arXiv subjects

Salvy P. Russo

Publications and source records attributed to Salvy P. Russo.

At least 19 recordsLinked to original sources

arXiv2025

Nitrogen-vacancy centre in lonsdaleite: a novel nanoscale sensor?

Hexagonal diamond, often called lonsdaleite, is an exotic allotrope of carbon, predicted to be harder than cubic (conventional) diamond with a wider bandgap. Due to its pure sp$^3$ bonded lattice, it should be expected to host sub-bandgap defect centres (colour centres). Here we perform \textit{ab initio} modeling of nitrogen-vacancy (NV) colour centres in hexagonal diamond nanocrystals; for both the neutral and negatively charged species (NV$^0$ and NV$^-$). We identify three distinct configurations for the NV center: two of which are analogous to NV in diamond, and one which is a configuration that can only exist in the hexagonal form. The diamond-like NV systems comprise three symmetry equivalent centers which reside on the same carbon plane, and one defect that is split across two planes and replaces a carbon-carbon bond. There is an additional NV centre where the N and V each have four nearest neighbour carbon atoms. The presence of this latter configuration would provide an unambiguous determination of the hexagonal nature of lonsdaleite. Quantum chemical analysis show all derivatives to be thermochemically stable, and each with their own unique photophysical properties, spectral profiles, and magneto-optical characteristics. By assuming that the ground state properties of the NV$^-$ in hexagonal diamond are comparable to those of NV$^-$ in cubic diamond, albeit with increased strain, we predict ground state fine structure splitting for two of the centres of 2.74~GHz and 4.56~MHz, compared with 2.87~GHz for cubic diamond. The possibility of optically detected magnetic resonance with NV$^-$ in lonsdaleite would provide a new carbon-based quantum sensing system, and an unambiguous method to resolve outstanding issues around the structure of lonsdaleite as hexagonal diamond.

quant-ph↗

arXiv2024

Embedding material graphs using the electron-ion potential: application to material fracture

At the heart of the flourishing field of machine learning potentials are graph neural networks, where deep learning is interwoven with physics-informed machine learning (PIML) architectures. Various PIML models, upon training with density functional theory (DFT) material structure-property datasets, have achieved unprecedented prediction accuracy for a range of molecular and material properties. A critical component in the learned graph representation of crystal structures in PIMLs is how the various fragments of the structure's graph are embedded in a neural network. Several of the state-of-art PIML models apply spherical harmonic functions. Such functions are based on the assumption that DFT computes the Coulomb potential of atom-atom interactions. However, DFT does not directly compute such potentials, but integrates the electron-atom potentials. We introduce the direct integration of the external potential (DIEP) methods which more faithfully reflects that actual computational workflow in DFT. DIEP integrates the external (electron-atom) potential and uses these quantities to embed the structure graph into a deep learning model. We demonstrate the enhanced accuracy of the DIEP model in predicting the energies of pristine and defective materials. By training DIEP to predict the potential energy surface, we show the ability of the model in predicting the onset of fracture of pristine and defective carbon nanotubes.

cond-mat.mtrl-sci↗

arXiv2023

Intrinsic defect engineering of CVD grown monolayer MoS$_2$ for tuneable functional nanodevices

Defects in atomically thin materials can drive new functionalities and expand applications to multifunctional systems that are monolithically integrated. An ability to control formation of defects during the synthesis process is an important capability to create practical deployment opportunities. Molybdenum disulfide (MoS$_2$), a two-dimensional (2D) semiconducting material harbors intrinsic defects that can be harnessed to achieve tuneable electronic, optoelectronic, and electrochemical devices. However, achieving precise control over defect formation within monolayer MoS$_2$, while maintaining the structural integrity of the crystals remains a notable challenge. Here, we present a one-step, in-situ defect engineering approach for monolayer MoS$_2$ using a pressure dependent chemical vapour deposition (CVD) process. Monolayer MoS$_2$ grown in low-pressure CVD conditions (LP-MoS$_2$) produces sulfur vacancy (Vs) induced defect rich crystals primarily attributed to the kinetics of the growth conditions. Conversely, atmospheric pressure CVD grown MoS$_2$ (AP-MoS$_2$) passivates these Vs defects with oxygen. This disparity in defect profiles profoundly impacts crucial functional properties and device performance. AP-MoS$_2$ shows a drastically enhanced photoluminescence, which is significantly quenched in LP-MoS$_2$ attributed to in-gap electron donor states induced by the Vs defects. However, the n-doping induced by the Vs defects in LP-MoS$_2$ generates enhanced photoresponsivity and detectivity in our fabricated photodetectors compared to the AP-MoS$_2$ based devices. Defect-rich LP-MoS$_2$ outperforms AP-MoS$_2$ as channel layers of field-effect transistors (FETs), as well as electrocatalytic material for hydrogen evolution reaction (HER). This work presents a single-step CVD approach for in-situ defect engineering in monolayer MoS$_2$ and presents a pathway to control defects in other monolayer material systems.

physics.app-ph↗

arXiv2023

All-electron $\mathrm{\textit{ab-initio}}$ hyperfine coupling of Si-, Ge- and Sn-vacancy defects in diamond

Colour centres in diamond are attractive candidates for numerous quantum applications due to their good optical properties and long spin coherence times. They also provide access to the even longer coherence of hyperfine coupled nuclear spins in their environment. While the NV centre is well studied, both in experiment and theory, the hyperfine couplings in the more novel centres (SiV, GeV, and SnV) are still largely unknown. Here we report on the first all-electron \textit{ab-initio} calculations of the hyperfine constants for SiV, GeV, and SnV defects in diamond, both for the respective defect atoms ($^{29}$Si, $^{73}$Ge, $^{117}$Sn, $^{119}$Sn), as well as for the surrounding $^{13}$C atoms. Furthermore, we calculate the nuclear quadrupole moments of the GeV defect. We vary the Hartree-Fock mixing parameter for Perdew-Burke-Ernzerhof (PBE) exchange correlation functional and show that the hyperfine couplings of the defect atoms have a linear dependence on the mixing percentage. We calculate the inverse dielectric constant to predict an \textit{ab-initio} mixing percentage. The final hyperfine coupling predictions are close to the experimental values available in the literature. Our results will help to guide future novel experiments on these defects.

cond-mat.mes-hall↗

arXiv2022

Naturally-meaningful and efficient descriptors: machine learning of material properties based on robust one-shot ab initio descriptors

Establishing a data-driven pipeline for the discovery of novel materials requires the engineering of material features that can be feasibly calculated and can be applied to predict a material's target properties. Here we propose a new class of descriptors for describing crystal structures, which we term Robust One-Shot Ab initio (ROSA) descriptors. ROSA is computationally cheap and is shown to accurately predict a range of material properties. These simple and intuitive class of descriptors are generated from the energetics of a material at a low level of theory using an incomplete ab initio calculation. We demonstrate how the incorporation of ROSA descriptors in ML-based property prediction leads to accurate predictions over a wide range of crystals, amorphized crystals, metal-organic frameworks and molecules. We believe that the low computational cost and ease of use of these descriptors will significantly improve ML-based predictions.

cond-mat.mtrl-sci↗

arXiv2021

Singlet exciton dynamics of perylene diimide and tetracene based hetero/homogeneous substrates via an \textit{ab initio} kinetic Monte Carlo model

Luminescent solar concentrators (LSCs) are devices that trap a portion of the solar spectrum and funnel it towards photon harvesting devices. The modelling of LSCs at a quantum chemical level however, remains a challenge due to the complexity of exciton and photon dynamic modelling. This study examines singlet exciton dynamics occurring within a typical LSC device. To do this, we use a rejection-free kinetic Monte Carlo method to predict diffusion lengths, diffusion coefficients, substrate anisotropy, and average exciton lifetimes of perylene diimide (PDI) and tetracene based substrates in the low concentration scheme. \textit{Ab initio} rate constants are computed using time-dependant density functional theory based methods. PDI type substrates are observed to display enhanced singlet exciton transport properties when compared to tetracene. Simulations show that substrates with dipole-aligned chromophores are characterised by anisotropic exciton diffusion, with slightly improved transport properties. Finally, a PDI-tetracene substrate is simulated for both disordered and dipole-aligned chromophore configurations. In this multi-dopant substrate transport is predominantly mediated by PDI due to the asymmetry in the transport rates between the two dyes considered. We conclude discussing the properties of multi-dopant substrates and how they can impact the design of next generation LSCs.

cond-mat.mtrl-sci↗

arXiv2021

Bright $\mathrm{\textit{ab-initio}}$ photoluminescence of NV+ in diamond

The positively charged nitrogen vacancy (NV+) centre in diamond has been traditionally treated as a dark state due to the experimental lack of an optical signature. Recent computational studies have shown that it is possible for the NV+ defect to have an excited state transition equivalent to that of the negatively charged (NV-) centre, but no PL predictions have been reported so far. We report the first $\mathrm{\textit{ab-initio}}$ calculation showing that the NV+ center presents quantum emission, with zero phonon line at 765 nm and a non-zero transition dipole moment, approximately 4x smaller than the transition dipole moment of NV-. We calculate the energy levels of the multielectron states under time-dependent density functional theory (singlet and triplet E states), and using our recently developed frequency cutoff method, we predict the full PL spectrum. Our results suggest that this state cannot be considered intrinsically 'dark' and charge specific quenching mechanisms should be investigated as the cause of the lack of optical activity in experimental characterizations.

cond-mat.mes-hall↗

arXiv2021

Accurate calculation of excitonic signatures in the absorption spectrum of BiSBr using semiconductor Bloch equations

In order to realize the significant potential of optical materials such as metal halides, computational techniques which give accurate optical properties are needed, which can work hand-in-hand with experiments to generate high efficiency devices. In this work a computationally efficient technique based on semiconductor Bloch equations (SBEs) is developed and applied to the material BiSBr. This approach gives excellent agreement with the experimental optical gap, and also agrees closely with the excitonic stabilisation energy and the absorption spectrum computed using the far more computationally demanding \textit{ab initio} Bethe-Salpeter approach. The SBE method is a good candidate for theoretical spectroscopy on large- or low dimensional systems which are too computationally expensive for an \textit{ab initio} treatment.

cond-mat.mtrl-sci↗

arXiv2020

An $\mathrm{\textit{ab-initio}}$ effective solid state photoluminescence by frequency constraint of cluster calculation

Measuring the photoluminescence of defects in crystals is a common experimental technique for analysis and identification. However, current theoretical simulations typically require the simulation of a large number of atoms to eliminate finite size effects, which discourages computationally expensive excited state methods. We show how to extract the room-temperature photoluminescence spectra of defect centres in bulk from an $\mathrm{\textit{ab-initio}}$ simulation of a defect in small clusters. The finite size effect of small clusters manifests as strong coupling to low frequency vibrational modes. We find that removing vibrations below a cutoff frequency determined by constrained optimization returns the main features of the solid state photoluminescence spectrum. This strategy is illustrated for an NV$^{-}$ defect in diamond, presenting a connection between defects in solid state and clusters; the first vibrationally resolved $\mathrm{\textit{ab-initio}}$ photoluminescence spectrum of an NV$^{-}$ defect in a nanodiamond; and an alternative technique for simulating photoluminescence for solid state defects utilizing more accurate excited state methods.

cond-mat.mes-hall↗

arXiv2020

Computational investigations of dispersion interactions between small molecules and graphene-like flakes

In this work, we investigate dispersion interactions in a selection of atomic, molecular, and molecule-surface systems, comparing high-level correlated methods with empirically-corrected density functional theory (DFT). We assess the efficacy of functionals commonly used for surface-based calculations, with and without the D3 correction of Grimme. We find that the inclusion of the correction is essential to get meaningful results, but there is otherwise little to distinguish between the functionals. We also present coupled-cluster quality interaction curves for \ce{H2} and \ce{NO2} interacting with large carbon flakes, acting as models for graphene surfaces, using novel absolutely localised molecular orbital based methods. These calculations demonstrate that the problems with empirically-corrected DFT when investigating dispersion appear to compound as the system size increases, with important implications for future computational studies of molecule-surface interactions.

physics.chem-ph↗

arXiv2019

First-Principles Calculation of Triplet Exciton Diffusion in Crystalline Poly($p$-phenylene vinylene)

Understanding and controlling exciton transport is a strategic way to enhance the optoelectronic properties of high-performance organic devices. In this article we study triplet exciton migration in crystalline poly($p$-phenylene vinylene) polymer (PPV) using comprehensive electronic structure and quantum dynamical methods. We solve the coupled electron-nuclear dynamics for the triplet energy migrating between two neighboring Frenkel sites in J- and H-aggregate arrangements. From the two-site model we extract key parameters for use with a master-equation approach that allows us to treat nanosize systems where time-dependent Schrödinger equation becomes intractable. We calculate the transient exciton density evolution and determine the diffusion constants along the principal crystal axes of the PPV. The triplet diffusion is characterized by two distinctive components: fast intrachain, and slow interchain. At room temperature the interchain diffusion coefficients are found to be $D_a=0.89\cdot10^{-2}$ cm$^2$s$^{-1}$ and $D_b=1.49\cdot10^{-2}$ cm$^2$s$^{-1}$ along the respective $\bar{a}$- and $\bar{b}$-axes, and the intrachain is $D_c=3.03$ cm$^2$s$^{-1}$ along the fast $\bar{c}$-axis. The exceptionally high exciton mobility along the $π$-conjugated backbone facilitates rapid triplet migration over long distances. Our results can be utilized in the design of efficient energy conversion and light-emitting devices with desired solid-state properties.

cond-mat.mtrl-sci↗

arXiv2018

Yang-Mills Structure for Electron-Phonon Interactions

This work presents a method of grouping the electron spinors and the acoustic phonon modes of polar crystals such as metal oxides into an SU(2) gauge theory. The gauge charge is the electron spin, which is assumed to couple to the transverse acoustic phonons on the basis of spin ordering phenomena in crystals such as V$_{2}$O$_{3}$ and VO$_{2}$, while the longitudinal mode is neutral. A generalization the Peierls mechanism is presented based on the discrete gauge invariance of crystals and the corresponding Ward-Takahashi identity. The introduction of a band index violates the Ward-Takahashi identity for interband transitions resulting in a longitudinal component appearing in the upper phonon band. Thus both the spinors and the vector bosons acquire mass and a crystal with an electronic band gap and optical phonon modes results. In the limit that the coupling of bosons charged under the SU(2) gauge group goes to zero, breaking the electron U(1) symmetry recovers the BCS mechanism. In the limit that the neutral boson decouples, a Cooper instability mediated by spin-wave exchange results from symmetry breaking, i.e. unconventional superconductivity mediated by magnetic interactions.

cond-mat.str-el↗

arXiv2017

Band structure and giant Stark effect in two-dimensional transition-metal dichalcogenides

We present a comprehensive study of the electronic structures of 192 configurations of 39 stable, layered, transition-metal dichalcogenides using density-functional theory. We show detailed investigations of their monolayer, bilayer, and trilayer structures' valence-band maxima, conduction-band minima, and band gap responses to transverse electric fields. We also report the critical fields where semiconductor-to-metal phase transitions occur. Our results show that band gap engineering by applying electric fields can be an effective strategy to modulate the electronic properties of transition-metal dichalcogenides for next-generation device applications.

physics.app-ph↗

arXiv2017

Electronic structure of tungsten-doped vanadium dioxide

A common method of adjusting the metal-insulator transition temperature of M$_{1}$ VO$_{2}$ is via disruption of the Peierls pairing by doping, or inputting stress or strain. However, since adding even small amounts of dopants will change the band structure, it is unclear how doped VO$_{2}$ retains its insulating character observed in experiments. While strong correlations may be responsible for maintaining a gap, theoretical evidence for this has been very difficult to obtain due to the complexity of the many-body problem involved. In this work we use GW calculations modified to include strong local $\textbf{k}$-space interactions to investigate the changes in band structure from tungsten doping. We find that the combination of carrier doping and the experimentally observed structural defects introduced by inclusion of tungsten are consistent with a change from band-like to Mott-insulating behavior.

cond-mat.str-el↗

arXiv2017

An Ab Initio Description of the Mott Metal-Insulator Transition of M$_{2}$ Vanadium Dioxide

Using an \textit{ab initio} approach based on the GW approximation which includes strong local \textbf{k}-space correlations, the Metal-Insulator Transition of M$_2$ vanadium dioxide is broken down into its component parts and investigated. Similarly to the M$_{1}$ structure, the Peierls pairing of the M$_{2}$ structure results in bonding-antibonding splitting which stabilizes states in which the majority of the charge density resides on the Peierls chain. This is insufficient to drop all of the bonding states into the lower Hubbard band however. An antiferroelectric distortion on the neighboring vanadium chain is required to reduce the repulsion felt by the Peierls bonding states by increasing the distances between the vanadium and apical oxygen atoms, lowering the potential overlap thus reducing the charge density accumulation and thereby the electronic repulsion. The antibonding states are simultaneously pushed into the upper Hubbard band. The data indicate that sufficiently modified GW calculations are able to describe the interplay of the atomic and electronic structures occurring in Mott metal-insulator transitions.

cond-mat.str-el↗

arXiv2017

A study of size-dependent properties of MoS2 monolayer nanoflakes using density-functional theory

Novel physical phenomena emerge in ultra-small sized nanomaterials. We study the limiting small-size-dependent properties of MoS$_{2}$ monolayer rhombic nanoflakes using density-functional theory on structures of size up to Mo$_{35}$S$_{70}$ (1.74~nm). We investigate the structural and electronic properties as functions of the lateral size of the nanoflakes, finding zigzag is the most stable edge configuration, and that increasing size is accompanied by greater stability. We also investigate passivation of the structures to explore realistic settings, finding increased HOMO-LUMO gaps and energetic stability. Understanding the size-dependent properties will inform efforts to engineer electronic structures at the nano-scale.

physics.app-ph↗

arXiv2016

Surface-gate-defined single-electron-transistor in a MoS$_{2}$ bilayer

We report the multi-scale modeling and design of a gate-defined single-electron transistor in a MoS$_{2}$ bilayer. By combining density-functional theory and finite-element analysis, we design a surface gate structure to electrostatically define and tune a quantum dot and its associated tunnel barriers in the MoS$_{2}$ bilayer. Our approach suggests new pathways for the creation of novel quantum electronic devices in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2016

Correlating the Energetics and Atomic Motions of the Metal-Insulator Transition of M1 Vanadium Dioxide

Materials that undergo reversible metal-insulator transitions are obvious candidates for new generations of devices. For such potential to be realised, the underlying microscopic mechanisms of such transitions must be fully determined. In this work we probe the correlation between the energy landscape and electronic structure of the metal-insulator transition of vanadium dioxide and the atomic motions occurring using first principles calculations and high resolution X-ray diffraction. Calculations find an energy barrier between the high and low temperature phases corresponding to contraction followed by expansion of the distances between vanadium atoms on neighbouring sub-lattices. X-ray diffraction reveals anisotropic strain broadening in the low temperature structure's crystal planes, however only for those with spacings affected by this compression/expansion. GW calculations reveal that traversing this barrier destabilises the bonding/anti-bonding splitting of the low temperature phase. This precise atomic description of the origin of the energy barrier separating the two structures will facilitate more precise control over the transition characteristics for new applications and devices.

cond-mat.mtrl-sci↗