Raman Spectroscopy: Unlocking Graphene's Potential (2024-205 Breakthroughs)

By Raimundas Juodvalkis
Raman Spectroscopy: Unlocking Graphene's Potential (2024-205 Breakthroughs)

Graphene, the revolutionary two-dimensional material, captivates researchers with its extraordinary properties like unparalleled strength and exceptional electrical conductivity, promising applications from advanced electronics to robust composites. Harnessing this potential requires precise characterization and control, making Raman spectroscopy an indispensable tool that is continuously evolving.

Recent scientific data from 2024 and projections into 2025 highlight significant advancements in how Raman spectroscopy tests and understands graphene, pushing the boundaries of research and development. Horiba, a key player, emphasized Raman spectroscopy's pivotal role in graphene research and development in 2025. Their instruments, such as the LabRAM HR Evolution (now LabRAM Odyssey) and LabRAM HR800, are used for precise characterization, often employing a 532 nm laser for optimal resolution and fluorescence suppression. Horiba's work, alongside broader scientific efforts—including a May 2024 study on 785 nm Raman spectroscopy for authenticity and property confirmation—confirms Raman spectroscopy's status as a powerful, non-destructive analytical technique essential for advancing graphene technology by distinguishing:

  • Layer number
  • Assessing disorder
  • Strain
  • Doping
  • Thermal properties
  • Overall uniformity and quality

Unveiling Graphene's Flaws: Advanced Defect & Functionalization Insights

The quality and functionality of graphene are profoundly influenced by its structural integrity, with defects and functionalization strategies playing critical roles. Raman spectroscopy leads this analysis.

Researchers routinely use the intensity ratio of the D and G bands (I(D)/I(G))—a key spectroscopic signature—as a crucial indicator of defect density and graphene's overall quality; lower ratios suggest fewer defects. The D band signifies defects, while the G band is characteristic of graphitic materials. The I(D)/I(D') ratio is highly sensitive to the defect's nature. Specifically, higher values are observed for vacancies compared to sp³ sites (carbon atoms with four single bonds, indicating hybridization or functionalization). A July 2024 review highlights the historical progress and future promise of Raman spectroscopy, especially Tip-Enhanced Raman Spectroscopy (TERS), for scrutinizing defects in graphene and other 2D materials. TERS offers nanoscale spectroscopic measurements with unprecedented resolution.

In 2024, TERS detailed graphene oxide (GO) reduction mechanisms, exposing structural and chemical nuances at defect sites with a nanoscale spatial resolution of 10 nm (achieved with gap-mode TERS). Non-gap mode TERS has mapped intrinsic defects in single-layer graphene with 20 nm resolution. TERS shows heightened sensitivity to defects located at:

  • Graphene edges
  • Folds
  • Overlapping regions

This technique provides simultaneous structural and spectral information at a localized scale, not achievable with conventional micro-Raman spectroscopy. Furthermore, 2025 research explored how TERS tips can induce reversible defects. Meanwhile, silver nanowires (Ag-nanowires) act as nano-antennas, significantly enhancing weak D-band signals from monolayer graphene defects.

Functionalization processes also alter the Raman spectrum, with new peaks emerging, such as a band at 1187 cm⁻¹ attributed to nitrogen-carbon (N-C) bonds, confirming successful functional group attachment. Pioneering low-temperature Raman studies on GO films in February 2024 (5–325 K) illuminated phonon dynamics and defect distribution. These studies attribute a slight deviation in the G mode below 50 K to an additional phonon decay channel and suggest surface functionalization effectively separates adjacent layers—insights crucial for advanced material design.

Beyond characterizing existing defects, the 2024-2025 period highlights interest in intentionally engineering and controlling graphene defects. Late 2025 studies report new methods for deliberately introducing structural defects, such as 5-7 ring defects using molecules like azupyrene, enhancing properties for electronics, sensors, and catalysts. Breakthroughs in AI-driven defect control are anticipated by 2025, aiming to reduce wafer-level graphene defect rates to below 0.1%. Research in 2025 also investigates how defect type and degree impact graphene quantum dots, influencing their HOMO-LUMO energy levels (the energies of the highest occupied and lowest unoccupied molecular orbitals). This research reveals a substantial reduction in thermal conductivity in defective graphene, even at low defect concentrations.

Graphene Under Pressure: Mapping Mechanical Strain

The mechanical environment profoundly impacts graphene's electronic and structural behavior. Understanding and mapping strain is vital for optimizing device performance, and Raman spectroscopy is highly sensitive to these stresses.

Shifts in the G and 2D bands of the Raman spectrum create detailed strain maps. Tensile strain (stretching) leads to a decrease (redshift) in the Raman shift for both bands due to carbon-carbon bond elongation. Uniaxial strain causes substantial red shifts and can split the G band into distinct sub-bands (G+ and G-), a characteristic arising from symmetry breaking, providing an optical method to determine crystallographic orientation.

In contrast, biaxial strain preserves hexagonal symmetry, leading to isotropic expansion/compression and a linear peak shift. The 2D band's FWHM (Full Width at Half Maximum – a measure of peak broadening) often remains constant under biaxial strain. Notably, 2024 research highlighted biaxial strain's critical role in structural transformations, such as the formation of 2D diamond from graphene stacks, where the G band blueshifts under pressure before the transition. Building on this, nano-Raman investigations in 2025—including an April study by Gustavo Soares and colleagues—explore anisotropic strain (strain varying with direction) in twisted bilayer graphene (tBG). This research uses TERS for nanometer-scale mapping of twist angle, strain distribution, and elastic energy, uncovering localized strain gradients that can lead to buckling. This analysis, supported by finite element modeling, differentiates uniaxial and biaxial strain effects. Additionally, polarized Raman spectroscopy is being advanced in 2025 to characterize spatial variations of uniaxial strain in monolayer graphene with sub-micron resolution.

Decoding Graphene's Structure: Layer Counting & Stacking Analysis

The number of graphene layers and their stacking order are fundamental to its electronic properties. Raman spectroscopy offers a standard, non-destructive approach to determine these characteristics.

The 2D band is particularly informative. Its position, shape, and FWHM, along with its intensity ratio relative to the G band (I(2D)/I(G)), distinguish between single-layer, bilayer, and multilayer graphene. An I(2D)/I(G) ratio exceeding 2 typically signifies single-layer graphene. New research, including projections into 2025, indicates that distinctive features of the 2D mode, alongside the G-mode peak's width and position, provide insights into different stacking orders in multilayer graphene, distinguishing Bernal (ABA) versus rhombohedral (ABC) stacking. Rhombohedral stacking can be identified by a peak around 1580 cm⁻¹. Peak positions and forms change with layer number and relative rotation, providing insights into evolving interlayer interactions crucial for advanced heterostructure design. A January 2025 publication details a deep-learning-enabled strategy for fast, non-destructive Raman identification of twist angle in bilayer graphene, decoding and predicting it across the full angular range and extending to 2D plane orientational mapping. This data-driven model addresses traditional challenges of large-scale sample preparation and identification at specific angles.

Tailoring Properties: The Impact of Doping

Introducing impurities or dopants into graphene tunes its electronic and vibrational properties. Raman spectroscopy is an active area of investigation for understanding these doping effects.

New 2025 data demonstrates that doping influences the intensities of the D and D' peaks, which typically decrease with increasing doping, especially in defected graphene. Doping also induces shifts in the G and 2D bands. The G peak tends to stiffen and sharpen with both hole (p-type) and electron (n-type) doping. This is due to the non-adiabatic removal of the Kohn anomaly (a unique electronic instability) at the Γ point (a specific high-symmetry point in the material's Brillouin zone). The 2D peak exhibits a blueshift with p-doping but redshifts with n-doping, with its intensity decreasing significantly. The ratio of the 2D to G peak intensities (I(2D)/I(G)) is crucial for determining doping level.

Polymer electrolyte gating experiments allow researchers to achieve significantly higher electron and hole doping, precisely tuning the Fermi level by forming an electric double layer for high gate capacitance. In-situ Raman measurements continuously monitor spectral effects. Applications include studying ambipolar behavior (conduction of both positive and negative charge carriers) in graphene transistors and developing biosensors. Earlier in 2024, electrolyte gating confirmed a Fermi-level tuning of 0.39 eV for terahertz amplitude modulators.

The successful incorporation of heteroatoms, such as nitrogen and boron, into the graphene lattice, each imparting distinct electronic effects, has been confirmed through detailed Raman analysis. Nitrogen (N) doping often results in a red shift of the G-band and a blue shift of the 2D-band. Boron (B) p-doping causes a blue shift in the G-band. Beyond N and B, phosphorus (P) and sulfur (S) are also explored. These elements, being more electron-rich than carbon, often lead to n-type doping effects, creating active sites and redistributing electron density, further characterized by their impact on Raman band shifts and intensities. These studies reveal changes in phonon frequencies directly attributable to charge-transfer processes induced by the dopants.

Novel Techniques and Future Applications

Beyond fundamental characterization, 2024-2025 is witnessing innovative techniques and applications leveraging Raman spectroscopy:

  • Femtosecond Laser Irradiation: A 2024 study showcased femtosecond (fs) laser irradiation for precise graphene modification, with Raman spectroscopy simultaneously monitoring this "soft modification" at sub-ablation threshold energies. This enables real-time analysis of changes in Raman parameters (central position, bandwidth, intensity) for controlled modification, creating non-thermally modified regions like oxidized nanoislands, optical forging, and nanopore networks. Femtosecond lasers are also used for micropatterning graphene, visualized through multi-parametric Raman mapping and AFM. This technology extends to e-textiles, creating laser-induced graphene on substrates like Kevlar for sensing and temperature monitoring.
  • Graphene-Enhanced Raman Spectroscopy (GERS): Research in 2025 demonstrates GERS as a cutting-edge metrology method, significantly enhancing Raman signals of analytes and quenching fluorescence. This facilitates detecting ultra-low concentrations of pharmaceuticals like ibuprofen and paracetamol in water, with enhancement factors up to 48 times for G-/2D lines and 200 times for the D-line. This opens avenues for medical diagnostics, identifying pharmaceutical spectra as biomarkers, detecting disease biomarkers, DNA/RNA fragments, bacteria, and viruses. Graphene-mediated SERS (G-SERS), often using gold/silver nanoparticles, shows promise for advanced DNA detection and hybridization. Handheld Raman instruments leveraging GERS are gaining traction for rapid, real-time point-of-care diagnostics (e.g., tuberculosis, pancreatic cancer), adaptable to various biomarkers and sample types. The integration of AI-powered SERS models is transforming traditional SERS protocols into AI-driven models for enhanced identification, quantification, classification, and discovery.
  • AI-driven Production and Characterization: A significant leap in 2024-2025 involves integrating AI-driven closed-loop process control into graphene manufacturing. Advanced Raman spectroscopy provides real-time feedback, allowing AI algorithms to precisely tune plasma parameters for targeted graphene characteristics. Projections include AI-driven atomic-level defect repair technologies to reduce wafer-level graphene defect rates to below 0.1% by 2025. AI-enhanced Raman analysis can automatically cluster thousands of Raman spectra with high accuracy (up to 99.95%), identifying different graphene layer numbers and stacking orders, heralding intelligent graphene fabrication generalizable to other 2D materials. A rapid, high-resolution slit-scanning Raman microscopy technique (November 2024) tracks Raman peaks to determine hole carrier density (nH) and pinpoint variations, often originating during wet-transfer processes crucial for scalable manufacturing. Beyond graphene, AI optimizes resource utilization, minimizes waste, and improves operational efficiency. Closed-loop AI optimization (AIO) is emerging for autonomous parameter adjustment, leading to significant gains in throughput and labor productivity.
  • Standardization Efforts: Progress is being made in standardizing protocols for Raman spectroscopy and TERS in defect metrology. The International Organization for Standardization (ISO) is leading efforts. ISO/TS 9651:2025 (July 2025) provides a framework for classifying graphene-related materials, including defect density. ISO/TS 23359:2025 (August 2025) details methods for analyzing chemical properties in graphene powders and dispersions. An international interlaboratory comparison (ILC) led by NPL through the Versailles Project on Advanced Materials and Standards (VAMAS) has outlined improvements in Raman spectroscopic measurements of CVD-grown graphene, forming the basis for new ISO/IEC international standards. The Graphene Flagship Standardisation Committee has pioneered new IEC standards, including one focused on Raman spectroscopy for quality control. Standardized TERS methods are crucial for consistent defect measurements as graphene devices shrink. The "Graphene Atlas" (March 2025) provides a universal Raman spectroscopy-based framework for quantifying defects in mono- to multilayer graphenic materials, aiming to set a new standard.

The Path Forward

Raman spectroscopy remains a cornerstone for fundamental graphene research and quality control. Innovations through 2024-2025 are expanding its capabilities, offering unprecedented depth in understanding graphene's multifaceted properties. These developments are pivotal for accelerating the creation of high-performance graphene-based materials and devices that will shape future technology.