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Publications
Publications (106)
We measure the thermal conductivity of solid and molten tungsten using Steady State Temperature Differential Radiometry. We demonstrate that the thermal conductivity can be well described by application of Wiedemann-Franz Law to electrical resistivity data, thus suggesting the validity of Wiedemann-Franz Law to capture the electronic thermal conduc...
We measure the thermal conductivity of solid and molten tungsten using Steady State Temperature Differential Radiometry. We demonstrate that the thermal conductivity can be well described by application of Wiedemann-Franz Law to electrical resistivity data, thus suggesting the validity of Wiedemann-Franz Law to capture the electronic thermal conduc...
Covalent-organic frameworks (COFs) are a highly promising class of materials that can provide an excellent platform for thermal management applications. In this Perspective, we first review previous works on the thermal conductivities of COFs. Then we share our insights on achieving high, low, and switchable thermal conductivities of future COFs. T...
Understanding the fundamental heat-transport mechanisms across interfaces comprised of two-dimensional (2D) materials is crucial for the further development of the next generation of optoelectronic devices based on 2D heterostructures for which one of the main factors affecting the device performance is their poor thermal management. Here we use sy...
The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary re...
Resulting from their remarkable structure-property relationships, metal halide perovskites have garnered tremendous attention in recent years for a plethora of applications. For instance, their ultralow thermal conductivities make them promising candidates for thermoelectric and thermal barrier coating applications. It is widely accepted that the "...
The understanding of the fundamental relationships between chemical bonding and material properties, especially for carbon allotropes with diverse orbital hybridizations, is significant from both scientific and applicative standpoints. Here, we elucidate the influence of the intermolecular covalent bond configuration on the mechanical and thermal p...
Ultrafast laser irradiation of metals can elevate the temperature of electrons to the same order as the Fermi temperature while the lattice remains cold. In this transient and highly nonequilibrium regime, the material properties can undergo drastic modifications, revealing insights into the physical phenomena of the condensed state that are otherw...
Dielectric amorphous multilayers (AMLs) play a critical role in a wide array of technologies such as optical coatings, nanoelectronics, energy harvesting, and recovery devices. However, despite their wide applications, a robust understanding of the effect of the interplay between chemical and structural disorder on the thermal properties of AMLs is...
We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm,...
The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that...
Covalent organic frameworks (COFs) are an emerging class of polymeric crystals with immense permanent porosities and large surface areas that are highly sought after for a variety of applications such...
Extreme pressure conditions reveal fundamental insights into the physical properties of elemental metals that are otherwise not evident under ambient conditions. Herein, we use the density functional perturbation theory to demonstrate that the change in thermal conductivity as a result of large hydrostatic pressures at room temperature for aluminum...
Proper thermal management of solar cells based on metal halide perovskites (MHPs) is key to increasing their efficiency as well as their durability. Although two-dimensional (2D) MHPs possess enhanced thermal stability as compared to their three-dimensional (3D) counterparts, the lack of comprehensive knowledge of the heat transfer mechanisms dicta...
Hydrogen-bonded organic frameworks (HOFs) are a class of nanoporous crystalline materials formed by the assembly of organic building blocks that are held together by a network of hydrogen-bonding interactions. Herein, we show that the dynamic and responsive nature of these hydrogen-bonding interactions endows HOFs with a host of unique physical pro...
Silicon carbide coatings and thin films are used for a wide array of applications ranging from thermal barrier coatings to microelectronics. In this paper, we report on the role of mass density and atomic coordination on the fundamental vibrational characteristics and thermal conductivity of amorphous silicon carbide systems through a combination o...
We discuss the role of atomic coordination in dictating the vibrational characteristics and thermal conductivity in amorphous carbon. Our systematic atomistic simulations on amorphous carbon structures at varying mass densities show the significant role played by the ratio of sp2 to sp3 hybridized bonds in dictating the contributions from propagati...
We experimentally and theoretically investigate the thermal conductivity and mechanical properties of polycrystalline HKUST-1 metal-organic frameworks (MOFs) infiltrated with three guest molecules: tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), and (cyclohexane-1,4-diylidene)dimalononitrile (H4-TCNQ...
We derive a theoretical model for phonon thermal boundary conductance across solid interfaces in the high temperature classical limit using quasi-harmonic thermodynamics, an approach that accounts for phonon anharmonicity effects on energy density changes via thermal expansion. Commonly used predictive models based on harmonic theory predict a ther...
The primary bottle neck that is inhibiting ideal operating efficiency and performance in wide bandgap (WBG) semiconductor-based power electronics is the inability to effectively mitigate temperature. This is due in part to the limiting heat transfer across GaN interfacial regions due to the increases in phonon thermal boundary resistances. It is th...
The complete understanding of the mechanical and thermal responses to strain in hybrid organic–inorganic perovskites holds great potential for their proper functionalities in a range of applications, such as in photovoltaics, thermoelectrics, and flexible electronics. In this work, we conduct systematic atomistic simulations on methyl ammonium lead...
As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covale...
Hydrogenated amorphous dielectric thin films are critical materials in a wide array of technologies. In this work, we present a thorough investigation of the thermal conductivity of hydrogenated amorphous silicon nitride (a−SiNx:H), a ubiquitously used material in which the stoichiometry plays a direct role in its functionality and application. In...
We experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the tot...
Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data processing to overcome the von Neumann bottleneck. In PCMs, data storage is driven by thermal excitation. However, there is limited research regarding PCM thermal properties at length scales close to...
Responses of materials to high pressures unveil important insight into their structure-property relationships that are otherwise masked under ambient conditions. Here, using atomistic simulations, it is shown that the thermal conductivity of methylammonium lead iodide can be drastically increased by more than an order of magnitude with applications...
This review introduces relevant nanoscale thermal transport processes that impact thermal abatement in power electronics applications. Specifically, we highlight the importance of nanoscale thermal transport mechanisms at each layer in material hierarchies that make up modern electronic devices. This includes those mechanisms that impact thermal tr...
Thin-film (5−20 μm) polymer dielectrics are a critical component in high energy density capacitors. Understanding thermal transport in these materials is critical to addressing thermally enabled dielectric breakdown, a primary failure mechanism. Here, we measure the anisotropy in thermal conductivity for strained polymer films using frequency-domai...
In 2D organic-inorganic hybrid perovskite materials, layers of conducting inorganic material are separated by insulating organic spacers whose length and composition can be tuned. We report the heat capacity and cross-plane thermal conductivity of 2D alkylammonium lead iodide single crystals with increasing chain length, (C
n
H2n+1NH3)2PbI4 (n = 4-...
We have fabricated a model system of precisely layer-engineered inorganic–organic thin-film structures using atomic/molecular-layer deposition (ALD/MLD). The samples consist of nanoscale polycrystalline ZnO layers and intervening benzene layers, covering a broad range of layer sequences. The samples characterized in this study combined with previou...
Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data storage and processing towards overcoming the von Neumann bottleneck. In PCMs, the primary mechanism for data storage is thermal excitation. However, there is a limited body of research regarding the...
We report on the thermal conductivities of alkyl- and indene-group functionalized fullerene derivative thin films as measured via time domain thermoreflectance and steady state thermoreflectance. The thermal conductivities vary from 0.064±0.007 W m−1 K−1 for [6,6]-phenyl C61-butyric acid methyl ester (PCBM) to 0.15±0.017 W m−1 K−1 for indene-C60 bi...
Aluminum nitride (AlN) has garnered much attention due to its intrinsically high thermal conductivity. However, engineering thin films of AlN with these high thermal conductivities can be challenging due to vacancies and defects that can form during the synthesis. In this work, we report on the cross-plane thermal conductivity of ultra-high-purity...
Finding a competitor for diamond as a good heat conductor remains challenging. Measurements on crystals of cubic boron nitride demonstrate a thermal conductivity of 1,600 W m−1 K−1 at room temperature, rivalling diamond.
We report on the thermal conductivities of two-dimensional metal halide perovskites films measured by time domain thermoreflectance. Depending on the molecular substructure of ammonium cations and owing to the weak van der Waals interactions in the layered structures, the thermal conductivity for our two-dimensional hybrid perovskites ranges from 0...
The fundamental scattering mechanisms between electrons and phonons in metals and metallic alloys dictate a wide range of phenomena in their materials’ physics. Using first-principles calculations carried out on dense electron and phonon wavevector grids, we determine the mode-level descriptions of electron–phonon interactions for nine characterist...
Charge and energy transfer at nanoscale metal/metal and metal/semiconductor interfaces are essential for modern electronics, catalysis, photovoltaics and other applications. Experiments show that a thin Ti adhesion layer deposited between an Au film and a semiconductor substrate greatly accelerates electron-vibrational energy transfer. We employ ab...
Interfacial thermal resistance is the primary impediment to heat flow in materials and devices as characteristic lengths become comparable to the mean‐free paths of the energy carriers. This thermal boundary conductance across solid interfaces at the nanoscale can affect a plethora of applications. The recent experimental and computational advances...
We investigate the effects of fullerene functionalization on the thermal transport properties of graphene monolayers via atomistic simulations. Our systematic molecular dynamics simulations reveal that the thermal conductivity of pristine graphene can be lowered by more than an order of magnitude at room temperature (and as much as by ∼93% as compa...
Responses of materials at high pressures can reveal fundamental insights into properties of condensed matter that are otherwise masked under ambient conditions. Using first-principles calculations to determine the electron-phonon interactions of free-electron metals at high pressures, we show that the electrical and thermal transport properties of...
Understanding the effects and limitations of solid/liquid interfaces on energy transport is crucial to applications ranging from nano-scale thermal engineering to chemical synthesis. Up to now, the majority of experimental evidence regarding solid/liquid interactions has been limited to macroscale observations and experiments. The lack of experimen...
We report on the lifetimes of zone-center longitudinal phonon modes in GaAs/AlAs superlattices with varying period thicknesses. By measuring the lifetimes at 78 K and room temperature, we separate the contributions of intrinsic and extrinsic contributions to the lifetime of a 223 GHz longitudinal phonon mode in a 12 nm x 12 nm period superlattice....
Carbon based materials have attracted much attention as building blocks in technologically relevant nanocomposites due to their unique chemical and physical properties. Here, we propose a new class of hierarchical carbon based nano-truss structures consisting of fullerene joints attached with carbon nanotubes as the truss forming a three-dimensiona...
We present experimental measurements of the thermal boundary conductance (TBC) from 77 − 500 K across isolated heteroepitaxially grown ZnO films on GaN substrates. This data provides an assessment of the underlying assumptions driving phonon gas based models, such as the diffuse mismatch model (DMM), and atomistic Green’s function (AGF) formalisms...
The temperature-dependent reflectivity of metals is quantified by the thermoreflectance coefficient, which is a material-dependent parameter that depends on the metallic band structure, electron scattering dynamics, and photon wavelength. After short-pulse laser heating, the electronic subsystem in a metal can be driven to temperatures much higher...
We report on the thermal resistances of thin films (20 nm) of hafnium zirconium oxide (Hf1-xZrxO2) with compositions ranging from 0 ≤ x ≤ 1. Measurements were made via time-domain thermoreflectance and analyzed to determine the effective thermal resistance of the films in addition to their associated thermal boundary resistances. We find effective...
Focused Ion Beam (FIB) technology has become a valuable tool for the microelectronics industry and for the fabrication and preparation of samples at the micro/nanoscale. Its effects on the thermal transport properties of Si, however are not well understood, nor do experimental data exist. This paper presents a carefully designed set of experiments...
Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons...
The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC). However, recent computational studies have suggested that interfacial defects can enhance this thermal boundary conductance through the emergence of...
The energy flux to a surface during plasma exposure and the associated surface heating are of long standing interest as they contribute to the physico-chemical changes that occur during plasma-based materials synthesis and processing. Indeed, the energy delivered to the surface, via a flux of particles and photons, in concert with a flux of reactiv...
Manipulating thermal transport by designing materials with control of their properties over the entire spectral range of vibrational frequencies would provide a unique path to create solids with designer thermal conductivities. Traditional routes of nanostructuring to reduce the vibrational thermal conductivity of solids typically target narrow ban...
We investigate the vibrational heat transfer mechanisms in amorphous Stillinger-Weber silicon and germanium-based alloys and heterostructures via equilibrium and nonequilibrium molecular dynamics simulations along with lattice dynamics calculations. We find that similar to crystalline alloys, amorphous alloys demonstrate large size effects in therm...
We use aluminum nano-inclusions in silicon to demonstrate the dominance of elastic modulus mismatch induced scattering in phonon transport. We use time domain thermoreflectance to measure the thermal conductivity of thin films of silicon co-deposited with aluminum via molecular beam epitaxy resulting in a Si film with 10% clustered Al inclusions wi...
Nanoscale superlattice structures are known to significantly suppress the thermal conductivity in thin films due to phonon scattering at the interfaces of the mutually different layers. Here it is demonstrated that in addition to the number of interfaces, their spacing within the film can lead to a reduction in thermal conductivity. The proof‐of‐co...
We investigate the effect of mass disorder, temperature, and pressure on the spectral thermal conductivity of multicomponent crystalline solid solutions via molecular dynamics simulations. The thermal conductivities of Lennard-Jones based solid solutions with one to five different atomic components in the crystalline lattice are simulated at a rang...
Fullerene condensed-matter solids can possess thermal conductivities below their minimum glassy limit while theorized to be stiffer than diamond when crystallized under pressure. These seemingly disparate extremes in thermal and mechanical properties raise questions into the pressure dependence on the thermal conductivity of C60 fullerite crystals,...
In this study, we calculate the steady-state temperature rise that results from laser heating of multilayer thin films using the heat diffusion equation. For time- and frequency-domain thermoreflectance (TDTR and FDTR) that rely on modulated laser sources, we decouple the modulated and steady-state temperature profiles to understand the conditions...
We investigate the influence of optical phonon coupling across interfaces comprised of different materials with varying crystallographic orientations on the overall thermal boundary conductance. We show that for interfaces formed between a fcc solid and a L10 solid (where L10 solids exhibit alternating atomic layers in certain orientations), coupli...
Contact Resistance (RC) is a major limiting factor in the performance of graphene devices. RC is sensitive to the quality of the interface and the composition of the contact, which are affected by the graphene transfer process and contact deposition conditions. In this work, a linear correlation is observed between the composition of Ti contacts, c...
Electron-phonon relaxation in thin metal films is an important consideration for many ultra-small devices and ultra-fast applications. Recent time-resolved experiments demonstrate a significant, more than a factor of five, increase in the electron-phonon coupling and an acceleration in the electron-phonon relaxation rate when a narrow Ti adhesion l...
The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC).$^1$ However, recent computational works have suggested that interfacial defects can enhance this thermal boundary conductance through emergence of u...
We present experimental measurements of the thermal boundary conductance (TBC) from $77 - 500$ K across isolated heteroepitaxially grown ZnO films on GaN substrates. These data provide an assessment of the assumptions that drive the phonon gas model-based diffuse mismatch models (DMM) and atomistic Green's function (AGF) formalisms for predicting T...
Time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) are common pump-probe techniques that are used to measure the thermal properties of materials. At elevated temperatures, transducers used in these techniques can become limited by melting or other phase transitions. In this work, time domain thermoreflectance is used...
We investigate the effect of mass disorder on the thermal conductivity of Lennard-Jones based multi-component solid solutions via classical molecular dynamics simulations. In agreement with Klemens' perturbation theory, the thermal conductivity reduction due to mass scattering alone is found to reach a critical point, whereby adding more impurity a...
For this paper, single-pulse ablation mechanisms of ultrafast laser pulses (25 ps) were studied for thin gold films (65 nm) on an array of substrates with varying physical properties. Using time-domain thermoreflectance, the interfacial properties of the thin-film systems are measured: in particular, the thermal boundary conductance. We find that a...
We investigate the effect of crystalline configuration on the thermal conductivity of binary Lennard-Jones based solid solutions via classical molecular dynamics simulations and harmonic lattice dynamics calculations. We show that the pronounced effect of Umklapp scattering causes the cross-plane thermal conductivity of the chemically ordered alloy...
Damage in the form of dewetting and delamination of thin films is a major concern in applications
requiring micro- or nano-fabrication. In non-contact nanoscale characterization, optical interrogation
must be kept to energies below damage thresholds in order to conduct measurements such as pumpprobe
spectroscopy. In this study, we show that the the...
Utilizing a combination of atomic layer deposition and dip-coating techniques, we have incorporated natural nanocellulose fibers into an inorganic matrix in order to create a layered hybrid inorganic–organic thin-film structure. Such layer-engineered hybrid materials with an unorthodox combination of components are highly potential candidates for e...