Substrate Characterized by Scanning Tunneling Microscopy, Tip

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Dual Binding Configurations of Subphthalocyanine on Ag(100) Substrate Characterized by Scanning Tunneling Microscopy, TipEnhanced Raman Spectroscopy and Density Functional Theory Philip J Whiteman, Jeremy F. Schultz, Zachary D. Porach, Hanning Chen, and Nan Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12068 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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The Journal of Physical Chemistry

Dual Binding Configurations of Subphthalocyanine on Ag(100) Substrate Characterized by Scanning Tunneling Microscopy, TipEnhanced Raman Spectroscopy and Density Functional Theory Philip J. Whiteman,‚ Jeremy F. Schultz,‚ Zachary D. Porach,‚ Hanning Chen,Á Nan Jiang* ‚ ‚

Á

Department of Chemistry, University of Illinois at Chicago, Illinois 60607, United States 'HSDUWPHQW RI &KHPLVWU\ 7KH *HRUJH :DVKLQJWRQ 8QLYHUVLW\ :DVKLQJWRQ 'LVWULFW RI &ROXPELD 8QLWHG 6WDWHV

Abstract In order to fully characterize interfacial systems at the smallest scales, advanced analytical surface techniques have to be employed to render a complete picture of molecular assemblies. In this study, we carried out ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) experiments on subphthalocyanine (SubPc) molecules, which are self-assembled on a Ag(100) substrate. The UHV STM experiments were complemented by tip-enhanced Raman spectroscopy (TERS), surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculations. The TERS spectrum shows a high signal intensity (> 600 ADU·mW-1·s-1) due to piezo-driven in-vacuo excitation and collection lenses with large numerical apertures (NA = 0.4). A new two-dimensional molecular superstructure of SubPc was discovered to consist of two distinct molecular binding configurations, both of which interact relatively weakly with the underlying metallic substrate as revealed by high-signal-to-noise enhanced Raman spectra. Our results demonstrate the necessity of advanced Raman techniques such as TERS when precisely probing molecule-molecule and molecule-substrate interactions.

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1. INTRODUCTION Self-assembly of organic molecules on metal surfaces offers exciting opportunities to fabricate nanomaterials with desirable electronic and optical properties. Designing such materials requires a fundamental understanding of the molecule-molecule and molecule-substrate interactions at the atomic level. It is thus instrumental to characterize the adsorption process, the growth mechanism, and the structure of ordered molecular arrays on metal substrates when perceiving the functions of these nanomaterials fabricated through a bottom-up approach. For example, the phthalocyanine family of molecules has been investigated in many surface studies with the aid of scanning tunneling microscopy (STM).1-5 Their remarkable optoelectronic properties make them well suited for light capturing and light generating applications as exemplified in organic photovoltaic devices (OPVDs)6-7 and organic light emitting diodes (OLEDs)8-9, respectively. Although STM can answer many questions with regard to the morphology of molecular arrangements, a thorough characterization of these nanomaterials demands additional experimental techniques to help understand molecular-surface couplings as well as moleculemolecule interactions. Tip-enhanced Raman spectroscopy (TERS) has been shown to be an effective tool for full characterization of nanoscale molecular systems.10-14 Vibrational spectroscopy of surface-bound molecules can be obtained with extreme energy (~6 cm-1)15 and spatial resolution (sub-nm)16-18. Recently, a ~15 cm-1 shift of vibrational peak due to strong molecule-substrate coupling was discovered by TERS, further validating it for interfacial characterization.19-20 TERS spectra, taken in tandem with STM imaging, allows us to investigate the local chemical environment of our chosen molecular system to answer the important questions of how these molecules interact with each other and with the surface that they are bound to.21


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In this article, we present our findings of ultrahigh vacuum (UHV) STM and TERS experiments on subphthalocyanine (SubPc), a unique member of the phthalocyanine family both in its molecular geometry and its chemical composition. As opposed to the typical 4-fold symmetric structure of a phthalocyanine22, SubPc only has three isoindolyl lobes which form a three-legged bowl structure that results in a non-planar

C3v point group.23 In addition, its central

boron atom is connected to a chlorine atom to form a rather polar B-Cl bond. Thus, the adsorption and subsequent self-assembly of SubPc molecules on atomically flat metal substrates may potentially facilitate the formation of unique molecular superstructures. In the present study, we use piezo-driven in-vacuo lenses with large numerical apertures (NA = 0.4) to raise the scattering photon collection efficiency. With this improvement, we were able to ascertain subtle moleculemolecule and molecule-substrate interactions of SubPc deposited on Ag(100) via STM in tandem with TERS to obtain a more comprehensive understanding of surface-bound SubPc under pristine UHV conditions. 2. EXPERIMENTAL AND THEORETICAL METHODS Experiments took place in a low-temperature to variable-temperature STM system (Unisoku) under ultrahigh vacuum at a base pressure of 5.0 × 10-11 torr. The Ag(100) single crystal (Princeton Scientific, 99.999% purity) was prepared in a preparation chamber (base pressure 1 × 10-10 torr) separated from an STM chamber by a gate valve. The crystal was cleaned by cycles of argon sputtering followed by indirect heat annealing to 800 K. The subphthalocyanine molecules were purchased from Sigma-Aldrich (85% purity) and deposited onto the Ag(100) surface within a preparation chamber via a K-cell style molecular evaporator (220 °C) at sub-monolayer coverage. The sample was kept near room temperature during deposition. The samples were then transferred to another chamber for STM imaging and Raman spectroscopy experiments. These 3 ACS Paragon Plus Environment


experiments took place at liquid nitrogen temperatures (78 K). The STM was ran under constant current mode. Biases were applied to the sample with respect to the grounded tip. Chemicallyetched Ag tips were used for STM imaging and TERS experiments. WSxM 5.0 was used for image processing.

Figure 1. Diagram of the UHV STM chamber and laser optics setup. As seen in Fig. 1, a 561 nm solid-state CW laser (LASOS) was used to excite molecules for SERS and TERS experiments. Most laser optics for laser excitation and Raman signal collection were housed in separate cage systems outside of the UHV chamber, with the important exception of the lenses used for laser focusing and scattered light collection at the tip-sample junction. A separate Raman signal collection lens located on the other side of the scanning tip21 was used as opposed to other reflection mode side-illumination24 or transmission mode bottomillumination25 setups. These in-vacuo lenses were positioned in close proximity to the tip and sample within the UHV chamber (NA = 0.4), controlled by piezoelectric motors to facilitate both 4 ACS Paragon Plus Environment

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focusing the laser spot at the tip-sample junction and collecting the Raman signal. The excitation and signal collection cage systems were fiber-coupled to the laser and spectrograph, respectively. Both the angle of incidence of the laser and the angle of signal collection were 55° with respect to the surface normal, positioned opposite to each other. An Isoplane SCT-320 spectrometer, coupled with a PIXIS 100 CCD (Princeton Instruments), was used for all Raman experiments. All spectra shown are an average of six sequential 10-second acquisitions, meaning each spectrum was acquired over a total time of 60 seconds. Ambient surface-enhanced Raman spectroscopy (SERS) experiments were performed on commercially available silver SERS substrates provided by OceanOptics. The SERS substrate was soaked in a dilute solution of SubPc in 1-chlorobenzene and lightly rinsed before taking SERS spectra. UHV SERS experiments took place on a silver film-over-nanosphere substrate. Molecules were deposited onto this substrate with parameters identical to our TERS experiments. An Ocean Optics Red Tide Spectrometer was used for UV-Vis measurements. The interpretation of experimental TERS and SERS spectra was facilitated by computational studies, which were performed using ADF package26 with PBE exchangecorrelation functional27 and TZP Slater-type basis set.28 In our simulation, all vibrational normal modes were ascertained by diagonalizing the mass-weighted Hessian matrix for the optimized geometry of SubPc, while their excited-state vibronic displacements were determined by the socalled independent-mode-displaced-harmonic-oscillator (IMDHO) model:29 Dj = -

1 § wE ex · ¸ ¨ w j © wq j ¹

q j =0

where E ex is the excite-state energy determined by the time-dependent density functional theory (TDDFT).30



3. RESULTS AND DISCUSSION 3.1. Scanning tunneling microscopy Molecular islands of SubPc on the Ag(100) substrate are visualized in our STM image (Fig. 2a) with individual molecules clearly resolved. In our experiments, isolated SubPc molecules that do not belong to any island are rarely seen. This has been the case for most other substrates as well, with the exception of the reconstructed Si(111) 7×7 surface wherein the abundance of isolated and noninteracting SubPc molecules was ascribed to strong molecule-substrate interactions.31 Fig. 2a also shows that some SubPc molecules have bright protrusions at their center due to the presence of their axial B-Cl bonds. In accord with earlier studies32, we attribute these bright molecules to SubPcµV ³FKORULQH-XS´ binding configuration in which the three-legged isoindolyl moiety of SubPC is bound to the substrate as shown in Fig. 2d and Fig. 2e. By contrast, molecules without a bright center are assumed to adopt tKH ³FKORULQH-GRZQ´ ELQGLQJ FRQILJXUDWLRQ ZLWK WKH PROHFXOH¶V

Figure 2. (a) STM image showing submonolayer coverage of molecular islands of SubPc on the Ag(100) substrate. (STM Imaging Conditions: V = +1.0 V, I = 150 pA). The molecular superstructure can be discerned, as well as the presence of bright spots at certain molecular locations FRUUHVSRQGLQJ WR WKH 6XE3F PROHFXOHV LQ WKH ³FKORULQH XS´ FRQILJXUDWLRQ E-e) Ball and stick models RI 6XE3F LQ WKH ³FKORULQH GRZQ´ E DQG F DQG ³FKORULQH XS´ G DQG H FRQILJXUDWLRQV atop a Ag(100) crystal lattice.


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axial chorine atom pointing towards the substrate as shown in Fig. 2b and Fig. 2c. The two configurations are roughly equivalent in their prevalence in our STM data. This 1:1 ratio is in line with the small difference of 0.09 eV between these two binding configurations on their calculated up down = 0.19eV vs. Eads = 0.10eV , as will be discussed in detail later. In adsorption energies, i.e., Eads

spite of the energetic near-equivalence of both configurations, molecules were found to aggregate into islands each consisting mostly of a single configuration. The formation of like-configuration islands is clearly illustrated in Fig. 2a, where the island at the right lower quadrant is on the ³chlorine-down´ configuration whereas its counterpart at the left upper quadrant is on the ³chlorine-XS´ FRQILJXUDWLRQ 0RUHRYHU RXU ')7 VLPXODWLRQ LGHQWLILHG WKH LQWHUVWLFHV RI WKH WRSmost layer of the Ag(100) substrate as the preferred binding sites for SubPc. This theoretical prediction was experimentally confirmed by our STM images at a finer spatial resolution (Fig. 3). It is interesting to note that the bright spots in the island at the left upper quadrant (Fig. 2) appear parallelly in 3-nm wide belts separated by ~1 nm. Although the driving force for the periodicity of the like-configuration molecules is not known yet, all STM images that we have gathered so far exhibit the dominance of a single configuration within a molecular island. In an earlier STM study of SubPc on Cu(100) substrate at liquid nitrogen temperatures, the configuration of the surface-bound SubPc molecule could be flipped by scanning a molecular island at varied biases.32 This phenomenon of molecular ³IOLSSLQJ´ ZDV QRW REVHUYHG LQ RXU experiments (Fig. S1 of the Supporting Information) and can be explained by a much tighter packing of SubPc molecules in our zipper pattern compared to the molecular lattice of SubPc on Cu(100). More specifically, the number density of SubPc on Ag(100) turns out to be 0.877 molecule/nm2 which is over 40% denser than that on Cu(100). As a result, the SubPC molecules on Ag(100) would not have the same degree of rotational flexibility as present in the square-



Figure 3. STM image of SubPc molecules on an atomically-resolved Ag(100) substrate. (STM Imaging Conditions: V = +0.5 V, I = 200 pA). The molecular unit cell is shown in blue. latticed superstructure on Cu(100), making them harder to flip during STM scanning. The selfassembled SubPc layer of such dense structure (Fig. 3), to the best of our knowledge, has not been observed in other STM studies on any substrate. Likewise, superstructures that have been previously reported on Cu(100), Ag(111), and Au(111) single-crystalline substrates are not found on our Ag(100) substrate. The SubPc layer on Ag(100) can be considered as stacked chains of molecules mechanically interlocked through their aligned isoindoline lobes (Fig. 3), resulting in a zipper-like pattern. Moreover, the SubPc molecules seem to be slightly tilted with respect to the Ag(100) substrate, making the interlocked isoindoline lobes between adjacent chains closer to the substrate than those interlocked along the same chain. As observed in STM and photoelectron spectroscopy studies, WKH VXEVWUDWH¶V FKHPLFDO LGHQWLW\ KDV D SURIRXQG HIIHct on the self-assembly, or lack thereof, of SubPc molecules on a surface.33 In DGGLWLRQ WR WKH VXEVWUDWH¶V FKHPLFDO LGHQWLW\ facet orientation is another deciding factor in terms of molecular superstructure. For instance, this 8 ACS Paragon Plus Environment

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³]LSSHU´ SDWWHUQ LV TXLWH GLIIHUHQW WKDQ WKH ³KRQH\FRPE´ SDWWHUQ UHSRUWHG RQ both Ag(111)33 and Au(111)34. Fig. 3 depicts the edge of a SubPc island as well as an atomically resolved Ag(100) underlying VXEVWUDWH 6XE3F PROHFXOHV ZLWKLQ WKH ³]LSSHU´ SDWWHUQ DUH VHHQ WR EH DOLJQHG ZLWK WKH lattice direction of the Ag(100) single crystal. This lattice direction match only happens on (100) surface. Thus, both the chemical identity and the facet orientation of the substrate are critical to the nanoscale topology of the self-assembled SubPc layer. 3.2. Resonance Raman spectroscopy (TERS and SERS) A UV-Vis absorption spectrum was collected from SubPc molecules dissolved in chlorobenzene, exhibiting a sharp absorbance peak centered at 566 nm (Fig. 4a). Encouragingly, its associated optical gap of 2.21eV is very close to our calculated E ex s of 2.12 eV (586 nm) for both the first and second lowest-lying electronic transitions. With regard to the near-degeneracy of these transitions in SubPc due to its

C3v molecular symmetry, the simulated resonance Raman

Figure 4. (a) Experimental (red line) and DFT-calculated (black line) UV-Vis absorption spectra of SubPc in chlorobenzene. (b) DFT-calculated exciton map of the first electronic excited state of SubPc. 9 ACS Paragon Plus Environment


spectrum presented in Fig. 5a is actually an equally weighted superposition of two spectra corresponding to

S0 o S1 and S0 o S2 excitations. Based on the absorption spectrum, a 561nm

solid state cw-laser was chosen for our Raman experiments as this photon energy results in real electronic excitation transitions of the SubPc molecule, leading to resonance Raman and a higher yield of inelastically scattered light per incident photon.35-36 As illustrated in Fig. 4b, both the calculated highest occupied natural transition orbital (HOMO) and its lowest unoccupied counterpart (LUMO) for the excitation are significantly delocalized onto the isoindolyl moiety, giving rise to a large transition dipole moment that is perpendicular to the axial B-Cl bond of SubPc. UHV SERS, UHV TERS, (Fig. 5a) and ambient SERS experiments (Supporting Info. Fig. S2) were performed with a 561 nm laser. As expected, the enhancement of the collected TERS signal is very impressive with an A.D.U. of over 600 counts·mW-1·s-1. This amplified signal is

Figure 5. (a) Experimental UHV TERS (black line = tip retracted, red line = tip engaged) and UHV SERS (pink line) vibrational spectra of SubPc compared to calculated (blue line) vibrational modes of an isolated SubPc molecule in free space. (b) 701 cm-1 vibrational mode model as calculated by DFT. 10 ACS Paragon Plus Environment

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attributed both to the already gainful signal-boosting benefits of resonance Raman as well as the unique design of our chamber, namely the large numerical apertures of our excitation and Raman signal collection lenses as discussed in the Experimental Methods Section. More than twenty discrete vibrational peaks were identified in our TERS spectra, which were acquired through STM scanning over molecular islands with likewise binding configurations to the Ag(100) substrate. Interestingly, the vibrational spectra exhibit no discernable differences in peak position and/or intensity between WKH ³FKORULQH-XS´ DQG ³FKORULQH-GRZQ´ ELQGLQJ FRQILJXUDWLRQV (Supporting Info. Fig. S3). In addition to our highly spatially resolved STM experiments, our UHV SERS experiments conducted an ensemble Raman measurement of many molecular islands over a large DUHD RI WKH VXEVWUDWH ODVHU IRFXV VSRW GLDPHWHU RI

P DV RSSRVHG WR Whe sub-nanometer spatial

resolution that TERS experiments are known for. Once again, the peaks in our UHV SERS spectra are aligned with those in our TERS spectra (Fig. 5a). This excellent agreement led us to conclude that the coupling between a SubPc molecule and its substrate are practically indistinguishable for both of its binding configurations. If they were distinguishable, the UHV SERS spectra would show some peaks other than those in TERS since both binding configurations should have been sufficiently sampled. Aiming to exclude the possibility of SubPc alteration and/or degradation during UHV deposition, we also performed ambient SERS measurements. In those controlled experiments, SERS samples were prepared by soaking a commercially available SERS substrate in a solution of SubPc dissolved in chlorobenzene while the UHV samples had SubPc sublimed onto them under ultrahigh vacuum. Because both ambient and UHV SubPc deposition methods yielded similar Raman spectra (Fig. S2 of the Supporting Information), SubPc did not seem to degrade during its high-temperature sublimation onto the Ag(100) single crystal and UHV silver film-over-nanosphere (AgFON) substrates.



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We then interpreted our TERS results by comparing them with our calculated Resonance Raman spectrum for an isolated SubPc molecule in free space. As shown in Fig. 5a, the alignment between these two spectra is quite good, suggesting that the underlying Ag(100) substrate does not interact strongly with SubPc as also implied by the indistinguishaELOLW\ RI WKH PROHFXOH¶V WZR binding configurations. Moreover, the intermolecular interactions between SubPc molecules within an island on the Ag(100) substrate are also indicated to be relatively weak. Otherwise, the strong couplings between SubPc molecules would have notably deviated their vibrational frequencies from those for an isolated molecule. Since the type of adsorption between SubPc molecule and Ag(100) substrate may decide the exact mechanism of TERS and SERS, we evaluated the adsorption energies,

configurations (Fig. 2):

Eads

IRU ERWK WKH ³FKORULQH-XS´ DQG ³FKORrine-GRZQ´

Eads = ESubPc + EAg(100) - ESubPc/ Ag(100) where ESubPc ,

E Ag(100) , and

ESubPc/ Ag(100) are the optimized energies of an isolated SubPc molecule, a 20.43 Å × 20.43 Å × 4.38

Å $J has

VODE DQG WKHLU DGGXFW UHVSHFWLYHO\ ,W ZDV IRXQG WKDW WKH ³FKORULQH-XS´ configuration a

slightly

higher

adsorption

energy

than

its

³FKORULQH-GRZQ´

counterpart

(

down up = 0.10eV ), its feeble affinity to the Ag slab is likely induced by their Van = 0.19eV vs. Eads Eads

der Waals interactions rather than through interfacial charge transfer or electron sharing. As a result, the amplified SERS signals of SubPc should primarily stem from the locally enhanced electromagnetic fields,37 WKXV ODUJHO\ SUHVHUYLQJ WKH PROHFXOH¶V UHODWLYH UHVRQDnce Raman intensities when placed onto the Ag slab. According to a short-time approximation,38 the relative resonance Raman intensity, I j , for a vibrational normal mode, q j , is proportional to where w j is its angular frequency, and D j is its excited-state vibronic displacement.


w 2j D2j

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Although most prominent peaks in our calculated Resonance Raman spectrum can find their counterparts in our experimental TERS spectrum, one interesting exception is the vibrational mode at 701 cm-1 (Fig. 5b). This peak appears to be blueshifted to 738 cm-1 in our TERS spectra, suggesting a substrate effect that is only remarkable for specific vibrational modes. If one takes a closer look at this blueshifted vibrational mode, it is easy to find that it is dominated by the stretching of the axial B-Cl bond. Therefore, this bond-stretching mode should be much more susceptible to frequency shift than others due to the plausibly strong coupling between the Ag(100) substrate and the highly polarizable chlorine atom of SubPc regardless of its binding configuration.39 Although the relative peak intensities of the experimental and calculated spectra show some discrepancies, particularly in the low-energy mode regime, it is critical to consider the enhancement mechanism present in the TERS and SERS experiments for a fair comparison. The polarization of the incident laser in our experiments is always parallel to the scanning tip to create D ³KRW VSRW´ EHWZHHQ WKe tip-sample junction. $OO ³RII-SODQH´ YLEUDWLRQDO PRGHV RI 6XE3F ZLWK prominent normal vectors perpendicular to the surface are subject to a much stronger HOHFWURPDJQHWLF HQKDQFHPHQW RI WKHLU 5DPDQ VLJQDOV ZKHQ FRPSDUHG WR WKHLU ³LQ SODQH´ counterparts.40 As a consequence of ignoring the directionality of incident light, our DFT calculation on an isolated SubPc molecule in free space might yield different relative Raman intensities.41 Please note that low-energy phonon modes of the molecular superstructure could also contribute to the SERS and TERS spectra.42 However, we do not think their contributions are significant in the present study as all peaks in the low-energy domain of our experimental spectra can be well resolved by free-space DFT calculations.



4. CONCLUSIONS Taken all together, the growth of SubPc deposited on a Ag(100) surface has been characterized by STM, TERS, and SERS experiments in conjunction with DFT calculations. Our STM images reveals a molecular superstructure consiVWLQJ RI ³chlorine-up´ and ³chlorine-down´ binding configurations in equal abundance. More interestingly, SubPc molecules of same binding configuration prefer to aggregate with each other to form like-configuration islands, resulting in highly polarized molecular domains with large collective dipole moments. Finally, both TERS and SERS experiments yielded nearly identical vibrational spectra for both binding configurations, consistent with their small adsorption energies (

Substrate Characterized by Scanning Tunneling Microscopy, Tip

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