Various aspects of the disclosure provide thermal interface materials having high thermal conductivities. In one aspect, a thermal interface material comprises metal-diamond composite nanoparticles, where each composite nanoparticle comprises a diamond core surrounded by a metal shell with a low fusion temperature. In one aspect, a thermal interface is formed between two surfaces (e.g., surfaces of a heat source and heat sink) by applying the thermal interface material between the two surfaces and heating the thermal interface material to the fusion temperature of the metal shells. The heating causes the metal shells to fuse together and to the two surfaces. The fusion results in a thermal interface between the two surfaces comprising a metal layer formed by the fused metal shells and the diamond cores embedded within the metal layer. The high thermal conductivity of the embedded diamond cores greatly enhance the thermal conductivity of the thermal interface.