Woo Lee

Structural engineering of nanoporous anodic aluminium oxide by pulse anodization of aluminium

Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodization or hard anodization. The first method produces self-ordered pore structures, but it is slow and only works for a narrow range of processing conditions; the second method, which is widely used in the aluminium industry, is faster, but it produces films with disordered pore structures. Here we report a novel approach termed pulse anodization that combines the advantages of the mild and hard anodization processes. By designing the pulse sequences it is possible to control both the composition and pore structure of the anodic aluminium oxide films while maintaining high throughput. We use pulse anodization to delaminate a single as-prepared anodic film into a stack of well-defined nanoporous alumina membrane sheets, and also to fabricate novel three-dimensional nanostructures.

In situ control of oxygen vacancies in TiO 2 by atomic layer deposition for resistive switching devices

Oxygen vacancies (V(O)) have profound effects on the physical and chemical performance of devices based on oxide materials. This is particularly true in the case of oxide-based resistive random access memories, in which memory switching operation under an external electrical stimulus is closely associated with the migration and ordering of the oxygen vacancies in the oxide material. In this paper, we report on a reliable approach to in situ control of the oxygen vacancies in TiOx films. Our strategy for tight control of the oxygen vacancy is based on the utilization of plasma-enhanced atomic layer deposition of titanium oxide under precisely regulated decomposition of the precursor molecules (titanium (IV) tetraisopropoxide, Ti[OCH(CH₃)₂]₄) by plasma-activated reactant mixture (N₂+O₂). From the various spectroscopic and microstructural analyses by using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, confocal Raman spectroscopy, and spectroscopic ellipsometry, we found that the precursor decomposition power (R(F)) of plasma-activated reactant mixture determines not only the oxygen vacancy concentration but also the crystallinity of the resulting TiO(x) film: nanocrystalline anatase TiO(x) with fewer oxygen vacancies under high R(F), while amorphous TiOx with more oxygen vacancies under low RF. Enabled by our controlling capability over the oxygen vacancy concentration, we were able to thoroughly elucidate the effect of oxygen vacancies on the resistive switching behavior of TiO(x)-based memory capacitors (Pt/TiO(x)/Pt). The electrical conduction behavior at the high resistance state could be explained within the framework of the trap-controlled space-charge-limited conduction with two characteristic transition voltages. One is the voltage (V(SCL)) for the transition from Ohmic conduction to space-charge-limited conduction, and the other is the voltage (V(TFL)) for transition from space-charge-limited conduction to trap-filled-limited conduction. In this work, we have disclosed for the first time the dependence of these two characteristic transition voltages (i.e., V(SCL) and V(TFL)) on the oxygen vacancy concentration.

Highly ordered porous alumina with tailor-made pore structures fabricated by pulse anodization.

A new anodization method for the preparation of nanoporous anodic aluminum oxide (AAO) with pattern-addressed pore structure was developed. The approach is based on pulse anodization of aluminum employing a series of potential waves that consist of two or more different pulses with designated periods and amplitudes, and provides unique tailoring capability of the internal pore structure of anodic alumina. Pores of the resulting AAOs exhibit a high degree of directional coherency along the pore axes without branching, and thus are suitable for fabricating novel nanowires or nanotubes, whose diameter modulation patterns are predefined by the internal pore geometry of AAO. It is found from microscopic analysis on pulse anodized AAOs that the effective electric field strength at the pore base is a key controlling parameter, governing not only the size of pores, but also the detailed geometry of the barrier oxide layer.

Quantitative analysis of the grain morphology in self-assembled hexagonal lattices.

We present a methodology for the analysis of the grain morphology of self-ordered hexagonal lattices and for the quantitative comparison of the quality of their grain ordering based on the distances between nearest neighbors and their angular order. Two approaches to grain identification and evaluation are introduced: (i) color coding the relative angular orientation of hexagons containing a central entity and its six nearest neighbors, and (ii) incorporating triangles comprising three nearest neighbors into grains or repelling them from grains based on deviations of the side lengths and the internal angles of the triangles from those of an ideal equilateral triangle. A spreading algorithm with tolerance parameters allows single grains to be identified, which can thus be ranked according to their size. Hence, grain size distributions are accessible. For the practical evaluation of micrographs displaying self-ordered structures, we suggest using the size of the largest identified grain as a quality measure. Quantitative analyses of grain morphologies are key to the systematic and rational optimization of the fabrication of self-assembled materials.

A versatile ultra-thin Au nanomesh from a reusable anodic aluminium oxide (AAO) membrane

We demonstrated multiple replications of versatile ultra-thin Au nanomeshes from a single porous anodic aluminium oxide (AAO) membrane as a reusable replication master. A bilayered film of Ag and Au was sequentially deposited on nanoporous AAO (i.e., Au/Ag/AAO), in which the Ag layer sandwiched in-between the Au layer and AAO serves as a sacrificial layer for separating the Au film from the oxide membrane during selective wet-chemical etching of Ag in HNO3 solution. The structure of the separated Au film is characterized by a free-standing nanomesh, the hole array pattern and pitch distance of which are defined by the AAO replication master. Ultra-thin Au meshes can be transferred onto substrates of choice without any structural failure. On the other hand, our repeated replication experiments have revealed that a single porous AAO replication master can be reused more than 30 times without significant structural changes. In the present work, we demonstrated the versatility of replicated Au nanomeshes in various nanofabrications, including preparation of extended arrays of Si nanowires (SiNWs) by metal-assisted chemical etching of silicon wafers and surface patterning of polymeric substrates with sub-100 nm resolution. The applicability of Au meshes as an electrode for flexible devices was manifested by comparable resistance values of Au meshes on the polymer substrate, viz., polyethylene terephthalate (PET), to a continuous Au film and stable I–V behaviours regardless of its bending state. The present approach of multiple replications of metal meshes from a single AAO membrane can be readily extended to other metallic materials and is time- and cost-effective compared to conventional lithographic methods.

Domain structures and piezoelectric properties of Pb(Zr0.2Ti0.8)O3 nanocapacitors

Epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) nanoislands and nanocapacitors were fabricated by stencil mask-assisted pulsed laser deposition. By x-ray diffraction reciprocal space mapping it was observed that PZT nanoislands contain mainly c-domains and residual fractions of tilted and nontilted a-domains, while extended thin films contain only c-domains and tilted a-domains. The presence of nontilted a-domains manifests clearly that the misfit strain is significantly reduced in PZT nanoislands, compared to the thin film. Some of the a-domains turned out to be switchable under an external electric field due to the strain relaxation in the PZT nanocapacitors. The piezoresponse of PZT nanocapacitors was higher than that of continuous thin-film capacitors.