arXiv subjects

Nicholas A. Kotov

Publications and source records attributed to Nicholas A. Kotov.

8 recordsLinked to original sources

arXiv2026

Raman and Terahertz Spectroscopy of Low-Frequency Chiral Phonons in Amino Acids

Chiral phonons are mirror-symmetric vibrations that correspond to twisting and rotational motions of atoms. In chiral biomolecules, they correspond to low-energy terahertz (THz)-range vibrations of the molecular segments involving dozens of atoms whose energies are sensitive to the chirality of the molecules and local atomic geometries. Here we present spectral signatures of chiral phonons in circularly polarized low-frequency Raman and Raman optical activity (ROA) spectra from crystals of several amino acids in different enantiomeric forms. Along with complementary THz circular dichroism (TCD) measurements, our ROA data reveal two sets of bisignate peaks in valine, alanine, tyrosine and proline between 1 and 4.5 THz that are more intense than the ROA peaks in the fingerprint region. Density functional theory (DFT) calculations on L-alanine attribute these modes to twisting and shearing molecular motions. The strong agreement between the ROA and TCD data demonstrates the power of these complementary vibrational spectroscopy techniques to identify chiral phonons in biomolecules, and offers new insights into their vibrational properties and interactions with circularly polarized light.

cond-mat.mtrl-sci↗

arXiv2025

Chiral quantum magnets with optically and catalytically active spin ladders

Chiral quantum magnets with spin-states separated by a large energy gap are technologically attractive but difficult to realize. Geometrically frustrated topological states with nanoscale chirality may offer a chemical pathway to such materials. However, room temperature spin misalignment, weakness of Dzyaloshinskii-Moriya interactions, and high energy requirements for lattice distortions set high physicochemical barriers for their realization. Here, we show that layered iron oxyhydroxides (LIOX) address these challenges due to chirality transfer from surface ligands into spin-states of dimerized FeO6 octahedra with zig-zag stacking. The intercalation of chiral amino acids induces angular displacements in the antiferromagnetic spin pairs with a helical coupling of magnetic moments along the screw axis of the zig-zag chains, or helical spin-ladders. Unlike other chiral magnets, the spin states in LIOX are chemically and optically accessible, they display strong optical resonances with helicity-matching photons and enable spin-selective charge transport. The static rather than dynamic polarization of spin ladders in LIOX makes them particularly suitable for catalysis. Room-temperature spin pairing, field-tunability, environmental robustness, and synthetic simplicity make LIOX and its intercalates a uniquely practical family of quantum magnets.

cond-mat.mtrl-sci↗

arXiv2024

Graph-Theoretical Description and Continuity Problems for Stress Propagation Through Complex Strut Lattices

Interconnected networks of rigid struts are critical for application in lightweight, load-bearing structures. However, accurately modeling stress distribution in these strut lattices poses significant computational challenges due to its strong dependence on organizational patterns, boundary conditions, and collective effects. Leveraging two-dimensional strut lattices that enable visualization of local elastic deformation, we investigate how graph theory (GT) provides a framework for stress prediction. We investigate how the geometric features often neglected by GT play a crucial role in the behavior of anisotropic networks. We also address the challenge of topological continuity that arises when applying discrete mathematics to physical structures. We show that modified centrality parameters combining lattice topology with geometry more accurately predict local stress, as validated through birefringence imaging and finite element modeling. Finally, we show how further improvements are made by incorporating strut lattice boundary conditions into the centrality definition, in a manner that simultaneously simplifies the computational cost.

cond-mat.mtrl-sci↗

arXiv2023

Layer-by-Layer Assembled Nanowire Networks Enable Graph Theoretical Design of Multifunctional Coatings

Multifunctional coatings are central for information, biomedical, transportation and energy technologies. These coatings must possess hard-to-attain properties and be scalable, adaptable, and sustainable, which makes layer-by-layer assembly (LBL) of nanomaterials uniquely suitable for these technologies. What remains largely unexplored is that LBL enables computational methodologies for structural design of these composites. Utilizing silver nanowires (NWs), we develop and validate a graph theoretical (GT) description of their LBL composites. GT successfully describes the multilayer structure with nonrandom disorder and enables simultaneous rapid assessment of several properties of electrical conductivity, electromagnetic transparency, and anisotropy. GT models for property assessment can be rapidly validated due to (1) quasi-2D confinement of NWs and (2) accurate microscopy data for stochastic organization of the NW networks. We finally show that spray-assisted LBL offers direct translation of the GT-based design of composite coatings to additive, scalable manufacturing of drone wings with straightforward extensions to other technologies.

physics.app-ph↗

arXiv2022

High temperature nanocomposites with photonic group velocity suppression of thermal emission

Quenching of thermal emission above 0 K is an unusual material property, essential for future energy, transportation, and space technologies. Despite the great effort invested, nearly complete quenching of thermal radiation rather than some reduction of its flux has only been achieved at low temperatures (below 373 K) and in narrow spectral windows using complex techniques suitable only for small scale objects. In this work, we present a light and flexible composite material that can suppress propagating photonic modes and, in this way, quench thermal radiation while preserving heat transfer (by thermal conduction) at a room and higher temperature below 600 K. This has been achieved by altering the local photonic density of states and consequentially the thermal properties of carbon nanotubes forming a percolating nanofiber network with a thermostable polymeric matrix.

physics.optics↗

arXiv2022

Chiral Assemblies of Pinwheel Superlattices on Substrates

The unique topology and physics of chiral superlattices make their self-assembly from nanoparticles a holy grail for (meta)materials. Here we show that tetrahedral gold nanoparticles can spontaneously transform from a perovskite-like low-density phase with corner-to-corner connections into pinwheel assemblies with corner-to-edge connections and denser packing. While the corner-sharing assemblies are achiral, pinwheel superlattices become strongly mirror-asymmetric on solid substrates as demonstrated by chirality measures. Liquid-phase transmission electron microscopy and computational models show that van der Waals and electrostatic interactions between nanoparticles control thermodynamic equilibrium. Variable corner-to-edge connections among tetrahedra enable fine-tuning of chirality. The domains of the bilayer superlattices display strong chiroptical activity identified by photon-induced near-field electron microscopy and finite-difference time-domain simulations. The simplicity and versatility of the substrate-supported chiral superlattices facilitate manufacturing of metastructured coatings with unusual optical, mechanical and electronic characteristics.

cond-mat.mtrl-sci↗

arXiv2018

Interpretable and efficient contrast in scanning transmission electron microscopy with a diffraction grating beamsplitter

Efficient imaging of biomolecules, 2D materials and electromagnetic fields depends on retrieval of the phase of transmitted electrons. We demonstrate a method to measure phase in a scanning transmission electron microscope using a nanofabricated diffraction grating to produce multiple probe beams. The measured phase is more interpretable than phase-contrast scanning transmission electron microscopy techniques without an off-axis reference wave, and the resolution could surpass that of off-axis electron holography. We apply the technique to image nanoparticles, carbon sub- strates and electric fields. The contrast observed in experiments agrees well with contrast predicted in simulations.

physics.ins-det↗

arXiv2007

Theory of plasmon-enhanced Foerster energy transfer in optically-excited semiconductor and metal nanoparticles

We describe the process of Foerster transfer between semiconductor nanoparticles in the presence of a metal subsystem (metal nanocrystals). In the presence of metal nanocrystals, the Foerster process can become faster and more long-range. The enhancement of Foerster transfer occurs due to the effect of plasmon-assisted amplification of electric fields inside the nanoscale assembly. Simultaneously, metal nanocrystals lead to an increase of energy losses during the Foerster transfer process. We derive convenient equations for the energy transfer rates, photoluminescence intensities, and energy dissipation rates in the please of plasmon resonances. Because of strong dissipation due to the metal, an experimental observation of plasmon-enhanced Foerster transfer requires special conditions. As possible experimental methods, we consider cw- and time-resolved photoluminescence studies and describe the conditions to observe plasmon-enhanced transfer. In particular, we show that the photoluminescence spectra should be carefully analyzed since the plasmon-enhanced Foerster effect can appear together with strong exciton energy dissipation. Our results can be applied to a variety of experimental nanoscale systems.

cond-mat.mes-hall↗