image: The multi-excitonic emission characteristics of monodisperse CdSe-based QDs under pulsed laser excitation.
Credit: ©Science China Press
Colloidal quantum dots (QDs) are semiconductor nanocrystals that have garnered extensive attention from both academic and industrial community in recent years, owing to their excellent and widely tunable optoelectronic properties as well as their high compatibility with low-cost solution-based synthetic and processing techniques. The 2023 Nobel Prize in Chemistry was awarded to the three inventors of QDs—Moungi G. Bawendi, Louis E. Brus, and Alexei I. Ekimov, marking a significant impact of QD technology on the development of human science and technology.
As the size of QDs is typically smaller than or close to their exciton Bohr radius, the distribution of excitons is constrained by the three-dimensional quantum confinement effect, resulting in an atomic-like discrete distribution of the density of states. Moreover, the bandgap can be adjusted by tuning their composition, size, and/or structure to meet requirement of various applications. Typically, the optical properties of the monodisperse QDs are relatively determined and stable after synthesis. For instance, the work presented here utilizes CdSe-based QDs with an average particle size of 12.6 nm, and the QD film constructed from these exhibits a spontaneous emission peak at 631 nm with a full width at half maximum (FWHM) of 23 nm at room temperature. Regardless of being driven electrically or illuminated by a blue LED, the luminescence of these QDs exhibits a bright red color, which remains essentially unchanged over time. However, under specific conditions, such as when exposed to pulsed laser irradiation with a very high peak power density, it is possible to simultaneously excite multiple electrons to the conduction band of the QDs, thereby triggering multi-excitonic radiative transitions. Due to the two-fold degeneracy of CdSe-based QDs, the band-edge 1Se (1Shh) orbital can accommodate up to two electrons (holes). Therefore, when the single pulse energy density of the pump laser is sufficiently high to excite more than two excitons simultaneously, electrons can be pushed to higher energy orbitals such as 1Pe, triggering higher energy radiative transitions (as shown in Figure 1b). At this point, the emission spectrum of the QDs will exhibit a broad band and multiple peaks, with the originally red-emitting QDs changing to an orange-yellow color. It is particularly noteworthy that the multi-exciton states of QDs not only support the generation of broadband, multicolored light but also essentially constitute a state of population inversion. This provides a prerequisite for broadband gain, amplification, and even lasing, broadening the tunable range of optical properties of monodisperse QDs and making full-color emission and even lasing based on monodisperse QDs possible.
However, despite in-depth research on the multi-excitonic characteristics of QDs by scholars such as V. Klimov since the end of the last century, the practical application of these properties remains challenging. Specifically, there are several challenges and issues that need to be addressed:
1. Exciting QDs to a multi-exciton state typically requires an extremely strong pumping source. For instance, the multi-excitonic states of CdSe-based QDs reported previously are often excited by femtosecond pulsed lasers, which have a large size (comparable to a computer tower), high power consumption (hundreds of watts), and a high cost (tens to hundreds of thousands of RMB), all of which hinder the practical applications of the multi-excitonic characteristics of QDs.
2. Temporal/spatial separation of various colors in the broadband multi-excitonic emission spectrum. For applications such as display and optical communication, a narrow emission linewidth is usually required. Therefore, it is necessary to separate the various colors from the broad multi-excitonic emission spectrum; at the same time, in display applications, to achieve full-color patterning, it is necessary to precisely adjust the spatial distribution of each color pixel. Thus, it is also necessary to spatially separate the various colors in the multi-exciton emission spectrum to achieve the definition and separation of each color pixel.
3. High-density, large-area, low-cost pixel patterning. Based on effective temporal/spatial separation of colors, how to efficiently define and arrange each color pixel based on monodisperse QDs to present a specific spatial distribution and arrangement is also a key issue that needs to be resolved for practical applications, including display technology.
Recently, a collaborative team led by Professor Kai Wang from Southern University of Science and Technology and Professor Hoi Wai Choi from University of Hong Kong has conducted a research on the manipulation of multi-excitonic emission properties of the monodisperse QDs. To address the challenges and bottlenecks such as the high excitation intensity required for multi-excitonic emission of QDs, the broad spectral linewidth, and the difficulty in patterning multicolor pixels, they proposed a method to manipulate the multi-excitonic behavior by coupling the QDs with a microcavity that possess strong Purcell effect and optical field modulation capabilities. Utilizing angle-resolved spectroscopy and numerical simulation, they studied the anisotropic polychromatic emission characteristic of the monodisperse QDs induced by the multi-excitonic emission coupled to microcavity. This work has overcome the technical bottlenecks for the practical application of multi-excitonic emission of QDs, achieving multi-exciton emission pumped by quasi-continuous laser and full-color micro-pixel array patterning based on monodisperse QDs. It has expanded the adjustable range of optical properties of monodisperse QDs and laid the foundation for the practical application of multi-excitonic emission of QDs. This achievement has been published in National Science Review , with the title "Low-threshold anisotropic polychromatic emission from monodisperse quantum-dots", where Professor Wang and Professor Choi are the corresponding authors, and doctoral student Tan Yangzhi is the first author.
To address the issues aforementioned, the research team designed an optical microcavity with unique angular dispersion characteristic and a strong Purcell effect. QDs were coupled into the microcavity, and a laser pump was used to excite the QDs to their multi-excitonic states. The red and green emissions from the multi-excitonic emission spectrum of QDs were extracted at different angles, thus enabling the observation of anisotropic polychromatic emission (APE) behavior (as shown in Figure 2). Further research revealed that the threshold and intensity of the APE are related to the optical loss and the Purcell effect within the cavity. By adjusting the field distribution of the optical modes within the microcavity, more than a threefold enhancement in the extraction efficiency and a reduction of over 34% in the APE threshold were achieved. Thanks to the reduced optical loss and the enhanced Purcell effect, APE with a threshold of no more than 5 W/cm2 was observed under quasi-continuous laser pumping. This threshold is significantly lower than previously reported for works related to the multi-excitonic emission of QDs, indicating that the multi-excitonic emission of QDs, could potentially be driven by portable and low-cost laser diodes or even LEDs, highlighting the application potential of the interesting multi-excitonic characteristics of QDs.
Furthermore, to achieve multicolor pixel patterning based on monodisperse QDs, the research team developed a method that patterning the optical microcavities to spatially separate and arrange red/green pixels. By employing a simple patterning process of silver thin film with fine metal mask, the team realized an array of red-green micro-pixels based on monodisperse QDs. Additionally, in conjunction with a blue LED, they further achieved a red-green-blue full-color micro-pixel array (as shown in Figure 3), demonstrating the feasibility of full-color display using only monodisperse QDs in combination with a blue light source. This showcases the potential for the display applications of multi-exciton emission from monodisperse QDs, offering new avenues of inspiration.