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

Scheme 1. Synthetic procedures from InP core to InP/ZnSe/ZnS core/shell/shell QDs. InP: Indium phosphide; QDs: quantum dots.

AVANCE II 400 MHz NMR system). The samples were packed into 3.2 mm zirconia rotors. The rotors
+
were spun 10 kHz at room temperature. P solid-state NMR was performed with a recycle delay of 100 s. P
31
31
chemical shifts were referenced to an external 85% H PO sample. Raman spectral analysis was conducted
4
3
utilizing a Raman imaging microscope (Thermo Scientific Inc., DXR2xi). The samples were prepared by
drop-casting the QDs solution on a silicon substrate and drying in ambient condition. All samples were
excited at a wavelength of 455 nm employing a laser diode. The resultant scattering signals were captured
via an electron-multiplying charge-coupled device (CCD).

RESULTS AND DISCUSSION
The present green-emissive InP cores were synthesized by heating up a mixture of (TMS) P, In acetate, Zn
3
acetate, palmitic acid, and ODE to 240 °C, where the formed Zn carboxylate plays a role in regulating the
reactivity of indium precursor with (TMS) P via Zn-P coupling toward narrow size distribution [37-39] . The
3
resulting InP cores displayed a well-defined absorption feature with the first excitonic peak at 440 nm along
with a high valley depth (VD) of 0.4 [Supplementary Figure 1A]. The VD value, defined as the ratio of
absorption between the first maximum and the minimum inflection point, is a convenient measure for
assessing the size distribution particularly for binary QDs, as their band gap (E ) variation is derived
g
exclusively from the degree of size distribution with the compositional deviation excluded. Nemoto et al.
reported an even higher VD of 0.51 from green-emissive InP cores , indicating there is still room for
[40]
synthetic optimization of our InP cores toward higher size homogeneity. The size of InP core required to
secure green emissions of ca. 525-535 nm in PL peak wavelength typically ranges in 2-2.5 nm
(corresponding to the first excitonic absorption peaks at ca. 430-460 nm), although the ultimate PL
wavelength nontrivially varies with details of core/shell heterostructure including shell chemical
composition and thickness. As seen in a TEM image of InP cores [Supplementary Figure 1B], the precise
size determination of green-emissive InP core is highly challenging from direct microscopic measurement
due to its extremely tiny size. Such size uncertainty is often tackled with the correlation between E versus
g
[41]
diameter (d) (i.e., sizing curve) such as E = 1.401 + 3.493/d for InP cores , by which the size of the
1.172
g
present InP cores was estimated to be 2.0 nm.
InP cores were placed in a two-step shelling process, which rules out the gradual incorporation of unreacted
species (yielding the non-radiative recombination channels) into the subsequent heteroepitaxial shell .
[42]
Distinct from the existing shelling protocol, where Zn(OA) is exclusively used for ZnSe inner and ZnS
2
outer shell growth, we uniquely adopted Zn halide precursors of ZnX (X = Cl, Br, I). For convenience’s
2
sake, we denoted InP QDs shelled with different types of Zn precursors as InP-ZnX and InP-Zn(OA) .
2
2
Figure 1A presents the Zn precursor-dependent variations of PL QYs and peak wavelengths of green InP/
ZnSe/ZnS QDs. InP-ZnX /ZnSe/ZnS QDs exhibited overall higher PL QYs (specifically, 75%-89%,
2
depending on the type of Zn halide) compared to InP-Zn(OA) /ZnSe/ZnS ones (71%), while use of ZnBr 2
2