We have demonstrated the cross over from incoherent to coherent phonon transport in superlattices through thermal conductivity measurements of strontium titanate (SrTiO3)/calcium titanate (CaTiO3) superlattices with different periodicities.  Our work was recently published in Nature Materials (“Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices,” Nature Materials doi:10.1038/nmat3826).  Congratulations to Ramez who is a joint first author on this work!!! This work summarizes a major collaboration with several groups around the US, including Profs. Ramamoorthy Ramesh, Arun Majumdar, Darrell Schlom, Mark Zurbuchen, David Muller, and Joel Moore.  We appreciate the generous support from the Army Research Office and the National Science Foundation.

Abstract

Elementary particles such as electrons^1, 2 or photons^3, 4 are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave–particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.   We are grateful for financial support from Army Research office (ARO) grant W911NF-13-1-0378.  TDTR measurements on the SrTiO3 /CaTiO3 superlattices at the University of Virginia were supported by the National Science Foundation (NSF) grant CBET-1339436.

In our recent paper published in Applied Physics Letters (Applied Physics Letters 102, 251912 (2013)), we measure the thermal conductivities of a range of organic semiconducting polymers common in photovoltaic devices.  We report on the thermal conductivity of PEDOT:PSS, P3HT, PCBM, and mixtures of P3HT and PCBM.  Thermal conductivities vary from 0.031±0.005 to 0.227 ± 0.014  W m⁻¹ K⁻¹ near room temperature and exhibit minimal temperature dependence across the range from 319 to 396 K. The thermal conductivities of P3HT/PCBM blend films follow a rule of mixtures, and no percolation threshold is found. We also find that thermal annealing of blend films has a variable effect on thermal conductivity.  Finally, we measure variations of thin film PCBM spun under different processing conditions, and show that even when accounting for large uncertainties in heat capacity (which we discuss in the supporting information of this article), PCBM still exhibits exceptionally low, “hyper-insulating” thermal conductivities, varying from 0.031 – 0.057 W/m/K.

This work was a collaborative effort with Dr. Mool Gupta in the ECE Department at U.Va.  This work was partially supported by the Army Research Office (Program Manager: Pani Varanasi).

In a collaboration with Sandia National Laboratories, we have developed Single Element Raman Thermometry, a new implementation of Raman thermometry is presented having lower uncertainty while also reducing the time and hard- ware needed to perform the experiment.  The details of this development our outline in our publication in Review of Scientific Instruments (Review of Scientific Instruments 84, 064903 (2013)).  The experimental platform was developed by Chris Saltonstall, a PhD candidate at U.Va. that is co-advised by Patrick and Associate Dean Pam Norris, while Chris was a graduate student intern at Sandia National Laboratories.  We collaborated with Drs. Justin Serrano and Thomas E. Beechem at Sandia on this project.

This work was partially supported by the National Science Foundation (CMMI Grant No. 1229603).

Brian Foley was officially awarded and ARCS Fellowship.  Nominated by UVA, Brian was selected to become one of the ARCS Foundations’ 2013 Scholar’s due to his impressive academic achievements and future research goals.  More inforation about the Metro Washington DC ARCS Chapter can be found here: https://www.arcsfoundation.org/metro_washington/

Our recent paper has demonstrated chemically exfoliated MoS2 as a near IR photothermal agent.  As described in our publication in Angewandte Chemie International Edition (Angewandte Chemie International Edition 52, 4160-4164 (2013)), we show that owing to an absorbance profile reaching into the NIR, ceMoS2 heats up rapidly upon NIR irradiation. Because of its high surface-area-to-mass ratio, ceMoS2 also possesses loading capacities on par with GO, the current best-in-class.  Clearly, ceMoS2 possesses many desirable traits present in the aforementioned photothermal agents.

The TDTT (time domain thermotransmission) measurements were conducted by Brian Foley during one of Visiting Professor Bryan Kaehr’s trips to our lab.  This work was in collaboration with Dr. Kaehr along with Professor Jeff Brinker at Sandia/UNM and Drs. Stanley Chou and Vinayak Dravid at Northwestern.

Patrick has been selected to receive an award from the Young Investigator Program of the Office of Naval Research.  The ONR YIP seeks to identify and support academic scientists and engineers who are in their first or second full-time tenure-track or tenure-track-equivalent academic appointment and for FY13, have begun their first appointment on or after Nov. 1, 2007, and who show exceptional promise for doing creative research. The program’s objectives are to attract outstanding faculty members of Institutions of Higher Education to the Department of Navy’s (DoN’s) research program, to support their research, and to encourage their teaching and research careers. Patrick’s proposed research is focused on developing the ability to measure the thermal boundary conductance, or Kapitza conductance, across the interface of a solid and a low thermal conductivity fluid, along with developing simultaneous diagnostics to measure the solid/fluid interfacial pressure and wetting.  This work will establish the relationship between nanoscale surface roughness, chemistry, and wetting on heat transport processes across solid/fluid interfaces via novel experimental measurements.

The title of the awarded project is “Surface chemistry and geometry effects on nanoscopic heat transfer processes at solid/fluid interfaces.”

The official announcement can be found here:  http://www.onr.navy.mil/en/Science-Technology/Directorates/office-research-discovery-invention/Sponsored-Research/YIP/2013-young-investigator-recipients-YIP.aspx

Our paper examining the effects of coherent domain walls on the thermal conductivity of bismuth ferrite (BiFeO3) was published in Applied Physics Letters (Appl. Phys. Lett. 102, 121903 (2013)). We show that coherent domain walls can scatter phonons as effectively as incoherent grain boundaries, opening up a new regime of strain engineering of phonon transport.

Abstract

Ferroelectric and ferroelastic domain structure has a profound effect on the piezoelectric, ferroelectric, and dielectric responses of ferroelectric materials. However, domain walls and strain field effects on thermal properties are unknown. We measured the thermal conductance from 100–400K of epitaxially grown BiFeO3 thin films with different domain variants, each separated primarily by 71 deg. domain walls. We determined the Kapitza conductance across the domain walls, which is driven by the strain field induced by the domain variants. This domain wall Kapitza conductance is lower than the Kapitza conductance associated with grain boundaries in all previously measured materials.

This work was partially funded  from the AFOSR Young Investigator Program (FA9550-13-1-0067).

Our paper – Duda et al. “Influence of interfacial properties on thermal transport at gold:silicon contacts,” was recently published in Applied Physics Letters (Appl. Phys. Lett. 102, 081902 (2013)).  In this paper, we measured the thermal boundary conductance across Au/Si interfaces with varying degrees of roughness and “adhesion” (i.e., Au/native oxide/Si, Au/Ti/native oxide Si and Au/Ti/Si).  We find that roughness affects the low frequency phonon transmission, as measured via picosecond ultrasonics.  We also demonstrate that a Ti adhesion layer, which increases the Au/Si bonding, can lead to a factor of 3 increase in thermal boundary conductance.

Abstract

We measure the Kapitza conductances at Au:Si contacts from 100 to 296 K via time-domain thermoreflectance. Contacts are fabricated by evaporating Au films onto Si substrates. Prior to Au deposition, the Si substrates receive pretreatments in order to modify interfacial properties, i.e., bonding and structural disorder. Through the inclusion of a Ti adhesion layer and the removal of the native oxide, Kapitza conductance can be enhanced by a factor of four at 296 K. Furthermore, interfacial roughness is found to have a negligible effect, which we attribute to the already low conductances of poorly bonded Au:Si contacts.

This work was funded by NSF (CBET Award #1134311) and Sandia National Laboratories through the LDRD Program Office.

Patrick’s recent review paper on thermal transport across solid interfaces with nanoscale imperfections was recently published in ISRN Mechanical Engineering: Hopkins, P.E., “Thermal transport across solid interfaces with nanoscale imperfections: Effects of roughness, disorder, dislocations and bonding on thermal boundary conductance,” ISRN Mechanical Engineering, 2013, 682586 (2013).

Abstract

The efficiency in modern technologies and green energy solutions has boiled down to a thermal engineering problem on the nanoscale. Due to the magnitudes of the thermal mean free paths approaching or overpassing typical length scales in nanomaterials (i.e., materials with length scales less than one micrometer), the thermal transport across interfaces can dictate the overall thermal resistance in nanosystems. However, the fundamental mechanisms driving these electron and phonon interactions at nanoscale interfaces are difficult to predict and control since the thermal boundary conductance across interfaces is intimately related to the characteristics of the interface (structure, bonding, geometry, etc.) in addition to the fundamental atomistic properties of the materials comprising the interface itself. In this paper, I review the recent experimental progress in understanding the interplay between interfacial properties on the atomic scale and thermal transport across solid interfaces. I focus this discussion specifically on the role of interfacial nanoscale “imperfections,” such as surface roughness, compositional disorder, atomic dislocations, or interfacial bonding. Each type of interfacial imperfection leads to different scattering mechanisms that can be used to control the thermal boundary conductance. This offers a unique avenue for controlling scattering and thermal transport in nanotechnology.

This work was funded by the National Science Foundation (CBET Grant no. 1134311) and Sandia National Laboratories through the LDRD program office.

Patrick has been selected to receive an award from the Young Investigator Program of the Air Force Office of Scientific Research.  The objective of this program is to foster creative basic research in science and engineering, enhance early career development of outstanding young investigators, and increase opportunities for the young investigators to recognize the Air Force mission and the related challenges in science and engineering.  Patrick’s proposed research is focused on the fundamental interactions between femtosecond laser pulses and solids, exploring the regimes of ballistic transport, electron distribution, and electron, phonon, and interfacial scattering mechanisms.

The title of the awarded project is “Electron Dynamics During High-Power, Short-Pulsed Laser Interactions with Solids and Interfaces.”

The official announcement can be found here:  http://www.eurekalert.org/pub_releases/2013-01/afoo-aag010913.php#