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Thermal properties of two-dimensional (2D) materials

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Thermal properties of two-dimensional (2D) materials are of fundamental interest for energy recovery and electron cooling at the nanoscale [1–3]. Exploring thermal transport in 2D systems is a challenging issue particularly in the substrate-supported architecture, which is the most relevant for applications. In this configuration, heat diffusion is strongly affected by thermal losses through the substrate and highly depends on interfacial properties.
We investigate thermal transport of supported few-layer 2D materials by the Joule self-heating method, which has been probed as a reliable approach for metallic nanowires.
We have used this approach in the case of graphene nanowires of 300 nm width and few µm long in the high-temperature range (400 K < T < 500 K), which is less explored in literature [4]. The case of graphene is particularly intruiging. Graphene is one of the most-studied 2D materials, it has high mobility and a large Seebeck coefficient [5,6], as well as very high thermal conductivity, reaching values up to 5000W/m K [7] when suspended.
By using a thick and rough oxide layer, we have found that thermal losses through the substrate are reduced by almost 1 order of magnitude with respect to the expected theoretical values. Most importantly, we have unveiled an effective reduction of the thermal conductivity, with values as low as 40W/m K, comparable to reported values for only supported graphene nanoribbons. Such a reduction can be induced by increased phonon scattering with the substrate and impurities in the system, which is inherent to the device fabrication. The used method is valuable for the substrate-supported sample configuration, which is a realistic one. Our findings should stimulate research on alternative solutions for energy conversion at the nanoscale using graphene and other 2D materials when deposited on substrates.

Fig. 1 : a) Scheme of a device based on a graphene nanowire. b) Optical image of the graphene flake before the etching defining the nanowires (mask in red). c) Thermal conductivity of graphene multilayer (10 layers) nanowires on substrate as function of temperature. The inset shows the temperature dependence of the thermal conductivity k following the Callaway model by increasing scattering with impurities.

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