Thermal Transport in a 2D Amorphous Material

Two-dimensional (2D) crystals proved revolutionary soon after graphene was discovered in 2004. However, 2D amorphous materials down to a single layer of atoms only became accessible in 2020, and they remain largely unexplored. In particular, the thermophysical properties of amorphous materials are of great interest upon transition from 3D to 2D. Here, we probe and simulate thermal transport in monolayer amorphous carbon (MAC). An ultralow cross-plane thermal conductivity (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>κ</a:mi></a:math>) is measured for van der Waals stacked multilayers, which is comparable to that of randomly stacked graphene despite the extra disorder in MAC. This result reveals the predominant role of the weak interlayer interactions in 2D materials. Meanwhile, an unexpectedly high in-plane <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>κ</c:mi></c:math> is obtained for freestanding monolayers, which is a few times higher than what is predicted by conventional wisdom for 3D amorphous carbon with a similar <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msup><e:mrow><e:mi>sp</e:mi></e:mrow><e:mn>2</e:mn></e:msup></e:mrow></e:math> fraction. This observation is primarily attributed to the dimensionality-induced reduction of anharmonicity and the unique low-frequency out-of-plane vibrational modes in MAC. Amorphous materials at the 2D limit open up new avenues for understanding and manipulating heat at the atomic scale.