CHARACTERIZATION OF THE SURFACES OF THE CONDUCTING SALTS ALPHA-(BEDT-TTF)(2)X (X=I3, IBR2) BY SCANNING TUNNELING AND ATOMIC-FORCE MICROSCOPY

We carried out scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements of the alpha-(BEDT-TTF)2X (X=I3 and IBr2) salts and interpreted the results on the basis of the partial density rho(r0, e(f)) and total electron density rho(r0) plots calculated by the extended Huckel tight-binding method. Large scale STM images show surface modifications induced by the tip-surface interactions. The STM and AFM images show the patterns of the cation layers. The rho(r0, e(f)) plots are essential in relating the features of the observed atomic-resolution STM images to the surface structures and show that the ethylene-group hydrogen atoms of the protruded cations dominate the STM images. The dependence of the STM images of the BEDT-TTF salts on the tunneling gap resistance is probably caused by the tip-induced conformational change of the ethylene groups of the surface cations.