Nanostructured and quantum materials are enabling revolutionary advances in nanoscience and nanotechnology. These engineered materials promise to go beyond what is offered by natural materials. Advances in materials growth now make it possible to synthesize 3D nanostructured materials with nanometer resolution creating nanosystems with tunable electronic, photonic, magnetic, and thermal properties for a variety of applications including nanoelectronics, thermoelectrics, photovoltaics, and sensors. However, understanding of the fundamental mechanism behind many of these applications is still incomplete, making the development of powerful theoretical and experimental tools a priority for many material research communities.
Understanding thermal transport in nanostructured systems has long been a challenge. In the last few years, experimental techniques have pushed the measurement limits on systems with characteristic dimensions and geometries down to tens of nanometers, often times finding new surprising physical behaviors . However, it is still challenging to apply appropriate models that connect novel data to meaningful thermal properties and fundamental mechanisms. Most researchers rely on using effective or phenomenological diffusive models or the linearized Boltzmann transport equation that are not able to capture all of the observed nanoscale effects and nonlinear interactions [1-3]. This is because it is very difficult to perform atomistic simulations over experimentally-relevant length scales, due to prohibitive computational requirements. As a result, there is a large gap in understanding of how the nanoscale phonon physics unfolds for different size and geometries to modify the transport properties. Moreover, possible coherent effects are often not adequately included in current models.
The focus of this work is understanding thermal transport in nanostructured materials by bridging the gap between theory and experiment. In particular, previous work done in our research group uncovered a new thermal transport regime named “collectively-diffusive” by tracking the heating and cooling of periodic arrays of nanoscale heat sources on bulk crystalline silicon and sapphire substrates . This regime emerges from the interaction between the nanoscale geometries and energy carriers in materials. In particular, when nanoscale heat sources are placed closer, they cool down faster than when farther apart—the opposite of heat dissipation dynamics of the macroscale. The observed increase in heat dissipation efficiency has large consequences in thermal management in microelectronic devices, and is currently not captured by models available to both researchers in academia and industry.
In this work, we present the results of steady-state molecular dynamics (MD) simulations that can model the experimentally-explored geometries, and how these results provide a global understanding of non-diffusive phonon dynamics that appear to dominate energy transport in the deep nanoscale regime. Our MD results of nanoscale heat sources indicate that we are able to successfully capture the novel collectively-diffusive thermal dynamics observed recently.
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Ziabari et al. Nature communications, 9(1):255, (2018).