Electrochemical Charging Effect on the Optical Properties of InP/ZnSe/ZnS Quantum Dots

Abstract

fabrication of blue-emitting Cd-free QDs are still under development. [7-10] Above all, the charging of QDs is an obstacle for manufacturing QD-LEDs with good performance. [11] The simple mechanism of EL is that electrons and holes are formed by charge injection, and light is generated by the electron-hole pair recombination. Hence, it is essential to understand the charging process by mimicking EL processes. The complementary combination of spectroscopic technology and electrochemical control helps emulate EL and explain the charging process whether the charges occupy quantum states or trap states. Recently, there has been an increase in research to improve QD luminescence capabilities and charge extraction properties by performing electrochemical charging studies on QD films. [1,12,13] Li et al. [13] have shown that electron injection into the ground excitonic state of CdSe/CdS QDs leads to the bleaching of 1S 3/2 1S e transition because of 1S e state filling. Qin et al. [14] have investigated how the photoluminescence (PL) lifetime changes in CdSeS/ZnS QD under electrochemical control with blinking dynamics. Gooding et al. [15] have studied the effects of hole and electron injection on the PL of CdSe/ CdS/ZnS QDs. Many studies have revealed the effect of electron injection on the optical properties in Cd-based QDs, which leads to environmental issues. Environmentally friendly III-V semiconductors, especially InP QDs, have recently emerged as the best alternative to toxic II-VI semiconductors. [16-17] Meanwhile, the surfaces of InP core QDs are easily oxidized when they are exposed to oxygen environments. As a countermeasure, overcoating nanocrystals with wider band-gap materials has proven to be the best option to increase stability against photo-oxidation and to enhance the photoluminescence quantum yield (PL QY). [5,18,19] For example, the inorganic core can be surrounded by the shell, such as ZnSe and ZnS. [20] Möbius et al. reported that the electrical charges generated by application of a mechanical load are injected into the QDs, leading to PL quenching in InP/ZnS-based device. [17] However, the spectroelectrochemical properties were not yet reported in InP QDs to the best of our knowledge. Recently, we have reported highly luminescent InP/ZnSe/ZnS QDs synthesized with as much high efficiency as state-of-the-art Cd-based QDs. [21] In this study, we analyze the spectroelectrochemical properties depending on mid-shell thickness to identify the effect of charging on highly luminescent InP Semiconductor quantum dots (QDs) are spotlighted as a key type of emissive material for the next generation of light-emitting diodes (LEDs). This work presents the investigation of the electrochemical charging effect on the absorption and emission of the InP/ZnSe/ZnS QDs with different mid-shell thicknesses. The excitonic peak is gradually bleached during electrochemical charging, which is caused by 1S e (or 1S h) state filling when the electron (or hole) is injected into the InP core. Additional charges also lead to photo...

Highly Bright, Narrow Emissivity of InP Quantum Dots Synthesized by Aminophosphine: Effects of Double Shelling Scheme and Ga Treatment

“…As an inner shell composition, a better lattice‐matched ZnSe with InP core compared to ZnSe 0.5 S 0.5 likely enables a more perfect surface passivation toward brighter PL. PL line width of multishelled InP QDs is somewhat intricately correlated with the composition, uniformity, [ 15,18 ] dimension, [ 24 ] and processing (i.e., one‐pot or two‐step) [ 18 ] of shell as well as the size dispersion of core. In InP QD heterostructure, a relatively S‐rich composition in ZnSe x S 1− x inner shell tends to provoke PL broadening, as it can lead to the nonuniform inner shell growth in thickness and shape, which is attributable to the disparities in lattice mismatch relative to InP and crystal bond energy between ZnS versus ZnSe.…”

“…Meanwhile, ZnSe (or Se‐rich ZnSeS in some cases) can afford a small lattice mismatch (3.3%) relative to InP, by which ZnSe inner shell growth with an extended thickness over 3 nm becomes allowable without a noticeable interfacial lattice relaxation. [ 23 ] Recent InP QDs displaying the outstanding PL figures of merit such as ≥90% in PLQY and ≤40 nm in FWHM in particular for both green [ 21,22 ] and red color [ 23,24 ] were indeed integrated commonly with ZnSe/ZnS double shells.…”

Aminophosphine-derived, high-quality red-emissive InP quantum dots by the use of an unconventional in halide

“…Kim's group recently reported that in (TMS) 3 P-based red InP/ZnSe/ZnS QDs, the thickening of the inner shell was effective in narrowing PL bandwidth, which is associated with the considerable reduction of exciton–phonon coupling strength with a thicker inner shell-induced larger localization length of excitons. 21 In this regard, we presume that a sharp bandwidth of 44 nm obtained from the present InP/thick-ZnSe/ZnS QDs was ascribable jointly to a relatively thick thickness as well as homogeneous composition of the inner shell. Besides, InP/thick-ZnSe/ZnS QDs yielded a higher PL QY (86%) than InP/thin-ZnSe/ZnS ones (81%), pointing to a more defective core/inner shell interface from the latter versus the former.…”

“…In the case of InP heterostructured QDs with double shells such as ZnSe/ZnS and ZnSeS/ZnS, their emission bandwidth is dependent on the composition 8,9,19 and thickness of the inner shell. 21–23 Given the identical inner shell ( i.e. , ZnSe) for both InBr 3 - and InCl 3 -based red QDs above, the equality in bandwidth may point to the growth of the ZnSe inner shell with the same thickness as well as the same degree of core size monodispersity.…”

“…Auger recombination, the most detrimental factor in limiting QLED performance, can be highly sensitive particularly to the thickness of the inner shell in the InP/ZnSe/ZnS QD system. 10,21,22 Thickening ZnSe inner shell was shown to be beneficial in slowing down the Auger rate of particularly negative trion by delocalizing an electron and thus weakening intercarrier Coulombic interaction, 21,22 by which the chance of multi-carrier radiative recombination becomes enhanced toward higher device efficiency.…”