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Jo et al. Soft Sci 2024;4:27 https://dx.doi.org/10.20517/ss.2024.19 Page 7 of 14

[36]
[32]
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direct HF or ZnF -assisted indirect HF treatment . A solid-state magic angle spinning (MAS) P NMR is
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a useful tool for examining the oxidation of InP QDs as it provides the distinction between InP and InPO
x
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(x = 2-4) along with the quantitative fraction of oxidized phosphorus [46,47] . As shown in P NMR spectra
[Figure 2D], the single peak at -200 ppm corresponds to unoxidized InP, while those between 60-0 ppm are
relevant to phosphate moieties. Combined analytic results of XPS and NMR point to the effective removal
of surface oxide species via adoption of a hybrid Zn process, under which HX is produced by the reaction of
ZnX + RCOOH → (RCOO) Zn + 2HX and the released HX takes part in the in-situ etching of surface
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oxide. Typical PL QY of as-synthesized InP core is highly poor (i.e., less than 1%) due to the lack of
passivation of surface defects [14,15,37,44] . A set of InP-ZnX samples exhibited low PL QYs of 1%-5%
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[Figure 2E], while another set of InP-ZnX -Zn(OA) ones possessed overall higher values of 4%-17%
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[Figure 2F], depending on the type of Zn halide. An increasing tendency in PL QY is commonly observable
in the order of Cl > Br > I from both sets of samples. ZnX as Z-type ligand can be involved with the
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passivation of undercoordinated surface P (hole trap state) [48,49] . Simultaneously, X ions released from ZnX
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can also serve as X-type ligands for the passivation of undercoordinated surface In (electron trap state) .
[50]
The above PL QY tendency as a function of halide can be understood from the consensus that the smaller
size of either ZnX or X is more advantageous in surface passivation from a perspective of steric
-
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hindrance [50,51] . Besides, higher PL QYs of InP-ZnX -Zn(OA) compared to InP-ZnX samples (with the type
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of ZnX identical) are ascribable to better ligand passivation enabled by the surface oxide removal, further
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validating the above analytical outcomes. Presence of surface halide species sitting on InP-ZnX -Zn(OA)
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samples can be verified by XPS analysis [Supplementary Figure 4].
Leveraging the above hybrid Zn process, subsequent ZnSe shelling was implemented for InP-ZnX -Zn(OA)
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by co-injecting the equimolar Se stock solution. Figure 3A-C presents TEM images of a set of InP/ZnSe
QDs with use of ZnCl , ZnBr , and ZnI , corresponding to the average sizes of 8.0, 5.6, and 5.1 nm,
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respectively. This size trend is in line with that from the previous InP-ZnX /ZnSe/ZnS QDs [Figure 1C-E],
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manifesting that the thickness of ZnSe shell is sensitively dependent on the type of ZnX despite use of the
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identical molar quantity of ZnX . In the case of synthesis of aminophosphine-derived InP cores, where InX
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as a cationic precursor is used in the presence of amines such as OLA, the size of the resulting cores tends to
[52]
increase in the order of Cl > Br > I , which is analogous to the present ZnX -dependent variation of ZnSe
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shell thickness. Based on the assumption of surface reaction-limited condition, ZnSe shell growth would be
expected to be facilitated under the enhanced solute solubility and the increased rate constant of surface
reaction . For ZnX , OLA, categorized as a hard base, may be able to produce the complexes by more
[52]
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strongly coordinating with ZnCl compared to ZnI , indicative of a higher solute solubility of the former
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versus the latter. Meanwhile, given the adsorption of halide on InP core surface (as evidenced from
Supplementary Figure 4), the least bulky surface halide of Cl ions will render ZnSe shell growth easier,
-
raising the rate of surface reaction. Disparity in ZnSe shell thickness as a function of ZnX is well reflected in
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absorption spectra normalized at the 1S peak [Figure 3D], where a greater absorbance in blue-to-near UV
region was observed in the sequence of Cl > Br > I. Accordingly, PL systematically shifted with a thicker
ZnSe shell from 531 (for ZnI ) to 539 nm (for ZnCl ) [Figure 3E], being related to the ZnSe shell thickness-
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dependent delocalization of electron wavefunction aforementioned. Even without ZnS outer shelling, PL
QYs of a series of InP/ZnSe QDs were relatively high. In InP-ZnX -Zn(OA) /ZnSe QDs ZnBr yielded the
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highest PL QY of 93%, while ZnCl and ZnI gave comparable values of 83%-85% [Figure 3F]. As argued
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above, use of ZnCl leads to the fastest ZnSe shell growth, which stochastically renders controlled
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heteroepitaxial shell growth unlikely, thus leaving some imperfect sites at hetero-interface and/or shell
interior. As an opposite case, the introduction of ZnI induces the slowest shell growth reaction entailing the
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formation of the thinnest ZnSe shell, which can often encounter the high probability of incomplete shell
coverage on core surface. Besides, HI is known as an unstable chemical as it spontaneously oxidizes to
molecular iodine (I ) during the reaction [53,54] , implying that in removing the surface oxide, it may not serve
2