Aggregation-induced emission (AIE) luminescent materials boosting optical storage into the new era of petabit-level capacity

Abstract

The recording of information stands as the most significant milestone in human civilization. Historically, the recording and storage of information have undergone a technological evolution from paintings to carvings, scribing, and digitization. The invention of optical compact discs (CDs) was one of the major landmarks in digital information technology. Over the past half-century, scientists have endeavored to enhance optical storage capacity by improving both optical systems and optical storage materials, as shown in Scheme 1. In terms of commercial products, the storage capacity has increased from 700 MB (CDs) to 27 GB (Blu-ray discs) by optimizing the optical system based on the same optical storage medium (polycarbonate). To surpass the conventional optical diffraction limit, the optical systems have evolved from traditional lasers to nonlinear two-photon absorption (TPA) and stimulated emission depletion (STED), which has minified the laser spot size from microns to approximately tens of nanometers, marking a remarkable achievement. However, luminescent materials are essential to enable TPA and STED technology to be applied in optical storage.

The traditional photopolymers or photoresists usually possess weak emission, and doped with rare earth phosphors, up-conversion nanoparticles, carbon dots, nanoclusters, and metal nanorods that can lead to enhanced fluorescence intensity, but the inorganic nanoparticles in photopolymers may induce light scattering. Traditional conjugated organic chromophores produce quenching upon solid-state aggregation. The green fluorescent protein (GFP) doped in photoresist achieved a reversibly switchable record and detected sub-diffraction site spacing of approximately 200 nm,^[1] but the stability of GFP is far from meeting the requirements of optical storage. The above methods only enhance the fluorescence intensity of photopolymer film but cannot improve the fluorescence contrast of recorded and unrecorded areas. Therefore, the fluorescence signal radiated by the super-resolution point is easily submerged in the background noise, and the information cannot be read correctly.

Writing in Nature, Zhao et al.^[2] reported that a photoresist film doped with aggregation-induced emission luminogens (AIEgens) enables a volumetric nanoscale optical storage system, achieving an impressive 1.6 petabit super disk.

The concept of aggregation-induced emission (AIE) was proposed in the year of 2001.^[3] AIEgens usually have rotors or isomerizable double bonds, resulting in weak or no emission in single molecular states (dilute solution) and enhanced emission in the aggregate form due to the restriction of intramolecular motion (RIM). After more than two decades of development, thousands of AIEgens have been designed and synthesized, and their emissions cover from ultraviolet to near-infrared. AIEgens-doped photoresist film enhances the fluorescence intensity and signal-to-noise ratio simultaneously because their fluorescence properties are greatly affected by the microenvironment. Hao's group first applied AIEgens (Tetraphenylethylene, TPE) to the optical storage and successfully realized 3D fluorescence storage with an areal density of 200 Gbits/cm³ by TPA in 2019.^[4-7] Further development in recent Nature work,^[2] they utilized another AIEgen (Hexaphenylsilole, HPS) doped photoresist (DDPR) and achieved a capacity of 1.6 Pb. The writing process is shown in Figure 1A, which could be labeled as three states. The first state was the as-prepared colloidal mixture of HPS and photoresist precursors, and the second state was UV-curing film. UV-curing resulted in initial polymerization to a homogeneous cross-linked thin film (HPS-DDPR), transforming from the colloidal first state to the solid second state, which enhanced fluorescence intensity due to solid state restriction of rotation of the peripheral phenyl rings of HPS and blocked the non-radiative pathway. Subsequently, under irradiation with a 515-nm femtosecond writing laser beam, the irradiated spots were further polymerized in the third state, in which the HPS fluorescence shows an exceptional brightness as the local microenvironment packing tighter and further inhibited intramolecular motion. The fluorescence intensity of the irradiated spots showed a good correspondence with the laser power. Last but not least, the unrecorded area showed weaker emissions than the recorded spot. This ultimately results in a recorded spot of super-resolution size, which can be read by all-optically differentiating the fluorescence intensities of recorded and unrecorded areas with a high signal-to-noise ratio and low bit error rate (Figure 1B).

The success of HPS is also because its excitation wavelength (480 nm) does not induce polymerization of the photoresist, and its emission wavelength is close to the emission of the photoresist. Therefore, multiple readings of the disc can be achieved. The stability of HPS in daylight and ambient conditions was also desired by optical storage. Besides, the high transmittance, good compatibility with photoresists, and flexibility and processable of HPS facilitated 100-layer volumetric nanoscale optical storage system (Figure 1C). HPS perfectly makes up for the shortcomings of other luminous materials in the field of optical storage, a finishing touch for boosting optical storage into the era of petabit-level capacity.

Due to the sensitivity of AIEgens’ fluorescence properties (both intensity and wavelength) to the microenvironment, it is believed that AIEgens-doped photopolymer can enable more extensive applications in optical storage. Furthermore, AIEgens with circular polarization light and room temperature phosphorescence may open new windows for anti-counterfeiting and encryption information storage. At the same time, we have also noticed that cluster luminescence, which has developed rapidly in recent years, is also one of the luminescence mechanisms applied in photoresists. Fluorescence with tunable wavelength and high quantum yield can be obtained based on cluster luminescence, which may be an improvement on traditional photoresists to achieve a high signal-to-noise ratio without any doping. In summary, due to the emergence of new materials and new emission mechanisms, especially the AIEgens and cluster luminescence, the selectivity of optical storage media is more abundant, which is a great opportunity to improve the capacity of optical storage.

The authors declare no conflicts of interest.