# Mini-LED and Micro-LED: Promising Candidates for the Next Generation Display Technology
## 3 Micro-LED
### Color Conversion Full-Color Display
#### 3.2.2 Quantum Dots  
To realize full-color display, colloidal QDs can be a great choice. The QDs are usually synthesized by chemical solution process [44] and possess unique properties such as high quantum yield, size-dependent emission wavelength, and narrow emission linewidth [45; 46]. Combining UV micro-LEDs and RGB QDs together, Prof. [PERSON] et al. demonstrated a method to achieve full-color high-quality micro-LED displays via the aerosol jet (AJ) technique [47; 48]. Additionally, a photoresist (PR) mold is fabricated to limit the optical cross-talk effect, and the QDs are deposited into the PR mold with clear separation between each other. Furthermore, a distributed  
Figure 11: Mechanism of color conversion full-color display. Figure 12: (**a**) One colorful pixel includes four sub-pixels in combination of RGGB and (**b**) a full-color display by excitation of RGB phosphors by UV LEDs [41]. Figure reproduced with permission from John Wiley and Sons. Bragg reflector (DBR) covered above the PR mold can significantly enhance both the utilization of UV light and the emission intensity of RGB QDs. As the core-shell type with ligands attached to the outer shells, RGB QDs in the experiment have the average particle sizes about 9.3 nm, 6.2 nm and 2.5 nm, and the emission spectra of them are 630 nm, 520 nm and 450 nm, respectively. Figure 13 shows the absorption and emission spectra as well as images of blue, green, and red QDs solution, respectively. Moreover, due to the narrow emission linewidths of RGB QDs, large color gamut can be readily achieved. In order to fully excite RGB QDs and greatly raise color quality, an UV micro-LED array is prepared as an excitation source on an UV epitaxial wafer. The emission wavelength of the excitation source is 395 nm, while the size of micro-LED chip is 35 um \(\times\) 35 um, as shown in Figure 14a. For reducing the cross-talk effect of QDs, a PR mold with open windows and blocking walls is fabricated via the simple lithography technique. The definition of the mask for the PR mold fabrication is adopted with the pitch size of 40 um as the same as the micro-LED array. By aligning the mold windows to the micro-LED mesa, as shown in Figure 14b-e, UV micro-LEDs can accurately excite RGB QDs without the cross-talk effect. The AJ system consists primarily of an ultrasonic atomizer and a spraying chamber (Figure 15). Inside the ultrasonic atomizer, ultrasonic vibrations first atomize the RGB QD suspension. Then, a nitrogen gas flow transfers the AJ to the nozzle to becoming small drops. Ultimately, QDs are deposited on the substrate with high resolution and narrow linewidth. Table 4 lists the optimized parameters of the AJ system. The working distance and stage speed are fixed according to the experimental experience. Different sizes of the QDs will affect the deposition conditions. Therefore, different flow rates are required to control the AJ system. Generally, as the particle size of QDs increases, both the carrier gas flow rate and the sheath gas flow rate need to be increased to maintain the linewidth of the deposited QDs, which is about 35 um in the study. Figure 13: Absorption and emission spectra of (**a**) blue, (**c**) green and (**e**) red QDs as well as solution photos of (**b**) blue, (**d**) green and (**f**) red QDs [47]. Figure reproduced with permission from Optical Society of America. Figure 16a demonstrates the RGB pixel of the micro-LED display after the deposition of QDs on the PR mold lying above micro-LEDs. In this manner, RGB pixels have very clear boundaries between one another Figure 16b, compared with the previous case where cross-talk effect is observed without any PR mold Figure 16c. Therefore, the blocking walls in the PR mold can efficiently confine QDs to avoid the cross-talk effect.