Spatial Tuning of the Metal Work Function by Means of Alkanethiol and Fluorinated Alkanethiol Gradients Nagaiyanallur V. Venkataraman, † Stefan Zu ¨ rcher, † Antonella Rossi, †,§ Seunghwan Lee, † Nicola Naujoks, ‡,| and Nicholas D. Spencer* ,† Laboratory for Surface Science and Technology, Department of Materials, and Materials Research Center, ETH Zurich, Wolfgang-Pauli-Str. 10, CH-8093 Zurich, Switzerland, Nanotechnology Group, ETH Zurich, Tannenstrasse 3, CH-8092 Zurich, Switzerland, and Dipartimento di Chimica Inorganica ed Analitica, UniVersita ` degli Studi di Cagliari, Cittadella UniVersitaria di Monserrato, I - 09100 Cagliari, Italy ReceiVed: October 16, 2008; ReVised Manuscript ReceiVed: January 30, 2009 Surface-chemical gradients composed of self-assembled monolayers (SAM) of decanethiol (DT) and a partially fluorinated decanethiol (PFDT) on gold, exhibiting gradual changes in surface concentration of one or both components, have been prepared by a simple, controlled-immersion process. Infrared spectroscopic studies on a single-component PFDT gradient indicate a change in average molecular orientation with increasing surface coverage, whereas on a two-component gradient, the orientation remains invariant over the entire length of the gradient. X-ray photoelectron spectroscopic measurements on a single-component PFDT gradient show a systematic decrease in the fluorine (F 1s) binding energy with increasing surface coverage, whereas a single-component DT gradient shows an increase in the carbon (C 1s) binding energy. In two-component (DT-PFDT) gradients, the molar ratios of the two components at any particular location on the sample surface determine the magnitude of the binding-energy shifts at that location. Such shifts, which are on the order of 1 eV, are shown to be a consequence of work-function changes in the underlying gold upon SAM formation. These results are discussed in light of the surface-potential measurements on a DT-PFDT gradient by Kelvin Probe Force Microscopy and XP spectra acquired on “floating” and grounded samples. Introduction Self-assembled monolayers (SAMs) of thiols on gold 1,2 have attracted great interest as a convenient route for surface modification in widely different fields, such as microelectronics, biosensing, single-molecule electronics, or photovoltaics. 3-6 Many of these applications can be attributed to the ability of SAMs to precisely modify interfacial properties by exposing a particular chemical functional group at the SAM/air or SAM/ liquid interface. For example, a simple change of terminal group in an alkanethiol SAM from -CH 3 to -CF 3 is sufficient to bring about dramatic changes in the interfacial dipole moment, wetting, and friction of these films. 7-9 Being able to adjust the surface potential and thereby the work function of a metal is of great interest, since it allows tuning of charge- and hole-injection properties in semiconducting devices, such as organic light- emitting diodes and other molecular electronic systems that involve a metal-organic junction. 10 Although the presence of any adsorbed species alters the work function of a metallic surface, SAMs are particularly attractive since they can lead to a highly ordered array of molecular dipoles on the surface, resulting in a net nonzero dipole moment normal to the surface, whose magnitude and sign can be adjusted by changing the chemical nature of the assembling molecules. Several recent experimental as well as theoretical studies have compared SAMs of alkanethiols and fluorinated alkanethiols on gold and silver with respect to their ability to modify the work function of the metal. 11-17 Campbell et al. have studied, by Kelvin-probe measurements, the change in the surface potential of a silver electrode upon SAM formation with different thiols and correlated them with the calculated molecular dipole moments. 17 Alloway et al. measured the work function of a series of partially fluorinated thiol SAMs on gold and compared them to their nonfluorinated analogs, assigning the differences to the changes in molecular dipoles. 16 While Ray et al. studied the organization-induced changes in the work function of a series of thiol monolayers with different end-groups, 18 Carbacos et al. have recently shown the effect of embedded dipoles at different locations along an alkyl thiol monolayer. 19 De Boer et al. have demonstrated that SAMs of fluoroalkyl or alkyl thiols dramatically alter the hole-injection barrier in a metal-polymer contact. 15 These studies have established a clear correlation between molecular properties of the adsorbed thiolate species with the electronic properties of the resulting interface. Most of the above studies, however, have focused on single- component SAMs. The present study demonstrates that by mixing an alkanethiol with a highly fluorinated alkanethiol, it is possible to precisely modify the work function of the resulting interface. Mixed monolayers can be readily obtained by self- assembly from solutions containing the two different thiols. However, the resulting surface compositions are often different from the composition of the solution, and to achieve precise control, it is necessary to study multiple samples. An alternative, more convenient way to explore a range of surface compositions is to deliberately prepare a gradient in surface composition of the two components, since it circumvents the need to prepare * To whom correspondence should be addressed. E-mail: spencer@ mat.ethz.ch. Fax: +41 44 633 10 27. † Laboratory for Surface Science and Technology, Department of Materials, and Materials Research Center, ETH Zurich. ‡ Nanotechnology Group, ETH Zurich. § Dipartimento di Chimica Inorganica ed Analitica, Universita ` degli Studi di Cagliari, Cittadella Universitaria di Monserrato. | Present Address: Applied Physics, Chalmers Institute of Technology, 412 96 Go ¨teborg, Sweden. J. Phys. Chem. C 2009, 113, 5620–5628 5620 10.1021/jp809156a CCC: $40.75  2009 American Chemical Society Published on Web 03/17/2009