Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆]

(EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] is a molecular solid with <i>R</i>3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic <i>P</i>1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties

Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆]

(EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] is a molecular solid with <i>R</i>3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic <i>P</i>1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties

Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆]

(EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] is a molecular solid with <i>R</i>3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic <i>P</i>1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties

Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆]

(EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] is a molecular solid with <i>R</i>3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic <i>P</i>1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties

Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆]

(EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] is a molecular solid with <i>R</i>3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic <i>P</i>1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties

Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆]

(EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] is a molecular solid with <i>R</i>3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic <i>P</i>1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH₂)₆[Re₆Se₈(CN)₆] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties

Two-Dimensional Hierarchical Semiconductor with Addressable Surfaces

Surfaces play a key role in determining material properties, and their importance is further magnified in the two-dimensional (2D) limit. Though monolayers are entirely composed of surfaces, there is no chemical approach to covalently address them without breaking intralayer bond. Here, we describe a 2D semiconductor that offers two unique features among 2D materials: structural hierarchy within the monolayer and surface reactive sites that enable functionalization. The 2D semiconductor is composed of a single layer of strongly interconnected Re₆Se₈ clusters arranged in an oblique lattice capped by substitutionally labile Cl atoms. We show that a simple ligand substitution strategy borrowed from traditional coordination chemistry can be used to modify the surface of the 2D material while preserving its internal structure. The potential generality of this approach establishes a promising route toward multifunctional 2D materials with tunable physical and chemical properties and may also facilitate better electrical top contact to 2D semiconductors

Superatomic Two-Dimensional Semiconductor

Structural complexity is of fundamental interest in materials science because it often results in unique physical properties and functions. Founded on this idea, the field of solid state chemistry has a long history and continues to be highly active, with new compounds discovered daily. By contrast, the area of two-dimensional (2D) materials is young, but its expansion, although rapid, is limited by a severe lack of structural diversity and complexity. Here, we report a novel 2D semiconductor with a hierarchical structure composed of covalently linked Re₆Se₈ clusters. The material, a 2D structural analogue of the Chevrel phase, is prepared via mechanical exfoliation of the van der Waals solid Re₆Se₈Cl₂. Using scanning tunneling spectroscopy, photoluminescence and ultraviolet photoelectron spectroscopy, and first-principles calculations, we determine the electronic bandgap (1.58 eV), optical bandgap (indirect, 1.48 eV), and exciton binding energy (100 meV) of the material. The latter is consistent with the partially 2D nature of the exciton. Re₆Se₈Cl₂ is the first member of a new family of 2D semiconductors whose structure is built from superatomic building blocks instead of simply atoms; such structures will expand the conceptual design space for 2D materials research

Reversible Control of Crystalline Rotors by Squeezing Their Hydrogen Bond Cloud Across a Halogen Bond-Mediated Phase Transition

We report on a crystalline rotor that undergoes a reversible phase transition at 145 K. Variable-temperature X-ray and ¹H spin–lattice relaxation experiments, and calculations of rotational barriers, provide a description (i) of the way in which the rotators’ dynamics changes back and forth at the onset of the phase transition and (ii) of the mechanism responsible for the abrupt switching of the crystalline rotors from a very low-energy 4-fold degenerate equilibrium state, in which the rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy state associated with a slower motion (2.3 GHz at 145 K). Our results provide evidence that the reversible change observed in the rotational barriers at the transition is due to a cooperative modulation of the C–H_rotator···I_stator hydrogen bond cloud across a C–I_stator···I_stator–C halogen bond-mediated phase transition. In addition, we report evidence for second-harmonic generation from this material, thereby confirming with a second example the benefit of using polarized light to probe the torsional degree of freedom of chiral helix blades, as well as symmetry and dimensionality of large collections of chiral rotors in the solid state

Design and Evaluation of a Crystalline Hybrid of Molecular Conductors and Molecular Rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis­(carboxyethynyl)­bicyclo[2.2.2]­octane (BABCO) functional rotor. One, [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂, is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH₂)₂⁺ (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO^–] rotors. Using variable-temperature (5–300 K) proton spin–lattice relaxation, ¹H T₁^–1, we were able to assign two types of Brownian rotators in [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂. We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol^–1. Rotors on the BABCO^– sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol^–1. In contrast, the BABCO^– rotors within the highly conducting crystals of (EDT-TTF-CONH₂)₂⁺[BABCO^–] are essentially “braked” at room temperature. Notably, these crystals possess a conductivity of 5 S cm^–1 at 1 bar, which increases rapidly with pressure up to 50 S cm^–1 at 11.5 kbar. Two regimes with different activation energies <i>E</i>_a for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C_sp3–H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol^–1. Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers