1：Experimental Design and Method Selection:

First principles methods based on the density functional theory were employed to study the electronic structure and optical properties of Ta-containing MXenes (Tant1Cn, n ? 1, 2, 3). The full-potential linearized augmented plane-wave method (FLAPW) as implemented in WIEN2K computer package was used. The effects of the approximation to the exchange-correlation energy were treated by the generalized gradient approximation (GGA).

2：3). The full-potential linearized augmented plane-wave method (FLAPW) as implemented in WIEN2K computer package was used. The effects of the approximation to the exchange-correlation energy were treated by the generalized gradient approximation (GGA). Sample Selection and Data Sources:

2. Sample Selection and Data Sources: The model of 2D structure was constructed appropriately by removing the Al atoms from its parent Tant1AlCn phase. A vacuum space of 30 A was introduced to eliminate the interaction between adjacent Tant1Cn layers.

3：List of Experimental Equipment and Materials:

WIEN2K computer package was used for all density functional theory (DFT) calculations.

4：Experimental Procedures and Operational Workflow:

The self-consistent cycle was iterated for each calculation until the total energy was converged to within 10-5 Ry/unit cell. The effect of metallicity for Tant1Cn was considered by adopting an empirical Drude term, and the plasma frequency of 0.001 eV was used in the calculations.

5：001 eV was used in the calculations. Data Analysis Methods:

5. Data Analysis Methods: The real and imaginary parts of the dielectric functions, absorption coefficients, reflectivity, refraction index, loss function, and transmittance were comprehensively described.