Data-driven discovery of high performance layered van der Waals piezoelectric NbOI2

Enhanced Carrier Lifetime and Mobility in Monolayer NbOI2

The lifetime and mobility of hot carriers are critical parameters for assessing the performance of optoelectronic materials, as they directly impact the response speed and operational efficiency of devices. Combining first-principles calculations with nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigated the electronic properties and carrier dynamics of monolayer NbOI2. Our findings indicate that, at room temperature, this material demonstrates a carrier lifetime of up to ∼13 ns and an electron mobility reaching as high as ∼9 × 103 cm2 V-1 s-1. The low Young's modulus makes it susceptible to deformation under external stress, and we found that the carrier lifetime extends to ∼40 ns under a 4% tensile strain, along with a significant increase in hole mobility. This study elucidates the carrier dynamics in monolayer NbOI2, facilitating its potential application in future flexible optoelectronic devices.

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.

Above-Room-Temperature Ferroelectricity and Giant Second Harmonic Generation in 1D vdW NbOI3

The realization of spontaneous ferroelectricity down to the one-dimensional (1D) limit is both fundamentally intriguing and practically appealing for high-density ferroelectric and nonlinear photonics. However, the 1D vdW ferroelectric materials are not discovered experimentally yet. Here, the first 1D vdW ferroelectric compound NbOI3 with a high Curie temperature TC > 450 K and giant second harmonic generation (SHG) is reported. The 1D crystalline chain structure of the NbOI3 is revealed by cryo-electron microscopy, whereas the 1D ferroelectric order originated from the Nb displacement along the Nb-O chain (b-axis) is confirmed via obvious electrical and ferroelectric hysteresis loops. Impressively, NbOI3 exhibits a giant SHG susceptibility up to 1572 pm V-1 at a fundamental wavelength of 810 nm, and a further enhanced SHG susceptibility of 5582 pm V-1 under the applied hydrostatic pressure of 2.06 GPa. Combing in situ pressure-dependent X-ray diffraction, Raman spectra measurements, and first-principles calculations, it is demonstrated that the O atoms shift along the Nb─O atomic chain under compression, which can lead to the increased Baur distortion of [NbO2I4] octahedra, and hence induces the enhancement of SHG. This work provides a 1D vdW ferroelectric system for developing novel ferroelectronic and photonic devices.

Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y2Se2BrF Monolayer

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y2Se2XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 104 cm2 V-1 s-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y2Se2BrF possesses outstanding d33 (d33 = -405.97 pm/V), greatly outperforming CuInP2S6 by 4.26 times. Overall, the nano Y2Se2BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.

Exciton-Driven and Layer-Independent Linear and Nonlinear Optical Properties in NbOCl2

We investigate the electronic structure and linear and nonlinear [second-harmonic generation (SHG)] spectra of the NbOCl2 monolayer, bilayer, and bulk by using a real-time first-principles approach based on many-body theory. First, the interlayer couplings between NbOCl2 layers are very weak, due to the relatively large interlayer distance, saturation of the p orbital of Cl atoms, and high degree of localization of charge density around the Nb atom for both the lowest conduction band and the highest valence band. Second, the quasiparticle gaps and exciton binding energy for the three systems show layer-dependent features and decrease with an increase in layer thickness. Most importantly, the linear and SHG spectra of the NbOCl2 monolayer, bilayer, and bulk are dominated by strong excitonic resonances and exhibit layer-independent features due to the weak interlayer couplings. Our findings demonstrate that excitonic effects should be included in studying the optical properties of not only two-dimensional materials but also layered bulk materials with weak interlayer couplings.

Exciton-Enhanced Spontaneous Parametric Down-Conversion in Two-Dimensional Crystals

We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down-conversion, a second-order optical response that generates entangled photon pairs. We benchmark our ab initio many-body calculations using experimental polar plots of second harmonic generation in NbOI_{2}, clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D NbOCl_{2} leads to giant enhancement in the second order susceptibility. Our work paves the way for the realization of efficient ultrathin quantum light sources.

Third Harmonic Generation in Thin NbOI2 and TaOI2

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI2 and TaOI2. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS2, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.

Ambient Degradation Anisotropy and Mechanism of van der Waals Ferroelectric NbOI2

The spontaneous centrosymmetry-breaking and robust room-temperature ferroelectricity in niobium oxide dihalides spurs a flurry of explorations into its promising second-order nonlinear optical properties, and promises potential applications in nonvolatile electro-optical and optoelectronic devices. However, the ambient stability of the niobium oxide dihalides remains questionable, which overshadows their future development. In this work, the chemical degradation of NbOI2 is comprehensively investigated using combined chemical and optical microscopies in conjunction with spectroscopies. We unveil the highly anisotropic degradation kinetics of NbOI2 driven by the hydrolysis process of the unstable dangling iodine bonds dominantly on the (010) facet and progressing along the c axis. Knowing its degradation mechanism, the NbOI2 flake can then be stabilized by the hexagonal boron nitride encapsulation, which isolates the air moisture. These findings provide direct insights into the ambient instability of NbOI2, and they deliver possible solutions to circumvent this issue, which are essential for its practical integration in photonic and electronic devices.

First-Principles Study on Out-of-Plane Piezoelectricity of Self-Assembled Ammonia Layers Confined in Two Vertically Stacked Graphene Oxide Nanosheets

Two-dimensional (2D) piezoelectric materials have attracted widespread attention due to their increasingly important niche applications in flexible nanoscale devices. The water-wetted graphene oxide papers exhibit scalable out-of-plane piezoelectricity induced by the hydrogen-bonded network within, and this system can be treated as a potential 2D piezoelectric candidate for future device applications. It triggered our interest to search for more 2D piezoelectric hydrogen-bonded networks. Ammonia (NH3) isoelectronic with water is introduced to generate NH3-wetted graphene oxide papers and realize their out-of-plane piezoelectricity. Their structures and piezoelectricity are investigated using first-principles calculations. They reveal ultrahigh piezoelectricity, compared to the best reported 2D materials. Their piezoelectricity is tuned by varying oxygen-containing functional groups in GO plates, confined NH3 layers, or orientations of NH3 molecules, and it could be applied to fabrication of ammonia sensors. Our study not only enriches the family of 2D piezoelectric nanosystems but also inspires their future experimental exploration.

Manipulating Peierls Distortion in van der Waals NbOX2 Maximizes Second-Harmonic Generation

Two-dimensional (2D) van der Waals (vdW) materials, featuring relaxed phase-matching conditions and highly tunable optical nonlinearity, endow them with potential applications in nanoscale nonlinear optical (NLO) devices. Despite significant progress, fundamental questions in 2D NLO materials remain, such as how structural distortion affects second-order NLO properties, which call for advanced regulation and in situ diagnostic tools. Here, by applying pressure to continuously tune the displacement of Nb atoms in 2D vdW NbOI₂, we effectively modulate the polarization and achieve a 3-fold boost of the second-harmonic generation (SHG) at 2.5 GPa. By introducing a Peierls distortion parameter, λ, we establish a quantitative relationship between λ and SHG intensity. Importantly, we further demonstrate that the SHG enhancement can be achieved under ambient conditions by anionic substitution to tune the distortion in NbO(I_1-xBr_x)₂ (x = 0-1) compounds, where the chemical tailoring simulates the pressure effects on the structural optimization. Consequently, NbO(I_0.60Br_0.40)₂ with λ = 0.17 exhibits a giant SHG of over 2 orders of magnitude higher than that in monolayer WSe₂, reaching the record-high value among reported 2D vdW NLO materials. This work unambiguously demonstrates the correlation between Peierls distortion and SHG property and, more broadly, opens new paths for the development of advanced NLO materials by manipulating the structure distortions.

Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study

2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX₂ (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX₂ was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl₂ has a stronger ferroelectric order than NbOI₂. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl₂. STM imaging reveals the unreconstructed atomic structures of NbOX₂ surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.

Intrinsic ferromagnetism and the quantum anomalous Hall effect in two-dimensional MnOCl2 monolayers

Due to their potential application in spintronic devices, two-dimensional (2D) ferromagnetic materials are highly desired. We used first-principles calculations and Monte Carlo simulations to investigate the electronic structure and magnetic characteristics of the MnOCl₂ monolayers. We discovered two stable monolayer structures, Pmna-MnOCl₂ and Pmmn-MnOCl₂. Our findings show that the Pmna-MnOCl₂ monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.152 eV and a Curie temperature (T_C) of 202 K, while the Pmmn-MnOCl₂ monolayer is an intrinsic ferromagnetic Dirac semimetal with a high T_C (910 K) and triaxial magnetic anisotropy. We also show that a Pmmn-MnOCl₂ monolayer with a nontrivial band gap of 6.2 meV can achieve the quantum anomalous Hall effect (QAHE) with Chern number C = 1. Additionally, the existence of a gapless edge state can be flexibly regulated by choosing the terminal edges. Our studies reveal that the Pmmn-MnOCl₂ monolayer can serve as a candidate material to achieve high-temperature QAHE.

Large piezoelectric response in ferroelectric/multiferroelectric metal oxyhalide MOX2 (M = Ti, V and X = F, Cl and Br) monolayers

Flexible two-dimensional (2D) piezoelectric materials are promising for applications in wearable electromechanical nano-devices such as sensors, energy harvesters, and actuators. A large piezo-response is required for any practical applications. Based on first-principles calculations, we report that ferroelectric TiOX₂ and multiferroelectric VOX₂ (X = F, Cl, and Br) monolayers exhibit large in-plane stress (e₁₁) and strain (d₁₁) piezoelectric coefficients. For example, the in-plane piezo-response of TiOBr₂ (both e₁₁ = 28.793 × 10^-10 C m^-1 and d₁₁ = 37.758 pm V^-1) is about an order of magnitude larger than that of the widely studied 1H-MoS₂ monolayer, and also quite comparable to the giant piezoelectricity of group-IV monochalcogenide monolayers, e.g., SnS. Moreover, the d₁₁ of MOX₂ monolayers - ranging from 29.028 pm V^-1 to 37.758 pm V^-1 - are significantly higher than the d₁₁ or d₃₃ of commonly used 3D piezoelectrics such as w-AlN (d₃₃ = 5.1 pm V^-1) and α-quartz (d₁₁ = 2.3 pm V^-1). Such a large d₁₁ of MOX₂ monolayers originates from low in-plane elastic constants with large e₁₁ due to large Born effective charges (Z_ij) and atomic sensitivity to an applied strain. Moreover, we show the possibility of opening a new way of controlling piezoelectricity by applying a magnetic field.