Reduced thermal conductivity of ALD-grown PbTe-PbSe nano-composite thin films published in Advanced Functional Materials – Congrats Mallory DeCoster!

Mallory DeCoster’s recent work on the thermal conductivity of ALD grown PbTe-PbSe nano-composite thin films was published today in Advanced Functional Materials.

DeCoster, M.E., Chen, X., Zhang, K., Rost, C.M., Hoglund, E.R., Howe, J.M., Beechem, T.E., Baumgart, H., Hopkins, P.E., “Thermal conductivity and phonon scattering processes of ALD grown PbTe-PbSe thermoelectric thin films,” Advanced Functional Materials DOI: 10.1002/adfm.201904073 (2019).

In this work, in a collaboration with Prof. Helmut Baumgart from ODU, Dr. Thomas Beechem from Sandia, and Prof. Jim Howe from UVA, we studied the thermal conductivity of a series of thickness varying, compositionally varying PbTe-PbSe nano-structured thin films grown by atomic layer deposition.  We show that the thermal conductivity of these films exhibit glass like trends, and ultimately their thermal conductivities are dictated by the crystalline quality of the films, which can change as a function of thickness due to the ALD growth mode of the PbTe (Volmer-Weber).  This work has pronounced impact on understanding the thermal conductivity of ALD-grown thin films and their applications for thin film thermoelectric and thermal barriers.



This work studies the thermal conductivity and phonon scattering processes in a series of n‐type lead telluride‐lead selenide (PbTe–PbSe) nanostructured thin films grown by atomic layer deposition (ALD). The ALD growth of the PbTe–PbSe samples in this work results in nonepitaxial films grown directly on native oxide/Si substrates, where the Volmer–Weber mode of growth promotes grains with a preferred columnar orientation. The ALD growth of these lead‐rich PbTe, PbSe, and PbTe–PbSe thin films results in secondary oxide phases, along with an increase microstructural quality with increased film thickness. The compositional variation and resulting point and planar defects in the PbTe–PbSe nanostructures give rise to additional phonon scattering events that reduce the thermal conductivity below that of the corresponding ALD‐grown control PbTe and PbSe films. Temperature‐dependent thermal conductivity measurements show that the phonon scattering in these ALD‐grown PbTe–PbSe nanostructured materials, along with ALD‐grown PbTe and PbSe thin films, are driven by extrinsic defect scattering processes as opposed to phonon–phonon scattering processes intrinsic to the PbTe or PbSe phonon spectra. The implication of this work is that polycrystalline, nanostructured ALD composites of thermoelectric PbTe–PbSe films are effective in reducing the phonon thermal conductivity, and represent a pathway for further improvement of the figure of merit (ZT), enhancing their thermoelectric application potential.


The authors appreciate funding from the Army Research Office, Grant No. W911NF‐16‐1‐0406. This work was supported in part by the NSF I/UCRC on Multi‐functional Integrated System Technology (MIST) Center IIP‐1439644, IIP‐1439680, and IIP‐1738752. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE‐NA0003525.

A new thermometry method for measuring the thermal conductivity of materials – Congrats Dr. Jeff Braun (and congrats on successful PhD defense on 4/12/19!)

The recently deemed “Dr.” Jeff Braun recently published his work on steady state thermoreflectance (SSTR), an optical pump-probe technique based on continuous wave lasers in which the steady state temperature rise of a solid is monitored as a function of laser power to measure the thermal conductivity of the material.  SSTR is capable of measuring the thermal conductivity of materials over length scales from just a few microns to hundreds of microns, filling an important void in the landscape of thermal conductivity measurement capabilities.


Braun, J.L., Olson, D.H., Gaskins, J.T., Hopkins, P.E., “A steady-state thermoreflectance method to measure thermal conductivity,” Review of Scientific Instruments 90, 024905 (2019). PDF.


We demonstrate a steady-state thermoreflectance-based optical pump-probe technique to measure the thermal conductivity of materials using a continuous wave laser heat source. The technique works in principle by inducing a steady-state temperature rise in a material via long enough exposure to heating from a pump laser. A probe beam is then used to detect the resulting change in reflectance, which is proportional to the change in temperature at the sample surface. Increasing the power of the pump beam to induce larger temperature rises, Fourier’s law is used to determine the thermal conductivity. We show that this technique is capable of measuring the thermal conductivity of a wide array of materials having thermal conductivities ranging from 1 to >2000 W m−1 K−1, in excellent agreement with literature values.


Thermal conductivity switch using materials derived from squid protein – Paper published in Nature Nanotechnology – Congrats John Tomko!

We have recently demonstrated the ability to control the thermal conductivity of a material derived from squid ring teeth protein by hydrating the material with water.  In collaboration with Prof. Melik Demirel’s group at Penn State, along with collaborations from Prof. Ben Allen at Penn State and Dr. Madhusudan Tyagi from NIST and UMN, we have develop a thermal conductivity switch with higher thermal conductivity on/off ratios than any other single material that has been observed to date.  Congrats to John Tomko who was a first author on this paper that appeared in Nature Nanotechnology (Tomko, J.A., Pena-Francesch, A., Jung, H., Tyagi, M., Allen, B.D., Demirel, M.C., Hopkins, P.E., “Tunable thermal transport and reversible thermal conductivity switching in topologically-networked bio-inspired materials,” Nature Nanotechnology 13, 959-964 (2018). PDF (Supporting Information).).

Paper information below


Tunable thermal transport and reversible thermal conductivity switching in topologically networked bio-inspired materials


The dynamic control of thermal transport properties in solids must contend with the fact that phonons are inherently broad- band. Thus, efforts to create reversible thermal conductivity switches have resulted in only modest on/off ratios, since only a relatively narrow portion of the phononic spectrum is impacted. Here, we report on the ability to modulate the thermal conductivity of topologically networked materials by nearly a factor of four following hydration, through manipulation of the displacement amplitude of atomic vibrations. By varying the network topology, or crosslinked structure, of squid ring teeth-based bio-polymers through tandem-repetition of DNA sequences, we show that this thermal switching ratio can be directly programmed. This on/off ratio in thermal conductivity switching is over a factor of three larger than the cur- rent state-of-the-art thermal switch, offering the possibility of engineering thermally conductive biological materials with dynamic responsivity to heat.


J.A.T. and P.E.H. acknowledge support from the Office of Naval Research (grant no. N00014-15-12769). M.C.D., B.D.A., A.P.-F. and H.J. were supported by the Army Research Office (grant no. W911NF-16-1-0019) and the Materials Research Institute of Pennsylvania State University. Access to the HFBS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Science Foundation and NIST under agreement no. DMR-1508249. Certain commercial material suppliers are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the NIST, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Massive reduction in thermal conductivity of silicon irradiated with silicon ions – Paper Published in Physical Review Materials – Congrats Ethan Scott!

Congratulations to Ethan Scott who recently published his paper on “Phonon scattering effects from point and extended defects on thermal conductivity studied via ion irradiation of crystals with self-impurities,” in Physical Review Materials (Scott, E. A., Hattar, K., Rost, C.M., Gaskins, J.T., Fazli, M., Ganski, C., Li, C., Bai, T., Wang, Y., Esfarjani, K., Goorsky, M., Hopkins, P.E., “Phonon-strain scattering effects from point and extended defects on thermal conductivity studied via ion irradiation of crystals with self-impurities,” Physical Review Materials 2, 095001 (2018). PDF).



Fundamental theories predict that reductions in thermal conductivity from point and extended defects can arise due to phonon scattering with localized strain fields. To experimentally determine how these strain fields impact phonon scattering mechanisms, we employ ion irradiation as a controlled means of introducing strain and assorted defects into the lattice. In particular, we observe the reduction in thermal conductivity of intrinsic natural silicon after self-irradiation with two different silicon isotopes, 28Si+ and 29Si+. Irradiating with an isotope with a nearly identical atomic mass as the majority of the host lattice produces a damage profile lacking mass impurities and allows us to assess the role of phonon scattering with local strain fields on the thermal conductivity. Our results demonstrate that point defects will decrease the thermal conductivity more so than spatially extended defect structures assuming the same volumetric defect concentrations due to the larger strain per defect that arises in spatially separated point defects. With thermal conductivity models using density functional theory, we show that for a given defect concentration, the type of defect (i.e., point vs extended) plays a negligible role in reducing the thermal conductivity compared to the strain per defect in a given volume.

DOI: 10.1103/PhysRevMaterials.2.095001


This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-18-1-0352. The authors would like to thank D. Buller and R. Sisson with their assistance with the implantation, as well as the Nuclear Regulatory Commission for their support of this research through the Jump Start in Nuclear Materials Education and Research Graduate Fellowship Program at the University of Virginia. We also appreciate support from the Office of Naval Research under a MURI program, Grant No. N00014-18-1-2429. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.


Thermal boundary resistance at thin ALD-grown high-k dielectric interfaces – Congrats Ethan Scott!

Congrats to Ethan Scott for his recent publication that experimentally determines the thickness regime in which the thermal boundary resistance at atomic layer deposited high-k dielectric interfaces dominates the total thermal resistance of the system.  In a collaboration with Dr. Sean King from Intel, we studied this with Al2O3, HfO2 and TiO2 ALD-grown thin films on silicon.

Scott, E.A., Gaskins, J.T., King, S.W., Hopkins, P.E., “Thermal conductivity and thermal boundary resistance of atomic layer deposited high-k dielectrics aluminum oxide, hafnium oxide, and titanium oxide thin films on silicon,” APL Materials 6, 058302 (2018). PDF.


The need for increased control of layer thickness and uniformity as device dimen- sions shrink has spurred increased use of atomic layer deposition (ALD) for thin film growth. The ability to deposit high dielectric constant (high-k) films via ALD has allowed for their widespread use in a swath of optical, optoelectronic, and electronic devices, including integration into CMOS compatible platforms. As the thickness of these dielectric layers is reduced, the interfacial thermal resistance can dictate the overall thermal resistance of the material stack compared to the resistance due to the finite dielectric layer thickness. Time domain thermoreflectance is used to interrogate both the thermal conductivity and the thermal boundary resistance of aluminum oxide, hafnium oxide, and titanium oxide films on silicon. We calculate a representative design map of effective thermal resistances, including those of the dielectric layers and boundary resistances, as a function of dielectric layer thickness, which will be of great importance in predicting the thermal resistances of current and future devices.

We appreciate support from the Army Research Office (Grant No. W911NF-16-1-0320).

Oxygen stoichiometry of adhesion layer dictates thermal boundary conductance: Paper published in APL – Congrats Hans Olson!

Congrats to Hans Olson for his recent publication demonstrating that the oxygen stoichiometry of a Ti adhesion layer between an Au films and a non-metal substrate will impact the thermal boundary conductance across Au/Ti/substrate interfaces.  To maximize TBC, the Ti layer should be a pure metal (i.e., no oxygen defects), which is rarely achieved in high vacuum conditions, but can be achieved by depositing Ti layers in ultra-high vacuum.

Olson, D.H., Freedy, K.M., McDonnell, S., Hopkins, P.E., “The influence of titanium adhesion layer layer oxygen stoichiometry on thermal boundary conductance at gold contacts,” Applied Physics Letters 112, 171602 (2018). PDF (Supporting Information).


We experimentally demonstrate the role of oxygen stoichiometry on the thermal boundary conductance across Au/TiOx/substrate interfaces. By evaporating two different sets of Au/TiOx/ substrate samples under both high vacuum and ultrahigh vacuum conditions, we vary the oxygen composition in the TiOx layer from 0 x 2.85. We measure the thermal boundary conductance across the Au/TiOx/substrate interfaces with time-domain thermoreflectance and characterize the interfacial chemistry with x-ray photoemission spectroscopy. Under high vacuum conditions, we speculate that the environment provides a sufficient flux of oxidizing species to the sample surface such that one essentially co-deposits Ti and these oxidizing species. We show that slower deposi- tion rates correspond to a higher oxygen content in the TiOx layer, which results in a lower thermal boundary conductance across the Au/TiOx/substrate interfacial region. Under the ultrahigh vacuum evaporation conditions, pure metallic Ti is deposited on the substrate surface. In the case of quartz substrates, the metallic Ti reacts with the substrate and getters oxygen, leading to a TiOx layer. Our results suggest that Ti layers with relatively low oxygen compositions are best suited to maximize the thermal boundary conductance.

D. H. Olson would like to thank the Virginia Space Grant Consortium (VSGC) for their continued funding and support. We also appreciate the support from the Army Research Office, Grant Nos. W911NF-16-1-0320 and W911NF-16-1-0406.

Low frequency phonon transport in C60 fullerite: Paper published in Phys. Rev. B – Congrats Ash Giri!

Congrats to Ash Giri for his recent publication in Phys. Rev. B on the pronounced low-frequency vibrational thermal transport in C60 fullerite realized through pressure-dependent molecular dynamics simulations.

Giri, A., Hopkins, P.E., “Pronounced low-frequency vibrational thermal transport in C60 fullerite realized through pressure-dependent molecular dynamics simulations,” Physical Review B 96, 220303(R) (2017). PDF.

Abstract: 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, and how the spectral contributions to vibrational thermal conductivity changes under applied pressure. To answer these questions, we investigate the effect of strain on the thermal conductivity of C60 fullerite crystals via pressure-dependent molecular dynamics simulations under the Green-Kubo formalism. We show that the thermal conductivity increases rapidly with compressive strain, which demonstrates a power-law relationship similar to their stress-strain relationship for the C60 crystals. Calculations of the density of states for the crystals under compressive strains reveal that the librational modes characteristic in the unstrained case are diminished due to densification of the molecular crystal. Over a large compression range (0–20 GPa), the Leibfried-Schlömann equation is shown to adequately describe the pressure dependence of thermal conductivity, suggesting that low-frequency intermolecular vibrations dictate heat flow in the C60 crystals. A spectral decomposition of the thermal conductivity supports this hypothesis.

We would like to thank the Army Research Office for support (Grant No. W911NF-16-1-0320).

High temperature TDTR up to 1000 K using HfN transducers: Paper published in Appl. Phys. Lett – Congrats Tina!

Dr. Tina Rost’s work has recently appeared in Applied Physics Letters.

Rost, C.M., Braun, J.L., Ferri, K., Backman, L., Giri, A., Opila, E., Maria, J.-P., Hopkins, P.E., “Hafnium nitride films for thermoreflectance transducers at high temperatures: Potential based on heating from laser absorption,” Applied Physics Letters 111, 151902 (2017). PDF.

In a typical TDTR experiment, a thin metal transducer is deposited on top of a sample to measure the sample’s thermal properties.  Ultimately, TDTR can be limited by the stability of this transducer.  In this work, we have demonstrated the ability to extend TDTR measurements up to 1000 K using HfN as a metal transducer.  Note only does HfN demonstrate one of the highest thermoreflectance coefficients at 800 nm measured to date, but it’s high temperature phase stability make it attractive for use as a metal transducer at high temperatures.


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 to determine the viability of HfN thin film transducers grown on SiO2 through measurements of the SiO2 thermal conductivity up to approximately 1000 K. Further, the reliability of HfN as a transducer is determined by measuring the thermal conductivities of MgO, Al2O3, and diamond at room temperature. The thermoreflectance coefficient of HfN was found to be 1.4 10^-4 K^-1 at 800 nm, one of the highest thermoreflectance coefficients measured at this standard TDTR probe wavelength. Additionally, the high absorption of HfN at 400 nm is shown to enable reliable laser heating to elevate the sample temperature during a measurement, relative to other transducers.

We acknowledge the financial support from the Office of Naval Research MURI program (Grant No. N00014-15-1- 2863).

Thermal boundary resistance limits the ablation thresholds of thin films: Paper published in Physical Review B – Congrats John Tomko!

John Tomko’s works has recently appeared in Physical Review B.

Tomko, J.A., Giri, A., Donovan, B.F., Bubb, D.M., O’Malley, S.M., Hopkins, P.E., “Energy confinement and thermal boundary conductance effects on short-pulsed thermal ablation thresholds in thin films,” Physical Review B 96, 014108 (2017). PDF.

In this work,we demonstrated a direct correlation between thermal ablation of thin gold films and the thermal boundary conductance across the film/substrate interface.  We used high energy pulsed lasers to induce the film ablation and showed a dependence on substrate.  However, the ablation threshold did not trend with the substrate thermal properties, but instead the thermal boundary conductance across the film/substrate interface.


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 an often used, and widely accepted relation describing threshold fluences of homogeneous bulk targets breaks down at the nanoscale. Rather than relying solely on the properties of the ablated Au film, the ablation threshold of these Au/substrate systems is found to be dependent on the measured thermal boundary conductance; we additionally find no discernible trend between the damage threshold and properties of the underlying substrate. These results are discussed in terms of diffusive thermal transport and the interfacial bond strength.

This material is based upon work supported by the Air Force Office of Scientific Research under Award No. FA9550-15-1-0079. D.M.B. and S.M.O. acknowledge Awards CMMI- 1531783 and CMMI-0922946 from the National Science Foundation.

Functional chemistry impacts interfacial heat transport from fullerene derivatives to liquids: Paper published in ACS Nano – Congrats Chet!

Chet Szwejkowski’s work on the thermal boundary conductance across fullerene derivative/liquid interfaces has recently appeared in ACS Nano.

Szwejkowski, C.J., Giri, A., Kaehr, B., Warzoha, R.J., Donovan, B.F., Hopkins, P.E., “Molecular tuning of the vibrational thermal transport mechanisms in fullerene derivative solutions,” ACS Nano 11, 1389-1396 (2017).

In this work, we showed that the functional group on fullerene derivatives can be used to control the thermal boundary conductance across the molecule/liquid interfaces.  Based on the geometry of the functional group, the density of states of low frequency modes will change and impact the transmission of thermal energy across the molecule/liquid interface.  We demonstrated this by measuring the thermal boundary conductance across these interfaces in samples of dilute fullerene derivative suspensions.  The measurements were conducted using time domain thermotransmission.  Our experimental results are supported with molecular dynamic simulations.


Control over the thermal conductance from excited molecules into an external environment is essential for the development of customized photothermal therapies and chemical processes. This control could be achieved through molecule tuning of the chemical moieties in fullerene derivatives. For example, the thermal transport properties in the fullerene derivatives indene-C60 monoadduct (ICMA), indene-C60 bisadduct (ICBA), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), [6,6]-phenyl C61 butyric acid butyl ester (PCBB), and [6,6]-phenyl C61 butyric acid octyl ester (PCBO) could be tuned by choosing a functional group such that its intrinsic vibrational density of states bridge that of the parent molecule and a liquid. However, this effect has never been experimentally realized for molecular interfaces in liquid suspensions. Using the pump−probe technique time domain thermotransmittance, we measure the vibrational relaxation times of photoexcited fullerene derivatives in solutions and calculate an effective thermal boundary conductance from the opto-thermally excited molecule into the liquid. We relate the thermal boundary conductance to the vibrational modes of the functional groups using density of states calculations from molecular dynamics. Our findings indicate that the attachment of an ester group to a C60 molecule, such as in PCBM, PCBB, and PCBO, provides low-frequency modes which facilitate thermal coupling with the liquid. This offers a channel for heat flow in addition to direct coupling between the buckyball and the liquid. In contrast, the attachment of indene rings to C60 does not supply the same low-frequency modes and, thus, does not generate the same enhancement in thermal boundary conductance. Understanding how chemical functionalization of C60 affects the vibrational thermal transport in molecule/liquid systems allows the thermal boundary conductance to be manipulated and adapted for medical and chemical applications.