Abstract
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 with Slow microsecond decay) has PLQY that has steadily improved from ∼3% in 1990 to 65 ± 5% by 2017. The "F-Band" (blue-yellow with Fast nanosecond decay) has reported PLQY values that have improved from ∼0.1% in 1994 to as high as ∼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.
Highlights
This review focuses on the visible photoluminescence of silicon nanostructures; collating both experimental and theoretical work on most types of nanostructure; providing quantitative performance metrics, on photoluminescence quantum yield (PLQY); photoluminescence wavelength tunability (PLWT) and photoluminescence decay times (PLDT)
Accurate measurement of PLQY is not as straightforward as measuring PL decay times and a number of different protocols exist, but they can be divided into comparative ones that utilize photoluminescence standards of known PLQY and absolute ones that directly measure the fraction of absorbed photons that are emitted as photoluminescence or the fraction of photons lost by non-radiative recombination
Remarkable progress has been made in raising the efficiency of visible emission from silicon-based quantum dots, superlattices and porous silicon over the last 30 years, with efficiencies of S-band red emission around 60–70% and green to blue F-band emission around 80–90% being the highest reported values
Summary
Nanostructuring has introduced an exciting new capability to semiconducting silicon: the ability to emit visible light very efficiently under photoexcitation. The review covers six application areas of efficient visible PL: sensing, photovoltaics, medical imaging, theranostics, white LEDs and nanothermometry. It proposes a number of ways in which the eld might further progress, and other application areas that might emerge if cost and scalability issues are surmounted. Electroluminescence, optical gain, and thereby optoelectronics, are omitted in order to entirely focus here on the potential of photoluminescent properties Such a platform property is o en combined with other crucial ones for speci c uses, such as biodegradability and biocompatibility for medical applications. This review collates and discusses some of the very large amount of data available on many types of photoluminescent silicon nanostructures and many applications thereof. It is hoped to be of use both as a broad introduction to the vast literature of this maturing eld, but should stimulate speci c areas of discussion and perhaps further research in highlighted topics
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