総説

先進セラミックス研究センター年報（2018）. Vol. 7, 19‑30

Synthesis of Photoluminescence Si Nanoparticles: Size Controlling and
Surface Functionalization
Yunzi Xin*, Takashi Shirai***

Advanced Ceramics Research Center, Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, JAPAN**Department of Life Science and Applied Chemistry, Graduate School of Engineering,
Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, JAPANSi nanoparticle (NP), as one of the most promising functional materials has attracted much attention in next-generation
optoelectronic device, photovoltaic and in-vivo/in-vitro bio-imaging technologies, based on its tunable band structure induced
optical properties. Due to the quantum confinement effect and large surface to core atom numbers, controlling of size andselective surface passivation during synthesis process, become the most important issue for designing of chemical and physicalproperties of Si NP. This paper reviews the synthesis method of Si NP with respect to size-controlling and surfacefunctionalization as well as its photoluminescence properties.
Keywords: Si, nanoparticle, photoluminescence, size controlling, surface functionalization

Si nanoparticles (NPs) with quantum confinement
effect induced unique electronic structure and opticalproperties [1-9] hold great promise in application of
next-generation optoelectronic device [10-17],
photovoltaic [19-24] and in-vivo and in-vitro bioimaging [25-29], also by taking advantages of its natureabundance, nontoxicity and biodegradability. Byutilizing crystalline Si NPs (Si nanocrystals, Si NCs) as
emissive material, F. Maier-Flaig et. al developed
multicolor light-emitting diodes (LEDs) which performs
orange to red electroluminescence (EL) with the highest
external quantum efficiency (EQE) of 1.1 % for red
emitter in 2013 [14]. The highest EQE for red region EL
achieved to 2.7 %, was further updated later by Pi andQinʼs groups by applying an inverted device structure[17]. In 2015, Xin et al. initially reported the white-blue
EL from a Si quantum dot (QD) hybrid LED with 77.8 %efficient emission from blue Si QDs, which enables thedevelopment of white emissive large-area solid-state
lighting technology for next-generation displays [16]. As
for photovoltaic applications, Kim et al. reported highly
efficient Si NC hetero-junction solar cells recently [18].
Meinardi et al. reported Si NC based highly efficientluminescent solar concentrators, and their flexible device
with comparable performance to at concentrators
provides new design freedom for building integrated
photovoltaic elements [22]. By applying water-soluble
Si NPs, Wang et al. reported the effective imaging ofliving cells with a fluorescence lifetime imaging
microscopy [24].
Since the first report on room-temperature
photoluminescence (PL) from nano-Si [29], theoretical[8,30-35] and experimental [8,9,30,36-88] studies onphotoluminescent Si NPs have been numerously
conducted. As for theoretical researches, theinvestigation of absorption and emission mechanism has
been discussed as a function of particle size, based on
the simulation of molecular orbital of Si NCs [8,30-35].
In the case of experimental researches, Si NPs have beensynthesized by different methods, and their sizes,
chemical structures and compositions, as well as opticalproperties have been extensively studied [8,9,39,36-88].
The PL mechanism of Si NPs has been mainly
summarized as the following two origins: (i) intrinsic
recombination of electrons and holes based on quantum
confined band structure in a core; (ii) extrinsic
recombination through an induced level on surface.
Therefore, the luminescence from Si NPs is not only
dependent of the size of NP core, but also strongly
affected by the chemical state on surface. Since thenumber of Si atoms on surface can reach nearly 50 % of
all atoms in a Si NP when its size is reduced to 2 nm,
surface chemical state may significantly affect theelectronic state of ultra-small Si NP. Thus, sizecontrolling and surface functionalization enable thematerial designing through the electronic and optical― 19 ―

Synthesis of Photoluminescence Si Nanoparticles

properties of Si NPs. It is also worth noting that aneffective surface functionalization can also optimize
chemical properties of Si NPs, such as solubility inhydrophilic and hydrophobic solvents, which might beimportant factor towards further applications of Si NPs.
Controlling of Si NP size has been achieved by adjusting
the experimental conditions during either synthesis oradditional size-reducing via chemical etching byhydrofluoric acid. Surface functionalization of various
inorganic and organic groups have been induced in bothsynthesis stage and further chemical reactions onhydrogen-terminated Si NPs.

[68-72]. For top-down process, laser ablation of Si wafer[73-82], electro-chemical etching of Si wafer [83-85]
and mechanochemical treatment of bulk Si [86-88] areutilized. The present paper reviewed the above synthesis
approaches of Si NPs in the point view of sizecontrolling and surface functionalization as well as its
optical properties.

Figure 1 . Summary of bottom-up and top-down synthesis

methods of Si NPs.

The synthesis process of Si NPs can be categorized asbottom-up and top-down approaches, as summarized in
Figure 1. Bottom-up process consists of plasma
decomposition of silane (SiH 4) [37-41], chemical
reduction of tetrachloride silane (SiCl4) [42-55], thermal
pyrolysis of hydrogen silsesquioxane (HSQ) and(HSiO 1.5 ) n (n>8) polymer [56-62], thermal andmechanochemical disproportionation of siliconmonoxide (SiO) [63-67], microwave-assisted reductionof (3-aminopropyl) trimethoxysilane (APTES) precursor

Figure 2. (a) Plasma reactor developed by Kortshagen groupand (b) emission of Si NCs synthesized by plasma
decomposition (Reprinted with permission from Nano Lett.
2005, 5, 655. Copyright © 2005 American Chemical Society.)(c) QY as a function of Si NCs size and emission wavelength(Reprinted from Appl. Phys. Lett. 88, 233116 (2006),
Copyright © 2006 American Institute of Physics).

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Yunzi Xin, Takashi Shirai

Plasma decomposition of SiH4. U. Kortshagenʼs group
initially reported the preparation of Si NPs via plasma
decomposition of SiH4 [37]. By inducing the flow of
SiH4 and argon (Ar) mixture into a non-thermal lowpressure plasma reactor (shown as Figure 2a), different
sized Si NCs can be obtained by changing the partialpressure of SiH4 or the residence time. The average sizeof primary Si NPs is tuned in 3-6 nm, which emitorange-red PL when excited by a UV lamp at 365 nm(see Figure 2b). Such PL is originated from the intersurface state formed native Si oxide during particle
synthesis. By further optimization of experimental
condition, Si NCs with size near 4 nm was synthesized,
which performs PL at 789 nm with highest quantum
yield of 62%±11%, as given by Figure 2c. The quantum
yield is dramatically decreased when the size of Si NCs
is decreased as the PL peak blue-shifted, however, thereason of this phenomenon is not clearly clarified [38].
Chemical reduction of SiCl4. The synthesis of Si NCs
via chemical reduction of SiCl4 has been reported byutilizing different reductant chemicals, including zintl
salt [42-46], LiAlH4 [47-51] or sodium naphthalide insolution [52-55]. For Zintl salt as reductant, Yang et al.
reported the synthesis of crystalline Si NPs under room
temperature [42]. The size of Si NPs is 3~4 nm when thelength of alkyl termination in Zintl salt is varied, while
the PL spectra show similar peak wavelength around360 nm. As another synthetic routine, Si NPs was
prepared via the chemical reduction of SiCl 4 with
LiAlH 4 [47-51]. Hydrogen-terminated Si NPs werefirstly observed, whose surface can be further selectivelyfunctionalized by alkene [47] or allylamine [48,51] byusing Pt catalyst in solution. Despites the above
heterogeneous reactions, researchers of Kauzlarichgroup demonstrated a homogenous reaction between
SiCl 4 and sodium naphthalenide as a new synthesis
routine for high crystalline Si NPs [52]. The synthesized
Si NPs perform PL in blue region (λ = 410~430 nm).
Interestingly, ultra-stable blue PL was obtained from asiloxane-coated Si NPs through multiple step surface
passivation, as indicated in Figure 3a [53]. Suchsynthesized Si NPs show air oxygen and moisture stableemission when stored in non-polar organic solvent up to
one year, as illustrated by Figure 3b. In addition, byusing carbazole as surface functionalization group,
Wang and coworkers reported the ultrabright PL from Si
NPs with a quantum yield of 70 % (see details in Figure
3c and 3d) [54,55].
Thermal pyrolysis of HSQ. In 2006, Hessel andcolleges firstly reported the thermal pyrolysis synthesis

Figure 3. (a) Multiple step surface passivation for siloxane-coated Si NPs and (b) PL and its stability of siloxane-coated Si NCs

synthesized by plasma decomposition (Reprinted with permission from Nano Lett. 2004, 4, 1181. Copyright © 2004 American
Chemical Society.). (c) synthesis and surface functionalization process and (d) PL and QY of carbozole-capped Si NPs. (Reprinted
with permission from J. Am. Chem. Soc. 2013, 135, 14924. Copyright: © 2013 American Chemical Society.).
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Synthesis of Photoluminescence Si Nanoparticles

of nanocrystalline Si-SiO2 composites and free-standing
Si NPs by utilizing HSQ as a novel molecular precursor[56]. Si NPs embedded SiO2 can be observed by onehour thermal pyrolysis of HSQ at temperature higher
than 500 °C under a mixed insert atmosphere of 4% H2
and 96% N2. The nanocrystalline Si NPs with size lessthan 5 nm can be obtained when the temperatureapproach to 1000 °C, and the size of Si NCs is increased
as the temperature increases to 1400 °C. With assistanceof chemical etching of SiO2 via HF, the free-standing Si
NCs with hydrogen terminated surface can be liberated
from matrix composite. The process is illustrated by
Figure 4a. The PL maximum peak is shifted in 500-700
nm by changing the period of chemical etching.
H o w e v e r, t h e h y d r o g e n p a s s i v a t e d S i N C s i ssignificantly unstable and easily oxides due to the very
low disassociation bond energy of Si-H. Efficientsurface passivation of different functional groups areachieved through different chemical reactions. Surface
passivation of alkyl groups can be induced throughhydrosilylation reaction between hydrogen terminated Si
NCs and alkene/alkyne under thermal treatment or UV
light irradiation [58-61]. Furthermore, Dasog et al.
reported the preparation of Si NCs with PL over thevisible region by varying the surface functional groups
on fix-sized (3-4 nm) hydrogen terminated Si NCs [58].

The PL of synthesized Si NCs covers entire visible
wavelength region, and different Si NCs perform
excitation wavelength dependent or independent PL,
respectively (as given by Figure 4b). Instead of utilizing
commercial HSQ as precursor, Xin et al. reported the
preparation of photoluminescent Si NCs by thermal
pyrolysis of selectively synthesized cross-linked(HSiO 1.5 ) n polymers [62]. The size of Si NCs iscontrolled in 1-10 nm by thermal pyrolysis at 1100 °C
of chemical structure controlled (HSiO1.5)n polymers,
whose process avoids the requirement of higher
temperature process for size-tuning, as demonstrated by
Figure 4c [56,57]. Liberated hydrogen-terminated Si
NCs show size dependent PL peaks, ranging from 550
nm to 700 nm.
Thermal disproportionation of SiO. As a commerciallyavailable material, SiO is also utilized as a promisingprecursor for Si NC synthesis through thermal
disproportionation reactions [63-66]. Rybaltovskiy et al.
previously reported the synthesis of Si/SiOx core/shellnano-particles by thermal disproportionation of SiO,
whose process is displayed as Figure 5a [63]. The sizeof crystalline Si core increases from 4.7 nm to 11.1 nm
when the temperature is increased from 350 ºC to 1100 ºC.
The synthesized Si/SiOx core/shell nanoparticles and Si
NCs perform PL in the infrared wavelength region.

Figure 4. (a) Synthesis of Si NPs by thermal pyrolysis of HSQ and following liberation of Si NPs via chemical etching of HF, and (b)PL images of same sized Si NPs passivated by different functional groups (Reprinted from ACS Nano, 2014, 8(9), 9636-9648.
Copyright: © 2014 American Chemical Society.) (c) Size-controlled synthesis of Si NPs by utilizing structure selectively synthesizedpolymers reported by Xin (Reprinted from Chem. Lett. 2017, 46, 5, 699-702. Copyright: © 2017 The Chemical Society of Japan.).
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Yunzi Xin, Takashi Shirai

Figure 5. (a) Thermal disproportionation of SiO and preparation of Si/SiOx core/shell NPs (Reprinted from J. Mater. Sci. 2015, 50, 5,2247-2256. Copyright: © Springer Science+Business Media New York 2014). (b) Infrared and red emission of monodispersed Si NPs
fractions separated by size-selective process (Reprinted from Nanoscale, 2016, 8, 3678-3684. Copyright: © The Royal Society ofChemistry 2016). (c) Mechanochemical disproportionation of SiO for synthesis of visible Si NPs (Reprinted from RSC Adv., 2019, 9,8310. Copyright: © The Royal Society of Chemistry 2019.).

Then, Sun et al. demonstrated the synthesis of freestanding Si NCs on a large-scale, based on thermal
disproportionation of SiO at 850–1100 °C. They
mentioned that the most significant disproportionationtransformations happened in the temperature range
between 900 and 950 °C. Based on the original
polydisperse Si NCs product, monodisperse Si NCs
fractions are selectively separated via a size-selective
precipitation. Such liberated Si NCs show corresponding
size of 3.5-4 nm with PL wavelength in infrared region
around 820-830 nm (Figure 5b) [64].
Mechanochemical disproportionation of SiO. In 2019,
Xin et al. reported the synthesis of visible
photoluminescent Si NCs via a novel and facile room
temperature mechanochemical disproportionation of
SiO, whose process is shown in Figure 5c [67]. Comparewith the requirement of high temperature (> 900 ºC) inthermal disproportionation process, this new strategy
enables room temperature disproportionation via
mechanochemical treatment. The size of Si NCs can bee a s i l y t u n e d i n 2 - 5 nm b y t h e d u r a t i o n o fmechanochemical treatment, while critical higher
temperature (~1400 ºC) is required for size-controllingin the case of thermal disproportionation. In this work,
free-standing Si NCs are liberated from the obtained Siembedded SiO2 matrix through chemical etching by HF
under dark condition. The Si NCs show bright blue PL( λ maximum= 450 nm) after efficient surface
passivation of alkyl-group with 1-decene. According to
the excitation-wavelength dependent PL, it indicates that
the PL of synthesized Si NCs origins from the electronhole recombination emission on wide bandgap of corestate induced from quantum confinement effect, while Si
NCs obtained from thermal disproportionation only
show PL in infrared region.
Microwave-assisted reductant of APTES. Zhong
presents aqueous synthetic approach via in situ growthof Si NPs under microwave irradiation at 160 ºC byutilizing APTES as Si source and trisodium citrate
dihydrate as reductant, see details in Figure 6a [68]. This
strategy enables large-scale (0.1 g) synthesis in a
relatively short time (10 mins). The obtained Si NPs― 23 ―

Synthesis of Photoluminescence Si Nanoparticles

Figure 6. (a) Reaction for microwave-assisted reduction of APTES by trisodium citrate dihydrate and (b) the size distribution, surface

chemical groups and PL properties of synthesized Si NPs (Reprinted from J. Am. Chem. Soc. 2013, 135, 8350-8356. Copyright: ©
2013 American Chemical Society.). (c) Reactions for Si NPs synthesis via microwave-assisted reduction of different Si precursors and(d) emission from Si NPs when excited by UV light. (Reprinted from Adv. Mater. Interf. 2015, 2, 16, 1500360. Copyright: © 2015
WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.).

show average size of 2.2 nm and PL at 450 nm with PL
QY up to 20-25 %. As for the surface chemical state, it
demonstrates that the obtained Si NPs contains large
amount of amino group on surface. By changing the Si
source from APTES to N-[3-(trimethoxysilyl) propyl]
ethylenediamine (DAMO) and 3-[2-(2-aminoethylamino)
ethylamino] propyl-trimethoxysilane (AEEA), the size,
surface chemical state, PL properties were investigatedin advance [69]. As a result, Si NPs synthesized from
DAMO, which exhibits size of 3.4 ± 0.6 and super high
QY up to 82.4 % at PL wavelength of 450 nm (as shownin Figure 6b). By replacing the reductant chemical from
trisodium citrate dihydrate to glucose, Jo et al. reported
a relative low temperature microwave-assisted reaction
system [70]. Their prepared Si NPs show Si-Si rich or
Si-O rich chemical composition when the reaction
temperature changed from 70 ºC to 50 ºC. The observed
Si NPs show relatively large size in 5-8nm. Although the
PL peak position is quite near to that of Zhongʼs previous
work, the QY is relatively low, 26.2 % for PL at 446 nm.
Wu et al. demonstrates the synthesis of dual-color
photoluminescent Si NPs via microwave-assisted

biomimetic synthetic strategy by utilizing diatom [71].
Red- (3.8 ± 0.8 nm) and blue- (2.1 ± 1.0 nm) emitting
Si NPs can be prepared via reaction under 150 ºC for
different duration, that is 10 or 180 mins, respectively.
Although the QY (15-20 %) is not so high to comparewith the previous strategy, the PL show narrower
emission spectral width (~30 nm), which indicates ahigh purity of the PL color saturation. For above
strategy, synthesized Si NPs with PL in blue and red
region is numerously accomplished. In 2018, Bose et al.
prepared green-emitting Si NPs successfully from themicrowave-assisted reaction of rice husk in NaOH
solution [72]. The diameter of synthesized Si NP is up to
4.9 nm and show green luminescence with a QY of ~60 %.
Laser ablation. Laser ablation has attracted muchattention in NPs synthesis due to several advantages,
such as 1) facile process such as one-step and one-pot
synthesis, 2) NPs with the size of a few nm scale isachieved in short time. Laser ablation has been applied
for different target materials in different surroundings. Si
NPs have been synthesized via laser ablation in liquid(LAL), inert gas and supercritical fluid, among which― 24 ―

Yunzi Xin, Takashi Shirai

LAL attracts much more attention with respects to theenabled surface passivation along synthesis process [7382]. As mentioned above, the functional groups on
surface not only controls the optical properties of NPs
via inducing of surface state, but also affects thesolubility of NPs. Liu et al. reported Si NPs with zincblende structure is prepared by LAL in water, exhibit
size of 7-14 nm and violet-blue PL (λ = 300~440 nm)[73]. The surface of synthesized Si NPs is partiallycapped with SiO 2 , which makes the NPs can bedispersed well in water. On the other hand, Xin et al.
reported the synthesis of alkyl-capped visible
photoluminescent Si NPs with altered size distributions
and optical properties via LAL in several organic
solvents with different length of carbon chain [74]. It
demonstrates that the average size of Si NPs can betuned from 1.8 nm to 5.9 nm as the carbon atom ratio inorganic solvent is increased. In addition, the absolute
QY of Si NPs generated in different solvents show goodcorrelation between size and carbon atom. It worth
noting that the synthesized Si NPs show stable PL withenhanced QY during aging in LAL solvent. The above
results are summarized as Figure 7. The low yield of NP
product is still a challenge due to the limited beam sizeof laser, although laser ablation is a facile one-step
accumulated process with easy size-controlling.

Figure 7. Synthesis of Si NPs via LAL in different organic

solvents reported by Xin et al, with the size and QY are
correlated as function of carbon atom ratio of organic solvents(Reprinted from Chem. Phys. Lett. 674 (2017) 90-97.
Copyright: © 2017 Elsevier B.V. All rights reserved.).

Mechanochemical of bulk Si. Heintz reports blue PL
from Si NPs prepared through planetary ball-milling ofbulk Si in alkene/alkylene solvents [86]. The synthesized
Si NPs surface is simultaneously passivated by stablealkyl/alkenyl groups through mechanochemicaltreatment, during where the radicals on solvent andsurface of bulk Si is covalently linked, such mechanism
is described by schematic image given by Figure 8a. The

size of Si NPs prepared through 24 hours ball-milling
process exhibits size smaller than 4 nm. Synthesized Si
NPs show excitation wavelength dependent PL in visible
range with wavelength varied in 350-650 nm, as shownin Figure 8b. Dhara el al. demonstrates that the size of Si
NCs prepared via the above mechanochemical process
can be controlled in 7-20 nm when the ball-milling
duration settled from 10-40 hours [88], however, thewide size distribution requires further optimization ofexperimental condition.
In summary, the size controlling of Si NP can beaccomplished by altering the experimental conditions
during different process. In the case of plasma of SiH4,
the size was tuned in the range of 2-8 nm by SiH4 flowrate as well as partial pressure in the Ar mixture. As forthe reduction of SiCl4, the size can vary from 1 to 5 nm
by changing of reductants and molar ratio of precursor
regents. For thermal pyrolysis of HSQ and (HSiO1.5)n(n>8), 4-10 nm sized Si NCs were obtained by changing
the temperature of thermal process or the chemical
structure of precursor. The size of Si NP produced fromlaser ablation was 5-8 nm by changing the laser fluences
or solvent. As for electrochemical etching of Si wafer,the size was tuned in 2-30 nm by current density andetching time. As for mechanochemical of bulk Si, theNP size was reduced from 20 nm to 7 nm when the
milling time was increased from 10 hours to 40 hours.
The surface functionalization of Si NPs by different
chemical groups has been investigated through different
reaction approaches. Most of all, the passivation of alkyl
chains has been numerously studied, since a goodsolubility of Si NPs in nonpolar organic solvent isessential for device fabrication through solution process.
In addition, Dohnalova suggested that the alkyl
passivation can give rise to the radiative transition rateof Si NP by modifying the electron and hole
wavefunctions [30]. The alkyl passivation is mainly
conducted on surface-active (dangling-bonds harvested),hydrogen-terminated and halogen-terminated Si NPs. Si
NPs with surface-active dangling bonds, prepared bylaser ablation or ball milling, they can be directly
passivated by alkene or alkyne solvent during highenergy synthesis. For hydrogen-terminated Si NPs,
obtained from plasma decomposition or chemical
etching, the hydrosilylation has been numerously
reported using alkene/alkyne via either thermal-assistant(heating) or photon-assistant (UV laser irradiation). As
for halogen-terminated Si NPs, usually obtained fromchemical reduction of SiCl4 system, the passivation of― 25 ―

Synthesis of Photoluminescence Si Nanoparticles

Figure 8. (a) Synthesis mechanism of Si NPs via mechanochemical treatment of bulk Si in alkene solvents. (b) PL spectra and TEM

images of obtained Si NPs (Reprinted from Adv. Mater. 2007, 19, 22, 3984-3988. Copyright © 2007 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim.). (c) Average size of Si NPs as a function of milling time (Reprinted from Nanoscale Res. Lett. 2011, 6:320.
Copyright: © 2011 Dhara and Giri; licensee Springer.).

alkyl chain was induced by chemical reaction with RLi,
RMgX and RSiX3 (R=alkyl, X=halogen). The synthesis
processes of photoluminescent Si NPs summarized in
the present paper might provide good understanding on
the size-controlling approaches and surfacefunctionalization, and the related chemical and opticalproperties are essential for material design ofnanomaterials.

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