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By means of ion bombardment of clean Au(111) films, atomically flat nanoparticles of various shapes and sizes were created, ranging from several tens of nm(2) down to only a few nm(2). Both two-dimensional Au islands as well as one-dimensional Au nanowire-like structures have been investigated by means of low-temperature scanning tunneling microscopy and spectroscopy. We were able to probe their local electronic structure in a broad energy range, which was found to be dominated by pronounced size-dependent confinement effects. Mapping of the local density of states revealed complex standing wave patterns that arise due to interference of scattered Au surface state electrons at the edges of the Au nanoparticles. The observed phenomena could be modeled with high accuracy by theoretical particle-in-a-box calculations based on a variational method that can be applied to '2D boxes' of arbitrary polygonal shape and that we have previously successfully applied to explain the electronic wave patterns on Co islands on Au(111). Our findings support the general validity of this particle-in-a-box model.

We successfully identified native point defects that occur in Bi 2 Te 3 crystals by combining high-resolution bias-dependent scanning tunneling microscopy and density functional theory based calculations. As-grown Bi 2 Te 3 crystals contain vacancies, antisites and interstitial defects that may result in bulk conductivity and therefore may change the insulating bulk character. Here, we demonstrate the interplay between the growth conditions and the density of different types of native near-surface defects. In particular, scanning tunneling spectroscopy reveals the dependence on not only the local atomic environment, but also on the growth kinetics and the resulting sample doping from n-type towards intrinsic crystals with the Fermi level positioned inside the energy gap. Our results establish a bias-dependent STM signature of the Bi 2 Te 3 native defects and shed light on the link between the native defects and the electronic properties of Bi 2 Te 3 , which is relevant for the synthesis of topological insulator materials and the related functional properties. Keywordstopological insulators, Bi 2 Te 3 , native point defects, scanning tunneling microscopy, density functional theory Topological insulators (TIs) represent one of the most active areas of quantum technology related research in condensed matter physics the last couple of years. The intense interest in TI materials soon after their theoretical prediction and experimental confirmation is justified as they realize a new electronic phase with promising applications ranging from advanced electronic and spintronic devices to quantum computing. The topological surface states (TSSs) of a three-dimensional TI provide robust conduction on the surface while the bulk is insulating. The low-energy electronic properties of these TSSs are dominated by massless Dirac fermion excitations, where the emerging Dirac cone defines the energy dispersion relations. Furthermore, a spin-momentum locking property is acquired due to spin-orbit interaction and time reversal symmetry that is present in these materials. As a result, backscattering from non-magnetic impurities is not allowed in TIs. [1][2][3][4][5][6][7][8][9][10][11] However, these fascinating properties experience many challenges caused by intrinsic point defects that exist naturally in binary tetradymites, including Bi 2 Te 3 and Bi 2 Se 3 . 12 Bi antisite defects denoted as Bi Te , Te antisite defects denoted as Te Bi , and Te vacancies denoted as V Te , are native defects with low formation energies that dope Bi 2 Te 3 crystals, which in turn exhibit bulk conductance that overshadows the TSSs. [13][14][15][16] On the other hand, identifying and investigating the impact of such native point defects close to the surface of the Bi 2 Te 3 is critical for improving the sample quality and the TI electronic properties necessary for TI-based

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We report on a combined scanning tunneling microscopy (STM) and density functional theory (DFT) based investigation of Co atoms on Ge(111)2×1 surfaces. When deposited on cold surfaces, individual Co atoms have a limited diffusivity on the atomically flat areas and apparently reside on top of the upper π-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a highly anisotropic electronic perturbation of the Ge surface surrounding these Co atoms and pronounced one-dimensional confinement along the π-bonded chains. DFT calculations reveal that the individual Co atoms are in fact embedded in the Ge surface, where they occupy a quasi-stationary position within the big 7-member Ge ring in between the 3 rd and 4 th atomic Ge layer. The energy needed for the Co atoms to overcome the potential barrier for penetration in the Ge surface is provided by the kinetic energy resulting from the deposition process. DFT calculations further demonstrate that the embedded Co atoms form four covalent Co-Ge bonds, resulting in a Co 4+ valence state and a 3d 5 electronic configuration. Calculated STM images are in perfect agreement with the experimental atomic resolution STM images for the broad range of applied tunneling voltages.