Large-Area Atomically Flat Monocrystalline Gold Flakes: Recent Advances, Applications, and Future Potential

High aspect ratio oblate polygonal gold crystals - such as triangular and hexagonal platelets - have attracted considerable interest due to their extraordinary physical, chemical, and mechanical properties. Commonly referred to as "gold flakes," these structures exhibit atomically flat surfaces, µm² areas with nanometric thickness, and a monocrystalline morphology. Since their first discovery by John Turkevich in 1951, considerable progress has been made in shape-controlled synthesis and large-scale production, unlocking steadily new opportunities for ever more advanced applications. This review explores large-area gold flakes with lateral dimensions spanning from hundreds of nanometers to millimeters, with an emphasis on their unique properties. We provide a comprehensive overview of key developments, from early discoveries, synthesis approaches, and fabrication techniques to recent breakthroughs in the integration of gold flake as functional building blocks in photonics (e.g., for nanoantennas), sensing, nanoelectronics, biomedicine, and beyond. We conclude with a discussion of existing challenges, emerging applications, and possible future developments of this unique and versatile class of materials.