bulletin cover story Quantum dots and nanocrystal- embedded glasses for display applications lasses are widely used in display applications, as substrates for thin By Yoon Hee Nam, Hansol Lee, and Woon Jin Chung G film transistor arrays and color filters or cover windows. In addition, glasses and glass-ceramics The current high demand and fast-growing market for more now also are being considered as active materi- realistic images and vivid motion pictures drives the need for als for manipulating the primary colors of light high-quality picture displays. Quantum dots and nanocrystal- sources, further expanding their potential appli- embedded glasses show a lot of promise for this purpose. cation in display technologies. Current cutting-edge systems, such as liquid crystal displays (LCD) using quantum dots (QDs) and organic light emitting diodes (OLED), rely on organic materials or QDs synthesized by wet-chemistry to produce and control the primary colors of lights, such as red, green, and blue (RGB). It is well known that organic materials suffer from inherently weak chemical and ther- mal stabilities and complicated synthesis process, while inorganic glasses or glass-ceramics possess high chemical and thermal stabil- ity. These properties give glasses durability as well as long-term stability, and their relatively easy fabrication process reduces production cost. These advantages are opening new opportunities for glass-based materials as light generating materials, replacing conventional QDs. In this overview, the recent use of conventional QDs in displays is reviewed, and the development and current status of quantum dots and nanocrystal-embedded glasses for display applications are dis- cussed, along with their challenges and future prospects. Color gamut of displays The current high demand and fast-growing market for more realistic images and vivid motion pictures is driving the need for high-quality picture displays, which rely on several key features, 28 Celebrating 100 years www.ceramics.org | American Ceramic Society Bulletin, Vol. 100, No. 1 including resolution, brightness, high dynamic range, and color gamut. Color gamut, which represents the range of (Ref. 1) color reproduction within the visible color space, is especially important as Mater. Today displays with ultrahigh-definition (UHD) resolution (3840 × 2160 in pixels) become more popular in the market. Credit: Yang et al., The color gamut of a display is defined by a triangle area enclosing the primary RGB colors. The standard color gamut for cathode-ray tubes was defined by the (Ref. 1) National Television Systems Committee (NTSC) within the CIE 1931 chro- Mater. Today maticity diagram in 1953. With the development of new display technologies since the 1990s, such as plasma dis- Credit: Yang et al., play panel and LCD, the International Credit: Nam, Lee, and Chung Telecommunication Union Radio com- Figure 1. (a) Electroluminescence and photoluminescence (EL+PL) spectra of RG-LED. munication organization recommended (b) EL spectra of QD-LEDs and OLEDs. (Reprinted with permission from Ref. 1. a new color gamut for high-definition Copyright 2018 Elsevier Ltd.) (c) Color gamuts of NTSC, Rec 709, Adobe RGB 1998, TVs (HDTV) with 1920 × 1080 resolu- and Rec 2020 standards within the CIE 1931 chromaticity diagram. tion, which is known as Rec. 709 (or BT. 709), as found in the chromaticity The RGB colors of conventional used with a combination of white OLED diagram of Figure 1c. LCDs are determined by a combination and color filters to produce RGB colors, The International Electrotechnical of color filters and the emission spectra and accordingly have a limited color 1 Commission announced standard RGB of a white LED, which is comprised of gamut following behind QD-based (sRGB) for computer displays, and it a blue LED with green and red ceramic LCDs in terms of brightness and color 2+ has a color reproduction range similar phosphors (RG-LED). b-Si6AlON8:Eu reproduction range. 2+ 2+ to Rec. 709. Adobe and Digital Cinema and CaAlSiN3:Eu (CASN:Eu ) were Initiatives, LLC also suggested definitions mostly used in commercial RG-LEDs QDs for displays for color gamut, those being Adobe RGB as the green and red phosphors, respec- QDs are semiconductor nanocrys- for publication and DCI-P3 for digital tively. However, because of their broad tals with diameters (typically < 10 nm) cinema colors, respectively. Thanks to emission bands, the LCD with RG-LED smaller than their Bohr radius, and they technological advances in the display have broad emission spectra, as drawn in include group II-VI (e.g., CdS, CdSe, industry, a new standard was required for Figure 1a. This emission spectra results in CdTe) and III-V (e.g., InP) compounds. the UHDTV environment, and thus the a color gamut of about 75% of NTSC. Since their first discovery by Alexey 2 International Telecommunication Union Scientists achieved significant improve- Ekimov in the 1980s, researchers exten- Radio suggested a wider color range, ment of the color gamut using QDs, sively investigated QDs and made several which is known as Rec. 2020 (or which have narrow emission bandwidth observations. For example, QDs have a BT. 2020), whose area corresponds to and tunable emission peak wavelengths. high theoretical quantum efficiency, up 150% of the NTSC area. As schematically illustrated in Figure 1, to 100%, and narrow emission band- It should be noted that the HDTV when QDs are used as color converting width of less than 50 nm, thanks to their color gamut corresponds to about 72% phosphors, well separated emission spec- strong quantum confinement effect. of NTSC, which most conventional tra can be obtained, providing a wide Their emission peak wavelength can be 1 LCD-based HDTVs provide. Rec. 2020 color gamut higher than 100% NTSC. easily tuned from ultraviolet to infrared for UHD was defined by monochromatic QDs thus were applied by major panel by varying their shape, size, or chemical 3 630, 532, and 467 nm lights for red, makers such as Samsung in advanced composition, as shown in Figure 2. green, and blue, respectively. Accordingly, LCD displays known as QLED-TVs. QDs are normally synthesized in a col- to realize UHD color gamut, it is highly Displays based on active-matrix loidal form by wet chemical approaches important to adjust the peak emission OLEDs (AMOLED) also can supply using chemical reactors, such as batch 4 wavelengths of the RGB colors and RGB colors with narrower emission reactors and continuous reactors. Among reduce their emission bandwidth or full bands than conventional RG-LEDs and batch reactors, hot injection organometal- width at half maximum for high color provide a high color gamut. However, lic synthesis is most widely used to obtain saturation (or color purity). for large sized panels, AMOLEDs are monodisperse colloidal QDs (cQDs), American Ceramic Society Bulletin, Vol. 100, No. 1 | www.ceramics.org Celebrating 100 years 29 Quantum dots and nanocrystal-embedded glasses for display applications ment film, which is then placed on top of a light guiding panel film, which delivers blue LED light.8 However, like (Ref. 3) OLEDs, QDs can also exhibit electro- Nano Lett. luminescence when they are used as an emission layer with an organic hole transport layer and electron transport layers, to create an LED (QD-LED or Credit: Anikeeva et al., EL-QD).5 Although QD-LEDs still need (Ref. 4) to overcome several obstacles, includ- (Ref. 8) ing low external quantum efficiency and long term stability, they are being extensively studied for next generation Ind. Eng. Chem. Res. Adv. Mater. Technol displays with high picture quality. Recently, perovskite structured Credit: Pu et al., quantum dots or nanocrystals based Credit: Lee et al., on CsPbX3 (X=Cl, Br, and I) have been Figure 2. (a) Schematic diagram (left), actual photographs (top), and electrolumines- extensively studied to replace conven- cence and photoluminescence (EL+PL) spectra (bottom) of colloidal QDs. Solid lines rep- tional semiconductor-based QDs. Unlike resent EL spectra while dashed lines represent PL spectra. (Reprinted with permission conventional cQDs, perovskite quantum from Ref. 3. Copyright 2009 American Chemical Society.) (b) Schematic diagram (left), dots or nanocrystals (PQDs or PNCs) actual photographs (top), and PL spectra (bottom) of PNCs. (Reprinted with permission have intrinsic defect tolerance due to from Ref. 4. Copyright 2015 American Chemical Society.) (c) Color gamuts of RG-LED (phosphorous LED), QD-LED (Cd-QD LED), and PNC-LED (PQD-LED). (Reprinted with per- their characteristic electronic band struc- mission from Ref. 8. Copyright 2020 Wiley.) ture, and thus exhibit high PLQY, up to 100%, without any shell-like passivation which are obtained by the pyrolysis of stability and photoluminescence quantum layers.8,9 As shown in Figure 2, they also organometallic precursors rapidly injected yield (PLQY). Multishell cQDs (e.g., have high color tunability by varying into a hot organic coordination solvent at CdSe/ZnSe/ZnS) or ternary/quaternary size and composition, as well as a nar- 4 a temperature of 120–360°C. alloyed cQDs (e.g., CdSxSe1–x, ZnxCd1–xSe, row emission bandwidth, covering wider To improve batch-to-batch reproduc- ZnxCd1–xSySe1–y) demonstrated significant range up to about 140% of the NTSC ibility, a one-pot noninjection colloidal enhancement of PLQY, up to 80~100%, color gamut.9 synthesis method at low temperature was by successfully decreasing defects caused PNCs have at least 2.5 times higher 4 5,6 also suggested for various QDs. An aque- by lattice mismatch. Although cadmi- absorption coefficient than cadmium- ous synthesis method, in which QDs are um-based QDs with core/shell structure based or InP-based QDs, and can be directly synthesized within a water-based showed highest QY, cadmium-free or easily fabricated without core/shell solvent, also was suggested for biologi- heavy metal-free QDs were required due structure even at room temperature, cal and clinical applications.