Design Principles for Negative Thermal Expansion in Two-Dimensional Materials

-boron nitride, transition-metal dichalcogenides, metal phosphides, arsenides and other emerging 2D materials. Beyond intrinsic mechanisms, we discuss tunability strategies unique to atomically thin systems, including pore size modulation, heteroatom substitution, defect modification, and magnetic or electronic-state control. Recent studies link NTE in 2D materials to phonon transport and topology-driven lattice responses and reveal trade-offs between thermal expansion and lattice thermal conductivity (TC). Most low-TC 2D systems exhibit pronounced NTE and vice versa. Spontaneous symmetry breaking in 2D-materials is associated with pseudo Jahn-Teller (PJT) distortions. The machine-learning (ML)-based predictions highlight the strong structural dependence of NTE in 2D materials. Despite growing interest in NTE materials across diverse applications, rational structural design and controlled tuning of lattice expansion remain challenging. This Account assesses current limitations and outlines future directions based on high-throughput calculations, ML, and topology-guided design for realizing enhanced NTE in 2D systems.