Role of Fluorine Interactions in the Self-Assembly of a Functionalized

Article pubs.acs.org/JPCC

Role of Fluorine Interactions in the Self-Assembly of a Functionalized Anthradithiophene Monolayer on Au(111) Shawn M. Huston,† Jiuyang Wang,‡ Marsha A. Loth,§ John E. Anthony,§ Brad R. Conrad,† and Daniel B. Dougherty*,‡ †

Department of Physics and Astronomy, Appalachian State University, Boone, North Carolina 28608, United States Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202, United States § Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States ‡

ABSTRACT: Scanning tunneling microscopy studies of the first monolayer of 2,8difluoro-5,11-(bis)triethylsilylethynyl anthradithiophene on Au(111) reveal two ordered structures with anthradithiophene planes parallel to the substrate. Submolecular resolution STM images demonstrate structures with a close approach of fluorine−sulfur and fluorine−fluorine atoms in the ordered structures. This provides evidence for the importance of noncovalent F−S and F−F in driving 2D self-assembly in the monolayer. Spectroscopic studies indicate a transport gap of 2.4 eV that is insensitive to the local domain structures, as expected for weak intermolecular interactions.

I. INTRODUCTION Organic electronic materials offer a promising route toward low-cost, large-area devices and are now commonly used in the form of organic light-emitting diode displays. The creation of solution processable, planar aromatic molecules with good electronic performance and tunable optical absorption is a crucial step toward simultaneously achieving these advantages.1−3 The functionalization of pentacene derivatives at the 6- and 13-positions with tri-iso-propylsilylethynyl (TIPS) side arms has proven to be an effective strategy for establishing solution processability in conjunction with high performance intrinsic carrier mobility limits.4−7 A related class of molecules, derivatives of 5,11-bis(triethylsilylethynyl) anthradithiophenes (TES-ADTs), has also shown particular promise. Their relatively high mobilities in solution-deposited thin-film devices are, in part, explained by their tendency to form well-ordered crystals given appropriate growth conditions.8−10 Solutioncasting and spray deposition11 of TES-ADT and its 2,8difluorinated derivative (diF-TES-ADT), as seen in the inset of Figure 1a, have been employed to create organic field-effect transistors with hole mobilities of up to 1 cm2/(V s).9,10 Partial fluorination leads to significantly improved material stability and has been argued to be a subtle route toward promoting crystallization without disrupting the crucial, albeit weak, π−π stacking interactions that allow for efficient transport in organic semiconductors.12 For the case of TES-ADT, fluorination has resulted in comparable mobilities for solution-deposited thinfilm devices and electrostatically bonded single crystals with mobilities as large as 6 cm2/(V s).13,14 To maximize the performance of these new materials, the specific intermolecular © 2012 American Chemical Society

Figure 1. Inset: DFT calculated structure of the anti isomer of diFTES-ADT generated with XCrySDen.31 The molecule is 14.5 Å from F to F and 15.8 Å from terminal H to terminal H on side-chain ethyl groups. (a) A (68 nm)2 STM image of a monolayer of diF-TES-ADT on Au(111) showing a combination of differently ordered domains and disordered regions. Taken at 10 pA with 1.33 V at 131 K. (b) A (31 nm)2 STM image of the linear (top) and zigzag (middle) structures. Au(111) herringbone reconstruction is visible and is highlighted by the solid blue lines. Taken at 10 pA and 0.5 V; pixel size is 0.10 nm. Maximum height variation is ∼150 pm.

interactions that drive crystallization and how these interactions can be modified for particular electrode materials require further study. In this paper, we report evidence for the importance of several unusual noncovalent interactions in Received: July 30, 2012 Revised: August 22, 2012 Published: August 30, 2012 21465

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driving the self-assembly of diF-TES-ADT in the first monolayer on a Au(111) substrate. The interaction energies for crystalline organic materials are much smaller than those of traditional semiconductors.15 This leads to an increase in the variety of relevant intermolecular interactions and, specifically for fluorine substitution, is suspected to result in several uncommon varieties of weak noncovalent bonds. First, direct interactions between fluorine atoms on adjacent molecules have been observed in some systems12 and are plausible for diF-TES-ADT. The microscopic origin of this kind of interaction remains unclear due to the expected low atomic polarizability of fluorine, but it nevertheless is empirically argued to be involved in a number of molecular crystal structures.15 Second, interactions between a fluorine atom and a less electronegative S atom on different molecules may also exist and have been implicated in the crystal structure formation in fluorinated thiophene derivatives.16 This latter interaction may be regarded as comparable in energy to “weak hydrogen bonding”, which is known to provide the driving force for monolayer molecular self-assembly in some cases.17,18 Finally, more conventional weak hydrogen-bondinglike interactions between F and very weakly acidic H atoms on the aromatic ADT backbone may occur as well as interactions between thiophene S atoms and H atoms on an adjacent aromatic backbone. Taken together, these relatively weak noncovalent interactions can enhance crystallization without disrupting the π−π stacking interactions that are favorable for carrier transport. In addition to the diverse intermolecular interactions that may be relevant to bulk crystallization, thin films of diF-TESADT grown from solution exhibit morphologies that are very strongly dependent on the metal electrode−molecule interactions.8,19 On Au source and drain electrodes treated with pentafluorobenzenethiol (PFBT), diF-TES-ADT films grow with (100) planes parallel to the substrate, leading to good π−π overlap along the transistor channel. In addition, this electrode treatment permits rapid and continuous growth of (100) crystallites outward from the electrodes into the channel.19 In contrast, untreated Au electrodes lead to localized molecular crystallization with randomly oriented domains. In particular, significant populations of small (111)-oriented crystallites grow in the conduction channel, resulting in markedly lower electronic performance. Differences in crystal orientation are expected, based on the relative interaction strength between planar aromatics and functionalized versus nonfunctionalized electrodes.19,20 However, the tendency to form polycrystalline structures and the device growth mode mechanism are more difficult to explain without a detailed microscopic characterization of the metal−organic interface. Such characterization has been carried out for some functionalized planar aromatics, including dichloropentacene21 and thienoanthracene.22 Two dimensional ordering is typically observed, driven by noncovalent interactions in multidomain assemblies often determined by substrate symmetry.21 In this paper, we report scanning tunneling microscopy (STM) observations of the molecular assembly of diF-TESADT in the first monolayer on a Au(111) surface. We find two coexisting ordered structures on the surface as well as a small population of poorly ordered domains. This is consistent with the small polycrystalline domains observed during growth from solution on untreated Au source and drain electrodes.19 Highresolution STM imaging of the ordered structures suggests the importance of unusual noncovalent intermolecular interactions

mediated by F and sulfur atoms that are similar to the Fmediated interactions that exist in the bulk 3D crystal. Local tunneling spectroscopy demonstrates a transport gap of 2.4 eV that is independent of the local domain structure and an interface dipole of ∼0.8 eV.

II. EXPERIMENTAL METHODS Synthesis of diF-TES-ADT was carried out as reported by Subramanian et al.12 and yielded a 50−50 mixture of syn and anti isomers. The material, in the form of a red powder, was loaded into a quartz crucible that could be heated by a tantalum filament coiled along its length. Evaporation of this material was carried out in situ in a deposition system with a base pressure of ∼1 × 10−7 Torr. The molecular flux from the evaporator was calibrated with a quartz crystal microbalance and was typically 0.27 Å/s. The Au(111) substrate was prepared in an ultra-high-vacuum chamber (base pressure ∼ 3 × 10−11 Torr) housing a commercial variable-temperature STM (Omicron VT-XA) connected to the organic deposition chamber by a gate valve. It was cleaned by sputtering with 1 keV Ar+ ions for ∼1 h and then annealing at approximately 700 K for 20 min. Deposition of diF-TES-ADT was carried out with the substrate at room temperature. The sample was then transferred to the STM stage, which was held at a temperature of ∼130 K by a braided connection to a liquid nitrogen bath cryostat for all imaging and spectroscopy experiments. STM imaging was carried out in constant current mode with electrochemically etched W tips, and spectroscopy was carried out in constant current distance versus voltage mode at a current set point of 10 pA in order to minimize tip-induced perturbations of the molecular layers that are likely to occur for traditional STS in constant height I(V) mode.23 The reported biases are applied to the sample with the STM tip held at ground potential. Image sizes are calibrated using the known dimensions of the 22 × √3 “herringbone” reconstruction of the Au(111) surface that could be imaged even beneath close-packed monolayer domains of molecules. STM images are processed by a plane subtraction using the commercial (SPIP) software package. Density functional theory calculations were undertaken on a single molecule of diF-TES-ADT (Figure 1a) to arrive at an accurate molecular geometry for the purposes of modeling the adsorption patterns. Plane-wave calculations were performed using PWscf24 with the Perdew−Burke−Ernzerhof (PBE)25 exchange-correlation functional as implemented in the generalized gradient approximation (GGA)26−28 using Vanderbilt ultrasoft pseudopotentials.29,30 The plane-wave kinetic energy cutoff and charge density cutoff for these calculations were 36 and 144 Ry, respectively. Relaxation of the molecule was performed with a single k-point at the high symmetry Γ point. Results of the DFT calculations are used to generate graphics in this paper with the XCrySDen software package.31 III. RESULTS AND DISCUSSION A. Monolayer Structures. Figure 1a shows a representative (66 nm)2 STM image of a nearly complete diF-TES-ADT film, containing several different structural domains in close proximity. Two ordered domains are apparent in the upper and lower parts of Figure 1a. Partially separating these regions, a poorly packed and structurally disordered domain can be seen in the central portion of the image. 21466

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Figure 1b shows two ordered domains whose orientation tracks the long-range 120° rotation of the Au(111) herringbone reconstruction. This relationship to the herringbone reconstruction was not always observed and is probably connected to the higher probability of defect creation near the herringbone elbows rather than a strong directional character to molecule− surface interactions that has been reported for other molecular assemblies.32 Nevertheless, van der Waals interactions between the molecule and the substrate are likely to be very significant in this system and the substrate enforces a “flat-lying” molecular geometry discussed in more detail below. The film structures reported here are interpreted as the first molecular layer on Au(111) based on thickness calibration with an in situ quartz microbalance (showing nominal thicknesses of less than 1 nm) and the observation of corrugation due to the well-known Au(111) herringbone reconstruction beneath the molecules (Figure 2a). Additionally, it is possible to find

Figure 3. (a) The linear structure of diF-TES-ADT. In the (4.5 nm)2 STM image (a), anthradithiophene backbones between bright protrusions can be clearly seen. Bright protrusions correspond to the molecular side groups. Full z-scale in the image is 170 pm. The molecules assemble with a packing density of 0.64 molecules/nm2 via F···S interactions with an approximate separation of 2.4 Å between F and S atoms on adjacent molecules. The F···F separation is 2.9 Å. (b) An overlaid molecular model with lattice vectors shown as described in the text. The image was taken at 50 pA with a −1.0 V bias.

flat on the Au(111) surface, showing no apparent tilt. In addition, the large apparent height of the TES arms is consistent with STM imaging of bulky tert-butyl functional groups in a number of other molecular adsorption systems, such as 3,3′,5,5′-tetra-tert-butylazobenzene/Au(111)33 and Cutetra(3,5-di-tert-butyl phenyl) porphyrin on Cu(100) and Ag(110).34 DFT calculations of molecular size, in combination with the observed molecular size in the STM images, suggest that the TES side arms are bent away from the plane of the Au surface in order to maximize the interactions between the metal surface and the aromatic ADT face. Moreover, this substrateinduced distortion (not depicted in the molecular models in Figure 3b and elsewhere) would prevent strong repulsive interactions due to the close approach of H atoms on adjacent molecules. An arrangement for the molecules in Figure 3a is proposed, as indicated by the molecular drawings, in Figure 3b. Molecule size is determined by DFT calculations and compares favorably to XRD experimental measurements.20 This structure is named “linear” because of the apparent long-range order in the rows of ethyl side groups, as seen in the bottom right-hand-side stripe of Figure 1a or Figure 3a. Since the raw starting material is an inseparable mixture of syn and anti isomers, a model for the linear structure where these two structures alternate along a linear row is proposed and shown in Figure 3. This allows maximal F-mediated noncovalent interactions while maintaining the isomeric composition of the starting material. In separate experiments with a larger total surface coverage of diFTES-ADT, nearly complete coverage of the linear domains is observed, suggesting that this isomeric mixture is essential. With an apparent packing density of 0.62 ± 0.08 molecules/ nm2, individual molecules occupy sites of an oblique lattice with dimensions of al = 1.49 ± 0.01 nm, bl = 1.25 ± 0.01 nm, and included angle = 56.8 ± 0.3°. This arrangement of molecules is very similar to the arrangement in the bc plane of the bulk 3D crystal structure.12 To identify likely atom-specific, noncovalent interactions, we use high-resolution STM images to consider whether interatomic distances between molecules are smaller than expected van der Waals radii. The average F−S separation measured from STM images in this structure is 2.4 Å, which is consistent with a specific nondispersive interaction between

Figure 2. (a) STM image (94 nm × 60 nm, −1 V, 10 pA) showing an island of (linear) ordered diF-TES-ADT on Au(111) surrounded by bare substrate. (b) Line profile through the diagonal white line shown in (a).

regions where the clean Au(111) surface is clearly visible with an apparent height difference between the molecular layer and the surface of ∼200 pm, as shown in the line profile near the molecular island edge in Figure 2b. Figure 3 shows higher-resolution (4.5 nm)2 STM images with submolecular contrast. This increased resolution allows for the molecular orientation of the diF-TES-ADT molecules on the Au(111) surface to be revealed and the bright protrusions visible in Figures 1b and 2a to be identified as the TES side groups. Molecular symmetry is apparent in Figure 3a, where two bright protrusions are assigned as the bulky ethyl groups on the TES side arms and the lower apparent height ovalshaped feature between the bright protrusions is the ADT backbone. Figure 3 illustrates that the ADT plane is essentially 21467

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these atoms.15 The perfect alternation in between isomers in the model presented could be modified in real monolayers by defects such as short sequences with consecutive syn or anti isomers. In addition, every third molecule is constrained to have modified S−F and F−F interactions compared to the optimal interactions depicted in Figure 3b. The high-resolution (8.0 nm)2 STM image shown in Figure 4a shows an entirely different ordered structure, referred to as

Table 1. Measured Distance between Atoms/Functional Groups in Different Ordered diF-TES-ADT Overlayers on Au(111)a structure

F−F

S−S

F−S

F−H

S−H

linear zig zag vdW distance

0.29 nm N/A 0.294 nm

N/A 0.25 nm 0.36 nm

0.24 nm 0.25 nm 0.327 nm

N/A 0.24 nm 0.267 nm

N/A 0.21 nm 0.30 nm

a

F−H and S−H are the distances between F or S atoms on one molecule and the nearest H atom on the ADT backbone of the adjacent molecule. The final row labeled “vdW distance” compiles expected van der Waals distances between the atoms inferred from standard radii.46

measurable difference in STS spectra between the two structures identified in Figures 3 and 4.

Figure 4. Zigzag structure of diF-TES-ADT on Au(111). (a) A (8 nm × 8 nm) STM image of a zigzag domain taken at 10 pA at 0.5 V with a 0.58 molecules/nm2 packing density. Full z-scale in the image is 170 pm. Lattice vectors shown as described in the text. (b) A (3.29 nm)2 zoom-in image from part (a) showing molecular models with close F− S and F−H distances.

“zigzag”. This structure displays a packing density of 0.58 ± 0.09 molecules/nm2, where molecules occupy points of an oblique lattice with dimensions az = 1.30 ± 0.01 nm, bz = 1.36 ± 0.02 nm, and included angle = 73 ± 1°. The observed zigzag structure assembles with close F-S contacts similar to fluorinated benzothiophenes16 as indicated by the molecular models overlaid on the STM image in Figure 4b. Close S−S and S−H contacts may be a byproduct of this interaction. In addition, small distances between terminal F atoms and H atoms on the ADT backbone may indicate a further weak hydrogen bonding stabilization due to fluorination. The side chain separation is difficult to infer due to the likelihood of outof-plane distortion of the side moieties, but can also contribute to dispersion interactions. It is important to note that the linear and zigzag structures coexist with essentially the same packing density on the surface. We interpret this observation as evidence for the competition between the various weak intermolecular interactions that create a number of local minima in the potential energy surface for adsorption. In all cases, we expect a significant molecule− substrate interaction between the ADT plane and the Au surface as well as a van der Waals interaction between the bulky ethyl side groups. In addition, a number of weak, but atomspecific noncovalent interactions, are inferred from measured distances, as indicated in Table 1. The situation we report here is similar to the coexistence of a number of coexisting monolayer ordered domains for pentacene on Au(111) and has been attributed to the competition between a large number of weak interactions in this system.35 B. Local Tunneling Spectroscopy. For organic electronic applications, a crucial task is to correlate molecular structure and morphology with electronic-level alignment at the metal− semiconductor interfaces. Scanning tunneling spectroscopy (STS) in constant current distance versus voltage mode was conducted on each of the ordered structures discussed here, and the results are summarized in Figure 5.23 We found no

Figure 5. (a) Raw z(V) spectra (10 pA) comparing regions of ordered diF-TES-ADT with adjacent regions of the bare Au(111) substrate. (b) Normalized dz/dV spectra (10 pA) showing the filled (V < 0) and empty (V > 0) frontier orbitals of diF-TES-ADT on Au(111). Each curve represents more than 50 coaveraged spectra. All STS measurements were taken at 131 K. The estimated transport gap is 2.4 eV measured from HOMO peak to LUMO peak.

Figure 5a shows raw tip displacement versus sample voltage spectra measured in the same local region for both the uncovered Au(111) substrate and monolayer ordered islands of diF-TES-ADT. The spectra measured on Au(111) exhibit higher noise levels than those on the molecular domains because regions of “bare” substrate most likely have a very small coverage of somewhat mobile molecules. Nevertheless, comparison of the spectra measured over these different surface regions allows identification of slope changes associated 21468

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with molecular-orbital-derived states.23,36, This is clarified for a larger, extensively coaveraged data set in Figure 5b that was measured over a molecular domain for which no bare Au was available for simultaneous comparison. More than 50 spectra are coaveraged to reduce noise and are plotted in Figure 5b as normalized dz/dV plots following the logarithmic derivative procedure described by Pronschinske et al.23 The HOMO-derived state can be located in Figure 5 at ∼0.95 eV below the Fermi level. This is in agreement with ultraviolet photoelectron spectroscopy (UPS) measurements of a much thicker film of a related fluorinated-ADT derivative on Au that showed a HOMO of ∼0.80 eV below the Fermi level.37 The small difference between STS and UPS results is expected and can be attributed to increased stabilization of the HOMO level of the monolayer due to a contribution from the electrostatic polarization of the nearby Au substrate in response to electron extraction to the STM tip.38 An additional broader peak (not labeled) at about 2.0 eV below the Fermi level is assigned as the HOMO−1 orbital. The LUMO level is observed at 1.5 eV above the Fermi level and has not yet been reported for this molecule. The STS observations of HOMO- and LUMO-derived levels indicate a transport gap of 2.4 eV. Such a value is consistent with expectations based on the observed optical gap of 2.26 eV, where the gap has been measured at the leading edge of the lowest-energy absorption peak.12 The difference between the optical and the transport gaps can be used36 to estimate an exciton binding energy of 0.1−0.2 eV for this system, which is important in determining the suitability of diF-TES-ADT and related compounds39 for optoelectronic applications, including the creation of organic solar cells. The observation of identical spectra for all of the ordered domains reported here is consistent with the expectation that the intermolecular interactions driving self-assembly in this system are indeed very weak and the frontier orbitals are not significantly altered by their different structural environments in the monolayer phases. In thicker devices, when there are significant π−π stacking interactions (some notably sensitive to crystallization rates 40 ) between the ac crystal planes, intermolecular polarization stabilization is expected and will partially compensate for the loss of substrate polarization due to the proximity of the metal−organic interface. Figure 6 shows a wider voltage range in the unoccupied density of states above an ordered molecular domain. In this

range, image potential−derived surface states can be observed that are connected with the local workfunction outside of a surface.41 We assign the very intense peak at ∼4.7 eV above the Fermi level as the n = 1 (Stark-shifted) image potential-derived state for diF-TES-ADT on Au(111). By comparison with published spectra that place the clean Au(111) image potential state at ∼5.5 eV above the Fermi level,42 we infer a workfunction change of about −0.8 eV due to diF-TES-ADT adsorption. This is similar in size to the interface dipole shift measured of the change in secondary electron cutoff in ultraviolet photoelectron spectroscopy (UPS).43 The small differences arise because our experiments are carried out for adsorption on a single-crystal Au(111) substrate compared to the polycrystalline Au used in the UPS study. The frontier orbitals computed from our DFT calculations are shown in Figure 7. They are remarkably delocalized, with

Figure 7. Isosurfaces generated with XCrySDen31 for the lowestenergy orbitals of the anti isomer of diF-TES-ADT: (a) HOMO, (b)HOMO−1, (c) LUMO, (d) LUMO+1.

density extending not only over the ADT backbone but also over the carbon triple bond and even somewhat onto the Si atom binding the ethyl groups. The energy difference between HOMO (Figure 7a) and LUMO (Figure 7c) calculated in DFT is smaller than the observed transport gap in STS due to the well-documented “gap problem”44,45 in this ground-state methodology. However, we find good agreement between the calculated energy spacing between the HOMO and HOMO−1 (Figure 7b) orbitals and the 1.1 eV spacing between the two peaks nearest the Fermi level in the occupied states in Figure 5b. Moreover, the LUMO+1 orbital (Figure 7d) is calculated to be ∼1.5 eV above the LUMO. This may be related to the onset leading to a broad shoulder on the low-energy side of the n = 1 IPS in the spectrum in Figure 6.

V. CONCLUSIONS In conclusion, we have identified two coexisting ordered monolayer assemblies of diF-TES-ADT on Au(111). These structures exhibit several unique features. Assembly is at least partly driven by uncommon noncovalent interactions that

Figure 6. Normalized dz/dV (10 pA) spectrum in a high bias range above the Fermi level showing the Stark-shifted image potential state at 4.7 eV. 21469

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include fluorine−fluorine and fluorine−sulfur interactions between alternating syn and anti isomers as well as possible weak hydrogen bonding. In addition, the local electronic structure of the various ordered domains has been measured showing a transport gap of 2.4 eV and an interface dipole inferred from the n = 1 image potential-derived state of ∼0.8 eV. This interfacial electronic structure is insensitive to the local domain geometry, as is expected for the weak intermolecular interactions that dominate the surface ordering. These results demonstrate that multiple domain nucleation phenomena are likely to contribute to the observed polycrystalline growth of diF-TES-ADT films on untreated transistor electrodes. Furthermore, they support the notion that weak noncovalent interactions, especially those involving fluorine, are heavily involved in the crystallization of these molecules, even in the submonolayer 2D assemblies. This particular idea represents an important strategy for controlling molecular crystals by incorporating uncommon weak intermolecular interaction motifs. Investigation of the relation between new surface interactions and electronic structure will be particularly important in the ongoing search for versatile, high-performance organic semiconductors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.B.D. acknowledges support from a National Science Foundation CAREER award (DMR-1056861). B.R.C. thanks ORAU for the Ralph E. Powe Junior Faculty Enhancement Award. J.E.A. and M.L. acknowledge support from the Office of Naval Research.

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