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J. Phys. Chem. C 2008, 112, 14464–14474
Adsorption and Desorption of Stilbene from the Ag/Ge(111)-3 Surface H. C. Wu,†,‡,§ L.-W. Chou,†,| L.-C. Wang,‡,§ Y.-R. Lee,§ C.-M. Wei,§,⊥ J.-C. Jiang,| C. Su,*,# and J.-C. Lin*,‡,§ Department of Chemistry, National Taiwan Normal UniVersity, Taipei 106, Taiwan, R.O.C., Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 106, Taiwan, R.O.C., Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei 106, Taiwan, R.O.C., Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, R.O.C., and Institute of Organic and Polymeric Materials, National Taipei UniVersity of Technology, Taipei 106, Taiwan, R.O.C. ReceiVed: March 26, 2008; ReVised Manuscript ReceiVed: May 23, 2008
The adsorption and desorption of stilbene on Ag/Ge(111)-(3 × 3)R30° (Ag/Ge(111)-3) were investigated using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (DFT). Both trans- and cis-stilbenes form a (2 × 1) overlayer structure on Ag/Ge(111)-3 at a coverage of ∼1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. At a coverage of less than 1 ML, the TPD of cis-stilbene shows only one peak, attributed to submonolayer desorption. The TPD peaks are indistinguishable for desorption of trans-stilbene from the surface submonolayer and multilayer. This is due to the simultaneous desorption and/or thinning of adsorbed multilayers during the TPD process, as determined from the STM analysis of adsorbed trans-stilbene structures before and after annealing. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(111)-3 with desorption energies of 20.1 (cis-) and 21.3 kcal/mol (trans-), which are comparable with the calculated values using the DFT method. A plausible explanation for the stilbene desorption process on Ag/Ge(111)-3 is proposed and discussed. 1. Introduction The study of highly ordered monolayers of oligomeric organic molecules on an inorganic single crystal surface is of both practical and fundamental interest.1–4 The research is interesting because it provides information necessary for the epitaxial growth of organic films and allows us to determine useful properties of organic-inorganic interfaces. This is important for the development of novel (opto-)electronic devices. In particular, for polymer-based organic materials, the interaction between the individual units and the substrate surface dominates the packing arrangement of polymers and affects the properties of devices. A study of oligomeric monolayers is of fundamental interest because a number of studies have shown that small oligomer adsorbed surfaces are much more ordered, e.g., well defined, than their corresponding polymers, which are much more difficult to understand in detail.5,6 Therefore, it is advantageous to first investigate monomer surface overlayers and then a range of oligomers, from dimer to trimer, etc., to mimic the complete polymer chain. The specific system that we are interested in is conjugated organic polymers due to their promising potential in optoelectronic technology.7–9 The most popular and simplest conjugated organic polymer is poly-p-phenylenevinylene (PPV), which has been used as the light-emitting layer in organic light-emitting * Corresponding authors (J.-C.L.): tel., 886-2-23668261; fax, 886-223620200; e-mail,
[email protected]. (C.S.): tel., 886-2-27712171 ext. 2435; fax, 886-2-27317174; e-mail,
[email protected]. † These authors contributed equally to this work. ‡ National Taiwan Normal University. § Institute of Atomic and Molecular Sciences, Academia Sinica. | National Taiwan University of Science and Technology. ⊥ Institute of Physics, Academia Sinica. # National Taipei University of Technology.
diodides (OLEDs).10 It cannot form ordered packed arrays upon deposition on the device substrate due to numerous olefinic links as well as various possible conformations in a single polymer chain. Stilbene is the shortest oligomer of PPV. Its adsorption and cis-trans photoisomerization on Ag/Ge(111)-(3 × 3)R30° (denoted as the Ag/Ge(111)-3 surface hereafter) have been previously studied using scanning tunneling microscopy (STM).11–13 It was found that the monolayer of transstilbene forms a commensurate (2 × 1) structure on Ag/Ge(111)3 with two phenyl groups adsorbed to the 3-fold site at the center of two alternative Ag-trimers. An important feature of the Ag/Ge(111)-3 surface is that the lattice constant of the surface unit cell (∼6.5 Å) is equal to the distance between the centers of two phenyl rings in stilbene. This makes Ag/Ge(111)3 a potential substrate for stilbene self-organization. Also, because Ag is a relatively inert metal due to its deep-lying 4d valence bands, no dissociation would occur upon the adsorption of stilbene on Ag/Ge(111)-3 to hinder the study of the monolayer structure.5 The extreme high resolution of STM allows us to in situ observe, at the molecular level, the photoisomerization of two-dimensional stilbene aggregates. It was found that the isomerization process proceeds pairwise through the biexciton-assisted mechanism.12 As a continuation of previous work, we present a more detailed investigation of stilbene adsorption and desorption on Ag/Ge(111)-3 using more surface techniques. The two most common tools for the study of the geometric structure of surface adsorbates are STM and low-energy electron diffraction (LEED). Whereas the use of STM appears straightforward, there are some inherent problems in LEED study. For instance, because of the large lattice constants of the adsorbate, which may be up to tens of angstroms, there are few unit cells within the coherence length
10.1021/jp802611y CCC: $40.75 2008 American Chemical Society Published on Web 08/23/2008
Adsorption/Desorption of Stilbene from Ag/Ge(111)-3 of the electron beam of a normal LEED optic, making the detection of broad spots difficult. In addition, organic molecules are expected to be very sensitive to the primary electron beam, which may induce damage and/or the desorption of organic adsorbates. Although the interaction between stilbene and Ag/ Ge(111)-3 is strong enough to immobilize the molecules onto the surface forming 2D layers, stilbene is mainly physisorbed on Ag/Ge(111)-3. In this work, using LEED, we have successfully acquired the long-range order of the stilbene molecules on Ag/Ge(111)-3 in perfect agreement with the STM images. For stilbene desorption from Ag/Ge(111)-3, temperatureprogrammed desorption (TPD) experiments were performed. The TPD study allows one to draw conclusions on the respective binding energies, the thermal stability, and the desorption mechanism of organic adsorbates from the substrate surface. Slayton et al. studied the desorption and photochemistry of stilbene on Al2O3(0001) using the combination of TPD and sticking model simulation.14 It was found that for both transand cis-stilbene (denoted respectively as TSB and CSB hereafter), the monolayer desorption peak overlapped substantially with the multilayer ones which grow before the first surface layer is completely full. When a multilayer of TSB is irradiated by a UV light of 313 nm, instead of photoisomerization to form CSB, molecular dimerization through an excimer intermediate occurred. It would be interesting to compare the stilbene desorption on the metallic Ag/Ge(111)-3 surface with that on the insulated Al2O3 surface. The present study was undertaken to investigate the adsorption and desorption of stilbene on Ag/Ge(111)-3. The CSB and TSB molecules were found to form highly ordered domains on Ag/Ge(111)-3. The structures of stilbene overlayers were observed with intramolecular resolution by STM, and the unit cell was confirmed by LEED. The results from the TPD study show that the molecular desorption of stilbene from Ag/Ge(111)3 follows half-order kinetics. The local interaction between adsorbed stilbenes (cis- and trans-) and Ag/Ge(111)-3 was further calculated using the conventional density functional theory (DFT). A plausible mechanism implicating domain shrinking upon desorption is then proposed and discussed. 2. Experimental Section The experiments were carried out in two separate ultrahigh vacuum (UHV) systems to employ different techniques to study the adsorption and desorption of stilbene on the Ag/Ge(111)3 surface. The system used for the adsorption study is a dualchamber consisting of a temperature variable scanning tunneling microscope (VT STM-Omicron Vakuum Physik) chamber and a sample preparation/surface analysis chamber equipped with an Auger electron spectroscope (AES, VG) for surface elemental analysis, a quadrupole mass spectrometer (QMS, AMETEKDYCOR, LC200M) for verifying background gas during the dosing, a homemade Ag evaporation gun for Ag deposition, and an ion sputtering gun (Physical Electronics) for both substrate cleaning and STM tip sharpening. An STM tip mechanically cut from a 0.3 mm diameter tungsten wire (Omega) was used in this study. All the STM images were recorded in constant current mode. The chamber base pressure was less than 2 × 10-10 Torr. The Ge samples (2 × 10 × 0.37 mm3) used in this study were cut from a single crystal, n-type (
