MXenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, exhibit unique optical properties that make them promising candidates for advanced optoelectronic and photonic applications. Their optical characteristics are highly tunable through compositional variations, surface terminations, and layer thickness, enabling precise control over light-matter interactions. Key aspects of MXenes' optical behavior include plasmon resonance, nonlinear optical responses, and efficient photothermal conversion, each of which is explored in detail below.

Plasmon resonance in MXenes arises from the collective oscillations of free electrons at the surface or within the material. Unlike noble metals, MXenes demonstrate highly adjustable plasmonic properties due to their variable electron densities and anisotropic structures. For instance, Ti3C2Tx, one of the most studied MXenes, exhibits localized surface plasmon resonance (LSPR) in the visible to near-infrared (NIR) range, typically between 600 nm and 900 nm, depending on synthesis conditions and surface functional groups (Tx denotes surface terminations such as -O, -F, or -OH). The LSPR peak can be shifted by altering the transition metal composition—substituting titanium with niobium or molybdenum results in a redshift or blueshift, respectively. Additionally, increasing the number of atomic layers reduces plasmon damping due to decreased surface scattering, leading to sharper resonance peaks. The ability to fine-tune plasmonic responses makes MXenes suitable for applications such as surface-enhanced spectroscopy and plasmonic sensing without requiring complex nanostructuring.

Nonlinear optical properties of MXenes are another area of significant interest. These materials exhibit strong nonlinear absorption and refraction, which are critical for ultrafast photonics and optical limiting applications. MXenes like Ti3C2Tx demonstrate saturable absorption under high-intensity laser excitation, transitioning from high absorption at low fluence to transparency at higher fluence. This behavior is attributed to Pauli blocking, where photoexcited electrons fill conduction band states, preventing further absorption. The nonlinear refractive index of MXenes is also composition-dependent; for example, Mo2CTx shows a higher nonlinear refractive index compared to Ti3C2Tx due to differences in band structure and carrier mobility. The magnitude of these effects can be further modulated by adjusting the flake size and layer count—thinner flakes exhibit stronger nonlinear responses because of enhanced quantum confinement and reduced interlayer screening. These properties position MXenes as viable materials for mode-locking in ultrafast lasers and optical switching devices.

Photothermal conversion efficiency is a standout feature of MXenes, particularly for biomedical and energy applications. When exposed to light, MXenes rapidly convert photon energy into heat due to their high absorption cross-section and strong electron-phonon coupling. Ti3C2Tx, for instance, achieves photothermal conversion efficiencies exceeding 40% under NIR irradiation, outperforming many conventional photothermal agents like gold nanoparticles. The efficiency is influenced by the material’s composition and structural parameters. Increasing the carbon-to-metal ratio enhances light absorption, while surface oxidation can either improve or degrade performance depending on the degree of oxidation and resulting bandgap changes. Layer thickness also plays a crucial role—monolayer or few-layer MXenes exhibit higher photothermal coefficients than their bulk counterparts because of reduced thermal conductivity and increased surface area for energy dissipation. These attributes make MXenes ideal for photothermal therapy, solar-driven water desalination, and light-triggered catalysis.

Tuning MXenes’ optical properties through composition involves strategic selection of transition metals and surface terminations. For example, replacing titanium with vanadium in V2CTx shifts the plasmon resonance to longer wavelengths due to changes in the free carrier concentration. Similarly, modifying surface terminations from -F to -O groups alters the electronic structure, leading to variations in interband transitions and optical absorption edges. The introduction of nitrogen into the lattice, forming carbonitrides like Ti3CNTx, further diversifies optical responses by modifying the density of states near the Fermi level. These compositional adjustments enable precise engineering of MXenes for specific optical applications without altering the underlying crystal structure.

Layer thickness is another critical parameter for optical property modulation. Few-layer MXenes exhibit quantum confinement effects that influence excitonic behavior and plasmonic damping. As the number of layers decreases, the bandgap can widen slightly, affecting absorption onset and photoluminescence characteristics. Thinner flakes also demonstrate stronger light-matter interactions due to reduced dielectric screening, enhancing nonlinear optical effects. However, excessive thinning may lead to oxidation and degradation under ambient conditions, necessitating careful optimization for practical use.

In summary, MXenes present a versatile platform for optical applications due to their tunable plasmonics, pronounced nonlinear responses, and efficient photothermal conversion. By strategically varying composition, surface chemistry, and layer thickness, their optical characteristics can be tailored to meet the demands of next-generation technologies. These materials bridge the gap between conventional semiconductors and metallic systems, offering unique advantages in fields ranging from biomedical imaging to ultrafast photonics. Continued research into their structure-property relationships will further unlock their potential for innovative optical applications.