Strongly Reshaped Organic-Metal Interfaces: Tetracyanoethylene on Cu(100)

The interaction of the strong electron-acceptor tetracyanoethylene with the Cu(100) surface is studied with scanning tunneling microscopy experiments and first-principles density functional theory calculations. We compare two different adsorption models with the experimental results and show that the molecular self-assembly is caused by a strong structural modification of the Cu(100) surface rather than the formation of a coordination network by diffusing Cu adatoms. Surface atoms become highly buckled, and the chemisorption of tetracyanoethylene is accompanied by a partial charge transfer.

Figure 1

DFT-LDA optimized structural geometry for the one-dimensional chain arrangement of TCNE on the Cu(100) surface (a) for the reconstructed model and (b) for the adatom model. A transparent plane is used to emphasize the magnitude of reconstruction of the topmost Cu layer.

Figure 2

Topographic STM images of a TCNE chain (a) obtained experimentally with a $\mathrm{Pt}/\mathrm{Ir}$ tip ($V=1\mathrm{mV}$, $I=5\mathrm{nA}$) from Ref. 10 and (b) calculated on a reconstructed Cu(100) surface with a Pt(111) tip ($V=1\mathrm{mV}$, $I=15\mathrm{nA}$). (c) Calculated charge density of the valence electrons for the reconstructed model (a black plane is used to highlight the density drop). (d) Calculated STM image of a TCNE chain for the adatom model. All image domains are $31\times 18{\AA }^2$.

Figure 3

(a) Total DOS and projected DOS for a buckled Cu surface without TCNE and (b) after adsorption of TCNE, and total DOS of an isolated TCNE one-dimensional chain. (c) Wave function contours of the molecular orbitals located at $-8.9$ and $-5.5\mathrm{eV}$ in (b) and HOMO and LUMO of isolated TCNE.