Ashutosh Giri

Efficiently suppressed thermal conductivity in ZnO thin films via periodic introduction of organic layers

A combination of atomic and molecular layer deposition techniques is used to fabricate thin films of hybrid inorganic–organic superlattice structures with periodically repeating single layers of hydroquinone within a ZnO or (Zn0.98Al0.02)O framework. A significant reduction of up to one magnitude in the thermal conductivity of the films as evaluated with the time-domain thermoreflectance technique is observed upon introduction of the organic layers, resulting in a greatly improved thermoelectric performance.

Analytical model for the effects of wetting on thermal boundary conductance across solid/classical liquid interfaces

We develop an analytical model for the thermal boundary conductance between a solid and a liquid. By infusing recent developments in the phonon theory of liquid thermodynamics with diffuse mismatch theory, we derive a closed form model that can predict the effects of wetting on the thermal boundary conductance across an interface between a solid and a classical liquid. We account for the complete wetting (hydrophilicity), or lack thereof (hydrophobicity), of the liquid to the solid by considering varying contributions of transverse mode interactions between the solid and liquid interfacial layers; this transverse coupling relationship is determined with local density of states calculations from molecular dynamics simulations between Lennard-Jones solids and a liquids with different interfacial interaction energies. We present example calculations for the thermal boundary conductance between both hydrophobic and hydrophilic interfaces of Al/water and Au/water, which show excellent agreement with measured v...

Experimental evidence of excited electron number density and temperature effects on electron-phonon coupling in gold films

The electronic transport properties of metals with weak electron-phonon coupling can be influenced by non-thermal electrons. Relaxation processes involving non-thermal electrons competing with the thermalized electron system have led to inconsistencies in the understanding of how electrons scatter and relax with the less energetic lattice. Recent theoretical and computational works have shown that the rate of energy relaxation with the metallic lattice will change depending on the thermalization state of the electrons. Even though 20 years of experimental works have focused on understanding and isolating these electronic relaxation mechanisms with short pulsed irradiation, discrepancies between these existing works have not clearly answered the fundamental question of the competing effects between non-thermal and thermal electrons losing energy to the lattice. In this work, we demonstrate the ability to measure the electron relaxation for varying degrees of both electron-electron and electron-phonon thermalization. This series of measurements of electronic relaxation over a predicted effective electron temperature range up to ∼3500 K and minimum lattice temperatures of 77 K validate recent computational and theoretical works that theorize how a nonequilibrium distribution of electrons transfers energy to the lattice. Utilizing this wide temperature range during pump-probe measurements of electron-phonon relaxation, we explain discrepancies in the past two decades of literature of electronic relaxation rates. We experimentally demonstrate that the electron-phonon coupling factor in gold increases with increasing lattice temperature and laser fluences. Specifically, we show that at low laser fluences corresponding to small electron perturbations, energy relaxation between electrons and phonons is mainly governed by non-thermal electrons, while at higher laser fluences, non-thermal electron scattering with the lattice is less influential on the energy relaxation mechanisms.

Spectral analysis of thermal boundary conductance across solid/classical liquid interfaces: A molecular dynamics study

We investigate the fundamental mechanisms driving thermal transport across solid/classical-liquid interfaces via non-equilibrium molecular dynamics simulations. We show that the increase in thermal boundary conductance across strongly bonded solid/liquid interfaces compared to weakly bonded interfaces is due to increased coupling of low-frequency modes when the solid is better wetted by the liquid. Local phonon density of states and spectral temperature calculations confirm this finding. Specifically, we show that highly wetted solids couple low frequency phonon energies more efficiently, where the interface of a poorly wetted solid acts like free surfaces. The spectral temperature calculations provide further evidence of low frequency phonon mode coupling under non equilibrium conditions. These results quantitatively explain the influence of wetting on thermal boundary conductance across solid/liquid interfaces.

Kapitza resistance and the thermal conductivity of amorphous superlattices

We report on the thermal conductivities of amorphous Stillinger-Weber and Lennard-Jones superlattices as determined by non-equilibrium molecular dynamics simulations. Thermal conductivities decrease with increasing interface density, demonstrating that interfaces contribute a non-negligible thermal resistance. Interestingly, Kapitza resistances at interfaces between amorphous materials are lower than those at interfaces between the corresponding crystalline materials. We find that Kapitza resistances within the Stillinger-Webber based Si/Ge amorphous superlattices are not a function of interface density, counter to what has been observed in crystalline superlattices. Furthermore, the widely used thermal circuit model is able to correctly predict the interfacial resistance within the Stillinger-Weber based amorphous superlattices. However, we show that the applicability of this widely used thermal circuit model is invalid for Lennard-Jones based amorphous superlattices, suggesting that the assumptions made...

Ultra-low thermal conductivity in TiO2:C superlattices

TiO2:C superlattices are fabricated from atomic/molecular layer deposited (ALD/MLD) inorganic–organic [(TiO2)m(Ti–O–C6H4–O–)k=1]n thin films via a post-deposition annealing treatment that converts the as-deposited monomolecular organic layers into sub-nanometer-thick graphitic interface layers confined within the TiO2 matrix. The internal graphitic layers act as effective phonon-scattering boundaries that bring about a ten-fold reduction in the thermal conductivity of the films with a decreasing superlattice period down to an ultra-low value of 0.66 ± 0.04 W m−1 K−1 – a finding that makes inorganic-C superlattices fabricated by the present method promising structures for e.g. high-temperature thermal barriers and thermoelectric applications.

Mechanisms of nonequilibrium electron-phonon coupling and thermal conductance at interfaces

We study the electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump-probe time-domain thermoreflectance. The coupling between the electronic and the vibrational states is increased by more than a factor of five with the inclusion of an ∼3 nm Ti adhesion layer between the Au film and the non-metal substrate. Furthermore, we show an increase in the rate of relaxation of the electron system with increasing electron and lattice temperatures induced by the laser power and attribute this to enhanced electron-electron scattering, a transport channel that becomes more pronounced with increased electron temperatures. The inclusion of the Ti layer also results in a linear dependence of the electron-phonon relaxation rate with temperature, which we attribute to the coupling of electrons at and near the Ti/substrate interface. This enhanced electron-phonon coupling due to electron-interface scattering is shown to have negligi...

Effect of crystalline/amorphous interfaces on thermal transport across confined thin films and superlattices

We report on the thermal boundary resistances across crystalline and amorphous confined thin films and the thermal conductivities of amorphous/crystalline superlattices for Si/Ge systems as determined via non-equilibrium molecular dynamics simulations. Thermal resistances across disordered Si or Ge thin films increase with increasing length of the interfacial thin films and in general demonstrate higher thermal boundary resistances in comparison to ordered films. However, for films ≲3 nm, the resistances are highly dependent on the spectral overlap of the density of states between the film and leads. Furthermore, the resistances at a single amorphous/crystalline interface in these structures are much lower than those at interfaces between the corresponding crystalline materials, suggesting that diffusive scattering at an interface could result in higher energy transmissions in these systems. We use these findings, together with the fact that high mass ratios between amorphous and crystalline materials can...

Influence of chemical ordering on the thermal conductivity and electronic relaxation in FePt thin films in heat assisted magnetic recording applications

We report on the out-of-plane thermal conductivities of tetragonal L10 FePt (001) easy-axis and cubic A1 FePt thin films via time-domain thermoreflectance over a temperature range from 133 K to 500 K. The out-of-plane thermal conductivity of the chemically ordered L10 phase with alternating Fe and Pt layers is ~23% greater than the thermal conductivity of the disordered A1 phase at room temperature and below. However, as temperature is increased above room temperature, the thermal conductivities of the two phases begin to converge. Molecular dynamics simulations on model FePt structures support our experimental findings and help shed more light into the relative vibrational thermal transport properties of the L10 and A1 phases. Furthermore, unlike the varying temperature trends in the thermal conductivities of the two phases, the electronic scattering rates in the out-of-plane direction of the two phases are similar for the temperature range studied in this work.

Influence of Hot Electron Scattering and Electron–Phonon Interactions on Thermal Boundary Conductance at Metal/Nonmetal Interfaces

It has recently been demonstrated that under certain conditions of electron nonequilibrium, electron to substrate energy coupling could represent a unique mechanism to enhance heat flow across interfaces. In this work, we present a coupled thermodynamic and quantum mechanical derivation of electron–phonon scattering at free electron metal/ nonmetal substrate interfaces. A simplified approach to the Fermi’s Golden Rule with electron energy transitions between only three energy levels is adopted to derive an electron–phonon diffuse mismatch model, that account for the electron–phonon thermal boundary conductance at metal/insulator interfaces increases with electron temperature. Our approach demonstrates that the metal-electron/nonmetal phonon conductance at interfaces can be an order of magnitude larger than purely phonon driven processes when the electrons are driven out of equilibrium with the phonons, consistent with recent experimental observations. [DOI: 10.1115/1.4027785]