Surface-enhanced Raman scattering of dipolar molecules by the graphene Fermi surface modulation with different dipole moments

Accepted Manuscript Title: Surface-enhanced Raman scattering of dipolar molecules by the graphene Fermi surface modulation with different dipole moments Authors: Mingjia Zhang, Yandan Leng, Jing Huang, JiaoJiao Yu, Zhenggang Lan, Changshui Huang PII: DOI: Reference:

S0169-4332(17)32004-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.015 APSUSC 36549

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18-5-2017 30-6-2017 3-7-2017

Please cite this article as: Mingjia Zhang, Yandan Leng, Jing Huang, JiaoJiao Yu, Zhenggang Lan, Changshui Huang, Surface-enhanced Raman scattering of dipolar molecules by the graphene Fermi surface modulation with different dipole moments, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Article type: Full Length Article

Surface-enhanced Raman scattering of dipolar molecules by the graphene Fermi surface modulation with different dipole moments Mingjia Zhang,a,† Yandan Leng,a,b,† Jing Huang,a JiaoJiao Yu,a,b Zhenggang Lan,a and Changshui Huanga,* †

These authors contributed equally to this work.

a

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R. China b University of Chinese Academy of Sciences, Beijing 100049, P. R. China. E-mail address: [email protected] (C. -S. Huang) Graphical abstract:

The table of contents entry: The Raman spectrum of chromophore/graphene hybrids with different dipole molecules has been investigated. It is shown that the graphene-enhanced Raman scattering (GERS) can be significantly changed by the dipole moment values. By the analysis of the Raman signals of graphene as well as the interfacial energy level match, a strong correlation between the GERS and graphene Fermi surface is revealed.

Highlights •

Raman scattering spectrums of chromophore/graphene hybrids with different dipole moment values by changing the terminal groups are investigated.

•

The surface-enhanced Raman scattering shows significant positive correlation with the dipole moment.

•

Polarization characteristics of chromophores are found to play an important role in the interfacial energy level matching by regulating graphene Fermi surface. 1

ABSTRACT We report the modulation of Raman scattering spectrum of chromophore/graphene hybrids by tunning the molecular polarization with different terminal groups (methyl, methoxy, nitrile, and two nitros). Based on the density functional theory, the specific dipole moment values of the chromophore molecules are calculated. An obvious surface-enhanced Raman scattering (SERS) was observed and the scattering intensity of molecule increases with enlarged dipole moment. According to the analysis of G band Raman shifts of graphene, the enhancement of the Raman signal can be attributed to strong electronic coupling between graphene and chromophore, which is closely related with the modulation of graphene Fermi surface by changing the dipole moment of the molecule. Besides, the optimization of the ground state geometry and the binding energy of the hybrids were also calculated with the Density Functional Based Tight Bonding (DFTB) method, which confirms that the enhanced Raman scattering of molecules on graphene arises from the improved energy level matching between graphene Fermi surface and molecular band, further providing a new way to design novel SERS devices.

Keywords: Surface-enhanced Raman scattering, graphene, dipole moment, chromophore, Fermi surface

1. Introduction Although many fantastic properties like unique electrical, optical, and mechanical properties have been found on graphene [1-5], functionalization of graphene surfaces is still a very important way to enrich the potential application of graphene with other properties in phototransistors, solar cells, electronic devices, etc [6-8]. In particular, dipolar molecules have been demonstrated as an efficiency functional group to interact with low-dimensional carbon materials such as single wall carbon nanotube (SWNT) and graphene [9-17]. The dipole 2

moment of molecules was caused by the asymmetric distribution of positive and negative electron clouds in molecules. Thus the dipole moment interaction can result in the molecular orbital mixing of dipolar molecule and graphene or SWNT, altering their electronic and optical properties [18]. More interesting, due to the high quantum capacitance properties of graphene, the dipole moment interaction can also lead to a large amplification of relatively small change in electrostatic potential when the dipolar molecules was latched on graphene [12, 18, 19]. In our previous studies, we have reported that dipolar molecule DR1P with azobenzenes functional unit showed reversible modulation of charge doping in SWNT and graphene [1925]. Besides being a dopant, the graphene-enhanced Raman scattering (GERS), which has been observed in many different hybrid systems[26, 27], was also observed when the dipolar molecules interacts with single-layer graphene [19, 25]. A large enhancement factor was obtained based on the dipolar molecule DR1P suggests that GERS could be an efficient and convenient method to study the interaction of dipolar molecules and graphene. Especially the Raman enhancement can be directly used to probe the dipole moment interaction, thus it is needed to make clear the influence of intensity of polarization on Raman spectrum. Besides, the specific modulation mechanism of graphene Fermi level by changing polarization magnitude is still unknown, which is also very important to not only reveal the role of energy level matching but also design new GERS devices. In this work, we design and synthesized four dipolar molecules with different dipole moment values and investigate the Raman spectrum of chromophore/graphene hybrids. GERS study was performed to directly observe the dipole moment interaction induced by those molecules on single layer graphene and it is found that the enhanced Raman signals are closely related with the dipole moment, which can be attributed to the change of graphene Fermi surface. Furthermore, the density functional tight-binding (DFTB) calculations were 3

also carried out to examine the molecular orbital variation of dipolar molecules and the Fermi’s level shift of graphene, which further confirms the contribution of interfacial energy level matching as well as the hybridization of the electronic states. 2. Experimental Section 2.1. Synthesis of Dipolar Molecules The four chromophores DRMP, DRUP, DRCP, and DRNP were synthesized according to previous reported literature [19, 20, 24]. 2.2. Synthesis of Graphene Single layer graphene was synthesized on the copper foil (99.9%, 25 um thick, Alfa Aesar) using atmospheric pressure chemical vapor deposition. The as-grown graphene sheet was subsequently transferred to (300 nm) SiO2/p++ Si substrate with a typical size of ~0.5×0.5cm via the typical wet transfer procedure [28] for chemical vapor deposition (CVD) grown graphene, including polymethyl methacrylate (PMMA) spin coating on copper foil, chemical etching of copper, rinsing in deionization (DI) water, and removal of PMMA capping layer. 2.3. Preparation of Samples 60 μL amount of DRMP, DRUP, DRCP, and DRNP solutions in dichloromethane with concentrations ranging from 5× 10−10 M, 5× 10−9 M, 5× 10−8 M, 5× 10−7 M, 5× 10−6 M, 5× 10−5 M, to 5×10−4 M were spin-coated separately onto the surface of prepared graphene samples at 3000 rpm within 60 s. The typical area size of the samples was about 1 cm × 1 cm. 2.4. Calculations The geometric and electronic structure properties were studied with the dispersioncorrected self-consistent charge density functional tight-binding (SCC-DFTB-D) method [29, 30]. This method is suitable for electronic-structure calculations of large molecular systems, but it contains some tunable parameters to achieve the reasonable accuracy. We used the mio 4

parameters set that was developed for the treatment of organic molecules containing O, N, C, and H [30]. In this work, the modified azobenzene based dipolar molecules, graphene and the molecule@graphene hybrids were studied. Dispersion was included via a Lennard-Jones potential between each pair of atoms. The parameters for the potential can either be entered by the user or the program can automatically take the parameters from the Universal Force Field (UFF) [31]. During the calculation, we employed a large planar model to mimic graphene, which contains 184 carbon atoms and 74 hexatomic rings. By construction, the edge of graphene is saturated with hydrogen atoms. This model is large enough to mimic the absorption interaction between target compounds and graphene. All calculations were performed using the DFTB+ package [32]. 2.5. General Procedures Raman spectra of the hybrids on the wafers were measured using the Thermo Scientific DXR Raman Confocal microscope with 532 nm diode laser excitation. The 20× objective lens was used to observe the samples. Wavenumber calibration was carried out using a Si peak at 520 cm−1 as a reference. 3. Results and Discussion The schematic illustration of dipolar molecules on graphene and on the bare SiO2/Si substrate was displayed in Fig. 1(a). Since the dipole moment of the dipolar molecules can be changed by varying the electro acceptor parts, four molecules with different electronwithdrawing unit were synthesized. The details of the synthesis were described in experimental section. Fig. 1(b) showed the structures of the four molecules. The dipolar molecules were identical except for their terminal groups, which are methyl (-CH3) for DRMP, hydrogen (-H) for DRUP, nitrile (-CN) DRCP and two nitros (-NO2) for DRNP. Here CVDprepared single layer graphene (SLG) was used to provides a flat surface for pyrene tether of the four molecules via π−π stacking interactions. The graphene was transferred to SiO2/Si 5

substrate as exhibited in Fig. 1(c), so that the Raman signal of molecules on graphene and on bare SiO2/Si substrate can be obtained at same time. Raman spectrum of this graphene without any treatment shows the typical characteristic signatures of G and 2D bands of SLG that we used here (Fig. 1(d)). In order to obtain comparable results, four of the dipolar chromophore molecules were designed as similar structure, but with different terminal parts as we have discussed in Fig. 1(b). Those terminal parts showed different electron-withdrawing ability. Since other parts of the dipolar chromophore molecules, especially the electron-drawing group, are the same, the chromophore molecules should show different dipole moment according to the ability of the electron-withdrawing units. The electron-withdrawing ability of the four terminal parts increased as the order of -CH3 < H < -CN < -NO2. The dipole moments of the chromophore molecules can be calculated through density functional theory (DFT), which is displayed in Table 1. Among the four dipolar molecules, DRMP showed the smallest dipole moment as 4.35 Debye (D) for trans- structure, while DRNP showed the largest dipole moment as 13.68 D for trans- structure. The dipole moment of those dipolar molecules followed the sequence as DRMP < DRUP < DRCP < DRNP, which was in accordance with the electronwithdrawing ability of the terminal parts. For molecule/graphene hybrid, the orientation of the dipolar molecules would be important especially when we expect to compare the affection of dipole moment with different value. Thus we investigated the orientation of DRMP, DRUP, DRCP and DRCP on graphene using X-ray absorption spectroscopy (XAS) and DFT, and found that those molecules were oriented with the azobenzene moiety tilted by a similar angle of ≤34°with graphene, as shown in Fig. 2, suggesting that all these dipolar molecules displayed almost the same orientation when interacted with graphene. To gain deeper insight into the detailed molecular geometry of DRMP, DRUP, DRCP and DRNP on graphene, The geometric and 6

electronic structure properties were studied with the dispersion-corrected self-consistent charge density functional tight-binding (SCC-DFTB-D) method [29, 30]. This SCC-DFTB-D method is suitable for electronic-structure calculations of large molecular systems, but it contains some tunable parameters to achieve the reasonable accuracy. We used the mio parameters set that was developed for the treatment of organic molecules containing O, N, C, and H. In this work, the DRP, the modified azobenzene moieties of DRP and the DRP@graphene complexes were studied. Dispersion is included via a Lennard-Jones potential between each pair of atoms. The parameters for the potential can either be entered by the user or the program can automatically take the parameters from the Universal Force Field (UFF) [31]. All calculations were performed using the DFTB+ package [33]. The noncovalent binding of DRMP, DRUP, DRCP and DRNP on graphene was modeled with the pyrene tether interacting with a large 5×5 rhombus graphene fragment via π−π stacking interactions. Unconstrained geometry optimizations for the entire system (all atoms for both azobenzene chromophores and the graphene fragment) were carried out, and the possible existence forms were obtained in Fig. 2 and Fig. 3. Two stable conformations are the same in addition to the azobenzeneparty of molecular. The stable minimum is the parallel formation in Fig. 2 that the terminal pyrene tethers via π−π stackinginteractions, and with direct contact of the azobenzene group to graphene. In contrast, azobenzene moiety at a certain angle relative to graphene substrate for the conformer in Fig. 3 possibly exists in the Raman enhancement.Similar as the surface-enhanced Raman scattering, the Raman scattering signal of the molecule adsorbed on graphene can be greatly enhanced, commonly termed as graphene-enhanced Raman scattering (GERS). We have reported that dipolar molecules with the same structure as DRNP on graphene exhibited GERS. Thus the different affection of dipolar molecules DRMP, DRUP, DRCP and DRNP on graphene can be directly estimated through the enhancement of Raman signal. The Raman measurements presented in this work 7

do not distinguish between cis or trans configurations. The synthesized dipolar molecules here were most keep in the trans configuration, which was normally the stable state of azobenzene structure. Fig. 4 displayed the Raman spectra of DRMP, DRUP, DRCP and DRNP on the single layer graphene and SiO2/Si substrate respectively with the concentration of 5×10-4 M. The experimental data were obtained upon excitation with the 532 nm laser. For all the four molecules, the Raman spectra with strong signals and high signal-to-noise ratio were observed on graphene, while only a very weak Raman signal with fluorescence background were observed when collecting from the SiO2/Si substrate. Those spectra clearly showed a GERS of the dipolar molecules. The peak at the position of 1580 cm-1 is covered by the G peak of graphene. Here, the intensity of the Raman signals was normalized using a reference Si peak (520 cm-1). It can be found that the Raman signal of DRNP is the strongest among these four molecules. The highest peak generated from DRNP reached to 400. The intensity of DRCP was a half of DRNP, while DRUP and DRMP almost showed the same Raman intensity. The results hinted at the possibility of GERS induced by different dipolar molecules. It should be noted that the Raman signal of DRNP on the SiO2 surface was the weakest compared with other three molecules, and the strongest was DRMP, which might be caused by the different resonant excitation effect. To estimate the enhanced Raman signal, Raman enhancement factor for four different characteristic peaks of those dipolar molecules was calculated. The intensity of the Raman signals of DRNP, DRCP, DRUP and DRMP with the concentration of 5×10-4 M on the SiO2/Si substrate was regarded as the normalization reference. Based on the Raman data shown in Fig. 4, the calculated Raman enhancement factors for the four dipolar molecules were displayed in Fig. 5. The peak at ~1621, ~1307, ~1398, and ~1436 cm−1 were assigned to the C=C, C-O, N=N, and C-C stretch mode, respectively. The Raman enhancement factors for 8

four different stretch modes on graphene varied between 1.8 and 16. The DRNP showed largest enhancement factors among all the four molecules, while the DRMP exhibited smallest enhancement factors. Here, the enhancement in our system was vibration dependent, which is related with the surface geometry of the molecule on graphene. The surface geometry of the molecule on graphene and the structure of the molecules were similar, as we discussed. So the Raman enhancement factors can straightforward exhibit the interaction between the dipole molecules and graphene. Next, we also investigated the influence of concentrations through spin coating on GERS, with the four molecules concentrations ranging from 5×10-9 M to 1×10-3 M. As shown in Fig. 6, the intensity of the Raman signals from the four molecules on single-layer graphene displayed obvious dependencies with the molecule concentrations. For azobenzene molecule, within low concentrations, from 5 × 10-10 M to 5 × 10-7 M the signal almost displays the Raman scattering characteristics of graphene. The signal of molecule begins to occur until the concentration reaches 5 × 10-6 M, and becomes more and more obvious with increasing the concentration to 1 × 10-3 M. On the other hand, when the concentrations reach up to 5 × 10-4 M and 1 × 10-3 M, the baselines for graphene are not so smooth, which may be due to the signal of fluorescence is so strong that the graphene-induced fluorescence quenching can’t balance out. Then we analyzed the strongest peak at 1332 for DRMP, 1332 for DRUP, 1332 for DRCP and 1332 for DRNP, and found that the intensity data approach saturation as the concentration increases. Herein, the increased Raman intensity can be well fitted by the BET model as fellows: [19]

I  I max  Cazo (Cazo  M )

(1)

where I, Imax, Cazo and M represent the corresponding Raman intensity, the maximum Raman intensity corresponding to the saturation adsorption, the azobenzene molecule concentration and the fitting parameter respectively, suggesting it is possible to employ the 9

chromophore/graphene hybrids for quantitative SERS-based detection. Under resonant excitation, the signals are attributed to three factors, the number of the molecules, resonant enhancement, and chemical enhancement [34]. As Fig. 6 shown, we can find that DRMP intensity increase slowly with concentration while DRNP intensity rises quickly. Considering the origin of Raman enhancement on graphene, we would attribute the change of Raman signal to the variation of molecule with different concentrations. First, the molecules we used are conjugated and macrocyclic with the basic structures similar to graphene. When deposited as a submonolayer on graphene, due to the π-π stacking, these aromatic molecules should lie parallel to the surface of graphene [35], which means that the distance between graphene and the molecules is small. Second, the position of the HOMO and LUMO of the molecules are all located on the two sides of the Fermi level of graphene. The charge transfer can easily occur between graphene and the molecules, which will induce chemical enhancement. Additionally, because of the similarity of the chemical structure between the molecules and graphene, the vibrational coupling between them may be another factor contributing to the Raman enhancement. Thus the number of the molecules, resonant enhancement, and chemical enhancement all contribute to the Raman enhancement. When the amount of molecules is small, the resonant enhancement is insufficient to observe the Raman signals of molecules, meanwhile there is no chemical enhancement on the SiO2/Si substrate, so we cannot see the signals. But on graphene, both resonant enhancement and chemical enhancement exist, which result in observable Raman signals. As for high concentration, the resonant enhancement and chemical enhancement may further strengthen, inducing a larger GERS signal. In order to further clarify the interaction mechanism between graphene and molecule for GERS, we investigated the change of G band energy of graphene with dipole moment, which is shown in Fig. 7(a)-(c). Under the certain concentration such as 5×10-5, 5×10-4 and 1×10-3, the G band energy values with different molecule dipole moment can be fitted by linear relation. It has 10

been reported that the graphene G band energy changes linearly with Fermi level [36]. As for our four molecule/graphene hybrids, the only change is dipole moment. In this case the equivalent electric field only depends on the dipole moment of coated dipolar molecule, which directly causes the Fermi surface change of graphene like hole-doping with different degrees. For DRNP, the Fermi surface down-shift is the most significant, which enhances the molecule charge transfer resonance since the energy gap becomes closer to the energy of the laser, which is sketched in Fig. 7(d). Thus we can conclude that the dipole moment dependent GERS are modulated by changing the Fermi level of graphene. To further confirm the interactions between dipolar molecules and graphene in theory, we examine the density of state (DOS) of isolated DRP and the partial DOS (PDOS) of DRP in the DRP@graphene (Fig. 8), as well as the energy changes of isolated graphene and graphene with chromophores on its surface (Fig. 9). As Fig. 8 shown, we can see that the purple line is the original energy state of chromophores and the gray line is the stable energy state after the interaction with graphene. We attribute the Fermi’s level to zero, and then select the nearest peak transition for the LUMO-HOMO transition. The shift of HOMO of DRP before and after absorbed on graphene follows the below order: DRMP (a, 0.134 eV) < DRUP (b, 0.150 eV) < DRCP (c, 0.158 eV) < DRNP (d, 0.174 eV). This order is also consistent with electron-withdrawing ability of the modified azobenzene moieties in DRP. Similarly, the energy level shift of graphene can also be obtained in Fig. 9. The black and red lines correspond to the energy state of graphene without and with chromophores molecules coating respectively. It is obvious the Fermi energy level of graphene shifts down with dipole interaction, and the shift range increases with larger dipole moment. Due to the similar structure of chromophores and graphene, the interface dipole interaction and bandgap arising from energy matching can promote the charge transfer process at the interface of hybrid. These calculation results are coincident with the GERS analysis discussed above, further 11

revealing the import role of modulating Fermi level of graphene in chromophores/graphene hybrids. 4. Conclusion In summary, we have examined the Raman signal of the chromophore on graphene and on bare SiO2/Si substrates through detailed comparison of DRNP DRCP DRMP and DRUP. The terminal groups influence the magnitude of dipole moment of the chromophores and the magnitude of dipole moment is an important factor in the Raman enhancement. Considering the electronic properties and polarity of graphene, we conclude that the strong charge transfer interaction between graphene and the chromophores by changing Fermi level of graphene can induce such an enhancement effect of Raman signal. Based on the theory calculation, we further confirm the dipole-dipole interaction on bandgap and charge transfer. This study benefits not only the future potential application of 2D materials such as graphene, but also their possible use in the observation and utilization of the SERS effect.

Ming-Jia Zhang and Yan-dan Leng contributed equally to this work.

Acknowledgements This study was supported by the “Hundred Talents Program” of the Chinese Academy of Sciences, the National Natural Science Foundation of China Youth Science Fund Project (21301184), and the Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201504).

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Fig. 1. (a) Schematic illustration of the dipolar molecules on graphene and SiO2/Si substrate. (b) Molecular structure of DRMP, DRUP, DRCP, and DRNP. Optical Microscope image(c) and Raman spectra (d) of as-prepared CVD graphene transferred on the SiO2/Si substrate.

Fig. 2. The most stable configuration of molecular on the graphene optimized by DFT.

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Fig. 3. The possible configuration of molecular on the graphene optimized by DFTB and the orientation of the chromophores labeled.

Fig. 4. (a)-(d) are the comparisons of Raman signals of DRNP, DRCP, DRUP and DRMP deposited on the graphene and on the SiO2/Si substrate at the concentration of 5×10-4 M.

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Fig. 5. Relative Raman intensity of DRNP DRCP DRMP and DRUP at the concentration of 5×10-4 M with the different stretch modes in the spectra.

Fig. 6. (a),(c),(e),(g) Raman spectra of the different molecules deposited on monolayer graphene by spin coated with different concentrations, from 5×10-9 M to 1×10-3 M. (b),(d),(f),(h) The corresponding plots of Raman intensity peaks as a function of concentration. The signals on the SiO2/Si substrate are set to 1. The black line is the nonlinear fitting by BET model. (i) The sketch map of change on molecular amount with increasing concentration.

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Fig. 7. (a)-(c) Dipole moment dependent G band energy of graphene with different concentrations. (d) The sketch of charge transfer at the hybrid interface.

Fig. 8. The enengy changes of isolated chromophores and chromophores on the graphene.

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Fig. 9. The enengy changes of isolated graphene and graphene with chromophores on its surface.

Table 1. The magnitude of the dipole moment of DRNP, DRCP, DRUP and DRMP at transand cis- state. Molecular Configuration

DRMP

DRUP

DRCP

DRNP

Trans- (D)

4.35

5.04

9.32

13.68

Cis- (D)

4.20

4.36

5.78

8.14

19