Probing Intermolecular Vibrational Symmetry Breaking in Self

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Probing Intermolecular Vibrational Symmetry Breaking in Self-Assembled Monolayers with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Naihao Chiang, Nan Jiang, Lindsey R. Madison, Eric A. Pozzi, Michael R. Wasielewski, Mark A. Ratner, Mark C Hersam, Tamar Seideman, George C. Schatz, and Richard P. Van Duyne J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10645 • Publication Date (Web): 03 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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Journal of the American Chemical Society

Probing Intermolecular Vibrational Symmetry Breaking in SelfAssembled Monolayers with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Naihao Chiang,‡,

#

Nan Jiang,§,

#, *

Lindsey R. Madison,†,

#

Eric A. Pozzi,† Michael R.

Wasielewski,† Mark A. Ratner,† Mark C. Hersam,†,‡,∥ Tamar Seideman,†,‡ George C. Schatz,† and Richard P. Van Duyne†,‡,* ‡

∥

Applied Physics Graduate Program, †Department of Chemistry, and Department of Materials Science and

Engineering, Northwestern University, Evanston, Illinois 60208, United States. §

Department of Chemistry, University of Illinois at Chicago, Illinois 60607, United States.

#

These authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: Ultrahigh Vacuum Tip-enhanced Raman spectroscopy (UHV-TERS) combines the atomic-scale imaging capability of scanning probe microscopy (SPM) with the singlemolecule chemical sensitivity and structural specificity of surface-enhanced Raman spectroscopy (SERS). Here, we use these techniques in combination with theory (TDDFT) to reveal insights into the influence of intermolecular interactions on the vibrational

spectra

of

a

N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-

bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates at room temperature. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule-molecule interaction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most important effect of intermolecular interactions on the vibrational modes of surface-bound molecules. Keywords: scanning tunneling microscopy, tip-enhanced Raman spectroscopy, intermolecular interaction, self-assembled monolayer, accidental degeneracy.


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INTRODUCTION. Self-assembled monolayers (SAMs) provide a unique way of tailoring interfacial properties of metals, oxides and semiconductor surfaces.1 The detailed behavior of molecular self-assembly is governed by molecule-molecule and molecule-substrate interactions.2 Strong molecule-substrate interactions, i.e. chemisorption or strong charge-transfer interaction with the substrate, may result in different packing structures on different substrates. On the other hand, strong molecule-molecule interactions can form similar assemblies on different surfaces since the structure is mainly stabilized by intermolecular forces. Molecular vibrations are extremely sensitive to the local molecular environment. A strong interaction with the surrounding environment can lead to vibrational symmetry breaking.3-4 Depending on the nature of the interaction, vibrational symmetry breaking in molecular systems may result in different spectroscopic selection rules,4 modifications to band intensity,5 and enhanced transfer of vibrational coherence in photodissociation.6 Tip-enhanced Raman spectroscopy (TERS) is a powerful technique for characterizing surface-bound molecular systems.7-8 It combines the chemical selectivity and sensitivity of surface-enhanced Raman spectroscopy9-10 (SERS) with the atomic resolution of scanning probe microscopy11 to achieve the ultimate spatial resolution: Ångstrom-scale resolution.12-18 By overcoming the optical diffraction limits, deeper insight into phenomena otherwise unattainable due to limited spatial resolution are revealed, including the sub-ensemble behavior of adsorbates on plasmonic surfaces,19-20 local electric field distribution,21 and single-molecule surface-induced chemistry.22-23 Ultrahigh vacuum TERS (UHV-TERS) interrogates fundamental surface interactions and



dynamics in the ultimate controlled environment.24-26 Recently, TERS under electrochemical environments has also been demonstrated.27-30 Herein, we report additional insights into the nature of intermolecular interactions in a perylene diimide monolayer adsorbed on single crystal Ag substrates at room temperature. More specifically, we have interrogated the lifting of the vibrational degeneracy

of

N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide)

(PDI) on Ag(111) and Ag(100) surfaces. In combination with time-dependent density functional theory (TDDFT) simulations, we assess the nature of the vibrational normal modes that are directly responsible for the doublets observed in the TER spectra reported herein. EXPERIMENTAL METHODS. in-vacuo Sample Preparation. The Ag(111) and Ag(100) single crystals (Princeton Scientific Corp.) were cleaned by repeated cycles of Ar+ ion sputtering (~2 × 10-6 Torr) and annealing at 750 K. PDI molecules were thermally sublimed in UHV at ~510 K onto the clean Ag(111) and Ag(100) samples. Electrochemically etched Ag tips31 were cleaned by Ar+ ion sputtering before all experiments. UHV-STM and TERS. STM imaging and TERS were performed with a homebuilt optical UHV-STM32 at room temperature with all optical elements located outside the UHV chamber (base pressure of ~2 × 10-11 Torr). All STM images were taken in constant-current mode. A 532 nm continuous wave laser (Spectra-Physics Excelsior) was focused onto the STM tip-sample junction with the excitation polarization parallel to the tip axis. The excitation laser angle of incidence was ~75° with respect to the single-


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crystal surface normal. The TER signal was detected at the same angle through a viewport on the opposite side of the chamber using a spectrograph (Princeton Instruments SCT 320) equipped with a thermoelectrically cooled CCD (Princeton Instruments PIXIS 400BR). A laser power stabilizer (Brockton Electro-Optics Corp. LPC) maintains the excitation power at 1.00 mW to avoid sample degradation due to the strong plasmonic field (Figure S1). All TER spectra were acquired for 30 seconds with 6 accumulations and at Vb = 0.1 V since it gave the strongest intensity (Figure S2). TDDFT Simulation. The Raman scattering cross sections of gas-phase PDI were calculated with the Amsterdam Density Functional (ADF) software package33-35 at the density functional level of theory. The triple ζ with polarization (TZP) basis set and the Becke-Perdew (BP86)36-37 exchange-correlation were chosen because harmonic vibrational frequencies calculated at this level of theory closely match experimental results without incorporating a scaling factor.38 The C2 point group was enforced during geometry optimization and the numerical calculation of the vibrational modes. The σh and inversion operations, although valid for gas phase PDI, would not be valid for the case when PDI is bound to the metal surface and that symmetry is broken. The AOResponse module implemented in ADF and developed by Jensen et al.39 was used to calculate the real and imaginary components of the polarizability tensors. In AOResponse module, the short time approximation proposed by Lee and co-workers4042

is made to the Kramers, Heisenberg, and Dirac formulation of Resonance Raman

scattering. The wavelength of the perturbation was 785 nm, the damping parameter was set to 0.004 au (approximately 0.1 eV), the zero-order regular approximation (ZORA)4344

was applied to the calculation, and the convergence for the AOResponse module was



set to 10−8 a.u. These parameters are consistent with previous work.39, 45-48 RESULTS AND DISCUSSION. The molecular structure of PDI (Figure 1a) consists of a center perylene-3,4,9,10tetracarboxylic-3,4,9,10-diimide (PTCDI) with two bis-2,6-diisopropylphenyl functional groups on the two ends. The center PTCDI governs the main electronic absorption in the UV-Vis spectrum (Figure 1b). This particular functionalization of PTCDI is chosen because the diisopropylphenyl groups are expected to lift the molecule away from the surface and consequently align better with the plasmonic dipole of the tip-sample junction.49 For a mostly planar molecule like this, the in-plane Raman modes have higher than expected intensities, probably because the molecules are thermally activated to tilt away from being parallel to the surface.48 Additionally, the diisopropylphenyl groups also fine-tune the absorption spectrum so that the main absorption peak is at ~532 nm which enables strong resonance enhancement in Raman scattering when excited with a 532 nm laser. Although PDI is known for its ability to generate excited triplet states through singlet fission, we do not expect to observe Raman spectra from the excited-states because no sensitizers were used50 and the PDI molecules were in direct contact with metal surfaces.


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Figure 1. Self-assembled PDI adlayer on Ag(111). (a) Chemical structure of PDI. (b) Solution-phase absorption spectrum of PDI. (c) Large scale STM image (50 nm × 50 nm) of a well-ordered PDI adlayer on a Ag(111) surface imaged by a W tip. (STM conditions: Vb = 2 V, it = 100 pA.) The formation of a large stable PDI molecular island (Figure 1c) at room temperature is a direct result of strong lateral molecule-molecule interactions.51 On open terraces (top right corner in the STM image), individual PDI molecules cannot be resolved due to surface diffusion. This rapid diffusion further reveals that the interaction between PDI and the Ag substrate is weak, as expected from the addition of diisopropylphenyl groups.



Figure 2. (a) Sub-molecular resolution topographic STM image (15 nm × 15 nm) of a PDI self-assembled monolayer on Ag(111) imaged with a Ag tip under 1 mW 532 nm laser excitation. (STM conditions: Vb = 2.5 V, it = 100 pA.), hardsphere models of PDI are inserted to show the unit-cell. (b) Detailed packing structure of PDI on Ag(111) reconstructed from (a). (c) Sub-molecular resolution topographic STM image (15 nm × 15 nm) of a PDI self-assembled monolayer on Ag(100) imaged with a Ag tip under 1 mW 532 nm laser excitation. (STM conditions: Vb = 2 V, it = 100 pA.), hardsphere models of PDI are inserted to show the unit-cell. (d) Detailed packing structure of PDI on Ag(100) reconstructed from (c). (e) UHV-TER spectra of a PDI adlayer on Ag(111) and Ag(100) with 1 mW 532 nm laser excitation. (STM conditions: Vb = 0.1 V, it = 500 pA.)


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The detailed molecular packing structure can be slightly altered by changing the underlying symmetry of the Ag single-crystal facet. Figure 2a and 2c show submolecular resolution STM images of PDI islands formed on Ag(111) and Ag(100), respectively. The PDI packing structures on different Ag facets were reconstructed based on the high resolution STM images (Figure 2b and 2d). The green lines were inserted to indicate the presumed interactions between an end diisopropylphenyl group of one molecule and a side of the perylene ring of the neighboring molecule. The red lines indicate the possible interactions between the methyl groups in the end diisopropylphenyl groups of the neighboring molecules. Even though the packing structures were altered by changing the Ag crystal facets, Figure 2e shows no significant spectral difference observed in the UHV-TER spectra of PDI on Ag(111) and Ag(100). We note that the small red-shifts in the TER spectra on Ag(111) and Ag(100) are inconclusive since it is beyond the spectral resolution of our instrument.20 From the reconstructed packing structures in Figure 2b and 2d, the main structural difference were shown as changes in the intermolecular distance between the end diisopropylphenyl groups in the adjacent molecules. The observation of no-spectral differences in UHV-TER spectra from the two different packing structures suggests that the direct π-π interaction between the phenyl rings is weak while the methyl-methyl (red lines) and methyl-π (green lines) interaction are strong. Therefore, we conclude that the strong PDI-PDI interactions between the methyl groups in the diisopropylphenyl groups of neighboring PDIs, and between the methyl groups in the diisopropylphenyl group of one PDI molecule and a side of the perylene ring of the other are the driving force for the self-assembling on Ag substrates.



Figure 3. TDDFT simulated Raman spectrum of a PDI molecule. TDDFT simulated Raman spectrum of a gas phase PDI molecule is shown in Figure 3. The simulated spectrum reveals several weak modes below 1100 cm-1, and most intense modes are located between 1200 cm-1 to 1600 cm-1. The relative intensities in the simulated spectrum are in good agreement with the experimentally observed TER spectra in Figure 2 and on other metal substrates (Figure S3). Since the simulation did not account for any substrate-effects, the simulation supports the experimental observation that the molecule-surface interaction is indeed weak. In order to gain detailed fundamental insights into the intermolecular interaction, Raman-active vibrational normal modes are assigned based on the naming conventions shown in Figure 4. The π systems were divided into four types of rings based on the molecular symmetry (Figure 4a). The vibrational modes of those rings can be characterized by 6 distinct motions (Figure 4b): ring breathing, Kekulé, quinoidal, ring distortion with bond stretching, bonds bending asymmetrically, and bonds bending symmetrically.


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Figure 4. PDI vibrational modes naming conventions. (a) Ring identifications. (b) Vibrational modes assignments. Figure 5a shows a zoomed-in UHV-TER spectrum of PDI on Ag(111) above the simulated spectrum in 1200 cm-1 to 1600 cm-1 spectral region. Three pairs of doublets were observed experimentally but only 5 peaks (assigned in Figure 5b) were present in the simulation. The doublet at ~1550 cm-1 is comprised of two distinct vibrational modes and is assigned as ring A breathing mode with ring C distortion bending (exp: 1587 cm1

, calc: 1565 cm-1) and ring A quinoidal like mode with ring C distortion bending mode

(exp: 1572 cm-1, calc: 1555 cm-1). The doublet at ~1350 cm-1 also consists of two different vibrational modes; the vibrational mode at ~1380 cm-1 (exp: 1383 cm-1, calc: 1384 cm-1) is assigned as a combination of symmetric ring A bending, ring B Kekulé, and symmetric ring C bending, and the vibrational mode at ~1370 cm-1 (exp: 1371 cm-1, calc: 1342 cm-1) is assigned as a combination of a ring A breathing, ring B quasi Kekulé, and ring C distortion bending. The ~30 cm-1 shifts of the 1371 cm-1 peak in the TDDFT simulation with respect to experiment could possibly be due to strong lateral



intermolecular interactions which were not considered in the simulation of the gasphase PDI.

Figure 5. (a) Zoomed-in UHV-TER spectrum of PDI on Ag(111) and the corresponding TDDFT simulated Raman spectrum. (b) Simulated Raman active normal modes of PDI for the experimentally observed doublets in the 1200 cm-1 to 1600 cm-1 region. Only one Raman peak was predicted near ~1300 cm-1 in the simulation although this consists of two degenerate vibrational modes, both involving ring A breathing, ring B distortion stretching, ring C no motion, and ring D Kekulé mode (exp: 1293 and 1301 cm-1, calc: 1286 cm-1). The top and bottom phenyl rings are perpendicular to the plane of the PDI in the optimized geometry, leading to weak interactions with the in-plane modes. Ring D was mixed in opposite way for the two modes (1286a and 1286b cm-1). The degeneracy is therefore an accidental degeneracy52 due to weak coupling of


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different vibrations, rather than a degeneracy that arises from intrinsic symmetry effects. Consequently, when ring D of one PDI molecule interacts strongly with ring B of another molecule, the degeneracy of the 1286 cm-1 modes was lifted due to an intermolecular coupling between the two. This prediction is consistent with the UHV-STM and TERS observations in Figure 2. The methyl-π (green-lines) and methyl-methyl (red-lines) interactions identified were mainly between the methyl groups in one diisopropylphenyl end group (ring D in Figure 4a) of a PDI molecule and a side of the perylene ring (ring B in Figure 4a) of another molecule. The vibrational mode amplitudes in Figure 5b show that the modes at 1342 cm -1 and the top mode at 1286 cm -1 have the largest amplitude at the periphery of the ring system that overlaps with the green/red lines in Fig. 2b,d. It is therefore understandable that these are the modes that will be most strongly perturbed by intermolecular interaction. The methyl-methyl interactions can be on the order of few kcal/mol53-54 which is strong enough to perturb the molecular vibrations. The methyl-π interaction is difficult to quantify. Other Raman-mode assignments can be found in Table 1 with the degenerate modes highlighted in red. Table 1. Raman Peak Assignments for PDI. The degenerated modes are highlighted in red. Experiment (cm-1)

TDDFT Simulation (cm-1)

540

533

1059

1046

1293, 1301

1286a, 1286b

1371

1343

Vibration Description PDI ring breathing mode. A, B, C breathing symmetrically A: “Quinoidal” like mode, B: ring distortion bending, C:bending asymmetrically A: breathing, B: ring distortion stretching, C: no motion, D: Kekulé mode. D mixed in opposite way for the two modes A: breathing, B: quasi Kekulé, C: ring distortion bending



1383

1384

1452

1439

1572

1555

1587

1566

A: Bending symmetric, B: Kekulé, C: symmetric bending A: symmetric bending, B: antisymmetric bending, C: ring distortion bending A: Quinoidal like mode, C: ring distortion bending mode from carbon #3 in the C ring. Rings B have very similar ring distortions as seen in 1566 cm-1 A: Ring breathing mode C: ring distortion bending mode from carbon #3 in the C ring. Rings B have very similar ring distortions as seen in 1555 cm-1

Conclusion. In summary, we have used a combination of UHV-TERS and TDDFT simulations to reveal vibrational symmetry breaking of a self-assembled PDI island, which was induced by the intermolecular interactions, on single-crystal Ag substrates. Through careful examinations of the packing structures on different Ag facets and with consideration of the nature of the Raman active vibrational normal modes, we conclude that lifting of the vibrational degeneracy of the 1286 cm-1 modes originates from strong lateral intermolecular interactions between the diisopropylphenyl end groups and the center perylene rings. This provides direct insight into the strength of intermolecular interactions for SAM formation at sub-molecular scales. We anticipate that this information will advance the fundamental understanding of the critical effect of intermolecular interactions on the vibrational modes of surface-bound molecules.


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ASSOCIATED CONTENT Supporting Information Laser excitation power-dependence and STM bias-dependence of UHV-TERS of PDI on Ag(111)/Ag(100), UHV-TERS of PDI on Cu(111) and Au(111), Lorentzian fitted TER spectra for Figure 2 and Figure 5. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *email: [email protected]. [email protected]. Author Contributions #

These authors contributed equally to this work.

Notes. The authors declare no competing financial interests. Acknowledgement. L.R.M., G.C.S., and R.P.V.D. acknowledge support from the National Science Foundation Center for Chemical Innovation dedicated to Chemistry at the Space−Time Limit (CaSTL) Grant CHE-1414466. N.C., N.J., T.S., M.C.H., and R.P.V.D. acknowledge support from the Department of Energy Office of Basic Energy Sciences (SISGR Grant DEFG02-09ER16109). E.A.P. and L.R.M. acknowledge support from the National Science Foundation Graduate Research Fellowship under Grant DGE-1324585 and the National Science Foundation Materials Research Science and Engineering Center (DMR-1121262). M.R.W. acknowledges the support of the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Department of Energy under grant No. DE-FG0299ER14999.



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Probing Intermolecular Vibrational Symmetry Breaking in Self

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