Tunability Limit of Photoluminescence in Colloidal Silicon Nanocrystals

Colloidal silicon nanocrystals (Si NCs) have attracted tremendous research interest due to their intriguing optical and electronic properties and thus promising applications from microelectronics to optoelectronics compatible with silicon-based optoelectronic integrated circuits1,2,3,4,5. One of the most important features of NCs/quantum dots (QDs) is the broad tunability of their photoluminescence (PL) energy induced through quantum confinement by changing the nanocrystal size3,6. The PL of Si NCs has been extensively investigated7,8,9. Two PL bands are prominent; one in the red/near infrared (referred to as S-band with μs lifetime) and one in blue/green PL (referred to as F-band with ns lifetime). PL quantum yield (QY) as high as 60% have been reported for colloidal ensemble of Si NCs10,11,12. It is generally accepted the PL in the red/NIR dominantly originates from the quantum confinement while the green/blue PL is arisen from the oxygen relevant defect/surface states1,4,5,13,14,15,16,17. The exciton emission exhibits a marked blue shift with decreasing the size which can be predicted based on quantum confinement in the large size regime18. Great effort has been made for broadening the tunability of the PL by changing the size of Si NCs19,20. However, it has been confirmed that the PL peak does not further blue-shift when the size decreases smaller than 2 nm or the PL peak approaches 590 nm in high uniformity Si NCs19,20,21,22, deviating significantly from the prediction of quantum confinement. Moreover, the PL of the small sized Si NCs exhibits decreased PL quantum yield (QY) and evidently larger nonradiative rate compared to the larger Si NCs, even with identical procedures of synthesis and surface passivation7,12,19. To date, it is still unknown why the blue shift of the S-band should stop at the “magic” wavelength ~590 nm, rather than continues further towards the green/blue19,21,22. To this end we investigate the electron dynamics in Si NCs using steady state and time-resolved techniques. For the first time we elucidate a nanosecond transient yellow PL band (Y-band) emitting around 590 nm that plays a critical role in influencing the electron dynamics and thus dominates the PL shift. When the PL peak approaches the yellow band the excited electrons are dominantly trapped at the surface states and thus the nonradiative component increases significantly. This inhabits any further shift of the PL peak to the blue/green predicted based on quantum confinement only7,10,19,20.