arXiv subjects

Patrick E. Hopkins

Publications and source records attributed to Patrick E. Hopkins.

At least 37 records · Page 2Linked to original sources

arXiv2024

Ultrafast evanescent heat transfer across solid interfaces via hyperbolic phonon polaritons in hexagonal boron nitride

The efficiency of phonon-mediated heat transport is limited by the intrinsic atomistic properties of materials, seemingly providing an upper limit to heat transfer in materials and across their interfaces. The typical speeds of conductive transport, which are inherently limited by the chemical bonds and atomic masses, dictate how quickly heat will move in solids. Given that phonon-polaritons, or coupled phonon-photon modes, can propagate at speeds approaching 1 percent of the speed of light - orders of magnitude faster than transport within a pure diffusive phonon conductor - we demonstrate that volume-confined, hyperbolic phonon-polariton(HPhP) modes supported by many biaxial polar crystals can couple energy across solid-solid interfaces at an order of magnitude higher rates than phonon-phonon conduction alone. Using pump-probe thermoreflectance with a mid-infrared, tunable, probe pulse with sub-picosecond resolution, we demonstrate remote and spectrally selective excitation of the HPhP modes in hexagonal boron nitride in response to radiative heating from a thermally emitting gold source. Our work demonstrates a new avenue for interfacial heat transfer based on broadband radiative coupling from a hot spot in a gold film to hBN HPhPs, independent of the broad spectral mismatch between the pump(visible) and probe(mid-IR) pulses employed. This methodology can be used to bypass the intrinsically limiting phonon-phonon conductive pathway, thus providing an alternative means of heat transfer across interfaces. Further, our time-resolved measurements of the temperature changes of the HPhP modes in hBN show that through polaritonic coupling, a material can transfer heat across and away from an interface at rates orders of magnitude faster than diffusive phonon speeds intrinsic to the material, thus demonstrating a pronounced thermal transport enhancement in hBN via phonon-polariton coupling.

cond-mat.mes-hall↗

arXiv2024

Disorder controlled sound speed and thermal conductivity of hybrid metalcone films

The multifaceted applications of polymers are often limited by their thermal conductivity. Therefore, understanding the mechanisms of thermal transport in polymers is of vital interest. Here, we leverage molecular layer deposition to grow three types of hybrid metalcone (i.e., alucone, zincone, and tincone) films and study their thermal and acoustic properties. The thermal conductivity of the hybrid polymer films ranges from 0.43 to 1.14 W/mK. Using kinetic theory, we trace the origin of thermal conductivity difference to sound speed change, which is dictated by the structural disorder within the films. Changing the disorder has negligible impacts on volumetric heat capacity and vibrational lifetimes. Our findings provide means to improve the thermal conductivity of organic, hybrid, and inorganic polymer films.

physics.app-ph↗

arXiv2023

Validation of the Wiedemann-Franz Law in solid and molten tungsten above 2000 K through thermal conductivity measurements via steady state temperature differential radiometry

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 conductivity of metals in their molten phase. We further support this conclusion using ab initio molecular dynamics simulations with a machine-learned potential. Our results show that at these high temperatures, the vibrational contribution to thermal conductivity is negligible compared to the electronic component.

cond-mat.mtrl-sci↗

arXiv2023

Predictions and Measurements of Thermal Conductivity of Ceramic Materials at High Temperature

The lattice thermal conductivity ($κ$) of two ceramic materials, cerium dioxide (CeO$_2$) and magnesium oxide (MgO), is computed up to 1500 K using first principles and the phonon Boltzmann Transport Equation (PBTE) and compared to time-domain thermoreflectance (TDTR) measurements up to 800 K. Phonon renormalization and the four-phonon effect, along with high temperature thermal expansion, are integrated in our \textit{ab initio} molecular dynamics (AIMD) calculations. This is done by first relaxing structures and then fitting to a set of effective force constants employed in a temperature-dependent effective potential (TDEP) method. Both three-phonon and four-phonon scattering rates are computed based on these effective force constants. Our calculated thermal conductivities from the PBTE solver agree well with literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency shift, and phonon linewidth also compare well with available experimental data. Our results show that high temperature softens phonon frequency and reduces four-phonon scattering strength in both ceramics. Compared to MgO, we find that CeO$_2$ has weaker four-phonon effect and renormalization greatly reduces its four-phonon scattering rates.

cond-mat.mtrl-sci↗

arXiv2023

Design rules for the thermal and elastic properties of rare-earth disilicates

Rare-earth silicates are the current standard material for use as environmental barrier coatings for SiC-based ceramic matrix composites as hot-section components in gas-turbine engines. Expanding the design space to all available rare-earth elements to facilitate optimizing functionality requires an understanding of systematic trends in $RE_2$Si$_2$O$_7$ properties. In this work, we combine first-principles calculations with experimental measurements of Young's modulus, coefficient of thermal expansion, and thermal conductivity for a range of different $RE_2$Si$_2$O$_7$ compositions and phases. Clear trends are observed in these properties as a function of the radius of the rare-earth cation. In the case of Young's modulus and thermal expansion, these trends also hold for multi-component systems; while the thermal conductivity of multi-component systems is noticeably lower, indicating the potential of such materials to also act as thermal barriers. These results provide design rules for developing new thermal and environmental barrier coatings with stiffness and thermal expansion engineered to match that of the substrate, while simultaneously having reduced thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2022

On the thermal and mechanical properties of Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O across the high-entropy to entropy-stabilized transition

As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek further understanding of property changes across the phase boundary between \enquote{high-entropy} and \enquote{entropy-stabilized}. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent X-ray diffraction, and nano-indentation. Thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ~2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24 % from 10.8 to 14.1 x 10$^{-6}$ K$^{-1}$. Mechanical softening is also observed across the transition.

cond-mat.mtrl-sci↗

arXiv2022

Direct Visualization of Localized Vibrations at Complex Grain Boundaries

Grain boundaries (GBs) are a prolific microstructural feature that dominates the functionality of a wide class of materials. The change in functionality at a GB is a direct result of unique local atomic arrangements, different from those in the grain, that have driven extensive experimental and theoretical studies correlating atomic-scale GB structures to macroscopic electronic, infrared-optical, and thermal properties. Here, we examine a SrTiO3 GB using atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) and ultra-high-energy-resolution monochromated electron energy-loss spectroscopy (EELS), in conjunction with density functional theory (DFT) calculations. This combination enables the direct correlation of the GB structure, composition, and chemical bonding with atomic vibrations within the GB dislocation-cores. We observe that nonstoichiometry and changes in coordination and bonding at the GB leads to a redistribution of vibrational states at the GB and its dislocation-cores relative to the bounding grains. The access to localized vibrations within GBs provided by ultrahigh spatial/spectral resolution EELS correlated with atomic coordination, bonding, and stoichiometry and validated by theory, provides a direct route to quantifying the impact of individual boundaries on macroscopic properties.

cond-mat.mtrl-sci↗

arXiv2021

Nanoscale Phonon Spectroscopy Reveals Emergent Interface Vibrational Structure of Superlattices

As the length-scales of materials decrease, heterogeneities associated with interfaces approach the importance of the surrounding materials. Emergent electronic and magnetic interface properties in superlattices have been studied extensively by both experiments and theory. $^{1-6}$ However, the presence of interfacial vibrations that impact phonon-mediated responses, like thermal conductivity $^{7,8}$, has only been inferred in experiments indirectly. While it is accepted that intrinsic phonons change near boundaries $^{9,10}$, the physical mechanisms and length-scales through which interfacial effects influence materials remain unclear. Herein, we demonstrate the localized vibrational response associated with the interfaces in SrTiO$_3$-CaTiO$_3$ superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy and density-functional-theory calculations. Symmetries atypical of either constituent material are observed within a few atomic planes near the interface. The local symmetries create local phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. The results provide direct visualization and quantification, illustrating the progression of the local symmetries and interface vibrations as they come to determine the vibrational response of an entire superlattice; stated differently, the progression from a material with interfaces, to a material dominated by interfaces, to a material of interfaces as the period decreases. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behavior. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids having emergent infrared and thermal responses.

cond-mat.mtrl-sci↗

arXiv2021

Plasma-induced surface cooling

Here we show that, despite a massive incident flux of energetic species, plasmas can induce transient cooling of a material surface. Using time-resolved optical thermometry in-situ with this plasma excitation, we reveal the novel underlying physics that drive this `plasma cooling' that is driven by the diverse chemical and energetic species that comprise this fourth state of matter. We show that the photons and massive particles in the plasma impart energy to different chemical species on a surface, leading to local and temporally changing temperatures that lead to both increases and decreases in temperature on the surface of the sample, even though energy is being imparted to the material. This balance comes from the interplay between chemical reactions, momentum transfer, and energy exchange which occur on different time scales over the course of picoseconds to milliseconds. Thus, we show that through energetically exciting a material with a plasma, we can induce cooling, which can lead to revolutionary advances in refrigeration and thermal mitigation with this new process that is not inhibited by the same limitations in the current state-of-the-art systems.

physics.plasm-ph↗

arXiv2021

Thermal conductivity measurements of sub-surface buried substrates by steady-state thermoreflectance

Measuring the thermal conductivity of sub-surface buried substrates are of significant practical interests. However, this remains challenging with traditional pump-probe spectroscopies due to their limited thermal penetration depths (TPD). Here, we experimentally and numerically investigate the TPD of recently developed optical pump-probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates typically inaccessible by traditional pump-probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries.

physics.ins-det↗

arXiv2020

Interface controlled thermal properties of ultra-thin chalcogenide-based phase change memory devices

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 thermal properties of PCMs at length scales close to the memory cell dimension and, thus, the impact of interfaces on PCM operation is unknown. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in thermal boundary resistance as GST transitions from one crystallographic structure (cubic) to another (hexagonal) and as the thickness of tungsten contacts is reduced from five to two nanometers. Simulations reveal that interfacial resistance between the phase change unit and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~40% and ~50%, respectively. The resultant phase-dependent and geometric effects on thermal boundary resistance dictate that the effective thermal conductivity of the phase change unit can be reduced by a factor of four, presenting a new opportunity to reduce operating currents in PCMs.

physics.app-ph↗

arXiv2020

Dual-Phase High-Entropy Ultra-High Temperature Ceramics

A series of dual-phase high-entropy ultrahigh temperature ceramics (DPHE-UHTCs) are fabricated starting from N binary borides and (5-N) binary carbides powders. >~99% relative densities have been achieved with virtually no native oxides. These DPHE-UHTCs consist of a hexagonal high-entropy boride (HEB) phase and a cubic high-entropy carbide (HEC) phase. A thermodynamic relation that governs the compositions of the HEB and HEC phases in equilibrium is discovered and a thermodynamic model is proposed. These DPHE-UHTCs exhibit tunable grain size, Vickers microhardness, Young' and shear moduli, and thermal conductivity. The DPHE-UHTCs have higher hardness than the weighted linear average of the two single-phase HEB and HEC, which are already harder than the rule-of-mixture averages of individual binary borides and carbides. This study extends the state of the art by introducing dual-phase high-entropy ceramics (DPHECs), which provide a new platform to tailor various properties via changing the phase fraction and microstructure.

cond-mat.mtrl-sci↗

arXiv2020

Long-lived modulation of plasmonic absorption by ballistic thermal injection

Energy and charge transfer across metal-semiconductor interfaces are the fundamental driving forces for a broad range of applications, such as computing, energy harvesting, and photodetection. However, the exact roles and physical separation of these two phenomena remains unclear, particularly in plasmonically-excited systems or cases of strong nonequilibrium. We report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, following optical excitation of a metal-semiconductor heterostructure. For the first time, we explicitly show that in cases of strong non-equilibrium, a novel energy transduction mechanism arises at the metal/semiconductor interface. We find that hot electrons in the metal contact transfer their energy to pre-existing electrons in the semiconductor, without transfer of charge. These experimental results findings are well-supported by both rigorous multilayer optical modeling and first-principle, ab initio calculations.

physics.app-ph↗

arXiv2020

Monolayer Vanadium-doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor

Dilute magnetic semiconductors, achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices, especially for two-dimensional systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room-temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here we describe room-temperature ferromagnetic order obtained in semiconducting vanadium-doped tungsten disulfide monolayers produced by a reliable single-step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation. These monolayers develop p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of a few atomic percent and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium-vanadium spacings, as supported by transmission electron microscopy, magnetometry and first-principles calculations. Room-temperature two-dimensional dilute magnetic semiconductors provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures them into the realm of practical application.

cond-mat.mtrl-sci↗

arXiv2019

Experimental Observation of High Intrinsic Thermal Conductivity of AlN

AlN is an ultra-wide bandgap semiconductor which has been developed for applications including power electronics and optoelectronics. Thermal management of these applications is the key for stable device performance and allowing for long lifetimes. AlN, with its potentially high thermal conductivity, can play an important role serving as a dielectric layer, growth substrate, and heat spreader to improve device performance. However, the intrinsic high thermal conductivity of bulk AlN predicted by theoretical calculations has not been experimentally observed because of the difficulty in producing materials with low vacancy and impurity levels, and other associated defect complexes in AlN which can decrease the thermal conductivity. This work reports the growth of thick AlN layers by MOCVD with an air-pocketed AlN layer and the first experimental observation of intrinsic thermal conductivity from 130 K to 480 K that matches density-function-theory calculations for single crystal AlN, producing some of the highest values ever measured. Detailed material characterizations confirm the high quality of these AlN samples with one or two orders of magnitude lower impurity concentrations than seen in commercially available bulk AlN. Measurements of these commercially available bulk AlN substrates from 80 K to 480 K demonstrated a lower thermal conductivity, as expected. A theoretical thermal model is built to interpret the measured temperature dependent thermal conductivity. Our work demonstrates that it is possible to obtain theoretically high values of thermal conductivity in AlN and such films may impact the thermal management and reliability of future electronic and optoelectronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Thermal Conductance Across Harmonic-matched Epitaxial Al-sapphire Heterointerfaces

A unified understanding of interfacial thermal transport is missing due to the complicated nature of interfaces which involves complex factors such as interfacial bonding, interfacial mixing, surface chemistry, crystal orientation, roughness, contamination, and interfacial disorder. This is especially true for metal nonmetal interfaces which incorporate multiple fundamental heat transport mechanisms such as elastic and inelastic phonon scattering as well as electron phonon coupling in the metal and across the interface. All these factors jointly affect thermal boundary conductance (TBC). As a result, the experimentally measured interfaces may not be the same as the ideally modelled interfaces, thus obfuscating any conclusions drawn from experimental and modeling comparisons. This work provides a systematic study of interfacial thermal conductance across well controlled and ultraclean epitaxial (111) Al parallel (0001) sapphire interfaces, known as harmonic matched interface. A comparison with thermal models such as atomistic Green s function (AGF) and a nonequilibrium Landauer approach shows that elastic phonon scattering dominates the interfacial thermal transport of Al sapphire interface. By scaling the TBC with the Al heat capacity, a nearly constant transmission coefficient is observed, indicating that the phonons on the Al side limits the Al sapphire TBC. This nearly constant transmission coefficient validates the assumptions in AGF and nonequilibrium Landauer calculations. Our work not only provides a benchmark for interfacial thermal conductance across metal nonmetal interfaces and enables a quantitative study of TBC to validate theoretical thermal carrier transport mechanisms, but also acts as a reference when studying how other factors impact TBC.

physics.app-ph↗

arXiv2019

Titanium Contacts to MoS2 with Interfacial Oxide: Interface Chemistry and Thermal Transport

The deposition of a thin oxide layer at metal/semiconductor interfaces has been previously reported as a means of reducing contact resistance in 2D electronics. Using X-ray photoelectron spectroscopy with in-situ Ti deposition, we fabricate Au/Ti/TiOx/MoS2 samples as well as Au/Ti/MoS2 and Au/TiOx/MoS2 for comparison. Elemental titanium reacts strongly with MoS2 whereas no interface reactions are observed in the two types of samples containing TiOx/MoS2 interfaces. Using time domain thermoreflectance for the measurement of thermal boundary conductance, we find that samples contacted with Ti and a thin TiOx layer at the interface (less than or equal to 1.5 nm) exhibit the same behavior as samples contacted solely with pure Ti. The Au/TiOx/MoS2 samples exhibit approximately 20% lower thermal boundary conductance, despite having the same MoS2 interface chemistry as the samples with thin oxide at the Ti/MoS2 interface. We identify the mechanism for this phenomenon, attributing it to the different interfaces with the top Au contact. Our work demonstrates that the use of thin interfacial oxide layers to reduce electrical contact resistance does not compromise heat flow in 2D electronic devices. We note that the thicknesses of the Ti and TiOx layers must be considered for optimal thermal transport.

cond-mat.mtrl-sci↗

arXiv2017

Titanium Contacts to Graphene: Process-Induced Variability in Electronic and Thermal Transport

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, characterized by X-ray photoelectron spectroscopy, and the Ti/graphene (Gr) contact resistance measured by the transfer length method. We find that contact composition is tunable via deposition rate and base pressure. Reactor base pressure is found to effect the resultant contact resistance. The effect of contact deposition conditions on thermal transport measured by time-domain thermoreflectance is also reported and interfaces with higher oxide composition appear to result in a lower thermal boundary conductance. Possible origins of this thermal boundary conductance change with oxide composition are discussed.

cond-mat.mtrl-sci↗