Single PbS Research Articles

Hybrid electronic properties between the molecular and solid state limits: Lead sulfide and silver halide crystallites

Tiny single PbS crystals of ∼25 Å diameter are synthesized and studied optically in low-temperature colloidal solutions. Electron microscopic examination shows a simple cubic rock salt structure with a lattice constant unchanged, within experimental error, from the bulk value. These crystallites lack the near infrared electronic absorption characteristic of bulk PbS. The small crystallite absorbance in the visible rises more steeply than does the bulk absorbance. These results reflect electron and hole localization if one considers the variation in effective mass across the band structure. A simple discussion of localization anywhere in the Brillouin zone is given. For the first time, crystallite syntheses are carried out in solvent mixtures that form transparent glasses upon cooling. The PbS spectra are independent of temperature (at current experimental resolution) down to 130 K, in contrast to earlier results for quantum size exciton peaks in ∼20 Å ZnS crystallites. Previously published observations of size dependence in the excited state electronic properties of AgI and AgBr are explained as consequences of electron and hole localization in the small crystallites. AgBr appears to be the first indirect gap semiconductor to be examined in the regime where bulk properties are not fully formed.

The growth of epitaxial single crystal PbS 1−xSe x films by hot wall evaporation

Heteroepitaxial films of semiconducting PbS 1− x Se x on rock salt and mica substrates have been prepared and studied. The films have good crystalline perfection but have low electron mobilities, probably due to deviations from stoichiometry.

LEED-AES study of sulfide single crystals

The clean surfaces obtained by cleaving the sulfide single crystals, PbS, CdS, MoS 2 and ZnS were examined by means of low energy electron diffraction and Auger electron spectroscopy. It was found that Auger electron yield of sulfur in sulfide crystals depended strongly on its concentration in the top atomic layer. The δ−η method revealed that the backscattering electrons contributed significantly to Auger electron intensities. Measurements were also made on the energy dependence of the ionization cross section, and the results showed, to a first approximation, a good coincidence with the theoretical model proposed by Gryzinski. An approach to the quantitative analysis of Auger electron spectroscopy was also discussed.

A Direct-Reading Comparison Spectroradiometer

A modification of the Cary Model 14 for radiometry is described which allows recording in microwatts per square mlllimeter per nanometer on the ordinate and linear wavelength on the abscissa through the range 250-2500 nm. A 5.08-cm diam rotating integrating sphere is used to compare standard to unknown sources at a 30 c/s rate. The undispersed radiation from the sphere is directed through the entrance slit of the monochromator. After dispersion by the monochromator the energy is received by a single phototube or PbS detector as alternate pulses of standard and unknown energy. This design results in common optical elements throughout the instrument except for the standard and unknown sources themselves. An electrical calibration system permits an ordinate scale reading irradiance directly in microwatts per square centimeter per nanometer with simple provisions for range change. The instrument provides a dynamic range of 2 x 10(6) to 1. A large open area is provided for mounting unknown sources. The entrance optical system is designed so that remote sources can be viewed conveniently.