KONA Powder and Particle Journal No. 41 (2024) 172–182/https://doi.org/10.14356/kona.2024001

Review Paper

Recent Progress in Controlled Nanostructure of Colloidal
Nanocrystal Powders for Efficient Light Emission †Naoto Shirahata
1 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Japan
2 Graduate School of Chemical Sciences and Engineering, Hokkaido University, Japan
3 Department of Physics, Chuo University, Japan

4 CNRS−Saint-Gobain−NIMS, IRL3629, Laboratory for Innovative Key Materials and Structures, JapanThe application of semiconductor nanocrystals containing cadmium, lead, selenium andmercury as constituent elements is strictly limited by concerns about environmental
pollution and health effects. Nanocrystals free of these toxic elements are being pushed to
the forefront of nanocrystal research because of their environmentally friendly advantages
and attractive photophysical properties on recent advances in colloidal synthesis, excellent
optical properties, optoelectronic device applications, and biological applications of theseenvironmentally friendly fluorescent nanocrystals. In this context, the first topic in this review
paper introduces group IV semiconductors. In particular, the unique light emitting properties
generated in silicon nanocrystals of diameters smaller than bulk exciton Bohr radius arehighlighted. Next the topic turns to the nanocrystals of group III–V semiconductors. After that,
attentions are paid to the lead-free perovskite nanocrystals such as tin-based halide perovskite
and double perovskite structures. Recent efforts on how to control nanostructures to enhancephotoluminescence quantum yields is highlighted for each semiconductor nanocrystal.
Finally, the remaining challenges that must be overcome to realize nontoxic optoelectronic
devices will be discussed.
Keywords: quantum dot, perovskite nanocrystals, photoluminescence, optoelectronics,
bioimaging, photothermal effect

1. Introduction

Colloidal semiconductor nanocrystals have attracted
huge interests over 30 years. The nanocrystals containhundreds to thousands of atoms and are covered withorganic molecules (i.e., ligands) to make them stabilize insolution (Calvin et al., 2022), providing high accessibility
to printable technology. They are called “quantum dots
(QDs)”, “artificial atoms” or “zero-dimensional quantum
boxes”. The effect of quantum confinement happens forthe nanocrystals of diameter smaller than the bulk exciton
Bohr radius for the bulk crystal. QDs are usually larger
than the conventional organic dyes but comparable to fluorescent proteins and other large biomolecules. Their opticalproperties are unique and can be adjusted with diameter,
chemical composition, shape, and surface state. Therefore,
the QDs as optically active sources have a strong potential
for use in a variety of optoelectronic applications including
light-emitting diodes, photodiodes and photovoltaics aswell as in biomedical detection such as fluorescence celReceived 25 July 2022; Accepted 19 October 2022
J-STAGE Advance published online 29 December 2022
1, 4 Add: 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2
Add: Kita 13, Nishi 8, Kita-ku, Sapporo 060-0814, Japan3
Add: 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan
E-mail: SHIRAHATA.Naoto@nims.go.jp
TEL: +81-29-859-2743 FAX: +81-29-859-2401†

172

lular imaging. In a typical, a single QD contains atoms ofgroup IV elements (e.g., Si, C or Ge), group II–VI elements(e.g., CdS, CdSe, CdTe, ZnS, ZnSe or ZnTe), group III–V
elements (e.g., InP, InSb or InAs), group I–III–VI2 elements (e.g., CuInS2, CuInSe2, AgInS2 or AgInSe2), group
IV–VI elements (e.g., PbS, PbSe or PbTe), ZnS–AgInS2(ZAIS) or AgInSe2–ZnSe (ZAISe). The QD as a core could
be often encapsuled with additional protective shells of
ZnS and/or ZnSe, which are well-known wide-band-gap
semiconductors, to form core/shell structures.
Metal-halide perovskite nanocrystals are optically active
materials whose chemical compositions and structural
spaces are constantly expanding for successive tuning of
optical absorption and emission spectra. Lead halide perovskite has emerged as an attractive semiconductor material due to its excellent optical and electronic properties andis expected to find applications in a wide range of optoelectronic fields, including light-emitting diodes (LEDs). Until
now, many efforts have been made to increase photoluminescence quantum yield (PLQY) to nearly 100 % in thevisible and near-infrared regions by optimizing synthesis
parameters, impurity doping, alloying and surface passivation (Chen et al., 2021a). Various high-performance optoelectronic devices with the lead perovskite nanocrystals asactive layers have been investigated by taking advantageof these excellent properties (Quan et al., 2019). Thus, theCopyright © 2024 The Author. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (https://creativecommons.org/licenses/by/4.0/).

Naoto Shirahata / KONA Powder and Particle Journal No. 41 (2024) 172–182

lead halide perovskite nanocrystals are expected to be thenext generation of light emitting devices, and the external
quantum efficiency of lead-based LEDs exceeds 20 % (Linet al., 2018). There are concerns of environmental pollution and health problems due to the toxicity of Pb. Recent
studies have shown that the bioavailability of lead fromlead halide perovskite is stronger than other sources of leadand that as the concentration of lead halide perovskite inplants increases, the capacity for lead uptake increases (Liet al., 2020). To overcome this short coming, lead-free perovskite nanocrystals such as tin halide perovskite or double
perovskite nanostructures have attracted attention over theyears. However, the luminescence performances of thelead-free perovskite nanocrystals currently far inferior to
the lead perovskite ones.
Controlling the fate of free-charge carriers generated in
nanocrystals of semiconductor through sequential absorption of photons in the photoexcited state determines optical
performance (Chen et al., 2021b). For example, radiative
recombination between electrons and holes should be
strengthened to increase the PLQY. Furthermore, defective
surface serving as electron trapper should be removed todecrease the nonradiative recombination rate (Ghosh et al.,2020). On the other hand, the formation of shallow energy
level is key to synthesize long persistent phosphors, which
are a class of optical batteries, allowing emission of luminescence continuously for a long time after the removalof excitation sources (Liu Q. et al., 2021a). In this review,
recent efforts on defect control to improve PLQY of typical
non-toxic nanocrystals for optical applications such as
light-emitting diodes with emission layer of QD (QLED),
namely, electrically driven luminescent (EL) devices will
be presented.

2. Group IV QDs

QDs in group IV semiconductors such as carbon, silicon

Review Paper(Si), and germanium have attracted attention in the applications of abundant and nontoxic fluorescence materials, with
an emphasis on their role in optoelectronics and nanomedicine. This chapter presents the research advances in fluorescent SiQD on controlled nanostructures giving a strong
light emission and various photonic applications such as
light-emitting diodes, biomarkers for cellular imaging andphotothermal therapy of cancer cells.

2.1 Improved PLQYs

Bulk Si crystal has an indirect band structure, which
results in very low PLQY (<10–4 %). However, the situation changes drastically when crystals are nanosized tonanoparticles of 5 nm or less in diameter. In the electronic
structure of 2–5 nm crystals, the quantum confinement
effect of photoexcited carriers relaxes the momentum conservation based on the uncertainty principle (ΔχΔρ ≥ ħ/2)
and interface scattering-induced Γ–X coupling, leading
to the zero-phonon optical interband transitions (Lee etal., 2016). In the 1.1–1.7 nm crystal size range, a diamondcubic crystalline Si can behave like a molecule in termsof energy structure (Shirahata et al., 2020). The mostconventional Si surface is terminated with hydrogen atoms(H-SiQD) as known from the wafer chemistry (Shirahata etal., 2005). Photoluminescence (PL) properties are dependent of the size of diamond cubic nanocrystal of Si. First,
the peak position of the PL spectrum depends on the sizeof the H-SiQD and can be modulated over a wide wavelength range of 530–1050 nm as shown in Fig. 1(a)–(c).
Second, the PLQYs monotonically increase with decreasein size of H-SiQD based on the quantum confinement
effect (see Fig. 1(d)). PL peak positions at 1050 nm forthe 5-nm diameter SiQD and does not shift to a longer
wavelength side anymore for the diameter range between
5 and 8 nm. The hydrogen atoms may cover 99 % of the
Si atoms in the outermost layer. In the oxidized SiQD,

Fig. 1 Optical properties of SiQD powders: (a) photographs of visible-light emitting H-SiQD powders excited at UV light (λ = 365 nm), (b) PL spectraof the H-SiQD powders listed in the Panel (a), (c) near-infrared PL spectra of the H-SiQDs excited at UV light (λ = 365 nm), (d) PLQYs of the H-SiQDs
before and after hydrosilylation of 1-decene. The PL spectral tuning could be realized by controlling diameters of SiQDs in the 1–5 nm range. Adaptedwith permission from Ref. (Shirahata et al., 2020). Copyright 2020, American Chemical Society.

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the electron-hole recombination involves O-related defectcenters (Dohnalová et al., 2013). In contrast, the hydrogentermination minimizes surface oxidation that forms SiOx.
Nevertheless, the resultant PLQYs are still less than 15 %as shown in Fig. 1(d). A significant increase in PLQY is
realized by replacement of surface hydrogen atom withalkyl monolayer through the hydrosilylation reaction (seeFig. 1(d)). The best value of PLQY is more than 73 % for
the near-infrared light emitting QD (Jurbergs et al., 2006).
Two possible mechanisms have been proposed to explain
the improved PLQY. Dohnalová et al. reported the theoretical study of the change in radiation rate and recombination
process due to the attachment of alkyl monolayers, whichswitches SiQD from an indirect-bandgap-like material toa direct-one, leading to the phonon-less character of excitonic recombination close to band edges (Poddubny andDohnalová, 2014). The increase in radiative recombination
rate resulting from the alkylation of surface Si atoms has
also been reported (Mastronardi et al., 2012), consistent
with the theoretical prediction (Lee et al., 2016). Furthermore, alkyl monolayers work to inhibit the formation of
defects as a nonradiative channel in the outermost layer
of QD. Specifically, combining scanning transmission
electron microscopic (STEM) observation with Raman
spectroscopy demonstrated that the hydrogenated surface
layer is distorted under structural reconstruction to generate an amorphous phase (Ghosh et al., 2016), whereas thepassivation with alkyl monolayers suppresses the surface
reconstruction to hold a bulk-like geometry (i.e., a diamondcubic lattice structure) in a broad range from the center

Review Paper

toward the near-surface in QD (Ghosh et al., 2018a). The
amorphous region might have many dangling bonds as anonradiative relaxation channel, resulting in a low PLQY,
but there are few defects around the surface terminatedwith alkyl monolayers. This suggests that the passivation
with alkyl monolayers works to prevent any undesired
surface reconstruction, leading to high PLQY.

2.2 Optoelectronic devices

The tremendous improvement in PLQY has attracted
interests of scientists and encouraged the fabrication ofdevices such as current-driven QLEDs in which alkyl
terminated SiQD works as an active layer. The reported device architectures have a multilayer structure where SiQD
are sandwiched between hole and electron transportation
layers as shown in Fig. 2(a)–(c). The electroluminescence(EL) signals are observed under low applied voltage.
Fig. 2(d)–(f) demonstrates that the EL spectral peak shapeand broadness are similar with those of PL spectra. Besides, the photocurrent depends on applied voltage, leading
to the increase of luminance at high device current density.
The EL spectral peak position precisely tuned by diameterof SiQD. To date, the highest values of external quantum
efficiency (EQE) at present were 4.8 %, 8.6 %, 3.36 %,6.2 %, for the EL spectral peaking at 1000 nm (Watanabe
et al., 2021), 853 nm (Cheng et al., 2011), 755 nm (Yamada
et al., 2020), 720 nm (Ghosh et al., 2018), 700 nm (Liu X.
et al., 2018), respectively. The EQEs of EL spectra peaking
at the wavelength shorter than 700 nm are less than 1.2 %(Yamada et al., 2020). These values are still lower thanFig. 2 (a) Schematic representation, (b) a cross-sectional TEM image with (c) EDS maps of the area indicated in the image (b) of the Si-QLED. Scalebars are 200 nm for the image (b) and 20 nm for the image (c). (d–f) Device current-voltage and luminance-voltage characteristics for each QLED.
Inset figure shows comparison of EL and PL spectra and photograph demonstrates a representative EL emission during operation. Adapted with permission from Ref. (Yamada et al., 2020). Copyright 2020, American Chemical Society.

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20 % which is commonly employed as a benchmark. Further improved EQE requires both improvement of carrier
mobility and charge balance (Watanabe et al., 2021).
By taking advantage of the large Stokes shift between
optical absorption and emission derived from SiQDs, awhite-light EL is realized with a superimposed spectrumof a red EL peak and a blue–green EL peak without lossof optical energy (Ghosh et al., 2014). The high PLQYs
are realized by various method such as nonthermal plasma(Kortshagen et al., 2016), annealing of Si-rich borophosphosilicate (Kano et al., 2018) and thermal disproportionation of hydrogen silsesquioxane (Dasog et al., 2016) orsilicon monoxide (Sun et al., 2015). Hydrogen silsesquioxane are commercially available from Dow Corning Corporation (trade name FOx-17) but could be synthesized byhydrolysis and condensation of triethoxysilane (Chandra etal., 2017) or trichlorosilane (Sun et al., 2016).

2.3 Bio imaging and analysis

Inherently nontoxic element such as Si is expected touse for fluorescence labeling of cells or tissues. Wang etal. reported the first ever biomedical use of SiQDs as afluorescence label for DNA (Wang et al., 2004). Since
then, many papers have been reported on modifying thesurface of QDs to give them water solubility (Xu et al.,2016). Swihart et al. further studied the labeling of cellswith SiQDs and found that SiQDs can be used for in vivo
imaging (Erogbogbo et al., 2010). Si as an element shows
cell cytotoxicity but the toxicity is strongly influenced bysurface terminal groups (Ruizendaal et al., 2009). Specifically, amine-terminated surfaces are known to have low
cell viability, while carboxyl-terminated surfaces are lesstoxic to cells than amine-terminated ones, since most living
cells are negatively charged. In many cases, amphiphilicmolecules such as Pluronic F127 or PEG are employed
to encapsulate the core of SiQDs because the PEGylated
surface is electronically neutralized due to hydroxyl groups
exposed on the QD surface (Chinnathambi et al., 2019)
resulting in high hydrophilicity and biocompatibility(Hessel et al., 2010). Such a biologically inert surface
demonstrates a potential for medical diagnosis withoutmajor conformational change of human blood plasma
proteins in the bloodstream (Chinnathambi et al., 2020).
Thus, water-soluble SiQDs have the potential to be used as
fluorescent labeling materials for cells and tissues due totheir low cytotoxicity, but there are still challenging topics
in terms of optical properties. Specifically, PL spectra aredependent of QD size in the near-infrared (NIR) wavelength range, but SiQDs exhibit low absorption efficiency
for NIR light. Therefore, it is difficult to realize NIR–NIR
excitation-emission bioimaging in a single-photon excitation environment. There are two methods to overcome
this difficulty. First one is the fluorescence imaging undermultiphoton excitation conditions (He et al., 2008). The

Review Paper

first multiphoton-excited fluorescence imaging of cells
labeled with SiQD was reported in 2016 (Chandra et al.,2016). Second one takes advantage of long radiative decay
time (~µs scale) originating from energy structure of SiQD.
That is, the time-gated fluorescence imaging in the NIR
wavelength range was investigated for the observation of
biological tissues labeled with SiQDs (Sakiyama et al.,2018).
The Si surface exhibits high chemical affinity for covalent bonding with carbon, oxygen, and nitrogen, and can
produce a variety of organic derivatives hybridized at themolecular level (Shirahata, 2011). Their bonding ability
can be used for post-synthesis purification by gel permeation chromatography (Shirahata et al., 2010). During
purification, QDs can be separated according to differencesin PLQY (Watanabe et al., 2021).

2.4 Photothermal effect

The photothermal effect refers to the ability to convert
photon energy from light absorption into heat. Thermal
conversion by photoexcited nanostructures can take placeby different mechanisms depending on the nature of thematerial. Unlike metallic nanoparticles, for semiconductor
QDs, photothermal conversion is explained by excess
carriers undergoing nonradiative recombination, whichis enhanced by maximizing the nonradiative decay rates
and manipulating nonradiative channels. Controlling theheat of the QDs expands their availability for applications
including phototheranostics and photoacoustic bioimaging.
The photothermal responses of crystalline Si have been
investigated for various forms, including nanoribbons,nanowires, films, porous particles, and nanoparticles.
Multiple structural parameters such as size, surface configuration, and shape affect the photothermal response,but size is the most influential parameter. The temperature
trend for larger crystals is explained by the Mie resonances(Ishii et al., 2016). As shown in Fig. 3(a), the photothermal
response was dependent of size of QD even in the narrow
range of 2–5 nm because the presence of a relatively highdensity of defect states for nonradiative thermal dissipation
for larger QDs (Özbilgin et al., 2021). The photothermal
conversion efficiency values of SiQDs are dependent of
their sizes and estimated to be 25–37 % which are comparable to the nanoparticles of mesoporous Si (33.6 %),Au (13–21 %), CuSe (21 %) Fe/Fe3O4 (20.3 %), black
phosphorus (24.8 %), Bi2S3 (28.1 %) or CdTe (14 %),suggesting a potential of SiQD available in photothermal
applications. One example is demonstrated in the thermotherapeutic study where carboxy-terminated SiQDs internalized in the HeLa cell generates the heat to kill the cells
under laser irradiation (Özbilgin et al., 2022). Fig. 3(b)shows the fluorescence image of HeLa cells labeled withthe water-soluble red-light-emitting SiQDs, confirming thecellular internalization of the QDs. Furthermore, there is no175

Naoto Shirahata / KONA Powder and Particle Journal No. 41 (2024) 172–182

Review Paper

Fig. 3 (a) Summary of temperature rise traces of SiQDs of different sizes and forms as a function of incident green-laser power. (b) HeLa cellular
uptake of QDs terminated with undecanoic acid under confocal fluorescence microscopy. (c–f) Observation of HeLa cells before and after laser irradiation: (c, d) HeLa cells without QDs and (e, f) HeLa cells with 50 μg/mL QDs. Adapted with permission from Ref. (Özbilgin et al., 2022). Copyright2022, American Chemical Society.

significant change in cell morphology. Fig. 3(c) and 3(d)show the differential interference contrast (DIC) image
where HeLa cells are observed throughout the microscope
field of view. Without QDs, the cells remained the sameeven after laser irradiation as shown in Fig. 3(d). However,there was a noticeable decrease in the number of cells observed for the irradiated cells in which the QDs with eventhe concentration as low as 50 μg/mL are internalized. As
evidenced in Fig. 3(f), the HeLa cells that take up the QDs
detached from the glass substrate. A few cells remaining
on the glass surface have also changed their morphology
to round and have lost their adhesive ability. This result
indicates that QDs can be effectively used for photothermal
therapy. Controlling the nonradiative channels allows
surgeons and medical/healthcare professions to manipulate
the photothermal heating in a relatively lower temperature
range, contributing to the development of therapeutic thermal phononics.

3. Group III–V QDsGroup III–V semiconductors are compounds formed
between group III and group V of the periodic table and176

contain InN, GaN, GaAs, InP, InAs and InSb. Group
III–V semiconductor devices are used as high electron
mobility transistors due to their high electron mobility andelectron density, which are essential for high-speed device
operation. For example, InN is a semiconductor with anarrow band gap of about 0.7 eV at room temperature.
When combined with gallium to make the ternary alloy
InGaN, the band gap is tuned in the range of 0.7–3.4 eV,covering the infrared to the UV. GaN offers a wide energy
bandgap and attract the development of nitride based blue
LEDs and ultraviolet laser diodes. Bulk crystal of InP has
a 1.35 eV of bandgap at room temperature, and its bulk
exciton Bohr radius and exciton binding energy are 11 nm
and 6 meV, respectively. Therefore, QD of InP crystal has
attracted attention for the spectral tunability of the emission
and the first exciton peak of absorption in a wide visible
wavelength range. This chapter describe recent strategies
for structural control of InP QDs to obtain high PLQY for
optoelectronic applications.

3.1 Improved PLQYs

To improve PL performance, it is necessary to eliminate

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Fig. 4

Types of Core/Shell energy structure.

surface defects, which work as nonradiative exciton
recombination centers (Coughlan et al., 2017). In a typical,
a semiconductor core/shell structure is employed to reduce
defects such as electron traps at the interface as depicted
in Fig. 4 (Reiss et al., 2009). A core QD (e.g., indium
phosphide, InP) is encapsulated by another semiconductor
shell whose band gap (Eg) is large enough for sandwichingthat of the core QD. Such a core/shell heterostructure isreferred to as a Type I band alignment. In this configuration, excitons are spatially confined to the core, minimizing
their interaction with surface states, and preventing theirdiffusion throughout the core/shell QD due to high energy
barrier of shell Eg (Tamang et al., 2016). This confinement
strength allows for enhanced radiative recombination rates
and high PLQY. A good example of Type I architecture is
InP/ZnSe/ZnS core/shell/shell structure. In the bulk state,
direct deposition of ZnS (lattice constant, aZnS = 0.541 nm)
on InP (aInP = 0.587 nm) results in a large lattice mismatch of 7.8 %, sufficient to cause defect formation at
the heterogeneous core/shell interface and relaxation of
the mismatch by lattice distortion is no longer thermodynamically feasible because of lattice mismatch relaxation.
There are three approaches to solve this problem. First is
to use the ZnSe (aZnSe = 0.566 nm) as the shell material;
the lattice mismatch (≤3.5 %) between ZnSe and InP is
reduced compared to the case of ZnS, so an energy-graded
core/shell interface is constructed, and defect generation issuppressed. As a result, PLQY is significantly enhanced up
to unity (Wu et al., 2020). Second, multielement alloying
for the core (e.g., InxZnyP) is effective to mediate the latticestrain at the heterogeneous core/shell interface, leading to
the improved PLQY. As expected, the strategy of alloying
core has inspired the concept of compositional alloying
between Se and S or Mg and Zn for shell materials (Limet al., 2013). In the InP/GaP/ZnS core/shell/shell system,
the strain energy that would have been induced at the interface between the inner shell and core is relaxed by cationexchange. During high-temperature synthesis for shellformation, Ga3+ ions diffuse into the core and are alloyedby replacing In3+ ions. This alloying QD achieves a PLQY
as high as 80 % by continuously changing the chemicalcomposition and reducing the strain energy at the interface(Srivastava et al., 2018). Third, Nemoto et al. reported aninteresting idea of using material strain to build a QD with a

Review Paper

Fig. 5 Illustration and HAADF-STEM image of InP/ZnS QDs ofcoherent core/shell structure with a single lattice parameter of 0.56 nm.
Adapted with permission from Ref. (Nemoto et al., 2022). Copyright2022, Royal Society of Chemistry.

coherent core/shell structure (Nemoto et al., 2022). Specifically, the core/shell QD allowed both epitaxial growth of
the shell layer and distortion of the spherical core, changing
the intrinsic interatomic distance of each crystal for coherency, where the core crystal is elastically compressed, andthe shell crystal is stretched by lattice distortion to a lattice
constant similar with that of the core. As depicted in Fig. 5,
the coherent growth of ZnS on a InP core QD yields InP/ZnS core/shell structure with a single lattice constant of
0.563 nm. Compared to the bulk crystal of zinc-blend (cubic) InP, the lattice of the core QD is compressed by 4.1 %.
In contrast, the lattice of the shell expands by 4.1 % relative
to the bulky ZnS crystal throughout the core/shell QD. This
could be because the change in lattice constant due to stressis thermodynamically more favorable than the formationof defect trapping sites at the core/shell interface to releasestrain energy. As expected, PLQY is enhanced up to 70 %and PL spectrum is also as narrow as 36 nm.

3.2 Optoelectronic devices

The first QLED that exceeded the benchmark was reported in winter of 2014 and used CdSe as the active layerof the device, emitting red light (Dai et al., 2014). The
second QLED exhibited the green EL with 20 % EQE andhad CsPbBr3/MABr quasi-core/shell structure as the active
layer (Lin et al., 2018). InP-based QDs as optically active
layers are the most promising alternative Cd- and Pb-free
QLEDs. Since the innovative work of the first InP-QLED
in 2011, intensive efforts have produced visible light emitting QLEDs. Finally, in 2019, Jang et al. reported blue,
green, and red emitting QLEDs with 20 % of EQE using
InP/ZnSe/ZnS QDs as active layers (Won et al., 2019).
Photoelectrode is also one of the optoelectronic applications. It is expected to realize the wavelength tunability ofdetected light by taking advantage of size dependence ofthe first exciton peak in the optical absorption spectrum,
but not yet reported before due to possible difficulty indissociation of photogenerated carriers into electrons andholes. Do-Hyun Kwak et al. reported a phototransistor witha light absorbing layer of InP QD in the hybrid structure of
InP QD/black phosphorus. They replaced the oleylamine

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on the QD surface with 1,2-ethanedithiol (EDT), which
has a shorter alkyl length, by ligand exchange to improvethe carrier mobility and enhance device characteristics(Kwak et al., 2019). Under violet-light illumination, thephotogenerated electrons from the InP QD layers arequickly transferred to the BP transport layer, resulting in ahigh photo response of 109 A/W and a high detectivity of
4.5 × 1016 Jones.

4. Lead-free perovskite nanocrystals

Lead halide perovskites are increasingly gaining attention for advanced optoelectronic applications due totheir excellent optoelectronic properties, but there are
two serious concerns for commercial use. First, metallic
lead ions are readily soluble in water to cause ecologicalthreats. Second, the current lead perovskite nanocrystals
have been found to have low stability to UV irradiation,
moisture, and heat. These disadvantages have stimulated
the development of research into the colloidal synthesisof lead-free perovskite nanocrystals. This chapter presents
recent progress in synthesis methods to enhance the PLQY
of lead-free perovskite nanocrystals.

4.1 Cesium tin halide perovskite nanocrystals

Inorganic metal halide perovskites have a generalchemical formula ABX3, where A is a monovalent cation(e.g., cesium, Ce+), B is a divalent metal ion (typicallytin, germanium or copper), and X is a halide anion (Cl–,
Br–, or I–). Tin halide perovskite exhibits a band gap of
1.2–1.4 eV, high carrier mobilities, and long carrier diffusion lengths (Pitaro et al., 2022). Therefore, nanocrystalsof cesium tin halide perovskite structure (i.e., Cs2SnX3)
are expected to be a representative of Pb-free perovskite
nanocrystals. Owing to a large absorption coefficient and awide bandgap-tunability between 1.3 and 2.8 eV, Cs2SnX3
has a potential of their use in various applications such as
photovoltaics, photoelectrode and light-emitting devices(Chen J. K., 2021c). So far, the synthetic development of its
nanocrystals lags far behind lead perovskite nanocrystals.
As a result, their optical properties are also inferior to thoseof Pb-based materials; Jellicoe et al. reported that a higher
density of trapped states results in lower PLQY (Jellicoe
T. C. et al., 2016); Wong et al. pointed that Sn vacancies
as the trap states work to lower PLQY (Wong et al., 2018).
How to reduce defect density to obtain high PLQY? To
answer this conundrum, Sun et al. proposed that precisely
adjusting the ratios of starting precursors corresponding to
Cs, Sn, and X over a wide range is crucial for suppressing
the formation of lattice defects in CsSnX3 nanocrystals
because the defect formation energy is closely related tothe chemical potential of each component. Wang et al.
tuned the molar ratios of precursors in a wide range to suppress the formation of defects contributing to nonradiative
charge-carrier recombination and found that addition of the

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small amount of zwitterionic phosphatidylcholine as a surfactant in the reaction is crucial for controlling the growthrate of the nanocrystals (Wang et al., 2021). According totheir experimental study, the growth rate of CsSnX3 nanocrystals was controlled by the formation of an intermediate
complex between the zwitterionic phosphatidylcholine
and the precursor and the steric hindrance effect of the
branched fatty acid side chain of phosphatidylcholine. The
resultant nanocrystal exhibited PLQY of 12.5 %.
Great attention has more recently been paid to the colloidal synthesis guiled by theoretical study using machinelearning. In this context, Sun et al. investigated the formation energy and the thermodynamical charge-transitionlevels of typical point defects, as well as the influenceof Sn vacancy on the photophysical properties based on
density functional theory (DFT) calculations and molecular
dynamics (MD) simulations (Liu et al., 2021b). Specifically, the stability regions of different compounds vs. I
and Sn chemical potentials of CsSnI3 was calculated at theoutset (see Fig. 6(a)). The result implied that the defectformation energy of CsSnI3 is sensitive to the chemical
potentials of reactants. Then, the defect formation energies
and defect charge-transition levels were calculated to show
Fig. 6(b) and 6(c). Clearly, tin vacancies (VSn) introduced
deep trap levels. Guided by this calculation result, CsSnI3
nanocrystals were colloidally synthesized through thecareful tuning of the ratio of precursors. It was found thatsuppressing the Sn vacancy strengthens the increase ofradiative recombination rate for enhanced PLQY alongwith a narrow emission spectral peak. By intentionallyadopting a Sn-rich reaction condition, along with judicious
choice of precursors with suitable reactivity, the resulting
narrow-band-emissive CsSnI3 nanocrystals exhibit a record
PLQY of 18.4 %. The development of synthetic methods,
guided by computational science, will facilitate the studyof tin-based perovskite nanocrystals and accelerate theirapplication in high-performance optoelectronic devices
such as solar cells, light-emitting diodes, and lasers.

4.2 Double perovskite nanocrystals

Inorganic double perovskite crystals have a chemical formula, A2B(I)B(III)X6 where A is a monovalent cation (typically Ce+), B(I) is a monovalent cation (Ag+ or Na+), B(III)is trivalent cation (In3+, Bi3+ or Sb3+). Since the first reporton the synthesis of lead-free double perovskite crystals, various chemical compositions of compound A2B(I)B(III)X6
have been investigated to tune the bandgap. For example,
when Cl– (2.77 eV), Br– (1.95 eV), and I– (1.75 eV) areintroduced as X sites in the Cs2AgBiX6 structure, the band
gap decreases in that order (Creutz et al., 2018). Among
the reported lead-free double perovskite structures, a bulk
crystal of white-light emitting Cs2AgxNa1–xInyBi1–yCl6
double perovskite structure was reported to show a high
PLQY of 86 ± 5 % (Luo et al., 2018). After that, the PLQY

Naoto Shirahata / KONA Powder and Particle Journal No. 41 (2024) 172–182

of Bi3+-doped Cs2Ag0.4Na0.6InCl6 microcrystals of about
10 μm in diameter was improved up to 97.33 % (Peng K.
et al., 2022). From now on, the double perovskite crystals
become a potential material for single-emitter layer appliedin optical applications (Zhang et al., 2022). However, a dramatic decrease in PLQY resulting from nanostructuration
happens due to the formation of defective structures.
Research to increase PLQY in nanocrystals has been
intensively pursued recently. Here, a typical method tofind the underlying mechanism of the enhancement of
the PLQY is demonstrated using Cs2Ag0.65Na0.35InCl6
nanocrystal as an example. HR-TEM image shown in
Fig. 7(a) demonstrates Cs2Ag0.65Na0.35InCl6 nanocrystals

Review Paper

having a square shape and an average diameter of about
10 nm. The estimated PLQY was 3 %. Improvementof PLQY was realized by impurity doping of Bi3+ ions.
Fig. 7(b) shows a typical HR-TEM image of the Bi-doped
nanocrystals. When the 1 % Bi3+ ion was doped, the
PLQY was enhanced up to 33.2 % (see Table 1). Fig. 7(c)shows PL spectra of the undoped sample measured in thetemperature range from 20 K to 300 K. As clearly seen,
the integrated PL intensity decreases with the increasing
temperature monotonously possibly due to the thermal
activation of the nonradiative channels present in the
nanocrystals. Doping of 1 % Bi3+ ions suppressed thedecreasing rate in PL intensity with temperature compared

Fig. 6 (a) Stability regions of different compounds vs I and Sn chemical potentials of CsSnI3. The shaded region indicates the equilibrium chemical
potential region. The representative points (A: Sn-poor/I-rich, B: moderate, C: Sn-rich/I-poor conditions) are chosen for the defect formation energy
calculations. (b) Calculated defect formation energies for 2 × 2 × 2 orthorhombic CsSnI3 supercell at Sn-poor/I-rich, moderate, and Sn-rich/I-poor
conditions, corresponding to the A, B, and C point in (a), respectively. (c) Defect charge-transition levels of CsSnI3. Adapted with permission from Ref.
(Liu Q. et al., 2021b). Copyright 2021, American Chemical Society.

Fig. 7 TEM images and temperature dependent PL spectra of Cs2Ag0.65Na0.35InCl6 nanocrystals (a, c) without and (b, d) with 1 % Bi3+ ions. Adaptedwith permission from Ref. (Huang et al., 2022). Copyright 2022, Royal Society of Chemistry.

179Naoto Shirahata / KONA Powder and Particle Journal No. 41 (2024) 172–182
Table 1 PL decay parameters * of the Cs2Ag0.65Na0.35InCl6 doped withand without Bi3+ ions.

Sample

PLQY (%) kr a(μs–1)

knr b(μs–1)

R2

Undoped

3.0

0.13

4.20

0.99

Doped

33.2

0.14

0.28

0.97

a k is the radiative recombination rate calculated by the formula:rkr = PLQY/τ(avg). τ(avg) means the value of average PL lifetime.
knr is the nonradiative recombination rate calculated by theformula: knr = 1/τ(avg) – kr.
* Other parameters for calculation are reported in the publishedpaper (Huang et al., 2022).
b

to undoped ones as shown in Fig. 7(d), suggesting thedecrease of nonradiative channels. As summarized inTable 1, the nonradiative recombination rate (knr) was
reduced by a factor of 15 in the doped sample compared tothe undoped sample. The possible defects as the nonradiative channel identified by investigating the temperaturedependence of PL spectral linewidth combined with chemical composition analysis were indium vacancies generated
in the crystal growth (Huang et al., 2022). The possible
mechanism of enhanced PLQY was that Bi3+ ions occupy
the indium vacancy to relax the strained lattice, yielding thedecreasing number of nonradiative channels. Thus, spectroscopic studies at low temperatures provide structural
insight into the cause of low PLQY.

5. Summary

Driven by the toxicity concerns of cadmium, lead, mercury, and selenium, research on the synthesis of colloidal
nanocrystals composed of nontoxic elements has madesignificant progress over the last decade. While previous
studies have already detailed the synthesis and excellent
properties of Cd- and Pb-based nanocrystals, non-toxic
nanocrystals, the subject of this review, have not yet been
fully described. The review was structured as follows:First, optical properties of SiQD were summarized. Dueto their low absorption coefficients, they do not work as
strong phosphors, but they find an application in the deicefield of current driven LED because carriers are given fromelectrode and does not need absorbing the light. Second,
InP was topicalized among III–V semiconductors. Metallic
In is a small amount of constituent element on earth but
does not exclude it from the discussion here. The pioneering work on red−green−blue (RGB) LEDs with beyond
20 % EQE spurs a lot of interests. Next target is narrowingof their EL spectra (<30 nm). For Pb-free perovskite nanocrystals, recent advances in colloidal chemical method
were presented here. The journey to find experimental parameters to reduce defects has just begun. Much research isstill needed to advance the field of non-toxic semiconductor
nanocrystals. Although technological advances in synthesis
methods, reports of prototype devices using light emission

180

Review Paper

from nanocrystals, and even induced emission and nanobio
applications have progressed over the past decade, it is safe
to say that nanophotonics is still in its infancy.

Acknowledgments

This work was supported by WPI program, JSPS
KAKENHI Grant-in-Aid for Scientific Research (B) Grant
Number 21H01910 and The Murata Science Foundation.

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Author’s Short Biography
Naoto Shirahata
Naoto Shirahata is Group Leader of Nanoparticle Group of National Institute for Materials Science (NIMS) in Japan, and Guest Professors of Hokkaido University and Chuo University. He earned
his Ph.D. in materials science from Kyoto Institute of Technology in 2001. He worked as postdoc
researchers at Nagoya University, AIST, and NIMS from 2001 to 2006. Since 2006 he has beenpermanent researcher at NIMS. He was holder of JST-PRESTO in 2009–14. He was visiting scientist
at University of Toronto in 2015–16. He has received awards for his research from the Division of
Colloid and Surface Chemistry, the Chemical Society of Japan, the Ceramic Society of Japan, andJapan Society of Powder and Powder Metallurgy.

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