Understanding and tuning thermal transport at the nanoscale [electronic resource]
Keywords: Materials Science and Engineering.

Thermal transport in nanoscale materials is highly sensitive to the presence of defects and imperfections. For applications in areas ranging from electronics thermal management to energy harvesting, it is crucial to understand how material microstructure impacts thermal conductivity. Microstructural features such as grain boundaries, interfaces, and point defects can all scatter vibrational energy carriers leading to a modulation of the local thermal resistance. This work examines these phenomena over a range of defect length scales, in materials ranging from isotropic three-dimensional (3D) to highly anisotropic two-dimensional (2D) layered systems, using ultrafast optical pump-probe spectroscopy (time-domain thermoreflectance, or TDTR) as a probe. First, we discuss thermal transport measurements and modeling of heat conduction in thin-films of polycrystalline diamond, down to ~500 nm in thickness, where phonon scattering at grain boundaries plays a crucial role. The complex nucleation and growth kinetics lead to the formation of a columnar grain structure, which results in a highly anisotropic (~5×) and inhomogeneous thermal conductivity tensor. We construct a geometrical model that relates the spatial evolution of the thermal conductivity to the underlying competition and survival of growing crystallites. Second, we describe thermal conductivity measurements of epitaxially-grown short-period (~4 nm) III-V superlattice thin-films, where phonon-interface scattering plays a central role. Detailed theoretical analysis using a phonon Boltzmann transport equation (BTE) model reveals that lattice-matching in this system leads to high-quality interfaces that scatter phonons specularly. The implications of these results for the design and operation of mid-infrared Quantum Cascade Lasers are discussed. Third, we shift our focus to understanding thermal transport in van der Waals layered 2D materials and devices. We describe measurements of the fundamental mean free path (MFP) spectrum of heat-carrying phonons along the c-axis of MoS2, via thickness-dependent cross-plane thermal conductivity measurements of films as thin as ~20 nm. We uncover a surprising fact about the prominent contribution of long MFP phonons (MFPs as long as ~100s of nm) to thermal transport. Combining our experimental data with first principles phonon calculations, we show that the spectrum of heat-carrying vibrational modes (traveling normal to the layers) extends from ~5 nm to ~1 µm in crystalline MoS2. Finally, we present a novel thermal application of a dynamically tunable microstructure based on a layered 2D material. We show how point-defects, namely intercalated Li ions, can be used to reversibly modulate cross-plane thermal transport in MoS2. These electrochemical thermal transistors, only ~10 nm thick, show large thermal "on/off" ratios approaching nearly one order of magnitude. Using a novel in operando thermal conductance microscopy technique, we demonstrate "live" visualization of Li ion redistribution during charging and discharging of the MoS2 thermal transistor, with a spatial resolution down to ~1 µm. Atomic force microscopy on chemically intercalated samples shows that Li insertion into the van der Waals gap in MoS2 increases its c-axis lattice constant, and introduces mesoscopic disorder. Using first principles density functional theory and molecular dynamics calculations, we sort out the relative contributions of different factors that lead to the large modulation in thermal conductivity, including increased phonon scattering by Li 'rattler' modes, acoustic mode-softening due to lattice-expansion, and disorder at the atomic and mesoscopic length scales.