Fig. 2: Spin-exchange CM.

a, Spin-exchange CM (SE-CM) in Mn-doped PbSe QDs. Spin directions are shown by short, black single-sided arrows. Spin-conserving transitions are shown by red arrows. Due to spin conservation, the biexciton produced via relaxation of the excited Mn ion is a combination of a dark and a bright exciton (spins 1 and 0, respectively) in two different L valleys of PbSe (L₁ and L₂). The final biexciton state comprises two band-edge electrons with co-aligned spins, which is impossible in a single-valley semiconductor due to Pauli exclusion. b, An excitonic representation of spin-exchange CM in Mn-doped PbSe/CdSe core/shell QDs. An exciton generated in the CdSe shell (X_CdSe) initiates the first step of spin-exchange CM, which is formation of the excited Mn state (Mn*) due to CdSe–Mn spin-exchange energy transfer (step 1). During step 2, Mn* undergoes spin-flip relaxation by creating two excitons in the PbSe core (X_PbSe) via the spin-exchange process depicted in a. c, A schematic depiction of QD synthesis. Mn dopants are incorporated into preformed PbSe QDs via diffusion doping. The CdSe shell is formed using a controlled cation exchange reaction during which the original cations in the peripheral region of the QD are replaced with the Cd²⁺ ions. d, A typical TEM image of Mn-doped PbSe/CdSe core/shell QDs (scale bar, 5 nm). Inset shows a higher magnification view of an individual QD, which displays a clear core/shell structure (scale bar, 2 nm). e, Absorption (Abs.; blue) and dual-band PL (red and black) spectra of the Mn-doped PbSe/CdSe QDs (sample Mn-1). The NIR (black; hv_PbSe = 0.83 eV) and visible-range (red; hv_CdSe = 2.38 eV) PL bands are due to emissions from the PbSe core and the CdSe shell, respectively (inset).

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