Size-dependent oxidation of monodisperse silicon nanocrystals with allylphenylsulfide surfaces

Article abstract of DOI:10.1002/smll.201401965

The synthesis and characterization of size-separated silicon nanocrystals functionalized with a heteroatom-substituted organic capping group, allylphenylsulfide, via photochemical hydrosilylation are described for the first time. These silicon nanocrystals form colloidally stable and highly photoluminescent dispersions in nonpolar organic solvents with an absolute quantum yield as high as 52% which is 20% above that of the allylbenzene analogue. Solutions of the size-separated fractions are characterized over time to monitor the effect of aging in air by following the change of their photoluminescence and absolute quantum yields, supplemented by transmission electron microscopy.

Full text of DOI:10.1002/smll.201401965

Silicon Nanocrystals
Size-Dependent Oxidation of Monodisperse Silicon
Nanocrystals with Allylphenylsulfide Surfaces
Julia Rinck,* Dirk Schray, Christian Kübel, Annie K. Powell, and Geoffrey A. Ozin*
T
he synthesis and characterization of size-separated silicon nanocrystals
functionalized with a heteroatom-substituted organic capping group,allylphenylsulfide,
via photochemical hydrosilylation are described for the first time. These silicon
nanocrystals form colloidally stable and highly photoluminescent dispersions in non-
polar organic solvents with an absolute quantum yield as high as 52% which is 20%
above that of the allylbenzene analogue. Solutions of the size-separated fractions are
characterized over time to monitor the effect of aging in air by following the change of
their photoluminescence and absolute quantum yields, supplemented by transmission
electron microscopy.
1
. Introduction
semiconductor nanocrystals, the advantage of silicon, the ele-
ment that dominates the microelectronics and photovoltaic
Luminescent, colloidally stable silicon nanocrystals are a new industry, is its nontoxicity,[18–20] earth abundance and low cost.
class of materials with a wide range of applications from med-
Although bulk silicon is an indirect bandgap semicon-
[1,2]
ical imaging
to optoelectronics such as light emitting diodes ductor, silicon nanocrystals with a size smaller than the Bohr
[
3–15]
[16,17]
(LEDs)
and solar cells.
In contrast to II–VI and III–V exciton radius of silicon show photoluminescence with a high
[
21,22]
quantum yield
in the size range of 1–5 nm due to spatial
[
23,24]
confinement effects.
Dr. J. Rinck
Karlsruhe Institute of Technology
Center for Functional Nanostructures
Wolfgang-Gaede Str. 1a, Karlsruhe 76131, Germany
E-mail: Julia.Rinck@kit.edu
Semiconductor nanocrystals with sizes between 1 and
00 nm consist of several tens to several thousand atoms.
1
Their physical and chemical properties are determined by
quantum size effects and differ from those of the corre-
sponding bulk material. The energy levels of the electrons are
quantized and the effect of decreasing size is to widen the
electronic band gap, causing their photoluminescence to shift
to smaller wavelength from the red towards the blue spectral
Prof. G. A. Ozin
University of Toronto
Lash Miller Chemical Laboratories
Department of Chemistry
8
0 St George Street, Toronto, Ontario M5S 3H6, Canada
[25]
region when excited within their optical absorption edge.
E-mail: gozin@chem.utoronto.ca
The chemical and, consequently, optical properties of the
nanocrystals can be altered by attachment of organic capping
groups onto the surface atoms of the nanocrystals to provide
them with colloidal stability and protect them from oxida-
Dr. D. Schray, Prof. A. K. Powell
Institute of Inorganic Chemistry
Engesserstr. 15, Karlsruhe 76131, Germany
Dr. C. Kübel, Prof. A. K. Powell
Karlsruhe Institute of Technology
Institute of Nanotechnology
Hermann-von-Helmholtz-Platz 1
Eggenstein-Leopoldshafen 76344, Germany
[
26]
tion,
which is important for advanced materials and bio-
medical applications. Thus, control over both the size and the
surface of silicon nanocrystals is highly desirable. Since opto-
electronics, and in particular LEDs, are currently attracting
great interest, the search for improvements in performance
Dr. C. Kübel
Karlsruhe Institute of Technology
Karlsruhe Nano Micro Facility
Hermann-von-Helmholtz-Platz 1
Eggenstein-Leopoldshafen 76344, Germany
and especially for the discovery of environmentally friendly
[27]
systems is an important goal. We showed previously
that
LEDs with high external quantum efficiencies and low turn-
on voltages can be built using silicon nanocrystals as the
active light emitting material. Using size-separated ncSi, we
DOI: 10.1002/smll.201401965
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J. Rinck et al.
found that not only the color can be tuned, but also that the polydisperse ensemble (sample 1) was dissolved in a small
lifetime of the device can be significantly improved. amount of toluene and size-separated by size-selective pre-
Indeed, the efficiency of the device can in principle be fur- cipitation into 15 fractions by stepwise addition of methanol
ther improved by modifying the ligand system, for example as an anti-solvent. This resulted initially in the precipitation
by introducing a heteroatom next to the aromatic node it of the largest nanocrystals. These nanocrystals were collected
should be possible to increase the electron density within the by centrifugation and re-dissolved in dry toluene for char-
aromatic system and thus to alter the influence of the ligand acterization. The photoluminescence (PL) of each fraction
on the photoluminescence (PL). This raises the question as was measured at an excitation wavelength of 350 nm and
to whether allylphenylsulfide (APS) can lead to an increased the curves were normalized. The PL maximum of each frac-
Photoluminescence absolute quantum yield (PL AQY) com- tion (increasing fractionation step) shifted continuously to a
pared with its allylbenzene (AB) analogue.
smaller wavelength as the band gap widens with decreasing
Various synthetic routes to silicon nanocrystals have been size. Each fraction shows two distinct emission peaks, one in
developed in recent years[28–31] and amongst these pyrolysis the red and one in the blue spectral region. With decreasing
of SiH is a favored method which can be carried out ther- nanocrystal size the red peak shifts to shorter wavelength and
4
[
32]
[33]
mally
or using microwave irradiation
or else within its intensity slowly decreases while the intensity of the blue
[
34]
a high-frequency plasma.
In our study we applied the peak correspondingly increases but without any discernible
method published by Henderson et al.,[35–37] in which the shift. Overall, with decreasing nanocrystal size, a continuous
hydrolysis of trichlorosilane is used to obtain a sol-gel. This shift of the measured optical extinction maxima to a shorter
was then dried prior to reduction to the crystalline silicon wavelength was observed, ranging from 374 nm (fraction 5) to
nanocrystals (ncSi) which are encapsulated within a matrix 325 nm (fraction 10).
of silicon dioxide which can be etched away using aqueous
The organic capping groups of the nanocrystals do not
hydrogen fluoride, resulting in hydrophobic hydride ter- only enhance the colloidal stability of the nanocrystals in
minated nanocrystals of ncSi:H. These could then be func- solution, but they also protect the surface of the nanocrys-
tionalized with allylphenyl sulfide (APS) in a light driven tals from oxidation, which would, in turn, lead to a change
hydrosilylation reaction to yield ncSi:APS.
of the chemical and optical properties of the nanocrystals.
A survey of the different ways of functionalizing ncSi:H With increasing oxidation, the band gap of the nanocrystals
has been provided by Veinot.[38] Despite the many different increases and the PL maxima shift to smaller wavelength into
approaches used to functionalize silicon nanocrystals, these the blue spectral region as the silicon core shrinks in size.
mostly result in a broad size distribution, leading to aver- This was investigated for the different fractions.
aging effects of the measured size-dependent properties. This
is mostly clearly exemplified by the absolute quantum yield of the spectrometer and were not used.Thus starting from frac-
AQY), which shows a distinct size effect and which can only tion 5 the emission was in the visible spectral region and was
The first four fractions emit in the NIR beyond the range
(
be understood when monodisperse samples are studied. As a investigated in detail. These fractions emit in the range 682 to
result, recent efforts have turned to post-synthetic size-sepa- 578 nm with colors ranging from deep red to pale yellow.
ration out of which fractionation by density gradient ultracen-
For the initial measurement (day 0) all samples were
[39]
trifugation (DGU)
using an anti-solvent
and simple size-selective precipitation transferred into a quartz glass cuvette in a glove box under
[40]
have proved successful means for argon atmosphere and the cuvette was sealed with a plastic
obtaining size-separated functionalized silicon nanocrystals lid to prevent contact with air. The spectra were recorded
and thus enabling the study of quantum size effects in the immediately thereafter. For these ten fractions the PL
NIR- and visible range. Such approaches can be used because maxima shifts from 682 (fraction 5) to 578 nm (fraction 15)
the nanocrystals are capped with organic ligands which in steps of approximately 10 nm, the first steps being some-
enable colloidal stability in non-polar solvents. This approach what larger. We assume that the fresh samples are only mini-
was used to investigate nanocrystals of silicon. The stepwise mally oxidized but then age to form a surface oxide that was
addition of an anti-solvent, such as methanol, was found to monitored by FT-IR spectroscopy. Thus as the samples age,
result in an aggregation of larger nanocrystals in solution and the overall shift between fractions becomes smaller starting
permits a stepwise precipitation according to size. at 105 nm for the initial spectra and reducing to 71 nm after
We find that in addition to the desirable property of a five days (Figure 1) .
narrow size for applications in optoelectronic devices, the
The full width at half maximum (FWHM) continuously
chemical stability of the silicon nanocrystals is an issue. For decreases from around 140 nm to almost half of this value,
example, adventitious oxidation will result in a change of the indicating a more effective size separation with decreasing
optical properties and, since the surface to volume ratio and particle size. This is not surprising taking into account the
also the surface curvature changes with size, is expected to be larger volume of the first fractions which steadily decreases
size dependent, the subject of the work reported herein.
with each step.
After the measurement the sample was transferred into a
glass beaker, allowing contact with air. The slow oxidation of
the nanocrystals was monitored over two weeks, re-measuring
the samples after 1, 2, 5, 7, and 13 days. Each fraction shows
2
. Results and Discussion
To investigate the quantum size effects and the size-dependent a monotonic shift of the emission maxima to smaller wave-
aging of the chosen APS-capped silicon nanocrystals, the length with an average of 30 nm over these 13 days. For most
2
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Size-Dependent Oxidation of Silicon Nanocrystals with Allylphenylsulfide Surfaces
Figure 2. Evolution of time-dependent photoluminescence spectra
and intensity for Sample 1 Fraction 9 of size separated ncSi:APS after
exposure to air.
Si–O, Si–O–Si and Si–OH vibration modes at 805, 1028–1067,
−
1
and 3450 cm , respectively. After exposure to air for two
weeks a significant increase of the Si–O, Si–O–Si and Si–OH
bands is observed and the Si–H peak vanishes, indicating that
the remaining surface hydride atoms have been replaced by
oxide in the form of either oxide or hydroxide (Figure S2).
Transmission electron microscopy (TEM) was used to
investigate the size distribution of three selected fractions,
namely fractions 6, 9, and 14.With increasing fraction number,
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) clearly shows a decrease in the
nanocrystal size with mean diameters of 2.53 ± 0.45 nm for
fraction 6, 2.20 nm ± 0.36 nm for fraction 9, 1.61 nm ± 0.34 nm
Figure 1. Size depend PL spectra normalized to 1 of the freshly prepared
ncSi sample (Sample 1) (top) and after exposure to air after 5 days
(
bottom).
samples almost no PL change was observed after one day but for fraction 14 (Figure 2). This decrease in the size of the
beyond this time the oxidation progressed slowly. Fraction 9 nanocrystals can be correlated with the increasing fraction
was chosen to demonstrate the decrease of the intensity of number and assigned to the PL peak values of 667, 618 and
the red peak that occurs over 13 days as well as the shift to 584 nm. This supports the previous statements suggesting a
smaller wavelength, in this particular case from 618 to 591 nm blue shift to smaller wavelength with decreasing particle size.
and a total shift of 27 nm, as the nanocrystals age (Figure 2
Photoluminescence absolute quantum yield (PL AQY)
and Table S1). As the surface of the silicon nanocrystals measurements were performed on size-separated ncSi:APS
begins to oxidize, the silicon core shrinks in size causing the dispersed in dry toluene (sample 2) that were transferred into
PL shift to the blue. This behavior was observed for all frac- a quartz glass ampoule in air prior to the measurement. With
tions although the effect is more distinct when looking at an excitation wavelength of 353 the highest AQY of 51.5%
the fractions with the largest and smallest nanocrystals. The was measured for the bright red emitting nanocrystals with a
total blue shift of fractions 6–11 is significantly smaller than PL maximum of 706 nm. The AQY dropped with decreasing
the shift of the fractions with larger or smaller nanocrystals nanocrystal size concomitant with a shift of the PL to smaller
(Figure S1) (and this trend is reproducible).
wavelength. For nanocrystals with a PL maximum of 671 nm,
Several proposals have been put forward to explain theAQY dropped to 35.8% and with a PL maximum of 637 nm
the origin of the blue PL although a quantum size effect is the AQY dropped abruptly to 15.3% and at 626 nm only
unlikely as the blue PL does not shift with increasing sur- 11.6% and less were recorded. Also for larger particle with a
face oxidation and decreasing size of the silicon nanocrystal PL of 760 nm the AQY drops to 36.7% and particles emitting
core. From earlier studies it is known that silicon dioxide in the NIR (958 nm) show an AQY of only 9.6% (Figure 3) .
shows a strong blue luminescence due to recombination of
There could be the possibility that between the fraction
[
41]
self-trapped excitons.
Therefore we prefer to suggest the that shows the highest AQY and the neighboring fractions in
surface oxidation of the silicon nanocrystals as the more which the AQY already dropped by a third (to roughly 36%),
likely explanation of the origin of the blue PL. This assump- that there is an additional fraction lying somewhere between
tion is supported by Fourier-transform infrared spectroscopy these values with an even higher AQY than 51.5%. With an
(
FTIR). The spectra of freshly etched nanocrystals show a AQY of 51.5% this material is highly interesting for optical
−
1
significant Si–H vibration band at 2070 cm and only weak applications, e.g., in light-emitting diodes and it proves that
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Figure 3. HAADF-STEM images and histograms of monodisperse ncSi (Sample 1) Fractions 6 (a), 9 (b), and 14 (c).
introduction of a heteroatom changes the electronic prop- AQY loss of ∼8% was measured. For fractions 2, 4 and 5 with
erties towards a higher AQY more than 20%. For the AB the highest initial AQYs the decrease is more pronounced
analogue an AQY for red emitting particles of 43% has although the shift of the PL to smaller wavelength is less pro-
[
21]
been reported.
The initial increase of the PL AQY from nounced compared to the fractions with smaller particles. For
the infrared to the visible region with decreasing size of the fractions 4–7 the PL shift was 25, 17, 31 and 46 nm and the
particles could indicate the transition from bulk properties to AQY decreased to ∼16, 5, 3, and 3%, corresponding to a loss
quantum confinement.
of 20, 18, 12, and 9%, respectively (Table S2). Values below
To investigate how oxidation affects the AQY, the frac- 10% are only approximate values. Unfortunately the solvent
tions of sample 2 were stored in air and their AQY was of fraction 3 evaporated over time (aging). After re-suspen-
re-measured after some time. A steady decrease for all frac- sion of the precipitated particles in toluene by ultra-sonica-
tions (IR and VIS-region) was observed (Figure 4, Table S2). tion the absorption was too low to obtain data of their AQY.
In fraction 2 particles started to precipitate over time and
Comparison with the TEM results allows us to give an
were filtered off. Although the PL shifted 23 nm (from 763 approximation for the size of the nanocrystals of sample
to 740 nm) towards the region where we initially measured 2 fraction 4 with an AQY of 35.8% and PL of 671 nm. We
the highest AQY (706 nm), a decrease and not an increase of find these are in the same size regime as the nanocrystals of
∼
24% was observed. For fraction 1 a PL shift of 46 nm and an sample 1 fraction 6 for which a PL of 667 nm and an average
particle size of 2.53 ± 0.45 nm was determined. The particles
with the highest measured AQY of 51.5% and a bright red
PL of 706 nm are thus larger in size.
3
. Conclusion
It has been shown that the introduction of a heteroatom
into the ligand system is a suitable method to change the
electronic properties of the silicon nanocrystals resulting in
an increase of the AQY. Using allylphenylsulfide as ligand
instead of allylbenzene the AQY can be increased more
than 20% to an overall AQY of 51.5% making this system
extremely interesting for optoelectronic applications like
LEDs especially as size-selective precipitation allows one to
tune the color. This increase of the AQY can be explained by
the lower frequency vibrations associated with the C-S bond
Figure 4. Wavelength-dependent photoluminescence AQY of fresh in APS compared to C–C in AB, that reduces the non-radia-
black) and aged (gray) of Fraction 1–7 of ncSi:APS (Sample 2).
tive vibrational relaxation in the former relative to the latter
(
4
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Size-Dependent Oxidation of Silicon Nanocrystals with Allylphenylsulfide Surfaces
and thereby enhances the PL AQY of ncSi:APS relative to The acetic mixture was stirred with a plastic spatula and was
ncSi:AB. The time dependent study of the size-dependant PL allowed to warm to room temperature. After 30 min the precipitate
and AQY shows their stability towards oxidation in air which was transferred into a Büchner funnel and washed with approxi-
is an important result for applications such as LEDs. It seems mately 2 L of deionised water until the pH was almost neutral.
that for the largest particles (emitting in the IR region) we The product was transferred into a Schlenk flask and dried in
already start to observe the transition from quantum con- vacuo, heating it to 100 °C in a water bath. The fine white powder
finement to bulk properties, resulting in a strong decrease of (HSiO1.5)n was stored under nitrogen to prevent unwanted oxida-
the PL AQY. The highest AQY was observed for ncSi:APS tion and hydrolysis of the Si-H.
particles emitting in the range of 650–600 nm. These turned
The sol-gel was heated to 1100 °C in a tube furnace flushing
out to also be the most stable particles towards oxidation in it with 5% H /95% Ar with 3 L/min. The temperature was kept for
2
air showing a smaller blue shift of their PL after aging. This 1 h before slowly cooling to room temperature.
could be a result of closer packing of the capping organics
0.3 g of the brown product was ground in an agate mortar and
on the surface of the larger particles that emit in the visible transferred into a plastic beaker. 5 mL ethanol and 10 mL HF (48%)
region. The converse is true for the smaller particles where were added. The mixture was stirred for 1 h and 45 min to etch
the higher surface curvature results in more open packing of the SiO matrix and gradually decrease the size of the nanoparticle
2
the organic capping groups, which results in faster oxidation core, before 15 mL dry toluene was added. The mixture was stirred
of the more accessible surface, resulting in a more rapid loss for a few minutes to allow the hydrophobic hydrate-terminated
of the PL and decrease of the AQY.
nanocrystals (ncSi:H) to transfer into the solvent phase before the
solution was separated into a Schlenk flask under inert gas.
After the addition of 12 mL allylphenylsulfide the stirred mix-
ture was exposed to UV-light (254 nm, 9 Watt, Pen-Ray Hg lamp)
at room temperature for 12 h. The solvent and any excess of the
ligand were distilled in vacuo and increasing heat resulting in a dry
residue of functionalized nanocrystals (ncSi:APS).
Size separation of the functionalized nanoparticles: Under
inert gas the functionalized particles were re-dissolved in 3 mL dry
toluene and gradually precipitated by stepwise addition of 2 mL dry
methanol.[21] The precipitate was centrifuged for 3 min (9000 rpm),
4
. Experimental Section
Synthesis Procedures: Due to the air sensitivity of the materials
to oxidation and/or hydrolysis all procedures were carried out under
an atmosphere of dry nitrogen or argon using Schlenk techniques on
a high vacuum line capable of being evacuated to 10− Torr and an
inert gas supply or using a glovebox from BRAUN company. Nitrogen
was used from the building supply and cleaned from traces of
oxygen and water by letting it flow over a Cu-catalyst from BASF com-
3
separated and re-dissolved in dry toluene.
pany loaded with hydrogen (at 140 °C), bubbling it through H SO , a
2
4
Characterization: The photoluminescence of the particles dis-
solved in toluene was collected with a Varian Cary Eclipse Fluorom-
eter. The emission was collected in the region of 375–800 nm with
an excitation of 350 nm. The cuvettes were made of Quartz glass.
For the determination of AQY ϕ, an absolute PL quantum yield meas-
urement system from Hamamatsu Photonics was used. The system
consisted of a photonic multichannel analyzer PMA-12, a model
C99200–02G calibrated integrating sphere and a monochromatic
light source L9799–02 (150 W Xe- and Hg-Xe-lamps). Data analysis
was done with the PLQY measurement software U6039–05, pro-
vided by Hamamatsu Photonics. TEM analysis was performed using
an image-corrected FEI Titan 80−300 microscope operated at an
acceleration voltage of 300 kV HAADF-STEM imaging was performed
with a nominal probe size of 0.15 nm. TEM sample preparation of
the ncSi:APS was done by diluting the material in toluene and dis-
persing it on a carbon-coated Au grid (Quantifoil) under inert gas
atmosphere. The infrared spectra were collected with a Perkin Elmer
mixture of pumice and P O and blue silica gel prior to use.
4
10
Toluene and methanol were dried prior to use. Toluene was
dried using sodium and adding benzophenone, methanol was
refluxed with magnesia and iodine for 2 h before distillation.
Synthesis of the Ligand Allylphenylsulfide (APS): To a stirred
solution of 25 mL (0.27 g, 0.24 mol) thiophenol and 100 mL dry
diethyl ether, cooled to 0 °C with an ice bath, 100 mL (0.25 mol) of
a 2.5 molar butyl lithium solution in pentene solution was added
dropwise over one hour. The mixture was stirred at room tempera-
ture for one hour. To remove the white precipitate the mixture was
filtered with a G4 glass frit before the solvent was removed by distil-
lation. The formed lithium thiophenol was dissolved in 100 mL dry
toluene and 24 mL (0.27 mol) allyl bromide was added dropwise
within two hours. The mixture was stirred for 12 h at room temper-
ature before it was filtered in air and the solvent was distilled off
under vaccum. Allyl phenyl sulfide was distilled at 130 °C and 10−
2
bar to yield a clear colourless liquid with a yield of 70% (26 g).
Spectum GX spectrometer in the region of 400 cm−1 to 4000 cm
−1
1
H NMR (300 MHz, CD CN, δ): 7.24 (m, 5H, Ar H), 5.95 (m, 1H,
−1
3
in transmission mode using 4 scans to a resolution of 4 cm . The
samples were prepared as KBr-discs by grinding the dry sample with
a small amount of KBr in an approx. ratio of 1:50 and then pressed
13
CH), 5.10 (m, 2H, CH ), 3.72 (m, SCH ); C NMR (300 MHz, CD CN,
2
2
3
δ): 136.12, 129.1, 126.10 (Ar C), 133,98 (-CH = ), 117.17 ( = CH2),
37.0 (-CH2-); IR (KBr): ν = 3062 (w), 3027 (w), 2958 (w), 2922 (s),
4
to a KBr disc of 1 cm in diameter under the pressure of 10·10 N.
2862 (w), 1700 (vw), 1653 (vw), 1560 (s), 1496 (w), 1483 (s), 1453
(
7
7
w), 1419 (s), 1375 (w), 1358 (w), 1229 (w), 1086 (vs), 917 (w),
43 (vs), 698 (s), 668 (vw), 623 cm− (vw); Anal. calcd for C H S: C
1
9
10
1.94, H 6.70, N 0, S 21.34; found: C 72.12, H 6.75, N 0.15, S 21.13.
Synthesis of the Functionalized Silicon Nanocrystals (ncSi:APS): Supporting Information
2
0 mL HSiCl were transferred into a beaker that was cooled with
3
an isopropanol/CO2(s) bath to −78 °C. 80 mL of deionised water Supporting Information is available from the Wiley Online Library
were added quickly and a white precipitate formed immediately. or from the author.
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J. Rinck et al.
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DOI: 10.1002/smll.201401965