Congratulations, Dr. Abhishek Shandilya!
We heartily congratulate Dr. Abhishek Shandilya for successfully defending his thesis titled “Modeling interfaces in polymer nanodielectrics”.
Abstract: Ab initio design of polymer nanocomposite materials for high breakdown strength requires prediction of localized trap states at the polymer–filler interface. Systematic first-principles calculations of realistic interfaces can be challenging, particularly for amorphous polymers and fillers that necessitate the calculation of ensembles of large unit cells with hundreds of atoms. We present a computational approach for automatically generating reasonable structures for amorphous polymer–filler interfaces, combining classical molecular dynamics and Monte Carlo simulations. We identify trap states by analyzing the localization of electronic eigenstates calculated using density functional theory on ensembles of interface structures, clearly distinguishing shallow trap states from delocalized band-edge states. Applying this approach to silica–polyethylene interfaces as an initial example, we find under-coordination and distorted coordination structures at amorphous silica surfaces contribute a combination of deep and shallow traps at these interfaces, whereas polyethylene does not generate localized interfacial states.
Nanofillers in polymer nanocomposites are functionalized to improve dielectric performance in both direct and indirect ways. For comprehensive design of polymer nanodielectrics, we include coupling agents with functional groups to our automated scheme of generating interface structure. In our initial study, we create ensembles of interfaces with three functional groups - thiophene, terthiophene, ferrocene. Analyzing their localized eigenstates reveals distinct distribution of hole and electron traps in energy and localization space, with ferrocene exhibiting the highest number of traps.
A combination of ab initio, classical molecular dynamics and Monte Carlo methods applied in investigating amorphous interfaces of polymer nanodielectrics can be extended to other areas too. Understanding electrochemical interfaces is an equally-complex problem. Controlling electrochemical reactivity requires a detailed understanding of the charging behavior and thermodynamics of the electrochemical interface. Experiments can independently probe the overall charge response of the electrochemical double layer by capacitance measurements, and the thermodynamics of the inner layer with potential of maximum entropy (PME) measurements. Relating these properties by computational modeling of the electrochemical interface has so far been challenging due to the low accuracy of classical molecular dynamics (MD) for capacitance and the limited time and length scales of ab initio MD (AIMD). Here, we combine large ensembles of long-time-scale classical MD simulations with charge response from electronic DFT to predict the potential-dependent capacitance of a family of ideal aqueous electrochemical interfaces with different peak capacitances. We calculate two charge-based benchmarks which indicate an asymmetric response of interfacial water that is stronger for negatively charged electrodes, while the difference between CME and CMC illustrates the richness in behavior of even the ideal electrochemical interface.