Radiation damage of mesoporous silica thin films monitored by X ray reflectivity and
Scanning electron microscopy
S. Dourdain1, X. Deschanels1, G. Toquer1, C. Grygiel2, I. Monnet2, S. Pellet-Rostaing1, A.
Grandjean1
1
Institut de Chimie Séparative de Marcoule, UMR5257 CEA-CNRS-UM2-ENSCM, 30207 Bagnols
sur Cèze, France
2
CEA CNRS ENSI CAEN, CIMAP GANIL, F-14070 Caen 5, France
Abstract
An innovative approach to investigate the effects of irradiation on mesoporous structure is
proposed from the characterisation of thin mesoporous silica films by X-Ray Reflectivity
measurements coupled with Scanning Electron Microscopy observations. Two different
geometries of mesoporous thin films were irradiated with Xe ion beam. A collapse or a
complete destruction of the porous structure depending on the pristine pore structure and an
increase of the porosity attributed to track formation are discussed.
Keywords: mesoporous thin film, irradiation, X ray reflectivity
Introduction
Two main solutions are usually proposed for conditioning nuclear wastes. For low level activity
wastes, contaminated particles or pieces are immobilized by cementation or bitumen processes, while
for high level activity wastes, radionuclides need to be incorporated in the network of mineral matrices
such as ceramics or glasses. Borosilicate glasses are able to immobilize a wide range of extremely
complex waste compositions, but they are obtained by a melting process at a temperature close to
1200°C that can be problematic for volatile species such as iodine or caesium. It is thus of interest to
develop new, low temperature, process to incorporate mobile species into inorganic matrices.
Over the past two decades, the development of nanoscale composites and mesoporous materials,
synthesized under mild conditions, provided new possibilities for encapsulating nuclear wastes [1-4].
These relative soft synthetic routes could be an alternative way to develop solids able to entrap volatile
species. Moreover, their porosity can be closed under soft conditions like mild thermal treatment of
suited chemical stresses, in order to use these solids as conditioning matrices. This ability of
mesoporous structure to collapse under given stresses is thus of great advantage regarding the foreseen
applications, but it also raises the question of the behaviour of such materials under irradiation.
For bulk or porous materials, the interaction of an ionized particle with a solid is expected to impact
the solid in two different ways: on one hand the incident particle transfers its energy to the electrons of
the target (resulting in ionization and electronic excitations processes), on the other hand its energy is
directly transferred to some nuclei of the target, leading to ballistic processes. None of these processes
are exclusive and their relative intensity is mainly directed by the properties of the incident particle
(mass, charge, velocity). In this paper we are interested in the first process. For low electronic stopping
powers, it leads to a temperature rise in the materials, and induce under certain conditions several
atomic displacements. When the electronic stopping power increases, the energy is predominantly
deposited through inelastic interactions and the resulting intense electronic excitation produces a
narrow tail of permanent damage along the ion path, a so called ion track. Tracks have been observed
in many materials including semiconductors [5,6], insulators [7,8,9], and various metals [10,11],
beyond a material dependent threshold value of electronic energy loss [12-14]. In vitreous silica the
threshold for track formation is observed in the range of values 2 to 4 keV/nm [15]. Densification and
plastic deformation can be induced by heavy ion irradiation in amorphous materials (SiO2 [15], glassy
metals [16]). More specific studies on the radiation effects on porosity show a shrinkage of Vycor
glasses nanopores under bombardment with track-generating ions. This phenomenon has been
observed by Klaumünzer [17] after the bombardment by various ions of such glasses at room
temperature (40Ar-70MeV, 86Kr-260MeV, 129Xe-340MeV), under fluences lower than 1014 at/cm2.
Silica gels with non organised nanopores (TMOS) were also studied, showing densification and
consolidation of the material after irradiation with He ions (E=3.6 MeV) at fluences ranging from 1015
to 5x1015 He.cm-2 [18].
The aim of this study was to investigate the effect of radiation damages onto mesoporous structures
and a potential radiation induced collapse. Irradiations were performed on mesoporous silica thin films
(~100nm thick). At this material scale, the evolution of the pores, the matrix deformations, the
densification as well as the top surface sputtering can be examined via X-Ray Reflectivity (XRR) and
Scanning Electron Microscopy (SEM) measurements. This paper relates the first results obtained from
irradiation implemented on the GANIL facility (Caen, France) with ions having a high electronic
stopping power, expected to induce electronic defects on the samples.
Results and discussion
Highly organized mesoporous silica thin films were synthesized by Evaporation Induced SelfAssembly (EISA) [19]. Thin films with 100 nm average thickness were obtained by dip coating in a
sol-gel process as described on Supplementary Information (SI). Two kinds of mesoporous thin films
have been investigated: (i) film having the 2D hexagonal p6m symmetry, with cylindrical pores
having a diameter of 5.3 nm and (ii) film having the 3D hexagonal P6/mmc symmetry, made up of
spherical pores of about 2.5 nm in diameter [20].
The irradiation by Xe ions (E=92MeV, 1014cm-2) was carried out at room temperature at GANIL
facility on Irrsud beam line. In these conditions, the electronic stopping power dE/dx = 8.2keV/nm
exceed the threshold for track formation of vitreous silica. Consequently, the depth of Xe ions
implantation is higher than the thickness of the silica layer (See SRIM calculations in SI).
The mesoporous films were characterized before and after irradiation by XRR measurements
performed on a Brucker D8 Advance diffractometer. The morphology of the samples was also
investigated by Scanning Electron Microscopy (SEM) (see SI).
Figure 1 shows the results obtained for S2D film before and after irradiation.
(a)
(b)
0
10
1
10
100 nm
Qc1
S2D
S2Di
-1
S2D
Qc2
Qc1
-2
Reflectivity
10
-3
0.1
0.01
10
0.02
0.03
-4
S2Di
10
100 nm
Cylindrical Pores
-5
10
T
-6
10
d
Substrate
Silica
-7
10
0.05
0.10
0.15
-1
Q (A )
0.20
0.25
Figure 1: XRR curves (a) and SEM micrographs (b) obtained for a 2D hexagonal mesoporous thin
film made up with 5 nm cylindrical pores, before (S2D) and after being irradiated (S2Di).
The XRR curve of pristine thin film exhibits Kiessig fringes (between 0.25 and 0.75 Å−1) that are
characteristic of a film having a measurable total thickness T. The periodicity d of the stacking of
pores produces Bragg reflections, located at Q=0.09, 0.16 and 0.23 Å−1. A cross section of S2D is
illustrated at bottom inset of Figure 1a, showing the hexagonal organisation of the cylindrical pores
and the parameters T and d.
It appears that the first Bragg reflection remains visible after irradiation, showing that the mesopores
are still present and organized in the z direction. The decrease of its intensity and its shift to the higher
Q values indicates however that the organisation of the pores is altered. The total thickness T of the
film, determined from the period of the Kiessig fringes, is decreased from T=88 nm to T=68 nm, while
the periodicity d of the stacking of pores (determined from the position of the Bragg reflection) is
decreased from d=7.3 nm to d=5.8 nm. The film is therefore made up with 10 and 9 layers of pores
before and after irradiation respectively. It can therefore be estimated that 20% of the total thickness
decrease is due to the collapse of the porous structure.
Another interesting feature is present at small angles (top insets of Fig. 1a) where two different critical
angles of total external reflection Qc are observed. The first one (Qc1) can be related to the average
electron density of the film, whereas the second one (Qc2) is assigned to the silicon substrate (~0.032
Å−1)[20]. A comparison of the panels before and after irradiation, clearly shows that Qc1 is strongly
decreased after irradiation, while, as expected Qc2 does not vary. The shift in Qc1 evolves from 0.027
Å−1 to 0.023 Å−1. This corresponds in turn to a change of average electron density  film , from 0.52
e−/Å3 to 0.38 e−/Å3. Taking into account the electron density of non porous silica  SiO2 =0.66 e−/Å3,
and using the equation (1) for the film porosity  [20], the decrease of electron density of the film can
be related to a porosity increase of about 50%.

 SiO   film
2
 SiO
(1)
2
While the total thickness decrease can easily be attributed to the collapse of the pores, XRR
measurements need to be completed by SEM observations in order to explain the increase of porosity.
SEM micrographs are shown on Figure 1b for the S2D sample before and after irradiation.
Cross-sectional view of the fractured film before irradiation displays the 2D hexagonal structure of
cylindrical pores. It supports the number or layers of pores and the average total thickness obtained
from XRR. After irradiation at fluence of 1014Xe.cm-2, some damaged areas resulting from recovery of
the ions tracks can clearly be identified on the SEM micrograph. They are homogenously distributed
over the irradiated surface and they pass through the overall thickness of the film until the underlying
silicon substrate, which does not appear to be damaged by irradiation. The damaged areas are
obviously responsible of the porosity increase evidenced by XRR.
Amazingly, no open organised mesopore have been observed on the SEM micrograph after irradiation,
while XRR measurements showed that some pores remain present and organised. This can be
explained by considering that the remaining pores are located into the safe pieces of film between the
damaged areas, and that irradiation induces a collapse of the pores located along the path of the
incident ion. This phenomenon could be compared to a densification of the matrix around the tracks
described in the literature [21], that leads in our case to a mesostructure amorphisation induced by
irradiation.
The same approach was applied on the S3D film, made up with smaller and spherical pores organised
in a 3D hexagonal symmetry. Figure 2 shows the XRR curves and SEM micrographs before and after
irradiation.
(a)
0
(b)
10
S3D
Qc1
-1
10
S3D
S3Di
-2
10
Qc2
0.1
-3
10
Reflectivity
100 nm
0.01
-4
10
0.02
0.03
100 nm
-5
10
Spherical Pores
S3Di
-6
10
T
-7
10
d
Substrate
Silica
-8
10
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
-1
Q (A )
Figure 2: XRR curves (a) and SEM micrographs (b) obtained for a 3D hexagonal mesoporous thin
film made up with 2.5 nm spherical pores, before (S3D) and after being irradiated (S3Di).
On XRR curves, the organisation of the pores produces a first Bragg peak located at Q=0.18A-1,
related to a periodicity of pore stacking d of 3.5 nm (Figure 2a). The disappearance of this peak after
irradiation evidences that the pores organisation is in this case, completely destroyed. A few Kiessig
fringes present at small Q indicate a total thickness reduced by 17% (from 84 nm to 70 nm).
While the porous structure appears to be collapsed, the plateau of total external reflection (bottom
inset of Figure 1a) shows here again a decrease of the average electron density of the film,
corresponding to an increase of porosity from 36% to 43%.
SEM measurements performed before and after irradiation are shown at figure 2b. The 2.5 nm pores of
S3D are nevertheless too small to be distinguished with this method. From our knowledge, only
transmission electron microscopy can reach such a resolution but it cannot be easily applied for crosssectional view of thin film. It is however possible to distinguish on the SEM micrograph, the
alignment of the pores at the top surface of the film. After irradiation, as for the irradiated S2D film,
cylindrical damaged areas are formed through the total thickness of the film as a consequence of the
ions tracks. In this case, the safe pieces of film are smaller than for the S2D film. It may explain why
the porous structure appears more damaged on the XXR curve for the S3D film than for the S2D.
Conclusion
Two mesoporous thin films corresponding to different pore sizes and shapes, were irradiated by Xe
ion beam. The damages were caused by electronic effects on the whole thickness of the samples.
Characterization by XRR shows a collapse of the film thickness (estimated to 20% for S2D and to
17% for S3D) and a significant damage of the pores organisation. The irradiation effect on the two
porous structures appears to be comparable concerning the deformation of the total thickness, but the
amorphisation of the mesostructure appears to be higher for smaller pores. Both samples show in
addition a decrease of the average density of the film corresponding to an increase of their porosity.
This feature was attributed to the formation of damaged areas as a consequence of the ions tracks
evidenced by SEM measurements.
This study shows the feasibility of monitoring the porosity and porous structure of mesoporous thin
film under irradiation, by coupling XRR with SEM measurements. XRR give quantitative information
on the pores size, average density of the films and therefore porosity, while SEM allows identifying
damaged areas..
Further fluences or radiation energy need to be tested to follow their impact on the total thickness and
porous volume deformation. The evolution of these parameters could therefore be simulated with the
Trinkhaus model [22] and related to the energy transfer in the material.
Acknowledgment:
This work was supported by CEA, CNRS, University Montpellier 2 and GNR PACEN. We
greatly thank O. Dugne, P. Allegri and A. Chocard for their help in the SEM characterization, and the
Irrsud beam line for the irradiation. . The authors thank also the MATINEX French Research Group
for its financial support.
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SUPPLEMENTARY INFORMATIONS
Thin films synthesis
The sol that was prepared in two steps. In the first step a stock solution of prehydrolysed silica
precursor was obtained by refluxing TEOS (TertaEthylOrthoSilicate), millipore water and
hydrochloric acid. In the second step the surfactant dissolved in ethanol in acidic condition was added
to the first solution. For irradiation, two kinds of films corresponding to two pores sizes and shapes
were subsequently prepared either templated by CTAB or P123.
For the P123 templated film, the second solution was aged for 2 hours, before adding, 4g of H2O (pH
= 1.25). The thin film was then made by dip-coating at a constant withdrawal velocity of 14cm/min a
silicon substrate in the final sol of molar composition 1TEOS:72C2H5OH:8H20:3HCl:0.012P123.
Such a composition was known to yield films having the 2D hexagonal p6m symmetry, with
cylindrical pores having a diameter of 5.3 nm [19].The film was thoroughly rinsed with ethanol at
80°C for 6h to remove the surfactant so as to produce a mesoporous film. This film is labelled S2D.
The CTAB templated film was obtained from a final solution in a molar ratio
1TEOS:20C2H5OH:0.004HCl:4H2O:0.10CTAB aged for 4 days. In order to obtain very thin films,
the sol was diluted by adding 21g of ethanol, and the films were dip-coated at a constant withdrawal
velocity of 3.6cm/min on a silicon substrate. Such a composition was known to yield films having the
3D hexagonal P6/mmc symmetry, made up of spherical pores of about 2.5 nm in diameter [19]. The
surfactant was finally removed by rinsing the film with a solution containing ethanol and hydrochloric
acid with a molar ratio: 1C2H5OH:0.007HCl for 3h at 60°C. This film will be labelled S3D in this
study.
Irradiation
The irradiation by Xe ions (E=92MeV, 1014cm-2) was carried out at room temperature at GANIL
facility on Irrsud beam line. In these conditions, we used SRIM 2008 to calculate the Xe (92 MeV)
implantation profile into a layer of SiO2 (100nm) deposited on a Si substrate.
It appears that the electronic stopping power dE/dx = 8.2keV/nm exceed the threshold for track
formation of vitreous silica. Consequently, the depth of Xe ions implantation is higher than the
thickness of the silica layer.
Characterizations
The mesoporous films were characterized before and after irradiation by XRR measurements
performed on a Brucker D8 Advance diffractometer, equipped with a motorised reflectivity stage that
allows vertical translation of the sample (see SI). The complete primary optics setup for XRR analysis
was composed of a Cu K (= 1.54 Å) source, a Göbel mirror, a motorised divergence slit, a fixed 0.2
mm slit, an automatic absorber, a fixed 0.1 mm slit after the absorber, and 2.5° Sollers slits. The
secondary optics included a motorised anti scattering slit, a graphite monochromator, 2.5° Sollers slits,
a 0.05 mm receiving slit and a point detector. Standard -2 scans were used for data collection. Step
sizes and counting times were adapted to the intensity collected by the detector. The resolution of this
setup was estimated to 0.03°.
The morphology of the samples was investigated before and after irradiation by Scanning Electron
Microscopy (SEM) using a ZEISS Supra 55 microscope at 10 kV.