- The paper demonstrates that FTIR and Raman spectroscopy can effectively distinguish stacking orders in trilayer graphene.
- It identifies unique IR spectral features for ABA, ABC, and mixed configurations, including a new low-energy signature in ABC stacking.
- Raman analysis confirms shifts in the 2D band linked to stacking order, advancing non-invasive methods for graphene characterization.
Overview of Stacking Order Imaging in Few-Layer Graphene via Infrared and Raman Spectroscopy
This paper investigates the stacking order in few-layer graphene (FLG), particularly trilayer graphene, using infrared (IR) and Raman spectroscopy techniques. The researchers focus on the ABA (Bernal) and ABC (rhombohedral) stacking orders, elucidating the impact of stacking sequences on the electronic and vibrational properties of FLG.
Key Findings
The paper primarily leverages micro-Fourier Transform Infrared spectroscopy (FTIR) to measure the IR optical conductivity of exfoliated FLG on SiO₂ substrates. The method of analysis employs the relative reflection spectra of graphene films against the bare substrates to extract optical conductivity as a function of photon energy (ℏω). The authors confirm that for high photon energies (>0.7 eV), the FLG behaves similarly to independent graphene layers, allowing for the detection of trilayer graphene by its conductivity, specifically the value of 3×πe²/2h.
The researchers identified two distinct IR spectra in trilayer graphene samples, corresponding to different stacking orders. Approximately 10% of the samples exhibited a unique IR spectrum, previously unreported, suggesting a new low-energy electronic structure in ABC-stacked trilayer graphene, contrasting with the typical ABA stacking. In addition, a third type of IR spectrum, noted in around 30% of samples, was consistent with a linear combination of ABA and ABC signals, indicative of mixed stacking.
Raman spectroscopy further differentiated between the stacking orders via the 2D-mode spectral lineshapes. The paper underscores the difference in stacking orders through distinct shifts in the Raman 2D band and the utilization of multiple Lorentzian functions to fit the spectra of both ABA and ABC trilayers. Notably, the results show a consistent mean 2D-band Raman shift for ABA-stacked trilayers compared to ABC-stacked ones across various excitation energies.
Additionally, experimental procedures involved evaluating the substrate effects by employing suspended graphene samples. This isolated the spectral attributes from substrate-induced phenomena, confirming that substrate impact on the 2D-mode spectral lineshape is minimal.
Implications and Future Directions
This research provides significant contributions to understanding the stacking-dependent optical and electronic properties of graphene. By revealing differing IR and Raman responses attributed to specific stacking orders, the paper advances the functional flexibility of FLG in optical and electronic applications. The detection of a previously unknown IR spectrum for the ABC stacking order suggests additional complexity and potential for unexplored electronic configurations in graphene structures.
From a theoretical perspective, these findings augment existing models of electronic band structure in layered materials. Practically, the research paves the way for characterizing FLG using non-invasive optical methods, critical for the development of graphene-based optoelectronic devices where layer-specific properties are essential.
Future research could explore the thermodynamic aspects of stacking order more extensively, particularly the annealing processes, to better understand how external stimuli affect stacking stability. Understanding the dynamics between mixed stacking orders might also illuminate how geometric variations impact electronic properties, offering deeper insights into tunable conductivity and other functional features of FLG. Furthermore, exploring the doping effects and electronic distributions could unravel additional applications and enhance our fundamental comprehension of multilayer graphene systems.
The paper stands as a pivotal reference for researchers keen on exploiting graphene's anisotropic stacking-dependent properties, pushing the envelope further in nanomaterials and nanoscale electronics research.