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H-and Cl-terminal groups of bicyclo[2.2.2]octane (BCO) derivatives in a mixedself-assembled monolayer (SAM) on Au(111) were imaged using a modified Si tip with aCaF2 nanocluster to differentiate the two terminals, which have different electronegativities.In order to achieve this we fabricated a new sample holder, on which aCaF2 single crystal and the mixed SAM on Au(111) could be mounted side byside. We transferred the holder with the two samples into a ultrahighvacuum (UHV) atomic force microscopy (AFM) chamber. Upon cleaving theCaF2 single crystal underUHV, a fresh and clean CaF2(111) surface parallel with the SAM surface appeared within 2mm of the separation. Themodified Si tip was prepared by repeatedly making contact between a Si tip and theCaF2(111) surface. The resulting modified tip could image the atomic periodicity of aCa2+ and anF− sublatticeon the CaF2(111) surface depending on the sign of the tip-terminating ion, i.e.anF− anda Ca2+ ion, respectively, as reported previously (Foster et al 2002 Phys.Rev.B 66235417). Using the modified Si tip with the known tip-terminating ion, weobserved the Cl-terminal in the surrounding H-terminals in the mixed SAM bynoncontact (NC) AFM. Here, the Cl-terminal is negatively charged due to itselectronegativity and thus the BCO moiety with the Cl-terminal is terminated by aCδ+–Clδ− permanent dipole, while the H-terminal is almost neutral. The Cl-terminal appearedbrighter (more attractive) and darker (more repulsive) than the surrounding H-terminalsin NC-AFM images depending on the sign of the tip-terminating ion, i.e.aCa2+ andan F− on the modified tip, respectively, although the relationship between the image contrast andthe sign of the tip-terminating ion was not always perfect because of the instability of thetip-terminating ion on the nanocluster. The present method can be used to distinguishterminal groups with different electronegativities. [ABSTRACT FROM AUTHOR]

The effect of the binding sites of the terminal groups [?]S on gold on currents through a single molecular junction (MJ) of Au/1,6-hexanedithiol/Au was studied by measuring current-separation (i-s) curves during repeated formation of a break junction in UHV-STM. Three different single molecular conductance (SMC) values (i.e. Gm(HC), Gm(MC) and Gm(LC)) were found by a careful analysis of corrected current histograms for background tunneling currents using a previously developed robust statistical analysis. Here, HC, MC and LC represent a single MJ with high, medium and low conductance, respectively. These three SMC values are attributed to three different contact modes (i.e. strong-strong, strong-weak (or weak-strong) and weak-weak bindings at the two ends). In addition to these three SMC values due to the different contacts, another lower SMC value was newly observed in the corrected histogram. The presence of the fourth SMC is specific to MJs of alkanedithiols and is attributable to LC of a single alkylene chain with gauche rich conformation, which has a lower SMC value than that of LC with all-trans conformation as proposed previously (Fujihira M et al 2006 Phys. Chem. Chem. Phys. 8 3876). Due to the effects of the contact and the conformational change, it was difficult to determine six different SMC values corresponding to two different conformations (i.e. gauche-rich versus all-trans) with three different contacts (i.e. HC, MC and LC). In addition to this complexity, the current steps corresponding to HC, MC and LC almost always appeared in this order in measured i-s curves during separation. The current step observed here could not only be a contribution from a single molecule, but also contributions from a few groups of molecules that happen to link gold atoms of the substrate with those of the tip apex. Therefore, the SMC value for HC obtained as a peak or a set of peaks in the current histogram could be based upon the sum of the current of HC and those of MCs and LCs coexisting in parallel, unless every MJ would change successively from HC to MC and MC to LC. Namely, the currents through coexisting MCs and LCs would raise the intrinsic current observed for HC itself, while those through coexisting LCs would raise the intrinsic current for MC. To avoid such errors in determining the true SMC, we demonstrate here a new method based upon analyses of individual i-s curves referred to as jump height analyses of individual i-s curves. By this method, the true SMC of LC(all-trans) was determined to be 1.6 nS (i.e. Gm (LC, all-trans) of 2.1 × 10[?]5Go) without ambiguity in spite of the possible presence of LCs(gauche rich) in parallel. [ABSTRACT FROM AUTHOR]

A point defect on a cleaved CaF2(111) surface is imaged by non-contact atomic force microscopy in ultrahigh vacuum at room temperature to identify the nature of the point defect. First, atomic contrast formation on the point defect is examined using a negatively and a positively charged tip (i.e. F?- and Ca2+-terminated tips), which are prepared by modifying a Si tip with a CaF2cluster. Analysis of the two types of atomic contrast imaged by the F?- and Ca2+-terminated tips reveals that the point defect has a positive charge and the positive charge is located on a Ca2+sublattice. Second, the lateral mobility of the point defect is investigated. We repeated imaging on the point defect by decreasing the tip-sample distance. At the smaller tip-sample distance, the point defect moved laterally to one of six neighbouring sites of the defect when the tip scanned the surface across the point defect. Taking into account (i) the atomic contrast on the point defect by the charged tips with a known terminating ion (i.e. F?or Ca2+) and (ii) the lateral mobility of the point defect, we find a probable simple model of the point defect among six possible candidates. [ABSTRACT FROM AUTHOR]