How ß-Phase Content Moderates Chain Conjugation and Energy Transfer in Polyfluorene Films.

Poly(9,9-dioctylfluorene) (PFO) is a blue-light-emitting polymer exhibiting two distinct phases, namely, the disordered "glassy" phase and a more ordered ß-phase. We investigate how a systematic increase in the fraction of ß-phase present in PFO films controls chain conformation, photoluminescence quantum efficiency (PLQE), and the resonant energy transfer from the glassy to the ß-phase. All films are prepared by the same technique, using paraffin oil as an additive to the spin-coating solution, allowing systematic tuning of the ß-phase fraction. The PFO films exhibit high PLQE with values increasing to 0.72 for increasing fractions of ß-phase present, with the ß-phase chain conformation becoming more planar and including more repeat units. Differences in Förster radii calculated from the overlap of steady-state absorptance and emission spectra and from time-resolved ultrafast photoluminescence transients indicate that exciton diffusion within the glassy phase plays an important role in the energy transfer process.

Ultraclean Single Photon Emission from a GaN Quantum Dot.

Wide bandgap III-nitride quantum dots (QDs) are promising materials for the realization of solid-state single-photon sources, especially operating at room temperature. However, so far a large degree of inhomogeneous broadening induced by spectral diffusion has compromised their use. Here, we demonstrate the ultraclean emission from single GaN QDs formed at macrostep edges in a GaN/AlGaN quantum well. As a likely consequence of the high growth temperature and hence a reduced defect density, spectral diffusion is heavily suppressed to levels at least 1 order of magnitude lower than conventional GaN QDs. A record narrow line width of as small as 87 µeV is obtained, while the low inhomogeneous broadening enables us to assess an upper limit of homogeneous broadening in the QDs (27 µeV). Furthermore, the uncontaminated emission facilitates the generation of ultraviolet single-photons with unprecedented purity (g(2)(0) = 0.02). The realization of high-quality GaN QDs will enable exploration of optoelectronic properties of III-nitrides, opening up the possibility of realizing single-photon quantum information systems operating at room temperature.