ATOMIC-FORCE MICROSCOPY OF MERCURY IODIDE CRYSTAL-GROWTH FROM POROUS-MEDIA AT ROOM-TEMPERATURE

A real time observation of the growth of, agglomerates of pyramidal cr ystallites of mercuric iodide on a porous glass surface using tapping mode atomic force microscopy is reported. Crystallites are observed to grow from the pores (5.0 nm diameter) onto the surface of the porous glass impregnated with mercuric iodide. immediately after fracturing t he impregnated porous glasses, topographical images reveal a surface w ith a root mean square roughness of 75-130 nm over a 5 mu m x 5 mu m a rea, which is typical for surfaces of fractured porous glass. After 3 h, 60-90 nm pyramidal structures with a high aspect ratio (pyramid bas e to height) begin to appear on the surface. Some of the pyramidal str uctures agglometrate and form structures much larger and higher than t he surrounding ones. The bottom of some of these agglomerated pyramids form equilateral triangles with dimensions of 300-500 nm, exhibiting a preferred orientation on the surface. 6 h after fracturing, micromet er-size crystallites are observed. Most of the pyramidal structures ar e no longer observed and mostly needlelike features are covering the s urface. Step heights consistent with multiples of the c axis of alpha- mercuric ioide are observed on the micrometer-sized crystallites. Some pyramidal structures are observed between the large crystallites, wit h sizes comparable to the early stage pyramidal structures. Raman and electronic spectra are also reported for mercuric iodide confined in p orous glasses. The Raman spectra have peaks located at 39 and 141 cm(- 1), indicating that the confined mercuric iodide is stabilized in the yellow, orthorhombic (beta phase) at room temperature. Confinement of mercuric iodide in pores with radii smaller than 3.75 nm results in th e appearance of a new band at 145 cm(-1) in the Raman spectra, suggest ing tine presence of a new or modified beta phase of mercuric iodide. Further evidence supporting the presence of a new phase of mercuric io dide is found in the electronic spectra where an energy gap of 2.7 eV is observed, and lies between the energy gaps of the red, tetragonal, alpha phase (2.2 eV) and the: yellow, orthorhombic beta phase (3.1 eV) of mercuric iodide. The atomic force microscopy measurements provide a unique method for real time monitoring of the crystal growth and mor phology changes of nanocrystalline mercuric iodide, while the spectros copy provides insight to the phonon structure and energy gaps of the c onfined material. (C) 1996 American Vacuum Society.