A pinboard by
Niranjan Ilawe

Graduate Student Researcher, University of California, Riverside


Plasmonic systems composed of nanoparticle arrays have fascinated researchers over the last couple decades, prompting comprehensive studies on plasmon-mediated excitation energy transfer (EET) processes. A theoretical understanding of these processes is crucial not only for the development of nanoscale photonic circuitry, but also for the design of highly efficient energy-guiding nanoantennas. Although theoretical methods based on Forsters resonance energy transfer (FRET) have been successfully applied to single donor/acceptor systems, these methods in their standard form fail for large multi-donor/acceptor assemblies in complex configurations. While ab-initio quantum-mechanical methods like density functional theory (DFT) go beyond the simple point-dipole and spectral overlap approximations of FRET, they are computationally intensive and limited to a few hundred atoms. Here, we describe our use of the density functional tight binding (DFTB) approach and its real-time time-dependent counterpart, RT-TDDFTB, to probe in detail the EET dynamics of plasmonic nanoantenna systems without recourse to the point-dipole or spectral overlap approximations. The computational efficiency of DFTB is due to additional integral approximations arising from the tight-binding approach, resulting in a linear scaling computational cost. In particular, we discuss the results obtained by the RT-TDDFTB calculations for a large plasmonic nanoantenna composed of 220 sodium atoms. We reveal a complex interplay of many-body interactions that govern the EET mechanism in the plasmonic nanoantenna that go beyond the single donor/acceptor interactions considered in traditional theories. We attribute these effects in part to the exceedingly long-range electrodynamic couplings, which are a result of the coherent nature of oscillating electrons in plasmonic nanoparticles. We also corroborate our findings via an analytical two-level system that captures many of the intricacies of the full quantum dynamical method. Most importantly, our time-domain studies provide an intuitive approach to probe in microscopic detail the real-time electron dynamics in large plasmonic nanoantennas.


Assessing backbone solvation effects in the conformational propensities of amino acid residues in unfolded peptides.

Abstract: Conformational ensembles of individual amino acid residues within model GxG peptides (x representing different amino acid residues) are dominated by a mixture of polyproline II (pPII) and β-strand like conformations. We recently discovered rather substantial differences between the enthalpic and entropic contributions to this equilibrium for different amino acid residues. Isoleucine and valine exceed all other amino acid residues in terms of their rather large enthalpic stabilization and entropic destabilization of polyproline II. In order to shed light on these underlying physical mechanisms, we performed high-level DFT calculations to explore the energetics of four representative GxG peptides where x = alanine (A), leucine (L), valine (V), and isoleucine (I) in explicit water (10 H2O molecules with a polarizable continuum water model) and in vacuo. We found that the large energetic contributions to the stabilization of pPII result, to a major extent, from peptide-water, water-water interactions, and changes of the solvent self-energy. Differences between the peptide-solvent interaction energies of hydration in pPII and β-strand peptides are particularly important for the pPII ⇌ β equilibria of the more aliphatic peptides GIG and GLG. Furthermore, we performed a vibrational analysis of the four peptides in both conformations and discovered a rather substantial mixing between water motions and peptide vibrations below 700 cm(-1). We found that the respective vibrational entropies are substantially different for the considered conformations, and their contributions to the Gibbs/Helmholtz energy stabilize β-strand conformations. Taken together, our results underscore the notion of the solvent being the predominant determinant of peptide (and protein) conformations in the unfolded state.

Pub.: 08 Sep '15, Pinned: 08 Jun '17

Breaking Badly: DFT-D2 Gives Sizeable Errors for Tensile Strengths in Palladium-Hydride Solids.

Abstract: Dispersion interactions play a crucial role in noncovalently bound molecular systems, and recent studies have shown that dispersion effects are also critical for accurately describing covalently bound solids. While most studies on bulk solids have solely focused on equilibrium properties (lattice constants, bulk moduli, and cohesive energies), there has been little work on assessing the importance of dispersion effects for solid-state properties far from equilibrium. In this work, we present a detailed analysis of both equilibrium and highly nonequilibrium properties (tensile strengths leading to fracture) of various palladium-hydride systems using representative DFT methods within the LDA, GGA, DFT-D2, DFT-D3, and nonlocal vdw-DFT families. Among the various DFT methods, we surprisingly find that the empirically constructed DFT-D2 functional gives extremely anomalous and qualitatively incorrect results for tensile strengths in palladium-hydride bulk solids. We present a detailed analysis of these effects and discuss the ramifications of using these methods for predicting solid-state properties far from equilibrium. Most importantly, we suggest caution in using DFT-D2 (or other coarse-grained parametrizations obtained from DFT-D2) for computing material properties under large stress/strain loads or for evaluating solid-state properties under extreme structural conditions.

Pub.: 18 Nov '15, Pinned: 08 Jun '17

Polarizabilities of π-Conjugated Chains Revisited: Improved Results from Broken-Symmetry Range-Separated DFT and New CCSD(T) Benchmarks.

Abstract: We present a detailed analysis of non-empirically tuned range-separated functionals, with both short- and long-range exchange, for calculating the static linear polarizability and second hyperpolarizabilities of various polydiacetylene (PDA) and polybutatriene (PBT) oligomers. Contrary to previous work on these systems, we find that the inclusion of some amount of short-range exchange does improve the accuracy of the computed polarizabilities and second hyperpolarizabilities. Most importantly in contrast to prior studies on these oligomers, we find that the lowest-energy electronic states for PBT are not closed-shell singlets, and enhanced accuracy with range-separated DFT can be obtained by allowing the system to relax to a lower-energy broken-symmetry solution. Both the computed polarizabilities and second hyperpolarizabilities for PBT are significantly improved with these broken-symmetry solutions when compared to previously published and current benchmarks. In addition to these new analyses, we provide new large-scale CCSD(T) and explicitly-correlated CCSD(T)-F12 benchmarks for the PDA and PBT systems, which comprise the most complete and accurate calculations of linear polarizabilities and second hyperpolarizabilities on these systems to date. These new CCSD(T) and CCSD(T)-F12 benchmarks confirm our DFT results and emphasize the importance of broken-symmetry effects when calculating polarizabilities and hyperpolarizabilties of π-conjugated chains.

Pub.: 23 Jun '16, Pinned: 08 Jun '17

A Non-Thermal Plasma Route to Plasmonic TiN Nanoparticles

Abstract: In this contribution, we present a high-throughput method for the synthesis of titanium nitride nanoparticles. The technique, based on a continuous-flow nonthermal plasma process, leads to the formation of free-standing titanium nitride particles with crystalline structures and below 10 nm in size. Extinction measurements of the as-synthesized particles show a clear plasmonic resonance in the near-infrared region, with a peak plasmon position varying between 800 and 1000 nm. We have found that the composition can be controllably tuned by modifying the process parameters and that the particle optical properties are strongly dependent upon composition. XPS and STEM/EDS analyses suggest that nitrogen-poor particles are more susceptible to oxidation, and the extinction spectra show a decrease and a red-shift in plasmon peak position as the degree of oxidation increases. The role of oxidation is confirmed by real-time, time-dependent density functional tight binding (RT-TDDFTB) calculations, which also predict a decrease in the localized surface plasmon resonance energy when a single monolayer of oxygen is added to the surface of a titanium nitride nanocrystal. This study highlights the opportunity and challenges presented by this material system. Understanding the processing-properties relationships for alternative plasmonic materials such as titanium nitride is essential for their successful use in biomedical, photocatalytic, and optoelectronic applications.

Pub.: 03 Jan '17, Pinned: 08 Jun '17