124 research outputs found
Defect-Driven Anomalous Transport in Fast-Ion Conducting Solid Electrolytes
Solid-state ionic conduction is a key enabler of electrochemical energy
storage and conversion. The mechanistic connections between material
processing, defect chemistry, transport dynamics, and practical performance are
of considerable importance, but remain incomplete. Here, inspired by studies of
fluids and biophysical systems, we re-examine anomalous diffusion in the iconic
two-dimensional fast-ion conductors, the - and
-aluminas. Using large-scale simulations, we reproduce
the frequency dependence of alternating-current ionic conductivity data. We
show how the distribution of charge-compensating defects, modulated by
processing, drives static and dynamic disorder, which lead to persistent
sub-diffusive ion transport at macroscopic timescales. We deconvolute the
effects of repulsions between mobile ions, the attraction between the mobile
ions and charge-compensating defects, and geometric crowding on ionic
conductivity. Our quantitative framework based on these model solid
electrolytes connects their atomistic defect chemistry to macroscopic
performance with minimal assumptions and enables mechanism-driven
'atoms-to-device' optimization of fast-ion conductors.Comment: 45 pages, 23 figures. Additional code is available at
https://github.com/apoletayev/anomalous_ion_conductio
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Light-Induced Currents at Domain Walls in Multiferroic BiFeO3.
Multiferroic BiFeO3 (BFO) films with spontaneously formed periodic stripe domains can generate above-gap open circuit voltages under visible light illumination; nevertheless the underlying mechanism behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters
The persistence of memory in ionic conduction probed by nonlinear optics
Predicting practical rates of transport in condensed phases enables the rational design of materials, devices and processes. This is especially critical to developing low-carbon energy technologies such as rechargeable batteries1,2,3. For ionic conduction, the collective mechanisms4,5, variation of conductivity with timescales6,7,8 and confinement9,10, and ambiguity in the phononic origin of translation11,12, call for a direct probe of the fundamental steps of ionic diffusion: ion hops. However, such hops are rare-event large-amplitude translations, and are challenging to excite and detect. Here we use single-cycle terahertz pumps to impulsively trigger ionic hopping in battery solid electrolytes. This is visualized by an induced transient birefringence, enabling direct probing of anisotropy in ionic hopping on the picosecond timescale. The relaxation of the transient signal measures the decay of orientational memory, and the production of entropy in diffusion. We extend experimental results using in silico transient birefringence to identify vibrational attempt frequencies for ion hopping. Using nonlinear optical methods, we probe ion transport at its fastest limit, distinguish correlated conduction mechanisms from a true random walk at the atomic scale, and demonstrate the connection between activated transport and the thermodynamics of information
The Persistence of Memory in Ionic Conduction Probed by Nonlinear Optics
Predicting practical rates of ion transport from atomistic descriptors
enables the rational design of materials, devices, and processes, which is
especially critical to developing low-carbon energy technologies such as
rechargeable batteries. The correlated mechanisms of ionic conduction,
variation of conductivity with timescale and confinement, and ambiguity in the
vibrational origin of translation, the attempt frequency, call for a direct
atomic probe of the most fundamental steps of ionic diffusion: ion hops.
However, such hops are rare-event large-amplitude translations, and are
challenging to excite and detect. Here we use single-cycle terahertz pumps to
impulsively trigger ionic hopping in battery solid electrolytes. This is
visualized by an induced transient birefringence enabling direct probing of
anisotropy in ionic hopping on the picosecond timescale. The relaxation of the
transient signal measures the decay of orientational memory, and the production
of entropy in diffusion. We extend experimental results using in silico
transient birefringence to identify attempt frequencies for ion hopping. Using
nonlinear optical methods, we probe ion transport at its fastest limit,
distinguish correlated conduction mechanisms from a true random walk at the
atomic scale, and demonstrate the connection between activated transport and
the thermodynamics of information.Comment: 41 pages, 22 figure
Picosecond electric-field-induced threshold switching in phase-change materials
Many chalcogenide glasses undergo a breakdown in electronic resistance above
a critical field strength. Known as threshold switching, this mechanism enables
field-induced crystallization in emerging phase-change memory. Purely
electronic as well as crystal nucleation assisted models have been employed to
explain the electronic breakdown. Here, picosecond electric pulses are used to
excite amorphous AgInSbTe. Field-dependent reversible
changes in conductivity and pulse-driven crystallization are observed. The
present results show that threshold switching can take place within the
electric pulse on sub-picosecond time-scales - faster than crystals can
nucleate. This supports purely electronic models of threshold switching and
reveals potential applications as an ultrafast electronic switch.Comment: 6 pages manuscript with 3 figures and 8 pages supplementary materia
Picosecond carrier recombination dynamics in chalcogen-hyperdoped silicon
Intermediate-band materials have the potential to be highly efficient solar cells and can be fabricated by incorporating ultrahigh concentrations of deep-level dopants. Direct measurements of the ultrafast carrier recombination processes under supersaturated dopant concentrations have not been previously conducted. Here, we use optical-pump/terahertz-probe measurements to study carrier recombination dynamics of chalcogen-hyperdoped silicon with sub-picosecond resolution. The recombination dynamics is described by two exponential decay time scales: a fast decay time scale ranges between 1 and 200 ps followed by a slow decay on the order of 1 ns. In contrast to the prior theoretical predictions, we find that the carrier lifetime decreases with increasing dopant concentration up to and above the insulator-to-metal transition. Evaluating the material's figure of merit reveals an optimum doping concentration for maximizing performance.Center for Clean Water and Clean Energy at MIT and KFUPMNational Science Foundation (U.S.) (Grant Contract ECCS-1102050)National Science Foundation (U.S.) (United States. Dept. of Energy Contract EEC-1041895
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Hidden phonon highways promote photoinduced interlayer energy transfer in twisted transition metal dichalcogenide heterostructures.
Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS2/WS2 at 4° twist angle and WSe2/MoSe2 heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations
Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites.
Femtosecond resolution electron scattering techniques are applied to resolve the first atomic-scale steps following absorption of a photon in the prototypical hybrid perovskite methylammonium lead iodide. Following above-gap photoexcitation, we directly resolve the transfer of energy from hot carriers to the lattice by recording changes in the mean square atomic displacements on 10-ps time scales. Measurements of the time-dependent pair distribution function show an unexpected broadening of the iodine-iodine correlation function while preserving the Pb-I distance. This indicates the formation of a rotationally disordered halide octahedral structure developing on picosecond time scales. This work shows the important role of light-induced structural deformations within the inorganic sublattice in elucidating the unique optoelectronic functionality exhibited by hybrid perovskites and provides new understanding of hot carrier-lattice interactions, which fundamentally determine solar cell efficiencies
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