Making better contacts: A key issue in fabricating transistors is making a good electrical contact to the semiconductor gate material. For two-dimensional materials, one route is through a phase transition that converts a hexagonally packed semiconductor phase into a distorted octahedrally packed metallic phase. Cho et al. show that laser heating of molybdenum telluride (MoTe ₂ ) achieves this conversion through the creation of Te vacancies. The phase transition improves charge carrier mobility while maintaining the low resistance necessary for improved transistor function. Science , this issue p. 625

The growth of the $\sqrt{3}\times \sqrt{3}$ reconstructed silicene on Ag substrate has been frequently observed in experiments while its atomic structure and formation mechanism is poorly understood. Here, by first-principles calculations, we show that $\sqrt{3}\times \sqrt{3}$ reconstructed silicene is constituted by dumbbell units of Si atoms arranged in a honeycomb pattern. Our model shows excellent agreement with the experimentally reported lattice constant and STM image. We propose a new mechanism for explaining the spontaneous and consequential formation of $\sqrt{3}\times \sqrt{3}$ structures from $3\times 3$ structures on Ag substrate. We show that the $\sqrt{3}\times \sqrt{3}$ reconstruction is mainly determined by the interaction between Si atoms and have weak influence from Ag substrate. The proposed mechanism opens the path to understanding of multilayer silicon.