[Paper Review] Effects of Layer Stacking on the Combination Raman modes in Graphene

Rahul Rao, Ramakrishna Podila+5 more|Figshare|Jun 6, 2011

Photonic and Optical Devices3 citations

TL;DR

This study investigates how layer stacking in graphene affects combination Raman modes in single-, bi-, few-, and incommensurate bilayer graphene on SiO2 substrates. Using Raman spectroscopy, the authors identify new modes at ~1860 cm⁻¹ (iTALO−), ~1880 cm⁻¹ (iTALO+), and ~2220 cm⁻¹ (iTOTA), showing that their frequencies shift linearly with layer count, while the iTALO− peak intensity decreases with increasing layers—offering insights into stacking-dependent electron-phonon coupling in 2D carbon allotropes.

ABSTRACT

We have observed new combination modes in the range from 1650 - 2300 cm-1 in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. The M band at ~1750 cm-1 is suppressed for both SLG and IBLG. A peak at ~1860 cm-1 (iTALO-) is observed due to a combination of the iTA and LO phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent in bulk graphite. Two previously unidentified modes at ~1880 cm-1 (iTALO+) and ~2220 cm-1 (iTOTA) in SLG are tentatively assigned as combination modes around the K point of the graphene Brillouin zone. The peak frequencies of the iTALO+ (iTOTA) modes are observed to increase (decrease) linearly with increasing graphene layers.

Motivation & Objective

• To investigate the influence of layer stacking on combination Raman modes in graphene across different layer counts.
• To identify and assign previously unreported Raman modes in single- and few-layer graphene.
• To understand the evolution of phonon coupling in graphene as a function of layer number and stacking order.
• To correlate observed Raman features with electronic and vibrational properties near the K point of the Brillouin zone.

Proposed method

• Raman spectroscopy was performed on single-, bi-, few-, and incommensurate bilayer graphene (IBLG) samples grown on SiO2 substrates.
• The study focused on the 1650–2300 cm⁻¹ spectral range to detect combination modes involving phonons.
• Peak frequencies and intensities were analyzed as functions of layer number and stacking configuration.
• Theoretical assignments were made based on symmetry and phonon coupling at the K point of the graphene Brillouin zone.
• Linear trends in peak frequency shifts were quantified with respect to layer count.
• Comparison with bulk graphite was used to confirm the absence of certain modes in highly ordered systems.

Experimental results

Research questions

• RQ1 How do combination Raman modes in graphene evolve with increasing layer count and stacking order?
• RQ2 What causes the suppression of the M band at ~1750 cm⁻¹ in single-layer and incommensurate bilayer graphene?
• RQ3 Why is the iTALO− mode at ~1860 cm⁻¹ absent in bulk graphite despite its presence in few-layer systems?
• RQ4 What is the origin of the newly observed modes at ~1880 cm⁻¹ (iTALO+) and ~2220 cm⁻¹ (iTOTA) in single-layer graphene?
• RQ5 How do the frequencies of iTALO+ and iTOTA modes scale with the number of graphene layers?

Key findings

• A new combination mode at ~1860 cm⁻¹ (iTALO−) is assigned to the coupling of iTA and LO phonons, with intensity decreasing as layer count increases.
• The iTALO+ mode at ~1880 cm⁻¹ and iTOTA mode at ~2220 cm⁻¹ are tentatively assigned as combination modes near the K point of the Brillouin zone in single-layer graphene.
• The frequency of the iTALO+ mode increases linearly with the number of graphene layers.
• The frequency of the iTOTA mode decreases linearly with increasing layer count.
• The M band at ~1750 cm⁻¹ is suppressed in both single-layer and incommensurate bilayer graphene, indicating stacking-dependent symmetry effects.
• The iTALO− peak is absent in bulk graphite, suggesting that long-range order suppresses this specific phonon coupling channel.