LAUSR

Controlling self‐assembly of nanocomposites is a fundamental challenge with exciting implications for next‐generation advanced functional materials. Precursors for composites can be generated photochemically, but limited insight in the underlying processes has hindered precise hands‐on guidance. In this study, light‐controlled nucleation and growth is demonstrated for self‐assembling composites according to precise user‐defined designs. Carbonate is generated photochemically with UV light to steer the precipitation of nanocomposites of barium carbonate nanocrystals and amorphous silica (BaCO3/SiO2). Using a custom‐built optical setup, the self‐assembly process is controlled by optimizing the photogeneration, diffusion, reaction, and precipitation of the carbonate species, using the radius and intensity of the UV‐light irradiated area and reaction temperature. Exploiting this control, nucleation is induced and the contours and individual features of the growing composite are sculpted according to micrometer‐defined light patterns. Moreover, moving light patterns are exploited to create a constant carbonate concentration at the growth front to draw lines of nanocomposites with constant width over millimeters with micrometer precision. Light‐directed generation of local gradients opens previously unimaginable opportunities for guiding self‐assembly into functional materials.