The application of X-ray photoelectron spectroscopy (XPS) to two-dimensional materials and van der Waals heterostructures provides critical insights into their electronic structure, chemical composition, and interfacial phenomena. As a surface-sensitive technique, XPS is particularly suited for studying atomically thin systems, where layer-dependent properties, doping effects, and charge transfer play pivotal roles in determining material behavior. The technique's ability to probe core-level shifts, valence band structure, and chemical bonding makes it indispensable for characterizing these advanced materials.
One of the primary advantages of XPS in studying 2D materials is its sensitivity to layer-dependent electronic states. In monolayer and few-layer systems such as graphene, transition metal dichalcogenides (TMDs) like MoS2, and hexagonal boron nitride (hBN), the electronic environment of atoms varies with thickness due to quantum confinement and interlayer interactions. For example, in MoS2, the core-level binding energies of molybdenum and sulfur exhibit shifts as the number of layers decreases from bulk to monolayer. These shifts arise from changes in dielectric screening and orbital hybridization, which XPS can resolve with high precision. The technique can distinguish between monolayer, bilayer, and bulk-like electronic structures by analyzing peak positions and line shapes, providing a direct measure of layer-dependent effects.
Doping and chemical modification of 2D materials are also effectively characterized using XPS. Intentional doping, whether through substitutional atoms, adsorption of molecules, or electrostatic gating, alters the core-level and valence band spectra. In graphene, for instance, nitrogen or boron doping introduces new chemical states detectable as distinct peaks in the C 1s spectrum. Similarly, oxidation or functionalization of TMDs generates additional spectral features corresponding to metal-oxygen or chalcogen-oxygen bonds. Quantitative analysis of these peaks allows determination of doping concentrations and identification of bonding configurations. Charge transfer doping, where electron density is redistributed between a 2D material and an adjacent layer or substrate, manifests as binding energy shifts in XPS spectra. By comparing spectra of isolated monolayers with those in heterostructures, researchers can deduce the direction and magnitude of interfacial charge transfer.
Van der Waals heterostructures, formed by stacking different 2D materials, present unique opportunities and challenges for XPS analysis. The technique can probe buried interfaces non-destructively due to the escape depth of photoelectrons, which typically ranges from a few nanometers. This enables investigation of interfacial chemistry and electronic coupling between layers. For example, in graphene-MoS2 heterostructures, XPS reveals charge transfer from graphene to MoS2, evidenced by shifts in the C 1s and Mo 3d peaks. Similarly, in TMD-based heterobilayers, interlayer hybridization and band alignment can be inferred from valence band spectra. However, the weak van der Waals interactions at these interfaces mean that beam-induced damage or surface contamination can significantly affect measurements, necessitating careful experimental design.
Despite its strengths, XPS analysis of ultrathin 2D materials presents several challenges. Beam damage is a critical concern, as prolonged exposure to X-rays can cause bond breaking, oxidation, or desorption of surface species. This is particularly problematic for sensitive materials like phosphorene or certain organic-inorganic hybrids. To mitigate damage, measurements should be performed at reduced beam fluxes or with short acquisition times. Another challenge is the limited escape depth of photoelectrons, which restricts analysis to the top few layers. While this is advantageous for surface studies, it complicates the characterization of buried interfaces in thick heterostructures. Angle-resolved XPS can partially address this by varying the take-off angle to enhance surface or bulk sensitivity.
Contamination and environmental effects further complicate XPS studies of 2D materials. Exposure to air can lead to surface oxidation or adsorption of hydrocarbons, which obscure intrinsic material properties. In-situ sample preparation and transfer under ultra-high vacuum conditions are often necessary to preserve pristine surfaces. Additionally, charging effects may arise in insulating substrates or thick heterostructures, leading to peak shifts and broadening. Charge neutralization techniques, such as low-energy electron flooding, are commonly employed to stabilize the sample potential during measurements.
Recent advancements in XPS instrumentation have expanded its capabilities for 2D material research. High-energy resolution spectrometers enable detection of subtle spectral features, such as spin-orbit splitting in TMDs or defect states in graphene. Operando XPS setups allow real-time monitoring of chemical and electronic changes during thermal annealing, gas exposure, or electrochemical processes. Combined with other techniques like Raman spectroscopy or scanning probe microscopy, XPS provides a comprehensive understanding of structure-property relationships in layered materials.
In summary, XPS serves as a powerful tool for investigating the electronic and chemical properties of 2D materials and van der Waals heterostructures. Its ability to resolve layer-dependent effects, doping mechanisms, and interfacial charge transfer is unmatched by many other techniques. However, careful attention must be paid to experimental conditions to avoid beam damage, charging artifacts, and surface contamination. As the field of layered materials continues to advance, XPS will remain indispensable for elucidating their fundamental characteristics and guiding the development of next-generation nanodevices.