Jolien Dendooven

Tailoring nanoporous materials by atomic layer deposition

Atomic layer deposition (ALD) is a cyclic process which relies on sequential self-terminating reactions between gas phase precursor molecules and a solid surface. The self-limiting nature of the chemical reactions ensures precise film thickness control and excellent step coverage, even on 3D structures with large aspect ratios. At present, ALD is mainly used in the microelectronics industry, e.g. for growing gate oxides. The excellent conformality that can be achieved with ALD also renders it a promising candidate for coating porous structures, e.g. for functionalization of large surface area substrates for catalysis, fuel cells, batteries, supercapacitors, filtration devices, sensors, membranes etc. This tutorial review focuses on the application of ALD for catalyst design. Examples are discussed where ALD of TiO(2) is used for tailoring the interior surface of nanoporous films with pore sizes of 4-6 nm, resulting in photocatalytic activity. In still narrower pores, the ability to deposit chemical elements can be exploited to generate catalytic sites. In zeolites, ALD of aluminium species enables the generation of acid catalytic activity.

Modeling the Conformality of Atomic Layer Deposition: The Effect of Sticking Probability

The key advantage of atomic layer deposition (ALD) is undoubtedly the excellent step coverage, which allows for conformal deposition of thin films in high-aspect-ratio structures. In this paper, a model is proposed to predict the deposited film thickness as a function of depth inside a hole. The main model parameters are the gas pressure, the deposition temperature, and the initial sticking probability of the precursor molecules. Earlier work by Gordon et al. assumed a sticking probability of 0/100% for molecules hitting a covered/uncovered section of the wall of the hole, thus resulting in a stepwise film-thickness profile. In this work, the sticking probability is related to the surface coverage θ by Langmuirs equation s(θ) = s 0 (1 - θ), whereby the initial sticking probability s 0 is now an adjustable model parameter. For s 0 ≅ 100%, the model predicts a steplike profile, in agreement with Gordon et al., while for smaller values of s 0 , a gradual decreasing coverage profile is predicted. Furthermore, experiments were performed to quantify the conformality for the trimethylaluminum (TMA)/H 2 O ALD process using macroscopic test structures. It is shown that the experimental data and the simulation results follow the same trends.

Conformality of Al2O3 and AlN Deposited by Plasma-Enhanced Atomic Layer Deposition

This paper focuses on the conformality of the plasma-enhanced atomic layer deposition (PE-ALD) of Al2O3 using trimethylaluminum [Al(CH3)(3); (TMA)] as a precursor and O-2 plasma as an oxidant source. The conformality was quantified by measuring the deposited film thickness as a function of depth into macroscopic test structures with aspect ratios of similar to 5, 10, and 22. A comparison with the thermal TMA/H2O process indicates that the conformality of the plasma based process is more limited due to the surface recombination of radicals during the plasma step. The conformality can slightly be improved by raising the gas pressure or the radio-frequency power. Prolonging the plasma exposure time results in further improvement of the conformality. Furthermore, there are indications that the H2O produced during the plasma step in the PE-ALD process for Al2O3 contributes to the observed conformality through a secondary thermal ALD reaction. The conformality of Al2O3 is also compared to the conformality of AlN deposited by PE-ALD from TMA and NH3 plasma. For the same exposure, O-2 plasma results in better conformality compared to NH3 plasma, suggesting a faster recombination of the radicals in the NH3 plasma.

In situ monitoring of atomic layer deposition in nanoporous thin films using ellipsometric porosimetry

Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO(2) in a mesoporous silica film.

Aluminium atomic layer deposition applied to mesoporous zeolites for acid catalytic activity enhancement

Atomic Layer Deposition (ALD) of aluminium is a new method for enhancing acidity and acid catalytic activity in mesoporous zeolites and hierarchical materials.

Air-based photoelectrochemical cell capturing water molecules from ambient air for hydrogen production

A system is demonstrated that autonomously produces hydrogen gas using sunlight and outside air as the only inputs. Oxygen and hydrogen formation reactions occur on either side of a monolithic “solar membrane” inserted in a two-compartment photoelectrochemical cell. A surface film of Nafion® serves as a solid electrolyte. This proof of concept invites further development of air-based cells.

In situ synchrotron based x-ray fluorescence and scattering measurements during atomic layer deposition : Initial growth of HfO2 on Si and Ge substrates

The initial growth of HfO2 was studied by means of synchrotron based in situ x-ray fluorescence (XRF) and grazing incidence small angle x-ray scattering (GISAXS). HfO2 was deposited by atomic layer deposition (ALD) using tetrakis(ethylmethylamino)hafnium and H2O on both oxidized and H-terminated Si and Ge surfaces. XRF quantifies the amount of deposited material during each ALD cycle and shows an inhibition period on H-terminated substrates. No inhibition period is observed on oxidized substrates. The evolution of film roughness was monitored using GISAXS. A correlation is found between the inhibition period and the onset of surface roughness.

Plasma enhanced atomic layer deposition of Fe2O3 thin films

Atomic layer deposition of Fe2O3 is generally performed at temperatures above 350 °C. In this work, Fe2O3 thin films were deposited by remote plasma enhanced atomic layer deposition using tertiary butyl ferrocene (TBF) and O2 plasma in a broad temperature range starting from 150 to 400 °C. A maximum growth rate of 1.2 A per cycle was achieved between 300 and 350 °C. Below 300 °C, the saturated growth per cycle was found to depend on the temperature of the sample. All the deposited films were pure with no significant amount of carbon contamination. Films deposited at 250 °C and above were crystalline with an α-Fe2O3 crystal structure, while the low temperature films were crystallized by a post-deposition annealing in He. Annealing in H2 induced the formation of metallic iron.

Synthesis of uniformly dispersed anatase nanoparticles inside mesoporous silica thin films via controlled breakup and crystallization of amorphous TiO2 deposited using atomic layer deposition

Amorphous titanium dioxide was introduced into the pores of mesoporous silica thin films with 75% porosity and 12 nm average pore diameter via Atomic Layer Deposition (ALD) using alternating pulses of tetrakis(dimethylamino)titanium and water. Calcination provoked fragmentation of the deposited amorphous TiO2 phase and its crystallization into anatase nanoparticles inside the nanoporous film. The narrow particle size distribution of 4 ± 2 nm and the uniform dispersion of the particles over the mesoporous silica support were uniquely revealed using electron tomography. These anatase nanoparticle bearing films showed photocatalytic activity in methylene blue degradation. This new synthesis procedure of the anatase nanophase in mesoporous silica films using ALD is a convenient fabrication method of photocatalytic coatings amenable to application on very small as well as very large surfaces.

Catalytic activation of OKO zeolite with intersecting pores of 10- and 12-membered rings using atomic layer deposition of aluminium

Tetrahedral framework aluminium was introduced in all-silica zeolite -COK-14 using Atomic Layer Deposition (ALD) involving alternating exposure to trimethylaluminium and water vapour. The modification causes permanent conversion of the originally interrupted framework of -COK-14 to a fully connected OKO type framework, and generates catalytic activity in the acid catalysed hydrocarbon conversion reaction.