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Local Atomic Configuration in Laser Synthesized Si/C/N Powders Studied by X-Ray Photoelectron
Spectroscopy
A. Gheorghiu, G. Dufour, C. Sénémaud, N. Herlin, E. Musset, M. Cauchetier, X. Armand
To cite this version:
A. Gheorghiu, G. Dufour, C. Sénémaud, N. Herlin, E. Musset, et al.. Local Atomic Configuration in Laser Synthesized Si/C/N Powders Studied by X-Ray Photoelectron Spectroscopy. Journal de Physique III, EDP Sciences, 1997, 7 (3), pp.529-535. �10.1051/jp3:1997140�. �jpa-00249598�
Local Atomic Configuration in Laser Synthesized Silc IN
Powders Studied by X-Ray Photoelectron Spectroscopy
A. Gheorghiu
(~j, G. Dufour (~), C. S6nAmaud (~), N. Herlin (~>*), E. Musset (~),
M. Cauchetier and X. Armand (~)
(~) Laboratoire de Chimie-Physique (**), Universit4 Pierre et Marie Curie,
ii rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
(~) CEA DRECAM, Service des Photons Atomes et Mo14cules, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
(Received 6 May 1996, re~.ised and accepted 9 December1996)
PACS 79.60 -I Photoemission and photoelectron spectra
PACS 61 43 -j Disordered solids
PACS 81.05.Ys Nauophase materials
Abstract. Nanometric Si/C IN powders have been obtained from the laser synthesis of a
liquid precursor hexamethydisilazane The chemical composition of the products is a function of the experimental parameters: the C/N ratio in the powders is controlled m the range 0.34 to 134 by varying the ammonia content in the flowing g~s (argon). This paper presents some
results obtained by X-ray Photoelectron Spectroscopy (XPS) about the local atomic structure of these powders. The evolution of the powders after annealing treatments at 1500 and 1600 °C
under nitrogen atmosphere has been studied. The chemical composition deduced from KPS
wide spectra are compared with chemical analysis. From the analysis of the Si-2p core level, information about the local atomic bonding around silicon atoms are obtained. The different
environments present in the system and their evolution with temperature have been deduced from the comparison with data obtained for stoichiometric compounds (SiC, S13N4 and Si02).
The existence of local chemical disorder is shown in the as-formed powders. For powders with
C/N ts 0.6, the samples remains amorphous until 1500 °C, crystallization which starts around 1500 °C is total at 1600 °C The existence of C-N bonds at 1500 and 1600 °C is evidenced This
result is m good agreement with results obtained by neutron diffraction and X-ray absorption (XAS).
Introduction
Since several years, there has been a growing interest in the production of nanoceramic powders for different applications [Ii. Silicon based powders are expected to find application in the field of ceramics materials with improved thermomechanical properties [2].
Among the different methods for producing nanosized powders, the laser synthesis appears as
a powerful method. Since the first experiments by Haggerty [3], a wide variety of powders has been obtained (Si, SiC, S13N4, BN. .) [4j. In this method, the reaction zone is well confined and
(* Author for correspondence (e-mail. herlin©drecam cea.fr)
(** URA CNRS 176
© Les (ditious de Physique 1997
530 JOURNAL DE PHYSIQUE III N°3
it allows the production of particles with controlled chemical composition; size, crystallinity
and with a high chemical purity. Most often, gaseous precursors are used but recently the versatility of the method has been improved by introducing liquid precursors [5,6].
In the past few years, several studies have concerned the investigation of local order in nanometric Silc/N powders obtained from gaseous or liquid precursors [7-10j but to our
knowledge few studies have dealt with the problem of the evolution of the local order in these
powders as a function of annealing treatments ii1-lsj.
In a previous paper, the possibility to produce nanometric amorphous or crystalline Silc/~T powders from gaseous mixtures has been shown and the study of local order by XPS IX-ray
Photoelectron Spectroscopy) and EXAFS (Extended X-ray Absorption for Fine Structure) has revealed that the powder is not a simple mixture of SIC and S13N4 tetrahedra but is composed of mixed Si-Cz-N4-z tetrahedra [7j.
More recently, amorphous Silc/N powders with controlled chemical composition (0.3 < C/N < 13 (at)) have been produced using a liquid precursor [16j. The chemical composition of the powder is a function of the chemical composition of the reactive mixture.
It has been shown that the crystallization of samples with C/N ts 0.6 occurs at higher tem-
perature compared to samples with a high or low C/N ratio ii?]. It would be interesting to understand if this chemical composition is related to a special chemical environment. Such information can be obtained from XPS measurements. This paper is focused on the evolution of the chemical analysis and the environment around silicon atoms in two powders as a function of annealing treatments under nitrogen atmosphere. The results obtained by bulk chemical
analysis and XPS are compared. Information on the local bonding are compared to the results
presented in other studies.
1. Experimental Procedures
The experimental set-up used for the production of Silc IN powders has been described else- where [6j. Briefly, a liquid precursor (hexamethyldisilazane. HMDS, (CH3)3SiNHSi(CH3)3) is
placed in a glass jar. An intense beam of ultrasounds is focused at the surface of the liquid which generates an aerosol. The aerosol droplets are carried out to the reaction zone by a
gaseous flow containing argon and ammonia. The relative concentration of ammonia increases
in six successive experiments labeled from HMDS40 to HMDS 45 [16j The powders HMDS
40-45 are collected under air and stored in a glove box under argon atmosphere after pumping under vacuum.
The annealing treatments were performed in a graphite oven (Pyrox) under nitrogen at-
mosphere at temperatures up to 1600 °C. The heating ramp is 670 °C/hour and the divell duration is 4 hours.
The bulk chemical analysis were achieved by conventional methods giving a relative uncer-
tainty (Ant /ni) equal to 2% for C, N and O elements, 3% for Si.
The photoelectron spectra were excited by using a lion-monochromatized Mg Ka source hv
= 1253.6 eV and analyzed with an electrostatic hemispherical analyzer used in fixed trans-
mission mode (FAT). Wide spectra in the binding energy range 0-1000 eV and core level
spectra Si-2p were recorded with 80 elf and 10 eV band-pass respectively The spectra were scanned with a I eV and 0.I eV step respectively.
The samples were pressed pellets prepared with HMDS 43 and 44 powders and introduced in the spectrometer under inert atmosphere.
Let us note that in XPS measurements. the probed thickness is limited to 5-10 nm. For
example, if the grain size is 30 nm, the probed volume is 20% of the total volume.
j ~~~°KLL
~ ~ ~ ~
~~~~~~
~
1500°C
(
as formed
1000 800 600 400 200 0
Binding energy (eV)
Fig 1 XPS wide spectra of as-formed and annealed HMDS 43 sample as-formed sample, after annealing at 1500 °C, after annealing at 1600 °C
Table Chemical coInposition of as-formed and annealed HMDS $3 and II saInpies.
analysis XPS measurement N
43 31.
1500 °C 40 21 37 2 31.6 32.6 28.2 7.6
1600 °C 45 27 27 1 35.3 29.8 15-2 19.6
31.4 20.2 35.7 12.7 23.8 41.9
1500 °C 41 16 41 2 30.0 28.7 28.6 12.7
1600 °C 45 20 34 1 36 6 28.9 16 0 18A
2. Results and Discussion
Let us recall that for CIN = 1.34 and 0.34 (samples HMDS 40 and 45) the chemical composition of the sample is strongly modified by annealing treatment: sample HMDS 40 moves towards silicon carbide while HIVfDS 45 moves towards silicon nitride. On the contrary, for intermediate compositions (samples HMDS 43-44), annealing treatments at 1500-1600 °C do not strongly modify the ratio C/N. After crystallization the chemical composition is modified and the major phase is silicon carbide in sample HMDS 43 and there is more silicon nitride in sample
HMDS44 [12j.
2. I CHEMICAL COMPOSITION. Figure I reports the XPS wide spectra of HMDS 43 samples before and after annealing treatments at 1500 and 1600 °C. An estimation of the chemical composition of the different samples can be deduced from these spectra. The results are
presented in Table I together with the measurements obtained from chemical analysis. A first remark concerns the chemical composition of the as-formed powders. The chemical analysis
532 JOURNAL DE PHYSIQUE III N°3
gives a hydrogen content in the range 25-30% (at% for both HMDS 43 and 44 samples in good qualitative agreement ~vith neutron diffraction data [18] and infra-red spectra where Si-H, C-H and N-H bonds are identified iiIi For an easier comparison with XPS results, the hydrogen
content has not been included in Table I and the sum of silicon, carbon, nitrogen and oxygen
atoms has been assumed to be 100%.
The most striking result shown by Table I is that the carbon content is always higher by XPS than by chemical analysis It must be noted than in the case of XPS measurements a contamination by hydrocarbonated species is often observed. Nitrogen content is al~vays
smaller from XPS measurements than by chemical analysis. As concerned silicon atoms, the difference between the two measurements remains constant. Qualitatively, the evolution of bulk and surface chemical composition with heat treatment is consistent: the nitrogen content
decreases and silicon content increases with increasing temperature which is in good agreement with mass-coupled TGA measurement [15]
The as-formed samples are very sensitive to oxygen contamination (Tab. I) Before chemical
analysis, they were exposed to air. On the contrary, before XPS measurement the powders
were stored under argon and exposed to air less than one hour. It can explain that for both as- formed powders the oxygen contamination measured by XPS is weaker than the one obtained from chemical analysis (Tab. I). This result is strikingly different from previous data [13] where the oxygen content measured by XPS is always much higher than by chemical analysis. In this last case the powders were obtained from the gas phase.
After annealing treatment, the powders were stored under air. So it is not surprising that surface contamination (XPS) is very high compared with bulk measurements. In the as-formed
powders, the high XPS carbon measurement can be partially attributed to the presence of carbon structures due to the formation of carbonated radicals during the pyrolysis process [15].
After annealing at 1500 °C, the XPS measurements for carbon content are still higher than the bulk measurements. At this temperature, HRTEM (High Resolution Transmission Electron
Microscope) pictures show the formation of carbon planes at the surface of the grains (m 2 nm
thick) in qualitative agreement with our XPS measurements which concern the superficial layer.
At 1600 °C, carbon is mostly incorporated in the SiC crystallized grains and the agreement between the measurements becomes better.
2.2. LOCAL BONDING. In HMDS 43-44 powders, information on the local bonding around silicon atoms has been obtained from the analysis of Si-2p core level In order to get more detailed data, a mathematical decomposition of spectral lines into a sum of Voigt functions has been performed, as previously published [7]. It has been assumed in each case that the
different components have BE (Binding Energy) close to those of stoichiometric compounds:
Si, SiC, S13N4 and Si02 species.
Figures 2a and 2b report the Si-2p lines for as-formed and annealed (1500 and 1600 °C)
HMDS 43 and 44 samples. Strong modifications of the spectra are observed after heat treat- ments.
In as-formed HMDS 43 sample (Fig. 2a), both Si-C4 and Si-N4 groups are present. After annealing the intensity ratio Si-C4 to Si-N4 increases appreciably and the peaks become sharper. The Full Width at Half-Maximum (FWH~) of the components of the Si-2p peak
decrease by about 15% between as-formed and annealed powders. This result reveals that a strong chemical and topological disorder is present m the as-formed powder, it decreases after
annealing. After heat treatment, a non negligible peak is present at BE higher than 103 eV, which can be attributed to S102.
In as-formed HMDS 44 sample (Fig. 2b), the presence of Si-N4 groups dominates the spec-
trum. After heat treatment, an increase of the Si-C4 to Si-N4 ratio is observed. After
4
2 SiO~
Si 2p H43 Si 2p H44
1600°C 1600°C
Iu ~
# a
$ #
f cWE
~ ~
# 1500°C )
j &[
( o
Z
104 100 104 100
a) Binding energy (evl
~j Binding energy (evl
Fig. 2. Si-2p XPS core level spectra of as-formed and annealed (1500 and 1600 °C) powders The dots correspond to experimental measurements. The continuous lines (Voigt functions) correspond to the contributions of the different sites of silicon. a) HMDS 43. b) HMDS 44.
annealing at 1600 °C, the contribution of Si-C4 becnmes more intense than Si-N4.
As previously noted for HMDS43 sample, the lines become sharper after annealing than for as-formed powders. This is consistent with a chemical reordering of the system.
The contribution of Si02 oxide is more important after heat treatment than for as-formed powders.
Let us remark than for 1600 °C samples, the FWHM of the SiC component is close to the value measured for well organized fl-SiC [7].
The study of C Is and N Is lines confirms that as-formed powders correspond to a chemically
disordered network. It is noteworthy that after annealing at 1500 °C both N-Si and N-C bonds are present m HMDS 43 and 44 samples. Around C atoms, C -C and C-N bonds can be observed in both samples at 1500 °C. After annealing at 1600 °C, N-S13 and C-S14 bonds
corresponding to stoichiometric compounds are dominant.
534 JOURNAL DE PHYSIQUE III N°3
Conclusion
In this paper, the local chemical environment of Si/C IN samples with a CIN ratio close to 0.6 has been studied. These samples have been chosen because they remain amorphous at tem-
perature higher that 1500 °C. In as-formed powders, the structure is highly disordered. This result is in good agreement with the presence of mixed tetrahedra observed in these systems by EXAFS [19] and Nuclear Magnetic Resonance (NIVIR) [15], a more detailed comparison will be undertaken. The CIN ratio of these samples is not appreciably modified by annealing up to 1500 °C. Although the chemical composition of the two as-formed samples is very close, the local chemical environment is rather different. After annealing treatment at 1500 and 1600 °C under nitrogen atmosphere, a strong evolution of the SiC4 to SiN4 ratio is observed. The proportion of SiC4 groups increases noticeably with temperature in both samples.
A reordering of the network occurs under annealing as revealed hy the sharpening of the
Si-2p XPS components. Let us note than m annealed sample, the presence of C-N bonds is evidenced by XPS, XAS, and neutron diffraction II?,19]. A detailed XPS analysis of carbon and nitrogen bonding in these systems will be published elsewhere.
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