No abstract

Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed-packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 Å thick, whose lateral dimensions vary from 12 to 90 Å depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six-membered rings. The optical absorption spectra of PbI2 and BiI3 can be quantitatively understood in terms of the measured cluster size distributions.

The electronic and structural properties of two different edge terminations in normal graphite layer edges were investigated at atomic resolution using scanning tunneling microscopy (STM) and electronic band structure calculations. An emphasis is placed on distinguishing the electronic structure of [1,1,2̄,0] (“armchair”) and [1,0,1̄,0] (“zigzag”) edge terminations. Experimental STM investigations reveal that the [1,1,2̄,0] termination exhibits a (3×3)R30° superstructure in the local electron density of states profile superimposed on the normal graphite lattice. The [1,0,1̄,0] edge shows a different superstructure typified by two to three atomic lattice cells of high electron density parallel to the edge. The armchair edge superstructure persists for 2–3 nm (equivalent to about 10 unit cells) into the bulk layer. By modeling these edge types using extended Hückel tight binding calculations, we show that the results are consistent with an unreconstructed edge geometry. The details of the edge states do not appear to be particularly sensitive to the chemical composition of the edge. The experimental results are in general agreement with previously reported modeling studies of these edge states.

No abstract

No abstract