Ultralow-power nanoscale optical data storage in a nanocomposite based on up-conversion nanoparticles and graphene oxide

As an inevitable outcome of the `information revolution', a huge amount of digital information needs to be stored and continuously accessed over long-term periods. Current data centres are insufficient to tackle this demand and researchers must deal with the formidable responsibility of developing data storage memories with greater capacity, extended lifetime and minimal energy consumption. Optical data storage is a particularly encouraging approach to supply the required storage needs because of its exceptional performance and durability. Nevertheless, it is crucial to increment the capacity of contemporary optical media. Further increments in capacity can be accomplished by overcoming the diffraction-limit barrier and thus shrinking the data bit size to the nanoscale. <br><br>The recent invention of stimulated emission depletion (STED) microscopy and super-resolution photoinduction-inhibited nanolithography (SPIN) have permitted nanoscale imaging and writing of features, respectively, thereby offering the potential for data storage towards Petabytes. However, a suitable medium for nanoscale optical data storage is still lacking. Among the potential candidates, up-conversion nanoparticles appear to be very promising because they exhibit a fluorescence lifetime which is hundreds to thousands of times longer than other fluorophores, and provide a pathway for super-resolution optical activation with extremely low power required. Further, graphene oxide enables data recording through its reduction. <br><br>The fundamental aim of this PhD thesis is to achieve ultralow-power nanoscale optical data storage in an up-conversion nanoparticle- and graphene oxide-based nanocomposite. The optical activation is enabled by the up-conversion nanoparticles, while the data recording is achieved in graphene oxide through its reduction via resonance energy transfer. Further, the metastability of the energy levels in the up-conversion nanoparticles with a lifetime of up to milliseconds enables SPIN and STED microscopy for data writing and read-out, respectively, with low laser intensities. Thus, optical data storage with ultralow energy consumption can be obtained, which also guarantees a long lifetime for the optical memory devices.<br><br>The photochemical reduction of thin-film graphene oxide integrated with up-conversion nanoparticles under high-energy laser irradiation for the modulation of up-conversion fluorescence is first demonstrated. This provides a stepping stone towards encoding and retrieval in an up-conversion nanoparticles-graphene oxide nanocomposite. Initially, a 375-nm CW laser is used to photochemically reduce graphene oxide. The absorption change in the nanocomposite is accompanied by the quenching of the fluorescence from the up-conversion nanoparticles. Subsequently, a CW laser at 980 nm is used to excite the up-conversion nanoparticles, and detection of the decreased up-conversion fluorescence up to ~90% associated with quenching by reduced graphene oxide is achieved. Further, the fast reduction speed of using high-energy laser allows a decrease in the time to modulate up-conversion fluorescence down to milliseconds for individual pixels. Optical patterns are successfully achieved and retrieved distinctly using the up-conversion fluorescence quenching by reduced graphene oxide.<br><br>High-energy up-conversion fluorescent nanoparticles are developed for application in dual-laser photoactivated super-resolution systems, including their combination with graphene oxide. Theoretical and experimental investigations are conducted to individuate the key features of the nanoparticles for efficient photoactivation in applications beyond the diffraction-limit barrier in energy transfer-driven systems. The nanoparticles are then tested under dual-beam super-resolution optical techniques, such as STED microscopy. First, optical depletion efficiency of 450-nm up-conversion fluorescence up to ~90% is achieved for 4% Tm-doped nanoparticles with a saturation intensity of ~375 kW cm/2 (~1.5 mW). Second, super-resolution imaging of the up-conversion nanoparticles is undertaken via STED microscopy using a Gaussian-shaped 980-nm CW laser for excitation and a doughnut-shaped 808-nm CW laser for depletion, reaching a resolution of 64 nm with an intensity of 11.25 MW cm-2. The values of intensity of the excitation and depletion beams are one hundredth to one thousandth of those required for typical STED imaging using other fluorophores, which is very attractive for applications with low power consumption in photoactivated systems.<br><br>The high-energy up-conversion fluorescent nanoparticles are then conjugated with single-layer graphene oxide nanosheets, obtaining an efficiency of ~83% of resonance energy transfer in the up-conversion nanoparticles¿graphene oxide system. Demonstration of the photochemical reduction of graphene oxide in the nanocomposite is then reported under near-infrared laser excitation. Specifically, a CW laser at 980 nm is used to excite the up-conversion nanoparticles and produce up-conversion to levels with high energy, which triggers the dissociation of the oxygen groups in graphene oxide for reduction through resonance energy transfer. This mechanism demonstrates a suitable pathway for optical data writing. Two channels are then used to monitor the reduction process, which can thus be used for optical data reading: the quenching of 450-nm fluorescence from the up-conversion nanoparticles, which is quenched by ~40%, and the decrease of 650-nm two-photon excited fluorescence from graphene oxide, which is reduced by ~50%.<br><br>The up-conversion nanoparticles-graphene oxide nanocomposite is tested under dual-laser irradiation comprising a CW laser at 980 nm for excitation and CW laser at 808 nm for depletion. A value of ~95% optical depletion efficiency of the 450-nm up-conversion fluorescence is achieved. The saturation intensity is ~250 kW cm¿2 (~1.0 mW), corresponding to several orders of magnitude reduction of energy consumption by using this novel nanocomposite to encode information bits in comparison with other nanomaterials used in super-resolution optical methods. In addition, the 808-nm CW laser can effectively inhibit up-conversion to high-energy levels in the up-conversion nanoparticles, resulting in prevented reduction of graphene oxide. Finally, ultralow-power nanoscale optical data storage is achieved in the nanocomposite. Nano-sized features with a size of 61 nm are produced by SPIN and subsequently retrieved by fluorescence quenching microscopy combined with STED microscopy, obtaining an estimated capacity of ~200 TB per disc. <br><br>The work included in this PhD thesis provides a comprehensive insight into the demonstration of ultralow-power nanoscale optical data storage in a novel nanocomposite based on up-conversion nanoparticles and graphene oxide. This achievement is a conceptual breakthrough compared with the current state-of-the-art optical data storage systems and will enable to encode optical data bits beyond the diffraction-limit barrier. Hence, increased capacity of individual optical data storage memories is expected for sustainable growth.