Fig. 1. (Color online) (a) Photographs of 2 to 7 ML CdSe CQWs under ultraviolet light, showing the variation in photoluminescence with increasing layer thickness. (b) AFM surface topography of a 4 ML CdSe CQW film on a quartz substrate, illustrating the smoothness of the film. Scale bar: 3 μm. (c) Scratch height profile of the 4 ML CdSe CQW film, showing the step height between the film and the substrate. (d) Visualization of the (220) crystal plane in the CdSe CQW model. (e) Static X-ray diffraction (XRD) patterns of 2 to 7 ML CdSe CQWs, indicating structural properties and phase purity. (f) Gaussian fitting of the (220) plane in the 4 ML CdSe CQWs. (g) Enlarged view of the (111) diffraction peaks for 2 to 7 ML CdSe CQWs.

Fig. 2. (Color online) (a) Absorption spectra of 2 to 7 ML CdSe CQWs, illustrating the layer-dependent shift in absorption peaks. HH and LH represent the heavy-hole and light-hole transitions in the absorption spectra. (b) Tauc plots for 2 to 7 ML CdSe CQWs, with dotted lines indicating the intercept method used to estimate the optical bandgap. (c) Relationship between the optical bandgap as derived from the Tauc plots (red line) and density functional theory (DFT) calculations (blue line) as a function of monolayer thickness. Simulated band structure obtained through first-principles calculations for (d) 2 ML CdSe CQWs, (e) 4 ML CdSe CQWs, and (f) 7 ML CdSe CQWs, highlighting the changes in electronic structure with increasing monolayers.

Fig. 3. (Color online) (a) Structural model of the CdSe CQW film on a quartz substrate. (b) Experimental spectroscopic ellipsometry spectra (solid lines) and best-fit curves (dots) for 4 ML CdSe CQWs at a 65° incidence angle. (c) Real part of the dielectric function for 2 to 7 ML CdSe CQWs, showing the variation with increasing monolayer thickness. (d) Imaginary part of the dielectric function for 2 to 7 ML CdSe CQWs, illustrating the evolution of optical transitions with layer thickness.

Fig. 4. (Color online) (a) Refractive index and (b) extinction coefficient of 2 to 7 ML CdSe CQW films, demonstrating their layer-dependent optical properties. HH and LH represent the two prominent absorption peaks of CdSe CQW films. (c) Variation of the refractive index peak values from (a) with monolayer numbers. The inset illustrates the change in extinction coefficient peak values from (b) with monolayer numbers. (d) Variation of the normalized refractive index and normalized extinction coefficient (the inset) with monolayer number. (e) (Top) Deconvolution of the absorption spectrum for 4 ML CdSe CQWs, showing contributions from light-hole (LH) and heavy-hole (HH) excitons as well as free carriers. The exciton binding energy, estimated as 171 meV, is indicated. (Bottom) Absorption coefficient of 4 ML CdSe CQWs, with the positions of the LH and HH exciton lines highlighted. (f) Relationship between exciton binding energy and optical bandgap as a function of monolayer thickness in CdSe CQWs.