Monolayers of MoS2 on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ

Abstract

The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral line shapes. Insertion of thin decoupling layers reduces the line widths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS₂ on Ag(111) exhibits a semiconducting bandgap, which may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetracyanoquinodimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck-Condon model, we reveal their vibronic origin and identify the modes with strong electron-phonon coupling.

Keywords: decoupling layer; molybdenum disulfide (MoS2); scanning tunneling microscopy, tetracyanoquinodimethane (TCNQ); vibronic states.

Figure 1

a) STM topography of MoS₂ on Ag(111) recorded at V = 1.2 V, I = 20 pA. Inset: Line profile of a monolayer MoS₂ island along the green line. b) Close-up view on the moiré structure. c) Atomically resolved terminating S layer (V = 5 mV, I = 1 nA). d) Constant-height dI/dV spectra of MoS₂/Ag(111) recorded on a top and on a hollow region of the moiré structure as shown on the inserted STM topography (feedback opened at V = 2.5 V, I = 0.5 nA, V_mod = 10 mV). The inset shows the gap region of MoS₂/Ag(111) on a logarithmic scale. We identify the valence band maximum (VBM) and the conduction band minimum (CBM) as the changes of the slope of the dI/dV signal. Dashed lines indicate the CBM at approx. 0.05 V and the VBM at approx. −1.55 V. The strong features in the dI/dV spectra are associated to the onset of specific bands, which are labeled by Q, Γ₁ and Γ₂ according to their location in the Brillouin zone. The assignment follows that in [38].

Figure 2

Constant-height dI/dV spectra recorded (a) on a top and (b) on a hollow site of the moiré structure of MoS₂ on Ag(111) (red curves) and on Au(111) (blue curves). Feedback opened at V = 2.5 V, I = 0.5 nA, V_mod = 10 mV (all spectra, except for hollow site on Au(111): V_mod = 5 mV).

Figure 3

a) Stick-and-ball model of TCNQ. Gray, blue, and white spheres represent C, N, and H atoms, respectively. b) STM topography of a TCNQ molecular island on MoS₂/Ag(111) recorded at V = 1 V, I = 10 pA. c) STM topography of a TCNQ island on MoS₂/Ag(111) recorded at V = 0.8 V, I = 200 pA, with superimposed molecular models suggesting intermolecular dipole–dipole interactions (dashed lines). White arrows represent the unit cell of the self-organized TCNQ domain with lattice vectors a₁ = 0.9 ± 0.10 nm and a₂ = 1.0 ± 0.10 nm and the angle between them of (96 ± 2)°.

Figure 4

a) STM topography of a self-assembled TCNQ island on MoS₂/Ag(111), recorded at V = 0.2 V, I = 20 pA. b) dI/dV spectra acquired on TCNQ molecules within the island in panel a, with the precise location marked by colored dots. The gray spectrum was recorded on a bare MoS₂ layer for reference. Feedback opened at V = 2 V, I = 100 pA, with V_mod =20 mV.

Figure 5

a–d) Constant-height dI/dV maps of a TCNQ island on MoS₂ recorded at the resonance energies derived in Figure 4b. Feedback opened in panels (a–c) V = 2 V, I = 100 pA and (d) V = −2 V, I = 30 pA on the center of the molecule with V_mod = 20 mV. Close-up images with enhanced contrast on one molecule are shown as inset for each map. e) Energy-level diagram of TCNQ determined from gas-phase DFT calculations (left). The isosurfaces of the frontier molecular orbitals are shown on the right. These have been used to calculate the tunneling matrix element M_ts with an s-wave tip at a tip–molecule distance of 7.5 Å, work function of 5 eV. The map of the spatial distribution of is shown in the middle panel.

Figure 6

a) STM topography image of a TCNQ island recorded at V = 1 V, I = 10 pA. b) Simulated (top panel) and experimental (bottom panel) dI/dV spectra at the position indicated by the blue dot in panel (a) with feedback opened at V = 2 V, I = 100 pA, with V_mod = 10 mV. The simulated spectrum is obtained from DFT calculations for all vibrational modes of the TCNQ⁻ molecule with a Huang–Rhys factor higher than 0.01 (dots associated with the right axis). A Lorentzian peak of 60 meV broadening is applied to all of these modes. c) Schematic representation of electron transport through a TCNQ molecule adsorbed on MoS₂/Ag(111): singly charged TCNQ⁻ is formed upon injecting an electron into a vibronic state of an unoccupied molecular electronic level. d–f) Visualization of the vibrational modes contributing to the satellite peak. The orange arrows represent the displacement of the atoms involved in these vibrations.