arXiv subjects

Elena Besley

Publications and source records attributed to Elena Besley.

8 recordsLinked to original sources

arXiv2026

Bottom-up Synthesis of Metastable 2D Hexagonal Copper(I) Iodide on Monolayer and within Bilayer Graphene

Copper(I) iodide (CuI) is a wide-bandgap semiconductor crystallizing in the 3D $\gamma$-phase under ambient conditions; its layered van der Waals bulk phase $\beta$-CuI is stable only between 643 and 673 K. The two-dimensional (2D) h-CuI form has been obtained via liquid-phase exfoliation of mechanochemically prepared precursors and via encapsulation between graphene sheets, whereas bottom-up growth of 2D h-CuI on open surfaces has not yet been demonstrated. Here, we report a vapor-phase synthesis of h-CuI directly on low-defect, large-area monolayer and within bilayer reduced oxo-graphene (r-oxo-G) at low temperatures. Using a copper TEM grid as the solid-state precursor for copper, HI-vapor exposure at 40 $\deg$C initiates nucleation, while annealing at 180 $\deg$C promotes the growth of extended h-CuI domains. Aberration-corrected HRTEM resolves the atomic structure, local twist angles, and lattice anisotropy of the CuI/r-oxo-G nanohybrid, while STEM-EDX yields a Cu:I ratio consistent with 1:1. First-principles calculations show that van der Waals adhesion to graphene stabilizes the supported hexagonal layer. Under the presented low-temperature precursor conditions, pathways for nucleation of the $\gamma$-phase are not available, allowing the hexagonal phase to form selectively at the graphene interface. Ab initio molecular dynamics simulations show that the heterostructure retains its hexagonal lattice order at 600 K, including on an open monolayer graphene support. The lateral extent of the growth is limited mainly by remaining interfacial adsorbates. These results establish a route to metastable 2D h-CuI on a chemically inert graphene template, which may be useful for wide-bandgap electronic and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Static heterogeneity generates apparent universality in first-passage bursty dynamics

Processes involving bursts of activity separated by quiescent periods occur across diverse systems and scales. In human dynamics, these phenomena have been described by power-law inter-event time distributions, $P(t)\sim t^{-\alpha}$, with putative universality classes $\alpha=1$ and $\alpha=\frac{3}{2}$ having been proposed. Whether the observed $\alpha = 1$ scaling reflects intrinsic scale-free dynamics or instead emerges from heterogeneous underlying rates has been debated at length. We address this question in a canonical physical system for first-passage dynamics: two-dimensional molecular diffusion detected by the tip of a scanning tunnelling microscope. The resulting inter-pulse time distributions exhibit the same apparent truncated power-law form reported for human activities such as email communication, web browsing, and library loans. Maximum-likelihood estimation and model comparison decisively favor a Kohlrausch-Williams-Watts--tempered power law, $P(t)\propto t^{-\alpha}\exp\left(-(t/t_c)^\beta\right)$, with $\alpha \sim 1$. Kinetic Monte Carlo simulations reproduce this behavior, showing that the apparent $\alpha \sim 1$ scaling is confined to a finite time window and arises from tip-induced spatial heterogeneity, not scale invariance.

cond-mat.other↗

arXiv2026

Three-dimensional Moir\'e crystallography

Moir\'e materials, typically confined to stacking atomically thin, two - dimensional (2D) layers such as graphene or transition metal dichalcogenides, have transformed our understanding of strongly correlated and topological quantum phenomena. The lattice mismatch and relative twist angle between 2D layers have shown to result in Moir\'e patterns associated with widely tunable electronic properties, ranging from Mott and Chern insulators to semi- and super-conductors. Extended to three-dimensional (3D) structures, Moir\'e materials unlock an entirely new crystallographic space defined by the elements of the 3D rotation group and translational symmetry of the constituent lattices. 3D Moir\'e crystals exhibit fascinating novel properties, often not found in the individual components, yet the general construction principles of 3D Moir\'e crystals remain largely unknown. Here we establish fundamental mathematical principles of 3D Moir\'e crystallography and propose a general method of 3D Moir\'e crystal construction using Clifford algebras over the field of rational numbers. We illustrate several examples of 3D Moir\'e structures representing realistic chemical frameworks and highlight their potential applications in condensed matter physics and solid-state chemistry.

cond-mat.mtrl-sci↗

arXiv2025

Multiplet structure of chromium(III) dopants in wide band gap materials

Transition metal doping is commonly used for altering the properties of solid-state materials to suit applications in science and technology. Partially filled $d$-shells of transition metal atoms lead to electronic states with diverse spatial and spin symmetries. Chromium(III) cations have shown great potential for designing laser materials and, more recently, for developing spin qubits in quantum applications. They also represent an intriguing class of chemical systems with strongly correlated multi-reference excited states, due to the $d^3$ electron configuration. These states are difficult to describe accurately using single-reference quantum chemical methods such as density functional theory (DFT), the most commonly used method to study the electronic structures of solid-state systems. Recently, the periodic effective Hamiltonian of crystal field (pEHCF) method has been shown to overcome some limitations arising in the calculations of excited $d$-states. In this work, we assess the suitability of DFT and pEHCF to calculate the electronic structure and $d$-$d$ excitations of chromium(III) dopants in wide band gap host materials. The results will aid computational development of novel transition metal-doped materials and provide a deeper understanding of the complex nature of transition metal dopants in solids.

cond-mat.mtrl-sci↗

arXiv2025

Timing the Escape of a Caged Electron

Charge transfer is fundamentally dependent on the overlap of the orbitals comprising the transport pathway. This has key implications for molecular, nanoscale, and quantum technologies, for which delocalization (and decoherence) rates are essential figures of merit. Here, we apply the core hole clock technique - an energy-domain variant of ultrafast spectroscopy - to probe the delocalization of a photoexcited electron inside a closed molecular cage, namely the Ar 2p54s1 state of Ar@C60. Despite marginal frontier orbital mixing in the ground configuration, almost 80% of the excited state density is found outside the buckyball due to the formation of a markedly diffuse hybrid orbital. Far from isolating the intracage excitation, the surrounding fullerene is instead a remarkably efficient conduit for electron transfer: we measure characteristic delocalization times of 6.6 $\pm$ 0.3 fs and $\lesssim$ 500 attoseconds, respectively, for a 3D Ar@C60 film and a 2D monolayer on Ag(111).

physics.chem-ph↗

arXiv2019

Triplet excitation and electroluminescence from a supramolecular monolayer embedded in a boron nitride tunnel barrier

We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/2D hybrid tunneling diode. Electrons can tunnel from through the hBN/molecular barrier under an applied voltage VSD and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy > eVSD, indicating up-conversion by energies up to ~ 1 eV. We show that tunnelling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry and quantum spin-based applications of triplet generation, emission and molecular photon up-conversion.

cond-mat.mes-hall↗

arXiv2018

Substrate-Induced Shifts and Screening in the Fluorescence Spectra of Supramolecular Adsorbed Organic Monolayers

We have investigated the influence of the substrate on the fluorescence of adsorbed organic molecules. Monolayer films of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI), a supramolecular network formed from PTCDI and melamine, and perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) have been deposited on hexagonal boron nitride (hBN). The principal peaks in the fluorescence spectra of these films were red-shifted by up to 0.37 eV relative to published measurements for molecules in helium droplets. Smaller shifts (~0.03 eV) arising from interactions between neighbouring molecules are investigated by comparing the fluorescence of distinct arrangements of PTCDI, which are templated by supramolecular self-assembly and determined with molecular resolution using atomic force microscopy under ambient conditions. We compare our experimental results with red-shifts calculated using a combination of a perturbative model and density functional theory which account for, respectively, resonant and non-resonant effects of a dielectric hBN substrate. We show that the substrate gives rise to a red-shift in the fluorescence of an adsorbed molecule and also screens the interactions between neighbouring transition dipole moments; both these effects depend on the refractive index of the substrate.

cond-mat.mtrl-sci↗

arXiv2018

Implanting germanium into graphene

Incorporating heteroatoms into the graphene lattice may be used to tailor its electronic, mechanical and chemical properties. Direct substitutions have thus far been limited to incidental Si impurities and P, N and B dopants introduced using low-energy ion implantation. We present here the heaviest impurity to date, namely $^{74}$Ge$^+$ ions implanted into monolayer graphene. Although sample contamination remains an issue, atomic resolution scanning transmission electron microscopy imaging and quantitative image simulations show that Ge can either directly substitute single atoms, bonding to three carbon neighbors in a buckled out-of-plane configuration, or occupy an in-plane position in a divacancy. First principles molecular dynamics provides further atomistic insight into the implantation process, revealing a strong chemical effect that enables implantation below the graphene displacement threshold energy. Our results show that heavy atoms can be implanted into the graphene lattice, pointing a way towards advanced applications such as single-atom catalysis with graphene as the template.

cond-mat.mtrl-sci↗