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Controlled atmosphere high-temperature scanning probe microscopy (CAHT-SPM)
Controlled atmosphere hightemperature scanning probe microscopy (CAHT-SPM)
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K.V. Hansen Technical University of Denmark, Lyngby, Denmark
9.1
Introduction to high-temperature SPM
Scanning probe microscopy (SPM) is a versatile methodology [1] comprising a number of techniques or modes for acquiring local information from a sample by the use of a fine probe. The probe may be scanned over a microscopic area obtaining lateral information and resulting in images, or it may be located in a point for performing local measurements of various kinds. Depending on the probe properties and the specific technique information on, e.g., topography, mechanical, electrical, electrochemical, and magnetic properties can be obtained. SPM includes scanning tunneling microscopy, which was the first technique invented by Binnig et al., demonstrating atomic resolution in vacuum [2]. A few years later, in 1986, the atomic force microscope working on insulating materials in air was demonstrated [3] and SPM is thus a relatively young technique. Danish Micro Engineering A/S (DME), established in 1979 by Dr. Curt Sander, was a Danish company dedicated to consultancy on microprocessor technology and software development, which began developing SPMs at the end of 1987, and through the next 27 years, they developed both standard atomic force microscopes and highly customized equipment, including the two controlled atmosphere high-temperature scanning probe microscopes (CAHT-SPMs). In Jul. 2015, DME was acquired by the Hungarian company Semilab Zrt. (ref. www.dme-spm.com). The first prototype CAHT-SPM, CAHT-I, was developed in 2006 to allow investigations of metals and metal oxides at temperatures relevant for solid oxide fuel cell applications. The scope was in operando electrical and electrochemical investigations on a micro- to nanoscale at 400–800°C in reducing and oxidizing atmospheres. A few years later and based on the experience with CAHT-I, but with a totally new design and a number of new modes, the second prototype, CAHT-II, was developed in 2011. The CAHT-SPMs were the first microscopes to be developed specifically for experiments at high temperature combined with a well-defined gas atmosphere. At that time, the literature concerning high-temperature SPM under ambient pressure conditions was very scarce. Since then, a number of different types of samples and techniques have been explored with the two CAHT-SPMs [4–10] and case stories that
Metal Oxide-Based Thin Film Structures. https://doi.org/10.1016/B978-0-12-811166-6.00009-1 © 2018 Elsevier Inc. All rights reserved.
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describe some of them are presented in the next sections along with some preliminary results from ongoing investigations. SPM at several hundred degrees Celsius is a challenging discipline as outlined below, and today the number of publications on high-temperature SPM is still very limited as a very recent and comprehensive review by Nonnenmann [11] shows. Nonnenmann and his group have themselves published a number of articles showing their work on developing an environmental chamber to be used with their commercial atomic force microscope (AFM) [12] and work on fuel cell materials at 600°C [13,14]. Their system has some advantages such as easy mounting of cross-sectional samples and the use of commercial AFM tips, which increases the resolution.
9.1.1
Challenges
The high temperature puts demands on the materials in proximity to the hot zone. This includes the piezo translator and the detector electronics which need to be cooled, the entire assembly of the furnace, and fixation of the sample on the furnace. Circulating cooling water driven by a pump may induce noise in the images. Probes for high-temperature in situ or in operando applications must be chemically and mechanically stable against the high temperature and the atmosphere. This limits the number of available materials, and the temperature also influences the possibilities of fixing the probe. The temperature of the cantilever does not exceed a few hundred degrees Celsius, but this leaves out gluing of the probes and only a mechanical solution is stable enough. Coated probes may pose a problem as the thin metal coating may be damaged, e.g., by island formation. Besides the stability, it is also essential for many of the applications that the probes have sufficient electrical conductivity. Pt-Ir probes that are mechanically and chemically stable in the relevant atmospheres and also resistant to even the highest temperatures appear to be a good choice. In the CAHT-SPMs, Pt-Ir probes made in-house [9] are used for the most hightemperature experiments, but commercial probes are currently also being tested and used. There is an established procedure for fabrication of the Pt-Ir probes involving electrochemical etching of the Pt-Ir wire to produce a sharp tip. The tips can be made quite sharp even though not as sharp as silicon-based commercial probes. They are attached to the holder by spot welding, which ensures that the probes are well fixed and has a good electrical contact to the holder. The cantilevers are quite long and wide and reflect the laser beam well, but their geometry keeps the resonant frequency very low, in general, below 10 kHz. In-house made, electrically conducting ceramic probes fabricated by laser micromachining and focused ion beam milling have been demonstrated at 350°C. Commercial probes are challenging due to the short cantilever length as it limits the space for positioning of the laser beam and more importantly, they are difficult to fix sufficiently to the holder. Sample drift is always decreasing the image quality for scanning probe techniques, and the elevated temperature contributes significantly to this as the different parts of the system are heated at different rates. After reaching the desired temperature, some time for stabilization and equilibration is necessary. In a drift study [8] at 600°C after temperature equilibration, the drift was less than 1 μm in X- and Y-direction over
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26 min. The scanned images were 5 5 μm2 and otherwise acceptable. Thermal drift in this range with can probably not be avoided with CAHT-II, and for smaller scan areas and longer scan times, it will be more problematic. Thermal gradients across and laterally in the sample are unavoidable with the geometry of the system where the sample is heated from one side only. Since the scanned area is close to the symmetry axis of the furnace, the temperature differences here are mainly due to cooling through the probe tip. Finite element calculations and experiments have shown that large temperature gradients are present in the vicinity of such contact points [15]. This means that during surface scanning, the effective temperature at the contact point will depend on the tip-sample contact area as well as the scan rate. In stationary electrochemical experiments where the distance between the counter and reference electrodes is in the millimeter range, the equilibrium potential of the working electrode may differ from that of the reference electrode by tens of millivolts. Another challenge in the investigation of solid oxide cell materials is to fabricate samples that are suitable for in situ or in operando measurement and that still resembles the original microstructures. Local performance measurements require physical access to the part of the cell/sample to be investigated and for multilayer structures, different geometries are necessary to obtain information of other layers that the top one. This often requires a model sample where preparation techniques and materials are different from those of the full cell and the results may reflect this. The contacting of electrodes with platinum wires on small samples as well as a correct positioning of counter and reference electrodes are not trivial. In situ and in operando experiments are always quite challenging especially when they involve temperatures in the range of several hundred degrees Celsius, and there will always be issues and conditions that are less optimal than desired, and especially so in prototype instruments.
9.2
Importance of in situ and in operando local probing measurements
The surfaces and interfaces of functional materials are often crucial for their performance as they are the critical locations for vital reactions or processes. Some properties are a function of temperature and/or atmosphere, e.g., conductivity, and can only be measured at certain conditions. The same is the case for electrochemical reactions that depend on the nature of the surfaces, such as chemical composition and the gas atmosphere that interacts with the electrode surface. The structure and composition of electrode surfaces are influenced by temperature and atmosphere and may quickly adjust to new conditions such as cooling and atmosphere changes. Performance characterization of solid oxide cells (SOC) is often performed by electrochemical impedance spectroscopy (EIS) where full cells, symmetrical cells, or model electrodes are investigated at relevant operating conditions. It is thus always an in operando technique where the bulk electrode/electrolyte is characterized. Gradients in temperature, atmosphere composition, and electrical load across the cell, however, lead to different conditions and mechanisms depending on the exact
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location in the cells. The local performance is dependent on the nature of the specific locations, and it is thus desirable to be able to perform local measurements in selected places. In the literature, it is found that there is an increasing but still a limited number of articles dealing with in situ or in operando measurements at temperatures relevant for solid oxide fuel cells. The difference between in situ and in operando experiments is that for in situ experiments some of the parameters temperature, atmosphere, and electrochemical environment must mimic operation conditions whereas for in operando experiments they all must be in the realistic regime. This leaves all techniques in vacuum or a very diluted atmosphere such as X-ray photoelectron spectroscopy, scanning photoelectron microscopy, transmission electron microscopy, as in situ techniques. The group of in operando techniques contains among others scanning probe, Raman spectroscopy, and X-ray diffraction. Both scanning probe techniques and Raman spectroscopy may provide local and spatially resolved information. The advantage of in situ or in operando measurements are obvious that as the conditions are similar to operating conditions, the results will more likely show the properties of the materials while they are functioning, and thus the active reaction and degradation mechanisms can be studied while they are at play. Using several in situ and in operando techniques to obtain different types of data is essential to gain the full insight into the complex processes taking place (spatially and temporally) at operating conditions. In situ and in operando materials characterization is thus a cornerstone for understanding how materials work but they are very demanding experiments, and even though a lot is gained by performing in situ and in operando experiments, complementary ex situ techniques, such as TOF-SIMS, XPS, or LEIS, are still valuable, combining and correlating the different results, preferably in three dimensions [16].
9.3
The CAHT-SPMs
The CAHT-SPMS are basically working as normal AFMs with a fine probe scanning the surface and a laser-detector feedback system [1]. The main difference is that the sample is located on a furnace and that cooling of different parts is implemented. The CAHT-SPMs are both sample scanners, and the probe is thus stationary. This is to keep the sensitive electronics as far from the high temperature zone as possible. The SPMs are designed to be very flexible regarding the types of probes that can be accommodated and the possibilities for connecting external equipment such as potentiostats or lock-in amplifiers.
9.3.1
CAHT-I
CAHT-I was the first SPM that was developed for temperatures exceeding 600°C in ambient conditions. As a prototype, it was tested and improved in several steps as limitations were discovered through actual experiments, and many improvements were made following the first publication [17]. It can reach a sample surface temperature
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of 650–700°C in dry or humidified mixtures of nitrogen and oxygen and in 9% hydrogen in nitrogen. The humidification is realized by bubbling the gas through water at room temperature. CAHT-I is working in contact mode only and is capable of performing topography scans and conductance mapping, and in stationary mode, all the functionality of a potentiostat is available. CAHT-I consists of a bottom scanner part where the furnace and sample are also located, and an upper part with the electronics. The upper part can be lifted off, and the probe is mounted on this part. The in-house made probes are welded to a stainless steel holder, but commercial probes can be accommodated on a special ceramic holder. Electrically conducting ceramic probes have been fabricated and demonstrated at elevated temperature for conductance mapping. The sample is heated by a Linkam furnace TS-1200 and is fixed with Pt springs on a stainless steel “hot plate” on top of the furnace. Surface (minimum) temperatures are measured by fixing a thermocouple on top of the sample. There is a humidity sensor in the chamber for measuring relative humidity, and the gas outlet can be connected to a potentiometric oxygen sensor, and both are continuously logged along with the temperature of the water, the furnace temperature, thermocouples, and piezo temperature. The last part is for safety reasons, and a relay will disconnect the furnace if the piezo temperature becomes critical. The piezo scanner and the electronics parts are water cooled. The cooling system uses the siphon principle, so that the cooling water does not need a pump for circulation.
9.3.2
CAHT-II
CAHT-II (Fig. 9.1) is a more advanced SPM [6] and possesses many advantages compared with CAHT-I. This includes a parallel scanner and a better camera for positioning the probe in selected locations (Fig. 9.2). It is more gas tight and has a smaller volume sample chamber which decreases the time for gas exchange. Fig. 9.3 shows a sketch of a cross-section of CAHT-II. The sample and lower chamber are separated by a flexible membrane that allows the movement of the furnacesample assembly by some millimeters in the X and Y directions and also during scanning, avoiding to disturb the precise movements by the piezo translator. The probe on the holder is inserted from the side through a hole in the wall and positioned at a small angle to the sample surface. The furnace from HeatWaveLabs Inc. is made of alumina and Pt wire, and the sample is fixed on top of it with alumina pins attached to metal springs (Fig. 9.1B) located away from the hot zone, a preferable set-up compared with CAHT-I. The maximum sample surface temperature is 850°C. CAHT-II works in contact mode, tapping mode, and lift mode and is capable of performing Kelvin probe force microscopy, scanning tunneling microscopy and spectroscopy, and conductive AFM in addition to the techniques available from CAHT-I. It is also possible to measure other properties depending on the external set-up. For stationary measurements, sample drift may be an issue but for shorter time spans (minutes), this is not a major issue. It was specifically demonstrated for CAHT-II [8] that drift is in the less than a micron in the X and Y directions over a period of 26 min.
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Fig. 9.1 (A) CAHT-II from the outside. (B) The sample chamber of CAHT-II. Reproduced from K.V. Hansen, Y. Wu, T. Jacobsen, M.B. Mogensen, L. Theil Kuhn, Improved controlled atmosphere high temperature scanning probe microscope, Rev. Sci. Instrum. 84 (2013) 073701, with the permission of AIP Publishing.
Fig. 9.2 A camera image of a probe situated on a microelectrode with a diameter of 50 μm.
Conductance mapping [8] has proved to be a technique that can be advantageous for many different types of samples. In the conductance maps, an AC voltage is applied between the probe and the counter electrode from the generator of the lock-in amplifier (LIA). Usually, an amplitude of 0.5 V rms and a frequency of 10 kHz are chosen. The conductance is calculated as the ratio of the in-phase current and the voltage. With a scan rate corresponding to 1 s per line, the result is a weighted average of 40 to 80 cycles depending on the resolution of either 128 or 256 pixels per
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Fig. 9.3 A sketch of a cross-sectional view of CAHT-II.
line and normally a good signal-to-noise ratio is obtained. Experience has shown that with the present set-up conductances ranging from a few nS to 100 μS can be determined. The lower limit is determined by stray capacities shunting the probe-sample resistance and the upper by the 1-MΩ resistor inserted to avoid overloading of the LIA current input amplifier. The property actually determined in the conductance maps is the integral of the local resistivity from the contact surface to infinity converted to conductance. In the case of a homogeneous medium, the contribution from a volume element decreases with the distance from the probe contact squared. However, in less-trivial heterogeneous systems, the conductance is the result of a complicated averaging of the local properties. Fig. 9.4 presents four different situations of local environments (blue semicircle) for a probe. The grayscale of the grains indicates high (light) or low (dark) conductivities. Fig. 9.4A shows the current field (red lines) and a homogeneous system with high-conductivity grains. In Fig. 9.4B, low-conducting grains are found at the surface but with higher conducting grains below a higher apparent conductivity compared with the surface grains will be measured. Fig. 9.4C shows a local environment with high-conducting grains at the surface but lower conducting grains below. The resulting conductance will be lower than what is expected from the high-conducting surface grain. In the case of a heterogeneous environment with low-conducting grains, the probe will record a low conductance. The CAHT-SPMs have proved to be very versatile instruments that may be used in a wide temperature and atmosphere range for studies of surface-related properties. This includes especially the electrical or electrochemical area where either scanning or stationary modes can probe a number of properties. It may also be used for studying surface-related processes on time scales ranging from hours to min. The slow scan axis is then used as a time axis as illustrated by one of the case stories below. Furthermore, processes where the topography of the samples changes because of the effect of
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Fig. 9.4 Local conductivity environments below a probe situated on a surface. The light grey color signifies high conductivity and dark grey indicates low conductivity. (A) The probe is situated in an area with high conductivity, which gives a high conductance in the map. The current field below the probes is indicated with red lines. (B and C) In a sample with both highand low-conducting grains, the measured conductance depends on the local environment around the probe and is a result of complicated averaging in the volume indicated by the blue semicircle. (D) The probe is situated in an area with uniform and low conductance, and the measured conductance will thus reflect the situation well.
temperature and atmosphere or as an impact of electric potential may also be investigated. The possibilities of the CAHT-SPMs are still far from being exhausted; as shown later in the chapter, a present topic that is researched intensively is hightemperature scanning Kelvin probe microscopy. A mode that is yet relatively unexplored is STM and STS at elevated temperatures leaving still interesting properties to be explored at in operando conditions.
9.4
In situ surface reduction of NiO by hydrogen between 312°C and 523°C
The CAHT-SPMs may be used to study high-temperature surface reactions such as reduction and oxidation of metal oxides while they are occurring as demonstrated by this study of in situ surface reduction [5]. The difference in conductance of NiO
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and Ni is easily detectable by conductance mapping and can thus be used to distinguish between them. The reduction of NiO starts at the surface and works its way into the NiO grains, creating porosity simultaneously with the Ni formation. Reduction of NiO has previously been studied by different techniques [18–21] and there is a general consensus about the mechanisms. When it comes to details, however, there is some variation in the findings. Reduction of NiO is important in solid oxide fuel cells, where the reduction typically takes place at 800°C and for Ni-based catalysts that works at much lower temperatures and where reduction temperatures are determining for the performance of the catalyst [22]. Reduction of NiO is very fast at high temperature, and this limits the range in which such a reaction can be studied due to the relatively slow scan rate of the CAHT-SPM and the atmosphere exchange rate from nitrogen to 9% H2 in N2. The possible temperature window for the reduction experiments was 300–525°C [5]. This resulted in surface reduction times from several hours to a couple of minutes. The literature shows that water vapor has an influence on the reduction rate [18,23] and the capability of the CAHT-SPM to work in a humidified atmosphere was exploited to study the reaction in both dry and humidified hydrogen. The investigated samples were for convenience purposes symmetric cells, i.e., an yttria-stabilized zirconia (YSZ) electrolyte with NiO-YSZ anode and NiO-YSZ-Al2O3 anode support on both sides. It was thus anode support surfaces on 5 5 mm2 samples that were studied. Before the experiments, the sample surfaces were polished to ensure a flat surface for ideal imaging conditions. During the experiments, the samples were heated in air and a conductance scan was performed to depict the NiO and YSZ (+Al2O3) grains. Subsequently, the atmosphere was changed to nitrogen, and after an hour of stabilization, the atmosphere was changed to 9% H2 in N2 while scanning to capture the transformation from NiO to Ni. Fig. 9.5A shows a conductance image obtained in air at 462°C. High conductance is shown by yellow colors, that is, the green and yellow grains are the NiO grains, and the blue, low-conductance grains are YSZ and alumina. NiO is a p-type electronic conductor due to the presence of Ni3+. The conductivity depends on impurities and previous treatment [24] and may not be very high but is easily distinguishable from that of YSZ, in which the bulk ionic conductivity at 450°C is 3.2 104 S/cm. The different shades of yellow and green for the conducting grains are related to the local environment around the grain (Fig. 9.4). The CAHT-SPM is thus well suited to distinguish the fine details in conductance as long as the sample is not a good conductor [8]. Fig. 9.5C shows a conductance image obtained during reduction at 462°C. Approximately after 5 μm (time ¼ 0), the atmosphere was, as indicated, changed from nitrogen to 9 %H2 in N2 and a few scan lines later the conductance starts to decrease. This decrease of almost an order of magnitude was found to be very consistent among more than 20 reduction experiments that were performed. In Fig. 9.5D, several crosssections through Fig. 9.5C are displayed and they show the evolution of the conductance including the decrease in section B. For lower temperatures, the decrease was slower, and for higher temperatures, it was faster. The decrease in conductivity was interpreted as a result of the initial reduction of Ni3+ to Ni2+, which continued until the redox potential defined by the Ni3+/Ni2+
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Fig. 9.5 Conductance maps form of a reduction performed at 462°C in dry 9% H2 in N2. (A) The unreduced NiO-YSZ-alumina surface. (B) After surface reduction, there is no NiO left. (C) During reduction. The image is scanned in the direction from bottom to top. (D) Crosssections of (C) from 0, 5, 10, 15, and 20 μm that show the evolution of conductance with time.
concentration ratio reached the redox potential corresponding to the coexistence of NiO and metallic Ni. When this occurred, the formation of Ni could proceed. The low conductance is thus followed by an increase to much higher conductances when a percolating metallic Ni network is formed. As seen in Fig. 9.5D (region D), two levels of conductance are present: the low level, which represents YSZ and alumina, and the high level, which is due to metallic Ni. For the lowest temperatures, it was possible to observe an increasing conductance but within the experiment time, the reaction did not end with metallic Ni. For the higher temperatures, it was much faster, and above 525°C, it was almost instant. When metallic Ni starts to form on the surface, the reaction is autocatalytic and occurs very quickly at higher temperatures. After the surface reduction was completed, the conductance maps of the surface show only the two levels (Fig. 9.5B). The quality of the image is compromised by two factors. One is that there is a volume reduction of 40% involved and upon Ni formation, it withdraws from the surface. The second is that the tip degrades during the scans and it may also pick up some Ni, which makes it less sharp. This fits well
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with the topography images in which the scan lines during the reduction becomes disturbed and there are indications that some particles may be moved by the probe. When the surface reduction is complete, the topography images become stable again. In some cases, the developed porosity may be observed. In the literature, the reduction process is described to be dependent on both temperature and the presence of water, and with the CAHT-SPM, it was investigated whether the reduction proceeds differently in dry and humidified hydrogen. The reductions were thus performed in both atmospheres at different temperatures. Fig. 9.6 illustrates both the temperature and atmosphere dependence. Fig. 9.6A and B show images obtained in dry H2 at 380°C and 437°C. At 437°C, the surface reduction goes to completion within the scanned image but only at 380°C, a slight indication of increasing conductance is seen in the uppermost 5 μm of the image. Fig. 9.6C and D present images obtained in humidified 9% H2 in N2. At 525°C, the reduction goes to the completion within the image, but at 425°C, this is not the case. Comparing Fig. 9.6B and C, the retarding effect of water is clear as they are obtained at almost the same temperature. The length of the period from where conductance starts to decrease and until it increases again is equivalent to the incubation period. The
Fig. 9.6 In situ surface reduction at different conditions (A) at 380°C in dry H2, (B) at 437°C in dry H2, (C) at 428°C in wet hydrogen, and (D) at 523°C wet hydrogen. The dotted line indicates the time for change from N2 to 9% H2 in N2.
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incubation periods for the four experiments in Fig. 9.6A and B were 4.8 and 1.4 min, respectively, and 8.7 and 1.6 min for Fig. 9.6C and D. This study demonstrates that besides image acquisition, the CAHT-SPM is suited for studies of time dependencies by detecting the incubation period for the reduction of NiO to Ni. Furthermore, from the data, it was possible to deduce that two temperature regimes with different activation energies exist and that the effect of water slowing the reduction resulted in a longer incubation period. The detailed findings from this investigation are described in Ref. [5].
9.5
Local electrochemical measurements at 650°C to 850°C
EIS is generally used for studying performance and degradation of fuel cells, electrolyzer cells, and batteries where it provides information on a global level. In the CAHTSPMs, electrochemical measurements can be performed locally by placing the probe in the point of interest. Microelectrodes are small electrodes typically fabricated by thin film deposition and photolithography and for fundamental studies where it is desirable that the triple phase boundary (TPB) is accessible and where a well-defined geometry is beneficial. Microelectrodes are ideal as the large number of electrodes on a substrate allows sequential measurements on adjacent electrodes, e.g., at similar conditions. Microelectrodes have previously been studied with other types of setups, e.g., a microprobe station [25,26]. Each of the CAHT-SPMs has a Gamry Instruments FAS2 Femtostat potentiostat attached and it is possible to conduct various electrochemical measurements with the probe as the working electrode. A limitation is that as the probe is very fine impedances ranging up to GΩs are common, however, the potentiostat is optimized for small currents and handles it well. The electrical contact between the probe and the sample must be quite good and this may necessitate very large contact forces to be applied to the probe. This typically means forces much larger than those used for performing scans, and it also means that the tip will not necessarily be as sharp after some measurements. During local impedance spectroscopy, the probe is used as the working electrode and can either be situated in the desired location by viewing with the camera or a scan can be made and the probe can be located very precisely in the chosen location. In a general evaluation of the technique, CAHT-I was used to study microelectrodes of gold in air at 650°C [9]. Gold is not a very good electrode, and high impedances and problems locating the electrodes with in the camera were the main challenges. In a second study, in CAHT-II, lanthanum strontium manganite, La1-xSrxMnO3 (LSM) was investigated. LSM is used as oxygen electrode material in solid oxide fuel and electrolysis cells. The microelectrodes can easily be contacted individually in the CAHT-II as shown in Fig. 9.2, where the image from the CAHT-II camera is shown, and they can be characterized by all electrochemical techniques available from a potentiostat.
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The electrochemical characterization of the LSM microelectrodes was performed in the range of 650–850°C, and the dependence on temperature, atmosphere, microelectrode size, and anodic and cathodic polarization was investigated. Fig. 9.7 shows some of the electrochemical measurement performed on LSM microelectrodes. The temperature dependence (Fig. 9.7A) of the high-frequency resistance and polarization resistance from impedance spectra can be obtained and used to estimate the conductivity of YSZ and for polarization resistance activation energy calculations, respectively [10]. The size dependence (Fig. 9.7B) can be investigated in different atmospheres and used to determine whether the polarization resistance scales with the microelectrode circumference or the microelectrode surface area thus giving information on whether the electrode reaction takes place at the triple phase boundary or whether the entire surface is involved [7,10]. Fig. 9.7C shows impedance spectra acquired under cathodic polarization in oxygen. They indicate that the electrode polarization decreases when current is drawn. Another type of measurement is cyclic voltammetry as shown in Fig. 9.7D, which can be used to assess current-voltage
Fig. 9.7 A selection of electrochemical measurements on LSM microelectrodes showing (A) impedance spectra obtained at different temperatures in air, (B) impedance spectra obtained from different sizes of microelectrodes in oxygen at 850°C, (C) impedance spectra obtained under cathodic polarization, and (D) cyclic voltammograms acquired in N2 from differently sized microelectrodes. The inset shows the peaks from oxidation and reduction of manganese oxides. Adapted from Hansen, Norrman, Jacobsen, et al., LSM microelectrodes: kinetics and surface composition, J. Electrochem. Soc. 162 (2015).
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characteristics and in this case also reduction and oxidation of manganese oxides (Fig. 9.7D, inset) [7], as seen from the minor peaks in the sweep for the 50-μm electrode.
9.6
Conductance mapping of LSM microelectrodes and correlation with complementary techniques
Several surface analytical techniques were used to characterize LSM microelectrode as the chemical stability is essential for the electrode performance [7,27]. Chemical changes will cause decrease in the electrochemical performance, and electrical conductivity is an important parameter. The most vulnerable location is at triple phase boundaries where electrode and electrolyte meets the atmosphere and where the electrochemical reactions take place, i.e., in this case, the microelectrode edges. Scanning electron micrographs showed that during sintering discrete secondary phases formed at the microelectrode edge and further that the microelectrode morphology changed during the high-temperature characterization. The secondary phases have a distinctively different morphology compared with the LSM electrode. Conductance scans were performed at 650°C in air on the sintered microelectrodes and showed a wellconducting surface as illustrated by the 20 μm microelectrode in Fig. 9.8A. It is noted that distributed along the edge are thin areas with a much lower conductance. The conductance is even lower than that of the surrounding YSZ. An SEM image of a similar micrograph is shown in Fig. 9.9D. Fig. 9.8B shows a 10 10 μm2 scan of a secondary phase. The low-conducting phase has a very sharp border to the surrounding YSZ and to the LSM. There is some effect of the probe scanning in contact mode over the surface, and some material is moved from the electrode to the surrounding areas. This was shown by TOF-SIMS imaging [8] but bearing in mind the extreme surface sensitivity of this technique, and as the electrode does not seem to be significantly changed, it is probably a minor issue for the microelectrodes. However, minimizing the
Fig. 9.8 Conductance maps obtained at 650°C in air of as-sintered microelectrodes (A) the entire surface of a microelectrode with a diameter of 20 μm and (B) detailed scan of a secondary phase located at the edge of a microelectrode.
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Fig. 9.9 TOF-SIMS composite ion images of an as-sintered 20 μm microelectrode showing the lateral distribution of (A) Mn, Zr, and Si, (B) LaO, Mn, and LaO + Mn, (C) Si, Sr, and Zr, and (D) SEM micrograph of a similar microelectrode with a diameter of 20 μm. Some of the secondary phases are marked with arrows.
number of scans is always advisable especially also because the contact force typically has to be higher at higher temperatures to keep the probe at the surface. The value of the (in situ) conductance images is increased significantly if they are compared with complementary information. Energy dispersive X-ray spectroscopy (EDS) in combination with SEM could not definitively distinguish the secondary phase but only substantiate that it had a different composition. The reason is that the thickness of the secondary phase is much below the probing depth and at lower acceleration voltage where that is minimized, the Si and Sr peaks are too close and the overlap prevented a precise determination. Sr is a component of the electrode material and thus present in large amounts just next to the secondary phase, and it was not possible to determine whether it was also a component of the secondary phase. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to investigate the chemistry of the secondary phases [27] as it easily distinguishes between the main components. From the composite ion images shown in Fig. 9.9, it is clear that the secondary phase contains Si, Sr, and La but not Mn, which shows that this oxide phase is distinctively different from the LSM. Silicon is an impurity present in the electrolyte material in ppm concentrations.
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Fig. 9.10 Conductance and topography scans a microelectrode after several heating sequences. (A) Conductance map obtained at 650°C. Notice the blue color at the microelectrode edge signifies a very low conductance of the edge phase. (B) Topography image obtained at 650°C.
The microstructure of the microelectrodes changed during the high-temperature experiments in the CAHT-SPM. The discrete particles at the edge disappeared and a microstructure with much coarser grains located along the circumference of the microelectrodes. The conductance images documented a correlating change in conductance in this area (Fig. 9.10) and the conductance of the outer few microns was very low along the entire circumference. These changes are most likely due to temperature even though it was documented that polarization also caused changes in the manganese distribution [7,27]. The low-conductance edge contained La, Sr, and Si but not Mn [8,27]. Conductance of the phases can only be performed at elevated temperatures even though the LSM is electrically conducting at room temperature, the YSZ is an insulator and the measurement would not be possible. The combination of several techniques validates the CAHT-SPM measurements and increases the knowledge on the degradation effects significantly.
9.7
Strong cathodic polarization of PtIr-YSZ microcontacts at 650°C
The Ni-YSZ cathode in solid oxide electrolysis cells experiences strong cathodic polarization during operation, and may consequently reach potentials that are low enough to significantly affect the stability of the materials. With the CAHT-SPM probe as a working electrode, it is possible to polarize microcontacts points and study the local effect of strong cathodic polarizations on the YSZ electrolyte and the probe material (Pt, Ni) itself in a hydrogen atmosphere. Previously, a somewhat similar but low-temperature study was conducted on ceria [28] in a combined study with CAHT-SPM and Kelvin probe force microscopy (KPFM). Macrocontact studies with Ni point electrodes have been conducted in a different
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experimental setup [29] and reveal significant changes in depth during both long and short strong cathodic polarizations, e.g., that ZrO2 is reduced but Y2O3 is not and that Ni and Zr form an intermetallic phase. Even a few minutes of strong cathodic polarization changes the local chemical and microstructural environment. To understand this better on a grain-to-grain scale, a series of experiments were designed for the CAHT-SPM. The experiments were performed in dry or humidified 9% H2 in N2 at a temperature of 650°C. Polarizations range from 100 mV to 2 V vs. a reference electrode in the same atmosphere, and for the lowest polarizations, the YSZ stability is compromised [30,31] by reaching the regime where reduction of ZrO2 occurs. During the polarization, electrodes are injected into the electrolyte thus increasing the electronic conductivity of ZrO2. Variation in time and polarization during impedance spectroscopy or chronoamperometry reveal that other processes than conductivity changes occur. Both the electrochemical measurements and conductance mapping of the contact point after the polarization was released contribute to the understanding of the effect. Fig. 9.11
Fig. 9.11 A preliminary study of a long-term polarized contact point of PtIr on YSZ correlated with AFM, SEM, and TOF-SIMS to show electrical, topographic, chemical, and microstructural data: (A) conductance map obtained at 650°C, (B) room temperature topography (note the different scale), (C) SEM micrograph, and (D) TOF-SIMS ion image showing the distribution of Na (bright color indicates high intensity). The arrow points to the actual contact point. The red circles in (A) and (C) indicate particles of low conductance correlated with particles in the SEM micrograph.
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shows the result of an initial experiment where a long-term polarization was performed. A relatively large spot with a high conductance was produced by a polarization of 2 V (Fig. 9.11A). In this area, particles with lower conductance are distributed, and in the center, where the actual contact point is visible, the conductance is also low. The low-conductance particles are also visible in the topography image (Fig. 9.11B), which was acquired at room temperature after the high-temperature experiment, and they are around 50 nm tall and from several nanometers to a few micrometers in diameter. The particles and the general pattern are also recognizable in the SEM image (Fig. 9.11C) and are further reflected in the TOF-SIMS ion image showing the distribution of Na (Fig. 9.11D). The actual contact point (arrow) shows different properties in all images when compared with the rest of the affected area. The Pt-Ir/YSZ contact is used as a model system to verify the technique, but ongoing work also includes similar investigations with Ni probes [32]. The Ni/YSZ system is more realistic as it simulates the actual hydrogen electrode composition. Based on the local information gathered with the CAHT-SPM combined with the complementary information from other techniques, detailed correlations can be made and several properties of the area on a micron-to-submicron range can be concluded.
9.8
High-temperature Kelvin probe force microscopy at 300–600°C
Many functional materials to a large extent rely on the surface functionality and the outer few atom layers may be determining for the performance of the material. The surface is always different from the bulk both due to the asymmetry of the crystal lattice and the chemical and structural reorganization related to this, and also because the chemistry due to adsorption or segregation of impurities differs significantly. KPFM [33] is a very surface-near technique and is used for studying work functions or surface potentials. Previously, high-temperature KPFM was demonstrated at 500°C [6], and presently, a Danish Independent Research Council project, ECoProbe (DFF—400500129) is involved in improving the high-temperature KPFM technique. The scope of this project is among other things to study electrode processes and degradation mechanisms using and developing localized probing techniques working at high temperatures. This study has yielded stable results with a good signal-to-noise ratio. A major task is to adapt the probe holder to be able to use commercial probes at high temperature and to explore the operating range regarding temperature and atmosphere of different commercial probes. Preliminary results show that Aspire conical probes (CFM) can be used up to 600°C in atmospheres ranging from 9% H2 in N2 to pure oxygen. These probes are made of highly doped Si and have a resonant frequency of 75 kHz. Higher temperatures pose a more difficult challenge, introducing instabilities in the topography measurements that negatively affect the KPFM signal. In addition, also the noise in the KPFM signal itself increases with temperature. An effect that could be partly due to a mechanically unstable mounting of the probe, which is a parameter that is under study and development.
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Fig. 9.12 Kelvin probe force microscopy images obtained at 300°C in 50% O2 and 50% N2 on LaNi0.6Fe0.4O3 (LNF) deposited on SrTiO3. The image of the nonpolarized sample is subtracted so that only the effect of polarization is seen. (A) Polarized positively and (B) polarized negatively.
Fig. 9.12 show KPFM images of a sample consisting of an approximately 500-nm thick film of LaNi0.6Fe0.4O3 (LNF) deposited on a SrTiO3 single crystal substrate. Using photolithography and wet etching in diluted HCl, a thin stripe was etched, forming a sample with two macroscopic LNF electrodes separated by an approximately 20 μm stripe of SrTiO3. The images were acquired at 300°C in 50% O2 and 50% N2 in dual-pass mode. This means that the topography is scanned in tapping mode first and then the tip is lifted by 10 nm over the sample, the piezoelectric shaker used in tapping mode is shut off and the electrostatic interaction is used to determine the KPFM signal. A relatively good signal is obtained over a large area and the effect of both negative and positive polarization is clear. It is thus a promising technique for in operando experiments studying the processes influencing the performance, lifetime, and degradation solid oxide cell materials at high temperatures.
9.9
Outlook
Localized probing is an advantageous technique for acquiring detailed topographical, electrical, or electrochemical information from electrode, electrolyte, or other surfaces of metals or metal oxides at in operando or in situ conditions. Today very few possibilities of reaching relevant conditions with SPMs exist but hightemperature SPM is a developing technique that will advance further in the coming years. All the presented examples are from solid oxide cells, but the techniques are obviously not limited to those materials or to the extremely high temperatures necessary for studying them. Furthermore, development of new categories of measurements, new electrical set-ups, and refining of the existing is an ongoing work to explore the possibilities and limitations of the CAHT-SPMs.
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References [1] D.A. Bonnell, Scanning Probe Microscopy and Spectroscopy: Theory, Techniques, and Applications, Wiley-VCH, New York, 2001. [2] G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel, Phys. Rev. Lett. 49 (1982) 57. [3] G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930. [4] K.V. Hansen, T. Jacobsen, A.-M. Nørgaard, N. Ohmer, M. Mogensen, Electrochem. SolidState Lett. 12 (2009) B144. [5] K.V. Hansen, T. Jacobsen, K. Thyden, Y. Wu, M. Mogensen, J. Solid State Electrochem. 18 (2014) 1869. [6] K.V. Hansen, Y. Wu, T. Jacobsen, M.B. Mogensen, L. Theil Kuhn, Rev. Sci. Instrum. 84 (2013)073701. [7] K.V. Hansen, K. Norrman, T. Jacobsen, Y. Wu, M.B. Mogensen, J. Electrochem. Soc. 162 (2015) F1165. [8] K.V. Hansen, K. Norrman, T. Jacobsen, Ultramicroscopy 170 (2016) 69. [9] Y. Wu, K.V. Hansen, T. Jacobsen, M. Mogensen, Solid State Ionics 197 (2011) 32. [10] Y. Wu, K.V. Hansen, K. Norrman, T. Jacobsen, M.B. Mogensen, ECS Trans. 57 (2013) 1673. [11] S.S. Nonnenmann, Nanoscale 8 (2016) 3164. [12] S.S. Nonnenmann, D.A. Bonnell, Rev. Sci. Instrum. 84 (2013) 073707. [13] S.S. Nonnenmann, R. K€ungas, J. Vohs, D.A. Bonnell, ACS Nano 7 (2013) 6330. [14] J. Zhu, C.R. Perez, T.-S. Oh, R. K€ungas, J.M. Vohs, D.A. Bonnell, S.S. Nonnenmann, J. Mater. Res. 30 (2016) 357. [15] T.M. Huber, A. Opitz, M. Kubicek, H. Hutter, J. Fleig, Solid State Ionics 268 (2014) 82. [16] M.L. Traulsen, C. Chatzichristodoulou, K.V. Hansen, L. Theil Kuhn, P. Holtappels, M. B. Mogensen, ECS Trans. 66 (2015) 3. [17] K.V. Hansen, C. Sander, S. Koch, M. Mogensen, J. Phys. Conf. Ser. 61 (2007) 389. [18] A.F. Benton, P.H. Emmett, J. Am. Chem. Soc. 46 (1924) 2728. [19] R.P. Furstenau, C. McDougall, M.A. Langell, Surf. Sci. 150 (1985) 55. [20] Q. Hu, T. Jacobsen, K.V. Hansen, M. Mogensen, Solid State Ionics 234 (2013) 1. [21] J.A. Rodriguez, J.C. Hanson, A.I. Frenkel, J.Y. Kim, M. Perez, J. Am. Chem. Soc. 124 (2002) 346. [22] H. Le, S. Zhang, W. Gong, D. Xu, Y. Wang, in: Proc. Annu. Int. Pittsburgh Coal Conf., 29 2012, p. 2402. [23] J. Bandrowski, C.R. Bickling, K.H. Yang, O.A. Hougen, Chem. Eng. Sci. 17 (1962) 379. [24] C.M. Osburn, R.W. West, J. Phys. Chem. Solids 32 (1971) 1343. [25] V. Brichzin, J. Fleig, H.-U. Habermeier, G. Cristiani, J. Maier, Solid State Ionics 152–153 (2002) 499. [26] J. Fleig, F.S. Baumann, V. Brichzin, H.R. Kim, J. Jamnik, G. Cristiani, H.-U. Habermeier, J. Maier, Fuel Cells 06 (2006) 284. [27] K. Norrman, K.V. Hansen, T. Jacobsen, M.B. Mogensen, RSC Adv. 5 (2015) 87679. [28] K. Neuhaus, M. Bernemann, K.V. Hansen, T. Jacobsen, G. Ulbrich, E.M. Heppke, M. Paun, M. Lerch, H.-D. Wiemh€ofer, J. Electrochem. Soc. 163 (2016) H1179. [29] K.V. Hansen, M. Chen, T. Jacobsen, K. Thyden, S.B. Simonsen, S. Koch, M. Mogensen, J. Electrochem. Soc. 163 (2016) F1217. [30] R.E.W. Casselton, J. Appl. Electrochem. 4 (1974) 25.
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[31] W. Weppner, J. Electroanal. Chem. 84 (1977) 339. [32] K. Kreka, K.V. Hansen, T. Jacobsen, K. Norrman, C. Chatzichristodoulou, M. Mogensen, In prep. 1111. [33] M. Nonnenmacher, M.P. O’Boyle, H.K. Wickramasinghe, Appl. Phys. Lett. 58 (1991) 2921.
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K.V. Hansen Technical University of Denmark, Lyngby, Denmark
9.1
Introduction to high-temperature SPM
Scanning probe microscopy (SPM) is a versatile methodology [1] comprising a number of techniques or modes for acquiring local information from a sample by the use of a fine probe. The probe may be scanned over a microscopic area obtaining lateral information and resulting in images, or it may be located in a point for performing local measurements of various kinds. Depending on the probe properties and the specific technique information on, e.g., topography, mechanical, electrical, electrochemical, and magnetic properties can be obtained. SPM includes scanning tunneling microscopy, which was the first technique invented by Binnig et al., demonstrating atomic resolution in vacuum [2]. A few years later, in 1986, the atomic force microscope working on insulating materials in air was demonstrated [3] and SPM is thus a relatively young technique. Danish Micro Engineering A/S (DME), established in 1979 by Dr. Curt Sander, was a Danish company dedicated to consultancy on microprocessor technology and software development, which began developing SPMs at the end of 1987, and through the next 27 years, they developed both standard atomic force microscopes and highly customized equipment, including the two controlled atmosphere high-temperature scanning probe microscopes (CAHT-SPMs). In Jul. 2015, DME was acquired by the Hungarian company Semilab Zrt. (ref. www.dme-spm.com). The first prototype CAHT-SPM, CAHT-I, was developed in 2006 to allow investigations of metals and metal oxides at temperatures relevant for solid oxide fuel cell applications. The scope was in operando electrical and electrochemical investigations on a micro- to nanoscale at 400–800°C in reducing and oxidizing atmospheres. A few years later and based on the experience with CAHT-I, but with a totally new design and a number of new modes, the second prototype, CAHT-II, was developed in 2011. The CAHT-SPMs were the first microscopes to be developed specifically for experiments at high temperature combined with a well-defined gas atmosphere. At that time, the literature concerning high-temperature SPM under ambient pressure conditions was very scarce. Since then, a number of different types of samples and techniques have been explored with the two CAHT-SPMs [4–10] and case stories that
Metal Oxide-Based Thin Film Structures. https://doi.org/10.1016/B978-0-12-811166-6.00009-1 © 2018 Elsevier Inc. All rights reserved.
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describe some of them are presented in the next sections along with some preliminary results from ongoing investigations. SPM at several hundred degrees Celsius is a challenging discipline as outlined below, and today the number of publications on high-temperature SPM is still very limited as a very recent and comprehensive review by Nonnenmann [11] shows. Nonnenmann and his group have themselves published a number of articles showing their work on developing an environmental chamber to be used with their commercial atomic force microscope (AFM) [12] and work on fuel cell materials at 600°C [13,14]. Their system has some advantages such as easy mounting of cross-sectional samples and the use of commercial AFM tips, which increases the resolution.
9.1.1
Challenges
The high temperature puts demands on the materials in proximity to the hot zone. This includes the piezo translator and the detector electronics which need to be cooled, the entire assembly of the furnace, and fixation of the sample on the furnace. Circulating cooling water driven by a pump may induce noise in the images. Probes for high-temperature in situ or in operando applications must be chemically and mechanically stable against the high temperature and the atmosphere. This limits the number of available materials, and the temperature also influences the possibilities of fixing the probe. The temperature of the cantilever does not exceed a few hundred degrees Celsius, but this leaves out gluing of the probes and only a mechanical solution is stable enough. Coated probes may pose a problem as the thin metal coating may be damaged, e.g., by island formation. Besides the stability, it is also essential for many of the applications that the probes have sufficient electrical conductivity. Pt-Ir probes that are mechanically and chemically stable in the relevant atmospheres and also resistant to even the highest temperatures appear to be a good choice. In the CAHT-SPMs, Pt-Ir probes made in-house [9] are used for the most hightemperature experiments, but commercial probes are currently also being tested and used. There is an established procedure for fabrication of the Pt-Ir probes involving electrochemical etching of the Pt-Ir wire to produce a sharp tip. The tips can be made quite sharp even though not as sharp as silicon-based commercial probes. They are attached to the holder by spot welding, which ensures that the probes are well fixed and has a good electrical contact to the holder. The cantilevers are quite long and wide and reflect the laser beam well, but their geometry keeps the resonant frequency very low, in general, below 10 kHz. In-house made, electrically conducting ceramic probes fabricated by laser micromachining and focused ion beam milling have been demonstrated at 350°C. Commercial probes are challenging due to the short cantilever length as it limits the space for positioning of the laser beam and more importantly, they are difficult to fix sufficiently to the holder. Sample drift is always decreasing the image quality for scanning probe techniques, and the elevated temperature contributes significantly to this as the different parts of the system are heated at different rates. After reaching the desired temperature, some time for stabilization and equilibration is necessary. In a drift study [8] at 600°C after temperature equilibration, the drift was less than 1 μm in X- and Y-direction over
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26 min. The scanned images were 5 5 μm2 and otherwise acceptable. Thermal drift in this range with can probably not be avoided with CAHT-II, and for smaller scan areas and longer scan times, it will be more problematic. Thermal gradients across and laterally in the sample are unavoidable with the geometry of the system where the sample is heated from one side only. Since the scanned area is close to the symmetry axis of the furnace, the temperature differences here are mainly due to cooling through the probe tip. Finite element calculations and experiments have shown that large temperature gradients are present in the vicinity of such contact points [15]. This means that during surface scanning, the effective temperature at the contact point will depend on the tip-sample contact area as well as the scan rate. In stationary electrochemical experiments where the distance between the counter and reference electrodes is in the millimeter range, the equilibrium potential of the working electrode may differ from that of the reference electrode by tens of millivolts. Another challenge in the investigation of solid oxide cell materials is to fabricate samples that are suitable for in situ or in operando measurement and that still resembles the original microstructures. Local performance measurements require physical access to the part of the cell/sample to be investigated and for multilayer structures, different geometries are necessary to obtain information of other layers that the top one. This often requires a model sample where preparation techniques and materials are different from those of the full cell and the results may reflect this. The contacting of electrodes with platinum wires on small samples as well as a correct positioning of counter and reference electrodes are not trivial. In situ and in operando experiments are always quite challenging especially when they involve temperatures in the range of several hundred degrees Celsius, and there will always be issues and conditions that are less optimal than desired, and especially so in prototype instruments.
9.2
Importance of in situ and in operando local probing measurements
The surfaces and interfaces of functional materials are often crucial for their performance as they are the critical locations for vital reactions or processes. Some properties are a function of temperature and/or atmosphere, e.g., conductivity, and can only be measured at certain conditions. The same is the case for electrochemical reactions that depend on the nature of the surfaces, such as chemical composition and the gas atmosphere that interacts with the electrode surface. The structure and composition of electrode surfaces are influenced by temperature and atmosphere and may quickly adjust to new conditions such as cooling and atmosphere changes. Performance characterization of solid oxide cells (SOC) is often performed by electrochemical impedance spectroscopy (EIS) where full cells, symmetrical cells, or model electrodes are investigated at relevant operating conditions. It is thus always an in operando technique where the bulk electrode/electrolyte is characterized. Gradients in temperature, atmosphere composition, and electrical load across the cell, however, lead to different conditions and mechanisms depending on the exact
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location in the cells. The local performance is dependent on the nature of the specific locations, and it is thus desirable to be able to perform local measurements in selected places. In the literature, it is found that there is an increasing but still a limited number of articles dealing with in situ or in operando measurements at temperatures relevant for solid oxide fuel cells. The difference between in situ and in operando experiments is that for in situ experiments some of the parameters temperature, atmosphere, and electrochemical environment must mimic operation conditions whereas for in operando experiments they all must be in the realistic regime. This leaves all techniques in vacuum or a very diluted atmosphere such as X-ray photoelectron spectroscopy, scanning photoelectron microscopy, transmission electron microscopy, as in situ techniques. The group of in operando techniques contains among others scanning probe, Raman spectroscopy, and X-ray diffraction. Both scanning probe techniques and Raman spectroscopy may provide local and spatially resolved information. The advantage of in situ or in operando measurements are obvious that as the conditions are similar to operating conditions, the results will more likely show the properties of the materials while they are functioning, and thus the active reaction and degradation mechanisms can be studied while they are at play. Using several in situ and in operando techniques to obtain different types of data is essential to gain the full insight into the complex processes taking place (spatially and temporally) at operating conditions. In situ and in operando materials characterization is thus a cornerstone for understanding how materials work but they are very demanding experiments, and even though a lot is gained by performing in situ and in operando experiments, complementary ex situ techniques, such as TOF-SIMS, XPS, or LEIS, are still valuable, combining and correlating the different results, preferably in three dimensions [16].
9.3
The CAHT-SPMs
The CAHT-SPMS are basically working as normal AFMs with a fine probe scanning the surface and a laser-detector feedback system [1]. The main difference is that the sample is located on a furnace and that cooling of different parts is implemented. The CAHT-SPMs are both sample scanners, and the probe is thus stationary. This is to keep the sensitive electronics as far from the high temperature zone as possible. The SPMs are designed to be very flexible regarding the types of probes that can be accommodated and the possibilities for connecting external equipment such as potentiostats or lock-in amplifiers.
9.3.1
CAHT-I
CAHT-I was the first SPM that was developed for temperatures exceeding 600°C in ambient conditions. As a prototype, it was tested and improved in several steps as limitations were discovered through actual experiments, and many improvements were made following the first publication [17]. It can reach a sample surface temperature
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of 650–700°C in dry or humidified mixtures of nitrogen and oxygen and in 9% hydrogen in nitrogen. The humidification is realized by bubbling the gas through water at room temperature. CAHT-I is working in contact mode only and is capable of performing topography scans and conductance mapping, and in stationary mode, all the functionality of a potentiostat is available. CAHT-I consists of a bottom scanner part where the furnace and sample are also located, and an upper part with the electronics. The upper part can be lifted off, and the probe is mounted on this part. The in-house made probes are welded to a stainless steel holder, but commercial probes can be accommodated on a special ceramic holder. Electrically conducting ceramic probes have been fabricated and demonstrated at elevated temperature for conductance mapping. The sample is heated by a Linkam furnace TS-1200 and is fixed with Pt springs on a stainless steel “hot plate” on top of the furnace. Surface (minimum) temperatures are measured by fixing a thermocouple on top of the sample. There is a humidity sensor in the chamber for measuring relative humidity, and the gas outlet can be connected to a potentiometric oxygen sensor, and both are continuously logged along with the temperature of the water, the furnace temperature, thermocouples, and piezo temperature. The last part is for safety reasons, and a relay will disconnect the furnace if the piezo temperature becomes critical. The piezo scanner and the electronics parts are water cooled. The cooling system uses the siphon principle, so that the cooling water does not need a pump for circulation.
9.3.2
CAHT-II
CAHT-II (Fig. 9.1) is a more advanced SPM [6] and possesses many advantages compared with CAHT-I. This includes a parallel scanner and a better camera for positioning the probe in selected locations (Fig. 9.2). It is more gas tight and has a smaller volume sample chamber which decreases the time for gas exchange. Fig. 9.3 shows a sketch of a cross-section of CAHT-II. The sample and lower chamber are separated by a flexible membrane that allows the movement of the furnacesample assembly by some millimeters in the X and Y directions and also during scanning, avoiding to disturb the precise movements by the piezo translator. The probe on the holder is inserted from the side through a hole in the wall and positioned at a small angle to the sample surface. The furnace from HeatWaveLabs Inc. is made of alumina and Pt wire, and the sample is fixed on top of it with alumina pins attached to metal springs (Fig. 9.1B) located away from the hot zone, a preferable set-up compared with CAHT-I. The maximum sample surface temperature is 850°C. CAHT-II works in contact mode, tapping mode, and lift mode and is capable of performing Kelvin probe force microscopy, scanning tunneling microscopy and spectroscopy, and conductive AFM in addition to the techniques available from CAHT-I. It is also possible to measure other properties depending on the external set-up. For stationary measurements, sample drift may be an issue but for shorter time spans (minutes), this is not a major issue. It was specifically demonstrated for CAHT-II [8] that drift is in the less than a micron in the X and Y directions over a period of 26 min.
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Fig. 9.1 (A) CAHT-II from the outside. (B) The sample chamber of CAHT-II. Reproduced from K.V. Hansen, Y. Wu, T. Jacobsen, M.B. Mogensen, L. Theil Kuhn, Improved controlled atmosphere high temperature scanning probe microscope, Rev. Sci. Instrum. 84 (2013) 073701, with the permission of AIP Publishing.
Fig. 9.2 A camera image of a probe situated on a microelectrode with a diameter of 50 μm.
Conductance mapping [8] has proved to be a technique that can be advantageous for many different types of samples. In the conductance maps, an AC voltage is applied between the probe and the counter electrode from the generator of the lock-in amplifier (LIA). Usually, an amplitude of 0.5 V rms and a frequency of 10 kHz are chosen. The conductance is calculated as the ratio of the in-phase current and the voltage. With a scan rate corresponding to 1 s per line, the result is a weighted average of 40 to 80 cycles depending on the resolution of either 128 or 256 pixels per
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Fig. 9.3 A sketch of a cross-sectional view of CAHT-II.
line and normally a good signal-to-noise ratio is obtained. Experience has shown that with the present set-up conductances ranging from a few nS to 100 μS can be determined. The lower limit is determined by stray capacities shunting the probe-sample resistance and the upper by the 1-MΩ resistor inserted to avoid overloading of the LIA current input amplifier. The property actually determined in the conductance maps is the integral of the local resistivity from the contact surface to infinity converted to conductance. In the case of a homogeneous medium, the contribution from a volume element decreases with the distance from the probe contact squared. However, in less-trivial heterogeneous systems, the conductance is the result of a complicated averaging of the local properties. Fig. 9.4 presents four different situations of local environments (blue semicircle) for a probe. The grayscale of the grains indicates high (light) or low (dark) conductivities. Fig. 9.4A shows the current field (red lines) and a homogeneous system with high-conductivity grains. In Fig. 9.4B, low-conducting grains are found at the surface but with higher conducting grains below a higher apparent conductivity compared with the surface grains will be measured. Fig. 9.4C shows a local environment with high-conducting grains at the surface but lower conducting grains below. The resulting conductance will be lower than what is expected from the high-conducting surface grain. In the case of a heterogeneous environment with low-conducting grains, the probe will record a low conductance. The CAHT-SPMs have proved to be very versatile instruments that may be used in a wide temperature and atmosphere range for studies of surface-related properties. This includes especially the electrical or electrochemical area where either scanning or stationary modes can probe a number of properties. It may also be used for studying surface-related processes on time scales ranging from hours to min. The slow scan axis is then used as a time axis as illustrated by one of the case stories below. Furthermore, processes where the topography of the samples changes because of the effect of
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Fig. 9.4 Local conductivity environments below a probe situated on a surface. The light grey color signifies high conductivity and dark grey indicates low conductivity. (A) The probe is situated in an area with high conductivity, which gives a high conductance in the map. The current field below the probes is indicated with red lines. (B and C) In a sample with both highand low-conducting grains, the measured conductance depends on the local environment around the probe and is a result of complicated averaging in the volume indicated by the blue semicircle. (D) The probe is situated in an area with uniform and low conductance, and the measured conductance will thus reflect the situation well.
temperature and atmosphere or as an impact of electric potential may also be investigated. The possibilities of the CAHT-SPMs are still far from being exhausted; as shown later in the chapter, a present topic that is researched intensively is hightemperature scanning Kelvin probe microscopy. A mode that is yet relatively unexplored is STM and STS at elevated temperatures leaving still interesting properties to be explored at in operando conditions.
9.4
In situ surface reduction of NiO by hydrogen between 312°C and 523°C
The CAHT-SPMs may be used to study high-temperature surface reactions such as reduction and oxidation of metal oxides while they are occurring as demonstrated by this study of in situ surface reduction [5]. The difference in conductance of NiO
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and Ni is easily detectable by conductance mapping and can thus be used to distinguish between them. The reduction of NiO starts at the surface and works its way into the NiO grains, creating porosity simultaneously with the Ni formation. Reduction of NiO has previously been studied by different techniques [18–21] and there is a general consensus about the mechanisms. When it comes to details, however, there is some variation in the findings. Reduction of NiO is important in solid oxide fuel cells, where the reduction typically takes place at 800°C and for Ni-based catalysts that works at much lower temperatures and where reduction temperatures are determining for the performance of the catalyst [22]. Reduction of NiO is very fast at high temperature, and this limits the range in which such a reaction can be studied due to the relatively slow scan rate of the CAHT-SPM and the atmosphere exchange rate from nitrogen to 9% H2 in N2. The possible temperature window for the reduction experiments was 300–525°C [5]. This resulted in surface reduction times from several hours to a couple of minutes. The literature shows that water vapor has an influence on the reduction rate [18,23] and the capability of the CAHT-SPM to work in a humidified atmosphere was exploited to study the reaction in both dry and humidified hydrogen. The investigated samples were for convenience purposes symmetric cells, i.e., an yttria-stabilized zirconia (YSZ) electrolyte with NiO-YSZ anode and NiO-YSZ-Al2O3 anode support on both sides. It was thus anode support surfaces on 5 5 mm2 samples that were studied. Before the experiments, the sample surfaces were polished to ensure a flat surface for ideal imaging conditions. During the experiments, the samples were heated in air and a conductance scan was performed to depict the NiO and YSZ (+Al2O3) grains. Subsequently, the atmosphere was changed to nitrogen, and after an hour of stabilization, the atmosphere was changed to 9% H2 in N2 while scanning to capture the transformation from NiO to Ni. Fig. 9.5A shows a conductance image obtained in air at 462°C. High conductance is shown by yellow colors, that is, the green and yellow grains are the NiO grains, and the blue, low-conductance grains are YSZ and alumina. NiO is a p-type electronic conductor due to the presence of Ni3+. The conductivity depends on impurities and previous treatment [24] and may not be very high but is easily distinguishable from that of YSZ, in which the bulk ionic conductivity at 450°C is 3.2 104 S/cm. The different shades of yellow and green for the conducting grains are related to the local environment around the grain (Fig. 9.4). The CAHT-SPM is thus well suited to distinguish the fine details in conductance as long as the sample is not a good conductor [8]. Fig. 9.5C shows a conductance image obtained during reduction at 462°C. Approximately after 5 μm (time ¼ 0), the atmosphere was, as indicated, changed from nitrogen to 9 %H2 in N2 and a few scan lines later the conductance starts to decrease. This decrease of almost an order of magnitude was found to be very consistent among more than 20 reduction experiments that were performed. In Fig. 9.5D, several crosssections through Fig. 9.5C are displayed and they show the evolution of the conductance including the decrease in section B. For lower temperatures, the decrease was slower, and for higher temperatures, it was faster. The decrease in conductivity was interpreted as a result of the initial reduction of Ni3+ to Ni2+, which continued until the redox potential defined by the Ni3+/Ni2+
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Fig. 9.5 Conductance maps form of a reduction performed at 462°C in dry 9% H2 in N2. (A) The unreduced NiO-YSZ-alumina surface. (B) After surface reduction, there is no NiO left. (C) During reduction. The image is scanned in the direction from bottom to top. (D) Crosssections of (C) from 0, 5, 10, 15, and 20 μm that show the evolution of conductance with time.
concentration ratio reached the redox potential corresponding to the coexistence of NiO and metallic Ni. When this occurred, the formation of Ni could proceed. The low conductance is thus followed by an increase to much higher conductances when a percolating metallic Ni network is formed. As seen in Fig. 9.5D (region D), two levels of conductance are present: the low level, which represents YSZ and alumina, and the high level, which is due to metallic Ni. For the lowest temperatures, it was possible to observe an increasing conductance but within the experiment time, the reaction did not end with metallic Ni. For the higher temperatures, it was much faster, and above 525°C, it was almost instant. When metallic Ni starts to form on the surface, the reaction is autocatalytic and occurs very quickly at higher temperatures. After the surface reduction was completed, the conductance maps of the surface show only the two levels (Fig. 9.5B). The quality of the image is compromised by two factors. One is that there is a volume reduction of 40% involved and upon Ni formation, it withdraws from the surface. The second is that the tip degrades during the scans and it may also pick up some Ni, which makes it less sharp. This fits well
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with the topography images in which the scan lines during the reduction becomes disturbed and there are indications that some particles may be moved by the probe. When the surface reduction is complete, the topography images become stable again. In some cases, the developed porosity may be observed. In the literature, the reduction process is described to be dependent on both temperature and the presence of water, and with the CAHT-SPM, it was investigated whether the reduction proceeds differently in dry and humidified hydrogen. The reductions were thus performed in both atmospheres at different temperatures. Fig. 9.6 illustrates both the temperature and atmosphere dependence. Fig. 9.6A and B show images obtained in dry H2 at 380°C and 437°C. At 437°C, the surface reduction goes to completion within the scanned image but only at 380°C, a slight indication of increasing conductance is seen in the uppermost 5 μm of the image. Fig. 9.6C and D present images obtained in humidified 9% H2 in N2. At 525°C, the reduction goes to the completion within the image, but at 425°C, this is not the case. Comparing Fig. 9.6B and C, the retarding effect of water is clear as they are obtained at almost the same temperature. The length of the period from where conductance starts to decrease and until it increases again is equivalent to the incubation period. The
Fig. 9.6 In situ surface reduction at different conditions (A) at 380°C in dry H2, (B) at 437°C in dry H2, (C) at 428°C in wet hydrogen, and (D) at 523°C wet hydrogen. The dotted line indicates the time for change from N2 to 9% H2 in N2.
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incubation periods for the four experiments in Fig. 9.6A and B were 4.8 and 1.4 min, respectively, and 8.7 and 1.6 min for Fig. 9.6C and D. This study demonstrates that besides image acquisition, the CAHT-SPM is suited for studies of time dependencies by detecting the incubation period for the reduction of NiO to Ni. Furthermore, from the data, it was possible to deduce that two temperature regimes with different activation energies exist and that the effect of water slowing the reduction resulted in a longer incubation period. The detailed findings from this investigation are described in Ref. [5].
9.5
Local electrochemical measurements at 650°C to 850°C
EIS is generally used for studying performance and degradation of fuel cells, electrolyzer cells, and batteries where it provides information on a global level. In the CAHTSPMs, electrochemical measurements can be performed locally by placing the probe in the point of interest. Microelectrodes are small electrodes typically fabricated by thin film deposition and photolithography and for fundamental studies where it is desirable that the triple phase boundary (TPB) is accessible and where a well-defined geometry is beneficial. Microelectrodes are ideal as the large number of electrodes on a substrate allows sequential measurements on adjacent electrodes, e.g., at similar conditions. Microelectrodes have previously been studied with other types of setups, e.g., a microprobe station [25,26]. Each of the CAHT-SPMs has a Gamry Instruments FAS2 Femtostat potentiostat attached and it is possible to conduct various electrochemical measurements with the probe as the working electrode. A limitation is that as the probe is very fine impedances ranging up to GΩs are common, however, the potentiostat is optimized for small currents and handles it well. The electrical contact between the probe and the sample must be quite good and this may necessitate very large contact forces to be applied to the probe. This typically means forces much larger than those used for performing scans, and it also means that the tip will not necessarily be as sharp after some measurements. During local impedance spectroscopy, the probe is used as the working electrode and can either be situated in the desired location by viewing with the camera or a scan can be made and the probe can be located very precisely in the chosen location. In a general evaluation of the technique, CAHT-I was used to study microelectrodes of gold in air at 650°C [9]. Gold is not a very good electrode, and high impedances and problems locating the electrodes with in the camera were the main challenges. In a second study, in CAHT-II, lanthanum strontium manganite, La1-xSrxMnO3 (LSM) was investigated. LSM is used as oxygen electrode material in solid oxide fuel and electrolysis cells. The microelectrodes can easily be contacted individually in the CAHT-II as shown in Fig. 9.2, where the image from the CAHT-II camera is shown, and they can be characterized by all electrochemical techniques available from a potentiostat.
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The electrochemical characterization of the LSM microelectrodes was performed in the range of 650–850°C, and the dependence on temperature, atmosphere, microelectrode size, and anodic and cathodic polarization was investigated. Fig. 9.7 shows some of the electrochemical measurement performed on LSM microelectrodes. The temperature dependence (Fig. 9.7A) of the high-frequency resistance and polarization resistance from impedance spectra can be obtained and used to estimate the conductivity of YSZ and for polarization resistance activation energy calculations, respectively [10]. The size dependence (Fig. 9.7B) can be investigated in different atmospheres and used to determine whether the polarization resistance scales with the microelectrode circumference or the microelectrode surface area thus giving information on whether the electrode reaction takes place at the triple phase boundary or whether the entire surface is involved [7,10]. Fig. 9.7C shows impedance spectra acquired under cathodic polarization in oxygen. They indicate that the electrode polarization decreases when current is drawn. Another type of measurement is cyclic voltammetry as shown in Fig. 9.7D, which can be used to assess current-voltage
Fig. 9.7 A selection of electrochemical measurements on LSM microelectrodes showing (A) impedance spectra obtained at different temperatures in air, (B) impedance spectra obtained from different sizes of microelectrodes in oxygen at 850°C, (C) impedance spectra obtained under cathodic polarization, and (D) cyclic voltammograms acquired in N2 from differently sized microelectrodes. The inset shows the peaks from oxidation and reduction of manganese oxides. Adapted from Hansen, Norrman, Jacobsen, et al., LSM microelectrodes: kinetics and surface composition, J. Electrochem. Soc. 162 (2015).
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characteristics and in this case also reduction and oxidation of manganese oxides (Fig. 9.7D, inset) [7], as seen from the minor peaks in the sweep for the 50-μm electrode.
9.6
Conductance mapping of LSM microelectrodes and correlation with complementary techniques
Several surface analytical techniques were used to characterize LSM microelectrode as the chemical stability is essential for the electrode performance [7,27]. Chemical changes will cause decrease in the electrochemical performance, and electrical conductivity is an important parameter. The most vulnerable location is at triple phase boundaries where electrode and electrolyte meets the atmosphere and where the electrochemical reactions take place, i.e., in this case, the microelectrode edges. Scanning electron micrographs showed that during sintering discrete secondary phases formed at the microelectrode edge and further that the microelectrode morphology changed during the high-temperature characterization. The secondary phases have a distinctively different morphology compared with the LSM electrode. Conductance scans were performed at 650°C in air on the sintered microelectrodes and showed a wellconducting surface as illustrated by the 20 μm microelectrode in Fig. 9.8A. It is noted that distributed along the edge are thin areas with a much lower conductance. The conductance is even lower than that of the surrounding YSZ. An SEM image of a similar micrograph is shown in Fig. 9.9D. Fig. 9.8B shows a 10 10 μm2 scan of a secondary phase. The low-conducting phase has a very sharp border to the surrounding YSZ and to the LSM. There is some effect of the probe scanning in contact mode over the surface, and some material is moved from the electrode to the surrounding areas. This was shown by TOF-SIMS imaging [8] but bearing in mind the extreme surface sensitivity of this technique, and as the electrode does not seem to be significantly changed, it is probably a minor issue for the microelectrodes. However, minimizing the
Fig. 9.8 Conductance maps obtained at 650°C in air of as-sintered microelectrodes (A) the entire surface of a microelectrode with a diameter of 20 μm and (B) detailed scan of a secondary phase located at the edge of a microelectrode.
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Fig. 9.9 TOF-SIMS composite ion images of an as-sintered 20 μm microelectrode showing the lateral distribution of (A) Mn, Zr, and Si, (B) LaO, Mn, and LaO + Mn, (C) Si, Sr, and Zr, and (D) SEM micrograph of a similar microelectrode with a diameter of 20 μm. Some of the secondary phases are marked with arrows.
number of scans is always advisable especially also because the contact force typically has to be higher at higher temperatures to keep the probe at the surface. The value of the (in situ) conductance images is increased significantly if they are compared with complementary information. Energy dispersive X-ray spectroscopy (EDS) in combination with SEM could not definitively distinguish the secondary phase but only substantiate that it had a different composition. The reason is that the thickness of the secondary phase is much below the probing depth and at lower acceleration voltage where that is minimized, the Si and Sr peaks are too close and the overlap prevented a precise determination. Sr is a component of the electrode material and thus present in large amounts just next to the secondary phase, and it was not possible to determine whether it was also a component of the secondary phase. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to investigate the chemistry of the secondary phases [27] as it easily distinguishes between the main components. From the composite ion images shown in Fig. 9.9, it is clear that the secondary phase contains Si, Sr, and La but not Mn, which shows that this oxide phase is distinctively different from the LSM. Silicon is an impurity present in the electrolyte material in ppm concentrations.
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Fig. 9.10 Conductance and topography scans a microelectrode after several heating sequences. (A) Conductance map obtained at 650°C. Notice the blue color at the microelectrode edge signifies a very low conductance of the edge phase. (B) Topography image obtained at 650°C.
The microstructure of the microelectrodes changed during the high-temperature experiments in the CAHT-SPM. The discrete particles at the edge disappeared and a microstructure with much coarser grains located along the circumference of the microelectrodes. The conductance images documented a correlating change in conductance in this area (Fig. 9.10) and the conductance of the outer few microns was very low along the entire circumference. These changes are most likely due to temperature even though it was documented that polarization also caused changes in the manganese distribution [7,27]. The low-conductance edge contained La, Sr, and Si but not Mn [8,27]. Conductance of the phases can only be performed at elevated temperatures even though the LSM is electrically conducting at room temperature, the YSZ is an insulator and the measurement would not be possible. The combination of several techniques validates the CAHT-SPM measurements and increases the knowledge on the degradation effects significantly.
9.7
Strong cathodic polarization of PtIr-YSZ microcontacts at 650°C
The Ni-YSZ cathode in solid oxide electrolysis cells experiences strong cathodic polarization during operation, and may consequently reach potentials that are low enough to significantly affect the stability of the materials. With the CAHT-SPM probe as a working electrode, it is possible to polarize microcontacts points and study the local effect of strong cathodic polarizations on the YSZ electrolyte and the probe material (Pt, Ni) itself in a hydrogen atmosphere. Previously, a somewhat similar but low-temperature study was conducted on ceria [28] in a combined study with CAHT-SPM and Kelvin probe force microscopy (KPFM). Macrocontact studies with Ni point electrodes have been conducted in a different
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experimental setup [29] and reveal significant changes in depth during both long and short strong cathodic polarizations, e.g., that ZrO2 is reduced but Y2O3 is not and that Ni and Zr form an intermetallic phase. Even a few minutes of strong cathodic polarization changes the local chemical and microstructural environment. To understand this better on a grain-to-grain scale, a series of experiments were designed for the CAHT-SPM. The experiments were performed in dry or humidified 9% H2 in N2 at a temperature of 650°C. Polarizations range from 100 mV to 2 V vs. a reference electrode in the same atmosphere, and for the lowest polarizations, the YSZ stability is compromised [30,31] by reaching the regime where reduction of ZrO2 occurs. During the polarization, electrodes are injected into the electrolyte thus increasing the electronic conductivity of ZrO2. Variation in time and polarization during impedance spectroscopy or chronoamperometry reveal that other processes than conductivity changes occur. Both the electrochemical measurements and conductance mapping of the contact point after the polarization was released contribute to the understanding of the effect. Fig. 9.11
Fig. 9.11 A preliminary study of a long-term polarized contact point of PtIr on YSZ correlated with AFM, SEM, and TOF-SIMS to show electrical, topographic, chemical, and microstructural data: (A) conductance map obtained at 650°C, (B) room temperature topography (note the different scale), (C) SEM micrograph, and (D) TOF-SIMS ion image showing the distribution of Na (bright color indicates high intensity). The arrow points to the actual contact point. The red circles in (A) and (C) indicate particles of low conductance correlated with particles in the SEM micrograph.
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shows the result of an initial experiment where a long-term polarization was performed. A relatively large spot with a high conductance was produced by a polarization of 2 V (Fig. 9.11A). In this area, particles with lower conductance are distributed, and in the center, where the actual contact point is visible, the conductance is also low. The low-conductance particles are also visible in the topography image (Fig. 9.11B), which was acquired at room temperature after the high-temperature experiment, and they are around 50 nm tall and from several nanometers to a few micrometers in diameter. The particles and the general pattern are also recognizable in the SEM image (Fig. 9.11C) and are further reflected in the TOF-SIMS ion image showing the distribution of Na (Fig. 9.11D). The actual contact point (arrow) shows different properties in all images when compared with the rest of the affected area. The Pt-Ir/YSZ contact is used as a model system to verify the technique, but ongoing work also includes similar investigations with Ni probes [32]. The Ni/YSZ system is more realistic as it simulates the actual hydrogen electrode composition. Based on the local information gathered with the CAHT-SPM combined with the complementary information from other techniques, detailed correlations can be made and several properties of the area on a micron-to-submicron range can be concluded.
9.8
High-temperature Kelvin probe force microscopy at 300–600°C
Many functional materials to a large extent rely on the surface functionality and the outer few atom layers may be determining for the performance of the material. The surface is always different from the bulk both due to the asymmetry of the crystal lattice and the chemical and structural reorganization related to this, and also because the chemistry due to adsorption or segregation of impurities differs significantly. KPFM [33] is a very surface-near technique and is used for studying work functions or surface potentials. Previously, high-temperature KPFM was demonstrated at 500°C [6], and presently, a Danish Independent Research Council project, ECoProbe (DFF—400500129) is involved in improving the high-temperature KPFM technique. The scope of this project is among other things to study electrode processes and degradation mechanisms using and developing localized probing techniques working at high temperatures. This study has yielded stable results with a good signal-to-noise ratio. A major task is to adapt the probe holder to be able to use commercial probes at high temperature and to explore the operating range regarding temperature and atmosphere of different commercial probes. Preliminary results show that Aspire conical probes (CFM) can be used up to 600°C in atmospheres ranging from 9% H2 in N2 to pure oxygen. These probes are made of highly doped Si and have a resonant frequency of 75 kHz. Higher temperatures pose a more difficult challenge, introducing instabilities in the topography measurements that negatively affect the KPFM signal. In addition, also the noise in the KPFM signal itself increases with temperature. An effect that could be partly due to a mechanically unstable mounting of the probe, which is a parameter that is under study and development.
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Fig. 9.12 Kelvin probe force microscopy images obtained at 300°C in 50% O2 and 50% N2 on LaNi0.6Fe0.4O3 (LNF) deposited on SrTiO3. The image of the nonpolarized sample is subtracted so that only the effect of polarization is seen. (A) Polarized positively and (B) polarized negatively.
Fig. 9.12 show KPFM images of a sample consisting of an approximately 500-nm thick film of LaNi0.6Fe0.4O3 (LNF) deposited on a SrTiO3 single crystal substrate. Using photolithography and wet etching in diluted HCl, a thin stripe was etched, forming a sample with two macroscopic LNF electrodes separated by an approximately 20 μm stripe of SrTiO3. The images were acquired at 300°C in 50% O2 and 50% N2 in dual-pass mode. This means that the topography is scanned in tapping mode first and then the tip is lifted by 10 nm over the sample, the piezoelectric shaker used in tapping mode is shut off and the electrostatic interaction is used to determine the KPFM signal. A relatively good signal is obtained over a large area and the effect of both negative and positive polarization is clear. It is thus a promising technique for in operando experiments studying the processes influencing the performance, lifetime, and degradation solid oxide cell materials at high temperatures.
9.9
Outlook
Localized probing is an advantageous technique for acquiring detailed topographical, electrical, or electrochemical information from electrode, electrolyte, or other surfaces of metals or metal oxides at in operando or in situ conditions. Today very few possibilities of reaching relevant conditions with SPMs exist but hightemperature SPM is a developing technique that will advance further in the coming years. All the presented examples are from solid oxide cells, but the techniques are obviously not limited to those materials or to the extremely high temperatures necessary for studying them. Furthermore, development of new categories of measurements, new electrical set-ups, and refining of the existing is an ongoing work to explore the possibilities and limitations of the CAHT-SPMs.
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