Raman Radial Mode Revealed from Curved Graphene - The Journal

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Letter

Raman Radial Mode Revealed from Curved Graphene Jae-Kap Lee, Kailash P.S.S. Hembram, Yeseul Park, Sang Gil Lee, Jin-Gyu Kim, Wooyoung Lee, and Dong Ju Moon J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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The Journal of Physical Chemistry Letters

Raman Radial Mode Revealed from Curved Graphene Jae-Kap Lee,*† K. P. S. S. Hembram,† Yeseul Park,†‡ Sang-Gil Lee,§ Jin-Gyu Kim,§ Wooyoung Lee,‡ and Dong Ju Moon¥ †

Opto-Electronic Materials and Devices Research Center, Korea Institute of Science

and Technology, Seoul 130-650, Republic of Korea. ‡

Department of Materials Science and Engineering, Yonsei University, 262

Seongsanno, Seoul 120-749, Republic of Korea. §

Division of Electron Microscopic Research, Korea Basic Science Institute, Daejeon

305-333, Republic of Korea. ¥

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul

130-650, Republic of Korea. *Correspondence should be addressed to [email protected]

ABSTRACT: One of the unsolved fundamental issues on graphene is establishing an appropriate way to discern layers of graphene structures. We report a simple methodology to analyze graphene structures using Raman signals in the range ~100~500 cm-1 comprising clear 118 or 175 cm-1 peaks. We demonstrate that the low energy signals on Raman spectra of plasma seeded grown graphene sheets are originated from nano-curvature (c) of mono- (175 and 325-500 cm-1 signal) (c ~1 nm) and bilayer (118 cm-1 peak) (c ~2 nm) graphene with Raman simulations, based on Raman radial mode (RM) Eigen vectors. Our RM model provides a standard way of identifying and evaluating graphene structures.

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Graphene, a monolayer of honeycomb sp2 carbon atoms, has unlimited potential due to unique and outstanding physical properties.1 One of the unsolved fundamental issues on graphene is establishing an appropriate way to discern layers of graphene structures. Atomic- or high-resolution transmission electron microscopy images revealing edge lines of planar view graphene provide uncontroversial direct evidence of the presence pure graphene.2-5 Previously reported Raman approaches (e.g. shape of 2D-peak6-8 or an intensity ratio of 2D-peak to G-peak, I2D/IG9) cannot be criterions to discern graphene because the shape of 2D-peak for monolayer graphene has been reported to be similar to those for multilayer graphene10-13 and the intensity ratio is just an indication of electronic decoupling of graphene layers.14 Practically the ratio reported in the literature ranges ~1.0-4.5,15 indicating limitation of the intensity ratio approach.

Figure 1. Schematic models explaining RM of Eigen vector. (a,b) Eigen vectors for flat (a) and curved (b) graphene (‘c’ and ‘r’ represent curvature and radius of curvature). (c-e) Simulated Raman-active mode Eigen vectors for concentric (c) and curved (d,e) graphene with the same curvature of d(~c) ~1.4 nm. The concentric tube (SWNT (10,10), d=1.37 nm) reveals coherent symmetric RBM at 184 cm-1 (c) and curved graphene reveals RM at 191 (d) and 166 cm-1 (e). The variation of Raman shifts is due to strain-induced deformation of graphene.



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Nevertheless, Raman spectroscopy is a simple and powerful tool to explore graphene structures due to the capability of discerning even slight changes in structure.16-18 Raman signals are originated from collective lattice vibration of atoms.19 For twodimensional (2D) materials like graphene, the long wave length fluctuation makes the net structure ripple.20 The coupling of long wavelength fluctuation with anharmnic out-of-plane vibration enhances the buckling of graphene. The rippled or buckled (i.e., curved) graphene produces radial mode (RM) Eigen vector (Figure 1b), and simulations show that a graphene sheet with nano-curvatures (c) reveal additional Raman signals in low energy range (Figure 1d,e).

Raman radial breathing mode (RBM) of single-wall carbon nanotubes (SWNTs) suggested by Rao et al21 is a type of RM. RBM condition occurs only when all RM Eigen vectors head for the center of a concentric tube (Figure 1c), and thus is known to be unique to SWNTs16,18,22-24 (or double-wall carbon nanotubes (DWNTs).25 Rao et al. also reported the relationship between RBM signals and diameter of SWNTs with the governing equation, wRBM=A/d+B, where wRBM is wave number, d is diameter of a tube, A and B are constant determined by experiment.16,22-24 For SWNTs including commercial HiPco material, the range of RBM wave number is reported to be 167301 cm-1 (~0.77 nm < d < ~1.44 nm).24

The low energy Raman signals have been observed experimentally from graphene samples prepared by different methods. Podila et al.26 observed the signals at ~150 and ~170 cm-1 from mechanically exfoliated mono- and bilayer graphene. Lui et al.27 observed the signals at ~80, ~113, ~174 and ~198 cm-1 from few layer graphene, prepared by micromechanical cleavage of kish graphite when heated up to 400-900 K


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by laser. Campos-Delgado et al.28 and He et al.29 separately confirmed the signals at ~100 cm-1 and 110-200 cm-1 from chemical vapor deposition (CVD) bilayer graphene, respectively. Wang et al.30 confirmed the signals at 228 and 355 cm-1 (100-500 cm-1) from corrugated graphite sheets comprising 1-3 graphene layers. Verzhbitskiy et al.31 also measured the signals at 130-150 cm-1 from cove-shaped graphene nanoribbons synthesized by solution-based processing. Cai et al.’s Raman spectrum for chevrontype graphene nanoribbons (0.74 nm width)32 revealed a peak at 396 cm-1. All the signals appear as bands with one or two (band-like) peaks. Lui et al.27 attributed the signals appearing at the elevated temperature to layer breathing mode, while CamposDelgado et al.28, and He et al.29 separately attributed the signals to twist of bilayer graphene without providing Moiré pattern33,34 for twisted bilayer graphene. Others interpreted the signals as RBM-like mode.30-32 These indicate that the origin of the low energy Raman signals for graphene is unclear.

Figure 2. HRTEM images and Raman spectrum of plasma seeded grown graphene. (a,b) HRTEM images revealing mono- and bilayer graphene sheets. Arrows in (b) indicate incompletely formed bilayer graphene. (c) Atomically resolved TEM image for the rectangular zone of (b), revealing clear hexagon lattices for monolayer graphene (center) and line or dot lattices for spatially overlapped graphene (edges). (d) Raman spectrum of graphene



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sheets revealing RM, D and G peaks. Clear RM signals at 118, 175 and 250-325 cm-1 are evident in inset. Blue dotted line in (d) is the base line of the spectrum.

In this letter, we synthesize graphene structure by plasma CVD seeded growth (see Methods for details of the technique), where mono- and bilayer graphene are evident and reveal Raman signals in the range ~100-~500 cm-1. We demonstrate that the low energy Raman signals are originated from curved graphene structures with our RM Eigen vector model, supported by Raman simulations.

Figure 2a-c show HRTEM images of plasma CVD seeded grown graphene samples (1,073 K, 30 min) which reveal wrinkled morphology (Figure 2a). Monolayer graphene sheets prevail and appear to be spatially overlapped due to HRTEM imaging principle (Figure 2b,c). Bilayer graphene is also evident in Figure 2b (‘2’ and arrows). The low energy Raman signals between ~100 and ~500 cm-1 are evident with strong G and D peaks on the Raman spectrum. The low energy Raman signals are relatively noisy, but comprise relatively strong peaks at 118 and 175 cm-1 (inset in Figure 2d) and some weak bands.

Clear and flat graphene sheets are synthesized at higher temperature (1,273 K, 30 min) as shown in Figure 3a-c. With the edge lines, mono- and bilayer graphene are clearly distinguishable. The majority area of the samples is composed of bi- or trilayer graphene and the top portion consists of monolayer graphene. The well-developed graphene samples also reveal a clear low energy Raman signals at 118 cm-1 and weak broad signals ranging from 165 to 500 cm-1 (comprising some weak bands), together with the strong D and G peaks (Figure 3d).


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Figure 3. HRTEM images, Raman spectrum of plasma seeded grown graphene and Raman

simulation. (a-c) HRTEM images revealing mono-, bi-, and trilayer graphene sheets (a,b) together with thin graphite (c). Arrows in (a,b) indicate curved edge of graphene sheets (1, 2 and 3 refer mono-, bi- and trilayer graphene, respectively). Spatially overlapped graphene sheets are also evident (c). (d) Raman spectrum of graphene sheets revealing RM, D and G peaks. A clear signal at 118 cm-1 and some bands ranging from 165 to 500 cm-1 are evident in inset. Blue dotted line in (d) is the base line of the spectrum. (e,f) Simulated Raman modes for mono- (e) and bilayer (f) graphene with different end curvatures ranging from 50 to 487 cm-1 (Figure S1, Supporting Information). oc and ic indicate outer and inner curvature, respectively.

The revelation of the edge lines of mono- and bilayer graphene (Figure 2b and 3a,b) indicates that their ends are curved (see schematic in Figure 3a). The graphite sheet (comprising four or five graphene layers, shown in Figure 3c) also reveals curved signature. Practically, edges of freestanding graphene tend to curve (or scroll) as



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confirmed by Meyer et al.,35 and such unique TEM images prevail in the literature.3-5 Well-developed CVD single-crystalline graphite plates (comprising six graphene layers)36 and high quality CVD WS2 films (comprising one-three layers) also possess curved edges.37 These data indicate that the end curvature is the nature of 2D freestanding structures.

The simulation data (Figure 1d,e and Figure 3e,f) demonstrate that nano-curvatures of mono- and bilayer graphene (arrow in Figure 3a,b) reveal the low energy Raman modes at 50-487 cm-1 (Figure S1, Supporting Information). Shifts in the Raman modes depend on arc length of the end curvatures. ~1/4 arc of c ~1.0 nm reveals at 221, and 316 and 363 cm-1 (Figure 3e). The RM modes become higher with ~1/2 arc of c ~1.0 nm (250-421 cm-1) as well as with steeper curvature (c ~0.8 nm) (379-487 cm-1), but become lower with increased curvature (c ~1.2 nm) (210-294 cm-1). From the simulation data of monolayer graphene (Fig. 3e), we deduce the relation of RM

peak frequency (ω) with curvature (c) to be ω=A +B where A and B are 358 and -58

cm-1, respectively (Figure S2, Supporting Information).

Simulations of bilayer

graphene structure (outer curvature (oc) ~2.0 nm) reveal the 118 cm-1 signal measured in Figure 2d and 3d as well as 274 and 290 cm-1 signals which belong to the range of monolayer graphene. Thus, we assign the band ranging 165-325 cm-1 to curvatures of both bi- and monolayer graphene (Figure 3e,f).

With clear HRTEM morphological evidence and additional Raman spectra (Figure 4), we assign the strong 118 cm-1 peak and the broad weak 325-500 cm-1 band in Figure 3d (which also appear in Figure 2d) to end curvatures of bi- (oc ~2.0 nm) and monolayer (c 0.8-1.2 nm) graphene, respectively. We attribute the weak broad band


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for monolayer graphene to its small area, confined on the tip of bilayer graphene (arrow for ‘1’ in Figure 3a,b). It is difficult for small sized graphene attached to bilayer graphene to make its own curvature, but produces plural end curvatures resulting in the broad and weak (band) signals in the range of 165-500 cm-1.

Figure 4. Raman RM signals from graphene samples synthesized at different temperature (Figure S3, Supporting Information). (a) 1,073 K. (b) 1,273 K. With HRTEM morphological evidence shown in Figure 2,3 and the profiles of the Raman spectra, we divide the RM range into three, ~100-165 cm-1, 165-325 and 325-500 cm-1, which are for curvatures of bi-, bi- + mono- and monolayer graphene, respectively. Red, blue and black dotted lines in (a,b) are the base lines of the spectra.

We also assign the 175 cm-1 peak, shown in Figure 2d (which does not appear in Figure 3d), to monolayer graphene with c ~1.4 nm because the Raman shift is similar to those of SWNTs. SWNTs can be considered to be a graphene sheet with a curvature (Figure 1c). The simulation data, shown in Figure 1d,e, demonstrate that curved (wrinkled) graphene with c ~1.4 nm at the central zone reveals low energy Raman signal at 166 and 191 cm-1, explaining the revelation of 175 cm-1 peak. We



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expect that the unique peak was obtained from one or few monolayer graphene sheets with the curvature of c~1.4 nm, because it is very weak for other spectra obtained from different place of the sample (Figure 4a). These analysis also support the geometry dependency of the low energy Raman signal, confirmed in the simulation for end curvatures of graphene (Figure 3e,f). Wrinkled graphene (Figure 2a,b) also produces diverse low energy Raman signals (Figure 1d,e) because the structure comprises numerous curvatures. This explains the band-like low energy Raman signals from corrugated30 or random nanoribbon-typed31 graphene samples.

We attribute the noisy signals at ~100-165 cm-1 (assigned as curvatures of bilayer graphene) (Figure 2d and 4a) to the wrinkled morphology of bilayer graphene appearing as ‘non-straight’ features (arrow in Figure 2b) (also compare with planar bilayer graphene as shown in Figure 3a). This is supported by Wang et al.’s data30 where their corrugated graphene structures reveal broad signals 150-400 cm-1. We also attribute the clear and strong signal of 118 cm-1 peak to well-developed bilayer graphene which makes a unique curvature (see ‘2’ in Figure 3a). On the other hand, the sample grown at 1,273 K (Figure 3) comprises thin graphite with a thickness of a few nm (three to ten graphene layers, as shown in Figure 3d) with stacking of graphene layers during the growth, explaining the strong G peak compared with that of the sample grown at 1,073 K (Figure 2d). The graphene sheets reveal welldeveloped single-crystalline feature with a size of several hundred nm2 as confirmed by HRTEM and Raman analysis.

The noisy signals in the low energy (significant noise to signal ratio) indicate the presence of graphene structures with many curvatures (Figure 2). The clear and strong


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peak indicates the crystalline graphene structures (Figure 3). Hence the area under the curve of the RM signals is useful for estimating flatness as well as crystallinity of mono- or bilayer graphene (i.e., the area between spectra and the base lines, shown in Figure 2d and 3d, where that of 1,073 K is much larger than that of 1,273 K). The RMs for bilayer graphene are unclear, compared with those for monolayer graphene, indicating that the modes of bilayer graphene are resulted from coupling of both layers, rather than from individual graphene layer. We expect that the unclear RMs for bilayer graphene may be due to disordered stacking between two graphene layers.

Our RM model addresses the revelation of RM (band) signals from other type of graphene structures,26-32 explained with layer breathing mode model for the revelation of the signals at the elevated temperature (>600 K)27 and twisted bilayer graphene model.28,29 We expect that their signals may be from curved mono- or bilayer graphene. At the elevated temperature (>600 K), vibration becomes active, and causes graphene to form curvatures. He et al.’s optical image (Figure 1a of ref. 29) also reveals the evidence for curved edges of graphene samples (due to peeling-off). Furthermore, the low energy ‘band’ signals strongly support that they are originated from curvatures. Even single curved graphene comprises continuous plural curvatures, explaining the unique band signal comprising one or two peaks reported by Podila et al..26 If the signals were originated from RBM-like mode,30-32 graphite should have revealed the low energy signals, which is contradictory. With the facts that our RM model includes the RBM hypothesis of SWNTs (Fig. 1c) and the range of low energy Raman signals for commercial SWNTs (167-301 cm-1)24 overlaps that of monolayer graphene (165-325 cm-1), our results can provide a new approach to understand the nature of SWNTs as well as graphene.



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EXPERIMENTAL METHODS

Plasma seeded growth of graphene. For plasma seeded growth of graphene, graphene nanopowders (~5 nm in size) were served as seeds, placed on a molybdenum substrate and were grown under a direct current plasma generated at 100 Torr with a 10%CH4-90%H2 gas mixture for 10 minutes.4

Total gas flow was

maintained at 200 sccm. Input power was 1.2 kW. The seed growth was performed at two temperatures of 1,073 K and 1,273 K which was measured by a pyrometer targeting the top of the substrate. The growth condition (similar to diamond growth) was stable throughout the experiment. Analysis of the graphene samples. Seeded grown graphene samples, which were grown on the molybdenum substrate and appeared as a powder form, were analysed by an aberration corrected energy-filtered TEM (200 kV, Libra200 HT Mc Cs TEM; Carl Zeiss) and Raman (Renishaw In-Via Raman Microscope with laser excitation of 532 nm and spot size of 1~2 ㎛). For Raman analysis, some of the sample were taken and put it on a slide glass. For HRTEM observation, other of the sample were taken and placed on a TEM grid. Raman simulations. We carried out density functional theory (DFT) calculation as implemented in QUANTUM ESPRESSO simulation package.38 Generalized gradient approximation (GGA) was used for exchange correlation energy of electrons and ultrasoft pseudopotentials to represent interaction between ionic cores and valence electrons.39 Kohn Sham wave functions were represented with a plane wave basis with an energy cutoff of 40 Ry and charge density cutoff of 240 Ry.40 Integration over Brillouin zone (BZ) was sampled with a mesh of 1x1x2 grid.41,42 Various curved graphene structures were taken for investigating the low energy Raman active modes


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(Figure S1, Supporting Information). For instance, for monolayer structure, a series with increase in radius of curvature (~0.8 to ~1.2 nm) were considered. Again with fixing the radius of curvature, many structures with increase in arc length (~1/4 to ~1/2 of an imaginary circle) were considered. Finally bilayer curved graphene structures were considered with varying the arc length at a fixed inner and outer radius of curvature (oc ~2.0, ic ~1.3 nm). All the structures were initially relaxed and hence the relaxed structures loo similar (Not the exact same). Then, dynamical matrices at the Γ point (q=0) in BZ were computed using perturbative linear response approach used in DFT for Raman simulations. The Eigen vectors of the respective modes are visualized to understand their local vibrational characteristics.

ASSOCIATED CONTENT

Supporting information

The Supporting information is available free of charge on the ACS Publications website at DOI: Detailed simulated Raman modes for mono- and bilayer graphene with different end curvatures ranging from 50 to 487 cm-1.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Present Address: Opto-Electronic Materials and Devices Research Center, Korea Institute of Science and Technology, Seoul 130-650, Republic of Korea.

ACKNOWLEDGMENTS



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This work was supported by KIST Future Resource Program (2E26390 and 2E27150).

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