5 research outputs found
Thermodynamic Control of Two-Dimensional Molecular Ionic Nanostructures on Metal Surfaces
Bulk molecular ionic solids exhibit
fascinating electronic properties,
including electron correlations, phase transitions, and superconducting
ground states. In contrast, few of these phenomena have been observed
in low-dimensional molecular structures, including thin films, nanoparticles,
and molecular blends, not in the least because most of such structures
have been composed of nearly closed-shell molecules. It is therefore
desirable to develop low-dimensional ionic molecular structures that
can capture potential applications. Here, we present detailed analysis
of monolayer-thick structures of the canonical TTF–TCNQ (tetrathiafulvalene
7,7,8,8-tetracyanoquinodimethane) system grown on low-index gold and
silver surfaces. The most distinctive property of the epitaxial growth
is the wide abundance of stable TTF/TCNQ ratios, in sharp contrast
to the predominance of a 1:1 ratio in the bulk. We propose the existence
of the surface phase diagram that controls the structures of TTF–TCNQ
on the surfaces and demonstrate phase transitions that occur upon
progressively increasing the density of TCNQ while keeping the surface
coverage of TTF fixed. Based on direct observations, we propose the
binding motif behind the stable phases and infer the dominant interactions
that enable the existence of the rich spectrum of surface structures.
Finally, we also show that the surface phase diagram will control
the epitaxy beyond monolayer coverage. Multiplicity of stable surface
structures, the corollary rich phase diagram, and the corresponding
phase transitions present an interesting opportunity for low-dimensional
molecular systems, particularly if some of the electronic properties
of the bulk can be preserved or modified in the surface phases
Enhanced Dynamics of Hydrated tRNA on Nanodiamond Surfaces: A Combined Neutron Scattering and MD Simulation Study
Nontoxic, biocompatible nanodiamonds
(ND) have recently been implemented
in rational, systematic design of optimal therapeutic use in nanomedicines.
However, hydrophilicity of the ND surface strongly influences structure
and dynamics of biomolecules that restrict <i>in situ</i> applications of ND. Therefore, fundamental understanding of the
impact of hydrophilic ND surface on biomolecules at the molecular
level is essential. For tRNA, we observe an enhancement of dynamical
behavior in the presence of ND contrary to generally observed slow
motion at strongly interacting interfaces. We took advantage of neutron
scattering experiments and computer simulations to demonstrate this
atypical faster dynamics of tRNA on ND surface. The strong attractive
interactions between ND, tRNA, and water give rise to unlike dynamical
behavior and structural changes of tRNA in front of ND compared to
without ND. Our new findings may provide new design principles for
safer, improved drug delivery platforms
High‑<i>T</i><sub>c</sub> Layered Ferrielectric Crystals by Coherent Spinodal Decomposition
Research in the rapidly developing field of 2D electronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as nonlinear dielectrics are needed to enable realistic device architectures. Candidate materials require tunable dielectric properties and pathways for heterostructure assembly. Here we report on a family of cation-deficient transition metal thiophosphates whose unique chemistry makes them a viable prospect for these applications. In these materials, naturally occurring ferrielectric heterostructures composed of centrosymmetric In<sub>4/3</sub>P<sub>2</sub>S<sub>6</sub> and ferrielectrically active CuInP<sub>2</sub>S<sub>6</sub> are realized by controllable chemical phase separation in van der Waals bonded single crystals. CuInP<sub>2</sub>S<sub>6</sub> by itself is a layered ferrielectric with a ferrielectric transition temperature (<i>T</i><sub>c</sub>) just over room temperature, which rapidly decreases with homogeneous doping. Surprisingly, in our composite materials, the ferrielectric <i>T</i><sub>c</sub> of the polar CuInP<sub>2</sub>S<sub>6</sub> phase increases. This effect is enabled by unique spinodal decomposition that retains the overall van der Waals layered morphology of the crystal, but chemically separates CuInP<sub>2</sub>S<sub>6</sub> and In<sub>4/3</sub>P<sub>2</sub>S<sub>6</sub> within each layer. The average spatial periodicity of the distinct chemical phases can be finely controlled by altering the composition and/or synthesis conditions. One intriguing prospect for such layered spinodal alloys is large volume synthesis of 2D in-plane heterostructures with periodically alternating polar and nonpolar phases
Influence of Nonstoichiometry on Proton Conductivity in Thin-Film Yttrium-Doped Barium Zirconate
Proton-conducting
perovskites have been widely studied because of their potential application
as solid electrolytes in intermediate temperature solid oxide fuel
cells. Structural and chemical heterogeneities can develop during
synthesis, device fabrication, or service, which can profoundly affect
proton transport. Here, we use time-resolved Kelvin probe force microscopy,
scanning transmission electron microscopy, atom probe tomography,
and density functional theory calculations to intentionally introduce
Ba-deficient planar and spherical defects and link the resultant atomic
structure with proton transport behavior in both stoichiometric and
nonstoichiometric epitaxial, yttrium-doped barium zirconate thin films.
The defects were intentionally induced through high-temperature annealing
treatment, while maintaining the epitaxial single crystalline structure
of the films, with an overall relaxation in the atomic structure.
The annealed samples showed smaller magnitudes of local lattice distortions
because of the formation of proton polarons, thereby leading to decreased
proton-trapping effect. This resulted in a decrease in the activation
energy for proton transport, leading to faster proton transport
Cation–Eutectic Transition <i>via</i> Sublattice Melting in CuInP<sub>2</sub>S<sub>6</sub>/In<sub>4/3</sub>P<sub>2</sub>S<sub>6</sub> van der Waals Layered Crystals
Single crystals of the van der Waals
layered ferrielectric material CuInP<sub>2</sub>S<sub>6</sub> spontaneously
phase separate when synthesized with Cu deficiency. Here we identify
a route to form and tune intralayer heterostructures between the corresponding
ferrielectric (CuInP<sub>2</sub>S<sub>6</sub>) and paraelectric (In<sub>4/3</sub>P<sub>2</sub>S<sub>6</sub>) phases through control of chemical
phase separation. We conclusively demonstrate that Cu-deficient Cu<sub>1–<i>x</i></sub>In<sub>1+<i>x</i>/3</sub>P<sub>2</sub>S<sub>6</sub> forms a single phase at high temperature.
We also identify the mechanism by which the phase separation proceeds
upon cooling. Above 500 K both Cu<sup>+</sup> and In<sup>3+</sup> become
mobile, while P<sub>2</sub>S<sub>6</sub><sup>4–</sup> anions
maintain their structure. We therefore propose that this transition
can be understood as eutectic melting on the cation sublattice. Such
a model suggests that the transition temperature for the melting process
is relatively low because it requires only a partial reorganization
of the crystal lattice. As a result, varying the cooling rate through
the phase transition controls the lateral extent of chemical domains
over several decades in size. At the fastest cooling rate, the dimensional
confinement of the ferrielectric CuInP<sub>2</sub>S<sub>6</sub> phase
to nanoscale dimensions suppresses ferrielectric ordering due to the
intrinsic ferroelectric size effect. Intralayer heterostructures can
be formed, destroyed, and re-formed by thermal cycling, thus enabling
the possibility of finely tuned ferroic structures that can potentially
be optimized for specific device architectures