Synthesis and Assembly of Core–Shell Nanorods with High Quantum Yield and Linear Polarization

Abstract

The seeded growth method offers an efficient way to design core–shell semiconductor nanocrystals in the liquid phase. The combination of seed and shell materials offers wide tunability of morphologies and photophysical properties. Also, semiconductor nanorods (NRs) exhibit unique polarized luminescence which can potentially break the theoretical limit of external quantum efficiency in light emitting diodes based on spherical quantum dots. Although rod‐in‐rod core–shell NRs present higher degree of polarization, most studies have focused on dot‐in‐rod core–shell NRs due to the difficulties in achieving uniform NR seeds. Here, this study prepares high‐quality uniform CdSe NRs by improving the reactivity of the Se source, using a secondary phosphine, namely diphenylphosphine, to dissolve the Se power, along with the conventional tertiary phosphine, namely trioctylphosphine. Starting from these high‐quality NR seeds, this study synthesizes CdSe/CdxZn1−xS/ZnS core–shell NRs with narrow emission bandwidth (29 nm at 620 nm), high PLQY (89%) and high linear polarization (p = 0.90). This study then assembles these core–shell NRs using the confined assembly method and fabricates long‐range‐ordered microarrays with programmable patterns and displaying highly polarized emission (p = 0.80). This study highlights the great potential of NRs for application in liquid crystal displays and full‐color light emitting diodes displays.

High-Efficiency and Stable Colloidal One-Dimensional Core/Shell Nanorod Light-Emitting Diodes

“…However, we note that the rod-in-rod NRs present a degree of polarization ∼1.5 times higher than that of dot-in-rod NRs, which is crucial for improving the photon outcoupling efficiency of NR-LEDs. 20,37 In addition to the PLQY, the aspect ratio of NRs also affects the performance of NR-LEDs by affecting the film morphology of the NR emission layer, 24,27,29,32,34,35 since it has been reported that defects or the unevenness of the NR film can lead to leakage current and a low EQE. 38,39 In addition, the imbalanced carrier injection caused by the high electron mobility and small electron injection barrier of the widely used ZnO electron transport layer further limits the EQE of NR-LEDs.…”

“…To overcome the challenge of synthesizing uniform CdSe NRs, we used a secondary phosphine, namely diphenylphosphine, to enhance the reactivity of the Se source and prepare high-quality homogeneous CdSe NRs. 24 The growth of the alloying shell was initiated by rapidly injecting a mixture of a sulfur precursor and a CdSe NR seed into the Cd precursor solution. The Zn precursor was then slowly injected.…”

“…Compared with CdSe/CdS NRs, CdSe/CdZnS/ ZnS NRs have a higher binding energy due to the CdZnS/ZnS alloying shells. 24,49 The introduction of CdZnS/ZnS alloying shells in CdSe/ CdZnS/ZnS NRs can effectively suppress energy transfer in the film of NRs. 24,27,50 For an accurate comparison, we synthesized traditional CdSe/CdS rod-in-rod NRs in which the CdS shell has the same thickness as the shell in CdSe/CdZnS/ ZnS NRs.…”

“…These effects can be attributed to the strongly confined excitons in the core by the CdZnS/ ZnS shell, so that the Forster resonant energy transfer is significantly reduced. 24,42,45 In addition to the PLQY, the anisotropic emission is also affected by the structure of the NRs, including the core morphology and shell composition. 24,37,45 We measured the polarized PL from single NRs deposited on a silicon substrate.…”

“…24,42,45 In addition to the PLQY, the anisotropic emission is also affected by the structure of the NRs, including the core morphology and shell composition. 24,37,45 We measured the polarized PL from single NRs deposited on a silicon substrate. The average polarization of 10 CdSe/CdZnS/ZnS rod/rod single NRs is 0.89 ± 0.01, which is higher than that of the CdSe/CdS rod/rod NRs (0.82 ± 0.02) and the CdSe/CdS dot/rod NRs (0.72 ± 0.02) (Figure 3d and Figure S2).…”