Cryogenic photomotion enabled by phonon-engineered sp–sp2 carbon frameworks via reversible photothermal buoyancy switching

Cryogenic liquids dissipate local heat rapidly, which makes contactless light-driven motion difficult to achieve. Here we show that hydrogen-substituted graphdiyne, a porous sp–sp2 carbon framework with suppressed lateral heat spreading, retains photothermal energy near an illuminated region and drives reversible buoyancy switching in liquid nitrogen. Multiscale analysis shows that reduced acoustic-phonon group velocities and an increased density of high-frequency vibrational modes sustain a localized hotspot even when the film is immersed in a bath at 77 K. Under illumination at 110 mW cm−2, this hotspot nucleates a buoyancy plume that drives a vertical displacement of 4.5 cm within 6.5 s, and the float–sink cycle is reproducible over ten consecutive on–off cycles. The same module also supports actuation selected by boundary conditions, including interfacial rotation, bulk rotation and confined translation. These results provide a framework for engineering materials that manipulate vibrational energy for autonomous systems operating at cryogenic temperatures. Cryogenic liquids rapidly remove local heat. Here, the authors use a phonon-engineered porous carbon film to retain photothermal energy and achieve reversible buoyancy switching and programmed motion at 77 K.