Chemistry Reference
In-Depth Information
ALD has proven to be an effective technique for the deposition of con-
formal coatings in Si trenches
19,20
and anodic alumina nanopores.
21,22
Nevertheless, it is clear that, because of the diffusion limited gas trans-
port, achieving a good conformality requires careful optimization of the ALD
process parameters.
23,24
This is especially true when ALD is applied to
nanoporous materials that are typically used as high surface area catalyst
supports. Conformal ALD requires the precursor molecules to penetrate into
the bulk of these materials often comprising a complex 3D network of
interconnected extremely narrow channels (pore sizes
o
10 nm). This pre-
sents a challenge in itself and, therefore, the next section is dedicated to the
conformality of ALD in nanoporous materials.
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7.1.2 Conformality of ALD in Nanoporous Materials
A good understanding of the conformality of ALD in nanoporous materials is
essential to the optimization of the deposition parameters for ALD pro-
cessing of supported catalysts. Section 7.1.2.1 addresses the ability of ALD to
conformally coat the interior surface of pores with diameters in the low
mesoporous regime. Section 7.1.2.2 focuses on the transport of precursor
molecules in nanoporous powder particles exhibiting a high surface area.
7.1.2.1 ALD in Nanometer-sized Mesopores
It is not straightforward to obtain direct information about the conformality
of ALD coatings deposited in nanometer-sized mesopores. In a recent study,
porous titania thin films were used as substrate for the detailed investigation
of HfO
2
ALD in ink-bottle shaped mesopores by means of advanced char-
acterization techniques such as quantitative electron tomography and syn-
chrotron-based X-ray fluorescence (XRF).
25
The main results of this work are
reviewed in the next paragraphs.
A mesoporous titania film was synthesized by spin coating a solution of
titanium precursor and polymeric templates on a Si substrate.
26
As revealed
by ellipsometric porosimetry (EP),
27,28
the titania film contained ink-bottle
shaped mesopores with an average pore diameter of 6.8 nm and an average
neck size of 4.8 nm. The mesoporous film was coated with HfO
2
using 30
cycles of the tetrakis(ethylmethylamino)hafnium (TEMAH)/H
2
O ALD process
at 200 1C.
29
Quantitative electron tomography enabled a direct and local 3D charac-
terization of the ultra-thin HfO
2
coating deposited in the titania mesopores.
Where conventional transmission electron microscopy (TEM) 'only' provides
a two-dimensional (2D) projection of a 3D object, electron tomography aims
at reconstructing the 3D structure of the object from a series of 2D TEM
images acquired at different tilt angles.
30-32
In order to obtain a full tilt
range of 2D projection images a nanopillar was cut from the sample and a
dedicated on-axis tomography holder was used.
33,34
Figure 7.4a shows a 3D
volume rendering of the porous film obtained using the total variation
.
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