Incorporation of N and O into the Shell of Silicon Nanoparticles Offers Tunable Photoluminescence for Imaging Uses

Abstract

Silicon nanoparticles (SiNPs) show tunable photoluminescence (PL), water-dispersibility, high photostability, and low cytotoxicity, thus constituting promising candidates for bioimaging applications. Because SiNP PL depends finely on particles’ crystallinity and surface composition, specific tuning of PL properties has remained elusive. Herein, using steady and time-resolved PL studies, absorbance spectroscopy, and electrochemical techniques, we have deeply analyzed the origin of the PL of SiNPs obtained from a wet chemical synthesis procedure based on the oxidation of Zintl salts in dimethyl formamide (DMF). Obtained SiNPs, surface-functionalized with propylamine terminal groups, were amorphous and 2.8–3.7 nm in size. The photophysical evidence, together with XPS and FTIR spectroscopy, supported a core–shell structure of the nanoparticles consisting of a silicon core surrounded by a 0.7–1.25 nm-thick oxidized silicon shell containing low concentrations of trapped iminium siloxyl ions or related compounds. The introduction of N-functionalities in the nanoparticle shell was assigned to the reaction of Si–Cl and Si–H bonds formed during synthesis, with DMF. The use of increasing amounts of NH4Cl in the synthesis procedure led to more oxidized shell structures of SiNPs. It is suggested that the presence of an oxidized silicon shell containing trapped iminium siloxyl ions provided a high density of localized states capable of quenching the core-state emission and of being themselves populated by absorption of visible light. Moreover, it was experimentally confirmed that emission preferentially takes place from localized states introduced by O-functionalities with a high quantum efficiency (ηPL‑trap ≅ 1). As fluorophores, the obtained SiNPs display tunable PL emission and an important red-edge shift, allowing the selection of the PL by changing the excitation wavelength without modification of its chemical composition and size, thus meeting the needs of various types of biosensing methods.

Mixed-ligand-functionalized silicon–germanium alloy nanocrystals with improved carrier mobilities

“…3b-d) are consistent with the anchored amine ligands and other germanium oxynitride species. 29,30 To deepen the understanding of the potential influences of ligand passivation on the photophysical properties of SiGe NCs, we performed the absorption and photoluminescence (PL) measurements on the ODE-capped, the OAm-capped, and the ML-capped samples. All three samples show similar absorption features, which can be attributed to the SiGe crystalline core absorption (Fig.…”

Size-tunable silicon nanoparticles synthesized in solution via a redox reaction

“…Photoluminescence has several origins in silicon particles, due to quantum size effects, excitonic radiation recombination pathways, surface traps, charge transfer from silicon to surface-bound ligands, and combinations of these effects. [1][2][3][4] Efforts to engineer both the silicon nanoparticle and the surface ligands have led to high quantum efficiencies, tunable emission colors, lifetimes and full width at half maximum band emissions. 5,6 Partially oxidized amorphous silicon nanostructures have been shown to display photoluminescence via radiative processes from surface or sub-surface defects.…”