Quantum Materials for Additive Manufacturing Applications

Abstract

Quantum materials’ limited emission spectrum and easily adjustable color through particle size modification make them a viable option for the next generation of displays. The emission spectrum of quantum materials is sharp and pure, which makes quantum materials ideal for display applications. As display technology advanced, self-emitting display technology eventually replaced liquid crystal display (LCD). Researchers design different types of RGB pixels in the self-emitting display area to achieve the best possible visual impact. However, different types of pixels need quantum color conversion films with various patterns. Additive manufacturing offers a novel method for quicker prototyping of red, green, and blue (RGB) pixels with a faster iteration cycle. With the additive manufacturing technique, especially the inkjet printing method, the sample is not in contact with the surfaces; only the essential components are dispensed and deposited there. The additive manufacturing technique generally reduces sample damage or containment and material waste. This chapter introduces inkjet-printing quantum materials for high-resolution display applications.

1. Introduction

1.1 Emerging quantum dots for display applications

When the material’s size is narrowed down to the nano size, the interaction at the atomic and subatomic scale makes the quantum materials the magic material. The solid material loses its conventional order and takes on wavelike properties on the atomic and subatomic scale. In this situation, quantum phenomena such as interface, tunneling, fluctuations, entanglement, and topological effects are found. All these quantum phenomena are indeed a fascinating and nearly spooky field; over the past decade, we have seen a significant change in the knowledge and utilization of multiple applications of quantum physics, especially when utilizing quantum materials’ remarkable optical properties in display areas.

Among the quantum materials, quantum dots and perovskite nanoparticles have been widely studied over the years. At the same time, quantum dots (QDs) were first discovered in the 1980s by Louis E. Brus [1]. At that time, QDs only had a core without any passivation included. Following years of research, scientists discovered that covering the light-emitting core of QDs with a monolayer or several monolayers of a larger band gap material shell can significantly improve the optical properties of QDs [2, 3, 4, 5]. QDs are frequently covered with organic ligands, commonly amine-based compounds such as ODA OR TOPO. The primary objective of this coating is twofold: to protect the shell surface from unfulfilled chemical interactions and to enable the dispersion of QDs in different solvents. Figure 1(a) is a typical structure of QDs, where the core of QDs is covered with shell and stabilizer (organic ligands).

Figure 1.

(a) Illustration of QD structure. (b) The illustration of Cd/Se QDs solution of various sizes with the responsive emitting wavelength. (c) The emission spectrum of QDs with different sizes. (d) CIE chromaticity diagram of HDTV vs. QDs. Figure reproduced with permission from (c) Ref. [2] ©2017 Royal Society of Chemistry. Figure reproduced with permission from (d) Ref. [3] © 2018 Springer Nature.

Quantum confinement is one of the most unique properties of QDs. De Broglie’s theory states that minuscule particles like electrons and holes can act like waves [6]. On the other hand, classical waves, like electromagnetic waves (light), can behave like particles (photons). This is called wave-particle duality. Thus, we can find that the difference of energy states is inverse to the square of the standing wave/diameter of QDs. Therefore, the different QD sizes will have different energy levels, which lead to different wavelengths. Figure 1(b) is a photograph of the Cd/Se QD solution in multiple sizes. When the size of QD is 2.5 nm, the emitting wavelength is 480 nm, while the diameter of QD is 6.3 nm, the emitting wavelength is 640 nm. It can be found that the smaller the size of QD, the shorter wavelength it emits.

Besides, the emitting wavelength of QDs can be changed by only changing the size of QDs. QDs are known to have a very narrow emission band. The full-width half-maximum (FWHM) value of the emission spectrum is applied to analyze the emission band; the smaller value of FWHM means a sharper emission spectrum peak, which results in a more saturated color [7]. Figure 1(c) is the emission spectrum of QDs with different sizes. The emission spectrum is narrow and sharp, with a smaller value of FWHM. Figure 1(d) illustrates that the color gamut value with QDs is much higher than HDTV’s standard color gamut value. The white dashed triangle clearly shows a smaller spanning color gamut area than the black dashed line, which shows that QDs can reach color coordinate points with wavelengths ranging from 460 to 620 nm.

1.2 Emerging perovskite quantum dots for display application

Besides quantum dots, perovskite has attracted attention due to its good optoelectronics properties and the various types of these kinds of materials. Perovskite, a mineral composed of calcium titanium oxide (CaTiO₃), was identified. A perovskite is a substance that has a crystal structure that can be described by the formula ABX₃. Due to the unique property, based on the radiative recombination, various types of materials together with different shapes like 0D nanocrystals (NCs), 1D nanowires (NWs), and 2D nanoplates (NPLs) are emerging for unique scenarios. Various types of perovskites have been applied in different optoelectronic applications such as solar cells, photodetectors, light-emitting diodes (LEDs), high-power lasers, and biosensors. Here, we introduce two methods to prepare the perovskite QDs: the hot injection method and the ligand-assisted reprecipitation method.

The hot injection technique was the first developed for the synthesis of II-VI group colloidal cadmium selenide (CdSe) QDs [8], and then, it has been widely used for synthesizing different colloidal quantum dots nanoparticles [9, 10, 11, 12]. During the hot injection synthesis, a cold stock solution with precursors is fast injected into the solution at a high temperature, which contains surfactant and a high boiling point solvent. The key to success is to maintain very little induction time between solution injection and particle precipitation. By this approach, bright colloidal perovskite with a defined size can be achieved in a supersaturated solution, which brings the development of large-scale application of QDs. In 2015, Protesescu et al. first proposed a highly efficient all-inorganic perovskite. These perovskites are highly luminescent colloidal quantum dots with narrow bandwidth and cube sizes around 4–15 nm. The materials composition with cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) has been produced by economic synthesis methods. By specific tuning of materials composition with quantum confinement effect. During the synthesis process, highly luminescent colloidal CsPbX₃ QDs were created by injecting a Cs-oleate precursor at temperatures ranging from 140 to 200°C. This injection induced the formation and enlargement of the QDs, which have adjustable emission spectra. The wavelength of the emitted light can be tuned across the entire visible range, making these QDs advantageous for display applications. The high luminescent CsPbX₃ QDs can achieve a photoluminescence quantum yield (PLQY) over ~90% for efficient display performance (Figure 2) [13].

Figure 2.

Illustration of synthesized perovskite for display applications.

Another synthesis method is the ligand-assisted reprecipitation method. High photoluminescence quantum yields (PLQYs) and intense electroluminescent (EL) emissions may only be attained with high excitation fluencies or current density due to nonradiative routes facilitated by sub-band defect states. The ligand-assisted reprecipitation technique is a powerful approach to the synthesis of organometal halide perovskite nanocrystals [14]. In 2015, Zhang et al. developed an approach by ligand-assisted reprecipitation strategy (LARP) to synthesize efficient colloidal CH₃NH₃PbX₃ (X = Br, I, Cl) quantum dots with 70% quantum yield at room temperature and low excitation fluencies, which can be applied for display application. The LARP synthesis involves the combination of CH3NH3PbBr3 precursors in a solvent (N-dimethylformamide, DMF) and mixing it with a vigorously stirred solvent (toluene, hexane, etc.) that has long-chain organic ligands. This process results in colloidal nanoparticles’ controlled formation through the precursors’ crystallization. The intense increase of exciton binding energy in QDs and their proper surface passivation account for the emission enhancements in CH₃NH₃PbX₃QDs [15]. Figure 3 is the synthesis method of the LARP strategy with the basic structure of the designed perovskite QDs.

Figure 3.

(a) Synthesis method of LARP strategy. (b) The basic structure of the designed perovskite.

2. Inkjet printing quantum materials for display applications

Printing quantum materials for display applications has been widely studied in recent years due to its low manufacturing cost and fast prototyping. Traditional techniques such as photolithography, microcontact printing, and nanoimprinting have drawbacks such as high cost, severe causticity, unpredictable transfer printing, and poor machining efficiency. On the other hand, inkjet printing, which was previously used to create full-color displays, has a straightforward procedure, is inexpensive, highly automated, and allows for selective patterning without the need for a mask [16]. Here, we review excellent high-resolution patterning methods that have been developed and are compatible with existing device manufacturing processes to achieve good optical properties.

The history of printing technology dates back more than 1700 years—furthermore, the majority of printing techniques deposit material on the substrate by use of several physical events. Relief is used in flexography and gravure, surface energy is used in offset lithography, masking is used in screen printing, material is replaced by high force and pressure in embossing, material is removed by laser ablation, and material is released by excessive force and energy in laser transfer [17]. A non-contact, mask-free, additive method for depositing films with any design is inkjet printing [7, 18, 19].

2.1 Inkjet printing

The inkjet printing method is divided into continuous and drop-on-demand (DOD). Within the DOD group, there are two different types of inkjet technologies: piezoelectric inkjet technology and thermal inkjet technology, often known as bubble jet technology. Regarding the droplet manufacturing method, thermal inkjet technology differs from piezoelectric inkjet technology. All the methods’ guiding concepts are shown in Figure 4.

Figure 4.

Schematic of three main categories of inkjet printing technology. (a) Continuous inkjet printing (b) Piezoelectric inkjet printing (c) Thermal inkjet printing.

The continuous inkjet printing method employs high-voltage deflection plates to redirect the steam of droplets electrically. Unsettled droplets are gathered by a gutter and then recirculated again, which is shown in Figure 4(a). The piezoelectric inkjet printing process utilizes a chamber containing a piezoelectric crystal that undergoes expansion or contraction in response to an applied bias. The deformation of this piezoelectric material generates a pressure wave that compels the material to expand and exit via an aperture, which refers to Figure 4(b). The thermal inkjet printing technology utilizes the heating of a vapor bubble to generate an enlarged bubble, generating an acoustic wave and ultimately expelling a droplet from the chamber [20]. Figure 4(c) is the schematic of thermal inkjet printing. Thermal inkjet printing is widely used for graphics and low-end color printing. Meanwhile, the piezoelectric inkjet printer has a higher level of sensitivity toward the ink used. It can print temperature-sensitive biological samples or inks with very low viscosity. As a result, it is the favored choice for printing functional materials. Furthermore, piezoelectric print heads are more capable of generating droplets of varying sizes compared to thermal inkjet printers.

The DOD piezoelectric printing method is widely applied in mass production of OLED and QD panel fabrication [19]. However, current DOD piezoelectric printing still needs to improve on two fundamental issues when stepping toward high-resolution pixels. First, the ink composition and ejection will affect the printing quality [18, 21, 22]. The precise control over the small jetting nozzles to prevent the dislocation of droplets is the critical parameter to ensure delicate patterning, which is strongly related to the jetting parameters. The inverse Ohnesorge number defines the jetting parameters:

$$Z=\frac{\sqrt{\sigma \rho d}}{\gamma }$$E1

Here, the Ohnesorge number depends on the ink viscosity (γ, mPa s), surface tension (σ, N m⁻¹), density (ρ, g cm⁻³), and printer nozzle diameter (d, μm); precise printing can be only realized by carefully designed jetting parameter with 1 < Z < 14. With a Z number larger than 15, the satellite droplets will come out together with the primary drop. Optimized solvents with binary or ternary solution systems have been designed for optimal printing conditions [23, 24].

Another problem lies in the formation of the coffee ring effect during solvent evaporation due to the different evaporation rates in the droplet profile. Figure 5 is the schematic of the coffee ring effect and the Marangoni flow. After the droplet lands on the substrate, the evaporation rate is greater near the edges of the droplet than near the center. Consequently, there is a greater dispersion of freedom from the central region to the periphery and from a capillary from the center to the edge. This capillary flow from the center to the edge brings the quantum materials to settle to the droplet edge and form the coffee ring effect. One of the methods to reduce the coffee ring effect is by adding a co-solvent into the quantum dots ink and creating a Marangoni flow from the edge to the center, against the capillary flow. In this way, the quantum dots distribute uniformly on the substrate without the coffee ring effect. Once the co-solvent has higher vapor pressure than the original solvent, and the surface tension of the co-solvent is also higher than the original solvent, and then the induced Marangoni flow would reduce the coffee ring effect. Figure 6 is the 3D morphology image of QD dots on the substrate with different ratios of co-solvent. In Figure 6, toluene was applied as the co-solvent in the decane QDs solution. It can be seen that when the co-solvents adding ratio is to a proper value, the coffee ring effect is solved [25, 26]. This is one step closer to inkjet printing high-resolution QD display.

Figure 5.

Schematic of coffee ring effect and Marangoni flow.

Figure 6.

The 3D morphology image of polymer QDs dots with different co-solvent ratios. (A) 3D morphology image of QDs dots. (B) QDs dots cross-section profile in x-direction and y-direction. Figure reproduced with permission from Ref. [25] ©2023, OAPA.

Besides directly printing QDs on the surface of the desired substrate, researchers also inkjet print the perovskite QDs into a polymer layer, known as in-situ inkjet printing (ISIP) strategy. SHI et al. utilized this technology to fabricate perovskite QDs patterns by printing perovskite precursor solution onto a polymeric layer, with PLQY around 80%. The high-resolution microdots array can be easily fabricated on the surface of the polymeric coating. This ISIP strategy paved the way for fabricating large area, bright luminescent, and color tunable micro-pattern APbX₃ (A = MA, FA, Cs, X = Cl, Br, I) PQDs, showing more advanced than the traditional inkjet printing [27], which requires carefully optimized ink composition and narrow process time window. Figure 7 is the process of in-situ inkjet printing of perovskite QDs polymer film.

Figure 7.

Process of in-situ synthesis of perovskite polymer.

2.2 Aerosol jet printing

Besides the inkjet printing method, aerosol jetting is also widely used in quantum materials deposition applications. The aerosol jetting stands out due to its high printing resolution and delivery speed. Figure 8 is the printing mechanism of aerosol jetting. As illustrated in Figure 8, the concentrated aerosol ink is generated in chamber ① using atomization techniques. Ultrasonic waves are used to create an aerosol from a modest amount of low-viscosity inks (1–500 cP) [28]. Next, the appropriate aerosol ink is conveyed to part ② using an inert carrier gas. The aerosol ink is then further transported to the focusing and deposition section of the aerosol jetting part ③. During the focusing and deposition process, an annular sheath gas is utilized to concentrate the aerosol gas, and the aerosol ink is subsequently applied onto the desired substrate. Kuo et al. demonstrated aerosol jet printing high-resolution CdSe/ZnS QDs color conversion film for full-color μLED display applications [29]. The research applied a photoresist (PR) mold to resolve the coffee ring effect problem. Figure 9(a-d) illustrates the process of reducing the coffee ring effect using PR mold. The RGB pixels display independent emission is shown in Figure 9(e).

Figure 8.

Illustration of aerosol jetting printing mechanism.

Figure 9.

(a)-(d) Process to prevent coffee ring effect by PR mold. (e) Illustration of RGB pixel emitting. Figure reproduced with permission from (a)-(e) Ref. [29] © 2017, Photonics Research.

3. Current and future trends in printing quantum materials in display applications

Quantum materials are functional semiconductor nanomaterials with special optical properties. Meanwhile, quantum materials are suitable for the solution process, which makes printing quantum materials a promising method in display manufacturing technology. In display technology, quantum materials have a sharp emission spectrum with tunable color by just changing the size. LCD display first applies quantum dots for color enhancement purposes, which further enhances the color performance and luminance value of LCD display. The market penetration of LCD displays equipped with QDs was estimated at about 5% in 2021.

Display markets are pursuing higher resolution, lower cost, and higher efficiency. Compared with traditional LCD TV panels, Samsung’s QD-OLED was first launched in 2022. With the quantum materials applied in OLED display, mini-LED display, and micro-LED display technology, using printing methods to deposit the quantum materials becomes a trend in new display technology. The inkjet printing method is first introduced to deposit quantum materials. However, through inkjet printing technology, there are three dominant challenges to obtaining high-quality printing results, which include the satellite effect problem affecting the jetting process, the coffee ring effect problem affecting the drying process once quantum materials deposit on the substrate, and stable layer to layer printing process involving the stable printing process [30, 31]. For the aerosol jetting method, the aerosol steam during the printing process always comes with satellite dots. However, these satellite dots cannot be controlled or predicted, affecting the display’s printing quality. Thus, people use masks to confine the printing area and control the printing process. Perovskite-based display systems, particularly those utilizing mini-LED/Micro-LEDs boosted by perovskite materials, will significantly improve display technology by offering superior quality with increased contrast, reduced power consumption, longer lifespan, and faster response times. Nevertheless, a perovskite-based panel is still needed to enhance the longevity [32, 33].

The remaining challenge of printing quantum materials for display applications is to achieve ultra-high-resolution patterning of quantum materials pixels. High-resolution displays require pixels that are more petite than the human eye’s resolution limit, about 300 pixels per inch (PPI) for average viewing distance. However, conventional methods of quantum materials patterning, such as inkjet printing and aerosol jetting, are limited by the nozzle size, the surface tension of the ink, and the substrate roughness, resulting in pixel sizes that are larger than 10 micrometers or about 100 PPI. Figure 10 shows the application of future trends in mini-LED/micro-LED with different panel sizes and resolutions. From digital display to AR, the panel size shows a downward trend. Meanwhile, the PPI shows a growing trend. Thus, to fulfill the requirement, a new printing method with a nanopipette was developed [34]. Different colors of quantum materials can be printed by varying the chemical composition of the ink. The nano-printing technique can produce quantum materials pixels with sizes as small as 100 nanometers, or about 2550 PPI, which is about 400 percent higher resolution than the latest high-end smartphones.

Figure 10.

Illustration of future trend mini-LED/micro-LED application with different panel sizes and resolutions.

Another method of quantum materials patterning is based on using a nanocrystal mask, a thin film of quantum materials nanocrystals that can be selectively etched by a laser beam. Scanning the laser beam over the nanocrystal mask allows a pattern of quantum materials pixels to be transferred to the underlying substrate. The nanocrystal mask technique can produce quantum materials pixels with sizes as small as 200 nanometers, or about 1270 PPI, comparable to a retina display’s resolution.

4. Conclusion

In conclusion, high-resolution quantum materials printing is a promising technique for display applications, as it can offer superior performance and functionality over conventional materials and methods. Future research and development trends would focus more on improving the lifetime of non-toxic quantum materials, stable printing ink development, and nano-printing techniques with higher stability and resolution. The commercial viability and widespread adoption of printing quantum materials for display applications will be achieved with the realization of stable nano-printing techniques with stable quantum materials ink.

Acknowledgments

The Foshan government funded this research through a grant to HKUST Foshan Research Institute for Smart Manufacturing, the project of the Hetao Shenzhen-Hong Kong Science and Technology Innovation Cooperation Zone (HZQB-KCZYB-2020083), and the Innovation and Technology Commission in Hong Kong under Grant (ITS/010/21).

Conflict of interest

The authors declare no conflict of interest.