Superlattice Formation Research Articles

Trimerization-induced zone-folded phonons in 1T′-TaTe2

The trimerized state is an intriguing structure in the superlattice with three unit cells distorted periodically. Here, we report on two kinds of featured phonon modes induced by the trimerization transition in 1T′-TaTe2. One is the almost temperature-independent zone-folded mode, stemming from the collective excitation due to the superlattice formation and the other is the normal mode with the step-like temperature-dependent intensity and frequency, which is the characteristic of the first-order transition of the trimerization. Furthermore, the trimerization can be totally suppressed when Mo-dopant content is less than 6 at. %. Our work not only unveils the featured phonon modes in trimerized system but also provides a pathway to identify the multimerization transition in solids by Raman spectroscopy.

Nanoscale Optical Conductivity Imaging of Double-Moiré Twisted Bilayer Graphene.

A central paradigm of moiré materials relies on the formation of superlattices that yield enlarged effective crystal unit cells. While a critical consequence of this phenomenon is the celebrated flat electronic bands that foster strong interaction effects, the presence of superlattices has further implications. Here we explore the advantages of moiré superlattices in twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (hBN) for passively enhancing optical conductivity in the low-energy regime. To probe the local optical response of TBG/hBN double-moiré lattices, we use infrared (IR) nano-imaging in conjunction with nanocurrent imaging to examine local optical conductivity over a wide range of TBG twist angles. We show that interband transitions associated with the multiple moiré flat and dispersive bands produce tunable transparent IR responses even at finite carrier densities, which is in stark contrast to the previously limited metallic near transparency observed only in undoped pristine graphene.

Hierarchical Nanostructures Constructed by Soft Epitaxial Self-Assembly of Organic-Inorganic Hybrid Giant Amphiphiles.

2D nanomaterials with ångström-scale thicknesses offer a unique platform for confining molecules at an unprecedentedly small scale, presenting novel opportunities for modulating material properties and probing microscopic phenomena. In this study, mesogen-tethered polyhedral oligomeric silsesquioxane (POSS) amphiphiles with varying numbers of mesogenic tails to systematically influence molecular self-assembly and the architecture of the ensuing supramolecular structures, are synthesized. These organic-inorganic hybrid amphiphiles facilitate precise spatial arrangement and directional alignment of the primary molecular units within highly ordered supramolecular structures. The correlation between molecular design and the formation of superlattices through comprehensive structural analyses, incorporating molecular thermodynamics and kinetics, is explored. The distinct intermolecular interactions of the POSS core and the mesogenic tails drive the preferential formation of a 2D inorganic sublattice while simultaneously guiding the hierarchical assembly of organic lamellae via soft epitaxy. The findings reveal the intricate balance between shape, size, and interaction strengths of the inorganic and organic components, and how these factors collectively influence the structural hierarchy of the superstructures, which consist of multiple sublattices. By controlling this unique molecular behavior, it is possible to modulate or maximize the anisotropy of optical, mechanical, and electrical properties at the sub-nanometer scale for nanotechnology applications.

Synthesis and Modular Engineering of Porous Graphene Nanoarchitectures

Bottom-up nanoarchitectonics has demonstrated the capability to control structural parameters of nanomaterials with atomic precision. The surface-assisted synthesis of graphene-based one-dimensional nanostructures à la carte distinctly illustrates the power of this concept. However, despite impressive advances in the synthesis of 1D homostructures, advancing in structural complexity faces major challenges. However, despite impressive advances in the synthesis of 1D homostructures, advancing in structural complexity faces major challenges. The functionalization of edges in nanorib-bons, an effective strategy to tailor their electronic properties and chemical interactions, is a clear example. The concept of inserting the desired functional groups or dopants in the molecular precursor often fails due to their lack of stability during the reaction path. The fabrication of heterostructures is a second example, where the challenge lies on the control of the size and distribution of their com-ponents. A third one is extending the on-surface strategy to two-dimensional structures, where examples of long-range ordered nanoarchitectures are very limited.In this talk I will present different strategies that we have developed to overcome each one of this challenges. I will show introducing our method to synthesize a 2D nanoporous graphene, where the long-range order is achieved by the sequential growth of 1D building blocks and their posterior coupling [1]. I will continue presenting two modular strategies that lead to more complex nanoporous structures. The first consist on the molecular engineering of the coupling bridges. I will show an example where the introduction of phenyl groups in the bridges brings tunability of the quantum electronic coupling and the corresponding in-plane electronic anisotropy [2]. In a final example I will focus on intercalating different 1D ribbon components in periodic arrays for the formation of lateral heterostructure superlattices where the junction is defined with atomic precision [3]. One of the components being a N-doped ribbon, the heterostructure can also be seen as a nanoporous graphene sheet bearing N-doped pores.

SI/GE Quantum Dot Channel FETs for Multi-Bit Computing

This paper presents quantum dot channel (QDC) FETs in quantum wire and coupled quantum dot configurations for cryogenic operation with multi-state operation. It also describes gate-all-around (GAA) quantum dot channel (QDC) FETs that exhibit potential multi-state characteristics at room temperature. FETs with cladded Si and Ge quantum dot layers as a transport channel have been fabricated. The formation of a quantum dot superlattice (QDSL) when SiOx-cladded Si and/or GeOx-cladded Ge quantum dots (QD) are assembled results in mini-energy sub-bands in the conduction and valence band. The intra-mini-energy band transitions results in significant changes in the drain current when gate and/or drain voltages are varied. This novel feature provides a pathway for 16-/32-state logic in CMOS-X configuration. The gate-defined Si quantum dot FETs, comprising of tunnel barrier coupled, have been reported for quantum computing at cryogenic temperatures.

Local Confinement Effects of Block Copolymers Driving Self-Assembly of Nanoparticles into Discrete Pancake-Shaped Superlattices.

The utilization of polymer conformations to construct a variety of superlattices is a common method within the field. However, this technique often results in only long-range ordering rather than the formation of distinct superlattices. In this study, a well-organized array of discrete pancake-shaped superlattices (DPSs) is successfully obtained through the utilization of air-liquid interface self-assembly, facilitated by the confined environment created by a block copolymer. It is crucial to note that both the self-assembly behavior and resulting morphologies of the DPSs can be precisely tuned by adjusting several experimental parameters, most notably the concentration and molecular architecture of the block copolymers. Furthermore, this work provides valuable insights into the formation processes and mechanisms underpinning the DPSs. The approach described here is both straightforward and efficacious, establishing a strong foundation for subsequent research and the development of non-close-packed superlattice structures.

NMR study of charge density wave phase in the kagome metal RbV3Sb5

The mechanism of charge-density-wave (CDW) phase in one-dimensional lattices can be well described in theory. Whether similar mechanism works in two-dimensional systems is still a mystery. In this work, we employed 51V solid-state nuclear magnetic resonance (NMR) spectroscopy combined with first-principles computations to probe atomic structure and electronic properties at V sites in a quasi-two-dimensional crystal RbV3Sb5. Clear evidence of the CDW phase transition was observed from the temperature-dependent 51V NMR measurements. The sharp transition of 51V Knight shifts and electric field gradients (EFG) across the CDW transition supports the view that energy gap is formed by the nesting of partial Fermi surface in accompany with the superlattice formation. The calculated EFG parameters were also compared with our experiments, and the results suggest that the charge modulation of the CDW order is inverse Star-of-David pattern.

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene

Understanding electron-phonon interactions is fundamentally important and has crucial implications for device applications. However, in twisted bilayer graphene near the magic angle, this understanding is currently lacking. Here, we study electron-phonon coupling using time- and frequency-resolved photovoltage measurements as direct and complementary probes of phonon-mediated hot-electron cooling. We find a remarkable speedup in cooling of twisted bilayer graphene near the magic angle: The cooling time is a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. Our experimental and theoretical analysis indicates that this ultrafast cooling is a combined effect of superlattice formation with low-energy moiré phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone. This enables efficient electron-phonon Umklapp scattering that overcomes electron-phonon momentum mismatch. These results establish twist angle as an effective way to control energy relaxation and electronic heat flow.

Possibility and Optimized Conditions for Spontaneous Superlattice Formation in Stabilized Zirconia Thin Films Grown Under In Situ Magnetic Field

Possibility and Optimized Conditions for Spontaneous Superlattice Formation in Stabilized Zirconia Thin Films Grown Under In Situ Magnetic Field

Interlayer engineering of Fe3GeTe2: From 3D superlattice to 2D monolayer

The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546, 265-269 (2017) and B. Huang et al., Nature 546, 270-273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14, 408-419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe3GeTe2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature (TC), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

Long live(d) CsPbBr3 superlattices: colloidal atomic layer deposition for structural stability.

Superlattice formation afforded by metal halide perovskite nanocrystals has been a phenomenon of interest due to the high structural order induced in these self-assemblies, an order that is influenced by the surface chemistry and particle morphology of the starting building block material. In this work, we report on the formation of superlattices from aluminum oxide shelled CsPbBr3 perovskite nanocrystals where the oxide shell is grown by colloidal atomic layer deposition. We demonstrate that the structural stability of these superlattices is preserved over 25 days in an inert atmosphere and that colloidal atomic layer deposition on colloidal perovskite nanocrystals yields structural protection and an enhancement in photoluminescence quantum yields and radiative lifetimes as opposed to gas phase atomic layer deposition on pre-assembled superlattices or excess capping group addition. Structural analyses found that shelling resulted in smaller nanocrystals that form uniform supercrystals. These effects are in addition to the increasingly static capping group chemistry initiated where oleic acid is installed as a capping ligand directly on aluminum oxide. Together, these factors lead to fundamental observations that may influence future superlattice assembly design.

Polaron-induced changes in moiré exciton propagation in twisted van der Waals heterostructures.

Twisted transition metal dichalcogenides (TMDs) present an intriguing platform for exploring excitons and their transport properties. By introducing a twist angle, a moiré superlattice forms, providing a spatially dependent exciton energy landscape. Based on a microscopic many-particle theory, we investigate in this work polaron-induced changes in exciton transport properties in the exemplary MoSe2/WSe2 heterostructure. We demonstrate that polaron formation and the associated enhancement of the moiré exciton mass lead to a significant band flattening. As a result, the moiré inter-cell tunneling and the propagation velocity undergo noticeable temperature and twist-angle dependent changes. We predict a reduction of the hopping strength ranging from 80% at a twist angle of 1° to 30% at 3° at room temperature. The provided microscopic insights into the spatio-temporal exciton dynamics in presence of a moiré potential further expand the possibilities to tune charge and energy transport in 2D materials.

Effect of number ratio and size ratio on the formation of binary superlattices assembled by two-sized polymer-tethered spherical nanoparticles

Binary superlattices (BNSLs) with unique configurations are of great interest, attributing to the interaction between two kinds of nanoparticles, providing potential applications in sensing, electronic and optical fields. Here, polystyrene...

Design of high-entropy films as ultra-violet light reflector

This study proposes the design and implementation of high-entropy alloy (HEA) thin films as materials for the reflector of deep ultraviolet light-emitting devices and marks the first application of HEA in optical field. In the material design phase, an Al-Co-Zn-Ni alloy is designed by considering the reflectivity, phase stability and binary formation energy. Reverse Monte Carlo calculation is employed to establish stable atomic structure. Frequency-dependent dielectric equations are derived from first principles calculations, with dielectric constants incorporating contributions from ions and electrons and combining classical Drude and Lorentz models. The dielectric equations yield optical constants that are used to calculate the reflectance. In the experimental phase, we systematically vary the aluminum content and prepare Al-Co-Zn-Ni thin films ranging from pure aluminum to equimolar compositions. In the deep ultraviolet region, the reflectance of thermally treated Al65Zn12Co12Ni11 and Al50Zn16Co17Ni17 thin films is found superior to that of gold by three times. Structural and crystalline changes with varying aluminum content are found, as well as intriguing phase separation and superlattice formation that persist until equimolar compositions. This unique nanoscale structural variation causes the Young's modulus and hardness of Al-Co-Zn-Ni thin films to initially increase and then decrease with decreasing aluminum content. There is a significant increase in the corrosion potential difference when reducing Al contents in Al-Co-Zn-Ni, indicating improved passivation effects as the equimolar composition is approached. By modulating the aluminum content, the nanoscale structure of Al-Co-Zn-Ni can be adjusted, thereby improving its optical, mechanical, corrosion resistance, and thermal stability properties. This demonstrates the high versatility of this material for practical optical applications.

In Situ Real-Time Observation of Formation and Self-Assembly of Perovskite Nanocrystals at High Temperature.

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have received much attention due to their outstanding optical and electronic properties, but the underlying growth mechanism remains elusive due to their rapid formation process. Here, we report an in situ real-time study of the growth of Cs4PbBr6 NCs under practical synthesis conditions in a custom-made reactor. Through the synchrotron-based small-angle X-ray scattering technique, we find that the formation of Cs4PbBr6 NCs is accomplished in three steps: the fast nucleation process accompanied by self-focusing growth, the subsequent diffusion-limited Ostwald ripening, and the self-assembly of NCs into the face-centered cubic (fcc) superlattices at high temperature and the termination of growth. The simultaneously collected wide-angle X-ray scattering signals further corroborate the three-step growth model. The influence of superlattice formation is also elucidated, which improves the uniformity of the final NCs.

Evidence of Xe-incorporation in the bubble superlattice in irradiated U-Mo fuel

For years, researchers have reported competing ideas on the formation of fission gas bubble superlattices in metallic fuels, which was postulated to be comprised of elemental xenon (Xe) atoms. However, the chemical and physical arrangement of these elements within this gas bubble superlattice had not been verified. In this contribution, Xe was chemically profiled using atomic resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT) techniques in irradiated uranium-molybdenum (U-Mo) fuels. These complementary techniques provide conclusive evidence of Xe presence in the bubble superlattice up to fission densities of 4.5 × 1021 fissions/cm3 and give a quantitative assessment of Xe content within the bubble superlattice. Based on the results of simultaneous imaging and spectroscopy using STEM, the chemical composition of the Xe bubble superlattice was measured and found to be up to 8±1.3 atomic %. APT data complemented STEM findings on Xe distribution in irradiated U-Mo fuel sample. This study provides conclusive evidence that Xe is not only present within an atomically arranged bubble superlattice but provides fundamental insight into the physical state of Xe in low enriched U-Mo monolithic fuel.

A hybrid mechanism of 3D optical trapping of particles immersed in a cold buffer gas

A novel mechanism of the formation of a deep optical superlattice (OSL) for three-dimensional (3D) trapping of resonant impurity atoms immersed in a transparent cold buffer gas is proposed and investigated theoretically. This mechanism is associated with the joint action of two factors of different physical nature: rectification effect of the gradient force of light pressure and effect of the light induced drift (LID) of atoms (a consequence of the difference in gas-kinetic cross sections of excited and unexcited atoms). Such a situation can be realised when a gas mixture is irradiated by a bichromatic combined light beam which is a special coaxial superposition of cosine-Gaussian optical beams. Here, the RcGF creates a periodic 1D array of deep potential wells (OSL) along the beam axes, and an effective force, causing LID, pulls resonant particles into the combined light beam and provides their transverse (with respect to the beam axes) confinement in OSL. As a result, there can occur a giant accumulation of impurity particles in the OSL cells, accompanied by a strong spatial particle localization far away from the walls of the reservoir, in which the gas mixture is located.

Computer simulation of spontaneous superlattice formation process by dynamic aurora pulsed laser deposition using phase field method

Computer simulation of spontaneous superlattice formation process by dynamic aurora pulsed laser deposition using phase field method

In-plane anisotropy of graphene by strong interlayer interactions with van der Waals epitaxially grown MoO3.

van der Waals (vdW) epitaxy can be used to grow epilayers with different symmetries on graphene, thereby imparting unprecedented properties in graphene owing to formation of anisotropic superlattices and strong interlayer interactions. Here, we report in-plane anisotropy in graphene by vdW epitaxially grown molybdenum trioxide layers with an elongated superlattice. The grown molybdenum trioxide layers led to high p-doping of the underlying graphene up to p = 1.94 × 1013 cm-2 regardless of the thickness of molybdenum trioxide, maintaining a high carrier mobility of 8155 cm2 V-1 s-1. Molybdenum trioxide-induced compressive strain in graphene increased up to -0.6% with increasing molybdenum trioxide thickness. The asymmetrical band distortion of molybdenum trioxide-deposited graphene at the Fermi level led to in-plane electrical anisotropy with a high conductance ratio of 1.43 owing to the strong interlayer interaction of molybdenum trioxide-graphene. Our study presents a symmetry engineering method to induce anisotropy in symmetric two-dimensional (2D) materials via the formation of asymmetric superlattices with epitaxially grown 2D layers.

Interfacial Molecular Compatibility for Programming Organic-Metal Oxide Superlattices.

Artificially programming a sequence of organic-metal oxide multilayers (superlattices) by using atomic layer deposition (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here, we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy, in situ quartz crystal microbalance measurements, and Fourier-transformed infrared spectroscopy. These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the -OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive -OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.