Research Article www.small-journal.com Tunable Molecular Electrodes for Bistable Polarization Screening Irena Spasojevic,* José Santiso, José Manuel Caicedo, Gustau Catalan, and Neus Domingo* 1. Introduction The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, The polar discontinuity at ferroelectric screening is done by the electrodes, while in bare ferroelectric surfaces it is surfaces and concomitant presence of a typically accomplished by atmospheric adsorbates. Although chemisorbed detrimental depolarization field can be mit- species can have even better screening efficiency than conventional elec- igated in several ways. One is the forma- tion of domains. Another is the provision trodes, they are subject to unpredictable environmental fluctuations and, of screening charge, supplied internally in moreover, dominant charged species favor one polarity over the opposite. the form of free charge-carriers, externally This paper proposes a new screening concept, namely surface functionaliza- by metallic electrodes, or, in the case of tion with resonance-hybrid molecules, which combines the predictability bare (electrode-free samples), by adsorbate and bipolarity of conventional electrodes with the screening efficiency of species (Figure 1a). External screening by adsorbates coming from the atmosphere adsorbates. Thin films of barium titanate (BaTiO3) coated with resonant (such as H2O and O2)[1–5] has been shown para-aminobenzoic acid (pABA) display increased coercivity for both signs of to be more effective in stabilizing the fer- ferroelectric polarization irrespective of the molecular layer thickness, thanks roelectric phase compared to metallic elec- to the ability of these molecules to swap between different electronic configu- trodes with finite screening length.[6] rations and adapt their surface charge density to the screening needs of the Atmospheric adsorbates cannot be ferroelectric underneath. Because electron delocalization is only in the vertical considered the most optimal screening species because: i) Their composition direction, unlike conventional metals, chemical electrodes allow writing local- and adsorption is difficult to control and ized domains of different polarity underneath the same electrode. In addition, predict, sometimes leading to undesir- hybrid capacitors composed of graphene/pABA/ferroelectric have been made able effects such as sample degradation,[7] with enhanced coercivity compared to pure graphene-electode capacitors. dependence on sample’s history[8] or polarization switching[4,5,9–11] and ii) The charge sign or preferred dipole orientation of screening species tends to stabilize one ferroelectric polarity over the opposite, i.e., screening species I. Spasojevic, J. Santiso, J. M. Caicedo, G. Catalan, N. Domingo[+] can be considered as unipolar, requiring the dipole rotation and Catalan Institute of Nanoscience and Nanotechnology (ICN2) change of the interfacial chemical bond upon switching.[12] CSIC and BIST Campus UAB, Bellaterra, Barcelona 08193, Spain In view of this, it is desirable to find a screening mechanism E-mail: Irena.Spasojevic@uab.cat; ndomingo@ornl.gov that combines the screening efficiency of chemical adsorbates I. Spasojevic with the predictability and bipolar screening ability of metallic Department of Chemistry electrodes. The strategy we propose is the chemical modifica- Universitat Autònoma de Barcelona tion of ferroelectric surfaces by functionalization with optimal Bellaterra, Barcelona 08193, Spain screening molecules. In order to be considered optimal, such G. Catalan ICREA- Institució Catalana de Recerca i Estudis Avançats “chemical electrodes” have to meet several requirements: i) to Catalonia, Barcelona 08010, Catalonia provide efficient electrical screening of depolarization fields, so The ORCID identification number(s) for the author(s) of this article that ferroelectric polarization is stable, ii) to be able to adapt can be found under https://doi.org/10.1002/smll.202207799. to the change in screening needs upon polarization reversal, so that polarization can be switched. In addition, and from a © 2023 The Authors. Small published by Wiley-VCH GmbH. This is an practical point of view, the availability of the material and its open access article under the terms of the C ­ reative Commons Attribu- suitability for nanofabrication must be considered. As will be tion-NonCommercial License, which permits use, distribution and repro- shown, all these criteria are met by para-aminobenzoic acid. duction in any medium, provided the original work is properly cited and is not used for commercial purposes. In this work we demonstrate the effective bipolar stabiliza- tion of ferroelectricity in hybrid organic–ferroelectric hetero­ [+]Present structures based on the functionalization of BaTiO3 (BTO) address: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA with pABA molecules. pABA molecules exhibit high degree of delocalization of electrons throughout the benzene ring and its DOI: 10.1002/smll.202207799 carboxylic and amino group substituents.[13] In addition, pABA Small 2023, 19, 2207799 2207799 (1 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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.small-journal.com Figure 1. a) Screening mechanisms of a monodomain ferroelectric material, which include (from left to right) internal screening by free charge car- riers and external screening by metallic electrodes or atmospheric adsorbates. The last one is screening by molecules with a delocalized electronic system able to internally relocate the electrons, i.e., chemical electrodes. Electrons’ relocation should be highly sensitive and specific to the polarization direction and bound polarization charge of the underneath ferroelectric substrate; b) some resonance structures (electronic configurations) of pABA molecule proposed in this work, contributing to the resonance hybrid. A blue cloud represents the location of negative charge carriers. molecules possess distinct configurations (“chemical reso- entated SrTiO3 (STO) single crystalline substrates (CrysTec nance”) whereby the electrons can be internally relocated along GmbH). Growth parameters and detailed preparation proce- the out-of-plane direction and thus tune the surface charge dure can be found elsewhere.[4] The structural characteriza- density without changing their chemical configuration/surface tion of BTO/SRO bilayers is shown in Figure S1 (Supporting bonding as shown in (Figure 1b). The free amino (–NH2) group Information). is an electron donor able to couple its lone electron pair with the Controlled nanopatterning of BTO thin films by pABA mole- high electron density of the benzene ring and further with the cules was performed by microcontact printing (µCP) method, lone electron pairs of oxygen atoms in the carboxylic (COOH) by brining polydimethylsiloxane (PDMS) stamp inked by pABA group,[14] enabling electrons to be fully delocalized. The structure molecules into conformal contact with BTO thin film as shown of pABA delocalized electronic system is depicted by the reso- in Figure 2a. The atomic force microscopy (AFM) topography nance hybrid,[13,15] representing weighted averaged contribution of pABA molecules on micropatterned BTO sample is shown in of several resonance structures with distinct electrons’ distribu- Figure 2b. From the topography image, the presence of mole- tion within the molecule which, as shown in Figure 1, may act as cules in the perimeter of the square-like reliefs is evident, how- a counterweight to the bipolar depolarization fields of ferroelec- ever it is not clear whether there are some molecular layers also trics: upward polarization should attract the delocalized electrons inside or outside the square-like patterns. In order to determine towards the BTO surface; conversely, downward polarization the presence of pABA molecules on BTO surfaces we employed should repel the delocalized electrons further from the surface Kelvin Probe Force Microscopy (KPFM) and Amplitude Modu- towards the benzene ring. Furthermore, we probe the screening lated Frequency Modulated (AM-FM) Viscoelastic Mapping. efficiency of pABA molecules in capacitor-like geometry, build up The contact potential difference image obtained by KPFM is from pABA functionalized BTO thin films sandwiched between plotted in Figure 2c. The observed contrast between inner and SrRuO3 (SRO) bottom and multilayer graphene top electrode. outer regions of square-like features is consistent with different chemical composition of these two areas of the surface, corre- sponding to bare BTO surface and pABA-functionalized BTO 2. Results and Discussion surface. In addition, the Young’s modulus and indentation of the material was obtained from AM-FM viscoelastic mapping and 2.1. Functionalization of BTO Thin Films with pABA Molecules shown in Figure 2d,e, respectively. The area surrounding the square-like features exhibits lower Young modulus (Figure 2d) Epitaxial ferroelectric BTO/SRO bilayers were grown by Pulsed and higher indentation values as compared to their inner area, Laser Deposition (PLD) technique on as received (100) ori- indicating the presence of soft molecules compared to bare Small 2023, 19, 2207799 2207799 (2 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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.small-journal.com Figure 2. a) Schematic representation of microcontact printing process (top); AFM imaging of microcontact printed patterns of pABA molecules on a BTO thin film (bottom) such as b) AFM tapping mode topography; c) contact potential difference as measured by KPFM; d) Young’s modulus and e) indentation obtained by AM-FM Viscoelastic Mapping. AFM images have dimensions of 45 µm × 45 µm. Detailed explanation of images can be found in the main text. BTO. The evidence thus indicates that the BTO surface inside used Piezoresponse Force Microscopy (PFM). Figure 3 shows the squares is bare while pABA molecules cover the rest of the topography, PFM phase images and PFM hysteresis loops (of film around the patterned squares. Furthermore, infrared char- PFM phase as a function of voltage) for as-grown BTO film acterization indicates that pABA molecules chemically attach to with naturally present atmospheric adsorbates (Figure 3a–c), the BTO surface by forming ester-like monodentate linkage with non-functionalized (i.e., bare) area of BTO film (Figure 3d–f) Ti atoms (Figure S2, Supporting Information), which is in agree- and pABA-functionalized area of the same BTO thin film ment with studies of pABA functionalized TiO2 surfaces.[16,17] (Figure 3g–i). A defining characteristic of ferroelectric materials is that their spontaneous ferroelectric polarization can be reversed 2.2. Coupling of pABA Molecules and Ferroelectric Polarization by the application of an external electric field. Figure 3b in BTO Thin Films shows the PFM phase image of electrically-written ferroelec- tric domains. In this case ferroelectric lithography is done by In order to explore the impact of pABA surface-chemical modi- applying a DC voltage of +8 and −8 V to the tip while scan- fication on the ferroelectric properties of BTO thin films, we ning in contact over square like areas denoted by black lines, Small 2023, 19, 2207799 2207799 (3 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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.small-journal.com Figure 3. Ferroelectric characterization of as-grown BTO film with naturally present atmospheric adsorbates, non-functionalized (bare) area of BTO film and pABA-functionalized area of the same BTO film; AFM topography image of a) as-grown BTO, d) non-functionalized and g) pABA-functionalized area of the same BTO thin film; PFM phase images of electrically written ferroelectric domains, by applying DC voltages of +8 and −8 V to the AFM tip while scanning areas denoted by black lines in contact with b) as-grown BTO, e) non-functionalized and h) pABA-functionalized area of BTO thin film. From the phase contrast of PFM phase images, it can be seen that BTO film is up-polarized in all three cases; SS-PFM hysteresis loops of PFM phase obtained on c) as-grown, f) non-functionalized and i) pABA-functionalized area of BTO thin film, where a drastic increase of the coercive voltage can be observed; j) Mean coercive voltage V = (|Vc-| + | Vc+|)/2 as a function of pABA layer thickness for two BTO samples. obtaining downward and upward polarized states, respec- measured on the non-functionalized area of pABA-modified tively. Phase-contrast images show that the as-grown BTO BTO thin film do not change significantly as compared to thin film is spontaneously up-polarized, which is typical for as-grown BTO as shown in Figure 3e,f. Finally, pABA-func- BTO films grown on SRO-buffered STO substrates.[6] The tionalized area of BTO film keeps its as-grown up-polarized hysteresis loop of the as-grown BTO sample screened by state (Figure 3h), however, a drastic widening of PFM hys- atmospheric adsorbates reveals a mean coercive voltage of teresis loops of the hybrid organic–inorganic pABA/BTO V = 1 V and Vbias = +0.6 V. The observed asymmetry of the heterostructure is observed, with a substantial enhancement loop, shifted to positive voltages, agrees with the preferential of the mean coercivity up to V = 6.1 V and Vbias = +1.5 V up-polarized state of the BTO film. Ferroelectric properties (Figure 3i,j). Small 2023, 19, 2207799 2207799 (4 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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.small-journal.com First, we consider whether the observed enhancement of down-polarized BTO thin films will repel negative charges in the coercive field may be caused by a trivial passive-layer effect, the vicinity of the surface and delocalize the electrons inside whereby the pABA molecules act as a layer of low dielectric of benzene ring in order to achieve more positive ferroelectric constant connected in series with the underneath ferroelectric. screening (Figure 4 right). The presence of such passive layer would affect the electric It has been reported that inorganic molecules physisorbed field sensed by the sample and therefore the measured coercive on ferroelectric surfaces such as NH3 are able to rotate upon voltage, which would be linearly proportional to the thickness of polarization reversal and therefore bring their electropositive the passive layer. Figure 3g shows the dependence of the mean (hydrogen) or electronegative (nitrogen) atoms closer or fur- coercive voltage (Vc = |Vc-| + | Vc+|)/2) on the pABA molecular ther from ferroelectric surfaces.[12] However, in our case pABA layer thickness as measured by switching spectroscopy PFM molecules are chemisorbed, which makes it impossible for them (SS-PFM) for two BTO samples. For both samples, hysteresis to rotate; instead it is the internal electronic density that moves loops were probed in several different positions corresponding along delocalized bonds in order to compensate the bound to different thicknesses of the pABA molecular layer as meas- charge of the ferroelectric surface, thus resulting in a more ured from AFM topography height profiles (Figure S3, Sup- robust ferroelectric polarization. This result is further supported porting Information). Figure 3g shows that the coercive voltage by Kelvin Probe Force Microscopy measurements of pABA-func- is essentially the same for both samples and independent of tionalized up- and down-polarized BTO thin films showing the the pABA layer thickness. Hence pABA molecules do not con- different polarity of the charge concentration close to the mole- tribute to an increase of the coercive voltage as a dielectric “pas- cular surface for both cases (Figure S5, Supporting Information). sive layer”. Since this screening mechanism comprises fast internal Instead, the increased coercive voltage is consistent with an relocation of delocalized electrons instead of slow dipole reori- enhancement of the stability of both up and down polarization entation, we expect a faster speed at which ferroelectric polari- states thanks to an effective suppression of the depolarizing zation can be screened and therefore switched. Moreover, fields.[18] In this context, pABA molecules act as an electroac- as observed from Figure 3e, electron delocalization happens tive layer that is able to supply/take negative charges from the only in the out-of-plane direction (i.e., along the vertical axis surface region as dictated by the polarization of the underneath of pABA molecules), while the in-plane direction remains suf- film, and provide a superior screening for both signs of the fer- ficiently insulating to ensure that the switching happens only roelectric polarization (hence bistability) compared to naturally under the writing tip and not under the entire pABA layer. This present “unipolar” atmospheric adsorbates. offers an advantage of pABA tunable chemical electrodes over In the case of up-polarized ferroelectric state which requires conventional metallic electrodes where the size of the written negative screening charge, delocalized electrons from electro- domains is predetermined by the lateral extension of the top donating amino group and benzene ring are attracted toward electrode and its in-plane conductivity. With molecular-based the carboxylic group and act as additional negative screening screening, one can even write localized domains of different agents in the vicinity of the surface (Figure 4 left). Oppositely, polarity under the same chemical electrode. Figure 4. Schematic representation of ferroelectric polarization screening mediated by two different pABA resonance structures, with distinct delocali- zation of electrons. In the case of up-polarized state (left), pABA molecules delocalize electrons from the electro-donating amino group and benzene ring towards the carboxylic group and act as negative screening agents in the vicinity of the surface, making up-polarized state highly stable. Delocal- ized electrons are represented by red dashed lines. Reversibly, in the case of down-polarized state, all negative charges in the vicinity of the surface are repelled and delocalized inside of the benzene ring. The carboxilic group at the bottom of the molecule thus becomes positively charged, making the down-polarized state highly stable. Thus pABA molecules act as tunable chemical electrodes for bistable ferroelectric polarization screening and stabilization. Small 2023, 19, 2207799 2207799 (5 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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.small-journal.com 2.3. Tunable Electroactive Ferroelectric/pABA/2D Interfaces is strictly local and only senses the piezoelectric deformation arising below the tip, meaning that the obtained hysteresis An hybrid scenario between conventional electrodes and chem- cycle corresponds uniquely to that of the graphene(ML)/pABA/ ical screening arises with the use of 2D electrodes. Consid- BTO/SRO stack under the AFM contact point. The obtained ering graphene as an archetypal 2D electrode material,[12,19] it hysteresis loop (Figure 5c) has almost the same characteristics is hard to control the interface composition and the presence as hysteresis loop measured directly on top of the pABA mole- of intercalated molecules due to the transfer process of the gra- cules as shown in Figure 3f. Therefore, the increased coercive phene itself and also due the ubiquitous presence of surface field remains when the PFM hysteresis cycles are measured adsorbates on ferroelectric surfaces, which moreover strongly using the ML graphene as top electrode, indicating that pABA depend on the polarization state.[2,4,8,20–22] In turn, this can have molecules are effective buffer layers to stabilize ferroelectric an unpredictable effect on the device performance. One way to polarization underneath 2D electrodes such as graphene. overcome this issue is the deliberate chemical functionaliza- tion of the ferroelectric surface, enabling a controlled interfacial chemical composition: since adsorbates are inevitable, let us at 3. Conclusions least choose them ourselves so that they work in our favor. As we have just argued in the previous sections, pABA is a prom- The resonance properties of pABA molecules (i.e., their dis- ising material in this respect, so the practical question emerges: tinct configurations of the electronic charge density within the can we combine the screening ability of pABA molecules with molecule) enable a versatile response to polarization changes in the high in-plane conductivity and mechanical robustness of the ferroelectric surface. The chemical bonding between pABA graphene? In other words, can we intercalate pABA between molecules and the BTO surface results in a robust ferroelectric a graphene electrode and the ferroelectric film, so that the polarization with increased coercive field, thanks to the ability resulting hybrid-electrode capacitor retains the same robust fer- of the pABA molecules to respond to the screening needs of roelectricity observed in the pure pABA case? the ferroelectric by internally rearranging its delocalized elec- In this study, we have transferred multilayer (ML) graphene trons. Contrary to other chemisorbed species, pABA can tune patches obtained by graphite exfoliation to pABA-functionalized its surface charge density without having to change the surface BTO thin films, in order to build hybrid organic–ferroelectric bonding. In this way, pABA molecules provide i) controllable capacitor-like structures with graphene as the 2D top electrode and more predictable chemistry of ferroelectric surfaces and ii) (Figure 5a). Beside other ways for graphene preparation and agile and effective screening for both polarization states com- transfer,[23] mechanical exfoliation[24] was chosen in this work pared to naturally present “unipolar” atmospheric adsorbates. since it doesn’t involve contact of the functionalized substrate In addition, pABA’s electrons are delocalized only along their with any kind of solvents during the graphene transfer and vertical axis, and this allows writing ferroelectric domains only avoids additional chemical modifications that might arise. The in the area under the biased tip, giving them a unique advan- 3D topography image of the deposited pABA molecules cap- tage over the conventional metallic electrodes, which distribute tured underneath ML graphene patch is presented in Figure 5b. the field uniformly across the entire electrode area. Conversely, In order to probe the hysteresis cycle of pABA molecules conventional electrode functionality can be recovered by depos- underneath the graphene patch, an AFM tip was placed in iting graphene on top of the pABA layer. The functionalization contact with it, as depicted in Figure 5a, while voltage was of ferroelectric interfaces using pABA molecules can there- cycled between −7.5 and 7.5 V. In this configuration, while the fore be a useful pathway for the stabilization of the polariza- electric excitation signal in PFM spreads over the whole gra- tion in thin film capacitors. This may be of particular interest phene electrode, the reading of the electromechanical response in the case of ultra-thin films employed in ferroelectric tunnel Figure 5. Capacitor structures of graphene(ML)/pABA/BTO/SRO; a) schematic representation of the different electron configurations that pABA molecules are going through during hysteresis loop measurements over capacitor; b) AFM 3D topography of pABA molecules (red) in contact with ML graphene sheet (blue) altogether deposited on BTO thin films (yellow) and c) SS-PFM hysteresis loops measured through ML graphene top electrode, closely resembling those measured directly on pABA molecules. Small 2023, 19, 2207799 2207799 (6 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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.small-journal.com junctions, whereby the polarization stability of ultra-thin films in amplitude of the second eigenmode A2 and supplied driving voltage could be enhanced. Altogether, the obtained results have a contained information about energy dissipation, while changes in the frequency f2 bear information about elastic tip-sample interaction. Using strong impact in the field of interface engineering of nanode- obtained information about amplitude, phase and frequency shift of vices, whereby hybrid interfaces can be designed to tune the both modes quantitative data such as stiffness, elasticity, dissipation overall device performance. and Young’s modulus could be extracted. It was important to notice that AM-FM Viscoelastic mapping was intermittent-contact mode and was therefore very useful, non-invasive technique for obtaining information about chemical and/or mechanical properties of functionalized samples 4. Experimental Section with high lateral resolution. In this case the value of fundamental SrRuO3(SRO)/BaTiO3 (BTO) Thin Film Preparation: Structural resonance frequency was f1 = 71.8 kHz and that of the second eigenmode properties of as-grown BTO thin films were studied by X-Ray diffraction f2 = 457.0 kHz, giving correct relationship of f2/f1 = 6.4 for the application in theta/2theta (θ/2θ) geometry, whereby θ and 2θ denote incident and of this AFM mode. diffracted angle, respectively. Coupled θ/2θ scans and X-Ray reciprocal Finally, KPFM measurements were employed to study the electronic space map (XRSM) measurements were performed on PANalytical structure of bare and pABA-functionalized BTO surfaces. Measurements X’Pert Pro diffractometer. Results of structural characterization of BTO/ were conducted in two-pass mode. In the first pass topography SRO bilayers are presented in Supporting Information. information was obtained, while in the second pass DC voltage was Microcontact Printing (µCP): Surface functionalization was achieved applied to the tip in order to compensate electric potential between tip by microcontact printing. Polymeric PDMS stamps created by a curing and the sample and to cancel vibration of the cantilever. The obtained process at elevated temperatures were used, which have opposite DC voltage represented contact potential difference (CPD) between tip features to the master template made of silica. Once cured, stamp was and the sample. peeled off and cut to necessary size. In the experiments, 1 cm × 1 cm PDMS stamps were used with square-like features of 5 µm × 5 µm separated by 10 µm. PDMS stamp was inked with 0.1 m pABA solution (pABA, purified by sublimation, ≥ 99%, Sigma–Aldrich) in gamma- Supporting Information butyrolactone (GBL, reagentplus ≥ 99%, Sigma–Aldrich) solvent by Supporting Information is available from the Wiley Online Library or spreading 200 µl droplet of solution over it. This step was necessary from the author. in order to let pABA molecules adsorb onto the PDMS stamps. After 20 min, pABA solution was removed from PDMS surface and slightly dried using N2 flow. The PDMS stamp was then placed in conformal contact with the BTO thin film during 15 min, after which it was carefully Acknowledgements separated from the BTO substrate. After functionalization, the BTO thin film was subjected to thermal annealing on a hot plate at 80 °C during The authors acknowledge N. Aliaga-Alcalde and A. Verdaguer for 30 min in order to evaporate the solvent. Since pABA is hygroscopic, all providing with the PDMS stamps and Prof. M. Lira and Dr. H. Xie for reagents were stored and later used for solution preparation inside of a providing the molecules. Financial support was obtained under projects controlled humidity glove box. from the Spanish Ministerio de Ciencia e Innovacion (MICINN) under Graphene Electrodes Deposition onto pABA-Functionalized BTO projects PID2019-108573GB- C21 and PID2019-109931GB-I00. In addition, Surfaces: In order to build hybrid organic ferroelectric capacitor-like this work was partially funded by 2017-SGR-579 from the Generalitat de structures, ML graphene sheets were used with thickness of about Catalunya. The ICN2 was funded by the CERCA programme / Generalitat 10–20 nm obtained by mechanical exfoliation of highly oriented pyrolytic de Catalunya. The ICN2 was supported by the Severo Ochoa Centres of graphite (HOPG, SPI Supplies) with adhesive tape. During this process, Excellence Programme, funded by the Spanish Research Agency (AEI, normal force was exerted on the HOPG surface in order to overcome grant no. SEV-2017-0706). I.S. acknowledges support of the Secretaria the van der Waals attraction forces and to micromechanically cleave d’Universitats i Recerca – Departament d’Empresa i Coneixement – graphite sheets. Achieving very thin layers required several exfoliation Generalitat de Catalunya and the European Social Fund (ESF) (FI grant steps. After exfoliation, the obtained multilayer graphene sheets were reference 2020 FI_B2 00157). deposited onto pABA-functionalized BTO and used as top electrodes for PFM characterization. Atomic Force Microscopy (AFM) Measurements: AFM imaging was performed on MFP-3D AFM (Asylum Research). In all the experiments, Ptlr5 Conflict of Interest coated PPP-EFM tips (Nanosensors) with a stiffness constant k = 2.8 N/m The authors declare no conflict of interest. were used. The topography of the samples was imaged in tapping mode, while in order to investigate ferroelectric, electronic and mechanical properties of bare and functionalized thin films at the nano scale, different AFM modes were employed such as Piezoresponse Force Microscopy Data Availability Statement (PFM), Kelvin Probe Force Microscopy (KPFM) or Multifrequency AFM The data that support the findings of this study are available from the imaging[25] (MF-AFM) like AM-FM viscoelastic mapping. corresponding author upon reasonable request. In the PFM mode, an AC voltage was applied to a conductive AFM tip used as top mobile electrode and then scanned over the surface in contact mode. The PFM amplitude image of the mechanical vibration of the tip was proportional to the magnitude of the electromechanical Keywords response of the sample, while the PFM phase gave information about the orientation of ferroelectric polarization in the material. BaTiO3 ferroelectric thin films, chemical electrodes, graphene In AM-FM Viscoelastic Mapping fundamental resonance frequency ferroelectric capacitors, molecular functionalization, para-aminobenzoic f1 was operated in amplitude modulation mode (AM) to track the acid (pABA) molecules, polarization screening topography, while the second eigenmode frequency f2 was operated in frequency modulated mode (FM). In this case amplitude of the second Received: December 20, 2022 eigenmode A2 was kept constant by adjusting driving voltage, while the Revised: February 27, 2023 frequency f2 was adjusted in order to keep phase ϕ2 at °90. Changes Published online: April 17, 2023 Small 2023, 19, 2207799 2207799 (7 of 8) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 30, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202207799 by Readcube (Labtiva Inc.), Wiley Online Library on [27/10/2023]. 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