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

Reducing the uncertainty caused by the laser spot radius in frequency-domain thermoreflectance measurements of thermal properties

In a frequency-domain thermoreflectance (FDTR) experiment, the phase lag between the surface temperature response and the applied heat flux is fit with an analytical solution to the heat diffusion equation to extract an unknown thermal property (e.g., thermal conductivity) of a test sample. A method is proposed to reduce the impact of uncertainty in the laser spot radius on the resulting uncertainty in the fitted property that is based on fitting to the quotient of the test sample phase and that of a reference sample. The reduction is proven analytically for a semi-infinite solid and was confirmed using numerical and real experiments on realistic samples. When the spot radius and its uncertainty are well known, the reference phase can be generated numerically. In this situation, FDTR experiments performed on Au-SiO₂-Si and PbS nanocrystal test samples demonstrate 32% and 82% reductions in the overall uncertainty in thermal conductivity. When the spot radius used in the test sample measurement is not well known, a real reference sample, measured under conditions that lead to the same unknown spot radius, is required. Although the real reference sample introduces its own uncertainties, the total uncertainty in the fitted thermal conductivity can still be reduced. A reference sample can also be used to reduce uncertainty due to other sources, such as the transducer properties. Because frequency-domain solutions to the heat diffusion equation are the basis for time-domain thermoreflectance (TDTR) analysis, the approach can be extended to TDTR experiments.

Structural order enhances charge carrier transport in self-assembled Au-nanoclusters

The collective properties of self-assembled nanoparticles with long-range order bear immense potential for customized electronic materials by design. However, to mitigate the shortcoming of the finite-size distribution of nanoparticles and thus, the inherent energetic disorder within assemblies, atomically precise nanoclusters are the most promising building blocks. We report an easy and broadly applicable method for the controlled self-assembly of atomically precise Au₃₂(ⁿBu₃P)₁₂Cl₈ nanoclusters into micro-crystals. This enables the determination of emergent optoelectronic properties which resulted from long-range order in such assemblies. Compared to the same nanoclusters in glassy, polycrystalline ensembles, we find a 100-fold increase in the electric conductivity and charge carrier mobility as well as additional optical transitions. We show that these effects are due to a vanishing energetic disorder and a drastically reduced activation energy to charge transport in the highly ordered assemblies. This first correlation of structure and electronic properties by comparing glassy and crystalline self-assembled superstructures of atomically precise gold nanoclusters paves the way towards functional materials with novel collective optoelectronic properties.

Glassy Porphyrin/C60 Composites: Morphological Engineering of C60 Fullerene with Liquefied Porphyrins

Morphological control of C₆₀ fullerene using liquefied porphyrins (1 and 2) as the host matrices was explored. Slow evaporation of the solvent of the equimolar mixture of porphyrin and C₆₀ in toluene afforded the porphyrin/C₆₀ composite with a 3:1 molar ratio. The stoichiometric binding behaviors suggest that specific porphyrin-C₆₀ interactions operate the formation of the porphyrin/C₆₀ composites, as corroborated by spectroscopic and thermal properties, and glazing-incidence wide-angle X-ray diffraction. Under the bulk conditions, the conventional thermodynamic advantage of multiple binding cooperativity for molecular recognition is unlikely to explain the stoichiometric binding behaviors. Instead, we propose a size-matching effect on the porphyrin-C₆₀ interaction in the bulk porphyrin matrices, i.e., "supramolecular solvation". The glassy nature of the porphyrin matrices was transmitted to C₆₀ through the specific interaction, and the porphyrin/C₆₀ composites adopted glassy states at room temperature.

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.

N4Mg6M (M = Li, Na, K) superalkalis for CO2 activation

Superatoms have exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids hold the promise of high tunability, atomic precision, and robust architectures. By utilizing adamantane-like clusters as building blocks, a new class of superatoms N₄Mg₆M (M = Li, Na, K) is proposed here. The studied superalkalis feature low adiabatic ionization energies, an antibonding character in the interactions between magnesium and nitrogen atoms, and highly delocalized highest occupied molecular orbital (HOMO). Consequently, the N₄Mg₆M superalkalis might easily lose their HOMO electrons when interacting with superhalogen electrophiles to form stable superatom [superalkali]⁺[superhalogen]^- compounds. Moreover, the studied superalkalis interact strongly with carbon dioxide, and the resulting N₄Mg₆M/CO₂ systems represent two strongly interacting ionic fragments (i.e., N₄Mg₆M⁺ and CO₂ ^-). In turn, the electron affinity of the N₂ molecule (of -1.8 eV) is substantially lower than that observed for carbon dioxide (EA = -0.6 eV) and consequently, the N₂ was found to form the weakly bound [N₄Mg₆M][N₂] complex rather than the desired ionic [N₄Mg₆M]⁺[N₂]^- product. Thus, the N₄Mg₆M superalkalis have high selectivity over N₂ when it comes to CO₂ reduction and also are themselves stable. We believe that the results described within this paper will be useful for understanding CO₂ activation, which is the first step for producing fuels from CO₂. Moreover, we demonstrate that designing novel superatomic systems and exploring their physicochemical features might be used to create desirable functional materials.

Stepwise Assembly of an Electroactive Framework from a Co6 S8 Superatomic Metalloligand and Cuprous Iodide Building Units

The design of metal-organic frameworks (MOFs) that incorporate more than one metal cluster constituent is a challenging task. Conventional one-pot reaction protocols require judicious selection of ligand and metal ion precursors, yet remain unpredictable. Stable, preformed nanoclusters, with ligand shells that can undergo additional coordination-driven reactions, provide a platform for assembling multi-cluster solids with precision. Herein, a discrete Co₆ S₈ (PTA)₆ (PTA=1,3,5-triaza-7-phosphaadamantane) superatomic-metalloligand is assembled into a three-dimensional (3D) coordination polymer comprising Cu₄ I₄ secondary building units (SBUs). The resulting heterobimetallic framework (1) contains two distinct cluster constituents and bifunctional PTA linkers. Solid-state diffuse reflectance studies reveal that 1 is an optical semiconductor with a band-gap of 1.59 eV. Framework-modified electrodes exhibit reversible redox behavior in the solid state arising from the Co₆ S₈ superatoms, which remain intact during framework synthesis.

Modulating Orientational Order to Organize Polyhedral Nanoparticles into Plastic Crystals and Uniform Metacrystals

In nanoparticle self-assembly, the current lack of strategy to modulate orientational order creates challenges in isolating large-area plastic crystals. Here, we achieve two orientationally distinct supercrystals using one nanoparticle shape, including plastic crystals and uniform metacrystals. Our approach integrates multi-faceted Archimedean polyhedra with molecular-level surface polymeric interactions to tune nanoparticle orientational order during self-assembly. Experiments and simulations show that coiled surface polymer chains limit interparticle interactions, creating various geometrical configurations among Archimedean polyhedra to form plastic crystals. In contrast, brush-like polymer chains enable molecular interdigitation between neighboring particles, favoring consistent particle configurations and result in uniform metacrystals. Our strategy enhances supercrystal diversity for polyhedra comprising multiple nondegenerate facets.

Modified Lennard-Jones potentials for nanoscale atoms

A classical 6-12 Lennard-Jones (LJ) equation has been widely used to model materials and is the potential of choice in studies when the focus is on fundamental issues. Here we report a systematic study comparing the pair interaction potentials within solid-state materials (i.e., [Co₆ Se₈ (PEt₃ )₆ ][C₆₀ ]₂ , [Cr₆ Te₈ (PEt₃ )₆ ][C₆₀ ]₂ , [Ni₉ Te₆ (PEt₃ )₈ ][C₆₀ ]) using density functional theory (DFT) calculations and LJ parametrization. Both classical (6-12 LJ) and modified LJ (mLJ) models were developed. In the mLJ approach, the exponents 6 and 12 are replaced by different integer number n and 2n, respectively, and an additional parameter (α) is introduced to describe intermolecular distance shift arising within the geometric centers' approach (instead of the shortest interatomic distance between particles). A general LJ approach reexamination reveals that in the case of nanoatoms, the attractive term decays with distance as the inverse fourth power, and the dominating at short distances repulsive term decays as the inverse eighth power. The modification of the LJ equation is even more prominent for interaction profiles, where intermolecular distance corresponds to separation between geometric centers of particles. In this approach, the attractive term decays with distance as the inverse 12th power, while the repulsive term decays rapidly (as the inverse 24th power). Thus, the mLJ models (e.g., 4-8 LJ) rather than the 6-12 classical ones seem to be a better choice for the description of binary interactions of nanoatoms. The developed mLJ models and electronic structure characteristics give an insight into the explanation of the unique physicochemical properties of superatomic-based solid-state materials.

Self-Assembly of a Chiral Cubic Three-Connected Net from the High Symmetry Molecules C60 and SnI4

The design of new chiral materials usually requires stereoselective organic synthesis to create molecules with chiral centers. Less commonly, achiral molecules can self-assemble into chiral materials, despite the absence of intrinsic molecular chirality. Here, we demonstrate the assembly of high-symmetry molecules into a chiral van der Waals structure by synthesizing crystals of C₆₀(SnI₄)₂ from icosahedral buckminsterfullerene (C₆₀) and tetrahedral SnI₄ molecules through spontaneous self-assembly. The SnI₄ tetrahedra template the Sn atoms into a chiral cubic three-connected net of the SrSi₂ type. Our results represent the remarkable emergence of a self-assembled chiral material from two of the most highly symmetric molecules, demonstrating that almost any molecular, nanocrystalline, or engineered precursor can be considered when designing chiral assemblies.

Hierarchical Coherent Phonons in a Superatomic Semiconductor

The coupling of phonons to electrons and other phonons plays a defining role in material properties, such as charge and energy transport, light emission, and superconductivity. In atomic solids, phonons are delocalized over the 3D lattice, in contrast to molecular solids where localized vibrations dominate. Here, a hierarchical semiconductor that expands the phonon space by combining localized 0D modes with delocalized 2D and 3D modes is described. This material consists of superatomic building blocks (Re₆ Se₈ ) covalently linked into 2D sheets that are stacked into a layered van der Waals lattice. Using transient reflectance spectroscopy, three types of coherent phonons are identified: localized 0D breathing modes of isolated superatom, 2D synchronized twisting of superatoms in layers, and 3D acoustic interlayer deformation. These phonons are coupled to the electronic degrees of freedom to varying extents. The presence of local phonon modes in an extended crystal opens the door to controlling material properties from hierarchical phonon engineering.