2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article www.advelectronicmat.de Effect of Humidity on the Writing Speed and Domain Wall Dynamics of Ferroelectric Domains Irena Spasojevic, Albert Verdaguer, Gustau Catalan,* and Neus Domingo* 1. Introduction The switching dynamics of ferroelectric polarization under electric fields depends on the availability of screening charges in order to stabilize the The interaction of adsorbed water with fer- roelectric (FE) materials such as BaTiO3 has switched polarization. In ferroelectrics, thin films with exposed surfaces been studied due to its dipolar character investigated by piezoresponse force microscopy (PFM), the main source and the possibility to split into ionic species of external screening charges is the atmosphere and the water neck, and (i.e., OH− and H+) (dissociation).[1–8] Conse- therefore relative humidity (RH) plays a major role. Here, it is shown how the quentially, water plays a crucial role in the dynamic writing of domains in BaTiO3 thin films changes by varying scanning screening of the depolarization field and the stabilization of FE polarization in the speeds in the range of RH between 2.5% and 60%. The measurements reveal absence of electrodes capable of supplying that the critical speed for domain writing, which is defined as the highest free charge.[9–12] This screening role is of speed at which electrical writing of a continuous stripe domain is possible, particular importance in thin films studied increases non-monotonically with RH. Additionally, the width of line domains by piezoresponse force microscopy (PFM), shows a power law dependence on the writing speed, with a growth rate where the stability of patterned domain coefficient decreasing with RH. The size of the written domains at a constant arrays (i.e., bits) is ruled by screening pro- cesses and is a prerequisite for informa- speed as well as the creep-factor μ describing the domain wall kinetics follow tion storage applications, where long-time the behavior of water adsorption represented by the adsorption isotherm, data retention is needed.[13] Besides stabi- indicating that the screening mechanism dominating the switching dynamics lizing polarization, it has been long known is the thickness and the structure of adsorbed water structure and its associ- that switching dynamics is also affected ated dielectric constant and ionic mobility. by adsorbed water and therefore relative humidity (RH),[14–20] since nucleation and growth of domains depend on the distribu- tion of electric fields around the AFM tip and the mobility of ionic screening charges. I. Spasojevic, G. Catalan, N. Domingo So far, however, studies concerning size evolution of written Catalan Institute of Nanoscience and Nanotechnology (ICN2) domains with RH are inconsistent. The majority of works dem- CSIC and BIST onstrate that the size of written domains at constant bias grows Campus UAB linearly with RH,[17,18,21] but some authors report on nonlinear Bellaterra, Barcelona 08193, Spain E-mail: gustau.catalan@icn2.cat; neus.domingo@icn2.cat decrease of domain size at high RH.[16,22] Up to know, switching I. Spasojevic experiments were conducted for high RH between 20% and 90%, Department of Chemistry and studies in low RH conditions have not yet been performed. Universitat Autònoma de Barcelona Studies of switching dynamics in the dry range are of particular Bellaterra, Barcelona 08193, Spain interest because important processes such as chemisorption of A. Verdaguer anchoring hydroxyl groups and physisorption of the first water Institut de Ciència de Materials de Barcelona (ICMAB) layers are expected to take place, having strong influence on dif- CSIC Campus UAB fusional mobility of ionic compensating species.[23–25] Bellaterra, Barcelona 08193, Spain It is also important to point out that all previous studies of G. Catalan switching dynamics were conducted in a static mode, by applying ICREA - Institució Catalana de Recerca i Estudis Avançats pulses of different intensities and lengths to a biased AFM tip Catalonia, Barcelona 08010, Spain placed in contact at a fixed point of the substrate.[13,14,16–18,21,26] Seminal works lead by Gruverman[27,28] and Paruch et al.,[13,29] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.202100650. studying the dynamics of ferroelectric domain growth kinetics under tip generated electric fields stablish the exponential © 2021 The Authors. Advanced Electronic Materials published by W ­ iley- dependence of the lateral domain wall velocity with the field and VCH GmbH. This is an open access article under the terms of the Crea- tive ­Commons Attribution License, which permits use, distribution and the domain size, which is further modulated by a dynamical expo- ­reproduction in any medium, provided the original work is properly cited. nent μ when domain walls motion is governed by a creep process. However, encoding of ferroelectric bits is a dynamic process, and DOI: 10.1002/aelm.202100650 in some proposed setups bit rate is limited by the successfulness Adv. Electron. Mater. 2022, 8, 2100650 2100650 (1 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de of switching events at the given scanning speed: as the speed of 2. Results and Discussion rotation of a “ferroelectric record disk” is increased, successful single bit writing by a PFM tip decreases.[30,31] This behavior might For this study, we used epitaxial 21 nm thick BaTiO3 (BTO) thin be related to the insufficient population of surface screening film grown on 33 nm thick SrRuO3 (SRO) conductive buffer charges capable of stabilizing the newly formed domains at suffi- layer on a (001) oriented SrTiO3 (STO) substrate by pulsed laser cient speed. Although kinetics of domain growth can be tuned by deposition (PLD). The preparation procedure can be found RH, studies that concern of the interplay between scanning speed elsewhere.[32] The BTO film was atomically flat, fully epitaxial and RH on domain switching are still lacking. and uniformly c-axis oriented with up-polarized as-grown state, In this work, we explore the influence of RH on the critical as evidenced by piezoresponse force microscopy (PFM).[32] The speed for the linear domain writing, by driving a biased AFM tip writing speed experiments were performed using an Asylum across the ferroelectric BaTiO3 thin film surface using different MFP3D AFM with a fluid cell integrated to a low-noise con- speeds and in distinct RH conditions. Further on, we study tinuous flow humidity controller that by combining dry N2 domain size dependence on ferroelectric writing speed and gas with distilled water allows high precision control of RH at applied tip voltage independently in different RH conditions. the cell with long term stability.[33] The minimum value of RH All measurements were performed in a wide range of writing achieved after exposure to pure N2 gas for 60 h was 2.5%. Once speeds and RH, including the low-humidity range all the way the desired RH was achieved, two sets of consecutive writing down to 2.5% humidity. This enabled us to construct the map experiments were performed: first by applying an increasingly of domain size evolution as a function of these parameters and positive voltage to the AFM tip at a constant writing speed, and correlate observed behavior with dominant screening mecha- second by setting a constant applied voltage and increasing the nisms involved, closely related to water adsorption processes. writing speed, respectively. All domains were written dynami- As we will show, relative humidity has a strong and positive cally in the shape of lines. For each voltage/scanning speed effect on the maximum speed at which ferroelectric domains combination, two consecutive lines were written to verify can be written, but the effect is non-monotonical, reflecting repeatability. In order to achieve stable tip-sample coupling, two three distinct regimes in the water adsorption isotherm. extra lines were written at the beginning of each experiment Figure 1. a) PFM amplitude and phase images of down-polarized lines written in different RH conditions by increasing writing voltage, using a Nanosensors EFM tip (k ≈ 2.8 N m−1, r = 30 nm) at the constant speed of 12.47 μm s−1 and contact force of 100 nN. b) Domain width evolution as a function of applied tip voltage for the constant speed of 12.47 μm s−1 and for different RH. Domain width increases with voltage and RH until reaching a saturation value; c) Hysteresis loops measured by SS-PFM at 0.2 Hz in different RH conditions showing decrement of coercive voltage with RH. Adv. Electron. Mater. 2022, 8, 2100650 2100650 (2 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de at the lowest speed/voltage conditions. In all the experiments, and related adsorbates facilitate switching and stabilize the a Nanosensors EFM tip (k ≈ 2 N m−1) was used at a constant switched domains by providing the requisite screening charge. force of 100 nN. Then, the obtained pattern of written lines This conclusion is also supported by the switching spectroscopy was measured by PFM as shown in Figure 1a, and the line PFM measurements[34] on up-polarized BTO thin film as a func- thicknesses that correspond to domain width were calculated tion of RH shown in Figure 1c. It can be seen that coercive field from the average number of pixels in the PFM phase images. decreases when the RH is raised from 5% to 60% RH. The sym- In order to characterize the switching response, we apply metry of the hysteresis curve is also in agreement with similar “writing” voltages in the range from 4.5 to 7.5 V to the AFM studies[20] and the observed imprint is well explained by the tip while scanning straight lines in contact mode at a constant influence of surface screening on the domian nucleation[35] and speed of 12.47 μm s−1 and in different RH conditions. When the initial growth stages during hysteresis loops.[36] voltage is sufficiently large, “line domains” appear in the tip’s wake, with a width that, like the diameter of point-domains,[18] grows proportionally to the excess voltage above coercivity. This 2.1. Effect of Relative Humidity on the Writing Speed of is shown in Figure 1a. Extracting the quantitative information Ferroelectric Domains from the maps, we see (Figure 1b) that indeed domain width increases with voltage, until reaching a saturation value. We also Having established that water facilitates FE switching both in observe that the width of the line domains not only depends hysteresis loops and in FE lithography, next we explore how on applied voltage but also on humidity: for any given voltage, humidity affects the FE the writing speed. To address this issue, domain width increases with increasing humidity. Moreover, we perform a new set of FE lithography measurements where, the saturation width is larger, and is reached at lower voltages, instead of fixing the scanning speed and increasing voltage, we as humidity is increased. The results indicate that surface water fix the voltage and increase the scanning speed. Figure 2a shows Figure 2. a) PFM amplitude and phase images of down-polarized lines written in different RH conditions by increasing writing speeds and at a con- stant tip bias of 6.5 V while scanning using a force of 100 nN. Critical writing speed is shifting towards higher values as RH is increased; b) Measured domain width as a function of writing speed for different RH showing a power law dependence (Equation (1)); c) Slopes of the fitted data from Figure 2b with Equation (1) as a function of RH. Absolute values of growth rate coefficient decrease with the rise of RH, achieving a plateau at high RH and revealing that the domain width is more sensitive to the writing speed in the low RH range. Note that the differences in writing at 2.5% RH using the speed of 12.5 μm s−1 between Figure 1a and Figure 2a, might stem from the slight changes in experimental parameters, most probably the stability of water meniscus between the tip and the sample caused by discontinous water coverage at low RH or small changes in the coercive voltages. In low RH conditions, slight changes of mentioned parameters can have strong influence on the delicate balance between depolarization fields and scarce external screening charges. Adv. Electron. Mater. 2022, 8, 2100650 2100650 (3 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de the PFM amplitude and phase images of down-polarized lines where w is the line width, K is a constant, S is the tip velocity and written in different RH conditions by using a constant bias of φ is the growth rate coefficient. The growth rate coefficient φ is 6.5 V and increasing writing speeds in the range from 12.47 to observed to decrease as RH increases, i.e., for 2.5% RH φ = 0.64 585.94 μm s−1. At each RH, the width of the lines decreases and and decreases to φ = 0.26 for 20% RH, achieving a plateau gradually disappears for increasing writing speeds. We define above 40% RH (Figure 2c). This means that the line width is the critical speed of writing as the highest speed for which more sensitive to the writing speed in the low RH range, i.e., homogeneous writing (a continuous line domain) is observed under 10%: the high values of the growth rate coefficient for for each RH condition. It can be seen that the critical speed low RH indicate that domain width decreases faster with speed. shifts towards higher values as RH is increased, and at 40% RH Figure 3a shows iso-width lines as a function of RH and all the lines are written successfully, independently of the used writing speed. Every symbol represents a combination of RH scanning speed within our experimental range. The increase in and writing speed for which a writing experiment has been the critical speed for writing confirms that humidity facilitates performed. Full, half-filled and empty symbols denote condi- switching. tions for which either complete, partial or unsuccessful writing To further study the RH influence in the dynamics of the has been observed, respectively. For our experiment, where we ferroelectric polarization switching, we need to distinguish used a tip with a nominal radius of R = 50 nm, the minimal two different axis for the dynamics: the direction along the tip observed widths of dashed and fully written lines are 30 and displacement, characterized by the tip scanning speed, i.e., the 48 nm, respectively. The iso-width lines represent combina- writing speed S, and the transversal axis perpendicular to the tions of RH and tip velocity for which a given domain width is writing direction, where the domain growth is determined by achieved. These lines were calculated from the fitting parame- a creep regime with a characteristic time depending on the ters of Figure 2b by choosing the desired input values of speed inverse of the tip scanning speed. Previous studies of domain and then calculating predicted values of domain width at each wall kinetics observed that the domain size scales approxi- RH. Iso-width lines are obtained by connecting the coordinates mately logarithmically with the writing time and in general it (RH, Speed) that give the same value of calculated domain is best fitted by a power law w ≈ t φ,[37] with the growth rate width. The color scale is the whole distribution map of domain coefficient φ taking values φ < 1.[15,19,38] Correspondingly, in our width vs writing speed and RH. The white region denotes com- case time scales with the inverse of the tip scanning speed S binations of RH and tip scanning speed for which scanning (t ≈ 1/S). We have fitted our experimental that in Figure 2b fol- is too fast or humidity too low for domains to be written at lowing Equation (1) all, and therefore the edge of the white region represents the critical writing speed for each RH where domains disappear log w = K − ∅ ⋅ log S (1) altogether. Figure 3. a) Color map showing the domain width dependence on the writing speed and RH. Symbols correspond to experimental points of RH and speed for which writing experiment has been performed with full, half-filled and empty symbols denoting conditions for which complete, partial and unsuccessful writing has been observed. Iso-width lines are obtained by connecting the coordinates (RH, Speed) that give the same value of calculated domain width, predicted by using fitting parameters of Figure 2b; b) Cross sections of domain width map at four arbitrary chosen writing speeds, showing non-monotonic growth of the domain width with RH at the constant writing speed. Shape of the domain width vs RH closely resembles the shape of adsorption isotherm of water on BaTiO3 and similar oxide materials in different RH conditions, which correlates amount of adsorbed water as a function of RH. c) Area of the water peak normalized by the area of bulk oxide peak in O1s spectrum obtained by AP-XPS measurements as a function of RH, representing adsorption isotherm of water on BTO surface in low to intermediate RH range. Adv. Electron. Mater. 2022, 8, 2100650 2100650 (4 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Cross sections along the black horizontal lines of the color domain size as a power law of the writing speed points to fact map in Figure 3a yield the dependence of domain width on that the diffusion of ionic charges at the surface is indeed the RH for arbitrarily chosen writing speeds. A few such lines are dominant factor in surface screening.[42,43]. In this case, the dif- plotted in Figure 3b. Several interesting features emerge from fusion of ionic charges depends on surface conductivity which this graph. Above 20% humidity, the line width is an almost- will be essentially dominated not only by the amount of water linear function of the RH, which is in good agreement with adsorbed on the surface but also on its structure. the only reported study so far, where the domain diameter was The process of water adsorption is governed by RH condi- examined as a function of RH for RH > 30% and different pulse tions via the adsorption isotherm,[44] which relates the thick- times.[18] However, at low RH the behavior is non-monotonic, ness of the adsorbed water layer to the relative pressure or RH showing a plateau of constant domain width between 10% and at constant temperature. Nonetheless, the contribution of water 20%, and a non-linear dependence at lower humidity that gets structure, dictated by water adsorption and RH, on ferroelectric increasingly steeper as RH approaches 0. domain screening has not been considered up to now. In order The physical meaning of the power law relationship between to delve into the adsorption of water on our BaTiO3 thin film line domain width and writing speed is more intuitive when surfaces, we measured a set of oxygen O 1s spectra in distinct looked at in reverse: in a creep regime, the time required for a RH conditions by the means of Ambient Pressure X-Ray Photo­ point-contact domain to reach a given diameter is a power law electron Spectroscopy (AP-XPS). With this technique, XPS of the diameter. Translated to line-writing, domain diameter measurements were conducted in the range of RH between 0% becomes line domain width, and the writing time for each point and maximum of ≈17% (low and intermediate RH ranges) by along the line is inversely proportional to writing speed. Hence, increasing the water vapor pressure of the XPS chamber from writing speed must be slowed down logarithmically in order UHV to 4 mbar. Obtained O 1s spectra were fitted following to hit a given domain width. Conversely, line domain width the model explained in detail in our previous works,[6,32] which decreases logarithmically with writing speed (Equation (1)) as enabled us to quantify the amount of molecular water adsorbed shown by the experimental results. This fact can be explained at every RH. The area of the molecular water peak normal- in terms of external screening dynamics: when there is no suf- ized by the area of bulk oxide peak, which gives the informa- ficient external screening charge available for the screening tion about the amount of water isothermally adsorbed on the of newly written domains, the insufficient amount of surface surface, has been plotted as a function of RH and shown in adsorbates strongly affects successfulness of line writing. Figure 3c. It can be seen that at low RH up to ≈ 10%, water In static conditions, the size of the written domains depends adsorbs on BTO surface in non-linear fashion, reaching a on the lateral size of the water meniscus formed between tip plateau of constant water content between 10% and ≈17% of RH. and sample and on the ability of ionic species (such as H+, OH−, The shape of the adsorption isotherm of water on our BTO sur- and H3O+) to adsorb and diffuse on the surface to perform the faces in the measured RH range (Figure 3c), is in good agreement necessary screening.[16,18] The water meniscus is formed by capil- with the trend observed on similar different oxide surfaces[24,45–47] lary condensation of water vapor in the small gap between the (note that in any case, the final calculation of water thickness in tip and the sample and typically grows until the thermodynamic monolayers depends on the structure of the adsorbates layer and equilibrium through surface adsorption and transport of water also the evolution of the other available species with RH, which molecules from the surface (see detailed description in the Sup- can lead to a more pronounced slope for low RH) and is remark- porting Information). The overall time scale for the meniscus ably similar to the shape of the domain width vs RH depicted formation is of the order of several milliseconds,[39] which is in in Figure 3b, suggesting a link between the two. Each of these the limit of the total time spent in our experiments to move the curves consists of three regions corresponding to different range tip one single step of Δx = 100 nm at the lowest writing speed of of RH: a first region of steep increase in adsorbate thickness/ 12.5 μm s−1. On the other hand, further increase of water neck domain width as a function of humidity, a second region with volume until saturation is observed to take up to hundreds of sec- a plateau, and a third region where adsorbates layer thickness onds,[40,41] which is well above the total time used to write a full and domain size grow again. This trend is depicted as well in the line of 4 μm, which takes 320 ms at the lowest speed. This fact growth rate coefficient as a function of RH. rules out the influence of water meniscus on the growth rate and The low-humidity range in our FE dynamic lithography size of our ferroelectric domains since in almost all our experi- experiments comprises RH between 2.5% and 10% RH. Pre- ments in dynamic conditions, even the water neck is formed, it vious studies established that hydroxyl (OH) groups available doesn’t have time to grow to saturation. Moreover, low influence from water-splitting reactions[2,3,6] get strongly chemisorbed of the water meniscus is also expected in the electric field distri- onto the surface immediately after exposing the sample to the bution around the AFM tip, as shown by previous simulations.[18] ambient conditions from the vacuum preparation chamber. Still, the water meniscus is a clear source of ionic charges Therefore, our starting point is an already hydroxylated sur- such as protons and hydroxil groups among others, since a por- face, even in the lowest RH conditions with a sub-monolayer tion of water molecules will dissociate under the application water layer below RH ≈ 10%,[48] indicating that in this regime of high voltages, thus becoming the closer sink of screening the surface water coverage is discontinuous at best. Water agents. However, the efficiency on surface polarization screening molecules in the sub-monolayer up to the first monolayer state by ions generated or concentrated at the water neck, similarly to are tightly bound to the surface (Figure 3b, Region I), leading Dip Pen nanolithography, will strongly depend on the transport to the formation of a highly structured water layer with very rates of charges through the neck and more specifically their low dielectric constant[24,49,50] which could reach values around diffusion rate at the surface. The obtained relationship of the ε ≈ 2, and where rotation or diffusion of water molecules is Adv. Electron. Mater. 2022, 8, 2100650 2100650 (5 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de the hydroxylated substrate and adsorb in a less ordered manner, such that some water molecules are becoming mobile on the surface and have a structure similar to the bulk liquid water. Therefore, the plateau between 10% and 20% RH represents coexistence between two water-growth structures, schematicaly represented in the Figure 4, with different kinetic behavior: 1) highly ordered, surface immobilized water and 2) mobile, liquid-like water molecules. Moreover, the barrier for protonic mobility by hoping is decreasing with increasing RH, giving the overall rise of the protonic conductivity of thicker water layers.[53,54] Therefore, higher protonic conductivity together with increased mobility and orientational freedom of water molecules in this region might play crucial role in the polariza- tion screening, finally enabling successful writing of ferroelec- tric domains even when high speeds are used. At higher RH, i.e., above 20%, water forms a highly mobile liquid structure that characterizes region III in Figure 3b. Pre- vious studies of domain size for different pulse duration and RH show similar monotonic increase in the range of RH from Figure 4. Schematic representation of the water structures expected on 28% to 65%.[18] The most natural explanation for this behavior BTO surface as a function of increasing RH. I) Highly structured ice-like water layer where hydrogen bonds are formed between chemisorbed OH is that, once a continuous layer of liquid-like water has con- groups and upcoming water molecules (represented by dashed lines). densed on the entire surface of the ferroelectric film, ions can This immobilized ice-like water layer is characterized by low dielectric easily migrate to assist the screening process. The approxi- constant of water molecules and hindered mobility of water and ionic mately linear relationship between domain size and RH for any species; II) Coexistence between highly structured and liquid-like water given scanning speed (range III in Figure 3b) (or, equivalently, growth structures with a higher dielectric constant and therefore higher for any given switching time[18]) would be a direct evidence of capacity for dipolar screening but still limited ionic mobility; III. Stabiliza- tion of liquid-like water layers with maximized ionic mobility. the increasing thickness of the transport layer (the liquid), since conductivity (ionic, in this case) is linearly proportional to the very limited.[51] Therefore, in this highly structured water layer thickness of the conductor. Therefore, for RH above 10% we can dipolar screening by water molecules reorientation is negli- consider that the dominating screening mechanisms are driven gible. On the other hand, mobility of ionic charges mediated by by the progressively increase of surface adsorbate layer such as hoping of H+ and OH− through the chain of water molecules water and associated improve of the conductivity and difussion bound by H-bonds is also strongly affected by water structure of its dissociation products such as H+, OH−, and H3O+ ions. and therefore RH.[52] Ab initio molecular simulations con- firmed that proton transfer rate substantially decreases going from a liquid-like multilayer to the rigid monolayer coverage on 2.2. Effect of the Relative Humidity on the Transversal a ZnO surface, following the associated change on the water Domain Wall Dynamics structure.[53] In Figure 4 are schematicaly represented different water structures that can be expected on BTO surface as a func- If we now consider the dynamics in the transversal axis, we can tion of increasing RH. calculate the domain growth rate of the domains perpendicular In this regime, polarization screening mechanisms assisting to the writing direction. To do so, we multiply the domain FE polarization switching and stabilization, mediated by the width by the tip scanning speed to obtain an average growth change of the chemisorbed layer and/or redistribution of ionic velocity v = w ·S, describing the domain wall dynamics per unit charges in the area of newly formed domain, will be highly of linear nanometer as a function of RH. The domain growth innefficient due to the limited water dipoles and ionic mobility velocity in creep regimes depends on the domain wall motion, in this low RH range. Moreover, existence of incomplete mon- which has been traditionally described as governed by wall pin- olayer of water could additionally affect the already limited ning in defect sites.[18,55] The motion of domain walls governed mobility of ionic charges over longer ranges. The experimental by creep processes is described by the velocity of observed lat- evidence indicates that, at sufficiently fast scanning speeds, eral domain expansion taking the form neither of these processes is fast enough to keep up with the screening demands of a switched polarization, and probably  U E µ ν = ν ∞ exp  − a  c   (2) writing efficiency is limited by domain nucleation and internal  kT  E   screening mechanisms. As humidity goes up, the increased mobility of ionic species in liquid-like water layers favors a where μ is the dynamical exponent, Ua is the characteristic faster screening-charge dynamics.[23,52] activation energy, Ec >> E is the critical electric field, T is the The region between 10% and 20% RH is characterized temperature and k is the Boltzmann’s constant. If we consider by stagnation of the initial growth of domain width vs RH that E ≈ 1/w, then Equation (2) can be simplified and we obtain (Figure 3b, region II). After the first monolayer of water is the following logarithmic relationship for the domain growth formed, subsequent water layers feel weaker influence from velocityν and the final domain size w[56] Adv. Electron. Mater. 2022, 8, 2100650 2100650 (6 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Figure 5. a) Exponential dependence of the domain growth velocity along the transversal axial direction on the domain width for different relative humidities. The continuous lines correspond to the fitting of the data with Equation (3), using the dynamic coefficient μ plotted in (b) for the different curves. The fittings share the same K values for all RH. The inset shows the scheme of the two axes where dynamics have been studied. ln ν = ln ν ∞ − K ⋅ w µ (3) caused by the restricted dipolar screening and/or low mobility of ionic species in this low RH conditions. In this framework, deviations of the dynamical exponent from For a given writing speed (i.e., electric field dwell time μ = 1, corresponding the the domain wall motion in a defect-free in dynamic conditions), we identify three distinct dynamic crystal lattice, have been explained by a creep-like motion of the regimes as a function of RH: i) a low humidity regime below domain boundary. Figure 5a shows the fitting of Equation (3) to 10% RH, characterized by a non-linear growth of the domain the experimental data, with the dynamical exponents plotted in width as a function of RH; ii) a transition regime between 10% Figure 5b as a function of the RH. In this case, a stiff increase and 20%, where domain width and growth coefficients as a of the dynamical exponent with increasing RH from 0% to 10% function of RH meets a plateau; and iii) a high humidity regime is observed, followed by an abrupt change between 10% and above 20% RH with an almost linear growth in domain width 20% of RH, until a plateau is finally reached for RH ≥ 40%. vs RH. Quantification of AP-XPS measurements performed on The enhanced mobility of the screening charges as a func- BTO surfaces in controlled RH conditions enabled us to con- tion of RH as discussed in the previous section seems to have a struct the adsorption isotherm for water in the low to interme- clear impact on the dynamical exponent μ describing the creep diate RH ranges. Noting that the size of the written domains motion of ferroelectric domain walls and domains as observed as a function of RH and at fixed speed follows the character- in Figure 5b. Traditionally, the divergence of the growth rate of istic behavior of water adsorption in different RH conditions, domains from a linear behavior given by μ =1 in Equations (2) represented by the adsorption isotherm on BTO and other and (3) has been explained in terms of domain wall pinning oxides,[24,45–47] we propose that these regimes are consistent in bulk defects during growth. For uniform electric fields, the with the initial presence of: ice-like highly structured water observed domain dynamics have been determined as a balance molecules in regime I, which besides providing low level of between the rate of screening charge accumulation and domain dipolar screening also prevent redistribution of surface charges boundary velocity.[57] The observed strong dependence of the by hopping, necessary in attending the screening demands dynamical exponent with RH, especially for RH < 10% suggests (and therefore screening becomes slow in a nominally insu- that albeit the density of defects and pinning sites can be influ- lating ferroelectric film); ice-like and incomplete liquid-like enced by the presence of surface water, there is a direct impact water coexisting in regime II, with a higher dielectric constant of surface conductivity on the domain wall kinetics probably and therefore higher capacity for dipolar screening but still mediated by the dependence of the electrostatic boundary con- limited ionic mobility, and a complete layer of liquid water on ditions on the amount of surface water. In this sense, a more the surface of the film in regime III, which provides efficient general domain wall friction model including the impact of sur- screening via fast ionic mobility along the surface. face water structure on surface ionic conductivity is needed.[58] These findings have further impact on the study of domain wall dynamics. Our measurements demonstrate that the fer- roelectric switching and the domain wall motion are governed 3. Conclusions by two different mechanisms below and above a RH ≈ 10%, even out of thermodynamical equilibrium with environmental To summarize, we find that the critical speed for domain water. For RH<10%, the dominant screening might be provided writing is highly dependent on RH. While for RH above 20% by internal mechanisms due to the low surface conductivity domains can be written at speeds of 0.6 mm s−1 or faster, in dry of charges, with a creep-like lateral motion of domain walls atmospheres with RH below 5% the critical writing speed slows pinned by internal defects. For RH>10%, external screening down to below 100 µm s−1. The size of written line domains by surface charges becomes more efficient for the stabilization depends logarithmically on the writing speed, showing higher of ferroelectric switched domains, and ferroelectric switching growth rate coefficient φ, and therefore higher sensitivity of dynamics depend on the surface charge conductivity, which domain size to the writing speed, for the lowest measured RH, is modulated not only by the presence of water but also by its Adv. Electron. Mater. 2022, 8, 2100650 2100650 (7 of 9) © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2022, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202100650 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de structure when isothermally adsorbed on a surface, leading to [2] J. L. Wang, F. Gaillard, A. Pancotti, B. Gautier, G. Niu, B. Vilquin, increased levels of conductivity with water thickness. In this V. Pillard, G. L. M. P. Rodrigues, N. Barrett, J. Phys. Chem. C 2012, regime, the dynamics of the domain wall motion should be 116, 21802. [3] X. Li, B. Wang, T. Y. Zhang, Y. Su, J. Phys. Chem. C 2014, 118, 15910. revisited to account for the water induced change in the electro- [4] H. Lee, T. H. 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