Thin metallic films are indispensable for modern electronics. However, when their thickness is reduced to the nanoscale, they can exhibit instabilities similar to those observed in liquids. One of these is dewetting, which causes the film to spontaneously retract, leading to roughness, rupture and eventual loss of continuity. These thermal instabilities limit the operating temperature of thin metallic films to around 300 °C, making their integration into two-dimensional (2D) devices challenging.
To overcome this limitation, scientists from several institutions in China developed approximately 40-nm-thick gold films through van der Waals (vdW) epitaxy on graphene, using electron-beam evaporation. Unlike conventional epitaxy—in which material layers are deposited onto a substrate with an ordered surface, allowing their atoms to align precisely—vdW epitaxy relies on the weaker vdW interactions between the layers. This approach enables materials to grow on substrates with different crystal structures without necessarily forming direct chemical bonds between the two surfaces, owing to van der Waals forces.
The results show that vdW epitaxy on graphene promotes the formation of a more uniform microstructure, characterized by preferentially oriented (111) grains and low-energy grain boundaries. This microstructure changes the behavior of the film at high temperatures: it reduces the capillary forces that normally promote film rupture and facilitates self-healing processes that help maintain its continuity. As a result, the thin gold films remain continuous even at temperatures approaching 850 °C, far above the conventional limit of approximately 300 °C. In contrast, films produced using conventional methods exhibit a more heterogeneous microstructure and grain boundaries that are less favorable for thermal stability.
These findings show that controlling the microstructure through vdW epitaxy can be a strategy for significantly improving the thermal stability of ultrathin metallic films and facilitating their integration into 2D devices. This new methodology makes it possible to increase fabrication temperatures and times, enabling the production of next-generation devices with high-quality components.
For further information go to: nature communications
