Dispersing, Stabilizing and Functionalizing Silicon
Nanoparticles
A. Reindl1), C. Cimpean2), C. Kryschi2), and W. Peukert1)1)2)Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nürnberg, Germany
Institute of Physical Chemistry I, Friedrich-Alexander-University Erlangen-Nürnberg

ABSTRACT
Silicon particles were dispersed in organic solvents using a variety of methods. Morphology, particle size andstructure of untreated and dispersed silicon nanoparticles (SiNP) were examined using high-resolution
transmission electron microscopy (HRTEM), dynamic light scattering (DLS) and X-ray diffraction (XRD). Theamorphous SiOx of the SiNP was removed by hydrofluoric acid (HF) etching. The resulting hydride (H) terminated
SiNP surfaces were functionalized with styrene via hydrosilylation of phenyl acetylene. Evidences of a successful
functionalization of the SiNP were obtained from diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS), solid state 13C nuclear magnetic resonance (NMR) and HRTEM.

1. INTRODUCTION
Future progress in electronics requires thedevelopment of more powerful and compact integrated
circuits and nanometer-sized devices. In the bottom-up
approach the building blocks are proposed to beformed by semiconductor nanoparticles via surfacecontrolled
self-organization
processes.
Since
nowadays all microprocessor chips in electronic
products are still built on crystalline silicon wafers, thedevelopment of printing techniques using semiconducting silicon nanoparticles (SiNP) promises
inexpensive manufacture of electronic devices onflexible substrates. The fabrication of inorganic thin filmtransistors out of a solution of either CdSe or PbSe
nanocrystals has been already reported in literature [1,2]. The untreated SiNP however are nearly insulating
because of their native passivation shell that iscomposed of amorphous SiOx. Hence, SiNP surfaces
which enable interfacial charge transfer do not only
require the complete removal of the insulating oxide
layer but also have to be functionalized with organicmonolayers that achieve both, the protection against
oxidation and the support of the charge carriertransport. To this end, silicon surface chemistry is offundamental importance for potential applications of
nanostructured silicon in modern technology and has
become an area of intense research interest.
The Si-C linkage is both, kinetically andthermodynamically stable, due to the high strength andlow polarity of the bond. Hence, the most successful
approach for an organic functionalization of Si surfaces
is accomplished via hydrosilylation of 1-alkynes or 1alkenes yielding surface (Si-C) bound vinyl and alkyl
groups, respectively. Hydrosilylation requires hydride-terminated Si surfaces as involving the insertion of theunsaturated C-C bond into the Si-H group so that Si-C
bonds are formed. For efficient functionalization ofporous Si with unsaturated organic compounds Buriakand coworkers developed more than five wet chemistrysyntheses towards Si-C formation on porous Si
surfaces [3, 4]. Since the purification of SiNP is very
difficult, the least contaminated products are expected
to be obtained directly via thermally or photochemicallyinduced hydrosilylation of 1-alkenes or 1-alkynes on
hydride-terminated Si surfaces.
As a subproject of the Research Training Group
1161/1 “Disperse Systems in Electronics”, in close
cooperation with Degussa, this work focuses on thedispersion of SiNP and subsequent functionalization ofthe particle surfaces with phenyl acetylene. The main
impetus is to achieve an in-depth insight into thealteration of the surface morphology of the oxidepassivated SiNP occurring in the course of the HF
etching procedure and the subsequent thermallyinduced hydrosilylation reaction. To this end, solid13
stateC
NMRandDRIFT
spectroscopy
measurements were conducted to obtain unambiguous
evidence for surface-bound styrene molecules.
Valuable information on size, bulk and surface
structure of untreated, ground and functionalized SiNP
were obtained from high-resolution transmission
electron microscopy (HRTEM).

2. EXPERIMENTALA suspension of 20 wt.% silicon particles in 1-butanol
was dispersed for 6 h with a stirred media mill (NetzschPE 0785). The temperature of the milling chamber waskept constant at 15°C, and the revolution velocity of3. RESULTS AND DISCUSSION
The size, bulk structure and surface morphology of theuntreated silicon particles were examined using XRD
and HRTEM. The XRD spectrum (Fig.1) shows
diffraction peaks that unambiguously identify latticeplanes of crystalline bulk silicon. As obvious from theTEM image in Fig.2 the SiNP have nearly sphericalshapes and a primary particle size of approximately
100 nm. They are interconnected via sinter bridges sothat µm-sized agglomerates are formed.
The SiNP obtained from Degussa Creavis were
fabricated by gas phase synthesis in a hot-wall reactor.
The solid bridges are expected to result from sinteringor surface growth in the course of the synthesis.

Si (111)Si (220)counts [a.u.]

the SiC stirrer was set at 1000 min-1. Beads of yttrium
stabilized ZrO2 with diameters between 0.25 and 0.4
mm were used as grinding media. For the sake ofsafety the oxygen concentration was kept below 7%
during the milling process. Every hour four drops ofsuspension were taken from the milling chamber anddiluted with 5 ml 1-butanol. These samples were used
to examine the evolution of the particles size
distribution during the milling process by dynamic lightscattering. After 6 h of dispersing the milling beads
were separated from the SiNP by sieving and a stablesuspension of SiNP in 1-butanol was received. 1Butanol was removed from the SiNP suspension by
distillation. 15 g of the ground SiNP material wereredispersed by stirring in 40 ml ethanol. H terminationof the SiNP was acquired by adding 50 ml of aqueous
40 wt. % HF solution and by heating the mixture at60°C under N2 for 1 hour. The etching was stopped byremoving the HF solution through distillation. The Hterminated SiNP were dried in a N2 stream. Styrenyl
functionalization was started by immersing the Hterminated SiNP in 100 ml phenyl acetylene. The
yellow suspension was refluxed under N2 atmosphereat ca. 150°C for 7 hours. Unreacted phenyl acetylene
was decanted. The styrenyl-functionalized SiNP werewashed 6 times with toluene and subsequently 20times with ethanol. Finally the SiNP were dried byevaporation of ethanol. The solid-state TOSS (totalsuppression of spinning sidebands) 13C NMRexperiments were carried out on a Jeol Alpha 500 MHzspectrometer (1H = 500 MHz) operating at 125.79 MHz
for 13C. DRIFT spectra of the SiNP were recorded ona Varian FTS 3100 FTIR spectrometer equipped with aPike Technologies EasiDiff accessory. The TEM andHRTEM images were obtained using a Phillips CM 30
T/STEM microscope and a Philips CM 300 UltraTwin
microscope, respectively. The measurements werecarried out at an acceleration voltage of 300 kV in thebright-field mode. A Siemens D 500 diffractometerequipped with a graphite secondary monochromator
was used to measure the XRD spectra.

Si (311)Si (422)Si (331)
Si (400)
before

after
20

30

40

5060

70

8090

2 θ [degrees]Fig. 1 XRD of untreated and disp. SiNP.

400 nm

Fig.2 TEM image of untreated SiNP.
Fig. 3 displays a high-resolution TEM (HRTEM) imageof a part of a single SiNP. The occurrence of equally
spaced fringes, arising from the interference between
the diffracted and undiffracted electron waves,indicates the crystalline bulk of the SiNP. On the other
hand, the amorphous (i.e. disordered) shell with athickness of about 5 nm is ascribed to the SiOx
passivation layer.
C-filmSiO X

SiFig. 3 HRTEM image of a part of an untreated SiNP.
The SiNP do not generate stable suspensions while
dispersing with ultrasound or ultraturrax due to theformation of µm-sized agglomerates during the gas
phase synthesis. Hence, successful processing
requires methods with a high specific energy input e.g.
a stirred media mill. To this end, the SiNP powder waswet-ground with 1-butanol as described before. The

temporal evolution of the volumetric SiNP mean size
(x1,3) during the milling process was determined using
dynamic laser scattering (see Fig.4). The mean sizex1.3 decays exponentially with a time constant of 32 min
and reaches a value of 166 nm after 6 h milling. For
longer dispersing times the mean size x1,3 stays nearly
constant due to the equilibrium of breakage,agglomeration and desagglomeration [5, 6].

amorphous passivation layer is now substituted bystyrenyl groups yielding a surface covering thinner than
0.5 nm. The styrenyl termination of the SiNP
significantly decreases the value of the zeta potential
to -22.36 mV. This indicates to significant electronicand structural changes of the SiNP surfaces arisingfrom the substitution of SiOx by surface-bound styrene.

Si
1.6
1.4

x1,3 [µm]1.2C-film

1.0
0.8
0.6
0.4
0.2
0.0
0100200300400t [min]Fig. 4 Evolution of x1,3 with disp. time.
At 6 h the milling process was stopped, the yttrium
stabilized ZrO2 beads were removed by sieving and astable suspension of SiNP was received. In Fig. 5 theTEM image of ground SiNP is depicted. Obviously, theadhesion forces of the particles are sufficiently strong
so that the SiNP organize as aggregates during
evaporation of the solvent on the carbon film. However,instead of solid sinter bridges weaker physicalinteraction forces act between neighboured SiNP. Thezeta potential of ground SiNP dispersed in 99:1
butanol-water mixture reaches a value of -46.6 mV that
resembles the zeta-potential of silica in 99:1 butanolwater mixture with a value of -45 mV [7]. This resultshows that the surface potential of SiNP is
predominately determined by the electronic propertiesof amorphous SiOx.

400 nm

Fig.5 TEM image of aggregated ground SiNP.
The SiOx passivation shells of ground SiNP wereremoved by HF etching, and the resulting H-terminatedsurfaces were functionalized via hydrosilylation withphenyl acetylene. Fig.6 presents a HRTEM image of apart of a single styrenyl-terminated SiNP. In
comparison with the untreated SiNP the 5-nm thickFig. 6 HRTEM image of a functionalized SiNP.
The DRIFT spectrum of styrenyl-terminated SiNP
(Fig.7) contains the characteristic vibrational spectrumof styrene consisting of bands at 700 cm-1 (1), 1448
cm-1 (7), 1492 cm-1 (8), 1598 cm-1 (10) with a shoulder
at 1592 cm-1 (9), 3029 cm-1 (14) and 3061 cm-1 (15)[8]. While aromatic in-ring ν(C-C) stretches give rise tothe bands at 1448 cm-1 and 1492 cm-1, the vibrationalbands at 1598 cm-1 and 1592 cm-1 presumably
originate from the overlap of the ν(C=C) stretching modeof the vinyl group with that of the phenyl ring [9]. Theband at 700 cm-1 is assigned to the δ(C=C-H) bending
mode of the vinyl group. The vinylic and aromatic ν(C-H)
stretching modes produce bands at 3027 cm-1 (11) and3061 cm-1 (12), respectively. The appearance of thetypical styrene bands in the DRIFT spectrum may betaken as an indirect evidence for styrenyl-terminated
SiNP surfaces. The clear indication of the Si-C bond bythe ν(Si-C) stretching mode predicted to occur (600 –800 cm-1) in the IR spectrum is very difficult to obtain,
since Si-H, Si-Si and Si-O vibrational modes appear in
the same spectral range [10]. Fortunately the broad
peak at 803 cm-1 (3) may be unambiguously assigned
to the δ(Si-O) bending mode so that the sharper one at
761 cm-1 (2) should emerge from the stretching modeof the Si-C bond. The observation of both, thevibrational spectrum of styrene and the Si-C stretch at
761 cm-1 indicates to styrenyl functionalization. Thebroad band between 3200 and 3500 cm-1 (16), thepeaks between 2930 and 2966 cm-1 (13) and band at
1376 cm-1 (6), as being characteristic for the O-H
stretching, the alkane C-H stretching and C-H rocking
modes, respectively, originate from either ethanol used
for the washing procedure or 1-butanol used for themilling procedure. The pronounced band at 1100 cm-1(5) is due to the ν (Si-O) and ν(Si-O-Si) stretching modes ofresidual SiOx, whereas the band at 930 cm-1 (4) isgenerated by the δ(Si-OH) bending mode [11]. ResidualH termination of the SiNP are responsible forvibrational structures at , 2100 cm-1 (11) and 2261 cm-1(12) which are assigned to stretching modes [12].

reflected intensity [a.u.]

12112687

14 3510 9161315 14

4000

3000

2000

1000-1

wavenumber [cm ]Fig. 7 DRIFT spectrum of styrenyl-terminated SiNP.
13
Solid-state TOSS C NMR provides complementary
information of the Si-C linkages founding the styrenyltermination of the SiNP. The 13C NMR spectrum (Fig.8)
exhibits a prominent peak at δ= 127.76 ppm and asmaller, broadened peak centered at δ = 143.34 ppm
with a shoulder at 138.2 ppm.

in 1-butanol using a stirred media mill. A stablesuspension of SiNP was obtained after the milling
procedure. Subsequently the SiNP were functionalizedwith phenyl acetylene via hydrosilylation. Morphology,size and bulk structure of untreated, ground andstyrenyl-terminated SiNP were examined using
HRTEM, DLS and XRD. XRD data and HRTEM
images of untreated and functionalized SiNP manifest
the crystallinity of the SiNP. Additionally HRTEM
revealed information of the amorphous SiOx shell of
the untreated SiNP and the alteration of the surface
morphology during the functionalization process.
Evidence for the styrenyl termination of the SiNP wasprovided by solid-state TOSS 13C NMR and DRIFTS
measurements. While the 13C NMR spectrum contains
the 13C resonance of the olefinic carbon atom that isbound to the SiNP surface, there are vibrational bands
in the DRIFT spectrum that are characteristic forstyrene and for the Si-C linkage.

ACKNOWLEDGEMENTS
Support of the Deutsche Forschungsgemeinschaft(Research Training Group 1161/1) and Degussa
Creavis is gratefully acknowledged.

1,2,4,5,6,7

REFERENCES:
75342HO68

1[1]HOSiNP[2]

3,8[3]
[4]

*
300

250

200

150

100

50

0-50

chemical shift [ppm][5]

Fig. 8 Solid-state TOSS 13C NMR spectrum of
styrenyl-terminated SiNP.

[6]

While the prominent peak results from thesuperposition of the 13C resonances of five aromatic
carbon atoms (1, 2, 4, 5, 7) as well as of one olefinic
carbon atom (6), the maximum (δ=143.34 ppm) andshoulder (δ=138.2 ppm) of the smaller peak areassigned to one aromatic carbon atom (3) and to theother olefinic carbon atom (8) that is bound to the SiNP
surface, respectively. In particular, the assignment ofthe shoulder at δ=138.2 ppm to the Si surface-boundolefinic carbon (8) is corroborated by a simulation of a13
C NMR spectrum of β-trimethyl-silylstyrene, wherethe trimethylsilyl-bound carbon atom exhibits a 13C
resonance at 134.57 ppm.

[7]

[8]

[9]

[10]

[11]

3. CONCLUSIONS
Since the untreated SiNP as received by Degussa
Creavis are interconnected via sinter bridges and thus
form µm-sized agglomerates, the SiNP were dispersed[12]

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