Chemical Insight into the Origin of Red and Blue Photoluminescence Arising from Freestanding Silicon Nanocrystals

Abstract

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms as well being biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared via solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue-emitters upon exposure to nitrogen containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the ppm level in Si NCs gives rise to the blue emission.

Optical Biosensing and Bioimaging With Porous Silicon and Silicon Quantum Dots (Invited Review)

“…There is increasing evidence to suggest that the blue emission is governed by electron transfer from the surface states [96,121]. Remarkably, recent observations suggest that certain electron donating, nitrogen containing species can drastically improve the blue emitting quantum yield of silicon quantum dots significantly.…”

Synthesis of Diagnostic Silicon Nanoparticles for Targeted Delivery of Thiourea to Epidermal Growth Factor Receptor-Expressing Cancer Cells

“…[14][15][16][17] The size tunable emission [18][19][20] , high brightness 21 , and high photostability compared to organic dyes 22 , have made SiNPs attractive materials as fluorescent cellular markers. In addition, silicon is abundant and economical 23 but more importantly silicon has low inherent toxicity.…”

Investigation into Effects of Instability and Reactivity of Hydride-Passivated Silicon Nanoparticles on Interband Photoluminescence

“…As can be seen in Figure 2.1, the IR spectrum of the dried polymer contains overlapping peaks at 1050 cm -1 and 1090 cm -1 , both of which are in the region that has been attributed to Si-O bonding. 9,34,40,47,58,63,69,72,[75][76][77][78][79][80][81][82][83] Evidence for the preservation of Si-H bonding is also shown by the peak at 2240 cm -1 in the IR spectrum.…”

“…61 Conversely, others have reported the development of high energy PL from originally red-emitting Si NPs concurrent with surface oxidation. 63,67 For example, in a study conducted by our group, we characterized changes in the photophysical properties of initially 690 nm-emitting (d = 4 nm, λ exc = 370 nm), alkylpassivated Si NPs following exposure to a series of n-alcohols varying in hydrocarbon chain length. 67 We reported that the development of surface oxide (observed by FT-IR)…”

“…51,54,55,[57][58][59][60][61][62][63][64][65]123,124 These studies have frequently linked changes in surface composition to the development of electronically active (defect) states within the bandgap. If the defect states that form are radiative recombination centers, the Si NP suspension would be expected to exhibit new photoluminescence (PL) bands.…”