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

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

ヒセッショクゲンシカンリョクケンビキョウ ニヨル CeO₂ 111 ヒョウメン ノ テイオンカンサツ

研究ノー

Distinct nonequilibrium plasma chemistry of C2 affecting the synthesis of nanodiamond thin films from C2H2 (1%)/H2/Ar-rich plasmas

6 pages, 5 figures, 6 tables.We show that the concentrations of the species C2 (X 1Σg+), C2 (a 3Πu), and C2H exhibit a significant increase when the argon content grows up to 95% in medium pressure (0.75 Torr) radio frequency (rf) (13.56 MHz) produced C2H2 (1%)/H2/Ar plasmas of interest for the synthesis of nanodiamond thin films within plasma enhanced chemical vapor deposition devices. In contrast, the concentrations of CH3 and C2H2 remain practically constant. The latter results have been obtained with an improved quasianalytic space–time-averaged kinetic model that, in addition, has allowed us to identify and quantify the relative importance of the different underlying mechanisms driving the nonequilibrium plasma chemistry of C2. The results presented here are in agreement with recent experimental results from rf CH4/H2/Ar-rich plasmas and suggest that the growth of nanodiamond thin films from hydrocarbon/Ar-rich plasmas is very sensitive to the contribution of C2 and C2H species from the plasma.This work was partially funded by CICYT (Spain) under a Ramón y Cajal project and under Project No. TIC2002- 03235. One of the authors (F.J.G.V.) acknowledges a Ramón y Cajal contract from the Spanish Ministry of Science and Technology (MCYT). One of the authors (J.M.A.) acknowledges partial support from CICYT (Spain) under Project No. MAT 2002-04085-C02-02.Peer reviewe

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

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

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