Lithium fluoride crystals were subjected to electron beam irradiation at 200 and 300 keV using transmission electron microscopy in order to study in situ fabrication of Li nanostructures. We observed that LiF crystals decompose in a unique way different to all other metal halides: Fluorine ablation and salt-to-metal conversion is non-local and due to a rapid lateral diffusion of Li, the life cycle from nucleation to annihilation of fresh Li nano-crystals can be observed at a distance from the Li-source, the irradiated salt. Growth, shape transition and annihilation of Li nanostructures follow at slow enough speed for live video recording with resolution of 25 frames per second. The equilibrium shapes of pure Li nano-crystals range from cubic to rod-shaped and ball-shaped and up to 300 nm size. By varying the e-beam flux of irradiation, transitions from cube to spherical shape can be induced cyclically.

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Dielectric microspheres with extraordinary microscale optical properties, such as photonic nanojets, optical whispering-gallery modes (WGMs), and directional antennas, have drawn interest in many research fields. Microsphere-enhanced Raman spectroscopy (MERS) is an alternative approach for enhanced Raman detection by dielectric microstructures. Unfortunately, fabrication of microsphere monolayer arrays is the major challenge of MERS for practical applications on various specimen surfaces. Here we report a microsphere-embedded film (MF) by immersing a highly refractive microsphere monolayer array in the poly(dimethylsiloxane) (PDMS) film as a flexible MERS sensing platform for one- to three-dimensional (1D to 3D) specimen surfaces. The directional antennas and wave-guided whispering-gallery modes (WG-WGMs) contribute to the majority of Raman enhancement by the MFs. Moreover, the MF can be coupled with surface-enhanced Raman spectroscopy (SERS) to provide an extra >10-fold enhancement. The limit of detection is therefore improved for sensing of crystal violet (CV) and Sudan I molecules in aqueous solutions at concentrations down to 10 M. A hybrid dual-layer microsphere enhancer, constructed by depositing a MF onto a microsphere monolayer array, is also demonstrated, wherein the WG-WGMs become dominant and boost the enhancement ratio >50-fold. The present work opens up new opportunities for design of cost-effective and flexible MERS sensing platforms as individual or associated techniques toward practical applications in ultrasensitive Raman detection.

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