Cyano-Functional Group as an Anchoring Tool for Organic Small

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Cyano-Functional Group as an Anchoring Tool for Organic Small Molecules on Gold Christopher Dobler, Christina Tönshoff, Holger F. Bettinger, Thomas Chassé, and Maria Benedetta Casu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02077 • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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The Journal of Physical Chemistry

Cyano-Functional Group as an Anchoring Tool for Organic Small Molecules on Gold Christopher Dobler,⊺ Christina Tönshoff, † Holger F. Bettinger,† Thomas Chassé,⊺ M. Benedetta Casu⊺*

⊺Institute

of Physical and Theoretical Chemistry, University of Tübingen, Auf der

Morgenstelle 18, D-72076 Tübingen †

Institute of Organic Chemistry, University of Tübingen, Auf der Morgenstelle 18, D-72076

Tübingen

ABSTRACT

We evidence the effect of the cyano-functional group in organic thin films investigating a δ4substituted pentacene derivative, by using X-ray photoemission (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopies, together with atomic force microscopy. We have found that the presence of the cyano-functional group marks the electronic structure, the interface phenomena, as well as, the film morphology in the CN-substituted pentacene deposited on Au(111) single crystals. This is due to the presence of the CN groups that is known to have a strong chemical affinity towards gold. We propose the cyano-functional group as an efficient tool to chemically modify and functionalize gold surfaces or to anchor a variety of organic semiconductors on gold. 1 ACS Paragon Plus Environment


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INTRODUCTION Semiconductor fabrication plants for manufacturing circuits are based on a well-established technology, with silicon being the still unsurpassed semiconductor material. However, beyond this technology that will certainly remain dominant for several decades ahead, alternatives are possible when looking at specific areas such as flexible electronics, large and transparent displays, or design of new devices. In this respect organic semiconductors are very successful and reached the market in a very short time,1 once their technological relevance was proved.2 There are several reasons for this success: One is their chemical flexibility by synthesis. This makes possible, for example, the chemical engineering of surfaces,3 or designing new organic semiconductors4 with specific properties. In this view, functionalisation is an efficient approach to pursue the goal of tuning the characteristics of organic materials.5 Pentacene and its derivatives are prototype molecules that are widely investigated for applications in organic thin film-based devices.6-11 We have focused our previous work on three different substituted pentacenes: δ4-substituted (2,3-X2-9,10-Y2) pentacene with X=Y=methoxy group (MOP), X=Y=F (F4PEN), and X=methoxy group, Y=F (MOPF) (see the Supporting Information for the respective molecular structures), elucidating the strong correlation of their electronic, structural and morphological properties.12 While substitution tunes the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)13, the three substituted pentacenes show several common features. First of all, depositing them on a single crystal gold substrate kept at room temperature, they assemble as nanorods (Volmer–Weber growth mode), rather than forming a homogenous film.12, 14-16 The nanorod morphology is due to the fact that the barriers to surface diffusion on a terrace are anisotropic: diffusing across the width of a nanorod involves energy barriers twice as high as diffusing along its length, favoring the formation of nanorods.15 Additionally, the nanorods do not grow randomly distributed on the 2 ACS Paragon Plus Environment

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gold surfaces. Only certain directions are favored, because of a template effect due to the geometry of the lattice substrate that steers the aggregation of islands/nanorods during their growth.17-18 All three pentacene derivatives investigated so far are physisorbed on gold and show the clear fingerprint of a fractional charge transfer from the metal substrate to the physisorbed molecules.16 In this work, we focus on the influence of the cyano-functional group as a functional group of a δ4-substituted (2,3-X2-9,10-Y2) pentacene with X = methoxy group, and Y=CN (MOP-CN, Figure 1, C26H16O2N2), when depositing MOP-CN on Au(111) single crystals at room temperature. We investigate our system by using X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). The combination of these techniques allows accessing occupied and unoccupied states, as well as, structural and morphological information.

EXPERIMENTAL SECTION Sample preparation and experiments were performed at the UE52-PGM undulator beamline at BESSY II (Berlin) (top up mode, cff = 10, 20 µm exit slit, analyzer resolution= 0.1 eV). The main chamber (base pressure 2x10-10 mbar) was equipped with a standard twin anode X-ray source, and a SCIENTA R4000 electron energy analyzer. A clean Au(111) single crystal was used as a substrate, prepared by several cycles of sputtering (Ar+ ion bombardment: 600 V) and annealing (830 K). Film preparation was carried out in-situ under ultra high vacuum (UHV) conditions by using organic molecular beam deposition (evaporation rate: 0.8 Å/min, substrate at room temperature, Tsub = 300 K). The evaporation rate was measured with a quartz crystal microbalance. The photoelectron spectra were taken with photon energies of 330, 640, and 1000 eV. We carried out NEXAFS measurements in the partial electron yield mode, in grazing incidence (70° with respect to the sample normal). 3 ACS Paragon Plus Environment


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The NEXAFS spectra were normalized by using the clean substrate signal and the ring current. All spectra were scaled to give an equal edge jump. Details on NEXAFS normalization and scaling are given elsewhere.19-20 AFM measurements (Digital Instruments Nanoscope III Multimode) were performed under ambient conditions in tapping mode. No degradation of the samples was observed on the time scale of all discussed experiments.

RESULTS AND DISCUSSION XPS spectroscopic lines are sensitive to the chemical environment of the emitting atoms, thus, the C 1s line in case of MOP-CN presents several features. This is clearly visible in the thickness-dependent XPS spectra (Figure 1). The C 1s line is characterized by contributions due to electrons emitted from carbon atoms bound only to carbon or also to hydrogen (at around 285.0 eV in the multilayer). The feature at higher binding energy (286.4 eV in the multilayer) is due to two further peaks related to electrons emitted from carbon atoms which are bonded to nitrogen (C-N) and oxygen (C-O), as it can be deduced by using a best fit procedure14 (Figure 2, see details in the Supporting Information). We also observe the typical shake-up satellites due to electronic relaxation effects, occurring as a response of the system to the core−hole creation upon photoemission.21-22 The N 1s and O 1s lines are characterized by a single main feature (at 399.4 and 533.3 eV, respectively, in the multilayer), as expected, because all nitrogen and oxygen atoms in MOP-CN have the same chemical environment. The XPS core level spectra and the fitting results are in agreement with MOP-CN stoichiometric ratio (note that best fit procedure has always to include the satellite intensity14, 23

to mirror the correct molecular stoichiometry) signaling that the evaporation of MOP-CN

was successfully achieved, i.e., without molecule degradation (see the Supporting Information for details and Table S1). A direct comparison of the monolayer spectrum with the multilayer spectra provides information on the ongoing phenomena at the interface. At a glance we can immediately 4 ACS Paragon Plus Environment

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recognize that the multilayer spectra show a shift of the peak positions toward higher binding energies. The shift is non-rigid meaning that the C 1s, N 1s and O 1s main lines are not shifted in a parallel manner, but slightly different when comparing mono- versus multilayer spectra (0.42, 0.38, 0.37 eV for C 1s, N 1s and O 1s, respectively). We also observe in the C 1s spectra a different behavior of the shake-up satellite intensity in the higher and in the lower binding energy range: higher intensity in the higher binding energy range for the multilayer while the lower binding energy satellites have intensities that are strongly enhanced at the interface. Changes in the satellite features are also recognizable in the N 1s main line and are evidenced by the fitting procedure (see Tables S2 and S3 in the Supporting Information). The behavior of the shake-up satellite intensity in the higher binding energy range can be explained in terms of screening effects assisted by the substrate: the core-hole screening at the interface has also a component owing to an image-charge formation leading to a negative charging of the substrate on the time scale of the photoemission event.24 On the contrary, understanding the behavior of the shake-up satellite intensity at lower binding energy requires further investigations, specifically, as it will be discussed later, the support of NEXAFS measurements. The line shape and width slightly change with thickness, because the XPS line shape is influenced by several parameters, such as structural changes25 (the molecules experience reorientation effects with thickness, see the NEXAFS results below), and presence of nitrogen and oxygen atoms that impact the electronic charge redistribution, due to their high electronegativity.14 Note that the beamline had different resolution at 330, 640, and 1000 eV that contributes to the different line broadening of the spectra in the various experiments (i.e., different Gaussian width in the Voigt profile of the spectroscopic line). Furthermore, the N 1s peak is characterized by an asymmetric shape in the spectra of the interfacial film. The observed differences in the core level spectra at the interface may be due to several mechanisms, ranging from chemisorption to different physisorption strength, and including a 5 ACS Paragon Plus Environment


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local image-charge at the interface that may give rise to a different screening of the various atoms if the molecule is not perfectly planar or it adopts a recumbent arrangement on the surface.3, 16, 26 In this respect, the combination of different X-ray-based techniques is certainly a powerful approach to achieve a precise description of the interface. NEXAFS spectroscopy can be used to shed light on the phenomena: for example, image-charge screening at the interface does not affect the intensity of the NEXAFS resonances.31, 39 Conversely, we expect clear differences comparing the thin and the thick layer NEXAFS spectra in case of chemisorption. This is because molecular orbitals are highly directional and, consequently, the transition matrix element, described by the Fermi’s Golden Rule, is perturbed if a strong chemical substrate-adsorbate interaction occurs. To decouple electronic structure information (unoccupied states) from structural information that are both contained in the NEXAFS spectra, we use two different polarizations of the incoming electromagnetic radiation (in plane and out of plane E vector with respect to the substrate) and, as done for XPS, we compare the spectra of the mono- and the multilayer (Figure 3). The C K edge NEXAFS spectra are characterised by strong resonances due to transitions from the C-K shell to π*-orbitals (up to around 290 eV) and to σ*-orbitals (in the photon energy range above 290 eV). We observe a strong dichroic behaviour for thin and thick films, indicating a predominant flat-lying orientation of MOP-CN in the monolayer and a reorientation of the molecules towards a more upright standing position of the pentacene backbone in the multilayer (the average calculated orientation of the pentacene backbone, as obtained from the NEXAFS spectra, is 33° with respect to the substrate19-20, 27). When looking at the N K edge NEXAFS spectra, we also observe a strong dichroic behavior. Additionally, at both edges the features in the spectra of the interfacial film present clear differences, when compared to the multilayer spectra taken under the same experimental conditions, i.e., for the same polarization direction of E. This suggests the occurrence of a strong interaction between MOP-CNs and the gold substrate that 6 ACS Paragon Plus Environment

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involves the nitrogen atoms. Correlating this finding with the XPS results, we can therefore infer that the difference between the interfacial film and the multilayer is due to MOP-CN chemisorption. Strong interactions at the organic/metal interface influence also the shake-up satellite intensities.28-29 This is evidenced by the fact that MOP-CN satellite intensities behave in countertendency with respect to the other investigated substituted pentacenes that are physisorbed on gold:12, 14-16 While MOP-CN C 1s XPS spectra present very strong satellite intensities at the interface in the lower binding energy range, MOP, MOPF and F4PEN spectra show reduced satellite intensities. This behavior can be understood recalling previous studies on the shake-up satellite structures of small molecular compounds:21, 30-31 the shake-up satellites transitions in the lower binding energy, including the first one, due to HOMOLUMO transitions,22 have contributions owing to screening effects comprising a non-local charge transfer from molecular regions also far away from the generated core-hole.21, 30-32 If those molecular orbitals in the interfacial film are perturbed by a strong interaction with the substrate, the charge redistribution is different than in multilayer, and the intensity of the related shake-up satellite is affected, as it happens for MOP-CN. The presence of the cyano-functional group not only heavily influences the electronic structure and the interface phenomena, but it has striking consequences on the film morphology, as it shown in Figure 4, where the AFM images of a MOP-CN and a MOP assembly are compared. The nanorod morphology shown by the other substituted pentacenes is completely absent in the case of MOP-CNs.12, 14-15 The film is typically characterized by island formation (root mean square (RMS) surface roughness = 11 nm). While island aggregation in MOP-CNs, contrary to the other pentacenes,12, 14-15 does not show any template effect due to the threefold geometry of the substrate,18 the islands decorate and follow the substrate step bunches (Figure 4a). This is a consequence of the fact that MOP-CNs are chemisorbed on gold, thus, we expect that at least one organic layer covers the substrate. The first layer(s), on the one hand, masks the geometrical influence of the substrate, and 7 ACS Paragon Plus Environment


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presumably favors Stranki-Krastanov growth mode, on the other hand, it hinders nanorod formation. As a matter of fact, pentacene has no barriers to surface diffusion and a very low Schwoebel barrier (7 kcal/mol),15, 33 while specific substitutions, because of steric hindrance, can decrease or increase both barriers, influencing the growth processes in its derivatives, hindering or favoring the nanorod formation.15 Consequently, the AFM results indicate that surface diffusion and Schwoebel barriers in MOP-CN are much lower than in its related substituted pentacenes.

CONCLUSIONS In conclusion, we have investigated a δ4-substituted pentacene derivative, focusing on the CN group as a substituent. We have found that the presence of the cyano-functional group marks the electronic structure, the interface phenomena, as well as, the morphology in MOP-CNs deposited on Au(111) single crystals. Our results correlate very well with other published works performed on perylene-derivatives,34 diisocyanides,35 and triarylamines36 on gold. Our work clearly indicates that this is due to the presence of the CN groups, known to have a strong chemical affinity towards gold,37-43 and it evidences the role of the cyano-group in anchoring the first molecular layer on gold surfaces. Therefore, using the cyano-functional group can be considered an efficient way to chemically modify and functionalize gold surfaces or to anchor a variety of organic semiconductors on gold. The CN functionalization has also significant effects on the morphology of the obtained thin films that have to be taken into account, when parameters such as film morphology play an important role. For example, in designing new devices, the presence of a homogenous first organic layer, due to the chemical anchoring, can be beneficial aiming at improved performances.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Molecular structures of the substituted pentacenes. Stoichiometric and experimental elemental ratios. Fit results for energy position and relative intensities of the photoemission lines. Supporting Information references.

AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the Helmholtz-Zentrum Berlin (HZB) for providing beamtime at BESSY II, R. Ovsyannikov and M. Gorgoi for beamtime support, S. Pohl, W. Neu and E. Nadler for technical support. We would like to thank C. Arantes and S.-A. Savu for taking part in the beamtime. Financial support from the Helmholtz-Zentrum Berlin is gratefully acknowledged. MBC acknowledges the support of the Institutional Strategy of the University of Tübingen (DFG, ZUK 63).

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α,β-Unsaturated Carbonyl Compounds. Chem. Commun. (Cambridge, U. K.) 2014, 50, 50465048. (36) Gottardi, S.; Müller, K.; Moreno-López, J. C.; Yildirim, H.; Meinhardt, U.; Kivala, M.; Kara, A.; Stöhr, M. Cyano-Functionalized Triarylamines on Au(111): Competing Intermolecular Versus Molecule/Substrate Interactions. Adv. Mater. Interf. 2014, 1, 1300025 (37) Kharasch, M. S.; Beck, T. M. The Chemistry of Organic Gold Compounds. V. Auration of Aromatic Nitriles. J. Am. Chem. Soc. 1934, 56, 2057-2060. (38) Fesser, P.; Iacovita, C.; Wackerlin, C.; Vijayaraghavan, S.; Ballav, N.; Howes, K.; Gisselbrecht, JP.; Crobu, M.; Boudon, C.; Stohr, M.; et al. Visualizing the Product of a Formal Cycloaddition of 7,7,8,8-Tetracyano-P-Quinodimethane (TCNQ) to an AcetyleneAppended Porphyrin by Scanning Tunneling Microscopy on Au(111). Chem.-Eur. J. 2011, 17, 5246-5250. (39) Gao, P.; Weaver, M. J., Surface-Enhanced Raman Spectroscopy as a Probe of Adsorbate-Surface Bonding: Benzene and Monosubstituted Benzenes Adsorbed at Gold Electrodes. J. Phys. Chem. 1985, 89, 5040-5046. (40) Xu, T.; Morris, T. A.; Szulczewski, G. J.; Amaresh, R. R.; Gao, Y.; Street, S. C.; Kispert,

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(43) O’Shea, J. N.; Saywell, A.; Magnano, G.; Perdigão, L. M. A.; Satterley, C. J.; Beton, P. H.; Dhanak, V. R. Adsorption of Ptcdi on Au(111): Photoemission and Scanning Tunnelling Microscopy. Surf. Sci. 2009, 603, 3094-3098.



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FIGURE CAPTIONS

Figure 1. Photon and thickness-dependent core level spectra of MOP-CNs deposited on Au(111) single crystals, as indicated. a)-b) C 1 s core level spectra. c)-d) N 1s core level spectra. e)-f) O 1s core level spectra. MOP-CN molecular structure is also shown.

Figure 2. C 1 s (upper panel) and N 1s (lower panel) core level spectra, as in Figure 1, together with their fit (see the Supporting Information for details).

Figure 3. C K edge and N K edge NEXAFS spectra of the interface and the thicker film, for two different polarization of the vector E, as indicated.

Figure 4. 3 µm × 3 µm AFM images of (a) MOP-CN (3.7 nm nominal thickness) and (b) MOP (3.2 nm nominal thickness). The RMS roughness is 11 nm in both cases.


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Figure 1

C 1s hν = 1000 eV

3.7 nm

XPS Intensity (a. u.)


C 1s hν = 330 eV

3.2 nm 2.4 nm 1.6 nm 0.8 nm

3.7 nm

3.2 nm 2.4 nm 1.6 nm 0.8 nm

0.3 nm 0.3 nm

292 290 288 286 284 282 Binding Energy (eV)

a)


b)

N 1s hν = 640 eV

N 1s hν = 1000 eV 3.7 nm



3.7 nm

3.2 nm 2.4 nm

1.6 nm

3.2 nm

2.4 nm 1.6 nm 0.8 nm

0.8 nm 0.3 nm

0.3 nm

402 400 398 Binding Energy (eV)

c)

402 400 398 Binding Energy (eV)

d)

O 1s hν = 1000 eV

O 1s hν = 640 eV


3.7 nm

XPS Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2 nm

2.4 nm 1.6 nm 0.8 nm

3.2 nm 2.4 nm 1.6 nm 0.8 nm

0.3 nm

0.3 nm


e)

3.7 nm


f) 17 ACS Paragon Plus Environment


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Photon and thickness-dependent core level spectra of MOP-CNs deposited on Au(111) single crystals, as indicated. a)-b) C 1 s core level spectra. c)-d) N 1s core level spectra. e)-f) O 1s core level spectra. MOP-CN molecular structure is also shown.


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Figure 2

C-C C-N C-O S1

S2



C 1s hν = 330 eV 0.3 nm C-H

C 1s hν = 330 eV 3.7 nm

S3

292 290 288 286 284 282

292 290 288 286 284 282

Binding Energy (eV)

Binding Energy (eV)

N 1s hν = 640 eV 3.7 nm XPS Intensity (a. u.)

N 1s hν = 640 eV 0.3 nm XPS Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S1

402

400

398

Binding Energy (eV)

402

400

398

Binding Energy (eV)

C 1 s (upper panel) and N 1s (lower panel) core level spectra, as in Figure 1, together with their fit (see the Supporting Information for details).



C K edge

C K edge

0.3 nm

3.7 nm

Intensity (a.u.)

Intensity (a.u.)

Figure 3

E 70° to surface E parallel to surface 290 300 310 Photon Energy (eV)

320

280

290 300 310 Photon Energy (eV)

N K edge

N K edge

0.3 nm

3.7 nm

E 70° to surface E parallel to surface

395


400 405 410 415 Photon Energy (eV)

420

Intensity (a.u.)

280

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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320


395

400 405 410 415 Photon Energy (eV)

420

C K edge and N K edge NEXAFS spectra of the interface and the thicker film, for two different polarization of the vector E, as indicated.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4

79.79 nm

85.87 nm

600nm

600nm 0.00 nm

(a)

0.00 nm

(b)

3 µm × 3 µm AFM images of (a) MOP-CN (3.7 nm nominal thickness) and (b) MOP (3.2 nm nominal thickness). The RMS roughness is 11 nm in both cases.



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TOC Graphic


Cyano-Functional Group as an Anchoring Tool for Organic Small

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