Particle size effects of nano- and polycrystalline metal tungstate MWO4 (M = Ca, Pb, Cd) scintillators were examined
through a comparison of commercially available powders and solution precipitation prepared nanoscaled materials. The
scintillation behaviors of nanoparticles and commercial powders were examined with ion beam induced luminescence
(IBIL), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy techniques. For commercial microns
sized MWO4 powders, spectral emission differences between CL and PL were only observed for Cd and Pb tungstates
when compared to reported single crystals. The IBIL wavelength emissions also differed from the commercial MWO4
CL and PL data and were red shifted by 28 and 14 nm for CaWO4 and CdWO4; respectively, while PbWO4 had no
significant change. IBIL analysis on CaWO4 nanoparticles produced a 40 nm blue shift from the commercial powder
emission. These preliminary results suggest that both size and cation Z may affect the emission properties of the MWO4
Solid state light sources based on integrating commercial near-UV LED chips with encapsulated CdS quantum dots are demonstrated. Blue, blue-green, and white quantum dot LEDs were fabricated with luminous efficiencies of 9.8, 16.6, and 3.5 lm/W, respectively. These are the highest efficiencies reported for quantum dot LEDs. Quantum dots have advantages over conventional micron-sized phosphors for solid state lighting, such as strong absorption of near-UV to blue wavelengths, smaller Stokes shift, and a range of emission colors based on their size and surface chemistry. Alkylthiol-stabilized CdS quantum dots in tetrahydrofuran solvent with quantum yields (QYs) up to 70% were synthesized using room temperature metathesis reactions. A variety of emission colors and a white spectrum from monodisperse CdS quantum dots (D~2 nm) have been demonstrated. The white emission was obtained from the CdS quantum dots directly, by altering the surface chemistry. When incorporated into an epoxy, the high solution phase QY was preserved. In contrast to other approaches, the white LED contains monodisperse CdS quantum dots, rather than a blend of different-size blue, green, and red-emitting quantum dots. The concentration of CdS quantum dots in epoxy can be increased to absorb nearly all of the incident near-UV light of the LED.
Solid state lighting devices that utilize semiconducting nanoparticles (quantum dots) as the sole source of visible light emission have recently been fabricated. The quantum dots in these devices have been demonstrated to have quantum efficiencies similar to those of conventional phosphors. The dispersion and concentration of the nanoparticles within a suitable polymeric matrix was found to be critical to device performance. Yet achieving suitable concentrations and adequate dispersion implies chemical compatibility between the nanoparticles and the matrix, which must be achieved without detrimental effect on either the physical/optical properties of the matrix or the stability/surface state of the quantum dots. A number of encapsulation strategies have been identified and are discussed with regard to their effect on nanoparticle dispersion and concentration within silicone and epoxy matrices.