In bulk materials superconductivity is remarkably robust with respect to nonmagnetic disorder [1]. In the two-dimensional limit however, the quantum condensate suffers from the effects produced by disorder and electron correlations which both tend to destroy superconductivity [2][3][4][5][6][7]. The recent discovery of superconductivity in single atomic layers of Pb, the striped incommensurate (SIC) and √ 7 × √ 3 Pb/Si(111) [8,9], opened an unique opportunity to probe the influence of well-identified structural disorder on two-dimensional superconductivity at the atomic and mesoscopic scale [10,11]. In these two ultimate condensates we reveal how the superconducting spectra loose their conventional character, by mapping the local tunneling density of states. We report variations of the spectral properties even at scales significantly shorter than the coherence length. Furthermore, fine structural differences between the two monolayers, such as their atomic density, lead to very different superconducting behaviour. The denser SIC remains globally robust to disorder, as are thicker Pb films [12-17], whereas in the slighly more diluted √ 7 × √ 3 system superconductivity is strongly fragilized. A consequence of this weakness is revealed at monoatomic steps of √ 7 × √ 3, which disrupt superconductivity at the atomic scale. This effect witnesses that each individual step edge is a Josephson barrier. At a mesoscopic scale the weakly linked superconducting atomic terraces of √ 7 × √ 3 form a native network of Josephson junctions. We anticipate the Pb/Si(111) system to offer the unique opportunity to tune the superconducting coupling between adjacent terraces [18], paving a new way of designing atomic scale quantum devices compatible with silicon technology.

We report on scanning tunneling microscopy (STM) studies performed with single crystalline W[001] tips on a graphite(0001) surface. Results of distance-dependent STM experiments with sub-ångström lateral resolution and density functional theory electronic structure calculations show how to controllably select one of the tip electron orbitals for highresolution STM imaging. This is confirmed by experimental images reproducing the shape of the 5dxz,yz and 5d x 2 −y 2 tungsten atomic orbitals. The presented data demonstrate that the application of oriented single crystalline probes can provide further control of spatial resolution and expand the capabilities of STM.

The structure of the [001]-oriented single crystalline tungsten probes sharpened in ultra-high vacuum using electron beam heating and ion sputtering has been studied using scanning and transmission electron microscopy. The electron microscopy data prove reproducible fabrication of the single-apex tips with nanoscale pyramids grained by the {011} planes at the apexes. These sharp, [001]-oriented tungsten tips have been successfully utilized in high resolution scanning tunneling microscopy imaging of HOPG(0001), SiC(001) and graphene/SiC(001) surfaces. The electron microscopy characterization performed before and after the high resolution STM experiments provides direct correlation between the tip structure and picoscale spatial resolution achieved in the experiments.

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Precise knowledge of the atomic and electronic structure of scanning tunneling microscopy (STM) tips is crucial for correct interpretation of atomically resolved STM data and improvement of spatial resolution. Here we demonstrate that tungsten probes with controllable electronic structure can be fabricated using oriented single crystalline tips. High quality of the [001]-oriented W tips sharpened in ultra high vacuum was proved by electron microscopy. Distance dependent STM studies carried out on graphite (0001) surface demonstrate that application of crystallographically oriented single crystalline tips allows one to control the tip electron orbitals responsible for high resolution imaging under specific tunneling conditions.

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The growth and ordering of C 60 molecules on the WO 2 /W(110) surface have been studied by low-temperature scanning tunnelling microscopy and spectroscopy (STM and STS), low-energy electron diffraction (LEED) and density functional theory (DFT) calculations. The results indicate the growth of a well-ordered C 60 layer on the WO 2 /W(110) surface in which the molecules form a close-packed hexagonal structure with a unit cell parameter equal to 0.95 nm. The nucleation of the C 60 layer starts at the substrate's inner step edges. Low-temperature STM of C 60 molecules performed at 78 K demonstrates wellresolved molecular orbitals within individual molecules. In the C 60 monolayer on the WO 2 /W(110) surface, the molecules are aligned in one direction due to intermolecular interaction, as shown by the ordered molecular orbitals of individual C 60 . STS data obtained from the C 60 monolayer on the WO 2 /W(110) surface are in good agreement with DFT calculations.