A custom-designed semiconductor with structural and compositional features spanning multiple length scales has set a record for thermoelectric performance (Nature, DOI: 10.1038/nature11439). Thermoelectric materials exhibit an unusual combination of properties that enables them to convert heat to electricity when a temperature difference develops across a chunk of such a material. Likewise, applying a potential causes these materials to heat or cool. Thermoelectric materials are used in a few applications, some for military and aerospace use. The materials could be used more broadly—for example, to generate power from automobile exhaust waste heat—if the materials’ thermoelectric figure of merit, ZT, a measure of energy conversion efficiency, could be boosted. Recently, ZT values have topped out at around 1.5 at temperatures near 600 °C. Northwestern University’s Kanishka Biswas, Mercouri G. Kanatzidis, and coworkers now report that a plasma-sintered PbTe material exhibits a record-setting ZT value of 2.2 at temperatures above 600 °C. They say that the material’s knack for controlling heat flow results from its structure, which combines micrometer-scale PbTe grains, nanosized SrTe crystallites, and atomic-scale sodium dopants.