Atomic force microscopy for transition metals and tip structure effects

We study the influence of tip morphology on the contrast in atomic force microscopy (AFM) for transition metals tips and samples (T/S). For this, we consider a model based on the real space tight binding approach taking into account the electronic structure of the T/S system. Images of an W(001) surface obtained with atomically sharp or blunt tips are considered. The resolution of AFM images, in the attractive force range, is discussed for monatomic (multiatomic) apex(es) tips. Images of clusters deposited on the W(001) surface are also obtained.     

Silicon carbide nanocones: Computational analysis of chemical shieldings for pristine and boron/nitrogen decorated models

Density functional theory (DFT) calculations were performed to investigate the electronic and structural properties of pristine and boron/nitrogen (B/N) decorated models of a representative silicon carbide nanocone (SiCNC). The atoms of apexes and tips were differently decorated by B/N atoms to make all possible decorations of the investigated SiCNC. The evaluated parameters by the optimization processes and nuclear magnetic resonance (NMR) calculations indicated that the overall and atomic scale properties of the investigated SiCNCs are significantly dependent on the ways of decorations of Si/C atoms by B/N atoms. The Si/C atoms close to the decorated regions also exhibited notable changes in comparison to the pristine model.     

Quantum confinement in monatomic Cu chains on Cu(111)

The existence of one-dimensional (1D) electronic states in Cu/Cu(111) chains assembled by atomic manipulation is revealed by low-temperature scanning tunneling spectroscopy and density functional theory (DFT) calculations. Our experimental analysis of the chain-localized electron dynamics shows that the dispersion is fully described within a 1D tight-binding approach. DFT calculations confirm the confinement of unoccupied states to the chain in the relevant energy range, along with a significant extension of these states into the vacuum region.     

The LEDO expansion for diatomic overlap densities: application to the density functional theory of molecules

The limited expansion of differential overlap (LEDO) approach for the expansion of diatomic overlap densities in terms of mono-centre densities is discussed in the context of density functional theory (DFT). It is shown that it leads to a particularly simple construction scheme for major parts of the secular matrix, i.e. the electron-electron interaction and the exchange-correlation potential: using the LEDO expansion coefficients, matrix elements between atomic orbitals located on different centres can be expressed in terms of the corresponding mono-centre elements, thus allowing the reduction of three-centre and four-centre integrals to two-centre integrals. This results in the first DFT method with formal N ² scaling for the construction of the secular matrix, with N being the dimension of the atomic orbital (AO) basis set. Test calculations show that numerical agreement with the results of conventional DFT calculations is excellent.     

Atomic spin-sensitive dissipation on magnetic surfaces

We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett-type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.     

Insulating method using cataphoretic paint for tungsten tips for electrochemical scanning tunnelling microscopy (ECSTM)

A new tip insulating process for active metal tips allowing atomic resolution ECSTM imaging has been developed. This new method using cathodic cataphoretic paint deposition has been tested successfully. The insulating deposited film appears homogeneous under optical microscopy and it has been characterised by infra red and SEM analysis. The depositing layer of the paint is sufficiently dense to effectively resist electrolyte ion penetration and resists corrosion in various acidic, basic aqueous or non-aqueous media. The coating film does not reduce the imaging capability of the ECSTM even to atomic resolution. This new insulating method adds to the approaches available to those preparing tips for ECSTM. This approach would also be of great utility for the preparation of microelectrodes using active metals.     

Confinement of the field electron emission to atomic sites on ultra sharp tips

The spatially controlled field assisted etching method for sharpening metallic tips, in a field ion microscope (FIM), is used to study the evolution of the field emission when the tip apex radius is decreased below 1 nm. Unlike the conventional image formation in a field emission microscope (FEM), we demonstrate that at this scale the field emission is rather confined to atomic sites. A single atom apex fabricated at the end of such tips exhibits an outstanding brightness compared to other atomic tips. The measurements have been repeated for two double atom tips, with different atom-atom separations, and images of atomic field emission localization have also been obtained. We have found that the field emission intensity alternates between adjacent atoms when the applied voltage is gradually increased beyond a threshold value.     

Humidity effect on the interaction between carbon nanotubes and graphite

An atomic force microscope is used to study the effect of humidity on the interaction between carbon nanotubes anchored to atomic force microscopy tips and various samples. Commercial silicon tips were also used for comparison. Adhesion force and dissipative energy were measured between these tips and highly oriented pyrolytic graphite (HOPG) and PMMA in contact mode. The data provides a detailed understanding of carbon nanotube interactions as a function of humidity.     

Chemisorption energetics and surface reactivity: UBI-QEP versus DFT projections

The wealth of information accumulated recently by ab initio/DFT calculations of adsorption energetics and reactivity on metal surfaces makes it possible to systematically compare projections of the DFT and UBI-QEP approaches. We make such comparisons covering both qualitative regularities in and the quantitative accuracy of binding energies and activation barriers. We focus on the following areas: (1) DFT verification of the UBI-QEP assumptions and rigorous projections concerning atomic and molecular binding energies at low coverage; (2) coverage effects revealing in the M-A bond energy competition (metal sharing A-M-A) under atomic co-adsorption; (3) reactivity patterns including both qualitative (periodic dependence of the activation barriers, barrier vs. reaction enthalpy relationships, geometry of the transition state, etc.) and quantitative aspects. We demonstrate the broad agreement between the DFT and UBI-QEP projections and point out the areas where further testing by the ab initio/DFT techniques might be necessary and illuminating. In particular, while discussing the ab initio/DFT absolute values of the atomic binding energies Q _A and their periodic trends, we revised our previous assumptions about monotonic changes of the Q _A values with the changing atom A valence and along the VIII and IB group metals, where the periodic trends appear to be complicated. Accordingly, we make corrections to the original UBI-QEP parameters, summarizing the current recommended values of Q _A.     

Spin excitations on finite lattices and the discrete Fourier transform

The method of discrete Fourier transform (DFT) is applied to obtain the exact one- or bi-magnonic states in several finite one-dimensional spin systems. The advantage of DFT method, compared to Bethe ansatz, is that no assumption is made on the form of the wave function and that its limit of infinite system reduces to the standard approach to excitation spectra in condensed matter. So the method has, in principle, no limitation on lattice dimensionality and its physical interpretation is relatively transparent. It is demonstrated that the two approaches (DFT and BA) give identical results for the solution of the Schrodinger equation on the 1D lattice, although the structure of the methods is rather different. The excitation spectrum of the XXZ chain with arbitrary end fields is analyzed in detail and an analogy with atomic wires is briefly discussed.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

Quantitative atomic force microscopy with carbon monoxide terminated tips

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes.     

Forces and currents in carbon nanostructures: are we imaging atoms?

First-principles calculations show that the rich variety of image patterns found in carbon nanostructures with the atomic force and scanning tunneling microscopes can be rationalized in terms of the chemical reactivity of the tip and the distance range explored in the experiments. For weakly reactive tips, the Pauli repulsion dominates the atomic contrast and force maxima are expected on low electronic density positions as the hollow site. With reactive tips, the interaction is strong enough to change locally the hybridization of the carbon atoms, making it possible to observe atomic resolution in both the attractive and the repulsive regime although with inverted contrast. Regarding STM images, we show that in the near-contact regime, due to current saturation, bright spots correspond to hollow positions instead of atomic sites, providing an explanation for the most common hexagonal pattern found in the experiments.     

Systematic discrepancy of theoretical predictions of NMR chemical shifts for chlorinated aromatic carbons using the GIAO DFT method

Theoretical calculations of carbon-13 NMR chemical shifts using the gauge including atomic orbitals (GIAO) DFT approach with a moderately large set of basis functions usually yield quite satisfactory results. In the case of chlorinated aromatic carbons, however, abnormally large differences between experimental and calculated values have been noticed. This discrepancy has been proven not to be caused by improper referencing, or the basis set effect, and probably not by neglect of vibrational corrections. One of the possible sources of the chlorine effect could be the impact of relativistic phenomena on electrons moving about the chlorine nucleus. The second, probably more important factor is the influence of electron correlations, ignored in Hartree–Fock SCF and only partially included in DFT calculations. Surprisingly, however, the observed divergence has been significantly larger for DFT than for Hartree–Fock results. In the latter case the observed divergence between theoretical and experimental ¹³C NMR chemical shifts of chlorine-bonded carbons is systematic but rather small (3.4–4.4ppm).     

Density functional theory versus quantum Monte Carlo simulations of Fermi gases in the optical-lattice arena

We benchmark the ground state energies and the density profiles of atomic repulsive Fermi gases in optical lattices (OLs) computed via density functional theory (DFT) against the results of diffusion Monte Carlo (DMC) simulations. The main focus is on a half-filled one-dimensional OLs, for which the DMC simulations performed within the fixed-node approach provide unbiased results. This allows us to demonstrate that the local spin-density approximation (LSDA) to the exchange-correlation functional of DFT is very accurate in the weak and intermediate interactions regime, and also to underline its limitations close to the strongly-interacting Tonks–Girardeau limit and in very deep OLs. We also consider a three-dimensional OL at quarter filling, showing also in this case the high accuracy of the LSDA in the moderate interaction regime. The one-dimensional data provided in this study may represent a useful benchmark to further develop DFT methods beyond the LSDA and they will hopefully motivate experimental studies to accurately measure the equation of state of Fermi gases in higher-dimensional geometries.     

Hydrogen dissociation over Au nanowires and the fractional conductance quantum

The dissociation of H2 molecules over stretched Au nanowires and its effect on the conductance are analyzed using a combination of density functional theory (DFT) total energy calculations and nonequilibrium Keldysh-Green function methods. Our DFT simulations reproduce the characteristic formation of Au monatomic chains with a conductance close to G0=2e2/h. These stretched Au nanowires are shown to be better catalysts for dissociation than Au surfaces. This is confirmed by the nanowire conductance evidence: while insensitive to molecular hydrogen, atomic hydrogen induces the appearance of fractional conductances (G approximately 0.5G0) as observed experimentally.     

Ab initio simulations on the atomic and electronic structure of single-walled BN nanotubes and nanoarches

To simulate the perfect single-walled boron nitride nanotubes and nanoarches with armchair- and zigzag-type chiralities and uniform diameter of ∼5 nm, we have constructed their one-dimensional (1D) periodic models. In this study, we have compared the calculated properties of nanotubes with those for both hexagonal and cubic phases of bulk: bond lengths, binding energies per B-N bond, effective atomic charges as well as parameters of total and projected one-electron densities of states. For both phases of BN bulk, we have additionally verified their lattice constants. In the density functional theory (DFT), calculations performed using formalism of the localized Gaussian-type atomic functions as implemented in the CRYSTAL-06 code we have applied Hamiltonians containing either PWGGA or hybrid (DFT+HF) B3PW exchange-correlation functionals. After calculation of Hessian matrix for the optimized structures of BN bulk (both phases) and nanotubes (both chiralities) using the CRYSTAL code we have estimated their normal phonon modes within the harmonic approximation. Applying both atomistic and continuum models we have calculated the elastic energies and moduli for SW BN nanoarches. Our calculations clearly show a reproducibility of the atomic structure, effective charges and total energy, as well as phonon and elastic properties when using either PWGGA or hybrid B3PW Hamiltonians. On other hand, there is a high sensitivity of the discrete energy spectra parameters (including band gap) to the choice of the first principles approach (the hybrid method reproduce them noticeably better).     

镍晶格中的晶格格林函数

在含缺陷体系的模拟中，格林函数作为一种重要的多尺度耦合方法，可以将缺陷产生的局域应变场和长程应力场耦合起来.在镍基高温合金蠕变的初期阶段，位错主要分布在基体相中，通过格林函数的多尺度桥接模式，可得到基体相中位错芯的平衡构型，为位错-杂质复合体间相互作用提供计算基础.基于晶格动力学理论和声子谱计算中的力常数矩阵，通过第一布里渊区特殊k点取样以及傅里叶变换，计算完整镍晶格中晶格格林函数，可用于镍基材料的多尺度模拟计算.     

Atomic-scale sharpening of silicon tips in noncontact atomic force microscopy

The atomic-scale stability of clean silicon tips used in noncontact atomic force microscopy (NC-AFM) is simulated by ab initio calculations based on density functional theory. The tip structures are modeled by silicon clusters with and termination. For the often assumed Si(111)-type tip we observe the sharpening of the initially blunt tip via short-range chemical forces during the first approach and retraction cycle. The structural changes corresponding to this intrinsic process are irreversible and lead to stable NC-AFM imaging conditions. In opposition to the picture used in literature, the Si(001)-type tip does not exhibit the so-called "two-dangling bond" feature as a bulklike termination suggests.