Light-emitting diodes [LEDs] have attracted much scientific and commercial interest since the realization of a practical LED device with emission frequencies in the visible region of the electromagnetic spectrum [1
]. Since then, research activities have been focusing on how to produce economical LEDs with the desired colors as well as white light sources [2
]. The strong demand has also driven materials technology, and new emitting materials and configurations have been proposed to enhance the performance. For example, the use of a polymer instead of small molecules opens the door to flexible, large-area, and stable organic LEDs [OLEDs] [
3]. In the past 15 years, low-dimensional emitting devices incorporating quantum dots [QDs] and quantum wells [QWs] have been extensively investigated in order to achieve the desirable emission color and enhance device efficiency [4
]. However, LEDs suffer from inherently low efficiency due to the sometimes low internal quantum efficiency [IQE] and difficulty extracting the generated photons out of the device. Although the use of electro-phosphorescent materials with proper management of both singlet and triplet excitons has brought IQE in OLEDs to almost unity [11
], that of LEDs with inorganic emitting materials such as GaN, CdSe, and Si QDs or QWs remains unsatisfactory because non-radiative electron/hole pair recombination dominates. Another channel of energy loss is total internal reflection at the emitter/air interface because of the typically high refractive index of the emitting materials. Several methods have been proposed to enhance the overall efficiency of LEDs, and they include substrate modification and incorporation of scattering medium, micro-lenses, nanogratings, corrugated microstructures, photonic crystals, and so on [14
]. In spite of some efficiency enhancement, spectral changes and angle-dependent colors associated with the substrate modification techniques, the high precision needed to produce nanogratings and the high cost of photonic crystals are still challenging issues plaguing commercial applications.
Surface plasmon polaritons [SPPs] were first exploited to enhance the efficiency of InGaN QW-based LEDs by Okamoto et al. in 2004 [18
]. Known as Purcell effect, when the resonant frequency of the silver SPPs overlaps the emission frequency of the InGaN QWs, the energy coupled to the SPP mode is significantly increased and thus the IQE is enhanced [19
]. Scattered by the rough silver film, the energy coupled to the SPP mode can be recovered as free space photons. In their work, the enhanced IQE η
is observed to increase 6.8 times with Ag coating, leading to a very desirable Purcell factor
represents the original IQE. Figure shows the wavelength-dependent η
, Purcell factor, and the emission spectrum of their sample. It is clear that greater enhancement can be obtained at shorter wavelengths (~440 nm). However, this wavelength does not perfectly overlap the GaN/InGaN emission peak, leaving space for better enhancement. In fact, the SPP resonant energy must be in the vicinity of the emission energy in order to achieve the best enhancement. This rule has since been verified by other experiments [20
]. Hence, only a small subset of LEDs can be enhanced via SPP/emitter coupling because the SPP resonant frequency of a metal film cannot be easily tuned. Another challenge is that the metal film is typically opaque, thereby making light extraction from the metal side of the device difficult. It has been shown that light can be effectively extracted from the metal side by exploiting the surface plasmon cross-coupling effect, but incorporation of the appropriately scaled nanostructures is necessary [24
Figure 1 Enhanced emission efficiency, Purcell factor, and PL spectrum of the sample. These are shown as red dashed line, blue solid line, and black dotted line, respectively. Nearly 100% emission efficiency can be obtained at around 440 nm; however, this does (more ...)
In comparison with the aforementioned technology, localized surface plasmon [LSP] offers a unique advantage in tunability; that is, the optical properties resulting from LSP can be easily varied by altering the type, size, geometry, and interparticle distance of the metallic nanoparticles [NPs]. The other advantage of LSP-enhanced LEDs over SPP ones is less dissipation since the induced wave is locally confined and cannot propagate along the metal surface. Furthermore, the metal layer is no longer opaque, making emission from the metal side possible, and so metallic NPs instead of a continuous metal film can be used to enhance the performance of LEDs. Figure schematically shows the story of this review: incorporation of noble metallic nanoparticles into LEDs leads to a new class of highly efficient solid-state light sources (top row); in order to get considerable enhancement, the extinction band of LSP must be close to the band-gap emission energy of the LED (middle row); and this new technology has found its applications in general lighting, flat panel displays, and ultrafast optoelectronic chips (bottom row). Recent improvements combined with the low cost and easy fabrication process make localized surface plasmon resonance [LSPR]-enhanced LEDs very attractive commercially.
Noble metallic NP layer deposited on or within a conventional LED to enhance efficiency of device. This new class of LEDs can be used in various compelling applications.