Atomistic Theory Research Articles

A theoretical approach on the ability of functionalized gold nanoparticles for detection of Cd2+

Cadmium (Cd) as a toxic element that is widely present in water, soil, and air has important effects on human health, therefore proposing an accurate and selective method for detection of this element is of importance. In this article, by employing full atomistic molecular dynamics (MD) simulations and density functional theory dispersion corrected (DFT-D3) calculations, the effects of 6-mercaptonicotinic acid (MNA) and l-cysteine (CYS) on the stability of gold nanoparticles (AuNPs) and their sensitivity against Cd2+ were investigated. The obtained results indicate that pure AuNPs are not stable in water, while functionalized AuNPs with CYS and MNA groups have considerable stability without aggregation. In other words, the functional groups on the surface of AuNPs elevate their resistance against aggregation by an increase in the repulsive interactions between the gold nanoparticles. Moreover, functionalized AuNPs have considerable ability for selective detection of Cd2+ in the presence of different metal ions. Based on the MD simulation results, MNA-CYS-AuNPs (functionalized AuNPs with both functional groups) have the maximum sensitivity against Cd2+ in comparison with MNA-AuNPs and CYS-AuNPs due to the strong electrostatic interactions. DFT-D3 calculations reveal that the most probable interactions between the metal ions and functional groups are electrostatic, and Cd2+ can aggregate functionalized AuNPs due to strong electrostatic interactions with MNA and CYS groups. Moreover, charge transfer and donor–acceptor analyses show that molecular orbital interactions between the functional groups and Cd2+ can be considered as the driving force for AuNPs aggregation. A good agreement between the theoretical results and experimental data confirms the importance of the molecular modeling methods as a fast scientific protocol for designing new functionalized nanoparticles for application in different fields.

Atomistic effect of laterally and vertically growth shell on physical behaviours of CdSe/CdTe type-II core/crown and core/shell nanoplatelets: tight-binding theory

Utilizing the atomistic tight-binding theory, the impact of the lateral and vertical potential confinement by the coated shell on the CdSe/CdTe core/crown and core/shell nanoplatelets (NPLs) is attained. The spatial charge separation and encapsulated shell have a noteworthy impact on the electronic structures and optical properties because of the type-II band profile. The reduced band gaps with the growing laterally and vertically passivated shell thicknesses are due to the quantum confinement phenomena. The optical band gaps adjusted across the visible light are achieved by the shell thickness change. The excitonic binding energies of CdSe/CdTe core/shell NPLs are larger than those of CdSe/CdTe core/crown NPLs. Thanks to the spatial charge separation, a shortening of the oscillation strengths is concomitant with an increase of the radiative lifetimes. Overall, this scientific research underlines the importance of the theoretical understanding and practical control by lateral and vertical confinement of heterostructure NPLs.

Spin-valley qubits in gated quantum dots in a single layer of transition metal dichalcogenides

We develop a microscopic and atomistic theory of electron spin-based qubits in gated quantum dots in a single layer of transition metal dichalcogenides. The qubits are identified with two degenerate locked spin and valley states in a gated quantum dot. The two-qubit states are accurately described using a multi-million atom tight-binding model solved in wavevector space. The spin-valley locking and strong spin-orbit coupling result in two degenerate states, one of the qubit states being spin-down located at the $+K$ valley of the Brillouin zone, and the other state located at the $-K$ valley with spin up. We describe the qubit operations necessary to rotate the spin-valley qubit as a combination of the applied vertical electric field, enabling spin-orbit coupling in a single valley, with a lateral strongly localized valley-mixing gate.

Electric-field control of exciton fine structure in alloyed nanowire quantum dot molecules

Alloyed ${\mathrm{InAs}}_{0.2}{\mathrm{P}}_{0.8}/\mathrm{InP}$ nanowire quantum dot molecules reveal nontrivial electric-field evolution of the bright-exciton spectra; this was studied here using the atomistic theory. For a quantum dot molecule composed of two nanowire quantum dots of dissimilar sizes, the overall field dependence resembles the typical self-assembled quantum dot molecule spectra with an avoided crossing of direct and indirect excitons. However, for coupled nanowire quantum dots of identical dimensions and chemical compositions---where the bright-exciton splitting is triggered by alloy randomness---the notion of direct/indirect excitons is mostly lost, with the bright-exciton splitting field evolution varying strongly between various random realizations of nominally identical systems. Nonetheless, for several random samples, lower-higher excitonic branch mixing leads to the reduction of bright-exciton splitting below the $1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ threshold but with the restoration of pronounced optical activity away from the crossing. Thus, a simultaneous reduction of the bright-exciton splitting, without the detrimental reduction in the lower excitonic branch optical activity, makes alloyed nanowire quantum dot molecules a possible platform for applications in quantum optics and information.

Atomistic tight-binding calculations of CdSe/CdS core/shell dot-in-hexagonal platelet nanocrystals with interesting electronic structures and optical properties

I theoretically determine the electronic and optical signatures of CdSe/CdS core/shell dot-in-hexagonal platelet nanocrystals with experimentally synthesized dimensions exploiting the atomistic tight-binding theory. With the increasing lateral sizes and thicknesses of the CdS hexagonal platelets, the reduction of excitonic band gaps is ascribed to the quantum confinement consequence. The excitonic band gaps emitting the visible spectra are carried out by CdS shell sizes. The optical characteristic of CdSe/CdS dot-in-hexagonal platelet nanocrystals is advanced when comparing with bare CdSe nanocrystal. The reduction of oscillator strengths, excitonic binding energies and stokes shift with the increasing sizes of CdSe shell is ascribed to the electron-hole wave function overlaps. The key findings from the tight-binding analysis agree well with the experimental observation. Finally, this theoretical study delivers the essential direction to predesign of core/shell dot-in-hexagonal platelet nanocrystals with specific characteristics for various optoelectronic technologies.

Preferred growth of molybdenum thin films during magnetron sputtering

Molybdenum films deposited on a fused silica substrate by magnetron sputtering were investigated. Two types of film textures were identified: nucleation textures and growth textures. The experimental results showed that the nucleation texture of molybdenum films is characterized by the <110> axis, perpendicular to the substrate surface. Theoretical studies based on the atomistic theory of nucleation showed that the work of nucleation with the (110) orientation is the smallest in comparison with the work of nucleation of all other orientations. It was found that during film growth under the characteristic deposition conditions of the structural zone T, a transformation of the (110) nucleation texture into the (211) growth texture is observed, which is probably due to minimization of the overall energy of the growing film. It was found that increasing the working gas pressure within the range of 0.39 ​Pa–0.74 ​Pa reduces the thickness of the molybdenum film at which the texture (110) changes to the texture (211) by 30–40%.

Symmetry of the Non-Atomic Interactions of N-Atomic Energy and the Atomistic Theory of High-Order Elastic Modules

Symmetry of the Non-Atomic Interactions of N-Atomic Energy and the Atomistic Theory of High-Order Elastic Modules

From atomistic tight-binding theory to macroscale drift–diffusion: Multiscale modeling and numerical simulation of uni-polar charge transport in (In,Ga)N devices with random fluctuations

Random alloy fluctuations significantly affect the electronic, optical, and transport properties of (In,Ga)N-based optoelectronic devices. Transport calculations accounting for alloy fluctuations currently use a combination of modified continuum-based models, which neglect to a large extent atomistic effects. In this work, we present a model that bridges the gap between atomistic theory and macroscopic transport models. To do so, we combine atomistic tight-binding theory and continuum-based drift–diffusion solvers, where quantum corrections are included via the localization landscape method. We outline the ingredients of this framework in detail and present first results for uni-polar electron transport in single and multi- (In,Ga)N quantum well systems. Overall, our results reveal that both random alloy fluctuations and quantum corrections significantly affect the current–voltage characteristics of uni-polar electron transport in such devices. However, our investigations indicate that the importance of quantum corrections and random alloy fluctuations can be different for single and multi-quantum well systems.

The Empathic Responses: A Neglected Field for Research

The more fully one comprehends the nature of the shifts in theoretical and methodological orientation which have been taking place in social psychology during the past two decades, the greater becomes his appreciation of the fundamental contributions made by Harry Stack Sullivan to this highly significant movement in social science. An appraisal of the changes here indicated has been made elsewhere 1 . Briefly stated the basic trend has been toward what may be called interactional theories of human behavior and personality and away from static atomistic attributive theories, with accompanying efforts to develop methods more appropriate for dealing with dynamic interactional situations. Sullivan’s importance in this development is due not only to the fact that he has made the most explicit formulation of interactional theory and practice in psychiatry as such, but that these formulations are highly relevant and significant for the whole field of social psychology. There can be no doubt that he will rank as one of the major contributors to a matured social psychology.

Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals

We present a detailed study regarding the bandgap dependence on diameter and composition of spherical Ge-rich GexSi1−x nanocrystals (NCs). For this, we conducted a series of atomistic density functional theory (DFT) calculations on H-passivated NCs of Ge-rich GeSi random alloys, with Ge atomic concentration varied from 50 to 100% and diameters ranging from 1 to 4 nm. As a result of the dominant confinement effect in the DFT computations, a composition invariance of the line shape of the bandgap diameter dependence was found for the entire computation range, the curves being shifted for different Ge concentrations by ΔE(eV) = 0.651(1 − x). The shape of the dependence of NCs bandgap on the diameter is well described by a power function 4.58/d1.25 for 2–4 nm diameter range, while for smaller diameters, there is a tendency to limit the bandgap to a finite value. By H-passivation of the NC surface, the effect of surface states near the band edges is excluded aiming to accurately determine the NC bandgap. The number of H atoms necessary to fully passivate the spherical GexSi1−x NC surface reaches the total number atoms of the Ge + Si core for smallest NCs and still remains about 25% from total number of atoms for bigger NC diameters of 4 nm. The findings are in line with existing theoretical and experimental published data on pure Ge NCs and allow the evaluation of the GeSi NCs behavior required by desired optical sensor applications for which there is a lack of DFT simulation data in literature.

Interaction between dislocation and vacancies in magnesium oxide: Insights from atomistic simulations and elasticity theory

We determine the interaction between an $1/2\ensuremath{\langle}110\ensuremath{\rangle}{\overline{1}10}$ edge dislocation and charged vacancies in MgO, using both molecular static simulations and the elasticity theory developed in the framework of the elastic dipole approach. In this study, the confrontation of these two methods highlights the specific role of the dislocation core structure on the interaction. We thus show that in MgO, the edge dislocation core, within a region across the glide planes that expands over several Burgers vector, is strongly attractive for vacancies, especially those of oxygen. However, the resulting pinning force on the dislocation remains weak and should not contribute to a significant hardening.

Tuning electronic structures and optical properties in CdSe/CdS Dot-in-rod colloidal nanostructures: Atomistic tight-binding theory

Using the atomistic tight-binding theory, in this study, the electronic structures and optical properties of CdSe/CdS dot-in-rod nanocrystals are modified by engineering the core and rod diameters. The ground electron states favor an s-like orbital, while the ground hole states are formed by light hole-like characteristics. The quantum confinement effect is attributed to the reduction in excitonic band gaps as the core and rod diameters increase. Changing the core and rod sizes produces excitonic band gaps across the visible light spectra. The highest oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting are witnessed at a 3.0 nm CdSe core diameter. The oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting decrease as the CdS rod diameters increase. This study highlights the importance of theoretically understanding and controlling the active materials’ structural parameters in optoelectronic technology.

Systematic study of the emission spectra of nanowire quantum dots

A systematic study of the emission spectra of single InAsP nanowire quantum dots in position-controlled InP photonic nanowire waveguides is presented. Using excitation power-dependent photoluminescence and correlation measurements, we distinguish between the different excitonic complexes responsible for s-shell emission. From measurements of over 40 nominally identical devices, we obtain a standard deviation of ∼5 meV in the emission energy of excitons, biexcitons, and charged exciton photons. The mean biexciton binding energy was 1.9 meV with a standard deviation of ∼0.8 meV. The experimental spectra are understood using atomistic multi-million atom theory of neutral and charged multi-exciton complexes implemented in the design tool QNANO.

Nanometer-size Na cluster formation in micropore of hard carbon as origin of higher-capacity Na-ion battery

Development of high-energy-density anode is crucial for practical application of Na-ion battery as a post Li-ion battery. Hard carbon (HC), though a promising anode candidate, still has bottlenecks of insufficient capacity and unclear microscopic picture. Usage of the micropore has been recently discussed, however, the underlying sodiation mechanism is still controversial. Herein we examined the origin for the high-capacity sodiation of HC, based on density functional theory calculations. We demonstrated that nanometer-size Na cluster with 3–6 layers is energetically stable between two sheets of graphene, a model micropore, in addition to the adsorption and intercalation mechanisms. The finding well explains the extended capacity over typical 300 mAhg−1, up to 478 mAhg−1 recently found in the MgO-templated HC. We also clarified that the MgO-template can produce suitable nanometer-size micropores with slightly defective graphitic domains in HC. The present study considerably promotes the atomistic theory of sodiation mechanism and complicated HC science.

Recent progress in atomistic modelling and simulations of donor spin qubits in silicon

Electron or nuclear spins associated with dopant atoms, such as phosphorus impurities in silicon (Si:P), have been shown to form excellent qubits with promising potential for scale-up towards a fault-tolerant quantum computer architecture. The remarkable progress in the design and characterisation of Si:P qubits and quantum gates has been led by recent experimental demonstrations. Equally importantly, advances in theoretical modelling and simulations over a number of years have underpinned the experimental efforts through the fundamental understanding of dopant physics and by providing crucial interpretation of the experimental evidence. This brief review article provides highlights of our research on developing atomistic theoretical methods and their application to the understanding, characterisation and scale-up of Si:P qubits in silicon. We have established a state-of-the-art theoretical framework which is capable of performing electronic structure simulations over millions of atoms. This includes a comprehensive set of central-cell corrections within atomistic tight-binding theory to simulate dopant energy spectra and electronic wave functions with high precision. When integrated with Bardeen’s tunnelling formalism and Chen’s derivative rule, the theoretical simulations were able to reproduce the measured spatially resolved scanning tunnelling microscope (STM) images of dopant wave functions, providing an unprecedented access to the dopant physics in silicon. A systematic examination of the STM image features (brightness and symmetry) allowed pinpointing of the dopant atom positions in silicon lattice with an exact atom precision and for dopant depths up to 5 nm below the silicon surface. The scale-up of the metrology technique was demonstrated by training a machine learning algorithm such as convolutional neural network. For the design and implementation of high-fidelity two-qubit quantum gates, we investigated exchange interaction between dopant pairs and showed that the application of a small lattice strain could provide a full control in the presence of one-lattice site donor position variations. The state-of-the-art computational capability developed by our team is a culmination of more than five years of research efforts – it has been well-benchmarked against several different experimental measurements and is expected to play an important role in design and characterisation of quantum gates and scale-up architectures in the coming years.

Ternary Cu2SnS3: Synthesis, Structure, Photoelectrochemical Activity, and Heterojunction Band Offset and Alignment.

Ternary Cu2SnS3 (CTS) is an attractive nontoxic and earth-abundant absorber material with suitable optoelectronic properties for cost-effective photoelectrochemical applications. Herein, we report the synthesis of high-quality CTS nanoparticles (NPs) using a low-cost facile hot injection route, which is a very simple and nontoxic synthesis method. The structural, morphological, optoelectronic, and photoelectrochemical (PEC) properties and heterojunction band alignment of the as-synthesized CTS NPs have been systematically characterized using various state-of-the-art experimental techniques and atomistic first-principles density functional theory (DFT) calculations. The phase-pure CTS NPs confirmed by X-ray diffraction (XRD) and Raman spectroscopy analyses have an optical band gap of 1.1 eV and exhibit a random distribution of uniform spherical particles with size of approximately 15–25 nm as determined from high-resolution transmission electron microscopy (HR-TEM) images. The CTS photocathode exhibits excellent photoelectrochemical properties with PCE of 0.55% (fill factor (FF) = 0.26 and open circuit voltage (Voc) = 0.54 V) and photocurrent density of −3.95 mA/cm2 under AM 1.5 illumination (100 mW/cm2). Additionally, the PEC activities of CdS and ZnS NPs are investigated as possible photoanodes to create a heterojunction with CTS to enhance the PEC activity. CdS is demonstrated to exhibit a higher current density than ZnS, indicating that it is a better photoanode material to form a heterojunction with CTS. Consistently, we predict a staggered type-II band alignment at the CTS/CdS interface with a small conduction band offset (CBO) of 0.08 eV compared to a straddling type-I band alignment at the CTS/ZnS interface with a CBO of 0.29 eV. The observed small CBO at the type-II band aligned CTS/CdS interface points to efficient charge carrier separation and transport across the interface, which are necessary to achieve enhanced PEC activity. The facile CTS synthesis, PEC measurements, and heterojunction band alignment results provide a promising approach for fabricating next-generation Cu-based light-absorbing materials for efficient photoelectrochemical applications.

Method to Construct Volcano Relations by Multiscale Modeling: Building Bridges between the Catalysis and Biosimulation Communities.

Understanding the complex interactions of different building blocks within a sophisticated drug-delivery system (DDS), aimed at targeted transport of the drug to malignant cells, requires modeling techniques on different time and length scales. On the example of the anthracycline antibiotic doxorubicin (DOX), we investigate a potential DDS component, consisting of a gold nanoparticle and a short peptide sequence as carriers of DOX. The combination of atomistic molecular dynamics simulations and density functional theory calculations facilitates compiling a volcano plot, which allows deriving general conclusions on DDS constituents for chemotherapeutic agents within the class of anthracycline antibiotics: the nanoparticle and peptide carrier moieties need to be chosen in such a way that the anthracycline body of the drug is able to intercalate between both entities or between two (π-stacking) residues of the peptide. Using the popular volcano framework as a guideline, the present article connects the catalysis and biosimulation communities, thereby identifying a strategy to overcome the limiting volcano relation by tuning the coordination number of the drug in the DDS component.

Multiscale simulations of the electronic structure of III-nitride quantum wells with varied indium content: Connecting atomistic and continuum-based models

Carrier localization effects in III-N heterostructures are often studied in the frame of modified continuum-based models utilizing a single-band effective mass approximation. However, there exists no comparison between the results of a modified continuum model and atomistic calculations on the same underlying disordered energy landscape. We present a theoretical framework that establishes a connection between atomistic tight-binding theory and continuum-based electronic structure models, here a single-band effective mass approximation, and provide such a comparison for the electronic structure of (In,Ga)N quantum wells. In our approach, in principle, the effective masses are the only adjustable parameters since the confinement energy landscape is directly obtained from tight-binding theory. We find that the electronic structure calculated within effective mass approximation and the tight-binding model differ noticeably. However, at least in terms of energy eigenvalues, an improved agreement between the two methods can be achieved by adjusting the band offsets in the continuum model, enabling, therefore, a recipe for constructing a modified continuum model that gives a reasonable approximation of the tight-binding energies. Carrier localization characteristics for energetically low lying, strongly localized states differ, however, significantly from those obtained using the tight-binding model. For energetically higher lying, more delocalized states, good agreement may be achieved. Therefore, the atomistically motivated continuum-based single-band effective mass model established provides a good, computationally efficient alternative to fully atomistic investigations, at least at when targeting questions related to higher temperatures and carrier densities in (In,Ga)N systems.

Understanding Doping of Quantum Materials.

Doping mobile carriers into ordinary semiconductors such as Si, GaAs, and ZnO was the enabling step in the electronic and optoelectronic revolutions. The recent emergence of a class of "quantum materials", where uniquely quantum interactions between the components produce specific behaviors such as topological insulation, unusual magnetism, superconductivity, spin-orbit-induced and magnetically induced spin splitting, polaron formation, and transparency of electrical conductors, pointed attention to a range of doping-related phenomena associated with chemical classes that differ from the traditional semiconductors. These include wide-gap oxides, compounds containing open-shell d electrons, and compounds made of heavy elements yet having significant band gaps. The atomistic electronic structure theory of doping that has been developed over the past two decades in the subfield of semiconductor physics has recently been extended and applied to quantum materials. The present review focuses on explaining the main concepts needed for a basic understanding of the doping phenomenology and indeed peculiarities in quantum materials from the perspective of condensed matter theory, with the hope of forging bridges to the chemists that have enabled the synthesis of some of the most interesting compounds in this field.

Fine structure of dark and bright excitons in vertical electric fields: Atomistic theory of alloyed self-assembled InGaAs quantum dots

The vertical electric field response of dark- and bright-excitonic fine structures in self-assembled quantum dots remains largely unexplored. Using an atomistic tight-binding model, combined with a configuration-interaction approach, we show that the fine structure of both bright and dark excitons can be effectively tuned with a vertical field. The dark-exciton splitting reveals parabolic evolution under an applied electric field, contrary to linearlike trends for the bright-exciton splitting, with a linear change rate of the latter related to the bright-dark splitting. Atomistic results are further investigated in terms of the hole-band-mixing term, which reverses its sign under the electric field leading to a vanishing bright-exciton fine structure, and a minimum of the dark-exciton splitting in a nonalloyed case. Surprisingly, we find that the dark-exciton optical activity is also highly tunable with the electric field, despite the quantum dot's cylindrical shape, with potential implications for applications involving dark excitons. Finally, we study different random realizations of the same alloyed quantum dot, showing that mere alloy randomness substantially affects the bright-exciton splitting both at zero field and at the splitting minimum.