Atomic Force Microscopy Reveals Bistable Configurations of Dibenzo[a,h]thianthrene and their Interconversion Pathway

Article (PDF Available)inPhysical Review Letters 108(8):086101 · February 2012with60 Reads
DOI: 10.1103/PhysRevLett.108.086101 · Source: PubMed
We investigated dibenzo[a,h]thianthrene molecules adsorbed on ultrathin layers of NaCl using a combined low-temperature scanning tunneling and atomic force microscope. Two stable configurations exist corresponding to different isomers of free nonplanar molecules. By means of excitations from inelastic electron tunneling we can switch between both configurations. Atomic force microscopy with submolecular resolution allows unambiguous determination of the molecular geometry, and the pathway of the interconversion of the isomers. Our investigations also shed new light on contrast mechanisms in scanning tunneling microscopy.
    • "The feature was very tentatively assigned to potential hydrogen bonding; however, it was noted that such a feature was not reproduced in the DFT simulations. Similarly, the conformational determination of DBTH [28,49] (shown inFigure 1D) shows a pronounced sharp feature connecting the two sulphur atoms of the molecule, despite no such bond being present. This is in addition to more subtle observations, such as the images of non-bonded Au-PTCDA clusters [48]. "
    [Show abstract] [Hide abstract] ABSTRACT: A major challenge in molecular investigations at surfaces has been to image individual molecules, and the assemblies they form, with single-bond resolution. Scanning probe microscopy, with its exceptionally high resolution, is ideally suited to this goal. With the introduction of methods exploiting molecularly-terminated tips, where the apex of the probe is, for example, terminated with a single CO, Xe or H2 molecule, scanning probe methods can now achieve higher resolution than ever before. In this review, some of the landmark results related to attaining intramolecular resolution with non-contact atomic force microscopy (NC-AFM) are summarised before focussing on recent reports probing molecular assemblies where apparent intermolecular features have been observed. Several groups have now highlighted the critical role that flexure in the tip-sample junction plays in producing the exceptionally sharp images of both intra- and apparent inter-molecular structure. In the latter case, the features have been identified as imaging artefacts, rather than real intermolecular bonds. This review discusses the potential for NC-AFM to provide exceptional resolution of supramolecular assemblies stabilised via a variety of intermolecular forces and highlights the potential challenges and pitfalls involved in interpreting bonding interactions.
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    • "Inelastic electron excitations from a STM tip could be used to switch between the two FE phases, as recently proposed for bistable molecular switches [38] [39]. It should be pointed out that our study applies only to freestanding monolayer, since the presence of a substrate could lead to energetically nonequivalent FE states in real device applications. "
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    • "Eventually, the energy barriers E a calculated for the AB→BA conversion are comparable with those estimated for conventional FE oxides, such as PbTiO 3 for which E a ∼ 0.1 − 0.2 eV /formula unit, as well as with that of the recently predicted 1T FE phase of MoS 2 [39], suggesting that the polarization reversal could be experimentally accessible. Inelastic electron excitations from a STM tip could be also used to switch between the two FE phases, as recently proposed for bistable molecular switches[40, 41]. We discuss then the electronic band structures of a representative example, GeSn, calculated without and with SOC and shown inFig. 2 (a-b). "
    [Show abstract] [Hide abstract] ABSTRACT: By means of density functional theory calculations, we predict that several two dimensional AB binary monolayers, where A and B atoms belong to group IV or III-V, are ferroelectric. Dipoles arise from the buckled structure, where the A and B ions are located on the sites of a bipartite corrugated honeycomb lattice with trigonal symmetry. We discuss the emerging valley-dependent properties and the coupling of spin and valley physics, which arise from the loss of inversion symmetry, and explore the interplay between ferroelectricity and Rashba spin-spitting phenomena. We show that valley-related properties originate mainly from the binary nature of AB monolayers, while the Rashba spin-texture developing around valleys is fully controllable and switchable by reversing the ferroelectric polarization.
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