Raman Spectroscopy of Fullerenes: From C60 to Functionalized Derivatives

Introduction

Since the discovery in 1985[1], fullerene, a carbon allotrope, has attracted considerable interest in the scientific community due to its stable chemical properties, distinctive physical characteristics (such as high electrical and thermal conductivity, along with excellent nonlinear optical properties), and environmental friendliness[2,3,4,5,6,7,8,9,10,11]. It has been widely applied in diverse fields including energy, superconductors, catalysis, mechanical engineering, life sciences, and material science[12,13,14,15,16,17,18,19]. The most notable fullerene is the C₆₀ molecule, recognized for its hollow, cage-like structure made up of 12 pentagonal and 20 hexagonal carbon rings[20]. As research has progressed, fullerenes with varying carbon atom counts, from C₂₀ to those with up to 300 carbon atoms, have been synthesized, greatly expanding the fullerene family[21,22,23,24]. These fullerene molecules exhibit diverse structural configurations and distinct physical and chemical properties, offering greater scope and flexibility for extensive applications. Recently, research has extended to fullerene derivatives, such as endohedral fullerenes, which contain metal atom clusters or molecules embedded within the fullerene structure. These derivatives exhibit unique electronic and magnetic properties, promising broad applications in fields like biomedicine, photo-induced electron transfer, and quantum computing[25,26,27,28,29,30]. Additionally, by modifying their surfaces with different functional groups, functionalized fullerenes can improve solubility, biocompatibility, and interactions with biological macromolecules, showing potential in biomedical applications such as drug delivery and photodynamic therapy[31,32,33,34,35,36]. Through chemical linking or "cage-opening" methods, fullerene molecules can form one-dimensional chains or two-dimensional layers with specific structures and functions, possessing unique topological and optoelectronic properties crucial for molecular electronics, machines, and low-dimensional devices [37,38,39,40,41,42].

As research progresses, fullerenes and their derivatives are playing increasingly important roles in various fields of study. However, the vast diversity and complexity of fullerenes and their derivatives necessitate advanced characterization techniques to understand their structures and properties. Raman spectroscopy is a non-destructive optical characterization technique which sensitively reflects the vibrational characteristics of molecular groups and subtle environmental influences by analyzing changes in vibrational modes, making it an ideal tool for studying fullerenes and their derivatives. The unique spherical structure of fullerene molecules is reflected in their characteristic Raman peaks, providing crucial information about their molecular structure. When organic functional groups are added, or molecules or atoms are embedded to create derivatives, significant changes occur in the molecular structure and properties, leading to the emergence of new characteristic peaks and shifts in the original Raman spectrum[43]. Raman spectroscopy can also analyze the charge transfer process between the fullerene cage and organic functional groups or embedded molecules[44].

With ongoing advancements in Raman spectroscopy, methods such as resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and tip-enhanced Raman spectroscopy (TERS) have increasingly improved the detection sensitivity of fullerene derivatives, even enabling single-molecule spectral resolution [45,46]. This enhanced sensitivity is crucial for the study of molecular adsorption characteristics, vibrational modes, interfacial behaviors, and fine structural details. Furthermore, TERS allows for the probing of optical and vibrational heating effects at the nanoscale. These advanced Raman-based approaches thus greatly expand the scope of fullerene research, providing deeper insights into their structures and dynamics.

In this review, we have summarized key studies on conventional and enhanced Raman spectroscopy in probing fullerenes and their derivatives, highlighting its advantages in characterizing molecular structures, evolution processes, and electron transfer properties.

Basic Raman Principles and Fullerene Resonant Modes

Raman spectroscopy, as an inelastic scattering technique, offers several notable advantages, including minimal sample consumption, high spatial resolution, rich informational output, non-destructive and contactless measurement, and straightforward, user-friendly instrumentation. These features make it a powerful tool for analyzing lattice vibrations and chemical bond dynamics. When materials are exposed to laser light, both elastic scattering—such as Rayleigh scattering, where the incident photon energy (ħω_i) equals the scattered photon energy (ħω_s)—and inelastic scattering—such as Raman scattering, where ħω_i differs from ħω_s—occur simultaneously. Raman scattering only occurs when the polarizability of the vibrating molecules or lattices changes during scattering. On the other hand, changes in the dipole moment of molecules during scattering correspond to infrared absorption or scattering process, making Raman spectroscopy a complementation of infrared spectroscopy. During Raman scattering, the energy difference between the incident and scattered photons corresponds to the energy of vibrational or rotational energy levels (or the phonon energy of lattice vibrations), denote it as ħω. Stokes scattering corresponds to the situation when ħω_i - ħω = ħω_s, while anti-Stokes scattering corresponds to the one when ħω_i + ħω = ħω_s. Raman peak positions act as a unique fingerprint spectrum for tach material, for the material has characteristic vibrational energy levels, thus own the unique group of ħω. Shifts in Raman peak positions indicate the changes in vibrational characteristics, which can further inform the alterations in molecular structure or electronic distribution. According to thermodynamic statistical laws, the distribution of electrons on energy levels follows the Boltzmann distribution, resulting in a relationship between Stokes and anti-Stokes scattering intensities as I_S/I_aS ~ exp(ħω/kT), making Raman scattering also utilized to measure sample temperature.

Fullerene materials, characterized by their three-dimensional structures composed of numerous carbon atoms, offer a wealth of Raman spectral information. C₆₀ and C₇₀ are two typical representatives of fullerenes. C₆₀ is a molecular cage consisting of 60 carbon atoms, exhibiting I_h point group symmetry and featuring 10 Raman-active vibrational modes (2A_g + 8H_g)[47]. In contrast, C₇₀ has an ellipsoidal molecular cage with D_5h point group symmetry and possesses 53 Raman-active vibrational modes (12${\mathrm{A}}_1^{\mathrm{\text{'}}}$ + 22${\mathrm{E}}_2^{\mathrm{\text{'}}}$ + 19${\mathrm{E}}_1^{\mathrm{\text{'}}\mathrm{\text{'}}}$)[48]. Due to its lower molecular symmetry, C₇₀ presents more intricate Raman information, posing challenges in capturing all Raman signals accurately during experiments. Table 1 lists all experimentally determined Raman peaks for C₆₀ and some captured peaks for C₇₀. In the presence of endohedral molecules or attached functional groups, the Raman peak positions of fullerene molecules would shift and broaden. These features are central to the Raman study of fullerene derivatives, which will be explored in more detail in the following sections.

Conventional Raman Spectroscopy Characterization

Since Raman spectroscopy fundamentally identifies the vibrational-rotational energy levels of molecules, its primary and most common application is in distinguishing and analyzing chemical bonds within materials. In the study of fullerene derivatives, Raman spectroscopy is particularly useful in the following ways: (1) Characterizing Intramolecular Properties: The spherical structure of fullerene molecules results in distinct Raman peaks, and shifts in these peak positions can indicate subtle structural changes. Various functional groups introduce specific vibrational modes in Raman spectra, serving as "fingerprints" for chemical modifications, thus aiding in the precise identification and analysis of the chemical structures of fullerene derivatives. This provides direct evidence for understanding the impact of derivatives. By examining the vibrational modes and their variations in Raman spectra, insights into the electronic structure and charge distribution of molecules can be gained, as well as the presence of charge transfer between functional groups. This is vital for comprehending the optoelectronic properties of fullerene derivatives, providing essential information on the intermolecular interactions of these derivatives. (2) Assessing Environmental Impact: Raman spectroscopy can evaluate how environmental factors affect fullerene derivatives.

By recording Raman spectra of molecules under varying solvent or temperature conditions, alterations in molecular vibrational modes can be observed, thus assessing the influence of environmental factors on molecular structure and properties. Interactions between derivatives and between derivatives and solid substrates can also be characterized. In this section, we will explore examples of conventional Raman spectroscopy applications in researching fullerene derivatives, focusing on the points mentioned above.

In 2011, D. -J. Chung et al. synthesized hydroxyl-functionalized fullerene (-OH) using γ-irradiation in a methanol/1,2-dichlorobenzene mixture solution[53]. The Raman features of this functionalized fullerene (F-fullerene) (Figure 1a) closely resembled those of the base fullerene, indicating that the fundamental structure was preserved. However, they noted shifts in peak positions from 1458 and 1590 cm⁻¹ in the original fullerene to 1462 and 1570 cm⁻¹, respectively, suggesting a slight distortion in the fullerene’s cage structure. Additionally, the disappearance of the fullerene D band and reduced intensity of the G band were attributed to the formation of a disordered sp² carbon structure. G. Rambabu and colleagues used a similar analytical approach for fullerene functionalized with sulfonic acid (-SO₃H) groups[43]. They observed a 6 cm⁻¹ downshift in the pentagonal pinch mode A_g(2) (1464 cm⁻¹) upon functionalization(Figure 1b). Furthermore, broadening of Raman peaks in F-fullerene was noted and attributed to asymmetry introduced by surface functional groups. Recently, ethylenediamine functionalized fullerene (EDA@C₆₀) was fabricated and characterized by S. S. Narwade, et al.[54]. They identified a distinct G band at 1588.02 cm⁻¹(Figure 1c), which attributed to the first-order scattering of the E_g(2) mode for sp² hybridization between carbon and nitrogen atoms, and a D band at 1344.71 cm⁻¹ due to sp³-hybridized carbon and its structural defects (reflecting the O-C₆₀ band in this study). As functionalization progressed, they reported a D to G band intensity ratio (I_D/I_G) of 0.84, indicating the sensitive distribution of functional groups.

In the last century, Raman spectroscopy was already being used to characterize endohedral fullerenes with their structural and electric properties. M. Krause et al. embedded Thulium and Gadolinium into C₈₂ molecular cages (Tm@C₈₂ and Gd@C₈₂, with Tm@C₈₂ having three isomers A, B and C) using a modified Krätschmer arc burning method, and characterized them with Raman spectroscopy(Figure 2a)[55]. They observed a band bellowing 200 cm⁻¹(117 cm⁻¹ for the isomers A and B, and 116 cm⁻¹ for C) attributed to vibration between metal and cage, and the significant difference in Raman spectra between three isomers at 1550 to 1650 cm^-1. In the plot of the wave number of the metal cage Raman mode below 200 cm⁻¹ versus the square root of the reciprocal metal mass, Gd@C₈₂, La@C₈₂ and Y@C₈₂ were found on one straight line, while three Tm@C₈₂ isomers lowered significantly (Figure 2b). This was explained by differences in attraction forces between the metal ion and the charged fullerene cage. They concluded that Gd@C₈₂ correlates with trivalent ions as Gd³⁺@${\mathrm{C}}_{82}^{3-}$, similar to La³⁺ and Y³⁺ and rather than Gd²⁺@${\mathrm{C}}_{82}^{2-}$ to Tm²⁺. In the same year, they also identified the molecule Eu@C₇₄ to be Eu²⁺@${\mathrm{C}}_{74}^{2-}$ from analyzing Raman data[56]. Similar methodology was also applied to endohedral fullerenes containing molecules such as La₂@C₈₀, Ti₂@C₈₀[44], Gd₂@C₇₉N[57], Sc₃N@C₈₀[58], Sc₄N₂@C₆₀[29] and U₂@Ih(7)-C₈₀[59] et al.

Interactions with other molecules or substrates significantly impact the structural or electrical properties of fullerene derivatives. For instance, fullerene nanowhiskers showed an ~8 cm⁻¹ downshift(Figure 3a) in the pentagonal pinch mode compared to powders, attributed to C₆₀ polymerization[60]. In fullerene-water solutions, shifts of up to 15 cm⁻¹(Figure 3b) were observed for C₆₀ Raman active A_g and H_g bands, with additional vibrational bands at 1304 and 1637 cm⁻¹ connected to the colloidal state of C₆₀[61]. The electron exchange behavior, typically seen in intermolecular interactions, can also be detected through Raman spectroscopy. S. Paul and colleagues mixed C₆₀ into insulating poly-vinyl-phenol (PVP) polymer and built a simple metal–organic–metal (MOM) sandwich structure device(Figure 3c) to test its memory effect[62]. Raman spectroscopy confirmed C₆₀ involvement in charge storage, showing a downward shift in the A_g(1) mode peak after a +2.5 V write step, indicating charge injection into C₆₀ molecules, which returned to 1469 cm⁻¹ after a -2.5 V erase operation, signifying charge release(Figure 3d). Similar Raman techniques were used to analyze interactions between C₆₀ and g-C₃N₄[63,64]. Slight shifts in H_g(7) and H_g(8) modes indicated a charge transfer, with the A_g(2) mode disappearing when physically mixed with g-C₃N₄(Figure 3e)[63], and significantly downshifting when covalently bonded(Figure 3f)[64]. Interactions with Layered double hydroxides (LDH)[65] resulted in electron transfer to C₆₀ evidenced by upshifted Raman peaks(Figure 4a), which is benefit for improving material’s catalytic ability. After covalent bonding with graphene, the A_g(2) mode of C₆₀ upshifted(Figure 4e) compared to pure molecules, indicating a strong interaction with graphene sheet[66]. However, when covalently attached to graphene nanoplatelet edges, the A_g(2) mode exhibited an 11 cm⁻¹ downshift(Figure 4f)[67]. Similarly, C₆₀-embedded cobalt/nitrogen-codoped porous carbon materials (C₆₀@Co-N-PCM) also showed a downshifted A_g(2) mode(Figure 4d), indicating the electron charge transfer to fullerene[68]. These experiments consistently highlight the charge transfer characterization capabilities of Raman spectroscopy in fullerene derivatives.

Resonance Raman Spectroscopy Characterization

Resonance Raman spectroscopy is a powerful analytical technique that capitalizes on the resonance phenomenon, which occurs when the frequency of the excitation light closely matches the energy levels of electronic transitions in a molecule. This resonance condition leads to a substantial enhancement of the Raman scattering signals for specific vibrational modes, thereby providing a more pronounced and selective insight into the molecular structure. Such selective enhancement is particularly beneficial for investigating the vibrational properties of fullerene derivatives, as it allows researchers to obtain detailed structural information about specific components of these complex molecules. In addition, resonance Raman spectroscopy allows for detection at lower concentrations of samples, which is especially valuable in biological and environmental contexts, where the presence of fullerene derivatives may be present in trace amounts. Despite the increased equipment requirements and operational complexity associated with resonance Raman spectroscopy compared to conventional ones, its superior selectivity and sensitivity render it an essential tool in studying intricate molecular systems like fullerene derivatives.

In 1991, M. Matus and colleagues conducted one of the first resonance Raman characterizations of a mixture containing 85% C₆₀ and 15% C₇₀ on a silicon substrate(Figure 5a)[69]. They obtained Raman spectra using various excitation lasers with energies ranging from 3.04 to 1.90 eV at temperatures between 10 K and 320 K. A notable resonance enhancement in the C₆₀ pinch mode at 1468 cm⁻¹ was observed, with an enhancement factor of 100 between 2 eV and 2.6 eV. Comparing the resonance Raman peaks with theoretical predictions revealed two electronic transitions at 2.6 and 3.4 eV. They also noted a distinct inflection point(Figure 5b) in the temperature dependence of the Raman line position and width of the pinch mode, and attributed it to the phase transition. In the same year, L.D. Lamb et al. characterized C₆₀ film on silicon surface and Solution of C₆₀ in CS₂ respectively using resonance Raman spectroscopy(Figure 5c)[70]. They analyzed excitation profiles (Raman intensity as a function of the laser photon energy) for A_g(2) mode, and observed a peak around 2.4 eV in C₆₀ thin film, suggesting optical transitions occurring well below the minimum absorption level anticipated for isolated icosahedral molecules. Moreover, this transition was not observed for C₆₀ in solution. They proposed that solid-state effects might lower C₆₀ symmetry and thus permits HOMO to LUMO transitions(Figure 5d). W. Plank et al. used resonance Raman spectroscopy to analyze single bonds between two C₆₀ or C₅₉N molecules and their influence on the molecules(Figure 5e,f)[71]. They observed strong resonances with red lasers, indicating the decreased electron transition energy positions in (C₅₉N)₂ and (C₆₀)₂ compared to individual C₆₀ molecules, which was further confirmed by a lowered absorption edge in absorption spectroscopy. This decrease was attributed to intermolecular electron interactions in the dimer which leads to an electronic energy level redistribution. Recently, F. Khorobrykh and coworkers observed similar phenomena, finding that varying the wavelength of exciting lasers could distinguish C₆₀ clusters owning covalent bonds with different force constants [72]. In their works, higher force constant values corresponded to shorter-wavelength excitation needed for resonant Raman scattering observation.

M. L. McGlashen and colleagues used resonance Raman spectroscopy to characterize C₆₀ monoanions in solution in 1993[73]. They prepared C₆₀ solutions in a 70%/30% toluene/acetonitrile mix and generated C₆₀ monoanions by electrolysis at -1.0 V. The pentagonal pinch mode was strongly enhanced and showed a 6 cm⁻¹ downshift compared to neutral molecules(Figure 6a). This is highly consistent with the shift in K doped film data, thus highly support the hypothesis of complete electron transfer from K to C₆₀. S. H. Gallagher et al. achieved resonance with the C₆₀ A₁(0-1) transition using 406.67 nm laser, and attaining a higher h_g band intensity in comparison to intensity excited with 413.10 nm laser, which resonant with A₀(0-0) transition(Figure 6b)[74]. The intensity ratio of I_A1/I_A0, where I_A1 and I_A0 are the intensities of H_g band at 1421 cm^-1 excited from resonances with the A₁ and A₀ electronic transitions, matched the calculated value for D-term scattering within experimental error. Given that D-term scattering is a type of non-adiabatic scattering, this suggests the significant non-adiabatic dynamic coupling between certain excited electronic states of the C₆₀ molecule. In another research conducted by S. H. Gallagher et al., a resonance Raman peak at 281 cm⁻¹ also followed the 0-1 absorption envelope[75]. Surprisingly, the anti-Stokes side of this peak had greater intensity than the Stokes region under 413nm laser irradiation in benzene or toluene solvent(Figure 6c), despite thermal populations of the first vibrational excited state not exceeding 20%. Solvent-fullerene interactions were also observed by other teams with resonant Raman spectroscopy in the form of newly emerged and split Raman peaks due to the decreased molecular symmetry[76,77].

The structures of fullerene peapods, such as C₆₀@single-walled carbon nanotubes (SWCNT), have become newly focused point in carbon nanomaterials research. L. Kavan et al. utilized the sensitivity of resonance Raman spectroscopy to structures to characterize changes of fullerene caused by the outside SWCNT and anode/cathode charging[78], found that the A_g(2) mode of C₆₀ in fullerene peapods showed considerable intensity increase upon anodic doping(Figure 7a), which was not seen with cathodic charging. This differed from previously observed charging-induced peak shifts and was unique to fullerene peapods. They conducted follow-up experiments two years later, preparing another kind of fullerene peapods —C₇₀@SWCNT, and characterizing it with resonance Raman spectroscopy[79]. Different from C₆₀, C₇₀@SWCNT shows symmetric quenching of the C₇₀ Raman modes at both cathodic and anodic potentials(Figure 7b), indicating that the electronic structure of C₇₀ owns differently respond to charging compared with C₆₀. M. Kalbac et al. used laser with seven different excitation energies for resonance Raman spectroscopy measurements to study A_g(2) mode of C₆₀ in SWCNT[80]. The A_g(2) mode intensity weakened only under 1.16 eV laser illumination, while increasing with other energies (2.70, 2.60, 2.54, 2.41, 2.33, 2.18 eV) (Figure 7c). An A_g(2) frequency upshift during anode charging was observed, similar to other charging experiments.

Fullerene modified with functional groups could also characterized with resonance Raman technique. M. Polomska and colleagues achieved resonance enhancement of low-frequency H_g modes of (Ph₄P)₂·C₆₀·Y(Y = Cl, Br, I) using a 1064 nm laser, with modes broadened and down shifted due to electron–phonon interactions[81]. Resonance spectroscopy's high-resolution capability allowed deconvolution of the H_g band into five components which were attributed to decreased fullerene symmetry(Figure 8a,b). M. H. Herbst et al. employed resonance Raman spectroscopy to trace the formation of dimer and oligomer of platinum-fullerene compounds(Pt_nC₆₀, where n=1 and 2)[82]. The simultaneous observation of symmetric and asymmetric fullerene vibration modes in the spectrum indicates the formation of dimers and oligomers, respectively. The doublet splitting of the F_1u(2) and F_1u(4) mode also signaled the formation of oligomer chain structure in these compounds.

Surface Enhanced Raman Spectroscopy Characterization

Surface-enhanced Raman spectroscopy (SERS) significantly amplifies the signal intensity by exciting Raman scattering of samples near metal surfaces like gold, silver, or copper, achieving detection limits at the single-molecule level and greatly expanding the application scope of Raman spectroscopy. Two main theories are widely accepted to explain this enhancement: localized surface plasmon resonance (LSPR) and charge transfer (CT).[83]. LSPR occurs when incident light hits particles much smaller than its wavelength. If the light frequency matches the electron oscillation frequency, LSPR is generated, creating an enhanced electric field near the particle surface, which amplifies Raman scattering of adsorbates within this field. Additionally, the CT process is believed to contribute to SERS through chemical enhancement theory. These mechanisms contribute together to the overall SERS signal, significantly increasing its intensity.

Early in 1991, R. L. Garrell and partners had used SERS to characterize the structure and surface interactions of C₆₀ on gold surface[84]. Twenty-two distinct SRES bands were observed in the spectrum, far exceed the number of modes in conventional Raman(Figure 9a). The newly observed bands were attributed to metal-molecule vibrations and reduced symmetry due to metal surface interactions, supported by the lower shifts of major C₆₀ bands in SERS compared to conventional Raman. S. J. Chase et al. characterized next year the A_g(2) band shift of C₆₀ on different noble metal surfaces[85]. The largest shift of 28 cm^-1 compared to pure C₆₀ was observed for C₆₀/Ag, followed by 23 cm^-1 for C₆₀/Cu, and the smallest of 15 cm^-1 for C₆₀/Au(Figure 9b). The increased value of shift follows the decrease in noble-metal work function, indicating a lowered degree of charge transfer from Ag to Au, which was further confirmed with the assistance of ultraviolet photoemission spectroscopy (UPS). L. Zhixun et al. observed single-molecule SERS(SM-SERS) behavior in sample of which ultralow concentration solutions (10^-15 M) of pyridine/C₆₀ was dropped on Au coated cover glass(Figure 9c)[45]. At this density, the statistical probability for an area of hot spot to hold a single C₆₀ is less than one in a thousand. However, only around one tenth in the intensity was observed for SM-SERS comparing to the signal taken from the other sample with a 13 orders higher concentration (10^-2 M). Several additional Raman bands were also observed and were attributed to the symmetry reduction and perturbation of electronic structure in the adsorbed single molecule. Upon serious observation of SM-SERS at different hot spot, some shown distinct band shifts, while the others have no change. Time-dependent spectral fluctuation of the SM-SERS taken at same hot spot also reflects changes in intensity and band positions(Figure 9e-g). These observations collectively reflect that the SM-SERS got a much larger enhancement factor at some specific position. The exact SM-SERS was confirmed by C. G. Artur et al. in 2012[46]. They differentiated SERS signals by using C₆₀ molecules with different carbon isotopes(Figure 9d,h). By carefully screening the data and analyzing SERS spectra containing different numbers of molecules, the signal corresponding to single molecule was successfully extracted, and the intensity and frequency distribution were consistent with theoretical expectations, indicating that single molecule events were indeed observed. This provides clear evidence for SM-SERS detection of C₆₀ molecules, demonstrating the universality and ability of SERS technology for single-molecule detection.

The enhancement of signal intensity in SERS makes it useful in characterizing interaction of fullerene with other molecules or functional groups. Y. Zhao and Y. Fang studied interactions between C₆₀ and pyridine molecules, confirming weak physical interactions due to unchanged C₆₀ structures from SERS spectroscopy comparing to the pure solid[86]. M. Baibarac et al. studied particular chemical interaction of the polyaniline/fullerene (PANI/C₆₀) composite with N-methyl-2-pyrrolidinone (NMP) using SERS[87]. During the interaction, de-doping of PANI was observed, leading to the transformation of leucoemeraldine salt (LS) repeating units into leucoemeraldine base (LB). Additionally, a gradual increase in the intensity of the C₆₀ A_g(2) pentagonal pinch mode was also observed, which indicates the formation of PANI-LB and charge transfer complex of the type [(H)⁺(NMP)⁺(C₆₀)²⁻].N. Mojarad et al. used SERS to study the amorphization process of C₆₀ molecules[88], as SERS is non-destructive and does not introduce external amorphization. All C₆₀ resonances were suppressed under the electron irradiation with the appearence of G peak, which represents bond stretching of all pairs of sp² hybridized atoms. The observation represents the C₆₀ bonds cleavage and the formation of new structure. K. Yasuraoka and colleagues constructed a single-molecule junction by placing C₆₀ molecule between Au electrodes(Figure 10)[89]. The SERS spectrum showed an enhanced observation opportunity under the highly conductive state, where molecular orbital and Au electronic state are intensely coupled, suggesting that high electron coupling states would benefit SERS observation.

Due to the diverse structures and types of fullerenes and derivatives, plenty of researches have conducted to detect new method of achieving SERS enhancement. L. Zhixun and F. Yan inserted the C₆₀/C₇₀ molecule into the gap between gold nano-particles and iron substrate using the pyridine as intermediate[50], and reached an enhancement factor is 10⁶. The next year they achieved SERS enhancement by depositing gold/C₆₀ (/C₇₀) nano-clusters on the Anodic alumina oxide nano-sieves[90], as well as constructing a newly nonaqueous colloidal systems[91]. Three years later, L. Zhixun, Z. Yongsheng and coworkers fabricated high-density ordered arrays of core-shell nanopillars of Au@C₆₀/C₇₀ systems, in which an intense SERS signal with fluorescence-free background was achieved[92]. This was maintained even under 514 nm excitation, at which the normal Raman of C₆₀/C₇₀ present poor signal-to-noise. N. Zhiqiang et al. connected C₆₀ molecules between silver nano-particles and silver film by using the pyridine as intermediate[93]. Several vibrational modes were greatly increased as well as showing significant band splitting and frequency shifting, meaning the existence of symmetry lowering and selection rule relaxing of C₆₀ from asymmetrical environment. I. I. S. Lim et al. found out that when assembled with 1-(4-methyl)-piperazinyl fullerene(MPF), larger-scale Au nanoparticles (30 nm) produce much stronger SERS intensity than smaller ones (11 nm), indicating the significant influence of metal particle size on SERS enhancement [94]. Recently, N. Khinevich et al. achieved SERS enhancement of C₆₀ using silvered porous silicon (Ag/PS) for the first time[95].

Tip Enhanced Raman Spectroscopy Characterization

Tip-enhanced Raman spectroscopy (TERS), a near-field spectroscopy that surpasses the optical diffraction limit, has shown unique advantages in characterizing single molecules in recent years. As an enhancement of SERS method, TERS focuses incident light onto a nanoscale metal tip (typically gold or silver) and utilizes the local surface plasmon resonance effect, resulting in a strong electromagnetic field enhancement that significantly boosts the detection sensitivity of Raman signals[96]. Charge transfer between the metal tip and sample also contributes to TERS signal enhancement. The enhancement effect enables TERS to detect the vibrational modes of individual molecules, providing extremely high resolution for studying molecular structure and chemical states. TERS technology holds great potential in researching fullerenes and their derivatives, offering extremely high spatial resolution beyond the optical diffraction limit and enhanced Raman vibrational mode detection ability, which allows for in-situ, dynamic, and quantitative analysis of fullerenes and their derivatives without damaging the sample. With utilizing TERS technology, researchers can analyze the molecular structure, chemical state, and interactions with the surrounding environment of fullerene derivatives in detail at the nanoscale. In this section, we will explore the application of TERS technology in fullerene derivative research in detail.

Early in 2000, R. M. Stöckle and coworkers had utilized TERS technology to characterize fullerene[97]. They drop-coated C₆₀ molecules onto a glass substrate and scanned with homemade tips fabricated by electrochemical etching with thin Au wire. 488 nm wavelength laser was focused on the tip from below to excite the TERS. The C₆₀ Raman signal was observed when gold tip close to the sample, with almost no signal detected without the tip. Considering the scale of the tip apex and laser beam diameter, the enhancement was estimated to be greater than 40000. Both shifted and unshifted Raman peaks were observed, and were attributed to be enhanced by electromagnetic and chemical effects, respectively. P. Verma et al. also achieved TERS enhancement of C₆₀ using a silver-coated silicon tip and placed molecules on a silver film-covered glass cover slip[98]. All the Raman modes were enhanced in TERS signals comparing to the far-field spectrum. In their subsequent investigation of tip-force effect, the frequency shift observed when tip applied a certain extent of force (2.7nN to 4.7nN) to C₆₀ was contrary to DFT calculations. This phenomenon was attributed to photopolymerization under high tip pressure and under light irradiation at the same time.

In recent years, numerous studies on TERS characterization of fullerene have been published, driven by advancements in TERS technology for analyzing single molecule. In 2022, B. Cirera and colleagues studied TERS intensities of a single C₆₀ molecule in Scanning Tunneling Microscope (STM) system under tunneling regime and molecular point contact (MPC) situations, respectively(Figure 11a)[99]. They adsorbed C₆₀ onto the Ag tip and observed TERS signals as the distance between the molecule and Ag(111) surface varied. When the molecule contacted the metal surface, characteristic TERS intensity enhancement was observed, enabling the observation of modes undetectable in the tunneling regime(Figure 11b). A red shift in most C₆₀ modes compared to solid-state values was also noted. These effects were observed on other metal surfaces like Au(111), Cu(111), and Pt(111), confirming their universality. The additional enhancement in MPC situation was attributed to the chemical effects rather than electromagnetic enhancement, which was caused by charge transfer in the single molecule junction. B. Cirera et al. conducted a similar MPC experiment to study vibrational heating effect by observing the anti-Stokes lines of C₆₀ Raman modes[100]. They identified optical heating as the main mechanism in the tunneling regime(Figure 11c), while Joule heating dominated in MPC through inelastic electron-vibration scattering (IEVS) (Figure 11d). The influence of the electronic structure of the electrode to vibrational heating was also examined on different metal surfaces. The heating efficiency showed asymmetry in opposite bias polarity for Au(111) and Cu(111) surfaces, and became lower when C₆₀ was in point contact with Pt(111) surface than other noble metals, although Pt exhibited the largest conductance. Q. Meng et al. invested local thermal effect of C₆₀ single molecule via anti-Stokes TERS technology[101]. They studied the heating phenomenon with the defined effective temperature (T^eff ~ I_S/I_aS) as the tunneling current increased and observed the molecule decomposition at around 1150K(Figure 11e,f). The research demonstrated the ability of TERS to noninvasively detect local heating effects in a single molecule under nonequilibrium conditions and to identify the possible reaction pathway and product through spectrum sensitivity to molecule structures, which was also applicable to other fullerene derivatives. B. Cirera et al. achieved TERS enhancement with a factor of ~10¹² for a C₆₀ single molecule even on a Si(111)-(7×7) semiconductor surface, and observed additional Raman signal enhancement in MPC condition[102]. This indicates TERS's applicability for characterizing organic molecules on semiconductor surfaces, with potential applications for single-molecule Raman studies of vibrational heating and dissipation in metal-molecule-semiconductor nano-heterojunctions.

Conclusions and Perspectives

To date, Raman spectroscopy have provided distinct vibrational signatures that offer valuable insights into the molecular structure and electronic characteristics of fullerenes and their derivatives. Beyond structural elucidation, Raman spectroscopy also enables the investigation of fullerene–molecule interactions and environmental influences. By monitoring shifts in Raman peaks under various conditions, scientists can quantify the impact of solvents, temperature, and other external factors on the structural integrity and properties of fullerenes. Furthermore, advanced techniques such as Resonance Raman Spectroscopy and SERS have significantly broadened the scope of fullerene research. Resonance Raman Spectroscopy enhances specific vibrational modes via resonance effects, while SERS facilitates single-molecule detection, thus providing deeper insights into fullerene structures and dynamics. TERS further pushes the frontier by enabling in situ analyses of fullerene derivatives under single-molecule conditions, delivering nanoscale spatial resolution and ultra-high detection sensitivity. Notably, TERS is also capable of detecting the optical and vibrational heating effects in individual molecule, as well as observing the thermal decomposition of molecule during the heating process.

Raman spectroscopy and its enhanced versions have already made significant progress in the study of fullerenes and their derivatives. In the following, we attempt to provide some possible future directions to further expand the application of Raman spectroscopy in this field:

1. Combining Raman Spectroscopy with Artificial Intelligence (AI)

The Raman spectra of fullerenes and their derivatives are relatively complex, and the orientation effects between the molecules and the SERS substrate further increase this complexity[103]. Machine learning is particularly well-suited for analyzing these intricate spectra, allowing for the extraction of key signals, identification of weak features, and elimination of unnecessary noise. Additionally, AI can be utilized to optimize substrates and optical pathways, potentially enhancing the SERS detection efficiency of fullerene materials[104,105]. Developing machine learning algorithms and big data analytics to establish a comprehensive theoretical and experimental fullerene database will accelerate our understanding of fullerene systems and facilitate the discovery of new fullerene materials.

2. Further Improving the Resolution of TERS

TERS offers ultra-precise molecular characterization. Currently, angstrom-scale spatial resolution has been achieved with TERS on planar molecules such as tetraphenyl porphyrin (TPP)[106,107]. However, TERS studies on fullerene materials are primarily limited to C₆₀, and the characterization of other fullerene derivatives remains relatively scarce. Achieving similar resolution for spherical fullerene structures presents additional challenges. Nevertheless, with ongoing advancements in TERS technology, attaining angstrom-scale spatial resolution for fullerene materials is likely within reach. Further improvements in the spatial resolution of TERS will enable the acquisition of more detailed structural and property information about fullerenes and their derivatives.

3. Combining Raman Spectroscopy with Time-Resolved Techniques

As femtosecond laser technology and ultrafast spectroscopic methods continue to mature, time-resolved Raman spectroscopy has been successfully applied to study ultrafast processes in various materials[108]. Examples include the ultrafast structural dynamics of photoreceptor proteins[109], the formation of polaron pairs and electron polarons in polymer photocatalysts[110], and the time dynamics of optical phonon anharmonic coupling and electron-phonon interactions in graphite[111,112]. Applying this technology to fullerene materials will enable real-time characterization of structural and property changes in fullerene molecules and their derivatives during dynamic processes such as electrochemistry, catalysis, thermal decomposition, and light-induced transformations. This will provide essential data for advancing the preparation and application of fullerene-related materials.