Researchers have developed a groundbreaking material that is both extremely stiff and an excellent thermal insulator, and it can be printed as a thin film at large scales. This material has one of the lowest thermal conductivity values of any dense, nonporous material, approaching the theoretical limit of how well a material can insulate against heat. Such a material could be useful in a wide range of applications, from cookware and electronic devices to space exploration, according to Dali Sun, a professor of physics at North Carolina State University and co-corresponding author of a study on the material. However, creating a material that is both stiff and an effective insulator is a significant challenge, as typically stiff materials conduct heat well, while softer materials are better insulators. This new material defies that trend. Jun Liu, an associate professor of mechanical and aerospace engineering at NC State and co-corresponding author, explains that the team previously observed unusual interactions between stiffness and thermal conductivity in certain materials. For this study, they employed more advanced molecular engineering to intentionally create a material with an extreme combination of these properties. The research, titled "Extremely Low Thermal Conductivity in Rigid Layered Hybrid Perovskites," will be published in the journal Science Advances on September 18. The material is a type of two-dimensional hybrid organic-inorganic perovskite, a class of thin-film semiconductors made of alternating organic and inorganic layers with a highly ordered crystalline structure. By replacing some of the carbon-carbon chains in the organic layers with a specific combination of benzene rings, the researchers were able to control both the stiffness and thermal conductivity of the material. The resulting material, an azobenzene ethyl ammonium lead iodine thin film, was tested and found to have a thermal conductivity of about 0.04 W m⁻¹ K⁻¹ at room temperature—five times lower than that of silicone, a common insulating material used in items like oven mitts. According to Liu, the material is not only five times more effective at insulating against heat than silicone but also 700 to 10,000 times stiffer. Additionally, the method used to produce the material is scalable, meaning it can be manufactured at large scales and applied as a coating. Liu emphasizes that while the material's properties are impressive, the study also highlights the potential of molecular engineering to fine-tune hybrid layered materials for applications that require unique combinations of stiffness and thermal insulation. This opens the door for future innovations in materials science and engineering.