Interlayer Charge Transport in 2D Lead Halide Perovskites from First Principles.

We report on the implementation of a versatile projection-operator diabatization approach to calculate electronic coupling integrals in layered periodic systems. The approach is applied to model charge transport across the saturated organic spacers in two-dimensional (2D) lead halide perovskites. The calculations yield out-of-plane charge transfer rates that decay exponentially with the increasing length of the alkyl chain, range from a few nanoseconds to milliseconds, and are supportive of a hopping mechanism. Most importantly, we show that the charge carriers strongly couple to distortions of the Pb-I framework and that accounting for the associated nonlocal dynamic disorder increases the thermally averaged interlayer rates by a few orders of magnitude compared to the frozen-ion 0 K-optimized structure. Our formalism provides the first comprehensive insight into the role of the organic spacer cation on vertical transport in 2D lead halide perovskites and can be readily extended to functional π-conjugated spacers, where we expect the improved energy alignment with the inorganic layout to speed up the charge transfer between the semiconducting layers.

Establishment and validation of an electron inelastic mean free path database for narrow bandgap inorganic compounds with a machine learning approach.

Narrow bandgap inorganic compounds are extremely important in many areas of physics. However, their basic parameter database for surface analysis is incomplete. Electron inelastic mean free paths (IMFPs) are important parameters in surface analysis methods, such as electron spectroscopy and electron microscopy. Our previous research has presented a machine learning (ML) method to describe and predict IMFPs from calculated IMFPs for 41 elemental solids. This paper extends the use of the same machine learning method to 42 inorganic compounds based on the experience in predicting elemental electron IMFPs. The in-depth discussion extends to including material dependence discussion and parameter value selections. After robust validation of the ML method, we have produced an extensive IMFP database for 12 039 narrow bandgap inorganic compounds. Our findings suggest that ML is very efficient and powerful for IMFP description and database completion for various materials and has many advantages, including stability and convenience, over traditional methods.

Mixed Sulfur/Selenium Anions Weaken Electron-Vibrational Interaction in Cu2ZnSn(S,Se)4 Photoabsorber.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar absorbers have attracted intensive investigations for next-generation photovoltaic applications. Here, by using ab initio static and molecular dynamics simulations, we investigated the anion compositional dependence of electron-vibration interaction in CZTSSe materials. We found that the conduction band fluctuates more than the valence band, and as a result, the band gap variation is more sensitive to the change of the former, which can be understood in terms of p-d hybridization in the valence bands. Electron-phonon coupling is smaller in CZTSSe alloy compared to pure S- or Se-containing structures, as evidenced by the smaller fluctuation of excitation energy, and can be attributed to the weaker structural dynamics of the metal-anion bond. Small electron-phonon coupling strength may lead to better charge transport in these materials. We also elucidated the interplay between disordered structures and S/Se stoichiometry through analysis of optical line width. The results highlight the importance of anion composition engineering and provide new insights into the rational design of high-performing kesterite absorbers for solar cells.

Heusler alloy catalysts for electrochemical CO2 reduction.

Developing efficient catalysts for electrochemical CO2 reduction reaction (ECO2RR) to hydrocarbons is becoming increasingly important but still challenging due to their high overpotential and poor selectivity. Here, the famous Heusler alloys are investigated as ECO2RR catalysts for the first time by means of density functional theory calculations. The linear scaling relationship between the adsorption energies of CHO (and COOH) and CO intermediates is broken and, thus, the overpotential can be tuned regularly by chemically permuting different 3d, 4d, or 5d transition metals (TMs) in Heusler alloy Cu2TMAl. Cu2ZnAl shows the best activity among all the 30 Heusler alloys considered in the present study, with 41% improvement in energy efficiency compared to pure Cu electrode. Cu2PdAl, Cu2AgAl, Cu2PtAl, and Cu2AuAl are also good candidates. The calculations on the competition between hydrogen evolution reaction and CO2RR indicate that Cu2ZnAl is also the one having the best selectivity toward hydrocarbons. This work identifies the possibility of applying the Heusler alloy as an efficient ECO2RR catalyst. Since thousands of Heusler alloys have been found in experiments, the present study also encourages the search for more promising candidates in this broad research area.

Sp-Hybridized Nitrogen as New Anchoring Sites of Iron Single Atoms to Boost the Oxygen Reduction Reaction.

Carbon supported single-atom catalysts with metal-Nx configuration are considered as one of the most efficient catalysts for the oxygen reduction reaction (ORR). However, most of the metal-Nx active sites are composed by pyridinic N at the defect locations of graphene-like supports. Here, we employ graphdiyne (GDY) as a new carbon substrate to synthesize an iron (Fe) single atom catalyst (Fe-N-GDY), showing excellent catalytic performance. Benefitting from the abundant acetylenic bonds in GDY, sp-N anchored metal atoms are created without forming defects. The sp-N and OH ligands regulate the electronic structure of Fe atoms and optimize the adsorption energy of ORR intermediates on the active sites by reducing the electron local density of Fe atoms, which accelerates the reaction kinetics and promotes the ORR activity of Fe-N-GDY. Furthermore, the practical application of Fe-N-GDY is corroborated by its high power density and long-term performance via assembling a zinc-air battery.

Li8MnO6: A Novel Cathode Material with Only Anionic Redox.

In Li-excess transition metal-oxide cathode materials, anionic oxygen redox can offer high capacity and high voltages, although peroxo and superoxo species may cause oxygen loss, poor cycling performance, and capacity fading. Previous work showed that undesirable formation of peroxide and superoxide bonds was controlled to some extent by Mn substitution, and the present work uses density functional calculations to examine the reasons for this by studying the anionic redox mechanism in Li8MnO6. This material is obtained by substituting Mn for Sn in Li8SnO6 or for Zr in Li8ZrO6, and we also compare this to previous work on those materials. The calculations predict that Li8MnO6 is stable at room temperature (with a band gap of 3.19 eV as calculated by HSE06 and 1.82 eV as calculated with the less reliable PBE+U), and they elucidate the chemical and structural effects involved in the inhibition of oxygen release in this cathode. Throughout the whole delithiation process, only O2- ions are oxidized. The directional Mn-O bonds formed from unfilled 3d orbitals effectively inhibit the formation of O-O bonds, and the layered structure is maintained even after removing 3 Li per Li8MnO6 formula unit. The calculated average voltage for removal of 3 Li is 3.69 V by HSE06, and the corresponding capacity is 389 mAh/g. The high voltage of oxygen anionic redox and the high capacity result in a high energy density of 1436 Wh/kg. The Li-ion diffusion barrier for the dominant interlayer diffusion path along the c axis is 0.57 eV by PBE+U. These results help us to understand the oxygen redox mechanism in a new lithium-rich Li8MnO6 cathode material and contribute to the design of high-energy density lithium-ion battery cathode materials with favorable electrochemical properties based on anionic oxygen redox.

Charge Compensation Mechanisms and Oxygen Vacancy Formations in LiNi1/3Co1/3Mn1/3O2: First-Principles Calculations.

Charge compensation mechanisms in the delithiation processes of LiNi1/3Co1/3Mn1/3O2 (NCM111) are compared in detail by the first-principles calculations with GGA and GGA+U methods under different U values reported in the literature. The calculations suggested that different sets of U values lead to different charge compensation mechanisms in the delithiation process. Co3+/Co4+ couples were shown to dominate the redox reaction for 1 ≥ x ≥ 2/3 by using the GGA+U 1 method (U 1 = 6.0 3.4 3.9 for Ni, Co, and Mn, respectively). However, by using the GGA+U 2 (U 2 = 6.0 5.5 4.2) method, the results indicated that the redox reaction of Ni2+/Ni3+ took place in the range of 1 ≥ x ≥ 2/3. Therefore, according to our study, experimental charge compensation processes during delithiation are of great importance to evaluate the theoretical calculations. The results also indicated that all the GGA+U i (i = 1, 2, 3) schemes predicted better voltage platforms than the GGA method. The oxygen anionic redox reactions during delithiation are also compared with GGA+U calculations under different U values. The electronic density of states and magnetic moments of transition metals have been employed to illustrate the redox reactions during the lithium extractions in NCM111. We have also investigated the formation energies of an oxygen vacancy in NCM111 under different values of U, which is important in understanding the possible occurrence of oxygen release. The formation energy of an O vacancy is essentially dependent on the experimental conditions. As expected, the decreased temperature and increased oxygen partial pressure can suppress the formation of the oxygen vacancy. The calculations can help improve the stability of the lattice oxygen.

Metal-Organic Framework-Derived Graphene Mesh: a Robust Scaffold for Highly Exposed Fe-N4 Active Sites toward an Excellent Oxygen Reduction Catalyst in Acid Media.

This study demonstrates a special ultrathin N-doped graphene nanomesh (NGM) as a robust scaffold for highly exposed Fe-N4 active sites. Significantly, the pore sizes of the NGM can be elaborately regulated by adjusting the thermal exfoliation conditions to simultaneously disperse and anchor Fe-N4 moieties, ultimately leading to highly loaded Fe single-atom catalysts (SA-Fe-NGM) and a highly exposed morphology. The SA-Fe-NGM is found to deliver a superior oxygen reduction reaction (ORR) activity in acidic media (half-wave potential = 0.83 V vs RHE) and a high power density of 634 mW cm-2 in the H2/O2 fuel cell test. First-principles calculations further elucidate the possible catalytic mechanism for ORR based on the identified Fe-N4 active sites and the pore size distribution analysis. This work provides a novel strategy for constructing highly exposed transition metals and nitrogen co-doped carbon materials (M-N-C) catalysts for extended electrocatalytic and energy storage applications.

Weak Anharmonicity Rationalizes the Temperature-Driven Acceleration of Nonradiative Dynamics in Cu2ZnSnS4 Photoabsorbers.

We report a time-domain ab initio investigation of the nonradiative electron-hole recombination in quaternary Cu2ZnSnS4 (CZTS) at different temperatures using a combination of time-dependent density functional theory and nonadiabatic molecular dynamics. Our results demonstrate that higher temperatures increase both inelastic and elastic electron-phonon interactions. Elevated temperatures moderately increase the lattice anharmonicity and cause stronger fluctuations of electronic energy levels, enhancing the electron-phonon coupling. The overall nuclear anharmonic effect is weak in CZTS, which can be ascribed to their stable bonding environment. Phonon-induced loss of electronic coherence accelerates with temperature, due to stronger elastic electron-phonon scattering. The enhanced inelastic electron-phonon scattering decreases charge carrier lifetimes at higher temperatures, deteriorating material performance in optoelectronic devices. The detailed atomistic investigation of the temperature-dependent charge carrier dynamics, with particular focus on anharmonic effects, guides the development of more efficient solar cells based on CZTS and related semiconductor photoabsorbers.

Identifying and Passivating Killer Defects in Pb-Free Double Cs2AgBiBr6 Perovskite.

Pb-free double perovskites, such as Cs2AgBiBr6, are alternatives to lead halide perovskites for photovoltaic applications due to superior stability, low toxicity, and promising optoelectronic properties. However, their performance is subpar. We combine nonadiabatic molecular dynamics and real-time time-dependent density-functional theory to show that the negatively charged Br vacancy in Cs2AgBiBr6 creates an extremely detrimental donor-yielded (DY) center, which is a typical defect in six-coordinated semiconductors. Ag+ and Bi3+ form a bond by attraction through the anisotropic vacancy charge, generating a midgap state that traps holes within tens of picoseconds. Substituting Ag with indium by doping produces a weak and long In-Bi bond, lifting the defect energy level to the conduction band. Hole trapping slows down by an order or magnitude, and trap-assisted charge recombination decreases 4-fold. The simulations bring atomistic insights into defects of Pb-free double perovskites and provide a defect mitigation strategy for rational design of high-performance optoelectronic devices.

Formation of oxygen vacancies in Li2FeSiO4: first-principles calculations.

The formation of oxygen vacancies could affect various properties of oxides. Herein we have investigated the formation energies of an oxygen vacancy (VO) with the relevant charge states in bulk Pnma-Li2FeSiO4 using first-principles calculations. The formation energies of the VO are essentially dependent on the atomic chemical potentials that represent the experimental conditions. The calculated formation energies of an oxygen vacancy in different charge states indicate that it would be energetically favorable to fully ionize the oxygen vacancy in Li2FeSiO4. The presence of VO is accompanied by a distinct redistribution of the electronic charge densities only around the Fe and Si ions next to the O-vacancy site, which shows a very local influence on the host material arising from VO. This local characteristic is also confirmed by the calculated partial densities of states (PDOS). We also studied the influence of substitutional (MnFe and CoFe) and cation vacancy defects (i.e., VFe and VLi) in the vicinity of an O-vacancy on the formation of an O-vacancy, respectively. We find that the calculated interaction energies between these defects and the oxygen vacancy are all negative, which implies that the formation of an oxygen vacancy becomes easier when the above defects are introduced. Compared to the substitutional defects, the interaction energies between the vacancy defects and the oxygen vacancy are significantly larger. Among them, the interaction energy between VFe and VO is the largest.

Facet Engineering to Regulate Surface States of Topological Crystalline Insulator Bismuth Rhombic Dodecahedrons for Highly Energy Efficient Electrochemical CO2 Reduction.

Bismuth (Bi) is a topological crystalline insulator (TCI), which has gapless topological surface states (TSSs) protected by a specific crystalline symmetry that strongly depends on the facet. Bi is also a promising electrochemical CO2 reduction reaction (ECO2 RR) electrocatalyst for formate production. In this study, single-crystalline Bi rhombic dodecahedrons (RDs) exposed with (104) and (110) facets are developed. The Bi RDs demonstrate a very low overpotential and high selectivity for formate production (Faradic efficiency >92.2%) in a wide partial current density range from 9.8 to 290.1 mA cm-2 , leading to a remarkably high full-cell energy efficiency (69.5%) for ECO2 RR. The significantly reduced overpotential is caused by the enhanced *OCHO adsorption on the Bi RDs. The high selectivity of formate can be ascribed to the TSSs and the trivial surface states opening small gaps in the bulk gap on Bi RDs, which strengthens and stabilizes the preferentially adsorbed *OCHO and mitigates the competing adsorption of *H during ECO2 RR. This study describes a promising application of Bi RDs for high-rate formate production and high-efficiency energy storage of intermittent renewable electricity. Optimizing the geometry of TCIs is also proposed as an effective strategy to tune the TSSs of topological catalysts.

Fe-Al-Si Thermoelectric (FAST) Materials and Modules: Diffusion Couple and Machine-Learning-Assisted Materials Development.

To lower the introduction and maintenance costs of autonomous power supplies for driving Internet-of-things (IoT) devices, we have developed low-cost Fe-Al-Si-based thermoelectric (FAST) materials and power generation modules. Our development approach combines computational science, experiments, mapping measurements, and machine learning (ML). FAST materials have a good balance of mechanical properties and excellent chemical stability, superior to that of conventional Bi-Te-based materials. However, it remains challenging to enhance the power factor (PF) and lower the thermal conductivity of FAST materials to develop reliable power generation devices. This forum paper describes the current status of materials development based on experiments and ML with limited data, together with power generation module fabrication related to FAST materials with a view to commercialization. Combining bulk combinatorial methods with diffusion couple and mapping measurements could accelerate the search to enhance PF for FAST materials. We report that ML prediction is a powerful tool for finding unexpected off-stoichiometric compositions of the Fe-Al-Si system and dopant concentrations of a fourth element to enhance the PF, i.e., Co substitution for Fe atoms in FAST materials.

Theoretical screening of VSe2 as support for enhanced electrocatalytic performance of transition-metal single atoms.

Single-atom catalysts (SACs) have attracted enormous attentions in heterogeneous catalysts due to the maximized atomic utilization and extraordinary catalytic performance. Similar to homogeneous catalytic ligands, the support in SACs plays a vital role in the catalytic properties. Herein, we present a series of transition-metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Os, Ir and Pt) anchored on a vanadium diselenide (VSe2) monolayer as electrocatalysts through density functional theory calculations. Pd@VSe2 stands out among the considered SACs with a low overpotential of 0.38 V, exhibiting the excellent performance of oxygen reduction reaction (ORR). Meanwhile, a liner trend between the adsorption Gibbs free energy of the OH (ΔGOH*) and the predicted ηORR is revealed, which may serve as a simple descriptor for the inherent ORR catalytic activity of SACs. Particularly, Pt@VSe2 shows extraordinarily low theoretical overpotential of -0.04/0.47 V for hydrogen/oxygen evolution reaction, which transcends the state-of-the-art Pt and IrO2 and thereby can be exploited as highly-efficient bifunctional electrocatalyst for overall water splitting. This work broadens the perception of designing multifunctional electrocatalysts based on two-dimensional VSe2 material and offers a new paradigm for investigating advanced SACs.

First-principles study of electronic structures and elasticity of Al2Fe3Si3.

Al2Fe3Si3intermetallic compound shows promising application in low-cost and non-toxic thermoelectric device because of its relatively high power factor of ∼700µW m-1 K-2at 400 K. Herein we performed the first-principles calculations with the projector augmented-wave (PAW) method to study the formation energies, elastic constants, electronic structures, and electronic transport properties of Al2Fe3Si3. We discussed the thermodynamical stability of Al2Fe3Si3against other ternary crystalline compounds in Al-Fe-Si phase. The band gap of Al2Fe3Si3was particularly examined using the semilocal and hybrid functionals and the on-site Hubbard correction, which were also applied to ß-FeSi2to calibrate the prediction reliability of our employed computational methods. Our calculations show that Al2Fe3Si3is a narrow-gap semiconductor. The semilocal functional within generalized gradient approximation (GGA) shows an exceptional agreement between the predicted band gap of Al2Fe3Si3and the available experiment data, which is in contrast to the typical trend and rationally understood through a comprehensive comparison. We found that both HSE06 and PBE0 hybrid functionals with a standard setup overestimated the band gaps of Al2Fe3Si3and ß-FeSi2too much. The underlying reasons may be ascribed to a large electronic screening, which arises from the unique characteristics of Fe 3dstates appearing in both sides of band gaps of Al2Fe3Si3and ß-FeSi2, and to a reduced delocalization error thanks to the covalent Fe-Si and Si-Si bonding nature. The chemical bonding and elasticity of Al2Fe3Si3were compared with those of ß-FeSi2and FeAl2. In Al2Fe3Si3the Fe-Al bonding is more ionic and the Fe-Si bonding is more covalent. The elastic moduli of Al2Fe3Si3are comparable to those of ß-FeSi2and larger than those of FeAl2. Our calculation results indicate that the mechanical strength of Al2Fe3Si3could be strong enough for the practical application in thermoelectric device.

Elucidating the Influence of Sulfur Vacancies on Nonradiative Recombination Dynamics in Cu2ZnSnS4 Solar Absorbers.

We report a time-domain ab initio simulation of charge carrier trapping and relaxation dynamics in pristine and defect-containing kesterite Cu2ZnSnS4 (CZTS) structures. Our simulations show that introduction of a neutral sulfur vacancy in the CZTS system leads to a decrease of the charge recombination rate by a factor of ∼4, and the doubly positively charged sulfur vacancy results in a minor decrease of carrier lifetime, as compared to the pristine CZTS system. The neutral sulfur vacancy weakens the nonadiabatic (NA) electron-phonon coupling by moderately localizing charge density and accelerates the pure dephasing process, extending charge carrier lifetime. Therefore, the neutral sulfur vacancy is electrically benign. The doubly positively charged sulfur vacancy introduces a subgap state which is hardly populated, and recombination of the electron and hole bypassing the trap state dominates. As a result, the recombination rate decreases in the doubly charged sulfur vacancy structure. The reported results identified the key role of the sulfur-related vacancy on charge carrier trapping and relaxation of CZTS materials, carrying important implications for further optimization of CZTS and other thin-film solar cell materials.

Epitaxial Growth of Rectangle Shape MoS2 with Highly Aligned Orientation on Twofold Symmetry a-Plane Sapphire.

Research on transition metal dichalcogenides (TMDs) has been accelerated by the development of large-scale synthesis based on chemical vapor deposition (CVD) growth. However, in most cases, CVD-grown TMDs are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs), which seriously degrade their electrical and photoelectrical properties. Here, the epitaxial growth of highly aligned MoS2 grains is reported on a twofold symmetry a-plane sapphire substrate. The obtained MoS2 grains have an unusual rectangle shape with perfect orientation alignment along the [1-100] crystallographic direction of a-plane sapphire. It is found that the growth temperature plays a key role in its orientation alignment and morphology evolution, and high temperature is beneficial to the initial MoS2 seeds rotate to the favorable orientation configurations. In addition, the photoluminescence quenching of the well-aligned MoS2 grains indicates a strong MoS2 -substrate interaction which induces the anisotropic growth of MoS2 , and thus brings the formation of rectangle shape grains. Moreover, the well-aligned MoS2 grains splice together without GB formation, and thus that has negligible effect on its electrical transport properties. The progress achieved in this work could promote the controlled synthesis of large-area TMDs single crystal film and the scalable fabrication of high-performance electronic devices.

Author Correction: Physical and chemical descriptors for predicting interfacial thermal resistance.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Physical and chemical descriptors for predicting interfacial thermal resistance.

Heat transfer at interfaces plays a critical role in material design and device performance. Higher interfacial thermal resistances (ITRs) affect the device efficiency and increase the energy consumption. Conversely, higher ITRs can enhance the figure of merit of thermoelectric materials by achieving ultra-low thermal conductivity via nanostructuring. This study proposes a dataset of descriptors for predicting the ITRs. The dataset includes two parts: one part consists of ITRs data collected from 87 experimental papers and the other part consists of the descriptors of 289 materials, which can construct over 80,000 pair-material systems for ITRs prediction. The former part is composed of over 1300 data points of metal/nonmetal, nonmetal/nonmetal, and metal/metal interfaces. The latter part consists of physical and chemical properties that are highly correlated to the ITRs. The synthesis method of the materials and the thermal measurement technique are also recorded in the dataset for further analyses. These datasets can be applied not only to ITRs predictions but also to thermal-property predictions or heat transfer on various material systems.

Pressure-induced superconductivity in SnSb2Te4.

Here we firstly report the pressure-induced superconductivity in phase change materials SnSb2Te4. Single crystals of SnSb2Te4 were grown using a conventional melting-down method. The resistance under pressure was measured using an originally designed diamond anvil cell with boron-doped diamond electrodes. The temperature dependence of the resistance under different pressures has been measured up to 32.6 GPa. The superconducting transition of SnSb2Te4 appeared at 2.1 K ([Formula: see text]) under 8.1 GPa, which was further increased with applied pressure to a maximum onset transition temperature 7.4 K under 32.6 GPa.