Thermal storage and transport in organic/inorganic superlattices; 2 papers published in PRB – Congrats Ash!

We have measured the vibrational heat capacity and thermal conductivity in organic/inorganic superlattices grown by a combination of atomic layer deposition and molecular layer deposition.  We have demonstrate unique aspects of thermal transport including nearly complete spectral phonon scattering at an inorganic/organic interface in these nanocomposites, and ballistic transport across the organic layer for molecular layer thicknesses that are less than the vibrational wavelength in the inorganic layer.  Furthermore, we have demonstrated an average vibrational heat capacity change that scales with the number of organic layers in the superlattice.

This study resulted in two recently publications that appeared in Physical Review B (Physical Review B 93, 024201 (2016) and Physical Review B 93, 115310 (2016)), and Ash Giri is first author on both papers.  Congrats Ash!

The work discussed in these aforementioned Physical Review B articles was supported by the Army Research Office (Grant No. W911NF-13-1-0378).  The abstracts for these two works appear below.

Abstract – Physical Review B 93, 024201 (2016)

We study the influence of molecular monolayers on the thermal conductivities and heat capacities of hybrid inorganic/organic superlattice thin films fabricated via atomic/molecular layer deposition. We measure the cross plane thermal conductivities and volumetric heat capacities of TiO2– and ZnO-based superlattices with periodic inclusion of hydroquinone layers via time domain thermoreflectance. In comparison to their homogeneous counterparts, the thermal conductivities in these superlattice films are considerably reduced. We attribute this reduction in the thermal conductivity mainly due to incoherent phonon boundary scattering at the inorganic/organic interface. Increasing the inorganic/organic interface density reduces the thermal conductivity and heat capacity of these films. High-temperature annealing treatment of the superlattices results in a change in the orientation of the hydroquinone molecules to a 2D graphitic layer along with a change in the overall density of the hybrid superlattice. The thermal conductivity of the hybrid superlattice increases after annealing, which we attribute to an increase in crystallinity.

Abstract – Physical Review B 93, 115310 (2016)

Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across an interfacial region between two materials. In hybrid composites composed of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibronic modes in the molecular films can provide a new avenue to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we systematically study the heat transfer mechanisms in hybrid superlattices of atomic- and molecular-layer-grown zinc oxide and hydroquinone with varying thicknesses of the inorganic and organic layers in the superlattices. We demonstrate ballistic energy transfer of phonons in the zinc oxide that is limited by scattering at the zinc oxide/hydroquinone interface for superlattices with a single monolayer of hydroquinone separating the thicker inorganic layers. The concomitant thermal boundary conductance across the zinc oxide interfacial region approaches the maximal thermal boundary conductance of a zinc oxide phonon flux, indicative of the contribution of long wavelength vibrations across the aromatic molecular monolayers in transmitting energy across the interface. This transmission of energy across the molecular interface decreases considerably as the thickness of the organic layers are increased.