SnO2:Snox core-shell QD in glass: charge transport and UV emission in fully inorganic electroluminescent devices

The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands UV-electroluminescent materials with high mechanical, chemical, and environmental stability and complete compatibility with silicon technology. A variety of solution-based synthesis methods are available for producing high quality nanocrystals (NCs), either as self standing colloidal nanoparticles or as highly controlled crystalline nanophases in host matrices. NC-light-emitting diodes (LEDs) with remarkable tunability across the visible spectrum have been reported. The extension of the emission into the UV region is problematic and increasing efforts are being devoted to this task due to the technological relevance of UV-emitting nanomaterials. The achievement of robust solution-processed UV-LEDs with reasonable EL efficiency (EQE) remains, however, a long-standing goal in this field of materials research. Tin dioxide -with a direct forbidden optical gap of about 3.6 eV- is particularly suitable for producing NCs in optical grade materials. Band-edge excitonic emission at 320 - 360 nm and radiative recombination mediated by shallow traps at 370 - 390 nm have been observed for a variety of SnO2 nanostructures, among which, thermally grown NCs by selective nucleation of the molecular precursors in sol-gel silica1. This synthesis opened the way to the design of robust UV-LEDs on silicon substrate, with benefits derived from a solution-based method, a full compatibility with silicon technology, and superior environmental stability resulting from the full inorganic architecture and the incorporation of the SnO¬2 NCs in silica. Tin oxides exhibit charge carriers with opposite signs depending on tin oxidation state and, ultimately, on tin coordination number. SnO is an intrinsic p-type semiconductor (Egap=2.7-3.4 eV) with fourfold coordinated Sn2+ cations, whereas SnO2 is an intrinsic n-type semiconductor with sixfold coordinated Sn4+ cations. Tin atoms at the NC/matrix interfaces substitute Si atoms in the tetrahedrally coordinated amorphous network, therefore promoting the occurrence of tin cation having an oxidation state of 2+.1-3 As a result, the nanophase consist of core/shell-like NCs with n-type SnO2 cores coated by a p-type SnO shell, which is protected against further oxidation by the dense SiO2 matrix.3 Fully inorganic UV-light emitting diodes emitting at 390 nm with EQE ~ 0.3%, based on SnO2 nanoparticles (~5 nm in diameter) embedded in SiO2 thin films have been obtained through a sol-gel method, which involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere.4 The UV-LEDs exhibit superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. This clearly demonstrates the effectiveness of the oxide-in-oxide architecture with a silica matrix that ultimately confers to the whole NS material the chemical inertness of silica glass while preserving the charge transport of both electrons and holes due to the controlled segregation of n-type NCs with substoichiometric p-type interfaces. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices.