Engineering thermal transport within Si thin films: The impact of nanoslot alignment and ion implantation.

In recent years, nanoporous Si films have been intensively studied for their potential applications in thermoelectrics and the thermal management of devices. To minimize the thermal conductivity, ultrafine nanoporous patterns are required but the smallest structure size is largely limited by the spatial resolution of the employed nanofabrication techniques. Along this line, an effectively smaller characteristic length of a nanoporous film can be achieved with offset nanoslot patterns. Compared with periodic circular pores, the nanoslot pattern can achieve an even lower thermal conductivity, where a much smaller porosity is required using ultra-narrow nanoslots. The obtained low thermal conductivity can be understood from the thermally dead volume revealed by phonon Monte Carlo simulations. To further minimize the contribution from short-wavelength phonons, an additional 25% thermal conductivity reduction can be achieved with Ga ions implanted using a focused ion beam.

The unrevealed 3D morphological evolution of annealed nanoporous thin films.

Nanoporous materials (sub-10 nm in diameter) have potential applications in chips, biosensors, thermoelectrics, desalination and other fields due to their large surface-to-volume ratio. Thermal annealing is a preferred technique to precisely control the ultra-fine nanopore size. Here, the 3D morphological evolution of a membrane with periodic nanopores by thermal annealing is studied. It is found that the evolution is determined by the combination of the membrane thickness, the initial nanopore radius and the periodic length of the porous pattern, rather than the previously suggested ratio between the membrane thickness and pore radius. High-temperature annealing experiments and molecular dynamics simulations are performed to confirm the rationality of the newly proposed model. Energy analysis demonstrates that surface energy minimization is the driving force of the morphological evolution. The local minimum of energy in the new model provides the possibility of thermal stability of nanoporous silicon as a thermoelectric material. This study provides guidance for the mass production of nanoporous membranes with high-temperature annealing.

Small-Nanostructure-Size-Limited Phonon Transport within Composite Films Made of Single-Wall Carbon Nanotubes and Reduced Graphene Oxides.

Nanocarbon materials have been widely used for nanoelectronics and energy-related applications. In this work, composite films consisting of reduced graphene oxides (rGOs) and single-wall carbon nanotubes (SWCNTs) are synthesized and studied for their in-plane thermal conductivities. Different from pristine carbon nanotubes or graphene with decreased thermal conductivities above 300 K, the in-plane thermal conductivities of these composite films are found to follow the trend of the specific heat of graphene from 100 to 400 K, i.e., monotonously increasing at elevated temperatures. Such a trend can often be found within amorphous solids but has seldom been observed for nanocarbon. This unique temperature dependence of thermal conductivities is attributed to the largely restricted phonon mean free paths within the graphene sheets that mainly contribute to the in-plane thermal transport. The highest in-plane thermal conductivity among samples with different synthesis conditions is 62.8 W/(m·K) at 300 K. Such a high thermal conductivity, combined with its unique temperature dependency, can be ideal for applications such as flexible film-like thermal diodes based on the junction between two materials with a large contrast for their temperature dependence of the thermal conductivity.