Photophysics of InAs@InP@ZnSe core@shell@shell Quantum Dots Synthesized Using Amino-Arsine and Amino-Phosphine Based Precursors

Colloidal quantum dots (QDs) that absorb and emit in the infrared (IR) region are gaining significant attention as potential building blocks for optoelectronic devices. Among IR materials, InAs QDs are particularly promising, as their bandgap can be tuned from the visible to the near-IR range through quantum confinement. However, improving the photoluminescence quantum yield (PLQY) remain challenging. Encapsulation of InAs QDs in suitable wide bandgap shell materials, such as ZnSe, can improve PL efficiency, but lattice mismatch leads to strained interfaces, thus limiting their PLQY. In this work, we introduce InAs@InP@ZnSe multi-shell QDs as an effective solution wherein the InP interlayer improves structural stability by matching the lattice constants between InAs and ZnSe, reducing strain and defects, and enabling a smoother energy barrier for excitons. Herein the synthesis of these multi-shell quantum dots has been achieved for the first time by employing cost-effective and environmentally friendly precursors as an alternative to the traditional pyrophoric reagents, offering a safer and more sustainable approach. A detailed spectroscopic study has been carried out in order to understand the mechanism of improved optical properties in the core@shell and core@sheel@shell quantum dots. Initial InAs QDs exhibited a low PLQY (~2%) and a long photoluminescence (PL) lifetime of 180 ns at 80 K, attributed to exciton decay involving indirect excitons through resonant surface traps. However, due to poor surface passivation, they exhibited a fast, multiexponential PL decay profile at room temperature. Overgrowth of an InP shell resulted in InAs@InP QDs with a red-shifted absorption and PL spectra and improved optical properties: a higher PLQY (~13%), and a faster PL decay (52 ns at 80 K). This essentially indicate a transition to free exciton recombination and a significant carrier delocalization to the shell region indicating that these core@shell systems cannot be accurately described as type-I or quasitype-II, contrary to earlier assumptions. The subsequent addition of a ZnSe shell led to the formation of InAs@InP@ZnSe QDs with atomically sharp interfaces and low lattice strain (<1%). These multi-shell QDs demonstrated further suppression of non-radiative recombination, achieving a PLQY as high as 55%. The PL lifetime remained similar to that of InAs@InP QDs, suggesting that the reduced PLQY in InAs@InP QDs, as well as the residual losses in the multi-shell QDs, are likely due to ultrafast surface trapping that depletes the band edges without affecting the PL kinetics. To investigate the multi-exciton dynamics, fluence-dependent transient absorption spectroscopic (TA) measurements were carried out by varying the pump fluence according to the desired value of the average number of excitons per QDs. In all the systems, the biexciton quantum yield was found to be below 0.1%, demonstrating that neither InP nor ZnSe shells effectively suppress Auger recombination losses. This observation further confirmed that in InAs@InP QDs, both carriers can delocalize within the shell, resulting in negligible impact on wavefunction overlap, and the subsequent overgrowth of a ZnSe shell establishes a type-I band alignment with the "InAs@InP" core, which does not substantially modify the carriers’ overlap. These experimental observations were further supported by our density functional theory calculations. Overall this work demonstrates that InAs based multi-shell quantum dots synthesized with amino-based precursors achieve enhanced optical performance, providing an effective strategy for overcoming the challenges of non-radiative recombination in IR-emitting quantum dot systems.