We characterize energy transfer between luminescent 1.5 nm diameter gold nanocrystal (AuNC) acceptors and three structurally/functionally-distinct classes of emissive donors including organic dyes, metal chelates and semiconductor quantum dots (QDs). Energy transfer efficiencies within the donor-AuNC assemblies were evaluated with steady-state and time-resolved measurements. Donor quenching was observed for every donor-acceptor pair although AuNC sensitization was only observed from metal-chelates and QDs. Results were analyzed with Förster’s dipole-dipole coupling model (FRET) and dipole-metal damping models including nanosurface energy transfer (NSET) and nanovolume energy transfer (NVET). FRET dramatically underestimated energy transfer efficiencies while the damping models provided qualitatively better fits to the data although neither fully reproduces the experimental data. Analysis suggests that organic dye donor quenching without corresponding AuNC sensitization results from enhanced intersystem crossing between dye singlet and triplet states driven by AuNC magnetic dipoles. We further consider factors that account for the unique electronic properties of the ultra-small luminescent AuNCs including the high excited state densities, rapid dephasing time and strong electron confinement as well as paramagnetic properties. Overall, the results provide insight into requirements necessary for realizing applications based on AuNC acceptor sensitization.
Graphene growth of high crystal quality and single-layer thickness can be achieved by low pressure sublimation (LPS) on SiC(0001). On SiC(0001), which is the C-terminated polar surface, there has been much less success growing uniform, single-layer films. In this work, a systematic study of surface preparation by hydrogen etching followed by LPS in an argon ambient was performed. Hydrogen etching is an important first step in the graphene growth process because it removes damage caused by polishing the substrate surface. However, for SiC(0001), etching at too high of a temperature or for too long has been found to result in pit formation due to the preferential etching of screw dislocations that intersect the surface. It was found that temperatures above 1450°C in 200mbar of hydrogen result in pitting of the surface, whereas etch temperatures at and below 1450°C can result in atomically at terraces of ~ 1 µm width. Following the hydrogen etch optimization, argon-mediated graphene growth was carried out at several different temperatures. For the growth experiments, pressure and growth time were both fixed. Regardless of growth temperature, all of the films were found to have non-uniform thickness. Further, x-ray photoelectron spectroscopy and low energy electron diffraction measurements reveal that trace amounts of oxygen, which may be present during growth, significantly affects the graphene growth process on this polar surface.
Large magnetic field effects (MFE) have been observed in organic light emitting diodes (OLED) based on a bilayer of tris (8-hydroxyquinoline) aluminum (Alq3) and N,N’-Di(naphthalen-1-yl)-N,N’diphenyl-benzidine (NPB). They consist of an increase in electroluminescence (EL) of a few percent at low magnetic fields followed by a decrease in EL of 20+% at high fields. Associated with these two effects is a decrease in resistance of typically 1-3% as the magnetic field is increased. The magnitude of the high field effect (HFE) varies with temperature and current density, while the low field effect (LFE) survives even when the HFE is not present. The HFE is enhanced at low temperature and/or high current density. These effects are similar to those reported for anthracene single crystals suggesting a large triplet-triplet annihilation (TTA) component for the EL in Alq3. However, transient EL studies fail to definitively identify a delayed luminescence component with a time scale appropriate for TTA in Alq3. We discuss this and other questions concerning the origin of MFE in this system.