We employed density functional theory (DFT) calculations with dispersion corrections to investigate energetically preferred alignments of certain p,p'-dimethylaminonitrostilbene (DANS) molecules inside an armchair (m,m) carbon nanotube (n × DANS@(m,m)), where the number of inner molecules (n) is no greater than 3. Here, three types of alignments of DANS are considered: a linear alignment in a parallel fashion and stacking alignments in parallel and antiparallel fashions. According to DFT calculations, a threshold tube diameter for containing DANS molecules in linear or stacking alignments was found to be approximately 1.0 nm. Nanotubes with diameters smaller than 1.0 nm result in the selective formation of linearly aligned DANS molecules due to strong confinement effects within the nanotubes. By contrast, larger diameter nanotubes allow DANS molecules to align in a stacking and linear fashion. The type of alignment adopted by the DANS molecules inside a nanotube is responsible for their second-order non-linear optical properties represented by their static hyperpolarizability (β values). In fact, we computed β values of DANS assemblies taken from optimized n × DANS@(m,m) structures, and their values were compared with those of a single DANS molecule. DFT calculations showed that β values of DANS molecules depend on their alignment, which decrease in the following order: linear alignment > parallel stacking alignment > antiparallel stacking alignment. In particular, a linear alignment has a β value more significant than that of the same number of isolated molecules. Therefore, the linear alignment of DANS molecules, which is only allowed inside smaller diameter nanotubes, can strongly enhance their second-order non-linear optical properties. Since the nanotube confinement determines the alignment of DANS molecules, a restricted nanospace can be utilized to control their second-order non-linear optical properties. These DFT findings can assist in the design of nanotube-based materials exhibiting stronger non-linear optical properties.
Dispersion-corrected density functional theory (DFT) calculations analyze potential energy surfaces (PESs) for the insertion of a certain polarized π-conjugated molecule, p-(dimethylamino)-p′-nitrostilbene (DANS), inside an (m,m) carbon nanotube (n×DANS@(m,m)), where n is the number of inner guests. The current study considered two types of DANS alignments as the dimer structures: linear and stacked alignments, where their dipole moments are oriented in parallel. DFT calculations revealed that a single DANS insertion into a tube with a diameter of approximately 1.0 nm spontaneously proceeds through attractive host–guest interactions. From an end to the middle of a tube, various metastable states and a global minimum structure appear. In 2×DANS@(m,m), which is assumed to be generated by the insertion of another molecule into the tube that already contains one guest, significant roles of intermolecular interactions are found to distinguish their PESs from the 1×DANS@(m,m) case. In addition, the strength of the intermolecular interactions varies depending on the inner DANS dimer alignment and host tube diameters, and thus, striking contrasts were found in PESs for 2×DANS@(m,m). In the formation of linearly aligned DANS molecules inside a tube, the PES is almost identical to that in 1×DANS@(m,m) due to very weak intermolecular interactions. By contrast, intermolecular interactions between the parallel stacked DANS molecules are sufficiently stronger to distinguish their PESs from those in 1×DANS@(m,m); attractive (repulsive) intermolecular interactions within the (8,8) ((7,7)) tube stabilize (destabilize) PESs that are involved in their metastable states and global minimum structure. More importantly, in the stacked alignments, we found the activation energy for the transformation from a metastable state to a global minimum, for which the value decreases in the following order: 20 kcal/mol for 2×DANS@(7,7) > 13 kcal/mol for 2×DANS@(8,8). According to the DFT findings, one can kinetically control the alignments of the DANS molecules inside a thicker tube (i.e., (8,8) tube). In fact, through temperature optimization in heat treatments where the above-mentioned activation energy cannot be overcome, a one-dimensional array of DANS molecules can be formed inside a thicker tube despite its thermodynamic instability. Since linear DANS alignments are responsible for maximizing static hyperpolarizability, the kinetic control technique can expand the diameter ranges of tubes as hosts for DANS guest aggregates exhibiting a significant second-order nonlinear optical response.
Dispersion-corrected density functional theory (DFT) calculations examined energetically preferable hybrid structures between [13]cycloparaphenylene ([13]CPP) and an armchair (m,m) tube (5 ≤ m ≤ 9). Tube-in-ring and ring-on-tube structures, denoted by (m,m)@[13]CPP and [13]CPP–(m,m), respectively, were considered. DFT-based energy-decomposition analyses revealed that the key factors determining the stability of the hybrid structures are attractive long-range interactions between [13]CPP and the (m,m) tube and the energy penalty required for the deformation of [13]CPP. The long-range interactions in (m,m)@[13]CPP become more significant with an increase in the tube diameter. The [13]CPP deformation in (m,m)@[13]CPP varies with the degree of deviation between the radius difference between [13]CPP and a tube (Δr) and the van der Waal contact distance between two carbon atoms (3.35 Å). Accordingly, the stability of (m,m)@[13]CPP depends on the tube diameter. Particularly, (8,8)@[13]CPP, whose Δr is close to 3.35 Å, is highly stabilized among (m,m)@[13]CPP because of a negligible energy penalty for the [13]CPP deformation. In contrast, the stability of [13]CPP–(m,m) is less sensitive to the tube-diameter, and their attractive long-range interactions are weaker than those in (m,m)@[13]CPP. Consequently, a tube-in-ring structure whose radius difference is close to 3.35 Å is the most preferable among hybrid structures between [13]CPP and an (m,m) tube.
The lithium-O2 systems capture worldwide attention as a possible battery for electric vehicle propulsion. However, there are numerous scientific and technical challenges that must be overcome if this alluring promise is to turn into reality. In this study, using first principles molecular dynamics (FPMD), we first time report the dynamical behavior of discharge deposit product such as Li2O2 and investigate the deposit film thickness effect on the electrical conductivity of the cathode surface. We show that how discharge products accumulate on the cathode surface with time and deposit on catalysts surface as well as influence of the species transportation process in Li-O2 battery.
In this work, molecular dynamics (MD) study of triglyme (G3) solution containing lithium bis (trifluoro methyl sulfonyl) amide (Li[TFSA]) were investigated using classical atomistic force fields. G3 is a typical solvent used in non-aqueous Li-air battery. It shows here coordination of Li+ with G3 and [TFSA]- does not significantly change with increasing the concentration of G3 but self-diffusion coefficient of all the ions increases with increasing G3 concentration. The density of [Li(G3)[TFSA] complex decreases with increasing G3 concentration which lead to accelerate diffusivity of ions. Bangladesh Journal of Physics, 27(1), 13-22, June 2020
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