Lithium Tin Sulfide—a High‐Refractive‐Index 2D Material for Humidity‐Responsive Photonic Crystals

Abstract Extending the portfolio of novel stimuli‐responsive, high‐refractive‐index (RI) materials besides titania is key to improve the optical quality and sensing performance of existing photonic devices. Herein, lithium tin sulfide (LTS) nanosheets are introduced as a novel solution processable ultrahigh RI material ( n = 2.50), which can be casted into homogeneous thin films using wet‐chemical deposition methods. Owing to its 2D morphology, thin films of LTS nanosheets are able to swell in response to changes of relative humidity. Integration of LTS nanosheets into Bragg stacks (BSs) based on TiO 2 , SiO 2 , nanoparticles or H 3 Sb 3 P 2 O 14 nanosheets affords multilayer systems with high optical quality at an extremely low device thickness of below 1 µm. Owing to the ultrahigh RI of LTS nanosheets and the high transparency of the thin films, BSs based on porous titania as the low‐RI material are realized for the first time, showing potential application in light‐managing devices. Moreover, the highest RI contrast ever realized in BSs based on SiO 2 and LTS nanosheets is reported. Finally, exceptional swelling capability of an all‐nanosheet BS based on LTS and H 3 Sb 3 P 2 O 14 nanosheets is demonstrated, which bodes well for a new generation of humidity sensors with extremely high sensitivity.