Imaging Photoelectron Transmission through Self-Assembled Monolayers: The Work-Function of Alkanethiols Coated Gold Despina Fragouli and Theofanis N. Kitsopoulos* Institute of Electronic Structure and Laser (FORTH) and Department of Chemistry, UniVersity of Crete, 71110 Heraklion-Crete, Greece Letizia Chiodo, Fabio Della Sala, and Roberto Cingolani National Nanotechnology Laboratory of CNR-INFM, Via Arnesano, 73100 Lecce, Italy Supratim G. Ray and Ron Naaman Department of Chemical Physics, Weizmann Institute of Science, RehoVot, 76100, Israel ReceiVed NoVember 29, 2006. In Final Form: March 8, 2007 In this paper, we present a new approach for studying the electronic properties of self-assembled monolayers and their interaction with a conductive substrate, the low-energy photoelectron imaging spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through organic monolayers. Using this method, we measure the relative work-function of alkanethiols of different length on gold substrate, and we are able to follow the changes occurring when the surface coverage is varied. We also computed the work-function of model alkanethiols using a plane-wave density functional theory approach, in order to demonstrate the correlation between changes in the work-function with the monolayer organization and density. Introduction Interfaces between metals and organic molecular assemblies play an important role in all organic electronic devices, such as sensors, 1 field-effect transistors, 2 light-emitting diodes, 3 and single-electron devices. 4 Understanding the electronic properties of metal-organic interfaces is thus of fundamental importance as metallic contacts are crucial for the ultimate device perfor- mance. There is therefore an increased interest in studying how the electronic properties of interfaces 5 and how the intrinsic properties of the metal, such as the work-function, can be tailored by the chemical modification at the surface. 6 Extensive studies have been performed in order to investigate the adsorbate- modified electronic properties both in the case of conductor and nonconductor surfaces. 7-12 Organic films and specifically self-assembled monolayers (SAMs) have been widely used for such studies, as they can form a reproducible well-defined organic-inorganic interface. A particular class of SAMs, that is probably the most widely and extensively studied, is constituted by the monolayers of orga- nothiols compounds adsorbed on noble metals, primarily gold surfaces. 13-17 These kinds of systems provide dense organic ordered adlayers with many tunable parameters, such as chemical composition of the terminal groups, chain length, film thickness, and organization. All of these parameters can be systematically varied in order to exploit the detailed mechanism of the interaction of the layer with the substrate and the mechanism of electron- layer interaction. For the determination of the changes induced by thin films on the effective work-function of the substrate, contact-potential difference (Kelvin probe) measurements, and photoelectron spectroscopies have been used. 5,7-10,18-20 UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) are used to follow changes in effective work-function induced by the formation of hetero-junctions, charge exchange at interfaces, and charge redistribution effects 5,18 . Angle-resolved photoemission measurements indicate shifts in the vacuum level due to the adsorption of a close-packed alkane layer on metal or semiconductor substrates. 21 Moreover, vacuum-STM, surface X-ray, helium scattering, UPS, and Kelvin probe studies of * Corresponding author. E-mail: theo@iesl.forth.gr. (1) Janata, J.; Josowicz, M. Nat. Mater. 2003, 2, 19-24. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99- 117. (3) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539-541. (4) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804. (5) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater 1999, 11 (8), 605- 625. (6) Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Bredas, J.-L. J. Am. Chem. Soc. 2002, 124, 8131-8141. (7) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462-466. (8) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528-3530. (9) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. Rev.B 1996, 54, 14321- 14324. (10) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121-1127. (11) Howell, S.; Kuila, D.; Kasibhatla, B.; Kubiak, C. P.; Janes, D.; Reifenberger, R. Langmuir 2002, 18, 5120-5125. (12) Ratner, M. Nature 2000, 404 (6774), 137-138. (13) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (14) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481-4483. (15) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir- Blodgett to Self-Assembly; Academic Press: New York, 1991. (16) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (17) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Colorado, R., Jr.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690-11699. (18) Seki, K.; Hayashi, N.; Oji, H.; Ito, E.; Ouchi, Y.; Ishii, H. Thin Solid Films 2001, 393, 298-303. (19) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166-168. (20) Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121-128. (21) Seki, K.; Ueno, N.; Karlsson, U. O.; Engelhardt, R.; Koch, E.-E. Chem. Phys. 1986, 105, 247-265. 6156 Langmuir 2007, 23, 6156-6162 10.1021/la063471t CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007