Effect of Dipolar Molecule Structure on the Mechanism of Graphene

Article pubs.acs.org/JPCC

Effect of Dipolar Molecule Structure on the Mechanism of GrapheneEnhanced Raman Scattering Yongho Joo,†,∥ Myungwoong Kim,‡,∥ Catherine Kanimozhi,† Peishen Huang,† Bryan M. Wong,§ Susmit Singha Roy,† Michael S. Arnold,† and Padma Gopalan*,† †

Department of Materials Science and Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States Department of Chemistry, Inha University, Incheon 22212, Korea § Department of Chemical and Environmental Engineering, and Materials Science and Engineering Program, University of CaliforniaRiverside, Riverside, California 92521, United States ‡

S Supporting Information *

ABSTRACT: Graphene-enhanced Raman scattering (GERS) is a promising characterization technique which uses a single layer of graphene. As the electronic coupling of adsorbates with graphene leads to enhancement in the Raman signal, it is of immense interest to explore the factors that affect the coupling of the adsorbates with graphene. To probe this effect we have designed and synthesized a series of dipolar molecules with the general structure, N-ethyl-N-(2ethyl(1-pyrenebutyrate)-4-(4-R-phenylazo)aniline) where the Rgroups are varied from methoxy (−OCH3), methyl (−CH3), hydrogen (−H), nitrile (−CN), nitro (−NO2) to tricyanofuran (TCF) groups. This systematically changes the dipole moments and electronic/optical band gap of the molecules. By noncovalently interfacing these molecules on graphene, the Raman signal is enhanced by a factor of 40−90 at the excitation wavelength of 532 nm. Measurements of the Raman enhancement factor and Raman cross section are complemented with DFT calculations to correlate the dipole moment and the energy level of the hybrid to the Raman scattering efficiency. These studies highlight the relevance of the dipolar nature of chromophores, which determines their dipole moment and the band gap, and the resulting electronic coupling to graphene which simultaneously alters the energy level of the orbitals in the molecule and the Fermi level in graphene, resulting in efficient Raman excitations and GERS.

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INTRODUCTION Surface-enhanced Raman scattering (SERS) is a powerful analytical tool for characterizing the structure of materials, by enhancing the Raman signal of adsorbed molecules by 10−107 on metal nanostructures.1−3 In general, SERS has been understood using two widely recognized mechanisms: (1) electromagnetic mechanism (EM),4 associated with a large Raman enhancement factor for molecules on the surface of metal substrates. This is mainly attributed to the enhancement of the electromagnetic fields by plasmon resonance excitation. (2) Chemical enhancement mechanism (CM) associated with chemisorption of molecules on the substrate.5−7 For SERS, nanostructured metal substrates are important to induce a large Raman enhancement factor of up to 107 or more by the EM effect.3 SERS on a metal substrate has been widely utilized in practical applications;8 however, a new type of Raman enhancement technique is needed to circumvent some possible detection problems. For example, for analyzing species that react with SERS substrates it has been shown that photoinduced surface reaction of adsorbates with metal substrates which occur in the course of Raman excitation leads to © XXXX American Chemical Society

photocarbonization and accordingly produces quite intense background noise and signal contamination.5,9−11 A Raman enhancement technique such as grapheneenhanced Raman scattering (GERS) has emerged as a promising nonmetal-based technique,12,13 as graphene can be easily grown and transferred to desired surfaces for Raman measurements9,14,15 and results in a reliable and consistently enhanced Raman signal.16,17 Furthermore, GERS effect occurs over the entire graphene surface, which is promising for largearea analyte detection platforms, whereas SERS effect on metal nanostructures occurs only over a small area. As a result, a number of applications for GERS have been introduced, for example, probing molecular orientation at an interface18 and surface characterization of flexible substrates.9 Also, it has been shown that GERS effect can be further amplified by combining with metallic substrates.5,9,19 Received: April 23, 2016 Revised: June 4, 2016

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DOI: 10.1021/acs.jpcc.6b04098 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic illustration depicting the Raman experimental setup for the dipolar molecules on graphene/SiO2/Si substrate in GERS using different types of dipolar molecules DROMeP, DRMP, DRHP, DRCP, DR1P, and TCFP with different functional groups on graphene.

Scheme 1. Synthesis of DROMeP (2a), DRMP (2b), DRHP (2c), DRCP (2d), DR1P (2e), and TCFP (2f)

from the chromophore on single-layer graphene by a factor of 10−70 in the trans form, with a corresponding increase in the absolute Raman cross section as well.6 Further density functional theory (DFT) studies on a large graphene fragment suggested that the electronic structure of the adsorbate and graphene significantly changes upon hybridization. Based on these studies we proposed that electronic coupling between the highly polarizable molecule and graphene led to Raman enhancement as well as doping in graphene.6,25 Our hypothesis based on previous studies was that the observed Raman scattering was enhanced by the ability of the adsorbed molecule to dope graphene. Since the dipolar nature of the molecule led to p-doping in graphene,6,24 we expected that the magnitude of Raman enhancement could be influenced by the magnitude of the dipole moment of chromophores. To test this initial hypothesis, we have designed and synthesized six different pyrene-tethered azobenzene chromophores with different tail groups at the para position of the benzene ring terminating the azobenzene, namely: methoxy (DROMeP), methyl (DRMP), hydrogen (DRCP), nitrile (DRHP), nitro (DR1P), and tricyanofuran group (TCFP). The rationale for the chemical modification of the molecular structure is to systematically tune the strength of the electron-withdrawing

Contrary to conventional SERS, GERS does not rely on electromagnetic enhancement since intrinsic graphene plasmons relevant to electromagnetic amplification cannot be generated in visible light.16 Rather, the effect of interfacial interactions between adsorbates and graphene is known to be more relevant to the mechanisms in GERS. Ling et al. have shown that Raman enhancement is affected by the position of HOMO and LUMO levels of adsorbed molecules with respect to the Dirac point of pristine graphene, which is a comparable explanation to that of CM in SERS.9,16,20 Strong quenching of adsorbate fluorescence has also been described as an additional mechanism to explain Raman enhancement on graphene.21,22 It is noted that most molecules examined for GERS are either phthalocyanines or metallophthalocyanine or R6G-type molecules. Recently, we have reported that the electronic properties of graphene can be modulated by latching a highly polarizable pyrene-tethered azobenzene chromophore (DR1P), on nanocarbon materials.23 Specifically, light-driven molecular transformation of the highly polarizable dipolar molecules on graphene provides an additional chemical handle to tailor the doping of graphene, i.e., charge carrier concentration.24 Interestingly, this system exhibited an enhanced Raman signal B


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The Journal of Physical Chemistry C Scheme 2. Synthesis of Diazonium Salt Precursor (Compound 4)

dicyclohexylcarbodiimide (DCC) (22 mmol), and 4(dimethylamino)pyridine (DMAP) (4 mmol) were mixed in anhydrous CHCl3 and then stirred for 18 h. After the precipitate was filtered, the crude solution was collected and concentrated with a rotary evaporator. Resulting sticky liquid was further purified in a silica gel column with hexane/CHCl3 (3/1 v/v) as the eluent. Pure products were obtained as different colored solids (yield: 2a 50%, 2b 40%, 2c 42%, 2d 35%, 2e 40%, and 2f 30%) Synthesis of Compound 1f. To a mixture of 3 (4.8 mmol) in 20 mL of acetic acid was added 0.6 mL of concentrated HCl aqueous solution. The resulting clear solution was stirred at 0 °C for 15 min. Aqueous NaNO2 (5.8 mmol) solution was added slowly to the mixture at 0 °C, resulting in a dark yellow orange color. The mixture was further stirred at 0 °C for 30 min. Aqueous NaPF6 (6.8 mmol) solution was added to the mixture, followed by stirring for another 30 min. The resultant yellow residue was filtered and washed with diethyl ether to obtain compound 4 as a yellow solid. Compound 1f was synthesized with the modified procedure from a previous report.26 2-(N-ethylanilino) ethanol (1 mmol) was dissolved in 10 mL of acetic acid at room temperature, and (1.5 mmol) of compound 4 was added to the mixture. NaOAc (2.90 mmol) was added in two portions, followed by stirring overnight. The reaction mixture was extracted with dichloromethane and washed with saturated aqueous NaHCO3 solution and water. The organic layer was dried over MgSO4, followed by evaporating the residual solvent. The crude solid was recrystallized from dichloromethane/hexane (1/5 v/v) to obtain compound 1f as a red solid (yield: 50%). Preparation and Characterization of Hybrid Samples. An amount of 20 μL of chromophore solution (DROMeP, DRMP, DRCP, DRHP, DR1P, and TCFP) in dichloromethane with concentration of 5 × 10−4 M was spin-coated at 4000 rpm for 60 s onto graphene samples on 90 nm SiO2/Si substrates, prepared by conventional mechanical exfoliation of graphite. Raman spectra of DR1P/graphene samples were obtained using an Aramis Horiba Jobin Yvon Confocal Raman Microscope. Excitation laser sources (6 W laser power) at 532 nm were used with a 100× objective lens providing a probe size of ∼1 μm2. Wavenumber calibration was carried out using a Si peak at 520 cm−1 as a reference. The spectra were deconvoluted by fitting with the Voigt function, and characteristic peaks were identified. All of the data points in the Raman analyses represent averages of characteristic peaks from five spots. X-ray photoelectron spectroscopy (XPS) spectra of hybrid samples were measured with a Thermo Scientific K-alpha XPS under an Al Ka X-ray source. Survey spectra were collected with pass energy of 50 eV. For XPS studies, a single-layer graphene sample was synthesized by chemical vapor deposition (CVD)

group (EWG) or electron-donating group (EDG) to alter the magnitude of the dipole moment and hence the electronic coupling of chromophores with graphene (Figure 1). The hybrid system of the synthesized chromophores and graphene exhibited a systematic enhancement of the Raman signal; i.e., the enhancement factor and absolute Raman cross section increase as dipole moment of adsorbate increases. Furthermore, DFT studies demonstrate that the required energy to excite electrons in hybridized electronic structure, i.e., energy gap between LUMO and Fermi level, is influenced by the variation of dipole moment of the adsorbates, supporting our experimental findings that correlate the molecular structure with Raman enhancement. Ultimately, we confirm our hypothesis, with this unique series of polarizable molecules with increasing strength of electron-withdrawing groups, that interfacial electronic coupling is one of the important parameters to enhance the Raman signal.

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EXPERIMENTAL SECTION Chromophore Synthesis. All chemicals were purchased from Sigma-Aldrich and used as received unless noted. Three different chromophores DRCP, DRMP, and DR1P were synthesized by the methods reported previously.23 Dipolar chromophores DROMeP, DRHP, and TCF were synthesized by following previous literature procedures23,26 (Scheme 1). Compound 1f (TCFP) was synthesized through a modified procedure following Scheme 2.26 All NMR characterization data of DRHP, DROMeP, and TCFP are reported in the Supporting Information. General Synthesis of Diazo Compounds (1a−e) via Diazonium Coupling. 4-R-Aniline (100 mmol) was suspended in 100 mL of H2O by vigorous stirring at room temperature. An amount of 19.5 mL of concentrated aqueous HCl was diluted in 50 mL of H2O and then added to the suspension, resulting in 4-R-anilinium chloride as a clear solution. An ice-cold solution of NaNO2 (110 mmol) in 40 mL of H2O was added slowly to the 4-R-anilinium chloride solution in 0 °C bath and stirred for 15 min. The resultant mixture was added to a cold solution of 2-(N-ethylanilino)ethanol (100 mL) in glacial acetic acid (40 mL) and H2O (100 mL) at 0 °C. The reaction was stirred in the cold bath for 5 min and then neutralized by saturated NaHCO3 solution. Precipitate was formed during the neutralization as the final product, followed by filtering and subsequent washing with water. All the compounds can be recrystallized in THF/hexane (1/10 v/v) to reach high purity (yield: 1a 60%, 1b 80%, 1c 75%, 1d 80%, 1e 60%). General Synthesis of Pyrene-Tethered Dipolar Chromophores (2a−f) via Steglich Esterification. Compound 1a−f (20 mmol), 1-pyrenebutyric acid (20 mmol), N,N′C


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Table 1. List of Chromophores, Their Chemical Structures, Computed HOMO/LUMO Levels without Graphene, Computed Fermi Level of Isolated Graphene, and Magnitude of the Dipole Moment from DFT Calculationsa

a

Computed energy level of the Dirac point in isolated graphene is −4.27 eV.

Figure 2. (a) Photograph showing six different chromophore solutions in dichloromethane (concentration of 5 × 10−4), (b) absorption spectra of the chromophore solutions (inset: absorption spectra between 300 and 360 nm, which is attributed to pyrene tether), and (c) optical (black dot) and electronic (red dot) band gap of chromophores. Optical band gap was calculated using the Tauc model,32 and electronic band gap was determined by DFT calculations.

structure of graphene (i.e., the Dirac point and electronic level crossings), and the current DFT calculations provide a more realistic description of the dipolar molecular interactions with a graphene surface. Furthermore, to capture the noncovalent interactions of the various adsorbates with the graphene sheet, we utilized the dispersion-corrected PBE-TS exchangecorrelation functional, which is specifically designed for accurately describing noncovalent interactions in a variety of chemical environments.28,29 Geometry optimizations for all the isolated dipolar molecules and the dipolar molecule/graphene system were performed, and high-quality triple-ζ basis sets (i.e., the “tier 2” basis sets from the FHI-AIMS repository) were

on copper foil. After the growth, graphene on copper foil was transferred to a 90 nm SiO2/Si substrate using poly(methyl methacrylate) (PMMA) as a sacrificial support, following the literature procedure.6,23,27 Density Functional Theory Calculations. To take into account both the electronic band structure of graphene and the molecular energies of the chromophores, first-principles periodic DFT calculations were carried out for all six hybrid systems. In a previous work, we utilized nonperiodic DFT methods to calculate the energy levels of an isolated DR1P molecule adsorbed on a finite-sized graphene flake.6 However, these previous calculations did not capture the electronic band D


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Figure 3. Raman spectra of six different chromophores on graphene with 532 nm laser excitation. (a) Red for DROMeP, (b) orange for DRMP, (c) green for DRHP, (d) blue for DRCP, (e) purple for DR1P, and (f) black for TCFP. Black lines in all spectra represent Raman spectra of chromophores on a SiO2/Si substrate without graphene. (g) Raman enhancement factor of the v(NN) mode of different chromophores on graphene with respect to Si/SiO2 substrate. Peak intensities of chromophores on the Si/SiO2 substrate without graphene are set to “1” to calculate Raman enhancement factor.

interaction of pyrene tether31 with graphene leads to the formation of a stable film of dipolar molecules on graphene. The optical properties of the synthesized chromophores were characterized using UV/vis absorption spectroscopy (Figure 2b). Strong absorption in the 300−350 nm range for all the chromophores was assigned to S0 −S 2 transition from pyrene.33,34 DROMeP, DRMP, and DRHP exhibit yellow color in dichloromethane solution which corresponds to the absorption at λmax about 410 nm, assigned to the absorption of the Benzenamine unit.35 Hence, the variation of EDG in the chromophore structure (terminal hydrogen, methyl, and methoxy groups) does not influence the optical band gap greatly (Figure 2c), which was also confirmed by a theoretically determined electronic band gap. However, introducing nitrile, nitro, and tricyanofuran groups rather than methyl or methoxy groups as a terminal group evokes a significant red-shift of 36, 112, and 148 nm, respectively, with a λmax of 446 nm for DRCP, λmax of 522 nm for DR1P, and λmax of 558 nm for TCFP due to their stronger polarization ability through the π-conjugated bridge. Consequently, both the optical band gap and electronic band gap decrease with increase in the strength of EWG in order of DROMeP ∼ DRMP ∼ DRHP > DRCP > DR1P > TCFP, as shown in Figure 2c. The calculated dipole moment of chromophores is also affected significantly by changing the functional group from EWG to EDG as shown in Table 1. Although the DROMeP chromophore has an electron-donating methoxy tail group, the direction of the dipole moment vector stays away from the pyrene tether (head) toward the negative

used to calculate dipole moments, orbital energy levels/band structures, and electron densities. For all of the hybrid systems, we modeled the graphene substrate as a 6 × 6 periodic rhombus fragment, which was sufficiently large enough to interact with the pyrene tether.

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RESULTS AND DISCUSSION Synthesis of Chromophores and Their Optical Properties. In dipolar molecules the molecular dipole moment is typically determined by the position and strength of EDG and EWG and the distance by which they are separated.30 Also, the bridge between the EDG and EWG should be electronically conjugated. Based on our earlier study of the molecule/ graphene hybrid system where the molecule was DR1P, we designed five other molecules (Figure 1 and S1) using the general synthetic route outlined in Scheme 1. This strategy allowed us to not only retain the pyrene-tethered azobenzene structure but also effectively tune the dipole moment by fixing the EDG to the tertiary amine and varying the EWG. Another essential feature of these molecules is the common NN in the azobenzene moiety that enables tracking the same characteristic Raman mode, i.e., v(NN) mode, in all the molecules for systematic Raman enhancement studies. The terminal end functionality (tail group) was tuned from EDGs such as methoxy (−OCH3), methyl (−CH3), and hydrogen (−H) to increasingly EWGs such as cyano (−CN), nitro (−NO2), and tricyanofuran (−TCF) to systematically control molecular dipole moment (Table 1). In addition, strong π−π E


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Figure 4. Deconvoluted Raman spectra of different chromophores on graphene. (a) DROMeP/G (graphene), (b) DRMP/G, (c) DRHP/G, (d) DRCP/G, (e) DR1P/G, and (f) TCFP/G. Voigt function was utilized to identify characteristic peaks. The G-bands from graphenes and the v(N N) vibration mode of chromophores are marked in the plot.

(R6G), and dipolar chromophore (DR1P).5,6,13,21 Remarkably, the highest EF in this study (TCFP/graphene) is about 1.5 times higher than typical literature values. As shown in Figure 3g, DR1P and TCFP exhibit far higher EFs than others, underscoring the significance of molecular structural effects on Raman enhancement. In conventional SERS, a Raman enhancement event occurs through a combination of dominant EM contribution and relatively weak CM contribution. It has been challenging to isolate the contribution of CM from the combined Raman signals in Raman scattering experiments. However, any contributions from EM enhancement are ruled out in GERS due to the absence of surface plasmon excitation in the visiblelight range for graphene; hence, the effect of the interaction between adsorbate and substrate can be isolated.16 Electronic coupling of the adsorbate to graphene has been predominantly evoked to explain GERS phenomena, as it modifies the electronic structure and subsequently changes HOMO, LUMO, and Fermi level upon functionalization. These changes result in efficient Raman scattering and, therefore, an increase in the Raman cross section.40−42 Quantitative Analyses for Raman Cross Section of Adsorbed Chromophores on Graphene. The Raman cross section of the chromophores on graphene was quantitatively analyzed using the method previously reported by Thrall et al.21 The characteristic peaks were identified by fitting the spectra with the Voigt function, and then peak intensities were obtained by integration of the identified peaks (Figure 4). We specifically analyzed the v(NN) mode to compare the Raman cross section of all the chromophores (DROMeP/ graphene: 1379 cm−1, DRMP/graphene: 1390 cm−1, DRHP/ graphene: 1382 cm−1, DRCP/graphene: 1374 cm−1, DR1P/

end of the molecule containing functionalized azo-benzene (tail). Furthermore, our DFT calculations confirm that all the chromophores exhibit the same direction of the dipole moment vector. Since DR1P p-dopes graphene when it is latched on graphene,6,24 we anticipate that the molecular dipoles from the other five molecules would also act as p-type dopants in the hybrid system. Enhanced Raman Signal from Chromophores on Graphene. Figure 3 represents the Raman spectra of dipolar molecules latched onto graphene on Si/SiO2 or on the Si/SiO2 substrate without graphene, measured with a 532 nm excitation wavelength. Chromophores were deposited on graphene samples by spin-coating of chromophore solutions (5 × 10−4 M), following the procedure reported earlier.6 The graphene pieces in Figure S2 were confirmed to be a single layer by observing a single symmetric peak with Lorentzian line shape of the 2D band exhibiting higher intensity compared to the G band.36,37 Figure 3 clearly shows intensity enhancement: the Raman intensities of DROMeP (red line), DRMP (orange line), DRHP (green line), DRCP (blue line), DR1P (purple line), and TCFP (gray line) on single-layer graphene are much higher than the intensities on the SiO2/Si surface without graphene (black spectra in Figure 3). We further calculated the enhancement factors (EFs) for the characteristic v(NN) mode of six different chromophores, which typically appear at around ∼1375 cm−1 from all chromophores.38,39 Normalized intensities of v(NN) with respect to the intensity on SiO2/Si without graphene (Figure 3g) show that the EFs of the v(NN) mode for chromophores on graphene vary from a minimum of 40 to a maximum of 90. Typical EFs in GERS have been reported to be between 0.3 and 70 with organic molecules such as CuPc, Rhodamine 6G F


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Figure 5. (a) Peak positions of the G and 2D band of pristine graphene and different chromophores on graphene. (b) G band shifts as a function of band gap of the hybrid system (red) and dipole moment of chromophores (black). (c) Raman cross section of chromophores functionalized with different dipolar molecules as a function of the magnitude of dipole moment (black), band gap of hybrid system (red), and energy gap between the LUMO and the Fermi level of hybrids, ELUMO − EF (blue).

graphene: 1382 cm−1, and TCFP/graphene: 1384 cm−1). In all spectra, the intense peaks around 1590 cm−1 were assigned as the G band. The absolute Raman cross section of an adsorbed chromophore on graphene is given by εchromophore =

Achromophore AG

εGCG 1 Cchromophore (αL + αR )

Cgeom =

1 [4 − 3 cos θmax − cos3 θmax 8 + 3(cos3 θmax − cos θmax )cos2 Δ]

where Δ is the angle describing the orientation of dipole moment with respect to the symmetry axis of the collection optics, and θmax is the maximum collection angle.45 We calculated the Cgeom value of 0.216 sr−1 considering the parameters of θmax = 48.56° and Δ = 90° for chromophores on graphene. Dividing the calculated differential cross-section of chromophores by Cgeom yields an integrated cross section of 3.29−5.39 × 10−24 for DROMeP, 3.47−5.69 × 10−24 for DRMP, 1.94−3.19 × 10−24 for DRHP, 7.29−12.0 × 10−24 for DRCP, 8.7−14.3 × 10−24 for DR1P, and 1.22−2.00 × 10−23 for TCFP (unit: cm2·molecule−1). All resulting values and the parameters used in the analysis are summarized in Table S1. Effect of Dipole Moment: Correlation of Molecular Structure to Raman Scattering. The highly polarizable dipolar molecule, DR1P, is able to strongly p-dope graphene, consequently leading to large shifts in G and 2D band positions.24,46 Since the dipole moment was controlled by structural variation, corresponding systematic peak shifts in G and 2D bands are expected (Figure 5a), Typical G and the 2D band positions of pristine graphene are at 1584 and 2671 cm−1; however, the G band up-shifted by 4−8 cm−1, and the 2D band also up-shifted by 5−10 cm−1 upon functionalization with chromophores. The upshift of G and 2D bands confirms the pdoping of graphene upon functionalization.46 The shifts in G band also scale with the dipole moment of the adsorbed molecule and the band gap of the hybrid system (Figure 5b). It is clear that the upshift in G band scales with the dipole moment of the adsorbate, indicating increased hole concentration. The hole concentration was estimated using the empirical relation between charge carrier concentration and G band position in the literature,46 to be in the range of 3−6 × 1012 cm−2. Therefore, our experimental findings show that stronger dipole moment in chromophores leads to higher doping level. We further correlated theoretically determined dipole moment and band gap of adsorbed molecules chemisorbed on graphene with experimentally measured Raman cross sections. The Raman cross section also increases with the dipole moment of the molecules (Figure 5c). Generally, a

(1)

where εchromophore and the εG are the differential Raman cross section; Cchromophore and CG are the concentrations; and Achromophore and the A G are the Raman intensities of chromophore and graphene, respectively. 6,21,43 The Achromophore/AG ratio was determined by dividing the integrated intensity of the NN stretch mode of the chromophore at around 1390 cm−1 with the integrated intensity of the G band of graphene at around 1590 cm−1, shown in Table S1. The surface coverages (DDROMeP = 2.21 × 1013; DDRMP = 1.13 × 1013; DDRHP = 1.82 × 1013; DDRCP = 9.77 × 1012; DDR1P = 9.62 × 1012; DTCFP = 8.25 × 1012 (unit: molecules·cm−2)) were calculated from XPS using the integrated intensity ratio of the N(1s) peak to C(1s) peak.6,24 These surface coverage values are close to the monolayer coverages reported in our previous report.6,24 The value of εGCG(αL + αR)−1 (αL and αR: the terms related to laser penetration depth and Raman scattered light penetration depth in carbon substrate) for graphene is 2.7−4.43 × 10−11 sr−1, which was estimated using the εGCG(αL + αR)−1 value of the highly ordered pyrolytic graphite (HOPG) found to be 3.3−5.4 × 10−11 sr−1 from Kagan et al.43 and Wada et al.44 By taking the optical interference effects of Raman scattering from graphene, the value from HOPG was used to find εGCG(αL + αR)−1 for graphene, resulting in 1.22 times smaller value than that of graphite.6 Using the value of 2.7−4.43 × 10−11 sr−1 for εGCG(αL + αR)−1 and eq 1, we calculated the differential Raman cross section of chromophores, εDROMeP = 0.71−1.16 × 10−24; εDRMP = 0.75−1.23 × 10−24; εDRHP = 4.20− 6.89 × 10−25; εDRCP = 1.57−2.58 × 10−24; εDR1P = 1.88−3.08 × 10−24; and εTCFP = 2.63−4.31 × 10−24 (unit: cm2·sr−1· molecule−1). The integrated Raman cross section was calculated by dividing the differential Raman scattering cross section by Cgeom, the geometrical collection efficiency, given by the equation below G


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Figure 6. Electronic band structures of the various hybrid systems obtained with DFT calculations. (a) DROMeP/G, (b) DRMP/G, (c) DRHP/G, (d) DRCP/G, (e) DR1P/G, and (f) TCFP/G.

tally determined (Figure 5c). It is also worth noting at this point that the DFT calculations only give electronic properties for geometries at a local minimum for a specific molecular configuration such as molecular orientation of chromophores or the intermolecular interaction and packing between on graphene. For example, the DFT calculations do not account for variations in molecular packing between chromophores, which comprises a high-dimensional phase space that cannot be routinely explored with the current DFT methods. Also, molecular orientation of chromophores taken in this calculation may not represent actual orientation.23,24,48 Nevertheless, the good agreement between theory and experiment for the dipole moments, energy levels, and shifts in the graphene Fermi levels gives us confidence that most of the underlying physics is accurately captured by the DFT calculations for each of the six hybrid structures. We also note that understanding contributions from other effects such as resonance Raman scattering or fluorescence quenching to the Raman enhancement in current systems and isolating electronic coupling effect solely from those effects are still a challenge. This is mainly because of two reasons: (i) electronic coupling of molecules to graphene will likely change their absorption spectra, which is hard to measure in monolayer concentrations, and (ii) the practical limitation of accessing a broad range of Raman excitation wavelengths.

stronger dipole moment in the chromophore tends to exhibit a higher Raman cross section. Hence, an increase in dipole moment leads to a simultaneous increase in hole concentration in graphene and an increase in the Raman cross section of the molecule. It is also worthwhile to note that the Raman cross section decreases when the band gap of an adsorbed molecule increases. The relationships between molecular structure and electronic structure (HOMO and LUMO levels, electronic band gap) and the Fermi level of graphene have been known as one of the important parameters dictating the efficiency of Raman scattering.20,40,47 In our system, an electronic coupling upon functionalization should lead to a modified electronic band structure. Modified Electronic Structure in a Chromophore/ Graphene Hybrid. Figure 6 shows the electronic structures within the first Brillouin zone for each of the six hybrids obtained by DFT calculations. Note that the Fermi level for each of the hybrids still lies close to the Dirac point of graphene in the molecular orientations employed for DFT calculations (Figure S4); however, the position of the Fermi level relative to vacuum is slightly different among the chromophores. We observed two changes in the energy levels of the hybrid: one minor effect due to the HOMO energy and a larger effect arising from the LUMO in their electronic structure upon functionalization (Table S2 and Figure S4): The first smaller effect is the progressive lowering of the HOMO energy of the molecule with increase in the dipole moment. A larger effect is the energy difference between the LUMO of the molecule and the Fermi level (i.e., ELUMO − EF) with increasing dipole moment (Table S2). Specifically, this energy gap decreases rapidly with increasing dipole moment, which is also in agreement with the observed trends in the G band shifts and previous studies of ELUMO − EF energies of phthalocyanine molecules by Dresselhaus et al.20 Similarly the energy difference (ELUMO − EF) modulation resulting in 30% enhancement in Raman intensities for a given metal phthalocyanine (M-Pc) molecule40 on graphene using gate voltage (with a fast sweep rate) has been demonstrated. We have shown using targeted synthesis of a series of molecules that a much larger (90 times) enhancement can be obtained by tuning the dipole moment of the molecule. Finally, all molecular characteristic parameters that have been discussed, i.e., molecular dipole moment, band gap, and energy gap (i.e., ELUMO − EF), in hybrids exhibit strong correlations with Raman cross sections that were experimen-

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CONCLUSIONS Through a carefully controlled study using both high-resolution spectroscopic and DFT calculations, we have shown the influence of adsorbate coupling effects in GERS. These studies were conducted on a unique set of dipolar chromophores with systematic variations in dipole moment, which allowed us to map out the mechanism of GERS. Experimental measurements of Raman enhancement factor and Raman cross section were complemented with DFT calculations of dipole moment and energy level of hybrids. Our results show that Raman cross section was affected by the dipole moment of adsorbates and the electronic structure of the hybrid. The dipole moments of the chromophores shift the energy levels of the hybrid structure as a dipole effectively serves as an external electrostatic perturbation. For the series of dipolar molecules studied as the tail group is varied, the increased dipole moment of the adsorbate was found to have a larger effect on the energy gap ELUMO − EF of the hybrid, resulting in higher efficiency of Raman scattering. We have also demonstrated one of the H


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highest Raman enhancement factors up to 90 by increasing the molecular dipole moment with tricyanofuran-functionalized chromophore (TCFP). Though these series of molecules were based on azobenzenes these studies can be potentially generalized to other classes of dipolar molecules such as those based on stilbenes. These studies emphasize the importance of the interfacial electronic coupling between the adsorbate and graphene, which is greatly affected by the molecular structure of the adsorbate, for highly enhanced Raman signal gained in GERS platform. Hence, the molecular structure can be rationally tuned and combined with back gate driven changes in energy levels to further amplify the Raman signal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04098. NMR data of synthesized chromophores, XPS data of chromophore/graphene hybrids, optical images of pristine graphene samples and corresponding Raman spectra, the parameters used for Raman cross section calculation, and DFT calculation results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-608-265-4258. Author Contributions ∥

Y.J. and M.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synthesis of the chromophores and physical characterization is supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Science, U.S. Department of Energy under award No. ER46590. B.M.W. acknowledges the National Science Foundation for the use of supercomputing resources through the Extreme Science and Engineering Discovery Environment (XSEDE), Project No. TGCHE150040. S.S.R. and M.S.A. acknowledge support from DOE Office of Science Early Career Research Program (Grant number DE-SC0006414) through the Office of Basic Energy Sciences for the growth of graphene substrates by CVD. The authors acknowledge use of instrumentation supported by the UW-MRSEC (DMR-1121288) and the UW-NSEC (DMR0832760).

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Effect of Dipolar Molecule Structure on the Mechanism of Graphene

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