We demonstrate the growth of twin-free Bi 2 Te 3 and Sb 2 Te 3 topological insulators by molecular beam epitaxy, and a sizable reduction of the twin density in Bi 2 Se 3 , on latticematched BaF 2 (111) substrates. Using x-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se-and Tebased alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches, in which twin-free Bi 2 Se 3 films are achieved by increasing the substrate roughness, the quality of our Bi 2 Te 3 is superior on the flattest BaF 2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab-initio calculations. Bismuth and antimony chalcogenides, Bi 2 Te 3 , Sb 2 Te 3 and Bi 2 Se 3 , are prototypical topological insulators (TIs), a new class of bulk insulators with conductive surface states. [1-3] Their surface hosts Dirac fermions protected by time-reversal symmetry and characterized by spinmomentum locking, a property that prevents backscattering and that is highly attractive for spintronics and fault-tolerant quantum computation. [3,4] However, bismuth and antimony chalcogenides are narrow gap semiconductors that are very sensitive to doping, and the surface currents are typically overwhelmed by bulk currents. In order to favour surface transport, it is necessary to suppress the bulk conductivity, which requires the growth of lowdefect thin films, a task that has proven to be very challenging. [5-9] Due to doping by impurities or crystalline defects, the Fermi level of Bi 2 Se 3 and Bi 2 Te 3 is typically shifted to the conduction band. [5,8,10-14] Although counter-doping or electrostatic gating may be used to tune the Fermi energy, structural defects impact negatively the transport properties by coupling surface and bulk channels, by reducing the carrier mobility or by suppressing the surface states in the case of strain localized at grain boundaries. [15] Some common imperfections are Se/Te vacancies, dislocations, mosaïcity, 30° rotational domains and twin domains. [5,7,15-18] In particular, mirror-symmetric twin domains are of special importance owing to the extended planar nature of their boundaries (see Figure 1a). They are known to introduce electron scattering, strain and doping in various semiconductors such as GaAs [19] or HgCdTe. [20] In TIs, it has been demonstrated that twin boundaries hold a spontaneous polarization responsible for self-doping of the surface states, which may reach several hundreds of meV. [21] TIs are usually grown on conventional substrates such as Al 2 O 3 (0001) [7,10,13,22] and Si(111), [8,13,14,23-26] despite l...

High-quality borophenes and borophene/hexagonal boron nitride heterostructures can be synthesized from molecular precursors.

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Controlling interfacial interactions in magnetic/ topological insulator heterostructures is a major challenge for the emergence of novel spin-dependent electronic phenomena. As for any rational design of heterostructures that rely on proximity effects, one should ideally retain the overall properties of each component while tuning interactions at the interface. However, in most inorganic interfaces, interactions are too strong, consequently perturbing, and even quenching, both the magnetic moment and the topological surface states at each side of the interface. Here, we show that these properties can be preserved using ligand chemistry to tune the interaction of magnetic ions with the surface states. By depositing Co-based porphyrin and phthalocyanine monolayers on the surface of Bi 2 Te 3 thin films, robust interfaces are formed that preserve undoped topological surface states as well as the pristine magnetic moment of the divalent Co ions. The selected ligands allow us to tune the interfacial hybridization within this weak interaction regime. These results, which are in stark contrast with the observed suppression of the surface state at the first quintuple layer of Bi 2 Se 3 induced by the interaction with Co phthalocyanines, demonstrate the capability of planar metal−organic molecules to span interactions from the strong to the weak limit.

An extended study on an advanced method for the cleaning of carbon contaminations on large optical surfaces using a remote inductively coupled low-pressure RF plasma source (GV10x downstream asher) is reported in this work. Technical as well as scientific features of this scaled up cleaning process are analysed, such as the cleaning efficiency for different carbon allotropes (amorphous and diamond-like carbon) as a function of feedstock gas composition, RF power (ranging from 30 to 300W), and source-object distances (415 to 840 mm). The underlying physical phenomena for these functional dependences are discussed.

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The self-assembly of metal−organic complexes and networks of increasing complexity on solid surfaces is important for their application in a variety of fields, such as catalysis, sensing, and molecular magnetism. Here, we have selected a low-symmetry, freebase macrocyclic biquinazoline ligand, H-Mabiq, which upon metalation has the potential to incorporate cations in two different coordination sites, affording multi-valency and multielectron transfer capacity. We show that H-Mabiq molecules readily self-assemble onto the Ag(111) surface at room temperature, forming a well-ordered monolayer of closely packed molecules. Upon increasing the temperature, a new phase with a different long-range order and molecular packing is obtained. By means of scanning tunneling microscopy and photoelectron spectroscopy, we show that this new phase is characterized by a distinctive silver-bridged dimeric motif, entailing a Ag adatom accommodated at the peripheral coordination site of two opposing H-Mabiq molecules. Thus, the present work reveals the ability of the bio-inspired Mabiq ligands to form surface-confined two-dimensional assemblies incorporating metal adatoms. The results bode promise for the use of metalcontaining Mabiq compounds to engineer regular bimetallic arrays with atomic precision.

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