Developing materials that can conduct electricity easily, but block the motion of phonons is necessary in the applications of thermoelectric devices, which can generate electricity from temperature differences. In converse, a key requirement as chips get faster is to obtain better ways to dissipate heat. Controlling heat transfer in these crystalline materials devices — such as silicon — is important. The heat is actually the motion or vibration of atoms known as phonons. Finding ways to manipulate the behavior of phonons is crucial for both energy applications and the cooling of integrated circuits.