bk-2016-1246.ch005

Chapter 5

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Enhanced Raman Scattering on Graphene and Beyond Jingjing Lin, Na Zhang, Lianming Tong, and Jin Zhang* Center for Nanochemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China *E-mail: [email protected]

Graphene has shown its unique advantages in surface-enhanced Raman scattering (SERS) due to both the exploration of the chemical mechanism and the potential in analytical applications for sensing. In this chapter, we will first introduce graphene-enhanced Raman scattering (GERS) and the chemical mechanism of the Raman enhancement, including the first-layer effect, the role of the molecules orientation and the energy alignment, and the molecular selectivity. Beyond graphene, other two-dimensional (2D) layered materials have also shown Raman enhancement and provided new insights into the understanding of the chemical effects, for example, if the in-plane symmetry of the materials is taken into consideration. To the end of sensing applications, graphene-based SERS and its combination with metal nanostructures have provided practical strategies to overcome several bottlenecks in conventional SERS, owing to the improvement of stability and repeatability, the possibility of SERS quantification and biocompatibility. In the outlook section, we will bring forward certain future directions of graphene-based Raman enhancement.

© 2016 American Chemical Society Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Introduction Since the first observation of anomalously strong Raman signals of pyridine molecules adsorbed on rough silver electrodes by Fleischmann et al. in 1974 (1), surface-enhanced Raman scattering (SERS) has been intensely studied due to its exploration as a non-destructive and ultra-sensitive detection technique down to single molecule level, as well as its abundant and sophisticated physical/chemical processes (2–7). Two main mechanisms attribute to the SERS effects, that is, the electromagnetic enhancement (EM) and the chemical mechanism (CM) (8, 9). In general, the EM governed by surface plasmon resonance dominates the overall enhancement. The CM, which is related to the changes in the electronic polarizability of molecules, is typically several orders of magnitude lower than EM. The CM contribution is usually overwhelmed by EM and is therefore more technically demanding to study, although a number of theoretical models have been proposed and experimental methods developed. From the application point of view, because of its ultra-high sensitivity, SERS has been widely used for the detection of trace species in many fields, including food safety, environment monitoring, medical quality, disease diagnosis and antique identification (10–12). However, even up to now, the realistic applications of SERS-based analytical technique still suffer from the spectral instability and the lack of reliable quantitative capability, mainly caused by the orientation change and chemical reactivity of molecules on the surface of noble metals. Therefore, solutions to the above problems are in great demand for urgent needs in application. Since its discovery, graphene-enhanced Raman Scattering (GERS) has gradually become a unique technique that not only provides a new SERS platform with pure CM enhancement, but also indicates special applications in practice of SERS, owing to graphene’s superior electronic property, chemical inert, excellent bio-compatibility and so on (13–15). The CM mechanism of GERS has been extensively studied, including the first-layer effect, the energy alignment between the Fermi level and the HOMO/LUMO of molecules, and the molecular orientation (16–19). In addition, the “GERS” system has also been expanded to other two-dimensional (2D) layered materials, such as graphene-like isotropic 2D materials including hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2), and anisotropic 2D layered materials with low symmetry such as black phosphorus (BP) and rhenium disulphide (ReS2), which provide new insights into the understanding of CM processes from the anisotropy point of view (20, 21). On the other hand, due to the fluorescence quenching, the separation between the molecules and metal, and the 2D single-crystal nature of graphene, GERS and its combination with metal nanostructures have also been proven potential to overcome certain bottlenecks in the application of conventional SERS, that is, the fluctuation of SERS signals, the spectral continuum background, and the possibility of quantification (12, 22). In this chapter, we will first introduce the exploration of the CM mechanism of GERS, followed by the enhanced Raman scattering on other 2D layered materials with a focus on the in-plane symmetry of material. Then we will move on to the potential applications of enhanced Raman scattering on 2D materials, in particular, the graphene-mediated SERS (G-SERS) 98 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

tape. At last, a few perspectives in this research topic will be given in the outlook section.


Graphene-Enhanced Raman Scattering (GERS) Early in 2009, the quenching of fluorescence from dye molecules by graphene was observed (23), as shown in Figure 1a. The blank graphene samples were prepared by mechanical exfoliation of Kish graphite on SiO2/Si (300 nm thick oxide), and then soaked in solution of dye molecules, such as rhodamine 6G (R6G) and protoporphyrin IX (PPP). In solution, the Raman signals of R6G could not be observed due to a strong background of fluorescence, as shown in Figure 1b. Surprisingly, if the R6G molecules were deposited on graphene, the strong fluorescence background was significantly suppressed. Without the disturbance of fluorescence, the Raman peaks became clearly observed. The similar results were also observed for PPP molecules. This fluorescence quenching effect was attributed to the electron transfer and energy transfer process between graphene and the dye molecules, which is similar to the fluorescence quenching of molecules in the vicinity of metal substrates. However, from these phenomena, it is still not clear that whether the appearance of Raman features is due to an enhancement effect or the suppress of the fluorescence background.

Figure 1. (a) Schematic illustration of graphene as a substrate to quench fluorescence of R6G. The inset is an optical image of a monolayer graphene on SiO2/Si. (b) Raman-fluorescence spectra of R6G in water (10 μM) (upper curve) and R6G on a monolayer graphene (lower curve) under 514 nm excitation. The integration time was 10 s for the upper curve and 50 s for the lower curve. The Raman peaks labelled by the asterisks were from the SiO2/Si substrate. The 1588 cm−1 peak was from graphene. Adapted with permission from ref. (23). Copyright 2009 American Chemical Society. 99 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Later in 2009, such investigation was extended to other dye molecules, such as phthalocyanine (Pc) and crystal violet (CV), which do not have strong fluorescence background (13). In these cases, the Raman signals of Pc molecules were clearly enhanced by graphene in comparison with the Raman spectra on the blank substrate, as shown in Figure 2a, although the enhancement factor (EF) was not so appealing (~2-17). This was the first direct evidence that graphene can enhance the Raman signal of molecules, and this phenomenon is termed as “graphene-enhanced Raman scattering (GERS)”. Since the surface plasmon resonance of graphene is in the THz region, the EM enhancement can be safely excluded as the wavelength of the laser is in the visible range. Thus, we consider GERS as a pure CM contribution. To further confirm the CM mechanism, the typical characteristics of CM have been proven in GERS. The magnitude of the Raman enhancement is quite different on monolayer, multilayer graphene, and graphite, as shown in Figure 2b. It illustrates that the GERS effects should be related to the electronic properties of the graphene substrates. By taking the Raman signals on the SiO2/Si substrate as a reference, the Raman enhancement factors (EFs) on monolayer graphene can be obtained. The Raman EFs differ for different vibration modes, changing from 2 to 17, as can be seen from Figure 2c, which means GERS is mode-dependent. As a distinct SERS system, GERS provides a unique platform to explore CM for its natural isolation from EM. A series of investigations on the CM processes have been carried out and will be discussed below.

Figure 2. (a) The Raman spectra of Pc molecules on monolayer graphene and a blank SiO2/Si substrate. (b) Raman spectra of Pc deposited using vacuum evaporation on different surfaces - monolayer graphene (upper curve), multilayer graphene (middle curve) and graphite (lower curve). Except the peaks marked by the asterisk (*), all the other peaks are from Pc molecules. (c) The relative Raman intensity of Pc deposited 1 Å on different surfaces using vacuum evaporation. The different curves represent the different peaks of Pc listed in the inset. The signals on the SiO2/Si substrate are set to “1”. Adapted with permission from ref. (13). Copyright 2009 American Chemical Society. 100 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


First-Layer Effect An important characteristic of CM is the first-layer effect since charge interactions only occur if the molecules are in direct contact with the substrate. The first-layer effect has also been verified in GERS. We utilized the Langmuir–Blodgett (LB) technique to transfer mono- and multilayer ordered PPP molecules on top of graphene in a controlled way (16). Raman spectra of PPP with different layer numbers of the LB film were compared and is shown in Figure 3a. It is found that the Raman signal did not increase linearly with the number of PPP layers, and the first monolayer LB film of PPP had the largest contribution to the Raman enhancement compared with the subsequent layers, seen from Figure 3b. This result indicates that the enhancement of GERS is indeed dominated by the first layer of molecules, which also proves the short-range effect of CM. Accordingly, we can further assure that the mechanism of GERS should be attributed to pure CM since an EM effect can still largely enhance the Raman scattering of molecules several nanometers away from the surface.

Figure 3. (a) Raman spectra of PPP with different layer numbers of the LB film on graphene. The peaks labelled by the asterisk “*” are the G-band of graphene. (b) Relative Raman intensities of PPP corresponding to (a) as a function of the layer number of PPP. The Raman signals corresponding to the monolayer LB film of PPP were set as “1”. The lines with different symbols correspond to the peaks labeled in the inset. The dotted line is a reference of linear relationship. Adapted with permission from ref. (16). Copyright 2010 John Wiley & Sons, Inc. Molecules Orientation The “first-layer” effect indicates the necessity of direct contact between the molecules and graphene for the charge transfer to occur. A closer look at the scenario implies that even the different functional groups of the molecule may be influenced differently by the charge transfer process. Taking PPP molecules as an example, the molecules on the top and at the bottom of graphene experience different enhancement, and the Raman spectra are also different, as shown in Figure 4 (16). The reason is straightforward: if the PPP molecules (Figure 4a) are deposited on graphene directly (in upstanding configuration by LB technique), the functional groups facing down, that is, in contact with graphene, are hydrophilic group (–COOH). However, if the PPP molecules are deposited on the substrate first and then covered by graphene, the functional groups in contact with graphene 101 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


are the hydrophobic group (–CH=CH2). The functional group in direct contact with graphene has a stronger enhancement than other groups, so that the Raman features are different in these two cases. If the probing molecules are CuPc, which is symmetric with D4h symmetry, such difference in enhanced Raman spectra does not appear, as shown in Figure 4b, because the functional groups in contact with graphene are the same no matter the molecules are on the top or at the bottom of graphene. The different dependence of the Raman enhancement effect for CuPc and PPP indicates that the vibrations related to the chemical group in direct contact with graphene have a stronger enhancement. In other words, the closer the chemical group is to graphene, the larger extent of charge transfer between them will be, inducing a larger polarizability tensor and thus a larger Raman scattering cross section for this chemical group.

Figure 4. Raman spectra of PPP (a) and CuPc (b) on the top of (upper curve) or below (lower curve) graphene. The peaks labelled by the asterisk “∗” are the G-band of graphene. The insets in (a) and (b) show the corresponding molecular configurations on the top (left) or the bottom (right) of graphene. Adapted with permission from ref. (16). Copyright 2010 John Wiley & Sons, Inc. To further confirm the important role of graphene-molecule distance for CM in GERS, the effect of molecular orientation on the Raman enhancement was investigated by using CuPc as a probe molecule (19). The molecules were deposited on graphene using LB technique, and the initial orientation was upstanding. However, after an annealing process, the molecules change from upstanding to lying-down orientation, as illustrated in Figure 5a. Comparing the enhanced Raman spectra of CuPc molecules in these two extreme orientations, it is seen that stronger enhancement of Raman intensity was obtained with CuPc molecules lying down on graphene after the annealing process (Figure 5b). Combing the UV–visible absorption characterization shown in Figure 5c, we can conclude that the π-π interaction between the CuPc molecule and graphene became much stronger when the orientation of CuPc molecule on graphene changed from upstanding to lying down. 102 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 5. (a) Schematic illustration of molecular orientation change after annealing. Inset: the molecular structure of CuPc. (b) Comparison of Raman spectra of as-prepared CuPc LB film (dashed) and that after annealing at 300 °C (solid) on graphene. (c) Comparison of the UV–visible absorption spectra of as-prepared CuPc LB film and that after annealing. Adapted with permission from ref. (19). Copyright 2012 John Wiley & Sons, Inc. (d) Feynman figure of the Raman scattering process. Adapted with permission from ref. (24). Copyright 2012 American Chemical Society. This can also be well understood from the Raman scattering process illustrated by the Feynman figure as shown in Figure 5d (24). The Raman scattering process experiences three steps as described by the quantum theory: (1) an incident photon interacts with an electron in the ground state and the electron jumps to the excited state; (2) the excited electron couples to a phonon (related to a certain vibrational mode); (3) the electron relaxes back to the ground state and radiates a scattered photon. Every step can be described with the corresponding Hamiltonians and contributes to Raman intensity, which will be described as follows. The magnitude of the Raman intensity is governed by the magnitude of the polarizability. According to Fermi’s gold rule, the Raman polarizability α can be expressed as:

where is the initial state, and are two intermediate excited states in the Raman scattering process, is the final state, Hlight is the Hamiltonian of the light radiation, Hel–ph is the Hamiltonian of the electron–phonon coupling, Eλ is the energy of the excitation laser, Eii is the electron transition energy, and Eph is the phonon energy. Equation (1) corresponds to the three steps described by the Feynman figure of the Raman scattering process. Firstly, according to Equation (1), the polarizability α depends explicitly on the quantity of available optical transition channels. Therefore, when CuPc molecules make contact with graphene, interfacial dipoles form at the CuPc 103 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and graphene interface, which induces a change of the energy level of the CuPc molecules at the interface as shown in Figure 6a. The formation of the interfacial electron states increases the available optical transition channels of CuPc, which leads to the overall Raman enhancement of CuPc molecules. However, this may not be modulated by the molecule orientation greatly. Secondly, based on Equation (1), the Raman polarizability α also depends on both the strength of the electron–phonon coupling (Hel–ph) and the resonance condition (Eλ = Eii or Eλ= Eii+ Eph ). Considering our experimental condition, the resonance condition may not be changed greatly after the annealing process and should not be the main reason for the observed difference of Raman intensities. Hence, the strength of the electron-phonon coupling can be regarded as the source of the Raman enhancement differences.

Figure 6. Understanding of the molecular orientation effect in GERS. Schematic illustrations of a) the change of the electron energy band between graphene and the molecule before and after contact, and b) the relative direction of the delocalized π orbital of graphene and the CuPc molecule before and after annealing. Ef = Fermi level. Adapted with permission from ref. (19). Copyright 2012 John Wiley & Sons, Inc. Since the energy level for the π electrons in the CuPc molecule is close to that in graphene (for CuPc, the a1u orbital is located at about –5 eV, and the Fermi level of graphene is about –4.6 eV), the π electrons of CuPc should contribute the most to the Raman enhancement in this system. While, the coupling between the π electrons in the CuPc molecule and graphene can be larger when the CuPc molecules are lying-down on graphene than that with upstanding orientations as shown in Figure 6b. Therefore, such different molecular orientations will induce a different interfacial dipole, and then a different magnitude of the electronic polarizability of the molecules. For the lying-down orientation of CuPc molecules on graphene, the larger π-π interaction between the CuPc molecules and graphene induces a larger interfacial dipole (or higher strength of 104 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the electron-phonon coupling), and then a larger Raman scattering cross section. In addition, when CuPc molecules are adsorb on graphene in a lying-down orientation, the delocalized π-orbital cloud of the CuPc molecule overlaps more with that of graphene, as shown in Figure 6b. This allows higher probability of electron transfer and thus enhances the Raman scattering cross section of the CuPc molecule. Both the interfacial dipole and the overlap of the π-orbital could magnify the Hamiltonian of the electron–phonon coupling Hel–ph and consequently the electronic polarizability. Energy Alignment According to Equation (1), the Raman intensity depends on the excitation laser energy Eλ and reaches a maximum when Eλ = Eii or Eλ= Eii+ Eph, which corresponds to the ordinary resonant Raman intensity profile. Therefore, for different vibrational modes (corresponding to different Eph), the Raman excitation resonant profile should be different. For a specific vibrational mode, if it involves an excited-state charge transfer process, the corresponding resonance peak in the resonant Raman intensity profile should be observed. If it only involves a ground-state charge transfer process, the profile should be consistent with ordinary resonant Raman intensity. Hence, by analyzing the Raman excitation resonant profile in GERS system, it should be beneficial for the understanding of the charge transfer process in CM. The excitation wavelength-scanning Raman spectroscopy has been used to investigate the Raman excitation resonant profiles of CuPc molecules on graphene, as shown in Figure 7a (in the range of 545-660 nm, i.e., 2.27-1.88 eV) (24). The normalized intensity vs excitation wavelength profile can be well fitted using the ordinary resonant Raman scattering expression (for example, see Figure 7b for the results of peak 1530 cm-1), where the incident and scattered resonance peaks were well-distinguished, with the energy difference equaling the energy of the molecular vibrations. In the CM of Raman enhancement, the excited-state and the ground-state charge-transfer mechanisms will show different dependence of the enhanced Raman signals on the excitation wavelength. The Raman excitation profile in GERS system shown above meets the condition of ground-state charge transfer, in which model the dependence of the enhanced Raman signals on the excitation wavelength is the same as that of the ordinary Raman scattering, proving that GERS involves a ground-state charge-transfer mechanism. Secondly, the electrical field modulation was used to study the variation of the Raman intensities of molecules with the shift of the Fermi level of graphene (17, 18). Similarly, the probing molecules are chosen to be a series of metal phthalocyanine (M-Pc), such as cobalt phthalocyanine (CoPc). The GERS spectra of molecules were collected by in-situ Raman measurements under ambient condition, vacuum, NH3 atmosphere, and O2 atmosphere, respectively, in which the Fermi level of graphene was modulated by an electrical field. Different atmospheres were used here to modulate the original Fermi energy position. It is known that NH3 molecules donate electrons to graphene, while O2 molecules accept electrons from graphene as illustrated in Figure 8a and 8b. The modulation of the GERS effect with the Fermi level of graphene is the result of the change 105 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of the interaction between graphene and the molecule, as illustrated in Figure 8c. Accordingly, under different atmosphere, this modulation of Raman intensity of molecules on graphene by the external electric field shows different effects (18).

Figure 7. (a) A series of Raman spectra of CuPc molecules on graphene excited by the laser lines from 660 nm to 545 nm (1.88 eV to 2.27 eV). Raman measurement was carried out every 5 nm in the corresponding range. For clear display, only the spectra in the range of 1400–1600 cm–1 were shown. (b) The excitation profile of peak 1530 cm-1 and its fitting results (lower left and lower right curves). Adapted with permission from ref. (24). Copyright 2012 American Chemical Society.

Figure 8. Schematic representation of (a) the n-doping effect of NH3 to graphene and (b) the p-doping effect of O2 to graphene. (c) Schematic of the possible mechanism for the modulation of the charge-transfer enhancement in GERS by using an electrical field measured under different types of doping conditions. The Fermi surface of graphene under –150, 0, and +150 V are represented by the dash, dot, and dash dot cycles, respectively. Adapted with permission from ref. (17). Copyright 2011 John Wiley & Sons, Inc. 106 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Molecules Selectivity Through the previous investigations about CM in GERS, we separately investigated how the strength of the electron–phonon coupling and energy alignment influence the enhanced Raman intensities in GERS system with different methods. In any case, the probe molecule is of essential importance in the charge interactions with graphene and the Raman enhancement. To further explore the deeper understanding for CM on the GERS platform, the systematic comparison of the Raman enhancement using a variety of molecules with different properties as probing molecules has been performed (25). Since the enhancement factor of GERS is relatively low, we chose molecules with large Raman scattering cross sections to investigate the molecular selectivity in GERS. These molecules can be categorized as: (1) molecules with similar molecular structures but different energy levels and includes different phthalocyanine (Pc) derivatives; (2) molecules with similar energy levels but different molecular structures, such as tetrathienophenazine (TTP) and tris(4-carbazoyl-9-ylphenyl) amine (TCTA); and (3) other molecules of interest, such as 3,5-tris(N-phenylbenzimiazole-2-yl)benzene (TPBi), bathocu- proine (BCP). By analyzing the enhancement effects of all these molecules on graphene (25), one can conclude that the enhancement involving molecular energy levels requires the HOMO and LUMO energies to be within a suitable range with respect to graphene’s Fermi level. A large GERS enhancement can occur if one or more of the following conditions are met:

On the other hand, the enhancement involving the choice of molecular structures indicates that molecular symmetry and substituents similar to that of the graphene structure are found to be favorable for GERS enhancement, which can magnify the π-π interaction between molecules and graphene, as well as the overlaps of delocalized π-orbital cloud of molecule and that of graphene. Both factors, involving the molecular energy levels and structural symmetry of the molecules, suggest that a remarkable GERS enhancement requires strong molecule−graphene coupling and thus effective charge transfer between the molecules and graphene.

107 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Enhanced Raman Scattering on Other 2D Materials The first-layer effect, molecular orientation and energy alignment effects and molecule selectivity have provided a deeper understanding of basic priciples for CM of enhanced Raman scattering on graphene. Nowadays, a large number of new members have been obtained in the 2D materials family. Owing to the novel physical and chemical properties of such materials, it is also interesting to look at their Raman enhancement effects, which in a way around would reveal the properties of the materials themselves. Therefore, the GERS system has been expanded to a variety of other 2D materials.

Material Matters We first expanded the available substrates of GERS system to 2D materials with graphene-like structures, such as hexagonal boron nitride (h-BN) and monolayer MoS2 (20, 26). The electronic properties of such 2D materials are vastly different from graphene. As shown in Figure 9, the Raman enhancement effect on graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2) was apparently observed with CuPc molecule as a probe. However, from Table 1, it is found that the Raman enhancement is different for the different vibrational modes of CuPc depending strongly on the materials. Higher-frequency phonon modes of CuPc (such as those at 1342 cm−1, 1452 cm−1, 1531 cm−1) are enhanced more strongly on graphene than that on h-BN, while the lower frequency phonon modes of CuPc (such as those at 682 cm−1, 749 cm−1, 1142 cm−1, 1185 cm−1) are enhanced more strongly on h-BN than that on graphene. MoS2 demonstrated the weakest Raman enhancement effect as a substrate among these three 2D materials. These differences are attributed to the different enhancement mechanisms related to the different electronic properties and chemical bonds exhibited by the three substrates: (1) graphene is zero-gap semiconductor and has nonpolar C−C bonds, which induce charge transfer; (2) h-BN is insulating and has a strong B−N bond, while (3) MoS2 is semiconducting with the sulfur atoms on the surface and has a polar covalent bond (Mo−S) with the polarity in the normal direction to the surface. Therefore, the different Raman enhancement mechanisms differ for each material: (1) charge transfer may occur for graphene; (2) strong dipole−dipole coupling may occur for h-BN, and (3) both charge transfer and dipole−dipole coupling may occur, although weaker in magnitude, for MoS2. The above models explain qualitatively the Raman enhancement on different 2D materials. However, one should note that an analytical model that can quantitatively describe the enhancement is urgently needed, which should be the necessity in the future work.



Figure 9. Raman spectra of the CuPc molecule on the blank SiO2/Si, graphene, h-BN, and MoS2 substrates, respectively. The Raman signal was excited by a 632.8 nm laser. Adapted with permission from ref. (20). Copyright 2014 American Chemical Society.


110


Table 1. Intensity Comparison of the CuPc Raman Vibrational Modes on Different Substrates. Adapted with permission from ref. (20). Copyright 2014 American Chemical Society. ωG (cm-1)

ωh-BN (cm-1)

682.1

682.2

749.2

749.3

832.6

832.5

1109.0

1108.7

1142.9

ISiO2/Si

IG

Ih-BN

291.5

697.3

398.9

1007.6

56.8

182.4

20.2

257.9

329.8

12.7

1143.2

44.7

912.0

1834.7

1196.1

1196.7

24.36

425.3

720.4

1207.4

1214.7

274.8

364.0

1217.6

1221.2

23.2

402.2

190.2

17.2

1306.6

1305.7

29.4

713.0

205.9

1342.0

1341.6

62.8

2331.6

1452.1

1452.1

61.1

1531.2

1532.1

106.2

75.4

EFG

EFh-BN

EFG/ EFh-BN

mode assighment

0.4

B1g, in plane full symmetric nonmetal bound N−M stretch and outer ring stretches

0.4

B2g, in plane ring symmetric N−M stretch

0.3

A1g, in plane full symmetric N−M stretch

16.3

0.8

A1g, in plane symmetric N−M−N bend

20.3

41.0

0.5

B2g, in plane ring symmetric and outer rings breathing

17.4

29.5

0.6

B1g, in plane symmetric N−M−N bend

0.7

A2g

8.1

2.1

B2g

24.2

6.9

3.4

B 2g , in plane symmetric outer ring rotation

1315.4

37.1

20.9

1.7

B1g, in plane full symmetric N−C stretch and ring C−C stretch

2606.9

590.7

42.6

9.6

4.4

B2g, in plane ring symmetric outer ring C−C stretch

6752.1

1405.8

63.5

13.2

4.8

B2g, ring C−C stretch and in plane ring symmetric non metal bound NC stretch

5.2

13.3

ωG: Raman shift of the CuPc molecule on graphene. ωh-BN: Raman shift of the CuPc molecule on h-BN. ISiO2/Si: Raman intensity of CuPc molecule on a blank SiO2/Si substrate. IG: Raman intensity of CuPc molecule on graphene. Ih-BN: Raman intensity of CuPc molecule on h-BN. EFG: Intensity ratio of the Raman signal of CuPc molecule on graphene and on a blank SiO2/Si substrate. EFh-BN: Intensity ratio of the Raman signal of CuPc molecule on h-BN and on a blank SiO2/Si substrate.

Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The In-Plane Symmetry From the structural symmetry point of view, the above materials, hexagonal boron nitride (h-BN) and monolayer MoS2, are both isotropic with graphene-like structures. Lately, several new types of promising 2D layered materials with lower symmetry, including black phosphorus (BP, see Figure 10a) and rhenium disulfide (ReS2), exhibit unprecedented anisotropic electrical and optical properties owing to their natural in-plane anisotropy (27–30). Such materials, as Raman enhancement substrate, could possibly reveal new insights into the chemical mechanism, for example, if the anisotropic electronic properties would lead to different charge transfer and eventually different Raman enhancement or not, and if the in-plane structural symmetry would affect the enhancement or not.

Figure 10. (a) Top and side view of orthorhombic BP. The armchair direction of BP is denoted as AC and marked by the double-arrow. (b) Schematic illustration of sample preparation procedure. (c) Raman spectra of CuPc molecules on a 300 nm SiO2/Si substrate with (upper curve) and without (lower curve) few-layer BP on top. (d, e) Angular dependence of the normalized Raman spectra of BP (d) and CuPc molecules with BP (e), respectively. (f) Polar plots of the normalized intensities of 468 cm−1 (BP, Ag2), 682 cm−1 (CuPc, A1g), 1450 cm−1 (CuPc, B2g), 1530 cm−1 (CuPc, B1g) modes as a function of sample rotation angle measured on BP. The AC direction of BP is marked by the double-arrow. Adapted with permission from ref. (21). Copyright 2015 American Chemical Society.



By utilizing CuPc molecules as Raman probe, distinct anisotropy of Raman enhancement was found on anisotropic few-layered orthorhombic BP, revealed by the angle-resolved polarized Raman spectroscopy (ARPRS) (21), even though the probe molecules are randomly distributed, as shown in Figure 10. The molecules were deposited on the SiO2/Si substrate via vacuum thermal deposition and were randomly orientated (Figure 10b). The Raman enhancement was clearly observed by comparing the Raman spectra obtained on BP and on the blank substrate (Figure 10c). The angular dependent polarized Raman spectra of BP and CuPc molecules were surf-plotted in Figure 10d and 10e, respectively, both exhibiting periodic variation. The intensities of different vibrational modes were polar-plotted in Figure 10f, which clearly showes the anisotropic Raman enhancement of CuPc molecules on BP. Similar phenomenon were observed on triclinic ReS2 substrate. Such anisotropic Raman enhancement is totally absent on isotropic graphene and h-BN substrates. To understand the angular dependence of the Raman spectra of CuPc on BP and ReS2, the detailed group theory analysis was carried out. CuPc is a planar molecule and belongs to the D4h space group. The A1g, B1g, B2g and Eg modes are Raman active. The Raman tensors of these modes under sample rotation can be obtained by introducing a transform matrix into the original Raman tensors. The measured Raman intensity can be expressed as a function of molecule orientation and polarization geometry as follows:

where I is the collected Raman intensity, ei and es are the unit polarization vectors of the electric field for the incident (ei) and scattered (es) light, respectively. represents the Raman scattering tensor of a specific vibrational mode. Under parallel polarization configuration, the theoretical angular-dependent Raman intensity for the D4h symmetry group is expressed as follows:

From the simulation results, it can be seen that the A1g mode shows no polarization dependence, see the A1g modes of group 1 (G1) as shown in Figure 10f. In contrast, the intensities of the B2g and B1g modes change with θ in a 90° periodicity. Further, the angle between the maxima of these two modes is 45°, agreeing very well with the experimental observations for group 2 (G2, B2g modes) and group 3 (G3, B1g modes) respectively. However, the simulated polarization dependence holds only for a single molecule or for a set of uniformly aligned CuPc molecules, but not for randomly oriented CuPc molecules that homogeneously contribute to the Raman intensities. 112 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 11. DFT calculations of charge (re)distributions for CuPc/2D layered materials system. (a, b) Charge distributions (in light gray) of electronic bands near the Fermi level for (a) CuPc/BP, (b) CuPc/graphene systems. For CuPc/BP system (left b), upon CuPc adsorption, the charges are redistributed into 1D chains along AC direction (the direction with highest carrier mobility), while for pristine BP, the charges are uniformly distributed across the surface. For graphene surface (b), even with CuPc presence, the charge distributions remain isotropic. (c, d) Schematic illustration of anisotropic/isotropic charge interaction process for CuPc/BP (c) and CuPc/graphene (d) systems, respectively. Adapted with permission from ref. (21). Copyright 2015 American Chemical Society. To explore the charge interaction between CuPc molecule and BP (ReS2), we performed first-principles DFT calculations for CuPc/BP, CuPc/ReS2, and CuPc/graphene composite systems. The results are shown in Figure 11. It is seen that for BP surface, upon the contact of the CuPc molecule, one-dimensional (1D) chain-like charge redistribution along the armchair (AC) direction appears (Figure 11a), which corresponds to the direction with the highest charge carrier mobility. For ReS2 surface, regardless of the CuPc’s presence, its charge distributions are always primarily along the zigzag (ZZ) Re atomic chain (the direction with the highest charge carrier mobility). Under laser irradiation, the charge carriers are more mobile and diffuse faster along the AC direction of BP or the ZZ direction of ReS2. Accordingly, the CuPc molecules with their primary axis aligned in these directions are expected to have the strongest charge interaction across the interface and thus the strongest Raman enhancement (Figure 11c). Therefore, the polarization dependence of the Raman spectra is mainly determined by this small portion of molecules with specific relative orientation. For comparison, the charge distribution on graphene (Figure 11b) is always isotropic with and without the presence of CuPc molecules, thus the charge interactions between molecules and graphene is also isotropic, leading to isotropic Raman enhancement (Figure 11d). Though the apparent overall Raman enhancement factor (EF) of CuPc molecules on BP and ReS2 is below 10, given the low proportion of the effective CuPc molecules, the highest EF of single CuPc molecules from the chemical 113 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

enhancement can be several times larger. These findings suggest a new way to reveal the fundamental principles of charge interactions between molecules and 2D materials, which are crucial in understanding the chemical effects of SERS, and may also suggest a spectroscopic method to explore the intrinsic electronic properties of anisotropic 2D materials.


Toward Sensing Applications Enhanced Raman scattering on graphene and other 2D materials has provided new insights into the deeper understanding of CM process as a unique and pure platform for CM. From the application point of view, graphene is the most mature material for enhanced Raman scattering in sensing due to its chemical inertness, flexibility and ease in growth in single crystal with large scale. On the other hand, graphene derivatives, such as doped graphene, (reduced) graphene oxide (GO or rGO), graphene quantum dots and such, have also great potential in SERS analytical analysis (12, 22, 31). Graphene and Its Derivative For graphene-enhanced Raman scattering, the enhancement factor was reported to be below 20 (13). However, by choosing appropriate probe molecules, the EF of graphene has approached 100 and the detection limit has also reached to 10-8 M (16, 24). Though the EF of GERS is still significantly lower than EM, enough for CM, for analytical applications, the detection limit can already satisfy the demands of some specific applications. On the other hand, substitutional doping of graphene with heteroatoms, such as nitrogen (N-doped), boron (B-doped) and silicon (Si-doped) atoms, will introduce new excited states, high electronic interaction and more mobile carriers, which might be beneficial for CM enhancement due to the increase of the available optical transition channels (32–34). For some specific dye molecules like R6G and CV, the detection limit of N-doped graphene can even reach to the concentration as low as 10-11 M. Different from doped graphene, GO and rGO have a number of functionalized chemical groups, in particular, the highly electronegative oxygen species, which can also lead to a large enhancement of Raman scattering, for example, the EF of 103-104 (35–40). In addition, graphene quantum dots (GQDs), graphene nanomesh, and nanocolloids, can also enhance the Raman scattering of molecules (41–43). Graphene-Mediated SERS and G-SERS Tape GERS provides a pure platform for CM, but its reachable EF is essentially limited. For conventional SERS with dominant EM from metal nanostructures, the charge transfer contributes slightly to the EF, but somehow leads to spectral instability and even chemical reaction of molecules. Hence, the combination of GERS and conventional SERS is expected to merit the advantages from both. Along this line, graphene-mediated SERS (G-SERS) has been developed, where 114 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


graphene serves as an atomically thin, seamless, and chemically inert spacer (12, 22). In G-SERS, graphene prevents the chemical interactions between molecules and metal, and thus greatly improves the spectral stability. In 2013, our group developed a graphene-veiled SERS substrate, as shown in Figure 12a, where the annealing processes were crucial to “activate” the SERS activity (44). Before annealing, the EM “hot spots” are between gold nanoparticles where graphene can not reach, so that the molecules adsorbed on graphene can not experience the “hot spots”. After an annealing process, gold nanoparticles reshaped and graphene can now reach the nanogaps, leading to significant Raman enhancement, which is shown in the right panel of Figure 12a. However, it is clear that the surface morphology of such substrate inherited the roughness of gold nanoislands, which might limit the certain practical applications where the analyte molecules can not diffuse into the active sites. To overcome this drawback, a G-SERS substrate with atomic flatness was developed as schematically shown in Figure12b. The left panel depicts the picture of the flat G-SERS substrate, and the simulated electric field distribution is shown in the middle panel. The atomic force microscope (AFM) image in the right panel shows that the roughness of the surface is less than 2 nm. Since the metal (Cu, Ag or Au) nanostructures are covered by graphene, this substrate has been proved to effectively prevent the surface oxidation of metals in ambient air, allowing stable enhancement in a long period.

Figure 12. (a)Schematic of graphene-veiled gold nanoparticles before (case 1) and after (case 2) an annealing process. The Raman spectra in the right panel clearly shows the SERS intensity was greatly improved after the annealing process. Adapted with permission from ref. (44). Copyright 2013 John Wiley & Sons, Inc. (b) G-SERS with a flat surface. The left panel shows the schematic of the substrate. The simulated electric field distribution is shown in the middle panel. In the right panel, the atomic force microscope (AFM) image of such a substrate is shown. Adapted with permission from ref. (50). Copyright 2012 National Academy of Sciences. 115 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In fact, a number of work have been reported using graphene/metal hybrid composite for SERS (45–49). Since the Raman enhancement is now dominated by EM, which depends on the type of metal, the morphology, and the laser wavelength, more degree of freedom is allowed in improving the EM, for example, using various metal nanostructures such as arrays of nanodisks, nanorods, and nanospheres as well as sandwiched nanostructure of metals and graphene. As mentioned before, SERS on a flat graphene surface inherits the merits of both GERS and traditional SERS. We developed a transparent, free-standing and flexible “G-SERS tapes”, which allows quick, non-invasive and sensitive detection on objects with any arbitrary morphology (50). The “G-SERS tape” can be fabricated following the four steps shown in Figure13a, and the “G-SERS tape” consists of three parts: from bottom to top are the polymer supporter, the sandwiched metal nanoparticles and the flat graphene monolayer, respectively. Such G-SERS tape can be used to detect molecules on a flat substrate, rough solid samples and in liquid environment, as shown in Figure13b-d , that is, self-assembled monolayer of p-aminothiophenol on a flat gold surface (Figure 13b), CuPc molecules adsorbed on the surface of a cauliflower (Figure 13c) and R6G in aqueous solution (Figure 13d).

Figure 13. (a) Schematic of the fabrication of the G-SERS tape. (b) Pristine (lower curve) and G-SERS (upper curve) measurements of a self-assembled monolayer of p-aminothiophenol on a flat gold surface. (c) Pristine (lower curve) and G-SERS (upper curve) spectra of a cauliflower surface with adsorbed CuPc (by soaking in a 1 × 10−5 M CuPc solution in ethanol for 10 min). (d) A real time and reversible G-SERS characterization of R6G directly in a 1 × 10−5 M aqueous solution. (I, II, III are the Raman spectra with the same G-SERS tape on H2O, R6G/H2O and replaced on H2O, respectively). Adapted with permission from ref. (50). Copyright 2012 National Academy of Sciences. Besides, the “G-SERS tape” can also be possibly used for quantitative analysis. The large-scale single-crystalline nature of graphene offers the distinctive advantage that the probe molecules are homogeneously adsorbed on the surface, rendering the possibility of reliable determination of the number of 116 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

molecules. And the Raman features of graphene have been used as an internal standard to calibrate the signal variation due to instrumental or measurement drift. In general, the “G-SERS tapes” substrate is reusable, low cost and easy to be used, which makes this substrate applicable for different target molecules in a variety of systems on different morphology surface.


Outlook In this chapter, we have discussed enhanced Raman scattering on graphene and other 2D layered materials, including h-BN and MoS2, and have discussed the chemical mechanism from the first-layer effect, molecules orientation, energy alignment and molecular selectivity aspects. Further, we extended the chemical enhancement to anisotropic 2D materials, such as BP and ReS2, considering the anisotropic electronic and optical properties of the materials. From the application point of view, we briefly discussed the sensing application of graphene and its derivatives. For a higher EF and universal application, graphene-veiled SERS and G-SERS tape have also been discussed. In particular, the G-SERS tape can be used for detection of trace species on objects of arbitrary morphology. The future directions in this field would cover firstly the expansion of SERS substrate to other 2D materials of more choice in electronic and optical properties, so that the chemical enhancement and the charge interactions between molecules and materials could be understood in more detail. Applications wise, quantitative detection of trace amounts has always been the goal to pursue of the SERS community. G-SERS tape, owing to the chemical inertness and large-scale single crystalline nature of graphene, the flexibility, transparency and high enhancement factor, has the potential to be used for quantitative analysis in the future. In addition, optical chirality has been recently reported in bilayer twisted graphene (51). Such a chiral substrate, if applied in G-SERS system, would be of great importance in the detection of chiral molecules in stereochemistry and biology.

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