Coexistence of Magnetic Order and Ferroelectricity at 2D Nanosheet Interfaces

Preparation, properties and applications of two-dimensional superlattices

As a combination concept of a 2D material and a superlattice, two-dimensional superlattices (2DSs) have attracted increasing attention recently. The natural advantages of 2D materials in their properties, dimension, diversity and compatibility, and their gradually improved technologies for preparation and device fabrication serve as solid foundations for the development of 2DSs. Compared with the existing 2D materials and even their heterostructures, 2DSs relate to more materials and elaborate architectures, leading to novel systems with more degrees of freedom to modulate material properties at the nanoscale. Here, three typical types of 2DSs, including the component, strain-induced and moiré superlattices, are reviewed. The preparation methods, properties and state-of-the-art applications of each type are summarized. An outlook of the challenges and future developments is also presented. We hope that this work can provide a reference for the development of 2DS-related research.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets

Bandgap tunable lanthanum niobium oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6 nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63 eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- and oxide [Ca₂ Nb₃ O₁₀ ]^- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

Fast and efficient shear-force assisted production of covalently functionalized oxide nanosheets

HYPOTHESIS

While controlled and efficient exfoliation of layered oxides often remains a time consuming challenge, the surface modification of inorganic nanosheets is of outmost importance for future applications. The functionalization of the bulk material prior to exfoliation should allow the application of tools developped for Van der Waals materials to directly produce functionalized oxide nanosheets.

EXPERIMENTS

The Aurivillius phase Bi₂SrTa₂O₉ is functionalized by a linear aliphatic phosphonic acid via microwave-assisted reactions. The structure of the hybrid material and the coordination of the phosphonate group is scrutinized, notably by Pair Distribution Function. This functionalized layered oxide is then exfoliated in one hour in organic solvent, using high shear force dispersion. The obtained nanosheets are characterized in suspension and as deposits to check their chemical integrity.

FINDINGS

The covalent functionalization decreases the electrostatic cohesion between the inorganic layers leading to an efficient exfoliation in short time under shearing. The functionalization of the bulk material is preserved on the nanosheets upon exfoliation and plays a major role to enable liquid-phase exfoliation and in the stability of the resulting suspensions. This strategy is very promising for the straighforward preparation of functionalized nanosheets, paving the way for versatile design of new (multi)functional hybrid nanosheets for various potential applications.

Construction of Multilayer Films and Superlattice- and Mosaic-like Heterostructures of 2D Metal Oxide Nanosheets via a Facile Spin-Coating Process

This study reports a design of a variety of nanostructured films of 2D oxide nanosheets. We systematically examined the deposition of perovskite-type Ca₂Nb₃O₁₀^- nanosheets by spin-coating their dimethyl sulfoxide dispersion. Neat and homogeneous monolayer tiling was attained on various substrates by selecting an optimum rotation speed, which was dependent on the nanosheet concentration. Repeating the optimized spin-coating process allowed for layer-by-layer deposition of the nanosheets into multilayer films with a designed layer number. Vertical superlattice heterostructures could also be assembled by alternately spin-coating the suspensions of Ca₂Nb₃O₁₀^- and Ti_0.87O₂^0.52- nanosheets. Furthermore, spin-coating of a mixed suspension of Ca₂Nb₃O₁₀^- and Ti_0.87O₂^0.52- nanosheets led to a mixed mosaic-like monolayer of these two nanosheets. The present study thus demonstrated spin-coating as a facile and powerful route to construct various nanostructures based on 2D oxide nanosheets.

High-Performance Two-Dimensional Perovskite Ca2Nb3O10 UV Photodetectors

We first report two-dimensional (2D) perovskite Ca₂Nb₃O₁₀ ultraviolet photodetectors (UV PDs), which are prepared via a facile calcination-exfoliation method. The 2D Ca₂Nb₃O₁₀ PDs demonstrate high performance at 3 V at 280 nm, high responsivity (14.94 A W^-1), high detectivity (8.7 × 10¹³ Jones), high spectral selectivity (R₂₈₀/R₄₀₀ = 8.84 × 10³), fast speed (0.08/5.6 ms), and long-term stability, exceeding those of most reported UV PDs. Furthermore, the Ca₂Nb₃O₁₀ PDs integrated with poly(ethylene terephthalate) (PET) show excellent flexibility and have high linear dynamic range (96 dB). Our work provides a general strategy for searching new UV PDs based on numerous layered niobates. The Ca₂Nb₃O₁₀ nanosheets may be one of the optimum semiconductor materials for next-generation high-performance UV PDs.

Single Droplet Assembly for Two-Dimensional Nanosheet Tiling

Recent advances in two-dimensional (2D) materials offer an opportunity for atomic layer engineering of functional thin films and superlattices. For future applications of 2D materials, there is an urgent need to develop convenient deposition processes that enable precise control of thin-film architectures while reducing the time, cost, and energy/sample consumption. Here, we demonstrate a strategy for nanosheet assembly using a simple drop casting with a pipet and a hot plate. By controlled thermal convection of a single droplet on a hot plate, a range of 2D nanosheets, such as Ti_0.87O₂^0.52-, Ca₂Nb₃O₁₀^-, Ru_0.95O₂^0.2-, and graphene oxide, can be neatly tiled to form an ideal monolayer on various substrates in ∼30 s over a wide area (i.e., a 50 mmϕ substrate). The mechanism and control strategies are discussed. We also demonstrate the production of various functional coatings such as conducting, semiconducting, insulating, magnetic, and photochromic coatings in multilayer, superlattice, and submicrometer-thick forms, offering the potential for a convenient way to produce high-quality 2D nanosheet films.

Metal Oxide Nanosheets as 2D Building Blocks for the Design of Novel Materials

Research into 2-dimensional materials has soared during the last couple of years. Next to van der Waals type 2D materials such as graphene and h-BN, less well-known oxidic 2D equivalents also exist. Most 2D oxide nanosheets are derived from layered metal oxide phases, although few 2D oxide phases can be also made by bottom-up solution syntheses. Owing to the strong electrostatic interactions within layered metal oxide crystals, a chemical process is usually needed to delaminate them into their 2D constituents. This Review article provides an overview of the synthesis of oxide nanosheets, and methods to assemble them into nanocomposites, mono- or multilayer films. In particular, the use of Langmuir-Blodgett methods to form monolayer films over large surface areas, and the emerging use of ink jet printing to form patterned functional films is emphasized. The utilization of nanosheets in various areas of technology, for example, electronics, energy storage and tribology, is illustrated, with special focus on their use as seed layers for epitaxial growth of thin films, and as electrochemically active electrodes for supercapacitors and Li ion batteries.

2D Superlattices for Efficient Energy Storage and Conversion

2D genuine unilamellar nanosheets, that are, the elementary building blocks of their layered parent crystals, have gained increasing attention, owing to their unique physical and chemical properties, and 2D features. In parallel with the great efforts to isolate these atomic-thin crystals, a unique strategy to integrate them into 2D vertically stacked heterostuctures has enabled many functional applications. In particular, such 2D heterostructures have recently exhibited numerous exciting electrochemical performances for energy storage and conversion, especially the molecular-scale heteroassembled superlattices using diverse 2D unilamellar nanosheets as building blocks. Herein, the research progress in scalable synthesis of 2D superlattices with an emphasis on a facile solution-phase flocculation method is summarized. A particular focus is brought to the advantages of these 2D superlattices in applications of supercapacitors, rechargeable batteries, and water-splitting catalysis. The challenges and perspectives on this promising field are also outlined.

2D Perovskite Sr2 Nb3 O10 for High-Performance UV Photodetectors

2D perovskites, due to their unique properties and reduced dimension, are promising candidates for future optoelectronic devices. However, the development of stable and nontoxic 2D wide-bandgap perovskites remains a challenge. 2D all-inorganic perovskite Sr₂ Nb₃ O₁₀ (SNO) nanosheets with thicknesses down to 1.8 nm are synthesized by liquid exfoliation, and for the first time, UV photodetectors (PDs) based on individual few-layer SNO sheets are investigated. The SNO sheet-based PDs exhibit excellent UV detecting performance (narrowband responsivity = 1214 A W^-1 , external quantum efficiency = 5.6 × 10⁵ %, detectivity = 1.4 × 10¹⁴ Jones @270 nm, 1 V bias), and fast response speed (t_rise ≈ 0.4 ms, t_decay ≈ 40 ms), outperforming most reported individual 2D sheet-based UV PDs. Furthermore, the carrier transport properties of SNO and the performance of SNO-based phototransistors are successfully controlled by gate voltage. More intriguingly, the photodetecting performance and carrier transport properties of SNO sheets are dependent on their thickness. In addition, flexible and transparent PDs with high mechanical stability are easily fabricated based on SNO nanosheet film. This work sheds light on the development of high-performance optoelectronics based on low-dimensional wide-bandgap perovskites in the future.

Two-Dimensional Ferroics and Multiferroics: Platforms for New Physics and Applications

Two-dimensional (2D) ferroics, including ferromagnets, ferroelectrics, ferroelastics, and multiferroics, recently have been theoretically proposed or experimentally revealed. The research has attracted tremendous attention because of the novel physics and promising applications for nanoelectronics, revealing ferroics in the 2D limit. In the present Perspective, we comprehensively review the recent research progress and also the proposed applications of 2D ferromagnetic, ferroelectric, and ferroelastic materials from theoretical and experimental viewpoints. We then introduce the coupling between ferroic orders and highlight the latest research on 2D multiferroic materials. The promising research directions and outlooks are discussed at the end of the Perspective. It is expected that the comprehensive overview of 2D ferroic materials can provide guidelines for researchers in the area and inspire further explorations of new physics and ferroic devices.

Tunable Chemical Coupling in Two-Dimensional van der Waals Electrostatic Heterostructures

Heterostructures of two-dimensional (2D) atomic crystals provide fascinating molecular-scale design elements for emergent physical phenomena and functional materials, as integrating distinct monolayers into vertical heterostructures can afford coupling between disparate properties. However, the available examples have been limited to either van der Waals (vdW) or electrostatic (ES) heterostructures that are solely composed of noncharged and charged monolayers, respectively. Here, we propose a "vdW-ES heterostructure" chemical design in which charge-neutral and charged monolayer-building blocks with highly disparate chemical and physical properties are conjugated vertically through asymmetrically charged interfaces. We demonstrate vdW-ES heteroassembly of semiconducting MoS₂ and dielectric Ca₂Nb₃O₁₀^- (CNO) monolayers using an amphipathic molecular starch, resulting in the emergence of trion luminescence observed at the lowest energy among MoS₂-related materials, probably due to interfacial confinement effects given by vdW-ES dual interactions. In addition, interface engineering leads to tailored exciton of the vdW/ES heterostructures owing to the pronounced dielectric proximity effects, bringing an intriguing interlayer chemistry to modify 2D materials. Furthermore, the current approach was successfully extended to create a graphene/CNO heterostructure, which verifies the versatility of the preparative method.

Topotactic Ion Exchange in a Three-Dimensional Close-Packed Trirutile Structure with an Octahedral Network

Topotactic ion exchange in open-framework solids and oxides with layered and tunnel structures has resulted in the formation of a variety of metastable functional materials that are inaccessible otherwise. These ion exchanges are primarily limited to the above structure types because of the presence of labile ions as loosely held charge-compensating cations/anions as in the framework or tunnel structures or the lability of the ions/charged motifs in interlayer galleries of layered oxides. While such topotactic exchanges are common in the above structure types, they are rare in the three-dimensional (3D) close-packed structures based solely on corner- and/or edge-connected polyhedral networks. Herein, we demonstrate divalent iron exchange in a close-packed all-octahedral-coordinated trirutile oxide. This has enabled the transformation of a near-ultraviolet-absorbing diamagnetic insulating oxide into a visible-light-active paramagnetic semiconductor. An ion exchange of this kind may open up avenues for the development of metastable functional oxides with a variety of other 3D structures and diverse properties.

Introduction of Fe2+ in Fe0.8Ti1.2O40.8− nanosheets via photo reduction and their enhanced electrochemical performance as a lithium ion battery anode

Fe²⁺ doped Fe_0.8Ti_1.2O₄^0.8− nanosheets were prepared via delaminating H_0.8Fe_0.8Ti_1.2O₄ precursor and further photo reduction. It shows improved electrochemical performance due to the enhanced electrical conductivity by the introduction of Fe²⁺.

Microwave-assisted functionalization of the Aurivillius phase Bi2SrTa2O9: diol grafting and amine insertion vs. alcohol grafting

The microwave-assisted functionalization of an Aurivillius phase is investigated towards various molecules – alcohols, diols and amino-alcohols – and the preferential reactivity of the various moieties is studied as a function of the reaction conditions.

Microwave-assisted functionalization of the layered Aurivillius phase Bi₂SrTa₂O₉ by alcohols is thoroughly investigated. The grafting of linear aliphatic and bulky alcohols is studied as a function of the starting material, underlining the importance of the prefunctionalization of the layered perovskite, for instance by butylamine. In addition, the functionalization by α,ω-alkanediols is explored. α,ω-alkanediols bearing long alkyl chains (n_C > 3) adopt an unprecedented pillaring arrangement, whereas 1,3-propanediol and ethyleneglycol adopt a bilayer arrangement, only one out of the two hydroxyl groups being coordinated. Finally, the reactivities of alcohols and amines towards insertion are compared: the preferential reactivity of the two functional groups appears to be strongly dependent of the reaction conditions, and especially of the water content. This study is further extended to the case of amino-alcohol insertion. In this case, the amine group is preferentially bound, but it is possible to control the grafting of the alcohol moiety, thus going from a bilayer arrangement to a pillaring one. This work is of particular importance to be able to functionalize easily and rapidly layered oxides with elaborated molecules, bearing several different potentially reactive groups.

Nanoarchitectonics in dielectric/ferroelectric layered perovskites: from bulk 3D systems to 2D nanosheets

We present an overview of recent investigations on the dielectric/ferroelectric properties of Dion-Jacobson-type perovskites, including bulk 3D layered systems and their exfoliated 2D nanosheets. In contrast to the Ruddlesden-Popper and Aurivillius phases, the Dion-Jacobson phases in bulk 3D systems have not been important targets for constructing dielectric/ferroelectric materials. However, recent investigations on Dion-Jacobson phases have provided new impetus to dielectric/ferroelectric materials. Dion-Jacobson perovskites can also facilitate delamination into 2D nanosheets. Layer-by-layer engineering of 2D perovskite nanosheets has a great potential for the rational design of new high-k dielectric/ferroelectric materials and nanodevices.

Soft chemistry of ion-exchangeable layered metal oxides

Disassembly and re-assembly of layered metal oxides by soft chemical approaches can be used to tailor functionalities in artificial photosynthesis, energy storage, optics, and piezoelectrics.

Neat monolayer tiling of molecularly thin two-dimensional materials in 1 min

Controlled arrangement of molecularly thin two-dimensional (2D) materials on a substrate, particularly into precisely organized mono- and multilayer structures, is a key to design a nanodevice using their unique and enhanced physical properties. Several techniques such as mechanical transfer process and Langmuir-Blodgett deposition have been applied for this purpose, but they have severe restrictions for large-scale practical applications, for example, limited processable area and long fabrication time, requiring skilled multistep operations. We report a facile one-pot spin-coating method to realize dense monolayer tiling of various 2D materials, such as graphene and metal oxide nanosheets, within 1 min over a wide area (for example, a 30-mmφ substrate). Centrifugal force drives the nanosheets in a thin fluid layer to the substrate edge where they are packed edge to edge all the way to the central region, without forming overlaps. We investigated the relationship between precursor concentration, rotation speed, and ultraviolet-visible absorbance and developed an effective method to optimize the parameters for neat monolayer films. The multilayer buildup is feasible by repeating the spin-coating process combined with a heat treatment at moderate temperature. This versatile solution-based technique will provide both fundamental and practical advancements in the rapid large-scale production of artificial lattice-like films and nanodevices based on 2D materials.