ABSTRACT
Alloy formation is ubiquitous in inorganic materials science, and it strongly depends on the similarity between the alloyed atoms. Since molecules have widely different shapes, sizes and bonding properties, it is highly challenging to make alloyed molecular crystals. Here we report the generation of homogenous molecular alloys of organic light emitting diode materials that leads to tuning in their bandgaps and fluorescence emission. Tris(8-hydroxyquinolinato)aluminium (Alq3) and its Ga, In and Cr analogues (Gaq3, Inq3, and Crq3) form homogeneous mixed crystal phases thereby resulting in binary, ternary and even quaternary molecular alloys. The M x M'(1-x)q3 alloy crystals are investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy on single crystal samples, and photoluminescence properties are measured on the exact same single crystal specimens. The different series of alloys exhibit distinct trends in their optical bandgaps compared with their parent crystals. In the Al x Ga(1-x)q3 alloys the emission wavelengths lie in between those of the parent crystals, while the Al x In(1-x)q3 and Ga x In(1-x)q3 alloys have red shifts. Intriguingly, efficient fluorescence quenching is observed for the M x Cr(1-x)q3 alloys (M = Al, Ga) revealing the effect of paramagnetic molecular doping, and corroborating the molecular scale phase homogeneity.
ABSTRACT
The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er3+-doped NaYF4 core as a function of the thickness of a NaLuF4 shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er3+ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCNC monolayers is further enhanced by electron-beam-lithographic-designed Au nanodiscs deposited either on top of or buried within the monolayer. It is observed that the improvement by the nanocrystal shells is greater than that of the Au nanodiscs.
ABSTRACT
A two-particle model is proposed which enables the assessment of particle-particle interactions in large, sparse arrays of randomly distributed plasmonic metal nanoparticles of arbitrary geometry in inhomogeneous environments. The two-particle model predicts experimentally observed peak splittings in the extinction cross section spectrum for randomly distributed gold nanocones on a TiO2:Er3+ thin film with average center-to-center spacings of 3-5 diameters. The main physical mechanism responsible is found to be interference between the incident field and the far-field component of the single-particle scattered field which is guided along the film.
