Our paper, “Manipulating thermal conductance at metal-graphene contacts via chemical functionalization,” was recently published in Nano Letters (Nano Lett. 12, 590 (2012)).  In this work, we use plasma functionalization to manipulate the bonding environment of graphene surfaces.  We metallize the graphene with aluminum, and show that the functionalization can increase the thermal boundary conductance between the Al and the graphene by a factor of two due to change in the bonding enviornment from oxygen functionalization and subsequent Al-O bonding.  This work was performed in collaboration with Thomas Beechem, Ed Barnat and Sean Kearney at Sandia National Laboratories and Scott Walton’s group at the Naval Research Laboratories.

Abstract

Graphene-based devices have garnered tremendous attention due to the unique physical properties arising from this purely two-dimensional carbon sheet leading to tremendous efficiency in the transport of thermal carriers (i.e., phonons). However, it is necessary for this two-dimensional material to be able to efficiently transport heat into the surrounding 3D device architecture in order to fully capitalize on its intrinsic transport capabilities. Therefore, the thermal boundary conductance at graphene interfaces is a critical parameter in the realization of graphene electronics and thermal solutions. In this work, we examine the role of chemical functionalization on the thermal boundary conductance across metal/graphene interfaces. Specifically, we metalize graphene that has been plasma functionalized and then measure the thermal boundary conductance at Al/graphene/SiO₂ contacts with time domain thermoreflectance. The addition of adsorbates to the graphene surfaces are shown to influence the cross plane thermal conductance; this behavior is attributed to changes in the bonding between the metal and the graphene, as both the phonon flux and the vibrational mismatch between the materials are each subject to the interfacial bond strength. These results demonstrate plasma-based functionalization of graphene surfaces is a viable approach to manipulate the thermal boundary conductance.

This work was funded by NSF (CBET Award #1134311)

Our paper,  “ Implications of cross-species interactions on the temperature dependence of Kapitza conductance”  was recently published in Physical Review B (Phys. Rev. B 84, 193301 (2011)).  This work was lead by Dr. John C. Duda and in collaboration with Drs. Bill Soffa and Leo Zhigilei in the Materials Science Department here at U.Va. along with Tim English and Dr. Ed Piekos at Sandia National Laboratories.

Dr. John C. Duda’s recent paper “Implications of cross-species interactions on the temperature dependence of Kapitza conductance” was recently accepted in Physical Review B.   In this work, we investigated the behavior of Kapitza conductance at interfaces between two Lennard-Jones fcc solids as a function of the range and strength of cross-species interactions via molecular dynamics simulations.  We found that decreasing either the range of strength of the bond at the interface leads to a reduction in the slope of linear temperature dependence of h_K, suggesting a corresponding decrease in the probability of inelastic phonon-phonon interactions.

This work was funded by NSF (CBET Award #1134311)

Astract

We investigate the behavior of Kapitza conductance at interfaces between two Lennard-Jones fcc solids as a function of the range and strength of cross-species interactions via molecular dynamics simulations. It is found that decreasing either of these quantities leads to a reduction in the slope of linear temperature dependence of Kapitza conductance, suggesting a corresponding decrease in the probability of inelastic phonon-phonon interactions. To further explore the mechanisms responsible for such behavior, we calculate the phonon density of states and spectral temperature of each of the monolayers adjacent to the interface. It is found that the reduction of the range and strength of cross-species interactions leads to a softening of the density of states near the interface, while the spectral temperature calculations provide further evidence that such reductions decrease the probability of inelastic phonon scattering. These findings help explain varying accounts of the temperature dependence of Kapitza conductance observed in previous works.

Our paper,  “Ultra-low thermal conductivity of ellipsoidal TiO₂ nanoparticle films ”  was recently published in Applied Physics Letters (Applied Physics Letters 99, 133106 (2011)).  In this work, we show that films comprised of close-packed titania nanoparticles exhibit thermal conductivities that are lower than the theoretical minimum limit and show a dependency on nanoparticle orientation and alignment.

Our paper, “Ultra-low thermal conductivity of ellipsoidal TiO2 nanoparticle films,” was just accepted into Applied Physics Letters.  In this work, we show that the orientational order of ellipsoidal titania nanoparticle films can affect the thermal transport.  In addition, the thermal conductivities of these films are lower than amorphous titania.  This work was performed in collaboration with Professor Eric Furst’s group at University of Delaware and Leslie Phinney and Anne Grillet at Sandia National Laboratories.

Abstract

We report on the thermal conductivity of a series of convectively assembled, anisotropic titania (TiO2) nanoparticle films. The TiO2 films are fabricated by flow coating a suspension of ellipsoidal colloidal nanoparticles, resulting in structured films with tailored orientational order. The thermal conductivities depend on nanoparticle orientation and exhibit thermal conductivities less than amorphous TiO2 films due to inter-nanoparticle boundary scattering. This nanoparticle ordering presents a unique method for manipulating the thermal conductivity of nanocomposites.

Our paper,  “Influence of anisotropy on thermal boundary conductance at solid interfaces”  was recently published in Physical Review B (Physical Review B 84, 125408 (2011)).  In this work, we show that the crystalline orientation can affect the thermal boundary conductance across solid interfaces when one of the solids comprising the interface has anisotropic phonon properties.

We are excited to welcome Brian Foley to the Lab, and to UVA!  Brian received his M.S. and B.E. degrees in Electrical & Computer Engineering from Worcester Polytechnic Institute in 2009 and 2007, respectively, where he focused on non-destructive evaluation technologies for General Motors.  Prior to joining the our group, he worked for two and a half years at Virginia Diodes, Inc. making vector network analyzer (VNA) extension modules for calibrated S-parameter measurements from 75-1100GHz.  As a PhD student, Brian will research the nano-scale heat transfer in materials used in RF & microwave semiconductor devices.

Our paper, “Influence of anisotropy on thermal boundary conductance at solid interfaces,” was just accepted into Physical Review B.  In this work, we show that crystalline anisotropy can affect the thermal boundary conductance across solid interfaces.  This is most prominent in materials with non-cubic crystal structures (i.e., Brillouin zones that can not be approximated as spherical).  The origin of the anisotropy is related to the phonon velocities in the different crystallographic directions.

Abstract

We investigate the role of anisotropy on interfacial transport across solid interfaces by measuring the thermal boundary conductance from 100 – 500K across Al/Si and Al/sapphire interfaces with different substrate orientations. The measured thermal boundary conductances show a dependency on substrate crystallographic orientation in the sapphire samples (trigonal conventional cell) but not in the silicon samples (diamond cubic conventional cell). The change in interface conductance in the sapphire samples is ascribed to anisotropy in the Brillouin zone along the principle directions defining the conventional cell. This leads to resultant phonon velocities in the direction of thermal transport that vary nearly 40% based on crystallographic direction.

Our paper,  “Controlling thermal conductance through quantum dot roughening at interfaces,”  was recently published in Physical Review B (Physical Review B 84, 035438 (2011)).  In this work, we show that the thermal boundary conductance at Al/S i interfaces can be controllably manipulated by patterning quantum dots on the Si surface.  This work was also selected to appear in the Virtual Journal of Nanoscale Science and Technology.

Our paper, “Controlling thermal conductance through quantum dot roughening at interfaces,” was just accepted into Physical Review B.  In this work, we control thermal transport across an Al/Si interface by patterning quantum dots on the Si surface to control the RMS roughness.  This work was done in collaboration with Jerry Floro’s group at U.Va.

Abstract

We examine the fundamental phonon mechanisms affecting the interfacial thermal conductance across a single layer of quantum dots (QDs) on a planar substrate. We synthesize a series of Ge_xSi_1-x QDs by heteroepitaxial self-assembly on Si surfaces and modify the growth conditions to provide QD layers with different RMS roughness levels in order to quantify the effects of roughness on thermal transport. We measure the thermal boundary conductance (h_K) with time-domain thermoreflectance. The trends in thermal boundary conductance show that the effect of the QDs on h_K are more apparent at elevated temperatures, where at low temperatures, the QD patterning does not drastically affect h_K. The functional dependence of h_K with RMS surface roughness reveals a trend that suggests that both vibrational mismatch and localized phonon scattering near the interface contribute to the reduction in h_K. We find that QD structures with RMS roughness greater than 4 nm decreases h_K at Si interfaces by a factor of 1.6. We develop an analytical model for phonon transport at rough interfaces based on a diffusive scattering assumption and phonon attenuation that describes the measured trends in h_K. This indicates that the observed reduction in thermal conductivity in SiGe quantum dot superlattices is primarily due to the increased physical roughness at the interfaces, which creates additional phonon resistive processes beyond the interfacial vibrational mismatch.