SCANNING TUNNELING MICROSCOPIC, OPTICAL, AND SCANNING TUNNELING SPECTROSCOPIC CHARACTERIZATION OF SIZE-QUANTIZED CADMIUM SELENIDE PARTICULATE FILMS INSITU GENERATED AT MONOLAYER INTERFACES

Cadmium selenide (CdSe) particulate semiconductor films were in situ generated at positively charged, dioctadecyldimethylammonium bromide (DODAB) monolayers floating on an aqueous subphase which contained appropriate concentrations of sodium selenosulfate and cadmium nitrilotriacetate. DODAB-monolayer-supported, 300-400 angstrom thick, CdSe particulate films were quantitatively transferred to solid supports by horizontal lifting. Absorption spectra showed shoulders at 605 nm and absorption edges at 646 nm, which led to the assessment of a direct band-gap of 1.92 eV and 65 +/- 10 angstrom for the diameter of the particles in the CdSe particulate film. Heating at 280-degrees-C for 5 min shifted the absorption edge to a longer wavelength and the direct band-gap to that corresponding to bulk crystalline CdSe (1.73 eV). Dark resistivities of a glass-supported CdSe particulate film were also observed to decrease from 10(7)-10(8) to 1-10 OMEGA cm upon heating. The increased conductance accompanying annealing, along with the recovery of the bulk band-gap, substantiates the proposed size quantization in CdSe particulate films prepared at DODAB interfaces. Scanning tunneling microscopic images indicated the presence of interconnected CdSe particles with 50-60 angstrom mean diameters. Current-voltage (I-V) and differential conductance vs voltage (dI/dV vs V) tunneling spectra of CdSe particulate films were determined in air. The obtained data indicated n-type Schottky behavior, sample- and tip-distance-dependent band bending, and approximately 2.0 eV band-gap energies.