Investigating Atomic Details of the CaF2_2(111) Surface with a qPlus Sensor

The (111) surface of CaF$_2$ has been intensively studied with large-amplitude frequency-modulation atomic force microscopy and atomic contrast formation is now well understood. It has been shown that the apparent contrast patterns obtained with a polar tip strongly depend on the tip terminating ion and three sub-lattices of anions and cations can be imaged. Here, we study the details of atomic contrast formation on CaF$_2$(111) with small-amplitude force microscopy utilizing the qPlus sensor that has been shown to provide utmost resolution at high scanning stability. Step edges resulting from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp structures and on flat terraces, the atomic corrugation is seen in high clarity even for large area scans. The atomic structure is also not lost when scanning across triple layer step edges. High resolution scans of small surface areas yield contrast features of anion- and cation sub-lattices with unprecedented resolution. These contrast patterns are related to previously reported theoretical results.Comment: 18 pages, 9 Figures, presented at 7th Int Conf Noncontact AFM Seattle, USA Sep 12-15 2004, accepted for publication in Nanotechnology, http://www.iop.or

Deposition Sequence Determines Morphology of C-60 and 3,4,9,10-Perylenetetracarboxylic Diimide Islands on CaF2(111)

Loske F, Reichling M, Kühnle A. Deposition Sequence Determines Morphology of C-60 and 3,4,9,10-Perylenetetracarboxylic Diimide Islands on CaF2(111). Japanese Journal of Applied Physics. 2011;50(8): 08LB07.The coadsorption of C-60 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 C-60 molecules, the PTCDI islands are completely covered by C-60. For the opposite deposition order, the initially grown C-60 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 C-60 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. (C) 2011 The Japan Society of Applied Physic

Role of tip structure and surface relaxation in atomic resolution dynamic force microscopy: CaF2(111) as a reference surface

By combining experimental dynamic scanning force microscope (SFM) images of the CaF2(111) surface with an extensive theoretical modeling, we demonstrate that the two different contrast patterns obtained reproducibly on this surface can be clearly explained in terms of the change of the sign of the electrostatic potential at the tip end. We also present direct theoretical simulations of experimental dynamic SFM images of an ionic surface at different tip-surface distances. Experimental results demonstrate a qualitative transformation of the image pattern, which is fully reproduced by the theoretical modeling and is related to the character of tip-induced displacements of the surface atoms. The modeling of the image transformation upon a systematic reduction of the tip-surface distance with ionic tips allows an estimate of the tip-surface distance present in experiment, where 0.28–0.40 nm is found to be optimal for stable imaging with well-defined atomic contrast. We also compare the modeling with ionic tips to results for a pure silicon tip. This comparison demonstrates that a silicon tip can yield only one type of image contrast and that the tip-surface interaction is not strong enough to explain the image contrast observed experimentally. The proposed interpretation of two types of images for the CaF2(111) surface can also be used to determine the chemical identity of imaged features on other surfaces with similar structure.Peer reviewe

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

Loske F, Reichling M, Kühnle A. Steering molecular island morphology on an insulator surface by exploiting sequential deposition. Chemical Communications. 2011;47(37):10386-10388.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

Cooperative mechanism for anchoring highly polar molecules at an ionic surface

Schütte J, Bechstein R, Rohlfing M, Reichling M, Kühnle A. Cooperative mechanism for anchoring highly polar molecules at an ionic surface. Physical Review B. 2009;80(20):205421.Structure formation of the highly polar molecule cytosine on the (111) cleavage plane of calcium fluoride is investigated in ultrahigh vacuum using noncontact atomic force microscopy at room temperature. Molecules form well-defined trimer structures, covering the surface as homogeneously distributed stable structures. Density-functional theory calculations yield a diffusion barrier of about 0.5 eV for individual molecules suggesting that they are mobile at room temperature. Furthermore, it is predicted that the molecules can form trimers in a configuration allowing all molecules to attain their optimum adsorption position on the substrate. As the trimer geometry facilitates hydrogen bonding between the molecules within the trimer, we conclude that the stabilization of individual diffusing molecules into stable trimers is due to a cooperative mechanism involving polar interactions between molecules and substrate as well as hydrogen bonding between molecules

Antibacterial activity of essential oils from Eucalyptus and of selected components against multidrug-resistant bacterial pathogens

Context: Eucalyptus globulus Labill (Myrtaceae) is the principal source of eucalyptus oil in the world and has been used as an antiseptic and for relieving symptoms of cough, cold, sore throat, and other infections. The oil, well known as ‘eucalyptus oil’ commercially, has been produced from the leaves. Biological properties of the essential oil of fruits from E. globulus have not been investigated much. Objective: The present study was performed to examine the antimicrobial activity of the fruit oil of E. globulus (EGF) and the leaf oils of E. globulus (EGL), E. radiata Sieber ex DC (ERL) and E. citriodora Hook (ECL) against multidrugresistant (MDR) bacteria. Furthermore, this study was attempted to characterize the oils as well as to establish a relationship between the chemical composition and the corresponding antimicrobial properties. Materials and methods: The chemical composition of the oils was analyzed by GLC-MS. The oils and isolated major components of the oils were tested against MDR bacteria using the broth microdilution method. Results: EGF exerted the most pronounced activity against methicillin-resistant Staphylococcus aureus (MIC ~ 250 μg/ml). EGF mainly consisted of aromadendrene (31.17%), whereas ECL had citronellal (90.07%) and citronellol (4.32%) as the major compounds. 1,8-cineole was most abundant in EGL (86.51%) and ERL (82.66%). Discussion and conclusion: The activity of the oils can be ranked as EGF > ECL > ERL ~ EGL. However, all the oils and the components were hardly active against MDR Gram-negative bacteria. Aromadendrene was found to be the most active, followed by citronellol, citronellal and 1,8-cineole

Concept for support and cleavage of brittle crystals

Tröger L, Schütte J, Ostendorf F, Kühnle A, Reichling M. Concept for support and cleavage of brittle crystals. Review of Scientific Instruments. 2009;80(6):063703.We report on sample holders for crystals to be cleaved for the preparation of surfaces with large atomically flat terraces. The concept for mounting sample crystals is based on a vicelike clamping mechanism to securely hold the crystal in position while reducing the risk of fragmentation. Sample holders based on this concept and made of suitable materials allow preparation and cleavage of crystals in the ultrahigh vacuum at high or low temperatures. To cleave the crystal, we employ a scalpel blade mounted on a wobble stick to generate a highly localized stress field initiating the cleavage process. The sample holders are used for experiments of highest resolution scanning force microscopy, however, the concept can be transferred to any other system where cleavage faces of crystals are of interest. Exemplarily, scanning force microscopy results demonstrate that (111) cleavage faces of CaF(2) crystals can be prepared with steps only a few F-Ca-F triple-layers high and atomically flat terraces extending over areas of several mu m(2)

Synergistic properties of the terpenoids aromadendrene and 1,8-cineole from the essential oil of Eucalyptus globulus against antibiotic-susceptible and antibiotic-resistant pathogens

The aim of the present study was to investigate the chemical composition of the essential oil of the fruits of Eucalyptus globulus and to examine the potential application of the fruit oil against multidrug-resistant bacteria. GLC/MS analysis in the fruit oil showed that aromadendrene was the main compound followed by 1,8-cineole and globulol. The three most abundant components of the fruit oil were also tested individually against microorganisms. In addition, the synergistic effects of combinations of the major constituents (aromadendrene and 1,8-cineole) of the fruit oil were also investigated. All Gram-positive bacteria were susceptible to the fruit oil with different degrees of susceptibility as determined by microdilution method. The oil exerted a marked inhibition against multidrug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) Enterococcus faecalis. The results indicated that aromadendrene might be responsible for the antimicrobial properties, whereas 1,8-cineole and globulol exhibited low activities. The checkerboard assay demonstrated that combinations of 1,8-cineole and aromadendrene reduce the MIC in most cases in an additive way, whereas the time-kill assay indicates a synergistic effect

Determining cantilever stiffness from thermal noise

Lübbe J, Temmen M, Rahe P, Kühnle A, Reichling M. Determining cantilever stiffness from thermal noise. Beilstein Journal of Nanotechnology. 2013;4:227-233.We critically discuss the extraction of intrinsic cantilever properties, namely eigenfrequency f(n), quality factor Q(n) and specifically the stiffness k(n) of the nth cantilever oscillation mode from thermal noise by an analysis of the power spectral density of displacement fluctuations of the cantilever in contact with a thermal bath. The practical applicability of this approach is demonstrated for several cantilevers with eigenfrequencies ranging from 50 kHz to 2 MHz. As such an analysis requires a sophisticated spectral analysis, we introduce a new method to determine kn from a spectral analysis of the demodulated oscillation signal of the excited cantilever that can be performed in the frequency range of 10 Hz to 1 kHz regardless of the eigenfrequency of the cantilever. We demonstrate that the latter method is in particular useful for noncontact atomic force microscopy (NC-AFM) where the required simple instrumentation for spectral analysis is available in most experimental systems

Quantitative description of C-60 diffusion on an insulating surface

Loske F, Lübbe J, Schütte J, Reichling M, Kühnle A. Quantitative description of C-60 diffusion on an insulating surface. Physical Review B. 2010;82(15): 155428.The diffusion of C-60 molecules on large, atomically flat terraces of the CaF2(111) surface is studied under ultrahigh vacuum conditions at various substrate temperatures below room temperature. The weak molecule-substrate interaction on this insulating surface makes a direct observation of hopping events difficult. Therefore, to determine a quantitative value of the diffusion barrier, we employ the so-called onset method. This method is based on the analysis of spatial properties of islands created by nucleation of diffusing C-60 molecules, as measured by noncontact atomic force microscopy. We first determine the critical cluster size to be i* = 1 from coverage-dependent island size distributions prepared at a fixed substrate temperature. The diffusion barrier of E-d=(214 +/- 16) meV and an attempt frequency of nu(0)=1.4 X 10(12 +/- 0.6) s(-1) are then obtained by analyzing the island densities at different substrate temperatures