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Thin films of BaFCl:Sm nanocrystals prepared using a polymer binder were used to create fluorescence images. The phosphor films were exposed to a UV-C mercury lamp light source via chromium-coated quartz greyscale masks to create 4 μm resolution greyscale fluorescence images. The mechanism relies on the highly efficient conversion of Sm³⁺ to Sm²⁺ ions upon exposure to UV-C light which displays a large linear dynamic range. The red fluorescence around 688 nm of the Sm²⁺ is then read-out using blue-violet illumination under a laser scanning confocal microscope. The greyscale images with 16 greyscale levels had a resolution equivalent to ∼125 line pairs per mm or ∼6400 dpi. Improvements in the resolution would be possible using collimated UV-C laser exposure of the film or the use of higher resolution photomasks. Ultra-high resolution binary fluorescence images were also created with resolutions down to 2 μm (∼250 line pairs per mm, ∼12 700 dpi). Downstream applications of the technology could include tailored covert or overt anti-counterfeiting labelling. Refereed/Peer-reviewed

Persistent spectral hole-burning is shown to be suitable for multiple bit per point optical data storage at room temperature. In a novel approach employing bespoke tailoring of the effective inhomogeneous line width and line shape, by using a physical mixture of chemically different matlockite nanocrystals, a relatively broad fluorescence emission line with a top-hat distribution can be realized at the diffraction limit and at room temperature. This top-hat profile enables multiple spectral holes to be burnt, all having the same baseline. We demonstrate this with two spectral holes burnt in a diffraction limited spot, with the prospects for multihole-depth or multilevel encoding to increase the number of bits stored per spectral hole. Using thermoelectrically cooled temperatures down to 175 K, a larger number of holes is also demonstrated. The technique presented is predicted to allow for optical data densities approaching terabits/cm2 by exploiting the unique properties of these inorganic phosphors. Refereed/Peer-reviewed

There is great interest in the use of highly-efficient all-inorganic halide perovskites CsₙPbBr₂₊ₙ for optoelectronic applications. There however remains considerable debate as to the origins of the green luminescence in the zero-dimensional phase of the perovskite Cs₄PbBr₆, with theories suggesting it originates either from defects in the Cs₄PbBr6 lattice or CsPbBr₃ impurities/inclusions. The confusion has arisen due to the two phases being miscible and typically co-existing. Moreover, low impurity levels of CsPbBr₃ in Cs₄PbBr₆ are difficult to detect by XRD measurements, yet have much stronger photoluminescence than bulk CsPbBr₃ that exhibits quenching, further contributing to the confusion as to the origins of the green photoluminescence. With the rise of significant debate and misconceptions, we provide conclusive evidence that the green emission from Cs₄PbBr₆ is indeed due to nanocrystalline CsPbBr₃ impurities. This is demonstrated by undertaking cathodoluminescence and EDX measurements on samples prepared mechanochemically by ball-milling. Cathodoluminescence imaging clearly shows the presence of small crystals embedded in/or between larger crystallites of Cs₄PbBr₆ and they emit around 520 nm. EDX shows that the smaller crystal inclusions have a Pb : Br ratio that is approximately 2 times higher, confirming the CsPbBr₃ phase, which has the expected size-dependent shift to shorter wavelengths (about 528 to 515 nm). These studies make significant inroads into understanding these lead halide perovskites for their use in a variety of optoelectronic and photovoltaic applications. Refereed/Peer-reviewed

Amongst significant debate we provide conclusive proof showing that the green luminescence from Cs4PbBr6 perovskites is in fact due to nanocrystalline CsPbBr3 impurities. This is verified with cathodoluminescence imaging and EDX measurements Refereed/Peer-reviewed

We present results demonstrating the prospects of samarium-doped nanocrystals for use in ultra-high capacity multilevel optical data storage. Optical data storage is demonstrated through fluorescence tuning of MeFCl:Sm (Me: Ba, Sr) in the deep UV with a 7 orders of magnitude linear dynamic range.

Novel approaches for digital data storage are imperative, as storage capacities are drastically being outpaced by the exponential growth in data generation. Optical data storage represents the most promising alternative to traditional magnetic and solid-state data storage. In this paper, a novel and energy efficient approach to optical data storage using rare-earth ion doped inorganic insulators is demonstrated. In particular, the nanocrystalline alkaline earth halide BaFCl:Sm is shown to provide great potential for multilevel optical data storage. Proof-of-concept demonstrations reveal for the first time that these phosphors could be used for rewritable, multilevel optical data storage on the physical dimensions of a single nanocrystal. Multilevel information storage is based on the very efficient and reversible conversion of Sm³⁺ to Sm²⁺ ions upon exposure to UV-C light. The stored information is then read-out using confocal optics by employing the photoluminescence of the Sm²⁺ ions in the nanocrystals, with the signal strength depending on the UV-C fluence used during the write step. The latter serves as the mechanism for multilevel data storage in the individual nanocrystals, as demonstrated in this paper. This data storage platform has the potential to be extended to 2D and 3D memory for storage densities that could potentially approach petabyte/cm³ levels. Refereed/Peer-reviewed

We demonstrate that exposure of nanoctystalline BaFCl:Sm3+ X-ray storage phosphor to blue laser pulses with peak power densities on the order of 10 GW/Cm-2 results in conversion of Sm3+ to Sm2+. This photoreduction is found to be strongly power-dependent with an initial fast rate, followed by a slower rate. The photoreduction appears to be orders of magnitude more efficient than that for previously reported systems, and it is estimated that up to 50% of the samarium ions can be photoreduced to the divalent state. The main mechanism is most likely based on multiphoton electron-hole creation, followed by subsequent trapping of the electrons in the conduction band at the Sm3+ centers. Nanocrystalline BaFCl:Sm3+ is an efficient photoluminescent X-ray storage phosphor with possible applications as dosimetry probes, and the present study shows for the first time that the power levels of the blue light have to be kept relatively low to avoid the generation of Sm2+ in the readout process. A system comprising the BaFCl:Sm3+ nanocrystallites embedded into a glass is also envisioned for 3D memory applications. Refereed/Peer-reviewed

We report on the highly efficient generation of stable Sm²⁺ in nanocrystalline BaFCl:Sm³⁺ by exposure in the deep UV (UV-C) below 250 nm. The generated Sm²⁺ can be read out via its distinct fluorescence signature which is efficiently excited around 425 nm by the relatively strong f-d transitions with ε ≈400 (l mol-1 cm-1). The generation of Sm²⁺ can also be reversed, erasing the fluorescence signal via two-photon ionization by increasing the power at 425 nm from ≤ 1 mW/cm2 to ~100 mW/cm2. It follows that the switching mechanism is based on oxygen impurities that are in close proximity to the Sm³⁺ ions. The photoionization kinetics indicate that the average Sm³+- oxide impurity separation is a few interionic spacings. The level of Sm³⁺- Sm²⁺ conversion in BaFCl is shown to be tunable over a large dynamic range, and therefore could serve as a platform for rewritable ultra-high density multi-level optical data storage. The present study also sheds light on BaFCl:Sm³⁺ as a photoluminescent X-ray storage phosphor. Refereed/Peer-reviewed