Figure 3: Spectral trends of polytypic CdSe NCs.

(a) Powder XRD pattern of a polytypic CdSe NC with ∼3.6 nm diameter (solid grey line) and simulated pattern with 65% WZ and 35% ZB (orange dotted line). The major diffraction peaks for the bulk ZB and WZ materials are also shown, with vertical dotted lines correspond to the shared features. (b) Absorption spectra of WZ (green), ZB (red) and polytypic (grey) NCs with matched E₀ energies, showing that the polytypic E₁ region is intermediate between those of WZ and ZB NCs. The inset shows the second derivative spectra, with red and green shades indicating the energy ranges of E_1,ZB and E_1,WZ peaks. (c) Structures of CdSe NCs with pure ZB or WZ phases or different degrees of polytypism (0, 36, 61, 85, 100% WZ) used in DFT simulations. ZB and WZ domains are separated by stacking faults in grey shading. (d) Simulated absorption spectra of nanocrystals shown in c. (e) Trend in the relative intensities of the two transitions, A_WZ/A_ZB, normalized to 0 for pure ZB and 1 for pure WZ, showing simulated data from c (blue circles) with a linear fit (blue line) and empirical data from b (orange crosses). (f) Spectrum of polytypic NCs (grey) in comparison with a summed spectrum from pure-phase ZB and WZ NCs in the correct mixture of phases (orange), showing that the experimental polytypic nanocrystal exhibits a depleted E_1,WZ peak. Red and green shades indicate the energy ranges of E_1,ZB and E_1,WZ peaks, respectively. (g) Theoretical spectra (imaginary component of the dielectric function) of the 36% WZ polytypic NC, broken down by domain region. All valence band states were used, but the conduction band was segregated into the indicated domains. In all domains, the ZB peak is evident, but the WZ peak has been substantially diminished.