2D Superlattices Research Articles

Channeling the exchanges of eg electrons by Co-implantation for magnetism enhancement in CrI3 monolayer

In recent few years, the two-dimensional (2D) magnets have emerged as one of the most important frontiers in materials physics and attracted much attention. As one of the earliest experimentally discovered 2D magnets, CrI3 shows a wealth of properties and has been extensively studied. In particular, an intriguing characteristic of the CrI3 monolayer is its octahedrally coordinated hollow within the unit-cell, which enables the implantation of a magnetic atom, thereby resulting in an artificial 2D superlattice with fertile physics to explore. In this work, using first-principles calculations, we investigate the Co-implanted CrI3 monolayer, denoted as Co-(CrI3)2, and demonstrate the vital roles of the exchange channels of eg electrons in enhancing magnetism. It is shown that the Co-(CrI3)2 monolayer has a half-metallic ferrimagnetic (FiM) ground-state with a net in-plane magnetic moment of 5.0μB/f.u. and a relatively high Curie point (TC) of ∼195 K, noting that TC of pristine CrI3 is only 45–61 K. The FiM ordering is established by the strong anti-ferromagnetic coupling in the t2g-eg exchange channels of the nearest-neighbor (NN) Cr–Co pair and the sizeable ferromagnetic coupling of the third NN Cr–Cr pair mediated by the itinerant eg electrons. In addition, an in-plane biaxial tensile strain of ∼2% may further enhance TC up to ∼210 K. This work offers unique insights into the magnetism enhancement of the CrI3 monolayer by atom-implantation, paving the way for the development of 2D magnets.

Unsupervised machine learning to classify the confinement of waves in periodic superstructures.

We propose a rigorous method to classify the dimensionality of wave confinement by utilizing unsupervised machine learning to enhance the accuracy of our recently presented scaling method [Phys. Rev. Lett.129, 176401 (2022)10.1103/PhysRevLett.129.176401]. We apply the standard k-means++ algorithm as well as our own model-based algorithm to 3D superlattices of resonant cavities embedded in a 3D inverse woodpile photonic band gap crystal with a range of design parameters. We compare their results against each other and against the direct usage of the scaling method without clustering. Since the clustering algorithms require the set of confinement dimensionalities present in the system as an input, we investigate cluster validity indices (CVIs) as a means to find these values. We conclude that the most accurate outcome is obtained by first applying direct scaling to find the correct set of confinement dimensionalities, and subsequently utilizing our model-based clustering algorithm to refine the results.

Conversion of chirality to twisting via sequential one-dimensional and two-dimensional growth of graphene spirals.

The properties of two-dimensional (2D) van der Waals materials can be tuned through nanostructuring or controlled layer stacking, where interlayer hybridization induces exotic electronic states and transport phenomena. Here we describe a viable approach and underlying mechanism for the assisted self-assembly of twisted layer graphene. The process, which can be implemented in standard chemical vapour deposition growth, is best described by analogy to origami and kirigami with paper. It involves the controlled induction of wrinkle formation in single-layer graphene with subsequent wrinkle folding, tearing and re-growth. Inherent to the process is the formation of intertwined graphene spirals and conversion of the chiral angle of 1D wrinkles into a 2D twist angle of a 3D superlattice. The approach can be extended to other foldable 2D materials and facilitates the production of miniaturized electronic components, including capacitors, resistors, inductors and superconductors.

Hybridizing Ti3C2Tx Layers with Layered Double Hydroxide Nanosheets at the Molecular Level: A Smart Electrode Material for H2O2 Monitoring in Cancer Cells.

Vertically stacked artificial 2D superlattice hybrids fabricated through molecular-level hybridization in a controlled fashion play a vital role in scientific and technological fields, but developing an alternate assembly of 2D atomic layers with strong electrostatic interactions could be much more challenging. In this study, we have constructed an alternately stacked self-assembled superlattice composite through integration of CuMgAl layered double hydroxide (LDH) nanosheets having positive charge with negatively charged Ti3C2Tx layers using well-controlled liquid-phase co-feeding protocol and electrostatic attraction and investigated its electrochemical performance in sensing early cancer biomarkers, i.e., hydrogen peroxide (H2O2). The molecular-level CuMgAl LDH/Ti3C2Tx superlattice self-assembly possesses superb conductivity and electrocatalytic properties, which are significant for obtaining a high electrochemical sensing aptitude. Electron penetration in Ti3C2Tx layers and rapid ion diffusion along 2D galleries have shortened the diffusion path and enhanced the charge transferring efficacy. The electrode modified with the CuMgAl LDH/Ti3C2Tx superlattice has demonstrated admirable electrocatalytic abilities in H2O2 detection with a wide linear concentration range and low real-time limit of detection (LOD) of 0.1 nM with signal/noise ratio (S/N) = 3. Practically, an electrochemical sensing podium based on the CuMgAl LDH/Ti3C2Tx superlattice has been effectively applied in real-time in vitro tracking of H2O2 effluxes excreted from different live cancer cells and normal cells after being encouraged by stimulation. The results exhibit that molecular-level heteroassembly holds great potential in electrochemical sensors to detect promising biomarkers.

Topological and Stacked Flat Bands in Bilayer Graphene with a Superlattice Potential.

We show that bilayer graphene in the presence of a 2D superlattice potential provides a highly tunable setup that can realize a variety of flat band phenomena. We focus on two regimes: (i)topological flat bands with nonzero Chern numbers, C, including bands with higher Chern numbers |C|>1 and (ii)an unprecedented phase consisting of a stack of nearly perfect flat bands with C=0. For realistic values of the potential and superlattice periodicity, this stack can span nearly 100meV, encompassing nearly all of the low-energy spectrum. We further show that in the topological regime, the topological flat band has a favorable band geometry for realizing a fractional Chern insulator (FCI) and use exact diagonalization to show that the FCI is in fact the ground state at 1/3 filling. Our results provide a realistic guide for future experiments to realize a new platform for flat band phenomena.

Spatiotemporal Changes in Atomic and Molecular Architecture of Mineralized Bone under Pathogenic Conditions

Mechanisms responsible for spatiotemporal changes in the atomic-molecular architecture of the human femur in intact and osteoarthritis-affected areas were studied using high-resolution X-ray diffraction and spectroscopic techniques. Comparison of the experimental data demonstrates strong deviations of core electron-binding energies, lattice constants of hydroxyapatite crystal cells, linear sizes of crystallites, and degrees of crystallinity for both intact and osteoarthritic areas. The quantitative values of these characteristics and their standard deviations in each area are measured and presented. A systematic analysis of the site-dependent deviations was carried out within the framework of the 3D superlattice model. It is argued that the main mechanism responsible for the deviations arises primarily as a result of carbonization and catalytic reactions at the mineral-cartilage interface. The impact of the mechanism is enhanced in the vicinities of the area of sclerosed bone, but not inside the area where mechanical loads are maximum. Restoration of the atomic-molecular architecture of mineralized bone in the sclerosis area is revealed. Statistical aspects of the spatiotemporal changes in mineralized bone under pathogenic conditions are discussed.

A General Approach to Stabilize Nanocrystal Superlattices by Covalently Bonded Ligands

Self-assembled inorganic nanocrystal (NC) superlattices are powerful material platforms with diverse structures and emergent functionalities. However, their applications suffer from the low structural stability against solvents and other stimuli, due to the weak interparticle interactions. Existing strategies to stabilize NC superlattices typically require the design and incorporation of special ligands prior to self-assembly and are only applicable to superlattices of certain NCs, ligands, and structures. Here we report a general method to stabilize superlattices of various NC compositions and structures via strong, covalently bonded ligands. The core is the use of light-triggered, nitrene-based cross-linkers that do not interfere the self-assembly process while nonspecifically and effectively bonding the native ligands of NCs. The stabilized 2D and 3D superlattices of metal, semiconductor, and magnetic NCs retain their structures when being exposed to solvents of different polarities (from toluene to water) and show high thermal stability and mechanical rigidity. This can further stabilize binary NC superlattices, beyond those achievable in previous methods. Stabilized superlattices show robust and reproducible functionalities, for instance, when serving as reusable substrates for surface enhanced Raman spectroscopy. These results create more possibilities in exploiting the impressive library of NC superlattices in a broad scope of applications.

2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles.

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

Superstructures of water-dispersive hydrophobic nanocrystals: specific properties.

Here, we describe water-soluble superstructures of hydrophobic nanocrystals that have been developed in recent years. We will also report on some of their properties which are still in their infancy. One of these structures, called "cluster structures", consists of hydrophobic 3D superlattices of Co or Au nanocrystals, covered with organic molecules acting like parachutes. The magnetic properties of Co "cluster structures" a retained when the superstructures is dispersed in aqueous solution. With Au "cluster structures", the longer wavelength optical scattered spectra are very broad and red-shifted, while at shorter wavelengths the localized surface plasmonic resonance of the scattered nanocrystals is retained. Moreover, the maximum of the long-wavelength signal spectra is linearly dependent on the increase in assembly size. The second superstructure was based on liquid-liquid instabilities favoring the formation of Fe3O4 nanocrystal shells (colloidosomes) filled or unfilled with Au 3D superlattices and also spherical solid crystal structures are called supraballs. Colloidosomes and supraballs in contact with cancer cells increase the density of nanocrystals in lysosomes and near the lysosomal membrane. Importantly, the structure of their organization is maintained in lysosomes for up to 8 days after internalization, while the initially dispersed hydrophilic nanocrystals are randomly aggregated. These two structures act as nanoheaters. Indeed, due to the dilution of the metallic phase, the penetration depth of visible light is much greater than that of homogeneous metallic nanoparticles of similar size. This allows for a high average heat load overall. Thus, the organic matrix acts as an internal reservoir for efficient energy accumulation within a few hundred picoseconds. A similar behavior was observed with colloidosomes, supraballs and "egg" structures, making these superstructures universal nanoheaters, and the same behavior is not observed when they are not dispersed in water (dried and deposited on a substrate). Note that colloidosomes and supraballs trigger local photothermal damage inaccessible to isolated nanocrystals and not predicted by global temperature measurements.

Tunable orientation of two-dimensional assembled Au octahedron superlattices in polymer films as flexible SERS substrates.

Anisotropic nanoparticles have been widely used as building blocks for preparing surface-enhanced Raman spectroscopy (SERS) substrates. However, tailoring the SERS activity at the self-assembly level through the anisotropic nanoparticle orientation is a big challenge, mainly due to the lack of simple assembly methods. In the present work, we report an air-water interface mediated co-assembly (AWIMCoA) strategy to prepare flexible 2D superlattices of Au octahedra with tunable orientations. We have demonstrated that Au octahedra can self-assemble into face-up, edge-up and vertex-up orientations on changing the surface wettability of Au octahedra, which determines the interparticle anisotropic interactions and the interaction between Au octahedra and the poly(styrene-ethylene-butylene-styrene) (SEBS) nanomembrane. The effect of assembly orientation on the SERS performance of 2D superlattices has been studied through correlated SEM characterization and SERS mapping. Among all the orientational modes, flexible 2D superlattices with the vertex-up orientation show the highest enhancement performance and uniformity, which is further demonstrated by theoretical simulation. Partially embedded 2D superlattices in the SEBS nanomembrane are robust to remove the surface ligands without breaking the whole nanostructure. This post-treatment process boosts the SERS performance of the 2D superlattice with the edge-up orientation by forming fused nanostructures among neighboring Au octahedra. We expect that the co-assembly method will be widely applied in the preparation of reusable and high-performance SERS substrates for broad application.

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.

Discovery of two-dimensional binary nanoparticle superlattices using global Monte Carlo optimization

Binary nanoparticle (NP) superlattices exhibit distinct collective plasmonic, magnetic, optical, and electronic properties. Here, we computationally demonstrate how fluid-fluid interfaces could be used to self-assemble binary systems of NPs into 2D superlattices when the NP species exhibit different miscibility with the fluids forming the interface. We develop a basin-hopping Monte Carlo (BHMC) algorithm tailored for interface-trapped structures to rapidly determine the ground-state configuration of NPs, allowing us to explore the repertoire of binary NP architectures formed at the interface. By varying the NP size ratio, interparticle interaction strength, and difference in NP miscibility with the two fluids, we demonstrate the assembly of an array of exquisite 2D periodic architectures, including AB-, AB2-, and AB3-type monolayer superlattices as well as AB-, AB2-, A3B5-, and A4B6-type bilayer superlattices. Our results suggest that the interfacial assembly approach could be a versatile platform for fabricating 2D colloidal superlattices with tunable structure and properties.

Approaching Bulk Mobility in PbSe Colloidal Quantum Dots 3D Superlattices.

3D superlattices made of colloidal quantum dots are a promising candidate for the next generation of optoelectronic devices as they are expected to exhibit a unique combination of tunable optical properties and coherent electrical transport through minibands. While most of the previous work wasperformed on 2D arrays, the control over the formation of these systems is lacking, where limited long-range order and energetical disorder have so far hindered the potential of these metamaterials, giving rise to disappointing transport properties. Here, it is reported that nanoscale-level controlled ordering of colloidal quantum dots in 3D and over large areas allows the achievement of outstanding transport properties. The measured electron mobilities are the highest ever reported for a self-assembled solid of fully quantum-confined objects. This ultimately demonstrates that optoelectronic metamaterials with highly tunable optical properties (in this case in the short-wavelength infrared spectral range) and charge mobilities approaching that of bulk semiconductor can be obtained. This finding paves the way toward a new generation of optoelectronic devices.

Dynamic Assembly of Polymer-Tethered Gold Nanoparticles into a 2D Superlattice at the Air–Liquid Interface: Influence of the Polymer Structure and Solvent Vapor

The two-dimensional (2D) superlattice obtained by self-assembly of gold nanoparticle (NPs) at the air–liquid interface shows a wide range of potential applications in catalysis, sensing, electronic storage, and molecular recognition. However, the dynamic assembly process of the gold NPs at the air–liquid interface is still unclear. In this study, gold NPs grafted with thiol-terminated polystyrene homopolymers (AuNPs@PS) were used as the assembly unit, and the 2D superlattice of AuNPs@PS was prepared by spreading and evaporating the AuNPs@PS toluene suspension at the air/diethylene glycol interface. The effects of polymer chain length, grafting density, and toluene vapor atmosphere on the microstructure of the superlattice were investigated as well. The microstructural evolution process of the AuNPs@PS superlattice at the air/diethylene glycol interface was monitored by the in situ grazing incident small-angle X-ray scattering (GISAXS) technique. The results demonstrate that the transition of long-chain polymer-tethered NPs from random packing to face-centered-cubic packed superlattice followed by two-dimensional hexagonal packing can be observed under a toluene vapor atmosphere. Additionally, by varying the polymer chain length and solvent vapor atmosphere, the spreading rate and superlattice interparticle spacing can be well mediated. This work provides a useful guide to prepare an ordered 2D superlattice with polymer-tethered inorganic NPs.

Elastic properties of moiré lattices in epitaxial two-dimensional materials

Unlike conventional two-dimensional (2D) semiconductor superlattices, moir\'{e} patterns in 2D materials are flexible and their electronic, magnetic, optical, and mechanical properties depend on their topography. Within a continuous+atomistic theory treating 2D materials as crystalline elastic membranes, we abandon the flat-membrane scenario usually assumed for these materials and address out-of-plane deformations. We confront our predictions to experimental analyses on model systems, epitaxial graphene, and MoS$_2$ on metals and reveal that compression/expansion and bending energies stored in the membrane can compete with adhesion energy, leading to a subtle moir\'{e} wavelength selection and the formation of wrinkles.

Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis.

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

Highly Efficient and Robust Full-Color Organic Afterglow through 2D Superlattices Embedment.

Purely organic afterglow (POA) originating from the slow radiative decay of stabilized triplet excited states has shown amazing potential in many fields. However, achieving highly stable POA with high phosphorescent quantum yield (PhQY) and long lifetime is still a formidable challenge owing to the intrinsically active and sensitive nature of triplet excitons. Here, triplet excitons of phosphors are protected and stabilized by embedding in tricomponent trihapto self-assembled 2D hydrogen-bonded superlattices, which not only enables deep-blue POA with high PhQY (up to 65%), ultralong lifetime (over 1300ms) and the highest figure-of-merit at room temperature, but also achieves excellent stability capable of resisting quenching effects of oxygen, solvent, pressure, light, and heat. In addition, the POA color is tuned from deep-blue to red via efficient Förster resonance energy transfer from the deep-blue POA emitters to the fluorophores. Moreover, with the high-performance, robust, and full-color POA materials, flexible anti-counterfeit displays and direct-current (DC)-driven lifetime-encrypted color Morse Code applications are facilely realized.

Visible Light Induced Ag-Polyoxometalate Coassembly into Single-Cluster Nanowires.

1D superlattices with long-ranged periodicity present extraordinary application properties due to their unique electronic structures. Here, the visible light driven synthesis of 1D single-cluster chains constructed by polyoxometalate (POM) and Ag clusters is reported, where two types of clusters align alternatively along the nanowire. Low symmetrical POM clusters of [P2 W17 O61 ]10- , [P2 W15 O56 ]12- , and [EuW10 O36 ]9- can be used as building blocks. The directly bonding cluster units result in interactive electronic structures of Ag and POM clusters, as well as the greatly promoted electron transfer during the redox reaction. The Ag-P2 W17 nanowires perform significantly enhanced activities in both electrochemical sensing and catalytic gasoline desulfurization compared with individual building blocks, demonstrating the extraordinary application properties and promising potentials of cluster-based heteroconstructions.

Slip-Line-Guided Growth of Graphene.

Manipulating the crystal orientation of emerging 2D materials via chemical vapor deposition (CVD) is a key premise for obtaining single-crystalline films and designing specific grain-boundary (GB) structures. Herein, the controllable crystal orientation of graphene during the CVD process is demonstrated on a single-crystal metal surface with preexisting atomic-scale stair steps resulting from dislocation slip lines. The slip-line-guided growth principle is established to explain and predict the crystal orientation distribution of graphene on a variety of metal facets, especially for the multidirectional growth cases on Cu(hk0) and Cu(hkl) substrates. Not only large-area single-crystal graphene, but also bicrystal graphene with controllable GB misorientations, are successfully synthesized by rationally employing tailored metal substrate facets. As a demonstration, bicrystal graphenes with misorientations of ≈21° and ≈11° are constructed on Cu(410) and Cu(430) foils, respectively. This guideline builds a bridge linking the crystal orientation of graphene and the substrate facet, thereby opening a new avenue for constructing bicrystals with the desired GB structures or manipulating 2D superlattice twist angles in a bottom-up manner.

Emission bandwidth control on a two-dimensional superlattice microcavity array.

Narrowband thermal emission at high temperatures is required for various thermal energy systems. However, the large lossy energy of refractory metals induces a broad bandwidth emission. Here, we demonstrated a two-dimensional (2D) superlattice microcavity array on refractory metals to control the emission bandwidth. A hybrid resonance mode was obtained by coupling the standing-wave modes and propagating surface-wave modes. The bandwidth emission was controlled by varying the superlattice microcavity array resulting from the change in electric field (E-field) concentration. The quality factor (Q-factor) improved by more than 3 times compared to that of a single-lattice array. A narrower band emission originating from the hybrid mode was observed and analyzed experimentally. This novel surface-relief microstructure method can be used to control the emission bandwidth of thermal emitters used in thermophotovoltaic (TPV) systems and other high-temperature thermal energy systems.