Selective Nontemplated Adsorption of Organic Molecules on Nanofacets and the Role of Bonding Patterns

A key element of functionalizing nanocrystals with organic molecules is the nontemplated selective adsorption of different molecules on different facets. Here we report scanning-tunneling-microscopy images of perylene-3,4,9,10-tetracarboxylic-dianhydride and 2,5-dimethyl-$N$,$N^{\prime }$-dicyanoquinonediimine on silver, demonstrating selective adsorption on different facets. We also report first-principles calculations that account for the data and show that bonding, which controls selectivity, occurs via the end atoms, while the molecule’s midregion arches away from the substrate. The results are also consistent with data that have been interpreted in terms of bonding via the midregion.

Figure 1

STM images of PTCDA and DMe-DCNQI on a Ag(775) substrate. (a) $30\mathrm{nm}\times 30\mathrm{nm}$ image of DMe-DCNQI and PTCDA, evaporating PTCDA first. The molecules are coadsorbed on (221) facets, while (111) facets are covered with PTCDA molecules (oval). (b) $50\mathrm{nm}\times 50\mathrm{nm}$ image of DMe-DCNQI and PTCDA, evaporating DMe-DCNQI first. DMe-DCNQI adsorbed on the (221) facet (dark arrow), while PTCDA adsorbed on the (111) facet (gray arrow). Uncovered (111) areas are visible (triangular frame). (c) $30\mathrm{nm}\times 30\mathrm{nm}$ image of $\mathrm{DMe}\mathrm{\text{−}}\mathrm{DCNQI}/\mathrm{Ag}(221)$ and $\mathrm{PTCDA}/\mathrm{Ag}(111)$. (d) $6\mathrm{nm}\times 6\mathrm{nm}$ image of DMe-DCNQI on Ag(221). (e) Schematic structure of $\mathrm{DMe}\mathrm{\text{−}}\mathrm{DCNQI}/\mathrm{Ag}(221)$ superimposed on the STM image (orientation determined from the STM image). (f) $6\mathrm{nm}\times 6\mathrm{nm}$ image of PTCDA on Ag(111). (g) Schematic structure of $\mathrm{PTCDA}/\mathrm{Ag}(111)$ superimposed on the STM image (orientation determined from the STM image). (h),(i) Images ($20\mathrm{nm}\times 20\mathrm{nm}$ and $30\mathrm{nm}\times 30\mathrm{nm}$, respectively) of only DMe-DCNQI evaporated, shown adsorbed only on Ag(221) facets [light (yellow) arrows]; Ag(111) facets are bare (black arrows). The tunneling current is 0.1 nA and the sample bias is 0.8 V for all the STM images.

Figure 2

Optimized configurations of (a) PTCDA and (b) DMe-DCNQI on the Ag(111) surface and on various stepped surfaces. Binding energies are indicated.

Figure 3

Experimental C $K$-edge x-ray absorption spectra (black lines) of (a) monolayer and (b) thick-film PTCDA on Ag(111). Theoretical spectra [light (red) lines] of (a) monolayer and (b) molecular crystal.

Figure 4

Spherically integrated valence electron density around different Ag atoms as a function of the integration radius. Ag atoms in the bulk (black line) and at the surface below the PTCDA midregion [light (cyan) line] are shown. The dotted line indicates the number of valence electrons in a neutral Ag atom.