ELECTRONIC ORIGIN OF STM IMAGES OF INSULATING MOLECULES ON A METALLICSURFACE - CATION AND ANION LAYER IMAGES OF BEDT-TTF SALTS

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) studies were carried out for three conducting salts kappa-(BEDT-TTF)2X , X = Cu(NCS)2, Cu[N(CN)2]Br and Cu[N(CN)2]Cl, and the STM and AFM ima ges were analyzed by calculating partial density rho(r0, e(f)) and the total density rho(r0) plots, respectively. The rho(r0, e(f)) plot of the cation layer is dominated by the ethylene groups of BEDT-TTF, whic h are the 'insulating part' of BEDT-TTF, due to their orbital mixing i nto the valence bands and their proximity to the scanning tip. The rho (r0, e(f)) plot calculated for the cation/anion double layer of kappa- (BEDT-TTF)2Cu(NCS)2 shows that, although the orbital mixing of the ins ulating anion layer into the valence bands is not strong, it is suffic ient enough for the anion layer to be detected by STM. Our study stron gly suggests that an orbital-mixing mechanism is responsible for the S TM imaging of insulating compounds on a metallic surface.