Vegetal melanin-induced radiative recombination centers in porous silicon

Abstract

Melanin has attracted significant attention in recent years due to its optoelectronic properties exploited in micro and nano-bioelectronic applications. Herein, were report on the photoluminescence tuning of porous silicon (PSi) functionalized with melanin. PSi was prepared through an electrochemical etching process applying a density current of 2.8 mA/cm2 to a silicon wafer using an electrolyte composed of HF and ethanol (1:1). The melanin/PSi heterojunction interface was created by drop casting a melanin solution (0.1 mg/mL in dimethyl sulfoxide) using different volumes of 10, 30 and 50 μl onto a PSi wafer. A remarkable increment of the photoluminescence intensity in the melanin/PSi interface as compared to the pristine PSi was observed through steady-state experiments. Moreover, a pronounced blue-shift (50 nm) of pristine PSi emission maximum as compared to melanin/PSi interfaces was also recorded. This behavior could be ascribed to the generation of radiative recombination centers as indicated by an increment of silicon-hydrogen (Si–H) and silicon-oxygen (Si–O) defects formed in the PSi upon interaction with the melanin, as proven by infrared (FT-IR) experiments. Time-resolved experiments revealed a remarkable decrease in the decay times of the melanin/PSi composites moving from microsecond-time components in the pristine PSi towards nanosecond decay times when the highest amount of melanin was employed, indicative of an increment of radiative recombination centers. Surface morphology changes of the composites were monitored by scanning electron microscopy (SEM), displaying the formation of small grains of melanin on the surface of the PSi up to the appearance of a melanin thin film when low and high amounts of melanin were used, respectively. This study demonstrates the enormous potential of melanin-PSi heterojunctions to provide a novel and sustainable solution for developing (bio)optoelectronic devices.