Quantum dots — tiny semiconductor crystals with diameters measured in billionths of a meter — have enormous potential for applications that make use of their ability to absorb or emit light and/or electric charges. Examples include more vividly colored light-emitting diodes (LEDs), photovoltaic solar cells, nanoscale transistors, and biosensors. But because these applications have differing — sometimes opposite — requirements, finding ways to control the dots’ optical and electronic properties is crucial to their success.