Two-dimensional heavy fermions in the van der Waals metal CeSiI.

Heavy-fermion metals are prototype systems for observing emergent quantum phases driven by electronic interactions1-6. A long-standing aspiration is the dimensional reduction of these materials to exert control over their quantum phases7-11, which remains a significant challenge because traditional intermetallic heavy-fermion compounds have three-dimensional atomic and electronic structures. Here we report comprehensive thermodynamic and spectroscopic evidence of an antiferromagnetically ordered heavy-fermion ground state in CeSiI, an intermetallic comprising two-dimensional (2D) metallic sheets held together by weak interlayer van der Waals (vdW) interactions. Owing to its vdW nature, CeSiI has a quasi-2D electronic structure, and we can control its physical dimension through exfoliation. The emergence of coherent hybridization of f and conduction electrons at low temperature is supported by the temperature evolution of angle-resolved photoemission and scanning tunnelling spectra near the Fermi level and by heat capacity measurements. Electrical transport measurements on few-layer flakes reveal heavy-fermion behaviour and magnetic order down to the ultra-thin regime. Our work establishes CeSiI and related materials as a unique platform for studying dimensionally confined heavy fermions in bulk crystals and employing 2D device fabrication techniques and vdW heterostructures12 to manipulate the interplay between Kondo screening, magnetic order and proximity effects.

Topological Radical Pairs Produce Ultrahigh Conductance in Long Molecular Wires.

Molecular one-dimensional topological insulators (1D TIs), which conduct through energetically low-lying topological edge states, can be extremely highly conducting and exhibit a reversed conductance decay, affording them great potential as building blocks for nanoelectronic devices. However, these properties can only be observed at the short length limit. To extend the length at which these anomalous effects can be observed, we design topological oligo[n]emeraldine wires using short 1D TIs as building blocks. As the wire length increases, the number of topological states increases, enabling an increased electronic transmission along the wire; specifically, we show that we can drive over a microampere current through a single ∼5 nm molecular wire, appreciably more than what has been observed in other long wires reported to date. Calculations and experiments show that the longest oligo[7]emeraldine with doped topological states has over 106 enhancements in the transmission compared to its pristine form. The discovery of these highly conductive, long organic wires helps overcome a fundamental hurdle to implementing molecules in complex, nanoscale circuitry: their structures become too insulating at lengths that are useful in designing nanoscale circuits.

A few-layer covalent network of fullerenes.

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces-a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60)∞ by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence spectroscopy reveals the existence of moiré-like superlattices3. More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

Superatom Regiochemistry Dictates the Assembly and Surface Reactivity of a Two-Dimensional Material.

The area of two-dimensional (2D) materials research would benefit greatly from the development of synthetically tunable van der Waals (vdW) materials. While the bottom-up synthesis of 2D frameworks from nanoscale building blocks holds great promise in this quest, there are many remaining hurdles, including the design of building blocks that reliably produce 2D lattices and the growth of macroscopic crystals that can be exfoliated to produce 2D materials. Here we report the regioselective synthesis of the cluster [trans-Co6Se8(CN)4(CO)2]3-/4-, a "superatomic" building block designed to polymerize and assemble into a 2D cyanometalate lattice whose surfaces are chemically addressable. The resulting vdW material, [Co(py)4]2[trans-Co6Se8(CN)4(CO)2], grows as bulk single crystals that can be mechanically exfoliated to produce flakes as thin as bilayers, with photolabile CO ligands on the exfoliated surface. As a proof of concept, we show that these surface CO ligands can be replaced by 4-isocyanoazobenzene under blue light irradiation. This work demonstrates that the bottom-up assembly of layered vdW materials from superatoms is a promising and versatile approach to create 2D materials with tunable physical and chemical properties.

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.

Polytypism, Anisotropic Transport, and Weyl Nodes in the van der Waals Metal TaFeTe4.

Layered van der Waals (vdW) materials belonging to the MM'Te4 structure class have recently received intense attention due to their ability to host exotic electronic transport phenomena, such as in-plane transport anisotropy, Weyl nodes, and superconductivity. Here we report two new vdW materials with strongly anisotropic in-plane structures featuring stripes of metallic TaTe2 and semiconducting FeTe2, α-TaFeTe4 and ß-TaFeTe4. We find that the structure of α-TaFeTe4 produces strongly anisotropic in-plane electronic transport (anisotropy ratio of up to 250%), outcompeting all other vdW metals, and demonstrate that it can be mechanically exfoliated to the two-dimensional (2D) limit. We also explore the possibility that broken inversion symmetry in ß-TaFeTe4 produces Weyl points in the electronic band structure. Eight Weyl nodes slightly below the Fermi energy are computationally identified for ß-TaFeTe4, indicating they may contribute to the transport behavior of this polytype. These findings identify the TaFeTe4 polytypes as an ideal platform for investigation of 2D transport anisotropy and chiral charge transport as a result of broken symmetry.

Surface Pyroelectricity in Cubic SrTiO3.

Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2 -terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3 . The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.

Solution-Processable Superatomic Thin-Films.

Atomically precise nanoscale clusters could assemble into crystalline ionic crystals akin to the atomic ionic solids through the strong electrostatic interactions between the constituent clusters. Here we show that, unlike atomic ionic solids, the electrostatic interactions between nanoscale clusters could be frustrated by using large clusters with long and flexible side-chains so that the ionic cluster pairs do not crystallize. As such, we report ionic superatomic materials that can be easily solution-processed into completely amorphous and homogeneous thin-films. These new amorphous superatomic materials show tunable compositions and new properties that are not achievable in crystals, including very high electrical conductivities of up to 300 S per meter, ultra low thermal conductivities of 0.05 W per meter per degree kelvin, and high optical transparency of up to 92%. We also demonstrate thin-film thermoelectrics with unoptimized ZT values of 0.02 based on the superatomic thin-films. Such properties are competitive to state-of-the-art materials and make superatomic materials promising as a new class of electronic and thermoelectric materials for devices.

Crystal structure of a new polymorph of (2S,3S)-2-amino-3-methyl-penta-noic acid.

A new polymorph of (2S,3S)-2-amino-3-methyl-penta-noic acid, l-isoleucine C6H13NO2, crystallizes in the monoclinic space group P21 with four independent mol-ecules in the asymmetric unit. The mol-ecules are zwitterions. In the crystal, N-H⋯O hydrogen bonds link two pairs of independent mol-ecules and their symmetry-related counterparts to form two types of layers stacked in an anti-parallel manner parallel to (001). The hydro-phobic aliphatic isopropyl groups protrude from these layers.

Polar Imperfections in Amino Acid Crystals: Design, Structure, and Emerging Functionalities.

Crystals are physical arrays delineated by polar surfaces and often contain imperfections of a polar nature. Understanding the structure of such defects on the molecular level is of topical importance since they strongly affect the macroscopic properties of materials. Moreover, polar imperfections in crystals can be created intentionally and specifically designed by doping nonpolar crystals with "tailor-made" additives as dopants, since their incorporation generally takes place in a polar mode. Insertion of dopants also induces a polar deformation of neighboring host molecules, resulting in the creation of polar domains within the crystals. The contribution of the distorted host molecules to the polarity of such domains should be substantial, particularly in crystals composed of molecules with large dipole moments, such as the zwitterionic amino acids, which possess dipole moments as high as ∼14 D. Polar materials are pyroelectric, i.e., they generate surface charge as a result of temperature change. With the application of recent very sensitive instruments for measuring electric currents, coupled with theoretical computations, it has become possible to determine the structure of polar imperfections, including surfaces, at a molecular level. The detection of pyroelectricity requires attachment of electrodes, which might induce various artifacts and modify the surface of the crystal. Therefore, a new method for contactless pyroelectric measurement using X-ray photoelectron spectroscopy was developed and compared to the traditional periodic temperature change technique. Here we describe the molecular-level determination of the structure of imperfections of different natures in molecular crystals and how they affect the macroscopic properties of the crystals, with the following specific examples: (i) Experimental support for the nonclassical crystal growth mechanism as provided by the detection of pyroelectricity from near-surface solvated polar layers present at different faces of nonpolar amino acid crystals. (ii) Enantiomeric disorder in dl-alanine crystals disclosed by detection of anomalously strong pyroelectricity along their nonpolar directions. The presence of such disorder, which is not revealed by accurate diffraction techniques, explains the riddle of their needlelike morphology. (iii) The design of mixed polar crystals of l-asparagine·H2O/l-aspartic acid with controlled degrees of polarity, as determined by pyroelectricity and X-ray diffraction, and their use in mechanistic studies of electrofreezing of supercooled water. (iv) Pyroelectricity coupled with dispersion-corrected density functional theory calculations and molecular dynamics simulations as an analytical method for the molecular-level determination of the structure of polar domains created by doping of α-glycine crystals with different l-amino acids at concentrations below 0.5%. (v) Selective insertion of minute amounts of alcohols within the bulk of α-glycine crystals, elucidating their role as inducers of the metastable ß-glycine polymorph. In conclusion, the various examples demonstrate that although these imperfections are present in minute amounts, they can be detected by the sensitive pyroelectric measurement, and by combining them with theoretical computations one can elucidate their diverse emerging functionalities.

The Contribution of Pyroelectricity of AgI Crystals to Ice Nucleation.

The pyroelectricity of AgI crystals strongly affects the icing temperature of super-cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super-cooled water, separately on each of the expressed Ag+ and I- hemihedral surfaces. The positive pyroelectric charge at the silver-enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide-enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.

Solvent-Induced Crystal Polymorphism as Studied by Pyroelectric Measurements and Impedance Spectroscopy: Alcohols as Tailor-Made Inhibitors of α-Glycine.

Metastable polymorphs commonly emerge when the formation of the stable analogues is inhibited by using different solvents or auxiliaries. Herein, we report that when glycine is grown in aqueous solutions in the presence of low concentrations of different co-solvents, only alcohols and acetone, unlike water and acetic acid, are selectively incorporated in minute amounts within the bulk of the α-polymorph. These findings demonstrate that although water binds more strongly to the growing face of the crystal, alcohols and acetone are exclusively incorporated, and thus serve as efficient inhibitors of this polymorph, leading to the precipitation of the ß-form. These solvents then create polar domains detectable by pyroelectric measurements and impedance spectroscopy. These results suggest that in the control of crystal polymorphism with co-solvents, one should consider also the different desolvation rates in addition to the energy of binding to the growing faces of the crystal.

Tetragonal CH3NH3PbI3 is ferroelectric.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

Nonclassical Crystal Growth as Explanation for the Riddle of Polarity in Centrosymmetric Glycine Crystals.

The riddle of anomalous polar behavior of the centrosymmetric crystal of α-glycine is resolved by the discovery of a polar, several hundred nanometer thick hydrated layer, created at the {010} faces during crystal growth. This layer was detected by two independent pyroelectric analytical methods: (i) periodic temperature change technique (Chynoweth) at ambient conditions and (ii) contactless X-ray photoelectron spectroscopy under ultrahigh vacuum. The total polarization of the surface layer is extremely large, yielding ≈1 µC·cm-2, and is preserved in ultrahigh vacuum, but disappears upon heating to 100 °C. Molecular dynamics simulations corroborate the formation of polar hydrated layers at the sub-microsecond time scale, however with a thickness of only several nanometers, not several hundred. This inconsistency might be reconciled by invoking a three-step nonclassical crystal growth mechanism comprising (i) docking of clusters from the supersaturated solution onto the evolving crystal, (ii) surface recognition and polar induction, and (iii) annealing and dehydration, followed by site-selective recrystallization.

Unusually Large Young's Moduli of Amino Acid Molecular Crystals.

Young's moduli of selected amino acid molecular crystals were studied both experimentally and computationally using nanoindentation and dispersion-corrected density functional theory. The Young modulus is found to be strongly facet-dependent, with some facets exhibiting exceptionally high values (as large as 44 GPa). The magnitude of Young's modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding network and the measured facet. Furthermore, we show computationally that the Young modulus can be as large as 70-90 GPa if facets perpendicular to the primary direction of the hydrogen-bonding network can be stabilized. This value is remarkably high for a molecular solid and suggests the design of hydrogen-bond networks as a route for rational design of ultra-stiff molecular solids.