Effective plasmonic enhancement of up-conversion photoluminescence from -NaGdF4: Yb3+, Er3+ nanoparticles by gold dendrites

The current paradigm of plasmon-enhanced up-conversion photoluminescence (PE-UCPL) lays in intimate coupling domains of noble metal and lanthanide-doped nanoparticles. An increase of incident electromagnetic field and radiative emission rates is the key reason for the UCPL enhancement by the primitive plasmonic architectures of metal nanospheres/nanorods/nanoshells. Based on the hypothesis of precise tuning surface plasmon resonance in more complicated nanoobjects by adjusting process-structure relationship, we engineered a PE-UCPL platform composed of α-NaGdF4:Yb3+,Er3+ nanoparticles and gold dendrites on macroporous silicon (macro-PSi). The uncertainty in contribution of metal dendrites to PE-UCPL is typically due to the structural unpredictability of their highly branched morphologies during formation. Remarkably, macro-PSi dramatically reduces such a barrier. An approach to manage dendritic morphology can be regarded as a controlled corrosive substitution of the silicon skeleton with gold atoms mediated by external fluorine ions from the HF-based electrolyte for gold deposition. Here, we present a comprehensive characterization of three types of Au dendrites grown on macro-PSi for selection of optimal geometry applied to enhance up-conversion of α-NaGdF4:Yb3+,Er3+ nanoparticles. We particularly examined structural/optical properties of Au dendrites and explored a role of incident electric field projection using computer simulation of a dendrite composed of hexagonal bipyramids. The selected Au dendrites provided a broadband PE-UCPL upon 780-990 nm excitation accompanied by a 35-fold increase in a 'red' peak integrated intensity and a 26-fold enhancement in a 'green' one compared to reference α-NaGdF4:Yb3+,Er3+ nanoparticles. Our results are prospective to advance many applications including bioimaging, solid-state lighting, and especially neutral-color solar cells based on macro-PSi.