Large-scale First-principles Calculations Research Articles

Exciton-Phonon Coupling Induces a New Pathway for Ultrafast Intralayer-to-Interlayer Exciton Transition and Interlayer Charge Transfer in WS2-MoS2 Heterostructure: A First-Principles Study.

Despite the weak, van der Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50 fs time scales. An early theoretical understanding of charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate optical properties of such materials. We employ an ab initio many-body perturbation theory approach, which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS2/MoS2 heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photoexcited excitons at the K valley of MoS2 and WS2 is 67 and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways that are otherwise inaccessible to noninteracting electrons and holes.

Maximally localized Wannier functions, interaction models, and fractional quantum anomalous Hall effect in twisted bilayer MoTe2

We investigate the moiré band structures and the strong correlation effects in twisted bilayer MoTe[Formula: see text] for a wide range of twist angles, employing a combination of various techniques. Using large-scale first-principles calculations, we pinpoint realistic continuum modeling parameters, subsequently deriving the maximally localized Wannier functions for the top three moiré bands. Simplifying our model with reasonable assumptions, we obtain a minimal two-band model, encompassing Coulomb repulsion, correlated hopping, and spin exchange. Our minimal interaction models pave the way for further exploration of the rich many-body physics in twisted MoTe[Formula: see text]. Furthermore, we explore the phase diagrams of the system through Hartree-Fock approximation and exact diagonalization (ED). Our two-band ED analysis underscores significant band-mixing effects in this system, which enlarge the optimal twist angle for fractional quantum anomalous Hall states.

Switching the Moiré Lattice Models in the Twisted Bilayer WSe2 by Strain or Pressure.

Moiré superlattices of twisted van der Waals heterostructures provide a promising and tunable platform for simulating correlated two-dimensional physical models. In twisted bilayer transition-metal dichalcogenides with twist angles close to 0°, the Γ and K valley moiré bands are described by the honeycomb and the triangular effective lattice models, respectively, with distinct physics. Using large-scale first-principles calculations, we show that in-plane biaxial strain and out-of-plane pressure provide effective knobs for switching the moiré lattice models that emerged at the band edges in twisted bilayer WSe2 by shifting the energy positions of the Γ and K valley minibands. The shifting mechanism originates from the differences in the orbital characters of the Γ and K valley states and their responses to strain and pressure. The critical strain and pressure for switching the Γ/K valleys are 2.11% and 2.175 GPa, respectively.

Coupling a Main-Group Metal with a Transition Metal to Create Biatom Catalysts for Nitric Oxide Reduction

Developing efficient biatom catalysts for renewable energies is becoming increasingly worthwhile, yet it remains a great challenge. Herein, by means of large-scale first-principles calculations, we report a design principle of coupling main-group metal with transition metal (TM) to explore biatom catalysts for nitric oxide reduction reaction (NORR), namely, $\mathrm{Mg}/\mathrm{Al}/\mathrm{Ga}\ensuremath{-}\mathrm{TM}$ dimers anchored on nitrogen-doped graphene. We propose a comprehensive screening strategy to recognize promising candidates of such catalysts. Following this strategy, we screen $\mathrm{Mg}\ensuremath{-}\mathrm{Ni}@\mathrm{NC}$ and $\mathrm{Ga}\ensuremath{-}\mathrm{Cr}@\mathrm{NC}$ out of 24 candidates as promising biatom catalysts with high activity and selectivity for direct $\mathrm{NO}$-$\mathrm{to}$-${\mathrm{NH}}_{3}$ conversion. Such high catalytic activity is related to the synergetic interatomic interactions. Moreover, based on the interplay between main-group metal and TM centers, we propose a ``donation-backdonation-redonation'' mechanism to characterize $\mathrm{NO}$ activation. In addition, $\mathrm{\ensuremath{\Delta}}{\ensuremath{\epsilon}}_{s/p\ensuremath{\rightarrow}d}$ is identified as an efficient quantitative descriptor to prereduce the number of such catalyst candidates. Our work opens a strategy for rational design of biatom catalysts toward NORR.

Comprehensive Screening of Halogen-Containing Oxide Double Perovskites A2BXO6 (X = Cl, Br, and I) for Photovoltaic Applications.

The new halogen-containing oxide double perovskites A2BXO6 (X = Cl, Br, and I) have attracted much attention because of their superb electronic properties in halide double perovskites and their high stability in oxide double perovskites. Herein, 408 A2BXO6 double perovskites have been systematically screened by high-throughput computation. Refer to the empirical structural factors phase diagram (t-u), which uses large-scale first-principles calculations. Fourteen stable perovskites are finally confirmed; moreover, 11 of them have never been reported before. Our results show that Ba2AgIO6 and Sr2AgIO6 are the most preferable candidates for photovoltaic applications, of which Sr2AgIO6 has balanceable electron and hole effective masses, a quasi-direct band gap, and strong optical absorption. Importantly, Sr2AgIO6 was successfully synthesized by the solution method. Our work enriches the family of double perovskites, and the tentative experimental evidence undoubtedly hints at their great potential applications in the near future.

Nontrivial spectral band progressions in electronic circular dichroism spectra of carbohelicenes revealed by linear response calculations.

We demonstrate that contemporary computational resources allow for accurate theoretical studies of systems matching recent advances in experimental helicene chemistry. Concerned with first-principles calculations of carbohelicenes, our work surpasses CH[12] as the largest system investigated to date and unravels trends in the electronic structure of the low-lying states of the homologous series. Utilizing a highly efficient implementation of linear response algorithms, we present electronic circular dichroism (CD) spectra of carbohelicenes ranging from CH[5] to CH[30] at the level of Kohn-Sham density-functional theory. Our results for a systematic increase in system size show the emergence of new CD bands that subsequently rise to intensities dominating the spectrum. The spectral band progressions exhibit a periodicity directly linked to the number of overlapping layers of conjugation. While our findings rectify the current understanding of the electronic structure of carbohelicenes, they also serve as a general call for caution regarding the extrapolation of trends from small system ranges.

The electronic properties of SrTiO3-\u03b4 with oxygen vacancies or substitutions

The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO3 perovskite ferroelectrics — strontium titanate (SrTiO3). The atomic and electronic structure of defects were theoretically investigated using the large-scale first-principles calculations for both bulk crystal and thin films. The results of calculations were experimentally verified by studies of the optical properties at photon energies from 25 meV to 8.8 eV for in-situ prepared thin films. It was demonstrated that substitutions and vacancies prefer locations at surfaces or phase boundaries over those inside crystallites. At the same time, local states in the bandgap can be produced by vacancies located both inside the crystals and at the surface, but by nitrogen substitution only inside crystals. Wide-bandgap insulator phases were evidenced for all defects. Compared to pure SrTiO3 films, bandgap widening due to defects was theoretically predicted and experimentally detected.

Covalent Atomic Bridges Enable Unidirectional Enhancement of Electronic Transport in Aligned Carbon Nanotubes.

Interconnecting the surfaces of nanomaterials without compromising their outstanding mechanical, thermal, and electronic properties is critical in the design of advanced bulk structures that still preserve the novel properties of their nanoscale constituents. As such, bridging the π-conjugated carbon surfaces of single-walled carbon nanotubes (SWNTs) has special implications in next-generation electronics. This study presents a rational path toward the improvement of the electrical transport in aligned semiconducting SWNT films by deposition of metal atoms. The formation of conducting Cr-mediated pathways between the parallel SWNTs increases the transverse (intertube) conductance while having a negligible effect on the parallel (intratube) transport. In contrast, doping with Li has a predominant effect on the intratube electrical transport of aligned SWNT films. Large-scale first-principles calculations of electrical transport on aligned SWNTs show good agreement with the experimental electrical measurements and provide insight into the changes that different metal atoms exert on the density of states near the Fermi level of the SWNTs and the formation of transport channels.

Transient Kinetic Selectivity in Nanotubes Growth on Solid Co–W Catalyst

Solid Co-W catalysts have been shown to yield single-walled carbon nanotubes (CNT) with high selectivity, simplistically attributed to CNT-catalyst symmetry match for certain chiral indices ( n, m). Here, based on large-scale first-principles calculations combined with kinetic Monte Carlo simulations, we show instead that such selectivity arises from a complex kinetics of growth. The solid Co7W6 catalyst strongly favors a restructured, asymmetric CNT edge which entails preferential nucleation of tubes with 2 m < n but much faster growth of chiral tubes with n ⩽ 2 m. We uncover a tendency of interface defects formation that, although rare, drive CNT type change from smaller to larger chiral angles (zigzag to armchair). Being both least prone to defects and fast growing, the (12,6) CNT appears as a transient, kinetics-selected type reaching highest abundance.

Optically inactive defects in monolayer and bilayer phosphorene: A first-principles study

Many-body excitonic effect is crucial in two-dimensional (2D) materials and can significantly impact their optoelectronic properties. Because defects are inevitable in 2D materials, understanding how they influence the optical and excitonic properties of the 2D materials is of significant scientific and technological importance. Here we focus on intrinsic point defects in monolayer and bilayer phosphorene and examine whether and how their optoelectronic properties may be modified by the defects. Based on large-scale first-principles calculations, we have systematically explored the optical and excitonic properties of phosphorene in the presence and absence of the point defects. We find that the optical properties of bilayer phosphorene depend on the stacking order of the layers. More importantly, we reveal that the dominant point defects in few-layer phosphorene are optically inactive, which renders phosphorene particularly attractive in optoelectronic applications.

Design of two-dimensional electron gas systems via polarization discontinuity from large-scale first-principles calculations

This work demonstrates an efficient approach to design perovskite-oxide-based two dimensional electron gas systems using large-scale first-principles calculations.

First-principles electron transport with phonon coupling: Large scale at low cost

Phonon-assisted tunneling plays a crucial role for electronic device performance and even more so with future size down-scaling. We show how one can include this effect in large-scale first-principles calculations using a single "special thermal displacement" (STD) of the atomic coordinates at almost the same cost as elastic transport calculations. We apply the method to ultra-scaled silicon devices and demonstrate the importance of phonon-assisted band-to-band and source-to-drain tunneling. In a diode the phonons lead to a rectification ratio suppression in good agreement with experiments, while in an ultra-thin body transistor the phonons increase off-currents by four orders of magnitude, and the subthreshold swing by a factor of four, in agreement with perturbation theory.

Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer

Organic–inorganic hybrid perovskite solar cells with a CdSe quantum dot/PCBM composite as an electron transport layer are reported by materials synthesis, characterization, device fabrication, performance measurements and large-scale first-principles calculations.

Adsorption and diffusion of Ru adatoms on Ru(0001)-supported graphene: Large-scale first-principles calculations.

Large-scale first-principles density functional theory calculations are performed to investigate the adsorption and diffusion of Ru adatoms on monolayer graphene (G) supported on Ru(0001). The G sheet exhibits a periodic moiré-cell superstructure due to lattice mismatch. Within a moiré cell, there are three distinct regions: fcc, hcp, and mound, in which the C6-ring center is above a fcc site, a hcp site, and a surface Ru atom of Ru(0001), respectively. The adsorption energy of a Ru adatom is evaluated at specific sites in these distinct regions. We find the strongest binding at an adsorption site above a C atom in the fcc region, next strongest in the hcp region, then the fcc-hcp boundary (ridge) between these regions, and the weakest binding in the mound region. Behavior is similar to that observed from small-unit-cell calculations of Habenicht et al. [Top. Catal. 57, 69 (2014)], which differ from previous large-scale calculations. We determine the minimum-energy path for local diffusion near the center of the fcc region and obtain a local diffusion barrier of ∼0.48 eV. We also estimate a significantly lower local diffusion barrier in the ridge region. These barriers and information on the adsorption energy variation facilitate development of a realistic model for the global potential energy surface for Ru adatoms. This in turn enables simulation studies elucidating diffusion-mediated directed-assembly of Ru nanoclusters during deposition of Ru on G/Ru(0001).

Static and dynamic buckling of reconstructions at triple steps on Si(111) surfaces

Triple steps on Si(111) surfaces are popular building blocks for bottom-up nanostructure assembly, conferring size uniformity and precise positioning of growing nanostructures. In this work, we employ the Si(7 7 10) regular stepped surface as model system to study the triple steps by scanning tunneling microscopy (STM) and large-scale first-principles calculations. We find a surprising cohabitation of reconstruction elements at the step edge that either buckles statically or dynamically at room temperature. The driving force for the observed sequence of buckling patterns is traced back to Coulomb interactions involving charged adatoms and rest-atoms lying on a mini-terrace. These results reconcile the Si(111) triple step model with the experimental STM data.

Stability and mobility of screw dislocations in 4H, 2H and 3C silicon carbide

Large-scale first-principles calculations were performed to determine the stability and mobility properties of screw dislocations in common silicon carbide polytypes (4H, 2H and 3C). There is a profound lack of knowledge regarding these dislocations, although experimental observations show that they govern the plastic behavior of SiC at low temperature. Numerical simulations reported in this paper indicate that these dislocations are characterized by a shuffle core, the associated Peierls stress of which ranges from 8.9 to 9.6GPa depending on the polytype. The only other stable dislocation core exhibits a reconstruction along the dislocation line, with a greater stability, but is also found to be sessile. Polytypism has a weak influence on these results, especially regarding dislocation core energies and Peierls stress. However, a qualitative difference is predicted between the cubic and the hexagonal systems regarding slip planes, with a possible dislocation displacement along a prismatic plane on average, which would result from a zigzag motion of the screw dislocations at the atomic scale.

Magnetic Nanostructures by Adaptive Twinning in Strained Epitaxial Films

We exploit the intrinsic structural instability of the Fe(70)Pd(30) magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials.

First-Principles Modeling of Oxygen Interaction with SrTiO3(001) Surface: Comparative Density-Functional LCAO and Plane-Wave Study

Large scale first-principles calculations based on density functional theory (DFT) employing two different methods (atomic orbitals and plane wave basis sets) were used to study the energetics, geometry, the electronic charge redistribution and migration for adsorbed atomic and molecular oxygen on defect-free SrTiO3(001) surfaces (both SrO- and TiO2-terminated), which serves as a prototype for many ABO3-type perovskites. Both methods predict substantial binding energies for atomic O adsorption at the bridge position between the oxygen surface ions and an adjacent metal ion. A strong chemisorption is caused by formation of a surface molecular peroxide ion. In contrast, the neutral molecular adsorption energy is much smaller, ∼0.25 eV. Dissociative molecular adsorption is energetically unfavorable, even at 0 K. Adsorbed O atoms migrate along the (001) direction with an activation energy of ∼1 eV which is much larger than that for surface oxygen vacancies (0.14 eV). Therefore, the surface O vacancies control encounter with the adsorbed O atoms and oxygen penetration to the surface which is the limiting step for many applications of ABO3-type perovskites, including resistive oxygen sensors, permeation ceramic membranes and fuel cell technology.

Adiabatic and nonadiabatic phonon dispersion in a Wannier function approach

We develop a first-principles scheme to calculate adiabatic and nonadiabatic phonon frequencies in the full Brillouin zone. The method relies on the stationary properties of a force-constant functional with respect to the first-order perturbation of the electronic charge density and on the localization of the deformation potential in the Wannier function basis. This allows for calculation of phonon-dispersion curves free from convergence issues related to Brillouin-zone sampling. In addition our approach justifies the use of the static screened potential in the calculation of the phonon linewidth due to decay in electron-hole pairs. We apply the method to the calculation of the phonon dispersion and electron-phonon coupling in ${\text{MgB}}_{2}$ and ${\text{CaC}}_{6}$. In both compounds we demonstrate the occurrence of several Kohn anomalies, absent in previous calculations, that are manifest only after careful electron- and phonon-momentum integration. In ${\text{MgB}}_{2}$, the presence of Kohn anomalies on the ${\text{E}}_{2g}$ branches improves the agreement with measured phonon spectra and affects the position of the main peak in the Eliashberg function. In ${\text{CaC}}_{6}$ we show that the nonadiabatic effects on in-plane carbon vibrations are not localized at zone center but are sizable throughout the full Brillouin zone. Our method opens perspectives in large-scale first-principles calculations of dynamical properties and electron-phonon interaction.

First-principles calculations of Fe-doped monolayer C60 on h-BN/Ni(111) surface

We have used large-scale first-principles calculations based on density functional theory to investigate the structure, energetics, electronic, and magnetic structures of Fe(n)-doped C(60) monolayers supported by h-BN monolayer-covered Ni(111) surfaces. A systematic study of n-dependent physical properties has been performed (n=1-4,15). Binding energies on Fe atoms to the Fe(n-1)-C(60) complex have been calculated for n=1-4 after a thorough configuration search and structural optimization. The binding energy, electron charge transfer (from Fe(n) to C(60)), and magnetic moment all increase monotonically as functions of n. The electron charge transfer, ranging from approximately 1e(-) to 5e(-), is from the spin minority population. This leads to a situation in which the net spin of the C(60) molecule aligns with the spin minority and the magnetic moment in C(60) is opposite to the total magnetic moment of the system. For n=2, a competing antiferromagnetic state has been found. In this state, the net spin of the system as well as the C(60) is zero. Density of states and projected density of states analysis indicate that the system becomes metallic upon metal doping regardless its magnetic state. In addition, we have also performed calculations with the Hubbard U term (DFT+U) for two systems, n=4 and 15, to investigate possible gap opening near the Fermi surface.