Fig. 1: Strain-graded CdSe-ZnSe core-shell QDs with compositionally abrupt interface.

a Schematic illustration of strain-graded (sg) wurtzite (wz) CdSe-ZnSe centered core-shell QDs with compositionally abrupt interface. The spherical CdSe core, under greater compression perpendicular to the C axis (red) than along the C axis (blue), becomes prolate spheroid-shaped. Near the interface, CdSe and ZnSe are mutually strained, creating smooth potential gradients in conduction and valence band edges. b EDS elemental mapping of Cd (red) and Zn (green) and c, HR-TEM image of CdSe (r = 2.5 nm)-ZnSe (H = 5.0 nm) sg-QDs. d Inverse FFT images of CdSe core (white square in (c)) showing the contraction of CdSe along [0002] (blue) and [11\(\bar{2}\)0] (red). Mean d-spacing and magnitude of compressive strain (β) along these directions are noted. e Lattice constant map obtained by geometric phase analysis. Scale bars in (b–e) are 10 nm, 5 nm, 1 nm, 1 nm and 5 nm, respectively. f Energy level diagram showing the changes in optically active exciton states upon the asymmetric compressive strain. |1S_HX > , |1S_LX> and |g> represent the state of heavy hole exciton, light hole exciton and ground, respectively. g Absorption spectra, h PL spectra and (I), PL decay dynamics of CdSe QDs (r = 2.5 nm) (grey) and CdSe (r = 2.5 nm)-ZnSe (H = 5.0 nm) sg-QDs (purple). Red and blue arrows in (g) indicate optical transitions from |g> to |1S_HX> and |1S_LX > , respectively. PL spectrum of sg-QDs in (h) is fitted with double gaussian curves, 1S_HX,em ( | 1S_HX> → |g > , red line with shading) and 1S_LX,em ( | 1S_LX> → |g > , line with blue shading). Single exciton decay times (τ_X) are noted in (I). j PL spectrum of an individual sg-QD (left) and a 2D contour plot showing PL spectra of 200 sequential frames (1 s per frame) (right) and (k), PL energy and linewidth of individual sg-QDs with Se-rich and Cd-rich CdSe core for CdSe (r = 2.5 nm)-ZnSe (H = 5.0 nm) sg-QDs (symbols: average, error bars: standard deviation, shaded region: range guidance) More structural information is provided in Supplementary Fig. 6.