Colloidal silica spheres with 200nm, 250nm, and 290nm diameters were self-assembled with single crystal crystallites 4-5mm wide and 10-15mm long. Larger spheres with diameters between 1000-2300nm were self-assembled with single crystal crystallites up to 1.5mm wide and 2mm long. The silica opals self-assembled vertically along the [100] direction of the face centered cubic lattice resulting in self-templated opals. Inverse opal photonic crystals with a partial band gap possessing a maximum in the near infrared at 3.8μm were constructed from opal templates composed of 2300nm diameter spheres with chalcogenide Ge33As12Se55 (AMTIR-1), a transparent glass in the near infrared with high refractive index. Inverse gold and gold/ polypropylene composite photonic crystals were fabricated from synthetic opal templates composed of 200-290nm silica spheres. The reflectance spectra and electrical conductance of the resulting structures is presented. Gold was infiltrated into opal templates as gold chloride and heat converted to metallic gold. Opals partially infiltrated with gold were co-infiltrated with polypropylene plastic for mechanical support prior to removal of the silica template with hydrofluoric acid.
Colloidal biomimetic disc shaped metallic gold shells with a uniform size distribution were synthesized using red blood cells as sacrificial templates. Red blood cells do not reproduce by dividing; hence they are truly colloidal particles. They are almost completely filled with hemoglobin allowing for an extremely dynamic work cycle with long intercellular vacations separated by self-destructive workloads on the cell surface. This method of exchange is emulated in the presented research. The colloidal disc shaped gold shells were coated with multiple layers of 50nm fluorescent polystyrene spheres followed by chemical removal of the gold core. This process yielded hollow synthetic biomimetic membranes with a strong optical signature that are diffusely permeable to water and impervious to particles larger than a few nanometers. Currently, the most successful synthetic intravascular oxygen carrying materials are perfluorocarbons; however, they break down quickly in roughly 50 hours from overexposure to their in vivo workload. The meso-porous membrane cages will be filled with hundreds of fibrous spheroid conglomerates composed of perfluorocarbon chains that can protrude through the meso-porous membrane as they thermally jostle about the cage. This is to statistically limit the exposure time of individual polymer strands to the self-destructive work at the surface and hopefully will greatly increase the effective functioning lifetime of the perfluorocarbon-based synthetic red blood cell. The artificial membranes are intentionally designed to be weak allowing them to flex under normal pressures and to hopefully burst under more extreme conditions such as blockage.
A technique for the fabrication of colloidal gold nano-wire and nano-dumbbell shaped particles using carbon nanotubes and rod shaped viruses as templates is described. The gold (Au) encapsulation process was accomplished by the precipitation of gold chloride from aqueous solutions. When this process was conducted in the presence of hydroxylated C60, small pieces of phase-separated composites of AuC60 appeared to have formed. These nano-clusters may turn out to be large noble metal analogs of the alkali metal fullerides with the smallest geometrically possible Au aggregate consisting of 55 gold atoms. The existence of noble metal fullerene composites has been previously theorized. The alkali metal fullerides are examples of phase separated solids and have exhibited superconductivity with temperatures as high 33K. The mechanism required for the binding energy between C60 and gold has been observed to exist between C60 and many of the mirror metals (Al, Ag, Au, Cu, Ni). This binding energy is a charge transfer from the metal Fermi level into the C60 LUMO. If this bonding energy, is greater than the metals coagulation energy an Au/C60 size terminated mechanism during the formation of the gold aggregates by the adhesion of C60 to the surface is energetically favorable.
Self assembled synthetic opals composed of spheres with 200-1500nm diameters were infiltrated with poly[2-methoxy-5-(20-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and Poly [(m-phenylenevinylene)-co- (2,5-dioctoxy-p-phenylenevinylene)] (PmPV) which are luminescent polymers. The luminescence spectra of the polymers was blue shifted when infiltrated deep into the nano-scopic opal pores relative to the luminescence spectra from thick films of the bulk polymers. Inverse composite plastic and luminescent polymer opals were constructed resulting in a significant blue shift in the peek luminescence of MEH-PPV from 568nm for the bulk polymer to 520nm in the inverse opal structure. We found that the majority of the blue shifting observed could be explained by the formation of isolated polymer strands within the opals. Silica spheres were coated with MEH-PPV, dispersed in H2O and coated with polyelectrolytes which recharged and sterically stabilized the colloidal surfaces. Luminescence spectra from individual and self-assembled silica spheres coated with MEH-PPV were also blue shifted.
The material constitution of modern photocathodes (i.e. third generation) has remained a constant for almost two decades. The active GaAs layer is grown by metal organic chemical vapor deposition (MOCVD) and processed to create a negative electron affinity (NEA) surface for photoemission. Thus, these types of cathodes are limited in their spectral response by the band gap energy of the GaAs. There is interest in extending this range past 1000nm while preserving a high quantum efficiency (QE). This would allow taking advantage of an increased luminescence of night sky in the infrared. MOCVD grown InGaAs photocathodes have a photoresponse (PR) in the near infrared. Still, a major drawback to date has been its low QE. We believe that the use of molecular beam epitaxy (MBE) to grow this alloy will permit the fabrication of a higher quality device beyond today's standards, with improved equivalent background illumination and higher QE over a 700nm to 1100nm spectral range. To demonstrate this concept two reflection mode InGaAs photocathode were grown. These cathodes were NEA activated with Cs:O in situ in the MBE reactor after their growth and their PR recorded. Following the activation, optical characterization techniques (i.e. photoluminescence, Raman spectroscopy) were employed to probe electron and phonon energy to relate fundamental material parameters to the observed PR. The collected information is being used to correct and enhance growth characteristics to increase spectral response and QE.
Modern image tube intensifier photocathodes rely on a GaAs active layer, which has traditionally been grown using metallorganic chemical vapor deposition (MOCVD) due to its high throughput and lower cost of operation. Molecular beam epitaxy (MBE) processes have not been thoroughly investigated in that context. The latter technique demonstrates greater structural interface control as well as an improved growth quality for a multitude of applications. Still, at this point it is uncertain, considering actual fabrication techniques for image intensifiers, that the higher growth quality will result in an improvement of devices. Studies are being carried out to compare fundamental optical parameters between GaAs photocathodes grown by both MOCVD and MBE following the same growth and fabrication guidelines. These experiments involve using photoluminescence and Raman spectroscopy to obtain electron and phonon energy information on the materials. An atomic force microscope (AFM) is employed to compare the surface roughness of both methods. In addition, the white light responses of the photocathodes are also evaluated during the creation of a negative electron affinity (NEA) surface to observe any differences between the two growth techniques.
We present the development of a near infrared image intensifier tube based on a Generation III configuration. The photocathode driving this tube is based on a negative-electron-affinity (NEA) InGaAs design. The surface and crystalline quality of the InGaAs active layer for this device is characterized by x-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive x-rays (EDX), and Auger spectroscopy. Room temperature and 77 K photoluminescence (PL) and transmission measurements indicate the near infrared responsivity (1.2 - 1.7 micrometers ) of the InGaAs active layer. Reflection mode measurements of the photoresponse (PR) in an ultra high vacuum environment produced white light sensitivities of 100 (mu) A/lumen and quantum efficiencies of approximately 1% for wavelengths of 1300 - 1600 nm at 300 K. Sealed image tubes were created, and early sealed tube results show low quantum efficiencies (approximately 0.1% 1550 nm) due to nonoptimized active layer thickness.
We extend our previous measurements of the lineshape of GaAs at the E1 transition (2.9 eV). This study covers the combined effect of temperature and carrier concentration along with a discussion of the effect of the electric field intensity and the field inhomogeneity within a depth of 20 nm from the surface. A systematic study of changes in the lineshape of the above bandgap transition, E1 as a function of temperature (80 - 400 K) and carrier concentration (CC) (2 - 200 X 1016cm-3umps with different penetration depths, so that the modulation has a gradient with respect to the depth within the sample. The application of this technique is demonstrated for a two-dimensional electron gas (2DEG) of a modulation doped heterojunction in comparison with conventional PR. In one case the heterojunction of interest was buried under two highly doped GaAs and AlGaAs layers 40 nm thick. We show that this heterojunction is barely distinguishable in a PR measurement. Nevertheless, at room temperatutric field on the photoreflectance lineshape is discussed. The observed effect may be applied as an optical measurement of the electric field and the carrier concentration within a depth of about 17 nm from the surface/interface.
We report photoreflectance studies ofMOCVD grown doped GaAs at the higher energy
transition E1( 2.9 eV). We are especially interested in the variation ofboth the energy position
and the broadening parameter F of the E1 transition with doping concentration. Above 1 x
10'8cin3 for Si:GaAs and ' 7 X 1018 for Zn:GaAs, we observe an increasing overlap of B1 and
E1 + Li structures. Evaluation of r based on curve fitting of the KramersKronig analysed
data shows a nearly linear relation between F and the logarithm of carrier concentration. This
observation has potential application in the determination of carrier concentration for heavily
doped films.
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