Evidence for Anionic Excess Electrons in a Quasi-Two-Dimensional Ca2N Electride by Angle-Resolved Photoemission Spectroscopy

Topological Fermi-arc surface state covered by floating electrons on a two-dimensional electride

Two-dimensional electrides can acquire topologically non-trivial phases due to intriguing interplay between the cationic atomic layers and anionic electron layers. However, experimental evidence of topological surface states has yet to be verified. Here, via angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM), we probe the magnetic Weyl states of the ferromagnetic electride [Gd2C]2+·2e-. In particular, the presence of Weyl cones and Fermi-arc states is demonstrated through photon energy-dependent ARPES measurements, agreeing with theoretical band structure calculations. Notably, the STM measurements reveal that the Fermi-arc states exist underneath a floating quantum electron liquid on the top Gd layer, forming double-stacked surface states in a heterostructure. Our work thus not only unveils the non-trivial topology of the [Gd2C]2+·2e- electride but also realizes a surface heterostructure that can host phenomena distinct from the bulk.

BaCu, a Two-Dimensional Electride with Cu Anions

The electron-rich characteristic and low work function endow electrides with excellent performance in (opto)electronics and catalytic applications; these two features are closely related to the structural topology, constituents, and valence electron concentration of electrides. However, the synthesized electrides, especially two-dimensional (2D) electrides, are limited to specific structural prototypes and anionic p-block elements. Here we synthesize and identify a distinct 2D electride of BaCu with delocalized anionic electrons confined to the interlayer spaces of the BaCu framework. The bonding between Cu and Ba atoms exhibits ionic characteristics, and the adjacent Cu anions form a planar honeycomb structure with metallic Cu-Cu bonding. The negatively charged Cu ions are revealed by the theoretical calculations and experimental X-ray absorption near-edge structure. Physical property measurements reveal that BaCu electride has a high electronic conductivity (ρ = 3.20 μΩ cm) and a low work function (2.5 eV), attributed to the metallic Cu-Cu bonding and delocalized anionic electrons. In contrast to typical ionic 2D electrides with p-block anions, density functional theory calculations find that the orbital hybridization between the delocalized anionic electrons and BaCu framework leads to unique isotropic physical properties, such as mechanical properties, and work function. The freestanding BaCu monolayer with half-metal conductivity exhibits low exfoliation energy (0.84 J/m2) and high mechanical/thermal stability, suggesting the potential to achieve low-dimensional BaCu from the bulk. Our results expand the space for the structure and attributes of 2D electrides, facilitating the discovery and potential application of novel 2D electrides with transition metal anions.

Charge-Transfer-Driven Phase Transition of Two-Dimensional MoTe2 in Donor-Acceptor Heterostructures

In this work, based on first-principles calculations, we propose that electrene can be considered as an electron-donating substrate to drive the phase transition of MoTe2 from the H to T' phase, which is a topic of long-standing interest and importance. In particular, new electrenes Ca2XN2 (X = Zr, Hf) are predicted with the existence of a nearly free two-dimensional (2D) electron gas and ultralow work functions. In MoTe2/Ca2XN2 donor-acceptor heterostructures, we find significantly large charge transfer (∼0.4e per MoTe2 unit cell) from Ca2XN2 to MoTe2, which stabilizes the T' phase and decreases the phase transition barrier (from ∼0.9 to ∼0.5 eV per unit cell). In addition, the phase transition of MoTe2 on Ca2XN2 remains effective as the interlayer distance varies. It therefore can be confirmed conclusively that our results open a new avenue for phase transition study and provide new insights for the large-scale synthesis of metastable high-quality T'-phase MoTe2.

Coexistence of ferromagnetism and charge density waves in monolayer LaBr2

Charge density waves (CDWs), a common phenomenon of periodic lattice distortions, often suppress ferromagnetism in two-dimensional (2D) materials, hindering their magnetic applications. Here, we report a novel CDW that generates 2D ferromagnetism instead of suppressing it, through the formation of interstitial anionic electrons as the charge modulation mechanism. Via first-principles calculations and a low-energy effective model, we find that the highly symmetrical monolayer LaBr2 undergoes a 2 × 1 CDW transition to a magnetic semiconducting T' phase. Concurrently, the delocalized 5d1 electrons of La in LaBr2 redistribute and accumulate within the interstitial space in the T' phase, forming anionic electrons, also known as 2D electride or electrene. The strongly localized nature of anionic electrons promotes a Mott insulating state and full spin-polarization, while the overlap of their extended tails yields ferromagnetic direct exchange between them. Such transition introduces a new magnetic form of CDWs, offering promising opportunities for exploring novel fundamental physics and advanced spintronics applications.

Exchange interactions in the 1T-VSe2 monolayer and their modulation via electron doping using alkali metal adsorption and the electride substrate

The modulation of exchange interactions in layered magnets has fundamental research value and potential applications in spintronics. Based on first-principles calculations, the exchange interactions in the experimentally controversial room-temperature ferromagnetic 1T-VSe₂ monolayer are systematically studied. It is found that three shells of nearest-neighbor Heisenberg exchange interactions and higher-order interactions are crucial for an accurate description of the magnetism and its thermal stability in the 1T-VSe₂ monolayer. Based on our understanding of tuning the magnetic interactions and the magnetic ground state in the 1T-VSe₂ monolayer via external factors, two modulation methods, involving adsorption of the alkali metal lithium and the electride Ca₂N substrate, are proposed. In both Li-VSe₂ and VSe₂/Ca₂N systems, the strongly frustrated Heisenberg exchange interaction competes with the Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy, leading to complex magnetic ground states, such as antiferromagnetic spin spiral and periodic antiferromagnetic cycloidal states. Moreover, the higher-order exchange interactions play a crucial role in the stabilization of long-range double-row-wise antiferromagnetic states in Li-VSe₂ and VSe₂/Ca₂N. These results highlight the effective manipulation of exchange interactions in two-dimensional magnets.

A new concept of atomically thin p-n junction based on Ca2N/Na2N donor-acceptor heterostructure: a first-principles study

In atomically thin p-n junctions, traditional strategies such as doping and implantation for realizing a p- or n-region will fail at the nanoscale, and the Schottky barrier and Fermi level pinning effect taking place in metal-semiconductor contacts seriously suppress the transport properties. In this work, based on first-principles calculations, we propose a new strategy for realizing an ultrathin p-n junction by vertically stacking nonstoichiometric Ca₂N and Na₂N monolayers, which represents a kind of donor-acceptor heterostructure with a natural Ohmic contact. It is of great interest to find that the tunneling barrier can be eliminated and the charge transfer quantity is one order of magnitude higher than that between polar monolayers by adjusting the interlayer distance. In addition, at equilibrium the interlayer tunneling can be turned into resonant transport due to the quasi-bonding, thus enabling excellent transmission performance. In accordance with the results, we believe that our new concept of an atomically thin p-n junction will provide an unprecedented possibility for the development of nanodevices.

First-Principles Study of Three-Dimensional Electrides Containing One-Dimensional [Ba3N]3+ Chains

Electrides, a unique type of compound where electrons act as anions, have a high electron mobility and a low work function, which makes them promising for applications in electronic devices and high-performance catalysts. The discovery of novel electrides and the expansion of the electride family have great significance for their promising applications. Herein, we reported four three-dimensional (3D) electrides by coupling crystal structure database searches and first-principles electronic structure analysis. Subnitrides (Ba₃N, LiBa₃N, NaBa₃N, and Na₅Ba₃N) containing one-dimensional (1D) [Ba₃N]³⁺ chains are identified as 3D electrides for the first time. The anionic electrons are confined in the 3D interstitial space of Ba₃N, LiBa₃N, NaBa₃N, and Na₅Ba₃N. Interestingly, with the increase of Na content, the excess electrons of Na₅Ba₃N play two roles of metallic bonding and anionic electrons. Therefore, the subnitrides containing 1D [Ba₃N]³⁺ chains can be regarded as a new family of 3D electrides, where anionic electrons reside in the 3D interstitial spaces and provide a conduction path. These materials not only are experimentally synthesizable 3D electrides but also are promising to be exfoliated into advanced 1D nanowire materials. Furthermore, our work suggests a discovery strategy of novel electrides based on one parent framework like [Ba₃N]³⁺ chains, which would accelerate the mining of electrides from the crystal structure database.

Ultralow work function of the electride Sr3CrN3

Electrides have valence electrons that occupy free space in the crystal structure, making them easier to extract. This feature can be used in catalysis for important reactions that usually require a high-temperature and high-pressure environments, such as ammonia synthesis. In this paper, we use density functional theory to investigate the behaviour of interstitial electrons of the 1-dimensional electride Sr₃CrN₃. We find that the bulk excess electron density persists on introduction of surface terminations, that the crystal termination perpendicular to the 1D free-electron channel is highly stable and we confirm an extremely low work function with hybrid functional methods. Our results indicate that Sr₃CrN₃ is a potentially important novel catalyst, with accessible, directional and extractable free electron density.

Boosting Photoredox Catalysis Using a Two-Dimensional Electride as a Persistent Electron Donor

Electrides, which have excess anionic electrons, are solid-state sources of solvated electrons that can be used as powerful reducing agents for organic syntheses. However, the abrupt decomposition of electrides in organic solvents makes controlling the transfer inefficient, thereby limiting the utilization of their superior electron-donating ability. Here, we demonstrate the efficient reductive transformation strategy which combines the stable two-dimensional [Gd₂C]²⁺·2e^- electride electron donor and cyclometalated Pt(II) complex photocatalysts. Strongly localized anionic electrons at the interlayer space in the [Gd₂C]²⁺·2e^- electride are released via moderate alcoholysis in 2,2,2-trifluoroethanol, enabling persistent electron donation. The Pt(II) complexes are adsorbed onto the surface of the [Gd₂C]²⁺·2e^- electride and rapidly capture the released electrons at a rate of 10⁷ s^-1 upon photoexcitation. The one-electron-reduced Pt complex is electrochemically stable enough to deliver the electron to substrates in the bulk, which completes the photoredox cycle. The key benefit of this system is the suppression of undesirable charge recombination because back electron transfer is prohibited due to the irreversible disruption of the electride after the electron transfer. These desirable properties collectively serve as the photoredox catalysis principle for the reductive generation of the benzyl radical from benzyl halide, which is the key intermediate for dehalogenated or homocoupled products.

Fluorides of Silver Under Large Compression*

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag⁺ and Ag²⁺ cations (Ag₃ F₄ and Ag₂ F₃ ) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag₂ F₅ , containing both Ag²⁺ and Ag³⁺ , we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed-valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF₄ as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed-valence compounds of silver at high pressure.

Water- and acid-stable self-passivated dihafnium sulfide electride and its persistent electrocatalytic reaction

Electrides have emerged as promising materials with exotic properties, such as extraordinary electron-donating ability. However, the inevitable instability of electrides, which is caused by inherent excess electrons, has hampered their widespread applications. We report that a self-passivated dihafnium sulfide electride ([Hf₂S]²⁺∙2e^-) by double amorphous layers exhibits a strong oxidation resistance in water and acid solutions, enabling a persistent electrocatalytic hydrogen evolution reaction. The naturally formed amorphous Hf₂S layer on the cleaved [Hf₂S]²⁺∙2e^- surface reacts with oxygen to form an outermost amorphous HfO₂ layer with ~10-nm thickness, passivating the [Hf₂S]²⁺∙2e^- electride. The excess electrons in the [Hf₂S]²⁺∙2e^- electride are transferred through the thin HfO₂ passivation layer to water molecules under applied electric fields, demonstrating the first electrocatalytic reaction with excellent long-term sustainability and no degradation in performance. This self-passivation mechanism in reactive conditions can advance the development of stable electrides for energy-efficient applications.

Tunable Topological State, High Hole-Carrier Mobility, and Prominent Sunlight Absorbance in Monolayered Calcium Triarsenide

Designing novel two-dimensional (2D) materials is highly desirable for material innovation. Here, we propose monolayered calcium triarsenide (1L CaAs₃) as a new 2D semiconductor with a series of encouraging functionalities. In contrast to the ∼33 meV small band gap in bulk CaAs₃, 1L CaAs₃ possesses a large direct band gap of 0.92 eV with a high hole mobility of ∼10⁴ cm² V^-1 s^-1. The electronic properties of 2D CaAs₃ can be manipulated significantly by the layer thickness and external strains. Remarkably, 2D CaAs₃ suggests a topologically nontrivial-trivial state transition under thickness reduction and strain engineering, which is attributed to the drastic surface relaxation and pinch effect under compression. A semiconductor-semimetal transition is also revealed when the layer thickness is greater than 3L. Furthermore, 1L CaAs₃ exhibits prominent visible-light absorption compared with the crystalline silicon. All these desired properties render 2D CaAs₃ a promising candidate for use in electronic and photovoltaic devices.

Metal-to-Semiconductor Transition and Electronic Dimensionality Reduction of Ca2N Electride under Pressure

The discovery of electrides, in particular, inorganic electrides where electrons substitute anions, has inspired striking interests in the systems that exhibit unusual electronic and catalytic properties. So far, however, the experimental studies of such systems are largely restricted to ambient conditions, unable to understand their interactions between electron localizations and geometrical modifications under external stimuli, e.g., pressure. Here, pressure-induced structural and electronic evolutions of Ca2N by in situ synchrotron X-ray diffraction and electrical resistance measurements, and density functional theory calculations with particle swarm optimization algorithms are reported. Experiments and computation are combined to reveal that under compression, Ca2N undergoes structural transforms from R3 ¯ m symmetry to I4 ¯ 2d phase via an intermediate Fd3 ¯ m phase, and then to Cc phase, accompanied by the reductions of electronic dimensionality from 2D, 1D to 0D. Electrical resistance measurements support a metal-to-semiconductor transition in Ca2N because of the reorganizations of confined electrons under pressure, also validated by the calculation. The results demonstrate unexplored experimental evidence for a pressure-induced metal-to-semiconductor switching in Ca2N and offer a possible strategy for producing new electrides under moderate pressure.

Electron-donor doping enhanced Li storage in electride Ca2N monolayer: a first-principles study

It was reported by Hu et al (2015 ACS Appl. Mater. Interfaces 7 24016) that Li adsorption on the Ca₂N monolayer is energetically unfavorable, although it was found to be a good candidate for Na storage. In this paper, from first-principles calculations, it is shown that compressive stain can greatly enhance the interactions between Li adsorbate and the Ca₂N host, which is beneficial to prevent Li from clustering on the surface and thus Li storage becomes possible. Charge distribution analysis further shows that the enhanced Li-adsorption is a result of the increased surface charge density and the more confined charge distribution under compressive strain. Inspired by this observation, several electron-donor-doped Ca₂N materials are considered as anode materials for Li-ion batteries. Results show that the O and F doped Ca₂N have the best performance, with predicted Li storage capacity reaching about 567.9 and 565.9 mAh g^-1 for O and F doped cases, respectively. It is also demonstrated that electron-donor doping does not change the metallic electronic structures and the low Li-ion migration energy barriers on the surface of the Ca₂N monolayer, and thus the rate performance of the doped Ca₂N can be as good as the undoped case. Our study offers a deep understanding of the Li interactions with 2D materials and provides an approach of material modification to 2D electrides for battery applications.

Topological Electride Y2C

Two-dimensional (2D) electrides are layered ionic crystals in which anionic electrons are confined in the interlayer space. Here, we report a discovery of nontrivial [Formula: see text] topology in the electronic structures of 2D electride Y₂C. Based on first-principles calculations, we found a topological [Formula: see text] invariant of (1; 111) for the bulk band and topologically protected surface states in the surfaces of Y₂C, signifying its nontrivial electronic topology. We suggest a spin-resolved angle-resolved photoemission spectroscopy (ARPES) measurement to detect the unique helical spin texture of the spin-polarized topological surface state, which will provide characteristic evidence for the nontrivial electronic topology of Y₂C. Furthermore, the coexistence of 2D surface electride states and topological surface state enables us to explain the outstanding discrepancy between the recent ARPES experiments and theoretical calculations. Our findings establish a preliminary link between the electride in chemistry and the band topology in condensed-matter physics, which are expected to inspire further interdisciplinary research between these fields.

Simple and Efficient Fabrication of Mayenite Electrides from a Solution-Derived Precursor

Mayenite (12CaO·7Al₂O₃, C12A7) electride with an anti-zeolite nanoporous structure has attracted intense attention due to its versatile promising application potentials. However, the synthesis difficulty because of extremely harsh conditions (e.g., reduction in sealed calcium or titanium vapor) significantly obstructs its realistic applications. In this work, we employed a simple, efficient, and cost-effective route for synthesizing mayenite electrides (C12A7:e^-) in both powder and dense ceramic. C12A7:e^- powders with efficient electron doping (3.5 × 10²⁰ cm^-3) were obtained via simple graphite reduction of a novel mixture precursor of CaAl₂O₄ (CA) and Ca₃Al₂O₆ (C3A) derived from a modified Pechini method. The structural evolution during the electride formation was investigated, and it was found that reduction below 1300 °C induced the formation of Ca₅Al₆O₁₄ (C5A3), while reduction above 1400 °C helped retain the mayenite structure. Fully dense C12A7:e^- ceramics were also fabricated via graphite reduction of presintered pellets with a relative density of 97.9% starting from the CA+C3A mixture. Careful studies improved the mechanism cognition of graphite treatment that the electrons injection was probably initiated by surface reduction with involatile C species (e.g., C₂^2-) rather than previously proposed CO, during which the mixed conduction of oxygen ions and electrons played an important role. Furthermore, the stability of C12A7:e^- in water as well as in the presence of moisture was discussed. These results not only suggest a novel precursor for fabricating high-quality mayenite electrides but also provide in-depth insights into the stability of the mayenite structure toward practical applications.

Experimental Demonstration of an Electride as a 2D Material

Because of their loosely bound electrons, electrides offer physical properties useful in chemical synthesis and electronics. For these applications and others, nanosized electrides offer advantages, but to-date no electride has been synthesized as a nanomaterial. We demonstrate experimentally that Ca₂N, a layered electride in which layers of atoms are separated by layers of a 2D electron gas (2DEG), can be exfoliated into two-dimensional (2D) nanosheets using liquid exfoliation. The 2D flakes are stable in a nitrogen atmosphere or in select organic solvents for at least one month. Electron microscopy and elemental analysis reveal that the 2D flakes retain the crystal structure and stoichiometry of the parent 3D Ca₂N. In addition, the 2D flakes exhibit metallic character and an optical response that agrees with DFT calculations. Together these findings suggest that the 2DEG is preserved in the 2D material. With this work, we bring electrides into the nanoregime and experimentally demonstrate a 2D electride, Ca₂N.