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32 Discussion because in a TENG there are many contacts and the current from each individual nanocontact will be averaged out. The temperature might also wash out the quan- tum effects. Stretching a nanocontact will give a large change in the diameter and consequently large differences in the quantum level that is readily detectable at room temperature as shown for example in the AFM experiments by Rubio et al. [11]. However, the nanowire cantilever studied in this thesis has a smaller shift in the levels during the bending because the change in the cross section due to bending is small. The mechanics of bending and stretching nanosized wire resembles nano con- tacting during triboelectrification as two surfaces are approaching or departing from each other. When the two surfaces are in close proximity, variations in the distribu- tion of charges, dipole-moments and induced dipole-moments, will create electric fields affecting neighbouring surfaces. This may lead to redistribution of charges and atoms at the neighboring surface in the event of nanocontact. A model of field induced surface diffusion of adatoms including electrostatic forces and van der Waals forces was developed. The electric field is analytically modelled as a point charge over an infinite conducting flat surface. The force act- ing on the adatoms is small for voltages commonly used, but due to thermal vibra- tions, adatoms are hopping on the surface. Under these circumstances even a small net force can be significant in the drift of adatoms. A novel formula for a polarity dependent threshold voltage for mound formation on the surface for a positively charged tip was developed. Knowing the voltage of the pulse, the radius of the formed mound could be calculated. A threshold electric field for mound formation of about 2 V/nm was also calculated. In addition, we found that the van der Waals force is of importance for shorter distances and its contribution to the radial force on the adatoms has to be considered for distances smaller than 1.5 nm for low voltages. Although this model of field induced matter transport was compared with sur- face modifications experiments using STM, the model has implications for the fun- damentals of triboelectricity. In the triboelectric system there is no STM tip, but each nanocontact formed during contact will be broken during detachment. During this breaking moment the remains from the broken nanocontact will form sharp protru- sions effectively acting as STM tips. The relevant voltage is different for the two cases: the typical voltage in the STM case is about 1 V, while it is 1-3 orders higher in the triboelectric case. One immediate observation concerns the wear of the contact areas of a triboelectric generator. Our field induced modification model show that material could be transfered from one electrode to the other one, potentially dimin- ishing the triboelectric effect. This process is polarity dependent as shown in the phase diagram Figure 1.5 in the introduction. The phase diagram seems to indicate that stable material transfer mainly occurs from a negative to a positive surface. As two different materials are brought into contact, the difference in triboelectric potentials between the materials results in electric charges transferred from one sur- face to the other. These mechanisms are not fully understood in the cases where one or both of the surfaces are non-metals. Charge transfer may then in some cases in- clude transfer of both electrons and ions. The dependency of charge transfer on vari- ables such as frequency and temperature should give valuable information aboutPDF Image | Nanomechanics Quantum Size Effects, Contacts, and Triboelectricity
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