Felix Loske

Growth of ordered C60 islands on TiO2(110)

Non-contact atomic force microscopy is used to study C(60) molecules deposited on the rutile TiO(2)(110) surface in situ at room temperature. At submonolayer coverages, molecules adsorb preferentially at substrate step edges. Upon increasing coverage, ordered islands grow from the decorated step edges onto the lower terraces. Simultaneous imaging of bridging oxygen rows of the substrate and the C(60) island structure reveals that the C(60) molecules arrange themselves in a centered rectangular superstructure, with the molecules lying centered in the troughs formed by the bridging oxygen rows. Although the TiO(2)(110) surface exhibits a high density of surface defects, the observed C(60) islands are of high order. This indicates that the C(60) intermolecular interaction dominates over the molecule-substrate interactions that may cause structural perturbations on a defective surface. Slightly protruding C(60) strands on the islands are attributed to anti-phase boundaries due to stacking faults resulting from two islands growing together.

Formation of metallic colloids in CaF2 by intense ultraviolet light

Highest purity CaF2 single crystals are irreversibly modified when irradiated with millions of pulses of 193 nm light at fluences of 120 mJ/cm2. Mie theory explains the observed haze by attributing the wavelength dependent extinction and the ratio between absorption and scattering to metallic colloids with radii in the range of 20 to 30 nm and a fractional volume of up to 2.8·10-7. Non-contact scanning force microscopy (NC-AFM) measurements performed on a surface produced by in-vacuo cleavage reveals that laser irradiation additionally produces a 104 times higher volume density of colloids with a radius of 1 to 2 nm.

Contrast inversion in non-contact atomic force microscopy imaging of C60 molecules

Non-contact atomic force microscopy (NC-AFM) was applied to study C(60) molecules on rutile TiO(2)(110). Depending on the tip-sample distance, distinctly different molecular contrasts are observed. Systematically decreasing the tip-sample distance results in contrast inversion that is obtained reproducibly on the C(60) islands. This change in contrast can be related to frequency shift versus distance (df(z)) curves at different sample sites, unraveling crossing points in the df(z) curves in the attractive regime. We have performed simulations based on a simple Morse potential, which reproduce the experimental results. This combined experimental and simulation study provides insight into the mechanisms responsible for molecular contrast in NC-AFM imaging. Moreover, this work demonstrates the importance of distance-dependent measurements for unambiguously identifying molecular positions within a molecular island using NC-AFM.

Manipulation of C60 islands on the rutile TiO2 (110) surface using noncontact atomic force microscopy

Regular, almost quadratic pits were created in an island of C60 molecules on a rutile TiO2 (110) surface using noncontact atomic force microscopy at room temperature. Upon gradually approaching the scanning tip toward the surface, the interaction between the tip and the C60 island was increased until manipulation was achieved. Analyzing the manipulation process unambiguously revealed that the manipulation was performed in the repulsive regime. Retracting the tip allowed for reproducible imaging the C60 island after the manipulation process. Moreover, whole islands could be reshaped or even removed when scanning with appropriate scanning parameters.

Steering molecular island morphology on an insulator surface by exploiting sequential deposition

Depending on the deposition order in coadsorption of C(60) and SubPc molecules on CaF(2) (111), distinctly different island morphologies can be obtained. We demonstrate that non-equilibrium processes can play a significant role in molecular structure formation and constitute a new route for complex molecular patterning of an insulating surface.

Deposition Sequence Determines Morphology of C60 and 3,4,9,10-Perylenetetracarboxylic Diimide Islands on CaF2(111)

The coadsorption of C60 and 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) molecules on atomically flat terraces of the CaF2(111) surface is studied under ultra-high vacuum conditions using non-contact atomic force microscopy (NC-AFM). Deposition of PTCDI molecules on CaF2(111) yields needle-shaped, molecularly well-ordered crystals. Upon following deposition of C60 molecules, the PTCDI islands are completely covered by C60. For the opposite deposition order, the initially grown C60 islands are not covered by PTCDI molecules, instead, most of the PTCDI molecules condense in pure islands, while only few PTCDI molecules nucleate at the edges of previously grown C60 islands. Simultaneous deposition of both molecules results in an intermixed phase with yet another island morphology. The observed fundamental differences in island morphology suggest that different dewetting barriers are involved in the formation process.