Design Rules for Obtaining White Light from Layered Perovskites and Related Lattices

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PART 1: NON-TECHNICAL SUMMARYThe transition to solid-state lighting (SSL) is predicted to halve global electricity consumption for lighting by the year 2025. SSL devices use light-emitting diodes (LEDs) coated with a phosphor material, which converts blue or ultraviolet light from the LED to white light. The emitted light must span the entire visible spectrum, similar to sunlight, in order for the lighting to accurately show the colors of illuminated objects. However, phosphors with such broadband emission are very rare. Commercial SSL devices mix LEDs and phosphors of different colors to achieve broadband white light, but a single material that emits broadband white light is highly desirable for artificial illumination. Layered hybrid perovskites are crystalline materials that contain alternating organic and inorganic layers. The inorganic layers of some layered metal-halide perovskites can convert ultraviolet light to broadband white light. The proposed work, supported by the Solid State and Materials Chemistry and Electronic and Photonic Materials programs in the Division of Materials Research, seeks to expand white-light emission to a much larger range of lattices. Nontoxic compositions, in particular, will be targeted. To date, the discovery of new white-light-emitting metal halides remains serendipitous. This research will identify and disseminate design rules for the predictable synthesis of white-light emitters. Undergraduate researchers will be involved in all aspects of the research. A new course will introduce general chemistry concepts to first-year students using examples from materials chemistry. Concepts in solid state and inorganic chemistry, which are typically not included in first-year courses, will be introduced in general chemistry in order to draw students to these fields early. Outreach activities at local high schools and at Stanford University will also emphasize the importance of materials discovery, with demonstrations including new materials synthesized in the PI's group. PART 2: TECHNICAL SUMMARYBroadband white-light emission from a single material without discrete chromophores is extremely unusual. Such an emission has been reported from the inorganic layers of 2D hybrid halide perovskites, attributed to exciton self-trapping. Although a growing number of metal-halide broadband emitters are now being reported, they remain as relatively rare, isolated examples driven by serendipitous discovery. This research, supported by the Solid State and Materials Chemistry and Electronic and Photonic Materials programs in the Division of Materials Research, seeks to probe the generality of exciton self-trapping in a large range of lattices to substantially increase the number of intrinsic white-light emitters. Synthetic and optical studies of 0D, 1D, and 2D lattices with varying composition and connectivity will allow for the articulation of overarching design rules for the rational synthesis of white-light emitters. Steady-state and time-resolved spectroscopies will probe correlations between the ground-state structure of the lattices and excited-state distortions associated with self-trapping. These methods will also interrogate the role of extrinsic self-trapping by studying the influence of native and chemically induced lattice defects on the emission. The fundamental studies outlined here are expected to set the stage for the synthesis of materials that can manipulate light in a predictable manner. Single-source white-light emitters eliminate problems associated with current methods of producing white light by mixing multiple different phosphors or light-emitting diodes. The fundamentally different emission mechanism in these hybrids compared to those of inorganic phosphors and organic LEDS may open new niches for their use in large-area displays and panels. Undergraduate researchers will be involved in all aspects of the research. New materials synthesized for this project will be included in high school outreach events. A new course will introduce general chemistry concepts to first-year students using examples from materials chemistry. Concepts in solid state and inorganic chemistry, which are typically not included in first-year courses, will be introduced in general chemistry in order to draw students to these fields early.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.