Silane control of the electron injection and oxygen sensitivity of dye-silane-GaN hybrid materials for luminescent chemical sensing

Abstract

UV to green light-emitting diodes (LEDs) based on III-nitride semiconductors have revolutionized areas as diverse as illumination, medical treatments and biophotonics. Chemical sensing also benefited, as many fluorophores that provide extreme sensitivity and selectivity absorb light in that region. Additionally, integration and smartphone-based sensing allow the sought portability and miniaturization for optosensor development. For manufacturing intrinsic O2-responsive GaN-based LEDs, we modified n-doped GaN substrates to tether a luminescent Ru(II) indicator dye. To achieve covalent attachment of the dye, the GaN surfaces were pre-activated with oxygen plasma and a ω-aminoalkyltrialkoxysilane layer was grown on top using each of six different silanes. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) allow evaluation of the coating thickness and morphology. Single-photon timing (SPT) microscopy provides the luminescence lifetime (τ) of the resulting dye-silane-GaN smart hybrid materials. The emission of the surface-bound Ru(II) dye is quenched by n-GaN, yielding a τ of 0.60μs for a 2.4nm-thick 3-aminopropyltriethoxysilane layer. Such quenching decreases exponentially with the silane layer thickness up to a τ of 2.30μs (18nm 4,7-diazaheptyltrimethoxysilane layer), thus confirming a previously suggested photoinduced electron transfer (PET) from the excited dye to the semiconductor. After eliminating the disturbing PET, the O2 sensitivity of the modified GaN reaches a τN2/τO2 ratio of 1.8 for the thickest silane layer. The ability to suppress the competitive PET photochemistry will allow luminescent chemical sensor developers based on hybrid materials to achieve the highest sensitivity regardless the indicator dye attached to the GaN-based semiconductor.