Oxygen adatom, charge state, noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM), tipsample distance, tip-induced electric field, density functional theory (DFT).

Structural superlubricity describes the state of virtually frictionless sliding if two atomically flat interfaces are incommensurate, that is, they share no common periodicity. Despite the exciting prospects of this low friction phenomenon, there are physical limitations to the existence of this state. Theory predicts that the contact size is one fundamental limit, where the critical size threshold mainly depends on the interplay between lateral contact compliance and interface interaction energies. Here we provide experimental evidence for this size threshold by measuring the sliding friction force of differently sized antimony particles on MoS. We find that superlubric sliding with the characteristic linear decrease of shear stress with contact size prevails for small particles with contact areas below 15 000 nm. Larger particles, however, show a transition toward constant shear stress behavior. In contrast, Sb particles on graphite show superlubricity over the whole size range. Ab initio simulations reveal that the chemical interaction energies for Sb/MoS are much stronger than for Sb/HOPG and can therefore explain the different friction properties as well as the critical size thresholds. These limitations must be considered when designing low friction contacts based on structural superlubricity concepts.

We study a low-temperature on-surface reversible chemical reaction of oxygen atoms to molecules in ultrahigh vacuum on the semiconducting rutile TiO2(110)-(1 × 1) surface. The reaction is activated by charge transfer from two sources, natural surface/subsurface polarons and experimental Kelvin probe force spectroscopy as a tool for electronic charge manipulation with single electron precision. We demonstrate a complete control over the oxygen species not attainable previously, allowing us to deliberately discriminate in favor of charge or bond manipulation, using either direct charge injection/removal through the tip-oxygen adatom junction or indirectly via polarons. Comparing our ab initio calculations with experiment, we speculate that we may have also manipulated the spin on the oxygens, allowing us to deal with the singlet/triplet complexities associated with the oxygen molecule formation. We show that the manipulation outcome is fully governed by three experimental parameters, vertical and lateral tip positions and the bias voltage.

The universality of the metal-insulator transition in three-dimensional disordered system is confirmed by numerical analysis of the scaling properties of the electronic wave functions. We prove that the critical exponent ν and the multifractal dimensions dq are independent on the microscopic definition of the disorder and universal along the critical line which separates the metallic and the insulating regime. 71.30., One of the main problem of the disorder induced metalinsulator transition (MIT) is the proof of its universality. In the pioneering work [1], it was conjectured that if the sample size exceeds all the length parameters of the model, then the conductance, g, is the only parameter which governs MIT. This scaling hypothesis has been confirmed by various numerical analysis, with the help of the finite-size scaling [2,3].Generally accepted scenario of the Anderson localization is that disorder broadens the conductance band. Electron states in the tail of the band become localized, separated from delocalized (metallic) states by the mobility edges, E c . System exhibits the MIT if the Fermi energy, E F , crosses the mobility edge. With increased disorder, E c moves towards the band center. There is a critical value of the disorder, W c , for which E c reaches band center, E c = 0. For disorder W > W c , all electronic states inside the band become localized. Phase diagram in the energy-disorder plane was calculated in [4] and is schematically shown in the upper panel of Fig. 1.At the band center, E = 0, the universality of the MIT was confirmed by detailed numerical analysis of the disorder and system size dependence of Lyapunov exponents in quasi-one dimensional systems [3,5], mean conductance [6], conductance distribution [7], and level statistics [8,9]. These studies determined the value of the critical exponent ν, which determines the divergence of the correlation length, ξ ∼ |W − W c | −ν , as ν = 1.57 ± 0.02 [5,6]. The analysis of MIT along the critical line (non-zero energy E) is more difficult because the critical region is narrower and finite size effects are stronger [10]. Critical exponent, ν, was obtained only in models with random hopping [11], and very recently in [12].In this paper, we present numerical proof of the universality of MIT. By scaling analysis of the electronic wave functions in the vicinity of two critical points, shown in the upper panel of Fig. 1, we prove that the critical exponent ν and fractal dimensions d q of the wave function (defined below) are universal along the critical line.Electron eigenenergies and wave functions are calculated for three-dimensional Anderson Hamiltonian,

Non-contact atomic force microscopy is used to measure the 3D force field on a dense-packed Cu(111) surface. An unexpected image contrast reversal is observed as the tip is moved towards the surface, with atoms appearing first as bright spots, whereas hollow and bridge sites turn bright at smaller tip-sample distances. Computer modeling is used to elucidate the nature of the image contrast. We find that the contrast reversal is essentially a geometrical effect, which, unlike in gold, is observable in Cu due to an unusually large stability of the tip-sample junction over large distances.

Antimony nanoparticles deposited under UHV conditions on HOPG are found to exhibit an intriguing frictional behavior characterized by a distinct clearly separated double dual behavior of dependence of the frictional force on contact area. We present the first realistic simulations, density functional modeling adapted to accommodate van der Waals interactions, of the (double) dual frictional behavior. The simulations provide insights into the physics/chemistry of all the frictional branches in terms of incommensurable interfaces, mobile spacer molecules as well as a novel concept of mobile oxidized multi-nanoasperities.

We study numerically the character of electron eigenstates of the three-dimensional disordered Anderson model. Analysis of the statistics of inverse participation ratio as well as numerical evaluation of the electronhole correlation function confirms that there are no localized states below the mobility edge, as well as no metallic states in the tail of the conductive band. We discuss also finite size effects observed in the analysis of all the discussed quantities.

Probing physical quantities on the nanoscale that have directionality, such as magnetic moments, electric dipoles, or the force response of a surface, is essential for characterizing functionalized materials for nanotechnological device applications [1][2][3] . Currently, such physical quantities are usually experimentally obtained as scalars. To investigate the physical properties of a surface on the nanoscale in depth, these properties must be measured as vectors. Here we demonstrate a three-force-component detection method, based on multifrequency atomic force microscopy on the subatomic scale [4][5][6][7][8][9] and apply it to a Ge(001)-c(4 × 2) surface. We probed the surface-normal and surface-parallel force components above the surface and their direction-dependent anisotropy and expressed them as a three-dimensional force vector distribution. Access to the atomic-scale force distribution on the surface will enable better understanding of nanoscale surface morphologies, chemical composition and reactions 10,11 , probing nanostructures via atomic or molecular manipulation 12,13 , and provide insights into the behaviour of nano-machines on substrates 14,15 . Noncontact atomic force microscopy (AFM) is an excellent tool not only for characterizing the atomic order on a surface but also for detecting the exchange, electrostatic, and chemical force interactions between the AFM tip and the sample surface [16][17][18][19] . However, the conventional AFM, in which the force sensor oscillates perpendicular to the surface, reflects only the surface-normal component of the tip force and ignores the surface-parallel components. Although the parallel component of force has been calculated from the normal component using a potential mapping extraction technique, this method is only indirect [20][21][22][23][24] . To obtain the distribution of the parallel components in three-dimensions (3D) with a higher accuracy, the force sensor should be oscillated also in the direction parallel to the surface 25,26 . Recently, a multi-frequency AFM method was developed 4 with the quest to investigate the physical properties of materials in deeper detail 5,6 . This method utilizes both flexural and torsional modes of the cantilever, thereby making it useful for determining the force in a vector form by allowing the surface-normal (Z direction) and one of the surface-parallel force components (along the X or Y direction) to be simultaneously measured 9 . Here we propose to detect all three components (X , Y and Z) of the total tip-surface force. For that purpose we use a clean Ge(001) surface. The surface has alternately aligned buckling dimers that form an anisotropic c(4 × 2) structure even at room temperature 19 .As shown in Fig. 1, this surface has a structure wherein two domains are separated by a single step; across this step, the dimers are at an angle of 90• to each other. Therefore, both X and Y surface-parallel components of the tip-surface interaction can be obtained from both domains without the need to rotate the tip or t...