Ag(111

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Control of Two-Dimensional Ordering of F16CuPc on Bi/Ag(111): Effect of Interfacial Interactions Kelvin Hong Liang Zhang,† Hui Li,†,§ Hongying Mao,‡ Han Huang,† Jing Ma,§ Andrew Thye Shen Wee,† and Wei Chen*,†,‡ Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, 117542, Singapore, Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, 117543, Singapore, and Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing UniVersity, Hankou Road 22, Nanjing 210093, People’s Republic of China ReceiVed: April 23, 2010

In-situ low-temperature scanning tunneling microscopy, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations have been used to systematically investigate the supramolecular assembly of copper hexadecafluorophthalocyanine (F16CuPc) on various Bi/Ag(111) surfaces, including metallic BiAg2 surface alloy, semimetal Bi-P ×
3 overlayer and Bi(110) monolayer. We demonstrate that the molecular ordering of F16CuPc is strongly affected by the molecule-substrate interfacial interactions on different substrates and the intermolecular interactions. At the monolayer region (lst layer), F16CuPc molecules interact strongly with BiAg2 and form a quasi-hexagonal unit cell with two alternative “R” and “β” in-plane orientations to minimize the repulsive electrostatic forces between neighboring F16CuPc. In contrast, a highly ordered quadratic monolayer structure with the same in-plane orientation forms on both P ×
3 overlayer and Bi(110) surface due to the relatively weak interfacial interactions. The molecular ordering in the second layer is largely governed by the delicate balance between the interlayer π-π interaction between the first two layers and the intermolecular interaction within the second layer. To reduce the electrostatic repulsion resulting from the fluorine atoms, the second layer F16CuPc adopts either a rotated or slipped geometry with respect to the first layer. It is also found that the second layer F16CuPc always adopts a 4-fold symmetry lattice regardless of the underlying substrates, consistent with our MD simulations. DFT calculations also demonstrate that for the second layer of F16CuPc molecules, the rotated geometry is the most favorable. 1. Introduction Organic thin film-based electronics have promising applications in low-cost, large-scale, and flexible devices such as organic light-emitting diodes (OLED), organic photovoltaic cells (OPV), organic field effect transistors (OFETs), and organic spintronics.1-4 The interface between organic and inorganic substrates is of particular interest. It is found that the molecular arrangement and the electronic energy alignment of the first few organic layers at the interface play crucial roles in determining the charge carrier transport, charge carrier injection, and molecular magnetism.5,6 For example, a high hole mobility close to 10 cm2 V-1 s-1 can be achieved on titanyl phthalocyanine (TiOPc)-based thin film transistors due to close intermolecular π-π contact;6 magnetic switching in various phthalocyanine thin films has been realized by manipulating the polymorphism or supramolecular packing structures.4 As such, the understanding and tailoring of the supramolecular packing of organic thin films at the molecular level is the key to the optimization and performance improvement of organic devices, prompting numerous investigations on the adsorption and growth of thin films of π-conjugated molecules on various surfaces.7-9 While a high level of sophistication has been reached in controlling the structural uniformity across the interfaces of inorganic materials such as silicon and gallium arsenide, the understanding and control of the organic-inorganic interface * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Physics, National University of Singapore. ‡ Department of Chemistry, National University of Singapore. § Nanjing University.

is still not well understood. This is attributed to the different properties inherent to organic materials, that is, large size, anisotropy, and relatively weak noncovalent intermolecular interaction. Two important factors determine the molecular ordering process in organic thin films: (i) molecule-substrate interfacial interactions and (ii) intermolecular interactions.8-13 Intensive research efforts have been devoted to the understanding and controlling of the molecular orientation of planar π-conjugated molecular thin films on various substrates. Planar π-conjugated molecules such as various phthalocyanine and pentacene molecules lie flat on metallic substrates due to the effective overlapping between the substrate electronic states and the π-orbitals in molecules,8,11,12 while they are found to adsorb in a “standing-up” configuration on surfaces such as SiO2 due to weak molecule-substrate interfacial interactions.13,14 It has been recently demonstrated that the out-of-plane orientation of pentacene thin films can be controlled by manipulating the semimetallic or metallic electronic structures of Au/Si(111) surfaces.15 It is also found that the molecular self-assembly and adsorption behaviors can be largely manipulated by controlling the surface density of states via quantum size effect of Pb(111) thin films on Si(111).16 However, it is less understood how interfacial and intermolecular interactions affect the twodimensional (2D) in-plane molecular ordering. Because of its remarkable chemical-, thermal-, and airstability, copper hexadecafluorophthalocyanine (F16CuPc) represents a promising n-type semiconducting molecule17 for use in organic semiconductor devices, in particular n-channel and bipolar OFETs.18 Previous studies focused on the growth of

10.1021/jp104034v  2010 American Chemical Society Published on Web 06/08/2010

Two-Dimensional Ordering of F16CuPc on Bi/Ag(111)

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Figure 1. High-resolution STM images of atomic structures of three Bi phases on Ag(111). (a) BiAg2 surface alloy with a
3 ×
3R30° superstructure. The bright dots represent Bi atoms which protrude out of the surface due to its large atomic size. (b) Bi-P ×
3 overlayer structure with lattice constant of a1 ) 4.74 Å, b1 ) 5.0 Å. (c) Large scale STM image showing the coexistence of BiAg2 surface alloy and P ×
3 overlayer. (d) Bi(110) monolayer structure (a2 ) 4.54 Å, b2 ) 4.75 Å).

F16CuPc on inert dielectrics such as SiO2, on which F16CuPc molecules adopt a standing up configuration with their molecular π-plane oriented nearly perpendicular to the substrate surface.19 F16CuPc molecules lie flat on single crystalline metal substrates such as Cu(111) and Ag(111)20 but with significant molecular distortion as revealed by an X-ray standing waves (XSW) investigation.21 In this article, we employ in situ low-temperature scanning tunneling microscopy (LT-STM, 77 K) to systematically investigate the adsorption behaviors of F16CuPc on several different surfaces constituted by various Bi phases on Ag(111). To understand how the interfacial interactions affect the inplane molecular ordering, we choose model systems of F16CuPc molecules grown on the metallic BiAg2 surface alloy substrate and the semimetal Bi-P ×
3 overlayers and Bi(110) monolayer. With the help of density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we compare and discuss the effect of molecule-substrate interfacial interactions on the molecular ordering of F16CuPc thin films on different substrates. 2. Experimental Section The STM experiments were conducted with an Omicron LTSTM housed in a multichamber ultrahigh vacuum (UHV) system with a base pressure better than 1.0 × 10-10 mbar and interfaced with a Nanonis controller (Nanonis, Switzerland).22 All STM imaging was performed at 77 K with a chemically etched W tip. A clean Ag(111) surface with large scale terraces was obtained after a few cycles of Ar+ ion bombardment and subsequent annealing at 800 K. Bi and F16CuPc molecules (Sigma-Aldrich) were thermally evaporated from two separated low-temperature Knudsen cells (MBE-Komponenten, Germany)

onto the samples held at room temperature (RT) in the UHV growth chamber (base pressure better than 3 × 10-10 mbar). Prior to deposition, F16CuPc was purified twice by gradient vacuum sublimation (Creaphys, Germany). The deposition rates of Bi and F16CuPc were monitored by a quartz-crystalmicrobalance (QCM), and were further calibrated by counting the adsorbed molecule coverage in the large-scale LT-STM images. In our experiments, all depositions were performed at constant rates of about 0.02 ML/min for Bi and 0.03 ML/min for F16CuPc. 3. Results and Discussion 3.1. Bi on Ag(111): BiAg2 Surface Alloy, Bi-P ×
3 Overlayer and Bi(110) Monolyer. To understand the effect of molecule-substrate interfacial interactions on molecular ordering, we choose the model Bi/Ag(111) system as it can transform into surface phases with different geometric and electronic structures, namely the metallic BiAg2 surface alloy, semimetal Bi-P ×
3 overlayer and Bi(110) monolayer. The transition between these three surface phases can be controlled by the Bi coverage on Ag(111).23 This is briefly summarized in Figure 1. At low coverage, deposited Bi atoms can either incorporate into the topmost layer of the Ag(111) to form an embedded BiAg2 surface alloy, or react with the Ag atoms released during the Bi incorporation to form BiAg2 surface alloy supported on top of Ag(111) surface.23,24 Both BiAg2 surface alloys possess a
3 ×
3R30° superstructure with a lattice constant of a ) 5.0 Å, as shown in Figure 1a. Upon increasing the coverage above a critical value, a surface dealloying process occurs. At the same time, the BiAg2 surface alloy phases gradually convert into an ordered Bi-P ×
3 overlayer

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Figure 2. (a) STM image of 0.2 ML F16CuPc deposited on surface with coexistence of the BiAg2 surface alloy and the Bi-P ×
3 overlayer, showing that F16CuPc molecules exclusively adsorb on the BiAg2 surface alloy. (b) Molecularly resolved STM image of the monolayer structure of F16CuPc adsorbing on the BiAg2 alloy and (c) its corresponding snapshot of the MD simulation at 500 ps. (d) Large-scale STM image of the monolayer F16CuPc on Bi(110), (e) its corresponding molecularly resolved image, and (f) the snapshot of the MD simulation at 500 ps.

structures with a rectangular lattice of a1 ) 4.7 Å and b1 ) 5.0 Å, as shown in Figure 1b. Figure 1c shows an STM image with coexistence of BiAg2 surface alloy and Bi-P ×
3 overlayer. Finally, increasing coverage more than 1 ML results in the formation of ordered Bi(110) monolayer with a rectangular lattice of a2 ) 4.54 Å, b2 ) 4.75 Å as shown in Figure 1d.25 DFT calculations have been performed to confirm the geometries of BiAg2 and Bi(110)/Ag surfaces, using the Vienna ab initio simulation package (VASP).26 The Perdew-Wang 199127 exchange-correlation functional and the ultrasoft pseudopotentials (USPP) were adopted. The calculation models contain three layers of Ag atoms as substrate, where the bottom layer was fixed, and the vacuum space is 10 Å. The cutoff energy of the plane-wave was set at 250.0 eV, and 3 × 2 × 1 k-point grids were used for summation over the surface Brillouin zone (SBZ). The calculated atomic structures of the BiAg2 surface alloy and Bi(110) on Ag(111) surface are shown in the inset of Figure 1a,d, respectively, in good agreement with the experimental results. Our calculations also reveal that the Bi atoms in BiAg2 surface alloy are slightly above the surface with a Ag-Bi bond length of around 2.97 Å. On Bi(110), Bi atoms form “zigzag” lines with Bi-Bi bond lengths of 3.07-3.10 Å. 3.2. F16CuPc Monolayer on Bi/Ag(111). F16CuPc molecules were thermally evaporated onto each surface phase at room temperature to evaluate the effects of molecule-substrate interfacial interactions on supramolecular assembly. Figure 2a shows a STM image of 0.2 ML F16CuPc deposited on a surface with coexistence of BiAg2 and Bi-P ×
3 phases. F16CuPc molecules exclusively adsorb on the BiAg2 surface alloy regions at this coverage. Adsorption of F16CuPc on the Bi-P ×
3 overlayer occurs only after the BiAg2 regions are fully terminated by F16CuPc. The preferential adsorption of F16CuPc on BiAg2 indicates a stronger molecule-substrate interfacial

interaction than that on the Bi-P ×
3 overlayer. It is known that the molecule-metal interfacial interaction is dominated by coupling between the molecular orbitals and substrate valence or conduction electronic states.8-15 Studies have shown that the BiAg2 surface is metallic and possesses a relative high density of states near the Fermi level, while the P ×
3 and Bi(110) surface phases exhibit low density of states near the Fermi level and are therefore semimetal.24,25 Hence, F16CuPc molecules are expected to interact more strongly with the BiAg2 surface due to the effective coupling between the molecular π-orbital and electronic states of the metallic BiAg2 surface. This scenario is consistent with our DFT calculation on the adsorption energy of F16CuPc on the two surfaces, which is defined by the terms of the energies, Ead ) E(mol/sub) - {E(sub) + E(mol)}. The calculation results shown that the adsorption energy of F16CuPc on AgBi2 surface is 15.6 kcal/mol lower than that on Bi(110) surface, indicating it is energetically favorable to adsorb on the metallic AgBi2 surface. Figure 2b shows a high-resolution STM image of the F16CuPc monolayer on BiAg2. The characteristic four-lobed shape of F16CuPc molecule with a central dark hole is clearly identified. The four lobes are assigned to the four F-substituted peripheral benzene rings and the center dark hole to the Cu atom.20 This image indicates that F16CuPc adsorbs “lying-down” with its molecular π-plane parallel to the surface.28,29 The F16CuPc molecular structure possesses a quasi-hexagonal unit cell on the 3-fold BiAg2 surface alloy with two types of in-plane orientations labeled “R” and “β”, as indicated in Figure 2b. The molecules with R and β orientations are rotated ∼60° with respect to each other. F16CuPc molecules possessing the same in-plane orientation (R or β orientations) stack together to form molecular rows along the Ag[11-2] directions, referred to as R or β molecular rows and the angle between the diagonal line of

Two-Dimensional Ordering of F16CuPc on Bi/Ag(111) either R or β F16CuPc molecules and Ag[11-2] directions is 60°. As such the BiAg2 surface alloy areas are dominated by alternatively stacked R and β molecular rows. The self-organization of organic molecules on metal substrates is controlled by the balance between the molecule-substrate interfacial interactions and intermolecular interactions.8-11 The effective molecule-metal interfacial interactions force molecules to assemble into monolayer structures with well-defined supramolecular arrangements, but constrained by the substrate surface periodicity and symmetry.8,30 We performed theoretical simulations to understand this assembly process. van der Waals (vdW) forces such as the dispersion force play an important role in stabilizing the molecular adsorption on substrates.31 Here we use the low-cost force field method to investigate the molecular packing structures of F16CuPc on the BiAg2 surface alloy. The classical MD simulations were performed in canonical (NVT) ensembles with modified consistent valence force field (CVFF)32 employed in the Discover module in the Material Studio package.33 The simulated annealing temperature was from 298 to 77 K, and the simulated time is 500 ps with time steps of 1.0 fs. MD simulations reproduce the molecular packing structures as observed in our LT-STM experiments. As shown by the high-resolution STM image in the inset of Figure 2b and the corresponding MD snapshot in Figure 2c, F16CuPc monolayers adopt a quasi-hexagonal arrangement on the 3-fold BiAg2 surface alloy, reflecting the symmetry of the underlying substrate. The intermolecular distance along either R or β molecular row (i.e., Ag[11-2]) is ∼14.5 Å and the molecular distance between the two neighboring rows is ∼15.0 Å, which are around three times of the lattice of the BiAg2. To this end, we can conclude that the strong F16CuPc and BiAg2 interaction locks F16CuPc to the specific adsorption sites on the substrate lattice. The large C-F bond polarity however causes a large electrostatic repulsive force between the neighboring F-substituted peripheral benzene rings. As illustrated in Figure 2c, by 60° rotation of F16CuPc in the unit cell the F-substituted benzene rings of R-oriented F16CuPc point into the hollow sites of β-oriented F16CuPc molecules. As such, the formation of the alternating R and β in-plane orientations minimizes the repulsive intermolecular electrostatic interaction between neighboring F16CuPc molecules. MD simulations show that the binding energy of the molecular structure with alternating R and β is slightly lower (∆E ) -0.9 kcal/mol) than pure R or β phases. In contrast, deposition of F16CuPc on the Bi(110) or Bi-P ×
3 overlayer leads to the formation of highly ordered molecular structures with a square lattice. F16CuPc on both surfaces exhibit the same growth behavior because of the similarity in electronic properties, although one lattice parameter of P ×
3 is 9% larger than that of Bi(110). As such, we will only discuss the results of F16CuPc on Bi(110) here. Figure 2d shows that F16CuPc molecules assemble into a highly ordered 4-fold symmetric monolayer structure on the rectangular Bi(110) unit cell. Figure 2e displays its corresponding high-resolution STM image with a square lattice constant of a ) 15.0 Å. The characteristic four-lobed shape of F16CuPc is clearly imaged, suggesting that the molecules adopt a “lying-down” configuration. In contrast to the case on BiAg2 surface alloy, all F16CuPc molecules on Bi(110) or Bi-P ×
3 overlayer have identical in-plane orientations. The monolayer structure is not commensurate with the substrate. The intermolecular distance (a ) 15.0 ( 1 Å) does not fit an integer multiple of the unit cell (a2 ) 4.54 Å, b2 ) 4.75 Å) of Bi(110). This suggests a relatively weak interfacial interaction between F16CuPc and Bi(110), attributed to the semimetal nature of Bi(110) and its low density

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Figure 3. STM images of the 2nd layer F16CuPc on the BiAg2 surface alloy. (a) Large scale image showing the second layer molecules assembling into molecular nanostripes, and (b) its corresponding detailed STM image showing that the 2nd layer possesses a nearly square unit cell.

of states near the Fermi level. The intermolecular interaction between 4-fold symmetric F16CuPc drives the molecules to take a quadratic configuration. To minimize repulsive intermolecular electrostatic interactions, the F-substituted benzene rings of each F16CuPc molecule point into the hollow sites of its neighboring molecules, as illustrated by the MD snapshots in Figure 2f. Such a monolayer packing structure with 4-fold symmetry has been observed for various “flat-lying” metal phthalocyanine molecules adsorbed on HOPG,34 Ag(111),35 and Au(111).36 3.3. Second Layer of F16CuPc on Bi/Ag(111). Turning now to the second layer, Figure 3a displays a large scale STM image of F16CuPc bilayer on the BiAg2 surface alloy, showing close packed molecular nanostripes with a few isolated molecules randomly distributed on top of the first layer. The molecular nanostripes preferentially align along the Ag[11-2] direction. It is clear from Figure 3b that the second monolayer retains the flat-lying configuration, driven by interlayer π-π interactions between the first and second layer. Neighboring molecules in the second layer tend to adopt the same in-plane orientation,

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Figure 4. STM images of the 2nd layer F16CuPc on Bi(110). (a) At coverage of 1.2 ML, F16CuPc molecules absorb as isolated molecules, randomly distributed on the 1st layer. (b) Molecularly resolved STM image showing that the in-plane orientation of the 2nd layer molecules are rotated by an angle of θ ≈ 45° with respect to those in the 1st layer. (c) The corresponding snapshot of the MD simulation at 500 ps. (d) STM image of 2nd layer molecules form a short-range
2 ×
2R45° superstructure with respect to the first layer at a coverage of 1.4 ML.

either R or β- orientation. This differs from the alternating R and β rows in the first layer. In contrast to the quasi-hexagonal structure in the first layer, the second layer exhibits a square unit cell. This difference can be explained by the different interfacial interactions, that is, the strong coupling between the metal electronic states and molecular π-orbitals for the first layer F16CuPc on the BiAg2 surface alloy,7,8 versus the weaker interlayer π-π interaction between the first and second F16CuPc layers.35-37 Thus, intermolecular interactions in the second layer dominate, and the square unit cell forms due to interactions between the 4-fold symmetric F16CuPc molecules. The initial growth behavior of the F16CuPc second layer on Bi(110) or the Bi-P ×
3 overlayer differs significantly from that on the BiAg2 surface alloy. As shown in Figure 4a, instead of forming a close-packed layer, the impinging F16CuPc initially adsorb as isolated molecules randomly distributed on top of the first layer. The high-resolution image in Figure 4b reveals that the four lobes of the second layer molecule are not stacked directly above those of the first layer molecules, but rotated by θ ≈ 45°. By increasing the F16CuPc coverage to 1.4 ML, the second layer molecules form a short-range ordered
2 ×
2R45° superstructure with respect to the first F16CuPc layer, as shown in Figure 4d. The intermolecular distance between the second layer F16CuPc molecules is measured to be 2.0 nm, which is significantly larger than the van der Waals radii of F16CuPc.19,20 Increasing the F16CuPc coverage above 1.5 ML induces a structural rearrangement in the second layer. The
2 ×
2R45° superstructure gradually transforms into a close packed phase at 2 ML coverage. Figure 5a represents a typical molecularly resolved STM image of the close packed second layer. As

marked by the red square in Figure 5a, the second layer has a 4-fold symmetric unit cell with intermolecular distance of 15.0 Å, identical to that of the first layer. In Figure 5b, both the first and second layer molecules are simultaneously resolved. The second layer possesses the same in-plane orientation as the first layer, but F16CuPc molecules in the second layer are laterally displaced by about 2.0 Å with respect to the first layer, as marked by the dashed line in Figure 5b. To understand this unique adsorption behavior of the second layer F16CuPc on Bi(110), we carried out MD simulations and DFT calculations. The nonbonded interactions in force fieldbased MD simulations are constructed by vdW and electrostatic interactions, justifying the reliability of using MD simulation to study the packing structures of the second layer F16CuPc. As shown in Figures 4c and 5c, the MD snapshots of the rotated and slipped second layers clearly reproduce our STM observations. To quantitatively evaluate the stability of the rotated and slipped structures and to understand the molecular packing structure transition from the rotated geometry in the
2 ×
2R45° superstructure (Figure 4) to the slipped close packed second layer (Figure 5), we perform DFT calculations of the F16CuPc dimer on B3LYP38,39/lanl2dz40 level with the Gaussian 03 package.41 Four optimized stacking geometries and relative energies are displayed in Figure 6 with one rotated and three different slipped geometries. Both DFT and MD results show that the 45° rotated geometry (Figure 6a) has the lowest binding energy (set to zero point energy as reference here) and thus is the most favorable adsorption configuration for the second F16CuPc layer on the first layer. The interlayer interactions between the first two layers of F16CuPc are dominated by intermolecular π-π interactions, which can be decomposed into

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Figure 5. (a) High-resolution STM image of the close packed 2nd layer F16CuPc on Bi(110). A square unit cell is highlighted by the red square. (b) An STM image with the first and second layer molecular structures simultaneously resolved. (c) The corresponding snapshot of the MD simulation at 500 ps.

Figure 6. Optimized geometries and relative energies (in the unit of kcal/mol) of various F16CuPc dimers. The first layer F16CuPc molecule is in navy color and the second layer is in gray color.

electrostatic and vdW dispersion forces. As such, the rotated stacking renders an effective way to minimize the repulsive intermolecular electrostatic force arising from the F-substituted benzene rings between the two F16CuPc layers.20 However, the rotated geometry causes the second layer F16CuPc to saturate at 1.5 ML coverage on Bi(110), that is, the
2 ×
2R45° superstructure. MD simulations and DFT calculations also identify another stable adsorption structure to be the slipped

stacking configuration, as shown by the snapshot in Figures 5c and 6b. The top F16CuPc adopts the same in-plane orientation as the underlying molecule but with a finite lateral displacement of about 2.0 Å. Similar slipped stacking geometry has also been observed in benzene dimers42 and in the multilayer H16CoPc film on Pb(111)43 and H16FePc on Au(111),8 as this also reduces the electrostatic repulsion between two stacked molecules. Such slipped stacking geometry is quite similar to the well-known

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R-type polymorph in H16MPc thin films.44 The slipped stacking geometry therefore facilitates the close packing of the second layer, revealing a layer-by-layer growth mode of F16CuPc thin film on Bi(110). We also identified another two optimized slipped geometries from the MD simulations and DFT calculations as shown in Figure 6c,d. Both of the stacking models have been determined as β-type polymorph in MPc.44 The stacking model in Figure 6c is the most unstable stacking geometry (Ebinding ) 1.85 kcal/mol). The stacking geometry in Figure 6d is analogous to the stacking structure of β phase F16CuPc thin film on SiO2,19c although the F16CuPc adopts a standing up geometry on SiO2 rather than lying-down on the surface. Nevertheless, the interlayer π-π interactions should remain same. 4. Conclusions In summary, we have demonstrated the control of in-plane supramolecular packing of F16CuPc thin films through the modification of interfacial interactions using different Bi/Ag surfaces, namely the metallic BiAg2 alloy surface with high density of states near the Fermi level, and the semimetallic of Bi-P ×
3 overlayer and Bi(110) monolayer. This detailed investigation of the molecular ordering processes of F16CuPc on different Bi-Ag substrates clarifies the role of interfacial interactions in determining the growth mechanisms of organic thin films. It helps us better understand the π-π stacking mechanism of π-conjugated planar molecules on surfaces. This will also enable us to better control the film properties such as supramolecular packing and molecular orientation for applications in organic electronic devices, in particular, air-stable n-channel OFETs based on F16CuPc or bipolar OFETs based on the combination of F16CuPc with CuPc or other p-type molecules. Acknowledgment. Authors acknowledge the support from Singapore ARF Grants R-143-000-392-133 and R-143-000-406112. References and Notes (1) (a) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (b) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (2) (a) Forrest, S. R. MRS Bull. 2005, 30, 28. (b) Brumbach, M.; Placencia, D.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 3142. (c) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. AdV. Mater. 2007, 19, 1551. (d) Rand, B. P.; Burk, D. P.; Forrest, S. R. Phys. ReV. B 2007, 75, 115327. (3) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (4) Heutz, S.; Mitra, C.; Wu, W.; Fisher, A. J.; Kerridge, A.; Stoneham, M.; Harker, T. H.; Gardener, J.; Tseng, H.-H.; Jones, T. S.; Renner, C.; Aeppli, G. AdV. Mater. 2007, 19, 3618. (5) (a) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605. (b) Kahn, A.; Koch, N.; Gao, W. Y. J. Polym. Sci. B 2003, 41, 2529. (c) Cahen, C.; Kahn, A. AdV. Mater. 2003, 15, 271. (d) Koch, N. ChemPhysChem 2007, 8, 1438. (e) Tang, J. X.; Lee, C. S.; Lee, S. T. J. Appl. Phys. 2007, 101, 064504. (f) Koch, N. J. Phys.: Condens. Matter 2008, 20, 184008. (6) Li, L. Q.; Tang, Q. X.; Li, H. X.; Yang, X. D.; Hu, W. P.; Song, Y. B.; Shuai, Z. G.; Xu, W.; Liu, Y. Q.; Zhu, D. B. AdV. Mater. 2007, 19, 2613. (7) (a) Witte, G.; Wo¨ll, C. J. Mater. Res. 2004, 19, 1889. (b) Eremtchenko, M.; Schaefer, J. A.; Tautz, F. S. Nature (London) 2003, 425, 602. (c) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (d) Schreiber, F. Phys. Status Solidi A 2004, 201, 1037. (8) (a) Cheng, Z. H.; Gao, L.; Deng, Z. T.; Liu, Q.; Jiang, N.; Lin, X.; He, X. B.; Du, S. X.; Gao, H. J. J. Phys. Chem. C 2007, 111, 2656. (b) Gao, L.; Deng, Z. T.; Ji, W.; Lin, X.; Cheng, Z. H.; He, X. B.; Shi, D. X.; Gao, H. J. Phy. ReV. B 2006, 73, 075424. (c) Yim, S.; Jones, T. S. J. Phys.: Condens. Matter 2003, 15, S2631.

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