Broadband Tunable Optical Gain from Ecofriendly Semiconductor Quantum Dots with Near-Half-Exciton Threshold

Abstract

Optical gain in solution-processable quantum dots (QDs) has attracted intense interest toward next-generation optoelectronics; however, the development of optical gain in heavy-metal-free QDs remains challenging. Herein, we reveal that the ZnSe 1−x Te x -based QDs show excellent optical gain covering the violet to near-red regime. A new gain mechanism is established in the alloy QDs, which promotes a theoretically threshold-less optical gain thanks to the ultrafast carrier localization and suppression of ground-state absorption by the Te-derived isoelectronic state. Further, we disclose that the hot-carrier trapping represents the main culprit to exacerbate the gain performance. With the increase of Te-to-Se ratio, a sub-band-gap photoinduced absorption (PA) appears and extinguishes the optical gain. To overcome this issue, we modulate the inner ZnSe shell thickness, and the gain is recovered by reducing the overlap between the gain and PA regions in the Te-rich QDs. Our finding represents a significant step toward sustainable QD-based optoelectronics.

Superior optical gain in zinc selenide colloidal nanocrystals induced by Coulomb-correlated electron–hole plasma

“…A series of ZnSe/ZnS NCs with different sizes are synthesized following a recipe slightly modified from the literature (see supplementary material, Note 1 for synthesis details). 14,22 The NCs exhibit a high PLQY of 84%-90%, indicating the samples are of high quality. Transmission electron microscope (TEM) images illustrate size ranges from 4 to 16 nm [Fig.…”

“…Interestingly, the occurrence of varied carrier density thresholds in the small ZnSe/ZnS NCs points to a nearly constant hN 0 i of $1.6 (the average electron-hole pairs per NC, hN 0 i), which is consistent with a biexcitonic gain character. 22 In stark contrast, the large ZnSe/ZnS NCs are found to share a nearly constant carrier density of 2.8 Â 10 18 cm À3 regardless of the size and excitation wavelength [Fig. 2(d)].…”

Interface Engineering of Water-Dispersible Near-Infrared-Emitting CuInZnS/ZnSe/ZnS Quantum Dots

“…It was hypothesized that the smaller band gap of ZnSe compared to ZnS could allow for greater wave function leakage, resulting in a red-shift of the PL. The use of a ZnSe layer has also been explored by Wang’s group, notably for shifting the optical gain in ZnSe 1– x Te x quantum dots. , …”

“…The use of a ZnSe layer has also been explored by Wang's group, notably for shifting the optical gain in ZnSe 1−x Te x quantum dots. 30,31 Growth of the ZnSe shell was achieved via a successive ion layer adsorption and reaction (SILAR) method in which the alternate addition of Zn and Se precursors resulted in the formation of one ZnSe monolayer at a time. This allowed for the controlled growth of a number of monolayers depending on the quantity of precursors used.…”

Blue-Emissive ZnSeTe Quantum Dots and Their Electroluminescent Devices

“…The first posits that the tail emission is associated with the generation of lattice defects serving as shallow traps above the valence band maximum (VBM), which originates from the notable ionic radius difference between Se and Te. − Another perspective suggests that the nearest-neighbor pairs of Te induce localized hole states, driven by the electronegativity difference between Se (2.4) and Te (2.1), as shown in Figure e. − Consequently, the observed tail emission results from spatially separated excitons characterized by delocalized electrons and localized holes confined within the traps of Te clusters. Intriguingly, the emission spectral shape of ZnSeTe QDs becomes symmetric with the tail emission eliminated upon applying a relatively heavier Te alloying, by which the emission range of the resulting QDs becomes beyond blue (e.g., green-to-near-red). − To address the disappearance of the tail emission in such heavily Te-alloyed ZnSeTe QDs, Wang’s group recently proposed that the charge density difference is no more localized around Te clusters, rather showing a unform distribution due to the decrease in the overall electronegativity difference between Te and surrounding atoms (Figure f) . To mitigate the tail emission, particularly existing in blue-emissive ZnSeTe QDs, researchers have focused on controlling defects in intragap states or achieving a more uniform Te distribution.…”