Uniform films occupying ~5 cm 5 cm were prepared by depositing 1000 lL of solution on a glass substrate. The solution was confined by inert sidewalls and allowed to dry. Films with thicknesses ranging from 1±3 lm were fabricated. Atomic force microscopy revealed a surface roughness of 4 nm over a 1 lm 1 lm region. The films exhibited interference fringes apparent in the absorption spectrum. Fabry±Perot fringe spacing indicated a phase index of 1.69 ± 0.03 between 1200 and 1500 nm. The films formed the core of planar waveguides on silica (refractive index ~1.5), in which light is well-confined to the DNA±PbS material.
TEM and EFTEM: TEM images were acquired using the transmitted-and scattered-electron detectors of a HD-2000 scanning transmission electron microscope operating at 200 kV (Hitachi). EFTEM imaging was performed with a Tecnai 20 (FEI, Eindhoven) transmission electron microscope equipped with an electron imaging spectrometer (Gatan, Pleasanton, CA) operated at 200 kV. Phosphorusenhanced images were collected at 155 eV energy loss, just beyond the L II,III phosphorus ionization edge. A 20 eV energy window was used. Bright-field images were collected at zero loss. For EFTEM sample preparation, carbon films 3±5 nm in thickness were floated onto 1000-mesh grids. The DNA±PbS solution was diluted in a mixture of Hepes (10 mM, pH 8) and MgCl 2 (1 mM). A small drop was placed onto the grid and allowed to adsorb to the film for 5 min. The grid was then rinsed three times with filtered distilled water and allowed to dry in air.