Introductory lecture: origins and applications of efficient visible photoluminescence from silicon-based nanostructures

A variety of silicon-based nanostructures with dimensions in the 1-5 nm range now emit tunable photoluminescence (PL) spanning the visible range. Achievement of high photoluminescence quantum efficiency (PLQY) relies critically on their surface chemistry passivation and an impressive tool box of options have been developed. Two distinct PL bands are dominant. The S-Band (red-green emission withSlowmicrosecond decay) has PLQY that has steadily improved from similar to 3% in 1990 to 65 +/- 5% by 2017. The F-Band (blue-yellow withFastnanosecond decay) has reported PLQY values that have improved from similar to 0.1% in 1994 to as high as similar to 90% by 2016. The vast literature on both bands is surveyed and for the S-band, size-structure-PL correlations and selective photo-excitation studies are highlighted. Resonant photoexcitation and single quantum dot studies have revealed the key role of quantum confinement and the excitonic phonon-assisted nature of the radiative transitions. For the F-band, in contrast, specific phenomenological studies are highlighted that demonstrate similar emission without the presence of silicon nanostructures. Low PLQY F-band emission from pure silicon-silica core shell systems is probably associated with oxide-related defects, but ultrahigh PLQY from many lower temperature synthesis routes is likely to be from carbon-based nanostructures or chromophores, not silicon nanostructures. Potential applications for both PL bands include sensing, medical imaging, theranostics, photovoltaics, LED colour converters and nano-thermometry. Emerging green synthesis routes are mentioned. If scalability and cost are significantly improved then a number of other proposed uses of ultra-efficient PL from nano-Si could become viable in cosmetics, catalysis, security and forensics.