Gu et al. Nanoscale Research Letters 2011, 6:199 http://www.nanoscalereslett.com/content/6/1/199 Page 5 of 12 Figure 4 LEE enhancement of a GaN-based LED. Schematic illustration of the structure of an electroluminescent LED in which LEE is enhanced via energy transfer. enhance light extraction from light emitters [38,39]. In the paper by Bakker et al. [40], a nanoantenna system consisting of two gold elliptical NPs is shown to enhance the LEE of a fluorescent dye by a factor of 20 to 100. Near-field measurements show that the enhanced emission is localized and polarized. Another group has reported an organic emitter coupled with sil- ver nanoantenna arrays [41]. In addition to increased light absorption in the ultraviolet [UV] range and enhanced PL efficiency, an energy transfer process responsible for light extraction is suggested. In fact, the effect of increased UV absorption is less dominant com- pared to the enhancement of LEE, as reported in similar experiments, and the results demonstrate the efficacy of nanoantennae in efficiently extracting generated photons. Fabrication Vacuum deposition such as sputtering or electron beam evaporation sometimes accompanied by post thermal processing [PTP], electron beam lithography [EBL], nanosphere lithography [NSL], nanoimprinting, and che- mical synthesis are common methods to produce metal- lic NP arrays. However, not all of these techniques have been used to make LEDs due to various technological considerations. For instance, many of these methods involve chemical processing, which can harm the LED structure. At present, vacuum deposition is the most successful in fabricating EL LEDs, whereas only PL results have been obtained from samples produced by other techniques. Nonetheless, the PL results still have considerable value as they enable better understanding of the nature of the LSPR-induced enhancement and spur further development in the fabrication methods. In this section, we not only describe these various methods but also discuss the compatibility of each method from the perspective of LED fabrication, ease of implementa- tion, and production cost. Vacuum deposition Under ultrahigh vacuum conditions, a thin metal film can be formed on the LED structure via the condensa- tion of atoms produced by evaporation. The film thick- ness is governed by factors such as the substrate type, ambient pressure, and sputtering time. One advantage of the technique is that it does not require chemical processing, and so chemical damage to the emitting region can be avoided. This method has been employed to produce Ag NP arrays on a silicon QD LED [42]. To alter the optical properties such as the extinction spec- trum of the materials for better enhancement, PTP is often applied. By conducting annealing at different tem- peratures and for different time durations on metal films with different thicknesses, NPs with the desirable sizes and heights can be produced in order to achieve the best coupling effect with the light emitters. Yeh et al. [43] have observed that the size and heights of Ag NPs are significantly increased after annealing three samples with initial film thicknesses of 5, 10, and 15 nm at 200°C for 30 min. Annealing enhances the PL inten- sity of the LSPR light emitters due to enhanced LSP/ QW coupling. Other PL experiments conducted on Pt/ ZnMgO films [44], Ag/ZnO films [45], and conjugated polymers [46] also demonstrate the constructive role of annealing after vacuum deposition in enhancing the coupling between the emitters and metallic NPs. Owing to the ease of implementation and reasonable cost, vacuum deposition together with PTP is the most common method in fabricating LEDs containing metal- lic NPs. Figure 5 shows a typical image of Ag NPs deposited on an n-GaN layer before and after annealing [33]. Similar to the results described in [43], after annealing at 750°C for 10 min in a metal/organic chemi- cal vapor deposition chamber, the size and height of the NPs change from 275 ± 50 and 8 ± 4 nm to 450 ± 50 and 15 ± 5 nm, respectively. Afterwards, a 20-nm-thick undoped GaN, 22-nm-thick InGaN/GaN MQW, and a 0.2-μm p-type GaN layer are deposited onto the Ag NPs and two electrodes are added to the LED structure. The output optical power is observed to increase by 32.2% at an input current of 100 mA due to the enhancement of IQE resulting from coupling between the MQW and Ag NPs. In addition, InGaN/GaN MQW LEDs with