Atomic force microscopy (AFM) is a critical tool for analyzing two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs). Its high spatial resolution and ability to operate in multiple modes make it indispensable for measuring thickness, identifying defects, and assessing layer uniformity at the nanoscale. Unlike electron microscopy techniques, AFM does not require conductive coatings or high-vacuum conditions, allowing for measurements in ambient air or liquid environments. This capability is particularly valuable for studying the structural integrity and surface morphology of atomically thin materials.

Thickness measurement is one of the most common applications of AFM in graphene and TMDC characterization. The technique relies on a sharp probe that scans the surface, detecting height variations with sub-nanometer precision. For mechanically exfoliated graphene, AFM can distinguish single-layer flakes from multilayer regions by measuring step heights. A single-layer graphene sheet typically exhibits a height of approximately 0.34 nm when adsorbed on a silicon oxide substrate, though this value may vary slightly due to interfacial interactions. In the case of TMDCs such as molybdenum disulfide (MoS2), AFM measurements reveal layer-dependent thicknesses, with each monolayer contributing around 0.65 nm. These measurements are crucial for verifying sample quality and ensuring consistency in device fabrication.

AFM also plays a key role in identifying defects within graphene and TMDC layers. Point defects, wrinkles, and tears can significantly alter electronic and mechanical properties. In contact mode, AFM detects topographical anomalies by tracking changes in cantilever deflection as the probe moves across the surface. For example, vacancies or adatoms in graphene appear as localized height deviations, while wrinkles manifest as elongated ridges with heights exceeding the nominal monolayer thickness. In TMDCs, sulfur vacancies or grain boundaries can be resolved with high precision. Tapping mode AFM further enhances defect detection by minimizing lateral forces, reducing the risk of sample damage during scanning. Phase imaging in this mode provides additional contrast, revealing variations in material stiffness or adhesion that correlate with defect sites.

Layer uniformity is another critical parameter assessed by AFM, particularly for large-area films produced by chemical vapor deposition (CVD). Unlike exfoliated samples, CVD-grown graphene or TMDCs often exhibit non-uniform thickness due to nucleation dynamics and growth conditions. AFM line profiles across these films can identify multilayer islands, incomplete coverage, or uneven terrace formation. For instance, CVD graphene on copper may show bilayer patches surrounded by monolayer regions, with AFM quantifying the area fraction of each domain. Similarly, wafer-scale MoS2 films may contain regions of discontinuous growth, where AFM pinpoints thickness fluctuations down to atomic-level precision.

Beyond topography, advanced AFM modes provide additional insights into material properties. Conductive AFM measures local current flow through graphene or TMDCs, correlating electrical behavior with structural features. Regions with high defect density often exhibit reduced conductivity, while grain boundaries in TMDCs may show increased contact resistance. Kelvin probe force microscopy (KPFM) maps surface potential variations, revealing doping inhomogeneities or charge trapping at defect sites. These measurements are essential for optimizing material performance in electronic devices.

Quantitative analysis of AFM data requires careful consideration of instrumental and environmental factors. Thermal drift can distort lateral measurements, while tip-sample convolution effects may exaggerate feature widths. Calibration using reference gratings ensures accurate height and lateral scale measurements. For statistical reliability, multiple scans across different sample regions are necessary to account for spatial variations. Automated image processing algorithms can extract metrics such as root-mean-square roughness or defect density, enabling objective comparisons between samples.

In graphene, AFM has been instrumental in studying the effects of substrate interactions on apparent thickness. The measured height of a graphene layer on silicon oxide often exceeds its theoretical value due to trapped water layers or interfacial forces. This phenomenon underscores the importance of environmental control during measurements. For TMDCs, AFM helps evaluate the impact of post-growth treatments such as annealing or chemical doping on layer morphology. Thermal processing may induce strain relaxation or defect migration, both detectable through changes in surface topography.

The ability to operate in liquid environments extends AFM's utility to in-situ studies of graphene and TMDCs under realistic operating conditions. Electrochemical AFM can monitor thickness changes during ion intercalation, relevant for energy storage applications. In biological studies, AFM tracks the interaction of two-dimensional materials with biomolecules in physiological buffers, providing real-time topographic feedback.

Despite its advantages, AFM has limitations in analyzing certain defect types. Subsurface defects or dopants may not produce sufficient topographic contrast for detection, requiring complementary techniques such as scanning tunneling microscopy or Raman spectroscopy. Additionally, soft or loosely adhered samples may deform under probe contact, necessitating low-force imaging protocols.

In summary, AFM serves as a versatile tool for characterizing graphene and TMDCs, offering nanometer-scale resolution for thickness profiling, defect identification, and uniformity assessment. Its compatibility with diverse environments and operational modes makes it indispensable for both fundamental research and industrial quality control. By correlating topographic data with electrical and mechanical properties, AFM provides a comprehensive understanding of structure-property relationships in these materials. Continued advancements in probe technology and imaging methodologies will further enhance its capabilities, ensuring its central role in two-dimensional material characterization.