Negative thermal expansion of two-dimensional materials: A review

Negative thermal expansion (NTE) in two-dimensional (2D) materials defies the traditional views of thermal expansion and holds promise for the development of advanced nanoelectronics and temperature-controlled devices. Recent advances in 2D NTE materials, particularly through studies of out-of-plane bending vibrations, structural unit rotations, spin-lattice interactions, and strain-modulated lattice responses, have enhanced our mechanistic understanding of NTE in ultrathin systems. This review summarizes recent progress in uncovering the characteristics and origins of NTE in key 2D materials such as graphene, hexagonal BN, transition metal dichalcogenides, and novel 2D frameworks. Although porous structures often exhibit NTE dominated by low-energy optical phonons, this work highlights the growing recognition of acoustic phonon contributions and direction-dependent elastic effects in atomically thin systems, further discussing NTE tuning strategies such as substrate-induced strain control, interlayer coupling in van der Waals heterostructures, and magnetic configuration engineering. Current challenges, including narrow operational temperature windows, environmental sensitivity, and incompatibility with conventional materials, are critically evaluated, and innovative design approaches combining machine learning predictions, defect modulation, and topology optimization are outlined to advance the development of 2D NTE materials with customized thermomechanical performance.