Monolayer Superlattices Research Articles

Phonon thermal transport in Bi2Te3/Sb2Te3 monolayer superlattice: a neural network potential study

Superlattice is a significant means to reduce the lattice thermal conductivity of thermoelectric material and optimize its performance. In this work, using high precision first-principles based neural network potential combined...

Monolayer Semiconductor Superlattices with High Optical Absorption.

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. A uniform film much thinner than the wavelength can only absorb up to 50% of the incident light when embedded in a symmetric and homogeneous environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption by artificially stacking WS2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the peak absorptance of a single WS2 monolayer, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach an absorptance of 27% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer with 25% absorptance while increasing photoluminescence thanks to doping. Finally, we exploit the atomic layer deposition of alumina spacers to boost the absorptance to 31% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve strong light-matter coupling and enhance the performance of nanophotonic devices such as modulators and photodetectors.

Superlattice assembly strategy of small noble metal nanoparticles for surface-enhanced Raman scattering

The self-assembly of noble metal nanoparticles into periodic structures has been a theme of great interest for surface-enhanced Raman scattering (SERS) and use in functional devices. However, small nanoparticle self-assembly faces numerous challenges in tunability, referring to controlling their structural properties like structure, gaps, and arrangement. These issues highlight the need for further research and development to enhance the tunability and stability of self-assembled small nanoparticles. Here, we report a general centimeter-scale superlattice assembly strategy for noble metal nanoparticles less than 15 nm in size. Not only is this monolayer superlattice assembly generally applicable to different kinds and sizes of noble metal nanoparticles, but also, the crystal plane spacing can also be quickly and conveniently controlled by changing the ethanol concentration. SERS results reveal that optimized superlattice membranes of noble metal nanoparticles possess high detection sensitivity and ordered hot spots. Therefore, our strategy offers prospects for high-performance SERS substrates based on small noble metal nanoparticle superlattices.

Exciton polariton interactions in Van der Waals superlattices at room temperature

Monolayer transition-metal dichalcogenide (TMD) materials have attracted a great attention because of their unique properties and promising applications in integrated optoelectronic devices. Being layered materials, they can be stacked vertically to fabricate artificial van der Waals lattices, which offer unique opportunities to tailor the electronic and optical properties. The integration of TMD heterostructures in planar microcavities working in strong coupling regime is particularly important to control the light-matter interactions and form robust polaritons, highly sought for room temperature applications. Here, we demonstrate the systematic control of the coupling-strength by embedding multiple WS2 monolayers in a planar microcavity. The vacuum Rabi splitting is enhanced from 36 meV for one monolayer up to 72 meV for the four-monolayer microcavity. In addition, carrying out time-resolved pump-probe experiments at room temperature we demonstrate the nature of polariton interactions which are dominated by phase space filling effects. Furthermore, we also observe the presence of long-living dark excitations in the multiple monolayer superlattices. Our results pave the way for the realization of polaritonic devices based on planar microcavities embedding multiple monolayers and could potentially lead the way for future devices towards the exploitation of interaction-driven phenomena at room temperature.

Rigid POSS Nanofiller Regulated the Interfacial Self‐assembly of Particles into 2D Monolayer Superlattice with Fixed Interparticle Spacing†

Comprehensive SummaryPlasmonic superlattices of nanoparticles (NPs) possess unique “surface lattice resonances” properties that facilitates their wide applications in plasmonic sensing, photocatalysis, and nanoscale light manipulation. However, it is still challenging to manufacture superlattices with precisely controllable NPs distance and break the size limitation of NPs. Herein, we provided an effective strategy to construct NPs superlattices via shape‐persistent polyhedral oligosilsesquioxane (POSS) molecular nanoparticles govern interfacial assembly. As a nanoscale molecule with diameter of 1.5 nm, the POSS‐SH molecule provides sufficient rigid steric hindrance and hydrophobic effect for tailoring the uniformity and controllable distance between NPs in superlattices. Interestingly, synergistically with hydrophilic ligands of polyethylene glycol (PEG‐SH) with optimized ratio, the rigid POSS ligands can effectively regulate the distance between NPs in a fixed range of 2.3—2.8 nm, which is independent of ligands molecular weight and particle size. Furthermore, the effective approach can be universal to anisotropic NPs for manufacturing monolayer films with high NPs density. We believe this nanoscale molecule tailored interfacial self‐assembly strategy can effectively break the size of NPs and assembly obstacles for superlattice monolayer film. Additionally, the definite distance between NPs in superlattices can minimize optical energy attenuation and facilitates the applications such as surface‐enhanced Raman spectroscopy and photocatalysis.

Self-assembly of anisotropy gold nanocubes into large area two-dimensional monolayer superlattices

The spontaneous self-assembly of metal nanocrystals into two-dimensional (2D) monolayer superlattices with highly ordered symmetry and configuration paves the way towards the fabrication of functional materials. However, there remains great challenge for anisotropic nanocrystals to self-assembly into high quality superlattice because of the orientation and configuration consistency. Here, a facile yet universal solvent annealing driven 2D interfacial assembly of synthetic dried metal nanocrystals is firstly developed to realize the construction of the non-close-packing 2D monolayer gold nanocube (AuNC) superlattice with tunable interparticle distance and internal configurations (i.e. face-to-face and hexagonally-packed arrangement), which is achieved by precisely controlling molecular weight of polymer ligands tethered on AuNCs and the van der Waals forces between the adjacent AuNCs. In addition, the scale of the generated 2D monolayer AuNC superlattice with highly ordered internal arrangement and orientation can reach up to hundreds of micrometers, thus acquiring significant surface-enhanced Raman scattering performance of the large scale superlattice due to the strong plasma coupling effect. This strategy not only provides a robust route to fabricate nanocrystal superlattice structures but also offers a promising platform for preparing diverse functional materials with potential applications in electronics, photonics, detections, and others.

Coherent and incoherent phonon thermal transport in group-III nitride monolayer superlattices with Tersoff type interatomic potential

The existence of a crossover from coherent to incoherent phonon transport for different monolayer superlattices such as h-BN/h-AlN, h-BN/h-GaN, h-AlN/h-GaN, and h-BN/h-AlN/h-GaN using large-scale non-equilibrium molecular dynamics simulations is unveiled. In this context, the lattice thermal conductivities of considered superlattice structures are obtained for varying period lengths, sample lengths, and temperatures. It is predicted that the thermal conductivities of superlattice structures can be lowered even up to 99% than that of their pristine counterparts. Also, it is revealed that a minimum thermal conductivity resulting from the competition between coherent, wave-like, and incoherent, particle-like, phonon transport can be achieved by altering the period length for all the superlattice structures. Moreover, it is observed that the thermal conductivity minimum at higher temperatures compared to low temperatures occurs at shorter period lengths for binary superlattices, and the minimum depths decrease with increasing temperature. Results point out that group-III nitride superlattices can be considered as an alternative option in controlling the heat flow for thermal management and thermoelectric device applications.

High-temperature ideal Weyl semimetal phase and Chern insulator phase in ferromagnetic BaEuNiOsO6 and its (111) (BaEuNiOsO6)/(BaTiO3)10 superlattice

Weyl semimetals (WSMs) have recently stimulated intensive interest because they exhibit fascinating physical properties and also promise exciting technological applications. So far, however, the few confirmed magnetic WSMs generally have a large number of Weyl points either located away from the Fermi level $({E}_{F})$ or shrouded by nontopological Fermi surface pockets. Based on first-principles density functional theory calculations, we establish cubic double perovskite ${\mathrm{BaEuNiOsO}}_{6}$ to be a high Curie temperature $({T}_{c})$ ferromagnetic (FM) WSM with magnetization along the [111] direction, just two pairs of Weyl points at the ${E}_{F}$ and ${T}_{c}=325$ K. The strong FM is attributed to the strong FM Ni $3d$-Eu $4f$-Os $5d$ coupling induced by the substitution of half of Ba atoms with Eu atoms in double perovskite ${\mathrm{Ba}}_{2}{\mathrm{NiOsO}}_{6}$. Moreover, the momentum separation of one Weyl point pair is large, thus giving rise to not only a long (001) surface Fermi arc but also large anomalous Hall conductivity. Intriguingly, as a unique physical result of bulk ${\mathrm{BaEuNiOsO}}_{6}$ being a FM WSM, the (111) ${\mathrm{BaEuNiOsO}}_{6}$ monolayer (ML) superlattice $({\mathrm{BaEuNiOsO}}_{6})/{({\mathrm{BaTiO}}_{3})}_{10}$ is found to be a high-temperature (${T}_{c}=210$ K) Chern insulator with a large band gap of $\ensuremath{\sim}90$ meV. Therefore, cubic double perovskite ${\mathrm{BaEuNiOsO}}_{6}$ will provide a superior high-temperature material platform for exploring fundamental physics of Weyl fermions, and its (111) ML superlattices will offer a high-temperature magnetic topological insulator for studying exotic quantum phenomena such as the quantum anomalous Hall effect.

The Fine-Structure Constant as a Ruler for the Band-Edge Light Absorption Strength of Bulk and Quantum-Confined Semiconductors.

Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of low-dimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals mπα (m = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of mπα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to mπα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.

Tunable magnetic anisotropy in multiferroic oxides

Weyl semimetals (WSMs) have recently stimulated intensive interest because they exhibit fascinating physical properties and also promise exciting technological applications. So far, however, the few conrmed magnetic WSMs generally have a large number of Weyl points either located away from the Fermi level (EF ) or shrouded by nontopological Fermi surface pockets. Based on first principles density functional theory calculations, we establish cubic double perovskite BaEuNiOsO6 to be a high Curie temperature (Tc) ferromagnetic WSM with magnetization along the [111] direction, just two pairs of Weyl points at the EF and Tc = 325 K. The strong ferromagnetism is attributed to the strong ferromagnetic Ni 3d-Eu 4f-Os 5d coupling induced by the substitution of half of Ba atoms with Eu atoms in double perovskite Ba2NiOsO6. Moreover, the momentum separation of one Weyl point pair is large, thus giving rise to not only a long (001) surface Fermi arc but also large anomalous Hall conductivity. Intriguingly, as a unique physical result of a ferromagnetic WSM, the (111) BaEuNiOsO6 monolayer superlattice (BaEuNiOsO6)/(BaTiO3)10, being its (111) quantum-well structure, is found to be a high temperature (Tc = 210 K) Chern insulator with a large band gap of 190 meV. Therefore, cubic double perovskite BaEuNiOsO6 will provide a superior high temperature material plotform for exploring fundamental physics of Weyl fermions and its (111) monolayer superlattices will offer a high temperature magnetic topological insulator for studying exotic quantum phenomena such as quantum anomalous Hall effect.

Individually Silica‐Embedded Gold Nanorod Superlattice for High Thermal and Solvent Stability and Recyclable SERS Application

AbstractA facile method to fabricate 2D hexagonal monolayer superlattices of gold nanorods (GNRs) individually embedded in silica matrix on a substrate is developed. In this method, the cetyltrimethylammonium bromide (CTAB) bilayer‐coated GNRs are self‐assembled into a hexagonally packed monolayer superlattice on the substrate by slow evaporation in the presence of silica precursors in the solution at a highly acidic condition. The GNR superlattices fabricated by this method show excellent structural stability at high temperature as high as 500 °C and in solvents of a wide range of polarities including water, ethanol, toluene, and cyclohexane. The structural stability makes the GNR superlattices highly reusable surface‐enhanced Raman scattering (SERS)‐active substrates for sensitive molecular detection, well‐maintaining the SERS intensity over 10 cycles.

Self-Assembled Nanoparticle Supertubes as Robust Platform for Revealing Long-Term, Multiscale Lithiation Evolution

Summary Herein, free-standing supertubes, composed of a single layer of close-packed carbon-coated nanoparticles, are fabricated by a confined-epitaxial-assembly strategy. Benefiting from the tubular geometry, monolayer superlattice structure, and uniform and conformal carbon coating, such free-standing supertubes promise high electrochemical performance while simultaneously serving as a robust platform for reliably elucidating the structure-performance relationship of lithium-ion batteries (LIBs). As a model, Fe3O4 supertubes, when used as LIB anodes, can deliver a capacity of ∼800 mAh g−1 after 500 cycles at 5 A g−1, outperforming most Fe3O4-based materials reported previously. More importantly, the structural evolution of Fe3O4 supertubes is revealed at meso-/nano-/atomic scales simultaneously upon lithiation and delithiation, which correlates well with the battery's capacity reactivation, stabilization, and degradation behaviors during the course of 500 cycles.

In Situ Atomic Force Microscopy of the Reconfiguration of On‐Surface Self‐Assembled DNA‐Nanoparticle Superlattices

AbstractThe ability to dynamically reconfigure superlattices in response to external stimuli is an intriguing prospect for programmable DNA‐guided nanoparticle (NP) assemblies, which promises the realization of “smart” materials with dynamically adjustable interparticle spacing and real‐time tunable properties. Existing in situ probes of reconfiguration processes have been limited mostly to reciprocal space methods, which can follow larger ordered ensembles but do not provide access to real‐space pathways and dynamics. Here, in situ atomic force microscopy is used to investigate DNA‐linked NP assemblies and their response to external stimuli, specifically the contraction and expansion of on‐surface self‐assembled monolayer superlattices upon reversible DNA condensation induced by ethanol. In situ microscopy allows observation and quantification of key processes in solution, e.g., lattice parameter changes, defects, and monomer displacements in small groups of NPs. The analysis of imaging data uncovers important boundary conditions due to DNA bonding of NP superlattices to a substrate. Tension in the NP–substrate DNA bonds, which can elastically extend, break, and re‐form during contraction/expansion cycles, counteracts the changes in lattice parameter and causes hysteresis in the response of the system. The results provide insight into the behavior of supported DNA‐linked NP superlattices and establish a foundation for designing and probing tunable nanocrystal‐based materials in solution.

Coherent strained superlattices

Nanomaterials Two-dimensional superlattices represent the atomic-thickness limit of heterostructures that enable technologies such as strain-engineered multiferroics and quantum-cascade lasers. Xie et al. were able to produce monolayer superlattices of transition metal dichalcogenides (WS2 and WSe2) with full lattice coherence, despite a 4% lattice mismatch. They used a modulated metal-organic chemical vapor deposition process that precisely controlled each precursor. Furthermore, the authors could strain-engineer the optical properties of the superlattices to observe out-of-plane rippling. Science , this issue p. [1131][1] [1]: /lookup/doi/10.1126/science.aao5360

Coherent, atomically thin transition-metal dichalcogenide superlattices with engineered strain.

Epitaxy forms the basis of modern electronics and optoelectronics. We report coherent atomically thin superlattices in which different transition metal dichalcogenide monolayers-despite large lattice mismatches-are repeated and laterally integrated without dislocations within the monolayer plane. Grown by an omnidirectional epitaxy, these superlattices display fully matched lattice constants across heterointerfaces while maintaining an isotropic lattice structure and triangular symmetry. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 millielectron volts. We present theoretical models to explain this coherent growth and the energetic interplay governing the ripple formation in these strained monolayers. Such coherent superlattices provide building blocks with targeted functionalities at the atomically thin limit.

Mesoscale Graphene-like Honeycomb Mono- and Multilayers Constructed via Self-Assembly of Coclusters

Honeycomb structure endows graphene with extraordinary properties. But could a honeycomb monolayer superlattice also be generated via self-assembly of colloids or nanoparticles? Here we report the construction of mono- and multilayer molecular films with honeycomb structure that can be regarded as self-assembled artificial graphene (SAAG). We construct fan-shaped molecular building blocks by covalently connecting two kinds of clusters, one polyoxometalate and four polyhedral oligomeric silsesquioxanes. The precise shape control enables these complex molecules to self-assemble into a monolayer 2D honeycomb superlattice that mirrors that of graphene but on the mesoscale. The self-assembly of the SAAG was also reproduced via coarse-grained molecular simulations of a fan-shaped building block. It revealed a hierarchical process and the key role of intermediate states in determining the honeycomb structure. Experimental images also show a diversity of bi- and trilayer stacking modes. The successful creation of SAAG and its stacks opens up prospects for the preparation of novel self-assembled nanomaterials with unique properties.

Tubular Monolayer Superlattices of Hollow Mn3O4 Nanocrystals and Their Oxygen Reduction Activity.

Self-assembled nanocrystal (NC) superlattices are emerging as an important class of materials with rationally modulated properties. Engineering the nanoscale structure of constituent building blocks as well as the mesoscale morphology of NC superlattices is a crucial step in widening their range of applications. Here, we report a template-assisted epitaxial assembly strategy, enabling growth of freestanding, carbon-coated tubular monolayer superlattices (TMSLs). Specifically, we design and construct TMSLs of hollow Mn3O4 NCs (h-Mn3O4-TMSLs) by exploiting structural evolution of MnO NCs. The tubular superlattices obtained possess a number of unique and advantageous structural features unavailable in conventional NC superlattices, rendering them particularly attractive for energy conversion applications. We demonstrate this by employing h-Mn3O4-TMSLs as electrocatalysts for oxygen reduction, the catalytic performance of which is comparable to that of state-of-the-art Pt/C catalysts and superior to that of most manganese oxide-based catalysts reported.

Trimetallic PtCoFe Alloy Monolayer Superlattices as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts.

A synthesis strategy for the preparation of trimetallic PtCoFe alloy nanoparticle superlattices is reported. Trimetallic PtCoFe alloy monolayer array of nanoparticle superlattices with a large density of high index facets and platinum-rich surface are successfully prepared by coreduction of metal precursors in formamide solvent. The concentration of cetyl trimethyl ammonium bromide plays a vital role for the formation of a monolayer array of nanoparticle superlattices, while the size of nanoparticles is determined by NaI. The prepared monolayer array of nanoparticle superlattices is the superior catalyst for oxygen reduction reaction as well as for ethanol oxidation owing to their specific structural and compositional characteristics.

Controlled Synthesis and Flexible Self-Assembly of Monodisperse Au@Semiconductor Core/Shell Hetero-Nanocrystals into Diverse Superstructures

Monodisperse nonepitaxially grown Au@MX (MX = Ag2S, ZnS, or CdS) core/shell hetero-nanocrystals (HNCs) with controlled crystallinity, composition, anisotropically shaped Au cores but isotropic overall morphologies were successfully prepared. By using C18 acid/alkali co-capping surface ligands, the as-prepared Au@MX HNCs can self-assemble into large-scale two-dimensional monolayer and three-dimensional multilayer superlattices (SLs) on both flexible (graphene) and rigid (Si wafer or glass) substrates. Moreover, unprecedented binary nanocrystal SLs can be achieved through synergistic self-assembly of Au@MX HNCs together with Au NCs (or CdS NCs) into different intricate patterns. Experimental evidence and finite-element method theoretical simulations indicate that the Au@CdS HNC SL film taking up a closest-packing mode has a rate of photocurrent generation much higher than that of its disordered–assembled counterpart, probably because of the enhanced plasmon–exciton coupling permitted by the large-scale orde...

Modeling of Electronic Structure of a Monolayer Superlattice SiC/GeC

Modeling of Electronic Structure of a Monolayer Superlattice SiC/GeC