Excitation-wavelength-dependent photoluminescence of silicon nanoparticles enabled by adjustment of surface ligands

Article abstract of DOI:10.1039/c8cc00047f

We herein present pioneering studies to reveal that excitation-wavelength-dependent photoluminescence properties of fluorescent silicon nanoparticles (SiNPs) can be realized by rationally designing surface ligands, i.e., several kinds of oxidized indole d

Full text of DOI:10.1039/c8cc00047f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
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Excitation-Wavelength-Dependent Photoluminescence of Silicon
Nanoparticles Enabled by Adjustment of Surface Ligands
Received 00th January 20xx,
Accepted 00th January 20xx
¶
,§
‡,§
‡
‡
,¶
,‡
Xiao-Bin Shen, Bin Song, Bei Fang, Ai-Rui, Jiang, Shun-Jun Ji,* and Yao He*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We herein present pioneering studies to reveal that excitation-
wavelength-dependent photoluminescence property
Herein, we reveal that distinct excitation-wavelength-
of dependent photoluminescence (PL) of SiNPs could be readily
fluorescent silicon nanoparticles (SiNPs) can be realized by realized via introducing new kinds of surface ligands, i.e.,
rationally designing surface ligand, i.e., several kinds of oxidized oxidized indole derivatives. Taken together with the
indole derivatives. The resultant ligands-decorated SiNPs exhibit measurement of photoluminescent energy and time-
strong fluorescence, with significant excitation-wavelength- correlated single photon counting (TCSPC), we deduce that
dependent emissive shifting from ~420 nm to ~550 nm. Taking photoexcited charge transfer process in different gap states is
advantages of their unique optical merits, we further exploit the responsible for the observed excitation-wavelength-dependent
resultant ligands-decorated SiNPs as novel fluorescent labels for PL. Taking advantage of the unique optical features, we further
anti-counterfeiting and cell imaging.
employ the resultant SiNPs for anti-counterfeiting and cell
imaging applications.
As the most important zero-dimensional silicon
nanomaterial, silicon nanoparticles (SiNPs) have received
intensive attention and shown high promise for myriad optical
applications due to their attractive optical properties and
In our experiment, the design of the surface ligands is
based on the following considerations. (1) The indole is the
core structure in many optoelectronic molecules and natural
products, which shows excellent photoelectric activity and
1
,2
5
negligible toxicity. Over the past two decades, extensive
efforts have been made for synthesizing high-performance
3
fluorescent SiNPs. It is worthwhile to mention that, recent
favourable biocompability. (2) The indole possesses a
nitrogen-containing aromatic electron-rich molecular system,
providing the feasibility for improving and modulating the PL
4
a
studies have revealed that surface ligands attached to SiNPs
play significant roles in optical properties of SiNPs. In particular,
the ligands containing nitrogen atoms (e.g., N-H group) have a
properties of SiNPs. It is worth noting that surface
modification with indole molecule produces little influence on
the maximum emission wavelength of SiNPs (Fig. S1). The
reason is that compared to N-H group, the C3 site possesses
more reactive position of indole towards electrophilic
substitution, thus hindering the introduction of nitrogen
4
large impact on the fluorescent properties of SiNPs. To date,
highly luminescent SiNPs have been successfully achieved
through modifying surface of SiNPs with proper ligands (e.g.,
diphenylamine, carbazole, 1,2,3,4-tetrahydrocarbazol-4-one,
5
a
defect. Therefore, with an aim to efficiently introduce
nitrogen defect, the indole molecule is passivated through the
4
and Cy dye series ). Despite above-mentioned tremendous
achievement, there still exists challenge to rationally control
emission spectra of SiNPs through the adjustment of their
surface ligands.
2
I /TBHP mediated oxidation reaction, producing four kinds of
5
b,c
oxidized indole derivatives. These resultant oxidized indole
molecules are then used for modifying the SiNPs surface.
The chemical structures of the as-synthesized oxidized
indole molecules are listed in Scheme 1b, namely isatin, 4-
methylisatin, (E)-N-(p-tolyl)-3-(p-tolylimino)-3H-indol-2-amine,
and (E)-6-methyl-N-(p-tolyl)-3-(p-tolylimino)-3H-indol-2-amine,
which are respectively shorted for IST, MIST, TTIA, and MTTIA,
¶Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
China
‡
Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices,
Institute of Functional Nano & Soft Materials (FUNSOM), and Collaborative
Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), Soochow
University, Suzhou, Jiangsu 215123, China
These authors contribute equally.
E-mail: yaohe@suda.edu.cn; shunjun@suda.edu.cn
(
see corresponding NMR characterizations in Fig. S2-S4). The
ligand-decorated SiNPs are formed through a two-step process
§
(Scheme 1a). Non-ligand decorated SiNPs (i.e., NH -SiNPs) are
2
†
Electronic Supplementary Information (ESI) available: Materials and devices,
methods, additional data (Fig. S1-S15), and corresponding discussions. See
DOI:10.1039/x0xx00000x
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nm (Fig. 1a-insert). A high-resolution TEM image (HRTEM) (Fig.
1
b) shows clear lattice spacing of ~0.31 nm for a single SiNP,
4
a,d
corresponding to the (111) facet of Si.
Moreover, as shown
in the selected-area electron diffraction pattern (Fig. 1b-insert),
the set of spots with a lattice spacing of ~0.31 nm are ascribed
to the (111) reflection of silicon, demonstrating good
crystallinity of the resultant SiNPs. The normalized UV-PL
spectra (Fig. S6a) show a resolved absorption peak at ~310 nm
and a PL peak at ~430 nm (excitation wavelength: 370 nm).
From the UV-vis absorption spectral analysis (Fig. S6b), we
deduce that the ~310 nm absorption in IST-SiNPs is produced
by the ligand. This result suggests the successful introduction
of ligand into SiNPs. Furthermore, the surface ligand of SiNPs is
characterized by Fourier Transform Infrared (FT-IR) spectra (Fig.
1
c), in which the characteristic stretching of the C=C bond (ca.
-1
~
1640 cm ) is observed for both the ligand and SiNPs. The
-1
strong and sharp absorbance peak at ~1030 cm , which is
4
a-c
Scheme 1. (a) Schematic illustration of synthesis of ligand- attributed to Si-O-Si , suggests the high oxidation state of
decorated SiNPs. Dashed line frame presents a proposed SiNPs. The EDX data confirms the existence of Si in the SiNPs
mechanism for excitation-wavelength-dependent fluorescent (Fig. S6c), and the oxidation state is further verified by X-ray
property of the SiNPs: the photon production originates from photoemission spectroscopy (XPS). As shown in Fig. 1d, the
exciton radiative recombination in different gap states under emissions at 99.4, 100.4, 101.4, 102.4 and 103.4 eV are
4
a,b
different excitation wavelengths, resulting in the excitation assigned to Si(0), Si(I), Si(II), Si(III) and Si(IV), respectively.
wavelength-dependent emission property of SiNPs. (b) The appearance of the Si(0) peak also confirms the existence
Chemical structures of the as-synthesized oxidized indole of a silicon core. Taken together, these results suggest the
molecules.
SiNPs have a crystalline Si/SiO
x y
N core/shell structure, whose
surface is covered by IST ligands. Significantly, the as-prepared
firstly formed under microwave irradiation via Ostwald SiNPs exhibit excitation-wavelength-dependent PL property
6
ripeningprocess. The resultant SiNPs are then modified by the (Fig. 1e). Typically, the emission peaks shift from ~415 to 550
4
a,b,c
above-mentioned ligands through self-assembly process.
nm with the increase of excitation wavelength from 350 to 520
Meanwhile, the surface of SiNPs is prone to be partially nm, respectively. In the short excitation wavelength window
oxidized to yield silica shell due to easy oxidization of silicon in (350-370nm), the emission intensity is enhanced with no
the ambient environment. Of particular significance, the as- obvious red-shift phenomenon, while gradually diminishes
prepared SiNPs exhibit excitation-wavelength-dependent PL
property, as shown in the frame in Scheme 1a. (I) When
excited at 365 nm, the resultant SiNPs emit strong blue light.
According to the energy gap law, the observed blue
luminescence could be attributed to the radiative
recombination of excitons in a charge transfer (CT) state at the
b,e
4
Si/SiO Ny interface.
x
(II) Cyan luminescence is observed
under 400 nm excitation. The red-shift phenomenon of PL
peak indicates the formation of a lower energy gap, which
might be localized in the interface of SiO N /ligand. This result
x
y
suggests the successful introduction of nitrogen defect. (III)
Green luminescence is visible under the excitation wavelength
of 450 nm. In this case, the blue PL is quenched due to the
mismatch between the excitation and emission wavelength
(excitation wavelength is larger than emission wavelength). As
shown in Fig. S5, the free ligands show a maximum emission
peak at ~530 nm through the microwave treatment; similarly,
the ligand-decorated SiNPs exhibit a ~530 nm maximum
emission peak when excited by ~450 nm. These results
indicate that the green PL of SiNPs is attributed by ligand
effect.
Transmission electron microscopy (TEM) image (Fig. 1a)
shows that the as-prepared IST-SiNPs are well-monodispersed
with a spherical structure and an average particle size of ~4.7
Fig. 1 (a) TEM image of the IST-SiNPs; the inset shows the TEM
size deviation (4.7 ± 0.6 nm) and DLS diameter (~5.1 nm) of the
2
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SiNPs. (b) HRTEM image of a single SiNP; the insert shows control experiment is performed, in which the silicon source
selected-area electron diffraction pattern. (c) FT-IR spectra of (i.e., APTMS) is absence during the microwave irradiation
free ligand (IST) and resultant SiNPs; (d) X-ray photoemission reaction. The resultant materials exhibit the maximum
spectra of Si 2p region; (e) Excitation-wavelength-dependent emission peak at ~530 nm, while show excitation-wavelength-
emission spectra of IST-SiNPs; (f) TCSPC spectrum of IST-SiNPs independent PL property (Fig. S5). Also, the products displays
(λ
ex = 370 nm;
λ
em = 430 nm, 25 °C).
feeble fluorescence, whose intensity is much weaker than that
of the ligand-decorated SiNPs. Besides, the excitation-
in the long excitation wavelength window. Previous study wavelength-dependent PL property could not be observed in
reveals that in the short excitation window, the σ* the absence of ligands (Fig. S12). Moreover, the PL lifetimes of
transition produced by the surface functional groups (e.g., - the resultant SiNPs excited at 450 nm are slightly shorter than
OH/NH ) leads to the strong blue emission of SiNPs. With the those excited at 370 nm (Fig. S13). This result is different from
excitation wavelength increases, the π* π transition the excitation-wavelength-independent NH -SiNPs, whose PL
→
n
2
→
2
7
contributed by the ligands is favored. Therefore, the surface lifetime dramatically decreases from 10.8 ns (370-nm
functional groups and ligands could act as the surface trap excitation) to 1.9 ns (450-nm excitation) (Fig. S14), indicating
states, which contribute to the observed excitation- that the resultant ligand-decorated SiNPs feature different
4
a
wavelength-dependent PL. The PL lifetime of the SiNPs is 11.3 pathways for the recombination of electrons and holes.
ns at 370 nm excitation, which is measured by time-correlated Taken together, we deduce that the excitation-wavelength-
single photon counting (TCSPC) technique (Fig. 1f). This dependent PL property is ascribed to the different CT states
nanosecond-scale emission further demonstrates that the PL is during ligand-actived process, thus producing the multiple
4
d,e
arisen from CT state,
recombination.
which allows fast radiative exciton energy levels in the band gap of SiNPs and generating
excitation-wavelength-dependent emission property (Scheme
1
).
Taking advantages of unique optical properties (e.g.,
excitation wavelength-dependent emission, as well as the
superior photostability against long-term high-power UV
irradiation (Fig. S15)), we further exploit the resultant SiNPs for
anti-counterfeiting applications. Assisted by inkjet printing
9
technique , silicon-based fluorescent ink is applied for pattern
printing (please see the detailed preparation of the ink and
instrument setup in ESI). Figure 3a displays the fluorescent
images of the butterfly-patterned paper printed by IST-SiNPs.
Blue, green, and yellow fluorescent images are observed,
when excited at 360, 455, and 523 nm, respectively. Quick
Response (QR) patterns (Fig. 3b (I)-(III)) are also readable
under different excitation wavelength. Furthermore, the SiNP-
and R6G-ink are simultaneously used to improve the anti-
Fig. 2 Excitation-wavelength-dependent PL spectra of (a) MIST-
SiNPs; (b) TTIA-SiNPs; (c) MTTIA-SiNPs. (d) TCSPC spectra of the
resultant SiNPs (
λex = 370 nm; λem = emission maximum
wavelength, 25 °C).
To test the generality of the ligand effect, other three kinds
of ligands are also employed for modifying SiNPs. The
characterization data for the resultant SiNPs are presented in
Fig. S7-9. Notably, all of the resultant SiNPs exhibit similar
excitation-wavelength-dependent PL property, as shown in Fig.
2a-c. The emission region ranges from ~420 nm to ~550 nm,
and the normalized PL spectra are displayed in Fig. S10.
Furthermore, the values of PL lifetimes are determined as 16.7,
8
.7, and 14.6 ns under 370 nm excitation (Fig. 2d). The
fluorescent quantum yields (QYs) are calculated as 12%, 20%,
%, and 16% for IST-, MIST-, TTIA, and MTTIA-SiNPs,
respectively (Fig. S11). Longer fluorescent lifetime is found to
8
be related to higher QY in these SiNPs, which suggests the Fig. 3 Fluorescent images of patterned commercially available
radiative recombination process can be promoted by the papers labeled with IST-SiNPs through inkjet printing. (a)
4
a,8
electron-richer ligands.
To interrogate the mechanism of Butterfly-patterned images captured with excitation at: (I) 360
the observed excitation-wavelength-dependent PL property, a nm, (II) 455 nm, and (III) 523 nm. (b) Quick Response (QR)-
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patterned images captured with excitation at: (I) 360 nm, (II) rationally designing high-quality SiNPs with unique optical
55 nm, and (III) 523 nm. (c) Rose- and bees-patterned images merits and understanding their luminescent mechanism,
4
composed from SiNP- and Rhodamine 6G (R6G)-solution facilitating the promotion of SiNPs-based widespread
captured with excitation at: (I) 455 nm, (II) 523 nm. Blue applications (e.g., laser, catalysis, solar cells, sensing and
emission window: 460-490 nm, exposure time: 800 ms; green bioimaging, etc.).
emission window: 490-520 nm, exposure time: 2000 ms; We express our thanks for the financial support provided by
yellow emission window: 540-590 nm, exposure time: 5000 ms. National Basic Research Program of China (973 Program
The thick white lines correspond to 1 cm.
2013CB934400), the National Natural Science Foundation of
China (21672157, 21542015, 21372174, 61361160412,
3
1400860, 21575096, and 21605109), the Ph.D. Programs
Foundation of the Ministry of Education of China
2013201130004), the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD),
11 Project, and the Collaborative Innovation Center of Suzhou
(
1
Nano Science and Technology (NANO–CIC).
Notes and references
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Fig. 4 Confocal images of HeLa cells distributed with the IST-
SiNPs. (a) Bright-field image. Cell images under excitation at (b)
405 nm, emission band pass: 430-470 nm; (c) 458 nm,
emission band pass: 470-510 nm; (d) 514 nm, emission band
pass: 530-570 nm.
3
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counterfeiting efficacy. In our case, rose and bees are coated
with IST-SiNPs and R6G, respectively. As shown in Fig. 3c, upon
excitation at 455 nm, only the rose printed by SiNP-ink appears
in the pattern, while bees printed by R6G-ink are invisible (Fig.
4
3
c (I)). When excited at 523 nm, yellow fluorescent signals
Kim, N. L. Rosi, Z. Z. Shao, R. C. Jin, ACS Nano, 2016, 10
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8
from both SiNPs and R6G are readily observed in the whole
pattern (Fig. 3c (II)). In addition to anti-counterfeiting
applications, these ligand-modified fluorescent SiNPs also
show potential feasibility for bioimaging applications. As proof-
of-concept demonstration, the fixed Hela cells distributed with
SiNPs exhibit strong and resolved fluorescent signals. As shown
in Fig. 4, obvious blue, green, and yellow fluorescent images
are observed upon excitation at 405, 458, and 514 nm,
respectively.
(
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Conclusions
9
In summary, we develop a kind of fluorescent SiNPs
featuring excitation-wavelength-dependent PL property, based
on introducing oxidized indole derivatives as surface ligands.
Taken together with the PL energy and TCSPC results, we
deduce the origin of the excitation-wavelength-dependent
fluorescent property is mainly attributed to the formation of
different CT states. We further exploit the resultant SiNPs as
novel fluorescent labels for anti-counterfeiting and bioimaging
applications. These results offer valuable information for
7
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9
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Table of Contents
Rational introduction of surface ligand leads to the
production of SiNPs with excitation-wavelength-dependent
fluorescence.
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