22 research outputs found
Unraveling the effects of inter-site Hubbard interactions in spinel Li-ion cathode materials
Accurate first-principles predictions of the structural, electronic,
magnetic, and electrochemical properties of cathode materials can be key in the
design of novel efficient Li-ion batteries. Spinel-type cathode materials
LiMnO and LiMnNiO are promising candidates
for Li-ion battery technologies, but they present serious challenges when it
comes to their first-principles modeling. Here, we use density-functional
theory with extended Hubbard functionals - DFT++ with on-site and
inter-site Hubbard interactions - to study the properties of these
transition-metal oxides. The Hubbard parameters are computed from
first-principles using density-functional perturbation theory. We show that
while is crucial to obtain the right trends in properties of these
materials, is essential for a quantitative description of the structural
and electronic properties, as well as the Li-intercalation voltages. This work
paves the way for reliable first-principles studies of other families of
cathode materials without relying on empirical fitting or calibration
procedures
Understanding the role of Hubbard corrections in the rhombohedral phase of BaTiO
We present a first-principles study of the low-temperature rhombohedral phase
of BaTiO using Hubbard-corrected density-functional theory. By employing
density-functional perturbation theory, we compute the onsite Hubbard for
Ti() states and the intersite Hubbard between Ti() and O()
states. We show that applying the onsite Hubbard correction alone to
Ti() states proves detrimental, as it suppresses the Ti()-O()
hybridization and drives the system towards a cubic phase. Conversely, when
both onsite and intersite are considered, the localized character of
the Ti() states is maintained, while also preserving the Ti()-O()
hybridization, restoring the rhombohedral phase of BaTiO. The generalized
PBEsol++ functional yields remarkable agreement with experimental results
for the band gap and dielectric constant, while the optimized geometry is
slightly less accurate compared to PBEsol. Zone-center phonon frequencies and
Raman spectra, being significantly influenced by the underlying geometry,
demonstrate better agreement with experiments in the case of PBEsol, while
PBEsol++ exhibits reduced accuracy, and the PBEsol+ Raman spectrum
diverges remarkably from experimental data, highlighting the adverse impact of
the correction alone in BaTiO. Our findings underscore the promise of
the extended Hubbard PBEsol++ functional with first-principles and
for the investigation of other ferroelectric perovskites with mixed
ionic-covalent interactions
Habituation based synaptic plasticity and organismic learning in a quantum perovskite
A central characteristic of living beings is the ability to learn from and respond to their environment leading to habit formation and decision making. This behavior, known as habituation, is universal among all forms of life with a central nervous system, and is also observed in single-cell organisms that do not possess a brain. Here, we report the discovery of habituation-based plasticity utilizing a perovskite quantum system by dynamical modulation of electron localization. Microscopic mechanisms and pathways that enable this organismic collective charge-lattice interaction are elucidated by first-principles theory, synchrotron investigations, ab initio molecular dynamics simulations, and in situ environmental breathing studies. We implement a learning algorithm inspired by the conductance relaxation behavior of perovskites that naturally incorporates habituation, and demonstrate learning to forget: A key feature of animal and human brains. Incorporating this elementary skill in learning boosts the capability of neural computing in a sequential, dynamic environment.United States. Army Research Office (Grant W911NF-16-1-0289)United States. Air Force Office of Scientific Research (Grant FA9550-16-1-0159)United States. Army Research Office (Grant W911NF-16-1-0042
Charge Transport in Molecular Junctions: A Study of Level-Alignment, Thermoelectric Properties, and Environmental Effects
Here, we use and develop first-principles methods based on density functional theory (DFT) and beyond to understand and predict charge transport phenomena in the novel class of nanostructured devices: molecular junctions. Molecular junctions, individual molecules contacted to two metallic leads, which can be systematically altered by modifying the chemistry of each component, serve as test beds for the study of transport at the nanoscale. To date, various experimental methods have been designed to reliably assemble and mea- sure transport properties of molecular junctions. Furthermore, theoretical methods built on DFT designed to yield quantitative agreement with these experiments for certain classes of molecular junctions have been developed. In order to gain insight into a broader range of molecular junctions and environmental effects associated with the surrounding solution, this dissertation will employ, explore and extend first-principles DFT calculations coupled with approximate self-energy corrections known to yield quantitative agreement with experiments for certain classes of molecular junctions.To start we examine molecular junctions in which the molecule is strongly hybridized with the leads: a challenging limit for the existing methodology. Using a physically motivated tight-binding model, we find that the experimental trends observed for such molecules can be explained by the presence of a so-called “gateway” state associated with the chemical bond that bridges the molecule and the lead. We discuss the ingredients of a self-energy corrected DFT based approach to quantitatively predict conductance in the presence of these hybridization effects.We also develop and apply an approach to account for the surrounding environment on the conductance, which has been predominantly ignored in past transport calculations due to computational complexity. Many experiments are performed in a solution of non-conducting molecules; far from benign, this solution is known to impact the measured conductance by as much as a factor of two. Here, we show that the dominant effect of the solution stems from nearby molecules binding to the lead surface surrounding the junction and altering the local electrostatics. This effect operates in much the same way adsorbates alter the work function of a surface. We develop a framework which implicitly includes the surrounding molecules through an electrostatic-based lattice model with parameters from DFT calculations, reducing the computational complexity of this problem while retaining predictive power. Our approach for computing environmental effects on charge transport in such junctions will pave the way for a better understanding of the physics of nanoscale devices, which are known to be highly sensitive to their surroundings
Determination of Energy Level Alignment and Coupling Strength in 4,4′-Bipyridine Single-Molecule Junctions
We measure conductance and thermopower
of single Au–4,4′-bipyridine–Au
junctions in distinct low and high conductance binding geometries
accessed by modulating the electrode separation. We use these data
to determine the electronic energy level alignment and coupling strength
for these junctions, which are known to conduct through the lowest
unoccupied molecular orbital (LUMO). Contrary to intuition, we find
that, in the high-conductance junction, the LUMO resonance energy
is further away from the Au Fermi energy than in the low-conductance
junction. However, the LUMO of the high-conducting junction is better
coupled to the electrode. These results are in good quantitative agreement
with self-energy corrected zero-bias density functional theory calculations.
Our calculations show further that measurements of conductance and
thermopower in amine-terminated oligophenyl–Au junctions, where
conduction occurs through the highest occupied molecular orbitals,
cannot be used to extract electronic parameters as their transmission
functions do not follow a simple Lorentzian form
Tunable Charge Transport in Single-Molecule Junctions via Electrolytic Gating
We modulate the conductance of electrochemically
inactive molecules
in single-molecule junctions using an electrolytic gate to controllably
tune the energy level alignment of the system. Molecular junctions
that conduct through their highest occupied molecular orbital show
a decrease in conductance when applying a positive electrochemical
potential, and those that conduct though their lowest unoccupied molecular
orbital show the opposite trend. We fit the experimentally measured
conductance data as a function of gate voltage with a Lorentzian function
and find the fitting parameters to be in quantitative agreement with
self-energy corrected density functional theory calculations of transmission
probability across single-molecule junctions. This work shows that
electrochemical gating can directly modulate the alignment of the
conducting orbital relative to the metal Fermi energy, thereby changing
the junction transport properties