Spotlight on Applications Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX
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Self-Assembly Driven Aggregation-Induced Emission of Copper Nanoclusters: A Novel Technology for Lighting Yi Liu, Dong Yao, and Hao Zhang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ABSTRACT: Because of the specific properties including HOMO−LUMO electronic transition, size-dependent fluorescent emission, and intense light absorption, metal nanoclusters (NCs) have been considered to be one of the most competitive color conversion materials in light-emitting diodes (LEDs). However, the monotonous emission color and the low emission stability and intensity of individual metal NCs strongly limit their universal application. Inspired by the concept of “aggregationinduced emission” (AIE), the utilization of highly ordered metal NC assemblies opens a door to resolve these problems. After self-assembly, the emission stability and intensity of metal NC assemblies are enhanced. At the same time, the emission color of metal NC assemblies become tunable. We termed this process as self-assembly driven AIE of metal NCs. In this review, we use Cu NCs as the example to convey the concept that the compact and ordered arrangement can efficiently improve the metal NCs’ emission stability, tunability, and intensity. We first introduce the synthesis of 2D Cu NC self-assemblies and their emissions. We further summarize some of the factors that can affect the emissions of 2D Cu NC self-assemblies. We then discuss the utilization of 2D Cu NC self-assemblies as color conversion materials for LEDs. At last, we outline current challenges and our perspectives on the development of this area. KEYWORDS: metal nanoclusters, self-assembly, aggregation-induced emission, 2D nanomaterials, light-emitting diodes
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INTRODUCTION Light is of essential importance to the universe and humankind. More than 20% of worldwide electric energy has been consumed for lighting every year.1 Thus, how to save electric energy and improve its efficiency are particularly appealing in the area of illumination. Compared with conventional light sources including fluorescent lamps and incandescent bulbs, solid-state lighting (SSL) in the form of light-emitting diodes (LEDs) is drawing more and more attention in laboratories and industries because of its ultrahigh-speed response time (microsecond-level on−off switching), attractive luminous efficiency (LE) of above 200 lm/W, wide range of controllable color temperatures (4500−12 000 K), operating temperature range (20 to 85 °C), and lack of low-temperature startup problems.2−8 So far, the commercial LEDs were usually fabricated by coating color conversion materials on the blue-violet light-emitting InGaN/GaN chip. The color conversion materials can completely or partially convert the chip emission to the desired emission color, thus producing LEDs with visible colorful or white light.9,10 Previously, rare-earth phosphors were usually used as the color conversion materials.11−14 However, their supply is facing a serious shortage, and the lack of recycling ability is destructive to the environment.15 Organic fluorophores with a wide range of chemical structures and spectral properties were another option.16−18 But they can be easily photobleached in practical application.19 Thanks to the unique optical properties, such as the broad absorption band, © XXXX American Chemical Society
narrow emission spectra, high photostability and photoluminescence quantum yields (PLQYs), and controllable emission and surface properties, quantum dots (QDs) are considered as competitive color conversion materials for fabricating LEDs.20−23 Unfortunately, most QD-related LEDs (QLEDs) rely on the use of QDs containing heavy-metal cations (for example cadmium, lead, and mercury, etc.). The European Union’s Restriction of Hazardous Substances Directive has severely limited the utilization of these heavy metals in consumer electronics in terms of the growing concern regarding the risks for the environment and health.8 Thus, to continue the advance of QLEDs, alternative heavy-metal-free QDs that meet the safety regulations are required. Recently, metal nanoclusters (NCs) have attracted a great deal of interest to be used as environmentally friendly and biocompatible color conversion materials in QLEDs owing to their low toxicity, low cost, and good biocompatibility.15,24 As novel luminescent nanomaterials, metal NCs usually consist of a few to a hundred atoms with sizes around the Fermi wavelength of an electron (
