UV laser-induced photolysis of 1,3-disilacyclobutane in oxygen for chemical vapour deposition of nano-sized polyoxocarbosilane films

Article abstract of DOI:10.1016/S0022-328X(01)01183-4

The multi-pulse ArF laser irradiation into gaseous 1,3-disilacyclobutane-O₂ mixture in excess of buffer gas occurs as non-explosive chemical vapour deposition of solid methylsilicone films, whereas the single-pulse irradiation into the gaseous

Full text of DOI:10.1016/S0022-328X(01)01183-4

Journal of Organometallic Chemistry 640 (2001) 170–176
www.elsevier.com/locate/jorganchem
UV laser-induced photolysis of 1,3-disilacyclobutane in oxygen for
chemical vapour deposition of nano-sized polyoxocarbosilane films
a,
a
b
c
&
Josef Pola *, Josef V´ıtek , Zdeneˇk Bastl , Jan Subrt
a Laser Chemistry Group, Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Roz6ojo6a Str. 135,
165 02 Prague 6, Czech Republic
b J. Heyro6sky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 182 23 Prague 8, Czech Republic
c
&
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25 068 Rezˇ near Prague, Czech Republic
Received 11 June 2001; accepted 10 August 2001
Dedicated to Professor M.G. Voronkov in recognition of his important contributions to organosilicon chemistry
Abstract
The multi-pulse ArF laser irradiation into gaseous 1,3-disilacyclobutane–O2 mixture in excess of buffer gas occurs as
non-explosive chemical vapour deposition of solid methylsilicone films, whereas the single-pulse irradiation into the gaseous
mixture in the absence of buffer gas results in explosive chemical vapour deposition of solid nano-structured polyoxocarbosilanes
poor in hydrogen. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: 1,3-Disilacyclobutane; Silene; Oxidation; Laser photolysis; Nanostructured polyoxocarbosilane
1. Introduction
split to silenes RHSiꢀCH2 in equilibrium with the iso-
meric silylenes: Si(R)CH3.
There is continuing interest in Si:C:O:H and Si:O:H
phases due to their importance in applied materials
research [1,2]. They can be produced by pyrolysis of
polysiloxanes [3] or polysiloxane gels [4], by pyrolytic
laser–aerosol interaction [5] and by plasma- [6] or laser-
[7] interaction with gaseous organosilanes, whereas the
Si:O:H phases can be formed in plasma of silane and
oxygen [2], by oxidation of Si:H films [8] and by UV
laser- [9] or IR laser- [10] induced decomposition of
disiloxane.
The photolytical or thermal reaction of both species
with 3O2 at low-temperatures of cryogenic matrices
[12–16] yields reactive oxygen-containing organosilicon
compounds like dioxasiliranes, silaesters, siladioxetanes
and silanones. The oxidation of silenes generated in the
gas phase by conventional low-pressure thermolysis of
1,1-dimethyl-1-silacyclobutane yields cyclic poly-
(dimethylsiloxanes) (Me2SiO)n (n=3, 4) and oxacar-
bosilanes [(Me2SiO)n(Me2SiCH2)m (n, m=1, 2) along
with formaldehyde and ethene and was interpreted [17]
as in Scheme 1.
Another route to these materials may be gas-phase
decomposition of silacyclobutanes or 1,3-disilacyclobu-
tanes (DSCB) (I)
We have reported that IR laser-induced thermolysis
and UV laser-induced photolysis of various silacyclobu-
tanes and DSCB in the gas phase yields transient silenes
(M=C, Si; R=alkyl, H) conducted in the presence of
molecular oxygen. These silacycles are known [11] to
* Corresponding author. Tel.: +420-2-203-90308; fax: +420-2-
209-20661.
E-mail address: pola@icpf.cas.cz (J. Pola).
Scheme 1.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01183-4

J. Pola et al. / Journal of Organometallic Chemistry 640 (2001) 170–176
171
programmed (20–150 °C) temperature, He carrier gas)
and also by comparison of their GC retention times
with those of authentic samples.
The depletion of DSCB was followed using a diag-
nostic band at 779 cm−1 and formation of ethyne and
methane was observed at 730 and 1305 cm−1, and that
of formaldehyde, MeOH and formic acid at 1745, 1034
and 1775 cm−1, respectively.
Quantitative GC analyses were performed using the
FID response factors as follows: hydrocarbons (1.0),
formaldehyde and formic acid (0.01) and methanol
(0.23). The sampling was conducted using a gastight
syringe.
The FTIR spectra of the deposits on KBr windows,
as well as on KBr substrates were recorded after evacu-
ation of the reactor.
Fig. 1. Pyrex reactor. 1, KBr window; 2, quartz window; 3, rubber
septum; 4, PTFE valve; 5, substrate.
(and silylenes) that, perhaps due to being generated in
high concentrations, do not cyclodimerize, but effi-
ciently gas-phase polymerize into solid saturated poly-
carbosilane films [18–22].
It was of further interest to examine one of these
reactions in the presence of oxygen and determine in
which way oxygen intervenes with formation of poly-
carbosilanes. Here we present the examination of ArF
laser-induced photolysis of DSCB [18] in the presence
of molecular oxygen and show that this reaction affords
solid polyoxocarbosilane deposit with H content af-
fected by the presence of buffer gas.
The X-ray photoelectron spectra were acquired using
a VG ESCA 3 Mk II electron spectrometer employing
Al–Ka radiation. X-ray excited Auger electron spectra
were obtained using bremsstrahlung radiation. High
resolution spectral scans were taken over Si 2p, Si KL23
L23, C 1s and O 1s regions. Pressure of residual gases
during spectra measurement did not exceed 10−6 Pa.
Quantification of the element surface concentration ra-
tio was accomplished by correcting integral intensities
of photoelectron spectra for pertinent cross-sections
[23] and accounting for the dependence of electron
analyzer transmission on the kinetic energy of electrons
[24].
2. Experimental
Laser photolysis experiments were carried out on
gaseous mixture of DSCB and O2 (both 0.7 kPa) in the
absence or at atmospheric pressure of helium or nitro-
gen using an ArF (ELI 94 model) laser operating at 193
nm with a repetition frequency of 1 Hz. The samples
were irradiated in a Pyrex reactor consisting of two
orthogonally positioned tubes (both 3 cm in diameter),
one (13 cm in length) furnished with KBr windows and
the other (9 cm in length) equipped with quartz win-
dows. The reactor had two side arms, one fitted with a
rubber septum and the other connecting to a standard
vacuum manifold (Fig. 1). The reactor accommodated
metal and KBr substrates, which, covered with the solid
material deposited in the course of irradiation, were
transferred for measurements of their properties by
photoelectron and FTIR spectroscopy as well as elec-
tron microscopy. The metal substrates were washed by
acetone in an ultrasonic bath. The laser beam at fluence
of 60 mJ effective on area of 1.7 cm2 was used.
Scanning electron microscopy of the deposits was
carried out with a Tesla BS 350 microscope and trans-
mission electron microscopy measurements were per-
formed on
a Philips 201 microscope. Materials
deposited on Al substrates were analyzed by SEM
technique and when scrubbed from the substrate and
held with a copper grid by TEM analysis.
DSCB was synthesized according to earlier reported
procedure [25] and its purity was better than 99%.
Compounds used as the standards for the analysis of
the FTIR spectra of the irradiated DSCB–O2–He mix-
ture were from the laboratory stock (CO, CH3OH) or
freshly prepared [26] by standard techniques (H2CO,
HCOOH). Their purity was checked by FTIR spec-
troscopy and gas chromatography.
3. Results and discussion
The progress of photolysis was monitored directly in
the reactor by infrared spectroscopy (an FTIR spec-
trometer Nicolet Impact 400) and also, after admitting
helium into the reactor to atmospheric pressure, by gas
chromatography (a Shimadzu 14A chromatograph with
Porapak P or SE-30 columns, programmed (20–150
°C) temperature, He carrier gas). The gaseous products
were identified by FTIR and GC–MS spectrometry (a
QP 1000 Shimadzu spectrometer, Porapak P column,
Gaseous DSCB exerts UV absorption above 190 nm
[18] with absorptivity 18×10−2 kPa−1 cm−1 at the
irradiating wavelength of ArF laser (193 nm). The
energy delivered by the 193 nm photons corresponds to
ca. 620 kJ mol−1 and is in excess of the activation
energy for cleavage of the four-membered ring of
DSCB (230 kJ mol−1, Ref. [27]) to yield silene. The
laser irradiation of gaseous DSCB–O2 mixture in the

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J. Pola et al. / Journal of Organometallic Chemistry 640 (2001) 170–176
Fig. 2. Typical FTIR spectrum of the initial DSCB–O2 mixture (a), DSCB–O2–He mixture after 300 pulses (b) and DSCB–O2 mixture after
single pulse (c) photolysis.
absence of inert gas (N2 or He) takes place as an
explosive reaction which is accomplished within a single
laser pulse, whereas the irradiation of the DSCB–O2
mixture in excess of helium or nitrogen is a much
slower process in which about 50% of DSCB depletes
with as many as 300 pulses. A discussion for each
mixture is given below.
3.3. Properties of deposited materials
X-ray photoelectron spectroscopy study of the solid
deposits from the explosive and non-explosive photoly-
sis confirms that both are composed of silicon, carbon
and oxygen and that their stoichiometry is
Si1.00C1.06O1.11 and Si1.00
C 0.90O1.33, respectively. The
observed values of Si 2 p core-level binding energies
(both 102.690.2 eV, Fig. 4), the calculated value of the
Auger parameters (both 1711.790.2 eV, Fig. 5), and
also the values of C 1s core level binding energies (both
284.790.2 eV) are consistent with data reported in the
literature for polymethylsilicone [28] and methyl(hy-
3.1. Multi-pulse non-explosi6e photolysis
The irradiation of the DSCB–O2 (each 0.7 kPa) in
excess of inert gas (He or N2, total pressure 53 kPa)
does not show any visible luminescence. It yields (in
relative molar per cent) formaldehyde (65–80),
methanol (5–10), formic acid (8–10), ethene (2–5) and
methane (2–3) as the volatile products detectable by
FTIR (Fig. 2) and GC–MS spectra and quantified by
GC analysis (Fig. 3). It also results in the deposition of
a whitish solid film. Typically, ca. 50% of DSCB was
depleted in as many as 300 pulses.
3.2. Single-pulse explosi6e photolysis
The irradiation of the DSCB–O2 (each 0.7 kPa) with
the single pulse results in an explosive reaction. A
whitish luminescence is observed and DSCB is com-
pletely depleted. Typical IR spectral changes are given
in Fig. 2. Ethyne, methane and carbon monoxide were
detected by FTIR spectra. GC and GC–MS analysis
revealed that the hydrocarbons are produced in ratios
C2H2:CH4:C2H4=1:0.5:0.03 (Fig. 3). Concomitantly,
the whitish solid film was deposited onto the reactor
surface.
Fig. 3. Typical GC trace of DSCB–O2–He mixture after 300 pulses
(a) and DSCB–O2 mixture after single pulse (b) photolysis. Porapak
P column, peak designation: 1, CH4; 2, C2H4; 3, C2H2; 4, H2CO; 5,
CH3OH; 6, HCO2H.

J. Pola et al. / Journal of Organometallic Chemistry 640 (2001) 170–176
173
consists of bands at w [cm−1] (relative absorbance) 760
(0.36), 862 (0.64), 930 (0.50), 963 (0.40), 1058 (1.0),
1258 (0.16), 2148 (0.50), 2233 (0.04) and 2924 (0.04)
which are assignable, in the given order, to w(SiꢁC),
l(SiHx), w(SiꢁO) (possibly contributed by w(SiCH2Si),
w(CꢁO), l(CH3Si), w(SiꢁH) and w(CꢁH) vibrations. The
latter shows a broad (major) band at 1087 cm−1 along
with weaker bands at 875 and 844 cm−1 of relative
absorbance 0.34. This pattern is in keeping with the
presence of SiꢁO and SiꢁC bonds and the absence of
SiꢁH and CꢁH bonds.
It is known that the oxygen and HnSi bending modes
below 1000 cm−1 in amorphous SiOx:H films (xB2)
and oxygenated polysilanes [29–32] are less informative
[33], while the SiꢁH stretching mode in polysilanes is
empirically related to the electronegativity/inductive ef-
fect of the neighbouring atoms [34,35]. It is also known
that fully oxidized (SiH2)x polysilanes exhibit [36] bands
at 2190 and 2245 cm−1. The bands at 2148 and 2233
cm−1 observed in the deposited solid from the non-ex-
plosive photolysis (Fig. 2b) can thus be assigned to the
H2Si(O) structural unit. The estimation of the relative
content of the SiꢁO and SiꢁH bonds by comparing the
Fig. 4. Photoelectron spectra of Si(2p) electrons. Deposit from single-
pulse explosive photolysis (1) and multi-pulse non-explosive photoly-
sis before (2) and after (3) Ar+ ion sputtering.
ASiꢁH:ASiꢁOꢁSi ratio in the deposit (0.5) and in cyclote-
trasiloxane (H2SiO)4 (ca. 0.5 [37]) is in line with an
important contribution of the (H2SiO)n formulae and
supports this view. The lack of bands at ca. 2100 and
2000 cm−1 indicates [36,38] the absence of (SiH2)n and
(SiH)n structures in which the SiꢁH bond is isolated
from oxygen by at least two silicon atoms.
Fig. 5. Spectra of Si (KL23L23) Auger electrons. Designation as in
Fig. 4.
The films deposited from the non-explosive photoly-
sis are more adhesive to glass, KBr and metal (Cu, Al)
and show different morphology than those produced in
the explosive photolysis (Fig. 7). TEM analysis reveals
that the solids produced during the explosive reaction
are composed of nano-structured agglomerates with
units size of 10 nm and of nanorods having 10–20 nm
diameter and ca. 200 nm length.
The materials from the explosive photolysis are not
soluble in common organic solvents (methanol, diethyl
ether, toluene, chloroform), whereas those from the
non-explosive photolysis are slightly soluble in toluene.
These features indicate that the materials have high
molecular weight and/or a cross-linked structure.
Fig. 6. Typical FTIR spectrum of the solid deposit from the single-
pulse (a) and multi-pulse (b) photolysis.
3.4. Hypothetical photolytic steps
drido)silicone [7] phases. Occurrence of the additional,
weak component in the spectra of the sample sputtered
by argon ions is most likely due to the inhomogeneity
of the static charging of this sample. This explanation is
corroborated by the fact that both components give the
same value of the Auger parameter thus showing that
identical chemical states of Si are involved.
3.4.1. Multi pulse non-explosi6e photolysis
The observation of formaldehyde is in accord with
the reaction scheme in which silene produced by DSCB
photolysis [10] undergoes addition of O2(3S) and yields
unstable siladioxetane which decomposes into formal-
dehyde and silanone [17]. The formation of formic acid
and methanol indicates partial oxidation of H2CO, as
well as its reaction with hydrogen arising from UV
photolytic dehydrogenation [9] of H2SiO species and/or
FTIR spectra of the solids from the non-explosive
and explosive photolysis differ (Fig. 6). The former

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J. Pola et al. / Journal of Organometallic Chemistry 640 (2001) 170–176
Fig. 7. SEM of solid deposit from non-explosive photolysis (a) and SEM (b) and TEM (c) of solid deposit from explosive photolysis. Al substrate,
film thickness ꢀ1 mm.

J. Pola et al. / Journal of Organometallic Chemistry 640 (2001) 170–176
175
of oligomeric (H2SiO)n. The gaseous silanone H2SiꢀO is
theoretically predicted to undergo no energy barrier
polymerization [39] and isomerization [40–42] to hy-
droxysilylene H(HO)Si: which it may react with to
produce hydridosilicone films [10].
in the oxidation of silenes and silylenes are highly
exothermic [15,16]. Their occurrence together with the
absence of cooling effect of buffer gas make the ho-
molytic contributions highly possible. We note similar
explosive character of gas-phase laser-driven oxidation
of tetrafluoroethene [45].
However, the observed composition of the solid films
shows the important incorporation of carbon. This can
primarily originate from reaction of silene with silanone
and from insertion of silene [11] into (H2SiO)n products,
but also from reactions [11] of intermediary silene
H2SiꢀCH2 with the minor nucleophilic products
(CH3OH, H2CO and HCO2H). The plausibility of the
cycloaddition of silene with H2CO is supported by the
occurrence of ethene, the product not observed in the
ArF laser photolysis of DSCB in the absence of oxygen
[18]. Other highly probable reactions leading to the
final solid methylsilicone product are insertion and
addition reactions of methylsilylene [43,44] (the isomer
of silene) to silanone and (H2SiO)n oligomers. Finally,
oxidation [16] of methylsilylene via dioxasilirane to
carbonyl compounds (CH3Si(O)OH or HSi(O)OCH3)
can also play a role. These reactions, except the last
one, are shown in Scheme 2.
4. Conclusion
The ArF laser photolysis of DSCB–O2 mixtures in
the presence buffer gas (N2, He) is a complex reaction
yielding formaldehyde, methanol, formic acid and
methane together with solid methylsilicone deposit. It is
judged to involve oxidation of silene and cleavage of
intermediary dioxetane and can be used for chemical
vapour deposition of methylsilicone films.
The ArF laser photolysis of DSCB–O2 mixtures in
the absence of the buffer gas occurs as a single-pulse
explosive reaction that yields methane, ethyne, ethene
and carbon monoxide along with unique solid nanos-
tructures of Si:C:O phase which can find use in applied
science.
3.4.2. Single pulse explosi6e photolysis
The observation of CH4, C2H2 (a typical high tem-
perature product) and CO together with the polycar-
bosilane deposit poor in H is in keeping with the view
that the paths assumed for the multi-pulse photolysis
are accompanied with high energy demanding ho-
molytic routes. The homolyses possibly include forma-
tion of methane via cleavage of intermediary species
with CH3ꢁSi bond, dehydrogenation of formaldehyde
into carbon monoxide and dehydrogenation of methyl-
silicone products. It is known that some steps occurring
Acknowledgements
The research was supported by the Ministry of Edu-
cation, Youth and Sports of the Czech Republic (Pro-
gram COST, No. OC 523.60) and by GACR (grant no.
203/00/1288).
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