Electron scattering in scanning probe microscopy experiments

Abstract

It has been shown that electron transitions, as measured in a scanning tunnelling microscope, are related to chemical interactions in a tunnelling barrier. Here, we show that the shape and apparent height of subatomic features in both, measurements of the attractive forces in an atomic force microscope, and measurements of the tunneling current between the Si(1 1 1) surface and an oscillating cantilever, depend directly on the available electron states of the silicon surface and the silicon tip. Simulations and experiments confirm that forces and currents show similar subatomic variations for tip-sample distances approaching the bulk bonding length. Ó 2005 Elsevier B.V. All rights reserved.The intense research into improving the local and energy resolution in scanning probe microscopes (SPM) over the last two decades have made the assignment of atomic features to the position of surface ions attainable in many cases by comparing experimental data to high level numerical simulations. The very high resolution of individual features in the experiments, today below the limit of 1 Å , makes it possible to carry out detailed studies of the electronic structure and their local extension, which a decade ago have been thought impossible. The greater simplicity of physical processes in an scanning tunnelling microscope (STM), where long-range forces and the dynamics of an oscillating cantilever do not enter the picture, seemed to allow accurate comparisons between STM experiments and theory quite early. However, also in this case interactions and atomic displacements in a tunneling junction may decisively alter the image [1]. The advent of the atomic force microscope (AFM) extended the range of experiments substantially, since AFM experiments do not rely on the transition of electrons through the tunnelling barrier, and can therefore in principle be performed on all materials [2]. Today, AFM is able to reveal finer details than STM [3][4][5][6]. Theoretically, the advance in quantitative models of AFM and their predictive power made it possible to analyze the differences between simple theoretical models and the obtained experimental results: by gradually obtaining a more realistic view of the experimental situation, the role of different interactions and their effects on SPM images was determined [7][8][9]. The simple initial models could thus be corrected, e.g., for the effects of atomic relaxations, of ionic charge, or dissipation processes during AFM scans [10].In this Letter, we show that the subatomic features, observed by AFM and STM measurements with oscillating cantilevers on Si(1 1 1) (7 · 7), which have been the subject of intense debate [3,4,11], are due to the scattering of electrons from a very narrow energy range at the surface into double dangling bonds of the silicon tip.The Si(1 1 1) surface was simulated by a four layer silicon film, the bottom layer was passivated with hydrogen. The ionic positions were determined by fully relaxing the surface layers until the Hellman-Feynman forces on individu...

Atomic Row Doubling in the STM Images of Cu(014)-O Obtained with MnNi Tips

“…1(b) and 1(c)] also correlates with the requirement of small tip-sample separation suggested in Ref. [10].…”

“…Similar effects may play a significant role in dynamic STM experiments on Si(111) where subatomic features could be explained by scattering of electrons into double dangling bonds of the silicon tip atoms [10]. The use of dynamic STM mode for atomic orbital imaging [10] was explained by the crucial importance of having small tipsurface separations (3-5 Å ), which could be achieved reproducibly without tip breaking due to lower lateral forces with an oscillating probe.In this Letter, we present new STM data on Cu(014)-O obtained by employing a MnNi probe in constant current mode (Fig. 1).…”

High resolution STM imaging with oriented single crystalline tips

“…Sums of these individual orbital contributions can be responsible for usual spherically symmetric atomic features but asymmetry effects can be observed in experiments if electron orbitals with non-zero orbital momentum (l) and momentum projection (m) on the quantization axis dominate at the tip apex. It took approximately two decades after the invention of STM to perform direct measurements of the asymmetric charge distribution related to apex atomic orbitals in atomic force microscopy (AFM) [6,7] and STM [8][9][10][11][12] experiments. Visualization of the f z 3 orbital (l=3, m=0) associated with Sm atom at the apex was claimed in Ref.…”

Application of single crystalline tungsten for fabrication of high resolution STM probes with controlled structure

“…The precise knowledge of its electronic structure is crucial for qualitative interpretation of atomically resolved STM data and for further enhancement of the spatial resolution because the electronic structure of the apex atom can modify the electron tunneling conditions in different surface regions. In some cases, the apex orbital structure can also be visualized directly at small tunneling gaps [3][4][5][6][7]. The knowledge of the probe atomic and electronic structure is important for explanation of chemical and spin contrast in atomic scale STM images.…”