Highly ordered TiO2 nanotube-stumps with memristive response

Electrochemistry Communications 34 (2013) 177–180

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Highly ordered TiO2 nanotube-stumps with memristive response JeongEun Yoo a, Kiyoung Lee a, Alexei Tighineanu a, Patrik Schmuki a,b,⁎ a b

Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 5 May 2013 Received in revised form 25 May 2013 Accepted 29 May 2013 Available online 7 June 2013 Keywords: TiO2 nanostump Anodization Memristor Self-organization

a b s t r a c t In the present work, we produce highly ordered and defined short aspect ratio self-organized TiO2 nanotubes – so called titania nanotube stumps [TNS] – by anodizing Ti in a HF/H3PO4 electrolyte. These TNS-layers are ideal for a conformal filling by metal sputter deposition. After decorating amorphous TNS with a Pt top contact, we investigate the solid-state current–voltage behavior and find a strongly pronounced memristic behavior. We ascribe the finding to the high vacancy density present in TNS bottoms prepared under high ion flux conditions. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Over the past decade, TiO2 nanotubes that are fabricated by selforganizing anodization of a Ti metal sheet have attracted wide scientific and technological attention [1]. On the one hand, investigations target a further increase in the control over defined tube geometries (tube length, wall thickness, diameter, and order). On the other hand, these directional high surface area structures are of interest in virtually any application where up to now TiO2 nanoparticles are used. Typical examples are Grätzel type DSSCs [2,3], photocatalysis [4,5], biomedical coatings [6,7], membranes [8,9], sensing devices [10–12], etc. For many of these applications high aspect ratio tubes are desired to increase the reactive surface area and to introduce a maximum of directionality to photon-, electron- or mass flow. After anodization, the as-grown tubes are amorphous, and for applications in electron transport devices, such as solar cells or other photoelectrochemical purposes, a conversion of the tubes to crystalline anatase or anatase/rutile is needed to generate a sufficient electron conductivity within the tube walls. However, there are potential applications where an amorphous, defective TiO2 layer is desired. In fact, a most interesting feature of “defective” or amorphous TiO2 – that is its pronounced memristor behavior – has surprisingly not been explored for TiO2 nanotubes. In 2008, Strukov et al. (revisiting some earlier electrochemical [13] and theoretical [14] work) demonstrated that a thin TiO2 film sandwiched between two platinum contacts shows a voltage dependent on/off resistive switching [15]. It is in the meantime generally accepted that the memristive effect is based on mobility of oxygen vacancies, i.e., some ⁎ Corresponding author. Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany. Tel.: + 49 9131 85 275 75; fax: + 49 9131 85 275 82. E-mail address: [email protected] (P. Schmuki). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.05.038

degree of defectiveness of the film is required [16]. Essentially vacancies, originally present in a layer of TiO2 can be distributed across the oxide using a sufficiently high applied voltage (field). A negative electrode attracts vacancies; a positive electrode rejects vacancies. A vacancy alignment throughout the oxide can lead to a conductive path in the oxide (a conductive filament) — i.e. as a result the oxide as a whole shows a high electron conductivity. By a sufficiently high reverse pulse, vacancies are repelled from the positive electrode, and the oxide as a whole shows a high resistivity. I.e. by sufficiently high voltage pulses that open or cut conductive filaments one can switch the resistivity state forth and back and hence this effect can be used as a data storage element. Such memristor effects have attracted tremendous attention for non-volatile memories. Accordingly a high density of oxygen vacancies is beneficial and fast field-aided transport can most easily be achieved in thin films. For a rapid and high magnitude memristive switching, thin oxide layers (some nm-thickness) are required to create high fields at low voltages. Frequently used method to fabricate thin film memristors involve atomic layer deposition or sputtering of TiO2 (or derivatives of it), followed by an adequate heat treatment (to adjust the vacancy concentration), and finally establishing a top contact (mostly Pt) [17–22]. In the present work, we present an alternative: we explore as memristive element the bottom layers of TiO2 nanotubes. These are formed under high voltage conditions (high vacancy flux), are amorphous, and show a precisely adjustable thickness in the few nm-range [23]. Thus, they seem an excellent candidate for inducing memristic switching. Hence, we fabricate Pt/TiO2/Ti nanotube structures and show that these structures indeed exhibit a distinct memristric switching. 2. Experimental To fabricate our memristive structure, we used a Ti substrate carrying anodic TiO2 nanotubes and decorated it by sputter deposition

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with a Pt top contact. To produce anodic TiO2 nanotubes, we used Ti foils (0.125 mm thick, 99.7% purity, Goodfellow, England) that were degreased by sonicating in acetone, ethanol, deionized water, and then dried with a nitrogen stream. In order to achieve sufficiently short tubes (titania nanotube stumps [TNS]) that easily could be fully coated by Pt sputter deposition we investigated as an electrolyte mixtures of different concentrations of hydrofluoric acid and phosphoric acid (Sigma-Aldrich). A two-electrode system was used in all anodization experiments with a platinum plate as the counter electrode and the Ti foil (2.25 cm2) as the working electrode. During anodization, the distance between two electrodes was kept at 2 cm. Anodization was carried out with a DC power supply (VLP 2403 pro, Voltcraft). After preparation, the samples were rinsed with ethanol and then dried in a nitrogen stream. The top Pt contact to the TiO2 nanotube was established by depositing a nominally 300 nm thick Pt layer using a plasma sputter device (EM SCD500, Leica) operated at 20 mA at 10−2 mbar vacuum, using Ar as moderating gas. To characterize the solid state current–voltage (I–V) characteristics (memristive behavior) we used an Agilent 4156C precision semiconductor parameter analyzer and a USMCO micromanipulator. Various potential steps ±0.5, ±1 and ±2 V were explored to trigger memristic switching. All I–V curves were measured with 20 mV/s sweep rate. For morphological characterization, a field-emission scanning electron microscope (Hitachi FE-SEM S4800) was used. X-ray diffraction analysis (XRD, X'pert Philips MPD with a Panalytical X'celerator detector) using graphite monochromized CuKα radiation (wavelength = 1.54056 Å), was used for determining the crystal structure of the samples. The composition and the chemical state of samples were characterized using X-ray photoelectron spectroscopy (XPS, PHI 5600, US). 3. Results and discussion In a first set of experiments, we evaluated the feasibility to reliably fabricate conformal Pt contacts onto various TiO2 nanotubes, and in particular to achieve a full coverage of their bottoms by classic sputter deposition. We found that for sufficiently reliable contacts, sufficiently short aspect ratio (b5) nanotubes are required. For producing these nanotube stump (TNS)-layers we used HF/H3PO4 electrolytes. Fig. 1 shows a suitable layer obtained with a 3 M HF/H3PO4 electrolyte

by anodization for 2 h [24]. From the top view of TiO2 nanotubes (Fig. 1 a) and cross-sectional view (Fig. 1 b), it is apparent that under these conditions highly organized TNS layers with a tube diameter of ~80 nm and a layer thickness ~200 nm have been formed. The TiO2 nanotubes show an open V-shape morphology which was found to be ideal for Pt coverage. Several Pt sputter deposition parameters were explored and various nominal Pt layer thicknesses were deposited. Fig. 1 c and d shows a Pt layer produced by sputtering under the conditions given in the experimental section. Under these conditions, conformal contacts along the tube walls and the bottoms were established without completely filling the tubes. We used this economically beneficial type of decoration for further experiments. From the inset it is clear that the produced nanotube bottom layer thickness, i.e., sandwiched between the metallic Pt and Ti contact, is approx. 30 nm. Fig. 2 shows XRD and XPS data taken before and after Pt deposition on the TiO2 nanotube layer. It is clear that after sputtering the Pt layer, all Ti peaks fully disappear in XPS. Hence, the sputtered Pt could completely cover the TiO2 nanotubes without any defects or holes (at least within the accuracy of XPS). XRD shows that after sputtering the TiO2 still does not show any crystalline peaks, i.e. the amorphous nature is maintained. The memristic behavior was investigated by voltage sweep and voltage step experiments as outlined in Fig. 3. A typical voltage sweep experiment is shown in the upper left inset of Fig. 3 for a Pt/TiO2/Ti structure using an amorphous TiO2 structure as shown in Fig. 1. When a sweep was started at −1 V in the positive direction, transition from low-resistance state (LRS) to high-resistance state (HRS) was observed at about 1.0 V (Fig. 3 upper left inset). As the applied voltage is reversed in the negative direction, the HRS was maintained down to a voltage of ≈−1 V when a sudden resistance decrease takes place. To establish a reliable and fast switching behavior we used voltage steps from ± 1.5 V, and then characterized the I–V curve from − 0.5 to + 0.5 V for the LRS established after a − 1.5 step; or a sweep from + 0.5 V to −0.5 V when the HRS was established after a step to pulse 1.5 V. Overall, the switching behavior of the Pt/TiO2/Ti device shows high degree of symmetry. After +1.5 V pulse was applied, the current curves were maintained at low current (μA range) with a narrow dispersion. On the other hand, after the −1.5 V pulse the LRS showed a widely increasing lower resistance. From the average slopes, extracted specific resistivities

Fig. 1. a) SEM top view image of highly ordered TNS obtained by anodization in 3 M HF/H3PO4. b) SEM cross-sectional view of TNS of Fig. 1 a. c) SEM top view image of TNS with sputter-deposited Pt layer. d) SEM cross-sectional view of Pt coated TNS.

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(ρ) are 38.04 Ω · m (LRS) and 551 Ω · m (HRS). Repeated switching (lower right inset) shows stable current with narrow dispersion for over 20 cycles. It should be noted that the experiment with annealed samples has no memristic response but rather a simple resistive to a mild reversible diode behavior was observed. This finding that this effect occurs only with as-formed tubes implies that the vacancy concentration in the amorphous material is sufficiently high to create a significant effect. This is in so far not surprising as anodic film growth is, to a large extent, governed by mobile vacancies [25]. In principle, the larger the anodic current in steady state growth, the higher is the vacancy flux and presumably also the content of “frozen” vacancies when the circuit is switched off. This may be the reason that nanotube bottoms (that usually are grown at much higher steady state current densities than classic anodic layers) show such a pronounced memristic behavior. In our example, anodization is carried out at high anodic potentials clearly above the positive switching potentials (vacancies are distributed throughout the oxide — the oxide is in its LRS) with applying a positive top voltage, that is higher than ≈+1.0 V, vacancies are driven way from the top contact, conductive filaments are interrupted and the oxide switches to the HRS. Applying a voltage b−1.0 V to the top contact attracts vacancies and a LRS is established again. The present explanations are also well in line with the observation that the inset in Fig. 3 the LR-behavior is ohmic, while the HR-behavior is more of a Schottky type — as expected from a classic memtristor setting [17]. 4. Conclusion The present work shows that bottoms of nanotubes can show a strong characteristic memristic switching. Prerequisite for a successful fabrication of a memristric Pt/TiO2 nanotube/Ti solid state device is that highly ordered nanotubes are grown with a sufficiently low aspect ratio to easily achieve Pt deposition with conformal wall and bottom coating. In line with the expectation that vacancy transport concentration and distribution are responsible for a strong memristic effect in TiO2 — we find that only amorphous tube bottoms are active while tubes annealed in air show a classic resistive to diode characteristics. Fig. 2. a) X-ray diffraction patterns and b) XPS results for as formed TiO2 and Pt sputtered TiO2. The annotations are referred to T; Ti and Pt; Platinum.

Acknowledgments The authors would like to acknowledge the DFG and the DFG cluster of excellence (EAM) for the financial support, and H. Hildebrand for the valuable technical help. References

Fig. 3. Memristic switching curves for the Pt/TiO2/Ti device (before switching, positive and negative pulses were applied). Normalized current–voltage curve (upper inset), endurance cycle characteristics (lower inset) of Pt/TiO2/Ti memristic device (20 mV/s sweep rate, 5 ms hold time).

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