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Abstract Cs₂SnI₆perovskite displays excellent air stability and a high absorption coefficient, promising for photovoltaic and optoelectronic applications. However, Cs₂SnI₆‐based device performance is still low as a result of lacking optimized synthesis approaches to obtain high quality Cs₂SnI₆crystals. Here, a new simple method to synthesize single crystalline Cs₂SnI₆perovskite at a liquid–liquid interface is reported. By controlling solvent conditions and Cs₂SnI₆supersaturation at the liquid–liquid interface, Cs₂SnI₆crystals can be obtained from 3D to 2D growth with controlled geometries such as octahedron, pyramid, hexagon, and triangular nanosheets. The formation mechanisms and kinetics of complex shapes/geometries of high quality Cs₂SnI₆crystals are investigated. Freestanding single crystalline 2D nanosheets can be fabricated as thin as 25 nm, and the lateral size can be controlled up to sub‐millimeter regime. Electronic property of the high quality Cs₂SnI₆2D nanosheets is also characterized, featuring a n‐type conduction with a high carrier mobility of 35 cm²V⁻¹s⁻¹. The interfacial reaction‐controlled synthesis of high‐quality crystals and mechanistic understanding of the crystal growth allow to realize rational design of materials, and the manipulation of crystal growth can be beneficial to achieve desired properties for potential functional applications. 

Expected to become mainstream in the electronic industry, flexible electronics still face major challenging issues. For polymeric based flexible electronic substrates in particular, these challenges include a lack of electromagnetic shielding capability and poor heat dissipation. Here, we report a highly flexible and thermally-conductive macroscopic polydimethylsiloxane (PDMS) polymer film embedded with copper-coated reduced graphene oxide (rGO) fiber meshes. rGO fibers are assembled into 3D fiber meshes and electroplated with micrometer-thick copper coatings, displaying excellent electrical and thermal conductivities. Oriented in the horizontal and perpendicular directions within the PDMS polymeric matrix, the fiber mesh severs as a highly electrically and thermally-conductive backbone through the in-plane direction. Meanwhile, the fiber mesh also effectively shields electromagnetic interference in the X-band without causing thermal damage. The macroscopic film maintains electrically-insulated in the through-plane direction. Utilizing both the favorable thermal and electrical properties of the graphene fiber-based mesh and the flexibility of the PDMS matrix, our film may exhibit potentials for flexible electronics applications such as wearable electronics thermal management and flexible microwave identification devices.