Few nanomaterials possess the flashiness of quantum dots. These colloidal nanocrystals glow in a rainbow of colors determined by their size, leading researchers to eye them for several technologies, including displays and solid-state lighting. The materials have yet to achieve their full promise because making them can be expensive and can generate a lot of waste. For example, metal sulfide quantum dots are usually made by adding a sulfide source to a metal salt solution and then stopping the reaction when the nanocrystals reach the desired size. This process can lead to variable yields and crystal sizes, as well as wasted starting material. Jonathan S. Owen and chemists at Columbia University have developed a library of inexpensive sulfide sources that produce quantum dots in quantitative yield (Science 2015, DOI: 10.1126/science.aaa2951). The library consists of thousands of thiourea compounds. By varying the thioureas’ substituents, the researchers tuned the rate at which the compounds produce sulfide, therefore giving the researchers control over nanoparticle size. Owen’s team also developed a similar synthetic strategy for making selenide quantum dots as well as nanocrystals with a metal-selenide core surrounded by a metal sulfide shell.