Gu et al. Nanoscale Research Letters 2011, 6:199 http://www.nanoscalereslett.com/content/6/1/199 is observed to increase 6.8 times with Ag coating, lead- ing to a very desirable Purcell factor (1) 1 − ηint Fp ≈ 1 − η∗int where hint represents the original IQE. Figure 1 shows the wavelength-dependent h*int, Purcell factor, and the emission spectrum of their sample. It is clear that greater enhancement can be obtained at shorter wave- lengths (~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 [20-23]. 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 diffi- cult. 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 [24,25]. In comparison with the aforementioned technology, localized surface plasmon [LSP] offers a unique advan- tage 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- 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 not perfectly match the emission peak. Reproduced from [18]. Copyright Nature Publishing Group, 2004. Page 2 of 12 enhanced LEDs over SPP ones is less dissipation since the induced wave is locally confined and cannot propa- gate 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 per- formance of LEDs. Figure 2 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 dis- plays, 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. Mechanism Pioneering experimental results have confirmed the importance of overlapping between the LSPR energy and emission energy, and some of them are presented in Table 1. The type, shape, height, and density of the NPs determine the degree of enhancement. In order to explore the cause and mechanism, experiments have been conducted carefully, often excluding other possible factors which may contribute to the enhancement such as reflection from the metallic NPs, emission from the NPs themselves, increased absorption of light in photo- luminescence [PL] enhancement, and quenching of defects emission. However, although it is generally agreed to stem from resonant coupling between the semiconductor band-gap emission and LSP generated by the metallic NPs, the exact mechanism is still debatable. In this section, we discuss two mechanisms that have been suggested to explain the resonant-coupling enhan- cing effect, namely, increase of IQE via emitter/plasmon coupling and increase of light extraction efficiency [LEE] by means of out-coupling of the generated photons. Enhancement of IQE The local electric field and magnitude of the extinction spectrum are significantly enhanced at the LSPR fre- quency [26]. This effect has been broadly studied and utilized in many fields such as surface-enhanced Raman spectroscopy, solar cells, and biosensors [27-30]. With regard to the efficiency improvement rendered by LSPR, it is supposed that the enhanced electric field interacts with the emitting materials, increasing the spontaneous emission rate and consequently enhancing the IQE of the device. This assumption can be partly verified by experiments showing that the radiative decay rate and