Understanding the Role of Single Molecular ZnS Precursors in the Synthesis of In(Zn)P/ZnS Nanocrystals

Abstract

Environmentally friendly nanocrystals (NCs) such as InP are in demand for various applications, such as biomedical labeling, solar cells, sensors, and light-emitting diodes (LEDs). To fulfill their potential applications, the synthesis of such high-quality "green" InP NCs required further improvement so as to achieve better stability, higher brightness NCs, and also to have a more robust synthesis route. The present study addresses our efforts on the synthesis of high-quality In(Zn)P/ZnS core-shell NCs using an air- and moisture-stable ZnS single molecular precursor (SMP) and In(Zn)P cores. The SMP method has recently emerged as a promising route for the surface overcoating of NCs due to its simplicity, high reproducibility, low reaction temperature, and flexibility in controlling the reaction. The synthesis involved heating the In(Zn)P core solution and Zn(S2CNR2) (where R = methyl, ethyl, butyl, or benzyl and referred to as ZDMT, ZDET, ZDBT, or ZDBzT, respectively) in oleylamine (OLA) to 90-250 °C for 0.5-2.5 h. In this work, we systematically studied the influence of different SMP end groups, the complex formation and stability between the SMP and oleylamine (OLA), the reaction temperature, and the amount of SMP on the synthesis of high-quality In(Zn)P/ZnS NCs. We found that thiocarbamate end groups are an important factor contributing to the low-temperature growth of high-quality In(Zn)P/ZnS NCs, as the end groups affect the polarity of the molecules and result in a different steric arrangement. We found that use of SMP with bulky end groups (ZDBzT) results in nanocrystals with higher photoluminescence quantum yield (PL QY) and better dispersibility than those synthesized with SMPs with the shorter alkyl chain groups (ZDMT, ZDET, or ZDBT). At the optimal conditions, the PL QY of red emission In(Zn)P/ZnS NCs is 55 ± 4%, which is one of the highest values reported. On the basis of structural (XAS, XPS, XRD, TEM) and optical characterization, we propose a mechanism for the growth of a ZnS shell on an In(Zn)P core.

Tuning the Lattice Parameter of InxZnyP for Highly Luminescent Lattice-Matched Core/Shell Quantum Dots

“…InP-based QDs are among the most investigated nontoxic Cd-free systems, owing to their comparable emission range. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] However, in terms of PL QY and PL stability they cannot yet compete with Cd-based QDs.…”

“…Although the addition of Zn to the synthesis of InP nanocrystals has become common practice in the last years, [11][12][13][14][15][16][17][21][22][23][24][25] very little has been reported on the actual role of the Zn. The few reports on this topic have focused on the effect on the optical properties, but structural aspects were not investigated.…”

The role of ion exchange in the passivation of In(Zn)P nanocrystals with ZnS

“…Although it overlaps with the ripple from the FT, its enhancement can be attributed to the formation of Zn-O bonds. At the early stage of shell growth, the oxide layer at the In(Zn)P NC surfaces 10 would not be eliminated completely by the etching process. The unstable oxide layers are likely to accept Zn atoms to form a small amount of Zn-O bonds at the surface.…”

“…This shell can also act as a physical barrier between the core and the surrounding medium rendering the NCs more stable to changes in the environment, more resistant to photo-oxidative degradation or changes in the surface chemistry. In fact, in the recent work by Xi et al , ZnS single molecular precursor complexes were decomposed during synthesis to give off hydrogen sulfide gas which results in etching of the InP surface, hence reducing the defects on the surface of these NCs 10 .…”

Synthesis and Functionalization of Copper-Doped Indium Phosphate Quantum Dots with Ratiometric Fluorescence

“…In recent years, doped QDs have emerged as a new class of°uorescent nanocrystals that intrinsically o®er dual-emission-based ratio-metric°uorescence. 15 The dual emission of doped QDs not only retains almost all the characteristics of traditional single-emission QDs, but also can avoid the self-quenching problem due to their substantial ensemble Stokes shift. By introducing trace impurity ions, the electron levels of the parent can be perturbed to generate new°uorescent properties without changing the crystal structure.…”