Porous silicon (PS) prepared by the appropriate electrochemical anodization consists of a nanopore structure and residual quantum-size nanocrystalline silicon (nc-Si). It has tunable physical properties that can be radically different from those of single-crystalline bulk silicon and plays a role as a platform of functional devices. Possible applications are discussed here and some recent developments highlighted: efficient route for nc-Si colloid formation, quasiballistic electron emission, and thermo-acoustic effect.
Using self-standing PS layer flakes as a base material, nc-Si dot colloid has been formed efficiently by either optical or thermal energy. Highly luminescent (35% in quantum efficiency) colloidal nc-Si dots can be produced with a considerably higher yield (85%) from target than the values (1-10%) in the conventional processes.
A multiple-tunneling transport mode through nc-Si dot chain induces efficient quasiballistic hot electron emission from an nc-Si diode. Being a relatively low operating voltage device compatible with silicon planar fabrication process, the emitter is applicable to mask-less parallel lithography under an active matrix drive. It has been demonstrated that the integrated 100×100 PS emitter array is useful for multibeam lithography and that the selected emitter pattern is delineated corresponding to the activated emitters [1]. In addition, highly reducing activity of emitted electrons is applicable to liquid-phase thin film deposition of metals and semiconductors under a printing mode [2,3]. Both the efficiency and the energy distribution are remarkably improved by using monolayer graphene as a surface electrode [4].
Due to a complete thermal insulating property of nc-Si layer retaining a low volumetric heat capacity, on the other hand, thermo-acoustic conversion is significantly enhanced. When a temperature fluctuation is produced at the surface of nc-Si layer, an acoustic wave is generated near the device surface without any mechanical vibrations. The non-resonant and broad-band emissivity with no harmonic distortions makes it possible to use the emitter for generating audible sound under a full digital drive [5] and reproducing complicated ultrasonic communication calls between mice [6].

References
1. A. Kojima et al., Proc. SPIE Adv. Lithography Symp. 9777, 977712 (2016).
2. R. Suda et al., J. Electrochem. Soc. 163(6), E162 (2016).
3. R. Suda et al., Mater. Sci. Semicond. Process 70, 44 (2017).
4. A. Kojima, R.Suda, and N. Koshida, Appl. Phys. Lett. 112 (2018) (accepted for publication).
5. N. Koshida et al., Appl. Phys. Lett. 102, 123504 (2013).
6. K. Mogi et al., Behav. Brain Res. 325, 138 (2017).