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Page 2 of 14 Jo et al. Soft Sci 2024;4:27 https://dx.doi.org/10.20517/ss.2024.19

INTRODUCTION
Today, colloidal quantum dots (QDs) are scientifically and industrially important nanomaterials
[1-4]
particularly as visible emitters for post-organic light-emitting diode next-generation displays . Besides,
high compatibility of QDs with solution processing and for inorganic-organic hybridization makes them
[5,6]
prospective candidates for flexible/stretchable display devices . To meet the high standards of state-of-the-
art display devices that require high efficiency and color gamut, bright and narrow emissivity of QDs must
be achieved. Pb-containing perovskite- and CdSe-based QDs with excellent photoluminescence (PL)
quantum yield (QY) (> 90%) and sharp linewidth [full-width-at-half-maximum (FWHM)] (< 20-30 nm) are
considered ideal light emitters [2,7-11] , but their use is strictly limited to the commercial display panels due to
the environmental regulations on toxic heavy metal elements [12,13] . In this regard, visible-emissive indium
phosphide (InP)-based QDs with an environmental benignity have emerged as the most promising
alternative [14-18] . A primary factor that limits the optical properties of InP QDs is the incomplete surface
passivation, by which unpassivated surface dangling bonds serve as the trap sites for photoexcited carriers,
detrimental to the radiative recombination. In heterovalent core/shell structure of InP QDs, ZnS has been
initially chosen as a shell due to its wide band gap which enables the effective confinement of charge carriers
within the InP core domain [19-22] . However, PL performance of the resulting InP/ZnS QDs was often
unsatisfactory mostly thanks to the considerable interfacial strain developed by a sizable InP-ZnS lattice
mismatch. Placement of better lattice-matched shells, such as GaP [23-25] , ZnSe [14,26] , and ZnSeS [27-29] , between
InP and ZnS led to significant enhancements of PL figures-of-merit (i.e., QY and FWHM). The state-of-the-
art heterostructured InP QDs for green and red colors displayed PL QYs of ≥ 95% and FWHMs of ca.
35 nm [30-32] . Meanwhile, single-particle measurements of InP QDs revealed the linewidth values comparable
to those of CdSe QDs (< 30 nm) [30,33] , suggesting there is still large room for further synthetic improvement
toward sharper, brighter emissivity of InP QDs.

In a typical synthesis of InP QDs, the carboxylic acid used to generate indium carboxylate produces water
through a ketonization reaction at high temperatures. This water, in turn, induces the unwanted formation
of a surface oxidation product on as-synthesized InP core [34,35] , which can hamper not only full passivation of
core surface but uniform growth of epitaxial shell. For an effort to etch out such surface oxide species,
[32]
hydrofluoric acid (HF) was directly introduced or indirectly in-situ generated via the reaction between
ZnF and carboxylic acid in synthesis of “red-emissive” InP/ZnSe/ZnS QDs, resulting in PL QYs of 90%-
[36]
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100% and FWHMs of 35-36 nm. Although the HF treatment is a useful means for the effective removal of
the surface oxide layer on InP core, its highly corrosive, toxic character may become a stumbling block for
eco-friendly, sustainable mass production in the future. Thus, an alternative safe synthetic method should
be unearthed. Moreover, judging from no noteworthy report on the efficacy of HF treatment on “green-
emissive” InP QDs, it is highly likely that HF treatment works well only to a relatively large-sized (i.e., red-
emissive) core, even though its cause appears still ambiguous.

In this contribution, a new and innovative means toward synthesis of green InP/ZnSe/ZnS QDs with a unity
PL QY together with a sharp emissivity is developed. The key to synthetic success involves adding zinc
oleate [Zn(OA) ] to the existing zinc halide (ZnX )-based shell growth process. We first explore the effects
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of different shell precursors of Zn(OA) and ZnX (X = Cl, Br, I) on growth outcomes and optical properties
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of the resulting heterostructured InP QDs. InP/ZnSe/ZnS QDs shelled with oxygen-free Zn precursors of
ZnX exhibit not only higher PL QYs (75%-89%, depending on the type of ZnX ) but more homogeneous
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particle morphologies compared to those shelled with a common Zn precursor of Zn(OA) . The subsequent
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hybrid shelling approach, where co-use of Zn(OA) and ZnX synergically leads to the effective removal of
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surface oxide species on InP core, further enables synthesis of green InP QDs having PL QYs of up to 100%
along with a narrow FWHM of 32 nm upon adopting an appropriate halide precursor of ZnBr and
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