Calculations.

By using first-principles electronic structure calculations, we studied electronic and energetic properties of perovskite oxide heterostructures with different epitaxial growth order between anatase TiO2 and LaAlO3. Two types of heterostructures, i.e., TiO2 film grown on LaAlO3 substrate (TiO2/LaAlO3) and LaAlO3 film grown on TiO2 substrate (LaAlO3/TiO2), were modeled. The TiO2/LaAlO3 model is intrinsically metallic and thus does not exhibit an insulator-to-metal transition as TiO2 film thickness increases; in contrast, the LaAlO3/TiO2 model shows an insulator-to-metal transition as the LaAlO3 film thickness increases up to 4 unit cells. The former model has a larger interfacial charge carrier density (n ∼ 1014 cm-2) and smaller electron effective mass (0.47me) than the later one (n ∼ 1013 cm-2, and 0.70me). The interfacial energetics calculations indicate that the TiO2/LaAlO3 model is energetically more favorable than the LaAlO3/TiO2 model, and the former has a stronger interface cohesion than the later model. This research provides fundamental insights into the different interfacial electronic and energetic properties of TiO2/LaAlO3 and LaAlO3/TiO2 heterostructures.

Interfacial Multiferroics of TiO2/PbTiO3 Heterostructure Driven by Ferroelectric Polarization Discontinuity.

Novel phenomena appear when two different oxide materials are combined together to form an interface. For example, at the interface of LaAlO3/SrTiO3, two-dimensional conductive states form to avoid the polar discontinuity, and magnetic properties are found at such an interface. In this work, we propose a new type of interface between two nonmagnetic and nonpolar oxides that could host a magnetic state, where it is the ferroelectric polarization discontinuity instead of the polar discontinuity that leads to the charge transfer, forming the interfacial magnetic state. As a concrete example, we investigate by first-principles calculations the heterostructures made of ferroelectric perovskite oxide PbTiO3 and nonferroelectric polarized oxide TiO2. We show that charge is transferred to the interfacial layer forming an interfacial ferromagnetic ordering that may persist up to room temperature. Especially, the strong coupling between bulk ferroelectric polarization and interface ferromagnetism represents a new type of magnetoelectric effect, which provides an ideal platform for exploring the intriguing interfacial multiferroics. The findings here are important not only for fundamental science but also for promising applications in nanoscale electronics and spintronics.

High-mobility two-dimensional electron gas in SrGeO3- and BaSnO3-based perovskite oxide heterostructures: an ab initio study.

We explored the possibility of producing a high-mobility two-dimensional electron gas (2DEG) in the LaAlO3/SrGeO3 and LaGaO3/BaSnO3 heterostructures using first-principles electronic structure calculations. Our results show that the 2DEG occurs at n-type LaAlO3/SrGeO3 and LaGaO3/BaSnO3 interfaces. Compared to the prototype LaAlO3/SrTiO3, LaAlO3/SrGeO3 and LaGaO3/BaSnO3 systems yield comparable total interfacial charge carrier density but much lower electron effective mass (nearly half the value of LaAlO3/SrTiO3), thus resulting in about twice larger electron mobility and enhanced interfacial conductivity. This work demonstrates that SrGeO3 and BaSnO3 can be potential substrate materials to achieve a high-mobility 2DEG in the perovskite-oxide heterostructures.

First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO3/SrTiO3 (A = Ca, Sr, and Ba) Heterostructures.

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) in nonpolar/nonpolar AHfO3/SrTiO3 (A = Ca, Sr, and Ba) heterostructures. Two types of nonpolar/nonpolar interfaces, (AO)0/(TiO2)0 and (HfO2)0/(SrO)0, each with AO and HfO2 surface terminations, are modeled, respectively. The polarization domain and resulting interfacial electronic property are found to be more sensitive to the surface termination of the film rather than the interface model. As film thickness increases, an insulator-to-metal transition is found in all the heterostructures with HfO2 surface termination: for (AO)0/(TiO2)0 interfaces, predicted critical film thickness for an insulator-to-metal transition is about 7, 6, and 3 unit cells for CaHfO3/SrTiO3, SrHfO3/SrTiO3, and BaHfO3/SrTiO3, respectively; for (HfO2)0/(SrO)0 interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the heterostructures with AO surface termination, CaHfO3/SrTiO3 exhibits a much larger critical film thickness about 11-12 unit cells for an insulator-to-metal transition; while SrHfO3/SrTiO3 and BaHfO3/SrTiO3 do not show any polarization behavior even film thickness increases up to 20 unit cells. The strain-induced polarization behavior was well-elucidated from energy versus polarization profile. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO3/SrTiO3 HS.

High-Throughput Design of Two-Dimensional Electron Gas Systems Based on Polar/Nonpolar Perovskite Oxide Heterostructures.

The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO3 and SrTiO3 (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO3-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO2)+/(SrO)0 interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion.

Creating Two-Dimensional Electron Gas in Polar/Polar Perovskite Oxide Heterostructures: First-Principles Characterization of LaAlO3/A(+)B(5+)O3.

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) at the polar/polar (LaO)(+)/(BO2)(+) interface in the LaAlO3/A(+)B(5+)O3 (A = Na and K, B = Nb and Ta) heterostructures (HS). Unlike the prototype polar/nonpolar LaAlO3/SrTiO3 HS system where there exists a least film thickness of four LaAlO3 unit cells to have an insulator-to-metal transition, we found that the polar/polar LaAlO3/A(+)B(5+)O3 HS systems are intrinsically conducting at their interfaces without an insulator-to-metal transition. The interfacial charge carrier densities of these polar/polar HS systems are on the order of 10(14) cm(-2), much larger than that of the LaAlO3/SrTiO3 system. This is mainly attributed to two donor layers, i.e., (LaO)(+) and (BO2)(+) (B = Nb and Ta), in the polar/polar LaAlO3/A(+)B(5+)O3 systems, while only one (LaO)(+) donor layer in the polar/nonpolar LaAlO3/SrTiO3 system. In addition, it is expected that, due to less localized Nb 4d and Ta 5d orbitals with respect to Ti 3d orbitals, these LaAlO3/A(+)B(5+)O3 HS systems can exhibit potentially higher electron mobility because of their smaller electron effective mass than that in the LaAlO3/SrTiO3 system. Our results demonstrate that the electronic reconstruction at the polar/polar interface could be an alternative way to produce superior 2DEG in the perovskite-oxide-based HS systems.