17 research outputs found
Revealing the free energy landscape of halide perovskites: Metastability and transition characters in CsPbBr and MAPbI
Halide perovskites have emerged as a promising class of materials for
photovoltaic applications. A challenge in these applications is how to prevent
the crystal structure from degradation to photovoltaically inactive phases,
which requires an understanding of the free energy landscape of these
materials. Here, we uncover the free energy landscape of two prototypical
halide perovskites, CsPbBr and MAPbI via atomic scale simulations using
umbrella sampling and machine-learned potentials. For CsPbBr we find very
small free energy differences and barriers close to the transition temperatures
for both the tetragonal-to-cubic and the orthorhombic-to-tetragonal transition.
For MAPbI, however, the situation is more intricate. In particular the
orthorhombic-to-tetragonal transition exhibits a large free energy barrier and
there are several competing tetragonal phases. Using large-scale molecular
dynamics simulations we explore the character of these transition and observe
latent heat and a discrete change in structural parameters for the
tetragonal-to-cubic phase transition in both CsPbBr and MAPbI
indicating first-order transitions. We find that in MAPbI the orthorhombic
phase has an extended metastability range and furthermore identify a second
metastable tetragonal phase. Finally, we compile a phase diagram for MAPbI
that includes potential metastable phases.Comment: 9 pages, 5 figure
icet - A Python library for constructing and sampling alloy cluster expansions
Alloy cluster expansions (CEs) provide an accurate and computationally
efficient mapping of the potential energy surface of multi-component systems
that enables comprehensive sampling of the many-dimensional configuration
space. Here, we introduce \textsc{icet}, a flexible, extensible, and
computationally efficient software package for the construction and sampling of
CEs. \textsc{icet} is largely written in Python for easy integration in
comprehensive workflows, including first-principles calculations for the
generation of reference data and machine learning libraries for training and
validation. The package enables training using a variety of linear regression
algorithms with and without regularization, Bayesian regression, feature
selection, and cross-validation. It also provides complementary functionality
for structure enumeration and mapping as well as data management and analysis.
Potential applications are illustrated by two examples, including the
computation of the phase diagram of a prototypical metallic alloy and the
analysis of chemical ordering in an inorganic semiconductor.Comment: 10 page
Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen
Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multiscale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical" sensitivity) is approximately constant at 180 nm/c(H) for nanodisk diameters of greater than or similar to 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic" sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures
Enhancing the sensitivity of magnetic sensors by 3D metamaterial shells
Magnetic sensors are key elements in our interconnected smart society. Their sensitivity becomes essential for many applications in fields such as biomedicine, computer memories, geophysics, or space exploration. Here we present a universal way of increasing the sensitivity of magnetic sensors by surrounding them with a spherical metamaterial shell with specially designed anisotropic magnetic properties. We analytically demonstrate that the magnetic field in the sensing area is enhanced by our metamaterial shell by a known factor that depends on the shell radii ratio. When the applied field is non-uniform, as for dipolar magnetic field sources, field gradient is increased as well. A proof-of-concept experimental realization confirms the theoretical predictions. The metamaterial shell is also shown to concentrate time-dependent magnetic fields upto frequencies of 100 kHz
High-throughput characterization of transition metal dichalcogenide alloys: Thermodynamic stability and electronic band alignment
Alloying offers a way to tune many of the properties of the transition metal
dichalcogenide (TMD) monolayers. While these systems in many cases have been
thoroughly investigated previously, the fundamental understanding of critical
temperatures, phase diagrams and band edge alignment is still incomplete. Based
on first principles calculations and alloy cluster expansions we compute the
phase diagrams 72 TMD monolayer alloys and classify the mixing behavior. We
show that ordered phases in general are absent at room temperature but that
there exists some alloys, which have a stable Janus phase at room temperature.
Furthermore, for a subset of these alloys, we quantify the band edge bowing and
show that the band edge positions for the mixing alloys can be continuously
tuned in the range set by the boundary phases.Comment: 10 pages, 6 figure
Revealing the Free Energy Landscape of Halide Perovskites: Metastability and Transition Characters in CsPbBr<sub>3</sub> and MAPbI<sub>3</sub>
Halide perovskites have emerged as a promising class
of materials
for photovoltaic applications. A challenge of these applications is
preventing the crystal structure from degrading to photovoltaically
inactive phases, which requires an understanding of the free energy
landscape of these materials. Here, we uncover the free energy landscape
of two prototypical halide perovskites, CsPbBr3 and MAPbI3, via atomic-scale simulations using umbrella sampling and
machine-learned potentials. For CsPbBr3, we find very small
free energy differences and barriers close to the transition temperatures
for both the tetragonal-to-cubic and orthorhombic-to-tetragonal transitions.
For MAPbI3, however, the situation is more intricate. In
particular, the orthorhombic-to-tetragonal transition exhibits a large
free energy barrier, and there are several competing tetragonal phases.
Using large-scale molecular dynamics simulations, we explore the character
of these transitions and observe the latent heat and a discrete change
in the structural parameters for the tetragonal-to-cubic phase transitions
in both CsPbBr3 and MAPbI3, indicating first-order
transitions. We find that in MAPbI3, the orthorhombic phase
has an extended metastability range, and we identify a second metastable
tetragonal phase. Finally, we compile a phase diagram for MAPbI3 that includes potential metastable phases
Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen
Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen
sensors. Here, a multi-scale modeling approach is employed to determine optimal
conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in
hydrogen pressure translate to changes in hydrogen content and eventually the
optical spectrum. At the single particle level, the shift of the plasmon peak
position with hydrogen concentration (i.e., the "optical" sensitivity) is
approximately constant at 180 nm/c_H for nanodisk diameters >~ 100 nm. For
smaller particles, the optical sensitivity is negative and increases with
decreasing diameter, due to the emergence of a second peak originating from
coupling between a localized surface plasmon and interband transitions. In
addition to tracking peak position, the onset of extinction as well as
extinction at fixed wavelengths is considered. We carefully compare the
simulation results with experimental data and assess the potential sources for
discrepancies. Invariably, the results suggest that there is an upper bound for
the optical sensitivity that cannot be overcome by engineering composition
and/or geometry. While the alloy composition has a limited impact on optical
sensitivity, it can strongly affect H uptake and consequently the
"thermodynamic" sensitivity and the detection limit. Here, it is shown how the
latter can be improved by compositional engineering and even substantially
enhanced via the formation of an ordered phase that can be synthesized at
higher hydrogen partial pressures.Comment: 14 pages, 8 figure