O. Pfeiffer

Cu-TBPP and PTCDA molecules on insulating surfaces studied by ultra-high-vacuum non-contact AFM

The adsorption of two kinds of porphyrin (Cu-TBPP) and perylene (PTCDA) derived organic molecules deposited on KBr and Al2O3 surfaces has been studied by non-contact force microscopy in ultra-high vacuum, our goal being the assembly of ordered molecular arrangements on insulating surfaces at room temperature. On a Cu(100) surface, well ordered islands of Cu-TBPP molecules were successfully imaged. On KBr and Al2O3 surfaces, it was found that the same molecules aggregate in small clusters at step edges, rather than forming ordered monolayers. First measurements with PTCDA on KBr show that nanometre-scale rectangular pits in the surface can act as traps to confine small molecular assemblies.

Atomic-resolution images of radiation damage in KBr

The first steps of electron irradiation induced modification of a KBr(1 0 0) surface have been studied by dynamic force microscopy with atomic resolution. Rectangular pits of monatomic depth with not more than one kink site per pit have been found. The atomic structure of KBr(1 0 0) is preserved at the bottom of the pits. Possible deexcitation and desorption mechanisms are discussed based on these results

Carbon nanotubes as tips in non-contact SFM

Abstract The demand for sharp and stable tips suggests the use of carbon nanotubes as probing tips in scanning force microscopy. Here, we report a comparison of the long-range forces of conventional tips and nanotube tips, topographical images of various surfaces, such as Cu(111), Si(111)7×7 and NaCl(100), as well as images of a bundle of multiwalled nanotubes, which was deposited by severe tip crashing. It is found that the long-range forces of carbon nanotube probing tips are reduced and that they are more resistant to wear than conventional silicon tips

Using higher flexural modes in non-contact force microscopy

The oscillation characteristics of higher flexural modes of a rectangular microfabricated silicon cantilever have been studied in ultra-high vacuum (UHV) for a free cantilever and for a typical situation in non-contact force microscopy. The results are discussed with respect to the use of such modes in dynamic force microscopy (DFM) and local dissipation measurements.

Noncontact atomic force microscopy simulator with phase-locked-loop controlled frequency detection and excitation

A simulation of an atomic force microscope operating in the constant amplitude dynamic mode is described. The implementation mimics the electronics of a real setup including a digital phase-locked loop (PLL). The PLL is not only used as a very sensitive frequency detector, but also to generate the time-dependent phase shifted signal driving the cantilever. The optimum adjustments of individual functional blocks and their joint performance in typical experiments are determined in detail. Prior to testing the complete setup, the performances of the numerical PLL and of the amplitude controller were ascertained to be satisfactory compared to those of the real components. Attention is also focused on the issue of apparent dissipation, that is, of spurious variations in the driving amplitude caused by the nonlinear interaction occurring between the tip and the surface and by the finite response times of the various controllers. To do so, an estimate of the minimum dissipated energy that is detectable by the instrument upon operating conditions is given. This allows us to discuss the relevance of apparent dissipation that can be conditionally generated with the simulator in comparison to values reported experimentally. The analysis emphasizes that apparent dissipation can contribute to the measured dissipation up to 15% of the intrinsic dissipated energy of the cantilever interacting with the surface, but can be made negligible when properly adjusting the controllers, the PLL gains and the scan speed. It is inferred that the experimental values of dissipation usually reported in the literature cannot only originate in apparent dissipation, which favors the hypothesis of “physical” channels of dissipation.

Formation of molecular wires on nanostructured KBr

The formation of molecular wires on a nanostructured KBr(001) substrate is observed by non- contact atomic force microscopy. Locally polarized cyano porphyrin molecules evaporated on this sample assemble in a linear fashion along straight pit edges forming one dimensional structures.

Atomic corrugation in nc-AFM of alkali halides

The atomic corrugation of alkali halides measured by non-contact force microscopy undergoes strong variations at low-coordinated sites like steps. We present experimental results on structured KBr surfaces and discuss the contrast mechanisms. Chemical sensitivity of the atomic corrugation is demonstrated for a mixed alkali halide crystal.

Force microscopy on insulators: imaging of organic molecules

So far, most of the high resolution scanning probe microscopy studies of organic molecules were restricted to metallic substrates. Insulating substrates are mandatory when the molecules need to be electrically decoupled in a electronic circuit. In such a case, atomic force microscopy is required. In this paper we will discuss our recent studies on different organic molecules deposited on KBr surfaces in ultra-high vacuum, and then imaged by AFM at room temperature. The distance between tip and surface was controlled either by the frequency-shift of the cantilever resonance or by the excitation signal required to keep the oscillation amplitude constant. Advantages and drawbacks of both techniques are discussed. The high mobility of the molecules, due to their weak interaction with the substrate, hinders the formation of regular self assembled structures. To overcome this problem we created artificial structures on the surface by annealing and by electron irradiation, which made possible the growth of the molecules onto step edges and their confinement into rectangular pits.

Dissipation Mechanisms Studied by Dynamic Force Microscopies

The dissipation mechanisms of contact force microscopy on solid surfaces are related to the fast motion during the slip process. Different degrees of freedom can be excited, such as phonons or electronic excitations. The dissipation mechanisms of dynamic force microscopy (DFM) were recently investigated due to the improvement in large amplitude DFM, also called dissipation force microscopy. Experimental methods to determine damping with DFM will be discussed. When an electrical field is applied between probing tip and sample, damping is observed, which depends on voltage. This type of damping is related to mirror charges, which move in the sample and/or tip because of the motion of the cantilever. When the contact potential is compensated, this long-range part is minimized. Under these conditions, only short-range damping can be measured, which appears at distances of about lnm and increases exponentially with closer separation. Recent models of this type of damping show, that there might be a relationship to the local phonon density.