Deciphering the Raman response of graphene nanoribbons

June 22, 2022 - Controlling the edge morphology and terminations of graphene nanoribbons (GNRs) enables tailoring their electronic properties and increases their application potential. Scientists from the Universities of Vienna (Austria), Aveiro (Portugal), and Ulm (Germany), as well as Sun Yat-sen University, Huzhou University, and ShanghaiTech University (China), and the National Institute of Advanced Industrial Science and Technology in Tsukuba (JP), have developed a crucial step in analyzing the nanoribbon nanotube system encapsulated in CNT to decipher the Raman signatures of individual CH modes for GNR with unprecedented detail.

Controlling the edge morphology and terminations of graphene nanoribbons (GNR) enables tuning of their electronic properties and increases their application potential. Here, terrylene is used as a precursor for fabricating long and ultrathin GNRs in nanotubes. The orientation and characteristic length of the encapsulated terrylene parallel to the tube axis facilitates GNR formation by polymerization with high stability up to 750 °C. This is not only a crucial step in the analysis of GNR-CNT systems, but also a study that allows for the first time to decipher the Raman signatures of the individual CH modes (the edge passivation signature) for GNR, in unprecedented detail.

Over the past three decades, the foundations have been laid for the inclusion of a variety of semiconducting nanostructures such as graphene nanoribbons (GNRs) as basic building blocks in a wide range of technologies. [2-5] These flat aromatic macromolecules arranged as narrow graphene strips are defined by the number N of dimer lines across their width. Their properties are determined by their edge structure (armchair or zigzag) and width [10], but the fabrication of GNR with atomically precise properties presents significant challenges. However, fabricating GNRs with atomically precise properties presents significant challenges.[13] Top-down methods struggle with edge quality and controlling the width of the ribbon, while bottom-up approaches are generally more expensive and require complex precursor molecules or substrates for growth.[14] Surface-based methods require metal substrates-such as gold-that serve as both catalysts and edge-control mechanisms.[20]

An alternative method for fabricating GNRs is to exploit the chemically inert environment inside carbon nanotubes (CNTs). This spatial confinement has proven effective in confining and stabilizing 1D structures such as inner concentric tubes[21-23], carbyne chains[24-27], and GNRs with doped ends[28-30]. In particular, the availability and use of single-walled CNTs (SWCNTs) has two major advantages: one-dimensional spatial confinement and the protective role of the tube.

Furthermore, GNRs can be tailored by choosing the SWCNT diameter[13, 31] and no transfer from a metallic substrate is required to eliminate any influence of the GNR environment when inside a tube.[19] Small molecules such as coronene, fullerene derivatives,[28] ferrocene[31] and perylene[29] have been successfully encapsulated and converted into GNRs within SWCNTs. Such hybrid systems offer the advantage of simultaneously investigating the properties of a SWCNT and a semiconducting encapsulated GNR. The latter encapsulated structure, perylene, is a short molecule belonging to a family of dyes. It has been observed to stack at an angle to the SWCNT axis, which hinders its effective polymerization to GNR inside the tubes[32]. On the other hand, there are other dye molecules such as terrylene or quaterrylene that have very similar aromatic configuration but differ in length.

In this work, the terrylene dye molecule was used as a precursor to prepare long and ultrathin (N = 5 C atoms) seated GNRs (5-AGNRs) in SWCNTs. The orientation and characteristic length of the terrylene molecules encapsulated parallel to the axis of SWCNTs facilitated the formation of 5-AGNRs, which exhibited high band stability when annealed at up to 750 °C and the hybrid system was maintained under high vacuum. Long and very narrow GNRs inside nanotubes with a diameter of 1.4 nm were detected by low-voltage transmission electron microscopy (LV-AC-TEM).

Results and discussion

SWCNTs (buckypapers) were filled with terrylene at 350 °C as outlined in Figure 1a. Without further thermal treatment, partial polymerization of the terrylene occurs to form short ribbons (Figure 1b). This material was studied by Raman spectroscopy as it was synthesized. Since molecules of unpolymerized terrylene are expected to remain outside the nanotube after the encapsulation process, as has been reported for other fillers,[29] it is reasonable to assume that large amounts of unencapsulated terrylene may interfere with the Raman signal of the newly formed hybrid structures.

The first approach to improve sample purity was to wash the hybrids (outlined in Figure 1c) with dichloromethane (DCM). Raman spectra were recorded before and after washing. The high-resolution microscope image taken in an aberration-corrected system (Figure 1f) shows long and broad structures in a hollow tube. Additional TEM images can be found in Figure 2a. The filling can be confirmed by the contrast profile in Figure 1g, based on the randomly selected cross sections with the color lines on the left. The theoretical width of a free-standing 5-AGNR is 0.49 nm, which is very close to the value estimated from the width profile in the top two panels of Figure 1g.

Identification of CH vibration signatures

Subsequent analysis including TEM (Fig. 2a) focuses primarily on revealing the signature of the CH modes based on an understanding of the Raman spectral features observed for terrylene, untreated nanotubes, and the encapsulated bands[20,35]. The three lower spectra in Figure 2b correspond to Raman signals recorded with an excitation wavelength of 785 nm. Using this laser line, it is possible to see a broader active response (more peaks), and it is clear that the spectrum of the filled structure is not simply an overlap of the first two, but consists of additional features that could be associated with nanoribbon formation. The position of the CH modes of the 5-AGNR@SWCNT at lower frequencies could be associated with the H terminations on the encapsulated terrylene edges. These terminations are particularly important for defining the physical properties of the GNR and implicitly for the overall Raman signal. However, this Raman shift may also be related to the formation of the expected nanoribbons,[37] in which case other strong CHnGNR modes with in-plane bending characteristic of the 5-AGNR@SWCNT must be considered.

A closer look at the region between 1200 and 1400 cm-1 reveals several peaks originating from the CH vibrations. The spectrum from the 633 nm excitation has a better resolved fine structure in the CH mode region, and peak deconvolution by a Voigt fit was performed (see Figure 2c).

The peak centers of the identified components, labeled 1 to 8, are listed in the table in Figure 2d. Most of the peaks found can be associated with known terrylene CH vibrations or the SWCNT-D band. However, the highlighted peaks at 1230 and 1247 cm-1 are most likely from the 5-AGNR@SWCNT hybrid system. In addition, the D-like mode (DLM) appears at ≈1365 cm-1[31,38] and small out-of-plane CH modes appear around ≈800 cm-1.[39]

Unbundling of GNR-related modes

Further investigation of the GNR edge modes requires the removal of adsorbed terrylene. For this purpose, the filled SWCNT samples were annealed for two hours in a vacuum of better than 10-5 mbar, each at a different temperature between 400 and 800 °C in 50 °C steps. The evolution of the intensity of the Terrylene G-band as a function of annealing temperature is shown in Figure 3a. The exponential decrease in intensity observed for the G-band indicates the removal of adsorbed terrylene with increasing temperatures. Therefore, the pattern plotted in blue can be used to further quantify the influence of the residual outer terrylene on the Raman spectra of the 5-AGNR@SWCNT. This curve can then be used as a scaling factor to analyze the spectral values for other excitation wavelengths, taking into account the laser line broadening, sensitivity, and resolution of the detector. In short, when using the Voigt fits of the G-band, the Gaussian width depends on the emission linewidth of the laser and on the resolution of the spectrometer, while the Lorentzian width depends on the vibrational lifetime and the Raman resonance. With this in mind, it is possible to subtract the Terrylene spectrum from the integrated signal of the 5-AGNR@SWCNT at a given annealing temperature. Further details on the empirical Terrylene model for the subtraction are given below*. Figure 3b illustrates the procedure for a sample measured immediately after filling and after a high temperature anneal at 600 °C. For this analysis, the spectra obtained with an excitation wavelength of 633 nm were used, since this is close to the optical gap of both Terrylene and 5-AGNR@SWCNT. The corresponding empirical Terrylene models, taking into account the broadening of the laser line and the sensitivity of the detector, were subtracted from the experimental data, resulting in spectra without Terrylene fingerprint. Only in this way were we able to decipher - for the first time - peaks that could not be assigned to either Terrylene or SWCNT, but responded exclusively to vibrational modes of 5-AGNR. Changes in CHnGNR modes upon annealing: looking again at Figure 3b, we see that there are two peaks at 492 and 1230 cm-1 that show high intensity after subtracting the Terrylene signal normalized to the G-band scaling factor. Their intensity evolution was analyzed as a function of annealing temperature, and no changes were observed up to temperatures between 550 and 600°C.

These GNRs were modeled encapsulated in metallic and semiconducting SWCNTS. For example, Figure 4a,b shows top and side views of a molecular model of a 5-AGNR in a SWCNT with (17,0) chirality (5-AGNR@(17,0)SWCNT).

Figure 4c shows the DFT-calculated spectra for a 5-AGNR@(19,0)SWCNT and a 5-AGNR@(18,0)SWCNT, which have a semiconducting and a metallic SWCNT host, respectively. Both tubes are close (±0.5 nm) to the experimental mean diameter of the nanotube samples. The spectrum recorded with an excitation wavelength of 785 nm on the sample annealed at 600 °C is compared with the calculated spectra in the figure. The signal with the highest intensity of the 5-AGNR@(19,0) SWCNT can be assigned to the G-band, followed by the CH modes at 1298, 1280 and 1234 cm-1. On the other hand, the G-band signal of the 5-AGNR@(18,0) is suppressed, while the highest contributions in this region are the modes at 1280 and 1298 cm-1, followed by the 1234 cm-1 mode. Other modes appear in the lower frequency ranges. For 5-AGNR@(19,0), it is also observed that the RBLM at 534 cm-1 and the RBM at 460 cm-1 (corresponding to the nanotube) are weaker than the G-line. The hybrid mode between 460 and 492 cm-1 in the RBLM region originates from an asymmetric vibration of the encapsulating SWCNT. This second mode from the mixed nanotube-nanoband vibration is found in the calculated spectrum of 5-AGNR@(19,0) compared to its counterpart at a lower frequency.

The plot in Figure 5 shows how their intensity decreases linearly at higher temperatures, confirming that the previous assignment of the 5-AGNR@SWCNT modes is correct. This also means that the GNR@SWCNT hybrid structure is stable up to 550-600 °C. After this temperature, the decrease in the signal-to-noise ratio indicates the decomposition of the GNR inside the tubes into amorphous carbon until the supplied thermal energy is sufficient to form internal SWCNTs.[24, 41] Considering the dependence of the signals in Figures 2b and 5 on the temperature of the terryl and the 5-AGNR@SWCNT, it is reasonable to use data recorded from samples heated above 600 °C for further peak analysis. At this temperature threshold, the 5-AGNR signal starts to decrease, while the Terrylene signal has already dropped below 20% of its original intensity. This means that annealing up to the described threshold defines the final conditions for obtaining long 5-AGNR@SWCNT before they shorten and transform into new encapsulated structures (as outlined in Figure 1e).

Resource: Milotti, V., Berkmann, C., Laranjeira, J., Cui, W., Cao, K., Zhang, Y., Kaiser, U., Yanagi, K., Melle-Franco, M., Shi, L., Pichler, T., & Ayala, P. (2022). Unravelling the complete Raman response of graphene nanoribbons discerning the signature of edge passivation. Small Methods, 6(8), 2200110.