Chemisorbed and Physisorbed Structures for 1,10-Phenanthroline and Dipyrido[3,2-a:2′,3′-c]phenazine on Au(111) Peter F. Cafe, ² Allan G. Larsen, ² Wenrong Yang, ² Ante Bilic, ² Iain M. Blake, ² Maxwell J. Crossley, ² Jingdong Zhang, ‡ Hainer Wackerbarth, ‡ Jens Ulstrup, ‡ and Jeffrey R. Reimers* ,² School of Chemistry, The UniVersity of Sydney, NSW, 2006, Australia, Department of Chemistry and NanoDTU, Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark ReceiVed: May 14, 2007; In Final Form: August 16, 2007 Scanning tunneling microscopy (STM) images of 1,10-phenanthroline (PHEN) and dipyrido[3,2-a:2′,3′-c]- phenazine (DPPZ) on Au(111) are recorded using both in situ and ex situ techniques. The images of PHEN depict regimes of physisorption and chemisorption, whereas DPPZ is only physisorbed. All physisorbed structures are not pitted and fluctuate dynamically, involving aligned (4 × 4) surface domains with short- range (ca. 20 molecules) order for PHEN but unaligned chains with medium-range (ca. 100 molecules) order for DPPZ. In contrast, the chemisorbed PHEN monolayers remain stable for days, are associated with surface pitting, and form a (4 ×13)R14° lattice with long-range order. The density of pitted atoms on large gold terraces is shown to match the density of chemisorbed molecules, suggesting that gold adatoms link PHEN to the surface. For PHEN, chemisorbed and physisorbed adsorbate structures are optimized using plane-wave density-functional theory (DFT) calculations for the surface structure. Realistic binding energies are then obtained adding dispersive corrections determined using complete-active-space self-consistent field calculations using second-order perturbation theory (CASPT2) applied to cluster-interaction models. A fine balance between the large adsorbate-adsorbate dispersive forces, adsorbate-surface dispersive forces, gold ligation energy, and surface mining energy is shown to dictate the observed phenomena, leading to high surface mobility and substrate/surface lattice incommensurability. Increasing the magnitude of the dispersive forces through use of DPPZ, rather than PHEN, to disturb this balance produced physisorbed monolayers without pits and/or surface registration but with much longer-range order. Analogies are drawn with similar but poorly understood processes involved in the binding of thiols to Au(111). 1. Introduction Molecular electronic devices typically are coupled with their ends tethered to electrodes. In experimental studies these electrodes are, in turn, connected to some instrumentation. One of the most common surfaces used for this purpose is Au(111), because of the relative ease of preparation of a clean, reproduc- ible surface and the geometric reliability of monolayer (ML) formation based upon the expected chemical bonding of the molecule. Precise and predictable surface-adsorbate bonding and geometries are elusive, however, and inconsistent contact geometries have given rise to large discrepancies in experimental results. 1,2 This report proposes geometries for the binding of 1,10-phenanthroline (PHEN) on the Au(111) surface, confirming its possible use as a molecular “alligator clip”. 3,4 The study also demonstrates enhanced properties for the binding of a laterally extended PHEN molecule, dipyrido[3,2-a:2′,3′-c]phenazine (DPPZ); both molecules are shown in Figure 1. The 1,10-phenanthroline MLs from aqueous solution on Au- (111) were first observed by Cunha, Jin, and Tao by means of electrochemical scanning tunneling microscopy (in situ STM) and were shown to form locally ordered, complex and variable structures. 5 Important features of the ML noted in that study include: (1) the ML forms spontaneously. (2) The molecules stand with their axes vertical to the substrate surface and the nitrogen atoms facing the surface. (3) They form long chains of stacked molecules (like rolls of coins). (4) The PHEN molecules in each chain are aligned at 60° to the chain, with each molecule stacked slightly off-center to its neighbors. (5) Stacking faults may occur along the chain, commonly forming groups of only 3, 6, or 9 fully aligned PHEN molecules. (6) Reversible phase transitions in the ML (order to disorder to order, etc.) occur with variations in the electrochemical potential of the substrate working electrode and are attributed 6 to the large molecular dipole moment. (7) The MLs can produce pits in the surface with a depth of typically one gold layer. (8) The pit sizes increase with increasing gold-substrate potential and decrease (and/or “islands” are formed) when this potential is * To whom correspondence should be sent. E-mail: reimers@chem. usyd.edu.au. ² The University of Sydney. ‡ Technical University of Denmark. Figure 1. Left: 1,10′-phenanthroline (PHEN), C12N2H8. h ) 8.0 Å, l ) 11.1 Å; right: dipyrido [3,2-a:2′,3′-c]phenazine (DPPZ), C18N4H10. h ) 12.9 Å, l ) 11.1 Å. BATCH: jp11b230 USER: emm29 DIV: @xyv04/data1/CLS_pj/GRP_jy/JOB_i44/DIV_jp0736591 DATE: September 22, 2007 10.1021/jp0736591 CCC: $37.00 © xxxx American Chemical Society PAGE EST: 11.9 Published on Web 00/00/0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63