Dmitri S. Kilin

Mathematical modeling of gas desorption from a metal–organic supercontainer cavity filled with stored N2 gas at critical limits

Metal–organic supercontainer (MOSC) molecules are ideal candidates for gas storage applications due to their construction with customizable ligands and tunable cavity and window sizes, which are found to be elastic in nature. Force field molecular dynamics (MD) are used to evaluate the utilization of MOSCs as nanoporous structures for gas storage. A MOSC, with nitrogen gas molecules filling the cavity, progresses through MD and releases gas molecules by applying temperature to the MOSC. It is the MOSCs elasticity which is responsible for the desorption of guests at elevated temperatures. Data obtained from MD serves as a guide for the derivation of analytical equations that can be used to describe and explain the mechanism of gas desorption from within the cavity. Mathematical modeling of gas desorption from the center cavity can provide a method of predicting MOSC behavior for a broader range of pressures and temperatures, which is challenging for direct atomistic modeling. The utilization of MD can provide data for a wide variety of properties and processes in various materials under different conditions for a broad range of technology-related applications.

Theoretical predictions on efficiency of bi-exciton formation and dissociation in chiral carbon nanotubes

Efficient multiple exciton generation (MEG) in chiral single-wall carbon nanotubes (SWCNTs) is present within the solar spectrum range as shown by the many-body perturbation theory calculations combined with the density functional theory simulations. To describe the impact ionization process, we calculate exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2) and (10,5) SWCNTs. Within the solar energy range, we predict R1→2 ∼ 1014 s-1, while biexciton-to-exciton recombination is weak with R2→1/R1→2 ≤ 10-2. Also we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, for which we find QE ≃ 1.2-1.6, that is 20%-60%. However, MEG strength in these SWCNTs varies strongly with the excitation energy due to highly non-uniform density of states at the low energy. We hypothesize that MEG efficiency in the chiral SWCNTs can be enhanced by altering the low-energy electronic spectrum via surface functionalization, or by mixing SWCNTs of different chiralities.

Molecular dynamics of laser-assisted decomposition of unstable molecules at the surface of carbon nanotubes: case study of CH2(NO2)2 on CNT(4,0)

ABSTRACT In this study, photoreactions of the dinitromethane molecule noncovalently adsorbed on the (4, 0) carbon nanotube (CNT) have been investigated by time-dependent, excited-state molecular dynamics, which takes into account simultaneous evolution of electronic excitation and nuclear positions under periodic optical excitations. It is found that desorption of molecular adsorbate from CNT surface can be controlled by UV−vis photoexcitations. In addition, it is shown that the presence of CNT substrate facilitates photodecomposition of the adsorbate molecule, related to optically controlled explosion. This model demonstrates potential of photoinduced charge transfer between the adsorbate and substrate, which can affect efficiency of desorption and decomposition reactions. This process has a potential use as a remote trigger for larger scale detonations, or as a mechanism for ‘cleaning’ CNTs of unwanted functionalisation. GRAPHICAL ABSTRACT

Singlet fission in chiral carbon nanotubes: Density functional theory based computation

Singlet fission (SF) process, where a singlet exciton decays into a pair of spin one exciton states which are in the total spin singlet state, is one of the possible channels for multiple exciton generation (MEG). In chiral single-wall carbon nanotubes (SWCNTs), efficient SF is present within the solar spectrum energy range which is shown by the many-body perturbation theory calculations based on the density functional theory simulations. We calculate SF exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2), (6,5), (10,5) SWCNTs, and in the (6,2) SWCNT functionalized with Cl atoms. Within the solar energy range, we predict R1→2∼1014-1015 s-1, while biexciton-to-exciton recombination is weak with R2→1∕R1→2≤10-2. SF MEG strength in pristine SWCNTs varies strongly with the excitation energy, which is due to highly non-uniform density of states at low energy. However, our results for the (6,2) SWCNT with chlorine atoms adsorbed to the surface suggest that MEG in the chiral SWCNTs can be enhanced by altering the low-energy electronic states via surface functionalization.

Multiple exciton generation in chiral carbon nanotubes: Density functional theory based computation

We use a Boltzmann transport equation (BE) to study time evolution of a photo-excited state in a nanoparticle including phonon-mediated exciton relaxation and the multiple exciton generation (MEG) processes, such as exciton-to-biexciton multiplication and biexciton-to-exciton recombination. BE collision integrals are computed using Kadanoff-Baym-Keldysh many-body perturbation theory based on density functional theory simulations, including exciton effects. We compute internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon in the course of the relaxation. We apply this approach to chiral single-wall carbon nanotubes (SWCNTs), such as (6,2) and (6,5). We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ∼1.6 at about 3Eg, where Eg is the electronic gap.

Molecular dynamics of reactions between (4,0) zigzag carbon nanotube and hydrogen peroxide under extreme conditions

ABSTRACT Single-wall carbon nanotubes (CNTs) have been suggested as potential materials for use in next-generation gas sensors. The sidewall functionalisation of CNTs facilitates gas molecule adsorption. In this study, density functional theory (DFT)-based ab initio molecular dynamics simulations are performed for a periodic zigzag single-wall (4,0) CNT surrounded by a monolayer of hydrogen peroxide molecules in an attempt to find conditions that favour sidewall functionalisation. The dependency of dynamics on charge states of the system is examined. It is found negative charges favour reactions that result in the functionalisation of the CNT. First principles molecular dynamics of defect formation yields chemically reasonable structure of stable defects, which can be reproduced in CNTs of any diameter and chirality. The explored hydroxyl and hydroperoxyl defects increase conductivity in a large diameter (10,0) CNT, while decrease conductivities in a small diameter (4,0) CNT.

Photoinduced dynamics to photoluminescence in Ln3+ (Ln = Ce, Pr) doped β-NaYF4 nanocrystals computed in basis of non-collinear spin DFT with spin-orbit coupling

ABSTRACT In this work, non-collinear spin DFT + U approaches with spin-orbit coupling (SOC) are applied to Ln3+ doped β-NaYF4 (Ln = Ce, Pr) nanocrystals in Vienna ab initio Simulation Package taking into account unpaired spin configurations using the Perdew–Burke–Ernzerhof functional in a plane wave basis set. The calculated absorption spectra from non-collinear spin DFT + U approaches are compared with that from spin-polarised DFT + U approaches. The spectral difference indicates the importance of spin–flip transitions of Ln3+ ions. Suite of codes for nonadiabatic dynamics has been developed for 2-component spinor orbitals. On-the-fly nonadiabatic coupling calculations provide transition probabilities facilitated by nuclear motion. Relaxation rates of electrons and holes are calculated using Redfield theory in the reduced density matrix formalism cast in the basis of non-collinear spin DFT + U with SOC. The emission spectra are calculated using the time-integrated method along the excited state trajectories based on nonadiabatic couplings.

Dynamics of Charge Transfer and Multiple Exciton Generation in the Doped Silicon Quantum Dot -- Carbon Nanotube System: Density Functional Theory Based Computation

We use the Boltzmann transport equation (BE) to study time evolution of a photoexcited state, including phonon-mediated exciton relaxation, multiple exciton generation (MEG), and energy-transfer processes. BE collision integrals are derived using Kadanoff-Baym-Keldysh many-body perturbation theory (MBPT) based on density functional theory (DFT) simulations, including exciton effects. We apply the method to a nanostructured p- n junction composed of a 1 nm hydrogen-terminated Si quantum dot (QD) doped with two phosphorus atoms (Si36P2H42) adjacent to the (6, 2) single-wall carbon nanotube (CNT) with two chlorine atoms per two unit cells adsorbed to the surface. We find that an initial excitation localized on either the QD or CNT evolves into a transient charge-transfer (CT) state where either electron or hole transfer has taken place. The CT state lifetime is about 40 fs. Also, we study MEG in this system by computing internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon during relaxation. We predict efficient MEG starting at 3 Eg ≃ 1.5 eV and with QE reaching QE = 1.65 at about 5 Eg, where Eg ≃ 0.5 eV is the lowest exciton energy, i.e., the gap. However, we find that including energy transfer and MEG effects suppresses CT state generation.

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by time-dependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. The identified pathway supports the chemistry proposed for the fabrication of silicon-based materials using CHS as a precursor. In addition, we find dimers have smaller HOMO-LUMO gaps and exhibit a red shift and line-width broadening in the computed photoluminescence spectra compared with a pair of CHS monomers.