Artificial Solids Research Articles

Layered hybrid superlattices as designable quantum solids.

Crystalline solids typically show robust long-range structural ordering, vital for their remarkable electronic properties and use in functional electronics, albeit with limited customization space. By contrast, synthetic molecular systems provide highly tunable structural topologies and versatile functionalities but are often too delicate for scalable electronic integration. Combining these two systems could harness the strengths of both, yet realizing this integration is challenging owing to distinct chemical bonding structures and processing conditions. Two-dimensional atomic crystals comprise crystalline atomic layers separated by non-bonding van der Waals gaps, allowing diverse atomic or molecular intercalants to be inserted without disrupting existing covalent bonds. This enables the creation of a diverse set of layered hybrid superlattices (LHSLs) composed of alternating crystalline atomic layers of variable electronic properties and self-assembled atomic or molecular interlayers featuring customizable chemical compositions and structural motifs. Here we outline strategies to prepare LHSLs and discuss emergent properties. With the versatile molecular design strategies and modular assembly processes, LHSLs offer vast flexibility for weaving distinct chemical constituents and quantum properties into monolithic artificial solids with a designable three-dimensional potential landscape. This opens unprecedented opportunities to tailor charge correlations, quantum properties and topological phases, thereby defining a rich material platform for advancing quantum information science.

Modular assembly of a library of hybrid superlattices and artificial quantum solids

Modular assembly of a library of hybrid superlattices and artificial quantum solids

Halide Perovskite Artificial Solids as a New Platform to Simulate Collective Phenomena in Doped Mott Insulators.

The development of quantum simulators, artificial platforms where the predictions of many-body theories of correlated quantum materials can be tested in a controllable and tunable way, is one of the main challenges of condensed matter physics. Here we introduce artificial lattices made of lead halide perovskite nanocubes as a new platform to simulate and investigate the physics of correlated quantum materials. We demonstrate that optical injection of quantum confined excitons in this system realizes the two main features that ubiquitously pervade the phase diagram of many quantum materials: collective phenomena, in which long-range orders emerge from incoherent fluctuations, and the excitonic Mott transition, which has one-to-one correspondence with the insulator-to-metal transition described by the repulsive Hubbard model in a magnetic field. Our results demonstrate that time-resolved experiments provide a quantum simulator that is able to span a parameter range relevant for a broad class of phenomena, such as superconductivity and charge-density waves.

Quantum simulations with cold atoms: Fundamentals, advances, and outlook

Since the first proposal of quantum simulation by Richard Feynman in 1982, multiple platforms have been explored to probe the behaviour of subatomic particles, exploiting the intrinsic nature of quantum systems. As one of the most promising candidates to realise large-scale simulation, quantum simulators with ultracold gases have attracted unprecedented attention. Featuring novel detection possibilities, a high degree of controllability and the extreme physical parameter regimes that can be reached in these ‘artificial solids’, quantum simulation with ultracold atoms has progressively matured to the point that it can be used to study exotic quantum phenomena. This review presents the theoretical fundamentals and recent advances in related technology and offers comment on future directions.

Temperature-Controlled Reversible Formation and Phase Transformation of 3D Nanocrystal Superlattices Through In Situ Small-Angle X-ray Scattering

For decades, the spontaneous organization of nanocrystals into superlattices has captivated the scientific community. However, achieving direct control over the formation of the superlattice and its phase transformations has proven to be a grand challenge, often resulting in the generation of multiple symmetries under the same experimental conditions. Here, we achieve direct control over the formation of the superlattice and its phase transformations by modulating the thermal energy of a nanocrystal dispersion without relying on solvent evaporation. We follow the temperature-dependent dynamics of the self-assembly process using synchrotron-based small-angle X-ray scattering. When cooled below -24.5 °C, lead sulfide nanocrystals form micrometer-sized three-dimensional phase-pure body-centered cubic superlattices. When cooled below -35.1 °C, these superlattices undergo a collective diffusionless phase transformation that yields denser body-centered tetragonal phases. These structural changes can be reversed by increasing the temperature of the dispersion and may lead to the direct modulation of the optical properties of these artificial solids.

Appearance, study and a possible correction for boron: a phenomenon in ultra-soft X-ray measurements using a synthetic multilayer crystal and the EPMA

Abstract. When measuring boron (B) in tourmalines calibrated with schorl, no deviations in the peak intensities could be detected with a proven analysis protocol and using the Mo/B4C multilayer crystal LDEB. It is only when boron is detected in natural and experimental samples, some with significantly lower boron concentrations than in tourmalines, that irregularities in the analysis become visible. This phenomenon is known but has not been analytically investigated so far. Using four natural and artificial solids with boron concentrations from 0.035 wt %–3.14 wt % B, an apparent linear trend line was drawn. The intersect of that trend line with the y axis represents the detection limit of boron, which is of about 0.25 wt % B. The discrepancy between the apparent and the true value trend lines at boron concentrations of 0.25 wt %–2.1 wt % B shows that a correction is necessary. At higher boron concentrations, the discrepancy between the apparent and true value trend lines is within the uncertainty of electron microprobe analysis (EPMA) and disappears completely up to boron concentrations of about 3 wt %.

Electron transport through supercrystals of atomically precise gold nanoclusters: a thermal bi-stability effect.

Nanoparticles (NPs) may behave like atoms or molecules in the self-assembly into artificial solids with stimuli-responsive properties. However, the functionality engineering of nanoparticle-assembled solids is still far behind the aesthetic approaches for molecules, with a major problem arising from the lack of atomic-precision in the NPs, which leads to incoherence in superlattices. Here we exploit coherent superlattices (or supercrystals) that are assembled from atomically precise Au103S2(SR)41 NPs (core dia. = 1.6 nm, SR = thiolate) for controlling the charge transport properties with atomic-level structural insights. The resolved interparticle ligand packing in Au103S2(SR)41-assembled solids reveals the mechanism behind the thermally-induced sharp transition in charge transport through the macroscopic crystal. Specifically, the response to temperature induces the conformational change to the R groups of surface ligands, as revealed by variable temperature X-ray crystallography with atomic resolution. Overall, this approach leads to an atomic-level correlation between the interparticle structure and a bi-stability functionality of self-assembled supercrystals, and the strategy may enable control over such materials with other novel functionalities.

Photoresponsive Covalent Organic Frameworks with Diarylethene Switch for Tunable Singlet Oxygen Generation

Incorporation of molecular switches with light, heat, and electricity responsibility into artificial solids has been developed as a successful strategy to construct stimuli-responsive functional materials. However, precise manipulation of their molecular geometries and electronic structures to control the properties of macroscopic materials still remains a fundamental challenge. Herein, a photoresponsive covalent organic framework (o-COF) with the square lattice was fabricated from the dynamic covalent chemistry reaction of ring-open dithienylethene–dialdehyde with 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (H2TAPP). UV irradiation of the dithienylethene-based units in o-COF afforded its reversible photoisomer (c-COF) in the ring-closed form. In addition to a range of diffraction, microscopic, and gas physical sorption characterizations, spectroscopic investigations with the help of theoretical simulations revealed different photocatalytic activities toward the evolution of singlet oxygen and corresponding photocatalytic oxidation of amines due to the different energy transfer pathways from the porphyrin unit to BBTP photoisomers in these two COFs. Most interestingly, such different photocatalytic behaviors for two COFs could be easily tuned in a reversible manner by adjusting the ring-closed/open form of dithienylethene units by means of UV and visible light.

Application of Dislocation Theory to Minimize Defects in Artificial Solids Built with Nanocrystal Building Blocks.

ConspectusOriented atomic attachment of colloidal inorganic nanocrystals represents a powerful synthetic method for preparing complex inorganic superstructures. Examples include fusion of nanocrystals into dimer and superlattice structures. If the attachment were perfect throughout, then the resulting materials would have single crystal-like alignment of the individual nanocrystals' atomic lattices. While individual colloidal nanocrystals typically are free of many defects, there are a multitude of pathways that can generate defects upon nanocrystal attachment. These attachment generated defects are typically undesirable, and thus developing strategies to favor defect-free attachment or heal defective interfaces are essential. There may also be some cases where attachment-derived defects are desirable. In this Account, we summarize our current understanding of how these defects arise, in order to offer guidance to those who are designing nanocrystal derived solids.The small size of inorganic nanocrystals means short diffusion lengths to the surface, which favor the formation of nanocrystal building blocks with pristine atomic structures. Upon attachment, however, there are numerous pathways that can lead to atomic scale defects, and bulk crystal dislocation theory provides an invaluable guide to understanding these phenomena. As an example, an atomic step edge can be incorporated into the interface leading to an extra half-plane of atoms, known as an edge dislocation. These dislocations can be well described by the Burgers vector description of dislocations, which geometrically identifies planes in which a dislocation can move. Our in situ measurements have verified that bulk dislocation theory predictions for 1D defects hold true at few-nanometer length scales in PbTe and CdSe nanocrystal interfaces. Ultimately, the applicability of dislocation theory to nanocrystal attachment enables the predictive design of attachment to prevent or facilitate healing of defects upon nanocrystal attachment. We applied similar logic to understand formation of planar (2D) defects such as stacking faults upon nanocrystal attachment. Again concepts from bulk crystal defect crystallography can identify attachment pathways that can prevent or deterministically form planar defects upon nanocrystal attachment. The concepts we discuss work well for identifying favorable attachment geometries for nanocrystal pairs; however it is currently unclear how to translate these ideas to near-simultaneous multiparticle attachment. Geometric frustration, which prevents nanocrystal rotation, and yet-to-be considered defect generation pathways unique to multiparticle attachment complicate defect-free superlattice attachment. New imaging methods now allow for the direct observation of local attachment trajectories and may enable improved understanding of such multiparticle phenomena. With further refinement, a unified framework for understanding and ultimately eliminating structural defects in fused nanocrystal superstructures may well be achievable in coming years.

Twistronics: A Recent Avenue in van der Waals Heterostructures

Two-dimensional (2D) materials, the thinnest form of materials to ever occur in nature, have the potential to dramatically alter and revolutionize our material world. After the discovery of graphene, the most prominent representative of this class of materials, many other 2D crystals have been identified. Even if individual 2D materials own various interesting and unexpected properties, the stacking of such layers leads to ‘artificial vdW solids’ called van der Waals heterostructures (vdW HSs) that result in the emergence of new states of matter with novel functionalities. The vdW HSs not only depend on the combination of different 2D crystal but also on their rotational alignment opening the avenue for a new field called twistronics. Coupling between the two layers depends on the stacking angle, which can be used as an external degree of freedom to tune their optical and electronic properties. Apart from excitonic ground states, 2D transition metal dichalcogenides (TMDs) and their heterostructures offer an excellent platform to explore fascinating higher-order excitations such as trion, biexciton, interlayer exciton, hybrid exciton, moiré exciton, and so on. The emergence of these higher-order excitations mostly depends on the symmetry, temperature, and the band alignment of the heterobilayer systems.

Spectroscopic ellipsometry of monolayered CdS nanoparticles assembled by layer-by-layer method

Semiconductor nanoparticles (NPs) are attractive for use as the constituent blocks of artificial solids. Furthermore, the layer-by-layer (LbL) method allows one to accurately arrange and stack NPs and is being explored widely for the fundamental research and development of artificial solids and devices based on NPs. In this study, we determined the absolute values of the dielectric function of monolayered semiconductor NPs deposited by the LbL method. The aim was to measure the structural parameters, such as the thickness, NP volume density, and NP number density, of the monolayered films composed of NPs. A novel fully optical method based on spectroscopic ellipsometry was used to evaluate thin films composed of monolayered CdS NPs deposited by the LbL method. We could successfully determine the absolute values of the dielectric function of the CdS NPs deposited as monolayered films based on the above-listed parameters with accuracy. Thus, the optical constants of monolayered semiconductor NPs as well as their structural information could be obtained successfully using the proposed method, which is highly suited for nondestructively determining the parameters of thin films, including those of semiconductor NPs.

Atomic reconstruction in twisted bilayers of transition metal dichalcogenides.

Van der Waals heterostructures form a unique class of layered artificial solids in which physical properties can be manipulated through controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic-resolution transmission electron microscopy to reveal the lattice reconstruction in twisted bilayers of the transition metal dichalcogenides, MoS2 and WS2. For twisted 3R bilayers, a tessellated pattern of mirror-reflected triangular 3R domains emerges, separated by a network of partial dislocations for twist angles θ < 2°. The electronic properties of these 3R domains, featuring layer-polarized conduction-band states caused by lack of both inversion and mirror symmetry, appear to be qualitatively different from those of 2H transition metal dichalcogenides. For twisted 2H bilayers, stable 2H domains dominate, with nuclei of a second metastable phase. This appears as a kagome-like pattern at θ ≈ 2°, transitioning at θ → 0 to a hexagonal array of screw dislocations separating large-area 2H domains. Tunnelling measurements show that such reconstruction creates strong piezoelectric textures, opening a new avenue for engineering of 2D material properties.

Nondestructive Measurements of the Mechanical and Structural Properties of Nanostructured Metalattices.

Metalattices are artificial 3D solids, periodic on sub-100 nm length scales, that enable the functional properties of materials to be tuned. However, because of their complex structure, predicting and characterizing their properties is challenging. Here we demonstrate the first nondestructive measurements of the mechanical and structural properties of metalattices with feature sizes down to 14 nm. By monitoring the time-dependent diffraction of short wavelength light from laser-excited acoustic waves in the metalattices, we extract their acoustic dispersion, Young's modulus, filling fraction, and thicknesses. Our measurements are in excellent agreement with macroscopic predictions and potentially destructive techniques such as nanoindentation and scanning electron microscopy, with increased accuracy over larger areas. This is interesting because the transport properties of these metalattices do not obey bulk predictions. Finally, this approach is the only way to validate the filling fraction of metalattices over macroscopic areas. These combined capabilities can enable accurate synthesis of nanoenhanced materials.

Energy Transport in CsPbBr3 Perovskite Nanocrystal Solids

Artificial solids of CsPbX3 perovskite nanocrystals (NC) are well known for their promising charge transport characteristics. The long-range diffusion of photoinduced charges in these materials is ...

Resilient Pathways to Atomic Attachment of Quantum Dot Dimers and Artificial Solids from Faceted CdSe Quantum Dot Building Blocks.

The goal of this work is to identify favored pathways for preparation of defect-resilient attached wurtzite CdX (X = S, Se, Te) nanocrystals. We seek guidelines for oriented attachment of faceted nanocrystals that are most likely to yield pairs of nanocrystals with either few or no electronic defects or electronic defects that are in and of themselves desirable and stable. Using a combination of in situ high-resolution transmission electron microscopy (HRTEM) and electronic structure calculations, we evaluate the relative merits of atomic attachment of wurtzite CdSe nanocrystals on the {11̅00} or {112̅0} family of facets. Pairwise attachment on either facet can lead to perfect interfaces, provided the nanocrystal facets are perfectly flat and the angles between the nanocrystals can adjust during the assembly. Considering defective attachment, we observe for {11̅00} facet attachment that only one type of edge dislocation forms, creating deep hole traps. For {112̅0} facet attachment, we observe that four distinct types of extended defects form, some of which lead to deep hole traps whereas others only to shallow hole traps. HRTEM movies of the dislocation dynamics show that dislocations at {11̅00} interfaces can be removed, albeit slowly. Whereas only some extended defects at {112̅0} interfaces could be removed, others were trapped at the interface. Based on these insights, we identify the most resilient pathways to atomic attachment of pairs of wurtzite CdX nanocrystals and consider how these insights can translate to the creation of electronically useful materials from quantum dots with other crystal structures.

Controlled Self-Assembly and Tuning of Large PbS Nanoparticle Supercrystals

Self-assembly of colloidal nanocrystals (NCs) into ordered superlattices (SLs) and supercrystals (SCs) enables new artificial NC solids for nanoelectronic and nanophotonic applications, which requires critical control of nucleation and growth conditions. Herein large SCs of PbS NCs up to ∼100 μm size were synthesized by two controlled self-assembly methods from NC solutions. Both translational symmetry and orientational ordering of the nanocrystals in the SCs were readily tuned by excess oleic acid ligands and antisolvents. Slow evaporation and the counterdiffusion method of solvents resulted in the formation of single SCs with two different SLs from the same PbS NCs: a face-centered cubic SL with weak yet complex orientational order or a body-centered cubic SL with strong and uniform particle orientation, respectively. The translational ordering was mainly determined by the effective shape of the NCs while the difference in orientational order was a result of the balance between ligand–ligand attraction and rotational entropy. The ease of the growth of large SC solids could lead to diverse NC systems and facilitate essential investigation of nanoparticle interactions and coupling based nanoelectronic and nanophotonic properties.

2D or Not 2D: Strain Tuning in Weakly Coupled Heterostructures

AbstractA route to realize strain engineering in weakly bonded heterostructures is presented. Such heterostructures, consisting of layered materials with a pronounced bond hierarchy of strong and weak bonds within and across their building blocks respectively, are anticipated to grow decoupled from each other. Hence, they are expected to be unsuitable for strain engineering as utilized for conventional materials which are strongly bonded isotropically. Here, it is shown for the first time that superlattices of layered chalcogenides (Sb2Te3/GeTe) behave neither as fully decoupled two‐dimensional (2D) materials nor as covalently bonded three‐dimensional (3D) materials. Instead, they form a novel class of 3D solids with an unparalleled atomic arrangement, featuring a distribution of lattice constants, which is tunable. A map to identify further material combinations with similar characteristic is given. It opens the way for the design of a novel class of artificial solids with unexplored properties.

Ultrafast interfacial energy transfer and interlayer excitons in the monolayer WS2/CsPbBr3 quantum dot heterostructure.

The idea of fabricating artificial solids with band structures tailored to particular applications has long fascinated condensed matter physicists. Heterostructure (HS) construction is viewed as an effective and appealing approach to engineer novel electronic properties in two dimensional (2D) materials. Different from common 2D/2D heterojunctions where energy transfer is rarely observed, CsPbBr3 quantum dots (0D-QDs) interfaced with 2D materials have become attractive HSs for exploring the physics of charge transfer and energy transfer, due to their superior optical properties. In this paper, a new 0D/2D HS is proposed and experimentally studied, making it possible to investigate both light utilization and energy transfer. Specifically, this HS is constructed between monolayer WS2 and CsPbBr3 QDs, and exhibits a hybrid band alignment. The dynamics of energy transfer within the investigated 0D/2D HS is characterized by femtosecond transient absorption spectrum (TAS) measurements. The TAS results reveal that ultrafast energy transfer caused by optical excitation is observed from CsPbBr3 QDs to the WS2 layer, which can increase the exciton fluence within the WS2 layer up to 69% when compared with pristine ML WS2 under the same excitation fluence. Moreover, the formation and dynamics of interlayer excitons have also been investigated and confirmed in the HS, with a calculated recombination time of 36.6 ps. Finally, the overall phenomenological dynamical scenario for the 0D/2D HS is established within the 100 ps time region after excitation. The techniques introduced in this work can also be applied to versatile optoelectronic devices based on low dimensional materials.

Application of thin-layer wicking method for surface free energy determination

The determination of solid surface free energy is still not a fully solved problem. There are several approaches to calculating the energy from the measured contact angles of probe liquids. Surface free energy changes accompany all processes occurring at a solid surface. Therefore, knowledge about these changes is very helpful for better understanding and prediction of their occurrences. In the case of a flat solid surface, contact angle measurement is a principal method for surface free energy determination, but many naturally occurring or artificial solids occur only as powders. In the case of such systems, the thin-layer wicking method, originally proposed by Giese, Constanzo and van Oss and further developed by Chibowski and Hołysz, is useful for determination of the surface free energy. The experimental methods for the determination of solid surface free energy bear some doubts, and the obtained results may be debatable. Nevertheless, even the determined relative changes in the energy shed light on the investigated process, helping in its handling. In this paper the theoretical fundamentals of the thin-layer wicking method as well as some practical guidelines are described. Moreover, literature data of the energy determination for some powdered solids and fabrics are reported.

Dry Cleaning of Fine Coal Based on Gas‐Solid Two‐Phase Flow: A Review

AbstractDry cleaning of coal becomes increasingly significant to improve energy efficiency and reduce air pollutant emissions. Fine coal dry cleaning processes are mostly based on gas‐solid two‐phase flow and mainly classified into the following three categories. Modified air‐dense medium‐fluidized bed cleaning has high cleaning accuracy, but confronts the problems of product purification and medium recovery. Density‐segregation gas‐fluidized bed cleaning does not use artificial medium solids and is characterized by a simple process and stable performance. However, its cleaning accuracy is relatively low. Air table cleaning also uses no artificial medium and separates fine coal depending on the differences in density and coefficient of friction. Its cleaning accuracy is approximate to that of density‐segregation gas‐fluidized bed cleaning but has more influential factors.