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.