Role of Intermolecular Interactions on the Electronic and Geometric Structure of a Large $\pi$-Conjugated Molecule Adsorbed on a Metal Surface

The organic semiconductor molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) exhibits two adsorption states on the Ag(111) surface: one in a metastable disordered phase, prepared at low temperatures, the other in the long-range ordered monolayer phase obtained at room temperature. Notably, the two states differ substantial in their vertical bonding distances, intramolecular distortions, and electronic structures. The difference is explained by intermolecular interactions, which are particularly relevant for the long-range ordered phase, and which hence require attention.

Figure 1

(a) He-I UPS spectra (45° detection angle) for successive annealing steps up to 355 K after growth at 150 K. The vertical guide lines illustrate the shift of the three topmost $\mathrm{PTCDA}/\mathrm{Ag}(111)$ hybride orbitals ($L0$, $L1$, and $L2$) upon the transition from the low temperature (LT) state to the room temperature (RT) state. The inset shows the structure of PTCDA and a hardsphere model of the herringbone structure in the RT phase. (b) High-resolution LEED scans of PTCDA superstructure spots (3rd and 4th order), taken for successive annealing steps, demonstrating the ordering. The original growth temperature was 147 K; the coverage 50%. The electron energy was 23.5 eV.

Figure 2

STM images (recorded at 6–9 K) of (a) the room temperature (RT) and (b),(c) the low temperature (LT) phases of PTCDA on Ag(111). (a) Ordered herringbone island prepared by evaporation at 300 K and subsequent annealing at 550 K (RT phase). Image size $100\AA \times 100\AA$, $I=16\mathrm{nA}$, $U_{\mathrm{bias}}=-0.34\mathrm{V}$. (b) Dendritic islands of a layer of $\sim 10\%$ coverage deposited at 100 K. Image size $755\AA \times 755\AA$, $I=100\mathrm{pA}$, $U_{\mathrm{bias}}=-0.34\mathrm{V}$. (c) Enlargement of part of a dendritic island prepared as in (b). Image size $100\AA \times 100\AA$, $I=55\mathrm{pA}$, $U_{\mathrm{bias}}=-0.34\mathrm{V}$. (d) LT phase: reduced angular distribution of the PTCDA orientation (long axis of the molecule) with respect to the $[1\overline{1}0]$ direction of Ag(111) (bottom panel) and correlation of molecular orientation with $L2$ binding energy (top panel). 178 molecules within 3 dendritic islands have been evaluated. Red lines mark averages. Green dots in panels (a) and (c) mark the typical positions where the STS spectra have been recorded.

Figure 3

Vertical configuration of PTCDA molecules in (a) the low temperature and (b) the room temperature phases as derived from NIXSW analysis. A side view onto the anhydride groups is presented. Gray: carbon atoms, blue: anhydride oxygen atoms, red: carboxylic oxygen atoms. The vertical length scale is expanded by a factor of 3. Data in (b) from Ref. 4. (c) Spectroscopy of $L2$: High-resolution UPS spectrum, background corrected (black dots), measured at 40 K, 21.2 eV excitation energy, 40° detection angle, of 0.5 ML RT phase (prepared at 300 K), compared with STS spectra of type $A$ (red, thick solid line) and type $B$ (blue, dashed line) molecules in RT phase [Fig. 2a]. Thin dotted line: Superposition of the spectra for both types of molecules. Thick green/brown solid lines: STS spectra of an isolated and a molecule in the LT phase, respectively. Bar diagrams (width 20 meV): Energy distribution of peak $L2$ for isolated molecules (top, 142 molecules) and molecules in LT-phase (bottom, 178 molecules). Black bars: Molecules aligned within $\pm 10°$ along $[1\overline{1}0]$. White bars: Molecules aligned within $\pm 20°$ along $[2\overline{1}\overline{1}]$; cf. the statistical distribution in Fig. 2d. STS spectra are taken under following conditions: $U_{\mathrm{mod}}=4\mathrm{mV}$, ${\nu }_{\mathrm{mod}}=1233\mathrm{Hz}$, $U_{\mathrm{bias}}=-340\mathrm{mV}$, $I=0.16\mathrm{nA}$; at the positions of highest intensity of LUMO lobe, marked with the green dots in Figs. 2a, 2c.