Silicene and germanene are conceptualised as monatomic two-dimensional (2D) crystals with a honeycomb lattice resembling that of graphene. The interest in silicene and germanene is motivated by the need to answer fundamental scientific questions. The most intriguing question is why silicene and germanene do not exist in nature in free-standing forms just like graphene? A related question is why silicon and germanium behave so differently compared to carbon despite the fact they belong to the same group IV of the periodic table? Is it possible to engineer silicene and germanene on suitable substrates and if so, what would be their structure? More specifically, are they going to be flat on the microscopic scale indicating a purely sp2-type of hybridisation or buckled instead meaning that a mixed sp2–sp3 hybridisation is favoured? Even more importantly, there are questions regarding their electronic properties. Are they semimetals like graphene featuring a Dirac cone, metallic or semiconducting/insulating instead and what would be the role of the substrate in determining their electronic identity? The effort in realising silicene and germanene is not merely a matter of scientific interest or curiosity but it is also driven by potential applications. The continuous lateral scaling of Si nanoelectronic devices and circuits suffers from insufficient electrostatic control which is mitigated mainly by reducing the gate equivalent oxide thickness and the channel thickness in fully depleted devices. Reducing channel thickness reduces also the physical (or geometrical) screening length, allowing lateral scaling without adverse short-channel effects. In fact, the …