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.

Extended iron phthalocyanine islands self-assembled on a Ge(001):H surface

“…Single-molecule prototypes or molecular nanostructures are often prepared on metals, which usually provide a sufficiently low diffusion barrier for efficient self-assembly and simultaneously allow for in-depth analysis through atomically precise tools from the family of scanning probe microscopes [1][2][3]. At the same time, however, metallic substrates usually influence the properties of adsorbed molecular species, leading to hybridization, charge transfer, or screening at the interface [4][5][6]. Also, metallic surfaces may provide relatively weak binding, dominated by van der Waals interactions [7], but the lack of a gap results in broadening and shifting of the molecular resonances.…”

“…Different insulating films have already been applied, ranging from ionic salts such as NaCl [ 8 – 9 ], KCl [ 10 – 11 ], or KBr [ 12 ], through oxide [ 13 ] or nitride [ 14 ] layers to molecular wetting layers [ 15 ] and two-dimensional materials, such as graphene [ 16 – 17 ], hBN [ 11 , 18 ], or even organic layers [ 19 ]. Recently, it has been proposed that a monolayer of transition metal dichalcogenides, for example, MoS 2 , may play a similar role [ 4 , 20 – 21 ].…”

Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation

“…To simulate the d I /d V spectrum, we considered individual and coupled modes with a significant Huang–Rhys factor, i.e., higher harmonics of individual and coupling of different modes and their harmonics (“progression of progressions”). Then, the intensities were convoluted with a Lorentzian lineshape. , A set of spectra with different Lorentzian widths is shown in Figure c (green). The best agreement with the experimental lineshape of the LUMO and LUMO + 1 (consisting of a broad peak and a shoulder) is obtained when applying a Lorentzian width of 65 mV (half-width at half-maximum).…”

“…Then, the intensities were convoluted with a Lorentzian lineshape. 59 , 67 A set of spectra with different Lorentzian widths is shown in Figure 3 c (green). The best agreement with the experimental lineshape of the LUMO and LUMO + 1 (consisting of a broad peak and a shoulder) is obtained when applying a Lorentzian width of 65 mV (half-width at half-maximum).…”

Molecular assemblies on surfaces: towards physical and electronic decoupling of organic molecules

“…The semiconducting 2D material MoS 2 may act as a decoupling layer for molecules from the underlying metal substrate if the molecular resonances lie within the MoS 2 bandgap. Hence, Yousofnejad et al [ 85 ] found using MoS 2 on Ag(111) as substrate that the HOMO of tetracyanoquinodimethane (TNCQ) is not decoupled because it is located in the MoS 2 valence band, while the lowest unoccupied molecular orbital narrows but still suffers from lifetime broadening because it is situated at the conduction band onset of MoS 2 . Despite this, the vibronic states of the transiently negatively charged TCNQ could be resolved by STS.…”