Abstract
New technologies are emerging which allow us to manipulate and assemble 2-dimensional (2D) building blocks, such as graphene, into synthetic van der Waals (vdW) solids. Assembly of such vdW solids has enabled novel electronic devices and could lead to control over anisotropic thermal properties through tuning of inter-layer coupling and phonon scattering. Here we report the systematic control of heat flow in graphene-based vdW solids assembled in a layer-by-layer (LBL) fashion. In-plane thermal measurements (between 100 K and 400 K) reveal substrate and grain boundary scattering limit thermal transport in vdW solids composed of one to four transferred layers of graphene grown by chemical vapor deposition (CVD). Such films have room temperature in-plane thermal conductivity of ~400 Wm−1 K−1. Cross-plane thermal conductance approaches 15 MWm−2 K−1 for graphene-based vdW solids composed of seven layers of graphene films grown by CVD, likely limited by rotational mismatch between layers and trapped particulates remnant from graphene transfer processes. Our results provide fundamental insight into the in-plane and cross-plane heat carrying properties of substrate-supported synthetic vdW solids, with important implications for emerging devices made from artificially stacked 2D materials.
Highlights
The past decade of graphene research has accelerated scientific discovery of 2D transition metal dichalcogenides,[1,2] phosphorene,[3] silicene,[4] and 2D hexagonal boron nitride.[5]
For samples with more than a single transferred layer of chemical vapor deposition (CVD) graphene the wet-transfer and anneal process is repeated in a LBL fashion to achieve artificial graphene van der Waals (vdW) solids with up to N = 4 CVD layers
High angle annular dark field scanning transmission electron microscopy (STEM) (HAADF-STEM) images and electron energy loss spectroscopy (EELS) analysis indicates the large trapped particulates are likely trapped copper particles, remnant from the etching and transfer process[38,39] (Supplementary Fig. S13). These measurements highlight the importance of material processing techniques on the structure–property correlations of thermal transport in LBL assembled graphene vdW solids
Summary
The past decade of graphene research has accelerated scientific discovery of 2D transition metal dichalcogenides,[1,2] phosphorene,[3] silicene,[4] and 2D hexagonal boron nitride.[5] These materials have unique electrical, thermal, optical, and mechanical properties as compared to their 3-dimensional (3D) counterparts. Such 2D building blocks exhibit metallic, semiconducting, and insulating behavior, providing novel material combinations for electronic device design.[1,6] For example, LBL assembly of graphene with other 2D materials has resulted in ultrathin heterostructures suitable for tunneling field effect transistors[7,8,9] and ultrathin optoelectronic devices.[10,11] the thermal properties of LBL assembled artificial vdW solids have received less attention. Similar to naturally occurring vdW solids, artificial vdW solids are expected to have strong in-plane bonds and weak inter-layer vdW interactions, resulting in anisotropic thermal properties between the in-plane and cross-plane directions.[12,13,14]
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