Back-and-Forth Energy Transfer during Electronic Relaxation in a Chlorin-Perylene Dyad.

Donor-acceptor dyads represent a practical approach to tuning the photophysical properties of linear conjugated polymers in materials chemistry. Depending on the absorption wavelength, the acceptor and donor roles can be interchanged, and as such, the directionality of the energy transfer can be controlled. Herein, nonadiabatic excited state molecular dynamics simulations have been performed in an arylethylene-linked perylene-chlorin dyad.

Excitation Energy Transfer between bodipy Dyes in a Symmetric Molecular Excitonic Seesaw.

We examine the redistribution of energy between electronic and vibrational degrees of freedom that takes place between a π-conjugated oligomer, a phenylene-butadiynylene, and two identical boron-dipyrromethene (bodipy) end-caps using femtosecond transient absorption spectroscopy, single-molecule spectroscopy, and nonadiabatic excited-state molecular dynamics (NEXMD) modeling techniques. The molecular structure represents an excitonic seesaw in that the excitation energy on the oligomer backbone can migrate to either one end-cap or the other, but not to both. The NEXMD simulations closely reproduce the characteristic time scale for redistribution of electronic and vibrational energy of 2.

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study.

We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs.

Photoinduced Energy Transfer in Linear Guest-Host Chromophores: A Computational Study.

Polymer-based guest-host systems represent a promising class of materials for efficient light-emitting diodes. The energy transfer from the polymer host to the guest is the key process in light generation. Therefore, microscopic descriptions of the different mechanisms involved in the energy transfer can contribute to enlighten the basis of the highly efficient light harvesting observed in this kind of materials.

Microcrystal Electron Diffraction for Molecular Design of Functional Non-Fullerene Acceptor Structures.

Understanding the relationship between molecular structure and solid-state arrangement informs about the design of new organic semiconductor (OSC) materials with improved optoelectronic properties. However, determining their atomic structure remains challenging. Here, we report the lattice organization of two non-fullerene acceptors (NFAs) determined using microcrystal electron diffraction (MicroED) from crystals not traceable by X-ray crystallography.

Photoexcitation dynamics in perylene diimide dimers.

We utilize first-principles theory to investigate photo-induced excited-state dynamics of functionalized perylene diimide. This class of materials is highly suitable for solar energy conversion because of the strong optical absorbance, efficient energy transfer, and chemical tunability. We couple time-dependent density functional theory to a recently developed time-resolved non-adiabatic dynamics approach based on a semi-empirical description.

Incommensurate Spin Fluctuations in the Spin-Triplet Superconductor Candidate UTe_{2}.

Spin-triplet superconductors are of extensive current interest because they can host topological state and Majorana fermions important for quantum computation. The uranium-based heavy-fermion superconductor UTe_{2} has been argued as a spin-triplet superconductor similar to UGe_{2}, URhGe, and UCoGe, where the superconducting phase is near (or coexists with) a ferromagnetic (FM) instability and spin-triplet electron pairing is driven by FM spin fluctuations. Here we use neutron scattering to show that, although UTe_{2} exhibits no static magnetic order down to 0.

Photoinduced Dynamics with Constrained Vibrational Motion: FrozeNM Algorithm.

Ab initio molecular dynamics (AIMD) simulation, analyzed in terms of vibrational normal modes, is a widely used technique that facilitates understanding of complex structural motions and coupling between electronic and nuclear degrees of freedom. Usually, only a subset of vibrations is directly involved in the process of interest. The impact of these vibrations can be evaluated by performing AIMD simulations by selectively freezing certain motions.

Machine learning approaches for structural and thermodynamic properties of a Lennard-Jones fluid.

Predicting the functional properties of many molecular systems relies on understanding how atomistic interactions give rise to macroscale observables. However, current attempts to develop predictive models for the structural and thermodynamic properties of condensed-phase systems often rely on extensive parameter fitting to empirically selected functional forms whose effectiveness is limited to a narrow range of physical conditions. In this article, we illustrate how these traditional fitting paradigms can be superseded using machine learning.

Electronic Energy Relaxation in a Photoexcited Fully Fused Edge-Sharing Carbon Nanobelt.

Carbon nanobelts are cylindrical molecules composed of fully fused edge-sharing arene rings. Because of their aesthetically appealing structures, they acquire unusual optoelectronic properties that are potentially suitable for a range of applications in nanoelectronics and photonics. Nevertheless, the very limited success of their synthesis has led to their photophysical properties remaining largely unknown.

From Trivial Kondo Insulator Ce_{3}Pt_{3}Bi_{4} to Topological Nodal-Line Semimetal Ce_{3}Pd_{3}Bi_{4}.

Using the density functional theory combined with dynamical mean-field theory, we have performed systematic study of the electronic structure and its band topology properties of Ce_{3}Pt_{3}Bi_{4} and Ce_{3}Pd_{3}Bi_{4}. At high temperatures (∼290  K), the electronic structures of both compounds resemble the open-core 4f density functional calculation results. For Ce_{3}Pt_{3}Bi_{4}, clear hybridization gap can be observed below 72 K, and its coherent momentum-resolved spectral function below 18 K exhibits an topologically trivial indirect gap of ∼6  meV and resembles density functional band structure with itinerant 4f state.

Determination of Structural Correlation Functions.

Determining the structural properties of condensed-phase systems is a fundamental problem in theoretical statistical mechanics. Here we present a machine learning method that is able to predict structural correlation functions with significantly improved accuracy in comparison with traditional approaches. The usefulness of this (from the machine) approach is illustrated by predicting the radial distribution functions of two paradigmatic condensed-phase systems, a Lennard-Jones fluid and a hard-sphere fluid, and then comparing those results to the results obtained using both integral equation methods and empirically motivated analytical functions.

Vibronic Quantum Beating between Electronic Excited States in a Heterodimer.

Energy transfer in multichromophoric molecules can be affected by coherences that are induced by the electronic and vibrational couplings between chromophore units. Coherent electron-vibrational dynamics can persist at the subpicosecond time scale even at room temperature. Furthermore, wave-like localized-delocalized motions of the electronic wave function can be modulated by vibrations that actively participate in the intermolecular energy transfer process.

Monitoring Electron-Phonon Interactions in Lead Halide Perovskites Using Time-Resolved THz Spectroscopy.

Lead halide perovskite semiconductors have low-frequency phonon modes within the lead halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting optoelectronic properties. Electron-phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes, among other important electronic characteristics.

Tuning Optical Properties of Conjugated Molecules by Lewis Acids: Insights from Electronic Structure Modeling.

Understanding and controlling the optoelectronic properties of organic semiconductors at the molecular level remains a challenge due to the complexity of chemical structures and intermolecular interactions. A common strategy to address this challenge is to utilize both experimental and computational approaches. In this contribution, we show that density functional theory (DFT) calculation is a useful tool to provide insights into the bonding, electron population distribution, and optical transitions of adducts between conjugated molecules and Lewis acids (CM-LA).

Ultrafast X-Ray Absorption Spectroscopy of Strongly Correlated Systems: Core Hole Effect.

In recent years, ultrafast pump-probe spectroscopy has provided insightful information about the nonequilibrium dynamics of excitations in materials. In a typical experiment of time-resolved x-ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) followed by an x-ray probe pulse after a time delay to measure the absorption spectra of the photoexcited systems. We present a theory for nonequilibrium x-ray absorption spectroscopy in one-dimensional strongly correlated systems.

Numerical tests of coherence-corrected surface hopping methods using a donor-bridge-acceptor model system.

Surface hopping (SH) is a popular mixed quantum-classical method for modeling nonadiabatic excited state processes in molecules and condensed phase materials. The method is simple, efficient, and easy to implement, but the use of classical and independent nuclear trajectories introduces an overcoherence in the electronic density matrix which, if ignored, often leads to spurious results, such as overestimated reaction rates. Several methods have been proposed to incorporate decoherence into SH simulations, but a lack of insightful benchmarks makes their relative accuracy unknown.

Photoinduced non-adiabatic energy transfer pathways in dendrimer building blocks.

The efficiency of the intramolecular energy transfer in light harvesting dendrimers is determined by their well-defined architecture with high degree of order. After photoexcitation, through-space and through-bond energy transfer mechanisms can take place, involving vectorial exciton migration among different chromophores within dendrimer highly branched structures. Their inherent intramolecular energy gradient depends on how the multiple chromophoric units have been assembled, subject to their inter-connects, spatial distances, and orientations.

Optical absorption spectroscopy in hybrid systems of plasmons and excitons.

Understanding the physics of light emitters in quantum nanostructures regarding scalability, geometry, structure of the system and coupling between different degrees of freedom is important as one can improve the design and further provide rigorous controls of quantum devices. The coupling between these degrees of freedom, in general, depends on the external field, the geometry of nano particles, and the experimental design. An effective model is proposed to describe the plasmon-exciton hybrid systems and its optical absorption spectra, which is studied in detail by exact diagonalization.

Orbital Selectivity Enhanced by Nematic Order in FeSe.

Motivated by the recent low-temperature experiments on bulk FeSe, we study the electron correlation effects in a multiorbital model for this compound in the nematic phase using the U(1) slave-spin theory. We find that a finite nematic order helps to stabilize an orbital selective Mott phase. Moreover, we propose that when the d- and s-wave bond nematic orders are combined with the ferro-orbital order, there exists a surprisingly large orbital selectivity between the xz and yz orbitals even though the associated band splitting is relatively small.

Ultrafast Spin-to-Charge Conversion at the Surface of Topological Insulator Thin Films.

Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states, endows topological insulators with superior spin-to-charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all-optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin-to-charge conversion in a prototypical topological insulator Bi Se /ferromagnetic Co heterostructure, down to the sub-picosecond timescale. Compared to pure Bi Se or Co, a giant terahertz emission is observed in the heterostructure that originates from spin-to-charge conversion, in which the topological surface states play a crucial role.

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics.

Coherence, signifying concurrent electron-vibrational dynamics in complex natural and man-made systems, is currently a subject of intense study. Understanding this phenomenon is important when designing carrier transport in optoelectronic materials. Here, excited state dynamics simulations reveal a ubiquitous pattern in the evolution of photoexcitations for a broad range of molecular systems.

Sb-Doped SnO Nanorods Underlayer Effect to the α-Fe O Nanorods Sheathed with TiO for Enhanced Photoelectrochemical Water Splitting.

Here, a Sb-doped SnO (ATO) nanorod underneath an α-Fe O nanorod sheathed with TiO for photoelectrochemical (PEC) water splitting is reported. The experimental results, corroborated with theoretical analysis, demonstrate that the ATO nanorod underlayer effect on the α-Fe O nanorod sheathed with TiO enhances the PEC water splitting performance. The growth of the well-defined ATO nanorods is reported as a conductive underlayer to improve α-Fe O PEC water oxidation performance.

Universal Scaling of Intrinsic Resistivity in Two-Dimensional Metallic Borophene.

Two-dimensional boron sheets (borophenes) have been successfully synthesized in experiments and are expected to exhibit intriguing transport properties. A comprehensive first-principles study is reported of the intrinsic electrical resistivity of emerging borophene structures. The resistivity is highly dependent on different polymorphs and electron densities of borophene.

Band Dependent Interlayer f-Electron Hybridization in CeRhIn_{5}.

A key issue in heavy fermion research is how subtle changes in the hybridization between the 4f (5f) and conduction electrons can result in fundamentally different ground states. CeRhIn_{5} stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn_{5}, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the 4f crystal field states and details on the 4f and conduction electron hybridization, which together determine the ground state.