This thesis presents a new low temperature scanning tunneling and field ion microscope (LT-STM/FIM). First STM measurements taken while cooling with liquid nitrogen demonstrate its good performance with stable tunneling conditions and atomic resolution on different sample systems. Likewise, the FIM works reliably and at 80 K and not only serves to characterize tips with atomic precision, but can also be used to sharpen the tip or make it blunter in a controlled fashion. While high voltages are applied to a tip it remains stable for hours. Without an applied voltage, the tip radius and its foremost facet can be preserved for several minutes. However, attempts to characterize a tip using FIM that stays stable throughout an STM experiment have remained unsuccessful. Yet, this doesn’t simply seem to be due to shortcomings of the setup or experimental conduct. In fact, tips that lead to good FIM images don’t seem to be any better for STM experiments than tips that haven’t been manipulated in a FIM setup. Respectively, the typical tip leading to good STM measurements appears to be rather blunt on the nanometer scale and achieves its high resolution by a sole adatom or small cluster.
For a monolayer CO on Cu(111) a reversible structural 2D phase transition that can be locally induced by the electric field of a tunneling tip, was documented and analyzed. A combination of high resolution STM images and DFT calculations was used to identify the atomic structure of both phases. The α-phase is made up of a 7 × 7 superstructure containing 25 CO molecules per unit cell whereas the β-phase exhibits a (3√3 × 3√3)R30° superstructure containing 13 CO molecules per unit cell.
The α-phase has a higher work function than the β-phase and it is effected more strongly by external electric fields. As the Gibbs free energies of both phases is extremely similar, any small change in the adsorption energies has a significant influence on the range of the thermodynamic variables stabilizing one phase or the other. Therefore, small electric fields suffice to induce a transition between the two phases.
A difference in voltage between the threshold for the formation of the β phase and the one for the formation of the α phase indicates a hysteresis, corresponding to a first order phase transi-tion. At a sample temperature of 80 K the required activation energy is overcome by thermal activation. Given suitable surface conditions, this hysteresis allows for the creation and imaging of almost arbitrary pattern of patches of the two phases. Furthermore, once coexisting domains of both phases are present on a terrace the balance between the two phases can be shifted by the electric field of the tunneling tip, causing the domain boundaries to move, increasing the area of the favored phase.
Given the relative ease of the experiment and the high precision of an STM, this is an ideal model system providing insight into the physics of structural phase transitions on the atomic scale.
|Advisor:||Möller , Rolf|
|School:||Universitaet Duisburg-Essen (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
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