Multiscale molecular simulations of the formation and structure of polyamide membranes created by interfacial polymerization

Large scale molecular simu lations to model the formation of polyamide membranes have been carried out using a procedure that mimics experimental interfacial polymerization of trimesoyl chloride (TMC) and metaphenylene diamine (MPD) monomers. A coarse - grained representation of the m onomers has been developed to facilitate these simulations, which captures essential features of the stereochemistry of the monomers and of amide bonding between them. Atomic models of the membranes are recreated from the final coarse - grained representatio ns. Consistent with earlier treatments, membranes are formed through the growth and aggregation of oligomer clusters. The membranes are inhomogeneous, displaying opposing gradients of trapped carboxyl and amine side groups, local density variations, and r egions where the density of amide bonding is reduced as a result of the aggregation process. We observe the interfacial polymerization reaction is self - limiting and the simulated membranes display a thickness of 5 – 10 nm. They also display a surface roughn ess of 1 – 4 nm. Comparisons are made with recently published experimental results on the structure and chemistry of these membranes and some interesting similarities and differences are found

Optical properties of charged defects in monolayer MoS₂

We present theoretical calculations of the optical spectrum of monolayer MoS2 with a charged defect. In particular, we solve the Bethe–Salpeter equation based on an atomistic tight-binding model of the MoS2 electronic structure which allows calculations for large supercells. The defect is modelled as a point charge whose potential is screened by the MoS2 electrons. We find that the defect gives rise to new peaks in the optical spectrum approximately 100–200 meV below the first free exciton peak. These peaks arise from transitions involving in-gap bound states induced by the charged defect. Our findings are in good agreement with experimental measurements

Electronic structure of monolayer and bilayer black phosphorus with charged defects

We use an atomistic approach to study the electronic properties of monolayer and bilayer black phosphorus in the vicinity of a charged defect. In particular, we combine screened defect potentials obtained from first-principles linear response theory with large-scale tight-binding simulations to calculate the wave functions and energies of bound acceptor and donor states. As a consequence of the anisotropic band structure, the defect states in these systems form distorted hydrogenic orbitals with a different ordering from that in isotropic materials. For the monolayer, we study the dependence of the binding energies of charged adsorbates on the defect height and the dielectric constant of a substrate in an experimental setup. We also compare our results with an anisotropic effective mass model and find quantitative and qualitative differences when the charged defect is close to the black phosphorus or when the screening from the substrate is weak. For the bilayer, we compare results for charged adsorbates and charged intercalants and find that intercalants induce more prominent secondary peaks in the local density of states because they interact strongly with electronic states on both layers. These insights can be directly tested in scanning tunneling spectroscopy measurements and enable a detailed understanding of the role of Coulomb impurities in electronic devices

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

Flat band properties of twisted transition metal dichalcogenide homo- and heterobilayers of MoS2, MoSe2, WS2 and WSe2

Twisted bilayers of two-dimensional materials, such as twisted bilayer graphene, often feature flat electronic bands that enable the observation of electron correlation effects. In this work, we study the electronic structure of twisted transition metal dichalcogenide (TMD) homo- and heterobilayers that are obtained by combining MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$ monolayers, and show how flat band properties depend on the chemical composition of the bilayer as well as its twist angle. We determine the relaxed atomic structure of the twisted bilayers using classical force fields and calculate the electronic band structure using a tight-binding model parametrized from first-principles density-functional theory. We find that the highest valence bands in these systems can derive either from $\Gamma$-point or $K$/$K'$-point states of the constituent monolayers. For homobilayers, the two highest valence bands are composed of monolayer $\Gamma$-point states, exhibit a graphene-like dispersion and become flat as the twist angle is reduced. The situation is more complicated for heterobilayers where the ordering of $\Gamma$-derived and $K$/$K'$-derived states depends both on the material composition and also the twist angle. In all systems, qualitatively different band structures are obtained when atomic relaxations are neglected

Atomistic Hartree theory of twisted double bilayer graphene near the magic angle

Twisted double bilayer graphene (tDBLG) is a moiré material that has recently generated significant interest because of the observation of correlated phases near the magic angle. We carry out atomistic Hartree theory calculations to study the role of electron–electron interactions in the normal state of tDBLG. In contrast to twisted bilayer graphene, we find that such interactions do not result in significant doping-dependent deformations of the electronic band structure of tDBLG. However, interactions play an important role for the electronic structure in the presence of a perpendicular electric field as they screen the external field. Finally, we analyze the contribution of the Hartree potential to the crystal field, i.e. the on-site energy difference between the inner and outer layers. We find that the on-site energy obtained from Hartree theory has the same sign, but a smaller magnitude compared to previous studies in which the on-site energy was determined by fitting tight-binding results to ab initio density-functional theory (DFT) band structures. To understand this quantitative difference, we analyze the ab initio Kohn–Sham potential obtained from DFT and find that a subtle interplay of electron–electron and electron–ion interactions determines the magnitude of the on-site potential

Large-scale density functional theory simulation of inorganic nanotubes : a case study on Imogolite nanotubes

The high experimental control over inorganic Imogolite-like open-ended nanotubes (Imo-NTs) composition, dimensions and monodispersity together with the potentially huge range of tuneable properties that can be introduced by chemical functionalisation and doping make Imo-NTs appealing substrates for nanotechnology, as artificial ion-channels and in chemical separation. Investigation of Imo-NTs electronic and spectroscopic properties has so far been hampered by the large size of the systems repeat unit (+300 atoms), which pose severe challenges and accuracy-viability compromises for standard plane-wave (fixed atomic basis set) density functional theory (DFT) simulations. These challenges can, however, be met by linear-scaling DFT (LS-DFT) approaches based on in situ optimisation of minimal basis sets. Here, we illustrate the applicability of LS-DFT to Imo-NTs by providing structural and electronic characterisation of periodic and finite models of aluminosilicate (AlSi) and methylated-AlSi Imo-NTs. It is shown that adoption of moderate kinetic energy cutoff (1000 eV) and basis set truncation radius (8 Bohr) leads to optimal accuracy-viability compromised for geometrical optimisation of Imo-NTs. These results should be useful for future LS-DFT investigation of Imo-NTs and other AlSi-based functional materials

Chiral valley phonons and flat phonon bands in moire materials

We investigate the chirality of phonon modes in twisted bilayer WSe 2 and demonstrate distinct chiral behavior of the K / K ′ valley phonons for twist angles close to 0 ∘ and close to 60 ∘ . In particular, multiple chiral nondegenerate K / K ′ valley phonons are found for twist angles near 60 ∘ whereas no nondegenerate chiral modes are found for twist angles close to 0 ∘ . Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moiré scale and find similar modes in moiré patterns of strain-engineered bilayers WSe 2 and MoSe 2 / WSe 2 heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in twisted bilayer WSe 2 . Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moiré materials and also for the design of phononic analogues of flat band electrons

Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube–polymer composites

We investigate the failure of carbon-nanotube/polymer composites by using a recently-developed hybrid quantum-mechanical/molecular-mechanical (QM/MM) approach to simulate nanotube pull-out from a cross-linked polyethene matrix. Our study focuses on the strength and failure modes of covalently-bonded nanotube–polymer interfaces based on amine, carbene and carboxyl functional groups and a [2+1] cycloaddition. We find that the choice of the functional group linking the polymer matrix to the nanotube determines the effective strength of the interface, which can be increased by up to 50% (up to the limit dictated by the strength of the polymer backbone itself) by choosing groups with higher interfacial binding energy. We rank the functional groups presented in this work based on the strength of the resulting interface and suggest broad guidelines for the rational design of nanotube functionalisation for nanotube–polymer composites

Combining embedded mean-field theory with linear-scaling density-functional theory

We demonstrate the capability of embedded mean field theory (EMFT) within the linear-scaling density-functional theory code ONETEP, which enables DFT-in-DFT quantum embedding calculations on systems containing thousands of atoms at a fraction of the cost of a full calculation. We perform simulations on a wide range of systems from molecules to complex nanostructures to demonstrate the performance of our implementation with respect to accuracy and efficiency. This work paves the way for the application of this class of quantum embedding method to large-scale systems that are beyond the reach of existing implementations