Controllable growth and heat-transporting properties of two-dimensional materials

Abstract

In this study, we first fabricate monolayer MoS₂ by chemical vapor deposition. The morphology evolution from triangular flakes to continuous thin films by changing the distance between source and growth substrates is observed. The effects of the nuclei density on the domain size and surface coverage are then systematically investigated. By optimizing the growth conditions, we successfully grow single-crystalline MoS₂ flake with the size larger than 300 m. We also develop a transfer process assisted with a Cu thin film, which allows us to transfer MoS₂ onto arbitrary substrates. The characterizations on transferred MoS₂ confirm observable residues and wrinkles are absent on the surface and that the MoS₂ maintains the properties as the as-grown one. To develop a deeper understanding of MoS₂ growth, we further observe the dynamic growth of MoS₂ in TEM by heating the solid precursor. The evolution process of in-situ MoS₂ growth is identified including the initial formation of vertically aligned layers, then a grain rotation towards a horizontally layered structure, precipitation and growth of nanocrystals, and formation of hexagonal MoS₂ nanoflakes by facet development. We also investigate the in-situ growth of MoxC. By adjusting the carbon concentration in the precursor, we attain MoC and Mo₂C in different phases and the growth pathways are also different. The developed understanding of MoS₂ and MXene formation using in-situ TEM technique provides fundamental knowledge in synthesis of the emerging 2D materials. Lastly, taking advantages of thermal conducting and electrical insulating h-BN, we develop the local thermal management for high electron mobility transistors, utilizing few-layer h-BN as heat spreaders and thick counterpart as heat sinks. Few-layer h-BN fully covers the transistors and rapidly dissipates the heat from the hotspot, while thick h-BN stores the intensive heat flux from few-layer h-BN and withdraws to the ambient without affecting the devices. With the developed thermal management, the performance of the transistor is enhanced. The simulations indicates ~40 °C temperature reduction when the transistor is operated at 4 W/mm on sapphire substrates after the application of the thermal management. The developed heat dissipation is highly potentially applicable in thermal management of power devices and integrated circuits because of the characteristics of thermal-conducting and electrical-insulating of h-BN.