Probing Intermolecular Vibrational Symmetry Breaking in Self-Assembled Monolayers with Ultrahigh Vacuum Tip-Enhanced RamanSpectroscopyNaihao 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, Chicago, Illinois 60607, United States

*S Supporting Information

ABSTRACT: Ultrahigh vacuum tip-enhanced Raman spectros-copy (UHV-TERS) combines the atomic-scale imaging capabilityof scanning probe microscopy with the single-molecule chemicalsensitivity and structural specificity of surface-enhanced Ramanspectroscopy. Here, we use these techniques in combination withtheory to reveal insights into the influence of intermolecularinteractions 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 substratesat room temperature. In particular, we have revealed the lifting ofa vibrational degeneracy of a mode of PDI on Ag(111) andAg(100) surfaces, with the most strongly perturbed mode beingthat associated with the largest vibrational amplitude on theperiphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule−moleculeinteraction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most importanteffect of intermolecular interactions on the vibrational modes of surface-bound molecules.

■ INTRODUCTION

Self-assembled monolayers (SAMs) provide a unique way oftailoring interfacial properties of metals, oxides, and semi-conductor 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 withthe substrate, may result in different packing structures ondifferent substrates. However, strong molecule−moleculeinteractions can form similar assemblies on different surfacessince the structure is mainly stabilized by intermolecular forces.Molecular vibrations are extremely sensitive to the local

molecular environment. A strong interaction with thesurrounding environment can lead to vibrational symmetrybreaking.3,4 Depending on the nature of the interaction,vibrational symmetry breaking in molecular systems may resultin different spectroscopic selection rules,4 modifications toband intensity,5 and enhanced transfer of vibrational coherencein photodissociation.6

Tip-enhanced Raman spectroscopy (TERS) is a powerfultechnique for characterizing surface-bound molecular sys-tems.7,8 It combines the chemical selectivity and sensitivity ofsurface-enhanced Raman spectroscopy9,10 (SERS) with the

atomic resolution of scanning probe microscopy11 to achievethe ultimate spatial resolution: Ångstrom-scale resolution.12−18

By overcoming the optical diffraction limits, deeper insight intophenomena otherwise unattainable due to limited spatialresolution are revealed, including the subensemble behaviorof adsorbates on plasmonic surfaces,19,20 local electric fielddistribution,21 and single-molecule surface-induced chemis-try.22,23 Ultrahigh vacuum TERS (UHV-TERS) interrogatesfundamental surface interactions and dynamics in the ultimatecontrolled environment.24−26 Recently, TERS under electro-chemical environments has also been demonstrated.27−30

Herein, we report additional insights into the nature ofintermolecular interactions in a perylene diimide monolayeradsorbed on single crystal Ag substrates at room temperature.More specifically, we have interrogated the lifting of thevibrational degeneracy of N-N′-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) on Ag(111) andAg(100) surfaces. In combination with time-dependent densityfunctional theory (TDDFT) simulations, we assess the nature

Received: October 5, 2017Published: December 3, 2017

Article

pubs.acs.org/JACSCite This: J. Am. Chem. Soc. 2017, 139, 18664−18669

© 2017 American Chemical Society 18664 DOI: 10.1021/jacs.7b10645J. Am. Chem. Soc. 2017, 139, 18664−18669

of the vibrational normal modes that are directly responsible forthe doublets observed in the TER spectra reported herein.

■ EXPERIMENTAL SECTIONIn Vacuo Sample Preparation. The Ag(111) and Ag(100) single

crystals (Princeton Scientific Corp.) were cleaned by repeated cycles ofAr+ ion sputtering (∼2 × 10−6 Torr) and annealing at 750 K. PDImolecules were thermally sublimed in UHV at ∼510 K onto the cleanAg(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 home-built optical UHV-STM32 at room temperature with alloptical elements located outside the UHV chamber (base pressure of∼2 × 10−11 Torr). All STM images were taken in constant-currentmode. A 532 nm continuous wave laser (Spectra-Physics Excelsior)was focused onto the STM tip−sample junction with the excitationpolarization parallel to the tip axis. The excitation laser angle ofincidence was ∼75° with respect to the single-crystal surface normal.The TER signal was detected at the same angle through a viewport onthe opposite side of the chamber using a spectrograph (PrincetonInstruments SCT 320) equipped with a thermoelectrically cooledCCD (Princeton Instruments PIXIS 400BR). A laser power stabilizer(Brockton Electro-Optics Corp. LPC) maintains the excitation powerat 1.00 mW to avoid sample degradation due to the strong plasmonicfield (Figure S1 of the Supporting Information, SI). All TER spectrawere acquired for 30 s with 6 accumulations and at Vb = 0.1 V since itgave 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 harmonicvibrational frequencies calculated at this level of theory closelymatch experimental results without incorporating a scaling factor.38

The C2 point group was enforced during geometry optimization andthe numerical calculation of the vibrational modes. The σh andinversion operations, although valid for gas phase PDI, would not bevalid for the case when PDI is bound to the metal surface and thatsymmetry is broken.The AOResponse module implemented in ADF and developed by

Jensen et al.39 was used to calculate the real and imaginarycomponents of the polarizability tensors. In AOResponse module,the short time approximation proposed by Lee and co-workers40−42 ismade to the Kramers, Heisenberg, and Dirac formulation ofResonance 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)43,44 was applied to the calculation, and the convergence forthe AOResponse module was set to 10−8 a.u. These parameters areconsistent with previous work.39,45−48

■ RESULTS AND DISCUSSION

The molecular structure of PDI (Figure 1a) consists of a centerperylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI)with two bis-2,6-diisopropylphenyl functional groups on thetwo ends. The center PTCDI governs the main electronicabsorption in the UV−vis spectrum (Figure 1b). This particularfunctionalization of PTCDI is chosen because the diisopropyl-phenyl groups are expected to lift the molecule away from thesurface and consequently align better with the plasmonic dipoleof the tip−sample junction.49 For a mostly planar molecule likethis, the in-plane Raman modes have higher than expectedintensities, probably because the molecules are thermallyactivated to tilt away from being parallel to the surface.48

Additionally, the diisopropylphenyl groups also fine-tune theabsorption spectrum so that the main absorption peak is at∼532 nm which enables strong resonance enhancement inRaman scattering when excited with a 532 nm laser. AlthoughPDI is known for its ability to generate excited triplet statesthrough singlet fission, we do not expect to observe Ramanspectra from the excited-states because no sensitizers wereused,50 and the PDI molecules were in direct contact withmetal surfaces.The formation of a large stable PDI molecular island (Figure

1c) at room temperature is a direct result of strong lateralmolecule−molecule interactions.51 On open terraces (top rightcorner in the STM image), individual PDI molecules cannot beresolved due to surface diffusion. This rapid diffusion furtherreveals that the interaction between PDI and the Ag substrate isweak, as expected from the addition of diisopropylphenylgroups.The detailed molecular packing structure can be slightly

altered by changing the underlying symmetry of the Ag single-crystal facet. Figure 2a,c shows submolecular resolution STMimages of PDI islands formed on Ag(111) and Ag(100),respectively. The PDI packing structures on different Ag facets

Figure 1. Self-assembled PDI adlayer on Ag(111). (a) Chemical structure of PDI. (b) Solution-phase absorption spectrum of PDI. (c) Large scaleSTM image (50 × 50 nm2) of a well-ordered PDI adlayer on a Ag(111) surface imaged by a W tip. (STM conditions: Vb = 2 V, it = 100 pA.).

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were reconstructed based on the high resolution STM images(Figure 2b,d). The green lines were inserted to indicate thepresumed interactions between an end diisopropylphenyl groupof one molecule and a side of the perylene ring of theneighboring molecule. The red lines indicate the possibleinteractions between the methyl groups in the enddiisopropylphenyl groups of the neighboring molecules.Even though the packing structures were altered by changing

the Ag crystal facets, Figure 2e shows no significant spectraldifference observed in the UHV-TER spectra of PDI onAg(111) and Ag(100). We note that the small red-shifts in theTER spectra on Ag(111) and Ag(100) are inconclusive since itis beyond the spectral resolution of our instrument.20 From thereconstructed packing structures in Figure 2b,d, the mainstructural differences were shown as changes in theintermolecular distance between the end diisopropylphenylgroups in the adjacent molecules. The observation of no-spectral differences in UHV-TER spectra from the two differentpacking structures suggests that the direct π−π interactionbetween 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 interactionsbetween the methyl groups in the diisopropylphenyl groups ofneighboring PDIs, and between the methyl groups in thediisopropylphenyl group of one PDI molecule and a side of theperylene ring of the other are the driving forces for the self-assembling on Ag substrates.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 modesare located between 1200 cm−1 to 1600 cm−1. The relativeintensities in the simulated spectrum are in good agreementwith the experimentally observed TER spectra in Figure 2 andon other metal substrates (Figure S3). Since the simulation didnot account for any substrate-effects, the simulation supportsthe experimental observation that the molecule−surfaceinteraction is indeed weak.In order to gain detailed fundamental insights into the

intermolecular interaction, Raman-active vibrational normalmodes are assigned based on the naming conventions shown inFigure 4. The π systems were divided into four types of ringsbased on the molecular symmetry (Figure 4a). The vibrationalmodes of those rings can be characterized by 6 distinct motions(Figure 4b): ring breathing, Kekule, quinoidal, ring distortionwith bond stretching, bonds bending asymmetrically, and bondsbending symmetrically.Figure 5a shows a zoomed-in UHV-TER spectrum of PDI on

Ag(111) above the simulated spectrum in the 1200 cm−1 to1600 cm−1 spectral region. Three pairs of doublets wereobserved experimentally but only 5 peaks (assigned in Figure5b) were present in the simulation. The doublet at ∼1550 cm−1

is composed of two distinct vibrational modes and is assignedas ring A breathing mode with ring C distortion bending (exp:1587 cm−1, calcd: 1565 cm−1) and ring A quinoidal like modewith ring C distortion bending mode (exp: 1572 cm−1, calcd:1555 cm−1). The doublet at ∼1350 cm−1 also consists of twodifferent vibrational modes; the vibrational mode at ∼1380cm−1 (exp: 1383 cm−1, calcd: 1384 cm−1) is assigned as acombination of symmetric ring A bending, ring B Kekule, andsymmetric ring C bending, and the vibrational mode at ∼1370cm−1 (exp: 1371 cm−1, calcd: 1342 cm−1) is assigned as a

Figure 2. (a) Submolecular resolution topographic STM image (15 ×15 nm2) of a PDI self-assembled monolayer on Ag(111) imaged with aAg tip under 1 mW 532 nm laser excitation. (STM conditions: Vb =2.5 V, it = 100 pA.), hardsphere models of PDI are inserted to showthe unit-cell. (b) Detailed packing structure of PDI on Ag(111)reconstructed from (a). (c) Submolecular resolution topographic STMimage (15 × 15 nm2) of a PDI self-assembled monolayer on Ag(100)imaged with a Ag tip under 1 mW 532 nm laser excitation. (STMconditions: Vb = 2 V, it = 100 pA.), hardsphere models of PDI areinserted to show the unit-cell. (d) Detailed packing structure of PDIon Ag(100) reconstructed from (c). (e) UHV-TER spectra of a PDIadlayer on Ag(111) and Ag(100) with 1 mW 532 nm laser excitation.(STM conditions: Vb = 0.1 V, it = 500 pA.).

Figure 3. TDDFT simulated Raman spectrum of a PDI molecule.

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combination of a ring A breathing, ring B quasi Kekule, andring C distortion bending. The ∼30 cm−1 shifts of the 1371cm−1 peak in the TDDFT simulation with respect toexperiment could possibly be due to strong lateral intermo-lecular interactions which were not considered in thesimulation of the gas-phase PDI.Only one Raman peak was predicted near ∼1300 cm−1 in the

simulation although this consists of two degenerate vibrationalmodes, both involving ring A breathing, ring B distortionstretching, ring C no motion, and ring D Kekule mode (exp:1293 and 1301 cm−1, calcd: 1286 cm−1). The top and bottomphenyl rings are perpendicular to the plane of the PDI in theoptimized 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 anaccidental degeneracy52 due to weak coupling of differentvibrations, rather than a degeneracy that arises from intrinsicsymmetry effects. Consequently, when ring D of one PDImolecule interacts strongly with ring B of another molecule, thedegeneracy of the 1286 cm−1 modes was lifted due to anintermolecular coupling between the two. This prediction isconsistent with the UHV-STM and TERS observations inFigure 2. The methyl−π (green-lines) and methyl−methyl(red-lines) interactions identified were mainly between themethyl groups in one diisopropylphenyl end group (ring D inFigure 4a) of a PDI molecule and a side of the perylene ring(ring B in Figure 4a) of another molecule. The vibrationalmode amplitudes in Figure 5b show that the modes at 1342 cm−1 and the top mode at 1286 cm −1 have the largest amplitudeat the periphery of the ring system that overlaps with thegreen/red lines in Figure 2b,d. It is therefore understandablethat these are the modes that will be most strongly perturbedby intermolecular interactions. The methyl−methyl interactionscan be on the order of few kcal/mol53,54 which are strongenough to perturb the molecular vibrations. The methyl−πinteraction is difficult to quantify. Other Raman-modeassignments can be found in Table 1 with the degeneratemodes highlighted in red.

■ CONCLUSIONS

In summary, we have used a combination of UHV-TERS andTDDFT simulations to reveal vibrational symmetry breaking ofa self-assembled PDI island, which was induced by theintermolecular interactions, on single-crystal Ag substrates.Through careful examinations of the packing structures ondifferent Ag facets and with consideration of the nature of theRaman active vibrational normal modes, we conclude thatlifting of the vibrational degeneracy of the 1286 cm−1 modesoriginates from strong lateral intermolecular interactionsbetween the diisopropylphenyl end groups and the centerperylene rings. This provides direct insight into the strength of

Figure 4. PDI vibrational modes naming conventions. (a) Ringidentifications. (b) Vibrational modes assignments.

Figure 5. (a) Zoomed-in UHV-TER spectrum of PDI on Ag(111) and the corresponding TDDFT simulated Raman spectrum. (b) SimulatedRaman active normal modes of PDI for the experimentally observed doublets in the 1200 cm−1 to 1600 cm−1 region.

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intermolecular interactions for SAM formation at submolecularscales. We anticipate that this information will advance thefundamental understanding of the critical effect of intermo-lecular interactions on the vibrational modes of surface-boundmolecules.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.7b10645.

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) andAu(111), Lorentzian fitted TER spectra for Figures 2and 5 (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*njiang@uic.edu*vanduyne@northwestern.eduORCIDNaihao Chiang: 0000-0003-3782-6546Nan Jiang: 0000-0002-4570-180XMichael R. Wasielewski: 0000-0003-2920-5440Mark C. Hersam: 0000-0003-4120-1426George C. Schatz: 0000-0001-5837-4740Richard P. Van Duyne: 0000-0001-8861-2228Author Contributions⊥These authors contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSL.R.M., G.C.S., and R.P.V.D. acknowledge support from theNational Science Foundation Center for Chemical Innovationdedicated 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 ofBasic Energy Sciences (SISGR Grant DE-FG02-09ER16109).E.A.P. and L.R.M. acknowledge support from the NationalScience Foundation Graduate Research Fellowship underGrant DGE-1324585 and the National Science FoundationMaterials Research Science and Engineering Center (DMR-1121262). M.R.W. acknowledges the support of the ChemicalSciences, Geosciences, and Biosciences Division, Office of BasicEnergy Sciences, Department of Energy under grant No. DE-

FG02-99ER14999. N.J. acknowledges support from startupfunding of the University of Illinois at Chicago.

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Table 1. Raman Peak Assignments for PDIa

experiment (cm−1) TDDFT simulation (cm−1) vibration description

540 533 PDI ring breathing mode. A, B, C breathing symmetrically1059 1046 A: “Quinoidal” like mode, B: ring distortion bending, C: bending asymmetrically

1293, 1301 1286a, 1286b A: breathing, B: ring distortion stretching, C: no motion, D: Kekule mode. D mixed in opposite way for the two modes1371 1343 A: breathing, B: quasi Kekule, C: ring distortion bending1383 1384 A: Bending symmetric, B: Kekule, C: symmetric bending1452 1439 A: symmetric bending, B: antisymmetric bending, C: ring distortion bending1572 1555 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

1587 1566 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

aThe degenerated modes are in italic.

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Journal of the American Chemical Society Article

DOI: 10.1021/jacs.7b10645J. Am. Chem. Soc. 2017, 139, 18664−18669

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