Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

W2125019185 · 수집 2026-10-04 18:12:21

T10321 Carbon and Quantum Dots Applications

CoT 라벨링

라벨링 전

PDF 파일

자동 다운로드한 PDF PDF 보기

기본 정보

DOI 제목 발행년도 저널 URL 페이지 열기 PDF URL PDF 열기 발행기관 피인용수 라이선스

추가 정보 초록

In this work, the fundamental mechanism of ultrabright fluorescence from surface-modified colloidal silicon quantum dots is investigated in depth using ultrafast spectroscopy. The underlying energy band structure corresponding to such highly efficient direct bandgap-like emissions in our surface-modified silicon quantum dots is unraveled by analyzing the transient optical spectrum, which demonstrates the significant effect of surface molecular engineering. It is observed that special surface modification, which creates novel surface states, is responsible for the different emission wavelengths and the significant improvement in the photoluminescence quantum yields. Following this essential understanding, surface-modified silicon quantum dots with deep blue to orange emission are successfully prepared without changing their sizes. Ultrafast spectroscopy shows that novel surface states give rise to ultrabright photoluminescence from surface-modified silicon quantum dots. Specifically, researchers from Jilin University and Fudan University in China used femtosecond transient absorption spectroscopy in combination with time-resolved fluorescence spectroscopy to investigate the underlying mechanism for the ultrabright, colour-tunable photoluminescence, which they had previously observed from nitrogen-containing, colloidal silicon quantum dots passivated by organic ligands. They discovered that the mechanism was related to exciton wave functions being modulated by surface molecular engineering. Based on this insight, they produced surface-modified silicon quantum dots that had identical sizes and yet whose emissions ranged in colour from deep blue to orange. This novel approach involves tailoring the surfaces of nanoparticles in contrast to the conventional approach of varying the nanoparticle size.

본문

www.nature.com/lsa

## ORIGINAL ARTICLE

# Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

Lei Wang 1,*, Qi Li 3,*, Hai-Yu Wang 1, Jing-Chun Huang 3, Ran Zhang 1, Qi-Dai Chen 1, Huai-Liang Xu 1, Wei Han 2,
Zheng-Zhong Shao 3 and Hong-Bo Sun 1,2

Keywords: quantum confinement; silicon quantum dots; surface molecular engineering; ultrafast spectroscopy; wave function modification

INTRODUCTION
Crystalline silicon has been the most important semiconductor material in the modern electronics industry due to its excellent electronic
properties. However, as a well-known indirect bandgap semiconductor, the optical properties of crystalline silicon are relatively
poor, which limits its applications in silicon photonics. To pursue the
desired optical performance in silicon materials, nanostructured silicon objects with enhanced photoluminescence (PL) have attracted
increasing interest. 1213 Most notably, due to the three-dimensional
quantum confinement effect in silicon nanocrystals (Si NCs), the
momentum conservation rule is relaxed, and the spatial distributions
of photogenerated exciton wave functions tend to extend to the surface of nanoparticles, which provides an efficient approach to manipulate the energy structure of silicon. 14218 Thus, the excitonic emission
from Si NCs are usually thought to follow three models: (i) ‘direct’
transition from a quantization-related bandgap; (ii) indirect approaches, i.e., with the help of other emission centers; and (iii) surface
and/or strain engineering. 19

For the ‘direct’ approach, the observed size-dependent PL behavior
in Si NCs can be well explained by the quantum confinement effect.
However, such Si NCs can hold strong PL only in the deep red
region with limited stability, and their PL lifetimes from these ‘direct’

transitions are usually on the microsecond time scale. 20222 For the
indirect approach, recently reported surface-modified Si NCs could also
provide highly effective emissive channels. 23 Furthermore, although the
possible modulation mechanisms of the carrier wave function in Si NCs
still remain controversial, some researchers have achieved the effect of
surface ligands on the low-energy electron states of Si NCs, which would
lead to phononless excitonic recombination. 24,25 Thus, the major drawback of Si NCs could be overcome by these effects solely or synergistically. Nevertheless, insight into the PL mechanism of Si NCs remains
elusive, 26231 in particular for ultrabright Si NCs. 23,32234 In addition, to
further tailor and improve the fluorescent properties of Si NCs, such as
their color-tunable fluorescence with high brightness, fundamentally
understanding the underlying mechanisms is also required.

Ultrafast spectroscopy is a powerful tool for investigating the electronic structure, excitonic properties and carrier dynamics for various
new types of fluorescent nanomaterials, for example, graphene
oxide, 35,36 graphene quantum dots (QDs), 37 carbon nanodots, 38,39

polymer nanoparticles, 40 direct bandgap semiconductor QDs 41243

and Si NCs. 44249 In our previous work, we produced novel Si NCs
with unprecedented ultrabright fluorescence and single-exponential
decay. 23 According to the time-resolved PL dynamics, we demonstrated that no fast PL decay component was present in the range of

1State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China; 2College of Physics, Jilin
University, Changchun 130012, China and 3 State Key Laboratory of Molecular Engineering of Polymers, Advanced Material Laboratory, Department of Macromolecular Science,
Fudan University, Shanghai 200433, China

*These authors contributed equally to this work

Correspondence: HY Wang,State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun
130012, China
E-mail: haiyu_wang@jlu.edu.cn
Or HB Sun, State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China
E-mail: hbsun@jlu.edu.cn
Received 16 August 2014; revised 26 November 2014; accepted 1 December 2014; accepted article preview online 4 December 2014

L Wang et al

sub-picosecond to hundreds of nanoseconds in these ultrabright Si
NCs. In addition, their steady-state optical properties, i.e., PL
quantum yield (QY), excitation- and temperature-dependent PL
behaviors, are closely related to the surface organic agents. Hence,
a synergic effect between attached organic ligands and surface
oxidized sites was suggested to explain these interesting phenomena;
however, a complete understanding of their PL mechanism remains
unknown.

Herein, in a quest to explore the fundamental photophysics in these
high-quality ultrabright Si NCs, we combined femtosecond transient
absorption (TA) spectroscopy with time-resolved fluorescence spectroscopy to systemically investigate the excited-state dynamics in a
series of nitrogen-containing organic ligand passivated Si NCs
(Figure 1a). After determining what would happen to these photogenerated excitons in the possible excited states, we further unraveled
the underlying photophysical model, which was related to the modulation of exciton wave functions due to the surface molecular engineering. We observed that the different emission wavelengths were
partly sensitive to their local environment (interaction), i.e., the
amount of N-linking phenyl rings, structure of capping agents or
linking mode of N atoms in the ligands anchored to the surface Si
atoms. Si NCs with deep blue to orange emission were then successfully prepared by tailoring their surface instead of changing their sizes.
In addition, a novel surface state in the visible range responding to
orange-red emission was first revealed using transient absorption
spectroscopy of one of the special surface-modified Si NCs.

CI-Si QDs

Te-Si QDs

Di-Si QDs

Ca-Si QDs

MATERIALS AND METHODS

Preparation of Si NCs
In a typical synthesis of unmodified Si NCs with surface Si–Cl bands
(Cl–Si NCs), 23 0.17 g of sodium and 0.75 g of naphthalene were
stirred in 20 mL of glyme for approximately 3 h to produce
sodium-naphthalide. Then, 0.20 mL of SiCl4 was added dropwise.
The surface treatment was performed by adding excess capping agents,
0.25-g 1,2,3,4-tetrahydrocarbazole (Te), 0.25-g diphenylamine (Di),
0.25-g carbazole (Ca), 0.25-g 4-methoxydiphenylamine (Me) and
0.25-mL aniline (An) to the as-synthesized Si NCs and stirring the
mixture at room temperature for several hours. The solvent and
naphthalene were removed under reduced pressure with heating in
an oil bath, leaving Si NCs with NaCl and organic byproducts. Finally,
water was added to dissolve the Si NCs. Excess organic byproducts and
inorganic salts can be removed from the Si NC products by a combination of ultrafiltration and dialysis. The sizes of the resulting Si NCs
were in the range of 223 nm, with an average value of only approximately 2.5 nm, both before and after surface modification.
Transmission electron microscopy images of these products are presented in Supplementary Fig. S1. Because their sizes are much smaller
than the exciton Bohr radius of Si crystals (approximately 4.2 nm), the
products were also so-called surface-modified Si QDs. A high-resolution transmission electron microscopy image of single organically
capped QD is also enlarged in each inset of Supplementary Fig. S1.
The diamond lattice of silicon can be easily observed, which is further
confirmed by the selected-area electron diffraction pattern, powder

1.0

0.5

0.0

CI-Si QDs

QY 8%

N
H

Te-Si QDs

QY 30%

N
H

Di-Si QDs

QY 50%

CI
CI

Ca-Si QDs

QY 75%

1.0

0.5

0.0

300 400 500 600 700 800

Wavelength (nm)

CI-Si QDs

Te-Si QDs

Di-Si QDs

Ca-Si QDs

400 500 600 700 800

Wavelength (nm)

Figure 1 (a) Surface-modified water-dispersible Si QDs with tunable PL. Steady-state absorption (b) and normalized emission spectra (c) for as-synthesized Clterminated and surface-modified Si QDs in water at 400-nm excitation. O.D., optical density; PL, photoluminescence; QD, quantum dot; QY, quantum yield.

X-ray diffraction pattern and X-ray photoelectron spectroscopy spectrum, as presented in Supplementary Fig. S2.

Femtosecond transient absorption experiments
In the transient absorption set-up, a mode-locked Ti:sapphire laser/
amplifier system (Solstice; Spectra-Physics, Santa Clara, CA, USA) was
used. The output of the amplifier of 1.5-mJ pulse energy, 100-fs pulse
width and 250-Hz repetition rate (this low frequency is set for matching the signal collection system) at 800-nm wavelength was split into
two parts. The stronger beam was used to generate the 400-nm pump
pulses (directly doubled from 800-nm laser pulses), and the other 800nm beam was used to generate the broadband white-light probe pulses
in the 4002800 nm range by focusing it into a 2-mm-thick water cell.
Both the white light and pump light are focused on the sample and
overlap with each other in space. The transient absorption data were
collected by a fiber-coupled spectrometer connected to a computer.
The group velocity dispersion of the transient spectra was compensated by a chirp program. All the samples were stored in a 2-mm
cuvette for the TA experiments at room temperature. Pump-power
dependent measurements were also performed. In the acceptable
range (1502350 nJ pulse 21 or 0.521.2 mJ cm 22 ), no pump intensity
dependent dynamics were observed.

Time-resolved fluorescence experiments
Subpicosecond time-resolved emission was measured using the femtosecond fluorescence upconversion method. A mode-locked Ti:sapphire
laser/amplifier system (Solstice, Spectra-Physics) was again used. The
output of the amplifier of 1.5-mJ pulse energy, 100-fs pulse width and
1-kHz repetition rate at 800-nm wavelength was split into two parts;
the stronger beam was used to generate excitation light. The resulting
fluorescence was collected and focused onto a 1-mm-thick BBO crystal with a cutting angle of 35 6 . The other part of the regenerative
amplifier output was sent into an optical delay line and served as
the optical gate for the upconversion of the fluorescence. The generated sum frequency light was then collimated and focused into the
entrance slit of a 300-mm monochromator. An ultraviolet-sensitive
photomultiplier tube 1P28 (Hamamatsu Photonics, Hamamatsu City,
Shizuoka Pref., Japan) was used to detect the signal. The full-width at
half-maximum of the instrument response function was approximately 400 fs.

L Wang et al

RESULTS AND DISCUSSION
The steady-state absorption and normalized PL spectra at 400-nm
excitation are shown in Figure 1b and 1c. For Cl-Si QDs, there is only
an absorption tail at approximately 400 nm. After surface modification,
the absorption value of the Te–Si QDs increases in this range, whereas
the Di–Si QDs and Ca–Si QDs also exhibit distinct absorption peaks.
Because no absorption occurs for these organic ligands in the wavelength range beyond 350 nm, this emerged absorption change in the
steady-state absorption spectra can be understood as enhanced interaction between ligands and the Si core to some extent. Excited at
360 nm (3.44 eV), the corresponding PL QY also gradually increases
in the sequence of Cl–Si QDs (approximately 5%), Te–Si QDs (approximately 30%), Di–Si QDs (approximately 50%) and Ca–Si QDs
(approximately 75%). The detailed PL QYs measurements are shown
in Supplementary Fig. S3. In addition, it is worth noting that additional
excitation-dependent PL behavior occurs for the Cl–Si QDs and Te–Si
QDs (Supplementary Fig. S4). According to these steady-state results, it
appears that the surface capping agents could lead to a different effect on
the organically capped Si QDs’ band structure, which would dramatically affect the optical properties of the surface-modified Si QDs.

To scrutinize the effect of the surface ligands on the surface-modified Si QDs’ band structure, the temporal resolution of the photogenerated carrier dynamics was investigated using femtosecond TA
spectroscopy at 400-nm excitation. In general, strong spectral overlapping occurs in the TA spectrum. Even in the relatively simplified TA
spectrum, which presented as one possible scenario in Figure 2a,
ground-state bleaching, excited-state absorption (ESA) and stimulated emission (SE) occur. Note that only spontaneous emission was
experimentally observed when the pump pulse was alone, whereas SE
was clearly induced when overlapping the white light probe pulse.
However, for the indirect bandgap Si NCs, the presence of a
ground-state bleaching signal in the visible range has rarely been
reported in previous TA experiments under 400-nm excitation. 44249

This phenomenon may occur because most reported Si NCs have large
Stokes shifts, and their photoluminescence excitation (PLE) peaks are
always below 400 nm. 16 As the PLE peaks of Cl–Si QDs, Te–Si QDs,
Di–Si QDs and Ca–Si QDs are also located below 400 nm (not shown
here), which is similar to previously reported results, it is reasonable to
ignore the ground-state bleaching signal in the 400–800 nm probe
window and safely assign the negative signals to stimulated emission

a b

–1

400

CI-Si QDs

Te-Si QDs

Di-Si QDs

Ca-Si QDs

500 600 700
Wavelength (nm)

800

GSB

Wavelength (nm)

ESA 1
ESA 2

Figure 2 (a) Schematic illustration of global analysis for simplified transient absorption data, ignoring the GSB (blue dot line) signal. The red dashed line represents the
TA data obtained experimentally, which can contribute to the overlapping among the two ESA signals and SE signal. (b) Typical transient absorption spectrum (at
1.5 ps) for four Si QDs with different surface modifications at 400-nm excitation. ESA, excited-state absorption; GSB, ground-state bleaching; O.D., optical density; QD,
quantum dot; SE, stimulated emission; TA, transient absorption.

L Wang et al

for the four samples in the following analysis. Thus, there are still two
signals, ESA and SE, overlapping with each other. Fortunately, if we
can obtain the PL lifetime (equal to the lifetime of the SE component
corresponding to the pure radiative recombination) using other
approaches, such as time-correlated single-photon counting or femtosecond time-resolved PL experiments, these signals (ESA and SE)
with the same lifetime components but different amplitudes at each
wavelength can be reasonably separated according to a global analysis.

In Figure 2b, the typical TA spectra (at 1.5 ps) for the as-synthesized
Cl-terminated Cl–Si QDs and surface-modified Si QDs with different
surface capping agents are displayed (presented as the entire transient
spectral evolution for each Si QD sample in Supplementary Fig. S5).
These apparent differences in the TA spectra imply that the underlying
excited-state processes in each surface-modified Si QD sample are
indeed largely affected by the surface ligands. The corresponding global analysis for the TA data of surface-modified Si QDs on the time
scale from 0.8 to 1330 ps is presented in Figure 3. As mentioned
before, one of the lifetime components is fixed and assigned to the
radiative recombination process according to the reported average PL
lifetimes, that is, 4, 15, 17 and 31 ns for Cl–Si QDs, Te–Si QDs, Di–Si
QDs and Ca–Si QDs, respectively. 23 These long-lifetime fluorescent
states containing stimulated emission (negative signal) accompanied
with the corresponding excited-state absorption (positive signal) are
well identified in the following global fitting results.

For Cl–Si QDs (Figure 3a), it is apparent that there are two lifetime
components related to emission (negative signals). One component is
the fixed lifetime component of 4 ns, contributing to blue fluorescence;

the other component is a very short-lifetime component of only 1.4 ps,
which is mainly attributed to green emission with very low PL QY due
to the fast decay. Thus, the remaining lifetime component of 21 ps can
be safely assigned to nonradiative recombination. Notably, this type of
the excited-state behavior is also suitable for Te–Si QDs (Figure 3b),
except that the long-lifetime PL component of 15 ns corresponds to
green emission, whereas the short-lifetime component of 1.0 ps corresponds to blue emission. Hence, these global fitting results are consistent with the previously observed excitation-dependent PL behaviors.
Such experimental results clearly demonstrate the intrinsic correlation
between TA spectra and PL data, which provides an exciting step
toward obtaining a comprehensive understanding of the photophysical
properties in our surface-modified colloidal Si QDs. Moreover, our
experiment results also directly demonstrate that although the surface ligands are completely different for Cl–Si QDs and Te–Si QDs,
their energy band structures are similar. The only difference is the
emission band, which can be effectively modulated by surface chemical
modification.

The question subsequently becomes how to explain the ultrabright
fluorescence from Di–Si QDs and Ca–Si QDs. Fortunately, the ultrafast spectroscopic analysis on the two samples can provide a crucial
clue in answering these questions. Figure 3c and 3d presents the global
analysis for the TA spectra of the Di–Si QDs and Ca–Si QDs, which
indicates that the spectral structures in the TA spectra for the latter
two types of surface-modified Si QDs mainly originated from the
radiative recombination states with lifetime components of 17 ns
and 31 ns, respectively. In addition, there are three remaining lifetime

a 0.8 b

0.3

0.2

0.1

0.0

Global fitting lifetime

1.0 ps 21 ps 15 ns

Te-Si QDs

0.6

0.4

0.2

0.0

–0.2

Global fitting lifetime

1.4 ps 21 ps 4 ns

CI-Si QDs

400 500 600 700 800
Wavelength (nm)

c 1.0 d

Global fitting lifetime

1.8 ps 34 ps 190 ps

400 500 600 700 800
Wavelength (nm)

0.4

Global fitting lifetime

0.3

0.2

0.1

0.0

1.2 ps 9 ps 100 ps

1 ns 31 ns

0.5

0.0

–0.5

17 ns

Di-Si QDs –0.1 Ca-Si QDs

400 500 600 700 800
Wavelength (nm)

Figure 3 Global analysis for the transient absorption data of (a) Cl–Si QDs, (b) Te–Si QDs, (c) Di–Si QDs and (d) Ca–Si QDs. O.D., optical density; QD, quantum dot.

Ex Em

components (1.8 ps, 34 ps and 190 ps) for Di–Si QDs and four lifetime components (1.2 ps, 9 ps, 100 ps and 1 ns) for Ca–Si QDs,
which lack an obvious spectral structure and thus are most likely
related to nonradiative excited-state processes.

Together with Te–Si QDs, the three types of surface-modified Si
QDs with cyan-green emission and single-exponential decay possess
almost the same size distributions and extremely similar surface
agents, as presented in Figure 1a. Compared with the surface ligands
in Ca–Si QDs, there is only a lack of a chemical bond linking the two
phenyl rings for Di–Si QDs and an increase of four hydrogen atoms in
one phenyl ring for Te–Si QDs. Therefore, it appears that surfacemodified Si QDs samples with higher PL QY exhibit longer and more
complex excited-state behaviors. The characterization dynamics
probed at the same wavelength (500 nm) extracted from femtosecond
time-resolved PL dynamics and TA spectra under 400-nm excitation
for Di–Si QDs and Ca–Si QDs are presented in Supplementary Fig. S6,
which clearly demonstrates that with the decay of TA signals, the PL
dynamics almost remain constant on the same time scale, which can be
indicated by a single-exponential PL lifetime. However, such a long
PL lifetime (17 ns for Di–Si QDs and 31 ns for Ca–Si QDs) leads to a
very low signal-to-noise ratio in the femtosecond time-resolved PL
experiments.

This finding indicates that the complex excited-state processes
observed in TA analysis are completely irrelevant to the pure exciton
radiative recombination obtained in time-resolved PL experiments. In
other words, these results unambiguously demonstrate that these
multi-exponential excited-state lifetime components arise from nonradiative recombination processes, i.e., intra-band hot-carrier relaxation. In general, higher temperature will lead to faster hot carrier
relaxation, which could shorten the PL lifetime and consequently
decrease the steady-state PL intensity, similar to the case of Di–Si
QDs. 23 However, due to the intrinsic slower hot-carrier relaxation
in Ca–Si QDs, this temperature effect could be greatly reduced.
Therefore, the temperature-dependent PL behavior for Ca–Si QDs is
not obvious compared with Di–Si QDs. These temperature-dependent steady-state PL spectra are presented in Supplementary Fig. S7. In
addition, it is worth noting that these nonradiative recombination
processes in Di–Si QDs and Ca–Si QDs are also different from those
in Cl–Si QDs and Te–Si QDs because these processes are irrelevant to
any spectral evolution. Hence, there are indeed two types of PL
mechanisms.

As stated above, due to the quantum confinement effect, larger
amplitudes of carrier wave functions in Si QDs are located at the
surface of nanoparticles. Thus, the effect of the surface ligands on
the Si QDs’ band structure (that is, the core–ligand interaction) must
be considered. We suggest that this wave function modification by
surface modification is common, whether for traditional O-, H- and
C-terminated Si NCs or, in our case, Cl-terminated Si QDs and Ncontaining organic ligands modified Si QDs; however, the effect on the
excited-state processes is different. The possible energy diagram for the
two types of surface-modified Si QDs at the lowest energy levels modified by surface ligands is presented in Figure 4.

For a moderate effect of the surface ligands, namely, the ‘weak’
interaction mode, this modification only creates some direct transition energy levels within the intrinsic indirect bandgap (Figure 4, left
panel). For this type of surface-modified Si QDs, there is a remarkable
sign, that is, a fast spectral evolution in transient absorption spectra
accompanied with no special absorption structure in steady-state
absorption spectra. However, for the cases where the excited-state
processes of surface-modified Si QDs can be strongly affected by the

L Wang et al

Ex Em

CI-/Te-Si QDs Di-/Ca-Si QDs

Direct transition energy levels

Indirect transition energy levels

Surface modified energy bands

Hot-carrier relaxation

Trap levels

Electron
Hole

Figure 4 Schematic illustration of the energy level arrangement for the two types
of surface-modified Si QDs. Em, emission; Ex, excitation; QD, quantum dot.

surface ligands, namely, the ‘strong’ interaction mode, the intrinsic
indirect bandgap of the Si QDs is almost filled with surface-modified
energy bands, which becomes an integrated direct transition energy
structure (Figure 4, middle panel). Therefore, there is a strong stimulated emission signal and no spectral evolution in transient absorption
spectra. In addition, special absorption structures in the band edge can
be easily observed in their steady-state absorption spectra. Recently,
Ku 6 sova´ et al. 50 reported the presence of tensile strain in organic passivated Si NCs. This effect could also exist in our studied surface-modified Si QDs, i.e., surface strain could also be one part of the so-called
core–ligand interaction.

After obtaining the energy band structure model of the surfacemodified Si QDs, the next issue is to further illustrate the nature of
such a dominant integrated band, which is the origin of ultrabright
fluorescence. Namely, is this phenomenon still a modified core-related
state or a novel surface state? To this end, we performed some additional experiments on the ultrabright surface-modified Si QDs that
were relevant to surface oxidation and the solvent effect. As previously
mentioned, 23 in addition to the crucial role of organic ligands, surface
oxidation is also necessary to achieve such ultrabright fluorescence in
Di–Si QDs and Ca–Si QDs. For example, we have investigated the
detailed evolution of Di–Si QDs’ fluorescence under various oxidation
degrees (Supplementary Fig. S8).

We observed that when Di–Si QDs were initially dissolved in water
with a low degree of oxidation, only a moderate blue photoluminescence was observed. After 100 h of storage, the ultrabright green fluorescence gradually occurred, and a higher degree of oxidation could be
observed in the Fourier transform infrared spectroscopy measurement. A controlled experiment was also conducted in which all the
oxygen during storage was removed. In this case, no ultrabright green
fluorescence was observed. In general, two types of fluorescence
changes can be observed when silicon nanoparticles are oxidized. If
the PL appears blueshifted, this PL is often regarded as core-emission
related because during the oxidation progresses, the size of the silicon
core will be reduced such that its PL band gap will increase based on
the quantum confinement effect. 51 However, some other researchers
hold a different viewpoint that such a change of steady-state PL in Si
QDs is caused by a blue emission surface center induced during the
oxidation process. 52 In our sample, a new red-shifted PL occurred
during oxidation. A possible explanation for this phenomenon is that
new surface states constitute during oxidation. 15,18,24

For the solvent effect of the fluorescence from our surface-modified
Si QDs, the steady-state PL in the surface-modified Si QDs exhibits a
more or less pronounced blueshift accompanied by the reduction of
the PL lifetime when the solvent is changed from water to acetone
(examples are presented in Supplementary Tables S1 and S2). These
results are consistent with recently reported data on the oxidation of

L Wang et al

N-containing Si NCs, where their photoluminescence was demonstrated to originate from the surface SiOxNy charge transfer state. 53

All these experimental results suggest a novel surface state as the dominant origin of the ultrabright fluorescence. Both organic ligands and
surface oxidation are necessary to form this state, and the structure of
ligands is crucial in determining whether such a state can dominate the
entire emission process. Herein, it should be noted that rigorous
theoretical modeling is needed to fully elucidate the effect of a particular surface capping on the Si QDs’ band structure, which will be
performed in the future.

Because the organic ligands containing N-linking phenyl rings have
exhibited great potential for modulating the energy structure of surface-modified Si QDs, we further use other simple N-containing
organic ligands–4-methoxydiphenylamine and aniline to modify Cl–
Si QDs. Noting that there are two N–H bonds in aniline, both of which
could be expected to react with the Si–Cl bonds on the surface of Si
QDs, the resulting steady-state absorption spectrum for aniline-modified Si QDs (An–Si QDs) exhibits a new absorption structure in the
range of 4502550 nm (Figure 5a). Surprisingly, the corresponding PL
spectra excited in this range exhibit a bright orange fluorescence
(Figure 5b). In contrast, for 4-methoxydiphenylamine modified Si
QDs (Me–Si QDs, Figure 5c), such a slight change in the surface ligand
in comparison with diphenylamine leads to a remarkable blueshift of
emission (Figure 5d).

The photoluminescence excitation spectrum corresponding to the
orange emission in the An–Si QDs peaks at approximately 500 nm

(Figure 6, top panel). This result is interesting because as we mentioned before, for most previously reported Si QDs, the PLE peaks are
located in the ultraviolet range because of the very low density of states
near their band edge. To obtain deeper insight into this phenomenon,
we performed a femtosecond transient absorption experiment and PL
dynamics measurement using the time-correlated single-photon counting technique for An–Si QDs under 500-nm excitation (Supplementary
Fig. S9). Interestingly, when this emission species is excited, there is a
large negative signal in the range of approximately 470–650 nm in the
TA spectra, which presents a fast decay followed by a long lifetime
component. In contrast, only a 4.5-ns PL lifetime was obtained, which
was probed at 600 nm and represents the pure radiative recombination
process. Fixing one of the lifetime components at 4.5 ns (denoting its
PL lifetime) in the global analysis of the An–Si QDs’ TA data (Figure 6,
bottom panel), we observed that in the obtained five lifetime components, two (88 ps and 4.5 ns) exhibited negative signals centered at
approximately 500 nm. This finding corresponds with the emission
species observed in the PLE spectrum. Therefore, these lifetime components of 88 ps and 4.5 ns are the ground-state bleaching signals of the
special emission species. For the other lifetime components (1.1 ps,
6.8 ps and 370 ps), their negative signals cover the entire PL emission
range of An–Si QDs at 500-nm excitation. Thus, these signals are
attributed to the stimulated emission.

Note that with the help of the PLE spectrum and time-resolved PL
dynamics, analyzing the TA data using global analysis in this case
remains reasonable. At least, this analysis cannot change the intrinsic

a 0.10 b

An-Si QDs

300

200

100

Orange
420
emitting

440

460

480

500

520

540

0.08

0.06

0.04

0.02

0.00

300 400 500 600

400 500 600 700 400 450 500 550 600 650

450 500 550 600 650 700 750

Wavelength (nm)

c 0.10 d

200

150

100

Wavelength (nm)

300

320

340

360

380

400

Blue
emitting

0.08

0.06

0.04

0.02

0.00

Me-Si QDs

300 400 500 600 700 350 400 450 500 550 600

400 500 600 700

350 400 450 500

Wavelength (nm)

Figure 5 (a) Steady-state absorption and (b) PL spectra at various excitation wavelengths for An–Si QDs. (c) Steady-state absorption and (d) PL spectra at various
excitation wavelengths for Me–Si QDs. O.D., optical density; QD, quantum dot.

Wavelength (nm)

400

–2

500 600 700 800

600

700

6.8 ps

4.5 ns

–4

–6

Global fitting lifetime

1.1 ps

370 ps

PLE

88 ps

400 500 600 700 800
Wavelength (nm)

Figure 6 (Top panel) Photoluminescence excitation spectrum (blue solid line)
corresponding to orange emission (probed at 600 nm) and steady-state emission
spectrum (red dot line) under 500-nm excitation in An–Si QDs. (Bottom panel)
Global analysis for the transient absorption data of An–Si QDs under 500-nm
excitation; the vertical dashed lines indicate the positions corresponding to the
PLE peak (approximately 500 nm) and PL peak (approximately 590 nm) in the
top panel. O.D., optical density; PL, photoluminescence; PLE, photoluminescence excitation; QD, quantum dot.

feature of each lifetime component. Hence, in accordance with our
proposed model, this TA experiment unambiguously demonstrates
that the observed emission state in the visible range originates from
the surface modification. Moreover, the results also suggest that these
lifetime components, except for the ground-state bleaching component of 4.5 ns in An–Si QDs, are nonradiative, which is consistent with
the recent results reported by Hannah et al. 54 It appears that there are
some common PL properties between carbon nanodots and surfacemodified Si NCs from the surface emission state viewpoint. These
photophysical studies on organically capped Si NCs may open an
intriguing window to obtain tunable PL in carbon nanodots and graphene QDs.

According to our criteria stated above, both the An– and Me–Si
QDs could belong to the ‘weak’ interaction mode; however, following
our proposed technical approach, deep blue, cyan-green, and orange
emissions from surface-modified Si QDs were achieved without changing their sizes. Recently, Dasog et al. 55 also reported Si NCs with
tunable PL across the visible spectrum via rational variation in the
surface moieties. Thus, our next goal is to further improve the PL QY
of the surface-modified Si QDs with orange-red emission (‘strong’
interaction mode) and illustrate the relationship between the organic

L Wang et al

agent structure and Si QDs’ emission color in more depth using other
methods such as quantum calculations.

CONCLUSIONS
In summary, our ultrafast spectroscopy study clearly demonstrates the
relationship between surface molecular engineering and the energy
band structure of Si QDs. The correlation between the excited-state
processes in transient spectra and the steady-state PL properties
(excitation-wavelength dependent and temperature-dependent PL
behaviors) is also fully discussed. The underlying PL mechanisms
are unraveled in detail, that is, there are two types of interaction modes
for surface-modified Si QDs: ‘weak’ interaction and ‘strong’ interaction, relying on the effect of surface ligands on the excited-state
processes (interaction strength of the core–ligand interaction) in Si
QDs. The former usually includes as-synthesized Cl–Si QDs with weak
blue emission and moderate fluorescence-emitting Me–, Te– and An–
Si QDs, in which the emission wavelengths can be tuned by different
surface modification. These emissions are simply common moleculelike emissions. For the latter, in the preparation of surface-modified Si
NCs, by specially designing the molecular structure of organically
capping agents to increase the density of state on the surface of Si
NCs (i.e., more phenyl rings in the surface ligands) and further
restricting the molecular vibrational/torsional motion, the ultrabright
photoluminescence can emerge, as for the cases of Di– and Ca–Si QDs,
in which the surface oxidation most likely also plays an important role.
This finding is attributed to the special integrated direct transition
energy structure, where the core–ligand interaction is so strong that
the entire intrinsic indirect bandgap of the Si QDs is almost filled by
the surface-modified energy bands. In this manner, these core-ligand
interactions could also be useful for the enhancement of fluorescent
emission in other Si nanostructures, for example, two-dimensional Si
nanosheets. 56 Finally, depending on the modification of exciton wave
functions by the core–ligand interaction, tunable emissions from deep
blue to orange can be successfully obtained from surface chemically
modified Si QDs without changing their sizes.

ACKNOWLEDGEMENTS
HBS thanks the Natural Science Foundation of China (NSFC) under Grant
Nos. 91423102, 21273096, 91323301 and 11104109, and the National Basic
Research Program of China (973 Program) under Grant No. 2014CB921302
for support.

1 Pavesi L, Dal Negro L, Mazzoleni C, Franzo` G, Priolo F. Optical gain in silicon
nanocrystals. Nature 2000; 408: 440–444.
2 Ding ZF, Quinn BM, Haram SK, Pell LE, Korgel BA et al. Electrochemistry and
electrogenerated chemiluminescence from silicon nanocrystal quantum dots.
Science 2002; 296: 1293–1297.
3 Cloutier SG, Kossyrev PA, Xu J. Optical gain and stimulated emission in periodic
nanopatterned crystalline silicon. Nat Mater 2005; 4: 887–891.
4 Liu CY, Holman ZC, Kortshagen UR. Optimization of Si NC/P3HT hybrid solar cells.
Adv Funct Mater 2010; 20: 2157–2164.
5 He Y, Su YY, Yang XB, Kang ZH, Xu TT et al. Photo and pH stable, highly-luminescent
silicon nanospheresand their bioconjugatesfor immunofluorescentcell imaging. J Am
Chem Soc 2009; 131: 4434–4438.
6 He Y, Zhong YL, Peng F, Wei XP, Su YY et al. One-pot microwave synthesis of waterdispersible, ultraphoto- and pH-stable, and highly fluorescent silicon quantum dots.
J Am Chem Soc 2011; 133: 14192–14195.
7 Erogbogbo F, Yong KT, Roy I, Hu R, Law WC et al. In vivo Targeted cancer imaging,
sentinel lymph node mapping and biocompatible silicon nanocrystals. ACS Nano
2011; 5: 413–423.
8 Puzzo DP, Henderson EJ, Helander MG, Wang ZB, Ozin GA et al. Visible colloidal
nanocrystal silicon light-emitting diode. Nano Lett 2011; 11: 1585–1590.
9 Wolf O, Dasog M, Yang Z, Balberg I, Veinot JG et al. Doping and quantum confinement
effects in single Si nanocrystals observed by scanning tunneling spectroscopy. Nano
Lett 2013; 13: 2516–2521.

L Wang et al

10 Vach H. Ultrastable silicon nanocrystals due to electron delocalization. Nano Lett
2011; 11: 5477–5481.
11 Wheeler LM, Neale NR, Chen T, Kortshagen UR. Hypervalent surface interactions for
colloidal stability and doping of silicon nanocrystals. Nat Commun 2013; 4: 2197.
12 Zhang YX, Han X, Zhang JM, Liu Y, Huang H et al. Photoluminescence of silicon
quantum dots in nanospheres. Nanoscale 2012; 4: 7760–7765.
13 Kang ZH, Liu Y, Lee ST. Small-sized silicon nanoparticles: new nanolights and
nanocatalysts. Nanoscale 2011; 3: 777–791.
14 Hybertsen MS. Absorption and emission of light in nanoscale silicon structures. Phys
Rev Lett 1994; 72: 1514–1517.
15 Wolkin MV, Jorne J, Fauchet PM, Allan G, Delerue C. Electronic states and
luminescence in porous silicon quantum dots: the role of oxygen. Phys Rev Lett
1999; 82: 197–200.
16 Hessel CM, Reid D, Panthani MG, Rasch MR, GoodfellowBW et al. Synthesis of ligandstabilized silicon nanocrystals with size-dependent photoluminescence spanning
visible to near-infrared wavelengths. Chem Mater 2012; 24: 393–401.
17 de Boer WD, Timmerman D, Dohnalova´ K, Yassievich IN, Zhang H et al. Red spectral
shift and enhanced quantum efficiency in phonon-free photoluminescence from
silicon nanocrystals. Nat Nanotechnol 2010; 5: 878–884.
18 Shirahata N. Colloidal Si nanocrystals: a controlled organic–inorganic interface and its
implications of color-tuning and chemical design toward sophisticated architectures.
Phys Chem Chem Phys 2011; 13: 7284–7294.
19 Priolo F, Gregorkiewicz T, Galli M, Krauss TF. Silicon nanostructures for photonics and
photovoltaics. Nat Nanotechnol 2014; 9: 19–32.
20 Wilson WL, Szajowski PF, Brus LE. Quantum confinement in size-selected, surfaceoxidized silicon nanocrystals. Science 1993; 262: 1242–1244.
21 Meier C, Gondorf A, Lu¨ttjohann S, Lorke A, Wiggers H. Silicon nanoparticles:
absorption, emission, and the nature of the electronic bandgap. J Appl Phys 2007;
101: 103112.
22 Miller JB, Van Sickle AR, Anthony RJ, Kroll DM, Kortshagen UR et al. Ensemble
brightening and enhanced quantum yield in size-purified silicon nanocrystals. ACS
Nano 2012; 6: 7389–7396.
23 Li Q, He Y, Chang J, Wang L, Chen HZ et al. Surface-modified silicon nanoparticles with
ultrabright photoluminescence and single-exponential decay for nanoscale fluorescence
lifetime imaging of temperature. J Am Chem Soc 2013; 135: 14924–14927.
24 Dohnalova´ K, Poddubny AN, Prokofiev AA, de Boer WD, Umesh CP et al. Surface
brightens up Si quantum dots: direct bandgap-like size-tunable emission. Light Sci
Appl 2013; 2: e47; doi:10.1038/lsa.2013.3.
25 Dohnalova´ K, Fucˇı´kova´ A, Umesh CP, Humpolı´cˇkova´ J, Paulusse JM et al. Microscopic
origin of the fast blue-green luminescence of chemically synthesized non-oxidized
silicon quantum dots. Small 2012; 8: 3185–3191.
26 Godefroo S, Hayne M, Jivanescu M, Stesmans A, Zacharias M et al. Classification and
control of the origin of photoluminescence from Si nanocrystals. Nat Nanotechnol
2008; 3: 174–178.
27 Ku 6 sova´ K, Cibulka O, Dohnalova´ K, Pelant I, Valenta J et al. Brightly luminescent
organically capped silicon nanocrystals fabricated at room temperature and
atmospheric pressure. ACS Nano 2010; 4: 4495–4504.
28 Anthony R, Kortshagen U. Photoluminescence quantum yields of amorphous and
crystalline silicon nanoparticles. Phys Rev B 2009; 80: 115407.
29 Hao HL, Shen WZ. Identification and control of the origin of photoluminescence from
silicon quantum dots. Nanotechnology 2008; 19: 455704.
30 Sa’ar A. Photoluminescence from silicon nanostructures: the mutual role of quantum
confinement and surface chemistry. J Nanophotonics 2009; 3: 032501.
31 Sa’ar A. On the origin of photoluminescence from silicon nanostructures: a new
perspective. Phys Status Solidi C 2011; 8: 1764–1768.
32 Belomoin G, Therrien J, Smith A, Rao S, Twesten R et al. Observation of a magic
discrete family of ultrabright Si nanoparticles. Appl Phys Lett 2002; 80: 841–843.
33 Jurbergs D, Rogojina E, Mangolini L, Kortshagen U. Silicon nanocrystals with
ensemble quantum yields exceeding 60%. Appl Phys Lett 2006; 88: 233116.
34 Hannah DC, Yang JH, Podsiadlo P, Chan MK, Demoritie`re A et al. On the origin of
photoluminescence in silicon nanocrystals: pressure-dependent structural and
optical studies. Nano Lett 2012; 12: 4200–4205.
35 Wang L, Wang HY, Wang Y, Zhu SJ, Zhang YL et al. Direct observation of quantumconfined graphene-like states and novel hybrid states in graphene oxide by transient
spectroscopy. Adv Mater 2013; 25: 6539–6545.
36 Zhang Q, Zheng HJ, Geng ZG, Jiang SL, Ge J et al. The realistic domain structure of assynthesized graphene oxide from ultrafast spectroscopy. J Am Chem Soc 2013; 135:
12468–12474.

37 Wang L, Zhu SJ, Wang HY, Wang YF, Hao YW et al. Unraveling bright molecule-like
state and dark intrinsic state in green-fluorescence graphene quantum dots via
ultrafast spectroscopy. Adv Opt Mater 2013; 1: 264–271.
38 Wen XM, Yu P, Toh YR, Hao XT, Tang J. Intrinsic and extrinsic fluorescence in carbon
nanodots: ultrafast time-resolved fluorescence and carrier dynamics. Adv Opt Mater
2013; 1: 173–178.
39 Wang L, Zhu SJ, Wang HY, Qu SN, Zhang YL et al. Common origin of green
luminescence in carbon nanodots and graphene quantum dots. ACS Nano 2014; 8:
2541–2547.
40 Wang L, Wu CF, Wang HY, Wang YF, Chen QD et al. Internal structure-mediated
ultrafast energy transfer in self-assembled polymer-blend dots. Nanoscale 2013; 5:
7265–7270.
41 Wang L, Wang HY, Gao BR, Pan LY, Jiang Y et al. Transient absorption spectroscopic
study on band-structure-type change in CdTe/CdS core–shell quantum dots.
IEEE J Quantum Electron 2011; 47: 1177–1184.
42 Pan LY, Zhang YL, Wang HY, Liu H, Luo JS et al. Hierarchical self-assembly of CdTe
quantum dots into hyperbranched nanobundles: suppression of biexciton Auger
recombination. Nanoscale 2011; 3: 2882–2888.
43 Wang L, Wang HY, Wei HT, Zhang H, Chen QD et al. Unraveling charge separation and
transport mechanism in aqueous-processed polymer/CdTe nanocrystal hybrid solar
cells. Adv Energy Mater 2014; 4: 1301882.
44 Sykora M, Mangolini L, Schaller RD, Kortshagen U, Jurbergs D et al. Size-dependent
intrinsic radiative decay rates of silicon nanocrystals at large confinement energies.
Phys Rev Lett 2008; 100: 067401.
45 Kuntermann K, Cimpean C, Brehm G, Sauer G, Kryschi C et al. Femtosecond transient
absorption spectroscopy of silanized silicon quantum dots. Phys Rev B 2008; 77:
115343.
46 Groenewegen V, Kuntermann V, Haarer D, Kunz M, Kryschi C. Excited-state relaxation
dynamics of 3-vinylthiophene-terminated silicon quantum dots. J Phys Chem C 2010;
114: 11693–11698.
47 Atkins TM, Thibert A, Larsen DS, Dey S, Browning ND et al. Femtosecond ligand/core
dynamics of microwave-assisted synthesized silicon quantum dots in aqueous
solution. J Am Chem Soc 2011; 133: 20664–20667.
48 Sommer A, Cimpean C, Kunz M, Oelsner C, Kupka HJ et al. Ultrafast excitation energy
transfer in vinylpyridine terminated silicon quantum dots. J Phys Chem C 2011; 115:
22781–22788.
49 Fuzell J, Thibert A, Atkins TM, Dasog M, Busby E et al. Red states versus blue states in
colloidal silicon nanocrystals: exciton sequestration into low-density traps. J Phys
Chem Lett 2013; 4: 3806–3812.
50 Ku 6 sova´ K, Hapala P, Valenta J, Jelı´nek P, Cibulka O et al. Direct bandgap silicon:
tensile-strained silicon nanocrystals. Adv Mater Interfaces 2014; 1: 1300042.
51 Kang ZH, Liu Y, Tsang CH, Ma DD, Fan X et al. Water-soluble silicon quantum
dots with wavelength-tunable photoluminescence. Adv Mater 2009; 21: 661–
664.
52 Pi XD, Mangolini L, Campbell SA, Kortshagen U. Room-temperature atmospheric
oxidation of Si nanocrystals after HF etching. Phys Rev B 2007; 75: 085423.
53 Dasog M, Yang Z, Regli S, Atkins TM, Faramus A et al. Chemical insight into the origin
of red and blue photoluminescence arising from freestanding silicon nanocrystals.
ACS Nano 2013; 7: 2676–2685.
54 Hannah DC, Yang JH, Kramer NJ, Schatz GC, Kortshagen UR et al. Ultrafast
photoluminescence in quantum-confined silicon nanocrystals arises from an
amprphous surface layer. ACS Photonics 2014; 1: 960–967.
55 Dasog M, De los Reyes GB, Titova LV, Hegmann FA, Veinot JG. Size vs surface: tuning
the photoluminescence of freestanding silicon nanocrystals across the visible
spectrum via surface groups. ACS Nano 2014; 8: 9636–9648.
56 Kim SW, Lee J, Sung JH, Seo DJ, Kim I et al. Two-dimensionally grown single-crystal
silicon nanosheets with tunable visible-light emissions. ACS Nano 2014; 8: 6556–
6562.

This work is licensed under a Creative Commons AttributionNonCommercial-ShareAlike 3.0 Unported License. The images or other third
party material in this article are included in the article’s Creative Commons license, unless
indicated otherwise in the credit line; if the material is not included under the Creative
Commonslicense,users willneedtoobtainpermissionfromthelicenseholdertoreproducethe
[material. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/](http://creativecommons.org/licenses/by-nc-sa/3.0/)

Supplementary information for this article can be found on the Light: Science & Applications’ website (http://www.nature.com/lsa/).

본문 출처 PDF: PDF 보기 · 다른 PDF는 위 [PDF 업로드]로 올릴 수 있습니다.

활용가능성

이력 0건

아직 편집 이력이 없습니다. 저장할 때마다 이전 내용이 여기에 백업됩니다.