Surface plasmon coupling effects on the photon color conversion behaviors of colloidal quantum dots in a GaN nanoscale hole with a nearby quantum-well structure.

To improve color conversion performance for color display application, we study the near-field-induced nanoscale-cavity effects on the emission efficiency and Förster resonance energy transfer (FRET) under the condition of surface plasmon (SP) coupling by inserting colloidal quantum dots (QDs) and synthesized Ag nanoparticles (NPs) into surface nano-holes fabricated on a GaN template and an InGaN/GaN quantum-well (QW) template. In the QW template, the inserted Ag NPs are close to either QWs or QDs for producing three-body SP coupling to enhance color conversion. Time-resolved and continuous-wave photoluminescence (PL) behaviors of the QW- and QD-emitting lights are investigated. The comparison between the nano-hole samples and the reference samples of surface QD/Ag NP shows that the nanoscale-cavity effect of the nano-hole leads to the enhancements of QD emission, FRET between QDs, and FRET from QW into QD. The SP coupling induced by the inserted Ag NPs can enhance the QD emission and FRET from QW into QD. Its result is further enhanced through the nanoscale-cavity effect. The relative continuous-wave PL intensities among different color components also show the similar behaviors. By introducing SP coupling to a color conversion device with the FRET process in a nanoscale cavity structure, we can significantly improve the color conversion efficiency. Simulation results confirm the basic observations in experiment.

Photon color conversion enhancement of colloidal quantum dots inserted into a subsurface laterally-extended GaN nano-porous structure in an InGaN/GaN quantum-well template.

To improve the color conversion performance, we study the nanoscale-cavity effects on the emission efficiency of a colloidal quantum dot (QD) and the Förster resonance energy transfer (FRET) from quantum well (QW) into QD in a GaN porous structure (PS). For this study, we insert green-emitting QD (GQD) and red-emitting QD (RQD) into the fabricated PSs in a GaN template and a blue-emitting QW template, and investigate the behaviors of the photoluminescence (PL) decay times and the intensity ratios of blue, green, and red lights. In the PS samples fabricated on the GaN template, we observe the efficiency enhancements of QD emission and the FRET from GQD into RQD, when compared with the samples of surface QDs, which is attributed to the nanoscale-cavity effect. In the PS samples fabricated on the QW template, the FRET from QW into QD is also enhanced. The enhanced FRET and QD emission efficiencies in a PS result in an improved color conversion performance. Because of the anisotropic PS in the sample surface plane, the polarization dependencies of QD emission and FRET are observed.

Nanoscale-cavity enhancement of color conversion with colloidal quantum dots embedded in the surface nano-holes of a blue-emitting light-emitting diode.

Although the method of inserting colloidal quantum dots (QDs) into deep nano-holes fabricated on the top surface of a light-emitting diode (LED) has been widely used for producing effective Förster resonance energy transfer (FRET) from the LED quantum wells (QWs) into the QDs to enhance the color conversion efficiency, an important mechanism for enhancing energy transfer in such an LED structure was overlooked. This mechanism, namely, the nanoscale-cavity effect, represents a near-field Purcell effect and plays a crucially important role in enhancing the color conversion efficiency. Here, we demonstrate the results of LED performance, time-resolved photoluminescence (TRPL), and numerical simulation to elucidate the nanoscale-cavity effect on color conversion by inserting a photoresist solution of red-emitting QDs into the nano-holes fabricated on a blue-emitting QW LED. Based on the TRPL study of the inserted QDs in a nano-hole structure fabricated on an un-doped GaN template of no QW, it is found that the emission efficiency of the inserted QDs is significantly increased due to the nanoscale-cavity effect. From the simulation study, it is confirmed that this effect can also increase the FRET efficiency, particularly for those radiating dipoles in the QWs oriented perpendicular to the sidewalls of the nano-holes. In the nanoscale-cavity effect, the enhanced near field distribution inside a nano-hole excited by a light emitter modifies its own radiation behavior through the Purcell effect such that its far-field emission becomes stronger.

Förster resonance energy transfer in surface plasmon coupled color conversion processes of colloidal quantum dots.

Förster resonance energy transfer (FRET) from a green-emitting quantum dot (GQD) into a red-emitting quantum dot (RQD) is an important mechanism in a multiple-color conversion process, particularly under the surface plasmon (SP) coupling condition for enhancing color conversion efficiency. Here, the dependencies of FRET efficiency on the relative concentrations of GQD and RQD in their mixtures and their surface molecule coatings for controlling surface charges are studied. Also, the SP coupling effects induced by two kinds of Ag nanoparticles on the emission behaviors of GQD and RQD are demonstrated, particularly when FRET is involved in the coupling process. FRET efficiency is reduced under the SP coupling condition. SP coupling can enhance the color conversion efficiency of either GQD or RQD. The combination of SP coupling and FRET can be used for controlling the relative converted light intensities in a multiple-color conversion process.

Combined effects of surface plasmon coupling and Förster resonance energy transfer on the light color conversion behaviors of colloidal quantum dots on an InGaN/GaN quantum-well nanodisk structure.

By forming nanodisk (ND) structures on a blue-emitting InGaN/GaN quantum-well (QW) template, the QWs become close to the red-emitting quantum dots (QDs) and Ag nanoparticles (NPs) attached onto the sidewalls of the NDs such that Förster resonance energy transfer (FRET) and surface plasmon (SP) coupling can occur to enhance the efficiency of blue-to-red color conversion. With a larger ND height, more QWs are exposed to open air on the sidewall for more QD/Ag NP attachment through QD self-assembly and Ag NP drop casting such that the FRET and SP coupling effects, and hence the color conversion efficiency can be enhanced. A stronger FRET process leads to a longer QD photoluminescence (PL) decay time and a shorter QW PL decay time. It is shown that SP coupling can enhance the FRET efficiency.