Integrated Effects of Near-Field Enhancement-Induced Excitation and Surface Plasmon-Coupled Emission of Elongated Gold Nanocrystals on Fluorescence Enhancement and the Applications in PLEDs

Localized surface plasmon resonance (LSPR) field enhancement effects of noble metallic nanoparticles can be exploited to enhance the performance of diverse luminescent materials and devices, in which the spectral proximity plays an important role in increasing near-field enhancement-induced excitation (NFEE) and surface plasmon-coupled emission (SPCE) efficiencies. In this work, we propose a scheme simultaneously utilizing the transversal and longitudinal SPR bands of elongated gold nanocrystals to match with the excitation and emission wavelengths of emitters, respectively, to achieve the most efficient enhancement. To demonstrate the idea, four types of gold nanoparticles (AuNPs, diameter = 20 nm), gold nanorods (AuNRs, length/width = 80/40 nm; 90/30 nm), and gold nano-bipyramids (AuNBPs, length/width = 80/40 nm) were employed as the plasmonic nanoantennas, according to spectral characteristics of the classic poly(2-methoxy-5-(2'-ethyl-hexoxy)-1,4-phenylene-vinylene) (MEH-PPV) chosen as the emitter. Due to the double SPR bands of both AuNBPs (80/40 nm) and AuNRs (80/40 nm) overlapping well with the excitation and emission wavelengths of MEH-PPV, a maximum 2.2 and 2.1 times enhancement in photoluminescence intensity was observed, respectively. This is attributed to the integrated effects of NFEE and SPCE that synchronously accelerate the excitation and radiation rate as made evident by steady-state and time-resolved photoluminescence measurements. Further, the above plasmonic nanoantennas were incorporated into the polymer light-emitting diodes (PLEDs), in which AuNBPs (80/40 nm)- and AuNRs (80/40 nm)-mediated devices exhibited approximately a 2.3 and 2.1 times enhancement in luminance, and 2.0 and 1.9 times enhancement in luminous efficiency compared with the pristine one, respectively. More notably, it is first demonstrated that the sharp-tip AuNBPs generating a stronger local electric field compared to AuNRs can efficiently enhance PLEDs' performance, showing a promising prospect for the development of high-performance red and near-infrared electroluminescence devices.