26 research outputs found
Mapping the perturbation potential of metallic and dipolar tips in tunneling spectroscopy on MoS2
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
Electronic and magnetic properties of single chalcogen vacancies in MoS/Au(111)
Two-dimensional (2D) transition-metal dichalcogenides (TMDC) are considered
highly promising platforms for next-generation optoelectronic devices. Owing to
its atomically thin structure, device performance is strongly impacted by a
minute amount of defects. Although defects are usually considered to be
disturbing, defect engineering has become an important strategy to control and
design new properties of 2D materials. Here, we produce single S vacancies in a
monolayer of MoS on Au(111). Using a combination of scanning tunneling and
atomic force microscopy, we show that these defects are negatively charged and
give rise to a Kondo resonance, revealing the presence of an unpaired electron
spin exchange coupled to the metal substrate. The strength of the exchange
coupling depends on the density of states at the Fermi level, which is
modulated by the moir\'e structure of the MoS lattice and the Au(111)
substrate. In the absence of direct hybridization of MoS with the metal
substrate, the S vacancy remains charge-neutral. Our results suggest that
defect engineering may be used to induce and tune magnetic properties of
otherwise non-magnetic materials
Ï-radical formation by pyrrolic H abstraction of phthalocyanine molecules on molybdenum disulfide
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 Ï bond after NâH 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 favors the stabilization of this radical state
Electronic Structure and Luminescence of Quasi-Freestanding MoS2 Nanopatches on Au(111)
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
resolution of electronic and vibronic states of TCNQ
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 FranckâCondon model, we reveal their vibronic origin and identify the modes with strong electronâphonon coupling
Observation of the Magnetic Ground State of the Two Smallest Triangular Nanographenes.
Fusion of three benzene rings in a triangular fashion gives rise to the smallest open-shell graphene fragment, the phenalenyl radical, whose Ï-extension leads to an entire family of non-KekulĂ© 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
Vibrational Excitation Mechanism in Tunneling Spectroscopy beyond the Franck-Condon Model
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 scanning tunneling microscopeâs tip over the molecule in great detail
Are environmental characteristics in the municipal eldercare, more closely associated with frequent short sick leave spells among employees than with total sick leave: a cross-sectional study
Background: It has been suggested that frequent-, short-term sick leave is associated with work environment
factors, whereas long-term sick leave is associated mainly with health factors. However, studies of the hypothesis of
an association between a poor working environment and frequent short spells of sick leave are few and results are
inconsistent. Therefore, we aimed to explore associations between self-reported psychosocial work factors and
workplace-registered frequency and length of sick leave in the eldercare sector.
Methods: Employees from the municipal eldercare in Aarhus (N = 2,534) were included. In 2005, they responded to
a work environment questionnaire. Sick leave records from 2005 were dichotomised into total sick leave days (0â14
and above 14 days) and into spell patterns (0â2 short, 3â9 short, and mixed spells and 1â3 long spells). Logistic
regression models were used to analyse associations; adjusted for age, gender, occupation, and number of spells or
sick leave length.
Results: The response rate was 76%; 96% of the respondents were women. Unfavourable mean scores in work
pace, demands for hiding emotions, poor quality of leadership and bullying were best indicated by more than 14
sick leave days compared with 0â14 sick leave days. For work pace, the best indicator was a long-term sick leave
pattern compared with a non-frequent short-term pattern. A frequent short-term sick leave pattern was a better
indicator of emotional demands (1.62; 95% CI: 1.1-2.5) and role conflict (1.50; 95% CI: 1.2-1.9) than a short-term
non-frequent pattern.
Age (= 40 years) statistically significantly modified the association between the 1â3 long-term sick leave
spell pattern and commitment to the workplace compared with the 3â9 frequent short-term pattern.
Conclusions: Total sick leave length and a long-term sick leave spell pattern were just as good or even better
indicators of unfavourable work factor scores than a frequent short-term sick leave pattern. Scores in commitment
to the workplace and quality of leadership varied with sick leave pattern and age. Thus, different sick leave
measures seem to be associated with different work environment factors. Further studies on these associations may
inform interventions to improve occupational health care