Self-Assembly of Bicomponent Molecular Monolayers: Adsorption Height Changes and Their Consequences

Codeposition of two molecular species [copper phtalocyanine (CuPc, donor) and perfluoropentacene (PFP, acceptor)] on noble metal (111) surfaces leads to the self-assembly of an ordered mixed layer with a maximized donor-acceptor contact area. The main driving force behind this arrangement is assumed to be the intermolecular $\mathrm{C}\text{−}\mathrm{H}\cdots \mathrm{F}$ hydrogen-bond interactions. Such interactions would be maximized for a coplanar molecular arrangement. However, precise measurement of molecule-substrate distances in the molecular mixture reveals significantly larger adsorption heights for PFP than for CuPc. Most surprisingly, instead of leveling to increase hydrogen-bond interactions, the height difference is enhanced in the blends as compared to the heights found in single-component CuPc and PFP layers. The increased height of PFP in mixed layers points to an overall reduced interaction with the underlying substrate, and its influence on electronic properties like the interface dipole is investigated through work function measurements.

Figure 1

(a) Chemical structure of CuPc (above) and PFP (below). (b) and (c) $11.5\mathrm{nm}\times 11.5\mathrm{nm}$ images of the $1:1$ $\mathrm{PFP}+\mathrm{CuPc}$ blend on Ag(111) and Cu(111), respectively. The unit cell, substrate directions, and molecular schemes are overlaid on the images.

Figure 2

(a) Left: High-resolution ${\mathrm{C}}_{1\mathrm{s}}$ photoemission intensity of single-component and mixed layers on Ag(111) ($\mathrm{CuPc}=\text{blue}$, $\mathrm{PFP}=\text{yellow}$, $\mathrm{mix}=\text{green}$). Right: Also on Ag(111), ${\mathrm{C}}_{1\mathrm{s}}$ photoemission intensity of the mix at photon energies corresponding to the minimum ($h{\nu }_1$) and close to the maximum ($h{\nu }_2$) of the reflectivity curve. The fitted curve (red) is made up of three Gaussians, each corresponding to the different chemical environments of the carbon atoms in the molecules: PFP’s ${\mathrm{C}}_{\mathrm{F}}$ component (yellow filling), CuPc’s ${\mathrm{C}}_{\mathrm{H}}$ and ${\mathrm{C}}_{\mathrm{C}}$ components (blue filling), and a convolution of PFP’s ${\mathrm{C}}_{\mathrm{C}}$ and CuPc’s ${\mathrm{C}}_{\mathrm{N}}$ (grey filling). The associated atoms are marked correspondingly in the molecular diagrams in the center. (b) Reflectivity curve (red triangles) and ${\mathrm{C}}_{1\mathrm{s}}$, ${\mathrm{N}}_{1\mathrm{s}}$, and ${\mathrm{F}}_{1\mathrm{s}}$ photoelectron yield curves (green curves; those of C and N are offset for clarity) for the molecular mixture on Ag(111) (left) and Cu(111) (right). (c) Molecular adsorption heights of CuPc monolayers (left) [12, 13], PFP monolayers (right) [10, 11], and the $\mathrm{PFP}+\mathrm{CuPc}$ mix (center) on the Ag(111) (top) and Cu(111) (bottom) surfaces, including height changes in the mixed layers referred to the single-component monolayers. Error in $d_H$ is about 0.05 Å [27]. Distance to the substrate is not to scale.

Figure 3

(a) Work function changes associated with CuPc (blue) and PFP (yellow) on Ag(111) (round markers) and Cu(111) (square markers) substrates as a function of coverage. (b) The work function change ($\mathrm{\Delta }\mathrm{\Phi }$) caused by deposition of a given amount of PFP is reduced by an amount $\delta$ when PFP is deposited upon a precovered submonolayer $\mathrm{CuPc}/\text{metal}$ system ($\mathrm{\Delta }{\mathrm{\Phi }}_{\mathrm{mix}}$), as compared to deposition on the clean metal ($\mathrm{\Delta }{\mathrm{\Phi }}_{\text{pure}}$). The effect is observed on both Ag(111) (left) and Cu(111) (right). The blue bar ($\mathrm{CuPc}/\text{metal}$) and the black bar (clean metal) have been aligned in order to more easily compare $\mathrm{\Delta }{\mathrm{\Phi }}_{\mathrm{mix}}$ and $\mathrm{\Delta }{\mathrm{\Phi }}_{\text{pure}}$.