Site-Selective Surface Modification of 2D Superatomic Re6Se8.

Coating two-dimensional (2D) materials with molecules bearing tunable properties imparts their surfaces with functionalities for applications in sensing, nanoelectronics, nanofabrication, and electrochemistry. Here, we report a method for the site-selective surface functionalization of 2D superatomic Re6Se8Cl2 monolayers. First, we activate bulk layered Re6Se8Cl2 by intercalating lithium and then exfoliate the intercalation compound Li2Re6Se8Cl2 in N-methylformamide (NMF). Heating the resulting solution eliminates LiCl to produce monolayer Re6Se8(NMF)2-x (x ≈ 0.4) as high-quality nanosheets. The unpaired electrons on each cluster in Re6Se8(NMF)2-x enable covalent surface functionalization through radical-based chemistry. We demonstrate this to produce four previously unknown surface-functionalized 2D superatomic materials: Re6Se8I2, Re6Se8(SPh)2, Re6Se8(SPhNH2)2, and Re6Se8(SC16H33)2. Transmission electron microscopy, chemical analysis, and vibrational spectroscopy reveal that the in-plane structure of the 2D Re6Se8 material is preserved through surface functionalization. We find that the incoming groups control the density of vacancy defects and the solubility of the 2D material. This approach will find utility in installing a broad array of chemical functionalities on the surface of 2D superatomic materials as a means to systematically tune their physical properties, chemical reactivity, and solution processability.

High-performance organic pseudocapacitors via molecular contortion.

Pseudocapacitors harness unique charge-storage mechanisms to enable high-capacity, rapidly cycling devices. Here we describe an organic system composed of perylene diimide and hexaazatrinaphthylene exhibiting a specific capacitance of 689 F g-1 at a rate of 0.5 A g-1, stability over 50,000 cycles, and unprecedented performance at rates as high as 75 A g-1. We incorporate the material into two-electrode devices for a practical demonstration of its potential in next-generation energy-storage systems. We identify the source of this exceptionally high rate charge storage as surface-mediated pseudocapacitance, through a combination of spectroscopic, computational and electrochemical measurements. By underscoring the importance of molecular contortion and complementary electronic attributes in the selection of molecular components, these results provide a general strategy for the creation of organic high-performance energy-storage materials.

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.

Shape Matching in Superatom Chemistry and Assembly.

Creating structures with superatomic nanoclusters rather than atoms offers the possibility of new hierarchical solids with collective properties. The variability of chemical compositions, sizes, and shapes of these superatomic building blocks provides great opportunities to access unknown assemblies. Herein we explore this concept by using geometrically anisotropic superatomic nanoclusters as building blocks. We reveal a series of novel superatomic architectures that are built from rod-shaped Co12Se16(PEt3)10 and C140 nanoclusters. More importantly, these assemblies show nonclose packings that afford voids to accommodate solvent molecules as a result of the shape anisotropy of the constituent building blocks. These intercalated small molecules act as "crystal modulators" to modulate the solid-state structures and properties. As a result, we are able to tune the crystal packings and optical gaps of the solids and see the moment when electrical conduction is "turned on". Our results demonstrate the vast potential of using anisotropic superatomic nanoclusters to create solid-state materials and provide a novel approach to configure their assemblies and properties.

Strong polaronic effect in a superatomic two-dimensional semiconductor.

Crystalline solids assembled from superatomic building blocks are attractive functional materials due to their hierarchical structure, multifunctionality, and tunability. An interesting example is Re6Se8Cl2, in which the Re6Se8 building blocks are covalently linked into two-dimensional (2D) sheets that are stacked into a layered van der Waals solid. It is an indirect gap semiconductor that, when heavily doped, becomes a superconductor at low temperatures. Given the finite electronic bandwidths (300-400 meV), carrier properties in this material are expected to be strongly influenced by coupling to phonons. Here, we apply angle-resolved photoemission spectroscopy to probe the valence band edge (VBE) of Re6Se8Cl2. We find that dispersion of the VBE is a strong function of temperature. The bandwidth is W = 120 ± 30 meV at 70 K and decreases by one order of magnitude to W ∼ 10 ± 20 meV as temperature is increased to 300 K. This observation reveals the dominant polaronic effects in Re6Se8Cl2, consistent with the Holstein polaron model commonly used to describe molecular solids.

Designing Three-Dimensional Architectures for High-Performance Electron Accepting Pseudocapacitors.

By storing energy from electrochemical processes at the electrode surface, pseudocapacitors bridge the performance gap between electrostatic double-layer capacitors and batteries. In this context, molecular design offers the exciting possibility to create tunable and inexpensive organic electroactive materials. Here we describe a porous structure composed of perylene diimide and triptycene subunits and demonstrate its remarkable performance as a pseudocapacitor electrode material. The material exhibits capacitance values as high as 350 F/g at 0.2 A/g as well as excellent stability over 10 000 cycles. Moreover, we can alter the performance of the material, from battery-like (storing more charge at low rates) to capacitor-like (faster charge cycling), by modifying the structure of the pores via flow photocyclization. Organic materials capable of stable electron accepting pseudocapacitor behavior are rare and the capacitance values presented here are among the highest reported. More broadly, this work establishes molecular design and synthesis as a powerful approach for creating tunable energy storage materials.

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.

Solution-Processed, Ultrathin Solar Cells from CdCl3(-)-Capped CdTe Nanocrystals: The Multiple Roles of CdCl3(-) Ligands.

Solution-processed CdTe solar cells using CdTe nanocrystal (NC) ink may offer an economically viable route for large-scale manufacturing. Here we design a new CdCl3(-)-capped CdTe NC ink by taking advantage of novel surface chemistry. In this ink, CdCl3(-) ligands act as surface ligands, sintering promoters, and dopants. Our solution chemistry allows obtaining very thin continuous layers of high-quality CdTe which is challenging for traditional vapor transport methods. Using benign solvents, in air, and without additional CdCl2 treatment, we obtain a well-sintered CdTe absorber layer from the new ink and demonstrate thin-film solar cells with power conversion efficiency over 10%, a record efficiency for sub-400 nm thick CdTe absorber layer.

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.

Room-temperature wavelike exciton transport in a van der Waals superatomic semiconductor.

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies. Using Re6Se8Cl2, a van der Waals (vdW) superatomic semiconductor, we demonstrate the formation of acoustic exciton-polarons, an electronic quasiparticle shielded from phonon scattering. We directly imaged polaron transport in Re6Se8Cl2 at room temperature, revealing quasi-ballistic, wavelike propagation sustained for a nanosecond and several micrometers. Shielded polaron transport leads to electronic energy propagation lengths orders of magnitude greater than in other vdW semiconductors, exceeding even silicon over a nanosecond. We propose that, counterintuitively, quasi-flat electronic bands and strong exciton-acoustic phonon coupling are together responsible for the transport properties of Re6Se8Cl2, establishing a path to ballistic room-temperature semiconductors.

Roll-To-Roll Friendly Solution-Processing of Ultrathin, Sintered CdTe Nanocrystal Photovoltaics.

Roll-to-roll (R2R) device fabrication using solution-processed materials is a cheap and versatile approach that has attracted widespread interest over the past 2 decades. Here, we systematically introduce and investigate R2R-friendly modifications in the fabrication of ultrathin, sintered CdTe nanocrystal (NC) solar cells. These include (1) scalable deposition techniques such as spray-coating and doctor-blading, (2) a bath-free, controllable sintering of CdTe NCs by quantitative addition of a sintering agent, and (3) radiative heating with an infrared lamp. The impact of each modification on the CdTe nanostructure and solar cell performance was first independently studied and compared to the standard, non-R2R-friendly procedure involving spin-coating the NCs, soaking in a CdCl2 bath, and annealing on a hot plate. The R2R-friendly techniques were then combined into a single, integrated process, yielding devices that reach 10.4% power conversion efficiency with a Voc, Jsc, and FF of 697 mV, 22.2 mA/cm2, and 67%, respectively, after current/light soaking. These advances reduce the barrier for large-scale manufacturing of solution-processed, ultralow-cost solar cells on flexible or curved substrates.

Superatomic solid solutions.

In atomic solids, substitutional doping of atoms into the lattice of a material to form solid solutions is one of the most powerful approaches to modulating its properties and has led to the discovery of various metal alloys and semiconductors. Herein we have prepared solid solutions in hierarchical solids that are built from atomically precise clusters. Two geometrically similar metal chalcogenide clusters, Co6Se8(PEt3)6 and Cr6Te8(PEt3)6, were combined as random substitutional mixture, in three different ratios, in a crystal lattice together with fullerenes. This does not alter the underlying crystalline structure of the [cluster][C60]2 material, but it influences its electronic and magnetic properties. All three solid solutions showed increased electrical conductivities compared with either the Co- or Cr-based parent material, substantially so for two of the Co:Cr ratios (up to 100-fold), and lowered activation barriers for electron transport. We attribute this to the existence of additional energy states arising from the materials' structural heterogeneity, which effectively narrow transport gaps.

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.

Modulating the hierarchical fibrous assembly of Au nanoparticles with atomic precision.

The ability to modulate nanoparticle (NP) assemblies with atomic precision is still lacking, which hinders us from creating hierarchical NP organizations with desired properties. In this work, a hierarchical fibrous (1D to 3D) assembly of Au NPs (21-gold atom, Au21) is realized and further modulated with atomic precision via site-specific tailoring of the surface hook (composed of four phenyl-containing ligands with a counteranion). Interestingly, tailoring of the associated counterion significantly changes the electrical transport properties of the NP-assembled solids by two orders of magnitude due to the altered configuration of the interacting π-π pairs of the surface hooks. Overall, our success in atomic-level modulation of the hierarchical NP assembly directly evidences how the NP ligands and associated counterions can function to guide the 1D, 2D, and 3D hierarchical self-assembly of NPs in a delicate manner. This work expands nanochemists' skills in rationally programming the hierarchical NP assemblies with controllable structures and properties.

The genomic landscape of diffuse intrinsic pontine glioma and pediatric non-brainstem high-grade glioma.

Pediatric high-grade glioma (HGG) is a devastating disease with a less than 20% survival rate 2 years after diagnosis. We analyzed 127 pediatric HGGs, including diffuse intrinsic pontine gliomas (DIPGs) and non-brainstem HGGs (NBS-HGGs), by whole-genome, whole-exome and/or transcriptome sequencing. We identified recurrent somatic mutations in ACVR1 exclusively in DIPGs (32%), in addition to previously reported frequent somatic mutations in histone H3 genes, TP53 and ATRX, in both DIPGs and NBS-HGGs. Structural variants generating fusion genes were found in 47% of DIPGs and NBS-HGGs, with recurrent fusions involving the neurotrophin receptor genes NTRK1, NTRK2 and NTRK3 in 40% of NBS-HGGs in infants. Mutations targeting receptor tyrosine kinase-RAS-PI3K signaling, histone modification or chromatin remodeling, and cell cycle regulation were found in 68%, 73% and 59% of pediatric HGGs, respectively, including in DIPGs and NBS-HGGs. This comprehensive analysis provides insights into the unique and shared pathways driving pediatric HGG within and outside the brainstem.