Fig. 1. Optical and EL properties of ccg-QDs.

a, The ground-state absorption spectrum (α₀) of the CdSe/Cd_1−xZn_xSe/ZnSe_0.5S_0.5/ZnS ccg-QDs (top-right inset). Vertical arrows mark the three lowest-energy transitions involving 1S and 1P electron and hole states (shown in the top-left inset). b, Transient absorption (TA) measurements of the ccg-QD solution sample conducted using 110-fs, 2.4-eV pump pulses with per-pulse fluence w_p = 1.6 mJ cm⁻² (⟨N⟩ = 25). The TA signal is presented as α(hv,t) = α₀(hv) + Δα(hv,t), where α₀ and α are the absorption coefficients of the unexcited and excited sample, respectively, and Δα is the pump-induced absorption change. The solid black line (α = 0) separates the regions of absorption (α > 0, brown) and optical gain (α < 0; green). The dashed black line is the second derivative of α₀ (panel a). c, Pump-intensity-dependent spectra of edge-emitted photoluminescence (PL) of a 300-nm-thick ccg-QD film on a glass substrate under excitation with 110-fs, 3.6-eV pump pulses. The pump spot is shaped as a narrow 1.7-mm-long stripe orthogonal to the sample edge. The emergence of narrow peaks at 1.93 eV and 2.08 eV (full width at half maximum 35 meV and 40 meV, respectively) at higher ⟨N⟩ indicates the transition to the ASE regime. On the basis of the onset of sharp intensity growth (inset), the 1S and 1P ASE thresholds are, respectively, about 1 and about 3 excitons per dot on average. d, A device stack of the reference LED comprises an L-ITO cathode, a ccg-QD layer and TFB/HAT-CN hole transport/injection layers separated by a LiF spacer with a current-focusing aperture. The device is completed with a Ag anode prepared as a narrow strip. e, The j–V (solid black line) and EL intensity–V (dashed blue line) dependences of the reference device. f, The j-dependent EL spectra of front (surface) emission of the reference device. The EL spectrum recorded at 1,019 A cm⁻² is deconvolved into three Lorentzian bands that correspond to the three ccg-QD transitions shown in a. AU, arbitrary units.