Enhancement of Graphene Phonon Excitation by a Chemically Engineered Molecular Resonance

The abstraction of pyrrolic hydrogen from a single phthalocyanine on graphene turns the molecule into a sensitive probe for graphene phonons. The inelastic electron transport measured with a scanning tunneling microscope across the molecular adsorbate and graphene becomes strongly enhanced for a graphene out-of-plane acoustic phonon mode. Supporting density functional and transport calculations elucidate the underlying physical mechanism. A molecular orbital resonance close to the Fermi energy controls the inelastic current while specific phonon modes of graphene are magnified due to their coupling to symmetry-equivalent vibrational quanta of the molecule.

Figure 1

(a) Ball-and-stick models of 2H-Pc and Pc. (b) STM image of molecule-covered graphene on Ir(111) (sample voltage: 0.78 V; tunneling current: 25 pA; size: $8\mathrm{nm}\times 8\mathrm{nm}$). The 2H-Pc and Pc molecules are marked by dashed circles. Inset: overview STM image with 2H-Pc-covered graphene (left) adjacent to pristine graphene (right) with the characteristic hexagonal Moiré pattern (0.81 V, 30 pA, $30\mathrm{nm}\times 19\mathrm{nm}$). (c)–(h) Spectra of $dI/dV$ acquired atop positions marked by dots in (b), i.e., atop (c),(d) graphene, and the center of (e),(f) 2H-Pc and (g),(h) Pc. Rectangles in panels (d), (f), and (h) mark the region of spectroscopic data in panels (c), (e), and (g). The asterisk in panel (d) indicates the signature of an Ir(111) surface resonance that exhibits a shift upon 2H-Pc adsorption [dashed lines in panels (d) and (f)]. Feedback loop parameters: (c),(e),(g) 0.1 V; (d),(f) 0.5 V; (h) 0.3 V and 50 pA.

Figure 2

Spectra of $dI/dV$ acquired atop (a),(b) a pyrrole and (c),(d) a benzene moiety of Pc on graphene-covered Ir(111). Inset to (c): STM image (1 V, 10 pA, $2.5\mathrm{nm}\times 2.5\mathrm{nm}$) of Pc embedded in a 2H-Pc island with marked (dots) positions of spectroscopy. Rectangles in panels (b) and (d) mark the region of spectroscopic data in panels (a) and (c). Feedback loop parameters are as follows: (a),(c) 0.1 V; (b),(d) 0.5 V and 50 pA.

Figure 3

(a) Side and (b) top views of the unit cell used in the simulations. (c) Elastic electron transmission function for Pc (solid line) and 2H-Pc (dashed line) as a function of energy ($E_F=0$). (d) Inelastic electron transmission for Pc and 2H-Pc. ${\mathrm{G}}_0$ denotes the quantum of conductance.

Figure 4

(a) Phonon inelastic transmission [$d{I}_i^X/dV$, Eq. (2)] for Pc decomposed according to phonon mode weights on atoms. Inset: close-up view of an energy range where a graphene out-of-plane phonon (reversed triangles) is particularly strong. (b) Mixing of graphene phonon and Pc vibrational modes with out-of-plane polarization.