Superatomic Crystals Research Articles

Super-defects in superatomic crystals

Superatoms hold great potential for creating novel crystals, yet the effect of resulting superatomic-based defects on the properties remains unclear. Here, we explore the effects induced by superatom-based defects, including typical vacant, substitutional and co-doped defects in the crystal formed directly from the metal superatom W@Cu12. Surprisingly, crystals with these defects retain the inherent properties of the perfect superatomic crystal in filtering light at approximately 27 eV while simultaneously being able to either reduce or broaden the filtered energy range. Moreover, substitutional and co-doped defects enhance the optical reflectivity and absorption below approximately 20 eV, and vacant defects decrease the specific heat capacity and elastic constants. Given that these defects originate from superatoms, they may be called “super-defects”. It is anticipated that this study of super-defects will not only reveal the potential for further tuning the unique properties of superatomic crystals but also facilitate the development of superatomic-based defect engineering.

Modulating the bandgap of Cr-intercalated bilayer graphene via combining the 18-electron rule and the 2D superatomic-molecule theory.

Bandgap engineering of graphene is of great significance for its potential applications in electronic devices. Herein, we used a sandwich compound Cr(C6H6)2 as the building block to construct Cr-intercalated bilayer graphene (BLG), namely a C12Cr monolayer. Chemical bonding analysis reveals that strong d-π interaction ensures π electrons of the graphene layers and d orbitals of the Cr atoms localized in C6CrC6 units to achieve the favored 18-electron rule, thus leading to a bandgap of 0.24 eV. Subsequently, a C48Cr monolayer with lower proportion of Cr is further designed using Cr(C54H18)2 as building units, where a newly developed two-dimensional (2D) superatomic-molecule theory is introduced to rationalize its electronic structure. The C48Cr monolayer not only satisfies the 18-electron rule, but also localizes extra π electrons to form two layers of 2D superatomic crystals composed of 2D superatoms (◊O and ◊N), resulting in a wider bandgap of 0.74 eV. This work opens an effective avenue to modulate the bandgap of BLG via combining the 18-electron rule and the 2D superatomic-molecule theory.

Cluster-assembled superatomic crystals for chirality-dependent charge-to-spin conversion

In chiral materials, spins and chirality are coupled via spin-orbit interaction, provoking a fast-growing field of chiral spintronics. Compared with the widely explored chiral molecules, exploration of chirality-dependent spin effects in crystals and supramolecules remain limited. Here we assemble chiral superatomic crystals MXTe4 (M = transition metal; X = Ga or Ge) using telluride tetrahedra clusters as building blocks. Distinct from atomic crystals, these assembled monolayers have tunable symmetries and electronic characteristics by tilting the tetrahedral units through the variation of inter-cluster interaction. Dresselhaus-type spin textures and anisotropic spin Hall effect with inversed sign of spin current under opposite geometrical handedness are demonstrated in these chiral monolayers by symmetry analysis and verified by ab initio calculations. These results provide an innovative paradigm for assembling superatomic crystals with designated symmetry and hierarchical structures to access the chirality-driven quantum effects.

Diselenide Dianion’s Dual Powers: PdSe2Polymorph Control and Pd3Se10Superatomic Crystal Creation

There is a diverse family of palladium selenide compounds, including semiconducting orthorhombic PdSe2 (O-PdSe2) and monoclinic PdSe2 (M-PdSe2), which are unusual among transition-metal dichalcogenides as they are composed of diselenide dianions Se22–. Thus far, the solution syntheses of materials with Se–Se bonds typically require the in situ reduction of Se precursors. Here, we explore the use of electrochemically precise reactions between Na2Se2 and a Pd2+ source, Na2PdCl4, as a solution-phase route to selectively form different PdSe2 polymorphs in the absence of surfactants. By altering the reactant molar ratios and time, we map out a synthetic phase space diagram that shows how to create a wide variety of palladium selenide phases. With increasing Se22–/Pd2+ molar ratios, regions are identified where Pd17Se15, M-PdSe2, and O-PdSe2 exist as the dominant or exclusive thermodynamic product. Additionally, we discover Pd3Se10, a superatomic crystal composed of Pd6Se20 cube-shaped clusters held together by van der Waals forces, which forms as a kinetic product under short reaction times. In total, the use of the diselenide dianion precursor allows for the selective solution-phase synthesis of M-PdSe2 or O-PdSe2, as well as the discovery of a previously unreported palladium selenide phase.

Low lattice thermal conductivity with two-channel thermal transport in the superatomic crystal PH4AlBr4

Designing novel crystalline materials composed of light and nontoxic elements, but with ultralow thermal conductivity insensitive to temperature, has been a long-standing challenge. One effective strategy is to utilize superatoms as building blocks to introduce hierarchical bonding and vibration for suppressing phonon velocity and enhancing high-order scattering. However, far fewer theoretical efforts have been made to understand the lattice dynamics and thermal transport in superatom-based materials. Herein, different from most of the existing works reported hitherto on atom-superatom hybrid systems, we carry out a comprehensive study on the phonon interaction and thermal transport in superatomic crystal $\mathrm{P}{\mathrm{H}}_{4}\mathrm{Al}{\mathrm{Br}}_{4}$ solely consisting of superatoms (superalkali and superhalogen), by employing homogeneous nonequilibrium molecular dynamics with active-learning potential, combined with density functional theory and unified theory of phonon thermal transport. We find that the supersalt $\mathrm{P}{\mathrm{H}}_{4}\mathrm{Al}{\mathrm{Br}}_{4}$ crystal exhibits amorphouslike ultralow lattice thermal conductivity of $0.329\ensuremath{-}0.286\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ from 200 to 600 K. In particular, due to the strong quartic anharmonicity, the contribution of four-phonon scattering reaches 38% of total scattering rates at 300 K, while the phonon coherence's contribution is comparable with that of population, resulting from significant phonon localization. These theoretical findings demonstrate that superatom-assembled materials exhibit features distinguished from atom-based crystals and provide a unique platform for exploring ultralow thermal conductivity with enhanced two-channel thermal transport.

Interweaving Polar Charge Orders in a Layered Metallic Superatomic Crystal

A 3D crystal of superatoms is the first known antipolar metal and possesses the first polar metallic state hosted in superatomic units, opening a category of quantum materials with versatile layered nanostructures.

Superatomic-Charge-Density-Wave in Cluster-Assembled Au6Te12Se8 Superconductors.

Superatomic crystals are a class of hierarchical materials composed of atomically precise clusters assembled via van der Waals or covalent-like interactions. Au6Te12Se8, an all-inorganic superatomic superconductor exhibiting superatomic-charge-density-wave (S-CDW), provides the first platform to study the response of its collective quantum phenomenon to the external pressure in superatomic crystals. We reveal a competition between S-CDW and superconductivity in an ultra-narrow pressure range. Distinct from conventional CDW ordering, S-CDW shows the lowest threshold (0.1 GPa) toward external pressure that is 1-2 orders of magnitude lower than other atomic compounds. Prominently, a second superconducting phase emerges above 7.3 GPa with a threefold enhancement in the transition temperature (Tc) to 8.5 K, indicating a switch of the conduction channel from the a- to b-axis. In situ synchrotron diffractions and theoretical calculations reveal a pressure-mediated mesoscopic slip of the superatoms and a 2D-3D transition of the Fermi surface topology, which well explains the observed dimensional crossover of conductivity and re-entrant superconductivity.

A unified superatomic-molecule theory for local aromaticity in π-conjugated systems.

Aromaticity is one of the most important concepts in chemistry. However, there is still no unified chemical insight for various systems with conjugated sp2 carbon. Herein, we proposed a superatomic-molecule theory to build a generalized electron rule for polycyclic conjugated hydrocarbons, fullerenes and 2D periodic materials. Taking benzenoid units as 2D superatoms, polycyclic conjugated hydrocarbons and C60 can be seen as superatomic molecules consisting of bonded superatoms, resulting in local aromaticity. In superatomic molecules, π electrons are not totally delocalized, but localized in a single superatom forming superatomic lone pairs or shared by two atoms forming a superatomic bond, mimicking rules in classical valence bond theory. Moreover, two 2D superatomic crystals (C18H6 and C54H18) are predicted to have fairly large band gaps (∼1.8eV), although the π electrons are conjugated and delocalized. The proposed superatomic-molecule theory provides generalized chemical insights into the nature of local aromaticity, which can be qualitatively evaluated by the chemical intuition given by superatomic Lewis structures.

Assembling Au4 Tetrahedra to 2D and 3D Superatomic Crystals Based on Superatomic-Network Model.

Thiolate-protected gold nanoclusters (denoted as Aum(SR)n or AunLm) have received extensive attention both experimentally and theoretically. Understanding the growth mode of the Au4 unit in Aum(SR)n is of great significance for experimental synthesis and the search for new gold clusters. In this work, we first build six clusters of Au7(AuCl2)3, Au12(AuCl2)4, Au16(AuCl2)6, Au22(AuCl2)6, and Au30(AuCl2)6 with the Au4 unit as the basic building blocks. Density functional theory (DFT) calculations show that these newly designed clusters have high structural and electronic stabilities. Based on chemical bonding analysis, the electronic structures of these clusters follow the superatom network (SAN) model. Inspired by the cluster structures, we further predicted an Au4 two-dimensional (2D) monolayer and a three-dimensional (3D) crystal using graphene and diamond as templates, respectively. The computational results demonstrate that the two structures have high dynamic, thermal, and mechanical stabilities, and both structures exhibit metallic properties according to the band structures calculated at the HSE06 level. The chemical bonding analysis by the solid-state natural density partitioning (SSAdNDP) method indicates that they are superatomic crystals assembled by two electron Au4– superatoms. With this construction strategy, the new bonding pattern and properties of AunLm are studied and the structure types of gold are enriched.

Superatoms in materials science

Clusters are ensembles of bound atoms intermediate in size between a molecule and a bulk solid. Some of these clusters form stable units with atomically precise structures that give rise to collective behaviours that mimic those of traditional atoms, essentially functioning as ‘superatoms’. Such superatoms are exciting, nanoscale building blocks for materials design. Sustained synthetic and theoretical efforts over the past 40 years have created a vast library of chemically tuneable superatoms that display unique physical and chemical properties. The use of superatoms as building blocks for materials offers opportunities to design materials with tailored functionalities; however, this potential has only begun to be realized in the past few years. The assembly of superatomic crystals presents numerous challenges, including the design of suitable building blocks, control of the self-assembly process, promotion of strong inter-superatom coupling and understanding the resulting collective properties. In this Review, we assess recent advances in meeting these challenges by focusing on metal chalcogenide and carbon-based clusters (for example, fullerenes) as superatomic building blocks. Molecular clusters can be used as tuneable, superatomic building blocks to create new functional materials. This Review outlines the synthesis and assembly of superatoms into superatomic crystals and the resulting collective material properties.

Doping-Induced Superconductivity in the van der Waals Superatomic Crystal Re6Se8Cl2.

Superatomic crystals are composed of discrete modular clusters that emulate the role of atoms in traditional atomic solids. Owing to their unique hierarchical structures, these materials are promising candidates to host exotic phenomena, such as doping-induced superconductivity and magnetism. Low-dimensional superatomic crystals in particular hold great potential as electronic components in nanocircuits, but the impact of doping in such compounds remains unexplored. Here we report the electrical transport properties of Re6Se8Cl2, a two-dimensional superatomic semiconductor. We find that this compound can be n-doped in situ through Cl dissociation, drastically altering the transport behavior from semiconducting to metallic and giving rise to superconductivity with a critical temperature of ∼8 K and upper critical field exceeding 30 T. This work is the first example of superconductivity in a van der Waals superatomic crystal; more broadly, it establishes a new chemical strategy to manipulate the electronic properties of van der Waals materials with labile ligands.

Fullerene rotational dynamics generate disordered configurations that suppress thermal conductivity in superatomic crystals.

The thermal conductivity of fullerene-based superatomic crystals (SACs) is investigated using molecular dynamics simulations. The temperature-dependent predictions agree with the trends of previous measurements. The thermal conductivity behavior emerges as a result of the C60 molecule rotational dynamics and orientation, which are quantified using the root mean square displacements of the carbon atoms and the relative orientations of the C60s. At low temperatures, the C60s exhibit small rotations around equilibrium positions (i.e., librations). When the librating C60s are orientationally-ordered, as in the [C60] and [Co6Se8(PEt3)6][C60]2 SACs, thermal conductivity decreases with increasing temperature, as is typical for a crystal. When the librating C60s are orientationally-disordered, however, as in the [Co6Te8(PEt3)6][C60]2 SAC, thermal conductivity is lower and temperature independent, as is typical for an amorphous solid. At higher temperatures, where the C60s in all three SACs freely-rotate and are thus dynamically disordered, thermal conductivity is temperature independent. The abrupt changes driven by the C60 dynamics suggest that fullerene-based SACs can be designed to be thermal conductivity switches based on a variety of external stimuli.

Hierarchical nanosheets built from superatomic clusters: properties, exfoliation and single-crystal-to-single-crystal intercalation.

Tuning the properties of atomic crystals in the two-dimensional (2D) limit is synthetically challenging, but critical to unlock their potential in fundamental research and nanotechnology alike. 2D crystals assembled using superatomic blocks could provide a route to encrypt desirable functionality, yet strategies to link the inorganic blocks together in predetermined dimensionality or symmetry are scarce. Here, we describe the synthesis of anisotropic van der Waals crystalline frameworks using the designer superatomic nanocluster Co3(py)3Co6Se8L6 (py = pyridine, L = Ph2PN(Tol)), and ditopic linkers. Post-synthetically, the 3D crystals can be mechanically exfoliated into ultrathin flakes (8 to 60 nm), or intercalated with the redox-active guest tetracyanoethylene in a single-crystal-to-single-crystal transformation. Extensive characterization, including by single crystal X-ray diffraction, reveals how intrinsic features of the nanocluster, such as its structure, chirality, redox-activity and magnetic profile, predetermine key properties of the emerging 2D structures. Within the nanosheets, the strict and unusual stereoselectivity of the nanocluster's Co edges for the low symmetry (α,α,β) isomer gives rise to in-plane structural anisotropy, while the helically chiral nanoclusters self-organize into alternating Δ- and Λ-homochiral rows. The nanocluster's high-spin Co edges, and its rich redox profile make the nanosheets both magnetically and electrochemically active, as revealed by solid state magnetic and cyclic voltammetry studies. The length and flexibility of the ditopic linker was varied, and found to have a secondary effect on the structure and stacking of the nanosheets within the 3D crystals. With these results we introduce a deterministic and versatile synthetic entry to programmable functionality and symmetry in 2D superatomic crystals.

Spontaneous Electronic Band Formation and Switchable Behaviors in a Phase-Rich Superatomic Crystal.

Structural phase transitions run in families of crystalline solids. Perovskites, for example, feature a remarkable number of structural transformations that produce a wealth of exotic behaviors, including ferroelectricity, magnetoresistance, metal-insulator transitions and superconductivity. In superatomic crystals and other such materials assembled from programmable building blocks, phase transitions offer pathways to new properties that are both tunable and switchable. Here we describe [Co6Te8(PEt3)6][C70]2, a novel superatomic crystal with two separate phase transitions that drastically transform the collective material properties. A coupled structural-electronic phase transition triggers the emergence of a new electronic band in the fullerene sublattice of the crystal, increasing its electrical conductivity by 2 orders of magnitude, while narrowing its optical gap and increasing its spin density. Independently, an order-disorder transition transforms [Co6Te8(PEt3)6][C70]2 from a phonon crystal to a phonon glass. These results introduce a family of materials in which functional phase transformations may be manipulated by varying the constituent building blocks.

Molecular Clusters: Nanoscale Building Blocks for Solid-State Materials.

The programmed assembly of nanoscale building blocks into multicomponent hierarchical structures is a powerful strategy for the bottom-up construction of functional materials. To develop this concept, our team has explored the use of molecular clusters as superatomic building blocks to fabricate new classes of materials. The library of molecular clusters is rich with exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids, which we term superatomic crystals (SACs), hold the promise of high tunability, atomic precision, and robust architectures among a diverse range of other material properties. Molecular clusters have only seldom been used as precursors for functional materials. Our team has been at the forefront of new developments in this exciting research area, and this Account focuses on our progress toward designing materials from cluster-based precursors. In particular, this Account discusses (1) the design and synthesis of molecular cluster superatomic building blocks, (2) their self-assembly into SACs, and (3) their resulting collective properties. The set of molecular clusters discussed herein is diverse, with different cluster cores and ligand arrangements to create an impressive array of solids. The cluster cores include octahedral M6E8 and cubane M4E4 (M = metal; E = chalcogen), which are typically passivated by a shell of supporting ligands, a feature upon which we have expanded upon by designing and synthesizing more exotic ligands that can be used to direct solid-state assembly. Building from this library, we have designed whole families of binary SACs where the building blocks are held together through electrostatic, covalent, or van der Waals interactions. Using single-crystal X-ray diffraction (SCXRD) to determine the atomic structure, a remarkable range of compositional variability is accessible. We can also use this technique, in tandem with vibrational spectroscopy, to ascertain features about the constituent superatomic building blocks, such as the charge of the cluster cores, by analysis of bond distances from the SCXRD data. The combination of atomic precision and intercluster interactions in these SACs produces novel collective properties, including tunable electrical transport, crystalline thermal conductivity, and ferromagnetism. In addition, we have developed a synthetic strategy to insert redox-active guests into the superstructure of SACs via single-crystal-to-single-crystal intercalation. This intercalation process allows us to tune the optical and electrical transport properties of the superatomic crystal host. These properties are explored using a host of techniques, including Raman spectroscopy, SQUID magnetometry, electrical transport measurements, electronic absorption spectroscopy, differential scanning calorimetry, and frequency-domain thermoreflectance. Superatomic crystals have proven to be both robust and tunable, representing a new method of materials design and architecture. This Account demonstrates how precisely controlling the structure and properties of nanoscale building blocks is key in developing the next generation of functional materials; several examples are discussed and detailed herein.

Dimerization of Endohedral Fullerene in a Superatomic Crystal.

We describe a solid state material created from the reaction of Ni9 Te6 (PEt3 )8 and Lu3 N@C80 . The resulting superatomic crystal, [Ni12 Te12 (PEt3 )8 ]2 [(Lu3 N@C80 )2 ], contains dimers of Lu3 N@C80 that form upon reduction of the fullerene through a single C-C bond at the triple hexagon junctions. The encapsulated Lu3 N cluster displays an unprecedented orientation that is collinear and coplanar with the intercage carbon bond. Density functional theory calculations rationalize this unique bonding and relative orientation of the Lu3 N clusters. Our structural and theoretical results provide new insights into the effect that the M3 N cluster species has on the dimerization process of endohedral fullerenes.

Single-crystal-to-single-crystal intercalation of a low-bandgap superatomic crystal.

The controlled introduction of impurities into the crystal lattice of solid-state compounds is a cornerstone of materials science. Intercalation, the insertion of guest atoms, ions or molecules between the atomic layers of a host structure, can produce novel electronic, magnetic and optical properties in many materials. Here we describe an intercalation compound in which the host [Co6Te8(PnPr3)6][C60]3, formed from the binary assembly of atomically precise molecular clusters, is a superatomic analogue of traditional layered atomic compounds. We find that tetracyanoethylene (TCNE) can be inserted into the superstructure through a single-crystal-to-single-crystal transformation. Using electronic absorption spectroscopy, electrical transport measurements and electronic structure calculations, we demonstrate that the intercalation is driven by the exchange of charge between the host [Co6Te8(PnPr3)6][C60]3 and the intercalant TCNE. These results show that intercalation is a powerful approach to manipulate the material properties of superatomic crystals.

Superatomic crystal emerging in transition-metal oxides: Molybdenum hollandite K2Mo8O16

Density-functional-theory-based electronic structure calculations are carried out to elucidate the origins of the observed electronic properties of molybdenum hollandite K${}_{2}$Mo${}_{8}$O${}_{16}$. We find that the Mo${}_{4}$ cluster in the double Mo chains behaves as a ``superatom,'' a hypothetical big atom with a single composite molecular orbital, and that the system can be regarded as a solid of the superatoms condensed into a simple monoclinic structure with four superatoms per unit cell, thereby yielding four energy bands near the Fermi level at half filling. Based on an effective model proposed, we argue that K${}_{2}$Mo${}_{8}$O${}_{16}$ is a Mott insulator with one electron per superatom, which exhibits strongly frustrated antiferromagnetic spin correlations in the superatomic crystal.