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.
This work describes full wafer encapsulation of released, self-assembled monolayer (SAM) coated, multi-level polysilicon surface micromachines using the anodic bonding technique. This process has been utilized to protect fragile surface micromachines from damage due to particles, moisture contamination, and post-release die handling. The anodic bonding process was optimized to ensure strong glass-to-wafer bonds, while maintaining the effectiveness of liquid-phase and vapor-phase deposited SAM coatings. The temperature, time, and voltage effects on each SAM coating was analyzed. Glass-to-silicon and glass-to-SAM coated silicon had shear strengths of approximately 18 MPa. Glass-to-polysilicon bonds had lower shear strengths of approximately 10 MPa. Bonds were hermetic to 5 X 10-8 atm-cm3/s.
A metal ion sensitive, fluorescent lipid-bilayer material (5% PSIDA/DSPC) was successfully immobilized in a silica matrix using a tetramethoxysilane (TMOS) sol-gel procedure. The sol- gel immobilization method was quantitative in the entrapment of self-assembled lipid-bilayers and yielded thin films for facile configuration to optical fiber platforms. The silica matrix was compatible with the solvent sensitive lipid bilayers and provided physical stabilization as well as biological protection. Immobilization in the silica sol-gel produced an added benefit of improving the bilayer's metal ion sensitivity by up to two orders of magnitude. This enhanced performance was attributed to a preconcentrator effect from the anionic surface of the silica matrix. Thin gels (193 micron thickness) were coupled to a bifurcated fiber optic bundle to produce a metal ion sensor probe. Response times of 10 - 15 minutes to 0.1 M CuCl2 were realized with complete regeneration of the sensor using an ethylenediaminetetraacetic acid (EDTA) solution.
Cathodoluminescence (CL) characterization in a demountable vacuum chamber is an important benchmarking tool for flat- panel display phosphors and screens. The proper way to perform these measurement is to minimize the effects of secondary electrons, excite the phosphor/screen with a uniform beam profile, and maintain a clean vacuum environment. CL measurements are important for preliminary evaluation and lifetesting of phosphor powders and screens prior to incorporation into the FPD. A survey of many CL characterization systems currently in use revealed the myriad of spectroradiometers, colorimeters, electron guns, vacuum pumps, mass spectrometers, etc. that introduce many avenues for error that are often difficult to isolate. A preliminary round-robin experiment was coordinated by Sandia and involved five other research groups. The purpose of this experiment was to obtain an indication of equipment capabilities and instrument variations, as well as reliability and consistency of results. Each group was asked to measure the luminance and chromaticity coordinates of a Y3Al2Ga3O12:Tb pellet and calculate the luminous efficiency. Pellets were chosen in order to reduce errors associated with processing and handling of powders or screens. Some of the data reported in this experiment were in good agreement, while others differed significantly. Determining sources of error in CL measurements is an ongoing effort. By performing this experiment, we were able to identify some of the causes of error and develop a characterization protocol for display phosphors.
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