Slow electron cooling dynamics mediated by electron–hole decoupling in highly luminescent CdS_xSe_1–x alloy quantum dots

Abstract

Spherical CdS_xSe_1–x alloy semiconductor QDs are receiving incredible research interest due to their composition-dependent optical tunability and charge carrier behavior. These highly luminescent alloy QDs can be used in several applications due to their very long excited state lifetime. Herein, we describe synthesis and characterization of highly luminescent CdSSe alloy QD using XRD, EDX, and HRTEM techniques. Steady state optical absorption and photoluminescence (PL) measurements show the nonlinear behavior of the alloy QDs with changing chalcogenide composition. Time-resolved photoluminescence (PL) measurements suggest CdS_0.7Se_0.3 alloy QD has much higher emission quantum yield (∼70%) and long radiative lifetime (24 ns) as compared to both CdS (ϕ_CdS = 24%, τ_CdS = 9.5 ns) and CdSe (ϕ_CdSe = 34%, τ_CdSe = 13.3 ns) QDs. Femtosecond transient absorption measurement has been carried out to unravel charge carrier dynamics in early and late time scale. Electron cooling time for CdS_0.7Se_0.3 alloy QD found to be extremely slow (τ_cooling = 8 ps) in contrast to both pure CdS (τ_cooling < 100 fs) as well as pure CdSe QD (τ_cooling = 600 fs) due to specially decoupled electron and hole in quasi type II core–shell structure. Charge recombination reaction found to be slowest in alloy QDs as compared to both CdS and CdSe QDs.