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Pd versus Au as evaporated metal contacts to molecules
4.R. M. Metzger, T. Xu, and I. R. Peterson, J. Phys. Chem. B 105, 7280 (2001);
4.H. Haick, M. Ambrico, T. Ligonzo, and D. Cahen, Adv. Mater. 16, 2145 (2004).
6.The dipole layer effect operates at both forward and reverse bias, as explained in Ref. 3 (cf. Fig. 1 in that reference).
7.The differences in shapes of the curves between the Pd and Au junctions [Figs. 1(a) and 1(b)] as well as the higher series resistance with Au, than with Pd, can be ascribed to the differences in metal film growth as described in the text. These lead to effective contact areas that, although still larger than the geometrical ones, are smaller with Au than with Pd, as well as to different densities of interface states [cf. D. R. Heslinga, H. H. Weitering, D. P. van der Werf, T. M. Klapwijk, and T. Hibma, Phys. Rev. Lett. 64, 1589 (1990);
7.E. Yablonovitch, T. Sands, D. M. Hwang, I. Schnitzer, T. J. Gmitter, S. K. Shastry, D. S. Hill, and J. C. C. Fan, Appl. Phys. Lett. 59, 3159 (1991)].
7.This interpretation is supported by low frequency capacitance–voltage measurements (unpublished).
8.The molecular dipole is taken as positive if the negative pole of the dipole points away from the binding group, viz. away from the surface after binding. As negative dipoles we used , and and as positive dipoles we used and . All values (in parentheses) are in Debye units, for the free molecules.
9.The difference in BHs of the bare and junctions can be ascribed to different pinning defects at their interfaces [cf. T. Drummond, Phys. Rev. B 59, 8182 (1999)]
9.and/or hydrogenation effects [cf. H. Y. Nie and Y. Nannichi, Jpn. J. Appl. Phys., Part 1 30, 906 (1991);
9.H. Y. Nie, Jpn. J. Appl. Phys., Part 2 32, L890 (1993)].
12.B. C. Haynie, A. V. Walker, T. B. Tighe, D. L. Allara, and N. Winograd, Appl. Surf. Sci. 203–204, 433 (2003).
14.Complementary experiments showed ICICE-made contacts to have higher effective than geometrical areas, whereas the effective LOFO-made junction areas are up to an order of magnitude smaller. As is clear from Refs. 1 and 3, the larger the effective contact area between deposited metal and molecules, the higher the dipole density due to the interaction of the metal spill-over electron density with the molecules, which is the dipole density that dictates the trend for junctions with LOFO-made contacts and with ICICE-made Pd ones.
15.The systematic effect of the molecules shown in Fig. 1, supported by TOF SIMS and XPS experiments, imply the absence of significant chemical interactions between the functional groups used here and the evaporated metals, even though different functional groups may lead to (minor) differences in the growth mode of the metal, on top of the dC–X films, as was shown recently for mono- and di-thiol conjugated molecules [cf. B. de Boer, M. M. Frank, Y. J. Chabal, W. Jiang, E. Garfunkel, and Z. Bao, Langmuir 20, 1539 (2004)].
16.Near-full coverage was obtained by adsorbing benzoic acid molecules [cf. S. Bastide, R. Butruille, D. Cahen, A. Dutta, J. Libman, A. Shanzer, L. Sun, and A. Vilan, J. Phys. Chem. B 101, 2678 (1997)]
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