186 research outputs found
The unique chemical reactivity of a graphene nanoribbon's zigzag edge
The zigzag edge of a graphene nanoribbon possesses a unique electronic state
that is near the Fermi level and localized at the edge carbon atoms. We
investigate the chemical reactivity of these zigzag edge sites by examining
their reaction energetics with common radicals from first principles. A
"partial radical" concept for the edge carbon atoms is introduced to
characterize their chemical reactivity, and the validity of this concept is
verified by comparing the dissociation energies of edge-radical bonds with
similar bonds in molecules. In addition, the uniqueness of the zigzag-edged
graphene nanoribbon is further demonstrated by comparing it with other forms of
sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged
graphene nanoribbon.Comment: 24 pages, 9 figure
Thermodynamic effects in morphological evolution of polymer-fullerene nanocomposites for photovoltaic applications
Polymer based photovoltaic devices promise solar technologies that are inexpensive enough to be widely exploited and therefore provide a significant fraction of the future energy needs. There are many promising polymer-fullerene mixtures that are promising materials candidates for achieving high performance devices, but their exploitation requires and improved understanding of their structure-property relationships. Of particular relevance is the phase behavior of the mixtures.
The phase behavior of donor-acceptor materials for photovoltaic applications is of key importance [1,2]: i) to gain a fundamental understanding and control of morphology development in the donor-acceptor blends; ii) to appropriately choose the operating window for thermal annealing; iii) to understand the long-term stability of the blended film morphology and consequently of the photovoltaic performance of the corresponding solar cells.
In this work the phase behavior of polymer-fullerene mixtures is being studied using Differential Scanning Calorimetry (DSC), Wide-Angle X-Ray Scattering (WAXS), Small-Angle Neutron Scattering (SANS) and theoretical ab initio Density Functional Theory (DFT) calculations
Scale-free switching of polarization in the layered ferroelectric material CuInPS
Using first-principles calculations we model the out-of-plane switching of
local dipoles in CuInPS (CIPS) that are largely induced by Cu
off-centering. Previously, a coherent switching of polarization via a
quadruple-well potential was proposed for these materials. In the super-cells
we considered, we find multiple structures with similar energies but with
different local polar order. Our results suggest that the individual dipoles
are weakly coupled in-plane and under an electric field at very low
temperatures these dipoles in CIPS should undergo incoherent disordered
switching. The barrier for switching is determined by the single Cu-ion
switching barrier. This in turn suggests a scale-free polarization with a
switching barrier of 203.6-258.0 meV, a factor of five smaller than that
of HfO (1380 meV) a prototypical scale-free ferroelectric. The mechanism of
polarization switching in CIPS is mediated by the switching of each weakly
interacting dipole rather than the macroscopic polarization itself as
previously hypothesized. These findings reconcile prior observations of a
quadruple well with sloping hysteresis loops, large ionic conductivity even at
250~K well below the Curie temperature (315~K), and a significant wake-up
effects where the macroscopic polarization is slow to order and set-in under an
applied electric field. We also find that computed piezoelectric response and
the polarization show a linear dependence on the local dipolar order. This is
consistent with having scale-free polarization and other polarization-dependent
properties and opens doors for engineering tunable metastability by-design in
CIPS (and related family of materials) for neuromorphic applications
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