Anomalous Interlayer Exciton Diffusion in WS2/WSe2 Moiré Heterostructure.

Stacking van der Waals crystals allows for the on-demand creation of a periodic potential landscape to tailor the transport of quasiparticle excitations. We investigate the diffusion of photoexcited electron-hole pairs, or excitons, at the interface of WS2/WSe2 van der Waals heterostructure over a wide range of temperatures. We observe the appearance of distinct interlayer excitons for parallel and antiparallel stacking and track their diffusion through spatially and temporally resolved photoluminescence spectroscopy from 30 to 250 K. While the measured exciton diffusivity decreases with temperature, it surprisingly plateaus below 90 K. Our observations cannot be explained by classical models like hopping in the moiré potential. A combination of ab initio theory and molecular dynamics simulations suggests that low-energy phonons arising from the mismatched lattices of moiré heterostructures, also known as phasons, play a key role in describing and understanding this anomalous behavior of exciton diffusion. Our observations indicate that the moiré potential landscape is dynamic down to very low temperatures and that the phason modes can enable efficient transport of energy in the form of excitons.

Effect of crystal facets in plasmonic catalysis.

While the role of crystal facets is well known in traditional heterogeneous catalysis, this effect has not yet been thoroughly studied in plasmon-assisted catalysis, where attention has primarily focused on plasmon-derived mechanisms. Here, we investigate plasmon-assisted electrocatalytic CO2 reduction using different shapes of plasmonic Au nanoparticles - nanocube (NC), rhombic dodecahedron (RD), and octahedron (OC) - exposing {100}, {110}, and {111} facets, respectively. Upon plasmon excitation, Au OCs doubled CO Faradaic efficiency (FECO) and tripled CO partial current density (jCO) compared to a dark condition, with NCs also improving under illumination. In contrast, Au RDs maintained consistent performance irrespective of light exposure, suggesting minimal influence of light on the reaction. Temperature experiments ruled out heat as the main factor to explain such differences. Atomistic simulations and electromagnetic modeling revealed higher hot carrier abundance and electric field enhancement on Au OCs and NCs than RDs. These effects now dominate the reaction landscape over the crystal facets, thus shifting the reaction sites when comparing dark and plasmon-activated processes. Plasmon-assisted H2 evolution reaction experiments also support these findings. The dominance of low-coordinated sites over facets in plasmonic catalysis suggests key insights for designing efficient photocatalysts for energy conversion and carbon neutralization.

One-Dimensional Magnetic Conduction Channels across Zigzag Graphene Nanoribbon/Hexagonal Boron Nitride Heterojunctions.

We examine the electronic structure of recently fabricated in-plane heterojunctions of zigzag graphene nanoribbons embedded in hexagonal boron nitride. We focus on hitherto unexplored interface configurations in which both edges of the nanoribbon are bonded to the same chemical species, either boron or nitrogen atoms. Using ab initio and mean-field Hubbard model calculations, we reveal the emergence of one-dimensional magnetic conducting channels at these interfaces. These channels originate from the energy shift of the magnetic interface states that is induced by charge transfer between the nanoribbon and hexagonal boron nitride. We further address the response of these heterojunctions to external electric and magnetic fields, demonstrating the tunability of energy and spin splittings in the electronic structure. Our findings establish that zigzag graphene nanoribbon/hexagonal boron nitride heterojunctions are a suitable platform for exploring and engineering spin transport in the atomically thin limit, with potential applications in integrated spintronic devices.

Atomistic Theory of Hot-Carrier Relaxation in Large Plasmonic Nanoparticles.

Recently, there has been significant interest in harnessing hot-carriers generated from the decay of localized surface plasmons in metallic nanoparticles for applications in photocatalysis, photovoltaics, and sensing. In this work, we develop an atomistic method that makes it possible to predict the population of hot-carriers under continuous wave illumination for large nanoparticles of relevance to experimental studies. For this, we solve the equation of motion of the density matrix, taking into account both the excitation of hot-carriers and subsequent relaxation effects. We present results for spherical Au and Ag nanoparticles with up to 250,000 atoms. We find that the population of highly energetic carriers depends on both the material and the nanoparticle size. We also study the increase in the electronic temperature upon illumination and find that Ag nanoparticles exhibit a much larger temperature increase than Au nanoparticles. Finally, we investigate the effect of using different models for the relaxation matrix but find that the qualitative features of the hot-carrier population are robust. These insights can be harnessed for the design of improved hot-carrier devices.

Influence of Atomic Relaxations on the Moiré Flat Band Wave Functions in Antiparallel Twisted Bilayer WS2.

Twisting bilayers of transition metal dichalcogenides gives rise to a moiré potential resulting in flat bands with localized wave functions and enhanced correlation effects. In this work, scanning tunneling microscopy is used to image a WS2 bilayer twisted approximately 3° off the antiparallel alignment. Scanning tunneling spectroscopy reveals localized states in the vicinity of the valence band onset, which is observed to occur first in regions with S-on-S Bernal stacking. In contrast, density functional theory calculations on twisted bilayers that have been relaxed in vacuum predict the highest-lying flat valence band to be localized in regions of AA' stacking. However, agreement with experiment is recovered when the calculations are performed on bilayers in which the atomic displacements from the unrelaxed positions have been reduced, reflecting the influence of the substrate and finite temperature. This demonstrates the delicate interplay of atomic relaxations and the electronic structure of twisted bilayer materials.

Theory of Hot-Carrier Generation in Bimetallic Plasmonic Catalysts.

Bimetallic nanoreactors in which a plasmonic metal is used to funnel solar energy toward a catalytic metal have recently been studied experimentally, but a detailed theoretical understanding of these systems is lacking. Here, we present theoretical results of hot-carrier generation rates of different Au-Pd nanoarchitectures. In particular, we study spherical core-shell nanoparticles with a Au core and a Pd shell as well as antenna-reactor systems consisting of a large Au nanoparticle that acts as an antenna and a smaller Pd satellite nanoparticle separated by a gap. In addition, we investigate an antenna-reactor system in which the satellite is a core-shell nanoparticle. Hot-carrier generation rates are obtained from an atomistic quantum-mechanical modeling technique which combines a solution of Maxwell's equation with a tight-binding description of the nanoparticle electronic structure. We find that antenna-reactor systems exhibit significantly higher hot-carrier generation rates in the catalytic material than the core-shell system as a result of strong electric field enhancements associated with the gap between the antenna and the satellite. For these systems, we also study the dependence of the hot-carrier generation rate on the size of the gap, the radius of the antenna nanoparticle, and the direction of light polarization. Overall, we find a strong correlation between the calculated hot-carrier generation rates and the experimentally measured chemical activity for the different Au-Pd photocatalysts. Our insights pave the way toward a microscopic understanding of hot-carrier generation in heterogeneous nanostructures for photocatalysis and other energy-conversion applications.

Dirac Half-Semimetallicity and Antiferromagnetism in Graphene Nanoribbon/Hexagonal Boron Nitride Heterojunctions.

Half-metals have been envisioned as active components in spintronic devices by virtue of their completely spin-polarized electrical currents. Actual materials hosting half-metallic phases, however, remain scarce. Here, we predict that recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity originates from the transfer of charges from hexagonal boron nitride to the embedded graphene nanoribbon. These charges give rise to opposite energy shifts of the states residing at the two edges, while preserving their intrinsic antiferromagnetic exchange coupling. Upon doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge controlling the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize tunable one-dimensional conducting channels of spin-polarized Dirac fermions seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.

Optimizing Hot Electron Harvesting at Planar Metal-Semiconductor Interfaces with Titanium Oxynitride Thin Films.

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (NFC ≈ 2.8 × 1019 m-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (NHE ≈ 1.6 × 1018 m-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Imaging Field-Driven Melting of a Molecular Solid at the Atomic Scale.

Solid-liquid phase transitions are basic physical processes, but atomically resolved microscopy has yet to capture their full dynamics. A new technique is developed for controlling the melting and freezing of self-assembled molecular structures on a graphene field-effect transistor (FET) that allows phase-transition behavior to be imaged using atomically resolved scanning tunneling microscopy. This is achieved by applying electric fields to 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-decorated FETs to induce reversible transitions between molecular solid and liquid phases at the FET surface. Nonequilibrium melting dynamics are visualized by rapidly heating the graphene substrate with an electrical current and imaging the resulting evolution toward new 2D equilibrium states. An analytical model is developed that explains observed mixed-state phases based on spectroscopic measurement of solid and liquid molecular energy levels. The observed nonequilibrium melting dynamics are consistent with Monte Carlo simulations.

Combining the Δ-Self-Consistent-Field and GW Methods for Predicting Core Electron Binding Energies in Periodic Solids.

For the computational prediction of core electron binding energies in solids, two distinct kinds of modeling strategies have been pursued: the Δ-Self-Consistent-Field method based on density functional theory (DFT), and the GW method. In this study, we examine the formal relationship between these two approaches and establish a link between them. The link arises from the equivalence, in DFT, between the total energy difference result for the first ionization energy, and the eigenvalue of the highest occupied state, in the limit of infinite supercell size. This link allows us to introduce a new formalism, which highlights how in DFT─even if the total energy difference method is used to calculate core electron binding energies─the accuracy of the results still implicitly depends on the accuracy of the eigenvalue at the valence band maximum in insulators, or at the Fermi level in metals. We examine whether incorporating a quasiparticle correction for this eigenvalue from GW theory improves the accuracy of the calculated core electron binding energies, and find that the inclusion of vertex corrections is required for achieving quantitative agreement with experiment.

Electrons Surf Phason Waves in Moiré Bilayers.

We investigate the effect of thermal fluctuations on the atomic and electronic structure of a twisted MoSe2/WSe2 heterobilayer using a combination of classical molecular dynamics and ab initio density functional theory calculations. Our calculations reveal that thermally excited phason modes give rise to an almost rigid motion of the moiré lattice. Electrons and holes in low-energy states are localized in specific stacking regions of the moiré unit cell and follow the thermal motion of these regions. In other words, charge carriers surf phason waves that are excited at finite temperatures. We also show that such surfing survives in the presence of a substrate and frozen potential. This effect has potential implications for the design of charge and exciton transport devices based on moiré materials.

Unveiling and Manipulating Hidden Symmetries in Graphene Nanoribbons.

Armchair graphene nanoribbons are a highly promising class of semiconductors for all-carbon nanocircuitry. Here, we present a new perspective on their electronic structure from simple model Hamiltonians and ab initio calculations. We focus on a specific set of nanoribbons of width n=3p+2, where n is the number of carbon atoms across the nanoribbon axis and p is a positive integer. We demonstrate that the energy-gap opening in these nanoribbons originates from the breaking of a previously unidentified hidden symmetry by long-ranged hopping of π electrons and structural distortions occurring at the edges. This hidden symmetry can be restored or manipulated through the application of in-plane lattice strain, which enables continuous energy-gap tuning, the emergence of Dirac points at the Fermi level, and topological quantum phase transitions. Our work establishes an original interpretation of the semiconducting character of armchair graphene nanoribbons and offers guidelines for rationally designing their electronic structure.

Combining Time-Dependent Density Functional Theory and the ΔSCF Approach for Accurate Core-Electron Spectra.

Spectroscopies that probe electronic excitations from core levels into unoccupied orbitals, such as X-ray absorption spectroscopy and electron energy loss spectroscopy, are widely used to gain insight into the electronic and chemical structure of materials. To support the interpretation of experimental spectra, we assess the performance of a first-principles approach that combines linear-response time-dependent density (TDDFT) functional theory with the Δ self-consistent field (ΔSCF) approach. In particular, we first use TDDFT to calculate the core-level spectrum and then shift the spectrum such that the lowest excitation energy from TDDFT agrees with that from ΔSCF. We apply this method to several small molecules and find encouraging agreement between calculated and measured spectra.

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors.

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V-1 s-1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V-1 s-1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics.

Predicting core electron binding energies in elements of the first transition series using the Δ-self-consistent-field method.

The Δ-Self-Consistent-Field (ΔSCF) method has been established as an accurate and computationally efficient approach for calculating absolute core electron binding energies for light elements up to chlorine, but relatively little is known about the performance of this method for heavier elements. In this work, we present ΔSCF calculations of transition metal (TM) 2p core electron binding energies for a series of 60 molecular compounds containing the first row transition metals Ti, V, Cr, Mn, Fe and Co. We find that the calculated TM 2p3/2 binding energies are less accurate than the results for the lighter elements with a mean absolute error (MAE) of 0.73 eV compared to experimental gas phase photoelectron spectroscopy results. However, our results suggest that the error depends mostly on the element and is rather insensitive to the chemical environment. By applying an element-specific correction to the binding energies the MAE is reduced to 0.20 eV, similar to the accuracy obtained for the lighter elements.

Imaging Reconfigurable Molecular Concentration on a Graphene Field-Effect Transistor.

The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.

Core Electron Binding Energies in Solids from Periodic All-Electron Δ-Self-Consistent-Field Calculations.

Theoretical calculations of core electron binding energies are required for the interpretation of experimental X-ray photoelectron spectra, but achieving accurate results for solids has proven difficult. In this work, we demonstrate that accurate absolute core electron binding energies in both metallic and insulating solids can be obtained from periodic all-electron Δ-self-consistent-field (ΔSCF) calculations. In particular, we show that core electron binding energies referenced to the valence band maximum can be obtained as total energy differences between two (N - 1)-electron systems: one with a core hole and one with an electron removed from the highest occupied valence state. To achieve convergence with respect to the supercell size, the analogy between localized core holes and charged defects is exploited. Excellent agreement between calculated and experimental core electron binding energies is found for both metals and insulators, with a mean absolute error of 0.24 eV for the systems considered.