NANO LETTERS
One-Dimensional Molecular Chains with Dispersive Electronic States
2009 Vol. 9, No. 12 4292-4296
Lan Chen,*, Hui Li,§ and Andrew Thye Shen Wee*,‡ Nanoscience & Nanotechnology InitiatiVe, Department of Physics, and Department of Chemistry, National UniVersity of Singapore, 2 Science DriVe 3, 117542, Singapore Received August 4, 2009; Revised Manuscript Received September 26, 2009
ABSTRACT One-dimensional molecular chains comprising self-assembled double-layer sexithiophene (6T) molecules on Ag(111) are studied with lowtemperature scanning tunneling microscopy. Despite the molecular chains being stabilized by van der Waals interactions and isolated from the metal substrate, dispersive electronic states are observed. The measurements of differential conductance and dI/dV maps along the chains reveal the end states forming at the ends of chains. Density functional theory calculations indicate the splitting and hybridization of unoccupied π molecular orbitals between neighboring layers of 6T molecules resulting in the formation of delocalized one-dimensional electronic states.
Strong metallic or covalent bonds between atoms in inorganic crystals can cause charge carriers to be delocalized and to move in wide bands with high mobility. This is in contrast to organic materials, which inherently have narrow bands and low-electron mobility due to the weak intermolecular interactions.1-3 Only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available,4-6 and the mechanism of electron delocalization in molecular materials is also not well understood. There are several reports on self-assembled molecular structures on metal or semiconductor surfaces (organic-metal hybrid materials) with delocalized electronic states observed.7-9 However, the dispersion of electronic states mainly arises as a result of strong metal-surface-mediated coupling between molecules within the overlayer.10 This means the high carrier mobility is mainly contributed by the crystal substrate. Another paradigm for the intermolecular electron delocalization involving isotropic interactions of atom-like electron orbitals centered on the nearly spherical C60 molecules was reported by M. Feng et al.11,12 They have demonstrated the delocalization for various C60 structures in the absence of a metal substrate. However, in common situations, molecule/substrate interactions also prevent the observation of electron delocalization within the overlayer. We introduce here a model approach to investigate the electron delocalization in molecular materials by constructing self-assembled molecular structures isolated from the substrate. The example demonstrated in this Letter is the one* To whom correspondence should be addressed. E-mail:
[email protected] (L.C.);
[email protected] (T.S.W.). † Nanoscience and Nanotechnology Initiative. ‡ Department of Physics. § Department of Chemistry. 10.1021/nl902527d CCC: $40.75 Published on Web 10/16/2009
2009 American Chemical Society
dimensional (1D) double-layer sexithiophene (6T) molecular chain with a brick-stacking structure on Ag(111). The interaction between the surface and second layer of 6T molecules can be neglected compared with the interaction between layers of molecules due to the intercalated first layer of molecules. As with strong a metallic-bonded 1D atomic chain,9 electronic states with dispersive features are observed. The different distribution of electronic states at the end of the molecular chains is attributed to the formation of an end state caused by a break in translational 1D symmetry. The dispersion of electronic states originates from splitting and hybridization of π* antibonding orbitals between different layer 6T molecules. Intermolecular dispersion of electronic states in our studies gives a detailed understanding of intermolecular interactions and electron delocalization in molecular materials. The single-crystal Ag(111) surface was prepared using Ar+ sputtering-annealing cycles in ultrahigh vacuum. 6T molecules were evaporated from a Knudsen cell at 175 °C onto Ag(111) kept at room temperature. The sample was then transferred in situ to a chamber (pressure < 5 × 10-11 Torr) housing a low-temperature scanning tunneling microscope (LT-STM, Omicrometer GmbH) and cooled to liquid nitrogen temperature (77 K) for STM experiments. Since a normal STM topographic image reflects the integrated local density of states (LDOS) of the sample from the Fermi level to the sample bias voltage V, a dI/dV spectra is approximately proportional to the LDOS.13 Our dI/dV spectra were measured as the in-phase ac component in the tunnelling current with a lock-in amplifier by superimposing an ac voltage of 20 mV and 600 Hz on the given dc bias of the substrate-tip gap. The dI/dV maps were obtained by positioning the STM
Figure 1. (a) STM topographic image of chain-like arrays corresponding to 1.5 ML of 6T molecules adsorbed on a Ag(111) surface. (b) dI/dV map of the same area as (a). The scanning parameters of (a) and (b) are -1.7 V and 100 pA. (c) Schematic map of the packing structure of the 6T double layer on a Ag(111) surface, which is obtained from a snapshot of MD simulations. The underlying gray balls and pink and green molecules present Ag atoms and 6T molecules in first and second layers, respectively. The white arrows mark the direction of chains. The inset shows the bilayer structure with high resolution. (d) The schematic side view of the 6T double layer on Ag(111) indicates the brick-stacking structure of the 6T bilayer.
tip at each point at constant current and then measuring dI/dV. In our experiments, the bias voltage is tip bias, which means the electrons tunnel from tip to sample when the voltage is negative. Deposition of 1.5 monolayers (ML) of 6T molecules onto Ag(111) results in the formation of highly periodic 6T bilayer nanostripes (Figure 1a).14 The width of the 6T row is measured to be 2.74 nm, corresponding to the long-axis length of 6T, which confirms the side-by-side packing of individual 6T molecules in each row. The strong moleculesubstrate interaction favors completely coplanar structures parallel to the surface in the first layer, which is different from any plane of a 6T single crystal.15,16 The 6T rows in the second layer are directly above the first layer. However, the precise related positions of 6T in the second layer to molecules in the first layer are not clear. Classic molecular dynamics (MD) simulations are performed to confirm the packing structure of two-layer 6T molecules on Ag(111) using the CVFF force field in Material Studio package.17 The MD snapshot (Figure 1c) shows that both the first and the second layers of 6T molecules lie parallel to the substrate with the same orientation, and each 6T molecule in the second layer is positioned directly above the midpoint between two neighboring molecules in the first layer. The vertical distance between the first layer and the second layer is about 3.0 Å. Such a brick-stacking chain structure in the 6T bilayer (Figure 1d) plays an important role in determining the electronic properties of the whole system, as will be discussed later. Inspection of the STM image in Figure 1a (tip bias of -1.7 V) reveals that the second-layer 6T rows appear unusually homogeneous and structureless, and single 6T molecules cannot be resolved. STM images taken at a tip bias > -1.0 V exhibit similar features. The vanishing molecular contrast of 6T bilayer chains in the STM images indicates dispersive electronic states in this structure.18 To better understand and elucidate the delocalized electronic states in the second 6T layer, we measure the STM image and dI/dV maps of one 6T row with different bias Nano Lett., Vol. 9, No. 12, 2009
Figure 2. (a) STM topographic image of a double-layer 6T chain with the end. (b, c) dI/dV maps of same area as (a). Scanning parameters: (a) -1.7 V, 100pA; (b) -1.5 V, 100pA; (c) -1.7 V, 100pA. The end part of the chain is highlighted by a blue ellipse. (d) dI/dV spectrum taken at the end and middle of the 6T chain, which are marked by red and blue dots shown in (a).
voltages, as shown in Figure 2. The magnified STM image taken at -1.7 V of Figure 2a indicates that the unoccupied states extend over the entire 1D assembly. The dI/dV maps of this 6T row at different negative tip bias show the different features at the middle and end of the row; at -1.5 V, the end of the row is darker than the middle (Figure 2b) but is 4293
Figure 3. Detailed analysis of LUMO peaks in spectra of Figure 2d for 6T at (a) the end of the chain and (b) the middle of the chain. The red lines, black dots, and green lines correspond to data from experiment and Gaussian fitting. The energy positions of subpeaks are indicated.
brighter than the middle at -1.7 V (Figure 2c). This end effect is observed on each 6T row in the second layer, as shown in the dI/dV image in Figure 1b, but not in the first layer. Scanning tunneling spectroscopy (STS) measurements provide a direct measure of the local DOS at the tip position. The differential conductivity (dI/dV) curves taken over the end and middle of the row in Figure 2d show the energy and density difference of the peaks and highlight this contrast reversal in the dI/dV maps. The main peak with lower energy corresponds to the lowest unoccupied molecular orbitals (LUMO) of 6T. The different distribution of the LUMO for molecules at the end or middle of the rows in the second layer is attributed to the formation of end states due to the break in translational symmetry,9,11 similar to two-dimensional (2D) surface states formed at the surface of a bulk 3D sample or a 1D edge state formed at the edges of a 2D structure.19 For molecules in the first layer, the energy shift and broadening of the LUMO peak (Figure 2d) result from wave function overlap between the π system of the flat-lying molecule and the metal surface.20 The LUMO peak of second-layer molecules is much narrower and sharper, which indicates that electronic coupling between second-layer molecules and substrates has been weakened a lot by the intercalated first-layer molecules. We find that there is a clearly fine structure in the LUMO peak, and Gaussian fitting to the LUMO peaks (Figure 3) shows each consisting of three subpeaks. The energy difference between the subpeaks with the lowest and highest energy is about 380 meV. The two subpeaks at lower energy are narrower than the third subpeak. At the middle of the rows, the subpeak with lowest energy has the highest intensity, but at the end of the rows, the middle subpeak has the highest intensity. Furthermore, the energy level of the three subpeaks at the end of the row is about 30 meV offset with respect to those at the middle. The fine structure of the LUMO peak and the formation of end states are believed to be related to the dispersive feature of electron states in the 6T bilayer molecular chains. To explore the origin of the dispersive LUMO states and explain the DOS and dI/dV maps, we performed DFT calculations of the electronic structure of the 6T structure. 4294
Due to weak electronic coupling between the second 6T layer and the Ag(111) substrate, the molecule-metal interface interaction should have little contribution to the fine structure of the delocalized state of the 6T bilayer chain, except for a shift of the energy levels; hence, we do not involve the substrate in our calculation model. Within the computational limit of the DFT method, our largest computational model comprises a brick-stacking chain structure of five coplanar 6T molecules in the first layer and four coplanar molecules in the second layer. The geometry is consistent with the STM images and MD simulated structure, in which the vertical distance between the two layers is set as 3.1 Å and the horizontal distance between neighboring 6T molecules is set as 6.5 Å. The calculation is performed on the B3LYP21,22/6-31G(d,p) level with the Gaussian03 package.23 The dispersion in electronic states is attributed to the strong intermolecular electronic coupling. In such parallel π-stacked arrangements of 6T bilayer molecular chains, van der Waals interactions between the first and second layer of 6T molecules are expected to be more significant than interactions between molecules in the same layer.24,25 Figure 4a shows the calculated results of LUMO states of the 6T bilayer chain, and we observe that π* antibonding LUMO orbitals of single 6T molecules split and re-form into a group of nondegenerate molecular orbitals (referred to as “split LUMOs”). There are four main orbitals with different energy shifts in the group of split LUMOs, which are mainly contributed by the second-layer molecules. The two orbitals with highest energy are nearly degenerate; therefore, there are three groups of split LUMOs, IM, IIM, and IIIM, which is consistent with the three subpeaks and the broader subpeak with highest energy (at -1.89 eV) observed in dI/dV curves (Figure 3a). The energy shift of LUMO with highest energy compared to the LUMO with lowest energy, EIIIM - EIM, that is, the dispersion of a band-like LUMO, is about 370 meV, which is also coincident with observations in our experiments. Since the large dispersion width suggests a strong intermolecular electronic coupling, which can infer a high charge transfer rate, we conclude that the π-π interaction between adjacent molecules is stronger than that in the herringbone structure of bulk 6T.5,6,25 From the spatial Nano Lett., Vol. 9, No. 12, 2009
Figure 4. (a) DFT calculation results of splitting LUMO states of molecules at the end and middle of a bilayer 6T molecular chain (marked by black ellipse), model as described in Figure 1c. Spectra have been aligned, in which the state with the lowest energy is assigned to zero. Left and right columns are the spatial distribution of corresponding split LUMO states. (b) Schematic illustration of 1D delocalized states formed in the bilayer 6T molecular chain.
distributions shown in Figure 4, we find that the split LUMO of the second-layer 6T with the lowest energy (IM) overlaps with that of first-layer 6T. The π* orbitals of two closest molecules in two different layers are not localized at the molecules, but the parts with opposite phases are connected together to form a σ-like orbital. 6T molecules are “polymerized” to form a 1D nearly delocalized state as shown in Figure 4b, similar to the case of conjugated conducting polymers.26 That is why the homogeneous and structureless 6T bilayer molecular chain image is observed in STM. The hybridization between LUMOs of different layers can efficiently decrease the LUMO energy level, and in our DFT calculation, the lowest LUMO energy level is indeed 0.2 eV lower than that of the single 6T molecule. Another important property of the 1D delocalized state is the “end state”, which corresponds to spectroscopic enhancement at the end of chains due to the break in translational symmetry. Our experimental observation of the end states can also be rationalized by the formation of a nearly delocalized 1D state, which is confirmed by the DFT calculations. As with the middle of the 6T molecular chains, there are also three groups of split LUMOs, IE, IIE, and IIIE, at the end of the 6T chains, consistent with the three subpeaks observed in experimental dI/dV curves. However, the energy levels of molecular orbitals at the end of the chains are blueshifted relative to the middle of the chains, similar to the experiments. The spatial distribution of the split LUMOs of 6T reproduces the different dI/dV maps for 6T at the end and middle of the chains. Although the orbital splitting caused by the interactions between neighboring molecules in the same layer is negligible compared to that between layers,25 the influence from neighboring molecules in the same layer is mainly through the interaction with the conjunct molecular underlayer. The formation of end states shows that the interaction between 6T molecules in different layers significantly influences their unoccupied states, even though they are packed in a self-assembled bilayer structure in the ground state by weak van der Waals interactions. Nano Lett., Vol. 9, No. 12, 2009
In conclusion, we have constructed a low-dimensional selfassembled molecular structure, double-layer 6T molecular chains with a brick-stacking structure on Ag(111), and experimentally and theoretically investigated the dispersive behavior of electronic states. Our results show the splitting and hybridization of π* antibonding orbitals between different 6T layers forming nearly delocalized 1D states. Unlike the hybridization of electronic states with high energy (>3.5 eV) into delocalized states in C60 molecular chains,11,27 the delocalized states of the 6T bilayer molecular chains come from the lowest unoccupied state, which can be easily observed and controlled. Our observations demonstrate that the stacking structure of molecules may provide a way of influencing intermolecular interactions and also give some insight into molecular design. Acknowledgment. The authors acknowledge funding support from A*STAR Grant Number R-398-000-036-305 and the Singapore Millenium Foundation (SMF) for a Fellowship to L.C. References (1) Coropceanu, V.; Cornil, J.; da Silva, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. L. Chem. ReV. 2007, 107, 926–952. (2) Tautz, F. S. Prog. Surf. Sci. 2007, 82, 479–520. (3) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99–117. (4) Koch, N.; Vollmer, A.; Salzmann, I.; Nickel, B.; Weiss, H.; Rabe, J. R. Phys. ReV. Lett. 2006, 96, 156803. (5) Koller, G.; Berkebile, S.; Oehzelt, M.; Puschnig, P.; Ambrosch-Draxl, C.; Netzer, F. P.; Ramsey, M. G. Science 2007, 317, 351–355. (6) Kakuta, H.; Hirahara, T.; Matsuda, I.; Nagao, T.; Hasegawa, S.; Ueno, N.; Sakamoto, K. Phys. ReV. Lett. 2007, 98, 247601. (7) Gonzalez-Lakunza, N.; Fernandez-Torrente, I.; Franke, K. J.; Lorente, N.; Arnau, A.; Pascual, J. I. Phys. ReV. Lett. 2008, 100, 156805. (8) Temirov, R.; Soubatch, S.; Luican, A.; Tautz, F. S. Nautre 2006, 444, 350–353. (9) Crain, J. N.; Pierce, D. T. Science 2005, 307, 703–706. (10) Ferretti, A.; Baldacchini, C.; Calzolari, A.; Di Felice, R.; Ruini, A.; Molinari, E.; Betti, M. G. Phys. ReV. Lett. 2007, 99, 046802. (11) Feng, M.; Zhao, J.; Petek, H. Science 2008, 320, 359–362. (12) Feng, M.; Lee, J.; Zhao, J.; Yates, J. T.; Petek, H. J. Am. Chem. Soc. 2007, 129, 12394–12395. 4295
(13) Chen, C. J. Introduction to Scanning Tunneling Microscopy; Oxford University Press: Oxford, U.K., 1993. (14) Chen, L.; Chen, W.; Huang, H.; Zhang, H. L.; Yuhara, J.; Wee, A. T. S. AdV. Mater. 2008, 20, 484–488. (15) Duncker, K.; Kiel, M.; Hofer, A.; Widdra, W. Phys. ReV. B 2008, 77, 155423. (16) Kiel, M.; Duncker, K.; Hagendorf, C.; Widdra, W. Phys. ReV. B 2007, 75, 195439. (17) Materials studio 4.0; Accelrys, Inc.: San Diego, CA, 2006. (18) Schafer, J.; Blumenstein, C.; Meyer, S.; Wisniewski, M.; Claessen, R. Phys. ReV. Lett. 2008, 101, 236802. (19) Bollinger, M. V.; Lauritsen, J. V.; Jacobsen, K. W.; Norskov, J. K.; Helveg, S.; Besenbacher, F. Phys. ReV. Lett. 2001, 87, 196803. (20) Sony, P.; Puschnig, P.; Nabok, D.; Ambrosch-Draxl, C. Phys. ReV. Lett. 2007, 99, 176401. (21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (22) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
4296
(24) (25) (26) (27)
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. Tsuzuki, S.; Honda, K.; Azumi, R. J. Am. Chem. Soc. 2002, 124, 12200–12209. Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 16866–16881. Akai-Kasaya, M.; Shimizu, K.; Watanabe, Y.; Saito, A.; Aono, M.; Kuwahara, Y. Phys. ReV. Lett. 2003, 91, 255501. Zhao, J.; Feng, M.; Yang, J. L.; Petek, H. ACS Nano 2009, 3, 853–864.
NL902527D
Nano Lett., Vol. 9, No. 12, 2009
