Billie Lynn Abrams

Catalytic Properties of Single Layers of Transition Metal Sulfide Catalytic Materials

Single layer transition metal sulfides (SLTMS) such as MoS2, WS2, and ReS2, play an important role in catalytic processes such as the hydrofining of petroleum streams, and are involved in at least two of the slurry‐catalyst hydroconversion processes that have been proposed for upgrading heavy petroleum feed and other sources of hydrocarbon fuels such as coal and shale oils. Additional promising catalytic applications of the SLTMS are on the horizon. The physical, chemical, and catalytic properties of these materials are reviewed in this report. Also discussed are areas for future research that promise to lead to advanced applications of the SLTMS.

Development of solid state light sources based on II-VI semiconductor quantum dots

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.

Encapsulation of Nanoparticles for the Manufacture of Solid State Lighting Devices

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.

Thermal quenching of cathodoluminescence from ZnS:Ag,Cl powder phosphors

Thermal quenching of cathodoluminescence (CL) was studied by incrementally increasing the temperature of a ZnS:Ag,Cl phosphor without exposure to a continuous electron beam and measuring the decreased CL intensity. A characteristic thermal quenching temperature of 150 °C with an activation energy (Ea) of 0.87 eV was measured. In addition to the reduced CL intensity, the spectra shifted to longer wavelengths and changed shape at elevated temperature due to band gap narrowing at high temperatures and to copper contamination from the heater stage. The CL spectral distribution and intensity were 100% recoverable upon cooling back to room temperature when the electron beam exposure was <1C∕cm2.