Surface brightens up Si quantum dots: direct bandgap-like size-tunable emission

Abstract

Colloidal semiconductor quantum dots (QDs) constitute a perfect material for ink-jet printable large area displays, photovoltaics, light-emitting diode, bio-imaging luminescent markers and many other applications. For this purpose, efficient light emission/absorption and spectral tunability are necessary conditions. These are currently fulfilled by the direct bandgap materials. Si-QDs could offer the solution to major hurdles posed by these materials, namely, toxicity (e.g., Cd-, Pb- or As-based QDs), scarcity (e.g., QD with In, Se, Te) and/or instability. Here we show that by combining quantum confinement with dedicated surface engineering, the biggest drawback of Si—the indirect bandgap nature—can be overcome, and a ‘direct bandgap’ variety of Si-QDs is created. We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling. The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain ‘direct bandgap-like’ (phonon-less) character. This results in efficient fast emission, tunable within the visible spectral range by QD size. These findings are fully justified within a tight-binding theoretical model. When the C surface termination is replaced by oxygen, the emission is converted into the well-known red luminescence, with microsecond decay and limited spectral tunability. In that way, the ‘direct bandgap’ Si-QDs convert into the ‘traditional’ indirect bandgap form, thoroughly investigated in the past. Surface engineering could provide silicon quantum dots with efficient and fast photoluminescence in the visible region. This is the finding of Kateřina Dohnalová and colleagues from the University of Amsterdam and Wageningen University in The Netherlands, who studied the optical properties of alkyl-capped silicon quantum dots. They believe that carbon surface termination can modify the energy band structure within the quantum dot to allow ‘direct-bandgap-like’ blue photoluminescence. The light emission can be tuned across the visible spectrum by changing the size of the quantum dot and has a radiative rate 100,000 times stronger than that of bulk silicon. In the future, such quantum dots could prove useful for making light-emitting diodes and displays, as a luminescent label for bio-imaging experiments, and for a wide range of other applications.