Julien Bachmann

A Practical, Self‐Catalytic, Atomic Layer Deposition of Silicon Dioxide

The outstanding chemical, electrical, and optical properties of silicon dioxide have made it ubiquitous in science and technology. The ability to create SiO2 nanostructures of well-defined geometry would broaden its range of applications even further, in particular in the chemical, electrokinetic, and biomedical realms. Atomic layer deposition (ALD) is especially suited to nanostructuring, since its kinetics are controlled by surface chemistry rather than mass transport from the gas phase. However, reports on the ALD of silica are few and far between in the open literature to date. All published reactions suffer from some weakness: a corrosive by-product or catalyst, poor reproducibility, or impurities in the deposited film. Herein, we describe a practical ALD process for SiO2 that overcomes such limitations. Based on the NH3-catalyzed hydrolysis of tetraethoxysilane (Si(OEt)4), chemical intuition dictates that a triethoxysilane bearing an aminoalkyl moiety be readily hydrolyzed without any extraneous catalyst. The basic functionality will labilize the strong Si!O bonds, a phenomenon that can be called “self-catalysis” to emphasize the fact that one chemical species is both substrate and catalyst. Subsequent oxidative cleavage of the tethered moiety should afford a silanol, amenable to further reaction with aminoalkyltriethoxysilane molecules. Accordingly, a three-step reaction sequence based on 3-aminopropyltriethoxysilane, water, and ozone (O3) can be envisioned for the ALD of SiO2 (Scheme 1).

Low temperature silicon dioxide by thermal atomic layer deposition: Investigation of material properties

SiO2 is the most widely used dielectric material but its growth or deposition involves high thermal budgets or suffers from shadowing effects. The low-temperature method presented here (150 °C) for the preparation of SiO2 by thermal atomic layer deposition (ALD) provides perfect uniformity and surface coverage even into nanoscale pores, which may well suit recent demands in nanoelectronics and nanotechnology. The ALD reaction based on 3-aminopropyltriethoxysilane, water, and ozone provides outstanding SiO2 quality and is free of catalysts or corrosive by-products. A variety of optical, structural, and electrical properties are investigated by means of infrared spectroscopy, UV-Vis spectroscopy, secondary ion mass spectrometry, capacitance-voltage and current-voltage measurements, electron spin resonance, Rutherford backscattering, elastic recoil detection analysis, atomic force microscopy, and variable angle spectroscopic ellipsometry. Many features, such as the optical constants (n, k) and optical transm...

Multilayered Core/Shell Nanowires Displaying Two Distinct Magnetic Switching Events

Adv. Mater. 2010, 22, 2435–2439 2010 WILEY-VCH Verlag G The size-dependent properties of pseudo-one-dimensional nano-objects have been abundantly documented for single-phase rods or wires. Elongated nanostructures that coaxially combine several phases of distinct physical properties could generate additional effects—spintronic, multiferroic, magnetoplasmonic, to name a few. Such core/shell wires have already been prepared and investigated: they combine two materials of a common class (epitaxial semiconductors to enhance confinement), they utilize a purely structural core as substrate for a functional tube, or they derive from a post-synthetic chemical reaction at a wire surface. To date, however, multiphase one-dimensional nanostructures incorporating several functionalities in a single object are rare. Indeed, a general preparative method is still missing to generate core/shell wires with the following four advantageous characteristics: (i) ability to combine two chemically and physically very different materials; (ii) possibility to introduce an inert layer (insulator, diffusion barrier, spacer); (iii) tunability of each geometric parameter (core radius, shell thickness, and separation between them); (iv) scalability. We propose the combination of atomic layer deposition (ALD) and electrodeposition in an ordered nanoporous template as one such preparative strategy. The template defines the order of the material and the diameter of the wires, ALD is used to conformally deposit one or several shells (including the inert layers), whereas electrodeposition furnishes the core. In this manner, the materials of core and shell can be chosen independently of each other and the thickness of every individual layer is accurately tunable. We demonstrate this method by synthesizing ordered nanostructures embedded in an alumina matrix and consisting of a nickel core and an iron oxide shell separated by a silica spacer layer. This combination of two coaxial nanomagnets is of interest for increasing data storage densities: it could either shield each object and decouple it from its neighbors, or store more than one bit of information per object. However, the occurrence of several distinct switching events in coaxially arranged magnetic phases has not been evidenced experimentally to date (except in larger microwires of diameter >10mm). This situation contrasts with the variety and practical importance of the effects described in multilayered magnetic films (in particular GMR and TMR). We start with porous alumina membranes prepared electrochemically as templates featuring hexagonally arranged pores of diameter (150 15) nm and length 20mm. The subsequent preparative steps are displayed in Figure 1. First, a thin acid-resistant SiO2 layer is coated onto the inner pore walls (Fig. 1a). After dissolution of the aluminum substrate, a long H3PO4 etch is performed to ensure complete dissolution of the barrier layer (Fig. 1b), without risk of pore widening. Then reactive ion etching, RIE (Fig. 1c), removes the exposed SiO2 tips and achieves a clean opening of the pores for the electrodeposition step. In the ALD processes that follow for Fe2O3 and SiO2 (Fig. 1d), near-perfect conformality of the tubes is ensured by long purge times, which prevent any undesired CVD side-reaction. The Ni cores are electrodeposited inside the multilayer nanotube array after definition of a gold electrode on one side of the sample (Fig. 1e). Here, the inner silica tube also serves as electrical insulator. Finally, gold is removed and Fe2O3 is converted to Fe3O4 by dihydrogen (Fig. 1f). [22] The thicknesses of the Fe3O4 shell and the silica spacer can be varied at will: this in turn defines the diameter of the Ni core. The quality and versatility of this preparative strategy are revealed from the microscopic investigations of the samples. Scanning electron microscopy (SEM) images taken in top view (Fig. 2a–d) evidence the hexagonal order of the tubes and the homogeneity of their diameters after various steps of elaboration. A clean opening is obtained after RIE (Fig. 2a) and remains after the ALD steps (Fig. 2b). Thus, electrodeposition results in complete filling of the pores (Fig. 2c,d). The uniform SEM contrast observed from the side (Fig. 2d) proves the homogeneous Ni deposition inside the nanotubes of the whole sample and along their whole length. Isolated tubes and core/shell

Magnetic reversal of cylindrical nickel nanowires with modulated diameters

Anodic alumina membranes with modulated pore diameters serve as template for the preparation of magnetic nanowires. Filling the pores with Ni by electrodeposition delivers wires replicating the variation in modulation in pore diameter from 80 to 160 nm. Such structures are of interest for the observation and control of magnetic domain wall motion. Single-object characterization utilizing the magneto-optical Kerr effect magnetometry evidences a strong correlation between geometric parameters and magnetic properties. Ensemble magnetization measurements with a superconducting quantum interference device show the effect of dipolar interactions. Analytical models can reproduce the lowering of coercivity due to the presence of enhanced stray fields within the array. Magnetic force microscopy at individual wires indicates the presence of a strong stray field in the vicinity of the diameter change. The preparation technique demonstrates a mass production method of nano-objects with designed geometric irregulariti...

Size effects in ordered arrays of magnetic nanotubes : Pick your reversal mode

Ordered arrays of magnetic nanotubes are prepared by combining a porous template (anodic alumina) with a self-limiting gas-solid chemical reaction (atomic layer deposition). The geometric parameters can thus be tuned accurately (tube length of 1–50 μm, diameter of 20–150 nm, and wall thickness of 1–40 nm), which enables one to systematically study how confinement and anisotropy effects affect the magnetic properties. In particular, the wall thickness of such ordered Fe3O4 nanotubes has a nonmonotonic influence on their coercive field. Theoretical models reproduce the size effects that are experimentally observed and interpret them as originating from a crossover between two distinct modes of magnetization reversal.

Nanocrystalline solar cells with an antimony sulfide solid absorber by atomic layer deposition

Extremely thin absorber solar cells are built in which an Sb2S3 absorber coating is created by atomic layer deposition (ALD). The material is distributed homogeneously along the depth axis and is free of oxide. Under our conditions, an optimal thickness of 10 nm, Sb2S3, yields efficiencies of up to 2.6%.

Understanding pore rearrangement during mild to hard transition in bilayered porous anodic alumina membranes.

We present a systematic study about the influence of the main anodization parameters (i.e., anodization voltage ramp and hard anodization voltage) on the pore rearrangement in nanoporous anodic alumina during mild to hard anodization regime transition. To cover the ranges between mild and hard regimes, the anodization parameters were each set to three levels (i.e., 0.5, 1.0, and 2.0 V s(-1) for the anodization voltage ramp and 80, 110, and 140 V for the hard anodization voltage). To the best of our knowledge, this is the first rigorous study about this phenomenon, which is quantified indirectly by means of a nickel electrodeposition. It is found that pore rearrangement takes place in a relatively random manner. Large areas of pores remain blocked when the anodization regime changes from mild to hard and, under certain anodization conditions, a pore branching takes place based on the self-ordering mechanism at work during anodization. Furthermore, it is statistically demonstrated by means of a design of experiments strategy that the effect of the anodization voltage ramp on the pore rearrangement is practically negligible in contrast to the hard anodization voltage effect. It is expected that this study gives a better understanding of structural changes in nanoporous anodic alumina when anodization is switched from mild to hard regime. Furthermore, the resulting nanostructures could be used to develop a wide range of nanodevices (e.g., waveguides, 1D photonic crystals, Fabry-Pérot interferometers, hybrid mosaic arrays of nanowires).

Reversal modes and magnetostatic interactions in Fe3O4/ZrO2/Fe3O4 multilayer nanotubes

Reversal modes and magnetostatic interactions of multilayered Fe(3)O(4)/ZrO(2)/Fe(3)O(4) nanotubes consisting of a ferromagnetic internal tube, an intermediate non-magnetic spacer and an external magnetic shell are investigated as a function of their geometric parameters and compared with those produced inside the pores of anodic alumina membranes by atomic layer deposition. Based on a continuum approach we obtained analytical expressions that underline the first experimental results and support their interpretation that the system of multilayer tubes behaves as the reversal of two isolated systems. It is observed that the magnetostatic interaction between both phases depends on the magnetic configurations in each phase and also on the geometrical parameters considered. These structures have potential applications in novel spintronics devices, ultra-small magnetic media and other nano-devices.

Unveiling the Hard Anodization Regime of Aluminum: Insight into Nanopores Self-Organization and Growth Mechanism

Pores growth mechanism and their self-ordering conditions are investigated for nanoporous alumina membranes synthesized by hard anodization (HA) of Al in a broad range of anodic conditions, covering oxalic acid electrolytes with concentrations from 0.300 M down to 0.075 M and potentiostatic anodization voltages between 120 and 225 V. The use of linear sweep voltammetry (LSV) and scanning and transmission electron microscopy, together with image analysis techniques allow one to characterize the intrinsic nature of the HA regime. HA of aluminum is explained on the basis of a phenomenological model taking into account the role of oxalate ions and their limited diffusion through alumina nanochannels from a bulk electrolyte. The depletion of oxalate ions at the bottom of the pores causes an increased growth of the alumina barrier layer at the oxide/electrolyte interface. Furthermore, an innovative method has been developed for the determination of the HA conditions leading to self-ordered pore growth in any given electrolyte, thus allowing one to extend the available range of interpore distances of the highly ordered hexagonal pore arrangement in a wide range of 240-507 nm, while keeping small pore diameters of 50-60 nm.

Confined crystallization of anatase TiO2 nanotubes and their implications on transport properties

Nanotubes of TiO2 (anatase) and their ordered arrays are emerging, promising candidates as efficient host materials in many applications such as photovoltaic cells, batteries, sensors and catalysts/catalytic supports, but the interplay between these structures and their transport properties has been reported only rarely. Monodisperse, stoichiometric TiO2 nanotubes with smooth morphology and controlled wall thickness were fabricated by template-directed low-temperature atomic layer deposition (ALD), followed by annealing at elevated temperatures. We present a study on the wall thickness-dependent crystallization behaviors due to physical and/or self-confinement, as well as on the corresponding electrical properties. Over certain wall thicknesses, unexpectedly, our TiO2 nanotubes were found to be a new type of mesoporous wide gap semiconductor in which they possess similar porosity, but in terms of conductivity differ from previously known mesoporous photoanodes (i.e., anodized surfaces of Ti films and sintered films consisting of TiO2 nanoparticles). These results were ascribed to the large, elongated anatase domains (by a factor of up to 15–40 wall thicknesses) that developed via boosted crystal growth on porous alumina templates (physical confinement) as well as to the highly curved tubular shape (self-confinement). Indeed, nanotube arrays with walls thicker than 10 nm exhibited an enhancement in conductivity, by more than three orders of magnitude, compared to sintered, mesoporous TiO2 (anatase) particles, approaching the bulk value. The nearly single-crystalline TiO2 nanotubes presented here should allow for a good model system to study TiO2-based surface chemistry and have potential for many applications in photovoltaic and/or catalytic systems.