Langmuir 2004, 20, 2713-2719 2713 Chiral Morphologies and Interfacial Electronic Structure of Naphtho[2,3-a]pyrene on Au(111) C. B. France† and B. A. Parkinson*,‡ Colorado State University, Department of Chemistry, Fort Collins, Colorado 80523, and Intel Corporation, Portland Technology Development, Hillsboro, Oregon 97124 Received August 19, 2003. In Final Form: January 12, 2004 The adsorption of the two-dimensionally chiral naphtho[2,3-a]pyrene molecule has been studied on Au(111). Both structural and electronic properties of the naphtho[2,3-a]pyrene (NP)/Au(111) interface have been measured. Ultraviolet and X-ray photoelectron spectroscopy have been employed to measure the energies of the molecular orbitals of the NP film with respect to the gold Fermi level. A Schottky junction with a large interface dipole (0.99 eV) is formed between Au(111) and NP. Temperature-programmed desorption was used to determine that adsorbed NP has a binding energy of 102.2 kJ/mol. Chiral domains have been observed with scanning tunneling microscopy due to the spontaneous phase separation of the 2-D enantiomers. Two distinct structural polymorphs have been observed, one of which has homochiral paired molecular rows. Models of the 2D structure are proposed that are in excellent agreement with experimental measurements. Introduction enantiomeric pure forms of the initial adsorbate must be Stereospecific chemistry is important for the synthesis, available. We are currently investigating the third method detection, and separation of enantiopure compounds. of producing a chiral surface based on the use of achiral Essential biomolecules, biological structures, and some adsorbates that have two-dimensional chirality. While naturally occurring minerals contain chiral configurations these molecules are not chiral in three dimensions, that are of scientific and industrial importance. A surface adsorption to a surface results in the loss of mirror plane that can sense, separate, or selectively catalyze these chiral symmetry, leading to nonsuperimposable forms of the centers would be of great utility. The production of chiral molecules on the surface. Local chiral pockets can be surfaces is limited by the ability to generate the handed- formed if the 2D enantiomers phase segregate; however, ness of the desired substrate materials. There are at least the entire surface will not be stereoselective unless one three different approaches that can be used to form a chiral enantiomeric domain is somehow favored over the other. surface. The first is the use of naturally occurring chiral Scanning tunneling microscopy (STM) has been used surfaces, such as certain high-index metal surfaces.1 This to characterize chiral structures on surfaces produced by approach has the advantage that the crystalline surface organic adsorbates. Racemic mixtures of chiral adsorbates is inherently chiral; however, it will be difficult to tailor have been found to segregate into pure R or S enantiomeric the surface to bind a specific adsorbate. These kinds of domains at the liquid/solid interface.4 DL-Cysteine has been surfaces can selectively adsorb one enantiomer,1 but observed to form racemic domains of homochiral pairs on further geometric specificity for molecular recognition and Au in a vacuum environment. Additionally, two-dimen- sensor applications will be difficult to achieve. sionally chiral molecules have also been observed to The second and third techniques for generating a chiral produce chiral domains. Systems using covalently bound surface involve the use of an achiral surface and the adsorbates,5 metal-organic complexes,6 organic acids with deposition of adsorbates that form chiral structures on long alkane tails,7 hydrogen bonding functional groups,8 the surface. The adsorption of chiral molecules can produce and helical heptahelicene molecules9 have been recently chiral patches of exposed substrate in the spaces between reported. the adsorbates. In these spaces, the substrate surface In this contribution, we have utilized X-ray and remains exposed; only chiral molecules of a desired ultraviolet photoelectron spectroscopy (XPS and UPS) to enantiomer with the correct geometric specificity can be measure an approximate energy level diagram of naphtho- bound at the asymmetric patches. The chiral surface [2,3-a]pyrene (NP) (the molecular structure of NP is pockets can then be tailored by changing the initial presented in the upper right of Figure 1) on Au(111). The adsorbate or its surface coverage, allowing for increased binding energy of NP to Au(111) was measured utilizing selectivity of the molecules being “sensed”. While more temperature-programmed desorption (TPD), and STM was primitive than nature’s enzyme/substrate lock and key mechanism, this method has the potential to allow for the (3) Nakanishi, T.; Yamakawa, N.; Asahi, T.; Osaka, T.; Ohtani, B.; development of similar catalytic stereospecificity. The Unosaki, K. J. Am. Chem. Soc. 2002, 124, 740. (4) Fang, H.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, formation of a chiral surface has been produced through 102, 7311. the deposition of 3D chiral molecules on low-index metal (5) Ohtani, B.; Shintani, T.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, surfaces.2,3 These procedures have the drawback that 6515. (6) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000. * To whom correspondence should be addressed. E-mail: (7) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B [email protected]. Fax: (970) 491-1801. 2000, 104, 7627. † Intel Corp. (8) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; De ‡ Colorado State University. Vita, A.; Cai, C.; Brune, H.; Gunter, P.; Kern, K. J. Am. Chem. Soc. (1) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2001, 123, 7953. 2002, 124, 7991. (2) Chen, Q.; Lee, C. W.; Frankel, D. J.; Richardson, N. V. Phy- (9) Fasel, R.; Parschau, M.; Ernst, K.-H. Angew. Chem. 2003, 115, sChemComm 1999, 9, 05986. 5336. 10.1021/la035532i CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004 2714 Langmuir, Vol. 20, No. 7, 2004 France and Parkinson Figure 1. (A) UPS He I spectra of naphtho[2,3-a]pyrene deposited on Au(111). The right portion shows the full spectra on a binding energy scale. The high binding energy cutoff is shown on the left. The molecular structure of NP is shown in the upper right of the figure. (B) A plot of the interface work function versus the NP film thickness. The total work function shift was determined to be 0.99 eV. used to observe the 2D structures formed by NP. Proposed Experimental Section structural models of the ordered domains are presented. Experiments were performed in a commercial Omicron Multi- While numerous morphological and electronic studies have probe ultrahigh vacuum (UHV) system (base pressure, 5 × 10-11 been undertaken on adsorbed symmetric aromatic mole- mbar). This system is equipped with a variable temperature cules,10-18 to our knowledge this is the first observation scanning tunneling microscope (VT-STM) for structural char- of a two-dimensional chiral aromatic system. acterization.19 XPS and UPS using a VSW EA125 single-channel hemispherical analyzer were used to study the electronic (10) France, C. B.; Parkinson, B. A. Appl. Phys. Lett. 2003, 82, 1194- structure of the interface. TPD has been added to the UHV system 1196. (11) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. (15) Schroeder, P. G.; France, C. B.; Parkinson, B. A.; Schlaf, R. J. A. Langmuir 2003, 19, 1274. Appl. Phys. 2002, 91, 9095. (12) France, C. B.; Schroeder, P. G.; Parkinson, B. A. Nano Lett. (16) Schroeder, P. G.; Nelson, M. W.; Parkinson, B. A.; Schlaf, R. 2002, 2, 693. Surf. Sci. 2000, 459, 349. (13) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. (17) Lackinger, M.; Griessl, S.; Heckl, W.; Hietschold, M. J. Phys. Appl. Phys. 2002, 91, 3010. Chem. B 2002, 106, 4482. (14) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. (18) Schlaf, R.; Parkinson, B. A.; Lee, P. A.; Nebesny, W.; Armstrong, Phys. Chem. B 2003, 107, 2253. N. R. J. Phys. Chem. B 1998, 103, 2984. Adsorption of Naphtho[2,3-a]pyrene on Au(111) Langmuir, Vol. 20, No. 7, 2004 2715 as previously described.11 A physical vapor deposition chamber (base pressure, 1 × 10-9 mbar) is attached to the UHV system allowing samples and films to be prepared in situ. The gold film was prepared by heatinga1cm× 1 cm mica sample, attached to the sample plate using molybdenum clips, for 24 h in UHV at 300 °C to evaporate surface contaminants. Gold was then evaporated from a resistively heated tungsten basket onto the heated mica substrate.20 The Mo clips provide electrical contact to the gold surface during STM imaging. Sputter (2 keV Ar+) and anneal (350 °C) cycles were used to clean and flatten the Au(111) surface. The chemical purity of the surface was determined with XPS (Mg KR, 50 eV pass energy), and the presence of the 23 ×x3 reconstruction was confirmed with STM.21 NP (Aldrich Chemical Co.) films were deposited under UHV (base pressure, 1 × 10-9 mbar) from a resistively heated boron nitride crucible (source temperature, ∼100 °C). The source was maintained at 75 °C for 12 h prior to deposition to remove any volatile contaminants. The crucible containing naphtho[2,3-a]- pyrene was heated to obtain a desired deposition rate that was monitored by a Leybold quartz crystal microbalance (QCM). The Au(111) substrate was maintained at room temperature during the deposition. Sequential depositions were performed up to a final film thickness of 50 Å. After each growth step, XPS (Mg KR,50eV pass energy) and UPS (HeI, 21.21 eV; and HeII, 40.81 eV; both with 10 eV pass energy) in normal emission were utilized to measure the electronic structure of the surface.