Design and fabrication of single-nanometer-scale graphene phononic crystals for thermal engineering by using focused helium ion beam

摘要

Thermal conductivity reduction at the nanoscale can be achieved by hindering the propagation of the thermal phonon waves. Specifically designed periodic arrays can reduce the heat propagation by the wave interference when the wavelength of thermal phonon (high frequency phonon in THz regime) is equal to the periodic spacing of nanopores. These type of materials are called phononic crystals (PnCs), exhibiting phononic bandgap (PnBG) where the propagation of certain phonon wavelength is suppressed. The study of phononic materials has garnered much interest due to the prospect of futuristicapplications like hypersound and heat control, acoustic and thermal cloaking, thermal diode etc.In comparison to the commonly used semiconductor material of silicon, graphene is regarded as a promising candidate for the phononic crystals. Owing to thehigh Young's modulus and Debye temperature in graphene, the minimum pore size and the spacing of nanopores to realize THz PnBG is larger than silicon.Moreover,we have developed a technique to fabricate suspended graphene phononic crystals with pore sizeof 3 to 4 nm and pitch 18 to 25 nm based on sub-1 nm helium ion beam milling technique.