Monolayer islands of molybdenum disulfide (MoS2) on Au(111) form a characteristic moiré structure, leading to locally different stacking sequences at the S-Mo-S-Au interface. Using lowtemperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we find that the moiré islands exhibit a unique orientation with respect to the Au crystal structure. This indicates a clear preference of MoS2 growth in a regular stacking fashion. We further probe the influence of the local atomic structure on the electronic properties. Differential conductance spectra show pronounced features of the valence band and conduction band, some of which undergo significant shifts depending on the local atomic structure. We also determine the tunneling decay constant as a function of the bias voltage by a height-modulated spectroscopy method. This allows for an increased sensitivity of states with non-negligible parallel momentum k and the identification of the origin of the states from different areas in the Brillouin zone.
Monolayers of transition metal dichalcogenides are interesting materials for optoelectronic devices due to their direct electronic band gaps in the visible spectral range. Here, we grow single layers of MoS2 on Au(111) and find that nanometer-sized patches exhibit an electronic structure similar to their freestanding analogue. We ascribe the electronic decoupling from the Au substrate to the incorporation of vacancy islands underneath the intact MoS2 layer. Excitation of the patches by electrons from the tip of a scanning tunneling microscope leads to luminescence of the MoS2 junction and reflects the one-electron band structure of the quasi-freestanding layer.
Tunneling spectroscopy is an important tool for the chemical identification of single molecules on surfaces. Here, we show that oligothiophene-based large organic molecules which only differ by single bond orientations can be distinguished by their vibronic fingerprint. These molecules were deposited on a monolayer of the transition metal dichalcogenide molybdenum disulfide (MoS2) on top of a Au(111) substrate. MoS2 features an electronic band gap for efficient decoupling of the molecular states. Furthermore, it exhibits a small electron–phonon coupling strength. Both of these material properties allow for the resolution of vibronic states in the range of the limit set by temperature broadening in our scanning tunneling microscope at 4.6 K. Using DFT calculations of the molecule in gas phase provides all details for an accurate simulation of the vibronic spectra of both rotamers.
Fusion of three benzene rings in a triangular fashion gives rise to the smallest open-shell graphene fragment, the phenalenyl radical, whose π-extension leads to an entire family of non-Kekulé triangular nanographenes with high-spin ground states. Here, we report the first synthesis of unsubstituted phenalenyl on a Au(111) surface, which is achieved by combining in-solution synthesis of the hydro-precursor and on-surface activation by atomic manipulation, using the tip of a scanning tunneling microscope. Single-molecule structural and electronic characterizations confirm its open-shell S = 1/2 ground state that gives rise to Kondo screening on the Au(111) surface. In addition, we compare the phenalenyl’s electronic properties with those of triangulene, the second homologue in the series, whose S = 1 ground state induces an underscreened Kondo effect. Our results set a new lower size limit in the on-surface synthesis of magnetic nanographenes that can serve as building blocks for the realization of new exotic quantum phases of matter.
Vibronic spectra of molecules are typically described within the Franck-Condon model. Here, we show that highly resolved vibronic spectra of large organic molecules on a single layer of MoS2 on Au(111) show spatial variations in their intensities, which cannot be captured within this picture. We explain that vibrationally mediated perturbations of the molecular wave functions need to be included into the Franck-Condon model. Our simple model calculations reproduce the experimental spectra at arbitrary position of the STM tip over the molecule in great detail.Vibronic excitations are resonant transitions from a molecular ground state to an electronically and vibrationally excited state. These excitations are typically described by the Franck-Condon model. The essence of it are fast electronic transitions treated in Born-Oppenheimer approximation, such that the excitations occur without changes in the nuclear coordinates or momentum. Vibronic excitations in single molecules on surfaces can be detected as resonant sidebands of positive or negative ion resonances in tunneling spectroscopy [1][2][3][4][5][6][7] with apparent submolecular variations due to distinct close-lying orbitals [8][9][10]. In contrast to these resonant excitations, inelastic vibrational excitations far below resonance [6,12,13] are described by a change in the nuclear coordinates, which leads to a modified tunneling matrix element and to the opening of a new tunneling path [7,15,16]. Hence, off-resonant inelastic tunneling and resonant vibronic transitions are treated in distinct and complementary models [17]. The combination of both models would be akin to phonon-mediated electronic transitions in crystal structures [18,19], where the activation of a phonon mode enables otherwise forbidden electronic transitions, as the initial and final state have different parallel momentum in the electronic band structure [20][21][22][23]. Signatures of such combined excitations in single molecules have not been reported to date.Recent tunneling experiments have revealed some limitations of the Franck-Condon model. In cases, where the electronic energy level spacing was similar to vibrational energies, it was found that avoided level crossings determine the resonant sidebands [24,25]. In other cases, intensity variations of the resonant sidebands along an organic molecule were interpreted in terms of coherent vibrational modes with different symmetries [8] or with vibration-assisted coupling of wave functions of different symmetry in molecule and tip [26]. Selection rules could not be derived, because the vibrational modes and associated nuclear displacements of the molecule could not be identified owing to an insufficient experimental energy resolution probably limited by non-adiabatic relaxation effects.Here, we show that vibration-assisted tunneling and Franck-Condon excitations are crucial for a complete vibronic model. To benchmark our model we use vibronic spectra of large organic molecules on a single layer of MoS 2 on Au(111). The van-der-Waals lay...
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