Silicon quantum dots are metal-free biologically compatible quantum dots with photoluminescence emission maxima that are tunable through the visible to near-infrared spectral regions. These quantum dots have unique properties arising from their indirect band gap, including long-lived luminescent excited-states and large Stokes shifts. A variety of disproportionation, pyrolysis, and solution protocols have been used to prepare silicon quantum dots. However, some solution-based protocols for preparing luminescent silicon quantum dots actually yield carbon quantum dots instead of the reported silicon. The unique properties of silicon quantum dots lend themselves to an array of potential applications: biological imaging, luminescent solar concentrators, light emitting diodes, sensors, and lithium-ion battery anodes.
History Silicon has found extensive use in electronic devices; however, bulk Si has limited optical applications. This is largely due to the vertical optical transition between the conduction band and valence band being forbidden because of its indirect band gap. In 1990, Leigh Canham showed that silicon wafers can emit light after being subjected to electrochemical and chemical dissolution. The light emission was attributed to the quantum confinement effect in the resulting porous silicon. This early work provided a foundation for several different types of silicon nanostructures including silicon nanoparticles (quantum dots), silicon nanowires, silicon nanoshells, silicon nanotubes, silicon aerogels, and mesoporous silicon. The first reports of silicon quantum dots emerged in the early 1990s demonstrating luminescence from freestanding oxidized silicon quantum dots. Recognizing the vast potential of their unique optical properties, many researchers explored, and developed methods to synthesize silicon quantum dots. Once these materials could be prepared reliably, methods to passivate the surfaces were critical to rendering these materials solution processable and minimize the effects of oxidation. Many of these surface passivation methods draw inspiration from methods that were first developed for silicon wafers and porous silicon. Currently, silicon quantum dots are being commercialized by Applied Quantum Materials Inc. (Canada).
Properties Silicon quantum dots (SiQDs) possess size-tunable photoluminescence that is similar to that observed for conventional quantum dots. The luminescence is routinely tuned throughout the visible and into the near-infrared region by defining particle size. In general, there are two distinct luminescence bands that dominate silicon quantum dot properties. Long-lived luminescence excited states (S-band, slow decay rate) are typically associated with size-dependent photoluminescence ranging from yellow/orange to the near-infrared. Short-lived luminescent excited states (F-band, fast decay rate) are typically associated with size-independent blue photoluminescence and in some cases nitrogen impurities have been implicated in these processes. The S-band is typically attributed to the size-dependent band gap of the silicon quantum dots. This emission can be tuned from yellow (600 nm) into the infrared (1000 to 1100 nm) by changing the diameter of the silicon quantum dots from about 2 to 8 nm. Some reports also describe the preparation of green-emitting silicon quantum dots prepared by decreasing the size, however, these materials are challenging to isolate and require further development. Silicon quantum dot luminescence may also be tuned by defining their surface chemistry. Attaching different surface species allows tuning of silicon quantum dot luminescence throughout the visible spectrum while the silicon quantum dot dimensions remain unchanged. This surface tuning is typically accompanied by the appearance of nanosecond lifetimes like those seen for F-band luminescence. Silicon quantum dot photoluminescence quantum yields are typically in the range of 10 to 40%, with a handful of synthetic protocols providing values in excess of 70%. The long-lived excited state of silicon quantum dot S-band luminescence that starkly contrasts photoemission from conventional quantum dots is often attributed to the inherent indirect band gap of silicon and lends itself to unique material applications. Combining long-lived excited states with the biological compatibility of silicon quantum dots enables time-gated biological imaging. The large Stokes shift allows them to convert photons from the ultraviolet range into the visible or infrared range and is particularly beneficial in the design and implementation of luminescent solar concentrators because it limits self-absorption while down converting the light. Importantly, SiQDs are biologically compatible and do not contain heavy metals (e.g., cadmium, indium, lead). The biological compatibility of these materials has been carefully studied both in vitro and in vivo. During in vitro studies, SiQDs have been found to exhibit limited toxicity in concentrations up to 72 μg/mL in HeLa cells and 30 μg/mL in epithelial-like cells (MDA-MB-231). In vivo studies assessing biological compatibility of SiQDs undertaken in mice and monkeys (rhesus macaques) found "no signs of toxicity clearly attributable to SiQDs." In bacteria, SiQDs have been shown to be less toxic than both CdSe and CdSe/ZnS quantum dots.
Synthesis
Synthesis methods Silicon quantum dots can be synthesized using a variety of methods, including thermal disproportionation of silicon suboxides (e.g., hydrogen silsesquioxane, a silsesquioxane derivative), and laser and plasma-induced decomposition of silane(s). These methods reliably provide high quality SiQDs exhibiting size/band gap dependent (S-band) photoluminescence. Top-down methods, such as laser ablation and ball-milling have also been reported. Several solution-based methods have also been presented that often result in materials exhibiting F-band luminescence. Recently, it has been determined that some of these methods do not provide silicon quantum dots, but rather luminescent carbon quantum dots.
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