Control of the dipole layer of polar organic molecules adsorbed on metal surfaces via different charge-transfer channels

Organic molecules with a permanent electric dipole moment have been widely used as a template for further growth of molecular layers in device structures. Key properties of the resulting organic films such as energy level alignment (ELA), work function, and injection/collection barrier are linked to the magnitude and direction of the dipole moment at the interface. Using angle-resolved photoemission spectroscopy (ARPES), we have systematically investigated the coverage-dependent work function and spectral line shapes of occupied molecular energy states (MESs) of chloroaluminium-phthalocyanine (ClAlPc) grown on Ag(111). We demonstrate that the dipole orientation of the first ClAlPc layer can be controlled by adjusting the deposition rate and postannealing conditions, and we find that the ELA at the interface differs by $\sim 0.4\mathrm{eV}$ between the Cl up and down configurations of the adsorbed ClAlPc molecules. These observations are rationalized by density functional theory (DFT) calculations based on a realistic model of the ClAlPc/Ag(111) interface, which reveal that the different orientations of the ClAlPc dipole layer lead to different charge-transfer channels between the adsorbed ClAlPc and Ag(111) substrate. Our findings provide a useful framework toward method development for ELA tuning.

Figure 1

The chemical structure of (a) ${\mathrm{H}}_2\mathrm{Pc}$ and (b) ClAlPc and (c) the corresponding side views of the ClAlPc/Ag(111) interface for the Cl up and down configuration. The pink, gray, brown, green, and blue spheres represent the H, N, C, Cl, and Al atoms, respectively.

Figure 2

VL shift as a function of coverage for (a) ${\mathrm{H}}_2\mathrm{Pc}$, AG ClAlPc, and AN ClAlPc on Ag(111), and (b) AG ClAlPc on Ag(111) for various deposition rates, i.e. 0.1, 3, and $5\AA /min$.

Figure 3

Coverage dependent angle-resolved photoemission spectra integrated from $-0.3{\AA }^{-1}$ to $0.3{\AA }^{-1}$ for AG ClAlPc at a deposition rate of (a) $0.1\AA /min$, (b) $3\AA /min$, and (c) $5\AA /min$. Blue, black, and green colors denote the EDCs corresponding to ClAlPc at 0, 1, and 2 ML.

Figure 4

Angle-resolved photoemission spectra integrated from $-0.3{\AA }^{-1}$ to $0.3{\AA }^{-1}$ for (a) 1 ML ${\mathrm{H}}_2\mathrm{Pc}$ and 1 ML AN ClAlPc on Ag(111), and (b) 1 ML AG ClAlPc on Ag(111) at the deposition rates of 3 and $0.1\AA /min$. The brown and green curves indicate two components from fitting. The gray curve indicates the fitting background. The red dashed curve represents the total fitting curve.

Figure 5

Angle-resolved photoemission spectra integrated from $-0.3{\AA }^{-1}$ to $0.3{\AA }^{-1}$ for (a) 1 ML AN ClAlPc on Ag(111) and (b) 1 ML AG ClAlPc at the deposition rate of $0.1\AA /min$ (upper panel), together with the calculated total DOS of the ClAlPc/Ag(111) interface and partial DOS of the OPGs and ILR (middle panel) as well as the partial DOS of the Cl and Al atoms (lower panel) as a function of energy relative to Fermi level (0 eV) for the Cl up and down configuration. An offset of $-1.3\mathrm{eV}$ has been imposed on the computational DOS. The insets show the magnified view for partial DOS of Cl and Al in the energy range from $-14$ to $-9.5\mathrm{eV}$.

Figure 6

Angle-resolved photoemission spectra at $k_{\parallel }=0.8{\AA }^{-1}$ for (a) 1 ML AG and AN ClAlPc and (b) 2 ML AG and AN ClAlPc on Ag(111). The brown and green curves indicate two fitting components. The gray curve indicates the fitting background. The red dashed curve represents the total fitting curve. The brown and green curves indicate two fitting components. Also shown are two-dimensional images of VL and HOMO position as a function of ClAlPc coverage for (c) AN ClAlPc and (d) AG ClAlPc on Ag(111).