High-Resolution Vibronic Spectra of Molecules on Molybdenum Disulfide Allow for Rotamer Identification

Krane, Nils; Lotze, Christian; Reecht, Gaël; Zhang, Lei; Briseño, Alejandro L.; Franke, Katharina J.

doi:10.1021/acsnano.8b07414

Cited by 31 publications

(55 citation statements)

References 42 publications

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“…Its width of ∼ 7 meV (inset of Figure 1c) signifies a long lifetime (∼ 100 fs) of the transiently charged molecular state. The resonance is followed by a small vibronic side peak at -0.905 V. Such a narrow linewidth and the observation of vibronic levels within the excited state are ascribed to an efficient electronic decoupling from the metal substrate combined with a small electron-phonon coupling strength in the underlying layer (MoS 2 ), preventing fast relaxation of the excitation [22]. Differential-conductance maps at the bias voltage of the sharp resonance show eight lobes -two lobes on each isoindole moiety ( Figure 1d).…”

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confidence: 97%

π-Radical Formation by Pyrrolic H Abstraction of Phthalocyanine Molecules on Molybdenum Disulfide

et al. 2019

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For a molecular radical to be stable, the environment needs to be inert. Furthermore, an unpaired electron is less likely to react chemically, when it is placed in an extended orbital. Here, we use the tip of a scanning tunneling microscope to abstract one of the pyrrolic hydrogen atoms from phthalocyanine (H2Pc) deposited on a single layer of molybdenum disulfide (MoS2) on Au(111). We show the successful dissociation reaction by current-induced three-level fluctuations reflecting the inequivalent positions of the remaining H atom in the pyrrole center. Tunneling spectroscopy reveals two narrow resonances inside the semiconducting energy gap of MoS2 with their spatial extent resembling the highest occupied molecular orbital (HOMO) of H2Pc. By comparison to simple density functional calculations of the isolated molecule, we show that these correspond to a single occupation of the Coulomb-split highest molecular orbital of HPc. We conclude that the dangling σ bond after N-H bond cleavage is filled by an electron from the delocalized HOMO. The extended nature of the HOMO together with the inert nature of the MoS2 layer favor the stabilization of this radical state.Molecular radicals hold one or more unpaired electrons in their valence orbitals. These molecules have attracted great interest for applications in organic optoelectronic [1, 2] and spintronic [3, 4] devices. Unfortunately, the open-shell nature renders these molecules highly reactive. Hence, the class of stable molecular radicals is limited. Most promising candidates for stable radicals are molecules with the unpaired electron being hosted in an extended orbital [3, 5]. Such molecules can be obtained, e.g., by dehydrogenation or dehalogenation, where the dissociation reaction involves the highest occupied molecular orbital (HOMO) [6][7][8][9][10][11]. Prime examples for H abstraction are porphyrin and phthalocyanine molecules, where one of the pyrrolic H atoms can be detached by a voltage pulse from the tip of a scanning tunneling microscope (STM) [12,13]. However, when the molecules are placed on a metal substrate the expected radical state is not preserved. Intramolecular relaxations and charge transfer with the substrate lead to a closed-shell configuration [8,14,15]. To suppress the interaction of radical molecules with the substrate, thin insulating layers of NaCl have been employed successfully for several radical species [7][8][9][10][11], but a radical state of H-abstracted Pc has not been reported to date.Here, we show that a monolayer of MoS 2 on Au(111) is an ideal system to study the H-abstraction reaction by STM. We prove the successful dehydrogenation reaction by three-level fluctuations of the tunneling current, reflecting the switching of the position of the single H atom in the pyrrolic center [12]. By mapping the spatial distribution of the singly occupied molecular orbital (SOMO) and singly unoccupied molecular orbital (SUMO), we find that the cleavage of an N-H bond does not lead to a localized dangling bond, but to an extended π rad...

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π-Radical Formation by Pyrrolic H Abstraction of Phthalocyanine Molecules on Molybdenum Disulfide

et al. 2019

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“…The spectrum taken on a BTTT molecule shows a narrow positive ion resonance (PIR) at −1 V, which can be ascribed to tunneling through the highest occupied molecular orbital (HOMO). The negative ion resonance (NIR) reflecting tunneling through the lowest unoccupied molecular orbital (LUMO) lies outside of the MoS 2 gap (> 2 V) 22 . Because the PIR is located inside the electronic gap of MoS 2 , it is electronically decoupled from the environment and exhibits a very narrow line width of 6 meV.…”

Section: Methodsmentioning

confidence: 99%

“…Recently, a single layer of MoS 2 has been suggested as a decoupling layer, where tunneling through electronic states of molecules revealed sharp resonances of only a few meV width 22 . In this work, we show that the molecular resonances exhibit an apparent shift along the molecular backbone by some tens of meV.…”

Section: Introductionmentioning

confidence: 99%

Mapping the perturbation potential of metallic and dipolar tips in tunneling spectroscopy on MoS2

et al. 2019

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Scanning tunneling spectroscopy requires the application of a potential difference between the sample and a tip. In metal-vacuum-metal junctions, one can safely assume that the potential is constant along the metallic substrate. Here, we show that the inhomogeneous shape of the electric potential has to be taken into account when probing spatially extended molecules on a decoupling layer. To this end, oligothiophene-based molecules were deposited on a monolayer of molybdenum disulfide (MoS2) on a Au(111) surface. By probing the delocalized molecular orbital along the thiophene-backbone, we found an apparent intramolecular shift of the positive ion resonance, which can be ascribed to a perturbation potential caused by the tip. Using a simple model for the electrostatic landscape, we show that such a perturbation is caused by the inhomogeneity of the applied bias potential in the junction and may be further modified by an electric dipole of a functionalized tip. The two effects can be disentangled in tunneling spectra by probing the apparent energy shift of vibronic resonances along the molecular backbone. We suggest that extended molecules on MoS2 can be used as a sensor for the shape of the electrostatic potential of arbitrary tips.

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“…Another type of two-dimensional materials is represented by single layers of transition metal dichalcogenides. A single layer of MoS 2 on Au(111) has recently been used to electronically decouple 2,5-bis(3-dodecylthiophen-2-yl)thieno [3,2-b]thiophene from the metal surface [10]. A rich vibronic fine structure was observed in the HOMO spectroscopic signature induced by several fundamental vibrational modes of the molecule together with their higher harmonics and combination vibrations.…”

Section: Introductionmentioning

confidence: 99%

Scanning tunneling microscopy and spectroscopy of rubrene on clean and graphene-covered metal surfaces

Rothe¹,

Mehler²,

Néel³

2020

Beilstein J. Nanotechnol.

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Rubrene (C42H28) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C42H28 consistently reflect the progressive reduction of the molecule–substrate hybridization. Separate C42H28 clusters are observed on Pt(111) as well as broad molecular resonances. On Au(111) and graphene-covered Pt(111) compact molecular islands with similar unit cells of the superstructure characterize the adsorption phase. The highest occupied molecular orbital of C42H28 on Au(111) exhibits weak vibronic progression while unoccupied molecular resonances appear with a broad line shape. In contrast, vibronic subbands are present for both frontier orbitals of C42H28 on graphene. They are due to different molecular vibrational quanta with distinct Huang–Rhys factors.

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High-Resolution Vibronic Spectra of Molecules on Molybdenum Disulfide Allow for Rotamer Identification

Cited by 31 publications

References 42 publications

π-Radical Formation by Pyrrolic H Abstraction of Phthalocyanine Molecules on Molybdenum Disulfide

π-Radical Formation by Pyrrolic H Abstraction of Phthalocyanine Molecules on Molybdenum Disulfide

Mapping the perturbation potential of metallic and dipolar tips in tunneling spectroscopy on MoS2

Scanning tunneling microscopy and spectroscopy of rubrene on clean and graphene-covered metal surfaces

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