I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Simple cubic self-assembly of PbS quantum dots by ﬁne ligand control†
J. Liu,∗1 K. Enomoto,∗1 K. Takeda,∗1 D. Inoue,∗1 and Y. -J. Pu∗1
Colloidal quantum dots (QDs) have attracted substantial attention due to their characteristic optoelectronic
properties based on the size conﬁnement eﬀect.1) They
are also known to self-assemble into highly ordered superlattices.2) The geometry of such self-assembled QDs
has been explored to better understand the ensemble
eﬀects on their optical and electrical properties.3) Accordingly, it is essential to prepare two-(2D) and threedimensional (3D) QD superlattices with diﬀerent geometry for their solid-state device applications.
Most colloidal quantum dots with a quasi -sphericalshape readily self-assemble into 3D face-centered cubic(fcc) or body-centered cubic (bcc) superlattices.4) In contrast, 3D simple cubic (sc) superlattices are diﬃcult toobtain from sphere-like QDs because of their relatively
low packing fraction of 0.52 and the low stability by entropy.5) Therefore, sc superlattices have been expected
to have diﬀerent optical and electronic properties fromother packing structures, and the quest has been on
to achieve 3D self-assembled sc superlattices. Here, we
achieve the selective control of the geometry of the quasi spherical PbS QDs in highly-ordered 2D and 3D superlattices: Disorder, sc, and fcc by selectively removing
the ligands from the QD surface through gel permeation
chromatography (GPC).
The uniform sphere-like PbS QDs with an average diameter of 7.3 nm were synthesized. The QD sample waspuriﬁed once by precipitation/redissolution process prior
to the GPC process (before-GPC). In the GPC process,the eluted QD solution was collected in 5 consecutive
portions (GPC-1 to GPC-5).
The thermogravimetric analysis (TGA) was carriedout to evaluate weight ratios of oleic acid (OA) ligands bound to QD surface (Fig. 1a) which gradually
increase from GPC-1 to GPC-5. To further determine
the ligand density, the atomic Pb/S ratios of GPC-2 andbefore-GPC were obtained from Rutherford backscattering (RBS) spectrometry in RIKEN Pelletron accelerator
facility (Fig. 1b).6) An average Pb/S ratio (1.27) combined with TGA results was used for estimating the ligand density. The TGA results (Fig. 1c) demonstrate thatthe ligand density of the PbS QDs gradually increases
from GPC-1 (0.6 nm−2 ) to GPC-5 (7.1 nm−2 ) and that
the GPC method can precisely and continuously control
the ligand density of the PbS QDs. We also estimatedthe ligand density by nuclear magnetic resonance spectroscopy. The obtained results are consistent with theligand density obtained from TGA (Fig. 1c).
Based on the diﬀerence in their ligand density, the 2D
self-assembly of these QDs was achieved by drop-casting
dilute solutions onto solid substrates. GPC-1–5 formed
2D superlattices with diﬀerent and unique geometries.
†∗1Condensed from the article in Chem. Sci. 12, 10354 (2021)
RIKEN Center for Emergent Matter Scienceabcde
100 nm

GPC-2

ϭϬʅŵ

100 nm

100 nm

GPC-5ϭϬʅŵ

100 nmFig. 1. (a) TGA curves of before-GPC and GPC1-5; (b) RBS
spectra of GPC-2 and before-GPC; (c) Ligand density ofPbS QDs as a function of the GPC fraction; The scanning electron microscopy (SEM) images of the supercrystal surfaces for GPC-2 (d) and GPC-5 (e); inset: their
2D self-assembled TEM images and SEM images of 3D
supercrystals.

GPC-1 QDs showed a random assembly due to insuﬃcient ligands on the QD surface. However, GPC-2 QDs(inset of Fig. 1d) formed a square self-assembled geometry without the fusion between QDs. Also, the GPC-2
QDs in the square superlattice are highly oriented along
QD [100] facets. The GPC-3–5 QDs with suﬃcient ligands formed the hexagonal assemblies with non-orientedfacets (inset of Fig. 1e).
Furthermore, the long-range-ordered 3D self-assembled
supercrystals for GPC-2 and GPC-5 QDs were preparedby solvent evaporation. GPC-5 QDs can form triangularor hexagonal fcc supercrystals with a hexagonal arrangement on the crystal surface (Fig. 1e). It is noteworthy
that GPC-2 QDs self-assembled in cubic supercrystals
with sc packing on account of ﬁnely controlled ligand
removal by GPC, which is usually diﬃcult for quasispherical QDs.
In this report, we develop GPC as a method to control
the ligand density of the quasi -spherical PbS QDs andachieve the selective control of the geometry of QDs in2D and 3D superlattices: Disorder, sc, and fcc. The precise control of QD self-assembled geometry is expected
to greatly improve the performance of next-generation
semiconductor devices and photocatalysis.
References
1) C. R. Kagan et al., Science 353, 885 (2016).
2) M. A. Boles et al., Chem. Rev. 116, 11220 (2016).
3) C. S. S. Sandeep et al., ACS Nano 8, 11499 (2014).
4) K. Bian et al., Chem. Mater. 30, 6788 (2018).
5) M. Boles, D. Talapin, J. Am. Chem. Soc. 137, 4494
(2015).
6) I. Moreels et al., ACS Nano 3, 3023 (2009).

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