The conjugated polymer poly(3,4-ethylenedioxythiophene)
polymerized
and stabilized in the presence of polystyrenesulfonate (best
known as PEDOT:PSS) is a working horse of organic electronics and
bioelectronics and one of the most important conductive polymers.
While its morphology is complex and depends on the details in synthesis
and post-treatment, its distinctive and common feature is a two-phase
granular structure attributed to PEDOT- and PSS-rich regions. Yet,
there is still no well-established consensus concerning the precise
nature of PEDOT- and PSS-rich regions as well as their chemical composition
and structure. In this study we perform coarse-grained MARTINI molecular
dynamics simulations of PEDOT:PSS focusing on understanding its two-phase
morphology as well as water intake and ion exchange. We demonstrate
that PEDOT:PSS is an essentially three-component system consisting
of positively charged PEDOT chains, PSS chains with mostly deprotonated
sulfonate groups, and protonated PSS chains. PEDOT-rich regions are
predominantly composed of PEDOT and deprotonated PSS chains, whereas
PSS-rich regions are composed of protonated PSS chains. Our calculations
unravel how PEDOT- and PSS-rich regions are formed from the solution
phase during the drying process. We show that when the dry polymer
film is immersed in water, its swells by nearly 60%, and we demonstrate
that the origin of swelling is related to deprotonation of the sulfonate
groups in the PSS-rich regions. It is mostly PSS-rich regions that
swell while the PEDOT-rich regions remain rather unchanged. We demonstrate
that swelling of the film is rather insignificant during reduction/oxidation
within the cyclic voltammetry (CV) conditions. We show that during
CV experiments each counterion brings on overage ≈4 water molecules
into the polymer region. Our simulations of swelling, CV experiments,
and π–π stacking formation in PEDOT and PSS match
well the experimental results. Our theoretical studies unravel the
most important morphological aspects of one of the most important
polymers for organic electronics, providing the essential insight
needed for the material and device design and improvements.
A Martini coarse-grained Molecular Dynamics (MD) model for the doped conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is developed. The morphology of PEDOT:Tos (i.e. PEDOT doped with molecular tosylate) and its crystallization in aqueous solution for different oxidation levels were calculated using the developed method and compared with corresponding all atomistic MD simulations. The diffusion coefficients of Na+ and Cl- ions in PEDOT:Tos are studied using the developed coarse-grained MD approach. It is shown that the diffusion coefficients decrease exponentially as the hydration level is reduced. It is also predicted that the diffusion coefficients decrease when the doping level of PEDOT is increased. The observed behavior is related to the evolution of water clusters and trapping of ions around the polymer matrix as the hydration level changes. The predicted behavior of the ionic diffusion coefficients can be tested experimentally, and we believe that molecular picture of ionic diffusion in PEDOT unraveled in the present study is instrumental for the design of polymeric materials and devices for better and enhanced performance.
First-principles calculations were performed to investigate the electronic structure of twodimensional (2-D) Ge, Sn, and Pb without and with the presence of an external electric field in combination with spin-orbit coupling. Tight-binding calculations based on four orbitals per atom and an effective single orbital are presented to match with the results obtained from firstprinciples calculations. In particular, the electronic band structure and the band splitting are investigated with both models. Moreover, the simple k · p model is also considered in order to understand the band splitting in the presence of an external electric field and spin-orbit coupling. A large splitting is obtained, which is expected to be useful for spintronic devices.The fair agreement between the first-principle, k · p model, and tight-binding approaches leads to a table of parameters for future tight-binding studies on hexagonal 2-D nanostructures. By using the tight binding parameters, the transport properties of typical 0-D triangular quantum dots between two semi-infinite electrodes in the presence of spin-orbit coupling are addressed.
The van der Waals (vdW) heterostructures are emerging as promising structures for future possible optoelectronic devices. Motivated by the recent studies on vdW heterostructures with their fascinating physical properties, we investigate the electronic and optical properties of boron phosphide/blue phosphorus heterostructures in the framework of density functional theory (DFT) and tight-binding (TB) approximations. We analyze the variation of the energy band gap, the characteristics of the energy band diagram, charge redistribution by stacking and the electrostatic potential along the perpendicular direction. The dynamical stability of these structures is ensured by the phonon spectra. We show that trilayer heterostructures of boron phosphide/bilayer blue phosphorus are in-direct band gap semiconductors while heterobilayers have a direct band gap at the K point. Moreover, we examine the optical properties of monolayer boron phosphide and heterostructures as part of DFT calculations. We conclude that the heterostructures have remarkable optical absorption over the UV range together with being transparent to the visible spectrum, and may be a prominent material for future optoelectronic devices.
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