Quantitative analysis of the electronic decoupling of an organic semiconductor molecule at a metal interface by a monolayer of hexagonal boron nitride

The adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a large height of 3.37 $\AA$ and only a minute downward displacement of the functional anhydride groups (0.07 Å) occurs, yielding adsorption heights that agree with the sum of the involved van der Waals radii. Thus, already a single hBN layer leads to a decoupled (physisorbed) molecule, contrary to the situation on the bare Cu(111) surface.

Figure 1

(a) Overview LEED pattern of one monolayer PTCDA on hBN/Cu(111) with superimposed calculated spot positions and reciprocal unit cell (green). Different colors refer to six symmetry equivalent domains. The pattern was recorded at an electron energy of 30.4 eV and a sample temperature $T$ of 110 K. (b) Zoom-in of the detail marked in (a). (c) STM image of PTCDA molecules on hBN/Cu(111) arranged in a herringbone pattern. The image was recorded at $U_{\mathrm{bias}}=+1.5\mathrm{V}$ and $I=500\mathrm{pA}$ at $T=300\mathrm{K}$. (d) Corresponding hard-sphere model of the unit cell. The covalent atom radii in the molecules are drawn at 75% of their sizes for clarity. The envelope of the vdW spheres and the unit cell of the PTCDA lattice (red) are indicated.

Figure 2

(a) XPS data for the C $1s$ level of PTCDA on different surfaces. Contributions from the different C atoms have been fitted and are indicated by different colors; red: different C atoms of the perylene core; blue: functional carbon atoms; dashed: respective satellites; dashed gray: nonassigned satellites. For details of the assignment, see Ref. [23] [Ag(100) spectra taken from Refs. [23, 34]; Cu(100) spectrum taken from Ref. 24]. (b) UPS data of the bare hBN/Cu(111) interface (black curve), and the PTCDA monolayer and bilayer on hBN/Cu(111) (red curves). The spectra are angle integrated over an emission angle between $8^{\circ }$ and ${25}^{\circ }$, and were normalized by the height of the Fermi edge. A zoom of the Fermi edge is shown. Spectra were measured using He II$\alpha$ radiation in ${45}^{\circ }$ incidence. For comparison, a gas phase spectrum of PTCDA is shown (blue curve, taken from Ref. [35]; the spectrum was aligned at the HOMO position).

Figure 3

(a) Electron yield curves from NIXSW data (angle integrated) for ${\mathrm{O}}_{\mathrm{carb}}$, ${\mathrm{O}}_{\mathrm{anh}}$, ${\mathrm{C}}_{\mathrm{funct}}$, and ${\mathrm{C}}_{\mathrm{peryl}}$ (vertically shifted). Photon energies are given relative to the Bragg energy of Cu(111). (b) Argand diagram showing the results of all data sets and the respective averaged values (heavy symbols). Shaded areas mark the envelope of the error bars of the data sets. The O $1s/\mathrm{C}1s$ level was evaluated on five/four different positions on the sample. Note that the coherent position of the ${\mathrm{C}}_{\mathrm{peryl}}$ signal is by $\sim 0.03$ larger than those of the other three species, which are about the same. (c) Side views of PTCDA/hBN/Cu(111) along the short and long molecular axes. The vertical distances within the PTCDA molecule are enlarged by a factor of 4 and the covalent radii of PTCDA are drawn at 75% of their sizes in order to better show the very small downwards bending of the terminal anhydride groups. For the hBN layer the vdW spheres are indicated.

Figure 4

Adsorption heights of chemically discernible groups in PTCDA on various surfaces as determined by NIXSW [22, 23, 24, 25] corrected (i.e., reduced) by the vdW radii of the respective surface atoms vs the adsorption energies $E_{\mathrm{ads}}$ (data marked by “a” and “b” taken from Ref. [23] and Ref. [38], respectively). For Cu(100) and Cu(111) (marked by $*$) $E_{\mathrm{ads}}$ values are not available and horizontal data point positions are only qualitatively placed according to the work-function trend [24]. The data illustrate the trend of the bonding height from stronger chemisorptive bonding (left) to weak physisorptive bonding (right). The vdW radii were taken from Ref. [39]: $r_{\mathrm{Ag}}^{\mathrm{vdW}}=1.72$ Å, $r_{\mathrm{Cu}}^{\mathrm{vdW}}=1.40$ Å, $r_{\mathrm{Au}}^{\mathrm{vdW}}=1.66$ Å. For the hBN substrate the boron radius was used, because it is larger ($r_{\mathrm{B}}^{\mathrm{vdW}}=1.65$ Å; $r_{\mathrm{N}}^{\mathrm{vdW}}=1.46$ Å) and because B is located by 0.03 Å higher than N [14]. For Ag(111), Cu(111), and Au(111), the positions of ${\mathrm{C}}_{\mathrm{peryl}}$ and ${\mathrm{C}}_{\mathrm{funct}}$ were not analyzed individually. Thus, averaged values for all C atoms (${\mathrm{C}}_{\mathrm{peryl}}+{\mathrm{C}}_{\mathrm{funct}}$) are shown there. For the Au(111) surface the position of the O atoms could not be determined for experimental reasons [22]. The lines linking the data points are guides for the eye. Dashed lines indicate incomplete data. The horizontal dashed lines mark the vdW radii of carbon and oxygen. The value $r_{\mathrm{C}}^{\mathrm{vdW}}=1.75$ Å is valid for $s{p}^2$ hybridized carbon in perylene, in the direction perpendicular to the molecular plane [39]. The value $r_{\mathrm{O}}^{\mathrm{vdW}}=1.65$ Å is the average of 1.6 and $1.7\AA$, the range covering the generally found radii for $s{p}^2$ hybridized oxygen parallel to its double-bond axis [39].