Search

Wide range binary and ternary thin film combinatorial libraries mixing Al, Cu, and Ga were screened for identifying alloys with enhanced ability to withstand electromigration. Bidimensional test wires were obtained by lithographically patterning the substrates before simultaneous vacuum co-deposition from independent sources. Current-voltage measurement automation allowed for high throughput experimentation, revealing the maximum current density and voltage at the electrical failure threshold for each alloy. The grain boundary dynamic during electromigration is attributed to the resultant between the force corresponding to the electron flux density and the one corresponding to the atomic concentration gradient perpendicular to the current flow direction. The screening identifies Al-8 at. % Ga and Cu-5 at. % Ga for replacing pure Al or Cu connecting lines in high current/power electronics. Both alloys were deposited on polyethylene naphthalate (PEN) flexible substrates. The film adhesion to PEN is enhanced by alloying Al or Cu with Ga. Electrical testing demonstrated that Al-8 at. % Ga is more suitable for conducting lines in flexible electronics, showing an almost 50% increase in electromigration suppression when compared to pure Al. Moreover, Cu-5 at. % Ga showed superior properties as compared to pure Cu on both SiO2 and PEN substrates, where more than 100% increase in maximum current density was identified.

Composite memristors based on anodic oxidation of Hf superimposed on Ta thin films are studied. A layered structure is obtained by successive sputtering of Ta and Hf thin films. The deposition geometry ensured components' thickness gradient profiles (wedges) aligned in opposite directions. Anodization in citrate buffer electrolyte leads to a nanoscale columnar structuring of Ta2O5 in HfO2 due to the higher electrical resistance of the latter. Following the less resistive path, the ionic current forces Ta oxide to locally grow toward the electrolyte interface according to the Rayleigh-Taylor principle. The obtained composite oxide memristive properties are studied as a function of the Hf/Ta thickness ratio. One pronounced zone prominent for memristive applications is found for ratios between 4 and 5. Here, unipolar and bipolar memristors are found, with remarkable endurance and retention capabilities. This is discussed in the frame of conductive filament formation preferentially along the interfaces between oxides.

The electrochemical behavior of a tungsten trioxide-nickel oxide (WO3-NiO) thin film library was investigated using scanning droplet cell microscopy (SDCM) in 0.1 mol dm-3 sodium perchlorate (NaClO4) solution. The WO3-Ni film library was deposited by thermal coevaporation on an indium tin oxide (ITO)-coated glass substrate in an atomic Ni concentration range from 2.8 to 15.6 at. %. After an oxidation/crystallization heat treatment, the Ni was oxidized and the crystal structure of WO3-NiO was transformed from monoclinic WO3 (3.5 at. % Ni) to cubic WO3 (up to 7.1 at. % Ni) and again to monoclinic WO3 when the Ni amount increased (>11.8 at. %). Proton (H+) intercalation (cathodic reaction) and deintercalation (anodic reaction) into the WO3-NiO mixed phases was induced. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) analysis revealed that the WO3-NiO film has n-type bilayer capacitive property, with the outer capacitive layer having a higher defect density than the inner capacitive layer. With a Ni concentration of 7.1 at. %, the WO3-NiO film was the most defective in the library. Introduction of the Ni cation into the WO3 network was associated with changes of the semiconducting properties of the film.

This study demonstrates the formation of barrier-type anodic films in a phosphate buffer (pH 8.4) on thermally evaporated aluminium-holmium alloys covering a 2.0–12.0 at.% Ho compositional spread. Scanning x-ray diffraction and scanning electron microscopy reveal that structure and morphology of the parent alloys library drastically change above 6.5 at.% Ho. Anodic films were formed when the as-evaporated alloys were stepwise anodized potentiodynamically to a maximum of 10 V in phosphate buffer regardless of film composition and morphology. Simultaneous in-situ electrochemical impedance spectroscopy discloses a good co-relation between film thickness and formation factor with the composition and morphology of parent alloys, suggesting a strong influence of holmium concentration on the structure, morphology of parent alloys as well as on the dielectric properties of resulting anodic films. Inductively coupled plasma optical emission spectroscopy measurements allowed a quantitative determination of the dissolved species during anodization.

Various oxides and especially their modifications are in actual scientific focus for memristors fabrication due to their applications in non-volatile memories. Such devices exceed most limits of conventional memory technology. Memristors are recognized as resistive random access memories (ReRAMs), in which the data storage profits from a non-volatile change in the material resistance. The switching between a high resistance state (HRS) and a low resistance state (LRS) depends on the selection of electrodes and active (oxide) layers. This generally impacts the conductive pathways (filaments) formation, which is mediated by oxygen vacancies and/or cations, and their field-activated movement inside the oxide. Anodic oxides of valve metals (Al, Hf, Ta, Ti, Nb, Zr) have shown remarkable performances as memristive elements. Studies on Hf- and Ta-based memristors reported excellent electrical and memory properties, such as multi-level switching, high endurance and data retention. Even though for research purposes the synthesis of such oxide layers is commonly done by atomic layer deposition or sputtering, the electrochemical anodization process should not be neglected. This is a faster, less complex and inexpensive method, with precise composition and oxide thickness control through electrochemical parameters. The value of this approach is clearly emphasized by its continuous industrial implementation in various sectors. Previous works have confirmed that the performance of Hf or Ta anodic memristors can be improved by carefully selecting the anodization electrolyte or other electrochemical parameters1,2. These play a crucial role in positioning and sizing of conductive filaments within the oxide. This approach directly leads to defect-engineered memristors fabrication, which is nowadays a major motivation for investigating devices based on mixed oxides formed in different electrolytes. Predicting the position and shape/thickness of a conducting filament may eventually lead to enhanced device stability and resistive states control. The focus of the current work is on the behavior of anodic memristors based on ultra-thin Hf superimposed on Ta films. The main idea linked to the control of resistive filaments is based on the particularities of the anodization process, when the interface between both oxides is dynamically changing. In situ oxide self-nanostructuring is already known for various superimposed valve metals, including Hf and Ta. Their anodization leads to nanoscale oxide columns (“fingers”) formation, when a metal producing a more resistive oxide is superimposed on a metal producing a less resistive one. This phenomenon is recognized as an electrical version of the Rayleigh-Taylor effect and results from the ionic current preferring the less resistive paths, enhancing the growth of the correspondent oxide. Oxide resistivities and structures, transport numbers, Pilling-Bedworth ratios are all considered as determining factors for the anodization process of such superimposed systems. In the current work, anodization of Hf/Ta system leads to Rayleigh-Taylor effects since HfO2 is the more resistive oxide. The boundary between Hf and Ta oxides may influence the conductive pathways required for the memristive effect, thus being most relevant for fabrication of highly stable and forming-free memristors. Additionally, the use of superimposed films with gradient but complementary thicknesses allows investigating the ideal Hf/Ta ratios for which the best memristive behavior is obtained.3 From this study, one pronounced zone prominent for memristive applications is found for Hf/Ta thickness ratios between 4 and 5. Here, unipolar and bipolar memristors are identified, with remarkable endurance and retention capabilities. The CFs positioning is predefined by the development of Ta2O5 columnar structures grown during the anodization process. It is also possible that few CFs may be found in parallel, according to TEM observations, showing more than one Ta2O5 “finger”. Previous studies on pure Hf anodic memristors have confirmed concurrent competing CFs formation.2 Thus, one could assume that the memristive switching mechanism can be conditioned by the formation of oxides with such structures. The identified Hf/Ta ratio could be an excellent choice for improved memristors fabrication. Controlled O vacancies generation is a critical factor in switching uniformity and reproducibility. Therefore, oxide “fingers” formation is a promising electrochemical approach towards defect-engineered memristors. Further investigation of the composite oxide formation, particularly in Hf/Ta superimposed system, is topical. Until now, such systems were not recognized in the literature for the ReRAM applications. This is highly promising since both memory and electrical characteristics are improved by the forming-free nature of the memristors with CFs mediated by oxide nanostructuring. I. Zrinski et al., Nanomaterials 11, 1 (2021) https://doi.org/10.3390/nano11030666 I. Zrinski et al. Appl. Surf. Sci. 565, 150608 (2021) https://doi.org/10.1016/j.apsusc.2021.150608 I. Zrinski et al., J. Phys. Chem. Lett. 12, 8917 (2021) https://doi.org/10.1021/acs.jpclett.1c02346 Figure 1

The conventional memory technology, based on von Neumann architecture, has challenged technical and scaling limitations. The architectures with separated memory and computing units have resulted in the low efficiency of computing systems. Hence, huge scientific attention has been recently drawn into the development of novel systems using the in-memory computing concept. Memristors are foreseen as a type of non-volatile memories that can fulfill such requirements but also as electronic components that can solve sneak path currents problems while consuming low power. The information storage mechanism is based on the resistance change of memristors which is driven by an applied external electric field. With this regard, the memory medium plays a crucial role, being an oxide layer sandwiched between the metallic bottom and top electrode. Valve metals and their oxides have shown promising switching properties when used as elements of metal-insulator-metal memristive (MIM) structures. The switching mechanism still depends on the nano-dimensional conductive filaments formation inside the oxide. Even though devices based on Hf, Nb, Ti, and Ta demonstrated a long lifetime, high data retention or resistive state ratio, devices are suffering from low lifetimes in the order of several thousand switching cycles, mainly due to the unpredictable size and position of conductive filaments being affected by the movement of cations or O species. Therefore, the development of materials for defect-engineered memristive applications is crucial. This was the main motivation for this study supported by the fact that memristors based on pure valve metals exhibited promising switching behavior. In this work, devices based on Ta, Nb, and their alloys were investigated. Since the goal was to improve the properties of devices based on pure metals, Nb and Ta were mixed and their alloys were used as bottom electrode for anodic memristors. It can be inferred that by mixing Ta and Nb, multifunctional devices can be produced. High number of Nb-Ta alloys were co-deposited by sputtering onto previously oxidized Si wafers. Afterwards, the samples were anodized for growing an insulating active layer for memristive devices. Onto the so-formed mixed oxides, Pt was deposited as top electrode. The compositional gradient was mapped by a scanning energy-dispersive X-ray spectroscopy (SEDX) system. The total compositional spread of the Nb-Ta library was ranging from Nb - 13 at.%Ta to Nb – 80 at% Ta over three Si wafers. Memristive devices based on Nb or Ta were also fabricated in identical conditions and used as reference samples. By using a combinatorial approach, the investigation of various electrical parameters could be established for more than 300 memristive devices produced per Si wafer. Electrical characterization was performed by a self-developed Gantry robot controlled via Labview software specifically designed for basic tests required for memristive characterization (I-U sweeps, endurance, retention tests). Electrical properties were screened with a compositional resolution of 1 at.%. The results have shown that memristive devices based on Ta reached a long lifetime and high resistive state ratio, whereas devices based on Nb indicated multi-level switching as well as non-volatile and threshold switching characteristics. The formation of Ta-oxyphosphate, responsible for conductive filaments pining, was confirmed by hard X-ray photoelectron spectroscopy. Similarly, multi-level switching characteristics were explained by the incorporation of electrolyte species in Nb2O5. However, Nb devices did not show reversible switching from unipolar to bipolar mode and vice versa. The improvement regarding reversible switching between unipolar and bipolar mode has been observed for memristive devices based on alloys containing Nb -35 at. % Ta. These memristors have also demonstrated longer endurance and data retention as compared to devices based on pure Nb and Ta. The switching mechanism was investigated by conductive filaments imaging using transmission electron microscopy. Conclusively, anodic memristors based on Nb and Ta exhibited non-volatile and threshold characteristics which can spread the range of memristive applications. Reversible switching behavior and multi-level switching possibilities are relevant for the development of memristive synapses and neuromorphic systems. The anodic growth of an insulating layer for devices based on Nb, Ta, and Nb/Ta alloys has simplified, speed up and decreased the cost of fabrication processes. The performance of anodic memristors based on metallic thin films containing Nb -35 at. % Ta indicated a substantial improvement. The switching mechanisms could be suggested based on the nanostructures found in oxide revealing the position and size of conductive filaments. I. Zrinski et al., Appl. Surf. Sci., 548, 149093 (2021) https://doi.org/10.1016/j.apsusc.2021.149093. I. Zrinski, et al., Appl. Mater. Today, accepted, (2021). I. Zrinski, et. al, J. Phys. Chem. Lett., 12, 8917 (2021) https://doi.org/10.1021/acs.jpclett.1c02346 Figure 1

A combinatorial screening approach has been designed for identification of Hf-Ta anodic memristors with enhanced properties. Three distinct compositional zones with similar characteristics have been identified, two of which are relevant for memristive applications. In a Hf-poor zone, (

In operando quantification of field-assisted ion release during high-voltage anodisation (up to 100 V SHE) of Nb in 0.1 M sulphuric acid was performed. Electrochemical high-field oxide formation under both potential and current control was studied separately. The quantification of in situ ion release via ICP-MS revealed an increase of the oxide dissolution factor (from 337 to 422 fm V−1) when decreasing the potential scan rate from 200 to 100 mV s−1. Dissolution rates measured during galvanostatic oxide formation allowed measuring oxide dissolution factors of 719 and 837 fm V−1 for current densities of 1.0 and 0.5 mA cm−2, respectively. As compared to the potentiodynamic case, higher dissolution rates and oxide dissolution factors were measured for galvanostatic anodisation. The overall fraction of the charge used for generation of soluble Nb species was below 0.4% for all oxide growth regimes. Cross-sectional SEM imaging proofs an oxide formation factor of 2.1 nm V−1. The surface of anodised films was extremely smooth and featureless without any cracks or voids. Based on X-ray diffraction, the films were found to be amorphous, indicating that no field crystallisation is occurring under the applied oxide growth conditions even at higher voltages.

Localized-plasmon voltammetry (LPV) bears great potential for electrochemical sensing applications beyond conventional cyclic voltammetry. In order to determine the limitations of this method, it is of utmost necessity to investigate the response toward chemical instability of the plasmonic electrode. We therefore investigated electrooxidation of a gold nanowire array with LPV in acidic electrolytes with different pH values. LPV shows excellent agreement with simultaneously recorded cyclic voltammograms up to the onset of oxygen evolution. Beyond that point, LPV still appears to provide meaningful signals. Further, with LPV the pH dependent reduction potentials of electrochemically grown gold oxides were determined and show a linear characteristic over the investigated pH range according to Nernst’s equation.

Anodic memristors obtained by electrochemical anodization of Ta in phosphate, borate and citrate buffer solutions are studied. Memristive behaviour is demonstrated by electrical switching between high and low conductive states. The endurance and retention of devices are analysed. The use of phosphate leads to 4 switching levels and the highest ratio between high and low resistive states. All studied oxides are stoichiometric Ta2O5 and only P is detected inside anodic memristors. The improved memristive characteristics of oxides anodized in phosphate are attributed to an increase of O vacancies due to the presence of Ta oxyphosphate, which is believed to mediate spatial pinning of conductive filaments positions during read/write. The anodic memristors show high stability, enhanced endurance and retention that combined with their active layer low fabrication cost makes them ideal candidates for industrial implementation.

We present an efficient strategy to identify anodizing electrolytes by coupling conventional Eh–pH diagrams with first-principles density functional theory calculations. Herein, the growth of ultra-thin films having a thickness of 10–20 nm is successfully demonstrated on thermally evaporated holmium. Considering only thermodynamic basis, simulated Eh-pH diagrams, and solubility analysis, electrolytes having different compositions and pH values are suggested for the efficient growth of anodic films. The Eh–pH diagrams are modified by incorporation of oxide–forming species in such a way to appreciably extend the stability domain. The predicted diagrams showed a strong agreement with the experimental observations and provide a better understanding of Ho–H2O based aqueous systems and can serve as a guide in other rare-earth elements anodizing. Grown film properties are investigated by using electrochemical impedance studies which disclose a linear increase in inverse capacitance with formation voltage indicating a uniform growth of anodic films irrespective of electrolyte selection. Formation factor, k, of anodic films grown up to 10 V varies from 1.16 to 1.95 nm V −1 in selected electrolytes. The k values greater than unity may contribute to the uniform film growth at different pH values.

A co-evaporation technique was used for depositing an Al-Er thin film combinatorial library on borosilicate glass substrates with Er concentrations between 3 and 21 at.% and a total compositional resolution of 0.25 at.% mm−1. Scanning droplet cell microscopy was employed for compositional mapping of several fundamental properties of mixed oxides grown on Al-Er thin film alloys. Microstructural and crystallographic particularities of Al-Er alloys were identified along the library, and a relevant compositional threshold at around Al-5 at.% Er was identified. Further, small surface grains in the nanometre range were found at Al grain boundaries, and an initial amorphization and/or hexagonal phase nucleation was identified for Er amounts around 5 at.%. Electrochemical studies revealed the importance of this compositional threshold, as distinct maximizations of oxide formation factors and oxide electrical permittivities were observed.

A simple and cost efficient glucose sensor was constructed using 3D printing having as active material a copper-15 at.% nickel thin film thermally co-evaporated on copper plated circuit boards. The glucose detection in alkaline solution was studied in detail by cyclic voltammetric and chronoamperometric measurements. The sensor suitability for being used in both quantitative and qualitative glucose detection was demonstrated and calibration of its response to various amounts of glucose revealed two linear regimes with different sensitivities. Glucose levels between 0 and 10 mM are most efficiently quantified as indicated by an amperometric signal increase of 240 μA cm−2 for each 1 mM increase of glucose concentration. The potentiostatic stability of the sensor was evaluated and its complete insensitivity after 7 h was solely attributed to the irreversible transformation of glucose into gluconolactone. A sensor life time of 20 cycles was demonstrated during potentiodynamic cycling when the sensor response remains constant at its maximum level. The magnitude of possible glucose quantification errors were evaluated as interferences induced by additions of ascorbic and uric acids. A worst case scenario of 96 % accuracy of glucose levels quantification was demonstrated using 25 times higher concentrations of interfering substances as compared to the glucose level.

Optical bandgap mapping of Nb-Ti mixed oxides anodically grown on a thin film parent metallic combinatorial library was performed via variable angle spectroscopic ellipsometry (VASE). A wide Nb-Ti compositional spread ranging from Nb-90 at.% Ti to Nb-15 at.% Ti deposited by cosputtering was used for this purpose. The Nb-Ti library was stepwise anodized at potentials up to 10 V SHE, and the anodic oxides optical properties were mapped along the Nb-Ti library with 2 at.% resolution. The surface dissimilarities along the Nb-Ti compositional gradient were minimized by tuning the deposition parameters, thus allowing a description of the mixed Nb-Ti oxides based on a single Tauc-Lorentz oscillator for data fitting. Mapping of the Nb-Ti oxides optical bandgap along the entire compositional spread showed a clear deviation from the linear model based on mixing individual Nb and Ti electronegativities proportional to their atomic fractions. This is attributed to the strong amorphization and an in-depth compositional gradient of the mixed oxides. A systematic optical bandgap decrease toward values as low as 2.0 eV was identified at approximately 50 at.% Nb. Mixing of Nb

Direct in situ Nb release measurements were performed by ICP-MS during Nb anodisation. In the potentiodynamic regime (0–8 VSHE, 10 mV s−1), constant electrochemical current (45 μA cm−2) and Nb dissolution rate (70 pg s−1 cm−2) confirm the high field oxide growth. The valve metal character of Nb is demonstrated during the reverse scan when both electrochemical current and dissolution rate decrease, indicating that Nb release is solely triggered by the high field conditions. It was shown that an extremely small fraction of the current (0.75%) was responsible for Nb dissolution. During 1000 s of potentiostatic anodisations up to 9 VSHE the dissolution rates decreased from 100 pg s−1 cm−2 to 10 pg s−1 cm−2 and were slightly potential dependent, with higher dissolution rates at higher anodisation potentials. The electrochemical current transients decayed faster than the dissolution rates, leading to an overall increase in the dissolution current fractions with time (up to 4.5% at 3 V). This fraction was also found to be slightly potential dependent. Interestingly, higher anodisation potentials resulted in lower dissolution fractions during potentiostatic oxide formation. Overall, the potentiostatic dissolution fractions were at least twice as high as in the potentiodynamic case. Keywords: Anodisation, Valve metals, Inductively coupled plasma mass spectrometry, Nb dissolution, Electrochemically induced corrosion

Co-evaporation of Cu and Pd was used for the deposition of a relatively small gradient concentration thin film combinatorial library with Pd amounts between 4 and 14 atomic percent (at%). Screening for electrocatalytic oxidation of formaldehyde was performed by scanning droplet cell microscopy along the Cu–Pd compositional spread in alkaline solution and a best material performance for this process was identified for 7.5 at% Pd in Cu confirming results from co-sputtering studies. However, the microstructure and crystallographic analysis of Cu–Pd thin film alloys showed a compositionally induced gradual change of properties without any significant discontinuity. This indicates that the Cu–Pd atomic ratio is the main factor defining the electrocatalytic activity of the investigated alloys. This finding is also confirmed for bulk Cu–Pd alloys where the reproducibility of significant formaldehyde oxidation electrocatalytic activity when using Cu-7.5 at% Pd was demonstrated. An amperometric formaldehyde sensor was fabricated and its reproducibility, repeatability and stability were assessed. During successive anodic formaldehyde current oxidation peak observations a standard deviation value of 8% was measured. Multiple efficient successive use of the same sensor (5 to 10 times) were demonstrated and a maximum of 5% decrease in the current density was observed after 21 days of normal environment storage during a shelf-lifetime evaluation of the sensor. Overall, the study reveals inexpensive approaches for fabrication of multiple use formaldehyde sensors via thermal evaporation or bulk alloy casting, as well as the transfer of the main feature (i.e. maximum current density for formaldehyde oxidation) from a thin film combinatorial library to bulk samples.

Doped ZnO thin films on ITO substrates were prepared by reactive co-sputtering of ZnO and several dopant metals, namely Al, Mn, Ti, W or Zr. To elucidate the influence of the dopant, morphological and compositional investigations were performed applying SEM/EDX, XRD and AFM. The optical band gaps of the materials were determined by UV-VIS measurements and the subsequent analysis of the derived Tauc plots. SKP (Scanning Kelvin Probe) measurements were performed under alternating illumination periods in order to measure the CPD (contact potential difference) response on UV irradiation; effective donor concentrations were calculated from the SKP results. The obtained X-ray diffractograms revealed that W : ZnO is amorphous, whereas all other dopants form crystalline structures with diffraction angles shifted towards lower values. SEM and AFM imaging revealed a significant influence of the dopant on the film morphology. The optical band gap values are in the range of the ZnO value (∼3.30 eV), with the lowest value of 3.29 eV being measured for Mn : ZnO. An exception was found for W : ZnO, which exhibits significant band gap widening reaching 4.35 eV. The effective donor concentrations are low for all samples under dark conditions, whereas they showed enhanced values under illumination. The sensitivity of all materials towards illumination makes them promising candidates for future research activities in the field of photovoltaics.

Cobalt-based alloys are used as total hip replacement prostheses due to their exceptional tribocorrosion properties in the human physiological environment[1]. Due to continuous mechanical and corrosive conditions, the dissolution of Cr and Co in the body fluid is unavoidable. These metallic particles and ions can cause cancer and gene mutation in the body, therefore their leaching should be diminished by proper passivation[2]. In this work, two combinatorial thin film libraries namely a binary Co-Cr and a ternary Co-Cr-Mo were prepared by means of co-sputtering. Both libraries were characterized by state of the art high resolution scanning electron microscopy (SEM) and X-ray diffraction (XRD). The chromium content for binary Co-Cr library varied from 14-40 at.% (balanced Co) while for ternary Co-Cr-Mo library a compositional spread of Co (46-79 at.%), Cr (17-48 at.%) and Mo (3-11 at.%) was obtained. In both libraries roundish spike shaped grains were revealed at high cobalt content which gradually changed to refined and compact grains with the increment of Cr. Similarly, molybdenum (>-7 at.%) brought additional grain refinement. Furthermore, weak, and broad XRD peaks of rt HCP and ht FCC pure Co were disclosed at nearly all compositions. No intermetallic phase was determined in either library. The real time electrochemical screening and corrosion stability were accomplished by a in house-built flow type scanning droplet cell microscope (FT-SDCM) in conjunction with an inductively couple plasma optical emission spectroscope (ICP-OES). Various electrochemical techniques i.e. open circuit potential, linear sweep voltammetry and Tafel extrapolation were utilized. No significant passive behavior was obtained for the alloys containing the Co content higher than (68-70 at.%) irrespective of Cr and Mo concentration. However, further increment in the Cr content significantly improved the alloy nobility and restricted the active to passive dissolution. The addition of Mo improved the transpassive dissolution. A graphical model was developed on the basis of this study and a composition range of (7-10) at.% Mo and (36-48) at.% Cr (complementary Co) is recommended as likely candidate for biomedical use. References [1] A. Aherwar, A. K. Singh, A. Patnaik, Cobalt Based Alloy: A Better Choice Biomaterial for Hip Implants, Trends Biomater. Artif. Organs, 30, 50–55, (2016); doi: 10.13140.2.1.2501.5284. [2] E. N. Codaro, P. Melnikov, I. Ramires, A. C. Guastaldi, Corrosion Behavior of a Cobalt Chromium-Molybdenum Alloy, Russ. J. Electrochem., 36, pp. 1263-1267, (2000); doi: 10.1007/02757531

In the field of semiconductor electronics, the metal-oxide- semiconductor technology (CMOS) has attracted a lot of scientific attention as one of the main representatives for non-volatile memories. The development of memory systems is crucial for a fast data processing for which purpose namely microprocessors are used, following the miniaturization trends at the same time. However, the limits of CMOS technology are already exhausted in terms of power consumptions, leakage current and processing speed. Surely, novel non-volatile elements and materials are necessary to fulfill such demands and memristors are in the front as the most promising candidates. Valve metals and their oxides are frequently used as the main constitutes of metal-insulator-metal (MIM) memristive structures. Recently, Hf and Ta became the most studied valve metals playing an important role in the next-generation of thin film micro- and nano-electronic devices primarily due to applications in redox-based resistive switching memories (ReRAMs), logic circuits, sensors and artificial neural networks. The operating mechanism of memristors is based on the electric field assisted conductive filaments (CFs) formation due to the ionic drift inside the insulating oxide layer. The understanding of CFs formation is a key point to ensure excellent switching and memory characteristics of memristive devices. In accordance with the reported studies, the regulated movement of oxygen vacancies or other mobile species through the CFs can allow their pining at the constant positions and consequently high switching reproducibility of a device. Therefore, the immediate findings on the oxygen enriched media that will ensure the controlled CFs formation is of high importance. With this regard, the selection of a proper electrolyte for the fabrication of the oxide layer sandwiched between two metallic electrodes may play a significant role. It can be assumed that incorporation of electrolyte species can increase the amount of oxygen vacancies leading to a possible manipulation with their movement. Tantalum and Hf were sputtered onto Si wafers to produce thin films. Both metals were electrochemically anodized to produce oxides responsible for the memristive properties. The aim was to replace expensive fabrication methods for oxides such as sputtering or laser deposition with cost-efficient, simple and rapid anodization. The bottom electrode was either Ta or Hf thin films, Ta2O5 or HfO2 were anodic oxide layers and Pt was patterned as the top electrode, that can be seen in the inserted Figure. Two groups of these samples were anodized in the same conditions, in three electrolytes (phosphate buffer (PB), borate buffer (BB), citrate buffer (CB)) to produce three different anodic oxide layers with the thickness in the range of 20 nm. For all cases, memristive switching between high and low resistance states was demonstrated and the corresponding I-U switching curves were compared. Additionally, writing (switching) and reading tests were done for high numbers of cycles and the endurance and retention of devices were analyzed. The same tests were performed when samples were thermally treated resulting in improved or worsened switching properties, depending on the solid electrolyte placed between the metallic electrodes. Unquestionably, the use of PB, CB and BB for anodization has differently influenced the electrical properties of the memristors likely due to the trapped electrolyte species inside the oxide layer. The incorporation was confirmed by the X-ray photoelectron spectroscopy (XPS) and the presence of CFs by transmission electron microscopy (TEM). Interestingly, Ta memristors formed in PB showed the presence of Ta oxyphosphate which has induced the increase of O vacancies and thus improved switching features. However, the presence of Hf-O-P bonds found in Hf memristors that was anodized in PB did not show any improvement of electrical properties. In summary, the current study offers a reliable, controllable and cost-efficient approach for the production of ReRAMs based on Hf and Ta anodic memristors as potential candidates for the future industrial implementation. Figure 1

The electrochemical fabrication of memristive devices based on Hf is demonstrated. Electrolyte incorporation in memristors is confirmed in oxides grown in 0.1, 0.5 and 1 M phosphate buffers. The impact of phosphate species on conductive filaments formation is described. The use of 1 M phosphate buffer allows formation of Hf-O-P compounds that hinder phosphate incorporation into the bulk of the memristors. Endurance, retention and memory characteristics of anodic Hf memristors suggest improved properties, as compared with previous reports, especially after mild heat treatments of devices. High resolution atomic imaging of conducting filaments allowed further understanding of memristive switching in HfO2.

An aluminium-samarium binary library with a varying Sm concentration between 4 to 14 at.% was produced using a thermal co-evaporation technique. Morphological and crystallographic characterization of the parent metal alloys revealed compositionally dependent surface structure and atomic arrangements. Grains resembling pure Al on the surface slowly disappeared with increasing Sm content and above 8 at.% Sm nucleation of the AlSm2 intermetallic phase was observed. Scanning droplet cell microscopy was used for a comprehensive electrochemical characterization along the Al-Sm compositional gradient. Anodic oxide formation under high field conditions was discussed for alloys below the compositional threshold of 8 at.% Sm. Above this threshold a continuous increase of Sm dissolution during anodization with increasing Sm concentration was proven by inductively coupled plasma optical emission spectroscopy. Coulometry followed by EIS allowed mapping of the oxide formation factors and oxide electrical permittivity as material constants for single Al-Sm alloys. A small increase of both material constants for alloys below the compositional threshold described the Sm contribution to the anodization process. An apparent enhancement of their values at alloys above the threshold was directly attributed to the increased Sm dissolution rates reaching values of 2 ng cm−2 s–1 at 12 at.% Sm.

Anodizing promises fast, cost-effective, and precise fabrication of mixed metal oxides from alloys of complex chemistries and compositions. Applicable to a whole class of valve metals and their alloys, this versatile fabrication method has found new dimensions of its applicability to a unique combination of valve and non-valve metals in the quest of finding new applications. Hereby the fabrication of anodic passive films on Al-Ce thin-film combinatorial alloy library covering 1.5 to 10.5 at.% Ce in a phosphate buffer electrolyte having pH = 8.3 is demonstrated. Few nanometre thin dielectric films were attained through the automated localized anodizing using scanning droplet cell microscopy (SDCM). Electrochemical measurements suggest a good correlation among alloy composition, film thickness, relative permittivity as well as film resistance. It is inferred that the formation factor, k greater than unity and stable growth of compact composite film is attainable through thermodynamic selection of electrolyte and Gibbs free energy minimization criteria.

A thin film dysprosium-magnesium-zinc combinatorial library has been deposited using a thermal co-evaporation technique. Scanning EDX, XRD, SEM and XPS were used to investigate the structure, surface morphology and species present. The entire library was found to be amorphous and morphological changes were visible by minor additions of Dy (approximately 1 at.%) and medium to high Zn contents (> 20 at.%). Open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarization measurements revealed a higher corrosion resistance for Zn-poor alloys (

Wide compositional range Mn–Si thin film combinatorial libraries were used for studying the influence of the oxidation potential of the annealing atmosphere (510–12,500 ppm H 2 O at 900 °C) on phase formation. For all annealing conditions, the formation of silicide phases was evidenced. A threshold for O incorporation in newly formed crystalline phases was found to be related to water contents between 510 and 2880 ppm. In spite of the influence of high potential for oxidation on phase formation, silicide phases were identified for Si-rich alloys. The formation of higher manganese silicide (HMS) phases was identified in the compositional zone ranging from 56 at.% to 80 at.% Si and the exact phases were determined to be influenced by the annealing conditions. With higher water content in the atmosphere, the formation of Mn 4 Si 7 was favored. However, independent on the increase in the water content the HMS formation was generally not inhibited by the applied annealing atmosphere. Given the wide range of possible application of HMS, this is highly relevant for the formation of HMS bulk or thin film materials.

Variable angle spectroscopic ellipsometry (VASE) was used for optical property mapping of anodic oxides grown on a wide spread Nb–Ta thin film combinatorial library (Nb content ranging between 8 and 84 at%) with 2 at% resolution. The microstructure of the parent metal alloys was tuned by modifying the library deposition conditions in order to avoid compositionally induced microstructure dissimilarities between the alloys. This allowed an interpretation of the mixed oxides properties based on a single Tauc–Lorentz oscillator for data fitting. Upon anodization up to 10 V vs. SHE, VASE was used to fit the oxide thickness in order to map the oxide formation factors with the results showing a good agreement with the electrochemical data. The measured bandgap values of the Nb–Ta oxides show a deviation from the linear model based on mixing individual Nb and Ta electronegativities proportional to their atomic fractions. This behavior was explained via an in-depth compositional gradient in the oxides. Minority species are depleted at the metal/oxide interface triggering a non-linear shift in the resulting alloy electronegativity. The Nb–Ta mixed oxides bandgap tuning via compositional tuning of the parent metal has been demonstrated and absorption close to the edge of the visible spectrum was identified above 30 at% Nb.

The oxide formation on valve metals such as Al, Nb, Ti, Ta has been studied for a long time. However one major drawback was always the unrealiability of the coulometric data. While the electrochemical current and thus the charge can be measured with very precision, it was simply not clear which reaction is actually consuming the charge or with which share are contributing concurrend reactions. The assumption of an almost complete consumption of the charge for oxide formation has often been read in papers. But especially the small losses which are usually neglected tell us a lot about the mechanism of oxide formation and lead to a modern basic understanding of the concurrent reactions. High purity valve metals namely Al, Hf, Nb, Ta, Ti, Zr were anodised in voltage step experiments, in cyclic voltammetry or galvanostatically using a flow type scanning droplet cell microscope (FT-SDCM) with an ultra trace analytical downstream electrolyte analysis using an inductively coupled mass spectrometer. Experiments reveal that even the most corrosion stable element tantalum undergoes a measurable dissolution during anodisation which clearly scales with the field strength in the oxide and in the interface between oxide and electrolyte. This dissolution is roughly 10 times higher for niobium and 100 times for titanium. A comparison of the electrochemical current during titanium anodisation and the corresponding chemical current proves that the onset of oxygen evolution at some 3.5 V SHE is definitely not an ionic but an electronic current. However this electronic current can be driven only under high field conditions in which a significant number of ionic charge carriers act as stepping stones for electron movement in the oxide. Spatial resolution and time resolution for these experiments are being analysed to put these method in the context of other corrosion evaluation tools. In particular when studying material libraries of laterally graded materials the spatial resolution of the FT-SDCM is of importance as it alloys allows adressing specific compositions.

A Co–Cu–Ni thin film material library was fabricated using a modified 3D-printed Hull cell. Rather than previous one by one attempts a combinatorial approach was here not to miss the exact optimum performance composition. An optimum activity at Co-16 at.% Cu-12 at.% Ni for the electrocatalytic glucose oxidation was found by screening along the compositional spread using flow-type scanning droplet cell microscopy (FT-SDCM). Crystallographic properties, surface microstructure and surface topography were investigated along the entire compositional spread using X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as atomic force microscopy (AFM), respectively. The entire compositional spread (Co 40–76 at.%, Cu 0–19 at.% and Ni 11–60 at.%) showed suitability for being used in glucose detection, as determined by cyclic voltammetric measurements. A maximum response for glucose electrooxidation was identified at a Ni content of 12 at.% with a current density value of 21.8 mA cm−2. The performance of a glucose sensor constructed using this identified thin film alloy was evaluated by localised amperometric detection. A linear behaviour was obtained for a wide range of glucose concentrations in the electrolyte solution between 1 mM and 100 mM. Additionally, the identified thin film alloy showed high stability, good electrooxidation reproducibility and high selectivity, thus indicating its suitability for being used as catalyst in glucose detection.

The lanthanoids are a consequent series of elements and with their 3 valence electrons they prefer the trivalent state while continuously filling the inner f-shell. A specific energetically stabilizing effect is observed so that the empty, the half-filled or the completely filled f-shell show extraordinary low energies resulting in a stabilizing effect so that neighboring valence states such as divalent or tetravalent can be also observed. Interestingly with increasing atomic number the atomic and ionic radii decrease - counter intuitively. As a result of the electronic shielding effect of the f-electrons the so-called lanthanoid contraction is observed. Aluminium is another element that forms preferentially trivalent ions. A combinatorial approach in which aluminium-lanthanoid-alloys are investigated both in terms of the alloy itself but in particular for the passivity based on the mixed oxides forming on these alloys is presented here. A complex attempt was made not only to cover the various elements themselves but also their concentration ratios in material thin film libraries. These material libraries are prepared in a large preparation and analysis cluster the so-called CALMAR. Asymmetrical physical codeposition of the constituting elements is used in HV and UHV chambers to prepare these libraries and to characterize them by a scanning EDX system as well as XRD. Theoretical considerations were used to compute the Pourbaix diagrams. Also phase diagrams of the Al-Lx alloys are consulted to identify intermetallics and regions of solid solution mixing. Passivation studies were performed using a flow type scanning droplet cell microscope FT-SDCM. This method allows the subsequent stepwise anodization and electrochemical impedance spectroscopy characterisation of a single spot of a library having a single composition. Spatial scanning of the library yields detailed information of the systematically changing alloy compositions. To understand not only the anodic oxide formation but also the chemical and electrochemical reactions during anodization a complementary investigation of the electrolyte is performed using inductively coupled plasma spectrometry. Various lanthanoids were mixed with aluminium and electrochemically studied. It is found that the large ionic radii mismatch is likely to cause strong amorphisation. In this way already binary alloys were found to form metallic glasses in some regions i.e. for some compositions. This has a strong impact on the passivation behavior and electrochemical stability. Some distinct intermetallic compounds in these binary alloys were found with the expected consequence for the stability.

Combinatorial thin film deposition from vapor phase combined with photolithography was involved in preparation of bidimensional wires for electromigration testing along Al-Cu, Al-Sm and Cu-Sm thin film libraries. Samarium was chosen as alloying element due to its larger atomic radius, directly impacting the crystallographic properties of the conducting alloy and decreasing the total grain boundary surface through amorphisation. As reference, pure Al and Cu wires were additionally deposited and studied. Empirical estimation of the experimental errors obtained during electromigration testing was performed by high throughput measurements and a value around 2% was found for pure Al and Cu metals. The resistance against electromigration of test wires was increased by electrochemical passivation (leading to void pinning), while the empirically evaluated experimental errors increased to 30%. Electrical screening of Al and Cu-based thin film combinatorial libraries was performed at extremely high current densities (MA cm−2). In the Al-Cu system, Al–14 at.% Cu was identified as the alloy able to withstand the highest power density of 2.10 MW cm−2 before failure. The Al-Sm and Cu-Sm thin film libraries showed compositional regions of total or partial amorphization likely impacting the electromigration resistance of the alloys. Electrical screening indicated that Al-6 at.% Sm is the alloy withstanding the highest power density before failure with a value of 2.48 MW cm−2. The highest current densities before electrical failure in conducting lines were measured on pure Al at 1.39 MA cm−2, with a value of maximum power density below 2.62 MW cm−2. Careful discrimination between these two variables is necessary before choosing one of the studied materials as ideal for specific applications.