Structural tuning of photoluminescence in nanoporous anodic alumina by hard anodization in oxalic and malonic acids

We report on an exhaustive and systematic study about the photoluminescent properties of nanoporous anodic alumina membranes fabricated by the one-step anodization process under hard conditions in oxalic and malonic acids. This optical property is analysed as a function of several parameters (i.e. hard anodization voltage, pore diameter, membrane thickness, annealing temperature and acid electrolyte). This analysis makes it possible to tune the photoluminescent behaviour at will simply by modifying the structural characteristics of these membranes. This structural tuning ability is of special interest in such fields as optoelectronics, in which an accurate design of the basic nanostructures (e.g. microcavities, resonators, filters, supports, etc.) yields the control over their optical properties and, thus, upon the performance of the nanodevices derived from them (biosensors, interferometers, selective filters, etc.)


S.1. Hard Anodization
The one-step anodization for fabricating nanoporous anodic alumina (NAA) under hard anodization conditions is divided into three stages (Scheme S1). First, when the acid electrolyte temperature is around 0ºC, the anodization process starts under constant voltage at 40 V. After this, the anodization voltage is increased at a rate of 1 V·s -1 until it reaches the target voltage (i.e. hard anodization voltage). Then, the anodization voltage is maintained constant until the desired NAA thickness is reached. Schemes S1b and d show a typical current density-time (Jt) transient of a one-step anodization process under hard anodization conditions at 140 V in oxalic and malonic acids, respectively. During the one-step anodization process, the following three different anodization regimes take place: This layer is about 500 nm thick and has two main functions: namely, i) to suppress breakdown effects due to high temperatures and ii) to enable uniform oxide film growth at high voltage. When the anodization voltage is increased, the pores are reorganized because the anodization conditions modify the interpore distance. In this way, some pores vanish and others continue growing by the self-ordering mechanism during the transition regime.
Finally, when the hard anodization voltage is reached, the pores growth uniformly at an exponential growth rate, which is characteristic of a hard anodization process. The result is a NAA film with disordered and ordered pores on the top and bottom sides, respectively (Schemes S1a and c).

Structural Tuning of Photoluminescence in Nanoporous Anodic Alumina by Hard Anodization in Oxalic and Malonic Acids
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S.2. Pore Opening by Current Control
The pore opening process was performed in a home-made electrochemical cell based on two containers connected each other by two tubes. The NAAMs were placed between these tubes with the top side (i.e. open pores) facing the container 1 and the bottom side (i.e. closed pores) facing the container 2. The container 1 was filled with an aqueous solution of KCl 0.2 M and the container 2 with H 3 PO 4 5 wt%. Both solutions were constantly stirred at 35 ºC. Two stainless steel electrodes immersed in the containers were connected to a power supply and a voltage of 2 V was applied between them. The current density (J) was monitored during the etching process. As Scheme S2 shows, the current density transient can be divided into three stages, which correspond to different stages of the pore opening process: namely, i) t 1 -J remains relatively constant, what denotes that the oxide barrier layer at the pore bottom tips is compact, ii) t 2 -there is a noticeable and progressive increase in J as a result of the ion permeation through some open pores and iii) t 3 -J becomes steady again but at a higher value because of the ion transport takes place through all the pores → Pore Opening Point. It was verified that the etching rate was always faster for those NAAMs fabricated in malonic than in oxalic acid.

S.3. Elemental Qualitative Analysis
An elemental qualitative analysis of some NAAMs (i.e. HA-Ox 1.1 -HA-Ox 1.4 and HA-Ml 1.1 -HA-Ml 1.4 ) was performed in order to estimate the effect of the carbon content on the PL behaviour.
This analysis was carried out using energy dispersive X-ray spectroscopy (EDXS) coupled with the ESEM equipment. The results are summarized in Figure S1 and Table S1.

S.4. Collapse of NAAMs by Excessive Pore Widening
From preliminary experiments, it was established that the total collapse of the NAAM structure of those samples fabricated in malonic acid takes place after 30 min of wet chemical etching in H 3 PO 4 5 wt% at 35ºC. Figure S2 shows the collapsed structure of two NAAMs after 30 ( Figure   S2a) and 45 min (Figure S2b) of pore widening.

S.5. Difference in Colour
It was observed with the naked eye that those NAAMs fabricated in oxalic acid presented a yellow colour and those produced in malonic acid were bright brown ( Figure S3). It is deduced