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1.4 Charged nanocontacts 7 1.4 Charged nanocontacts As we have seen above, usually quantum effects do not play any substantial role in nanomechanics except in special cases. If a voltage is applied between a scanning tunneling microscope tip and a sub- strate plane, a mound or a pit will be formed on the plane if the voltage is high enough. Many explanations of this effect has been proposed. Below the formation of the mound (another explanation for the pit), as a result of electric field induced dif- fusion of free surface atoms, so-called adatoms, towards the tip forming the mound under it, is discussed. The force that attracts the adatoms is the electrostatic force on a neutral particle with dipole moment in an inhomogeneous (position dependent) electric field. This force is the same as the one which deflects a thin water ray when holding an electri- cally charged hair brush close to the ray. This happens because the water molecule has a dipole moment and the electric field around the hair brush is inhomogeneous, that is the field is stronger near the brush and weaker further away from it. If the adatom or molecule is free to rotate or the dipole moment is induced by to the elec- tric field, the dipole force is always attractive. But for the adatoms on the plane sub- strate surface, a part of the dipole moment is surface induced. This component of the dipole moment of the atom always points away from the surface that it sits on. The total dipole force on the adatoms may, in this case, be either attractive or repulsive depending on the strength of the electric field, that is, on the applied voltage. Depending on the tip polarity and tip-to-surface distance, different things will occur. Studying the literature, the phase diagram shown in Figure 1.5 will emerge, discussed below. In area A, with positive tip, we obtain a mound, made up of sample material most likely created by field enhanced surface diffusion of adatoms. The mounds created using a positive tip are unstable, lasting only an hour in the experiment of Hsiao et al. [15]. Mayer et al. [19] made a computer simulation of such a field enhanced diffusion of adatoms under an STM tip. In area B, at short distances and negative tip voltage, mounds made of tip ma- terials are formed as found by Hsiao et al. [15]. The transport of tip material is due to field enhanced diffusion on the tip toward the gap, leading to a neck formation, which results in a mound when retracting the tip [15]. If the voltage applied is in- creased sufficiantly, field evaporation of tip material will occur instead [15]. In area C, the experiments by Kondo et al. [17, 20], show a strong correlation be- tween the threshold voltage for pit formation and the binding energy for ten differ- ent materials: for example gold (Au) with a binding energy of 3.8 eV, has a threshold voltage of U = 3.5 V, while tungsten (W) with a binding energy of 8.8 eV, has U = 8.7 V. Their explanation for this mechanism is sublimation induced by tunneling elec- trons. In area D, for short distances and low electric fields, Erts et al. [18] measured the force between a gold coated atomic force microscope (AFM) tip and a gold tipPDF Image | Nanomechanics Quantum Size Effects, Contacts, and Triboelectricity
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