In a crystal of bismuth tungsten oxide, there are empty spots where oxygen atoms should sit. Usually they are scattered randomly, and the film does not conduct electricity. But if you hold it in a cloud of nitrogen plasma for three minutes, the vacancies line up in neat rows, like soldiers at attention. Then electrons begin hopping from one vacancy to the next, and current flows.
After treatment, the film turns black, and its resistance drops by a factor of a billion. Oxygen plasma returns oxygen atoms to the vacant spots, the rows fall apart, and the film becomes transparent and insulating again. This cycle can be repeated dozens of times. Previously, ordering the vacancies required heating, but now room temperature suffices. This could be used to make switches for memory and sensors.
A resistance change of nine orders of magnitude is comparable to the difference between an insulator and a metal: if a wire were as thick as a human hair, its resistance would change from a value comparable to that of a kilometer of copper cable to that of a short piece of wire.
In Arthur C. Clarke's novel 'Childhood's End', aliens manipulate matter at the atomic level to change the properties of objects. Here a similar idea is realized without science fiction: plasma rearranges oxygen vacancies, and the material changes color and conductivity.