Obtaining two-dimensional electron gas in free space without resorting to electron doping: an electride based design

Van der Waals Electrides

ConspectusElectrides make up a fascinating group of materials with unique physical and chemical properties. In these materials, excess electrons do not behave like normal electrons in metals or form any chemical bonds with atoms. Instead, they "float" freely in the gaps within the material's structure, acting like negatively charged particles called anions (see the graph). Recently, there has been a surge of interest in van der Waals (vdW) electrides or electrenes in two dimensions. A typical example is layered lanthanum bromide (LaBr2), which can be taken as [La3+(Br1-)2]+•(e-). Each excess free electron is trapped within a hexagonal pore, forming dense dots of electron density. These anionic electrons are loosely bound, giving vdW electrides some unique properties such as ferromagnetism, superconductivity, topological features, and Dirac plasmons. The high density of the free electron makes electrides very promising for applications in thermionic emission, organic light-emitting diodes, and high-performance catalysts.In this Account, we first discuss the discovery of numerous vdW electrides through high-throughput computational screening of over 67,000 known inorganic crystals in Materials Project. A dozen of them have been newly discovered and have not been reported before. Importantly, they possess completely different structural prototypes and properties of anionic electrons compared to widely studied electrides such as Ca2N. Finding these new vdW electrides expands the variety of electrides that can be made in the experiment and opens up new possibilities for studying their unique properties and applications.Then, based on the screened vdW electrides, we delve into their various emerging properties. For example, we developed a new magnetic mechanism specific to atomic-orbital-free ferromagnetism in electrides. We uncover the dual localized and extended nature of the anionic electrons in such electrides and demonstrate the formation of the local moment by the localized feature and the ferromagnetic interaction by the direct overlapping of their extended states. We further show the effective tuning of the magnetic properties of vdW electrides by engineering their structural, electronic, and compositional properties. Besides, we show that the complex interaction between the multiple quantum orderings in vdW electrides leads to many interesting properties including valley polarization, charge density waves, a topological property, a superconducting property, and a thermoelectrical property.Moreover, we discuss strategies to leverage the unique intrinsic properties of vdW electrides for practical applications. We show that these properties make vdW electrides potential candidates for advanced applications such as spin-orbit torque memory devices, valleytronic devices, K-ion batteries, and thermoelectricity. Finally, we discuss the current challenges and future perspectives for research using these emerging materials.

Predictable Gate-Field Control of Spin in Altermagnets with Spin-Layer Coupling

Spintronics, a technology harnessing electron spin for information transmission, offers a promising avenue to surpass the limitations of conventional electronic devices. While the spin directly interacts with the magnetic field, its control through the electric field is generally more practical, and has become a focal point in the field. Here, we propose a mechanism to realize static and almost uniform effective magnetic field by gate-electric field. Our method employs two-dimensional altermagnets with valley-mediated spin-layer coupling (SLC), in which electronic states display valley-contrasted spin and layer polarization. For the low-energy valley electrons, a uniform gate field is approximately identical to a uniform magnetic field, leading to predictable control of spin. Through symmetry analysis and ab initio calculations, we predict altermagnetic monolayer Ca(CoN)_{2} and its family materials as potential candidates hosting SLC. We show that an almost uniform magnetic field (B_{z}) indeed is generated by gate field (E_{z}) in Ca(CoN)_{2} with B_{z}∝E_{z} in a wide range, and B_{z} reaches as high as about 10^{3}  T when E_{z}=0.2  eV/Å. Furthermore, owing to the clean band structure and SLC, one can achieve perfect and switchable spin and valley currents and significant tunneling magnetoresistance in Ca(CoN)_{2} solely using the gate field. Our work provides new opportunities to generate predictable control of spin and design spintronic devices that can be controlled by purely electric means.

Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al2Te3 and Al2Te5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al2Te5, a square planar chain (SPC) known as polytelluride [Te3]2- is neutralized by the covalently bonded [Al2Te2]2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al2Te5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al2Te5 monolayer. The calculated κL values of 1.8 and 0.5 W m-1 K-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al2Te3, and even lower than monolayers that contain heavy cations like Bi2Te3. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al2Te5, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al2Te5. The pioneering zT of Al2Te5 compared to that of Al2Te3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.

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.

Magnetic phase transition regulated by an interface coupling effect in CrBr3/electride Ca2N van der Waals heterostructures

Compared with ferromagnetic (FM) materials, antiferromagnetic (AFM) materials have the advantages of not generating stray fields, resisting magnetic field disturbances, and displaying ultrafast dynamics and are thus considered as ideal candidate materials for next-generation high-speed and high-density magnetic storage. In this study, a new AFM device was constructed based on density functional theory calculations through the formation of a CrBr3/Ca2N van der Waals heterostructure. The FM ground state in CrBr3 undergoes an AFM transition when combining with the electride Ca2N. In such a system, since the metal Ca atoms form the exposed layer in the electride, the heterostructure interface has a high binding energy and a large amount of charge transfer. However, for individual electron doping, the FM ground state in the CrBr3 monolayer is robust. Therefore, the main factor in magnetic phase transition is the interface orbital coupling caused by the strong binding energy. Furthermore, the interface coupling effect was revealed to be a competition between direct exchange and superexchange interactions. Additionally, different pathways of orbital hybridization cause a transition of the magnetic anisotropy from out-of-plane to in-plane. This work not only provides a feasible strategy for changing the ground state of magnetic materials on electride substrates but also brings about more possibilities for the construction and advancement of new AFM devices.

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.

First principles prediction of novel quantum topological insulator state in two-dimensional XMg2Bi2(X=Eu/Yb)

Topological insulator (TIs), a novel quantum state of materials, has a lot of significance in the development of low-power electronic equipments as the conducting edge states display exceptional endurance against back-scattering. The absence of suitable materials with high fabrication feasibility and significant nontrivial bandgap, is now the biggest hurdle in their potential applications in devices. Here, we illustrate using first principles density functional calculations that the quintuplet layers of EuMg2Bi2and YbMg2Bi2crystals are potential two-dimensional TIs with a sizeable nontrivial gaps of 72 meV and 147 meV respectively. Dynamical stability of these quintuplet layers of EuMg2Bi2and YbMg2Bi2is confirmed by our phonon calculations. The weakly coupled layered structure of parent compounds makes it possible for simple exfoliation from a three-dimensional structure. We observed gapless edge states inside the bulk band gap in both the systems which indicate their TI nature. Further, we observed the anomalous and spin Hall conductivities to be quantized in two dimensional EuMg2Bi2and YbMg2Bi2respectively. Our findings predict two viable candidate materials as two dimensional quantum TIs which can be explored by future experimental investigations and possible applications of quantized spin and anomalous Hall conductance in spintronics.

Constructing Tunable Electrides on Monolayer Transition Metal Dichalcogenides

Electrides have emerged as promising materials with exotic properties due to the presence of localized electrons detached from all atoms. Despite the continuous discovery of many new electrides, most of them are based on atypical compositions, and their applications require an inert surface structure to passivate reactive excess electrons. Here, we demonstrate a different route to attain tunable electrides. We first report that monolayer transition metal dichalcogenides (TMDCs) exhibit weak electride characteristics, which is the remainder of the electride feature of the transition metal sublattice. By introducing chalcogen vacancies, the enhanced electride characteristics are comparable to those of known electrides. Since the precise tailoring of the chalcogen vacancy concentration has been achieved experimentally, we proposed that TMDCs can be used to build electrides with controllable intensities. Furthermore, we demonstrate that the electride states at the chalcogen vacancy of monolayer TMDCs will play an important role in catalyzing hydrogen evolution reactions.

Theoretical Study of the Ternary Compound Monolayer CuP2Se for Photocatalytic Water Splitting with Efficient Optical Absorption

Utilizing photocatalytic method to produce hydrogen by splitting water is an efficient strategy to solve the hotspot issues of energy crisis and environmental pollution. Herein, we systematically investigate the corresponding properties of the reported Cu-bearing ternary compound monolayer CuP2Se by using the first-principle calculations. The monolayer CuP2Se has quite small cleavage energy of 0.51 J/m2, indicating it can be easily produced by the mechanical exfoliation method experimentally. In addition, it is an indirect bandgap semiconductor material which has a moderate value of 1.91 eV. The conduction band minimum (CBM) and valence band maximum (VBM) can perfectly straddle the redox potentials of water when a biaxial strain of -4% to 4% is applied, unveiling the high photocatalytic thermodynamic stability of monolayer CuP2Se in response to the effect of solvent tension. Remarkably, the monolayer CuP2Se also demonstrates significant sunlight capturing ability in the visible region. The outstanding electronic and optical properties suggest that the monolayer CuP2Se is undoubtedly a viable material for photocatalytic water splitting.

Strain and Electric Field Engineering of G-ZnO/SnXY (X, Y = S, Se) S-Scheme Heterostructures for Photocatalyst and Electronic Device Applications: A Hybrid DFT Calculation

Using hybrid density functional theory calculations, we systematically study the biaxial strain and electric field modulated electronic properties of g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe S-scheme van der Waals heterostructures (vdWHs). g-ZnO/SnS2 and g-ZnO/SnSSe are found to be promising photocatalysts for water splitting with high solar-to-hydrogen efficiencies, even under acidic, alkaline, and high-stress conditions. The strain effect on the bandgaps of g-ZnO/SnXY is explained in detail according to the correlation between geometry structure and orbital hybridization of SnXY, which could help understand the strain-induced band structure evolutions in other SnXY (X, Y = S, Se)-based vdWHs. It is surprising that under an external electric field, g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe can offer the occupied nearly free-electron (NFE) states. In many materials, NFE states are usually unoccupied and is not conducive to the charge transport. The NFE state in g-ZnO/SnSe2 is the most sensitive to the electric field and might be promising electron transport channel in nanoelectronic devices. g-ZnO/SnSe2 might also have application potential in gas sensors and high-temperature superconductors.

Alkaline Earth Metal Superatom of W@Si16: Characterization of Group 6 Metal Encapsulating Si16 Cage on Organic Substrates

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si16) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si16 superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si16, comprising a central atom from group 6 (MVI = Cr, Mo, and W) were deposited on C60 substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si16, the extent of charge transfer to the C60 substrate is approximately doubled, while the oxidative reactivity is subdued for MVI@Si16 on C60, especially for W@Si16. The results show that a divalent state of MVI@Si162+ appears on the C60 substrate, which is consistently calculated to be a symmetrical cage structure of W@Si162+ in C3v, revealing insights into the "periodic law" of M@Si16 superatoms pertaining to the characteristics of alkaline earth metals.

Precise Engineering of the Electrocatalytic Activity of FeN4-Embedded Graphene on Oxygen Electrode Reactions by Attaching Electrides

Using first-principles calculations combined with a constant-potential implicit solvent model, we comprehensively studied the activity of oxygen electrode reactions catalyzed by electride-supported FeN4-embedded graphene (FeN4Cx). The physical quantities in FeN4Cx/electrides, i.e., work function of electrides, interlayer spacing, stability of heterostructures, charge transferred to Fe, d-band center of Fe, and adsorption free energy of O, are highly intercorrelated, resulting in activity being fully expressed by the nature of the electrides themselves, thereby achieving a precise modulation in activity by selecting different electrides. Strikingly, the FeN4PDCx/Ca2N and FeN4PDCx/Y2C systems maintain a high oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activity with the overpotential less than 0.46 and 0.62 V in a wide pH range. This work provides an effective strategy for the rational design of efficient bifunctional catalysts as well as a model system with a simple activity-descriptor, helping to realize significant advances in energy devices.

High-throughput computational materials screening of transition metal peroxides

Semiconductor materials of abnormal stoichiometric ratio often exhibit unique properties, yet it is still a challenge to determine the structures of such materials in an efficient way. Herein, we propose a method for structurally biased screening according to the coordination numbers and the numbers of Wyckoff positions, balancing the atom local environment and the global symmetry of structures. Based on first-principles calculations, we have predicted two metastable peroxides P21/c-ScO2 and Pmmn-TiO3 with more than six coordination points. For these two structures, the most stable intrinsic defect is the oxygen vacancy (VO) at the peroxide anion (O2-2), which induces the absence of antibonding orbital formed by O2-2 near the valence band maximum. With the introduction of VO, the decrease of coordination numbers leads to charge recombination, and results in the appearance of an ordered phase TiO2.5 with stronger Ti-O orbital hybridization. The proposed method presents a promising and feasible approach for the screening of novel compounds.

Low thermal expansion of layered electrides predicted by density-functional theory

Layered electrides are a unique class of materials with anionic electrons bound in interstitial regions between thin, positively charged atomic layers. While density-functional theory is the tool of choice for computational study of electrides, there has to date been no systematic comparison of density functionals or dispersion corrections for their accurate simulation. There has also been no research into the thermomechanical properties of layered electrides, with computational predictions considering only static lattices. In this work, we investigate the thermomechanical properties of five layered electrides using density-functional theory to evaluate the magnitude of thermal effects on their lattice constants and cell volumes. We also assess the accuracy of five popular dispersion corrections with both planewave and numerical atomic orbital calculations.

Metallic CrP2 monolayer: potential applications in energy storage and conversion

Phosphorus-rich compounds have emerged as a promising class of energy storage and conversion materials due to their interesting structures and electrochemical properties. Herein, we propose that a metallic CrP2 monolayer, isomorphic to 1H-phase MoS2, is a good prospect as an anode for K-ion batteries and a catalyst for hydrogen evolution through first-principles calculations. The CrP2 monolayer demonstrates not only a desirable high K storage capacity (940 mA h g-1) but also a low K-ion diffusion barrier (0.10 eV) and average open circuit voltage (0.40 V). On the other hand, its Gibbs free energy (0.02 eV)/active site density is superior/comparable to that of commercial Pt, resulting from the contribution of the lone pair electrons of the P atom. Its high structural stability and intrinsic metallicity can ensure high safety and performance during the cyclic process. These interesting properties make the CrP2 monolayer a promising multifunctional material for energy storage and conversion devices.

Photocatalytic water splitting for hydrogen production with high efficiency monolayer In2Te5: a theoretical study

Employing density functional theory (DFT) calculations, we explore the excellent performance of two-dimensional (2D) semiconductor In2Te5 in photocatalytic water splitting at the theoretical level. The calculated results illustrate that 2D In2Te5 is a direct band gap semiconductor with a moderate band gap value and an ultrahigh optical absorption coefficient in the visible light region. It was found that its conduction band edge is higher than the reduction potential of water (-4.44 eV), which proves that it can split water to produce hydrogen. Furthermore, its excellent hydrogen evolution activity can be tuned under an appropriate biaxial strain. In addition, 2D In2Te5 shows a remarkable photo-generated current, suggesting that electrons and holes can be separated efficiently. Our results offer a superior candidate material for realizing photocatalytic water splitting for hydrogen evolution.

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.

Ohmic contacts of the two-dimensional Ca2N/MoS2 donor-acceptor heterostructure

In the current stage, conventional silicon-based devices are suffering from the scaling limits and the Fermi level pinning effect. Therefore, looking for low-resistance metal contacts for semiconductors has become one of the most important topics, and two-dimensional (2D) metal/semiconductor contacts turn out to be highly interesting. Alternatively, the Schottky barrier and the tunneling barrier impede their practical applications. In this work, we propose a new strategy for reducing the contact potential barrier by constructing a donor-acceptor heterostructure, that is, Ca₂N/MoS₂ with Ca₂N being a 2D electrene material with a significantly small work function and a rather high carrier concentration. The quasi-bond interaction of the heterostructure avoids the formation of a Fermi level pinning effect and gives rise to high tunneling probability. An excellent n-type Ohmic contact form between Ca₂N and MoS₂ monolayers, with a 100% tunneling probability and a perfect linear I-V curve, and large lateral band bending also demonstrates the good performance of the contact. We verify a fascinating phenomenon that Ca₂N can trigger the phase transition of MoS₂ from 2H to 1T'. In addition, we also identify that Ohmic contacts can be formed between Ca₂N and other 2D transition metal dichalcogenides (TMDCs), including WS₂, MoSe₂, WSe₂, and MoTe₂.

A Novel Two-Dimensional TiClO as a High-Performance Anode Material for Mg-Ion Batteries: A First-Principles Study

Searching for efficient electrode materials with excellent electrochemical performance is of great significance to the development of magnesium-ion batteries (MIBs). Two-dimensional Ti-based materials are appealing for use in MIBs due to their high cycling capability. On the basis of density functional theory (DFT) calculations, we comprehensively investigate a novel two-dimensional Ti-based material, namely, TiClO monolayer, as a promising anode for MIBs. Monolayer TiClO can be exfoliated from its experimentally known bulk crystal with a moderate cleavage energy of 1.13 J/m². It exhibits intrinsically metallic properties with good energetical, dynamical, mechanical, and thermal stabilities. Remarkably, TiClO monolayer possesses an ultra-high storage capacity (1079 mA h g^-1), a low energy barrier (0.41-0.68 eV), and a suitable average open-circuit voltage (0.96 V). The lattice expansion for the TiClO monolayer is slight (<4.3%) during the Mg-ion intercalation. Moreover, bilayer and trilayer TiClO can considerably enhance the Mg binding strength and maintain the quasi-one-dimensional diffusion feature compared with monolayer TiClO. All these properties indicate that TiClO monolayers can be utilized as high-performance anodes for MIBs.

Magnetic Electrides: High-Throughput Material Screening, Intriguing Properties, and Applications

Electrides are a unique class of electron-rich materials where excess electrons are localized in interstitial lattice sites as anions, leading to a range of unique properties and applications. While hundreds of electrides have been discovered in recent years, magnetic electrides have received limited attention, with few investigations into their fundamental physics and practical applications. In this work, 51 magnetic electrides (12 antiferromagnetic, 13 ferromagnetic, and 26 interstitial-magnetic) were identified using high-throughput computational screening methods and the latest Materials Project database. Based on their compositions, these magnetic electrides can be classified as magnetic semiconductors, metals, or half-metals, each with unique topological states and excellent catalytic performance for N₂ fixation due to their low work functions and excess electrons. The novel properties of magnetic electrides suggest potential applications in spintronics, topological electronics, electron emission, and as high-performance catalysts. This work marks the beginning of a new era in the identification, investigation, and practical applications of magnetic electrides.

Interlayer Charge Transfer Regulates Single-Atom Catalytic Activity on Electride/Graphene 2D Heterojunctions

Single-atom catalysts with structure and activity tunability have attracted significant attention for energy and environmental applications. Herein we present a first-principles study of single-atom catalysis on two-dimensional graphene and electride heterostructures. The anion electron gas in the electride layer enables a colossal electron transfer to the graphene layer, with the degree of transfer being controllable by the selection of electride. The charge transfer tunes the d-orbital electron occupancy of a single metal atom, enhancing the catalytic activity of hydrogen evolution reactions and oxygen reduction reactions. The strong correlation between the adsorption energy E_ads and the charge variation Δq suggests that interfacial charge transfer is a critical catalytic descriptor for the heterostructure-based catalysts. The polynomial regression model proves the importance of charge transfer and accurately predicts the adsorption energy of ions and molecules. This study provides a strategy to obtain high-efficiency single-atom catalysts using two-dimensional heterostructures.

In-gap states and strain-tuned band convergence in layered structure trivalent iridate K0.75Na0.25IrO2

Iridium oxides (iridates) provide a good platform to study the delicate interplay between spin-orbit coupling (SOC) interactions, electron correlation effects, Hund's coupling and lattice degrees of freedom. An overwhelming number of investigations primarily focus on tetravalent (Ir⁴⁺, 5d⁵) and pentavalent (Ir⁵⁺, 5d⁴) iridates, and far less attention has been paid to iridates with other valence states. Here, we pay our attention to a less-explored trivalent (Ir³⁺, 5d⁶) iridate, K_0.75Na_0.25IrO₂, crystallizing in a triangular lattice with edge-sharing IrO₆ octahedra and alkali metal ion intercalated [IrO₂]^- layers, offering a good platform to explore the interplay between different degrees of freedom. We theoretically determine the preferred occupied positions of the alkali metal ions from energetic viewpoints and reproduce the experimentally observed semiconducting behavior and nonmagnetic (NM) properties of K_0.75Na_0.25IrO₂. The SOC interactions play a critical role in the band dispersion, resulting in NM J_eff = 0 states. More intriguingly, our electronic structure not only uncovers the presence of intrinsic in-gap states and nearly free electron character for the conduction band minimum, but also explains the abnormally low activation energy in K_0.75Na_0.25IrO₂. Particularly, the band edge can be effectively modulated by mechanical strain, and the in-gap states feature enhanced band-convergence characteristics by 6% compressive strain, which will greatly enhance the electrical conductivity of K_0.75Na_0.25IrO₂. The present work sheds new light on the unconventional electronic structures of trivalent iridates, indicating their promising application as a nanoelectronic and thermoelectric material, which will attract extensive interest and stimulate experimental works to further understand the unprecedented electronic structures and exploit potential applications of the triangular trivalent iridate.

First-principles calculations of monolayered Al2Te5: a promising 2D donor semiconductor with ultrahigh visible light harvesting

Atomically thin two-dimensional (2D) crystals have piqued the curiosity of researchers due to their unique features and potential applications, such as catalysis and ion batteries. One essential and desirable aspect of 2D materials is that they have a large photoreactive contact surface for optical absorption. Here, a 2D crystal is proposed that possesses a moderate adjustable indirect band gap of 1.95 eV (HSE06) and exhibits ultrahigh visible light harvesting with a absorption coefficient of up to 10⁸ cm^-1 in the ∼380 to 800 nm range of the visible light spectrum. Besides that, the indirect band gap can be converted to a direct one under biaxial strain. By means of density functional theory, the 2D Al₂Te₅ monolayer displays great stability and promise of experimental fabrication. These advantages will provide considerable application potential for future photovoltaics (PV) devices.

Monolayer Kagome metals AV3Sb5

Recently, layered kagome metals AV₃Sb₅ (A = K, Rb, and Cs) have emerged as a fertile platform for exploring frustrated geometry, correlations, and topology. Here, using first-principles and mean-field calculations, we demonstrate that AV₃Sb₅ can crystallize in a mono-layered form, revealing a range of properties that render the system unique. Most importantly, the two-dimensional monolayer preserves intrinsically different symmetries from the three-dimensional layered bulk, enforced by stoichiometry. Consequently, the van Hove singularities, logarithmic divergences of the electronic density of states, are enriched, leading to a variety of competing instabilities such as doublets of charge density waves and s- and d-wave superconductivity. We show that the competition between orders can be fine-tuned in the monolayer via electron-filling of the van Hove singularities. Thus, our results suggest the monolayer kagome metal AV₃Sb₅ as a promising platform for designer quantum phases.

Conductive C3NS Monolayer with Superior Properties for K Ion Batteries

K-ion batteries (KIBs) have been considered as appealing alternatives to Li ion batteries due to the high abundance of K, their high working voltages, and allowing the use of mature LIB technology. Thus far, anode materials that can meet the rigorous requirements of KIBs are still rather rare. Here, we have identified a desirable anode material, a metallic C₃NS monolayer with high stability, a high storage capacity of 980 mAh/g, a low diffusion barrier of 0.24 eV, and a low open-circuit voltage of 0.36 V, through first-principles calculations. Metallic C₃NSK_n (n = 1-3) can ensure a high electron conductivity during the charge/discharge process. Valence electrons of the N atom in a triangular bipyramid configuration favor the formation of a planar edge-sharing hexagonal C₄N₂ unit and delocalized π bonding with C 2p electrons. The lone pair electrons of the S atom induce strong interactions with K atoms, facilitating storage capacity. These interesting properties make the C₃NS monolayer a promising anode for KIBs.

Designing a ferrimagnetic-ferroelastic multiferroic semiconductor in FeMoClO4 nanosheets via element substitution

Exploring two-dimensional multiferroic semiconductors, combined with ferro-/ferrimagnetism and ferroelasticity as well as large spin polarization around the valence band maximum (VBM) and conduction band minimum (CBM), is highly desirable but remains a challenging task. Here, via first-principles calculations, we predict such a material based on the square phase FeMoClO₄ nanosheet, which is experimentally accessible by exfoliating its layered bulk. Pristine FeMoClO₄ nanosheets are a weak antiferromagnet with zero spin polarization. After substituting nonmagnetic Mo with magnetic Mn, the resulting FeMnClO₄ nanosheet becomes ferrimagnetic with magnetic ordering temperature significantly enhanced from 14 to 127 K. Besides, the FeMnClO₄ nanosheet is a half semiconductor with its VBM and CBM 100% spin-polarized in the same spin direction. Interestingly, the initial square lattice is distorted into a rectangular one, inducing an in-plane ferroelasticity in the FeMnClO₄ nanosheet with a switching barrier of 27 meV per atom. Moreover, under ferroelastic transition, the orientation of the magnetic easy axis can be reversibly rotated by 90°, indicating a strong magnetoelastic coupling.

Intrinsic Ferromagnetism in 2D Fe2H with a High Curie Temperature

The rational design of ferromagnetic materials is crucial for the development of spintronic devices. Using first-principles structural search calculations, we have identified 73 two-dimensional transition metal hydrides. Some of them show interesting magnetic properties, even when combined with the characteristics of the electrides. In particular, the P3̅m1 Fe₂H monolayer is stabilized in a 1T-MoS₂-type structure with a local magnetic moment of 3 μ_B per Fe atom, whose robust ferromagnetism is attributed to the exchange interaction between neighboring Fe atoms within and between sublayers, leading to a remarkably high Curie temperature of 340 K. On the other hand, it has a large magnetic anisotropic energy and spin-polarization ratio. Interestingly, the above room-temperature ferromagnetism of the Fe₂H monolayer is well preserved within a biaxial strain of 5%. The structure and electron property of surface-functionalized Fe₂H are also explored. All of these interesting properties make the Fe₂H monolayer an attractive candidate for spintronic nanodevices.

The nearly free electron states and the conductivity limited by electron-phonon scattering of an OH-terminated MXene material, a case study of the Hf2C(OH)2 monolayer

Reducing the electron-phonon scattering is always desirable for realizing high conductivity of actual materials at room temperature. It is seemingly feasible in some OH-terminated MXenes such as the Hf₂C(OH)₂ monolayer, which hosts the so-called nearly free electron states (NFESs) near the Fermi energy. The NFESs are characterized by a large separation between the major electronic probability distribution and the atomic layer of MXenes. This implies that the NFESs suffer from a very weak electron-phonon scattering, hence the high conductivity at room temperature of these materials. We perform first principles calculations on the conductivity limited by the electron-phonon (e-ph) scattering of the Hf₂C(OH)₂ monolayer. Our results indicate that the conductivity of the Hf₂C(OH)₂ monolayer at room temperature is indeed higher than those of most of the MXene materials. However, such a high conductivity cannot be attributed to the existence of the NFESs because of their relatively low electronic band velocity. This conclusion is applicable to other OH-terminated MXene materials such as Zr₂C(OH)₂ since their band structures around the Fermi energy are highly analogous. Our study suggests that both large band velocity and weak e-ph coupling are important for realizing ultrahigh conductivity facilitated by the NFESs in materials.

Excellent Thermoelectric Performance of 2D CuMN2 (M = Sb, Bi; N = S, Se) at Room Temperature

2D copper-based semiconductors generally possess low lattice thermal conductivity due to their strong anharmonic scattering and quantum confinement effect, making them promising candidate materials in the field of high-performance thermoelectric devices. In this work, we proposed four 2D copper-based materials, namely CuSbS₂, CuSbSe₂, CuBiS₂, and CuBiSe₂. Based on the framework of density functional theory and Boltzmann transport equation, we revealed that the monolayers possess high stability and narrow band gaps of 0.57~1.10 eV. Moreover, the high carrier mobilities (10²~10³ cm²·V^-1·s^-1) of these monolayers lead to high conductivities (10⁶~10⁷ Ω^-1·m^-1) and high-power factors (18.04~47.34 mW/mK²). Besides, as the strong phonon-phonon anharmonic scattering, the monolayers also show ultra-low lattice thermal conductivities of 0.23~3.30 W/mK at 300 K. As results show, all the monolayers for both p-type and n-type simultaneously show high thermoelectric figure of merit (ZT) of about 0.91~1.53 at room temperature.

Photocatalytic properties of anisotropic β-PtX2 (X = S, Se) and Janus β-PtSSe monolayers

The highly efficient photocatalytic water splitting process to produce clean energy requires novel semiconductor materials to achieve a high solar-to-hydrogen energy conversion efficiency. Herein, the photocatalytic properties of anisotropic β-PtX₂ (X = S, Se) and Janus β-PtSSe monolayers were investigated based on the density functional theory. The small cleavage energy for β-PtS₂ (0.44 J m^-2) and β-PtSe₂ (0.40 J m^-2) endorses the possibility of mechanical exfoliation from their respective layered bulk materials. The calculated results revealed that the β-PtX₂ monolayers have an appropriate bandgap (∼1.8-2.6 eV) enclosing the water redox potential, light absorption coefficient (∼10⁴ cm^-1), and exciton binding energy (∼0.5-0.7 eV), which facilitates excellent visible-light-driven photocatalytic performance. Remarkably, the inherent structural anisotropy leads to an anisotropic high carrier mobility (up to ∼5 × 10³ cm² V^-1 S^-1), leading to a fast transport of photogenerated carriers. Notably, the required small external potential to realize hydrogen evolution reaction and oxygen evolution reaction processes with an excellent solar-to-hydrogen energy conversion efficiency for β-PtSe₂ (∼16%) and β-PtSSe (∼18%) makes them promising candidates for solar water splitting applications.

Two-dimensional IV-VA3 monolayers with enhanced charge mobility for high-performance solar cells

High-performance photovoltaics (PVs) constitute a subject of extensive research efforts, in which silicon (Si)-based solar cells (SCs) have been widely commercialized. However, the low carrier mobility of Si-based SCs can limit the effective charge separation, thereby negatively impacting the device performance. Here, via calculating the physicochemical and PV performance based on density functional theory, we demonstrate SCs based on two-dimensional (2D) group IV and V compounds with an AX₃ configuration. Firstly, the cleavage energies of AX₃ (A = Si, Ge; X = P, As, and Sb) are calculated to be less than 1 J m^-2, providing an experimental feasibility to be exfoliated from the corresponding bulk. Secondly, electronic and optical properties have been systematically investigated. To be specific, the band gap of monolayer AX₃ falls in the range of 1.11-1.27 eV, which is comparable with that of Si. Significantly, the electron mobility of monolayer AX₃ can reach as high as ∼30 000 cm² V^-1 s^-1, which is one order of magnitude higher than that of Si. Furthermore, the optical absorbance of monolayer SiAs₃, SiP₃ and GeAs₃ exhibits high coefficients in visible light. Therefore, we believe that our designed AX₃-based PV systems with power conversion efficiency of 20% can offer great potential in the application of high-performance two-dimension-based PVs.

Enhanced shift current bulk photovoltaic effect in ferroelectric Rashba semiconductorα-GeTe:ab initiostudy from three- to two-dimensional van der Waals layered structures

The ferroelectric Rashba semiconductors (FERSCs) are endowed with a unique combination of ferroelectricity and the spin degree of freedom, resulting in a long carrier lifetime and impressive bulk photovoltaic (BPV) efficiency that reached 25% in organometal halide perovskites. The BPV efficiency can be further improved by using low-dimensional ferroelectrics however, it is inhibited by the ferroelectric instability in low-dimensional perovskites and toxicity along with phase instability of the lead-halide perovskites. To address these challenges, theα-GeTe could be of great importance which is the simplest known lead-free FERSC with an intrinsic layered structure. Therefore, in this work, we investigate the BPV properties of three- to two-dimensional van der Waals structures ofα-GeTe by calculating the shift current (SHC). We predict that the mono (1.56 Å) and bi-layers (5.44-6.14 Å)α-GeTe with the buckled honeycomb structure are dynamically stable and possess the characteristic features of the bulk up to the nanoscale limit. The SHC of ∼70μA V^-2is calculated in bulk α-GeTe which is 20 times larger than that obtained in organometal halides in the visible light. The SHC increases with decreasing the number of layers, reaching a maximum amplitude of ∼300μA V^-2at 2.67 eV in the monolayer which is more than double that obtained in monolayer GeS. We find that the SHC in monolayer α-GeTe can be further enhanced and redshifted by applying a compressive strain; which is correlated with the strong absorption of thexx-polarized light, stimulated by the more delocalized p_x/yorbital character of the density of states. Furthermore, in the bilayer structures, the magnitude of the SHC is sensitive to the layers' stacking arrangement and a maximum SHC (∼250μA V^-2) can be achieved with an AB-type stacking arrangement. Combining these results with the benefits of being environmental-friendly material makesα-GeTe a good candidate for next-generation solar cells application.

Monolayer GaOCl: a novel wide-bandgap 2D material with hole-doping-induced ferromagnetism and multidirectional piezoelectricity

Two-dimensional (2D) materials with excellent properties are emerging as promising candidates in electronics and spintronics. In this work, a novel GaOCl monolayer is proposed and studied systematically based on first-principles calculations. With excellent thermal and dynamic stability at room temperature, its wide direct bandgap (4.46 eV) can be further modulated under applied strains. The 2D semiconductor exhibits high mechanical flexibility, and anisotropy in Poisson's ratio and carrier mobilities, endowing it with a broad spectrum of electronic and optoelectronic applications. More importantly, the GaOCl monolayer has spontaneous magnetization induced by hole doping and shows outstanding multidirectional piezoelectricity, which are comparable with those of either magnetic or piezoelectric 2D materials. Our calculations indicate that the GaOCl monolayer with wide bandgaps and tunable piezoelectricity and ferromagnetism could be promising for applications in multifunctional integrated nano-devices with high performance.

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.

Strain-induced multigap superconductivity in electrene Mo2N: a first principles study

Superconductivity in low dimensional materials and 2D electrides are topics of great interest with possible applications in next generation electronic devices. Using density functional theory (DFT) associated with Migdal-Eliashberg approach and maximally localized Wannier functions this study shows how biaxial strain affects superconductivity in a monolayer of Mo₂N. Results indicate that 2D Mo₂N presents strong electron-phonon coupling with large anisotropy in the superconducting energy gap. It is also proposed that, at low temperatures, a single layer of Mo₂N becomes an electride with localized electron gas pockets on the surface, resembling anions adsorbed on an atomic sheet. Calculations point to T_c = 24.7 K, a record high transition temperature for this class of material at ambient pressure. Furthermore, it is shown that when biaxial strain is applied to a superconducting Mo₂N monolayer, a new superconductivity gap starts at 2% strain and is enhanced by continuum strain, opening additional coupling channels.

Anisotropic electrene T'-Ca2P with electron gas magnetic coupling as anode material for Na/K ion batteries

There is an urgent need for high-performance rechargeable electrical storage devices as a supplement or a substitution for lithium ion batteries (LIBs) due to the shortage of lithium in nature. Herein we propose a stable 2D electrene T'-Ca₂P as an anode material for Na/K ion batteries developed using first principles calculations. Our calculated results show that the T'-Ca₂P monolayer is an antiferromagnetic semiconducting electrene with a spin-polarized electron gas. It exhibits suitable adsorption for both Na and K atoms, and its anisotropic migration energy barriers are 0.050/0.101 eV and 0.037/0.091 eV in the b/a direction, respectively. The theoretical capacities for Na and K are both 482 MA h g^-1, whereas the average working voltage platforms are 0.171-0.226 V and 0.013-0.267 V, respectively. All the results reveal that the T'-Ca₂P monolayer has promising prospects for application as an anode material for Na/K ion batteries.

Stability, electronic and magnetic properties of the penta-CoAsSe monolayer: a first-principles and Monte Carlo study

Using density functional theory (DFT) and ab initio molecular dynamics (AIMD) we predict the existence of a new 2D monolayer, namely Penta-CoAsSe with robust mechanical, thermal and dynamical stabilities. The electronic and magnetic properties of this monolayer are investigated within the generalized gradient approximation including the Hubbard interaction (U) on the localized d orbitals of Co. We show that this material is an antiferromagnetic (AFM) narrow-gap semiconductor and exhibits in-plane magnetic anisotropy energy with a sizable magnetocrystalline anisotropy (MCA) of -1.08 erg cm^-2. Furthermore, this system is found to have a substantial intrinsic piezoelectric response with an out-of-plane coefficient d₃₆ of 0.34 pm V^-1, surpassing other previously reported Penta-2D piezoelectric materials. By combining our DFT calculations with the Monte Carlo simulations, we find that CoAsSe has a transition temperature four times higher than that of Penta-CoS₂. The effects of biaxial strain and electron-electron correlation on the magnetic properties and electronic structure are also examined. These fascinating properties make the Penta-CoAsSe monolayer a promising candidate for a wide range of technological applications.

A new MoCN monolayer containing stable cyano structural units as a high-efficiency catalyst for the hydrogen evolution reaction

In the hydrogen evolution reaction (HER), it is essential to find a high-efficiency and nonprecious electrocatalyst comparable to Pt, which needs to have rich inherently active sites and good conductivity. By combining a global minimum structure search and first-principles calculations, a hitherto unknown 2D MoCN monolayer was found, which can be considered as a structure in which Mo atoms interact with the stable CN units through triple bonds. The resultant MoCN monolayer possesses superior thermodynamic, dynamic, thermal, and mechanical stabilities, as well as inherent metallicity. In particular, it can exhibit outstanding HER catalytic activity due to the presence of many active sites with near-zero ΔG_H* values, whose density totals 1.80 × 10¹⁵ sites per cm², even more than Pt. In addition, we also propose a series of other 2D monolayers containing stable CN units (i.e., MoC₂N, MoCN₂ and MoC₂N₂), all of which can uniformly show high stability and good HER catalytic activity. Applying strain can further effectively improve the activities of C-rich (MoC₂N) and N-rich (MoCN₂) monolayers, inducing considerably high HER catalytic performance. For the MoCN, MoC₂N and MoCN₂ monolayers, the most active sites are located at the Mo-C-N chain involved. All these fascinating findings can not only provide new excellent candidates but also new insights into the design of highly efficient and nonprecious HER electrocatalysts as an alternative to Pt in the near future.

Realizing Efficient Catalytic Performance and High Selectivity for Oxygen Reduction Reaction on a 2D Ni2SbTe2 Monolayer

One of the immediate challenges for the large-scale commercialization of hydrogen-based fuel cells is to develop cost-effective electrocatalysts to enable cathodic oxygen reduction reaction (ORR). Herein, we focus on the potential of the two-dimensional (2D) ternary chalcogenide Ni₂SbTe₂ monolayer as a high-performance electrocatalyst for the ORR using density function theory. Our computed results reveal that there are an obvious hybridization and electron transfer between the O 2p and Te 5p orbitals, which can activate the adsorbed oxygen and trigger the whole ORR process, with an overpotential as low as 0.33 V. In addition, the adsorption capacity of the monolayer surface for oxygen molecules can be effectively enhanced by doping with Fe or Co atoms. The Ni₂SbTe₂ monolayers doped with Fe or Co atoms not only maintain their original excellent ORR catalytic activity but also improve selectivity toward the four-electron (4e) reduction pathway. We highly anticipate that this work can provide excellent candidates and new ideas for designing low-cost and high-performance ORR catalysts to replace noble metal Pt-based catalysts in fuel cells.

SiCP4 Monolayer with a Direct Band Gap and High Carrier Mobility for Photocatalytic Water Splitting

Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP₂ and SiCP₄ monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP₄ monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼10⁴ cm^-1) and high carrier mobility (∼10⁵ cm² V^-1 s^-1) of SiCP₂ and SiCP₄ facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP₂ and SiCP₄ monolayers promising candidates for applications in photocatalytic water splitting.

Two-dimensional β-PdSeO3 monolayer as a high-efficiency photocatalyst for solar-to-hydrogen conversion

The β-PdSeO3 monolayer is semiconducting with a considerable band gap and shows appropriate band edge positions for photocatalytic water splitting.

Novel two-dimensional beta-XTe (X = Ge, Sn, Pb) as promising room-temperature thermoelectrics

In this paper, we designed novel low-symmetry two-dimensional (2D) structures based on conventional XTe (X = Ge, Sn, Pb) thermoelectrics with large average atomic mass. The first-principles calculations combined with Boltzmann transport theory show that the beta-XTe exhibit good stability, high electron carrier mobility, and ultralow Κ_L. The subsequent analyses show that the ultralow Κ_L stems from the coexistence of resonant bonding, weak bonding, and lone-pair electrons in beta-XTe, which leads to large anharmonicities. On the other hand, the lowest energy conduction band of beta-GeTe and beta-SnTe show the convergence of the low-lying Ʃ band, which is the source of the high-power factor in the two systems. The calculated maximum ZT of beta-XTe (X = Ge, Sn, Pb) are 3.08, 1.60, and 0.57 at 300 K, respectively, which is significantly greater than that of the previously reported high-symmetry 2D alpha-XTe and the commercial thermoelectrics. We hope that this work can provide important guidance for the development of thermoelectric materials.

Novel Braceletlike BiSbX3 (X = S, Se) Monolayers with an In-Plane Negative Poisson's Ratio and Anisotropic Photoelectric Properties

In this work, we predict two novel two-dimensional (2D) auxetic materials, BiSbX₃ (X = S, Se) monolayers, through first-principles calculations. Attributed to their special braceletlike structure, the in-plane negative Poisson's ratio (NPR) of BiSbS₃ and BiSbSe₃ monolayers are as high as -0.25 and -0.26, respectively. The phonon dispersion calculations, ab initio molecular dynamics simulations, and elastic constants calculations demonstrate that these two monolayers possess excellent dynamic, thermal, and mechanical stabilities. The band gap values of BiSbS₃ and BiSbSe₃ calculated at the HSE level by considering the spin-orbit coupling (SOC) effect are 1.68 and 1.20 eV. The anisotropic carrier mobility and superior optical absorption indicate that they may shine in the next generation of electronic and optoelectronic devices. All of these discoveries not only enrich the types of auxetic materials but also provide a structural reference for designing new auxetic materials on the molecular level. Furthermore, they can provide theoretical guidance for future applications of BiSbX₃ (X = S, Se) monolayers in various fields.

Simultaneous and Accurate Quantification of Multiple Antibiotics in Aquatic Samples by Surface-Enhanced Raman Scattering Using a Ti3C2Tx/DNA/Ag Membrane Substrate

Rapid and accurate analysis of multiple targets in complex samples is still a big challenge in the fast detection field. Herein, we developed a rapid and accurate strategy for simultaneous quantification of trace multiple antibiotic residues in complex aquatic samples by surface-enhanced Raman scattering (SERS) using a Ti₃C₂T_x/DNA/Ag membrane substrate. This membrane substrate was proven to have good uniformity, reproducibility, stability, and SERS activity by a series of characterizations. Also, this substrate combined excellent electromagnetic enhancement and chemical enhancement effects, which endowed it with good sensitivity and selectivity during SERS analysis. It achieved the integration of multitarget separation, enrichment, and in situ detection, which significantly improved the selectivity, sensitivity, accuracy, and detection throughput by membrane substrate coupling with SERS for real-sample analysis. Finally, this rapid SERS analysis strategy was successfully applied to the simultaneous quantification of trace nitrofurantoin (NFT) and ofloxacin (OFX) in aquatic samples. It was observed that trace NFT and OFX were actually detected and simultaneously quantified to be 8.0-13.7 and 42.6-49.1 μg/kg in aquatic samples, respectively, with good recoveries of 88.0-107% and relative standard deviations of 0.3-5.5%. The results were verified by a traditional high-performance liquid chromatography method with relative errors of -9.8 to 5.3%. This strategy provided a methodological reference for accurate SERS quantification of multiple targets in complex samples.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P₃S and P₂S monolayers. The P₃S monolayer, consisting of n-bicyclo-P₆ units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 10⁴ cm² V^-1 s^-1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 10⁵ cm^-1, respectively. Additionally, the P₃S monolayer has a high structural stability and resistance to air oxidation.

CrSbS3 monolayer: a potential phase transition ferromagnetic semiconductor

Two dimensional intrinsic ferromagnetic semiconductors with controllable magnetic phase transition are highly desirable for spintronics. Nevertheless, reports on their successful experimental realization are still rare. Herein, based on first principles calculations, we propose to achieve such a functional material, namely CrSbS₃ monolayer by exfoliating from its bulk crystal. Intrinsic CrSbS₃ monolayer is a ferromagnetic half semiconductor with a moderate bandgap of 1.90 eV. It features an intriguing magnetic phase transition from ferromagnetic to antiferromagnetic when applying a small compressive strain (∼2%), making it ideal for fabricating strain-controlled magnetic switches or memories. In addition, the predicted strong anisotropic absorption of visible light and small effective masses make the CrSbS₃ monolayer promising for optoelectronic applications.

Spin-Gapless States in Two-Dimensional Molecular Ferromagnet Fe2(TCNQ)2

Two-dimensional van der Waals magnetic atomic crystals have provided unprecedented access to magnetic ground states due to a quantum confinement effect. Here, using first-principles calculations, we demonstrate a spin-gapless molecular ferromagnet, namely, Fe₂(TCNQ)₂, with superior mechanical stability and a remarkable linear Dirac cone, which can be exfoliated from its already-synthesized van der Waals crystal. Especially, Young's modulus has values of 175.28 GPa·nm along the x- and y-directions with a Poisson's ratio of 0.29, while the Curie temperature within the Ising model is considerably higher than room temperature. Furthermore, spin-orbit coupling can open a band gap at the Dirac point, leading to topologically nontrivial electronic states characterized by an integer value of the Chern number and the edge states of its nanoribbon. Our results offer versatile platforms for achieving plastic spin filtering or a quantum anomalous Hall effect with promising applications in spintronics devices.

Self-Powered Broadband Photodetector and Sensor Based on Novel Few-Layered Pd3(PS4)2 Nanosheets

Optoelectronics and sensing devices are of enormous importance in our modern lives, which has propelled the scientific community to explore new two-dimensional (2D) nanomaterials to meet the requirements of future devices. Herein, we present the exfoliation of palladium thiophosphate (Pd₃(PS₄)₂) by mechanical shear force exfoliation. The Pd₃(PS₄)₂-based photoelectrochemical (PEC) device demonstrated self-powered broadband photodetection in the range of 385-940 nm with an unprecedented responsivity of 2 A W^-1 and a specific detectivity of about 8.67 × 10¹¹ cm Hz^1/2 W^-1 under the illumination of 420 nm LED light. The crucial parameters such as photoresponsivity, response, and recovery time of the device can be controlled by an externally applied voltage and the analyte concentration. Moreover, Pd₃(PS₄)₂-based vapor-sensing devices exhibited frequency-dependent selective acetone sensing in the presence of other organic vapors with an ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity of Pd₃(PS₄)₂ was revealed, which can be attributed to the presence of an appropriate band alignment with the catalytic activity of Pd. This novel material with the aforementioned fascinating phenomenon will pave the way toward practical future applications in optoelectronics and sensing.