Mapping Atomic Contact between Pentacene and a Au Surface using

Feb 4, 2010 - structure at the atomic scale, a number of model studies of metal atoms ... insets in Figure 1 are STM images of different pentacene mol...
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Mapping Atomic Contact between Pentacene and a Au Surface using Scanning Tunneling Spectroscopy Young Jae Song,†,‡ Kyuho Lee,†,§ Seong Heon Kim,† Byoung-Young Choi,† Jaejun Yu,† and Young Kuk* Department of Physics, Seoul National University, Seoul 151-747, Korea ABSTRACT We mapped spatially varying intramolecular electronic structures on a pentacene-gold interface using scanning tunneling spectroscopy. Along with ab initio calculations based on density functional theory, we found that the directional nature of the d orbitals of Au atoms plays an important role in the interaction at the pentacene-gold contact. The gold-induced interface states are broadened and shifted by various pentacene-gold distances determined by the various registries of a pentacene molecule on a gold substrate. KEYWORDS Pentacene, interface states, scanning tunneling spectroscopy

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rganic light-emitting devices (OLED)1,2 and organic solar cells3-6 are attracting a large amount of interest for next generation opto-electronic applications. A fundamental step in their development has been the study of organic semiconductor devices, to understand not only the nonlinear transport and optical characteristics of the molecule channel, but also the interface barrier between the molecule and the contact metal. As a reproducible lowbarrier metallic contact is one of the determining factors for good device performance, the interfaces between organic semiconductors and metal electrodes have been widely investigated.7 Traditionally, the geometric structure of an interface has been probed with a transmission electron microscope (TEM)8 and its electronic properties have been measured using scanning Kelvin probe microscopy (SKPM),9 I-V measurement,10 or photoemission spectroscopy.11 With the need for understanding the geometric and the electronic structure at the atomic scale, a number of model studies of metal atoms and aromatic molecules have been performed using STM measurements on the atomic scale. Nazin et al. modeled an atomic level molecule-metal bridge on NiAl(110) by manipulating gold atoms to form a bond to a Cu(II) phthalocyanine (CuPc) molecule along the planar direction. They found that the electronic structure of the contact varied with the bonding length.12 Repp et al. observed a similar metal-molecule bonding along the planar direction on an

insulating NaCl layer. The insulating layer was used to get rid of the metal-substrate effect.13 Because of its high field-effect mobility,14 chemical stability, and compatibility with low-temperature silicon processing, pentacene thin film has been regarded as one of the most promising candidates for an organic thin film transistor. In a thin film, the pentacene molecules stand up in a near-perpendicular direction to the substrate and are stacked to form a monolayer. Therefore, the molecule-metal contact geometry resembles the case when a pentacene molecule is flatly adsorbed on a metal substrate. In this study, we mapped spatially the local density of states of a pentacene molecule on Au(001) using scanning tunneling spectroscopy (STS) with subangstrom resolution. A recent STM study showed that pentacene adsorbed on Au(111) shows very similar molecular orbitals as observed on an NaCl thin insulator.15 In contrast, we found that that pentacene-gold contact has an intramolecular electronic structure strongly depending on the adsorption distance and registries, which is confirmed with the help of density functional theory (DFT) calculations. The experiments were performed with a homemade lowtemperature STM that can be cooled down to 10 K. The Au(001) single crystal substrate was cleaned until a wellordered ∼5 × 20 surface was obtained without any trace of impurities, and an electrochemically etched tungsten tip was also cleaned in situ in UHV by e-beam heating. A pentacene source in a Knudsen cell was outgassed at 130 °C overnight, and subsequently evaporated thermally within a very narrow temperature window of 140 ( 5 °C onto the substrate, which was kept at room temperature. Then the pentacenedeposited substrate was postannealed at 100 °C for 30 min. Figure 1 shows an STM image at a submonolayer coverage (, 0.1 ML) of pentacene molecules adsorbed on Au(001). With post annealing at 100 °C, most molecules diffused and

* To whom correspondence should be addressed. E-mail: [email protected]. † Author e-mail addresses: (Y.J.S.) [email protected]; (K.L.) [email protected]; (S.H.K.) [email protected]; (B.Y.C.) [email protected]; (J.Y.) [email protected]; (Y.K.) [email protected]. ‡ Present address: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899. § Present address: Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019. Received for review: 12/12/2009 Published on Web: 02/04/2010

© 2010 American Chemical Society

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DOI: 10.1021/nl904119y | Nano Lett. 2010, 10, 996–999

spectra (red spectra in the bottom of Figure 2a) of dI/dV versus energy (or sample bias) were measured at 512 points along the longitudinal direction (blue arrow in Figure 2a) over a pentacene molecule. We plotted all these dI/dV spectra by choosing the abscissa as the 512 spatial points, the ordinate as energy from -1.5 to +1.5 eV (top-half for the unfilled state, bottom-half for the filled state) and color level for the measured dI/dV signal height (Figure 2b-d).18 Every horizontal gray arrow in each figure indicates the location where a single pentacene molecule is adsorbed. Compared to elsewhere (bare gold substrate), only the pentacene region (gray arrow) shows gold-induced filled states, which is similar to the case of C60 adsorption on Ag(100).19 These gold-induced filled states are the interface states at the pentacene-gold junction. All these observed interface states in Figure 2 reveal different modulation patterns of the peak intensities, which can be attributed to different adsorption sites. These imply that the interface states are very sensitive to the adsorption geometries, which will be analyzed in more detail in the context of Figure 3 and Figure 4. To understand the observed SR-STS data, the electronic structure of pentacene on a corrugated Au(001) surface is calculated. We used ab initio DFT calculations with the generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof20 using the SIESTA code.21 The corrugated Au surface is modeled by a slab of four atomic layers with (1 × 5) reconstruction. The slab is separated by a vacuum region of about 12 Å. The reconstructed hexagonal surface has 20% higher atomic density than the unreconstructed flat cubic surface and has a unique surface state which cannot be seen from the unreconstructed cubic surface.22 The atoms in the top two layers are relaxed until the forces are smaller than 0.01 eV/Å. The relaxed surface shows a corrugation amplitude of 0.44 Å, in good agreement with previous studies.23 The most stable adsorption structure is shown in Figure 3 (a), where the molecule aligns along the 〈110〉 direction. At an estimated adsorption height z (3.2 Å), measured by the distance between the average plane of the molecule and the bottom of the trough, we find the highest occupied molecular orbital (HOMO) at -0.43 eV (all the energy levels are given with respect to the metal Fermi level) and HOMO-1

FIGURE 1. With the post annealing (100 °C for 30 min) after the deposition, pentacene molecules diffuse and adsorb under the step edge or along the trough of direction (IT ) 0.2 nA, VS ) 1.0 V, 40 nm × 40 nm). Insets are STM images of various pentacene molecules adsorbed on different adsorption sites with the same parameters (IT ) 0.2 nA, VS ) 1.0 V).

filled into the troughs along the 〈11¯0〉 direction or at lower step edges due to the Erlich-Schwobel barrier. The three insets in Figure 1 are STM images of different pentacene molecules at various sites, which are all measured with the same parameters (IT ) 0.2 nA, VS ) 1.0 V). The observed molecular structure does not reveal any hint of five benzene rings but does show an elongated pear shape, which is a signature of pentacene-gold hybridized structures, as reported previously.15-17 Because the STM images of many pentacene molecules at different adsorption sites show barely any differences, we measured the spatial distribution of the energy resolved density of states spectrum within the molecule to reveal the adsorption geometry-dependent characteristics. Figure 2a describes the principle of a one-dimensional (1D) spatially resolved scanning tunnelling microscopy (SRSTS) measurement and panels b-d show three different 1D SR-STS data sets from single pentacene molecules adsorbed at three different adsorption sites. The scanning tunnelling

FIGURE 2. (a) Schematic of one-dimensional spatially resolved scanning tunneling spectroscopy (1D SR-STS), (b-d) 1D SR-STS data for three different molecules. The X-axis means the distance along the long axis of the molecule and the Y-axis is the sample bias (from -1.5 to +1.5 V). Each gray arrows indicates the adsorbed pentacene molecule region. The inset in Figure 2b is a scale bar indicating the peak intensities at the position and the energy level. © 2010 American Chemical Society

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metal conduction bands, but also a clear indication of π-d interaction. Unlike the other planar orbitals, only the upright Au 5dz2 state (black dashed line in the lower panel of Figure 3 (b)) is altered by another upright C 2pz orbital of π state after the adsorption of pentacene (black solid line). The different modulation patterns of the HOMO level on the various adsorption sites in the SR-STS data can be explained by the directional nature of the orbitals involved in the bonding between pentacene and the substrate, in combination with the incommensurability between the pentacene molecule and the Au lattice. To show this effect more clearly, we plotted the density-of-states projected to each carbon atom of the pentacene molecule in Figure 3c. The center carbon atom (C0 in the figure), laterally the most matched to the gold atom on the surface, shows the highest peak height, while less matched carbon atoms like C2 and C4 exhibit decreasing peak height. On the other hand, the carbon atoms at each nodal point like C1, C3, and C5 have depressed peak heights. The mismatch between the positions of the carbon atoms in pentacene and the Au atoms of the substrate determines the peak height modulations. Figure 4a is a three-dimensional plot of Figure 2b, which makes the spatial variation of the pentacene electronic structure visible clearly. As discussed in Figure 3, the HOMO and HOMO-1 states at the different carbon sites (at -0.4 and -1.2 eV) in Figure 4a show that their peak heights are modulated spatially by the incommensurability between the pentacene structure and the substrate lattice, like the other cases in Figure 2. Figure 4b shows four vertical crosssectional plots, measured at three feature-rich points over the molecule (marked by blue, red, and orange arrows in Figure 4a) and on the bare gold substrate (marked by a green arrow). The Au surface state22 appears at -0.1 eV and is not quenched by the pentacene adsorption and can be observed over the whole molecule. The energy levels of the Au surface states in the region of the molecule are the same as that in the bare surface regions (green in the figure). This observation suggests a weak physisorption of the pentacene molecules and is quite similar to ZnEthioI adsorption on NiAl(110).25

FIGURE 3. (a) Theoretical model of the single pentacene molecule adsorbed on Au(001). (b) The calculation result (d orbitals only) of the local density of states projected to the gold atom (solid lines) under the C0 atom of the pentacene molecule in Figure 3a. All the dotted lines for each component mean the result on bare surface without a pentacene molecule on the surface. (c) A set of the local density of states projected to each carbon atoms of the pentacene molecule.

at -1.51 eV. Although there is no visible level splitting, as would be expected for weak physisorption, there is a clear indication of electronic rehybridization between the substrate states and the molecular orbitals, as will be shown below. The bonding character is analyzed by decomposing the density-of-states into each atomic orbital of Au atoms in the substrate (Figure 3 (b)). We have found not only π-s resonance24 between localized molecular orbitals and itinerant

FIGURE 4. (a) 3D presentation of 1D SR-STS data for Figure 2b. (b) The cross-section of panel a at three feature-rich regions (indicated by red, orange, and blue) within the molecule and one bare Au surface (green). (c) DFT calculations with different adsorption distance between the pentacene and the Au surface at the filled states. © 2010 American Chemical Society

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SRC program (CNNC of Sungkyunkwan University). We thank Joseph Stroscio, Jabez McClelland, Mark Stiles, and Suyong Jung at Center for Nanoscale Science and Technology, National Institute of Standards and Technology (CNSTNIST) for valuable comments and encouragements.

The shift of the HOMO peaks (indicated by the arrows of the corresponding color in Figure 4b) is due to the change of the adsorption height. The dependence of level alignment on adsorption height has been calculated and is shown in Figure 4c. When the adsorption height decreases there are three main changes in the interface electronic structure. First, the molecular levels are moved down with respect to the Fermi level of the substrate, due to the increased interface dipole by exchange repulsion.26 Second, the DOS of the Au 5dz2 near the pentacene levels is increased. Third, a broadening of molecular levels is caused by the increased overlap of molecular orbitals with the delocalized s band electrons.24 In experimental studies on complex molecules adsorbed on surfaces, it is difficult to assign the exact adsorption site of atoms in the molecule from an STM image. But, the PDOS or the simulated STM image based on DFT calculation can be utilized to explain the detailed registry and the local electronic structural variation. For example, the red spectrum in Figure 4b shows the strongest peak, suggesting it has a better registry than the other carbon sites (e.g., the orange and the blue spectra sites). Therefore, clear 1D SRSTS data combined with DFT calculations make it possible to illuminate the electronic structure of a single molecule on the metal surface and its energy level alignment variations according to the atomic contact distance and registries between the molecule and substrate. In summary, post-annealed pentacene molecules on an Au(001) surface show various atomic adsorption geometries, and we mapped the intramolecular electronic structure by high resolution SR-STS. The measured local density of states combined with DFT calculations identified two factors that determine the interface electronic structure. The interatomic mismatch caused by the incommensurability between the pentacene structure and the gold lattice determines the modulation patterns of the peak intensities, and the distance between pentacene and the substrate determines the peak shift and the broadening. The former is because of the directional nature of π-d interaction, and the latter is ascribed to the Pauli repulsion and π-s resonance. These nanoscopic observations imply that the barrier between a pentacene channel and a metal electrode in an organic transistor is determined by the ensemble average of these states and the resultant band alignment.

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Acknowledgment. This work was supported by National Research Foundation of Korea (3348-20090042) and the

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DOI: 10.1021/nl904119y | Nano Lett. 2010, 10, 996-–999