Two-dimensional Superlattice Research Articles

Two-dimensional superlattice films of gold nanoparticle-polystyrene composites: a bioactive platform for bone homeostasis maintenance

Osseo-integration between the implant and bone is a crucial factor to create a strong, durable bond that allows the implant to function effectively. However, regular implant surface with poor osseo-integration ability may cause aseptic loosening, resulting in the failure of implants. Herein, a serial of macroscopic one-particle thick superlattice films generated by self-assembly of diverse size of gold nanoparticles (GNPs) were termed as SFGs and were considered as bioactive implant coatings for enhancing osseo-integration. A hydroquinone-assisted seed method is established to fabricate homogenous GNPs with controllable sizes (20, 60, and 90 nm), which were further employed as building blocks to generate macroscopic one-particle thick superlattice films of GNPs (SFGs-20, SFGs-60, and SFGs-90) with the assistance of ploystryrene. The SFGs present a size-dependent performance on bone homeostasis, where SFGs-90 demonstrated the most pronounced facilitation of osteogenic differentiation of osteoblasts as well as deactivation of osteoclasts compared with SFGs-20 and SFGs-60. Considering the universal applicability of SFGs for depositing on various substrates, these SFGs with enhanced osseo-integration capabilities could serve as a bioactive platform for surface modification of orthopedic implants, effectively addressing the issue of aseptic loosening.Graphical abstractTwo-dimensional superlattice films of gold nanoparticle-polystyrene composites exhibit enhanced osteogenic-stimulation and osteoclastic-inhibition effects for regulating bone homeostasis maintenance.

Probing miniband structure and Hofstadter butterfly in gated graphene superlattices via magnetotransport

The presence of periodic modulation in graphene leads to a reconstruction of the band structure and formation of minibands. In an external uniform magnetic field, a fractal energy spectrum called Hofstadter butterfly is formed. Particularly interesting in this regard are superlattices with tunable modulation strength, such as electrostatically induced ones in graphene. We perform quantum transport modeling in gate-induced square two-dimensional superlattice in graphene and investigate the relation to the details of the band structure. At low magnetic field the dynamics of carriers reflects the semi-classical orbits which depend on the mini band structure. We theoretically model transverse magnetic focusing, a ballistic transport technique by means of which we investigate the minibands, their extent and carrier type. We find a good agreement between the focusing spectra and the mini band structures obtained from the continuum model, proving usefulness of this technique. At high magnetic field the calculated four-probe resistance fit the Hofstadter butterfly spectrum obtained for our superlattice. Our quantum transport modeling provides an insight into the mini band structures, and can be applied to other superlattice geometries.

FexSey@C superlattice nanocrystals for peroxymonosulfate activation: Intrinsic nature of Fe spin state

Two-dimensional layered carbon-coated FexSey nanocrystal superlattices with different crystalline structure (2D FexSey@C NS) are prepared and tested in Fenton-like reaction via PMS activation. The intrinsic nature and properties that govern the activity of 2D FexSey@C NS are deeply elucidated. Experimental analysis and theoretical calculations indicated that the spin state of Fe strongly decided the catalytic activity of 2D FexSey@C NS, and that a high spin state of Fe (4.16 μB) with a large effective magnetic moment not only regulated PMS adsorption (−3.06 eV) for increasing electronic density (0.79 e), but also accelerated interfacial charge transfer for HSO5− decomposition to produce SO4•−. Therefore, among 2D FexSey@C NS, 2D FeSe2 @C NS exhibited the most active performance in PMS activation with brilliant durability and adaptability. The study illustrates a new application of superlattice nanomaterials in environmental remediation and supplies fundamental insight to the intrinsic chemical structure of Fe-based catalysts for PMS activation.

Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly. Here we report high conductivity and the consequent metallic behaviour of semiconducting colloidal quantum dots of lead sulphide. Precise facet orientation control to forming highly-ordered quasi-2-dimensional epitaxially-connected quantum dot superlattices is vital for high conductivity. The intrinsically high mobility over 10 cm2 V−1 s−1 and temperature-independent behaviour proved the high potential of semiconductor quantum dots for electrical conducting properties. Furthermore, the continuously tunable subband filling will enable quantum dot superlattices to be a future platform for emerging physical properties investigations, such as strongly correlated and topological states, as demonstrated in the moiré superlattices of twisted bilayer graphene.

Synthesis of two-dimensional van der waals superlattices, heterostructures, and alloys from conversion of sequentially layered sub-nanometer metal films

Manipulation of bulk material properties by controlling layer-by-layer chemistry and structure of nanomaterials has remained an overarching goal of nanoscience and nanoengineering. In the case of two-dimensional (2D) materials, heterostructures consisting of different compositions, stacking, and orientation can serve as a platform for designing bulk material properties; however, fundamental challenges associated with materials processes have limited material quality and impeded scalability. Most attempts to overcome this limitation have relied on slow layer-by-layer growth or meticulous transfer of grown single layers. Our work describes a method to directly convert easily fabricated sub-nanometer metal multilayer heterostructures on both sapphire and SiO2 substrates followed by conversion to 2D van der Waals superlattices, heterostructures, and alloys. These materials exhibit novel bulk properties compared to individual 2D layers themselves including reduced bandgap, enhanced light-matter coupling, and improved catalytic performance. The process versatility enables tunable orientation, layer structure, and chemistry in an exciting class of 2D nanomaterials and provides an opportunity to generate a wide range of artificially stacked multicompositional 2D structures with controlled morphologies.

Superlattice-based Plasmonic Catalysis: Concentrating Light at the Nanoscale to Drive Efficient Nitrogen-to-Ammonia Fixation at Ambient Conditions.

Plasmonic catalysis promises green ammonia synthesis but is limited by the need of co-catalysts and poor performances due to weak electromagnetic field enhancement. Here, we use two-dimensional plasmonic superlattices with dense electromagnetic hotspots to boost ambient nitrogen-to-ammonia photoconversion without needing co-catalyst. By organizing Ag octahedra into a square superlattice to concentrate light, the ammonia formation is enhanced by ~15-fold and 4-fold over hexagonal superlattice and disorganized array, respectively. Our unique catalyst achieves superior ammonia formation rate and apparent quantum yield up to ~15-fold and ~103-fold, respectively, better than traditional designs. Mechanistic investigations reveal the abundance of intense plasmonic hotspots is crucial to promote hot electron generation and transfer for nitrogen reduction. Our work offers valuable insights to design electromagnetically hot plasmonic catalysts for diverse chemical and energy applications.

Nonlinear Hall effects in two-dimensional moiré superlattices

The Hall effect refers to the generation of a voltage in a direction perpendicular to the applied current. Since its discovery in 1879, the Hall effect family has become a huge group, and its in-depth study is an important topic in the field of condensed matter physics. The newly discovered nonlinear Hall effect is a new member of Hall effects. Unlike most of previous Hall effects, the nonlinear Hall effect does not need to break the time-reversal symmetry of the system but requires the spatial inversion asymmetry. Since 2015, the nonlinear Hall effect has been predicted and observed in several kinds of materials with a nonuniform distribution of the Berry curvature of energy bands. Experimentally, when a longitudinal alternating current (AC) electric field is applied, a transverse Hall voltage will be generated, with its amplitude proportional to the square of the driving current. Such a nonlinear Hall signal contains two components: one is an AC transverse voltage oscillating at twice the frequency of the driving current, and the other is a direct current (DC) signal converted from the injected current. Although the history of the nonlinear Hall effect is only a few years, its broad application prospects in fields of wireless communication, energy harvesting, and infrared detectors have been widely recognized. The main reason is that the frequency doubling and rectification of electrical signals via some nonlinear Hall effects are achieved by an inherent quantum property of the material - the Berry curvature dipole moment, and therefore do not have the thermal voltage thresholds and/or the transition time characteristic of semiconductor junctions/diodes. Unfortunately, the existence of the Berry curvature dipole moment has more stringent requirements for the lattice symmetry breaking of the system apart from the spatial inversion breaking, and the materials available are largely limited. This greatly reduces the chance to optimize the signal of the nonlinear Hall effect and limits the application and development of the nonlinear Hall effect. The rapid development of van der Waals stacking technology in recent years provides a brand new way to design, tailor and control the symmetry of lattice, and to prepare artificial moiré crystals with certain physical properties. Recently, both theoretical results and experimental studies on graphene superlattices and transition metal dichalcogenide superlattices have shown that artificial moiré superlattice materials can have larger Berry curvature dipole moments than those in natural non-moiré crystals, which has obvious advantages in generating and manipulating the nonlinear Hall effect. On the other hand, abundant strong correlation effects have been observed in two-dimensional superlattices. The study of the nonlinear Hall effect in two-dimensional moiré superlattices can not only give people a new understanding of the momentum space distribution of Berry curvatures, contributing to the realization of more stable topological transport, correlation insulating states and superfluidity states, but also expand the functional space of moiré superlattice materials which are promising for the design of new electronic and optoelectronic devices. This review paper firstly introduces the birth and development of the nonlinear Hall effect and discusses two mechanisms of the nonlinear Hall effect: the Berry curvature dipole moment and the disorder. Subsequently, this paper summaries some properties of two-dimensional moiré superlattices which are essential in realizing the nonlinear Hall effect: considerable Berry curvatures, symmetry breaking effects, strong correlation effects and tunable band structures. Next, this paper reviews theoretical and experimental progress of nonlinear Hall effects in graphene and transition metal dichalcogenides superlattices. Finally, the future research directions and potential applications of the nonlinear Hall effect based on moiré superlattice materials are prospected.

Two-dimensional graphene superlattice conductivity control by transverse electric fields

The possibility of controlling the conductivity of a graphene superlattice by transverse constant and alternating electric fields due to the nonadditivity of its energy spectrum is investigated. A technique is proposed for identifying the parameter areas of the applied fields in which conductivity control is most effective.

Controlling the Conductivity of a Two-Dimensional Graphene Superlattice by Means of Transverse Electric Fields

Controlling the Conductivity of a Two-Dimensional Graphene Superlattice by Means of Transverse Electric Fields

Interaction of solitary electromagnetic waves in a 2D graphene superlattice

The mutual influence of solitons propagating along the axes of a two-dimensional graphene superlattice in mutually perpendicular directions has been studied. It is shown that due to the nonadditivity of the energy spectrum of the structure, the overlap region of solitons undergoes characteristic changes associated with the formation of pulses propagating along the fronts of interacting solitary waves.

Ultraflexible two-dimensional Janus heterostructure superlattice: a novel intrinsic wrinkled structure.

The recently reported two-dimensional Janus transition metal dichalcogenide materials present promising applications such as in transistors, photocatalysts, and thermoelectric nanodevices. In this work, using molecular dynamics simulations, the self-assembled in-plane MoSSe/WSSe heterostructure superlattice is predicted with a natural sinusoidal structure constructed by an asymmetric interface. Such a sinusoidal structure shows extraordinary mechanical behavior where the fracture strain can be enhanced up to 4.7 times than that of the symmetrical interface. Besides, the deformational structure of all these MoSSe/WSSe heterostructure superlattice are in accordance with the Fourier function curve; the fracture strength and fracture strain also demonstrate pronounced size dependence. Our investigations proposed an ultrastretchable assembled heterostructure superlattice and provided a desirable strategy to tune the mechanical properties of such an in-plane two-dimensional heterostructure.

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.

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.

Dual effects of hetero-interfaces on phonon thermal transport across graphene/C[formula omitted]N lateral superlattices

Two-dimensional (2D) lateral superlattices, a typical artificial nano-phononic crystal, have stimulated widespread interests and potential application prospects in terms of their physically interesting features. Herein, we have found wave-particle crossover of phonon transport in the graphene (Gr)/2D polyaniline (C3N) lateral superlattices, which is an indication of a transition in the phonon transport mechanism from the incoherent to coherent regime. Due to the high structural similarity of C3N to Gr, the thermal conductivity of the Gr/C3N lateral superlattice can achieve an ultra-wide modulation range of about 500∼1100 W m−1 K−1, which is far beyond that of other superlattices. The analysis shows that an increase of the interface density will on the one hand weaken the thermal conductivity by increasing phonon-interface scattering, and on the other hand increase it by lowering the phonon transport barriers and allowing more long-wavelength phonons to participate in the transport. This determines the parabolic trend of thermal conductivity containing the minimum, and also reflects the dual effects of hetero-interfaces on phonon thermal transport. In addition, phonon calculations show that the above variation in thermal conductivity is entirely attributable to differences in the phonon mean free path for different interface densities and is not related to their group velocity.

Enhanced thermoelectric properties of p-type Bi1−xCaxCuSeO achieved via microwave synthesis combined with spark plasma sintering

In this work, the nearly single-phase Bi1−xCaxCuSeO bulks are fabricated via 4 min microwave synthesis combined with 5 min spark plasma sintering (SPS). The phase composition, microstructure, electrical and thermal transport performance of the samples are studied, and the effect of Ca doping contents on thermoelectric performance is discussed in the temperature range of 298–873 K. The conductivity and power factor increase with increasing Ca concentration on account of synchronously increase of carrier concentration and mobility. The two-dimensional layered superlattice structure allows the sample to maintain relatively low lattice thermal conductivity, and the total thermal conductivity of all samples is less than 1.0 W/mK. As a result, a maximum ZT value of 0.85 in Bi0.93Ca0.07CuSeO is achieved at 873 K, and which is ∼70 % higher than that of the undoped BiCuSeO. This indicates that the synthesis method is not only simple and efficient, but also has improved performance compared with traditional synthesis methods.

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.

Intercalated chiral-molecular 2D superlattice for spintronics

Solid-state spintronic devices are energy-efficient for data storage and energy harvesting. This field is revolutionized by the discovery of giant magnetoresistance. Recently, Qian et al. fabricated an intercalated chiral-molecular two-dimensional superlattice that exhibits highly efficient chiral-induced spin selectivity. They demonstrated a tunneling magnetoresistance ratio of >300% and a spin polarization ratio of >60%, which opens up promising future applications in spintronic devices.

Erratum: Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Erratum: Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Quantitative determination of interlayer electronic coupling at various critical points in bilayer MoS2

Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the moire pattern to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide (TMD) homo- and hetero-bilayers. However, a largely unexplored factor is the interlayer distance, d, which can impact the interlayer coupling strength exponentially. In this letter, we quantitatively determine the coupling strengths as a function of interlayer spacing at various critical points of the Brillouin zone in bilayer MoS2. The exponential dependence of the coupling parameter on the gap distance is demonstrated. Most significantly, we achieved a 280% enhancement of K-valley coupling strength with an 8% reduction of the vdW gap, pointing to a new strategy in designing a novel electronic system in vdW bilayers.

Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation

Theoretical study of photo-stimulated thermo-magnetoelectric effects in two-dimensional compositional superlattices using quantum kinetic equation