Radical-Induced Hydrosilylation Reactions for the Functionalization of Two-Dimensional Hydride Terminated Silicon Nanosheets

Article abstract of DOI:10.1002/chem.201505134

Herein we present the functionalization of freestanding silicon nanosheets (SiNSs) by radical-induced hydrosilylation reactions. An efficient hydrosilylation of Si-H terminated SiNSs can be achieved by thermal initiation or the addition of diazonium salts with a variety of alkene or alkyne derivatives. The radical-induced hydrosilylation is applicable for a wide variety of substrates with different functionalities, improving the stability and dispersibility of the functional SiNSs in organic solvents and potentially opening up new fields of application for these hybrid materials.

Full text of DOI:10.1002/chem.201505134

DOI: 10.1002/chem.201505134
Communication
&
Surface Chemistry
Radical-Induced Hydrosilylation Reactions for the
Functionalization of Two-Dimensional Hydride Terminated Silicon
Nanosheets
Tobias Helbich,[a] Alina Lyuleeva,[b] Ignaz M. D. Hçhlein,[a] Philipp Marx,[a] Lavinia M. Scherf,[c]
Julian Kehrle,[a] Thomas F. Fässler,[c] Paolo Lugli,[b] and Bernhard Rieger*[a]
tovoltaic solar cells[9] and have already been proven to exhibit
potential for lithium ion battery application.[11–13]
Abstract: Herein we present the functionalization of free-
Several routes have been reported for the production of
standing silicon nanosheets (SiNSs) by radical-induced hy-
drosilylation reactions. An efficient hydrosilylation of SiÀH
terminated SiNSs can be achieved by thermal initiation or
the addition of diazonium salts with a variety of alkene or
alkyne derivatives. The radical-induced hydrosilylation is
applicable for a wide variety of substrates with different
functionalities, improving the stability and dispersibility of
the functional SiNSs in organic solvents and potentially
opening up new fields of application for these hybrid ma-
terials.
SiNSs. They can be synthesized by magnesiothermic reduction
[14,15]
of SiO2
or grown on supports such as Ag and Si by chemi-
cal vapor deposition.[8,16,17] Another straightforward and easily
scalable route is chemical exfoliation from CaSi2.[18,19] The Zintl
salt CaSi2 consists of interconnected Si6 rings which form puck-
ered two dimensional, anionic silicon sheets and planar mono-
layers of Ca2+ ions. Through treatment with hydrochloric acid
the Ca2+ ions can be deintercalated giving exfoliated hydride
terminated SiNSs (Figure 1).
Silicon nanosheets (SiNSs) are two-dimensional semiconduct-
ing materials with a layer thickness in the nano-regime and
sheet sizes up to the micro-scale.[1] Like graphene they com-
bine outstanding structural and electronic characteristics. As
the electronic properties of these two materials differ from
each other, SiNSs should hence expand the field of applica-
tions for 2D nanomaterials.
Figure 1. Synthesis of layered polysilanes by chemical exfoliation from CaSi2.
According to theoretical and experimental studies the band
gap of SiNSs is tunable by physical strain,[2] the degree of hy-
drogenation, termination with heteroatoms,[3,4] and surface
functionalization with different substrates.[5,6] Additionally,
SiNSs exhibit unique optical properties such as a layer-thick-
ness-dependent absorption edge[7] and photoluminescence
(PL) emission arising from an enhanced direct band gap transi-
tion.[8,9] Hence SiNSs are promising candidates for microelec-
tronics such as novel field effect transistors (FET)[5,10] and pho-
Due to their structural properties (i.e., sp3 character), hydride
terminated SiNSs are also referred to as layered polysilanes.[20]
They are prone to oxidation under ambient conditions and
must therefore be functionalized to be of further use. Addi-
tionally functionalization breaks up the stacked structure of
non-functionalized SiNSs, rendering freestanding monolayers.
Functionalized SiNSs are dispersible in organic solvents which
facilitates their handling and characterization.
So far only three different methods have been published in
which the functionalization of hydride terminated SiNSs with
organic compounds is described. The reaction with n-alkyl-
amines leads to stable SiÀNÀSi derivatives which exhibit blue
PL,[21,22] while the conversion of SiNSs with the Grignard com-
pound PhMgBr renders (Si6H4Ph2)n layers.[1] Additionally Pt-cata-
lyzed hydrosilylation with 1-hexene enables the functionaliza-
tion of SiNSs and yields stable colloidal dispersions in chloro-
form.[23]
[a] T. Helbich, Dr. I. M. D. Hçhlein, P. Marx, J. Kehrle, Prof. Dr. B. Rieger
Wacker-Lehrstuhl für Makromolekulare Chemie
Technische Universität München, Lichtenbergstrasse 4
85747 Garching (Germany)
E-mail: rieger@tum.de
[b] A. Lyuleeva, Prof. Dr. P. Lugli
Institute for Nanoelectronics, Technische Universität München
Arcisstrasse 21, 80333 Munich (Germany)
[c] L. M. Scherf, Prof. Dr. T. F. Fässler
Lehrstuhl für Anorganische Chemie mit Schwerpunkt Neue Materialien
Technische Universität München, Lichtenbergstrasse 4
85747 Garching (Germany)
Hydrosilylation reactions can be used to react molecular and
surface SiÀH groups with various unsaturated substrates. The
reaction does not take place spontaneously, but needs initia-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201505134.
Chem. Eur. J. 2016, 22, 6194 – 6198
6194
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
tion. On the molecular level it can be mediated by a catalyst,
while on surfaces, thermally or initiator-induced radical reac-
tions are also possible.[24–26] The latter are easily feasible, cost-
efficient and tolerant towards a broad variety of functional
groups. Another advantage is that radical-induced hydrosilyla-
tion is conducted without the introduction of transition metal
impurities which are known to alter the PL properties of silicon
nanomaterials[27,28] and lead to increased surface oxidation.[29]
Despite their advantages, radical-induced hydrosilylation reac-
tions have not yet been examined for the surface functionaliza-
tion of SiNSs.
In this work the reactivity of SiNSs towards unsaturated
compounds with different functionalities was examined
(Scheme 1). The reactions were conducted in the absence of
a catalyst under thermal initiation (1308C) or by the addition
of a diazonium salt as radical initiators with a variety of func-
tional substrates. The resulting hybrid materials were fully char-
acterized by FTIR, NMR spectroscopy, AFM, TGA, XRD and PL
measurements.
The hydride-terminated SiNSs used in this work were synthe-
sized following a literature known procedure.[19] CaSi2 was
chemically exfoliated with HCl (conc.) at À208C for 7 days. To
remove residual oxide the obtained SiNSs were then etched
with hydrofluoric acid and extracted with dichloromethane. As
can be seen from the SiÀH stretching- (ꢀ2100 cmÀ1), scissor-
ing- (ꢀ900 cmÀ1) and wagging bands (ꢀ860 cmÀ1) in FTIR
spectra of the freshly etched SiNSs, the resulting silicon sheets
are hydride-terminated with only slight traces of SiÀO
(ꢀ1100 cmÀ1) and no SiÀOH (Figure 2a).
Figure 2. FTIR spectra of: a) freshly etched SiNSs, b) 1-dodecene and 1-dode-
cene-functionalized SiNSs, c) without the addition of an initiator, d) with 4-
DDB, and e) DT-induced hydrosilylation.
prevent stacking of the SiNSs (Figure S8 in the Supporting In-
formation). Additionally, after the workup strong SiÀO and re-
sidual SiÀH bands, visible in IR spectra indicate, that the func-
tionalization only takes place to a minor extent.
To understand the reactivity of the SiNSs towards unsaturat-
ed carbon compounds, control experiments were conducted
without a radical initiator at room temperature in the dark.
Therefore the HF treated SiNSs (15 mg) were dispersed in
a mixture of dry toluene (2 mL) and 1-dodecene (3 mmol), de-
gassed by three freeze-thaw-pump cycles and subsequently
stirred overnight under argon in the dark.
Neither molecular silanes[30,31] (see the Supporting Informa-
tion) nor hydride terminated silicon surfaces of bulk silicon[32]
or nanostructured silicon[33] show functionalization under these
conditions. Therefore we were surprised to observe a significant
amount of dodecyl groups on the SiNSs in FTIR measurements
(Figure 2c) and TGA (Figure S7 in the Supporting Information).
However this functionalization method is not sufficient to fully
The cause for this unprecedented reactivity is not fully un-
derstood and remains part of the ongoing research in our
group. Potential explanations are the very high surface area
and energy which results in a higher reactivity of the nanoma-
terial[34] or the surface activation by traces of oxygen which
were shown to initiate hydrosilylations on SiNCs.[35]
To achieve a higher degree of surface functionalization, the
SiNSs were subsequently treated with 1-dodecene under radi-
cal initiation conditions (Scheme 1).
Diazonium salts are known to be mild radical initiators for
hydrosilylation reactions on nanostructured silicon.[36] Especially
4-decylbenzene diazonium tetrafluoroborate (4-DDB) is suita-
ble due to its solubility in nonpolar solvents.[26] Additionally
elevated temperatures can initiate hydrosilylation.[35]
Scheme 1. Schematic overview of the executed functionalization methods: Thermal and diazonium salt induced hydrosilylations are applicable on a broad va-
riety of substrates.
Chem. Eur. J. 2016, 22, 6194 – 6198
www.chemeurj.org
6195
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Exfoliated SiNSs (15 mg) were etched with HF (48%), extract-
ed and dispersed in a mixture of dry toluene (2 mL) with 1-do-
decene (3 mmol). After degassing, 4-DDB (15 mmol) was added
and the reaction mixture was stirred at room temperature,
overnight. For thermal functionalization SiÀH-terminated SiNSs
were dispersed in 1-dodecene (1 mL), degassed and subse-
quently stirred at 1308C, overnight.
The mechanism for both initiation conditions is proposed to
proceed by the formation of silyl-radicals.[37] The 4-DDB-in-
duced functionalization occurs through reduction of the diazo-
nium compound, release of nitrogen and hence the formation
of an aryl radical. After deprotonation a silyl surface radical is
formed which then reacts with the unsaturated compound,
leading to the SiÀC bond and a carbon centered radical. Ab-
straction of a neighboring hydrogen radical again forms a sili-
con surface radical and the hydrosilylation proceeds as a radical
chain reaction (Scheme 2a).[26,38]
Additionally we confirmed the 4-DDB and thermally induced
functionalization with 1H NMR experiments in solution
(Figure 3). Due to the restricted freedom of movement of the
organic substrates attached to the Si surface the spectra can
only qualitatively be used to show the successful functionaliza-
tion and purification. The peaks of the double bond (A: d=
5.82 ppm; B+C: d=4.96 ppm) and the methylene group next
to it (D: d=2.04 ppm) have vanished and the spectra still
show the peaks of the terminal CH3 group (F: d=0.85 ppm)
and of the ÀCH2 groups (E: d=1.25 ppm) of the alkyl chain.
Atomic force microscope (AFM) images of the 1-dodecene-
functionalized SiNSs show freestanding monolayers with
a thickness of about 2.0 nm (Figure 4) which is similar to the
Thermally induced hydrosilylation is stated to be dependent
on the reaction temperature. While temperatures ꢁ1508C pro-
vide enough energy to homolytically cleave SiÀH bonds and
thus induce the radical chain propagation (Scheme 2b), for
temperatures between 80–1508C traces of oxygen are pro-
posed to accelerate hydrogen abstraction (Scheme 2c).[35]
After the reaction, the FTIR spectra of the functionalized
SiNSs (Figure 2d and e) show strong Csp3ÀH bands
(ꢀ2920 cmÀ1, ꢀ1460 cmÀ1) while only weak bands of residual
SiÀH (ꢀ2100 cmÀ1) and SiÀO (ꢀ1100 cmÀ1) are present. This
indicates a higher coverage of the SiNSs with dodecyl groups
in contrast to functionalization without radical initiation, which
is also supported by differences in the weight loss of the differ-
ently functionalized material in TGA measurements (Figure S7
in the Supporting Information).
Figure 3. NMR spectra of: a) 1-dodecene, 1-dodecene-functionalized SiNSs
with b) 4-DDB, and c) DT-induced hydrosilylation.
Scheme 2. Proposed mechanism for the: a) diazonium-compound induced mechanism and thermally induced hydrosilylation at temperatures between b) 80–
1508C, and c)ꢁ1508C.[25,35,39]
Chem. Eur. J. 2016, 22, 6194 – 6198
www.chemeurj.org
6196
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
flections of the stacked, non-functionalized sheets decrease
significantly in intensity when the SiNSs are functionalized
without the initiator and even further with radical initiation
(Figure S8 in the Supporting Information). The long dodecyl
chains on the surface seem to stabilize the monolayers and
hinder the SiNSs from stacking, resulting in a less ordered ma-
terial.
Besides the model substrate 1-dodecene, functional sub-
strates are promising candidates for further applications of the
hybrid nanomaterials. Hence, to demonstrate the broad applic-
ability of thermally and diazonium salt-induced hydrosilylations
with hydride terminated SiNSs, a number of functional alkenes
and alkynes were used. In this context we explored the func-
tionalization with 10-undecenoic acid, 1-dodecyne, 9-decen-1-
ol and tert-butyl methacrylate (tBuMA). In the case of tBuMA
thermal hydrosilylation leads to a vast excess of polymerized
alkene. No such side reaction could be observed in the 4-DDB-
induced functionalization of the SiNSs, demonstrating the ad-
vantage of this mild method.
FTIR spectra of the functionalized SiNSs (4-DDB: Figure 5;
DT: Figure S9 in the Supporting Information) with the different
substrates confirm the successful reaction exhibiting the
expected bands such as Csp3ÀH (ꢀ2920 cmÀ1), C=C
(ꢀ1640 cmÀ1), C=O (ꢀ1700 cmÀ1), OH (ꢀ2920 cmÀ1), CÀO
(ꢀ1150 cmÀ1) while only traces of SiÀH (ꢀ2100 cmÀ1), and SiÀ
O (ꢀ1100 cmÀ1) can be detected. SiNSÀC10H20OH and SiNSÀ
C10H20COOH are dispersible in protic solvents such as ethanol
and methanol while for SiNSÀC12H25 and SiNSÀCHCHÀC10H21
nonpolar hydrocarbons such as toluene and benzene are the
solvents of choice. Further reactions are possible with the thus
introduced functionalities on the SiNSs surface. This approach
is currently under investigation and should open up new fields
of applications.
Figure 4. Non-contact mode AFM image of 4-DDB-induced 1-dodecene-
functionalized SiNSs (top) and their line profile (bottom) taken along the
line in the left image.
thickness of the 1-hexene-functionalized SiNSs reported by
Nakano et al.[23]
XRD patterns recorded from the non-functionalized and 1-
dodecene-functionalized SiNSs show that the characteristic re-
The hybrid materials exhibit similar photoluminescence
properties to the non-functionalized materials, namely an emis-
Figure 5. FTIR spectra of functionalized SiNSs by 4-DDB-induced hydrosilylation.
Chem. Eur. J. 2016, 22, 6194 – 6198
www.chemeurj.org
6197
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[4] F. Zheng, C. Zhang, Nanoscale Res. Lett. 2012, 7, 422.
[5] N. Gao, W. T. Zheng, Q. Jiang, Phys. Chem. Chem. Phys. 2012, 14, 257.
[6] Y. Ding, Y. Wang, Appl. Phys. Lett. 2012, 100, 083102.
[7] Y. Liu, H. Shu, P. Liang, D. Cao, X. Chen, W. Lu, J. Appl. Phys. 2013, 114,
094308.
[8] U. Kim, I. Kim, Y. Park, K.-Y. Y. Lee, S.-Y. Y. Yim, J.-G. G. Park, H.-G. G. Ahn,
S.-H. H. Park, H.-J. J. Choi, ACS Nano 2011, 5, 2176–2181.
[9] H. Okamoto, Y. Sugiyama, H. Nakano, Chem. Eur. J. 2011, 17, 9864–
9887.
[10] O. D. Restrepo, R. Mishra, J. E. Goldberger, W. Windl, J. Appl. Phys. 2014,
115, 033711.
[11] Y. Kumai, S. Shirai, E. Sudo, J. Seki, H. Okamoto, Y. Sugiyama, H. Nakano,
J. Power Sources 2011, 196, 1503–1507.
[12] Y. Kumai, H. Kadoura, E. Sudo, M. Iwaki, H. Okamoto, Y. Sugiyama, H.
Nakano, J. Mater. Chem. 2011, 21, 11941.
[13] Y. Kumai, H. Nakano, Jpn. J. Appl. Phys. 2015, 54, 035201.
[14] W.-S. Kim, Y. Hwa, J.-H. Shin, M. Yang, H.-J. Sohn, S.-H. Hong, Nanoscale
2014, 6, 4297–4302.
[15] Z. Lu, J. Zhu, D. Sim, W. Zhou, W. Shi, H. H. Hng, Q. Yan, Chem. Mater.
2011, 23, 5293–5295.
Figure 6. PL spectra of freshly etched and functionalized SiNSs by 4-DDB-in-
duced hydrosilylation.
[16] B. Lalmi, H. Oughaddou, H. Enriquez, A. Kara, S. Vizzini, B. Ealet, B.
Aufray, Appl. Phys. Lett. 2010, 97, 223109.
[17] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M. C. Asen-
sio, A. Resta, B. Ealet, G. Le Lay, Phys. Rev. Lett. 2012, 108, 155501.
[18] O. Bçhm, J. Hassel, Z. Anorg. Allg. Chem. 1927, 160, 152; Hassel, Z.
Anorg. Allg. Chem. 1927, 160, 152.
[19] S. Yamanaka, H. Matsu-ura, M. Ishikawa, Mater. Res. Bull. 1996, 31, 307–
316.
[20] H. Nakano, T. Mitsuoka, M. Harada, K. Horibuchi, H. Nozaki, N. Takahashi,
T. Nonaka, Y. Seno, H. Nakamura, Angew. Chem. Int. Ed. 2006, 118, 6451–
6454; Angew. Chem. 2006, 118, 6451–6454.
[21] H. Okamoto, Y. Kumai, Y. Sugiyama, T. Mitsuoka, K. Nakanishi, T. Ohta, H.
Nozaki, S. Yamaguchi, S. Shirai, H. Nakano, J. Am. Chem. Soc. 2010, 132,
2710–2718.
sion at lmax ꢀ500 nm (Figure 6, Figure S10 in the Supporting
Information). This is in accord with the functionalized SiNCs
which show similar PL for SiÀHÀ and SiÀC-capped SiNCs.[26]
In summary we present the first radical-induced hydrosilyla-
tion approach for the efficient functionalization of hydride ter-
minated SiNSs. High temperatures (1308C) or the addition of
diazonium compounds successfully initiate the reaction. With
this method SiNSs are functionalized with (non-)functional al-
kenes and alkynes, introducing alkyl, hydroxyl-, carboxy- and
ester groups which improve the dispersibility properties. The
free-standing SiNSs were found to have a thickness of about
2 nm and exhibit PL (ꢀ500 nm). Additionally, we demonstrate
that NMR experiments in solution can be used to show the
successful functionalization of the SiNSs. The obtained func-
tional hybrid materials should be valuable for subsequent reac-
tions and open up possibilities for new applications, for exam-
ple, in electronics or optoelectronic devices.
[22] H. Okamoto, Y. Sugiyama, K. Nakanishi, T. Ohta, T. Mitsuoka, H. Nakano,
Chem. Mater. 2015, 27, 1292–1298.
[23] H. Nakano, M. Nakano, K. Nakanishi, D. Tanaka, Y. Sugiyama, T. Ikuno, H.
Okamoto, T. Ohta, J. Am. Chem. Soc. 2012, 134, 5452–5455.
[24] J. G. C. Veinot, Chem. Commun. 2006, 4160–4168.
[25] J. M. Buriak, Chem. Commun. 1999, 1051–1060.
[26] I. M. D. Hçhlein, J. Kehrle, T. Helbich, Z. Yang, J. G. C. Veinot, B. Rieger,
Chem. Eur. J. 2014, 20, 4212–4216.
[27] D. Andsager, J. Hilliard, J. M. Hetrick, L. H. AbuHassan, M. Plisch, M. H.
Nayfeh, J. Appl. Phys. 1993, 74, 4783.
[28] S. Li, I. N. Germanenko, M. S. El-Shall, J. Phys. Chem. B 1998, 102, 7319–
7322.
[29] J. M. Holland, M. P. Stewart, M. J. Allen, J. M. Buriak, J. Solid State Chem.
1999, 147, 251–258.
Acknowledgements
[30] I. Ojima, N. Clos, R. Donovan, P. Ingallina, Organometallics 1990, 9,
3127–3133.
[31] B. Marciniec, in Comprehensive Handbook on Hydrosilylation (Ed.: B.
Marciniec), Pergamon, Amsterdam, 1992.
[32] R. L. Cicero, M. R. Linford, C. E. D. Chidsey, Langmuir 2000, 16, 5688–
5695.
[33] Y. Yu, C. M. Hessel, T. D. Bogart, M. G. Panthani, M. R. Rasch, B. A. Korgel,
Langmuir 2013, 29, 1533–1540.
M. Knaus, R. Csiki, M. Kloberg and K. Melzer are thanked for
their support as well as TUM Graduate School and IGSSE. The
authors thank the DFG for financial support of Alberta/Techni-
sche Universität München Graduate School for Functional
Hybrid Materials ATUMS (IRTG2022). T.H. and L.M.S. are thankful
for their fellowship from Studienstiftung des deutschen Volkes.
I.M.D.H. and L.M.S. gratefully acknowledge the funding from
Fonds der Chemischen Industrie.
[34] E. Roduner, Chem. Soc. Rev. 2006, 35, 583–592.
[35] Z. Yang, M. Iqbal, A. R. Dobbie, J. G. C. Veinot, J. Am. Chem. Soc. 2013,
135, 17595–17601.
[36] D. Wang, J. M. Buriak, Langmuir 2006, 22, 6214–6221.
[37] M. Woods, S. Carlsson, Q. Hong, S. N. Patole, L. H. Lie, A. Houlton, B. R.
Horrocks, J. Phys. Chem. B 2005, 109, 24035–24045.
[38] J. M. Buriak, Chem. Rev. 2002, 102, 1271–1308.
[39] I. M. D. Hçhlein, J. Kehrle, T. K. Purkait, J. G. C. Veinot, B. Rieger, Nano-
scale 2015, 7, 914–918.
Keywords: hydrosilylation
nanotechnology · silicon nanosheets · surface chemistry
·
layered
polysilane
·
[1] Y. Sugiyama, H. Okamoto, T. Mitsuoka, T. Morikawa, K. Nakanishi, T.
Ohta, H. Nakano, J. Am. Chem. Soc. 2010, 132, 5946–5947.
[2] F. Li, R. Lu, Q. Yao, E. Kan, Y. Liu, H. Wu, Y. Yuan, C. Xiao, K. Deng, J. Phys.
Chem. C 2013, 117, 13283–13288.
Received: December 22, 2015
[3] T. H. Osborn, A. a. Farajian, O. V. Pupysheva, R. S. Aga, L. C. Lew Yan
Voon, Chem. Phys. Lett. 2011, 511, 101–105.
Published online on March 15, 2016
Chem. Eur. J. 2016, 22, 6194 – 6198
www.chemeurj.org
6198
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim