15 research outputs found
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
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>1Ģ
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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] 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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
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>1Ģ
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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] 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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
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>1Ģ
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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] 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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
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>1Ģ
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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] 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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
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>1Ģ
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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] 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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
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>1Ģ
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-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] 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
Re<sub>6</sub>Se<sub>8</sub> 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 Re<sub>6</sub>Se<sub>8</sub> clusters. The material, a 2D structural
analogue of the Chevrel phase, is prepared via mechanical exfoliation
of the van der Waals solid Re<sub>6</sub>Se<sub>8</sub>Cl<sub>2</sub>. 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.
Re<sub>6</sub>Se<sub>8</sub>Cl<sub>2</sub> 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 <sup>1</sup>H spinālattice relaxation experiments,
and calculations of rotational barriers, provide a description (i)
of the way in which the rotatorsā 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
CāH<sub>rotator</sub>Ā·Ā·Ā·I<sub>stator</sub> hydrogen
bond cloud across a CāI<sub>stator</sub>Ā·Ā·Ā·I<sub>stator</sub>āC 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-bisĀ(carboxyethynyl)Ābicyclo[2.2.2]Āoctane (BABCO)
functional rotor. One, [<i>n</i>Bu<sub>4</sub>N<sup>+</sup>]<sub>2</sub>[BABCO]Ā[BABCO<sup>ā</sup>]<sub>2</sub>, 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-CONH<sub>2</sub>)<sub>2</sub><sup>+</sup> (EDT-TTF = ethylenedithiotetrathiafulvalene)
and anionic [BABCO<sup>ā</sup>] rotors. Using variable-temperature
(5ā300 K) proton spinālattice relaxation, <sup>1</sup>H T<sub>1</sub><sup>ā1</sup>, we were able to assign two types
of Brownian rotators in [<i>n</i>Bu<sub>4</sub>N<sup>+</sup>]<sub>2</sub>[BABCO]Ā[BABCO<sup>ā</sup>]<sub>2</sub>. 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<sup>ā1</sup>. Rotors on the BABCO<sup>ā</sup> sites experience stochastic
32 GHz jumps at the same temperature over a rotational barrier of
2.72 kcal mol<sup>ā1</sup>. In contrast, the BABCO<sup>ā</sup> rotors within the highly conducting crystals of (EDT-TTF-CONH<sub>2</sub>)<sub>2</sub><sup>+</sup>[BABCO<sup>ā</sup>] are essentially
ābrakedā at room temperature. Notably, these crystals
possess a conductivity of 5 S cm<sup>ā1</sup> at 1 bar, which
increases rapidly with pressure up to 50 S cm<sup>ā1</sup> at
11.5 kbar. Two regimes with different activation energies <i>E</i><sub>a</sub> 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<sub>sp3</sub>āHĀ·Ā·Ā·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<sup>ā1</sup>. 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