Ganesh Balasubramanian

Pyrolysis reaction networks for lignin model compounds: unraveling thermal deconstruction of β-O-4 and α-O-4 compounds

Although lignin is one of the main components of biomass, its pyrolysis chemistry is not well understood due to complex heterogeneity. To gain insights into this chemistry, the pyrolysis of seven lignin model compounds (five β-O-4 and two α-O-4 linked molecules) was investigated in a micropyrolyzer connected to GC-MS/FID. According to quantitative product mole balance for the reaction networks, concerted retro–ene fragmentation and homolytic dissociation were strongly suggested as the initial reaction step for β-O-4 compounds and α-O-4 compounds, respectively. The difference in reaction pathway between compounds with different linkages was believed to result from thermodynamics of the radical initiation. The rate constants for the different reaction pathways were predicted from ab initio density functional theory calculations and pre-exponential literature values. The computational findings were consistent with the experiment results, further supporting the different pyrolysis mechanisms for the β-ether linked and α-ether linked compounds. A combination of the two pathways from the dimeric model compounds was able to describe qualitatively the pyrolysis of a trimeric lignin model compound containing both β-O-4 and α-O-4 linkages.

Atomistic clustering-ordering and high-strain deformation of an Al0.1CrCoFeNi high-entropy alloy

Computational investigations of structural, chemical, and deformation behavior in high-entropy alloys (HEAs), which possess notable mechanical strength, have been limited due to the absence of applicable force fields. To extend investigations, we propose a set of intermolecular potential parameters for a quinary Al-Cr-Co-Fe-Ni alloy, using the available ternary Embedded Atom Method and Lennard-Jones potential in classical molecular-dynamics simulations. The simulation results are validated by a comparison to first-principles Korringa-Kohn-Rostoker (KKR) - Coherent Potential Approximation (CPA) [KKR-CPA] calculations for the HEA structural properties (lattice constants and bulk moduli), relative stability, pair probabilities, and high-temperature short-range ordering. The simulation (MD)-derived properties are in quantitative agreement with KKR-CPA calculations (first-principles) and experiments. We study AlxCrCoFeNi for Al ranging from 0 ≤ x ≤2 mole fractions, and find that the HEA shows large chemical clustering over a wide temperature range for x < 0.5. At various temperatures high-strain compression promotes atomistic rearrangements in Al0.1CrCoFeNi, resulting in a clustering-to-ordering transition that is absent for tensile loading. Large fluctuations under stress, and at higher temperatures, are attributed to the thermo-plastic instability in Al0.1CrCoFeNi.

Thermal Conductivity of Suspended Graphene with Defects

We investigate the thermal conductivity of suspended graphene as a function of the density of defects, ND, introduced in a controllable way. Graphene layers are synthesized using chemical vapor deposition, transferred onto a transmission electron microscopy grid, and suspended over ~7.5-micrometer size square holes. Defects are induced by irradiation of graphene with the low-energy electron beam (20 keV) and quantified by the Raman D-to-G peak intensity ratio. As the defect density changes from 2.0x10^10 cm-2 to 1.8x10^11 cm-2 the thermal conductivity decreases from ~(1.8+/-0.2)x10^3 W/mK to ~(4.0+/-0.2)x10^2 W/mK near room temperature. At higher defect densities, the thermal conductivity reveals an intriguing saturation behavior at a relatively high value of ~400 W/mK. The thermal conductivity dependence on defect density is analyzed using the Boltzmann transport equation and molecular dynamics simulations. The results are important for understanding phonon - point defect scattering in two-dimensional systems and for practical applications of graphene in thermal management.We investigate the thermal conductivity of suspended graphene as a function of the density of defects, ND, introduced in a controllable way. High-quality graphene layers are synthesized using chemical vapor deposition, transferred onto a transmission electron microscopy grid, and suspended over ∼7.5 μm size square holes. Defects are induced by irradiation of graphene with the low-energy electron beam (20 keV) and quantified by the Raman D-to-G peak intensity ratio. As the defect density changes from 2.0 × 10(10) cm(-2) to 1.8 × 10(11) cm(-2) the thermal conductivity decreases from ∼(1.8 ± 0.2) × 10(3) W mK(-1) to ∼(4.0 ± 0.2) × 10(2) W mK(-1) near room temperature. At higher defect densities, the thermal conductivity reveals an intriguing saturation-type behavior at a relatively high value of ∼400 W mK(-1). The thermal conductivity dependence on the defect density is analyzed using the Boltzmann transport equation and molecular dynamics simulations. The results are important for understanding phonon - point defect scattering in two-dimensional systems and for practical applications of graphene in thermal management.

Optimizing isotope substitution in graphene for thermal conductivity minimization by genetic algorithm driven molecular simulations

We present results from a computational framework integrating genetic algorithm and molecular dynamics simulations to systematically design isotope engineered graphene structures for reduced thermal conductivity. In addition to the effect of mass disorder, our results reveal the importance of atomic distribution on thermal conductivity for the same isotopic concentration. Distinct groups of isotope-substituted graphene sheets are identified based on the atomic composition and distribution. Our results show that in structures with equiatomic compositions, the enhanced scattering by lattice vibrations results in lower thermal conductivities due to the absence of isotopic clusters.

Electrical Stimulus Controlled Binding/Unbinding of Human Thrombin-Aptamer Complex.

The binding/unbinding of the human thrombin and its 15-mer single stranded DNA aptamer, under the application of external stimulus in the form of electrostatic potential/electric field, is investigated by a combination of continuum analysis and atomistic molecular dynamics simulation. In agreement with the experiments that demonstrate the influence of electrostatic potential on the thrombin/aptamer complex, our computations show that the application of positive electric field successfully unbinds the thrombin from the aptamer. Results from umbrella sampling simulations reveal that there is a decrease in the free energy of binding between the thrombin and aptamer in presence of positive electric fields. Hydrogen bonding and non-bonded interaction energies, and hence the free energy of binding, between the thrombin and its aptamer reduce as the applied electric field is shifted from negative to positive values. Our analyses demonstrate that application of electrical stimulus modifies the molecular interactions within the complex and consequently, electrical field can be used to modulate the association between the thrombin and its aptamer.

Reduced Thermal Transport in the Graphene/MoS2/Graphene Heterostructure: A Comparison with Freestanding Monolayers

The thermal conductivity of the graphene-encapsulated MoS2 (graphene/MoS2/graphene) van der Waals heterostructure is determined along the armchair and zigzag directions with different twist angles between the layers using molecular dynamics (MD) simulations. The differences in the predictions relative to those of the monolayers are analyzed using the phonon power spectrum and phonon lifetimes obtained by spectral energy density analysis. The thermal conductivity of the heterostructure is predominantly isotropic. The out-of-plane phonons of graphene are suppressed because of the interaction between the adjacent layers that results in the reduced phonon lifetime and thermal conductivity relative to monolayer graphene. The occurrence of an additional nonzero phonon branch at the Γ point in the phonon dispersion curves of the heterostructure corresponds to the breathing modes resulting from stacking of the layers in the heterostructure. The thermal sheet conductance of the heterostructure being an order of magnitude larger than that of monolayer MoS2, this van der Waals material is potentially suitable for efficient thermal packaging of photoelectronic devices. The interfacial thermal conductance of the graphene/MoS2 bilayer as a function of the heat flow direction shows weak thermal rectification.

Impeding phonon transport through superlattices of organic–inorganic halide perovskites

Superlattice structures present a strategy to impede lattice thermal transport through organic–inorganic halide perovskites and improve their potential for thermoelectric applications. We investigate the phonon characteristics of such novel configurations and compare against predictions of the simple perovskite lattices using first principle calculations. Our results show the existence of structural instabilities due to distortions in the octahedral cage surrounding the methylammonium ion. In the superlattices, a strong phonon incoherence reduces the group velocities while interfacial resistance enhances scattering and limits the phonons impeding heat conduction. Although heat transfer is anisotropic in these perovskites, the interfaces in the superlattice obstruct phonon transport along all directions.

Aptamer based electrostatic-stimuli responsive surfaces for on-demand binding/unbinding of a specific ligand

We report an aptamer functionalized stimuli responsive surface that can controllably switch between binding and releasing its specific ligand under application of electrical stimuli. The high affinity of the aptamer for thrombin makes the surface undergo specific binding while electrostatic field induced actuation of the aptamer is utilized to release the ligand from the surface. Atomic force microscopy (AFM) was utilized to determine the characteristic height change, associated with the specific binding of thrombin, on anti-thrombin aptamer coated surfaces. Subsequently, the thrombin/aptamer complex covered surfaces were subjected to different magnitudes of electrostatic field and height changes on the surface were measured to investigate the influence of an electrical field. Application of positive electrical potential led to the removal of thrombin from the aptamer-covered surface. While under moderate magnitude of negative electrical potential the binding complexes were maintained, increasing the magnitude led to the removal of both molecules from the surface. Molecular dynamics (MD) simulations of the thrombin/aptamer complex under electrostatic fields show that thrombin dissociates from the aptamers in the presence of a positive electric field. These results demonstrate that aptamer covered surfaces undergo specific binding to the ligand and an electrostatic field may be used to disrupt the binding and on-demand release of the ligand from the surface.