Growth of Metal Phthalocyanine on Deactivated Semiconducting Surfaces Steered by Selective Orbital Coupling

Using scanning tunneling microscopy and density functional theory, we show that the molecular ordering and orientation of metal phthalocyanine molecules on the deactivated Si surface display a strong dependency on the central transition-metal ion, driven by the degree of orbital hybridization at the heterointerface via selective $p-d$ orbital coupling. This Letter identifies a selective mechanism for modifying the molecule-substrate interaction which impacts the growth behavior of transition-metal-incorporated organic molecules on a technologically relevant substrate for silicon-based devices.

Figure 1

(a)–(c) Large scale STM topography images of submonolayer coverage of (a) ZnPc, (b) CuPc, and (c) CoPc deposited on the deactivated Si surface at the same condition. (d)–(f) Expanded STM topography images of (d) ZnPc, (e) CuPc, and (f) CoPc showing the packing of $M\mathrm{Pc}$ molecules within a typical domain. ZnPc exhibits the highly ordered tilted molecular configuration. CuPc displays shifts in the molecular packing which are outlined by the dashed white lines and gap defects as highlighted by the dashed red ovals. CoPc shows both tilted and flat-lying molecular orientations, with the tilted domain outlined by the dashed green lines. The green and blue bars in (d)–(f) represent individual Pc molecules with tilted orientation. Molecular assignment in (f) is further corroborated in Fig. S2 [39]. Scanning conditions for (a)–(f): $V_s=+2.0–2.5\mathrm{V}$, $I_t=4–30\mathrm{pA}$.

Figure 2

(a) and (b), STM topography images of flat-lying (a) CuPc and (b) CoPc molecules taken in regions between tilted molecular domains. The three stable flat-lying CoPc orientations allowed by the substrate symmetry are highlighted by the red, blue, and green dashed rectangles, exhibiting registration with the underlying surface (solid rectangles). The dashed white circle highlights a single molecule rotating between all three orientations. (c) and (d), STM topography image of a single flat-lying CuPc molecule (c), and CoPc molecule (d) on the deactivated Si surface (overlaid unit cell grid). Scanning conditions for (a)–(d): $V_s=+1.3–2.0\mathrm{V}$ [$V_s=-2.0\mathrm{V}$ in (c)], $I_t=3–25\mathrm{pA}$.

Figure 3

(a) Schematic diagram of $d$-orbital filling for ZnPc, CuPc, and CoPc. (b) Schematic diagram of the orbital hybridization mechanism between a CoPc molecule and the Si(111)-B surface. Note that the back-bond surface state (the discrete energy level in pink) is in resonance with the bulk Si valence band. (c)–(e) Calculated charge density difference maps shown for the molecular adsorption of (c) ZnPc, (d) CuPc, and (e) CoPc on the deactivated Si surface. The same isosurface charge density value ($\pm 0.001e/{\text{Bohr}}^3$) is selected for all of these plots. Red regions denote electron accumulation while blue regions indicate electron depletion. The green balls represent ad-Si, and pink balls for subsurface borons.

Figure 4

(a) The energy landscapes of CuPc along the different diffusion pathways. Lines connecting the data points are shown to guide the view. Inset: Depicts the linearly independent minimum energy pathways for molecular diffusion. Orange circles denote the positions of ad-Si. Depending on the initial molecular orientation, the roles of the two pathways can be exchanged. (b) The energy landscape of CoPc along the solid path as shown in inset in (a).